International Journal of Civil Engineering and Technology (IJCIET) Volume 7, Issue 2, March-April 2016, pp. 128 139, Article ID: IJCIET_07_02_010 Available online at http://www.iaeme.com/ijciet/issues.asp?jtype=ijciet&vtype=7&itype=2 Journal Impact Factor (2016): 9.7820 (Calculated by GISI) www.jifactor.com ISSN Print: 0976-6308 and ISSN Online: 0976-6316 IAEME Publication EXPERIMENTAL STUDY ON SHEAR BEHAVIOR OF REINFORCED RECYCLED AGGREGATE CONCRETE BEAMS Pinal C. Khergamwala PhD Scholar, I. K. G. Punjab Technical University, Jalandhar, Punjab, India Dr. Jagbir Singh Associate Professor, Department of Civil Engineering, GNDEC, Ludhiana, India Dr. Rajesh Kumar Professor and Head, Department of Civil Engineering, CCET, Chandigarh, India ABSTRACT The use of recycled aggregates (RA) for structural concrete in construction, to the maximum possible limit, is becoming a necessity more than a desire. One such mechanical property, shear resistance of recycled aggregate concrete (RAC) beams is an intensive area of research. Three parameters i.e. compressive strength, percentage of tension steel and shear span to depth ratio were considered. An attempt has been made to study shear strength of RA concrete beams of M 20 grade with 25 and 50 % weight replacement of natural aggregate (NA) with recycled aggregate (RA) for different shear span to depth ratios a/d = 1.5, 2.5 and 3.5 with 1 % tension steel without shear reinforcement and compare the test results with the available shear models. Seven shear models for comparison were considered namely ACI 318, Canadian Standard, IS Code, CEB-FIP Model, Zsutty Equation, Bazant Equation and Okamura and Higai equation. The results revealed that Shear capacity of a RAC beams with 25 and 50 % RA is comparable, or sometimes superior, to that of a controlled beam made of conventional concrete. Equations proposed by Zsutty and Bazant gave relatively more accurate results in terms of the similar pattern as compared to other models but still considerably lower values as compared to experimental results and hence these models can be used effectively for recycled aggregare concrete also. Key words: Recycled aggregates, Parameters, Recycled aggregate concrete, Shear resistance, Shear models, Shear span to depth Ratio (a/d). http://www.iaeme.com/ijciet/index.asp 128 editor@iaeme.com
Experimental Study on Shear Behavior of Reinforced Recycled Aggregate Concrete Beams Cite this Article: Pinal C. Khergamwala, Dr. Jagbir Singh and Dr. Rajesh Kumar, Experimental Study on Shear Behavior of Reinforced Recycled Aggregate Concrete Beams, International Journal of Civil Engineering and Technology, 7(2), 2016, pp. 128 139. http://www.iaeme.com/ijciet/issues.asp?jtype=ijciet&vtype=7&itype=2 1. INTRODUCTION Under the goal of sustainability, the use of recycled aggregate concrete (RAC) has become an important issue in the field of civil engineering. Continuous efforts are being made to improve the mechanical properties of RAC as compared to normal aggregate concrete. There are several modes of failure in concrete structural members. Due to the fragility of concrete structures, shear failure is one of the most important and undesirable modes of failure. Shear strength of concrete depends significantly on the ability of the coarse aggregate to resist shearing stresses. RA used is relatively weaker than NA in most cases and yielded reduced shear strength. Shear force is present in beams at sections where there is a change in bending moment along the span. It is equal to the rate of change of bending moment. An exact analysis of shear strength in reinforced concrete beam is quite complex. The reuse of hardened concrete as aggregate is a proven technology - it can be crushed and reused as a partial replacement for natural aggregate in new concrete construction. The use of 100% recycled coarse aggregate in concrete, unless carefully managed and controlled, is likely to have a negative influence on most concrete properties but literature shows that the compressive strength of concrete up to 50 % RA have strength in close proximity to that of normal concrete. 2. EXPERIMENTAL PROGRAMME Nine reinforced concrete beams were cast and tested, under two point loading for varying shear span to effective depth ratio (a/d). The section of all the beams (width thickness) was kept constant at 150 300 mm. To investigate the effect of shear spanto-depth ratio, a/d values of 1.5, 2.5, and 3.5 were selected to cover short, intermediate, and long beams. Accordingly the overall length of the beam specimens was varied in the range 1.60 m, 2.20 m and 2.70 m. The percentage of tension 100A reinforcement, st was kept constant 1.1%. Concrete of grade M 20 having bd nominal crushing strength of 20 N/ mm 2 was used for investigation. Keeping in view the lower compressive strength of concrete with more than 50 % of recycled aggregates, concrete mix with more than 50 % recycled aggregates were not taken in to account for shear investigations and only 25 and 50 % weight replacement of natural aggregate with recycled aggregate for M 20 grade was considered. Controlled beams with 100 % natural aggregates (0 % RA) were also cast and tested to compare the results. The details of the specimens for shear test are listed in the Table 1 below: http://www.iaeme.com/ijciet/index.asp 129 editor@iaeme.com
Pinal C. Khergamwala, Dr. Jagbir Singh and Dr. Rajesh Kumar Table 1: Details of the specimens for shear tests Specimen Name Overall length L (mm) Effective Depth d (mm) P t of steel % A st (mm 2 ) Shear span-to-depth Ratio a/d No. of specimens Mix M20 M20R25A1.5P1 1600 265 1.1 452.16 1.5 01 M20R50A1.5P1 1600 265 1.1 452.16 1.5 01 M20R25A2.5P1 2200 265 1.1 452.16 2.5 01 M20R50A2.5P1 2200 265 1.1 452.16 2.5 01 M20R25A3.5P1 2700 265 1.1 452.16 3.5 01 M20R50A3.5P1 2700 265 1.1 452.16 3.5 01 Controlled beams with 100 % NA M20A1.5P1 1600 265 1.1 452.16 1.5 01 M20A2.5P1 2200 265 1.1 452.16 2.5 01 M20A3.5P1 2700 265 1.1 452.16 3.5 01 Note: In Colum. (1), M stands for Conventional mix type, R indicates % of recycled aggregate, A stands for a/d ratio and P indicates % of tension reinforcement. 2.1. Test Materials The concrete test specimens were cast using cement, fly ash, fine aggregate, natural coarse aggregate, recycled coarse aggregate, water and steel. The materials, in general, confirmed to the specification laid down in the relevant Indian Standard Codes. Ordinary Portland Cement of 53 Grade from a single source with specific gravity 3.14, confirming to IS: 8112-1989 was used. A low-calcium fly ash obtained from the combined fields of the electrostatic precipitator of the thermal power plants with specific gravity 2.24 was used. Locally available natural river sand having a specific gravity of 2.58, water absorption of 1.10% and a fineness modulus of 2.68 was used as fine aggregate. Portable water free from any harmful amounts of oils, alkalis, sugars, salts and organic materials was used for proportioning and curing of http://www.iaeme.com/ijciet/index.asp 130 editor@iaeme.com
Experimental Study on Shear Behavior of Reinforced Recycled Aggregate Concrete Beams concrete. Deformed steel bars of 10 mm and 12 mm nominal diameters and with nominal yield strength of 423 MPa were used as tension reinforcement in the beams. Shear reinforcement in the form of stirrups were not provided. All the steel reinforcement bars confirmed to IS 1786: 1985. Two types of coarse aggregates named Natural Aggregate (NA) and recycled aggregates (RA) were used in the RAC mixes. Locally available crushed granite having a specific gravity of 2.70 was used as NA. RA was derived from the tested concrete cubes in the laboratory that contained well-graded crushed granite stone. Specific gravity of RA was found 2.48, which is lower than NA. The concrete cubes were crushed manually to the specified size using a hammer and gradation was achieved through sieving of RA. The maximum size of coarse aggregate used was 20 mm in both recycled and natural aggregate concrete. 2.2. Concrete Mix Design The concrete mix M 20 of characteristic strength of 20 N/ mm 2 with constant water to cement ratio (w/c) 0.5 was used in this investigation which is commonly used in construction of structural members. The mix design was done according to the IS: 10262-2009 and numerous trial mixes were conducted to obtain the optimum mix. Once the optimum mix was determined, it was used to produce concrete with 25% and 50% recycled coarse aggregate by weight replacement of natural coarse aggregate. Due to the higher water absorption capacity of RA as compared to natural aggregate, both the aggregates are maintained at saturated surface dry (SSD) conditions before mixing operations. Fly ash was used as 25% by weight replacement of cement to achieve proper workability of the mix. The details of optimum mix are given in Table 2. Mix Mix proportion by weight Table 2: Mix proportion for optimum mix Fly Ash % Cement Constituents (kg/m 3 ) Fly Ash Sand Aggregates W/C ratio M 20 1:1.5:3.4 25 289 96 578 1310 0.5 2.3. Instrumentation and Testing Procedure In the present study beams were cast in steel forms with the tension reinforcement near the bottom. No stirrups (shear reinforcement) were provided in the beams. Lifting lugs were also provided for transporting the finished specimen to the test platform. The concrete was compacted with needle vibrator. Form work was removed after 48 hours. The beams were cured with wet hessian and sand for 28 days. To facilitate the tracing of cracks, the beams were distempered white prior to testing. For investigation of the shear behavior, beams designed only for adequate flexural strength and without any web reinforcement were tested under monotonically increasing loads in a four point loading configuration to study the shear failure mechanism. The beam specimens were tested as simply supported beam by using a manually operated hydraulic Jack that applied load gradually on the mid-span of the beam specimens until shear failure which pre-empted flexural failure. Diagonal cracking along with the formation of a dominant inclined crack is indicative of shear http://www.iaeme.com/ijciet/index.asp 131 editor@iaeme.com
Pinal C. Khergamwala, Dr. Jagbir Singh and Dr. Rajesh Kumar failure. Seven deflection gauges were employed to record deflection. The arrangement of 4 LVDT s attached diagonally in pairs on the side-face of the beams in the shear zone were done to detect diagonal cracking. The test setup configuration for the shear tests is shown in Figure. 1. Figure 1 Test setup configuration for the shear tests Spreader beam 300 300 300 Hinge LVDT Steel sleeve Roller LVDT Effective span All dimensions are in mm Figure 2 Shear failure of actual beam specimen http://www.iaeme.com/ijciet/index.asp 132 editor@iaeme.com
Experimental Study on Shear Behavior of Reinforced Recycled Aggregate Concrete Beams 3. DISCUSSION ON TEST RESULTS The cracking and ultimate shear stress calculations of RAC beams are presented in Table 3. Graphical representation of comparison of cracking and ultimate shear strength of RAC beams with controlled (NAC) beams is shown in Figs. 3 and 4. In the case of short beams (a/d < 2.5), a very significant amount of additional loading can be resisted by the reinforced recycled aggregate concrete beams beyond the formation of a first diagonal crack before ultimate failure in shear-compression occurs. This redistribution of stresses in short beams takes place because of the relatively short distance between the supports and the applied loads, and is evidenced by the large spread in magnitude that exists between V cr and V u (in short beams, the redistribution of stresses is due to the transferring of the applied loads directly to the supports by arch action). Conversely, the failure mode in long beams (a/d 2.5) is in diagonal tension with the formation and propagation of the first fully developed inclined crack. As a/d increases from 2.5 to 3.5, this failure mode becomes very sudden in RAC beams as total shear failure occurs almost immediately after the formation of a first major diagonal cracking. Table 3 Shear test results of the RAC beams Specimen ID Area of tension steel, A st (mm 2 ) Reinforcement ratio, ρ=a st /bd a/d Measured characteristics strength, fck (MPa) Diagonal cracking shear, V cr (kn) Ultimate shear, V u (kn) Cracking shear stress, v cr =V cr /bd (MPa) Ultimate shear stress, v u =V u /bd (MPa) M20A1.5P1 452.16 0.011 1.5 24.17 106.69 191.36 2.684 4.814 M20R25A1.5P1 452.16 0.011 1.5 23.43 88.83 181.97 2.235 4.578 M20R50A1.5P1 452.16 0.011 1.5 24.93 92.43 189.84 2.325 4.776 M20A2.5P1 452.16 0.011 2.5 24.17 50.15 88.16 1.262 2.218 M20R25A2.5P1 452.16 0.011 2.5 23.43 43.95 80.82 1.106 2.033 M20R50A2.5P1 452.16 0.011 2.5 22.67 48.15 89.57 1.211 2.253 M20A3.5P1 452.16 0.011 3.5 23.80 44.95 61.79 1.131 1.554 M20R25A3.5P1 452.16 0.011 3.5 23.43 40.26 58.46 1.013 1.471 M20R50A3.5P1 452.16 0.011 3.5 22.67 43.84 54.78 1.103 1.378 http://www.iaeme.com/ijciet/index.asp 133 editor@iaeme.com
Ultimate shear strength, kn Cracking shear strength, kn Pinal C. Khergamwala, Dr. Jagbir Singh and Dr. Rajesh Kumar 5. COMPARISON OF SHEAR STRENGTH OF RAC BEAMS WITH CONTROLLED (NAC) BEAM 120 100 80 60 40 20 0 % RA 25% RA 50% RA 0 1.5 2.5 3.5 Shear span to depth ratio, a/d Figure 3 Cracking shear strength V cr versus a/d 200 180 160 140 120 100 80 60 40 20 0 1.5 2.5 3.5 0 % RA 25% RA 50% RA Shear span to depth ratio, a/d Figure 4 Ultimate shear strength V u versus a/d Results indicated that with the increase in a/d ratio, there is sharp decrease in the shear capacity of the beam. At a/d ratio 1.5, the first cracking load as well as the ultimate diagonal shear load was observed to be almost double than that at a/d ratio 2.5 and 3.5. At a/d ratios 3.5 and concrete with 50 % of RA, the failure was observed to be sudden as compared to failure pattern observed for lower a/d ratio 1.5. Crack width for RAC beams was wider as compared to control beam due to weak bonding of RA with new concrete. Results also showed that the shear capacity of a RAC beams with 25 and 50 % RA is comparable, or sometimes superior, to that of a controlled beam made of conventional concrete. http://www.iaeme.com/ijciet/index.asp 134 editor@iaeme.com
Experimental Study on Shear Behavior of Reinforced Recycled Aggregate Concrete Beams 4. COMPARISON OF EXPERIMENTAL SHEAR RESULTS OF RAC BEAMS WITH AVAILABLE SHEAR MODELS Based on experimental observations, different researchers have developed different equations for the prediction of shear capacity of the NAC rectangular beams. Shear models used for NAC beams have been used here to predict shear strength of RAC beams because till date no specific code and models for RAC is formulated. Table 4 presents the experimental and predicted shear strength values of all the test beams with different percentage of recycled aggregate and with different a/d ratios. It can be observed that regardless of the a/d ratio, all the empirical equations gave a conservative estimate of the actual ultimate shear strength for all the replacement level of recycled aggregate of RAC beams. For all the models used to calculate V c, the calculated strength values became more conservative as the a/d ratio decreases. T C Zsutty collected the test data of about 200 beams from different responsible sources and developed equations by combining the techniques of dimensional and statistical regression analysis for the prediction of shear strength of longitudinally reinforced beams. Table 4 Experimental and Predicted Results of shear strength Specimen ID Experime ntal Failure Load Vexp (kn) ACI code Equatio n Canadia n Code Equatio n IS: 456-2000 code Equatio n Vpredicted (kn) CEB- FIP Mode l Zsutty Equatio n Bazant Equatio n Okamur a &Higai Equatio n M20A1.5P1 106.69 32.46 39.01 25.91 40.71 79.91 78.71 42.69 M20R25A1.5P1 88.83 32.13 38.40 25.81 40.31 78.72 78.26 42.28 M20R50A1.5P1 92.43 33.02 39.62 26.00 41.16 80.37 79.17 43.15 M20A2.5P1 50.15 30.76 39.01 25.91 34.33 40.44 43.01 39.27 M20R25A2.5P1 43.95 30.44 38.40 25.81 34.01 39.83 42.56 38.87 M20R50A2.5P1 48.15 29.98 37.78 25.70 33.63 39.41 42.10 38.44 M20A3.5P1 44.95 29.82 38.70 25.86 30.53 35.96 34.93 37.41 M20R25A3.5P1 40.26 29.71 38.40 25.81 30.42 35.61 34.71 37.21 M20R50A3.5P1 43.84 29.25 37.78 25.70 30.12 35.23 34.24 36.81 http://www.iaeme.com/ijciet/index.asp 135 editor@iaeme.com
Shear strength, kn Shear strength, kn Pinal C. Khergamwala, Dr. Jagbir Singh and Dr. Rajesh Kumar 120 100 80 60 40 20 0 M 20 R 0-1 % 1.5 2.5 3.5 Shear span to depth ratio, a/d Experimental ACI code Canadian IS: 456-2000 CEB- FIP Zsutty Bazant Okamura &Higai 100 80 60 40 20 0 (a) For M 20 (NAC beams) M20 R25-1 % 1.5 2.5 3.5 Shear span to depth ratio, a/d Experimental ACI code Canadian IS: 456-2000 CEB- FIP Zsutty Bazant Okamura &Higai (b) For M 20 R 25 http://www.iaeme.com/ijciet/index.asp 136 editor@iaeme.com
Shear strength, kn Experimental Study on Shear Behavior of Reinforced Recycled Aggregate Concrete Beams M20 R50-1 % 100 80 60 Experimental ACI code Canadian 40 20 0 1.5 2.5 3.5 Shear span to depth ratio, a/d IS: 456-2000 CEB- FIP Zsutty Bazant Okamura &Higai (c) For M 20 R 50 Figure 5: Comparison between predicted and Experimental Results The comparison of the experimental results with predicted values for all the seven models are presented in Fig. 5 (a), (b) and (c) for M 20 with 0, 25 and 50 % RA. Almost similar trend of normal aggregate concrete members is followed by RAC beams. There is no negative impact of the replacement of 25 and 50 % RA. Analytical values and experimental results revealed that a/d ratio significantly affects the shear capacity of recycled aggregate concrete beams. Most of the equations are under estimating the shear capacity at lower a/d ratios. When the a/d ratio is less than 1.5, strut action prevails and the shear resistance is very high. For a/d ratio 1.5 the experimental values showed remarkable increase in shear strength compared to various design models. Only predicted shear capacity using Zsutty and Bazant Equation had followed the same pattern for all the three a/d ratios but the values were still lower than experimental values for all concrete mixes. For a/d ratios 2.5 and 3.5 almost all the models followed the same trend but with quite lower values. 5. CONCLUSION Shear capacity of a RAC beams with 25 and 50 % RA is comparable, or sometimes superior, to that of a controlled beam made of conventional concrete. For a/d ratio 1.5, there is sharp increase (almost double) in shear capacity of RAC beams as compared to a/d ratio 2.5 and 3.5. There is not much difference in the shear capacity of RAC beams for a/d ratio 2.5 and 3.5. For higher a/d ratio 3.5, sudden shear failure of RAC beams were observed as compared with a/d ratio 1.5. There is less difference between first crack load and ultimate shear load for a/d ratio 3.5. Crack width for RAC beams was wider as compared to control beam due to weak bonding of RA with new concrete. ACI as well as IS code give overly conservative shear capacity predictions of recycled aggregate concrete beams without web reinforcement at all a/d ratios because ACI code presented a formula for the prediction of shear cracking load in 1963, which was developed by the linear regression based on thousands of beam test results subjected to UDL only. http://www.iaeme.com/ijciet/index.asp 137 editor@iaeme.com
Pinal C. Khergamwala, Dr. Jagbir Singh and Dr. Rajesh Kumar The Canadian code considered only compressive strength of concrete. It has not taken into account the effect of shear span to depth ratio and longitudinal tension reinforcement on shear strength of beams. The shear resistance of RAC member predicted based on Canadian code underestimates the actual shear capacity of member at all a/d ratios. Shear capacity of the RAC members predicted based on CEB-FIP model and Okamura- Higai equation showed conservative values at all a/d ratios. Zsutty equation is more appropriate and simple to predict the shear strength of both shorter and long beams as it takes into account size effect and longitudinal steel effect for RAC beams also. The Bazant equation has better agreement with the test data. In this equation five parameters (fc, ρ, a,d, d and da) are correlated with ultimate shear strength of rectangular beams, especially the effect of aggregate size, which plays very important role in the shear strength. ACKNOWLEDGMENTS I express my sincere thanks to I. K. G. Punjab Technical University, Kapurthala, India for providing strong platform for pursuing Ph.D. Authors acknowledge the help received from Head and faculty members of the Civil Engineering Department, Guru Nanak Dev Engineering College, Ludhiana, Punjab, for making testing facilities available to them. The invaluable cooperation of the laboratory staff of Heavy Testing Laboratory and Concrete Testing Laboratory of Civil Engineering Department is gratefully acknowledged. REFERENCES [1] ACI 318-02, Building code requirements for reinforced concrete, (ACI 318-02) and commentary, (ACI 318R-02). Detroit: American Concrete Institute, 2002. [2] Angelakos D, Bentz E C and Collins M P, Effect of concrete strength and minimum stirrups on strength of large members, ACI Structural Journal; 2001, 98, 290 300. [3] Brito J D and Richardo R, Recycled aggregate concrete methodology for estimating its long term properties, Indian journal of engineering and material sciences, 2010, volume 17, 449-462. [4] Etxeberria M, Marí AR, Vázquez E, Recycled aggregate concrete as structural material. Mater Struct; 2007, 40:529 41. [5] Fathifazl G, Razaqpur A G, Burkan Isgor O and Abbas A, Shear capacity evaluation of steel reinforced recycled concrete (RRC) beams, Engineering Structures journal, 2011, volume 33, 1025 1033. [6] González F, Martínez A and Eiras L, Structural shear behavior of recycled concrete with silica fume, Construction and Building Materials journal, 2009, volume 23, 3406 3410. [7] Han B C, Yun H D and Chung SY, Shear capacity of reinforced concrete beams made with recycled-aggregate, ACI Special Publication SP 200-31, Farmington Hills, MI, USA: American Concrete Institute; 2001, 503-515. [8] Hansen TC, Narud H. Strength of recycled concrete made from crushed concrete coarse aggregate, Concrete International, 1983, No. 1, 579-83. [9] I. Gull, Testing of strength of recycled waste concrete and its applicability, Journal of Construction Engineering and Management, 2011, vol. 137, 1 5. http://www.iaeme.com/ijciet/index.asp 138 editor@iaeme.com
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