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Assessment of the mechanical performance of crumb rubber concrete
Construction and Building Materials 125 (2016) 175–183
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Assessment of the mechanical performance of crumb rubber concrete Osama Youssf a,b,⇑, Julie E. Mills a, Reza Hassanli a a b
University of South Australia, Adelaide, Australia Mansoura University, Mansoura, Egypt
h i g h l i g h t s Enhancement study of CRC mechanical performance was carried out. Effect of pre-treatment period, SF additives, and cement content was assessed. 3
0.5 h pre-treatment, 0% SF, and 350 kg/m cement were the best alternatives. More than 0.5 h pre-treatment did not significantly affect CRC properties. 3
Higher than 350 kg/m cement did not significantly affect CRC properties.
a r t i c l e
i n f o
Article history: Received 4 February 2016 Received in revised form 7 June 2016 Accepted 10 August 2016 Available online 17 August 2016 Keywords: Rubberized concrete Crumb rubber Rubber treatment Silica fume Cement content
a b s t r a c t Crumb rubber concrete (CRC) has some known shortcomings in mechanical performance compared to conventional concrete, particularly with respect to compressive strength. Many previous researchers have tried to overcome the material deficiencies using different methods; however, the results have often been contradictory and highly variable. In this research, three methods to improve and then assess the mechanical performance of CRC have been examined namely, rubber pre-treatment using sodium hydroxide (NaOH) solution, using silica fume additives, and increasing concrete cement content. The effect of the rubber pre-treatment time (0–2 h), silica fume content (0–15%), and cement content (300–400 kg/m3) on CRC slump, short and long term compressive strength, and tensile strength were measured for fifteen concrete mixes prepared with 0% and 20% rubber content. Six 100 200 mm cylinders were prepared from each mix for evaluating the compressive strength at 7 and 28 days. Four additional 100 200 mm cylinders were prepared from two mixes for evaluating the compressive strength at 56 and 84 days. In addition, two 150 300 mm cylinders for each mix were prepared and tested to determine the indirect tensile strength at 28 days. The results showed that 0.5 h of rubber pre-treatment using NaOH solution, 0% of silica fume replacing cement by weight, and 350 kg/m3 cement content were the best alternatives in this assessment range to enhance the CRC performance. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Due to the health and environmental risks presented from used tyre waste [1] as well as the scarcity and cost of natural mineral aggregates [2], a significant body of recent research has focussed on utilizing used tyre rubber in concrete as a partial replacement of its mineral aggregates, resulting in a class of concrete called crumb rubber concrete (CRC). The recycling of used rubber conserves valuable natural resources and reduces the amount of rubber entering landfill [3]. Previous experimental studies on CRC materials have shown that using rubber in concrete enhanced its ductility, toughness, impact resistance, energy dissipation, and ⇑ Corresponding author at: University of South Australia, Adelaide, Australia. E-mail address: [email protected] (O. Youssf). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.040 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
damping ratio [4–7]. However, it reduced its compressive strength, tensile strength, and modulus of elasticity compared to conventional concrete [8–13]. Some of the main reasons for this strength reduction are the low hydraulic conductivity and the smooth surface of rubber particles, which both result in poor rubber/cement interface adhesion [14–17]. This poor adhesion is also attributed to the existence of zinc stearate which is used in tyre formulation during manufacturing. This zinc stearate migrates and diffuses to the rubber surface creating a soap layer that repels water [9]. To increase the effectiveness of using rubber in concrete, several approaches have been previously introduced. Of these approaches; the chemical pre-treatment of rubber particles using sodium hydroxide (NaOH) solution, replacing part of the cement by silica fume (SF) additive, and increasing the rubberized concrete cement content are the most common. However, the degrees of
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treated CRC. Turatsinze et al. [27] mentioned that the strength benefit due to the NaOH rubber pre-treatment was not substantial. Youssf et al. [9] investigated the effect of cement content, NaOH pre-treatment of rubber for 30 min, and replacing 10% of cement weight by SF on the mechanical properties of CRC. Their results showed that the losses in CRC compressive strength with 425 kg/m3 cement content were less than when using 350 kg/m3 cement content. In addition, they reported that when using pre-treated rubber, the concrete slump and tensile strength decreased by 25% and 13%, respectively. But, the compressive strength and modulus of elasticity increased by 15% and 12% respectively, compared to non-treated rubber. No effect was observed in their results when using SF except a slight increase in the compressive strength at rubber content of 20% by sand volume. Albano et al. [28] studied concrete composites containing scrap rubber previously treated with NaOH and SILAN coupling agent in order to enhance the adhesion between the rubber and the cement paste, but did not notice any significant changes when compared to the non-treated rubber composites. The contradictions and variations in the previous research findings indicate the need for future research in CRC performance enhancement. This paper investigates the mechanical properties of fifteen CRC mixes. The effects of rubber pre-treatment period, SF content, and cement content on the fresh and hardened properties of CRC were examined with the aim of assessing the CRC mechanical performance. These data provide additional information necessary to support the further development of CRC.
effectiveness using these approaches have been inconsistent and scattered in research published to date. Balaha et al. [18] experimented with three different cement contents namely, 300 kg/m3, 400 kg/m3, and 500 kg/m3 in rubberized concrete mixes containing up to 20% rubber replaced by sand volume, and they treated the rubber particles using NaOH solution for 30 min. In addition, they replaced 15% of cement by weight with SF. Their results showed that the CRC properties improved with cement content increase up to 400 kg/m3. Beyond 400 kg/m3 cement content, only slight improvements were observed. However, the slump was negatively affected when using 400 kg/m3. Using SF and NaOH pre-treatment, increased concrete slump by 77% and 7%, respectively, increased compressive strength by 18% and 15%, respectively, and increased tensile strength by 9% and 6%, respectively. Eldin and Senouci [19] treated rubber particles in NaOH solution for 5 min before use in CRC and achieved 16% increase in the compressive strength. Pelisser et al. [20] used NaOH pre-treated rubber combined with adding 15% SF by cement weight to the concrete mix. They reported almost total recovery of the concrete compressive strength. Güneyisi et al. [21] have observed lower workability but higher compressive strength of CRC by using SF. In addition, the positive effect of SF on the strength decreased as the rubber content increased. Mohammadi et al. [22] used pre-treated rubber in NaOH solution for 20 min, 2 h, 24 h, 48 h, and 7 days. Their results showed that 24 h is the best treatment period for the rubber as it resulted in the highest compressive strength and flexural strength. However, this pre-treatment had no effect on concrete slump. Hamza and Ghedan [23] washed rubber particles in NaOH solution before adding a coupling agent called SILAN to the rubberized concrete. Their results showed that the compressive strength improved by 74% compared to non-treated rubber mix. Other researchers reported less positive or contradictory results from these approaches. Deshpande et al. [24] used modified rubber by saturating it in NaOH solution for 20 min. Their results showed almost no difference between the compressive and tensile strengths of pre-treated and non-treated rubber mixes. However, 12% increase in the flexural strength was reported for the pretreated rubber mix. Tian et al. [25] reported 3.7% reduction in CRC compressive strength using NaOH pre-treated rubber for 24 h followed by tap water wash for 3 h compared to nontreated rubber. Li et al. [26] treated rubber particles using NaOH solution for 30 min and found that the properties of pre-treated rubberized concrete were nearly the same as those of non-
2. Experimental program Several factors could potentially affect the concrete properties, including water to cement ratio, rubber content, and cement content. This experimental programme focuses on the factors affecting the adhesion at the rubber/cement interface in the concrete matrix, such as the cement content, rubber pre-treatment, and SF content. The poor rubber/cement interface adhesion is one of the main sources of the deficiencies in the rubberized concrete properties [14–17]. Increasing the rubber content in concrete enhances its dynamic properties [4–7]. However, using more than 20% rubber in concrete may magnify the adverse effects on concrete characteristics, as recommended by Khatib and Bayomy [16]. In this research the performance of concrete mixes incorporating 0 and
Table 1 Proportions of concrete mixes. Mix code
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15
Rs (%)
0 20 0 20 0 20 20 20 20 0 20 0 20 0 20
Pre-treatment of rubber (period)
– NaOH (0.5 h) – NaOH (0.5 h) – NaOH (0.5 h) No treatment NaOH (1.0 h) NaOH (2.0 h) – NaOH (0.5 h) – NaOH (0.5 h) – NaOH (0.5 h)
RS, Per cent of sand volume replaced by rubber. SF, Per cent of cement replaced by silica fume. SP, Superplasticizer dosage.
SF (%)
Mix proportions (kg/m3) Cement
– – – – – – – – – 5 5 10 10 15 15
350 350 400 400 300 300 350 350 350 333 333 315 315 298 298
SF
– – – – – – – – – 18 18 35 35 53 53
Sand
866 693 814 651 916 733 693 693 693 862 690 859 687 856 685
Dolomite 10 mm
20 mm
311 311 293 293 330 330 311 311 311 310 310 309 309 308 308
727 727 684 684 769 769 727 727 727 724 724 722 722 719 719
Rubber
Water
SP
– 55.5 – 41.8 – 58.8 55.5 55.5 55.5 – 55.3 – 55.1 – 54.9
175 175 200 200 150 150 175 175 175 175 175 175 175 175 175
6.3 6.3 7.2 7.2 5.4 5.4 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3
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20% of crumbed scrap tyre rubber as a partial volume replacement of fine aggregates was experimentally investigated. The effect of rubber pre-treatment period, silica fume content, and cement content on CRC slump, short and long term compressive strength, and tensile strength were examined for fifteen concrete mixes by testing 128 standard concrete cylinders. 2.1. Concrete materials and variables Table 1 summarises the different components of all concrete mixes used in this study. General purpose cement type with specific gravity of 3.15, according to Australian Standard (AS) AS 3972 [29], was used as the binder material in the concrete mixes. Densified SF with specific gravity of 2.2 was used as a partial replacement of concrete cement by weight. Dolomite stone having nominal maximum sizes of 10 mm and 20 mm was used as coarse aggregates. River sand with a maximum aggregate size of 5 mm was used as fine aggregate. The crumb rubber used during the course of this study had two particle sizes of 1.18 and 2.36 mm and was used as a partial replacement of sand by volume (Table 1). The sieve analyses for all used aggregates are shown in Fig. 1. The specific gravity, unit weight, and fineness modulus were 2.72, 1570 kg/m3, and 6.02 respectively for dolomite; 2.65, 1630 kg/m3, and 2.36 respectively for sand; and 0.85, 530 kg/m3, and 4.53 respectively for rubber. Polycarboxylic ether type superplasticizer (SP) with a specific gravity of 1.08 was added to the concrete mixtures to achieve the required concrete workability. The variables in this study were; the pre-treatment period of rubber particles using 10% NaOH solution for 0.0 h, 0.5 h, 1.0 h, and 2.0 h; the SF content as a partial replacement of cement weight by 0%, 5%, 10% and 15%; and the concrete cement content of 300 kg/m3, 350 kg/m3, and 400 kg/m3. 2.2. Pre-treatment of rubber The pre-treatment of rubber particles can play an important role in improving the rubber/cement interface adhesion. Pretreatment of rubber using NaOH solution removes the zinc stearate layers that are present on the rubber surface of tyres as a result of the manufacturing process [30]. The NaOH solution is able to eliminate the additives on the rubber surface, leaving voids on the rubber outer surface that lead to a relatively rough and porous surface, compared with non-treated rubber. Because of the eroding effect of this acid solution on rubber particles, the surface of these particles is scraggy, which can improve the cohesion strength between rubber particles and cement [25]. In addition, it increases the hydraulic conductivity, rubber/cement water transfer rate, and
Fig. 1. Sieve analysis of the used aggregates.
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hydration at the interface, which improves the rubber/cement adhesion [19,31–37]. Youssf et al. in previous research [9,12,38] have successfully pre-treated the rubber particles by NaOH solution before using them in concrete. In the present study, pre-treatment of the rubber particles with 10% concentration NaOH solution was used with different treatment periods, namely 0.0 h, 0.5 h, 1.0 h, and 2.0 h. The process commenced with the rubber particles being washed by tap water to remove any impurities and dust. They were then submerged into NaOH solution for the required period in a container. Finally, the rubber particles were washed again by stirring in water until its pH became 7, and then they were left to air dry. The final washing is essential to remove any remaining NaOH solution to prevent any negative effect on the concrete durability. Fig. 2 shows the procedure of the rubber pre-treatment process. It was observed that submerging the rubber into the NaOH solution increased its pH to 14. Every 5 min during washing, the pH of the rubber solution was recorded using a pH meter. The total time that was needed to wash the rubber to fully remove the NaOH solution after the treatment process ranged from 30 to 45 min. 2.3. Concrete mix designs The concrete mixes were designed according to AS 1012.2 [39]. The target compressive strength of the control mix (M1) was 50 MPa. All mixes were designed with constant water to binder (W/B) ratio of 0.5 and SP content of 1.8% by Binder weight. The fine/coarse aggregate ratio was 1/1.2 by weight. The 10 mm /20 mm coarse aggregate ratio was 1/2.34 by weight. The mixing procedure for the control mixes was as follows: mix dry sand and dolomite for 1 min; add half of the water and mix for 1 min; rest for 2 min; add cementitious materials, water, and admixtures, and then mix for 2 min. The same procedure was followed for the CRC mixes; except that the rubber aggregate was first mixed with the dry cementitious materials for 1 min in an external container, aiming to increase the rubber-cement interface adhesion, which is one of the known main factors affecting CRC strength. 2.4. Specimen preparation and testing The standard slump test according to AS 1012.3.1 [40] was used to measure the concrete workability for each mix, as shown in Fig. 3(a). Six 100 200 mm cylinders were prepared from each mix: three for evaluating the compressive strength at 7 days; and three for evaluating the compressive strength at 28 days. Four additional 100 200 mm cylinders were prepared from each mix of M10 and M11; two for evaluating the compressive strength at 56 days; and two for evaluating the compressive strength at 84 days. This resulted in a total of ninety-eight cylinders from the fifteen mixes. In addition, two 150 300 mm cylinders for each mix were prepared and tested to determine the indirect tensile strength at 28 days, resulting in a total of thirty cylinders from the fifteen mixes. The traditional compaction method for the cast concrete was done using a standard compaction rod and hammer. All specimens were de-moulded after 24 h, and labelled for the various tests. Then they were cured in a water bath at 23 ± 2 °C, according to AS1012.8.1 [41]. At the test day, the tests were conducted according to the appropriate Australian Standard under monotonic loading until failure occurred. The compression test was carried out according to AS 1012.9 [42] using an 1800 kN capacity testing machine with a constant loading rate of 20 ± 2 MPa/min. Indirect tensile tests were performed according to AS 1012.10 [43] using a constant loading rate of 1.5 ± 0.15 MPa/min. Fig. 3 shows the concrete tests conducted in this study.
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1
2
3
4
Fig. 2. Rubber particle pre-treatment process.
Fig. 3. CRC tests: (a) slump, (b) compressive strength, and (c) indirect tensile strength.
3. Experimental results and discussion In this section, the effects of rubber pre-treatment period, silica fume content, and cement content on CRC slump, short and long term compressive strength, and tensile strength are discussed. Table 2 shows the experimental results of all concrete mixes in this study. 3.1. Effect of rubber pre-treatment period The effects of rubber pre-treatment using NaOH solution on concrete slump, 7 and 28 days compressive strength, and tensile strength were determined through comparison of the results of mixes M1, M2, M7, M8, and M9. Mixes M1 and M7 are comparable
in terms of the effect of using non-treated rubber on the concrete properties. Mixes M7, M2, M8, and M9 are comparable in terms of the effect of pre-treatment periods of 0.0 h, 0.5 h, 1.0 h, and 2.0 h on concrete properties, respectively. As shown in Fig. 4(a), using non-treated rubber in M7 (0.0 h pre-treatment) increased the concrete slump by 26.4% compared to that of the control mix M1. This was attributed to the poorlygraded rubber used in this study with a 4.53 fineness modulus that increased the fineness modulus of the hydride aggregates overall (sand and rubber) compared to sand with 2.36 fineness modulus in the conventional concrete. Increasing the fineness modulus of the concrete aggregates increases its workability [44]. On the other hand, pre-treatment of rubber particles had a minor negative effect on the concrete slump compared with no pre-treatment.
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O. Youssf et al. / Construction and Building Materials 125 (2016) 175–183 Table 2 Experimental results. Mix code
Rs (%)
Pre-treatment of rubber (period)
SF (%)
Slump (mm)
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15
0 20 0 20 0 20 20 20 20 0 20 0 20 0 20
– NaOH – NaOH – NaOH – NaOH NaOH – NaOH – NaOH – NaOH
– – – – – – – – – 5 5 10 10 15 15
170 207 230 210 5 12 215 205 205 165 210 65 165 20 155
(0.5 h) (0.5 h) (0.5 h) (1.0 h) (2.0 h) (0.5 h) (0.5 h) (0.5 h)
Compressive strength (MPa) 7 Days
28 Days
56 Days
84 Days
44.9 36.0 47.3 37.9 41.8 31.7 31.2 33.5 32.3 44.4 32.3 38.1 30.8 37.7 29.5
53.5 42.1 56.1 45.9 50.3 35.0 35.9 38.6 37.2 55.4 39.7 50.6 37.3 47.7 36.8
– – – – – – – – – 54.2 35.8 – – – –
– – – – – – – – – 57.6 42.5 – – – –
Tensile strength (MPa) 4.1 3.1 4.4 3.2 3.6 2.9 2.7 3.1 3.2 4.3 3.4 4.2 3.1 3.9 3.1
RS, Percent of sand volume replaced by crumb rubber. SF, Percent of cement replaced by silica fume.
Pre-treatment of rubber particles for 0.5 h in M2, 1.0 h in M8, and 2.0 h in M9 increased the concrete slump by 21.8%, 20.6% and 20.6%, respectively, compared with the conventional concrete mix but decreased the concrete slump by 3.7%, 4.6%, and 4.6%, respectively, compared with the non-treated rubber mix. This was attributed to the relatively rough surface of pre-treated rubber compared to non-treated rubber that resulted from the surface eroding caused by the acid solution. This eroding effect resulted in relatively slow movement of rubber particles in the concrete matrix and hence slump reduction. Increasing the rubber pretreatment period had no significant effect on the CRC slump. As the rubber exposure time to the NaOH solution increases, the solution penetrates the rubber particle and decreases its stiffness rather than eroding its outer surface. This lesser stiffness of the rubber particles at longer pre-treatment period has an insignificant effect on the concrete workability. Fig. 4(b) shows the variation of the CRC compressive strengths at ages of 7 and 28 days. As shown in the figure, using nontreated rubber particles in concrete mix M7 decreased its compressive strength by 30.5% and 32.8% at 7 and 28 days, respectively, compared with conventional concrete mix M1. However, the NaOH pre-treatment of rubber was able to recover part of the strength reduction as shown in Fig. 4(b). The most effective strength recovery was observed when using 0.5 h pre-treated rubber in M2 that resulted in compressive strength increase of 15.3% and 17.2% at 7 and 28 days, respectively, compared to non-treated rubber in M7, but still a reduction of 19.8% and 21.3% at 7 and 28 days, respectively, compared to the conventional concrete mix M1. The texture and stiffness of the rubber particles by nature mainly affect the CRC strength [9]. The relatively smooth texture of rubber particles results in low bond between rubber particles and cement mortar. The lower stiffness of rubber particles produces higher internal tensile stresses that cause early failure in the cement mortar and then strength reduction, and finally, the poor absorption of rubber particles compared with sand reduces the penetration of the rubber aggregate by cement paste which then results in poor rubber/cement interface adhesion. Using NaOH pre-treatment for rubber particles increases the roughness of their outer surface which leads to increased adhesion between rubber and the surrounding cement paste and hence strength increase. Increasing the rubber pre-treatment period beyond 0.5 h in M8 and M9 had a negative effect on compressive strength. The compressive strengths at 7 and 28 days decreased by 6.9% and 8.3%, respectively, for 1.0 h pre-treated rubber in M8 and by 10.2% and 11.6%, respectively, for 2.0 h pre-treated rubber in M9 compared to
0.5 h pre-treated rubber in M2. This was attributed again to the higher penetration of the NaOH solution into the rubber particles that decreased their stiffness and hence decreased the overall stiffness of the concrete matrix. The effect of rubber pre-treatment period on CRC tensile strength is shown in Fig. 4(c). This figure shows that the NaOH pre-treatment of rubber in M2, M8, and M9 was able to recover part but not all of the 34.1% tensile strength reduction caused by using non-treated rubber in M7, and the tensile strength reduction enhanced to only 24.4% compared with conventional concrete mix M1. Increasing the rubber pre-treatment period for longer than 0.5 h in both M8 and M9 had no further effect on the tensile strength. From the above results, it can be concluded that the best rubber pre-treatment period using 10% NaOH solution in this assessment range is 0.5 h, as it resulted in higher compressive and tensile strength compared to the other pre-treatment periods, and improved the slump compared with conventional concrete by about 22%. 3.2. Effect of silica fume additive The effects on concrete slump, short and long term compressive strength, and tensile strength of partially replacing cement by SF additive were determined through comparison of the results of mixes M1, M2, M10, M11, M12, M13, M14 and M15. Mixes M1, M10, M12, and M14 are comparable in terms of the effect of replacing 0%, 5%, 10%, and 15% of concrete cement by SF, respectively, at no rubber content on concrete properties. Mixes M2, M11, M13, and M15 are comparable in terms of the effect on concrete properties of replacing 0%, 5%, 10%, and 15% of concrete cement by SF, respectively, at 20% rubber content that was pretreated for 0.5 h using NaOH solution. As shown in Fig. 5(a), at 0%, 5%, 10%, and 15% SF content, using 20% rubber (in concrete mixes M2, M11, M13, and M15) increased the concrete slump 1.22, 1.27, 2.53 and 7.75 times, respectively, compared with using 0% rubber (in concrete mixes M1, M10, M12, and M14). This was again attributed to the increase in the fineness modulus of the hydride aggregates overall (sand and rubber). By increasing the SF content up to 5% the slumps of both CRC and conventional concrete were not significantly affected. At 10% and 15% SF contents, the CRC slump decreased by 20% (mix M13) and 25% (mix M15), respectively, compared to 0% SF (mix M2). However, at 10% and 15% SF contents, the conventional concrete slump severely decreased by 62% (mix M12) and 88% (mix M14),
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Fig. 4. Effects of rubber pre-treatment on concrete: (a) slump, (b) 7 and 28 days compressive strength, and (c) tensile strength.
respectively, compared to 0% SF (mix M1). The reduction in concrete workability with SF increase was attributed to the relatively small particle size of the SF that increases the tendency of cement particles to agglomerate in the paste, trapping some of the mixing water and hence resulting in slump reduction. Fig. 5(b) shows the effect of SF content on concrete compressive strengths at 7 and 28 days. As shown in the figure, the patterns of the compressive strength for the conventional concrete and CRC at 7 and 28 days are almost the same, with the compressive strength decreasing as the SF content increased. However, the rate of strength reduction in CRC was less than that in the conventional
Fig. 5. Effects of silica fume on concrete: (a) slump, (b) compressive strength, and (c) tensile strength.
concrete. Compared to the CRC without SF, using 5%, 10%, and 15% SF (in mixes M11, M13, and M15, respectively) decreased the CRC compressive strength by 10%, 14%, and 18%, respectively, at 7 days; and by 6%, 11%, and 12%, respectively, at 28 days. Compared to the conventional concrete without SF, using 5%, 10%, and 15% SF (in mixes M10, M12, and M14, respectively) decreased the conventional concrete compressive strength by 1%, 15%, and 16%, respectively, at 7 days. However, at 28 days concrete age, the conventional concrete compressive strength slightly increased by 3% for 5% SF and then decreased by 5% and 11% for 10% and 15% SF, respectively, compared to the conventional concrete without SF. Adding SF to concrete has physical and chemical effects that could improve its strength. The physical effect is the micro filling
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of the concrete voids by SF particles which improves the microstructure of the concrete matrix and results in more stronger and durable concrete. The chemical effect is the utilizing of the whole or part of the calcium hydroxide generated during the hydration process of cement and producing calcium silicate hydrate. This calcium silicate hydrate is able to increase the concrete compressive strength [44]. Previous researchers reported 6–67% increase in concrete compressive strength when replacing 5–40% of cement by SF. According to the results of this research, at 5% SF, the compressive strength marginally enhanced or diminished especially at 28 days; however, at higher SF contents, the compressive strength decreased. This reduction may be due to the amount of SF that was added being more than the required amount for the filling mechanism and pozzolanic chemical action, hence strength reduction occurred. More experimental research is recommended using SF content of less than 5%. Fig. 5(c) shows the effect of using SF on concrete tensile strength which indicates that 5% SF improved the tensile strength for the conventional concrete (mix M10) and the CRC (mix M11) by 5% and 10%, respectively. However, increasing SF content beyond 5% decreased the tensile strength for both the conventional concrete and the CRC. In order to investigate the effect of SF at later ages on CRC compressive strength, mixes M10 and M11 that included 5% SF were tested at 56 and 84 days. Fig. 6 shows the evolution of the concrete strength with time. As shown in the figure, the increase in the compressive strength at ages longer than 28 days was insignificant. However, the rate of the CRC compressive strength increase at 84 days was slightly higher than that of the conventional concrete. At 84 days, the compressive strength increased by 4% for the conventional concrete mix M10 and by 7% for the CRC mix M11, compared to the corresponding 28 day compressive strengths. ACI 234-96 [45] suggests that the main contribution of silica fume to concrete strength development takes place between about 3–28 days and that the contribution of silica fume to strength development after 28 days is minimal. From the above results and based on this assessment range, it can be concluded that using SF to replace part of the cement with the intention of enhancing CRC performance was not effective and in fact resulted in some negative effects on the concrete slump and compressive strength, with only a very marginal improvement in tensile strength at 5% SF.
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through comparison of the results of mixes M1, M2, M3, M4, M5 and M6. Mixes M1, M3, and M5 are comparable in terms of the effect of cement content on concrete properties containing no rubber. Mixes M2, M4, and M6 are comparable in terms of the effect of cement content on concrete properties containing 20% rubber content. The cement contents used in this assessment were 300 kg/m3, 350 kg/m3, and 400 kg/m3. As shown in Fig. 7(a), at 300 kg/m3 and 350 kg/m3 the CRC (M6 and M2) showed slump 2.4 and 1.2 times that showed by the conventional concrete (M5 and M1), respectively. This was attributed to the increase of the fineness modulus that occurred by using
3.3. Effect of cement content The effects of cement content on concrete slump, 7 and 28 day compressive strength, and tensile strength were determined
Fig. 6. Evolution of concrete compressive strength for 5% SF content.
Fig. 7. Effects of cement content on concrete: (a) slump, (b) 7 and 28 day compressive strength, and (c) tensile strength.
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rubber with higher fineness modulus than the replaced sand. At 400 kg/m3 the CRC (M4) showed slump 0.9 times that showed by the conventional concrete (M3). This slump reduction was attributed to the reduction in the aggregates to cement ratio at a given concrete volume compared to lower cement contents (300 kg/m3 and 350 kg/m3). This increased the effect of the cement paste rather than the concrete aggregates’ fineness modulus because the quantity of cement paste that became available for providing lubrication per unit surface area of aggregate increased. However, the irregular shape and surface texture of rubber particles in addition to their low adhesion with cement compared to sand particles resulted in slower mobility of the concrete matrix and hence slump reduction. Although the increase of cement content increased concrete slump as shown in Fig. 7(a), this increase was less significant in the CRC compared to the conventional concrete. Compared to 300 kg/m3 (M5 and M6), using 350 kg/m3 and 400 kg/m3, increased slump by 34 and 46 times (M1 and M3), respectively, for the conventional concrete and by 17 and 18 times (M2 and M4), respectively, for the CRC. This indicated that increasing cement content in CRC mixes beyond 350 kg/m3 is not worthwhile in terms of enhancing its workability. Fig. 7(b) shows the effect of cement content on concrete compressive strengths at 7 and 28 days. As shown in the figure, the patterns of the compressive strength for the conventional concrete and CRC at 7 and 28 days are the same, namely that the compressive strength increased with cement content increase, as expected. However, the rate of strength increase in the CRC was higher than that in the conventional concrete. Compared to 300 kg/m3 (M5 and M6), using 350 and 400 kg/m3 increased the compressive strength by 7% and 13% (M1 and M3), respectively, for the conventional concrete at 7 days and by 13% and 19% (M2 and M4), respectively, for CRC at 7 days. At 28 days these increases were 6% and 11%, respectively, for the conventional concrete and 20% and 31%, respectively, for CRC. The higher rate of strength increase using rubber was attributed to the low water absorption of rubber compared to sand [22]. This provides more water for cement hydration reaction and early curing, hence rapid strength development. In the case of CRC, increasing the cement content from 300 to 350 kg/m3 was more significant than from 350 to 400 kg/m3. Using 400 kg/m3 increased the CRC compressive strength by only 5% and 9% at 7 and 28 days, respectively, compared to compressive strength using 350 kg/m3. On the other hand, the compressive strength increased by 13% and 20% at 7 and 28 days, respectively, when the cement content increased from 300 to 350 kg/m3. This was attributed to the fact that the cement paste fills the voids between the aggregates in the concrete matrix. Using rubber in concrete increases its voids (air content) [46] compared to conventional concrete with the same mixing and compacting conditions. This indicates that CRC needs more cement to fill these voids. However, the nature of the rubber surface texture and the low cement/rubber interface adhesion keeps the voids around rubber particles constant with increasing cement content. Thus, once the cement content reached the required minimum amount to fill other voids, the effect of increasing its content was less significant [47]. Due to the high correlation between the concrete compressive and tensile strengths, the effect of cement content on the tensile strength was almost the same as that on the compressive strength, as shown in Fig. 7(c). For the CRC, the tensile strength increased by 7% with cement content increase from 300 to 350 kg/m3. Beyond that, increasing cement content had no significant effect on CRC tensile strength. From the above results, it can be concluded that the best cement content in this assessment range that enhanced the CRC performance was 350 kg/m3 as it showed the highest significant improvement in concrete slump, compressive strength and tensile strength. Cement content higher than 350 kg/m3 was able to
further improve the CRC performance but not at a significant enough rate to warrant the increased cost that might be incurred. 3.4. Recommendations for CRC mix design Based on the results obtained in this experimental study, the most significant improvements in the CRC fresh and hardened mechanical performance for a CRC mix containing 20% rubber replacement of sand volume resulted from 0.5 h pre-treatment period with NaOH, 0% SF content, and 350 kg/m3 cement content. The target conventional concrete compressive strength of 50 MPa was reduced to 42.1 MPa and the slump increased from 170 mm to 207 mm. The crumb rubber used in the mix had two particle sizes of 1.18 and 2.36 mm with a sieve analysis as shown in Fig. 1. The recommended design of the CRC mix is: Cement = 350 kg/m3 Superplasticizer = 1.8% by cement weight Dolomite 10 mm = 311 kg/m3
W/C = 0.5 Sand = 693 kg/m3 Dolomite 20 mm = 727 kg/m3 Pre-treated rubber for 0.5 h in 10% NaOH solution = 55.5 kg/m3
4. Summary and conclusions This paper investigates the mechanical properties of fifteen CRC mixes aiming to assess the mechanical performance of a 20% rubber content CRC mix by measuring the effect of the rubber pretreatment period, SF content, and cement content on its slump, short and long term compressive strength, and tensile strength. The results of this investigation are summarised as follows: 1. Using 20% rubber in concrete increased the concrete slump by 1.22, 1.27, 2.53 and 7.75 times that of 0% rubber mixes at 0%, 5%, 10%, and 15% SF content, respectively. 2. Pre-treatment of rubber particles for 0.5 h in 10% NaOH solution was the best pre-treatment period in this study. It increased the CRC slump by 22% compared to the conventional concrete slump. In addition it recovered 15.3% and 17.2% of the compressive strength lost by using non-treated rubber at 7 and 28 days, respectively. Furthermore, it increased the CRC tensile strength by 15% compared to non-treated rubber. Increasing the rubber pre-treatment period more than 0.5 h had no significant effect on the concrete slump and tensile strength and it decreased the compressive strength. 3. Using SF as partial replacement of cement was not useful and showed some negative effects on the concrete slump and compressive strength. It also made negligible improvement on the long term compressive strength. 4. The cement content that most enhanced the CRC performance in this study was 350 kg/m3. It showed the highest significant improvement in concrete slump, compressive strength and tensile strength. Although cement content greater than 350 kg/m3 was able to improve the CRC performance slightly further, the rate of improvement was not significant. At 300 kg/m3, 350 kg/m3, and 400 kg/m3 cement contents, the CRC showed slump 2.4, 1.2, and 0.9 times, respectively, that shown by the conventional concrete. The CRC compressive and tensile strengths at 28 day increased by 20% and 7%, respectively, with cement content increase from 300 to 350 kg/m3. Beyond that, the increases in the compressive and tensile strengths were less significant and not worthwhile economically. 5. Based on the results obtained in this experimental study, the recommended design of the CRC mix is 350 kg/m3 cement content, 0.5 water to binder ratio, 693 kg/m3 sand, 55.5 kg/m3
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pre-treated crumb rubber for 0.5 h in 10% NaOH solution, 311 kg/m3 of 10 mm dolomite, 727 kg/m3 of 20 mm dolomite, and superplasticizer dosage of 1.8% by cement weight. A cost analysis of the proposed CRC assessment is planned to be undertaken by the authors in future work. The upfront cost investigation of the CRC shows that it is slightly more expensive than the conventional concrete depending on the availability of shredded tyres and their local price. However, looking at the life cycle cost, a rubberized concrete structure can provide cost effectiveness performance compared to a conventional concrete structure [48]. This is due to the advantages of using rubber in concrete that including lower maintenance cost, providing environmentallyfriendly solutions in structures with high dynamic properties, increasing the structure sustainability, saving energy and natural resources, and reducing/eliminating the adverse effects of dumping end-of-life tyres to land fill. Acknowledgment The authors gratefully acknowledge the donation of the following: sand by Premix Concrete SA Pty. Ltd, aggregate by Hallett Concrete Australia Pty. Ltd, cement by Adelaide Brighton Cement Pty. Ltd, superplasticizer by BASF Construction Chemicals Australia Pty. Ltd. The authors also acknowledge the contributions of the following Honours’ students who assisted in the experimental work reported in this paper: William Tiong, Yang Wang, and Licheng Yang. References [1] K.S. Son, I. Hajirasouliha, K. Pilakoutas, Strength and deformability of waste tyre rubber-filled reinforced concrete columns, Constr. Build. Mater. 25 (1) (2011) 218–226. [2] T. Gupta, S. Chaudhary, R.K. Sharma, Assessment of mechanical and durability properties of concrete containing waste rubber tire as fine aggregate, Constr. Build. Mater. 73 (2014) 562–574. [3] G. Li, G. Garrick, J. Eggers, C. Abadie, M.A. Stubblefield, S.S. Pang, Waste tire fiber modified concrete, Compos. B Eng. 35 (4) (2004) 305–312. [4] A.O. Atahan, A.Ö. Yücel, Crumb rubber in concrete: static and dynamic evaluation, Constr. Build. Mater. 36 (2012) 617–622. [5] K.B. Najim, M.R. Hall, Mechanical and dynamic properties of self-compacting crumb rubber modified concrete, Constr. Build. Mater. 27 (1) (2012) 521–530. [6] M.M. Al-Tayeb, B.H. Abu Bakar, H. Ismail, H.M. Akil, Effect of partial replacement of sand by recycled fine crumb rubber on the performance of hybrid rubberized-normal concrete under impact load: experiment and simulation, J. Clean. Prod. 59 (2013) 284–289. [7] A.O. Atahan, U.K. Sevim, Testing and comparison of concrete barriers containing shredded waste tire chips, Mater. Lett. 62 (21–22) (2008) 3754– 3757. [8] M. Bravo, J. de Brito, Concrete made with used tyre aggregate: durabilityrelated performance, J. Clean. Prod. 25 (2012) 42–50. [9] O. Youssf, M.A. ElGawady, J.E. Mills, X. Ma, An experimental investigation of crumb rubber concrete confined by fibre reinforced polymer tubes, Constr. Build. Mater. 53 (2014) 522–532. [10] G.N. Kumar, V. Sandeep, C. Sudharani, Using tyres wastes as aggregates in concrete to form rubcrete–mix for engineering applications, Int. J. Res. Eng. Technol. 3 (11) (2014) 500–509. [11] H. Huynh, D. Raghavan, Durability of simulated shredded rubber tire in highly alkaline environments, Adv. Cem. Based Mater. 6 (3–4) (1997) 138–143. [12] O. Youssf, M.A. ElGawady, J.E. Mills, Experimental Investigation of Crumb Rubber Concrete Columns Under Seismic Loading, Elsevier, Structures, 2015, pp. 13–27. [13] M. Elchalakani, High strength rubberized concrete containing silica fume for the construction of sustainable road side barriers, Structures 1 (2015) 20–38. [14] D. Fedroff, S. Ahmad, B.Z. Savas, Mechanical properties of concrete with ground waste tire rubber, Trans. Res. Rec. J. Trans. Res. Board 1532 (-1) (1996) 66–72. [15] E. Ganjian, M. Khorami, A.A. Maghsoudi, Scrap-tyre-rubber replacement for aggregate and filler in concrete, Constr. Build. Mater. 23 (5) (2009) 1828–1836. [16] Z.K. Khatib, F.M. Bayomy, Rubberized Portland cement concrete, J. Mater. Civ. Eng. 11 (3) (1999) 206–213.
183
[17] T.C. Ling, Effects of compaction method and rubber content on the properties of concrete paving blocks, Constr. Build. Mater. 28 (1) (2012) 164–175. [18] M. Balaha, A. Badawy, M. Hashish, Effect of using ground waste tire rubber as fine aggregate on the behaviour of concrete mixes, Indian J. Eng. Mater. Sci. 14 (6) (2007) 427. [19] N.N. Eldin, A.B. Senouci, Rubber-tire particles as concrete aggregate, J. Mater. Civ. Eng. 5 (4) (1993) 478–496. [20] F. Pelisser, N. Zavarise, T.A. Longo, A.M. Bernardin, Concrete made with recycled tire rubber: effect of alkaline activation and silica fume addition, J. Clean. Prod. 19 (6) (2011) 757–763. [21] E. Güneyisi, M. Gesog˘lu, T. Özturan, Properties of rubberized concretes containing silica fume, Cem. Concr. Res. 34 (12) (2004) 2309–2317. [22] I. Mohammadi, H. Khabbaz, K. Vessalas, Enhancing mechanical performance of rubberised concrete pavements with sodium hydroxide treatment, Mater. Struct. 1–15 (2015). [23] D.M. Hamza, R.H. Ghedan, Effect of rubber treatment on compressive strength and thermal conductivity of modified rubberized concrete, J. Eng. Dev. 15 (4) (2011) 21–29. [24] N. Deshpande, S.S. Kulkarni, T. Pawar, V. Gunde, Experimental investigation on strength characteristics of concrete using tyre rubber as aggregates in concrete, Int. J. Appl. Eng. Res. Dev. 4 (2) (2014) 97–108. [25] S. Tian, T. Zhang, Y. Li, Research on modifier and modified process for rubberparticle used in rubberized concrete for road, Adv. Mater. Res. 243–249 (2011) 4125–4130. [26] G. Li, M.A. Stubblefield, G. Garrick, J. Eggers, C. Abadie, B. Huang, Development of waste tire modified concrete, Cem. Concr. Res. 34 (12) (2004) 2283–2289. [27] A. Turatsinze, S. Bonnet, J.-L. Granju, Potential of rubber aggregates to modify properties of cement based-mortars: improvement in cracking shrinkage resistance, Constr. Build. Mater. 21 (1) (2007) 176–181. [28] C. Albano, N. Camacho, J. Reyes, J.L. Feliu, M. Hernández, Influence of scrap rubber addition to Portland I concrete composites: destructive and nondestructive testing, Compos. Struct. 71 (3–4) (2005) 439–446. [29] Standards Australia, General purpose and blended cements, AS 3972, 2010. [30] F. Pacheco-Torgal, Y. Ding, S. Jalali, Properties and durability of concrete containing polymeric wastes (tyre rubber and polyethylene terephthalate bottles): An overview, Constr. Build. Mater. 30 (2012) 714–724. [31] L.H. Chou, C.N. Lin, C.K. Lu, C.H. Lee, M.T. Lee, Improving rubber concrete by waste organic sulfur compounds, Waste Manage. Res. 28 (1) (2010) 29–35. [32] T. Naik, S. Singh, Utilization of Discarded Tyres as Construction Materials for Transportation Facilities Report No. CBU-1991-02, 16, UWM Centre for byproducts utilization, University of Wiscosin, Milwaukee, 1991. [33] N. Oiknomou, M. Stefanidou, S. Mavridou, Improvement of the bonding between rubber tire particles and cement paste in cement products (2006) 234–42. [34] N. Segre, I. Joekes, Use of tire rubber particles as addition to cement paste, Cem. Concr. Res. 30 (9) (2000) 1421–1425. [35] R. Siddique, T.R. Naik, Properties of concrete containing scrap-tire rubber-an overview, Waste Manage. 24 (6) (2004) 563–569. [36] T. Naik, S. Singh, R. Wendorf, Applications of Scrap Tire Rubber in Asphaltic Materials: State of the Art Assessment A Technical Report, Wisconsin DNR, Center for By-Products Utilization, University of Wisconsin, Milwaukee, 1994. [37] O. Youssf, M.A. ElGawady, An overview of sustainable concrete made with scrap rubber, The 22nd Australasian Conference on the Mechanics of Structures and Materials (ACMSM 22), CRC Press, 2012, pp. 1039–1044. [38] O. Youssf, M.A. ElGawady, J.E. Mills, Static cyclic behaviour of FRP-confined crumb rubber concrete columns, Eng. Struct. 113 (2016) 371–387. [39] Standards Australia, Methods for testing concrete, Preparation of concrete mixes in the laboratory AS 1012.2, 1994. [40] Standards Australia, Methods for testing concrete, Determination of properties related to the consistency of concrete—Slump test AS 1012.3.1, 2014. [41] Standards Australia, Methods for sampling and testing aggregates, Method for making and curing concrete – compression and indirect tensile test specimens, AS1012.8.1, 2000. [42] Standards Australia, Methods of testing concrete – Compressive strength tests – Concrete, mortar and grout specimens, AS 1012.9, 2014. [43] Standards Australia, Methods for testing concrete, Determination of indirect tensile strength of concrete cylinders (Brasil or splitting test), AS 1012.10, 2000. [44] Neville A.M. Properties of Concrete Book, fifth ed., 1996. [45] ACI-234-96, Guide for the Use of Silica Fume in Concrete, American Concrete Institute, 1996. [46] M. Nehdi, A. Khan, Cementitious composites containing recycled tire rubber: an overview of engineering properties and potential applications, Cem. Concr. Aggr. 23 (1) (2001) 3–10. [47] P. Taylor, F. Bektas, E. Yurdakul, H. Ceylan, Optimizing Cementitious Content in Concrete Mixtures for Required Performance, 2012. [48] J. Van Kirk, G. Holleran, Reduced thickness asphalt rubber concrete leads to cost effective pavement rehabilitation, in: 1st International Conference World of Pavements, Sydney, Australia, 2000.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Assessment of the mechanical performance of crumb rubber concrete Osama Youssf a,b,⇑, Julie E. Mills a, Reza Hassanli a a b
University of South Australia, Adelaide, Australia Mansoura University, Mansoura, Egypt
h i g h l i g h t s Enhancement study of CRC mechanical performance was carried out. Effect of pre-treatment period, SF additives, and cement content was assessed. 3
0.5 h pre-treatment, 0% SF, and 350 kg/m cement were the best alternatives. More than 0.5 h pre-treatment did not significantly affect CRC properties. 3
Higher than 350 kg/m cement did not significantly affect CRC properties.
a r t i c l e
i n f o
Article history: Received 4 February 2016 Received in revised form 7 June 2016 Accepted 10 August 2016 Available online 17 August 2016 Keywords: Rubberized concrete Crumb rubber Rubber treatment Silica fume Cement content
a b s t r a c t Crumb rubber concrete (CRC) has some known shortcomings in mechanical performance compared to conventional concrete, particularly with respect to compressive strength. Many previous researchers have tried to overcome the material deficiencies using different methods; however, the results have often been contradictory and highly variable. In this research, three methods to improve and then assess the mechanical performance of CRC have been examined namely, rubber pre-treatment using sodium hydroxide (NaOH) solution, using silica fume additives, and increasing concrete cement content. The effect of the rubber pre-treatment time (0–2 h), silica fume content (0–15%), and cement content (300–400 kg/m3) on CRC slump, short and long term compressive strength, and tensile strength were measured for fifteen concrete mixes prepared with 0% and 20% rubber content. Six 100 200 mm cylinders were prepared from each mix for evaluating the compressive strength at 7 and 28 days. Four additional 100 200 mm cylinders were prepared from two mixes for evaluating the compressive strength at 56 and 84 days. In addition, two 150 300 mm cylinders for each mix were prepared and tested to determine the indirect tensile strength at 28 days. The results showed that 0.5 h of rubber pre-treatment using NaOH solution, 0% of silica fume replacing cement by weight, and 350 kg/m3 cement content were the best alternatives in this assessment range to enhance the CRC performance. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Due to the health and environmental risks presented from used tyre waste [1] as well as the scarcity and cost of natural mineral aggregates [2], a significant body of recent research has focussed on utilizing used tyre rubber in concrete as a partial replacement of its mineral aggregates, resulting in a class of concrete called crumb rubber concrete (CRC). The recycling of used rubber conserves valuable natural resources and reduces the amount of rubber entering landfill [3]. Previous experimental studies on CRC materials have shown that using rubber in concrete enhanced its ductility, toughness, impact resistance, energy dissipation, and ⇑ Corresponding author at: University of South Australia, Adelaide, Australia. E-mail address: [email protected] (O. Youssf). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.040 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
damping ratio [4–7]. However, it reduced its compressive strength, tensile strength, and modulus of elasticity compared to conventional concrete [8–13]. Some of the main reasons for this strength reduction are the low hydraulic conductivity and the smooth surface of rubber particles, which both result in poor rubber/cement interface adhesion [14–17]. This poor adhesion is also attributed to the existence of zinc stearate which is used in tyre formulation during manufacturing. This zinc stearate migrates and diffuses to the rubber surface creating a soap layer that repels water [9]. To increase the effectiveness of using rubber in concrete, several approaches have been previously introduced. Of these approaches; the chemical pre-treatment of rubber particles using sodium hydroxide (NaOH) solution, replacing part of the cement by silica fume (SF) additive, and increasing the rubberized concrete cement content are the most common. However, the degrees of
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treated CRC. Turatsinze et al. [27] mentioned that the strength benefit due to the NaOH rubber pre-treatment was not substantial. Youssf et al. [9] investigated the effect of cement content, NaOH pre-treatment of rubber for 30 min, and replacing 10% of cement weight by SF on the mechanical properties of CRC. Their results showed that the losses in CRC compressive strength with 425 kg/m3 cement content were less than when using 350 kg/m3 cement content. In addition, they reported that when using pre-treated rubber, the concrete slump and tensile strength decreased by 25% and 13%, respectively. But, the compressive strength and modulus of elasticity increased by 15% and 12% respectively, compared to non-treated rubber. No effect was observed in their results when using SF except a slight increase in the compressive strength at rubber content of 20% by sand volume. Albano et al. [28] studied concrete composites containing scrap rubber previously treated with NaOH and SILAN coupling agent in order to enhance the adhesion between the rubber and the cement paste, but did not notice any significant changes when compared to the non-treated rubber composites. The contradictions and variations in the previous research findings indicate the need for future research in CRC performance enhancement. This paper investigates the mechanical properties of fifteen CRC mixes. The effects of rubber pre-treatment period, SF content, and cement content on the fresh and hardened properties of CRC were examined with the aim of assessing the CRC mechanical performance. These data provide additional information necessary to support the further development of CRC.
effectiveness using these approaches have been inconsistent and scattered in research published to date. Balaha et al. [18] experimented with three different cement contents namely, 300 kg/m3, 400 kg/m3, and 500 kg/m3 in rubberized concrete mixes containing up to 20% rubber replaced by sand volume, and they treated the rubber particles using NaOH solution for 30 min. In addition, they replaced 15% of cement by weight with SF. Their results showed that the CRC properties improved with cement content increase up to 400 kg/m3. Beyond 400 kg/m3 cement content, only slight improvements were observed. However, the slump was negatively affected when using 400 kg/m3. Using SF and NaOH pre-treatment, increased concrete slump by 77% and 7%, respectively, increased compressive strength by 18% and 15%, respectively, and increased tensile strength by 9% and 6%, respectively. Eldin and Senouci [19] treated rubber particles in NaOH solution for 5 min before use in CRC and achieved 16% increase in the compressive strength. Pelisser et al. [20] used NaOH pre-treated rubber combined with adding 15% SF by cement weight to the concrete mix. They reported almost total recovery of the concrete compressive strength. Güneyisi et al. [21] have observed lower workability but higher compressive strength of CRC by using SF. In addition, the positive effect of SF on the strength decreased as the rubber content increased. Mohammadi et al. [22] used pre-treated rubber in NaOH solution for 20 min, 2 h, 24 h, 48 h, and 7 days. Their results showed that 24 h is the best treatment period for the rubber as it resulted in the highest compressive strength and flexural strength. However, this pre-treatment had no effect on concrete slump. Hamza and Ghedan [23] washed rubber particles in NaOH solution before adding a coupling agent called SILAN to the rubberized concrete. Their results showed that the compressive strength improved by 74% compared to non-treated rubber mix. Other researchers reported less positive or contradictory results from these approaches. Deshpande et al. [24] used modified rubber by saturating it in NaOH solution for 20 min. Their results showed almost no difference between the compressive and tensile strengths of pre-treated and non-treated rubber mixes. However, 12% increase in the flexural strength was reported for the pretreated rubber mix. Tian et al. [25] reported 3.7% reduction in CRC compressive strength using NaOH pre-treated rubber for 24 h followed by tap water wash for 3 h compared to nontreated rubber. Li et al. [26] treated rubber particles using NaOH solution for 30 min and found that the properties of pre-treated rubberized concrete were nearly the same as those of non-
2. Experimental program Several factors could potentially affect the concrete properties, including water to cement ratio, rubber content, and cement content. This experimental programme focuses on the factors affecting the adhesion at the rubber/cement interface in the concrete matrix, such as the cement content, rubber pre-treatment, and SF content. The poor rubber/cement interface adhesion is one of the main sources of the deficiencies in the rubberized concrete properties [14–17]. Increasing the rubber content in concrete enhances its dynamic properties [4–7]. However, using more than 20% rubber in concrete may magnify the adverse effects on concrete characteristics, as recommended by Khatib and Bayomy [16]. In this research the performance of concrete mixes incorporating 0 and
Table 1 Proportions of concrete mixes. Mix code
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15
Rs (%)
0 20 0 20 0 20 20 20 20 0 20 0 20 0 20
Pre-treatment of rubber (period)
– NaOH (0.5 h) – NaOH (0.5 h) – NaOH (0.5 h) No treatment NaOH (1.0 h) NaOH (2.0 h) – NaOH (0.5 h) – NaOH (0.5 h) – NaOH (0.5 h)
RS, Per cent of sand volume replaced by rubber. SF, Per cent of cement replaced by silica fume. SP, Superplasticizer dosage.
SF (%)
Mix proportions (kg/m3) Cement
– – – – – – – – – 5 5 10 10 15 15
350 350 400 400 300 300 350 350 350 333 333 315 315 298 298
SF
– – – – – – – – – 18 18 35 35 53 53
Sand
866 693 814 651 916 733 693 693 693 862 690 859 687 856 685
Dolomite 10 mm
20 mm
311 311 293 293 330 330 311 311 311 310 310 309 309 308 308
727 727 684 684 769 769 727 727 727 724 724 722 722 719 719
Rubber
Water
SP
– 55.5 – 41.8 – 58.8 55.5 55.5 55.5 – 55.3 – 55.1 – 54.9
175 175 200 200 150 150 175 175 175 175 175 175 175 175 175
6.3 6.3 7.2 7.2 5.4 5.4 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3
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20% of crumbed scrap tyre rubber as a partial volume replacement of fine aggregates was experimentally investigated. The effect of rubber pre-treatment period, silica fume content, and cement content on CRC slump, short and long term compressive strength, and tensile strength were examined for fifteen concrete mixes by testing 128 standard concrete cylinders. 2.1. Concrete materials and variables Table 1 summarises the different components of all concrete mixes used in this study. General purpose cement type with specific gravity of 3.15, according to Australian Standard (AS) AS 3972 [29], was used as the binder material in the concrete mixes. Densified SF with specific gravity of 2.2 was used as a partial replacement of concrete cement by weight. Dolomite stone having nominal maximum sizes of 10 mm and 20 mm was used as coarse aggregates. River sand with a maximum aggregate size of 5 mm was used as fine aggregate. The crumb rubber used during the course of this study had two particle sizes of 1.18 and 2.36 mm and was used as a partial replacement of sand by volume (Table 1). The sieve analyses for all used aggregates are shown in Fig. 1. The specific gravity, unit weight, and fineness modulus were 2.72, 1570 kg/m3, and 6.02 respectively for dolomite; 2.65, 1630 kg/m3, and 2.36 respectively for sand; and 0.85, 530 kg/m3, and 4.53 respectively for rubber. Polycarboxylic ether type superplasticizer (SP) with a specific gravity of 1.08 was added to the concrete mixtures to achieve the required concrete workability. The variables in this study were; the pre-treatment period of rubber particles using 10% NaOH solution for 0.0 h, 0.5 h, 1.0 h, and 2.0 h; the SF content as a partial replacement of cement weight by 0%, 5%, 10% and 15%; and the concrete cement content of 300 kg/m3, 350 kg/m3, and 400 kg/m3. 2.2. Pre-treatment of rubber The pre-treatment of rubber particles can play an important role in improving the rubber/cement interface adhesion. Pretreatment of rubber using NaOH solution removes the zinc stearate layers that are present on the rubber surface of tyres as a result of the manufacturing process [30]. The NaOH solution is able to eliminate the additives on the rubber surface, leaving voids on the rubber outer surface that lead to a relatively rough and porous surface, compared with non-treated rubber. Because of the eroding effect of this acid solution on rubber particles, the surface of these particles is scraggy, which can improve the cohesion strength between rubber particles and cement [25]. In addition, it increases the hydraulic conductivity, rubber/cement water transfer rate, and
Fig. 1. Sieve analysis of the used aggregates.
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hydration at the interface, which improves the rubber/cement adhesion [19,31–37]. Youssf et al. in previous research [9,12,38] have successfully pre-treated the rubber particles by NaOH solution before using them in concrete. In the present study, pre-treatment of the rubber particles with 10% concentration NaOH solution was used with different treatment periods, namely 0.0 h, 0.5 h, 1.0 h, and 2.0 h. The process commenced with the rubber particles being washed by tap water to remove any impurities and dust. They were then submerged into NaOH solution for the required period in a container. Finally, the rubber particles were washed again by stirring in water until its pH became 7, and then they were left to air dry. The final washing is essential to remove any remaining NaOH solution to prevent any negative effect on the concrete durability. Fig. 2 shows the procedure of the rubber pre-treatment process. It was observed that submerging the rubber into the NaOH solution increased its pH to 14. Every 5 min during washing, the pH of the rubber solution was recorded using a pH meter. The total time that was needed to wash the rubber to fully remove the NaOH solution after the treatment process ranged from 30 to 45 min. 2.3. Concrete mix designs The concrete mixes were designed according to AS 1012.2 [39]. The target compressive strength of the control mix (M1) was 50 MPa. All mixes were designed with constant water to binder (W/B) ratio of 0.5 and SP content of 1.8% by Binder weight. The fine/coarse aggregate ratio was 1/1.2 by weight. The 10 mm /20 mm coarse aggregate ratio was 1/2.34 by weight. The mixing procedure for the control mixes was as follows: mix dry sand and dolomite for 1 min; add half of the water and mix for 1 min; rest for 2 min; add cementitious materials, water, and admixtures, and then mix for 2 min. The same procedure was followed for the CRC mixes; except that the rubber aggregate was first mixed with the dry cementitious materials for 1 min in an external container, aiming to increase the rubber-cement interface adhesion, which is one of the known main factors affecting CRC strength. 2.4. Specimen preparation and testing The standard slump test according to AS 1012.3.1 [40] was used to measure the concrete workability for each mix, as shown in Fig. 3(a). Six 100 200 mm cylinders were prepared from each mix: three for evaluating the compressive strength at 7 days; and three for evaluating the compressive strength at 28 days. Four additional 100 200 mm cylinders were prepared from each mix of M10 and M11; two for evaluating the compressive strength at 56 days; and two for evaluating the compressive strength at 84 days. This resulted in a total of ninety-eight cylinders from the fifteen mixes. In addition, two 150 300 mm cylinders for each mix were prepared and tested to determine the indirect tensile strength at 28 days, resulting in a total of thirty cylinders from the fifteen mixes. The traditional compaction method for the cast concrete was done using a standard compaction rod and hammer. All specimens were de-moulded after 24 h, and labelled for the various tests. Then they were cured in a water bath at 23 ± 2 °C, according to AS1012.8.1 [41]. At the test day, the tests were conducted according to the appropriate Australian Standard under monotonic loading until failure occurred. The compression test was carried out according to AS 1012.9 [42] using an 1800 kN capacity testing machine with a constant loading rate of 20 ± 2 MPa/min. Indirect tensile tests were performed according to AS 1012.10 [43] using a constant loading rate of 1.5 ± 0.15 MPa/min. Fig. 3 shows the concrete tests conducted in this study.
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1
2
3
4
Fig. 2. Rubber particle pre-treatment process.
Fig. 3. CRC tests: (a) slump, (b) compressive strength, and (c) indirect tensile strength.
3. Experimental results and discussion In this section, the effects of rubber pre-treatment period, silica fume content, and cement content on CRC slump, short and long term compressive strength, and tensile strength are discussed. Table 2 shows the experimental results of all concrete mixes in this study. 3.1. Effect of rubber pre-treatment period The effects of rubber pre-treatment using NaOH solution on concrete slump, 7 and 28 days compressive strength, and tensile strength were determined through comparison of the results of mixes M1, M2, M7, M8, and M9. Mixes M1 and M7 are comparable
in terms of the effect of using non-treated rubber on the concrete properties. Mixes M7, M2, M8, and M9 are comparable in terms of the effect of pre-treatment periods of 0.0 h, 0.5 h, 1.0 h, and 2.0 h on concrete properties, respectively. As shown in Fig. 4(a), using non-treated rubber in M7 (0.0 h pre-treatment) increased the concrete slump by 26.4% compared to that of the control mix M1. This was attributed to the poorlygraded rubber used in this study with a 4.53 fineness modulus that increased the fineness modulus of the hydride aggregates overall (sand and rubber) compared to sand with 2.36 fineness modulus in the conventional concrete. Increasing the fineness modulus of the concrete aggregates increases its workability [44]. On the other hand, pre-treatment of rubber particles had a minor negative effect on the concrete slump compared with no pre-treatment.
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O. Youssf et al. / Construction and Building Materials 125 (2016) 175–183 Table 2 Experimental results. Mix code
Rs (%)
Pre-treatment of rubber (period)
SF (%)
Slump (mm)
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15
0 20 0 20 0 20 20 20 20 0 20 0 20 0 20
– NaOH – NaOH – NaOH – NaOH NaOH – NaOH – NaOH – NaOH
– – – – – – – – – 5 5 10 10 15 15
170 207 230 210 5 12 215 205 205 165 210 65 165 20 155
(0.5 h) (0.5 h) (0.5 h) (1.0 h) (2.0 h) (0.5 h) (0.5 h) (0.5 h)
Compressive strength (MPa) 7 Days
28 Days
56 Days
84 Days
44.9 36.0 47.3 37.9 41.8 31.7 31.2 33.5 32.3 44.4 32.3 38.1 30.8 37.7 29.5
53.5 42.1 56.1 45.9 50.3 35.0 35.9 38.6 37.2 55.4 39.7 50.6 37.3 47.7 36.8
– – – – – – – – – 54.2 35.8 – – – –
– – – – – – – – – 57.6 42.5 – – – –
Tensile strength (MPa) 4.1 3.1 4.4 3.2 3.6 2.9 2.7 3.1 3.2 4.3 3.4 4.2 3.1 3.9 3.1
RS, Percent of sand volume replaced by crumb rubber. SF, Percent of cement replaced by silica fume.
Pre-treatment of rubber particles for 0.5 h in M2, 1.0 h in M8, and 2.0 h in M9 increased the concrete slump by 21.8%, 20.6% and 20.6%, respectively, compared with the conventional concrete mix but decreased the concrete slump by 3.7%, 4.6%, and 4.6%, respectively, compared with the non-treated rubber mix. This was attributed to the relatively rough surface of pre-treated rubber compared to non-treated rubber that resulted from the surface eroding caused by the acid solution. This eroding effect resulted in relatively slow movement of rubber particles in the concrete matrix and hence slump reduction. Increasing the rubber pretreatment period had no significant effect on the CRC slump. As the rubber exposure time to the NaOH solution increases, the solution penetrates the rubber particle and decreases its stiffness rather than eroding its outer surface. This lesser stiffness of the rubber particles at longer pre-treatment period has an insignificant effect on the concrete workability. Fig. 4(b) shows the variation of the CRC compressive strengths at ages of 7 and 28 days. As shown in the figure, using nontreated rubber particles in concrete mix M7 decreased its compressive strength by 30.5% and 32.8% at 7 and 28 days, respectively, compared with conventional concrete mix M1. However, the NaOH pre-treatment of rubber was able to recover part of the strength reduction as shown in Fig. 4(b). The most effective strength recovery was observed when using 0.5 h pre-treated rubber in M2 that resulted in compressive strength increase of 15.3% and 17.2% at 7 and 28 days, respectively, compared to non-treated rubber in M7, but still a reduction of 19.8% and 21.3% at 7 and 28 days, respectively, compared to the conventional concrete mix M1. The texture and stiffness of the rubber particles by nature mainly affect the CRC strength [9]. The relatively smooth texture of rubber particles results in low bond between rubber particles and cement mortar. The lower stiffness of rubber particles produces higher internal tensile stresses that cause early failure in the cement mortar and then strength reduction, and finally, the poor absorption of rubber particles compared with sand reduces the penetration of the rubber aggregate by cement paste which then results in poor rubber/cement interface adhesion. Using NaOH pre-treatment for rubber particles increases the roughness of their outer surface which leads to increased adhesion between rubber and the surrounding cement paste and hence strength increase. Increasing the rubber pre-treatment period beyond 0.5 h in M8 and M9 had a negative effect on compressive strength. The compressive strengths at 7 and 28 days decreased by 6.9% and 8.3%, respectively, for 1.0 h pre-treated rubber in M8 and by 10.2% and 11.6%, respectively, for 2.0 h pre-treated rubber in M9 compared to
0.5 h pre-treated rubber in M2. This was attributed again to the higher penetration of the NaOH solution into the rubber particles that decreased their stiffness and hence decreased the overall stiffness of the concrete matrix. The effect of rubber pre-treatment period on CRC tensile strength is shown in Fig. 4(c). This figure shows that the NaOH pre-treatment of rubber in M2, M8, and M9 was able to recover part but not all of the 34.1% tensile strength reduction caused by using non-treated rubber in M7, and the tensile strength reduction enhanced to only 24.4% compared with conventional concrete mix M1. Increasing the rubber pre-treatment period for longer than 0.5 h in both M8 and M9 had no further effect on the tensile strength. From the above results, it can be concluded that the best rubber pre-treatment period using 10% NaOH solution in this assessment range is 0.5 h, as it resulted in higher compressive and tensile strength compared to the other pre-treatment periods, and improved the slump compared with conventional concrete by about 22%. 3.2. Effect of silica fume additive The effects on concrete slump, short and long term compressive strength, and tensile strength of partially replacing cement by SF additive were determined through comparison of the results of mixes M1, M2, M10, M11, M12, M13, M14 and M15. Mixes M1, M10, M12, and M14 are comparable in terms of the effect of replacing 0%, 5%, 10%, and 15% of concrete cement by SF, respectively, at no rubber content on concrete properties. Mixes M2, M11, M13, and M15 are comparable in terms of the effect on concrete properties of replacing 0%, 5%, 10%, and 15% of concrete cement by SF, respectively, at 20% rubber content that was pretreated for 0.5 h using NaOH solution. As shown in Fig. 5(a), at 0%, 5%, 10%, and 15% SF content, using 20% rubber (in concrete mixes M2, M11, M13, and M15) increased the concrete slump 1.22, 1.27, 2.53 and 7.75 times, respectively, compared with using 0% rubber (in concrete mixes M1, M10, M12, and M14). This was again attributed to the increase in the fineness modulus of the hydride aggregates overall (sand and rubber). By increasing the SF content up to 5% the slumps of both CRC and conventional concrete were not significantly affected. At 10% and 15% SF contents, the CRC slump decreased by 20% (mix M13) and 25% (mix M15), respectively, compared to 0% SF (mix M2). However, at 10% and 15% SF contents, the conventional concrete slump severely decreased by 62% (mix M12) and 88% (mix M14),
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Fig. 4. Effects of rubber pre-treatment on concrete: (a) slump, (b) 7 and 28 days compressive strength, and (c) tensile strength.
respectively, compared to 0% SF (mix M1). The reduction in concrete workability with SF increase was attributed to the relatively small particle size of the SF that increases the tendency of cement particles to agglomerate in the paste, trapping some of the mixing water and hence resulting in slump reduction. Fig. 5(b) shows the effect of SF content on concrete compressive strengths at 7 and 28 days. As shown in the figure, the patterns of the compressive strength for the conventional concrete and CRC at 7 and 28 days are almost the same, with the compressive strength decreasing as the SF content increased. However, the rate of strength reduction in CRC was less than that in the conventional
Fig. 5. Effects of silica fume on concrete: (a) slump, (b) compressive strength, and (c) tensile strength.
concrete. Compared to the CRC without SF, using 5%, 10%, and 15% SF (in mixes M11, M13, and M15, respectively) decreased the CRC compressive strength by 10%, 14%, and 18%, respectively, at 7 days; and by 6%, 11%, and 12%, respectively, at 28 days. Compared to the conventional concrete without SF, using 5%, 10%, and 15% SF (in mixes M10, M12, and M14, respectively) decreased the conventional concrete compressive strength by 1%, 15%, and 16%, respectively, at 7 days. However, at 28 days concrete age, the conventional concrete compressive strength slightly increased by 3% for 5% SF and then decreased by 5% and 11% for 10% and 15% SF, respectively, compared to the conventional concrete without SF. Adding SF to concrete has physical and chemical effects that could improve its strength. The physical effect is the micro filling
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of the concrete voids by SF particles which improves the microstructure of the concrete matrix and results in more stronger and durable concrete. The chemical effect is the utilizing of the whole or part of the calcium hydroxide generated during the hydration process of cement and producing calcium silicate hydrate. This calcium silicate hydrate is able to increase the concrete compressive strength [44]. Previous researchers reported 6–67% increase in concrete compressive strength when replacing 5–40% of cement by SF. According to the results of this research, at 5% SF, the compressive strength marginally enhanced or diminished especially at 28 days; however, at higher SF contents, the compressive strength decreased. This reduction may be due to the amount of SF that was added being more than the required amount for the filling mechanism and pozzolanic chemical action, hence strength reduction occurred. More experimental research is recommended using SF content of less than 5%. Fig. 5(c) shows the effect of using SF on concrete tensile strength which indicates that 5% SF improved the tensile strength for the conventional concrete (mix M10) and the CRC (mix M11) by 5% and 10%, respectively. However, increasing SF content beyond 5% decreased the tensile strength for both the conventional concrete and the CRC. In order to investigate the effect of SF at later ages on CRC compressive strength, mixes M10 and M11 that included 5% SF were tested at 56 and 84 days. Fig. 6 shows the evolution of the concrete strength with time. As shown in the figure, the increase in the compressive strength at ages longer than 28 days was insignificant. However, the rate of the CRC compressive strength increase at 84 days was slightly higher than that of the conventional concrete. At 84 days, the compressive strength increased by 4% for the conventional concrete mix M10 and by 7% for the CRC mix M11, compared to the corresponding 28 day compressive strengths. ACI 234-96 [45] suggests that the main contribution of silica fume to concrete strength development takes place between about 3–28 days and that the contribution of silica fume to strength development after 28 days is minimal. From the above results and based on this assessment range, it can be concluded that using SF to replace part of the cement with the intention of enhancing CRC performance was not effective and in fact resulted in some negative effects on the concrete slump and compressive strength, with only a very marginal improvement in tensile strength at 5% SF.
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through comparison of the results of mixes M1, M2, M3, M4, M5 and M6. Mixes M1, M3, and M5 are comparable in terms of the effect of cement content on concrete properties containing no rubber. Mixes M2, M4, and M6 are comparable in terms of the effect of cement content on concrete properties containing 20% rubber content. The cement contents used in this assessment were 300 kg/m3, 350 kg/m3, and 400 kg/m3. As shown in Fig. 7(a), at 300 kg/m3 and 350 kg/m3 the CRC (M6 and M2) showed slump 2.4 and 1.2 times that showed by the conventional concrete (M5 and M1), respectively. This was attributed to the increase of the fineness modulus that occurred by using
3.3. Effect of cement content The effects of cement content on concrete slump, 7 and 28 day compressive strength, and tensile strength were determined
Fig. 6. Evolution of concrete compressive strength for 5% SF content.
Fig. 7. Effects of cement content on concrete: (a) slump, (b) 7 and 28 day compressive strength, and (c) tensile strength.
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rubber with higher fineness modulus than the replaced sand. At 400 kg/m3 the CRC (M4) showed slump 0.9 times that showed by the conventional concrete (M3). This slump reduction was attributed to the reduction in the aggregates to cement ratio at a given concrete volume compared to lower cement contents (300 kg/m3 and 350 kg/m3). This increased the effect of the cement paste rather than the concrete aggregates’ fineness modulus because the quantity of cement paste that became available for providing lubrication per unit surface area of aggregate increased. However, the irregular shape and surface texture of rubber particles in addition to their low adhesion with cement compared to sand particles resulted in slower mobility of the concrete matrix and hence slump reduction. Although the increase of cement content increased concrete slump as shown in Fig. 7(a), this increase was less significant in the CRC compared to the conventional concrete. Compared to 300 kg/m3 (M5 and M6), using 350 kg/m3 and 400 kg/m3, increased slump by 34 and 46 times (M1 and M3), respectively, for the conventional concrete and by 17 and 18 times (M2 and M4), respectively, for the CRC. This indicated that increasing cement content in CRC mixes beyond 350 kg/m3 is not worthwhile in terms of enhancing its workability. Fig. 7(b) shows the effect of cement content on concrete compressive strengths at 7 and 28 days. As shown in the figure, the patterns of the compressive strength for the conventional concrete and CRC at 7 and 28 days are the same, namely that the compressive strength increased with cement content increase, as expected. However, the rate of strength increase in the CRC was higher than that in the conventional concrete. Compared to 300 kg/m3 (M5 and M6), using 350 and 400 kg/m3 increased the compressive strength by 7% and 13% (M1 and M3), respectively, for the conventional concrete at 7 days and by 13% and 19% (M2 and M4), respectively, for CRC at 7 days. At 28 days these increases were 6% and 11%, respectively, for the conventional concrete and 20% and 31%, respectively, for CRC. The higher rate of strength increase using rubber was attributed to the low water absorption of rubber compared to sand [22]. This provides more water for cement hydration reaction and early curing, hence rapid strength development. In the case of CRC, increasing the cement content from 300 to 350 kg/m3 was more significant than from 350 to 400 kg/m3. Using 400 kg/m3 increased the CRC compressive strength by only 5% and 9% at 7 and 28 days, respectively, compared to compressive strength using 350 kg/m3. On the other hand, the compressive strength increased by 13% and 20% at 7 and 28 days, respectively, when the cement content increased from 300 to 350 kg/m3. This was attributed to the fact that the cement paste fills the voids between the aggregates in the concrete matrix. Using rubber in concrete increases its voids (air content) [46] compared to conventional concrete with the same mixing and compacting conditions. This indicates that CRC needs more cement to fill these voids. However, the nature of the rubber surface texture and the low cement/rubber interface adhesion keeps the voids around rubber particles constant with increasing cement content. Thus, once the cement content reached the required minimum amount to fill other voids, the effect of increasing its content was less significant [47]. Due to the high correlation between the concrete compressive and tensile strengths, the effect of cement content on the tensile strength was almost the same as that on the compressive strength, as shown in Fig. 7(c). For the CRC, the tensile strength increased by 7% with cement content increase from 300 to 350 kg/m3. Beyond that, increasing cement content had no significant effect on CRC tensile strength. From the above results, it can be concluded that the best cement content in this assessment range that enhanced the CRC performance was 350 kg/m3 as it showed the highest significant improvement in concrete slump, compressive strength and tensile strength. Cement content higher than 350 kg/m3 was able to
further improve the CRC performance but not at a significant enough rate to warrant the increased cost that might be incurred. 3.4. Recommendations for CRC mix design Based on the results obtained in this experimental study, the most significant improvements in the CRC fresh and hardened mechanical performance for a CRC mix containing 20% rubber replacement of sand volume resulted from 0.5 h pre-treatment period with NaOH, 0% SF content, and 350 kg/m3 cement content. The target conventional concrete compressive strength of 50 MPa was reduced to 42.1 MPa and the slump increased from 170 mm to 207 mm. The crumb rubber used in the mix had two particle sizes of 1.18 and 2.36 mm with a sieve analysis as shown in Fig. 1. The recommended design of the CRC mix is: Cement = 350 kg/m3 Superplasticizer = 1.8% by cement weight Dolomite 10 mm = 311 kg/m3
W/C = 0.5 Sand = 693 kg/m3 Dolomite 20 mm = 727 kg/m3 Pre-treated rubber for 0.5 h in 10% NaOH solution = 55.5 kg/m3
4. Summary and conclusions This paper investigates the mechanical properties of fifteen CRC mixes aiming to assess the mechanical performance of a 20% rubber content CRC mix by measuring the effect of the rubber pretreatment period, SF content, and cement content on its slump, short and long term compressive strength, and tensile strength. The results of this investigation are summarised as follows: 1. Using 20% rubber in concrete increased the concrete slump by 1.22, 1.27, 2.53 and 7.75 times that of 0% rubber mixes at 0%, 5%, 10%, and 15% SF content, respectively. 2. Pre-treatment of rubber particles for 0.5 h in 10% NaOH solution was the best pre-treatment period in this study. It increased the CRC slump by 22% compared to the conventional concrete slump. In addition it recovered 15.3% and 17.2% of the compressive strength lost by using non-treated rubber at 7 and 28 days, respectively. Furthermore, it increased the CRC tensile strength by 15% compared to non-treated rubber. Increasing the rubber pre-treatment period more than 0.5 h had no significant effect on the concrete slump and tensile strength and it decreased the compressive strength. 3. Using SF as partial replacement of cement was not useful and showed some negative effects on the concrete slump and compressive strength. It also made negligible improvement on the long term compressive strength. 4. The cement content that most enhanced the CRC performance in this study was 350 kg/m3. It showed the highest significant improvement in concrete slump, compressive strength and tensile strength. Although cement content greater than 350 kg/m3 was able to improve the CRC performance slightly further, the rate of improvement was not significant. At 300 kg/m3, 350 kg/m3, and 400 kg/m3 cement contents, the CRC showed slump 2.4, 1.2, and 0.9 times, respectively, that shown by the conventional concrete. The CRC compressive and tensile strengths at 28 day increased by 20% and 7%, respectively, with cement content increase from 300 to 350 kg/m3. Beyond that, the increases in the compressive and tensile strengths were less significant and not worthwhile economically. 5. Based on the results obtained in this experimental study, the recommended design of the CRC mix is 350 kg/m3 cement content, 0.5 water to binder ratio, 693 kg/m3 sand, 55.5 kg/m3
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pre-treated crumb rubber for 0.5 h in 10% NaOH solution, 311 kg/m3 of 10 mm dolomite, 727 kg/m3 of 20 mm dolomite, and superplasticizer dosage of 1.8% by cement weight. A cost analysis of the proposed CRC assessment is planned to be undertaken by the authors in future work. The upfront cost investigation of the CRC shows that it is slightly more expensive than the conventional concrete depending on the availability of shredded tyres and their local price. However, looking at the life cycle cost, a rubberized concrete structure can provide cost effectiveness performance compared to a conventional concrete structure [48]. This is due to the advantages of using rubber in concrete that including lower maintenance cost, providing environmentallyfriendly solutions in structures with high dynamic properties, increasing the structure sustainability, saving energy and natural resources, and reducing/eliminating the adverse effects of dumping end-of-life tyres to land fill. Acknowledgment The authors gratefully acknowledge the donation of the following: sand by Premix Concrete SA Pty. Ltd, aggregate by Hallett Concrete Australia Pty. Ltd, cement by Adelaide Brighton Cement Pty. Ltd, superplasticizer by BASF Construction Chemicals Australia Pty. Ltd. The authors also acknowledge the contributions of the following Honours’ students who assisted in the experimental work reported in this paper: William Tiong, Yang Wang, and Licheng Yang. References [1] K.S. Son, I. Hajirasouliha, K. Pilakoutas, Strength and deformability of waste tyre rubber-filled reinforced concrete columns, Constr. Build. Mater. 25 (1) (2011) 218–226. [2] T. Gupta, S. Chaudhary, R.K. Sharma, Assessment of mechanical and durability properties of concrete containing waste rubber tire as fine aggregate, Constr. Build. Mater. 73 (2014) 562–574. [3] G. Li, G. Garrick, J. Eggers, C. Abadie, M.A. Stubblefield, S.S. Pang, Waste tire fiber modified concrete, Compos. B Eng. 35 (4) (2004) 305–312. [4] A.O. Atahan, A.Ö. Yücel, Crumb rubber in concrete: static and dynamic evaluation, Constr. Build. Mater. 36 (2012) 617–622. [5] K.B. Najim, M.R. Hall, Mechanical and dynamic properties of self-compacting crumb rubber modified concrete, Constr. Build. Mater. 27 (1) (2012) 521–530. [6] M.M. Al-Tayeb, B.H. Abu Bakar, H. Ismail, H.M. Akil, Effect of partial replacement of sand by recycled fine crumb rubber on the performance of hybrid rubberized-normal concrete under impact load: experiment and simulation, J. Clean. Prod. 59 (2013) 284–289. [7] A.O. Atahan, U.K. Sevim, Testing and comparison of concrete barriers containing shredded waste tire chips, Mater. Lett. 62 (21–22) (2008) 3754– 3757. [8] M. Bravo, J. de Brito, Concrete made with used tyre aggregate: durabilityrelated performance, J. Clean. Prod. 25 (2012) 42–50. [9] O. Youssf, M.A. ElGawady, J.E. Mills, X. Ma, An experimental investigation of crumb rubber concrete confined by fibre reinforced polymer tubes, Constr. Build. Mater. 53 (2014) 522–532. [10] G.N. Kumar, V. Sandeep, C. Sudharani, Using tyres wastes as aggregates in concrete to form rubcrete–mix for engineering applications, Int. J. Res. Eng. Technol. 3 (11) (2014) 500–509. [11] H. Huynh, D. Raghavan, Durability of simulated shredded rubber tire in highly alkaline environments, Adv. Cem. Based Mater. 6 (3–4) (1997) 138–143. [12] O. Youssf, M.A. ElGawady, J.E. Mills, Experimental Investigation of Crumb Rubber Concrete Columns Under Seismic Loading, Elsevier, Structures, 2015, pp. 13–27. [13] M. Elchalakani, High strength rubberized concrete containing silica fume for the construction of sustainable road side barriers, Structures 1 (2015) 20–38. [14] D. Fedroff, S. Ahmad, B.Z. Savas, Mechanical properties of concrete with ground waste tire rubber, Trans. Res. Rec. J. Trans. Res. Board 1532 (-1) (1996) 66–72. [15] E. Ganjian, M. Khorami, A.A. Maghsoudi, Scrap-tyre-rubber replacement for aggregate and filler in concrete, Constr. Build. Mater. 23 (5) (2009) 1828–1836. [16] Z.K. Khatib, F.M. Bayomy, Rubberized Portland cement concrete, J. Mater. Civ. Eng. 11 (3) (1999) 206–213.
183
[17] T.C. Ling, Effects of compaction method and rubber content on the properties of concrete paving blocks, Constr. Build. Mater. 28 (1) (2012) 164–175. [18] M. Balaha, A. Badawy, M. Hashish, Effect of using ground waste tire rubber as fine aggregate on the behaviour of concrete mixes, Indian J. Eng. Mater. Sci. 14 (6) (2007) 427. [19] N.N. Eldin, A.B. Senouci, Rubber-tire particles as concrete aggregate, J. Mater. Civ. Eng. 5 (4) (1993) 478–496. [20] F. Pelisser, N. Zavarise, T.A. Longo, A.M. Bernardin, Concrete made with recycled tire rubber: effect of alkaline activation and silica fume addition, J. Clean. Prod. 19 (6) (2011) 757–763. [21] E. Güneyisi, M. Gesog˘lu, T. Özturan, Properties of rubberized concretes containing silica fume, Cem. Concr. Res. 34 (12) (2004) 2309–2317. [22] I. Mohammadi, H. Khabbaz, K. Vessalas, Enhancing mechanical performance of rubberised concrete pavements with sodium hydroxide treatment, Mater. Struct. 1–15 (2015). [23] D.M. Hamza, R.H. Ghedan, Effect of rubber treatment on compressive strength and thermal conductivity of modified rubberized concrete, J. Eng. Dev. 15 (4) (2011) 21–29. [24] N. Deshpande, S.S. Kulkarni, T. Pawar, V. Gunde, Experimental investigation on strength characteristics of concrete using tyre rubber as aggregates in concrete, Int. J. Appl. Eng. Res. Dev. 4 (2) (2014) 97–108. [25] S. Tian, T. Zhang, Y. Li, Research on modifier and modified process for rubberparticle used in rubberized concrete for road, Adv. Mater. Res. 243–249 (2011) 4125–4130. [26] G. Li, M.A. Stubblefield, G. Garrick, J. Eggers, C. Abadie, B. Huang, Development of waste tire modified concrete, Cem. Concr. Res. 34 (12) (2004) 2283–2289. [27] A. Turatsinze, S. Bonnet, J.-L. Granju, Potential of rubber aggregates to modify properties of cement based-mortars: improvement in cracking shrinkage resistance, Constr. Build. Mater. 21 (1) (2007) 176–181. [28] C. Albano, N. Camacho, J. Reyes, J.L. Feliu, M. Hernández, Influence of scrap rubber addition to Portland I concrete composites: destructive and nondestructive testing, Compos. Struct. 71 (3–4) (2005) 439–446. [29] Standards Australia, General purpose and blended cements, AS 3972, 2010. [30] F. Pacheco-Torgal, Y. Ding, S. Jalali, Properties and durability of concrete containing polymeric wastes (tyre rubber and polyethylene terephthalate bottles): An overview, Constr. Build. Mater. 30 (2012) 714–724. [31] L.H. Chou, C.N. Lin, C.K. Lu, C.H. Lee, M.T. Lee, Improving rubber concrete by waste organic sulfur compounds, Waste Manage. Res. 28 (1) (2010) 29–35. [32] T. Naik, S. Singh, Utilization of Discarded Tyres as Construction Materials for Transportation Facilities Report No. CBU-1991-02, 16, UWM Centre for byproducts utilization, University of Wiscosin, Milwaukee, 1991. [33] N. Oiknomou, M. Stefanidou, S. Mavridou, Improvement of the bonding between rubber tire particles and cement paste in cement products (2006) 234–42. [34] N. Segre, I. Joekes, Use of tire rubber particles as addition to cement paste, Cem. Concr. Res. 30 (9) (2000) 1421–1425. [35] R. Siddique, T.R. Naik, Properties of concrete containing scrap-tire rubber-an overview, Waste Manage. 24 (6) (2004) 563–569. [36] T. Naik, S. Singh, R. Wendorf, Applications of Scrap Tire Rubber in Asphaltic Materials: State of the Art Assessment A Technical Report, Wisconsin DNR, Center for By-Products Utilization, University of Wisconsin, Milwaukee, 1994. [37] O. Youssf, M.A. ElGawady, An overview of sustainable concrete made with scrap rubber, The 22nd Australasian Conference on the Mechanics of Structures and Materials (ACMSM 22), CRC Press, 2012, pp. 1039–1044. [38] O. Youssf, M.A. ElGawady, J.E. Mills, Static cyclic behaviour of FRP-confined crumb rubber concrete columns, Eng. Struct. 113 (2016) 371–387. [39] Standards Australia, Methods for testing concrete, Preparation of concrete mixes in the laboratory AS 1012.2, 1994. [40] Standards Australia, Methods for testing concrete, Determination of properties related to the consistency of concrete—Slump test AS 1012.3.1, 2014. [41] Standards Australia, Methods for sampling and testing aggregates, Method for making and curing concrete – compression and indirect tensile test specimens, AS1012.8.1, 2000. [42] Standards Australia, Methods of testing concrete – Compressive strength tests – Concrete, mortar and grout specimens, AS 1012.9, 2014. [43] Standards Australia, Methods for testing concrete, Determination of indirect tensile strength of concrete cylinders (Brasil or splitting test), AS 1012.10, 2000. [44] Neville A.M. Properties of Concrete Book, fifth ed., 1996. [45] ACI-234-96, Guide for the Use of Silica Fume in Concrete, American Concrete Institute, 1996. [46] M. Nehdi, A. Khan, Cementitious composites containing recycled tire rubber: an overview of engineering properties and potential applications, Cem. Concr. Aggr. 23 (1) (2001) 3–10. [47] P. Taylor, F. Bektas, E. Yurdakul, H. Ceylan, Optimizing Cementitious Content in Concrete Mixtures for Required Performance, 2012. [48] J. Van Kirk, G. Holleran, Reduced thickness asphalt rubber concrete leads to cost effective pavement rehabilitation, in: 1st International Conference World of Pavements, Sydney, Australia, 2000.