INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 2, No 1, 2011

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1 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 2, No 1, 2011 Copyright 2010 All rights reserved Integrated Publishing services Research article ISSN Direct compressive strength and Elastic modulus of recycled aggregate concrete Gupta Arundeb 1, Mandal Saroj 2, Ghosh Somnath 3 1- Research Scholar, Department of Civil Engineering, Jadavpur University, Kolkata , 2, 3- Professor, Department of Civil Engineering, Jadavpur University, Kolkata arundeb_gupta@yahoo.co.in doi: /ijcser ABSTRACT An experimental investigation was conducted to study the change in direct compressive strength and elastic modulus of recycled aggregate concrete in presence of fly ash (as replacement of cement). Concrete cubes, cylinder and prisms were prepared as test specimens with varying percentage of fly ash using natural aggregate and recycled aggregate. These specimens were tested at different age of concrete to have compressive strength, elastic modulus, split tensile strength as well as flexural strength and their changes were noticed. MIP ( Mercury intrusion porosimetry ) test was conducted to estimate the percentage of micro voids in recycled aggregate concrete and also to appreciate the change of these micro voids due to presence of fly ash.empirical formulas involving major parameters such as fly ash content, water cement ratio, age of concrete etc., have been developed to predict elastic modulus and compressive strength of recycled aggregate concrete. Keywords: Recycled Aggregate Concrete, Natural Aggregate Concrete, Fly ash, Mercury intrusion porosimetry, Compressive strength, Elastic modulus 1. Introduction Recycling of waste concrete is now quite popular in the construction industry. Recycled coarse aggregate are widely used as sub base in road project and also for making lean concrete. Research works are in progress to establish it to be fit for structural concrete [all the research works mentioned under reference]. Successful use of recycled aggregates in regular concrete works would lead to a considerable reduction in demand of natural aggregates. Recycled aggregate may be considered as a green material to some extent. It is quite well known that the porosity of recycled aggregate concrete is more than conventional natural aggregate concrete. It is also known to us that is strength of concrete reduces with increase in porosity. Addition of fly ash to some extent will provide better micro structure as well as strength. This paper deals with an experimental investigation on recycled aggregate concrete made in presence of fly ash (as cement replacement). MIP (Mercury intrusion porosimetry) test was conducted to estimate the percentage of voids in recycled aggregate concrete and also to study the change of micro voids due to presence of fly ash. Empirical formulas involving major parameters such as fly ash content, water cement ratio, age of concrete etc., have been developed to predict elastic modulus and compressive strength of recycled aggregate concrete. 2. Materials and Method 2.1 Materials Received on September, 2011 Published on November

2 Cement Direct compressive strength and Elastic modulus of recycled aggregate concrete Ordinary Portland cement of Grade 53 Conforming to IS Fine aggregate Locally available natural sand of Zone III as per IS Coarse aggregate Two types of coarse aggregate were used 1. Natural stone aggregate of Basalt variety ii) Recycled aggregate processed from Laboratory waste concrete cubes of 150mm x 150mm x150mm of mean compressive strength 23.5 MPa. The recycled coarse aggregates were designated as Type-I and Type-II. The coarse aggregate passing through 40mm sieve and retained on 20mm sieve was designated as Type-I and the fraction of coarse aggregate passing through 20mm sieve and retained on 4.75mm sieve was designated as Type-II. Fly ash: The fly ash was directly obtained from Bandel thermal power station near Kolkata, India. Chemical composition of fly ash is shown in Table I and compared with standards as prescribed by IS 3812 part-i. Chemical Composition 2.2 Concrete Mix Proportion Table 1: Chemical composition of Fly ash Fly ash Specified requirement weight (%) IS 3812 part-i (% weight) SiO minimum Al 2 O CaO 8.0 MgO maximum TiO Loss of maximum ignition Na2O/K2O 1.0 There is no standard mix design procedure for recycled aggregate concrete. Hence, trial mixes as per ACI 14 for natural aggregate concrete were prepared and tested. Same proportion was used for recycled aggregate concrete also. The mixture proportion by weight for natural aggregate concrete (NAC) and recycled aggregate concrete (RAC) are furnished in Table 2. Table 2: Mix Proportion of NAC and RAC specimens Sl no. Mix Mix Proportion (by weight ) Cement Fly ash Sand Coarse aggregates Water cement 293

3 designation Type I Type ratio II 1 NAC NAC RAC RAC RAC NACa NAC1a RACa RAC1a RAC2a NACb NAC1b RACb RAC1b RAC2b Test Specimens Cubes of 150x150x150 mm, cylinders 150 dia x300 mm height and prisms 100x100x500mm long, were casted from each mix to have compressive strength, tensile strength, flexural strength and elastic modulus of recycled aggregate concrete. Elastic modulus in direct compression has been determined as per DIN 1048 Part 1. It may be noted here that the specimens were cured under water for 28 days and then left in air till the date of testing. 3. Results and discussion 3.1 Workability, wet density and yield of concrete Detailed results of workability, density and yield for NAC and RAC are presented in Table 3 It is noticed that workability of RAC is less than that of NAC. Similar results have been reported by other researchers [Topcu, 1997, Oliviea, 1996] also. Addition of fly ash to both RAC and NAC marginally reduces the compacting factor values. MIX NO Mix ID Table 3: Result of fresh Concrete mix Compacting FACTOR Unit Weight. (KG/ M 3 ) Yield / 50Kg of Cement 1 NAC NAC RAC RAC RAC NACa NAC1a RACa RAC1a RAC2a NACb

4 12 NAC1b RACb RAC1b RAC2b It is also observed that wet densities of recycled aggregate concrete are less than that of natural aggregate concrete. This is due to presence of low density old mortar attached on recycled aggregates. Results indicate that 10% addition of fly ash as replacement of cement in RAC and NAC marginally increases the unit weight. However, 20% addition of fly ash as replacement of cement in RAC and NAC marginally decreases the unit weight. Addition of fly ash reduces workability of concrete as observed in compaction factor test. 3.2 Relation between Elastic modulus of Recycled aggregate concrete and other parameters of concrete Elastic modulus in direct compression of recycled aggregate concrete having different ratio of cement to fly ash are presented in Figure 1 and an equation is obtained by regression analysis. Similarly, in Figure 2, the relationship between elastic modulus and 28 days compressive strength are shown. In Figure 3 the relationship between elastic modulus and water cement ratio has been shown. Figure 1: Elastic Modulus vs Percentage of fly ash Figure 2: Elastic Modulus vs 28 days compressive strength 295

5 Figure 3: Elastic Modulus vs Water-cement ratio A relation between modulus of elasticity, compressive strength, percentage of fly ash and water cement ratio has been determined from regression analysis, Assuming a relation: E f ck f a / w Or, E = f ck f a / w (3.2.1) Where, E = Modulus of Elasticity f ck = Compressive strength of concrete f a = % fly ash replacement in RAC sample w = Water cement ratio = Constant of proportionality From Figure 1, E = K 1 f a 2 + K 2 f a + K (3.2.2) K 1 = , K 2 =0.1005, K 3 = 2.26 From Figure 2, E = K 4 f CK 2 + K 5 f CK + K (3.2.3) K 4 = , K 5 =8.725, K 6 = From Figure 3, E = K 7 w 2 + K 8 w + K (3.2.4) 296

6 K 7 = -6, K 8 =4, K 9 = 1.76 Combining above equations E = (K 1 f a 2 + K 2 f a + K 3 )( K 4 f CK 2 + K 5 f CK + K 6 ) / ( K 7 w 2 + K 8 w + K 9 ) (3.2.5) Or, Ē = Ĕ (3.2.6) Where, Ĕ = (K 1 f a 2 + K 2 f a + K 3 )( K 4 f CK 2 + K 5 f CK + K 6 ) / ( K 7 w 2 + K 8 w + K 9 ) (3.2.7) Computing average for water cement ratio 0.4 and 0.5, derived value of modulus of elasticity from above relation are computed below Table 4: Comparison between predicted and experimental value of modulus of elasticity SL w/c = 0.4 w/c = 0.5 Exp. E value Calc. E value % deviation Exp. E value Calc. E value % deviation It is observed that the value of modulus of elasticity calculated from the derived formula tallies with the experimental results, so the relation could be used to predict the modulus of elasticity of recycled aggregate concrete of different mixes. 3.3 Effect of fly ash on Compressive strength at different ages Table 5 shows the compressive strength at various ages (7, 28, 90 days) for recycled aggregate concrete and natural aggregate concrete. Compressive strength of RAC specimens are found lower than that of NAC specimens. The percentage decrease in compressive strength at all ages and all water cement ratio for the RAC specimens varies from 11% to 31%. However, reduction in compressive strength for recycled aggregate concrete with 10% fly ash replacement (RAC1) is much lower (4% to 26%). Table 5: Result of Compressive Strength Mix NO. Mix Designation 7 days Strength MPa 28 days Strength MPa 1 N.A.C N.A.C R.A.C R.A.C R.A.C NACa days Strength MPa 297

7 7 NAC1a RACa RAC1a RAC2a NACb NAC1b RACb RAC1b RAC2b Effect of fly ash on Split tensile and flexural strength Table 6 presents the flexural and split tensile strength of specimens at 28 days. Comparison with NAC specimen reveals a reduction of 6% in RAC specimen. However, significant improvement can be noticed in the RAC1 specimen which show only 3% reduction in both flexural strength and split tensile strength. Mix No. Table 6: Split tensile and Flexural strength at 28 days Mix Designation Split tensile Strength ( MPa) Percentage change Flexural Percentage change 1 NAC NAC1 3.7 (3.4) 6.2 (3.3) 3 RAC 3.37 (- 5.86) 5.64 (-6) 4 RAC (-2.8) 6.18 (3) 5 RAC2 3.0 (-16.2) 4.78 (-20.3) Note: The value in the bracket ( ) indicates the percentage change of compressive strength with respect to Mix No.1 It is noted that optimum replacement of cement by fly ash for RAC recovers the compressive strength significantly. It shows that 10% fly ash replacement gives the maximum compressive strength among RAC with normal fine aggregate. This may be due to optimal gel formation, modified pore structure and improved interface bond between new cement paste and recycled aggregate with old mortar attached to it. 3.5 Relation between compressive strength, age, percentage replacement of Fly ash and water cement ratio The relation between compressive strength of concrete, percent fly ash replacement, water cement ratio and age of concrete are assumed as follows: Assuming a relation: f ck f a ǡ / w Or, f ck = f a ǡ / w ---- ( 3.5.1) Where, = Constant of proportionality 298

8 f ck = Compressive strength of recycled concrete f a = % fly ash replacement in RAC sample ǡ = Age of concrete w = Reciprocal of Water cement ratio From Figure 4 to 6, regression equations of different relationships between the above factors have been obtained. Figure 4: Strength of concrete vs Age of concrete Figure 5: Strength of concrete vs Percentage Fly ash 299

9 From Figure 4, Figure 6: Strength of concrete vs Reciprocal of water content f ck = K 1 ǡ 2 + K 2 ǡ + K (3.5.2) Where, K 1 = , K 2 = , K 3 = From Figure 5, f ck = K 4 f a 2 + K 5 f a + K (3.5.3) Where, K 4 = , K 5 = , K 6 = From Figure 6, f ck =K 7 w 2 -K 8 w+k (3.5.4) Where, K 7 = , K 8 = , K 9 = Combining regression equations from Figure 1, 2 & 3 as per eqn (3.5.1) f ck = (K 1 ǡ 2 + K 2 ǡ + K 3 ) (K 4 f a 2 + K 5 f a + K 6 ) / (K 7 w 2 -K 8 w+k 9 ) For Water content = 0.4 = For Water content = 0.5 = For Water content = 0.6 = Table 7: Comparison between predicted & experimental compressive strength of RAC mixes Sl % fly Age of Water Exp. f ck Calc. f ck % ash conc. content (MPa) (MPa) deviation

10 From table 7, it is found that, the test results and the calculated values are quite agreeable. 3.6 Mercury Intrusion Porosimetry (MIP) test A typical plot between intruded mercury volume and pore diameter for recycled aggregate concrete specimens is shown in Figure 7. Total pore volume of RAC is 8.4 cc/gm. However, addition of 10% fly ash results in significant decrease of pore volume ( 5.2 cc/gm).major portion of pores in RAC lies between 0.10 µm to 4 µm while in case of RAC1, most of the pores lies between 0.03 µm to 1 µm. Results indicate reduced diameter of pore with addition of fly ash ( 10% ) which explains in favour of getting higher strength of RAC1 over that of RAC specimens.incremental intrusion of mercury of RAC and RAC1 specimen is presented in Figure 8. It can be clearly observed that RAC specimen exhibit noticeably high incremental intrusion volume when compared to that of RAC1 specimen. 301

11 Figure 7: Comparison of Intruded Hg volume Vs pore diameter curve for RAC & RAC1 4. Conclusion Figure 8: Comparison of Incremental intrusion of Hg volume vs Pore diameter Based on the results obtained in the present study, following conclusions have been drawn: 1. Partial replacement of cement by fly ash leads to a encouraging result for the Recycled aggregate concrete, which advocates the utilization of fly ash and Recycled 302

12 aggregates (both are waste materials and endanger the environment) for making concrete. 2. Recycled aggregate concrete with 10% fly ash gives higher compressive strength over Recycled aggregate concrete. 3. Though split tensile and flexural strength are lower in recycled aggregate concrete, it could be improved to an extent by adding 10% by replacement of cement by fly ash. 4. Pore structure of recycled aggregate concrete could be improved by addition of fly ash with respect of total pore volume as well as pore diameter. 5. Developed equations could predict the value of elastic modulus in direct compression and compressive strength of recycled aggregate with reasonable accuracy. 5. References 1. Yamato TakeshiI, Emoto Yukio, Soeda Masashi and Sakamoto Yoshifumi (1988), Some Properties Of Recycled Aggregate Concrete. Proceedings of the Second International Symposium held by RILEM on Demolition and Reuse of Concrete and Masonry, pp Ravindrararajah R. and Tam C.T. (1988), Methods of Improving the Quality of Recycled Aggregate Concrete. Proceedings of the Second International Symposium held by RILEM on Demolition and Reuse of Concrete and Masonry. pp Topcu I.B. (1997), Physical and Mechanical Properties of Concretes Produced With Waste Concrete. Cement and Concrete Research, 27(12), pp Bairagi N.K., Ravande Kishore and Pareek V.K. (1992), Properties of Recycled Aggregate Concrete. Resources, Conservation and Recycling, 9 (1993) pp Mondal S and Ghosh A Study on properties of recycled aggregate concrete, ICI bulletin No.68, Indian Concrete Institute, July-September 1999, pp M. Barra De Oliviea, E Vasquz (1996), The influence of retained moisture in aggregates from recycling on the properties of new hardened concrete, Waste manage, 16(1-3), pp A.Katz (2003), Properties of concrete made with recycled aggregate from partially hydrated old concrete Cement and Concrete Research 33, pp H.J Chen, T Yen, K.H.Chen (2003), Use of building nibbles as recycled aggregate Cement and Concrete Research 33, pp J.M.Khatib (2005), Properties of concrete incorporating fine recycled aggregate Cement and Concrete Research, 35, pp IS 12269:1987, Specification for 53 grade ordinary Portland cement,, Bureau of Indian Standards, New Delhi 303

13 11. IS: 383: 1970, Specification for Coarse and Fine Aggregates from Natural Sources for Concrete (Second Revision). Bureau of Indian Standards, New Delhi 304