Use of Recycled Concrete Aggregates in Structural Concrete in Mauritius

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1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 6(1): Scholarlink Research Institute Journals, 2014 (ISSN: ) jeteas.scholarlinkresearch.com Journal of Emerging Trends Engineering and Applied Sciences (JETEAS) 6(1):71-81 (ISSN: ) Use of Recycled Concrete Aggregates in Structural Concrete in Mauritius Abdus Salaam Cadersa and Mahendra Ramchuriter Faculty of Engineering, University of Mauritius, Reduit Corresponding Author: Abdus Salaam Cadersa Abstract In the face of a possible scarcity of natural aggregates in the future in Mauritius and in line with sustainable construction, this research investigates the feasibility of the use of recycled coarse concrete aggregate of 5-20 mm fraction as an alternative to natural coarse aggregates in structural concrete. The recycled coarse concrete aggregate used in the research was processed from waste concrete at a major local concrete batching plant. Three pairs of grade 40 concrete mix each consisting of a mix and a test mix, were batched. The percentage of recycled concrete coarse aggregates by weight of all in aggregates in the test mixes were 15%, 25% and 35% respectively. The properties of both natural and recycled coarse aggregates and the fresh and hardened properties of both and trial concrete mixes were investigated. The results showed that the recycled coarse concrete aggregates had poor mechanical and physical properties but the chemical contents were within limits. Compressive and flexural strengths as well as modulus of elasticity were lowered with an increase in recycle aggregate content. The recycled coarse aggregates (RCA) had little influence on the hardened density of concrete and an increase in recycled aggregate content led to a decrease in bleeding capacity. The main drawback of the RCA was the high permeability of the recycled aggregate concrete. The research concludes that the use of RCA in structural concrete is not technically feasible in Mauritius since the most desired concrete properties such as strength and durability are affected. However, since the properties of RCA vary highly among sources, results of this research must not be taken as absolutes. It is recommended that more testing need to be carried out to make sure the conclusions that have been drawn in this paper are applicable. Keywords: recycled coarse concrete aggregate, recycled aggregate concrete, natural aggregate replacement, graded aggregate mixture, structural concrete, INTRODUCTION The global demand for construction aggregates exceeds 26.8 billion tons per year (Wagih et al. 2013). A critical shortage in the sources of natural aggregates is becoming a worldwide problem, especially in the face of the development of major urban centres. The application of recycled aggregates is important in providing alternative material sources to reduce the dependence of the construction industry on natural aggregates (Ismail et al. 2013). Indeed, many governmental bodies throughout the world have introduced a number of policies to sensitize people on the importance of preserving our natural resources and also encouraging the use of recycled materials (Limbachiya et al. 2004). Recycled concrete aggregate (RCA) is generally produced by the crushing of concrete rubble, screening, then removal of contaminants such as reinforcement, paper, wood, plastics and gypsum. Concrete made with such recycled concrete aggregate is called recycled aggregate concrete (RAC) (Wagih et al. 2013). According to the US Environmental Protection Agency (2011), re-use and recycling of concrete waste provides sustainability in various ways. The production of aggregates from concrete debris reduces the extraction of natural rocks, which is a useful and practical way of protecting the environment. The recycled aggregates produced can be reused in construction projects and thus the economic impact associated through purchase and disposal cost is reduced. Besides landfill space is also saved while using recycled concrete waste. Though recycled concrete aggregates (RCAs) have been studied for more than 30 years, their use as a source of aggregate in concrete has still not been widely implemented. The chemical and physical properties of RCAs can vary widely depending on the source of original concrete from which they are derived and the consequences of this variability are not well defined. As a result, the use of such materials in structural applications has been limited. However, the general conclusion is that RAC has lower properties than corresponding natural aggregate concrete (NAC) and that this decrease is proportional to the replacement level of NA with RCA (Butler et al. 2013). Li (2008b) stated that an increase in RCA content led to a decrease in compressive strength. In addition, recycled 71

2 aggregates from concrete require more water for the same workability than conventional concrete. Density, compressive strength and modulus of elasticity are relatively lower than that of the parent concrete; and for a given water/cement ratio, permeability, rate of carbonation and risk of reinforcement corrosion are higher (Padmini et al. 2009). Doming-Caboa et al. (2009) have found that RAC with 100% replacement level of coarse aggregate had considerably higher shrinkage and creep than those of NAC, being 70% and 51% higher respectively, after 180 days. In response to the global energy crisis in 2007, the Government of Mauritius announced the Maurice Ile Durable (MID) concept, which includes economic, social and environmental aspects of development, as being the new long term vision for making Mauritius a sustainable island. The recycling of waste therefore forms an integral part of this concept. However the level of awareness towards recycling in production or application of recycled aggregates is still generally low in Mauritius such that most of the waste concrete is directly landfilled and only a small amount is used as a backfilling material. A survey carried out recently by a major local stone crushing and concrete production company showed that the consumption of natural aggregates in the local construction industry increased from one million tons in 1986 to 4 million tons in The only source for aggregate up till now was from rocks piled in fields or buried in the earth and stone quarries. According to this survey, these natural sources of aggregates are becoming scarce and other sources must be explored to meet the increasing demand for the future (Quatre-Bornes Town Portal, 2011). In line with sustainable construction and in the face of a possible scarcity of natural aggregates in the future in Mauritius, it is imperative that the feasibility of the use of recycled aggregates be investigated. It is in this context that this research attempts to assess whether recycled concrete coarse aggregates can be used as partial replacement of natural coarse aggregates in structural concrete in Mauritius. The objectives include the following; 1. Assess the physical, mechanical and chemical properties of the recycled coarse concrete aggregates of 5-20 mm fractions. 2. Investigate the effect of the recycled coarse aggregates of 5-20 mm fractions on the properties of structural concrete such as bleeding, permeability, compressive strength, flexural strength, hardened density and elastic modulus. MATERIALS AND METHODOLOGY Selection of Materials Cement Ordinary Portland cement was used as binder throughout the study. Natural Aggregates The natural aggregates (NA) used in the project were obtained from a single batching plant. The aggregates are produced by processing locally available basaltic rocks. The coarse aggregates were available in three sized fractions and were single sized. Table 1: Types of Natural Aggregates Used Aggregate type Fraction used (mm) Fine NA Coarse NA Water Tap water was used for mixing the raw materials. Recycled Coarse Aggregates Concrete waste obtained at a concrete batching plant was hammered in the laboratory to produce recycled concrete aggregates. The concrete waste was free of dirt and impurities such as plastic and woods and the strength class of the concrete was unknown. Since the use of fine RCA is not covered in BS 8500, only the 5-20 coarse fraction was used in this research. Hansen (1986) has also shown that fine RCA are also coarser and rounder than required to produce good quality concrete. In addition, ungraded RCA was used due to the cost implication involved with the grading of aggregates into single sizes (Sowerby et al. 2004). The following steps were carried out in the laboratory to obtain the recycled coarse aggregates; Stage 1: The concrete rubbles of size mm were placed one by one in the compression testing machine. The concrete was loaded until it crushes into fragments of size approximately mm. This was collected for stage 2 for further processing. Stage 2: The fragments collected were here broken using an ordinary hammer. The broken pieces was then passed through two sieves (20 and 5mm), stacked together, to remove the unwanted size fractions. The undersize fraction was discarded (fines) and the oversized fraction was broken and sieved again. Stage 3: The processed recycled concrete aggregates retained on the 5mm sieve were washed to remove 72

3 the smaller cement particles which were attached to the aggregate. The final product is shown in figure1. Figure 1: Processed RCA (5-20mm) Testing of Aggregates Preliminary tests were performed on both natural and recycled aggregates. These tests include sieve analysis, determination of density and water absorption. The graphs and data obtained were then used in the concrete mix design process. The sampling of aggregates was done according to BS 812. The aim was to have a representative sample of the whole mass of aggregate available and this was done using a riffle box. Table 2 gives the densities and water absorption for both NA and RCA and figure 3 gives the grading curves of the aggregates. Table 2: Densities and Water Absorption for both NA and RCA used in the Study Relative Relative Water Aggregate Type and Density density absorption Size (Oven (SSD (%) Dried) Basis) NA Fine Coarse RCA In addition, tests to determine Flakiness Index; Aggregate Impact Value; Aggregate Crushing Value, Los Angeles Value, Sulphate Content and Chloride Content were carried out. Results are given at section 3. CONCRETE MIXES Classification of Concrete Mixes Three pairs of grade 40 mixes namely A, B and C were designed and batched in the laboratory. Each pair consisted of a mix (CM) containing 0% of recycled coarse aggregates by weight of all in coarse aggregates and a test mix (RACM) where natural coarse aggregates have been partially replaced by recycled coarse aggregates. Each test mix was compared with its corresponding mix. The mixes classification is given in table 3. Table 3: Classification of Mixes Pair A B C Mix CM1 RACM1 CM2 RACM2 CM3 RACM3 Mixture proportions of each pair were determined in accordance to the following conditions: Same concrete grade (40MPa) Same targeted workability (100 ± 10mm) Same maximum grain size (20mm) Same type and quantity of fine aggregate Same particle size distribution of all in coarse aggregates was maintained for the mix and test mix as far as possible. The following properties of the fresh and hardened properties of concrete were investigated; Fresh Concrete: Plastic Density and Bleeding Capacity Hardened Concrete: Compressive Strength; Hardened Density; Flexural Strength; Static Modulus of Elasticity and Initial Surface Absorption. The test procedures were carried out according to British Standards and ASTM only. Particle Size Distribution (PSD) Table 4 gives the fraction of aggregates used for the research. A continuous range of all in aggregates was used ranging from 0-20mm. Table 4: Aggregate Types Used Aggregate type Fraction used (mm) Fine NA Coarse NA Coarse RCA 5-20 The coarse NA was available in three sized fractions and was single sized. The method described in ACI Education Bulletin (2007) for combining two or more 73

4 aggregates using their respective grading was used to determine the percentage of each fraction of aggregates to be used so as to obtain a graded natural coarse aggregate. It was ensured that the PSD of the natural graded aggregates complied with the upper and lower limits of grading as given in tables 3, 4 and 5 of BS 882. In order to obtain a test mix having the same PSD as the mix, the same method and standard code were used for the replacement of coarse NA by processed RCA. However, in this procedure, three different ratios of the four fractions were combined such that each of the resulting three PSD obtained corresponded to the PSD used in the respective mix. Table 5 gives the ratios of coarse aggregates used for the three pair of mixes. The grading for all-in-aggregate and fineness modulus was also computed. The fineness modulus confirmed the small grading variation in each pair of mixes as shown in table 5. The percentage replacement obtained for RACM1, RACM2 and RACM3 was 15, 25 and 35% respectively. Table 5: Ratio of Coarse Aggregates Used RCA Fineness Modulus A CM RACM B CM RACM C CM RACM Mix Design The British method also known as the DoE method was used for the mix design process. This method of design comprises of tables and charts available at the Building Research Establishment (BRE). The following data were used for the mix design: Design slump: mm Target Strength: 40MPa NA fine: Crushed rock sand with 38% passing 600 µm sieve size NA coarse: Crushed basaltic rock with maximum 20mm size RCA coarse: Crushed concrete with maximum 20mm size Relative densities (SSD basis) and water absorption from table 2.2 were used. Note: No factor of safety was added to the characteristic strength. Trial Mix Trial mixes were carried out in order to obtain a workable mix. This was achieved by ling the amount of free water added until the targeted slump was obtained (100 ± 10mm). The adjusted free water refers to the amount of water remaining or extra added to obtain the targeted slump. The modified water content was calculated by either subtracting the remaining water or adding the extra free water. The wet density and yield of the resulting concrete was also evaluated (Neville & Brooks, 2010). The wet density was done according to BS 1881: Part 107. Table 6: Mix Proportions for Trial Mixes- Mix Cement content Fine aggregate Coarse aggregate 74 Free water (L/m 3 ) Absorbed water (L/m 3 ) Total water (L/m 3 ) CM CM CM RCAM RCAM RCAM Table 7: Results of Trial Mixes Mix Free remaining water (L/m 3 ) Modified free water content (L/m 3 ) Slump obtained (mm) Total mass of mixture per batch (kg) Actual Wet density Yield of concrete Actual Ratio CM CM CM RCAM RCAM RCAM Neville and Brooks (2010) states that trial mixes are used to adjust the free water needed to provide the required workability. Since the density and yield of concrete was adequate, only free water content for the 6 mixes was re-adjusted.

5 Table 8: Re-Adjusted Mix Proportions based on Trial Mix Results Mix Cement content Fine aggregate Coarse aggregate Free water (L/m 3 ) Absorbed water (L/m 3 ) Total water (L/m 3 ) CM CM CM RCAM RCAM RCAM RESULTS AND ANALYSIS Aggregates Particle Shape and Surface Texture Table 9 provides information on shape and surface texture of the aggregates. Particle shape and texture were assessed visually. The result of the flakiness index test on NA and RCA used in the study is given in table 10. Table 9: shape and surface texture of the aggregates Aggregate Shape Surface Texture Coarse NA Coarse RCA Less angular but irregular in shape Angular aggregate with well defined edges The aggregates had a rough surface texture but visible pores and cavities were noted on the surface of the smaller fraction (4-10mm) Rough texture. Presence of mortar covering the aggregate particles easily distinguished Table 10: Flakiness Index Aggregate Type Flakiness Index (%) Coarse NA (6.3-20mm) 21 Coarse RCA (6.3-20mm) 24 It was observed that the variation in flakiness index between the two types of aggregate was not significant. The flakiness index for both aggregate types was within the limit set by BS 882:1992, i.e 40%. The small difference in flakiness showed that the method used to crush concrete debris produced aggregate of approximately the same shape as that of aggregate produced from a crushing plant. However, the RCA was more angular than the NA, a factor which can increase the void content of concrete. The rough texture of NA implies good bonding between aggregate and cement paste. On the contrary, for the RCA, the mortar attached on the aggregate surface can result in weak bonds with the cement paste. Table 11: ACV, AIV and Los Angeles of Aggregates Aggregate Type ACV (%) (10-14mm) Aggregate Crushing Value (ACV), Impact Value (AIV) and Los Angeles Test AIV (%) (10-14mm) NA RCA % increase w.r.t NA Los Angeles (%) (Grade B Aggregate Sizes) ACV tests are used to assess the strength of aggregates. The results given in table 11 showed that NA was stronger that the RCA. The resistance of the RCA to wear and impact was also lower than NA. However the Los Angeles of both aggregate types was within the limit set by the American Standard Test Method (ASTM C33). The results showed that the RCA has a very low resistance to breakdown, which can have a negative impact on the strength and stiffness of the resulting concrete. The low resistance of the RCA to crushing and impact was due to the two- phase material. The attached mortar was easily reduced to pieces upon loading. Chloride and Sulphate Ions The chloride and sulphate content of coarse aggregates are given in table 12. Table 12: Chloride and Sulphate Content Aggregate Chloride Sulphate Ions (%) Ions (%) NA (5-20mm) * 10-4 RCA (5-20mm) 8.28 * * 10-5 It can be seen that no chloride ions were present in the coarse NA but a very small amount was present in the RCA. However, the chloride content of the latter was within the requirement of the first standard for RCA in China; DG/TJ07-008, i.e 0.25%. The sulphate content was also very low for both aggregate types and was less than 1%. The sulphate content of the the RCA was within the limits set in table 1 of DG/TJ

6 Grading Figure 3: Grading Curve for Aggregates Figure 4: Coarse Aggregate Grading for Control Mixes Figure 5: Coarse Aggregate Grading for RAC Mixes 76

7 Figure 6: Grading for All In Aggregate Table 13 Grading Curve Observations Figures Observations 3 Fine aggregate: A smooth line and a continuous range of particle sizes were observed representing clearly the fine aggregates fraction. Singled sized coarse aggregate: The coarse fraction was also well graded. RCA: The grading curve for RCA was between the 10-14mm and 14-20mm NA fraction. A smooth curve was obtained showing the particle size distribution between the sieve sizes 5mm to 20mm. A smooth line and a continuous range of sizes was observed for each mixes. 4 & 5 6 The difference in grading was easily distinguished whereby CM3 represented the coarser fraction of graded aggregate. The same trend was observed with the RACMs and here RACM3 represented the coarser fraction. Fine aggregates were not replaced; therefore fine aggregates grading curve was same for the six mixes. The difference in aggregate grading can be clearly seen between sieve sizes 5-20mm. As specified CM1-RACM1 (A), CM2-RACM2 (B) and CM3-RACM3 (C) had approximately the same grading and same fineness modulus. FRESH CONCRETE Plastic Density The densities and calculated yield for the 6 mixes are given in table 13. Table 13: Plastic Density and Yield of Concrete Mix Total mass of mixture per batch (kg) Actual Wet density Average Density Yield of concrete CM CM CM RACM RACM RACM concrete. However, since yield was satisfactory in this study, this property of concrete was not modified when performing the trial mixes. The results show that the ratio of yield for all the six mixes was above 1.0. There was a slight decrease of 2.6% in the average wet density of RACMs as compared to that of the mixes. Yield showed that the volumetric quantity of concrete produced per batch was higher than calculated. This tendency was observed with all the three pairs of mixes. Yield was computed to make sure that the right quantity of materials was being used to get a given volume of 77 The angular shape of the RCA rendered compaction more difficult resulting in a concrete with higher air voids and lesser density. In addition, the higher water/cement ratio and resulting yield values of the RACMs showed that not all water was removed upon

8 compaction. This could also affect the permeability of the resulting recycled concretes. Bleeding The bleeding capacity and bleeding rate of each concrete pairs are given in table 14. Table 14: Bleeding Capacity Mix Bleeding Capacity (% Free water) CM A RACM B C % increase w.r.t 69 CM RACM % increase w.r.t 24 CM RACM % increase w.r.t 16 The bleeding of the RACMs was higher than the CMs due to a higher water/cement ratio. However, in this research, bleeding water decreased from RCAM1 to RCAM3, that is with increasing recycled aggregate content. This discrepancy is explained by the fact that RCAM1 contains a higher proportion of smaller sized fraction (4-10 mm) than RCAM3 and therefore a higher existing/attached mortar content such that water absorbed by the latter is higher. Hardened Concrete Compressive Strength The compressive test results on 100mm cubes are given in table 15 and figure 4. Table 15 Compressive Strength Results (N/mm 2 ) Mixes 7days 28days CM RACM C A B w.r.t CM RACM w.r.t CM RACM w.r.t Figure 7: Variation of Compressive Strength for each Pair of Mixes Results show that the strengths of the recycled aggregate concretes are lower that their corresponding mixes. The strength attained by the RACMs were below the target strength and indicated that the RACMs were behaving more like grade 30 concrete. This behavior can be attributed to the weak bonding in the transition zones. For the CMs, the cement paste was bonded to the aggregate surface, whereas for the RACs, the cement paste was bonded to the existing mortar attached on the aggregate particles instead of the aggregate itself. The bond was hence weak and failure planes occurred between cement pastes and existing mortar. Moreover, the high water/cement ratio of RACMs contributed to the decrease in strength observed. As expected, the difference in strength was most observed in pair A mixes at both 7 and 28 days due to the high difference in water/cement ratio between CM1 and RACM, namely 0.53 as compared to Please refer to table 7 above. Flexural Strength Flexural strength results are given in table 16 and figure 5. Table 16: Flexural Strength (N/mm 2 ) Mixes Flexural Predicted Flexural Strength: Strength 0.7 (N/mm 2 ) (N/mm 2 ) CM RACM w.r.t 24 CM RACM w.r.t 15 CM RACM w.r.t 18 78

9 Figure 8: Flexural Strength for Control and RAC Mixes Jackson and Dhir (1988) states that as a guide, flexural strength may be taken as 0.7. Therefore, as given in figure 9, the flexural strengths of both CMs and RACMs were above the predicted value. A general decrease in flexural strength was observed with RACMs when compared to their respective CMs. Higher water/cement ratio and weaker bonding at the transition zone resulted in poor flexural strength of RACMs. However, the difference was higher in Set A mixes (24%) due to the high water/cement ratio and higher content of weak aggregates (4-10 mm) fraction in the mix. Hardened Density The hardened densities for the 6 mixes are given in table 17. Table 17: Hardened Densities (Kg/m 3 ) Mix 7 Days 28 Days CM A slight decrease in density was observed when the RACMs were compared to their respective due to the lower specific gravity of RCA in the mixes and also due to presence of trapped water that was confirmed from yield results. Elastic Modulus Results of elastic modulus are given in table 18 and variation of elastic modulus for each pair of mixes is given in figure 6. Table 18: Elastic Modulus (GPa) Mixes Elastic Modulus CM RACM w.r.t 6 CM RACM w.r.t 9 CM RACM w.r.t 14 RACM A w.r.t CM RACM B w.r.t C CM RACM w.r.t Figure 9: Modulus Elasticity for Control and RAC Mixes Typical values for the modulus of elasticity of normal concrete lie in the range of GPa. A high value indicates a stiff concrete (Neville & Brooks, 2010).

10 Therefore both the CMs and the RACMs have good elastic modulus values and are stiff concretes. However, the values of modulus of elasticity of the RCAMs are lower than those of the CMs. This behavior can be explained by the weak RCA content in the mixes. Results also show that the decrease in modulus of elasticity was 6%, 9% and 14% with set A, set B and set C mixes respectively. This is firstly due to the increase in amount of RCA and secondly due to the increase in Fineness Modulus of aggregates from set A to C mixes, resulting in an extension of the interface transition zone. Initial Surface Absorption The results of the permeability test are given in table 19 and figure 7. Table 19: ISAT (ml/m 2 /s) Reference 10min 30min 1hour 2hours Total water (ml/m 2 /s) (ml/m 2 /s) (ml/m 2 /s) (ml/m 2 /s) absorbed (ml/m 2 /s) CM RACM CM RACM Ratio of RACM/CM CM RACM However, the chemical composition of the RCA was not an issue as both chlorides and sulphate content were within limits specified in DG/TJ Modulus of elasticity of the concrete decreased with increasing RCA content, but nevertheless was still within the range of typical values for stiff concrete. Since the most desired concrete properties such as strength and durability are affected, it is concluded in that the use of RCA is not technically feasible in Mauritius. However, since the properties of RCA vary highly among different sources, more testing should be carried out to ensure that conclusions that have been drawn in this paper are applicable. Figure 10: Initial Surface Absorption The ISAT values given in table 19 show that the permeability of the RACMs were twice higher than that of the mixes. In addition, the total water absorbed was also almost doubled for the RCAMs. Therefore, RACs are much more permeable and therefore less durable that normal concrete mixes. The water/cement ratio, aggregate porosity and voids content all contributed to the high permeability of RACMs. CONCLUSIONS The research has demonstrated that the RCA processed from waste concrete at a batching plant has poor mechanical and physical properties as compared to NA. The existing mortar content of the RCA was the main cause for poor quality since it lowered its strength and increased its water absorption. In addition, the research demonstrates that the method described in the ACI Education Bulletin (2007) can be used effectively to combine different single aggregate fractions to produce a graded aggregate mixture which satisfies the limits set in BS 882 for both graded aggregates and all in aggregates. REFERENCES ACI (AMERICAN CONCRETE INSTITUTE) (2007). Aggregates for Concrete. ACI Education Bulletin. BUTLER, L., WEST, B., TIGHE, L.S. (2013). Effect of recycled concrete coarse aggregate from multiple sources on the hardened properties of concrete with equivalent compressive strength. Construction and Building Materials 47 (2013) DOMINGO-CABOA A., LAZAROA C., LOPEZ- GAYARRE F., SERRANO-LOPEZ M.A, SERNAA P., CASTANO-TABARES J.O. (2009). Creep and shrinkage of recycled aggregate concrete. Construction and Building Materials 23 (2009)

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