- Email: [email protected]
Influence of recycled concrete aggregates on strength properties of concrete
Construction and Building Materials 23 (2009) 1163–1167
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of recycled concrete aggregates on strength properties of concrete Sami W. Tabsh *, Akmal S. Abdelfatah Department of Civil Engineering, American University of Sharjah, P.O. Box 26666, Sharjah, United Arab Emirates
a r t i c l e
i n f o
Article history: Received 13 May 2006 Accepted 22 June 2008 Available online 8 August 2008 Keywords: Aggregate Compressive strength Concrete Recycled Tensile strength Waste
a b s t r a c t Many structures in the middle-east’s Gulf region are now either reaching the end of their design life or were not constructed according to the specifications. Demolition or maintenance work on such structures results in large amount of concrete rubbles. Recycling concrete wastes will lead to reduction in valuable landfill space and savings in natural resources. The objective of this study is to investigate the strength of concrete made with recycled concrete coarse aggregate. The variables that are considered in the study include the source of the recycled concrete and target concrete strength. The toughness and soundness test results on the recycled coarse aggregate showed higher percentage loss than natural aggregate, but remained within the acceptable limits. The compressive and splitting tensile strengths of concrete made with recycled coarse aggregate depend on the mix proportions. In general, the strength of recycled concrete can be 10–25% lower than that of conventional concrete made with natural coarse aggregate. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Many structures that were constructed in the Gulf (GCC) region, in the middle-east, during the construction boom of the 1970s are now in need of either major repairs or possible replacement. This is because some of such structures are now reaching the end of their design life, may not have been constructed according to the specifications, or did not receive the required maintenance while in service. New zoning laws in large urban centers have also contributed to the premature replacement of adequately performing buildings. The repair and replacement activities result in large quantities of construction waste that are usually dumped in the desert. Concrete rubbles generated from demolition works constitute a substantial proportion of the waste quantity. They yields fragments in which the aggregate is contaminated with hydrated cement paste, gypsum, and minor quantities of other substances. Since aggregate makes up most of the concrete by volume, it makes sense to investigate the use of concrete waste as aggregate in new concrete. The size fraction that corresponds to fine aggregate usually contains large amounts of hydrated cement and gypsum; therefore, it is not suitable for making fresh concrete mixtures. However, the size fraction that corresponds to coarse aggregate, although coated with cement paste, has been used successfully in many laboratory investigations and field studies. The advantages of recycling coarse aggregates from discarded old concrete include lower environmental pollution, reduction in valuable landfill space, and savings in natural aggregate resources. While economy * Corresponding author. Tel.: +971 6 515 2957; fax: +971 6 515 2979. E-mail address: [email protected] (S.W. Tabsh). 0950-0618/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2008.06.007
is not currently a major factor in recycling concrete in all regions of the world, however, it may become in the future due to the lower transportation cost and energy consumption that are commonly associated with recycled materials. The use of old construction materials in new projects is not a new concept. Recycling construction waste and demolition debris dates back to the time of the Romans, who often reused stones from previous roads in rebuilding newer ones. The industry has become well established in Europe since the end of WWII due to depletion in the supplies of high grade natural aggregates [1]. Today, recycling of construction materials is a successful research program supported by the European Commission on Management of Construction and Demolition Waste. The European Demolition Association estimates that out of the 200 million tons of waste produced annually in Europe, about 30% of this quantity is currently being recycled. Studies in this area, however, show large region differences in the amount of recycled material. Some early adapting countries, like the Netherlands and Belgium, achieve recycling rates of about 90%, but in other European countries, like Italy and Spain, the recycling rate is below 10% [2]. In North America, today construction waste and demolition makes up about 25–45% of the waste stream, depending on the region. The Construction Materials Recycling Association (CMRA) estimates that 25% of this quantity is recycled. The United States’ Environmental Protection Agency recently performed a study on building-related construction and demolition waste that estimated about 136 million tons of material is generated by building work, and that does not count the millions of tons coming from road, bridge, and airport construction and renovation. The CMRA estimates that about 100 million tons of concrete from all sources is
1164
S.W. Tabsh, A.S. Abdelfatah / Construction and Building Materials 23 (2009) 1163–1167
recycled annually in North America. The finished products are used largely as base material for road products, over which either a concrete or asphalt finish is placed [3].
2. Literature review In 1977, Frondistou-Yannas evaluated and compared the mechanical properties of conventional concrete and concrete containing pieces of concrete from demolition waste in the place of natural coarse aggregate [4]. He found out that recycled concrete best matches the mechanical behavior of conventional concrete when the recycled concrete is enriched in gravel at the expense of mortar. The recycled aggregate concrete has a compressive strength of at least 76% and modulus of elasticity from 60% to 100% of the control mix. Hansen and Narud [5] found that the compressive strength of recycled concrete is strongly correlated with the water–cement ratio of the original concrete if other factors are kept the same. When the water–cement ratio of the original concrete is the same or lower than that of the recycled concrete, the new strength will be as good as or better than the original strength, and vice versa. Later in 1984, Hansen and Hedegkd showed that the addition of a plasticizing, an air entraining, a retarding, and an accelerating admixture to the original concrete had little or no effect on the properties of recycled concrete [6]. Test results by Tavakoli and Soroushian indicated that the strength of recycled aggregate concrete is affected by the strength of the original concrete, percentage of the coarse aggregate in the original concrete, the ratio of top size of aggregate in the original concrete to that of the recycled aggregate, and the Los Angeles abrasion loss as well as the water absorption of the recycled aggregate [7]. It was shown that the conventional relationships between splitting tensile, flexural and compressive strengths are different for recycled concrete. In a study by Sagoe-Crentsil and Brown [8], it was found that the processing of recycled concrete aggregates commercially produces smoother spherical particles than those produced in the laboratory, which improves concrete workability. Tests on the compressive and tensile strengths of hardened concrete showed no significant difference between recycled concrete and concrete made with natural aggregates. Investigation of the durability indicated that the recycled aggregates caused a higher drying shrinkage values and reduced the abrasion resistance by about 12%. The water absorption and carbonation rates showed little difference between the recycled concrete and conventional one. Ajdukiewicz and Kliszczewicz examined the mechanical properties of high performance and high strength concretes made with recycled aggregates [9]. In their work, they considered recycled aggregates produced from concrete with compressive strength 40–70 MPa. They concluded that the water content should be modified in the recycled concrete mix design to obtain the same workability. The results indicated that the compressive strength dropped by about 10% when using recycled aggregates, while the bond stress at failure dropped by 8–20%, depending on the type of fine aggregate used in the concrete. The porosity of recycled concrete made with substitution of recycled concrete aggregate was studied by Gomez-Soberon [10]. The distribution of the theoretical pore radius, critical pore ratio, surface area of concrete, threshold ratio, and average pore ratio were investigated at 7, 28, and 90 days. The results showed that porosity increases when natural aggregate is replaced by recycled concrete aggregate. The increase in porosity is accompanied by a reduction in compressive and tensile strengths, as well as in modulus of elasticity.
Olorunsogo and Padayachee [11] investigated the durability of concrete made with different percentages of recycled concrete coarse aggregates (0%, 50%, and 100%). They showed that durability quality of recycled concrete is reduced with increases in the quantities of recycled aggregate, and the quality improved with the age of curing. They concluded that this phenomenon is due to cracks and fissures created within the recycled aggregate during processing, which make the aggregate susceptible to ease of permeation, diffusion and absorption of fluid. 3. Problem statement In recycled concrete, the reclaimed concrete used to make coarse aggregate for new concrete may come from different sources. It can be obtained through the demolition of concrete elements of roads, bridges, buildings and other structures, or it can come from the residue of fresh and hardened rejected units in precast concrete plants. The quality of the recycled concrete aggregate will normally vary depending on the properties of the recovered concrete. Variations between concrete types result from differences in aggregate quality, aggregate size and texture, concrete compressive strength, and uniformity [12]. Therefore, there is a need to investigate the effect of the origin of the recycled concrete aggregate on the strength properties of the new concrete. Specifically, it is desired to quantify the consequences of using recycled concrete coarse aggregate with lower, equal, or higher strength than the target strength of the new concrete. 4. Objectives and scope The objectives of the study are to investigate the quality of crushed old concrete and determine the factors that influence the compressive and tensile strengths of concrete made with coarse aggregates obtained from crushed old concrete with either predetermined or unknown strength. The scope of the investigation covers two different concrete mix designs, one that results in low strength and another that yields moderate strength. 5. Methodology 5.1. Test method The experimental program consisted of testing recycled coarse aggregates and hardened concrete specimens. The aggregate tests included abrasion (ASTM C131) and soundness (ASTM C88) tests. The hardened concrete tests consisted of testing 150 mm by 300 mm concrete cylindrical specimens at the age of 28 days in accordance with the relevant ASTM standards. The compressive strength test was carried out according to ASTM C39, while the tensile strength test was conducted following ASTM C496. All concrete cylinders were capped with a thin layer of sulphur to ensure smoothness of the end surface (ASTM C617). Curing and testing of the concrete specimens were performed by an independent material testing laboratory. 5.2. Aggregate gradation The origin of the recycled concrete aggregate came from two different sources. One recycled concrete had a known strength that was cast in the laboratory at an earlier time, while the other came from a dump site. For consistency, the gradation of the recycled concrete aggregate was the same as that of the natural aggregate. At the beginning, the gradation of the natural aggregate was obtained by sieving, as shown in Table 1. After crushing the hardened concrete, the recycled coarse aggregates were then sieved. The fi-
1165
S.W. Tabsh, A.S. Abdelfatah / Construction and Building Materials 23 (2009) 1163–1167
Opening size (mm)
Percent retained
25 19 12.5 9.5 6.3 4.75 2.36
0 8.9 48.0 10.5 14.9 9.7 8.0
nal gradation of the recycled aggregate matched that of the natural aggregates. 5.3. Mix proportions In order to compare results objectively, a control mix made from natural coarse aggregate is needed to benchmark the results. Two concrete mixes with different 28-days target strengths were considered: (a) Mix 1 with target cylinder strength fc0 = 30 MPa, and (b) Mix 2 with target cylinder strength fc0 = 50 MPa. All concrete mixes were proportioned for a 100 mm slump. Admixtures were not used in the different concrete mixes. Table 2 shows a summary of the mix proportions for the two considered target strengths. To achieve the same slump, concrete made with recycled aggregate required about 10% more water than that made with natural aggregate due to its higher porosity. 5.4. Experiment design Four different kinds of coarse aggregate were used in the new concrete. These kinds included: (a) aggregates from natural origin, (b) recycled from unknown source (dump site), (c) obtained by crushing 30 MPa concrete (based on Mix 1), and (d) aggregate obtained by crushing 50 MPa concrete (based on Mix 2). It should be noted that the source of the recycled coarse aggregate that was obtained from a dump site was mostly sidewalk curbs and roadway medians, where low strength concrete is usually used in such elements. Eight different concrete mixes were used in the experimental program of this study, as follows: 1. Concrete specimens based on Mix 1, with coarse aggregate from: (i) (ii) (iii) (iv)
Natural origin (denoted as Mix 1-N) Recycling of concrete from unknown origin (Mix 1-U) Recycling of concrete with low strength (Mix 1-L) Recycling of concrete with moderate strength (Mix 1-M)
2. Concrete specimens based on Mix 2, with coarse aggregate from: (i) (ii) (iii) (iv)
Natural origin (denoted as Mix 2-N) Recycling of concrete from unknown origin (Mix 2-U) Recycling of concrete with low strength (Mix 2-L) Recycling of concrete with moderate strength (Mix 2-M)
Table 2 Proportions of concrete mixes Constituent
Mix 1a (fc0 = 30 MPa)
Mix 2b (fc0 = 50 MPa)
Sand Coarse aggregate Cement Watera
27.3 Kg 29.1 Kg 9.1 Kg 5.3 Kg
20.4 Kg 29.1 Kg 14.3 Kg 5.3 Kg
a b
Slump of fresh concrete is 100 mm. This amount is increased by about 10% for the recycled concrete.
In order to consider the variability within each mix, ten 150 mm by 300 mm cylinders of each of the above mentioned eight mixes were cast, cured and tested, for a total of 80 specimens. Five specimens were tested in compression and the other five were tested in tension using the relevant ASTM standards. 6. Results Toughness and soundness tests were carried out on the recycled concrete coarse aggregate to investigate the quality of the recycled product. Also, compressive and tensile strength tests were conducted on concrete specimens made with the recycled coarse concrete aggregate. The results were compared with those of natural aggregates. 6.1. Aggregate toughness Crushing strength, abrasion resistance, and elastic modulus of aggregate are all interrelated properties that are greatly influenced by porosity. Experience has shown that natural coarse aggregates are generally dense and strong; therefore they are seldom a limiting factor to the strength of concrete. The Los Angeles degradation test (ASTM C131) is used to evaluate the resistance of coarse natural and recycled aggregates for possible degradation due to the effects of abrasion, wear and impact. The results are presented in Fig. 1. They show that the recycled concrete aggregates had on average 30% more losses than the natural aggregate. As expected, the strength of the crushed concrete affects the abrasion capability of the aggregate, i.e. stronger concrete results in less loss. Further, recycled aggregates from unknown source had the highest percentage of loss among the all the considered recycled aggregates (34%). This is because the old concrete was of inferior quality. Nevertheless, all of the results of recycled coarse aggregates are within the acceptable limit of 50% for structural applications according to ASTM C33. 6.2. Aggregate soundness The soundness test is an indication of the resistance of the aggregates to weathering. An aggregate is considered unsound when the volume changes in aggregate induced by weather (e.g. alternate cycles of wetting and drying, or freezing and thawing) result in deterioration of concrete. Lack of soundness is related more to the pore size distribution than to the total porosity of the aggregate. ASTM C88 measures the resistance of the coarse aggregate to disintegration by saturated solution of sodium sulfate. The test results, based on five cycles, are illustrated in Fig. 2. They show a similar trend to the toughness test results of Fig. 1. However, only the percentage loss of the recycled aggregates from known source (50
40
30
% Loss
Table 1 Gradation of coarse aggregates
20
10
0 Natural
50 MPa Concrete 30 MPa Concrete
Source of Coarse Aggregate Fig. 1. Results of aggregate toughness.
Unknown
1166
S.W. Tabsh, A.S. Abdelfatah / Construction and Building Materials 23 (2009) 1163–1167
and 30 MPa) is within the acceptable limit of 12%. The performance of recycled aggregate of concrete obtained from a dump site was the worst among all the considered aggregates. 6.3. Concrete compressive strength Experience has shown that concrete made with recycled concrete aggregate has high bonding strength between the coarse aggregate and the surrounding paste. This is because of the angularity of the coarse aggregate and the residual cementation on the surface of the recycled aggregate. Fig. 3 shows the compressive strength test results for testing five specimens of each of the Mix 1 (30 MPa concrete) cases considered in the experiment design (for a total of 20 cylinders). Fig. 4 depicts the results for Mix 2 cases (50 MPa concrete). From Fig. 3, it is clear that the strength for the concrete mix made with recycled coarse aggregates from stronger concrete produced almost the same compressive strength as that of the con-
15
19-12.5 mm 12.5-6.3 mm
% Loss
12 9 6 3 0
Natural
50 MPa Concrete 30 MPa Concrete
Unknown
Source of Coarse Aggregate
Compressive Strength (MPa)
Fig. 2. Results of aggregate soundness.
6.4. Concrete tensile strength
40 35 30 25 20 15 10 5 0 Natural
50 MPa Concrete 30 MPa Concrete
Unknown
Source of Coarse Aggregate Fig. 3. Compressive strength results of Mix 1.
Tensile strength in this study is measured using the splitting tensile test procedure according to ASTM C496. This test involves application of diametrically opposite compressive loads acting on the sides of a concrete cylinder. The size of the considered cylinders in this study was 100 mm by 200 mm. Figs. 5 and 6 show the results of the tensile strength tests for Mix 1 and 2 (fc0 = 30 and 50 MPa, respectively). The tensile strength results for Mix 1 indicate that concrete made with recycled coarse aggregate from 50 MPa concrete is as strong in tension as corresponding concrete made with natural coarse aggregate. However, about 25–30% drop in tensile strength was observed in concrete made with recycled coarse aggregate ob5
60
Tensile Strength (MPa)
Compressive Strength (MPa)
crete made with natural coarse aggregate. The concrete made with recycled aggregates from weak or unknown strength, however, resulted in lower strength. For the concrete that has coarse aggregate produced from 30 MPa concrete, the loss in compressive strength is about 30%. The corresponding loss in strength is about 40% when using recycled coarse aggregates from unknown source. A similar pattern is noticed in Fig. 4 for Mix 2 (50 MPa concrete), with the exception that the drop in compressive strength, for the concrete made with recycled coarse aggregate from either weak or unknown concrete, is less than that for Mix 1 (30 MPa concrete). For the concrete that has coarse aggregate made from 30 MPa concrete, the loss in compressive strength is about 10%, while the loss in strength is approximately 15% when using recycled coarse aggregates from unknown source. Note that Mix 2 concrete in this study was achieved by replacing a part of the fine aggregate of Mix 1 with extra cement, while keeping the same amount of coarse aggregate in both mixes. The results of Figs. 3 and 4 indicate two important findings. First, if the amount of coarse aggregate is kept the same in two mixes, the loss in strength due to the use of recycled aggregate is more significant in the weaker mix than the stronger one. This is because the strength of concrete is depends on both the coarse aggregate and cement; therefore if more cement is used, then the effect of the coarse aggregate is reduced. Second, the use of coarse aggregate made from recycled concrete with strength equal to 50 MPa will result in concrete strength comparable with that achieved when using natural coarse aggregate. While it is not investigated in this study, this conclusion is obviously valid if the strength of the recycled coarse aggregate is higher then 50 MPa. As mentioned earlier, about 10% extra water was used in the concrete made with recycled aggregate than in the concrete with natural aggregate to maintain the same slump. This increase in water content has an effect on the strength. It is believed that if admixtures were used to increase workability, while maintaining the same amount of water in the two concretes, then the recycled concrete would have had higher strength than observed.
50 40 30 20 10 0
4 3 2 1 0
Natural
50 MPa Concrete 30 MPa Concrete
Source of Coarse Aggregate Fig. 4. Compressive strength results of Mix 2.
Unknown
Natural
50 MPa Concrete 30MPa Concrete
Source of Coarse Aggregates Fig. 5. Tensile strength results of Mix 1.
Unknown
S.W. Tabsh, A.S. Abdelfatah / Construction and Building Materials 23 (2009) 1163–1167
Tensile Strength (MPa)
5 4 3 2 1 0 Natural
50 MPa Concrete
30 MPa Concrete
Unknown
1167
(3) The percentage loss in compressive or tensile strength due to the use of recycled aggregate is more significant in a weak concrete than in stronger one. (4) The use of coarse aggregate made from recycled concrete with strength equal to 50 MPa will result in concrete compressive and tensile strengths comparable with that achieved when using natural coarse aggregate. (5) Recycled concrete mixes require more water than conventional concrete to maintain the same slump without the use of admixtures. This affects the quality and strength of the concrete, resulting in lower concrete strength.
Source of Coarse Aggregate Acknowledgements Fig. 6. Tensile strength results of Mix 2.
tained from concrete with a target strength based on Mix 1 (fc0 = 30 MPa) or coarse aggregate obtained by crushing concrete from unknown source. The test results for Mix 2 show a similar trend to that of Mix 1, except for the drop in the strength, which is smaller. The concrete made with recycled coarse aggregate produced from Mix 2 shows almost the same tensile strength as that of concrete made with natural coarse aggregate. A drop of about 10–15% is observed when using concrete made with recycled coarse aggregates obtained either from concrete proportioned according to Mix 1 or obtained from an unknown source.
This research was supported by an AUS Research Grant. The authors would like to thank the American University of Sharjah for the support. Mr. Salam Abdallah and Fadi Abu-Houssa helped in the processing and testing of the natural and recycled aggregates and concrete samples. Their help is greatly appreciated. References [1] [2] [3] [4] [5]
7. Conclusions The results obtained in this study lead to the following conclusions: (1) The aggregate toughness test results, based on the Los Angeles degradation test, indicate that the percentage loss of the recycled concrete aggregate is within the acceptable limit of 50% for structural applications, irrespective of its origin. (2) The aggregate soundness test results, based on five cycles in saturated solution of sodium sulfate, show that the percentage loss of the recycled aggregates made from concrete with minimum strength of 30 MPa are within the acceptable limit of 12%.
[6] [7] [8]
[9] [10]
[11] [12]
Buck AD. Recycled concrete as a source of aggregate. ACI J 1997;74(5):212–9. Collepardi M. Letters to the Editor. Concrete Int 2002;24(12):19–21. CMRA 2004,. Frondistou-Yannas S. Waste concrete as aggregate for new concrete. ACI J 1977;74(8):373–6. Hansen TC, Narud H. Strength of recycled concrete made from crushed concrete coarse aggregate. Concrete Int 1983;5(1):79–83. Hansen TC, Hedegkd SE. Properties of recycled aggregate concretes as affected by admixtures in original concretes. ACI J 1984;81(1):21–6. Tavakoli M, Soroushian P. Strengths of recycled aggregate concrete made using field-demolished concrete as aggregate. ACI Mater J 1996;93(2):182–90. Sago-Crentsil KK, Brown T, Taylor AH. Performance of concrete made with commercially produced coarse recycled concrete aggregate. Cement Concrete Res 2001;31:707–12. Ajdukiewicz A, Kliszczewicz A. Influence of recycled aggregates on mechanical properties of HS/HPC. Cement and Concrete Comp 2002;24:269–79. Gomez-Soberon JMV. Porosity of recycled concrete with substitution of recycled concrete aggregate – an experimental study. Cement Concrete Res 2002;32:1301–11. Olorunsogo FT, Padayachee N. Performance of recycled aggregate concrete monitored by durability indexes. Cement Concrete Res 2002;32:179–85. Yrjanson WA. Recycling of Portland cement concrete pavements. National cooperative highway research program synthesis of highway practice No. 154. Transportation Research Board, National Research Council; 1989. p. 46.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of recycled concrete aggregates on strength properties of concrete Sami W. Tabsh *, Akmal S. Abdelfatah Department of Civil Engineering, American University of Sharjah, P.O. Box 26666, Sharjah, United Arab Emirates
a r t i c l e
i n f o
Article history: Received 13 May 2006 Accepted 22 June 2008 Available online 8 August 2008 Keywords: Aggregate Compressive strength Concrete Recycled Tensile strength Waste
a b s t r a c t Many structures in the middle-east’s Gulf region are now either reaching the end of their design life or were not constructed according to the specifications. Demolition or maintenance work on such structures results in large amount of concrete rubbles. Recycling concrete wastes will lead to reduction in valuable landfill space and savings in natural resources. The objective of this study is to investigate the strength of concrete made with recycled concrete coarse aggregate. The variables that are considered in the study include the source of the recycled concrete and target concrete strength. The toughness and soundness test results on the recycled coarse aggregate showed higher percentage loss than natural aggregate, but remained within the acceptable limits. The compressive and splitting tensile strengths of concrete made with recycled coarse aggregate depend on the mix proportions. In general, the strength of recycled concrete can be 10–25% lower than that of conventional concrete made with natural coarse aggregate. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Many structures that were constructed in the Gulf (GCC) region, in the middle-east, during the construction boom of the 1970s are now in need of either major repairs or possible replacement. This is because some of such structures are now reaching the end of their design life, may not have been constructed according to the specifications, or did not receive the required maintenance while in service. New zoning laws in large urban centers have also contributed to the premature replacement of adequately performing buildings. The repair and replacement activities result in large quantities of construction waste that are usually dumped in the desert. Concrete rubbles generated from demolition works constitute a substantial proportion of the waste quantity. They yields fragments in which the aggregate is contaminated with hydrated cement paste, gypsum, and minor quantities of other substances. Since aggregate makes up most of the concrete by volume, it makes sense to investigate the use of concrete waste as aggregate in new concrete. The size fraction that corresponds to fine aggregate usually contains large amounts of hydrated cement and gypsum; therefore, it is not suitable for making fresh concrete mixtures. However, the size fraction that corresponds to coarse aggregate, although coated with cement paste, has been used successfully in many laboratory investigations and field studies. The advantages of recycling coarse aggregates from discarded old concrete include lower environmental pollution, reduction in valuable landfill space, and savings in natural aggregate resources. While economy * Corresponding author. Tel.: +971 6 515 2957; fax: +971 6 515 2979. E-mail address: [email protected] (S.W. Tabsh). 0950-0618/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2008.06.007
is not currently a major factor in recycling concrete in all regions of the world, however, it may become in the future due to the lower transportation cost and energy consumption that are commonly associated with recycled materials. The use of old construction materials in new projects is not a new concept. Recycling construction waste and demolition debris dates back to the time of the Romans, who often reused stones from previous roads in rebuilding newer ones. The industry has become well established in Europe since the end of WWII due to depletion in the supplies of high grade natural aggregates [1]. Today, recycling of construction materials is a successful research program supported by the European Commission on Management of Construction and Demolition Waste. The European Demolition Association estimates that out of the 200 million tons of waste produced annually in Europe, about 30% of this quantity is currently being recycled. Studies in this area, however, show large region differences in the amount of recycled material. Some early adapting countries, like the Netherlands and Belgium, achieve recycling rates of about 90%, but in other European countries, like Italy and Spain, the recycling rate is below 10% [2]. In North America, today construction waste and demolition makes up about 25–45% of the waste stream, depending on the region. The Construction Materials Recycling Association (CMRA) estimates that 25% of this quantity is recycled. The United States’ Environmental Protection Agency recently performed a study on building-related construction and demolition waste that estimated about 136 million tons of material is generated by building work, and that does not count the millions of tons coming from road, bridge, and airport construction and renovation. The CMRA estimates that about 100 million tons of concrete from all sources is
1164
S.W. Tabsh, A.S. Abdelfatah / Construction and Building Materials 23 (2009) 1163–1167
recycled annually in North America. The finished products are used largely as base material for road products, over which either a concrete or asphalt finish is placed [3].
2. Literature review In 1977, Frondistou-Yannas evaluated and compared the mechanical properties of conventional concrete and concrete containing pieces of concrete from demolition waste in the place of natural coarse aggregate [4]. He found out that recycled concrete best matches the mechanical behavior of conventional concrete when the recycled concrete is enriched in gravel at the expense of mortar. The recycled aggregate concrete has a compressive strength of at least 76% and modulus of elasticity from 60% to 100% of the control mix. Hansen and Narud [5] found that the compressive strength of recycled concrete is strongly correlated with the water–cement ratio of the original concrete if other factors are kept the same. When the water–cement ratio of the original concrete is the same or lower than that of the recycled concrete, the new strength will be as good as or better than the original strength, and vice versa. Later in 1984, Hansen and Hedegkd showed that the addition of a plasticizing, an air entraining, a retarding, and an accelerating admixture to the original concrete had little or no effect on the properties of recycled concrete [6]. Test results by Tavakoli and Soroushian indicated that the strength of recycled aggregate concrete is affected by the strength of the original concrete, percentage of the coarse aggregate in the original concrete, the ratio of top size of aggregate in the original concrete to that of the recycled aggregate, and the Los Angeles abrasion loss as well as the water absorption of the recycled aggregate [7]. It was shown that the conventional relationships between splitting tensile, flexural and compressive strengths are different for recycled concrete. In a study by Sagoe-Crentsil and Brown [8], it was found that the processing of recycled concrete aggregates commercially produces smoother spherical particles than those produced in the laboratory, which improves concrete workability. Tests on the compressive and tensile strengths of hardened concrete showed no significant difference between recycled concrete and concrete made with natural aggregates. Investigation of the durability indicated that the recycled aggregates caused a higher drying shrinkage values and reduced the abrasion resistance by about 12%. The water absorption and carbonation rates showed little difference between the recycled concrete and conventional one. Ajdukiewicz and Kliszczewicz examined the mechanical properties of high performance and high strength concretes made with recycled aggregates [9]. In their work, they considered recycled aggregates produced from concrete with compressive strength 40–70 MPa. They concluded that the water content should be modified in the recycled concrete mix design to obtain the same workability. The results indicated that the compressive strength dropped by about 10% when using recycled aggregates, while the bond stress at failure dropped by 8–20%, depending on the type of fine aggregate used in the concrete. The porosity of recycled concrete made with substitution of recycled concrete aggregate was studied by Gomez-Soberon [10]. The distribution of the theoretical pore radius, critical pore ratio, surface area of concrete, threshold ratio, and average pore ratio were investigated at 7, 28, and 90 days. The results showed that porosity increases when natural aggregate is replaced by recycled concrete aggregate. The increase in porosity is accompanied by a reduction in compressive and tensile strengths, as well as in modulus of elasticity.
Olorunsogo and Padayachee [11] investigated the durability of concrete made with different percentages of recycled concrete coarse aggregates (0%, 50%, and 100%). They showed that durability quality of recycled concrete is reduced with increases in the quantities of recycled aggregate, and the quality improved with the age of curing. They concluded that this phenomenon is due to cracks and fissures created within the recycled aggregate during processing, which make the aggregate susceptible to ease of permeation, diffusion and absorption of fluid. 3. Problem statement In recycled concrete, the reclaimed concrete used to make coarse aggregate for new concrete may come from different sources. It can be obtained through the demolition of concrete elements of roads, bridges, buildings and other structures, or it can come from the residue of fresh and hardened rejected units in precast concrete plants. The quality of the recycled concrete aggregate will normally vary depending on the properties of the recovered concrete. Variations between concrete types result from differences in aggregate quality, aggregate size and texture, concrete compressive strength, and uniformity [12]. Therefore, there is a need to investigate the effect of the origin of the recycled concrete aggregate on the strength properties of the new concrete. Specifically, it is desired to quantify the consequences of using recycled concrete coarse aggregate with lower, equal, or higher strength than the target strength of the new concrete. 4. Objectives and scope The objectives of the study are to investigate the quality of crushed old concrete and determine the factors that influence the compressive and tensile strengths of concrete made with coarse aggregates obtained from crushed old concrete with either predetermined or unknown strength. The scope of the investigation covers two different concrete mix designs, one that results in low strength and another that yields moderate strength. 5. Methodology 5.1. Test method The experimental program consisted of testing recycled coarse aggregates and hardened concrete specimens. The aggregate tests included abrasion (ASTM C131) and soundness (ASTM C88) tests. The hardened concrete tests consisted of testing 150 mm by 300 mm concrete cylindrical specimens at the age of 28 days in accordance with the relevant ASTM standards. The compressive strength test was carried out according to ASTM C39, while the tensile strength test was conducted following ASTM C496. All concrete cylinders were capped with a thin layer of sulphur to ensure smoothness of the end surface (ASTM C617). Curing and testing of the concrete specimens were performed by an independent material testing laboratory. 5.2. Aggregate gradation The origin of the recycled concrete aggregate came from two different sources. One recycled concrete had a known strength that was cast in the laboratory at an earlier time, while the other came from a dump site. For consistency, the gradation of the recycled concrete aggregate was the same as that of the natural aggregate. At the beginning, the gradation of the natural aggregate was obtained by sieving, as shown in Table 1. After crushing the hardened concrete, the recycled coarse aggregates were then sieved. The fi-
1165
S.W. Tabsh, A.S. Abdelfatah / Construction and Building Materials 23 (2009) 1163–1167
Opening size (mm)
Percent retained
25 19 12.5 9.5 6.3 4.75 2.36
0 8.9 48.0 10.5 14.9 9.7 8.0
nal gradation of the recycled aggregate matched that of the natural aggregates. 5.3. Mix proportions In order to compare results objectively, a control mix made from natural coarse aggregate is needed to benchmark the results. Two concrete mixes with different 28-days target strengths were considered: (a) Mix 1 with target cylinder strength fc0 = 30 MPa, and (b) Mix 2 with target cylinder strength fc0 = 50 MPa. All concrete mixes were proportioned for a 100 mm slump. Admixtures were not used in the different concrete mixes. Table 2 shows a summary of the mix proportions for the two considered target strengths. To achieve the same slump, concrete made with recycled aggregate required about 10% more water than that made with natural aggregate due to its higher porosity. 5.4. Experiment design Four different kinds of coarse aggregate were used in the new concrete. These kinds included: (a) aggregates from natural origin, (b) recycled from unknown source (dump site), (c) obtained by crushing 30 MPa concrete (based on Mix 1), and (d) aggregate obtained by crushing 50 MPa concrete (based on Mix 2). It should be noted that the source of the recycled coarse aggregate that was obtained from a dump site was mostly sidewalk curbs and roadway medians, where low strength concrete is usually used in such elements. Eight different concrete mixes were used in the experimental program of this study, as follows: 1. Concrete specimens based on Mix 1, with coarse aggregate from: (i) (ii) (iii) (iv)
Natural origin (denoted as Mix 1-N) Recycling of concrete from unknown origin (Mix 1-U) Recycling of concrete with low strength (Mix 1-L) Recycling of concrete with moderate strength (Mix 1-M)
2. Concrete specimens based on Mix 2, with coarse aggregate from: (i) (ii) (iii) (iv)
Natural origin (denoted as Mix 2-N) Recycling of concrete from unknown origin (Mix 2-U) Recycling of concrete with low strength (Mix 2-L) Recycling of concrete with moderate strength (Mix 2-M)
Table 2 Proportions of concrete mixes Constituent
Mix 1a (fc0 = 30 MPa)
Mix 2b (fc0 = 50 MPa)
Sand Coarse aggregate Cement Watera
27.3 Kg 29.1 Kg 9.1 Kg 5.3 Kg
20.4 Kg 29.1 Kg 14.3 Kg 5.3 Kg
a b
Slump of fresh concrete is 100 mm. This amount is increased by about 10% for the recycled concrete.
In order to consider the variability within each mix, ten 150 mm by 300 mm cylinders of each of the above mentioned eight mixes were cast, cured and tested, for a total of 80 specimens. Five specimens were tested in compression and the other five were tested in tension using the relevant ASTM standards. 6. Results Toughness and soundness tests were carried out on the recycled concrete coarse aggregate to investigate the quality of the recycled product. Also, compressive and tensile strength tests were conducted on concrete specimens made with the recycled coarse concrete aggregate. The results were compared with those of natural aggregates. 6.1. Aggregate toughness Crushing strength, abrasion resistance, and elastic modulus of aggregate are all interrelated properties that are greatly influenced by porosity. Experience has shown that natural coarse aggregates are generally dense and strong; therefore they are seldom a limiting factor to the strength of concrete. The Los Angeles degradation test (ASTM C131) is used to evaluate the resistance of coarse natural and recycled aggregates for possible degradation due to the effects of abrasion, wear and impact. The results are presented in Fig. 1. They show that the recycled concrete aggregates had on average 30% more losses than the natural aggregate. As expected, the strength of the crushed concrete affects the abrasion capability of the aggregate, i.e. stronger concrete results in less loss. Further, recycled aggregates from unknown source had the highest percentage of loss among the all the considered recycled aggregates (34%). This is because the old concrete was of inferior quality. Nevertheless, all of the results of recycled coarse aggregates are within the acceptable limit of 50% for structural applications according to ASTM C33. 6.2. Aggregate soundness The soundness test is an indication of the resistance of the aggregates to weathering. An aggregate is considered unsound when the volume changes in aggregate induced by weather (e.g. alternate cycles of wetting and drying, or freezing and thawing) result in deterioration of concrete. Lack of soundness is related more to the pore size distribution than to the total porosity of the aggregate. ASTM C88 measures the resistance of the coarse aggregate to disintegration by saturated solution of sodium sulfate. The test results, based on five cycles, are illustrated in Fig. 2. They show a similar trend to the toughness test results of Fig. 1. However, only the percentage loss of the recycled aggregates from known source (50
40
30
% Loss
Table 1 Gradation of coarse aggregates
20
10
0 Natural
50 MPa Concrete 30 MPa Concrete
Source of Coarse Aggregate Fig. 1. Results of aggregate toughness.
Unknown
1166
S.W. Tabsh, A.S. Abdelfatah / Construction and Building Materials 23 (2009) 1163–1167
and 30 MPa) is within the acceptable limit of 12%. The performance of recycled aggregate of concrete obtained from a dump site was the worst among all the considered aggregates. 6.3. Concrete compressive strength Experience has shown that concrete made with recycled concrete aggregate has high bonding strength between the coarse aggregate and the surrounding paste. This is because of the angularity of the coarse aggregate and the residual cementation on the surface of the recycled aggregate. Fig. 3 shows the compressive strength test results for testing five specimens of each of the Mix 1 (30 MPa concrete) cases considered in the experiment design (for a total of 20 cylinders). Fig. 4 depicts the results for Mix 2 cases (50 MPa concrete). From Fig. 3, it is clear that the strength for the concrete mix made with recycled coarse aggregates from stronger concrete produced almost the same compressive strength as that of the con-
15
19-12.5 mm 12.5-6.3 mm
% Loss
12 9 6 3 0
Natural
50 MPa Concrete 30 MPa Concrete
Unknown
Source of Coarse Aggregate
Compressive Strength (MPa)
Fig. 2. Results of aggregate soundness.
6.4. Concrete tensile strength
40 35 30 25 20 15 10 5 0 Natural
50 MPa Concrete 30 MPa Concrete
Unknown
Source of Coarse Aggregate Fig. 3. Compressive strength results of Mix 1.
Tensile strength in this study is measured using the splitting tensile test procedure according to ASTM C496. This test involves application of diametrically opposite compressive loads acting on the sides of a concrete cylinder. The size of the considered cylinders in this study was 100 mm by 200 mm. Figs. 5 and 6 show the results of the tensile strength tests for Mix 1 and 2 (fc0 = 30 and 50 MPa, respectively). The tensile strength results for Mix 1 indicate that concrete made with recycled coarse aggregate from 50 MPa concrete is as strong in tension as corresponding concrete made with natural coarse aggregate. However, about 25–30% drop in tensile strength was observed in concrete made with recycled coarse aggregate ob5
60
Tensile Strength (MPa)
Compressive Strength (MPa)
crete made with natural coarse aggregate. The concrete made with recycled aggregates from weak or unknown strength, however, resulted in lower strength. For the concrete that has coarse aggregate produced from 30 MPa concrete, the loss in compressive strength is about 30%. The corresponding loss in strength is about 40% when using recycled coarse aggregates from unknown source. A similar pattern is noticed in Fig. 4 for Mix 2 (50 MPa concrete), with the exception that the drop in compressive strength, for the concrete made with recycled coarse aggregate from either weak or unknown concrete, is less than that for Mix 1 (30 MPa concrete). For the concrete that has coarse aggregate made from 30 MPa concrete, the loss in compressive strength is about 10%, while the loss in strength is approximately 15% when using recycled coarse aggregates from unknown source. Note that Mix 2 concrete in this study was achieved by replacing a part of the fine aggregate of Mix 1 with extra cement, while keeping the same amount of coarse aggregate in both mixes. The results of Figs. 3 and 4 indicate two important findings. First, if the amount of coarse aggregate is kept the same in two mixes, the loss in strength due to the use of recycled aggregate is more significant in the weaker mix than the stronger one. This is because the strength of concrete is depends on both the coarse aggregate and cement; therefore if more cement is used, then the effect of the coarse aggregate is reduced. Second, the use of coarse aggregate made from recycled concrete with strength equal to 50 MPa will result in concrete strength comparable with that achieved when using natural coarse aggregate. While it is not investigated in this study, this conclusion is obviously valid if the strength of the recycled coarse aggregate is higher then 50 MPa. As mentioned earlier, about 10% extra water was used in the concrete made with recycled aggregate than in the concrete with natural aggregate to maintain the same slump. This increase in water content has an effect on the strength. It is believed that if admixtures were used to increase workability, while maintaining the same amount of water in the two concretes, then the recycled concrete would have had higher strength than observed.
50 40 30 20 10 0
4 3 2 1 0
Natural
50 MPa Concrete 30 MPa Concrete
Source of Coarse Aggregate Fig. 4. Compressive strength results of Mix 2.
Unknown
Natural
50 MPa Concrete 30MPa Concrete
Source of Coarse Aggregates Fig. 5. Tensile strength results of Mix 1.
Unknown
S.W. Tabsh, A.S. Abdelfatah / Construction and Building Materials 23 (2009) 1163–1167
Tensile Strength (MPa)
5 4 3 2 1 0 Natural
50 MPa Concrete
30 MPa Concrete
Unknown
1167
(3) The percentage loss in compressive or tensile strength due to the use of recycled aggregate is more significant in a weak concrete than in stronger one. (4) The use of coarse aggregate made from recycled concrete with strength equal to 50 MPa will result in concrete compressive and tensile strengths comparable with that achieved when using natural coarse aggregate. (5) Recycled concrete mixes require more water than conventional concrete to maintain the same slump without the use of admixtures. This affects the quality and strength of the concrete, resulting in lower concrete strength.
Source of Coarse Aggregate Acknowledgements Fig. 6. Tensile strength results of Mix 2.
tained from concrete with a target strength based on Mix 1 (fc0 = 30 MPa) or coarse aggregate obtained by crushing concrete from unknown source. The test results for Mix 2 show a similar trend to that of Mix 1, except for the drop in the strength, which is smaller. The concrete made with recycled coarse aggregate produced from Mix 2 shows almost the same tensile strength as that of concrete made with natural coarse aggregate. A drop of about 10–15% is observed when using concrete made with recycled coarse aggregates obtained either from concrete proportioned according to Mix 1 or obtained from an unknown source.
This research was supported by an AUS Research Grant. The authors would like to thank the American University of Sharjah for the support. Mr. Salam Abdallah and Fadi Abu-Houssa helped in the processing and testing of the natural and recycled aggregates and concrete samples. Their help is greatly appreciated. References [1] [2] [3] [4] [5]
7. Conclusions The results obtained in this study lead to the following conclusions: (1) The aggregate toughness test results, based on the Los Angeles degradation test, indicate that the percentage loss of the recycled concrete aggregate is within the acceptable limit of 50% for structural applications, irrespective of its origin. (2) The aggregate soundness test results, based on five cycles in saturated solution of sodium sulfate, show that the percentage loss of the recycled aggregates made from concrete with minimum strength of 30 MPa are within the acceptable limit of 12%.
[6] [7] [8]
[9] [10]
[11] [12]
Buck AD. Recycled concrete as a source of aggregate. ACI J 1997;74(5):212–9. Collepardi M. Letters to the Editor. Concrete Int 2002;24(12):19–21. CMRA 2004,