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Properties of treated recycled aggregates and its influence on concrete strength characteristics
Construction and Building Materials 111 (2016) 611–617
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Properties of treated recycled aggregates and its influence on concrete strength characteristics P. Saravanakumar ⇑, K. Abhiram, B. Manoj School of Civil Engineering, SASTRA University, Thanjavur 613401, India
h i g h l i g h t s The surface treatment by presoaking the recycled aggregates in acids significantly improves the properties of RA. Acid treated and silica fume impregnated recycled aggregate gave better concrete strength in the later age. The strength development of recycled aggregate concrete with treated RA was better than untreated RA.
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Article history: Received 20 June 2015 Received in revised form 17 December 2015 Accepted 17 February 2016
Keywords: Recycled aggregate Presoaking surface treatment method Silica fume impregnating method Recycled aggregate concrete Compressive strength
a b s t r a c t Utilization of recycled aggregate (RA) from crushed concrete wastes as alternative to natural aggregate in construction industry solve the construction and demolition waste (C&DW) disposal problems and reduces the gap between the demand and supply. The adhered mortar affects the properties of RA to significant level. This paper has studied the characteristics of recycled aggregates retrieved from crushed old concrete obtained from demolished structures, and five different presoaking surface treatment method and silica fume impregnating method to improve the properties of the recycled aggregates and its effect on recycled aggregate concrete (RAC). From experimental results it was observed that after treatment there was a significant improvement in the physical and mechanical properties of RA because of adhered mortar removal. The compressive strength was also significantly improved by using treated RA in RAC. Hence it is concluded that these treatment methods can be effectively used for the recycled aggregates to improve its characteristics. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction For environmental reasons and availability of increased volumes of C&DW, the use of recycled aggregates as a supplement to natural aggregate in construction industry and producing good quality concrete having similar performance characteristics of NAC is considered very valuable, from different prospects. Many attempts were made by researchers to develop a structural concrete with RA [1–14]. The properties of recycled aggregates were generally inferior to NA mainly because of the existence of mortar Abbreviations: NA, natural aggregate; NAC, natural aggregate concrete; RA, recycled aggregate; RAC, recycled aggregate concrete; RAH2 SO4 , H2SO4 treated recycled aggregate; RAHNO3 , HNO3 treated recycled aggregate; RAHCl , HCl treated recycled aggregate; RACHCl&SF , HCl and silica fume treated recycled aggregate; RACH2 SO4 , H2SO4 treated recycled aggregate concrete; RACHNO3 , HNO3treated recycled aggregate concrete; RACHCl , HCl treated recycled aggregate concrete. ⇑ Corresponding author. E-mail address: [email protected] (P. Saravanakumar). http://dx.doi.org/10.1016/j.conbuildmat.2016.02.064 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
and impurities [15]. Most of the knowledge and experience with RAC showed a decrease in strength properties when compared with natural aggregate concrete and NA with RA replacements also limited to 20% as per RILEM TC 121 DRG [16] report for all strength classes. It was observed that, the major reason for the strength reduction of RAC was because of its adhered mortar which forms weak interfacial transition zone leads to cracks in the concrete. Many attempts were made by researchers to improve the characteristics of RAC by varying w/c ratios, adding mineral and chemical admixtures like silica fume, slag etc., and blending recycled and natural aggregate with various replacement percentages. Similarly the properties of RA also enhance by giving beneficiation treatment to RA. Recycled aggregates treatment mainly involves the reduction of adhered mortar present on the surface of the aggregate. To remove the adhered mortar, mechanical treatment (ultrasonic cleaning, ball milling), chemical treatment (presoaking RA in an acidic environment) and thermal treatment
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(aggregate heating through micro wave etc.,) were reported in literatures [4,17]. Amnon [4] studied the microstructure of RA and found that the presence of loose particles in RA affects significantly the bonding capacity of the RAC. Treatment of RA by impregnation of silica fume solution and by ultrasonic cleaning improves the compressive strength of RAC by 15% and 7% respectively. Purushothaman et al. [18] studied the influence of mechanical and chemical treatment methods such as acid scrubbing treatment, heating and scrubbing treatment and acidic treatment (HCl and H2SO4) on RA. They found that the aggregates treated with H2SO4 and heating and scrubbing gave better quality RA than other acid and mechanical treatments. Surface modification by aggregate coating results better bonding characteristics of RA. Polymers and pozzalanic materials were tried for surface coating by many researchers. Ismail and Ramli [19] attempted to improve the physical and mechanical strength of RA by soaking the RA in hydrochloric (HCl) acid at 0.5 mol (M) concentrations and impregnated with calcium metasilicate (CM) solution to coat their surface with CM particles. They found that combination of these two surface treatment methods modify the surface and improve RA properties. Li et al. [20] reported that surface coating with pozzalanic materials (silica fume and fly ash) improved the strength of RAC. The silicon-based polymer impregnation treatments were carried out on RA and reported that these treatment methods significantly reduce the rate of water absorption [21]. In the present study, the characteristics of recycled aggregates retrieved from crushed old concrete obtained from demolished structures were studied, and several presoaking surface treatment methods to improve the properties of the recycled aggregate were evaluated. The influence of treated recycled aggregates on concrete strength characteristics also studied.
2. Experimental investigations 2.1. Materials and methods Crushed granite recycled concrete aggregates were obtained from 20-year-old demolished structure. The reinforcements and aggregates were separated from the demolished concrete by crushing and cleaning. The required size of the recycled aggregate was attained by further crushing and the loose particles were removed through water washing. Physical and mechanical properties for RA such as specific gravity and water absorption, bulk density, aggregate crushing, impact and abrasion value were measured by the methods proposed by ASTM C127, ASTM C29 and ASTM C131 [22–24] respectively. The microstructure of RA was studied through Scanning electron microscope (SEM) analysis and its chemical composition was evaluated by XRF spectrometer. The XRF results were shown in Table 1. The recycled aggregates were subjected to various presoaking surface treatment methods and the performance of each treatment methods on physical and mechanical properties were estimated. For presoaking surface treatment, the recycled aggregates were presoaked in three different acidic solutions namely, hydrochloric acid (HCl), nitric acid (HNO3) and sulfuric acid (H2SO4) in room temperature (27– 30 °C) for 24 h. To provide suitable acidic environment and to improve the quality by means of removal of adhered mortar from recycled aggregates, 10% normality
was chosen for the acidic solutions. After presoaking the aggregates were thoroughly washed with distilled water to remove the acid solvents and loose particles. The HCl treated RA was again treated with silica fume by soaking the HCl treated RA into the silica fume solution for 24 h at room temperature. After that it was allowed to dry for 24 h. Fig. 1 shows the aggregates before and after treatment. 2.2. Experimental procedure 2.2.1. Aggregate crushing value In this test a cylindrical measure was filled with specified quantity of aggregate in three layers and compacted by tamping of 25 strokes using tamping rod for each layer. Using compression testing machine a uniform rate of 40 kN load was applying to the aggregate sample in a steel cylinder for 10 min. The crushing value of the aggregate sample was estimated by finding the passing percentage of resulting crushed aggregate through a No. 12 sieve. 2.2.2. Aggregate impact value In this test a steel cup shall be fixed firmly in position on the base of the impact test machine and the test sample will be placed in it and compacted by a single tamping of 25 strokes. The hammer shall be raised until its lower face is 375 mm above from the upper surface of the aggregate in the cup, and allowed to fall freely on the aggregate. The test sample shall be subjected to a total 15 such blows each being delivered at an interval of not less than one second. The crushed aggregate shall then be removed from the cup and the whole of it sieved on No. 7 B.S. sieve until no further significant amount passes in one minute. The percentage of aggregate passing through the sieve gives the impact strength of the aggregate. 2.2.3. Aggregate abrasion value In this test a specified quantity of aggregate is placed in the steel drum along with 6–12 steels spheres weighing approximately 420 g each. The drum is rotated for 500 revolutions with a shelf inside the drum causing a tumbling and dropping of the aggregate and balls. The percentage of the aggregate worn away is determined by sieving the resulting sample over a No. 12 sieve. As per ASTM C 33, ‘‘Concrete Aggregates,” specifies a maximum mass loss of 50% for gravel, crushed gravel, or crushed stone. 2.3. Concrete specimen preparation and testing Concrete mix design was done as per ACI method and 1:1.4:2.3 mix proportions was arrived with 0.45 water cement ratio. Ordinary Portland cement ASTM type 1 with a specific surface area 3960 cm2/g and specific gravity of 3.15 was used throughout this work. Treated and untreated recycled aggregates, natural crushed granite aggregates and locally available river sand were used for the specimen preparation. The maximum size of coarse aggregate used for this work was less than 20 mm and all aggregates were used in saturated surface dry state. Hence there was not much difference found in the slump value. The mix proportion of each concrete and its slump value was detailed in a Table 2. The particle size distribution for coarse and fine aggregates were carried out based on BS EN 933-1 [25] and the results were shown in Table 3. To increase the workability of concrete and to reduce the water content, super plasticizer Conplast SP-40 was used at 2% by mass of cement content. Concrete specimens made from the natural coarse aggregates, untreated and treated recycled aggregates in the forms of 100 mm sized cube according to British Standard (BS 1881: Part 116, 1983) were prepared to assess their compressive strength. The specimens were demolded after 24 h and further water cured in a curing tank at 27 ± 1 °C until the ages of 90 days were reached. The crystalline phases present in the concrete were found through XRD analysis. The pH values are also examined to find the alkalinity of the concrete. The properties of the concrete specimen were found at the age of 7, 28, 56 and 90 days and the average of three specimen values were taken for result comparison.
3. Results and discussion 3.1. Properties of recycled aggregate
Table 1 Chemical composition of coarse aggregates found by XRF analysis. Description
NA (%)
RA (%)
RAH2 SO4 (%)
RAHNO3 (%)
RAHCl (%)
RAHCl&SF (%)
SiO2 Al2O3 CaO Fe2O3 MgO Na2O K2O TiO2 SO3
56.54 17.81 6.17 6.07 2.91 4.20 2.65 0.66 0.14
53.44 11.9 18.84 5.9 0.94 2.19 3.89 1.00 1.03
53.41 12.99 13.12 7.18 2.78 2.66 1.86 0.69 1.57
52.46 15.19 14.80 7.80 2.94 2.93 1.44 0.63 0.84
52.44 16.78 11.70 8.19 4.45 3.91 0.70 0.64 0.36
56.90 14.77 11.55 7.15 3.35 3.56 0.93 0.54 0.51
3.1.1. Specific gravity and mass loss The specific gravity of natural aggregate and recycled aggregate were found as 2.71 and 2.47 respectively. The major factor for getting lower specific gravity in recycled aggregate was its source, mix proportion and age of the concrete [11]. For this work the recycled aggregates were collected from a single source and before demolition the compressive strength of the concrete was estimated by taking concrete cube samples and NDT (Rebound Hammer test) technique. From that it was confirmed that the existing concrete has a compressive strength of 25 MPa. Since the RA having adhered mortar, it affects its specific gravity very much. The specific gravity
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Fig. 1. Recycled aggregates before and after treatment.
Table 2 Mix proportion.
Table 4 Specific gravity and mass loss.
Type of aggregate
W/C ratio
Mix proportions(kg/m3) Water
Cement
Fine aggregate
NA RA RAHCl RAH2 SO4 RAHNO3 RAHCl&SF
0.45 0.45 0.45 0.45 0.45 0.45
170 170 170 170 170 170
378 378 378 378 378 378
727 727 727 727 727 727
Coarse aggregate NA
Slump value
RA
1180 – – – – –
1180 1180 1180 1180 1180
90 80 83 88 85 92
Aggregate type
Specific gravity
Mass loss in %
Standard deviation Specific gravity
Mass loss
NA RA RAHCl RAH2 SO4 RAHNO3 RAHCl&SF
2.71 2.47 2.49 2.65 2.55 2.62
– – 1.52 5.63 2.50 3.37
0.08 0.15 0.13 0.09 0.12 0.10
– – 1.25 1.05 1.13 1.08
1.5%, 2.5%, and 5.6% for HCl, HNO3 and H2SO4 treatment respectively. Effective acid solution treatment for RA was identified in H2SO4 acid. The lesser mortar removal was observed in HCl treated aggregates and because of the higher pores presence in the aggregate lower density was observed. Hence to get better densified recycled aggregate, the pores in RA were filled with SF by impregnating the HCl treated recycled aggregates in SF solution.
and mass loss values for the natural and recycled aggregates before and after treatment were found by taking 10 samples of 3 kg each and the average values were given in Table 4. Standard deviations of specific gravity and mass loss were also specified in Table 4 and it showed how much variability or diversity in the experimental data. The treated aggregates having better specific gravity than the untreated RA. After treatment there was a remarkable mass loss was found on the recycled aggregate (Table 4). The major reason for the improvement in specific gravity and mass loss was the removal of adhered mortar from RA. The mass loss was found to be
3.1.2. Density and water absorption The bulk density of RA was 15% lesser than that of NA (1635.55 kg/m3) and after treatment a reasonable improvement
Table 3 Particle size distribution of aggregates. Aggregate
Sand Natural coarse aggregates Recycled coarse aggregates
Aggregate passing (%) according to sieve size (mm)
Fine modulus
0.15
0.3
0.6
1.18
2.36
4.75
10
12
16
20
0.8 0.0 0.0
7.9 0.0 0.0
24.6 0.0 0.0
44.8 0.0 0.0
76.9 0.4 0.5
100 1 0.9
100 23 34
100 49.2 58.4
100 74.5 85.3
100 100 100
3.45 7.52 7.21
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Fig. 2. Bulk density variation and water absorption of recycled aggregates before and after treatment.
Fig. 3. Aggregate Crushing, impact, and abrasion value.
was found. Among the all three chemical treatments lower density variation (10%) was observed in H2SO4 treated recycled aggregates with respect to NA. The improvement in the RA density indicates a higher percentage adhered mortar removal from the recycled aggregates. The Higher density variation (13%) was observed in HCl treated recycled aggregates. Hence it was again retreated with silica fume impregnation and improvement in density was found. After silica fume impregnation treatment the density variation reduced from 13% to 11% for HCl treated recycled aggregates. The percentage variation in density and water absorption for RA before and after treatment was shown in Fig. 2. Natural and recycled aggregates water absorption characteristics were assessed by keeping 3 kg of oven dried aggregates in water for 24 h. Average of three experimental data was taken and from experimental results it was found that the recycled aggregates have higher water absorption than natural aggregates. The major factor for higher water absorption property of RA was its adhered mortar and source. After chemical treatment the adhered mortar was removed to the possible extent and hence the water absorption value gets considerably decreased in RA. Among the above treatment methods effective result was obtained in H2SO4 solution followed by HNO3 and HCl solutions. The SF impregnation for the HCl acid treated aggregates showed better improvement in the test results. From the above results it was concluded that the RA have higher water absorption value than NA after treatment also and hence for concrete preparation it is suggested that to use the RA in saturated surface dry (SSD) state. The SSD state can be obtained for coarse aggregate particles by saturating them in water and then drying the surfaces with absorbent cloth [22]. 3.1.3. Crushing, impact and abrasion value The quality of the aggregate and resistance to degradation due to handling, stockpiling, or mixing is generally estimated through
Fig. 4. SEM image of recycled aggregate before and after treatment.
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Fig. 5. Strength development of concrete specimen as a function of curing time.
abrasion, crushing and impact resistance tests. The results were expressed as percentage of total weight and stronger aggregates have lower percentage value. As per ASTM C33 [26], for concrete aggregates the crushing and impact values are limited to 50% for gravel, crushed gravel and crushed stone. From the experimental results it was observed that, the crushing value of NA is 19%, and for RA it was 33%. Similarly, the impact and abrasion value of natural aggregate were 15% and 24% respectively and for recycled aggregates it was 20% and 47% respectively. After acid treatment the strength of the recycled aggregate gets improved by removing the adhered mortar from the recycled aggregate. The crushing value for HCl, HNO3 and H2SO4 treated aggregates were improved by 7%, 3% and 2% respectively than RA. Similarly the impact resistance was improved by 9%, 10% and 7% and the abrasion value by 19%, 24% and 34% respectively than RA. The higher resistance against crushing, impact and abrasion (19%, 23% and 29% respectively) was observed in HCl&SF impregnated aggregates. The average of three test sample results for aggregate crushing, impact and abrasion values were shown in Fig. 3. It is clear that, the properties of RA were inferior to NA but well within the limits.
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3.1.4. Microstructure of recycled aggregates The microstructure and the morphological character of recycled aggregates were studied by scanning electron microscope (SEM). Fig. 4(a–e) shows the microstructure of RAC before and after treatment. From Fig. 4(a–e) it was found that higher amount of mortar removal was observed in H2SO4 acid treatment and lesser mortar removal was observed in HCl treated aggregates. The HCl treated and silica fume impregnated aggregates showed better surface characteristics. Many large pores were filled with silica fume in the HCl treated aggregate and hence the Fig. 4(e) showed only small pores. This ensured the densification of RA after silica fume impregnation treatment. The chemical composition of aggregates were examined through XRF was presented in Table 1. From results it was observed that every acidic treatment has its own merits and demerits. The sulfuric treated aggregates contain higher percentage of SO3 and hydrochloric acid treated aggregates contain higher percentage of Fe2O3 than other treated aggregates. As per the previous studies stated in literatures higher amount of SO3 presence leads to strength loss and increases expansion in lime and sulfate. Similarly higher amount of Fe2O3 improves the strength of concrete [27].
3.2. Properties of recycled aggregate concrete 3.2.1. Compressive strength of hardened concrete The concrete compressive strength developments at different ages for the different mixtures with NA and RA were found and presented in Fig. 5. It was found that irrespective of the age the compressive strength the recycled aggregate concrete (RAC) was inferior to natural aggregate concrete (NAC). Based on previous research reports and the authors earlier research reports it was observed that the presence of adhered mortar in recycled aggregate affects the strength of the concrete. The compressive strength of concrete made with recycled aggregates was 25% lesser than that of concrete made with natural aggregates at the age of 28 days. In treated aggregates the loose and the weak adhered
Fig. 6. XRD patterns of concrete at the age of 28 days. (a) Natural aggregate concrete, (b) recycled aggregate concrete, (c) H2SO4 treated recycled aggregate concrete, (d) HNO3 treated recycled aggregate concrete, (e) HCl treated recycled aggregate concrete, (f) HCl&SF treated recycled aggregate concrete. P = portlandite, E = ettringite, C = calcite, G = gypsum, Q = quartz, F = feldspar, D = dolomite and PC = Portland cement.
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Table 5 pH values for different concrete mixes. Mix
pH
NAC RAC RACH2 SO4 RACHNO3 RACHCl RACHCl&SF
12.32 11.96 12.12 12.20 12.14 12.28
mortar were removed to the possible extent and the aggregate surface characteristics were improved. The contact at the interfacial transition zone between treated RA and new cement paste gets improved and thereby the compressive strength of treated recycled aggregate concrete was improved by 8–18% at the age of 28 days than untreated RAC. The strength development of treated recycled aggregate concrete in the later age was found good. The percentage strength development for the HCl, HNO3, H2SO4 and HCl + SF treated aggregate concrete was 18%, 18.5%, 20% and 22% respectively. Among all the four treated aggregate concrete, the strength development was found comparatively lesser for HCl treated concrete. But if the same HCl treated aggregate was subjected to silica fume impregnation gave better strength improvement in concrete at the age of 90 days than other acid treated aggregate concrete. During silica fume impregnation the voids in the recycled aggregates was filled and there by improve the impermeable characteristics of recycled aggregates. The presence of silica in the recycled aggregate react with CH and produce additional C–S–H gel which improves the bond between the recycled aggregate and the cement paste. Hence, from the experimental results it was concluded that the silica fume impregnation treatment for RA improves the ITZ between the RA and new cement mortar. 3.2.2. XRD analysis XRD is the direct method to find the crystalline phases present in the concrete. The powder sample for testing was prepared by crushing the concrete samples taken from the 28 days cured concrete specimens and sieved through 150 lm sieve. The powdered samples were scanned for 10°–60°. The peaks in the XRD graph (Fig. 6) indicate the presence of portlandite, ettringite, calcite, gypsum, quartz, feldspar, dolomite and Portland cement. In the XRD pattern of natural aggregate concrete Feldspar, calcite and portlandite were the dominated crystalline phases. There was a broad and diffuse hump appears between 10° and 20°. This hump could be of the amorphous nature of the concrete. For RAC the XRD analysis data showed major peaks of feldspar and minor peaks of portlandite, gypsum, quartz, and dolomite. The presence of unhydrated Portland cement also found in RAC. Similar trend of XRD pattern was found in HCl treated RAC. But in all other treated RAC quartz dominated than other minerals. It is also worthy to note that RA has higher percentage of CaO compared to NA resulting into acceleration of CaCl2 attack. 3.2.3. Alkalinity The pH vale of the concrete indicates the degradation level of the concrete. It should be 12–13 for a good concrete and below 9 for failure concrete. If the pH value of the concrete is lowered, it affects the bonding capability of cement and leads to crack initiation and corrosion. To measure the alkalinity 28 days hardened concrete powder samples of 20 g was put into 100 ml of distilled water and the solution was allowed to stand for 72 h. The solution was agitated often, to enable more free lime of hydrated cement paste to get dissolved in water. The pH of the aqueous solution was measured by pH meter. The pH values of the all concrete mix specimens were shown in Table 5. From the experimental
results it was understood that the alkalinity of the RAC was well within the limit and it was very well improved and by using treated recycled aggregates in concrete. 4. Conclusions This paper discusses the surface treatment methods of recycled aggregate concrete based on physical and mechanical properties and the effects of treated recycled aggregate on concrete strength were also investigated. Results of this study can be summarized as follows. 1. The surface treatment method effectively removed the loose mortar particles and thereby significantly improves the properties of RA. 2. Silica fume impregnation after chemical immersion treatment gave better densified recycled aggregate. 3. From the results it was suggested that to use the RA in saturated surface dry (SSD) state before concrete mixing. 4. The compressive strength of RAC was lower than that of NAC at all ages. However, the development of concrete strength with treated RA was better than untreated RA. 5. The presence of silica in silica fume impregnated aggregate gave better strength in the later age. 6. Overall, the surface treatment by presoaking the RA in acids significantly improves the properties of RAC and hence this method can be employed in the application on large scale RAC projects.
Acknowledgement The authors would like to thank the Vice Chancellor of SASTRA UNIVERSITY for providing facilities to do this work and for the continuous support and encouragement given throughout this research work. References [1] B.I. Topcu, F.N. Guncan, Using waste concrete as aggregate, Cem. Concr. Res. 25 (7) (1995) 1385–1390. [2] A. Van, Recycling of concrete at a precast concrete plant, Concr. Precast. Plant Technol. 6 (1996) 91–101. [3] M.A. Mansur, T.H. Wee, L.S. Cheran, Crushed bricks as coarse aggregate for concrete, ACI Mater. J. 96 (4) (1999) 478–484. [4] K. Amnon, Treatments for the improvement of recycled aggregate, J. Mater. Civ. Eng. 16 (6) (2004) 597–603. [5] K. Amnon, Properties of concrete made with recycled aggregate from partially hydrated old concrete, Cem. Concr. Res. 33 (5) (2003) 703–711. [6] J.C. How, Y. Tsong, K.H. Chen, Use of building rubbles as recycled aggregates, Cem. Concr. Res. 33 (1) (2003) 125–132. [7] F.M. Khalaf, A.S. DeVenny, Recycling of demolished masonry rubble as coarse aggregate in concrete: a review, J. Mater. Civ. Eng. 16 (4) (2004) 331–340. [8] X. Jianzhuang, L. Jiabin, C. Zhang, Mechanical properties of recycled aggregate concrete under uniaxial loading, Cem. Concr. Res. 35 (6) (2005) 1187–1194. [9] R. Khaldoun, Mechanical properties of concrete with recycled coarse aggregate, Build. Environ. 42 (1) (2007) 407–415. [10] P.L. Yong, D.C.L. Teo, Utilization of recycled aggregate as coarse aggregate in concrete, UNIMAS e-J. Civil Eng. 1 (1) (2009) 29–36. [11] P. Saravanakumar, G. Dhinakaran, Effect of admixed recycled aggregate concrete on properties of fresh and hardened concrete, J. Mater. Civ. Eng. 24 (4) (2012) 494–498. [12] P. Saravanakumar, G. Dhinakaran, Strength characteristics of high-volume fly ash–based recycled aggregate concrete, ASCE J. Mater. Civil Eng. 25 (8) (2013) 1127–1133. [13] P. Saravanakumar, G. Dhinakaran, Durability characteristics of recycled aggregate concrete, Struct. Eng. Mech. 47 (5) (2013) 701–711. [14] P. Saravanakumar, G. Dhinakaran, Durability aspects of HVFA based recycled aggregate concrete, Mag. Concr. Res. 66 (4) (2014) 186–195. [15] C.K.K. Sagoe, T. Brown, A.H. Taylor, Performance of concrete made with commercially produced coarse recycled concrete aggregate, Cem. Concr. Res. 31 (5) (2001) 707–712. [16] RILEM TC 121 DRG, Specification for concrete with recycled aggregates, Mater. Struct. 27 (173) (1994) 557–559.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Properties of treated recycled aggregates and its influence on concrete strength characteristics P. Saravanakumar ⇑, K. Abhiram, B. Manoj School of Civil Engineering, SASTRA University, Thanjavur 613401, India
h i g h l i g h t s The surface treatment by presoaking the recycled aggregates in acids significantly improves the properties of RA. Acid treated and silica fume impregnated recycled aggregate gave better concrete strength in the later age. The strength development of recycled aggregate concrete with treated RA was better than untreated RA.
a r t i c l e
i n f o
Article history: Received 20 June 2015 Received in revised form 17 December 2015 Accepted 17 February 2016
Keywords: Recycled aggregate Presoaking surface treatment method Silica fume impregnating method Recycled aggregate concrete Compressive strength
a b s t r a c t Utilization of recycled aggregate (RA) from crushed concrete wastes as alternative to natural aggregate in construction industry solve the construction and demolition waste (C&DW) disposal problems and reduces the gap between the demand and supply. The adhered mortar affects the properties of RA to significant level. This paper has studied the characteristics of recycled aggregates retrieved from crushed old concrete obtained from demolished structures, and five different presoaking surface treatment method and silica fume impregnating method to improve the properties of the recycled aggregates and its effect on recycled aggregate concrete (RAC). From experimental results it was observed that after treatment there was a significant improvement in the physical and mechanical properties of RA because of adhered mortar removal. The compressive strength was also significantly improved by using treated RA in RAC. Hence it is concluded that these treatment methods can be effectively used for the recycled aggregates to improve its characteristics. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction For environmental reasons and availability of increased volumes of C&DW, the use of recycled aggregates as a supplement to natural aggregate in construction industry and producing good quality concrete having similar performance characteristics of NAC is considered very valuable, from different prospects. Many attempts were made by researchers to develop a structural concrete with RA [1–14]. The properties of recycled aggregates were generally inferior to NA mainly because of the existence of mortar Abbreviations: NA, natural aggregate; NAC, natural aggregate concrete; RA, recycled aggregate; RAC, recycled aggregate concrete; RAH2 SO4 , H2SO4 treated recycled aggregate; RAHNO3 , HNO3 treated recycled aggregate; RAHCl , HCl treated recycled aggregate; RACHCl&SF , HCl and silica fume treated recycled aggregate; RACH2 SO4 , H2SO4 treated recycled aggregate concrete; RACHNO3 , HNO3treated recycled aggregate concrete; RACHCl , HCl treated recycled aggregate concrete. ⇑ Corresponding author. E-mail address: [email protected] (P. Saravanakumar). http://dx.doi.org/10.1016/j.conbuildmat.2016.02.064 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
and impurities [15]. Most of the knowledge and experience with RAC showed a decrease in strength properties when compared with natural aggregate concrete and NA with RA replacements also limited to 20% as per RILEM TC 121 DRG [16] report for all strength classes. It was observed that, the major reason for the strength reduction of RAC was because of its adhered mortar which forms weak interfacial transition zone leads to cracks in the concrete. Many attempts were made by researchers to improve the characteristics of RAC by varying w/c ratios, adding mineral and chemical admixtures like silica fume, slag etc., and blending recycled and natural aggregate with various replacement percentages. Similarly the properties of RA also enhance by giving beneficiation treatment to RA. Recycled aggregates treatment mainly involves the reduction of adhered mortar present on the surface of the aggregate. To remove the adhered mortar, mechanical treatment (ultrasonic cleaning, ball milling), chemical treatment (presoaking RA in an acidic environment) and thermal treatment
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(aggregate heating through micro wave etc.,) were reported in literatures [4,17]. Amnon [4] studied the microstructure of RA and found that the presence of loose particles in RA affects significantly the bonding capacity of the RAC. Treatment of RA by impregnation of silica fume solution and by ultrasonic cleaning improves the compressive strength of RAC by 15% and 7% respectively. Purushothaman et al. [18] studied the influence of mechanical and chemical treatment methods such as acid scrubbing treatment, heating and scrubbing treatment and acidic treatment (HCl and H2SO4) on RA. They found that the aggregates treated with H2SO4 and heating and scrubbing gave better quality RA than other acid and mechanical treatments. Surface modification by aggregate coating results better bonding characteristics of RA. Polymers and pozzalanic materials were tried for surface coating by many researchers. Ismail and Ramli [19] attempted to improve the physical and mechanical strength of RA by soaking the RA in hydrochloric (HCl) acid at 0.5 mol (M) concentrations and impregnated with calcium metasilicate (CM) solution to coat their surface with CM particles. They found that combination of these two surface treatment methods modify the surface and improve RA properties. Li et al. [20] reported that surface coating with pozzalanic materials (silica fume and fly ash) improved the strength of RAC. The silicon-based polymer impregnation treatments were carried out on RA and reported that these treatment methods significantly reduce the rate of water absorption [21]. In the present study, the characteristics of recycled aggregates retrieved from crushed old concrete obtained from demolished structures were studied, and several presoaking surface treatment methods to improve the properties of the recycled aggregate were evaluated. The influence of treated recycled aggregates on concrete strength characteristics also studied.
2. Experimental investigations 2.1. Materials and methods Crushed granite recycled concrete aggregates were obtained from 20-year-old demolished structure. The reinforcements and aggregates were separated from the demolished concrete by crushing and cleaning. The required size of the recycled aggregate was attained by further crushing and the loose particles were removed through water washing. Physical and mechanical properties for RA such as specific gravity and water absorption, bulk density, aggregate crushing, impact and abrasion value were measured by the methods proposed by ASTM C127, ASTM C29 and ASTM C131 [22–24] respectively. The microstructure of RA was studied through Scanning electron microscope (SEM) analysis and its chemical composition was evaluated by XRF spectrometer. The XRF results were shown in Table 1. The recycled aggregates were subjected to various presoaking surface treatment methods and the performance of each treatment methods on physical and mechanical properties were estimated. For presoaking surface treatment, the recycled aggregates were presoaked in three different acidic solutions namely, hydrochloric acid (HCl), nitric acid (HNO3) and sulfuric acid (H2SO4) in room temperature (27– 30 °C) for 24 h. To provide suitable acidic environment and to improve the quality by means of removal of adhered mortar from recycled aggregates, 10% normality
was chosen for the acidic solutions. After presoaking the aggregates were thoroughly washed with distilled water to remove the acid solvents and loose particles. The HCl treated RA was again treated with silica fume by soaking the HCl treated RA into the silica fume solution for 24 h at room temperature. After that it was allowed to dry for 24 h. Fig. 1 shows the aggregates before and after treatment. 2.2. Experimental procedure 2.2.1. Aggregate crushing value In this test a cylindrical measure was filled with specified quantity of aggregate in three layers and compacted by tamping of 25 strokes using tamping rod for each layer. Using compression testing machine a uniform rate of 40 kN load was applying to the aggregate sample in a steel cylinder for 10 min. The crushing value of the aggregate sample was estimated by finding the passing percentage of resulting crushed aggregate through a No. 12 sieve. 2.2.2. Aggregate impact value In this test a steel cup shall be fixed firmly in position on the base of the impact test machine and the test sample will be placed in it and compacted by a single tamping of 25 strokes. The hammer shall be raised until its lower face is 375 mm above from the upper surface of the aggregate in the cup, and allowed to fall freely on the aggregate. The test sample shall be subjected to a total 15 such blows each being delivered at an interval of not less than one second. The crushed aggregate shall then be removed from the cup and the whole of it sieved on No. 7 B.S. sieve until no further significant amount passes in one minute. The percentage of aggregate passing through the sieve gives the impact strength of the aggregate. 2.2.3. Aggregate abrasion value In this test a specified quantity of aggregate is placed in the steel drum along with 6–12 steels spheres weighing approximately 420 g each. The drum is rotated for 500 revolutions with a shelf inside the drum causing a tumbling and dropping of the aggregate and balls. The percentage of the aggregate worn away is determined by sieving the resulting sample over a No. 12 sieve. As per ASTM C 33, ‘‘Concrete Aggregates,” specifies a maximum mass loss of 50% for gravel, crushed gravel, or crushed stone. 2.3. Concrete specimen preparation and testing Concrete mix design was done as per ACI method and 1:1.4:2.3 mix proportions was arrived with 0.45 water cement ratio. Ordinary Portland cement ASTM type 1 with a specific surface area 3960 cm2/g and specific gravity of 3.15 was used throughout this work. Treated and untreated recycled aggregates, natural crushed granite aggregates and locally available river sand were used for the specimen preparation. The maximum size of coarse aggregate used for this work was less than 20 mm and all aggregates were used in saturated surface dry state. Hence there was not much difference found in the slump value. The mix proportion of each concrete and its slump value was detailed in a Table 2. The particle size distribution for coarse and fine aggregates were carried out based on BS EN 933-1 [25] and the results were shown in Table 3. To increase the workability of concrete and to reduce the water content, super plasticizer Conplast SP-40 was used at 2% by mass of cement content. Concrete specimens made from the natural coarse aggregates, untreated and treated recycled aggregates in the forms of 100 mm sized cube according to British Standard (BS 1881: Part 116, 1983) were prepared to assess their compressive strength. The specimens were demolded after 24 h and further water cured in a curing tank at 27 ± 1 °C until the ages of 90 days were reached. The crystalline phases present in the concrete were found through XRD analysis. The pH values are also examined to find the alkalinity of the concrete. The properties of the concrete specimen were found at the age of 7, 28, 56 and 90 days and the average of three specimen values were taken for result comparison.
3. Results and discussion 3.1. Properties of recycled aggregate
Table 1 Chemical composition of coarse aggregates found by XRF analysis. Description
NA (%)
RA (%)
RAH2 SO4 (%)
RAHNO3 (%)
RAHCl (%)
RAHCl&SF (%)
SiO2 Al2O3 CaO Fe2O3 MgO Na2O K2O TiO2 SO3
56.54 17.81 6.17 6.07 2.91 4.20 2.65 0.66 0.14
53.44 11.9 18.84 5.9 0.94 2.19 3.89 1.00 1.03
53.41 12.99 13.12 7.18 2.78 2.66 1.86 0.69 1.57
52.46 15.19 14.80 7.80 2.94 2.93 1.44 0.63 0.84
52.44 16.78 11.70 8.19 4.45 3.91 0.70 0.64 0.36
56.90 14.77 11.55 7.15 3.35 3.56 0.93 0.54 0.51
3.1.1. Specific gravity and mass loss The specific gravity of natural aggregate and recycled aggregate were found as 2.71 and 2.47 respectively. The major factor for getting lower specific gravity in recycled aggregate was its source, mix proportion and age of the concrete [11]. For this work the recycled aggregates were collected from a single source and before demolition the compressive strength of the concrete was estimated by taking concrete cube samples and NDT (Rebound Hammer test) technique. From that it was confirmed that the existing concrete has a compressive strength of 25 MPa. Since the RA having adhered mortar, it affects its specific gravity very much. The specific gravity
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Fig. 1. Recycled aggregates before and after treatment.
Table 2 Mix proportion.
Table 4 Specific gravity and mass loss.
Type of aggregate
W/C ratio
Mix proportions(kg/m3) Water
Cement
Fine aggregate
NA RA RAHCl RAH2 SO4 RAHNO3 RAHCl&SF
0.45 0.45 0.45 0.45 0.45 0.45
170 170 170 170 170 170
378 378 378 378 378 378
727 727 727 727 727 727
Coarse aggregate NA
Slump value
RA
1180 – – – – –
1180 1180 1180 1180 1180
90 80 83 88 85 92
Aggregate type
Specific gravity
Mass loss in %
Standard deviation Specific gravity
Mass loss
NA RA RAHCl RAH2 SO4 RAHNO3 RAHCl&SF
2.71 2.47 2.49 2.65 2.55 2.62
– – 1.52 5.63 2.50 3.37
0.08 0.15 0.13 0.09 0.12 0.10
– – 1.25 1.05 1.13 1.08
1.5%, 2.5%, and 5.6% for HCl, HNO3 and H2SO4 treatment respectively. Effective acid solution treatment for RA was identified in H2SO4 acid. The lesser mortar removal was observed in HCl treated aggregates and because of the higher pores presence in the aggregate lower density was observed. Hence to get better densified recycled aggregate, the pores in RA were filled with SF by impregnating the HCl treated recycled aggregates in SF solution.
and mass loss values for the natural and recycled aggregates before and after treatment were found by taking 10 samples of 3 kg each and the average values were given in Table 4. Standard deviations of specific gravity and mass loss were also specified in Table 4 and it showed how much variability or diversity in the experimental data. The treated aggregates having better specific gravity than the untreated RA. After treatment there was a remarkable mass loss was found on the recycled aggregate (Table 4). The major reason for the improvement in specific gravity and mass loss was the removal of adhered mortar from RA. The mass loss was found to be
3.1.2. Density and water absorption The bulk density of RA was 15% lesser than that of NA (1635.55 kg/m3) and after treatment a reasonable improvement
Table 3 Particle size distribution of aggregates. Aggregate
Sand Natural coarse aggregates Recycled coarse aggregates
Aggregate passing (%) according to sieve size (mm)
Fine modulus
0.15
0.3
0.6
1.18
2.36
4.75
10
12
16
20
0.8 0.0 0.0
7.9 0.0 0.0
24.6 0.0 0.0
44.8 0.0 0.0
76.9 0.4 0.5
100 1 0.9
100 23 34
100 49.2 58.4
100 74.5 85.3
100 100 100
3.45 7.52 7.21
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Fig. 2. Bulk density variation and water absorption of recycled aggregates before and after treatment.
Fig. 3. Aggregate Crushing, impact, and abrasion value.
was found. Among the all three chemical treatments lower density variation (10%) was observed in H2SO4 treated recycled aggregates with respect to NA. The improvement in the RA density indicates a higher percentage adhered mortar removal from the recycled aggregates. The Higher density variation (13%) was observed in HCl treated recycled aggregates. Hence it was again retreated with silica fume impregnation and improvement in density was found. After silica fume impregnation treatment the density variation reduced from 13% to 11% for HCl treated recycled aggregates. The percentage variation in density and water absorption for RA before and after treatment was shown in Fig. 2. Natural and recycled aggregates water absorption characteristics were assessed by keeping 3 kg of oven dried aggregates in water for 24 h. Average of three experimental data was taken and from experimental results it was found that the recycled aggregates have higher water absorption than natural aggregates. The major factor for higher water absorption property of RA was its adhered mortar and source. After chemical treatment the adhered mortar was removed to the possible extent and hence the water absorption value gets considerably decreased in RA. Among the above treatment methods effective result was obtained in H2SO4 solution followed by HNO3 and HCl solutions. The SF impregnation for the HCl acid treated aggregates showed better improvement in the test results. From the above results it was concluded that the RA have higher water absorption value than NA after treatment also and hence for concrete preparation it is suggested that to use the RA in saturated surface dry (SSD) state. The SSD state can be obtained for coarse aggregate particles by saturating them in water and then drying the surfaces with absorbent cloth [22]. 3.1.3. Crushing, impact and abrasion value The quality of the aggregate and resistance to degradation due to handling, stockpiling, or mixing is generally estimated through
Fig. 4. SEM image of recycled aggregate before and after treatment.
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Fig. 5. Strength development of concrete specimen as a function of curing time.
abrasion, crushing and impact resistance tests. The results were expressed as percentage of total weight and stronger aggregates have lower percentage value. As per ASTM C33 [26], for concrete aggregates the crushing and impact values are limited to 50% for gravel, crushed gravel and crushed stone. From the experimental results it was observed that, the crushing value of NA is 19%, and for RA it was 33%. Similarly, the impact and abrasion value of natural aggregate were 15% and 24% respectively and for recycled aggregates it was 20% and 47% respectively. After acid treatment the strength of the recycled aggregate gets improved by removing the adhered mortar from the recycled aggregate. The crushing value for HCl, HNO3 and H2SO4 treated aggregates were improved by 7%, 3% and 2% respectively than RA. Similarly the impact resistance was improved by 9%, 10% and 7% and the abrasion value by 19%, 24% and 34% respectively than RA. The higher resistance against crushing, impact and abrasion (19%, 23% and 29% respectively) was observed in HCl&SF impregnated aggregates. The average of three test sample results for aggregate crushing, impact and abrasion values were shown in Fig. 3. It is clear that, the properties of RA were inferior to NA but well within the limits.
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3.1.4. Microstructure of recycled aggregates The microstructure and the morphological character of recycled aggregates were studied by scanning electron microscope (SEM). Fig. 4(a–e) shows the microstructure of RAC before and after treatment. From Fig. 4(a–e) it was found that higher amount of mortar removal was observed in H2SO4 acid treatment and lesser mortar removal was observed in HCl treated aggregates. The HCl treated and silica fume impregnated aggregates showed better surface characteristics. Many large pores were filled with silica fume in the HCl treated aggregate and hence the Fig. 4(e) showed only small pores. This ensured the densification of RA after silica fume impregnation treatment. The chemical composition of aggregates were examined through XRF was presented in Table 1. From results it was observed that every acidic treatment has its own merits and demerits. The sulfuric treated aggregates contain higher percentage of SO3 and hydrochloric acid treated aggregates contain higher percentage of Fe2O3 than other treated aggregates. As per the previous studies stated in literatures higher amount of SO3 presence leads to strength loss and increases expansion in lime and sulfate. Similarly higher amount of Fe2O3 improves the strength of concrete [27].
3.2. Properties of recycled aggregate concrete 3.2.1. Compressive strength of hardened concrete The concrete compressive strength developments at different ages for the different mixtures with NA and RA were found and presented in Fig. 5. It was found that irrespective of the age the compressive strength the recycled aggregate concrete (RAC) was inferior to natural aggregate concrete (NAC). Based on previous research reports and the authors earlier research reports it was observed that the presence of adhered mortar in recycled aggregate affects the strength of the concrete. The compressive strength of concrete made with recycled aggregates was 25% lesser than that of concrete made with natural aggregates at the age of 28 days. In treated aggregates the loose and the weak adhered
Fig. 6. XRD patterns of concrete at the age of 28 days. (a) Natural aggregate concrete, (b) recycled aggregate concrete, (c) H2SO4 treated recycled aggregate concrete, (d) HNO3 treated recycled aggregate concrete, (e) HCl treated recycled aggregate concrete, (f) HCl&SF treated recycled aggregate concrete. P = portlandite, E = ettringite, C = calcite, G = gypsum, Q = quartz, F = feldspar, D = dolomite and PC = Portland cement.
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Table 5 pH values for different concrete mixes. Mix
pH
NAC RAC RACH2 SO4 RACHNO3 RACHCl RACHCl&SF
12.32 11.96 12.12 12.20 12.14 12.28
mortar were removed to the possible extent and the aggregate surface characteristics were improved. The contact at the interfacial transition zone between treated RA and new cement paste gets improved and thereby the compressive strength of treated recycled aggregate concrete was improved by 8–18% at the age of 28 days than untreated RAC. The strength development of treated recycled aggregate concrete in the later age was found good. The percentage strength development for the HCl, HNO3, H2SO4 and HCl + SF treated aggregate concrete was 18%, 18.5%, 20% and 22% respectively. Among all the four treated aggregate concrete, the strength development was found comparatively lesser for HCl treated concrete. But if the same HCl treated aggregate was subjected to silica fume impregnation gave better strength improvement in concrete at the age of 90 days than other acid treated aggregate concrete. During silica fume impregnation the voids in the recycled aggregates was filled and there by improve the impermeable characteristics of recycled aggregates. The presence of silica in the recycled aggregate react with CH and produce additional C–S–H gel which improves the bond between the recycled aggregate and the cement paste. Hence, from the experimental results it was concluded that the silica fume impregnation treatment for RA improves the ITZ between the RA and new cement mortar. 3.2.2. XRD analysis XRD is the direct method to find the crystalline phases present in the concrete. The powder sample for testing was prepared by crushing the concrete samples taken from the 28 days cured concrete specimens and sieved through 150 lm sieve. The powdered samples were scanned for 10°–60°. The peaks in the XRD graph (Fig. 6) indicate the presence of portlandite, ettringite, calcite, gypsum, quartz, feldspar, dolomite and Portland cement. In the XRD pattern of natural aggregate concrete Feldspar, calcite and portlandite were the dominated crystalline phases. There was a broad and diffuse hump appears between 10° and 20°. This hump could be of the amorphous nature of the concrete. For RAC the XRD analysis data showed major peaks of feldspar and minor peaks of portlandite, gypsum, quartz, and dolomite. The presence of unhydrated Portland cement also found in RAC. Similar trend of XRD pattern was found in HCl treated RAC. But in all other treated RAC quartz dominated than other minerals. It is also worthy to note that RA has higher percentage of CaO compared to NA resulting into acceleration of CaCl2 attack. 3.2.3. Alkalinity The pH vale of the concrete indicates the degradation level of the concrete. It should be 12–13 for a good concrete and below 9 for failure concrete. If the pH value of the concrete is lowered, it affects the bonding capability of cement and leads to crack initiation and corrosion. To measure the alkalinity 28 days hardened concrete powder samples of 20 g was put into 100 ml of distilled water and the solution was allowed to stand for 72 h. The solution was agitated often, to enable more free lime of hydrated cement paste to get dissolved in water. The pH of the aqueous solution was measured by pH meter. The pH values of the all concrete mix specimens were shown in Table 5. From the experimental
results it was understood that the alkalinity of the RAC was well within the limit and it was very well improved and by using treated recycled aggregates in concrete. 4. Conclusions This paper discusses the surface treatment methods of recycled aggregate concrete based on physical and mechanical properties and the effects of treated recycled aggregate on concrete strength were also investigated. Results of this study can be summarized as follows. 1. The surface treatment method effectively removed the loose mortar particles and thereby significantly improves the properties of RA. 2. Silica fume impregnation after chemical immersion treatment gave better densified recycled aggregate. 3. From the results it was suggested that to use the RA in saturated surface dry (SSD) state before concrete mixing. 4. The compressive strength of RAC was lower than that of NAC at all ages. However, the development of concrete strength with treated RA was better than untreated RA. 5. The presence of silica in silica fume impregnated aggregate gave better strength in the later age. 6. Overall, the surface treatment by presoaking the RA in acids significantly improves the properties of RAC and hence this method can be employed in the application on large scale RAC projects.
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