A comprehensive review on the mechanical properties of waste tire rubber concrete

Construction and Building Materials 237 (2020) 117651

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Review

A comprehensive review on the mechanical properties of waste tire rubber concrete Rajeev Roychand a, Rebecca J. Gravina a,⇑, Yan Zhuge b, Xing Ma b, Osama Youssf b, Julie E. Mills b a b

School of Engineering, RMIT University, Melbourne, Australia School of Natural and Built Environments, University of South Australia, Adelaide, Australia

h i g h l i g h t s  More than 100 scientific papers published in last 30 years have been reviewed.  Effect of rubber on 12 different mechanical properties have been studied.  Effect of 20 different rubber treatment methods have been explored.  Presented comparative graphs on the effect of rubber particle sizes and w/c ratios from the database.  Presented a comparative graph on the effect of various rubber treatment methods from the database.

a r t i c l e

i n f o

Article history: Received 19 July 2019 Received in revised form 15 November 2019 Accepted 18 November 2019

Keywords: Waste tyre Crumb rubber Rubber particle size Rubber content Rubber treatment Rubber concrete

a b s t r a c t Recycling of ‘End of Life Tyres’ (ELT) is one of the major environmental concerns faced by the scientific community and the government organisations, worldwide. Every year, an estimated one billion tyres reach their end of life, out of which only about 50% are currently being recycled and the remaining form part of the landfills. Therefore, there is an urgent need to improve the existing and develop new applications of recycled tyre products to address this shortfall in the utilisation rate of the ELT. One application which is actively being researched is the use of waste tyre rubber as a partial replacement of conventional aggregates in concrete applications. Although it shows tremendous potential, it comes with its own challenges such as weak inherent strength of the rubber and poor bond performance with the cement matrix, which hinders its use as an aggregate in large quantities. To overcome this challenge, researchers have looked at various rubber treatment methods that not only improve the bond performance but also significantly improve the mechanical properties of rubber concrete. This review paper considers the effect of rubber particle size, percentage replacement and various treatment methods on different mechanical properties of rubber concrete, studied over the last 30 years. However, to be accepted by the concrete industry, the researchers have to come up with a rubber treatment method that can address the concerns of high flammability and the resultant release of noxious gases from the rubber particles, when exposed to fire. Ó 2019 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Research methodology and a snapshot of literature review database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties and shredding cost of waste tyre rubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Chemical composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Categories and physical properties of shredded rubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 3 3 3 4

⇑ Corresponding author. E-mail addresses: [email protected] (R. Roychand), [email protected] (R.J. Gravina), [email protected] (Y. Zhuge), [email protected] (X. Ma), [email protected] (O. Youssf), [email protected] (J.E. Mills). https://doi.org/10.1016/j.conbuildmat.2019.117651 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

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R. Roychand et al. / Construction and Building Materials 237 (2020) 117651

4.

5.

3.3. Processing and shredding cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Properties of rubber concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Fresh concrete (Workability) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.2. Hardened concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2.1. Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2.2. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.1. Workability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.2. Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.3. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.4. Flexural, split tensile strength and modulus of elasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.5. Modulus of rigidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.6. Abrasion resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.7. Fatigue life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.8. Fracture energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.9. Fracture toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.10. Micro/Macro cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.11. Static impact resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.12. Dynamic impact resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.13. Bond behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1. Introduction As per the 2018 world bank report ‘‘What a Waste 2.0: A Global Snapshot of Solid Waste Management to 2050” [1] global annual waste generation is expected to grow by 70 percent to 3.4 billion tonnes over the next 30 years, up from 2.01 billion tonnes in 2016. Globally, it has been recognized that the make, use and throw model is unsustainable and has detrimental impact on the economic, environmental and public health fronts. Realising this concern the government organisations, private stakeholders and the scientific community have joined hands to look for the scientific solutions for the recycling of all forms of waste materials that can support closed loop circular economy. Recycling solutions for various forms of waste materials are currently being investigated [2–6] that not only would create new business and employment but would also help in minimizing the generation of waste materials. Recycling of the ‘end of life tyres’ (ELT) is one of the major concerns shared by the scientific community and the environmental organisations because of their large volume of production and non-biodegradable properties. It is estimated that worldwide, about one billion end of life tyres are produced annually [7]. Of these, <50% are recycled and the remaining form part of landfills [8]. They not only consume valued space in landfills but also pose dangers of accidental fires, leaching of toxic substances into the ground and act as a breeding ground for mosquitoes. Therefore, governments around the world are introducing schemes and regulations to encourage the recycling of ELT to improve their utilization rate. With extensive research on the use of ELT, various applications have been discovered. They include: retreading of ELT; manufacture of rubber-molded products; tyre pyrolysis to produce carbon black and oil/gas that can be used as a fuel; use as an alternate fuel in cement kilns; in geotechnical applications such as sub-grade fill in roads and embankments; in rubber modified asphalt pavements and as partial replacement of aggregates in concrete [9,10]. Although the use of waste tyre rubber provides great potential as a fine and coarse aggregate replacement material, it poses a significant challenge in the performance of its bond behavior within the cement matrix [11,12]. Poor bond performance of rubber

particles with the cement paste results in a significant reduction in its mechanical [13,14] and potentially its durability properties [15,16]. To overcome this challenge, researchers have looked at various ways to improve the bond performance of rubber particles and to improve the mechanical and durability properties of rubber concrete. They have studied the effect of (i) particle size of rubber [17,18], (ii) percentage of rubber content [19,20], and the treatment of the rubber particles by/with (iii) water washing [21,22],

Table 1 Research methodology. Databases Searched

Google Scholar, Scopus, Science Direct, Web of science

Keywords

Rubber and ‘‘tyre or tire” and ‘‘cement or concrete” 1990–2018 English Original research articles, conference papers, review papers, reports  All publications showing under the search results, using the above selection criteria, up to the last page of the search index were downloaded  A database was created in excel using the following: paper title, year published, crumb rubber particle size, replacement levels, w/c ratio, rubber treatment methods used, fresh and hardened concrete properties studied.  Priority was given to a journal article where similar studies were reported by various authors in a journal and conference paper  Where there were > 3 articles available on the same type of treatment and properties studied, an oldest, a mid-range and a latest article was selected, having the highest citations  Studies on asphaltic concrete were excluded

Year range Language Types of publications Criteria used to produce the preliminary database for initial review

Criteria used to shortlist the final database for a detailed comprehensive review that is presented in this manuscript

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R. Roychand et al. / Construction and Building Materials 237 (2020) 117651 Table 2 Basic composition of tyre rubber [4].

Table 3 Potential tyre processing and shredding costs in Canada [10].

Material

Main ingredients

Composition

Particle size

Cost per tonne

Processing rate (tonnes/hour)

Rubber Reinforcing agent Softener

Natural rubber, synthetic rubber Carbon black, silica Petroleum process oil, petroleum synthetic resin, etc. Sulphur, organic vulcanizers Thiazole accelerators, sulfenic amide accelerator Zinc oxide, stearic acid

51% 25% 19.5%

>5 cm <5 cm <1.25 cm

$12 $31 $31 - $68

10–12 7 2–3

Amine antioxidants, phenol antioxidants, wax Calcium carbonate, clay

1.5%

Vulcanizing agent Vulcanizing accelerator Vulcanizing accelerator aid Antioxidant Filler

1.0% 1.5% 0.5%

(iv) water soaking [23], (v) cement paste and mortar coating [21], (vi) NaOH [22,24–26], (vii) Silane coupling agent [11,27], (viii) polyvinyl alcohol (PVA) [26,28], (ix) partial oxidation [29,30], (x) organic sulfur compounds [31], (xi) acrylic acid and polyethylene glycol [32], (xii) UV [33] and gamma [34] radiation, (xiii) solvents like methanol, ethanol and acetone [35], (xiv) CSBR latex [36], and (xv) KMnO4 & NaHSO3 [37], (xvi) heat treatment [38], (xvii) acid treatments with H2SO4 [22,39], HCl [40], HNO3 [41], CH3COOH [39,40], (xviii) Ca(OH)2 [39], (xix) CaCl2 [22], (xx) H2O2 [22], (xxi) CS2 [42]. The mechanical and durability properties of rubber concrete that have been investigated to date have included: (i) workability [17,19,43], (ii) bulk density [17,19], (iii) compressive strength [19,21,24,29,44], (iv) flexural strength [24,29,45], (v) split tensile strength [19,21,29,46], (vi) modulus of elasticity [24,47,48], (vii) modulus of rigidity [21], (viii) abrasion resistance [12,24], (ix) fatigue life [43,49,50], (x) fracture energy and toughness [19,24,51], (xi) crack resistance [51–53] (xii) impact resistance [32,47,51,54] (xiii) bond behavior [55,56], (xiv) water absorption [24,46,57,58], (xv) porosity [16,21], (xvi) chloride ion permeability [36,54,59], (xvii) carbonation [16,59] (xviii) shrinkage [48,60,61] and expansion [48], (xix) freeze thaw resistance [18,19,62], (xx)

acid/sulfate attack [16,63], (xxi) effect of sea water [64], (xxii) corrosion performance [65], (xxiii) thermal conductivity [28,64] and acoustic [28,66] properties and (xxiv) electrical resistivity [67]. Although, there have been a few review papers published in the past that cover the effect of some rubber treatment methods on the mechanical and durability properties, they do not cover all the treatment methods and all of the mechanical and durability properties studied so far [68–74]. Therefore, to address this gap in the review of available literature on ELT rubber concrete, this paper covers all of the various rubber treatment methods and their effect on the mechanical properties of crumbed rubber concrete studied in the last 30 years.

2. Research methodology and a snapshot of literature review database Table 1 provides the methodology used in compiling the database for the review of literature.

3. Properties and shredding cost of waste tyre rubber 3.1. Chemical composition Table 2 outlines the chemical composition of waste tyre rubber showing various constituent materials with their corresponding options of main ingredients and their percentage material compositions.

Fig. 1. Different stages of tyre shredding (modified image from original Source [77,78]).

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R. Roychand et al. / Construction and Building Materials 237 (2020) 117651

3.2. Categories and physical properties of shredded rubber Recycled tyres that are shredded to be used as aggregates in cement concrete are classified into three categories: (i) shredded or chipped rubber, used as coarse aggregate, is produced in a two stage process in which the primary stage shreds the tyre rubber to a size of 300–460 mm long by 100– 230 mm wide, followed by the secondary stage that produces particle sizes ranging between 13 mm and 76 mm [75] (ii) crumb rubber, used as fine aggregate, is produced by two methods: (a) at ambient temperature using cracker mills and (b) at temperatures less than 80 °C using liquid nitrogen by a cryogenic process, to produce particle sizes ranging from 4.75 mm to < 0.075 mm [75,76] (iii) fine ground rubber, used as very fine aggregate, of particle size ranging from 0.5 mm to as small as 0.075 mm, produced using micro milling or wet grinding process [75,76]. Fig. 1 shows the different stages of tyre shredding. 3.3. Processing and shredding cost The cost of processing and shredding of scrap tyres involves labour, power, equipment and its maintenance. The smaller the particle size, the higher is the cost associated with its production [10]. Table 3 summarises the potential shredding cost based on particle size in Canada, with costs in other countries likely to be comparable/proportional. 4. Properties of rubber concrete 4.1. Fresh concrete (Workability) Workability is a very important property of fresh concrete that has a significant impact on its final strength. It largely depends upon the properties of the raw material used in the concrete mix design. Workability of a rubber concrete mix decreases with the increase in rubber content [19,20,51,59,79], however conflicting results about the rubber particle size effect have been reported by various researchers. Khatib and Bayomy [20] reported that the workability of rubber concrete decreases with the decrease in rubber particle size because of the increase in surface area of the angular sized particles. Similar results were reported by Su et al. [80]. Interestingly, Eldin and Senouci [19] and Reda Taha et al. [51] observed a totally contradictory results. They found that the the workability of rubber concrete decreases with the increase in particle size because of the increase in friction between the angular rubber particles, that also reduces the flowability of the larger sized rubber particles [81]. Grinding of tyre rubber also plays an important role in influencing the workability of concrete. The mechanically ground rubber aggregates provide higher surface area and roughness to the rubber particles, thereby showing lower slump values compared to that of the cryogenic ground rubber [20,59]. However, lower workability of the rubber concrete can be improved with the addition of an appropriate quantity of superplasticizer and its required quantity increases with the increase in rubber content [82]. Soaking of rubber particles in water for 24 hrs also shows a positive effect on the workability of rubber concrete, possibly because of the adsorbed water helping in the interparticle movement of the rubber particles relative to the other elements in concrete [43]. The treatment of rubber with a chemical blend of anhydrous ethanol, acrylic acid and polyethylene glycol was shown to provide a significant improvement in the workability of the treated rubber con-

Table 4 Effect of various rubber treatment methods on the workability of fresh concrete. Rubber treatment method

Workability

References

Particle Size

Decreases with the decrease in particle size Decreases with the increase in particle size Decreases with the increase in percentage replacement Increases workability Increases workability

[20,80,88,89]

Particle Size

Percentage replacement

Soaking of rubber particles Treatment with a chemical blend of anhydrous ethanol, acrylic acid and polyethylene glycol Saturated NaOH for 30 min followed by water wash and drying 10% NaOH for 120 min followed by water wash and drying 10% NaOH for 120 min followed by water wash and drying Saturated NaOH for 30 min followed by water wash and drying Treatment with H2O2 Treatment with CaCl2 Treatment with KMNO4 & NaHSO4 Coating of rubber particles with potassium permanganate Coating of rubber particles with cement Coating of rubber particles with silica fume 10% H2SO4 for 120 min followed by water wash and drying 95% H2SO4 for 1 min followed by water wash and drying 35% H2SO4 for 24 h followed by water wash and drying 32% H2SO4 for 1 h followed by water wash and drying Treatment with SCA Treatment with SCA Treatment with CS2 Saturated Ca(OH)2 solution 32% acetic acid solution Ultra violet (UV) – A radiation

[19,51]

[19,20,51,59,79,88– 93] [43] [32]

No considerable change

[46]

No considerable change

[85]

Decreases workability

[22,84]

Decreases workability

[39]

Decreases workability Decreases workability Decreases workability

[22] [22] [22]

No considerable change

[85]

No considerable change Decreases workability

[85] [85]

Increases workability

[85]

Increases workability

[86]

Decreases workability

[22]

Decreases workability

[39]

Decreases workability No considerable change Decreases workability Decreases workability Decreases workability Increases workability

[87] [54] [42] [39] [39] [86]

crete compared to that of the untreated rubber concrete. This improvement in workability is attributed to the water reducing effect of the modifier that has a similar molecular structure to that of polycarboxylate based water reducer [32]. NaOH has also been investigated by some researchers to improve the mechanical properties of the concrete [21,22,46,83– 85] but its effect on the workability of concrete varied among these researchers. Marques et al. [46] observed that treating the rubber by soaking it in saturated NaOH followed by water washing did not bring about any considerable change in the workability of rubber concrete, compared to that of the untreated rubber concrete. Similar observations were reported by Kashani et al. [85]. However, the studies conducted by Youssf et al. [22,84] and MuñozSánchez et al. [39] reported a decrease in workability. The rubber content used by Marques et al. [46] and Kashani et al. [85] was 10 and 12 vol% compared to that used by Youssf et al. [22,84] was 20%. This indicates that the negative effect of NaOH treatment gets more pronounced at higher replacement levels.

R. Roychand et al. / Construction and Building Materials 237 (2020) 117651

Youssf et al. [22] also investigated the individual effects of H2O2, CaCl2, H2SO4, and a combination treatment of KMNO4 & NaHSO4, but all these treatment methods showed a negative effect on the workability of rubber concrete compared to that of the untreated rubber. However, the study conducted by Kashani et al. [85], and Alawais and West [86] reported an increase in workability with the treatment of rubber particles with H2SO4 solution. The difference in workability reported in the two publications is most likely because of the difference in concentration of H2SO4 solution used in the treatment process. Youssf et al. [22] soaked the rubber particles in 35% H2SO4 solution, whereas the one used by Kashani et al. [85] was10% H2SO4 solution, indicating that the higher concentration of H2SO4 solution brings about negative effect on the surface morphology of the rubber particles that result in the reduction in workability of the treated rubber concrete. Kashani et al. [85] in the same study also looked at the individual effects of coating of rubber particles with potassium permanganate, cement and silica fume. They observed no considerable change in workability by coating the rubber particles with potassium permanganate, and cement coating, however, coating the rubber particles with silica fume considerably reduced the workability of treated rubber concrete compared to that of the untreated rubber concrete, because of the high surface area of silica fume that consumes considerable amount of water of hydration during the pozzolanic reaction. Su et al. [87] looked at the effect of treating the rubber partilcles with silane coupling agent on the workability of rubber concrete. They observed a significant reduction in the slump of SCA treated rubber concrete compared to that of the untreated rubber concrete, which they ascribed to the sticky nature of the treating material. Table 4 provides the summary of the effect of various rubber treatment methods on the workability of concrete/mortar. Emam and Yehia [42] found that treating the crumb rubber with carbon disulfide (CS2) increasing friction between treated crumb rubber and mortar resulting in decreasing the workability of the treated rubber concrete compared to that of the untreated rubber concrete. Muñoz-Sánchez et al. [39] studied effect of treating crumb rubber with sodium hydroxide, sulfuric acid, calcium hydroxide, and acetic acid solutions on fresh and hardened properties of crumb rubber concrete. They noted that all the methods used to treat rubber had an adverse effect on the workability of the rubber concrete. The reduction in the workabilities of the treated rubber concrete were in the order of untreated rubber concrete > saturated NaOH, saturated Ca(OH)2, 32% CH3COOH (Similar slump) > 32% H2SO4. The highest reduction in workability was with the use of H2SO4 solution as it causes deeper changes in rubber, leading to a more porous, smaller and rougher particles, that severely affects its workability. Alawais and West [86] noted an increase in workability with 98% concentrated sulfuric acid treatment. They also observed a significant increase in the workability of UV – A radiated rubber concrete exposed to UV-A radiation for 120 h. 4.2. Hardened concrete 4.2.1. Density Low density or lightweight concrete can help in reducing the dead load, thereby reducing the size of structural elements and the overall cost of construction. The average specific gravity of tyre rubber varies between 0.6 and 1.15 [51,94–96], which is significantly lower than conventional aggregates that have an average specific gravity of ~ 2.65 [16,45,94]. This results in decreases in the density of concrete with the increase in rubber content, as reported by many authors [17,19,52] 4.2.2. Mechanical properties 4.2.2.1. Compressive strength. Compressive strength of concrete is one of the most important properties considered in the construc-

5

tion industry. Any new concrete mix proposed can only be considered by the construction industry if it satisfies their minimum requirement of the required compressive strength for a structural element. The addition of rubber particles as partial replacement of conventional aggregates has an adverse effect on the compressive strength of concrete. The strength of the rubber concrete decreases with the increase in rubber content [19,20,51,55,97,98]. In addition, the size of the rubber particles also play an important role in affecting the strength properties. The compressive strength of the rubber concrete decreases with the increase in particle size [19,20,51,55,97]. The reduction in strength with the increase in rubber content is attributed to three main reasons: (i) deformability of the rubber particles relative to the surrounding cement microstructure, resulting in crack initiation in a pattern similar to that of the air voids in normal concrete, citing [13,20,99], (ii) weak interfacial bond between the tyre rubber particles and the cement matrix, citing [99,100] and (iii) possible reduction in the concrete matrix density that further depends upon the size, density and the hardness of the aggregates. A majority of the studies showed the same trend in decrease of compressive strength with an increase in particle size, however one of the studies showed a contradictory result with the compressive strength decreasing with the decrease in rubber particle size [101]. A data comparison of the studies conducted by various authors of the effect of particle size on the compressive strength of rubber concrete at different w/c ratios and percentage rubber contents is presented in Fig. 2. A more detailed view of the data presented in Fig. 2 has been presented in Table 5 (fine rubber) and Table 6 (coarse rubber) that provides additional information about the particle size, w/c ratio, percentage replacement, strength of the control mix, relative strength of rubber concrete at various replacement levels and particle sizes, and their associated references. The data presented in these graphs are for untreated rubber only. The overall trend observed in Fig. 2 and Table 7 shows that typically fine rubber performs better in improving the compressive strength compared to that of the coarse rubber aggregates. However, there are some results where coarse aggregate particles show higher strength compared to that of some of the fine rubber concrete results at various replacement levels. It is to be noted that there are a large number of variables that can affect the relative strength of the rubber concrete compared to that of the control mix, like mixing procedure [22], concrete slump/flow [102], curing conditions [103], particle size distribution effect and the chemical composition of the waste tyre rubber, etc. The information about all of these variables is not available in all the referred publications and even covering all these variables in the graphs is very difficult, this may be the reason for not getting a sharp and clear distinction line between the effects of groups of fine and coarse aggregates on the relative compressive strengths of rubber concrete, compared to that of their respective control mix. The mean relative strength results of FR and CR concretes at various replacement levels has been presented in Table 7. Raw untreated rubber has weak inherent strength and poor bond performance, therefore numerous rubber pre-treatment methods have been investigated to improve upon its properties in rubber concrete. Najim and Hall [21] studied the comparative effect of untreated tyre rubber aggregates with water washed, cement pre-coated, mortar pre-coated and NaOH pre-treated rubber aggregates on the compressive strength of rubber concrete. All the methods proved to be effective in improving the compressive strength of the rubber concrete with varying degrees of improvement. Water washed and NaOH pretreated rubber provided very small improvement i.e. 4.7% and 3.1% respectively, followed by a significant improvement by using cement and mortar pre-coated rubber particles i.e. 15.6% and 40.6% respectively. Use of NaOH pre-treatment showed a varying degree of improvement according

6

R. Roychand et al. / Construction and Building Materials 237 (2020) 117651

Fig. 2. Relative compressive strength of fine rubber (FR) [12,13,20,50,51,55,57,59,60,93,96,97,104–110] and coarse rubber (CR) [13,20,45,51,55,59,93,97,104,111–113] concrete at different w/c ratios and rubber contents.

to different researchers [26,46,83,84]. A study conducted by Balaha et al. [26] showed about 13% improvement in compressive strength in rubber concrete containing NAOH pre-treated crumb rubber. Another study by Li et al. [83] looked at the effect of pre-treating the rubber particles with NaOH solution followed by pre-coating them with cement powder at 6, 12 and 18% by volume of sand. They observed a reduction of 11.5, 23.3 and 31.9% in the 28-day compressive strength of treated rubber concrete at 6, 12 and 18% replacement levels respectively, compared to the control mix not containing any rubber. Although the strength results follow a typical trend of reduction in compressive strength with the increase in crumb rubber, no test was available on the untreated rubber concrete to identify the comparative benefit of the treatment method used. Soaking of rubber particles in water is a most cost effective method of treatment. Mohammadi et al. in their two studies [43,61] found that water soaking of rubber aggregates for 24 h provides a significant improvement in the 28 day compressive strength, as the soaking of rubber particles helps in eliminating the air entrapped in the rubber particles, thereby improving the bond performance of rubber to cement matrix.

Treating the rubber with polyvinyl acetate is another pretreatment method that shows a significant improvement in the compressive strength of rubber concrete as its surface coating onto the rubber particles helps in improving the cement to rubber bond properties [26,28]. Balaha et al. [26] as part of the same study also looked at the effect of SF on mechanical properties of the rubber concrete, though SF was not used as rubber treatment method but just as a pozzolanic material to improve the strength properties. It significantly improved the 28 day compressive strength of the rubber concrete because of the increase in the density of the interfacial transition zone (ITZ) between the aggregate and the cement paste, which is typical of the effect of SF in improving the concrete strength properties [114–116]. However, Youssf et al. [84] found a totally contrary result to the typical behaviour of the addition of SF to concrete, showing a reduction in the strength of rubber concrete with the increase in SF content. This could possibly be because of the quality of the SF used and may not be reflective of its typical behaviour. No XRD study was available in their study that could provide an insight into the material properties of the SF used.

7

R. Roychand et al. / Construction and Building Materials 237 (2020) 117651 Table 5 Effect of fine rubber size and replacement level on the 28-day compressive strength of rubber concrete. Rubber Size (mm)

Replacement Level (%)

w/c ratio

Control mix Strength (MPa)

Strength relative to the control mix (%)

Reference

<0.3 <0.6 1.0 1.0 1–1.32 2.0 2.0 <2.0 <2.5 <4.0 0.8–4.0 <4.0 <4.75 Rubber Size (mm) 0.8–4.0 <4.0 <4.0 0.8–4.0 <4.0 1–4 <5.0 <5.0 1–5

5, 10, 15, 20 5, 10, 15, 20 5, 10, 20 15, 30, 45 15, 30 5, 10, 15 22.2, 33.3 25, 50, 75, 100 5, 10, 20, 30, 40, 50 5, 15, 25 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20 5, 10, 15, 20, 25, 30 10, 20, 30, 40 Replacement Level (%) 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20 5, 10, 15 10, 20, 30, 40, 50 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20 20, 30 15, 30, 45 10, 20, 40, 60, 80, 100 5, 10, 15, 20, 25, 30, 40 25, 50, 75, 100

0.35 0.35 0.48 0.62 0.52 0.31 0.53 0.48 0.48 0.35 0.40 0.40 0.49 w/c ratio 0.45 0.45 0.48 0.50 0.50 0.62 0.35 0.40 0.57

32.1 30.8 37.2 29.5 39.1 57.8 33.0 34.1 37.5 71.5 42.5 54.0 35.0 Control mix Strength (MPa) 39.0 55.0 33.6 36.5 38.0 31.7 61.7 53.0 26.5

96.5, 73.4, 73, 68.3 90.3, 77.6, 83.9, 71.1 94.6, 85.5, 79.8 66.8, 56.6, 43.4 77.2, 54.3 87.5, 78.4, 65.2 74.8, 61.2 71.8, 57.8, 44, 37.8 80, 65.3, 38.7, 18.1, 10.1, 5.3 81.1, 62.9, 37.8 96.5, 88.2, 87.1, 78.8, 70.6, 58.8, 54.8, 47.1 92.6, 82.4, 75.6, 64.8, 63.9, 55.6 128.6, 102.9, 80, 68.6 Strength relative to the control mix (%) 97.4, 84.6, 78.2, 70.5, 64.1, 55.1, 55.1, 51.3 81.5, 65.5, 49.1 79.2, 75.7, 57.3, 35, 20.8 92.3, 84.1, 80.3, 65.8, 58.4, 50.1, 47.9, 46.6 42.1, 21.1 57.1, 41.3, 28.4 86.5, 70, 50.6, 33.4, 23.8, 15.6 84.1, 79.5, 70.5, 58, 54.4, 46.7, 33.4 84.9, 74.7, 49.8, 32.1

[107] [107] [108] [96] [109] [50] [57] [13] [20] [106] [12] [55] [93] Reference [12] [59] [105] [12] [60] [96] [97] [110] [51]

Table 6 Effect of coarse rubber size and replacement level on the 28-day compressive strength of rubber concrete. Rubber Size (mm)

Replacement Level (%)

w/c ratio

Control mix Strength (MPa)

Strength relative to the control mix (%)

Reference

4–10 4–11.2 <12.7 <13 <15 5–20 5–20 5–20 <38 10–40 10–50 4.75–25

10, 15, 20, 25 5, 10, 15 25, 50, 75, 100 20, 40, 60, 80, 100 12.5, 25, 37.5, 50 10, 20, 40, 60, 80, 100 25, 50, 75 25, 50, 75, 100 25, 50, 75, 100 5, 10, 15, 20, 25, 30 5, 10, 20, 30, 40, 50 10, 20, 30, 40, 50

0.40 0.45 0.50 0.49 0.45 0.35 0.52 0.57 0.48 0.40 0.48 0.49

43.5 55.0 31.9 9.4 30.8 61.7 45.8 26.5 33.7 54.0 37.5 35.0

69, 46, 34.5, 26.4 85.8, 68.5, 51.8 61.4, 43.3, 31, 23.5 41.5, 34, 23.4, 10.6, 5.3 20.6, 4, 2.6, 1.8 74.4, 53, 41, 25.6, 23.2, 14.1 52.2, 45.6, 38 60.4, 52.1, 25.3, 21.5 55.8, 36.2, 26.4, 19.9 88, 81.5, 70.4, 62.4, 57, 50.9 73.3, 56, 33.3, 16, 9.9, 6.7 71.4, 51.4, 34.3, 8.6, 14.3

[111] [59] [45] [113] [104] [97] [112] [51] [13] [55] [20] [93]

Table 7 Average relative strengths of fine and coarse rubber concretes at various replacement levels. Rubber type

Fine rubber Coarse rubber

Replacement levels of conventional aggregtaes with waste tyre rubber (%) 5 10 20 30 Relative strength of rubber concrete at various replacement levels ± S.D.

40

50

87.1 ± 5.2 82.4 ± 6.5

32.3 ± 14.5 28.3 ± 13.3

39.7 ± 27.8 31 ± 19.5

76 ± 7 69.9 ± 8.4

59.5 ± 13.4 42.5 ± 10.5

43.9 ± 14.9 33.5 ± 17.5

Table 8 Contact angles and Intermolecular interaction forces [31]. Material

Untreated crumb rubber Treated crumb rubber

Contact angles

Intermolecular interaction forces

Advancing

Receding

Interaction forces (nN)

Mode (nN)

103.23 99.88

59.46 31.11

15–30 50–70

25 55

Silane coupling agent, a mixture of Z- 6020 (H2NCH2 CH2NH CH2 CH2 CH2Si(OCH3)3) and Z-6040 (O CH2 CH2 CH2O CH2 CH2 CH2 Si(OCH3)3) at 1:1 ratio by weight is another treatment method that shows a significant improvement in compressive strength of rubber concrete [27,54], because of the formation of a stronger chemical bond between the treated rubber particles and the cement paste. A two staged treatment of rubber particles with the first treatment with the silane coupling agent followed by

coating the treated rubber particles with OPC further improves the compressive strength of the rubber concrete because of the formation of a hard shell around rubber particles due to cement hydration [27]. Silane coupling agent has been used in combination with other chemicals as well to treat the rubber particle to improve its performance in rubber concrete. When used in combination with carboxylated styrene-butadiene rubber (CSBR) to modify the properties of crumb rubber it shows a considerable improvement

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R. Roychand et al. / Construction and Building Materials 237 (2020) 117651

in 28 day compressive strength of treated rubber concrete compared to that of the untreated one. The formation of hydrogen bonds as well as the increase of van der Waals’ forces at the treated rubber and cement paste interface, help in improving the performance of the treated rubber concrete [36]. The soft and elastic properties of tyre rubber are very sensitive to temperatures and can become stiff and brittle at higher temperatures [117]. However, this stiffness in waste tyre rubber can be beneficial for its application as replacement of conventional fine aggregates in concrete. Chou et al. [29] exploited this property and studied the effect of partial oxidation (oxidation temperature 150, 200 and 250 °C) of tyre rubber particles on the properties of rubber mortar containing 6% by mass of rubber content. The 28-day compressive strength of the rubber mortar partially oxidized at 150 °C was lower than that of the untreated rubber mortar samples. The one that was partially oxidized at 200 °C showed a slight improvement but was still lower than that of the untreated rubber mortar. Interestingly, partial oxidizing the rubber particles at 250 °C made a significant improvement in its compressive strength, which was 18.4% higher than that of the control mix not containing any waste tyre rubber. The SEM image of the rubber mortar samples containing partially oxidized rubber at 250 °C showed a much smaller crystal structure of hydration compared to that of all other treated and untreated rubber mortar samples. In addition, the morphology of the crystals of the hydration products changed from long and thin in asreceived crumb rubber to short and compact needles in 250 °C treated rubber mortar samples. A similar study conducted by Chen and Lee [118] on the partial oxidation of crumb rubber at 250 °C temperature used 5% crumb rubber by weight of cement and found the relative compressive strength of 82% compared to that of the cement paste. Yu et al. [119] investigated the effect of precipitating reinforcing silica on the rubber powder by cross-linking and bybrid modifying it by sol–gel technique with reactive precursor tetraethoxy- silane and c-glycidyloxypropyl trimethoxysilane (A-187). They observed a significant improvement of 42.5% in the 28-day compressive strength of the treated rubber concrete compared to that of the untreated rubber concrete. Chou et al. [31] found that treating the crumb rubber, used as a replacement of fine aggregates, with waste organic sulphur compounds brought about ~20% increase in the compressive strength of rubber concrete compared to that of the untreated rubber concrete. The atomic force microscopy results (Table 8) showed a reduction in the contact angles of rubber in water due to the adsorption of organic sulphur compounds onto the surface of rubber particles. This reduction in contact angles of the treated rubber reflected the improvement in its hydrophilic properties, reducing its inhibition to water availability for the hydration reaction of the cement, resulting in the improvement in its compressive strength. Moreover, the intermolecular interaction force between the treated rubber particles and the C-S-H gel was found to be significantly higher than that of the untreated rubber particles, which contributed towards the improvement in its strength properties. Zhang et al. [32] investigated the effect of treating tyre rubber particles with a chemical blend of 17.2% acrylic acid, 13.8% polyethylene glycol and 69% anhydrous ethanol by weight. They found a considerable improvement in the compressive strength results of the treated rubber concrete compared to that of the untreated rubber. The surface contact angle study showed a significant improvement in hydrophilic properties of the treated rubber that showed a change in contact angle from 105.13° in the case of raw crumb rubber to 68° for that of the treated rubber, which was also supported by the improvement in the bond between the rubber particle and the cement paste in the SEM images. Herrera-Sosa et al. [34] looked at modifying the properties of waste tyre rubber particles with gamma radiation at 10, 20 and 30% of fine aggregate replace-

ment levels, with particle sizes of (i) mesh 7 (<2.83 mm) and (ii) mesh 20 (<0.84 mm). They found that at 10% replacement level the irradiated rubber concrete with smaller particle size showed no difference but the one with a larger particle size showed a reduction of 18.6% at 28-day compressive strength compared to that of the untreated rubber concrete. At 20% replacement level, the smaller particle size rubber concrete showed a reduction of 19.2% at 28-day compressive strength; however, the larger particle size rubber concrete showed a 22.3% increase in its compressive strength, compared to that of the untreated rubber concrete. At 30% replacement level both the small and the large rubber particle concrete samples showed an increase in 28-day compressive strengths of 40 and 28.9% respectively, compared to that of their corresponding rubber concrete samples. Rivas-Vazquez et al. [35] studied the effect of treating the crumb rubber with Ethanol, Methanol and Acetone, prepared at 50% concentration of solvent by volume of water. They found that the treatment of crumb rubber with Ethanol did not show any significant difference in 7-day compressive strength, however the 21 and 28-day strengths showed a slight increase compared to that of the untreated rubber. The Methanol treated rubber showed a considerably higher improvement in strength at 3, 7, 21 and 28 days of curing compared to that obtained with Ethanol treatment. The best treatment method they found was that with Acetone, which provided the highest improvement in 3, 7, 21 and 28 day compressive strengths, compared to those of the untreated and the ones treated with the other solvents. The Fourier transform infrared (FTIR) spectroscopy results showed increase in the peak intensities between 2850 and 2950 cm 1 wavelength ranges, indicating the presence of stretching CAH bonds and incorporation of additional functional groups that improved the adhesion between the rubber and the cement paste interface of various treated rubber particles, resulting in the improvement in strength properties. He et al. [37] looked at the effect of treating the rubber particles with a combination of KMnO4 and NaHSO3 on the strength properties of rubber concrete containing 2, 4 and 6% of rubber powder by mass of concrete. They found 19.7, 48.7 and 35% improvement in the 28-day compressive strength of the treated rubber concrete samples at 2, 4 and 6% replacement levels, respectively. The FTIR results showed oxidation (by KMnO4) and sulphonation (by NaHSO3) of the rubber particles introducing polar carbonyl, hydroxyl and sulphonate groups onto the rubber surface, which produced a large number of hydrogen and ionic bonds between the rubber and the cement matrix. In addition, the contact angle study showed a change in contact angle of treated rubber from 95° in the case of the untreated rubber to 71°, indicating a fundamental change in the surface properties of the rubber, from strongly hydrophobic to hydrophilic. These changes on the surface of the rubber particles resulted in a significant improvement in the adhesive strength of the rubber and cement matrix, thereby improving the mechanical properties of the rubber concrete. Youssf et al. [22] used the similar treatment method of combination of KMnO4 and NaHSO3 at 20 vol% replacement level of fine aggregates. They did not obseve any noticeable change in the compressive strength of the treated and untreated rubber concrete. They also investigated the effect of the other rubber treatment methods like hydrogen peroxide (H2O2), calcium chloride (CaCl2), and sulphuric acid (H2SO4). Out of all these treatment methods only calcium chloride treatment method showed a small improvement in the 28 day strength of ~ 6%. No other treatment method showed any noticeable change in the strength between the treated and untreated rubber concrete. CaCl2 is a well known cement accelerator [120– 122], the accelerating effect on the cement surrounding CaCl2 treated rubber particles mostlikely would have contributed towards its strength improvement compared to that of the untreated rubber concrete.

R. Roychand et al. / Construction and Building Materials 237 (2020) 117651

Abd-Elaal et al. [38] investigated the effect of heating different sizes (0.6, 1–3, 2–5 mm) of rubber particles at 200 °C temperature for 1 hr at sand replacement level of 20 vol%. They observed that the 0.6 mm and 1–3 mm heat treated rubber concrete showed an improvement of 28% and 17.7%, however the rubber concrete containing the heat treated rubber particle size of 2–5 mm did not show any considerable improvement in the 28-day strength compared to that of the untreated rubber concrete. Also, the 28day compressive strength of the heat treated rubber concrete decreased with the increase in rubber particle size, however the untreated rubber showed no noticeable change among the different sizes of rubebr particles. They also looked at varying the duration (1 h, 1.5 h, 2 h) of the heat treatment for 2–5 mm rubber particle concrete. They observed an improvement of ~ 6.5% by increasing the heating duration from 1 to 1.5 h but the further increase in the heating duration did not provide any noticeable improvement in the 28-day compressive strength. They attributed the improvement in the compressive strength of heat treated rubber concrete to the two main factors (i) the removal of impurities on the crumb rubber surface and (ii) the formation of a hard shell on the surface that provide stiffness to the rubber particles. They also observed an improvement in the bond performance between the heat treated rubber particles and the cement paste that also contributed towards strength improvement. Muñoz-Sánchez et al. [39] studied effect of treating crumb rubber with sodium hydroxide, sulfuric acid, calcium hydroxide, and acetic acid solutions on the mechanical properties of crumb rubber concrete. They noted an improvement in 28 day compressive strength with all the treatment methods compared to that of the untreated rubber concrete. The highest improvement was observed with the NaOH treatment, followed by Ca(OH)2 > H2SO4 >CH3COOH > untreated rubber concrete. They noted that the surface roughness of the rubber particles were improved by using all the treatment methods with acid solutions providing the higher surface roughness compared to that of the alkaline solutions. The increase in the surface roughness of the rubber particles improved the bond performance thereby resulting in the improvement in the 28 day compressive strength results of the treated rubber concretes compared to that of the untreated rubber concrete. Abdulla and Ahmed [40] looked at treating crumb rubber with different solutions of 5% HCl, 35% HCl, 5% H2SO4, 35% H2SO4, 5% CH3COOH and replacing sand content of cement mortar with 30 vol% of crumb rubber of particle size 2 – 2.36 mm. They observed a reduction in the 28 day compressive strength in the order of Control > 35% H2SO4 > CH3COOH > 5% H2SO4 > 5% HCl > untreated rubber > 35% HCl. Leung and Grasley [41] studied the effect of treating rubber particles with 1 mol H2SO4 and 3 mol Nitric acid (HNO3) solutions on rubber cement mortar (no sand) containing 12.2% of rubber content by weight of cement. They observed an improvement in 28 day compressive strength of 33.3 and 2.5% with the rubber treatments of 1 mol H2SO4 and 3 mol HNO3 solutions respectively, compared to that of the untreated rubber mortar. Emam and Yehia [42] investigated the effect of treating crumb rubber with carbon disulfide (CS2) on rubber mortar containing 3 and 6 vol% of rubber contents replacing sand. They observed that their untreated rubber mortars showed a reduction of 2 and 1% respectively at 3 and 6% replacement levels, compared to that of the control mix. However their CS2 treated samples showed an improvement of 10 and 21% compared to that of the control mix, which they attributed it to the enhanced bond performance with CS2 treatment that improved the capability of crumb rubber to absorb more energy, resulting in higher strength. Pham et al. [123] looked at the effect of coating crumb rubber particles with the styrene-butadiene copolymer on rubber mortar containing 30% of crumb rubber (size < 4 mm) by volume of sand. They observed a small improvement in the loss of strength by coating

9

of rubber particles with styrene-butadiene copolymer. The relative strength of the untreated rubber mortar at 28 days was 32.1%, which increased to 34.6% with styrene-butadiene copolymer coating onto the rubber particles, because of the improved interfacial bond and a denser microstructure at the interfacial transition zone. Table 9 shows the detailed information about the effect of various treatment methods on the compressive strength of rubber mortar/concrete at various rubber particle sizes, replacement levels and water cement ratios. 4.2.2.2. Flexural strength. Khatib and Bayomi [20] investigated the effect of particle size (<2.5 mm and 10–50 mm) and the percentage by volume (5–100%) of untreated tyre rubber content on the flexural properties of rubber concrete. They found that the flexural strength decreased with the increase in rubber content irrespective of the particle size. However, they noticed much higher deflections in the rubber concrete before failure, compared to that of the control mix not containing rubber. The smaller particle size rubber performed better in providing higher flexural strength compared to that of the coarser particle size, although, the difference in the flexural strength decreased with the increase in rubber content. Skripkiunas et al. [101] found a similar trend in the reduction in strength with the increase in rubber content. However, they observed a reduction in flexural strength with the decrease in particle size (0–1, 1–2 and 2–3 mm), contrary to what Khatib and Bayomi [20] found in their study. This contrary observation on the effect of particle size between two studies could possibly be due to a large gap in the particle size studied by Khatib and Bayomi [20] compared to no gap between the particle size ranges studied by Skripkiunas et al. [101]. Najim and Hall [21] found that washing the rubber crumb slightly improved the flexural strength of the rubber concrete compared to the unwashed rubber concrete. They also studied: (i) pre-coating the rubber particles with cement paste and cement mortar and (ii) pre-treating the crumb rubber with saturated NaOH solution. They observed that the pre-coating of the rubber particles improved the 28-day flexural strength by 7%, whereas, pre-coating with mortar improved the strength by 10.5%, compared to that of untreated rubber concrete. However, they found that pre-treating the crumb rubber with saturated NaOH solution for 20 mins and then water washing, reduced the 28-day flexural strength of the rubber concrete by 6.7% compared to that of the untreated crumb rubber. Similarly, a reduction in flexural strength of rubber concrete containing NaOH treated and subsequently water washed rubber particles was reported by Segre and Joekes [24] and Ligang et al. [119]. Segre and Joekes [24], also found that treating the rubber particles with sodium silicate solution reduced the 28-day flexural strength of the rubber concrete by 45.9%, which was significantly worse than that of the NaOH treated rubber. Li et al. [36] assessed the effect of treating NaOH pretreated crumb rubber particles with a second stage treatment using a combination of silane coupling agent and carboxylated styrene-butadiene rubber (CSBR) on the properties of rubber concrete at sand replacement levels of 5, 10, 15, 20 and 30%. They observed a small improvement in flexural strength of NaOH treated rubber concrete at 5% replacement level, which decreased with the increase in rubber content, compared to that of the control mix not containing any rubber. However, the two staged treatment performed much better in improving the flexural strengths of the treated rubber concrete, which showed an improvement of 8.8, 12.8 and 2.9% in treated rubber concrete at 5, 10, 15% replacement, but at higher replacement levels the flexural strength showed a reduction of 7.4 and 25% respectively, compared to that of the control sample not containing any rubber. Mohammadi et al. [43] found that soaking the rubber particles for 24 hrs removes the entrapped air and improves the flexural strength of rubber concrete by 9 and 11% at replacement levels

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R. Roychand et al. / Construction and Building Materials 237 (2020) 117651

Table 9 Effect of various treatment methods on the compressive strength of mortar/concrete at various rubber particle sizes and replacement levels. Rubber Size (mm)

Treatment Method

Replacement Level (%)

w/c ratio

Control mix Strength (MPa)

Strength relative to the control mix (%)

Ref.

<4.75 <4.75 <6.0 <6.0 <6.0 <6.0

10, 20, 30, 40 20 38 38 38 38

0.40 0.45 0.48 0.48 0.48 0.48

63.0 55.6 52.5 52.5 52.5 52.5

85.9, 70.3, 49, 36.3 62.8 63.8 70.5 85.7 62.9

[61] [43] [21] [21] [21] [21]

6, 12, 18

0.50

51.1 (0% rubber concrete)

88.5, 76.7, 68.1

[83]

15

0.50

39.1 (0% rubber concrete)

59.4

[124]

12

0.50

47.5 (0% rubber mortar)

50.5

[46]

20

0.50

51.4 (0% rubber concrete)

81.7

[26]

20 20

0.50 0.50

51.4 (0% rubber concrete) 53.5 (0% rubber concrete)

84.8 78.7

[26] [84]

20

0.50

53.5 (0% rubber concrete)

69.5

[84]

0.6

24 hr water soaking 24 hr water soaking Water washing Cement paste coating Mortar coating Soaking in saturated NaOH solution for 20 mins, followed by water washing Pre-treating with NaOH solution followed by pre-coating them with cement powder Soaking in saturated NaOH solution for 30 mins, followed by water washing Soaking in saturated NaOH solution for 30 mins, followed by water washing Soaking in 10% NaOH solution for 30 mins, followed by water washing Polyvinyl alcohol treatment Soaking in saturated NaOH solution for 30 mins, followed by water washing Soaking in saturated NaOH solution for 120 mins, followed by water washing NaOH + CSBR latex + SCA

5, 10, 15, 20, 30

0.45

52.3 (0% rubber concrete)

[36]

<4.75 0.6–2.5 Rubber Size (mm)

Saturated NaOH treatment Saturated NaOH solution Treatment Method

38.0 (0% rubber concrete) 38.0 (0% rubber concrete)

20 20 20 20 2, 4, 6 12.5, 16.5, 21,26.5, 32 15 5% by cement wt. 2.9, 5.7 15, 30 5, 10,15, 20

0.50 0.50 0.50 0.50 0.46 0.50

41.5 41.5 41.5 41.5 49.2 55.2

concrete) concrete) concrete) concrete) concrete) mortar)

66.3 70.1 66.0 63.9 87.6, 71.3, 54.5 89.9, 85, 78.8,73, 67

[22] [22] [22] [22] [37] [119]

0.62 0.35 0.50 0.45 0.40

34.8 (0% rubber mortar) 87 (cement paste) 31.2 (0% rubber concrete) 37.6 (0% rubber concrete) 51.4 (0% rubber concrete)

118.4 82 90.4, 70.5 92, 76.1 83.7, 73.3, 68.5, 63.8

[29] [118] [31] [54] [32]

0.85 2.80 0.85 2.80 <1.18 <1.18 <1.18 Rubber Size (mm)

Untreated Soaking in saturated NaOH solution for 24 hrs, followed by water washing H2O2 treatment CaCl2 treatment H2SO4 treatment KMnO4 + NaHSO4 KMnO4 + NaHSO4 (ATRP) A-187 c-glycidyloxypropyl trimethoxysilane treated rubber powder Partial oxidation @ 250 °C Partial oxidation @ 250 °C Treatment with organic sulfur compounds Silane coupling agent (SCA) Treatment with Acrylic acid and polyethylene glycol Untreated Untreated Treatment with gamma radiation Treatment with gamma radiation Ethanol treatment Methanol treatment Acetone treatment Treatment Method

0.50 0.60 w/c ratio 0.50 0.50

41.5 (0% rubber concrete) 39.2 (0% rubber mortar) Control mix Strength (MPa)

0.15–0.3 0.15–0.3

20 10 Replacement Level (%) 2.5, 5, 7.5, 10 2.5, 5, 7.5, 10

101.1, 100.2, 95.6, 89.5, 71.9 70.4 88.0 Strength relative to the control mix (%) 79, 73.8, 59.8, 50 89.7, 97.2, 86.4, 82.2

24 (0% rubber concrete) 24 (0% rubber concrete) 24 (0% rubber concrete) 24 (0% rubber concrete) 188 (0% rubber concrete) 188 (0% rubber concrete) 188 (0% rubber concrete) Control mix Strength (MPa)

Rubber heat treatment–1 hr Rubber heat treatment–1 hr Rubber heat treatment–1 hr Rubber heat treatment–1.5 hr Rubber heat treatment–2 hr Untreated 32% H2SO4 solution 5% H2SO4 solution 35% H2SO4 solution Saturated Ca(OH)2 solution 32% CH3COOH solution 5% CH3COOH solution Untreated 5% HCl solution 35% HCl solution 1 mol H2SO4 solution

67, 50.8, 21.3 88.8, 53.8, 47.7 66.7, 41.7, 30.8 73.3, 66.7, 62.5 95.7 109 117 Strength relative to the control mix (%) 81.7 77.0 68.0 72.5 73.1 63.1 80.4 63.8 73.2 84.0 69.8 66.6 43.8 45.2 13.2 133.3

[34] [34] [34] [34] [35] [35] [35] Ref.

0.6 1–3 2–5 2–5 2–5 0.6–2.5 0.6–2.5 2–2.36 2–2.36 0.6–2.5 0.6–2.5 2–2.36 2–2.36 2–2.36 2–2.36 <0.42

0.54 0.54 0.54 0.54 0.50 0.50 0.50 w/c ratio 0.50 0.50 0.50 0.50 0.50 0.60 0.60 0.40 0.40 0.60 0.60 0.40 0.40 0.40 0.40 0.40

<0.42

3 mol HNO3 solution

102.5

[41]

3.0 3.0 <4.0 <4.0

Untreated Treatment with CS2 Untreated Styrene-butadiene copolymer coating

98, 99 110, 121 32.1 34.6

[42] [42] [123] [123]

<2.36 25 0.8 <4.0 <4.0 <5.0 <5.0

<4.75 <4.75 <4.75 <4.75 0.4 0.18 0.6 0.3–0.6 0.3 <4.75 0.42

10, 20, 30 10, 20, 30 10, 20, 30 10, 20, 30 10 10 10 Replacement Level (%) 20 20 20 20 20 10 10 30 30 10 10 30 30 30 30 12.2% by weight of cement 12.2% by weight of cement 3, 6 3, 6 30 30

0.40 0.50 0.50

(0% (0% (0% (0% (0% (0%

(0% (0% (0% (0% (0% (0%

rubber rubber rubber rubber rubber rubber

rubber rubber rubber rubber rubber rubber

concrete) concrete) concrete) concrete) concrete) concrete)

50.9 (0% rubber concrete) 50.9 (0% rubber concrete) 50.9 (0% rubber concrete) 50.9 (0% rubber concrete) 50.9 (0% rubber concrete) 39.2 (0% rubber mortar) 39.2 (0% rubber mortar) 36.5 (0% rubber mortar) 36.5 (0% rubber mortar) 39.2 (0% rubber mortar) 39.2 (0% rubber mortar) 36.5 (0% rubber mortar) 36.5 (0% rubber mortar) 36.5 (0% rubber mortar) 36.5 (0% rubber mortar) 16.2 (12.2 wt% untreated rubber mortar–no sand) 16.2 (12.2 wt% untreated rubber mortar–no sand) 23.4 (0% rubber mortar) 23.4 (0% rubber mortar) 61.4 (0% rubber mortar) 61.4 (0% rubber mortar)

[22] [39] Ref. [125] [125]

[38] [38] [38] [38] [38] [39] [39] [40] [40] [39] [39] [40] [40] [40] [40] [41]

11

R. Roychand et al. / Construction and Building Materials 237 (2020) 117651 Table 10 Effect of various treatment methods on the flexural strength of mortar/concrete at various rubber particle sizes and replacement levels. Rubber Size (mm)

Treatment Method

Replacement Level (%)

w/c ratio

Control mix Strength (MPa)

Strength relative to the control mix (%)

Ref.

<2.5 10–50 0–1 1–2 2–3 <6

No Treatment No Treatment No Treatment No Treatment No Treatment Water wash

5, 10, 5, 10, 5, 10, 5, 10, 5, 10, 38

0.48 0.48 0.35 0.35 0.35 0.48

37.3, 31.8, 36.4, 26.4, 63.1, 47.7, 78.5, 72.3, 81.5, 75.4, 101.6

[20] [20] [101] [101] [101] [21]

<6

Pre-coating with cement paste

38

0.48

<6

Pre-coating with cement mortar

38

0.48

<6

Pre-treating with NaOH solution

38

0.48

500 mm

Pre-treating with NaOH followed by water washing NaOH + CSBR latex + SCA

Addition as 10% by mass of cement paste 5, 10, 15, 20, 30

0.36

11.0 (0% rubber concrete) 11.0 (0% rubber concrete) 6.5 (0% rubber concrete) 6.5 (0% rubber concrete) 6.5 (0% rubber concrete) 6.26 (38% untreated rubber concrete) 6.26 (38% untreated rubber concrete) 6.26 (38% untreated rubber concrete) 6.26 (38% untreated rubber concrete) 5.6 (cement paste)

0.45

6.8 (0% rubber concrete)

24 hr water soaking 24 hr water soaking Partial oxidation @ 250 °C Untreated Treatment with organic sulfur compounds Untreated Treatment with Acrylic acid and polyethylene glycol Untreated Treatment with UV radiation

20 30 15 3, 6 3, 6

0.45 0.40 0.62 0.50 0.50

6.0 6.9 6.1 6.4 6.4

5, 10, 20 5, 10, 20

0.40 0.40

Treatment Method

15 15 (Exposure time 20, 40, 60hr) Replacement Level (%)

Untreated Untreated Treatment with gamma radiation Treatment with gamma radiation Untreated Saturated NaOH solution 32% Sulfuric acid solution Saturated Ca(OH)2 solution 32% acetic acid solution Untreated 5% HCl solution 35% HCl solution 5% H2SO4 solution 35% H2SO4 solution 5% CH3COOH solution

10, 10, 10, 10, 10 10 10 10 10 30 30 30 30 30 30

0.6 <4.75 <4.75 0.6 0.3 0.3 0.42 0.42 <840 lm <840 lm Rubber Size (mm) 0.85 2.80 0.85 2.80 0.6–2.5 0.6–2.5 0.6–2.5 0.6–2.5 0.6–2.5 2–2.36 2–2.36 2–2.36 2–2.36 2–2.36 2–2.36

20, 20, 20, 20, 20

20, 20, 20, 20,

30, 40 30, 40 30 30

30 30 30 30

of 20 and 30% respectively, compared to that of their corresponding untreated rubber concrete samples. Chou et al. [29] observed that partially oxidizing the rubber particles at 150 and 200 °C further reduced the 28-day flexural strength of rubber mortar. However, increasing the partial oxidation temperature to 250 °C brought a significant improvement in the flexural strength of the rubber mortar, that was approximately the same as that of the control mix. Chou et al. [31] looked at treating the crumb rubber particles with organic sulfur compounds and observed an improvement in flexural strength of ~ 15% in the rubber concrete containing 6% of crumb rubber. Zhang et al. [32] investigated the effect of modifying the properties of the tyre rubber particles (<4 mm) by treating them with a chemical blend of 17.2% acrylic acid, 13.8% polyethylene glycol and 69% anhydrous ethanol by weight. They found an improvement in 28-day flexural strength of 13.5, 18.2 and 9.7% in the treated rubber concrete at 5, 10 and 20% of replacement levels respectively, compared to that of the untreated rubber concrete. Ossola and Wojcik [33] investigated the effect of treating the rubber particles with UV radiation for 20, 40 and 60 hrs of exposure. They observed an improvement in rubber to cement bond

26.4, 23.6, 33.8, 55.4, 60

15.5, 9.1 17.3, 10.9 27.7 40

107.0

[21]

110.5

[21]

93.3

[21]

176.8

[24]

108.8, 113.2, 102.9, 92.6, 89.7 83.3 75.4 101.6 79.4, 76.3 90.6, 88.1

[36]

4.07 (0% rubber concrete) 4.07 (0% rubber concrete)

51.8, 30.9, 12.2 95.2, 80, 37.9

[32] [32]

0.32 0.32

6.33 (0% rubber mortar) 6.33 (0% rubber mortar)

78.7 82.3, 94.2, 93.8

[33] [33]

w/c ratio 0.54 0.54 0.54 0.54 0.6 0.6 0.6 0.6 0.6 0.40 0.40 0.40 0.40 0.40 0.40

Control mix Strength (MPa)

Strength relative to the control mix (%) 96.7, 93.3, 66.7 98, 88, 83.3 77.3, 66, 57.3 81.3, 70, 67.3 82.7 102.4 110.6 104.7 92.1 83.3 80.5 75.0 89.9 88.9 97.1

Ref.

(0% (0% (0% (0% (0%

rubber rubber rubber rubber rubber

concrete) concrete) mortar) concrete) concrete)

7.5 (0% rubber concrete) 7.5 (0% rubber concrete) 7.5 (0% rubber concrete) 7.5 (0% rubber concrete) 6.35 (0% rubber mortar) 6.35 (0% rubber mortar) 6.35 (0% rubber mortar) 6.35 (0% rubber mortar) 6.35 (0% rubber mortar) 3.6 (0% rubber mortar) 3.6 (0% rubber mortar) 3.6 (0% rubber mortar) 3.6 (0% rubber mortar) 3.6 (0% rubber mortar) 3.6 (0% rubber mortar)

[43] [43] [29] [31] [31]

[34] [34] [34] [34] [39] [39] [39] [39] [39] [40] [40] [40] [40] [40] [40]

behaviour, thereby improving the strength properties of the rubber concrete. The flexural strength of the rubber concrete increased with the increase in UV exposure period, to the maximum level reaching at 40 hrs of exposure, with no further improvement in flexural strength with the increase in the UV exposure period. Herrera-Sosa et al. [34] found that treating the rubber particles with gamma radiation reduced the flexural strength of the treated rubber concrete, in spite of its positive influence on improving the compressive strength on some of the mix designs containing gamma irradiated rubber. Muñoz-Sánchez et al. [39] noted an improvement in 28 day flexural strength in all the different rubber treatment methods they adopted i.e. treatment with NaOH, Ca (OH)2, H2SO4, and CH3COOH solutions, compared to that of the untreated rubber concrete. The highest improvement in flexural strength was observed with the H2SO4 treatment, followed by Ca (OH)2 > NaOH > CH3COOH > untreated rubber concrete. Another study conducted by Abdulla and Ahmed [40] looked at treating crumb rubber with different solutions of 5% HCl, 35% HCl, 5% H2SO4, 35% H2SO4, 5% CH3COOH and replacing sand content of cement mortar with 30 vol% of crumb rubber of particle size 2– 2.36 mm. They observed a reduction in the 28 day flexural strength

12

R. Roychand et al. / Construction and Building Materials 237 (2020) 117651

in the order of Control > CH3COOH > 5% H2SO4 > 35% H2SO4 > untreated rubber > 5% HCl > 35% HCl. Table 10 shows the detailed information about the effect of various treatment methods on the flexural strength of rubber mortar/concrete at various rubber particle sizes, replacement levels and water cement ratios. 4.2.2.3. Split tensile strength. Split tensile strength of concrete is one of the most important properties of the concrete because it helps in ascertaining the load at which the concrete members may crack and, in some cases, to design a structural member as an uncracked section. Eldin and Senouci [19] studied the effect of size (38, 25, 19, 6.4 and 2 mm) and percentage volume (0, 25, 50, 75 and 100%) of untreated rubber aggregates on the split tensile strength of concrete. They observed a 36% loss in strength at 28 days with 25% tyre rubber content as coarse aggregate, which increased to 75% loss at 100% replacement level. They found a similar trend in loss of strength with fine rubber aggregates, but the 28-day strengths with fine rubber aggregates were considerably higher than those of coarse rubber aggregates. They noticed a 19% loss in strength at 28 days with 25% tyre rubber content as fine aggregate, which increased to 49% loss at 100% replacement level. A similar trend in the reduction in split tensile strength due to the effect of particle size and percentage volume of tyre rubber content was observed by Topcu [96]. Najim and Hall [21] studied the comparative effect of untreated tyre rubber aggregates with water washed, cement pre-coated, mortar pre-coated and NaOH pre-treated rubber aggregates on the split tensile strength of concrete. The rubber replacement in their mix designs was 12% by mass of total aggregates (6% CA + 6% FA), i.e. 38% by total aggregate volume. They found no significant difference in the 28-day split tensile strengths of untreated, water washed, cement paste pre-coated and NaOH pre-treated rubber concrete samples, which were 2.6, 2.7, 2.7 and 2.55 MPa respectively. However, the cement mortar precoated rubber improved the split tensile strength of the rubber concrete by 19.2% compared to that of the untreated rubber concrete. Albano et al. [17] looked at the effect of two treatment methods (i) NaOH and (ii) silane coupling agent A-174 on rubber particle sizes of 0.29 mm and 0.59 mm and replacement levels of 5 and 10% by mass of fine aggregates. They found that the treatment of the rubber particles with maximum size of 0.29 mm, with NaOH at 5% replacement level showed a reduction in split tensile strength of 15% but an increase of 57% at 10% replacement level. However, the one containing 0.59 mm rubber particle size, showed an increase of 8.3% at 5% replacement level and a decrease of 30% at a replacement level of 10%. Their SCA treated rubber particles with a maximum size of 0.29 mm showed a reduction in split tensile strength of 5% at 5% replacement level and no difference in strength at a replacement level of 10%. The SCA treated rubber particles with a maximum size of 0.59 mm showed an increase of 11% at 5% replacement level and a negligible difference at a replacement level of 10%. Marques et al. [46] observed an improvement in 28-day split tensile strength of 17.1% in NaOH treated rubber concrete, compared to that of the untreated rubber concrete at 12% sand replacement level. Youssf et al. [84] investigated the effect of soaking rubber particles in 10% NaOH solution for 1 and 2 hrs duration. They found an improvement in 28-day split tensile strength of 14.8 and 18.5% in NaOH treated rubber concrete samples soaked for 1 and 2 hrs respectively, compared to that of the untreated rubber concrete. Chou et al. [29] looked at the effect of partially oxidizing the tyre rubber particles at 150, 200 and 250 °C temperatures, on the properties of rubber mortar containing 6% by mass of rubber content. They observed a 43.8% drop in 28-day split tensile strength of

untreated rubber compared to the control sample not containing any rubber. The samples containing rubber particles partially oxidized at 150 and 200 °C showed a similar reduction in strength to that of the untreated rubber. However, partially oxidizing the rubber particles at 250 °C brought a significant improvement in the 28-day rubber mortar strength, which was approximately similar to that of the control mix. Chou et al. [31] found that treating the crumb rubber with organic sulfur compounds resulted in a considerable improvement in the 28-day split tensile strength of the rubber concrete containing 3–6% of crumb rubber. They observed an improvement of 12.8 and 13.5% in 28-day split tensile strengths of rubber concrete containing 3 and 6% of treated rubber, respectively, compared to that of the corresponding untreated rubber concrete. Herrera-Sosa et al. [34] looked at the effect of gamma radiation on crumb rubber at 10, 20 and 30% of fine aggregate replacement levels, with particle sizes of mesh 7 (<2.83 mm) and mesh 20 (<0.84 mm). They found that at 10% replacement level the irradiated rubber concrete with smaller particle size showed no significant difference but the one with a larger particle size showed a reduction of 22% in 28-day split tensile strength, compared to that of the non-irradiated rubber concrete. At 20% replacement level, the irradiated rubber concrete with smaller particle size showed a reduction of 25% and the one with a larger particle size showed an increase of 16.2%, compared to that of the nonirradiated rubber concrete. At 30% replacement level, the irradiated rubber concrete with smaller particle size showed no considerable difference but the one with a larger particle size showed an improvement of 15% in 28-day split tensile strength, compared to that of the non-irradiated rubber concrete. Table 11 shows the detailed information about the effect of various treatment methods on the split tensile strength of rubber mortar/concrete at various rubber particle sizes, replacement levels and water cement ratios. 4.2.2.4. Modulus of elasticity. Li et al. [52] looked at the effect of the particle size and percentage of rubber content on the static elastic modulus of rubber concrete. They found that the elastic modulus increases with the increase in particle size and decreases with the increase in rubber content. Similar observations on the reduction of elastic modulus with the increase in rubber content were recorded by Atahan and Yucel [113] and Li et al. [83]. Zheng et al. [126] studied the effect of 2.36 mm and 15–40 mm rubber particle sizes and rubber contents of 15%, 30%, and 45 vol%. They found that both the static and dynamic elastic moduli decreased with the increase in rubber content but the dynamic elastic modulus was significantly higher than that of the static modulus at all replacement levels. The effect of particle size on the elastic modulus was only observed at the replacement level of 15%. They found that the concrete containing smaller sized rubber particles showed an increase of 17.1% and 17.4% in the dynamic and the static elastic modulus respectively, compared to that of the larger sized rubber particles. However, at higher replacement levels they did not see any significant difference in the effect of particle size on both the static and the dynamic elastic moduli. Najim and Hall [21] found that washing the rubber showed no change in the static elastic modulus at 28 days, but a 2.9% increase was seen in the dynamic elastic modulus, compared to that of the unwashed rubber concrete. They also studied the effect of: (i) precoating the rubber particles with cement paste and cement mortar and (ii) pre-treating the crumb rubber with saturated NaOH solution. They observed that pre-coating the rubber particles with cement paste improved the 28-day static elastic modulus of the rubber concrete by 10% and dynamic elastic modulus by 5.9%. Whereas, pre-coating with mortar improved the static elastic modulus by 15% and dynamic elastic modulus by 11.8%. Treatment of the rubber particles with saturated NaOH solution for 20 mins fol-

13

R. Roychand et al. / Construction and Building Materials 237 (2020) 117651 Table 11 Effect of various treatment methods on the split tensile strength of mortar/concrete at various rubber particle sizes and replacement levels. Rubber size (mm)

Treatment method

Replacement level (%)

w/c ratio

Control mix strength (MPa)

Strength relative to the control mix (%)

Ref.

<2 <38 0–1 1–4 <6

Particle size effect Particle size effect Particle size effect Particle size effect Water wash

25, 25, 15, 15, 38

0.48 0.48 0.62 0.62 0.48

80.8, 68.6, 63.1, 43.9, 67.6, 47.7, 46.7, 33.0, 103.8

[19] [19] [96] [96] [21]

<6

Pre-coating with cement paste

38

0.48

<6

Pre-coating with cement mortar

38

0.48

<6

Pre-treating with NaOH solution

38

0.48

<0.6

NaOH Treatment

5, 10

0.49

<0.6

Silane coupling agent treatment

5, 10

0.49

<0.8

Soaking in saturated NaOH solution for 30 mins, followed by water washing Untreated Partial oxidation @ 250 °C Untreated Treatment with organic sulfur compounds Non irradiated Non irradiated Treatment with gamma radiation Treatment with gamma radiation

12

0.5

3.4 (0% rubber concrete) 3.4 (0% rubber concrete) 3.21 (0% rubber concrete) 3.21 (0% rubber concrete) 2.6 (38% untreated rubber concrete) 2.6 (38% untreated rubber concrete) 2.6 (38% untreated rubber concrete) 2.6 (38% untreated rubber concrete) 3, 0.5 (5%, 10% untreated rubber) 3, 0.5 (5%, 10% untreated rubber) 6.8 (0% rubber mortar)

15 15 3, 6 3, 6 10, 20, 10, 20, 10, 20, 10, 20,

0.62 0.62 0.50 0.50 0.54 0.54 0.54 0.54

3.2 (0% rubber mortar) 3.2 (0% rubber mortar) 8.1 (0% rubber concrete) 8.1 (0% rubber concrete) 2.03 (0% rubber concrete) 2.03 (0% rubber concrete) 2.03 (0% rubber concrete) 2.03 (0% rubber concrete)

0.6 0.6 0.3 0.3 0.85 2.80 0.85 2.80

50, 50, 30, 30,

75, 100 75, 100 45 45

30 30 30 30

58.1, 47.1 33.4, 23.5 35.2 25.5

103.8

[21]

119.2

[21]

98.1

[21]

85.1, 160

[17]

93.6, 100

[17]

51.5

[46]

56.3 103.1 81.2, 71 91.6, 80.6 66.5, 59.1, 36 93.6, 55.2, 50.7 69, 43.3, 36.5 72.9, 62.6, 57.6

[29] [29] [31] [31] [34] [34] [34] [34]

Table 12 Effect of various treatment methods on the elactic modulus of mortar/concrete at various rubber particle sizes and replacement levels. Rubber size (mm)

Treatment method

Replacement level (%)

w/c ratio

Control elastic modulus (GPa)

Relative elastic modulus to the control mix (%)

Ref.

0.173 0.221 0.535 2 4 0–13 1.2–2.4 <2.38 15–40 <6

Particle size effect Particle size effect Particle size effect Particle size effect Particle size effect No treatment NaOH treatment Particle size effect Particle size effect Water wash

2, 4, 6, 8, 10 2, 4, 6, 8, 10 2, 4, 6, 8, 10 2, 4, 6, 8, 10 2, 4, 6, 8, 10 20, 40, 60, 80, 100 6, 12, 18 15, 30, 45 15, 30, 45 38

0.49 0.49 0.49 0.49 0.49 0.52 0.50 0.45 0.45 0.48

72.4, 69.5, 63.4, 59.3, 58 73.2, 72.4, 67.8, 62.4, 61 75.6, 75.1, 70, 67.8, 65.9 79.5, 76.8, 75.6, 76.1, 68.3 82, 78, 76.8, 76.1, 71.2 94, 88, 39.8, 12, 3.6 95.4, 94.1, 86.1 84.4, 75, 70.3 71.9, 75, 67.2 100

[52] [52] [52] [52] [52] [113] [83] [126] [126] [21]

<6

Pre-coating with cement paste

38

0.48

110

[21]

<6

Pre-coating with cement mortar

38

0.48

115

[21]

<6

Pre-treating with NaOH solution

38

0.48

105

[21]

<500 mm

No treatment

0.36

123.7

[24]

<500 mm

Pre-treating with NaOH solution followed by water wash 24 hr water soaking Untreated Untreated Treated with gamma radiation Treated with gamma radiation

10% by mass of cement 10% by mass of cement 10, 20, 30, 40 10, 20, 30 10, 20, 30 10, 20, 30 10, 20, 30

4.1 (0% rubber concrete) 4.1 (0% rubber concrete) 4.1 (0% rubber concrete) 4.1 (0% rubber concrete) 4.1 (0% rubber concrete) 16.6 (0% rubber concrete) 30.3 (0% rubber concrete) 32 (0% rubber concrete) 32 (0% rubber concrete) 20 (38% untreated rubber concrete) 20 (38% untreated rubber concrete) 20 (38% untreated rubber concrete) 20 (38% untreated rubber concrete) 5.9 (Cement paste)

0.36

5.9 (Cement paste)

105.1

[24]

0.40 0.54 0.54 0.54 0.54

46.5 10.7 10.7 10.7 10.7

92, 80.4, 71.8, 64.3 74.8, 58.9, 47.3 84.1, 74.8, 71.1 72.9, 59.8, 62.6 69.2, 62.6, 72

[43] [34] [34] [34] [34]

<4.75 0.85 2.80 0.85 2.80

lowed by water washing showed a 5% increase in the static elastic modulus and no change in the dynamic elastic modulus. Marques et al. [46] did not find any significant change in the 28-day elastic modulus of NaOH treated and water washed rubber (particle size < 800 mm) concrete at 12% by volume of sand replacement, compared to that of the untreated rubber concrete. Segre and Joekes [24], found a 15% reduction in static elastic modulus of rubber concrete containing 10% by mass of NaOH treated and subsequently water washed rubber particles with a particle size smaller than 500 mm.

(0% (0% (0% (0% (0%

rubber) rubber) rubber) rubber) rubber)

Mohammadi et al. [43] found that the elastic modulus decreases with the increase in w/c ratio and increases with the 24 h water soaking treatment. The 28-day static elastic modulus they observed for the 24 h water soaked samples at 0.4 w/c ratio were 10.1, 18.8, 24.6 and 33.3% lower than that of the control mix at the replacement levels of 10, 20, 30 and 40% respectively. Herrera-Sosa et al. [34] studied the properties of waste tyre rubber particles treated with gamma radiation at 10, 20 and 30% of fine aggregate replacement levels, with particle sizes of 2.83 mm and 0.84 mm. They found that at 10 and 20% replacement levels, the

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irradiated rubber concrete with smaller particle size showed no significant difference but the one with a larger particle size showed a reduction of 17.3% and 17.4% respectively, in 28-day static elastic modulus, compared to that of the untreated rubber concrete. At 30% replacement level, the irradiated rubber concrete with smaller particle size showed an improvement of 30.4% but the one with a larger particle size showed a minor improvement of 2.6% in elastic modulus, compared to that of the untreated rubber concrete. This indicates that the gamma radiation treatment of rubber has a small improvement on the stiffening of the rubber particles that improves the elastic modulus of rubber concrete, but to see a significant difference between the treated and untreated rubber concrete properties the replacement level has to be large i.e. at least 30% of rubber content. Table 12 shows the detailed information about the effect of various treatment methods on the modulus of elasticity of rubber mortar/concrete at various rubber particle sizes, replacement levels and water cement ratios. Considering the results for compressive, flexural, tensile strength and modulus of elasticity indicates that overall NaOH treatment followed by water washing does not yield any significant positive improvement in the mechanical properties. The little improvement it shows is at a significantly high rubber content which anyway drastically reduces the 28 day compressive strength of rubber concrete and at lower replacement levels, it shows negligible or negative results, indicating that it is not an effective rubber treatment method. 4.2.2.5. Modulus of rigidity. Najim and Hall [21] found that the washing the crumb rubber showed a minor improvement of 1.9% in the modulus of rigidity (dynamic shear modulus) at 28 days, compared to that of the unwashed rubber concrete. They observed a similar 1.9% improvement in the modulus of rigidity of the rubber concrete containing rubber particles treated with saturated NaOH solution for 20 mins followed by water washing. The precoating of the rubber particles with cement paste and cement mortar improved the modulus of rigidity by 3.8 and 7.5% respectively, compared to that of the untreated rubber concrete. 4.2.2.6. Abrasion resistance. Thomas et al. [12] studied the abrasion resistance of rubber concrete by substituting the fine aggregates with crumb rubber at 0–20% of replacement levels in the increments of 2.5%. They found that the addition of crumb rubber improved the abrasion resistance of concrete at all replacement levels. However, Bisht and Ramana [127] found that the abrasion resistance decreases with the increase in rubber content, with a particle size of 0.6 mm. They investigated the replacement level of 4, 4.5, 5 and 5.5% by mass of fine aggregates. They observed no significant change in the abrasion resistance of the concrete containing 4% of crumb rubber, compared to that of the control mix. However, they noticed a 5.7, 7.6 and 17.7% increase in the wear of the rubber concrete containing 4.5, 5, 5.5% of rubber crumb respectively, indicating a reduction in their respective abrasion resistance, compared to that of the rubber concrete containing 4% of rubber particles. Segre and Joekes [24], found a significant improvement in the abrasion resistance of rubber concrete by treating the rubber particles (size < 500 mm) with saturated NaOH solution for 20 mins and subsequently washing with water. Their control specimen was cement paste and the design mixes contained rubber content of 10% by mass of cement. They observed an increase in mass loss due to abrasion by 380, 240, 242, 257, 240 and 214% in untreated rubber mortar samples tested at 100, 200, 300, 400, 500 and 600 cycles respectively, compared to that of the corresponding control cement paste samples. However, the NaOH treated samples showed a mass loss of 100, 60, 42, 56, 50 and 30% higher than

the corresponding cement paste control samples tested at 100, 200, 300, 400, 500 and 600 cycles respectively, which is a significant improvement in the abrasion resistance. 4.2.2.7. Fatigue life. Liu et al. [50] studied the fatigue performance of rubber concrete containing 0, 5, 10 and 15% of crumb rubber as sand replacement, with a particle size of 2 mm. They conducted the three-point bending fatigue testing on a sample size of 150  150  550 mm with center to center span length of 400 mm. The rubber particle size, cement content and the w/c ratio in their mix design was 2 mm, 420 kg/m3 and 0.31 respectively. They found that the fatigue life of concrete increased with the increase in rubber content and decreased with the increase in stress level. However, at all stress levels, rubber concrete performed better than the control mix, which improved with the increase in the rubber content. They further described that the rubber concrete can absorb energy by deformation when subjected to external loads, which reduces the probability of internal crack propagation in the concrete, thereby absorbing the strain energy and eventually preventing the spreading of the cracks across the whole volume. Pacheco-Torres et al. [128] looked at the effect of particle size (1–4, 10 and 16 mm) and rubber content (10, 20 and 30%) on the fatigue performance of rubber concrete. They observed that intermediate size (10 mm) added in a moderate proportion 10 to 20% provided an improved relationship between the resistance loss and the increase in deformation. They also observed an increased fatigue resistance to a higher number of load cycles with the intermediate size (10 mm) rubber particles containing 30% of rubber content. In case of the largest sized (16 mm) rubber particles an improved behaviour of the material was observed only when added in the lowest proportion (10%), as its resistance was significantly affected with the further increase in the rubber content. In case of the smallest sized (1–4 mm) rubber particles an improvement in deformation was observed only with the intermediate proportion (20%). They observed a premature rupture with the smallest proportion (10%), and a loss of deformability with the largest proportion (30%). Mohammadi et al. [43] investigated the effect of 24 hr water soaking of rubber aggregates on the fatigue performance of rubber concrete containing 10, 20, 30 and 40% of rubber content as a replacement of fine aggregates at two w/c ratios i.e. 0.40 and 0.45. They observed that at w/c ratio of 0.45, the fatigue life (number of cycles) of the rubber concrete at 10, 20, 30 and 40% replacement levels were 19.7, 23.6, 10.3 and 8.3% lower than that of the control mix. With the lowering of the w/c ratio to 0.40, they noticed a similar reduction in fatigue life (cycles) of rubber concrete at 10 and 20% replacement levels, compared to the control mix, however, at higher replacement levels of 30 and 40% they noticed a considerable improvement. The fatigue life of rubber concrete at 30% replacement level was approximately similar to that of the control mix and at 40% replacement level it was 16.3% higher than that of the control mix. This indicates that the fatigue life of the rubber concrete can be considerably improved by reducing the w/c ratio of the concrete mix. Interestingly, the study conducted by Liu et al. [50] showed an improvement in the fatigue life of rubber concrete at all replacement levels but the one conducted by Mohammadi et al. [43] showed a considerable reduction in the fatigue life at 10 and 20% replacement levels, which improved at 30 and 40% replacement levels. The possible difference between their results could be linked to the w/c ratio, cement content and the rubber particle size, although no information about the rubber particle size was provided by Mohammadi et al. [43]. 4.2.2.8. Fracture energy and toughness. Gesoglu et al. [55] studied the effect of size and percentage content of the tyre rubber on the fracture energy of the rubber concrete, as per the recommenda-

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tion of the RILEM 50-FMC Technical Committee [129]. They looked at two rubber particle sizes (i) crumb rubber - particle size smaller than 4 mm and (ii) rubber chips – elongated rubber particles size varying from 10 to 40 mm and their percentage of rubber content varied from 5 to 30%. They observed that the fracture energy of the crumb rubber concrete performed better than that of the tyre chip concrete at all replacement levels. In addition, the fracture energy increased with the increase in rubber content up to the maximum of 15% and with the further increase in rubber content, the fracture energy decreased with the increase in rubber content. Segre and Joekes [24], found a significant increase in the fracture energy of rubber mortar containing rubber at 10% by mass of cement with a particle size < 500 mm, compared to that of the control cement paste mix. They observed a 311% increase in fracture energy of untreated rubber mortar compared to that of the control sample. However, by treating the rubber with saturated NaOH solution for 20 mins and subsequently water washing, the fracture energy of the treated rubber concrete decreased to 249% of that of the control sample. Overall the results indicate that the NaOH treatment followed by water washing is not an effective rubber treatment method. Reda Taha et al. [51] studied the effect of increasing the tyre rubber content on the fracture toughness parameters of rubber concrete; KIC – critical stress intensity, GIC – critical energy release rate, JIC – elastic plastic toughness parameter and Gf – fracture energy, of rubber concrete. They observed that the fracture toughness parameters KIC and GIC showed an increase up to the replacement level of 25% compared to that of the control mix. However, with the further increase in rubber content, both the KIC and GIC parameters decreased with the increasing amount of rubber content. Interestingly, they found that the elastic–plastic toughness parameter (JIC), showed a consistent increase with the increasing amount of rubber content up to 75% of rubber content and beyond that it showed a significant drop in value. They attributed this increase in the fracture toughness with the increase in rubber content to the ability of the tyre rubber particles to add toughening mechanisms like crack bridging, bending, compressing and twisting. In addition, the tyre rubber particles absorb part of the energy to which the cement matrix is subjected, thereby increasing the energy absorption capacity of the composite material before fracturing, compared to that of the control mix. They also found that the fracture energy Gf showed a similar trend to that of KIC and GIC i.e. the fracture energy increased up to the replacement level of 25% and then decreased with the further increase in rubber content up to 100%. The fracture energy showed a dependence on the maximum deformation and the load capacity of the material. They found that the total deformation of concrete increased with the increase in rubber content, however its maximum load capacity decreased. They found Gf to be a good measure, reflecting a trade-off between deformability and the strength development in rubber concrete, as the rubber replacement level increased. 4.2.2.9. Crack resistance. Waste tyre rubber aggregates help in hindering the formation and growth of micro-cracks in concrete [51] and delay the appearance of macro-cracks. Li et al. [52] investigated the effect of the particle size and percentage of rubber content on the formation of visible cracks in rubber concrete containing 2, 4, 6, 8 and 10% of rubber content as a replacement of fine aggregates by mass. They found that the crack stress in rubber concrete decreased with the increase in rubber content and was more prominent and clearly defined in the finer particle rubber concrete compared to that of the coarser rubber particles. Their observation was also supported by their crack strain results, which showed that the crack strain of rubber concrete increased with the increase in rubber content and the increasing trend of the crack strains was more prominent in the rubber concrete containing

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smaller sized rubber particles than that with the coarser rubber particles. They noted that the waste tyre rubber significantly delayed the formation and growth of visible cracks in concrete and the effectiveness of its performance increased with the increase in rubber content and the decrease in the size of rubber particle. Kang and Jiang [53] looked at the effect of waste tyre crumb rubber particles on the cracking resistance of cement mortar specimens using the ring test method, replacing sand with crumb rubber at 10, 15, 20, 30, 40 and 50% by volume. They found that the cracking time of the mortar increased with the increase in rubber content up to a maximum of 20% rubber content. Beyond the 20% rubber content, they noticed a significant drop in cracking time i.e. at 30% rubber volume fraction, which decreased further with the increase in rubber content. This may be due to the fact that the addition of crumb rubber leads to a reduction in the tensile strength and the shrinkage stresses, however their degree of reduction varies with the increase or decrease of rubber content. When the rubber fraction is less than or equal to 20% by volume of sand, the reduction in shrinkage stress is higher than that of the tensile strength, resulting in the retardation of the cracking time. However, at rubber content of higher than 20% there is a greater reduction in tensile strength that results in the advancement of the cracking time. Li et al. [83], who investigated the crack development and failure pattern of NaOH treated and cement coated rubber particles, found that the width, length and the number of cracks decreased with the increase in rubber content (6, 12, 18% by volume of sand) and the cracks were more uniform and scattered compared to that of the control mix, which showed very wide and concentrated cracks. 4.2.2.10. Impact resistance. Reda Taha et al. [51] found that the resistance to impact energy increased with the increase in rubber content up to a maximum replacement level of 50%. With the further increase in rubber content, the impact resistance decreased with the increase in rubber content and the performance of chipped tyre rubber in impact resistance was significantly higher than that of the crumb rubber. At low to medium level of rubber content, a reasonable trade-off between the strength and flexibility of the composite matrix provided higher energy absorption capacity to the rubber cement composite, compared to that of the normal concrete. However, at higher replacement levels i.e. > 50% of rubber content, the strength component reduced further and went lower than an optimal threshold level that could provide both the strength and the energy absorption capacity, thereby reducing the energy absorption capacity of the rubber cement composite. Atahan and Yucel [113] studied the energy dissipation capacity of rubber concrete subjected to a dynamic impact test on rubber concrete at replacement levels of 20, 40, 60, 80 and 100% by volume of total aggregates. They found that the maximum load to failure decreases with the increase in rubber content, whereas the energy absorption by the specimens increases with the increase in rubber content up to a maximum of 80% replacement level. No further increase in energy dissipation was observed past the 80% replacement level. They noted that there was a 71.6% decrease in the maximum load and 160.8% increase in the energy dissipated at the maximum load in 100% replacement level rubber concrete compared to that of the control mix. The rubber particles having lower brittleness and elastic modulus began to control the dynamic properties of the rubber concrete at higher replacement levels, leading to the increase in the contact durations. The impact forces decreased with the increase in the rubber content in the concrete. They highlighted that these results are the desirable traits for the concrete safety barriers as they lead to smaller deceleration forces resulting in reducing the vehicle damage and occupant injury risks.

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Al-Tayeb et al. [108] investigated the effect of rubber content on the behaviour of rubber concrete subjected to impact bending at replacement levels of 5, 10 and 20% by volume of aggregates. They observed that the peak bending load and the fracture energy increased with the increase in rubber content up to the maximum replacement level of 20% because of the high plastic energy capacity of the rubber particles that improved the ductility and the ability to absorb the impact load. Aliabdo et al. [28] investigated the effect of 0, 20, 40, 60, 80 and 100% replacement levels of sand with tyre crumb rubber on the impact performance of rubberized concrete. They found that the first crack and the impact resistance to failure decreased with the increase in rubber concrete, because of the poor adhesion between the rubber particles and the cement paste. However, the number of blows resulting in the first visual crack and that causing failure increased with the increase in rubber content because of the ability of the rubberized concrete to absorb energy. Dong et al. [54] studied the effect of coating the rubber particles (size < 4.75 mm) with silane coupling agent on the energy absorption capacity of concrete subjected to low intensity impact bending load. The rubber content in their blended mix was 15 and 30% by volume of aggregates. They noted that although the absorbed energy and the ratio of energy over peak load of the rubber concrete containing coated rubber were lower than those of the control samples, they were slightly higher than those of the uncoated samples. They attributed this small improvement in the dynamic properties of the rubber concrete to the formation of strong bonds between the SCA coated rubber particles and the cement paste. Their observation of the reduction in the energy absorption by rubber concrete with the increase in rubber content was somewhat similar to what Aliabdo et al. [28] discovered. However, the observations made by Dong et al. [54] on both the coated and uncoated rubber concrete were quite contrary to that of all other investigations [51,108,113], where a significant improvement in the energy absorption capacity of the rubber concrete subjected to an impact load was observed. Zhang et al. [32] studied the effect of a chemical blend of 17.2% acrylic acid, 13.8% polyethylene glycol and 69% anhydrous ethanol by weight, on the impact performance of treated rubber. They found a considerable improvement in the impact performance of the treated rubber concrete compared to that of the untreated rubber. The treated rubber concrete was able to withstand the impact more times and demonstrated lesser variation between three replicates compared to the untreated rubber concrete. He et al. [37] looked at the effect of a combination treatment of KMnO4 and NaHSO3 on the impact resistance of crumb rubber concrete containing 4% of rubber powder by mass of concrete. They noted that the number of blows and the impact energy (J) to the first visible crack increased by 66.7% and 56.3% respectively with the addition of 4% of untreated rubber, compared to that of the control mix not containing any rubber. The treated rubber performed considerably better than that of the untreated rubber and improved the number of blows and the impact energy (J) to the first visible crack by 100% and 90.2% respectively, compared to that of the control mix. They attributed this improvement in the untreated rubber to the elastic property of the rubber particles that helped in dispersing the local stresses, and the treated rubber performed better because of its stronger interfacial chemistry, which provided a better stress dispersion compared to that of the untreated rubber. 4.2.2.11. Bond behaviour. Gesoglu et al. [55] studied the effect of size and percentage content of the tyre rubber on the bond behaviour of N16 rebar in rubber concrete containing two particle sizes (i) crumb rubber 4 mm and (ii) rubber chips – elongated particles size varying from 10 to 40 mm at aggregate replacement levels

of 5–30%. They observed that the bond strength of the reinforcement bar decreased with the increase in rubber content irrespective of the particle size. However, the smaller sized rubber particles performed better than the larger sized tyre chips and provided a higher bond performance at all replacement levels. The difference in the bond strength between the crumb rubber concrete and that of the rubber chips concrete increased with the increase in rubber content. The negative performance of tyre rubber particles on rebar-concrete bond strength was ascribed to its weak adherence to the cement matrix, cracking that occurs in the ITZ between the rubber and the cement paste and its lower friction with the reinforcement bar as reported by [75,130]. Bompa and Elghouli [131] looked at the bond slip behaviour of N16 and N20 reinforcement bars in confined and unconfined rubber concrete keeping the confinement pressure between 0.5 and 3.0 MPa and the rubber content of 0, 20, 40 and 60 vol% of aggregates. They found that the bond strength decreased with the increase in rubber content in all confinement levels (unconfined, low confinement and high confinement) irrespective of the size of bonded reinforcement bars and it increased with the increase in confinement pressure, proportionally up to about 20% of confinement pressure to concrete strength ratio and a constant behaviour thereafter. 4.2.2.12. Stress-strain behaviour. Li et al. [83] investigated the effect of NaOH treated and cement pre-coated rubber particles on the stress-strain behaviour of rubber concrete at 0, 6, 12 and 18% by volume of sand replacement levels. They observed that the peak stress and the strain at peak stress decreased with the increase in rubber content. They also recommended two popular concrete stress–strain relationship models (i) Popovic’s model and (ii) Carreira & Chu’s model, as they fit better than any other model with their rubber concrete experimental data. 5. Conclusions This review paper presents the findings from 100 research studies published in the last 30 years on 25 different rubber treatment methods to improve the mechanical properties of rubber concrete. This extensive review of research data led to the following conclusions 5.1. Workability It is evident from the literature that the workability of rubber concrete decreases with an increase in size and percentage volume of rubber aggregates. In addition, the mechanically ground tyre rubber provides lower workability compared to that of the cryogenic ground tyre rubber. However, this issue of lower workability could be addressed by the addition of an appropriate amount of superplasticizer. Interestingly, some rubber treatment methods like (i) 24-hour water soaking and (ii) treating with a blended mix of anhydrous ethanol, acrylic acid and polyethylene glycol (iii) UV-A radiation also proved to be very helpful in improving the workability of the rubber concrete mix. H2SO4 acid treatment showed conflicting results among various researchers, showing both increase and decrease in workability. All other treatments like, NaOH, Ca(OH)2, H2O2, CaCl2, KMnO4 & NaHSO4, SCA, CS2, and CH3COOH, have been used to treat tyre rubber particles in many studies but they do not provide any improvement in workability of the rubber concrete. 5.2. Density Since the specific gravity of tyre rubber is significantly lower than that of the conventional aggregates, it considerably reduces

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the density of the rubber concrete, which decreases with the decrease in rubber particle size and increases with the increase in rubber content.

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cement paste, cement mortar treatment methods provide elastic modulus at par or higher than the untreated rubber concrete. 5.5. Modulus of rigidity

5.3. Compressive strength The compressive strength of rubber concrete decreases with the increase in particle size and rubber content. A majority of the studies showed the same trend in decrease of compressive strength with an increase in particle size, however one of the studies showed a contradictory result with the compressive strength decreasing with the decrease in rubber particle size. Tyre rubber aggregates, because of their soft material property and poor bond behaviour with the cement matrix, significantly hamper the mechanical properties of the rubber concrete. Researchers have overcome some of these negative properties of the rubber aggregates, using various rubber treatment methods with varying degree of success. The simplest methods of rubber treatment that can be easily adopted in the industry (in the increasing order of difficulty) are water washing, 24-hour water soaking, NaOH treatment and the treatment with solvents like ethanol, methanol and acetone. Among these methods of treatment, the solvent treatment was the only method that resulted in the compressive strength of the rubber concrete being higher than that of the control mix, with acetone providing the highest improvement and ethanol the lowest, at 10% rubber replacement level. Among the various other rubber treatment methods that come with a higher degree of complexity, but considerably improve the mechanical properties at varying degrees of improvement were: a combination of tetrahydrofuran, tetraethyl orthosilicate and c-glycidyloxypropyl trimethoxysilane; partial oxidation at 250 °C; organic sulfur compounds; silane coupling agent; blend of acrylic acid and polyethylene glycol; and gamma radiation treatment, calcium disulfide. Among all these methods the partial oxidation of rubber particles at 250 °C provided the highest improvement in the compressive strength of the rubber concrete, which was higher than that of the control mix at 15% aggregate replacement level. 5.4. Flexural, split tensile strength and modulus of elasticity The effect of rubber particle size, percentage content and various treatment methods reflected similar trends in reducing the flexural, split tensile strengths and modulus of elasticity with the increase in rubber content as well as with the increase in rubber particle size, as that observed in the compressive strength properties. A couple of studies showed the opposite effect of particle size on the flexural strength improvement; however, the majority of the studies followed a trend of decrease in flexural strength with the increase in particle size. The only additional observation in regard to the flexural strength is that the concrete containing rubber shows much higher deflections before failure compared to that of the control mix not containing rubber. Among various rubber treatment methods studied todate water washing, precoating with cement paste, cement mortar, H2SO4 and CH3COOH treatment methods provide flexural strengths at par or higher than the untreated rubber concrete at replacement levels greater that 20%. At replacement levels lower than 20%, partial oxidation @ 250 °C, H2SO4, CH3COOH, Ca(OH)2, NaOH, organic sulfur compounds, and UV radiation treatment methods provided flexural strengths at par or better than the untreated rubber concrete. In regards to the split tensile strength, water wash, precoating with cement paste, cement mortar, partial oxidation @ 250 °C, organic sulfur compounds treatment methods provide split tensile strengths at par or higher than the untreated rubber concrete. In regards to the elastic modulus, water wash, water soaking, precoating with

Only one study has been conducted on the effect of tyre rubber on the modulus of rigidity of rubber concrete to the authors’ knowledge i.e. by Najim and Hall [21]. Water washing and treatment of rubber aggregates with NaOH showed a small improvement in the modulus of rigidity. Pre-coating the rubber particles with cement paste performed better than water washing and NaOH treatment in improving the modulus of rigidity of the rubber concrete. However, the highest improvement in the modulus of rigidity was provided by coating the rubber particles with cement mortar. 5.6. Abrasion resistance There are very few studies available on the effect of tyre rubber on the abrasion resistance of the rubber concrete, with the available studies showing conflicting results with the use of untreated rubber. However, from the limited data available, the treatment of rubber particles with saturated NaOH solution followed by washing with water significantly improved the abrasion resistance of the rubber concrete, compared to that of the untreated rubber. 5.7. Fatigue life Fatigue life of the concrete increases with the increase in rubber content and decreases with the increase in stress level. However, at all stress levels the rubber concrete performs better than that of the control mix, and the performance improves with the increase in the rubber content. 5.8. Fracture energy The fracture energy of the rubber concrete increases with the decrease in rubber particle size and increase in rubber content up to a maximum of 15%. With a further increase in rubber content the fracture energy decreases with the increase in rubber content. The treatment of rubber particles with saturated NaOH solution for 20 mins followed by water washing significantly improves the fracture energy of the rubber concrete, compared to that of the control mix containing no rubber, however, its performance is relatively lower than that of untreated rubber concrete. 5.9. Fracture toughness Fracture toughness parameters KIC (critical stress intensity), GIC (critical energy release rate) and Gf (fracture energy) increases with the increase in rubber content up to a maximum of 25%, compared to that of the control mix. With the further increase in rubber content, both the KIC and GIC parameters decrease with the increasing in rubber content. The fracture energy shows a dependence on the maximum deformation and the load capacity of the material. The total deformation of the concrete increases with the increase in rubber content, however its maximum load capacity decreases. Interestingly, the elastic–plastic toughness parameter (JIC), increases with the increase in rubber content up to a maximum of 75% and beyond that it shows a considerable drop in the JIC value. The tyre rubber particles absorb a part of the energy to which the cement matrix is subjected, thereby increasing the energy absorption capacity of the composite material before fracturing.

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5.10. Micro/Macro cracking Waste tyre rubber aggregates help in hindering the formation and growth of micro-cracks in concrete and delay the appearance of macro-cracks. Crack stress in rubber concrete decreases with the increase in rubber content and is more prominent and clearly defined in the finer particle rubber concrete compared to that of the coarser rubber particles. The cracking time of the cement composites containing rubber aggregates increases with the increase in rubber content up to a maximum of 20% and with a further increase in rubber content the cracking time drops significantly.

low and high concrete confinement levels. The concrete fails in three ways (i) fully split into two parts across the cross-section at failure in case of unconfined concrete (ii) pull-out with some splitting cracks across the cross-section in case of low level of confinement and (iii) straight pull-out failure in case of high level of concrete confinement. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

5.11. Static impact resistance Acknowledgements The resistance to impact energy to failure increases with the increase in rubber content up to a maximum of 50% in a static impact test. With the further increase in rubber content, the impact resistance decreases with the increase in rubber content. Moreover, the larger sized rubber particles perform significantly better in improving the impact resistance of the rubber concrete compared to that of the smaller sized rubber particles. The first crack and the impact resistance to failure decreases with the increase in rubber concrete containing finer rubber particles (i.e.  2 mm). However, the difference between the number of blows resulting in the first visual crack and that causing failure increases with the increase in rubber content. When the rubber particles are treated with a chemical blend of 17.2% acrylic acid, 13.8% polyethylene glycol and 69% anhydrous ethanol by weight, the impact performance of the treated rubber improves noticeably compared to that of the untreated rubber. By treating the rubber particles (size < 420 mm) with KMnO4 and NaHSO3 the number of blows and the impact energy to the first visible crack increases considerably compared to that of the untreated rubber concrete, which itself shows better performance than that of the control mix. 5.12. Dynamic impact resistance In a dynamic impact test, the maximum load to failure decreases with the increase in rubber content, whereas the energy absorbed by the specimens increases with the increase in rubber content up to a maximum of 80% replacement level. The decrease in maximum load and the increase in dissipated energy at maximum load, in addition to the increase in total impact time leads to smaller deceleration forces resulting in the reduction of the severity of vehicle damage and occupant injury risks associated with rubber concrete safety barriers. In regard to the impact bending, the peak bending load and the fracture energy increases with the increase in rubber content up to the maximum replacement level of 20%. 5.13. Bond behaviour Bond strength of the reinforcement bar decreases with the increase in rubber content irrespective of the particle size and the concrete confinement level (i.e. unconfined, low confinement and high confinement). However, the smaller sized rubber particles perform better than the larger sized particles at all replacement levels. Varying the diameter of the reinforcement bar does not show any significant effect on the performance of bond strength in rubber concrete up to an aggregate replacement level of 20%, at any level of confinement. However, at rubber content of 40–60% by volume of fine aggregates, the high confinement concrete shows a significant increase in bond strength of 16Ø reinforcement bars. The 20Ø bars show a similar behaviour to that of 16Ø at 40% rubber content, but at rubber content of 60% the bond strength of the 20Ø bar shows a significant improvement at both

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