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  1. 1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME 268 STUDIES ON COMPRESSION AND FLEXURAL STRENGTH CHARACTERISTICS OF TRIPLE BLENDED HIGH STRENGTH RECYCLED AGGREGATE CONCRETE M.V.S.S. Sastri1 , Dr. K. Jagannadha Rao2 , Dr. V. Bhiksma3 1 (Assoc.Professor, Department of Civil Engineering, Vasavi College of Engineering, Ibrahimbagh, Hyderabad- 500031 (AP), India) 2 (Professor, Department of Civil Engg. Chaitanya Bharathi Institute of Technology, Hyderabad-500075 (AP) India) 3 (Professor, Department of Civil Engg, O.U.College of Engg (A). Osmania University, Hyderabad-500007 (AP) India) ABSTRACT The suitability of recycled coarse aggregate (RCA) in the production of a high-strength concrete using triple blended industrial by-products is tested in laboratory. The by-products used are fly ash and condensed silica fume as binders at different percentages and recycled aggregates as partial replacement to natural aggregates. The concrete mixtures containing both supplementary cementitious materials and recycled aggregates had shown high compressive strength (>70 MPa), high flexural strength and split tensile strength compared to control concrete. Keywords: Triple Blending, High Strength Concrete, Recycled Aggregate, Sustainability. 1.0 INTRODUCTION In order to reduce resource depletion from the construction sector, an effort to use recycled and secondary materials in concrete production has been introduced decades ago. The use of secondary materials in concrete is still largely limited to low-strength concrete products such as base courses for roads and 80% of the fly ash ends up in low value applications [1]. However, some industrial by-products show excellent properties as construction materials, which means that they could be used in concrete production not only for resource preservation but also to improve the final product but exhibits different properties compared to conventional materials. In order to safely use INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2014): 7.9290 (Calculated by GISI) www.jifactor.com IJCIET ©IAEME
  2. 2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME 269 them in concrete production they should undergo thorough quality control testing and their properties must be taken into account in the concrete mixture design.The changes in material properties or in production techniques generally take place for strengths more than 40 MPa. Earlier studies on conventional strength concrete [6, 7, 8] reported that there is no significant variation in the strength and other mechanical properties of recycle aggregate concrete compared to the natural aggregate concrete. 1.1 Recycled aggregate In the present scenario of construction, building demolition waste (BDW) concrete handling and management is the new primary challenge faced by the countries all over the world. The problem has to be tackled in an indigenous manner, it is desirable to completely recycle the waste in order to protect natural resources and reduce environmental pollution. Recycled concrete aggregates contain not only the original aggregates, but also a little hydrated cement paste. This paste reduces the specific gravity and increases the porosity compared to similar virgin aggregates. Higher porosity of recycled aggregates leads to a higher absorption [9, 10, 11, 12]. Quality requirements of recycled aggregates produced from the poorest quality concrete have to be same as that of conventional aggregates. 1.2 Recycled Aggregate Triple Blended Concrete Mixes In the present experimental investigation triple blending has been carried out by mixing fly ash and condensed silica fume in various proportions as replacements to ordinary Portland cement. Three percentages of fly ash (20, 30 and 40) and four percentages of CSF (0, 5, 10 and 15) were used as replacement to cement for triple blending. Recycled aggregate also varied at 0, 25 and 50% by weight. In all 28 concrete mixes were cast and tested. The objective of the present investigation is to find out the strength parameters, in specific, the compressive, flexural and split tensile strength of recycled aggregate triple blended high strength concrete and compare the same with that of ordinary concrete. In turn, the project is aimed towards experimentally proving the usage of recycled aggregate in structural usage over ordinary concrete and thus fostering its usage for not only greater strength and durability but also in view of the economic and environmental considerations involved. 2.0 EXPERIMENTAL INVESTIGATION 2.1 Cement The Ordinary Portland Cement (OPC) of UltraTech 53 grade confirming to Indian standard IS 12269-1987 was used. 2.2 Fine aggregate Fine aggregate used for this entire study of investigation for concrete was river sand confirming to zone-1 of IS: 383-1987. 2.3 Coarse aggregate Crushed hard granite chips of maximum size 20 mm were used in concrete mixes. 2.4 Water Potable water available in the college was used for casting and curing. 2.5 Condensed Silica Fume The CSF was obtained from M/s V.B. Ferro Alloys Pvt. Ltd., Hyderabad.
  3. 3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME 270 2.6 Fly Ash The material was procured from Ramagundam Thermal Power Plant (A.P). 2.7 Super Plasticizer (SP) of M/s Fosroc Industries Ltd Conplast SP 430 was used. 2.8 Recycled aggregate The building demolition waste was collected from a school building at the time of road widening and the age of the building is about 20 years. The concrete debris was broken into pieces of approximately 80 mm size with the help of hammer & drilling machine. The foreign matters were sorted out from the pieces. Further, those pieces were hand crushed in the lab and mechanically sieved through sieve of 4.75 mm to remove the finer particles. The recycled coarse aggregates were washed and dried and collected for use in concrete mix. Table-1: Summary of Physical properties of Coarse Aggregate Water absorption% Impact strength % Los Angeles abrasion value % Aggregate crushing value % Voids % Specific gravity Fineness Modulus NA 1.25 26 27 28 42 2.66 6.98 RA 2.71 31 31 47 51 2.50 6.925 2.9 Reference Concrete Mix Design of M-80 grade concrete mix was carried out by using Design of Experiments method. Quantity of cement is 650 kg/m3 with a water cement ratio of 0.28. The details of mix proportions are given in table-2. Table-2: Summary of Mix proportions Mix w/c ratio Water (litre) Cement(kg) NA(kg) FA(kg) Mix M80 0.28 182 650 1316.25 406.25 1:0.625:2.025 with 0.28 w/c and 1.5% SP 3.0 CASTING AND CURING OF SPECIMENS Casting of Specimens was done by batching of materials, preparation of moulds and placing of concrete in the moulds. Vibrator was used after every 1/3 filling of material into the mould and the top surface was properly leveled at the end. They were allowed to dry for 24 hrs and proper identification marks were written and kept into the curing tank for various ages of testing. 4.0 TESTS CONDUCTED ON HARDENED CONCRETE 4.1 Compressive strength Three specimens of size 100 mm x 100 mm x 100 mm were used for compression testing for each batch of mix. 4.2 Split Tensile strength Test Split tensile test was conducted on cylinders of size 100 mm diameter and 200 mm height. 4.3 Flexural strength The prisms of size 100x100x500mm were tested to evaluate the flexural strength of the concrete by two point loading. All the above tests are conducted as per IS specifications.
  4. 4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME 271 5.0 TEST RESULTS AND DISCUSSIONS The test results on hardened concrete are reported in tables 3 and 4 and figures 1 to 6. 5.1 Workability of Recycled Triple Blended, High Strength Concrete Mix When various percentages of condensed silica fume along with recycled aggregate was added the workability was becoming very low. Hence, superplasticizer was added up to a maximum percentage of 1.5% to maintain workability. A higher dosage of superplasticizer is required for high strength concrete mixes particularly when recycled aggregate and mineral admixtures like condensed silica fume is used. But the increment of fly ash has shown improvement in workability. Fig.1: Compressive strength of 25% RA at various ages in MPa Fig.2: Compressive strength of 50% RA at various ages in MPa Fig.3: Flexural strength of 25% RA in MPa Fig.4: Flexural strength of 50% RA in MPa Fig.5: Split tensile strength of 25% RA in MPa Fig.6: Split tensile strength of 50% RA in MPa Mix: first numerical in the parenthesis indicates %fly ash; second is %condensed silica fume and third is % recycled aggregate.
  5. 5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME 272 5.2 Influence of the Mineral Admixtures on the Compressive Strength The variation of compressive strength at 7, 28, 56 and 90 days with recycled aggregate triple blended concrete along with percentage increment over control mixes is shown in table 3 and in fig 1 and 2. It is observed that condensed silica fume contributes towards increase in the compressive strength of triple blended, high strength concrete mix. The compressive strength of the concrete is showing increasing trend when fly ash is added along with condensed silica fume. Fly ash is pozzolanic in nature and is reacting slowly as it needs longer curing periods hence even beyond 28 days the strength of concrete is improving particularly when percentage is more. Fly ash content of 20 percent and 5 percent condensed silica fume was found to be optimum for all the ages without recycled aggregate. Highest compressive strength was obtained at 5% condensed silica fume with 20% fly ash. This value is 94 MPa. The compressive strength of the reference mix without any mineral admixtures was obtained as 90.2 MPa at 90 days. There is an increase of nearly 4% in compressive strength over the reference mix. It is observed from the tables that as the Fly ash percentage increases, the compressive strength is gradually decreasing. This happened in the case of all other combinations. The strength of the triple blended recycled aggregate concrete mixes with 20% fly ash along with various percentages of condensed silica fume and recycled aggregates considered are above the design strength i.e. 80 MPa, but the observed strengths are lower than the control concrete. When the cement is replaced by fly ash at 30% along with various percentages of condensed silica fume and recycled aggregates considered the strength of recycled aggregate triple blended concrete mixes are in the range of 10 to 29% less than the control concrete. When the percentage replacement of fly ash at 40% and condensed silica fume at its maximum the strength of 50% recycled aggregate concrete mix is of 50% of control mix which is significant as the total amount of cement used is 357.5 kg/m3 only. M80 grade concrete with 25% recycled aggregate could meet the design strength requirement with 20% and 5% replacement of cement with fly ash and condensed silica fume respectively. 5.3 Influence of the Mineral Admixtures on the Flexural Strength Referring to table 4 and figure 3 and 4, it can be seen that silica fume contributes towards increase in the flexural strength up to 10% but after that the strength is decreasing drastically and it shows that optimum level of condensed silica fume has reached along with fly ash. Highest flexural strength of 9.2 MPa was obtained at 5% CSF with 20% fly ash which is equivalent to control concrete strength. Table 3: Average compressive strength at all ages in MPa for typical combinations and corresponding increase /decrease over control concrete MIX CF compressive strength (MPa) 7 day % 28 day % 56 day % 90 day % (0,0,0) 0.86 54.5 0% 85.0 0% 88.6 0% 90.2 0% (20,5,0) 0.88 58.2 7% 85.7 1% 88.5 0% 94.0 4% (20,10,0) 0.84 51.0 -6% 80.3 -6% 85.6 -3% 90.2 0% (20,15,0) 0.816 45.8 -16% 73.5 -14% 82.3 -7% 84.5 -6% (30,5,0) 0.89 46.5 -15% 57.4 -32% 78.3 -12% 89.2 -1% (30,10,0) 0.84 43.5 -20% 68.1 -20% 73.3 -17% 80.9 -10% (30,15,0) 0.88 33.8 -38% 64.3 -24% 66.5 -25% 67.9 -25% (40,5,0) 0.91 35.6 -35% 66.0 -22% 70.3 -21% 72.5 -20% (40,10,0) 0.89 38.6 -29% 67.2 -21% 71.3 -20% 73.5 -19% (40,15,0) 0.88 28.2 -48% 42.6 -50% 48.4 -45% 54.5 -40%
  6. 6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME 273 It can be seen from figure 3 and 4 that as the fly ash, condensed silica fume and recycled aggregate percentage increases, the flexural strength is gradually decreasing. As discussed earlier the optimum percentage of mineral admixture is obtained as 20% fly ash with 5% CSF without recycled aggregate. The flexural strength of concrete mix with 20% fly ash, 10% condensed silica fume along with recycled aggregate by 50% replacement is less by 9 percent to control mix, hence can be neglected. 5.4 Influence of the Mineral Admixtures on the Split Tensile Strength Referring to table 5 and figures 5 and 6 it is observed that condense silica fume contributes towards increase in the split tensile strength without recycled aggregate. Highest split tensile strength of 4.5 MPa was obtained at 10% CSF with 20% fly ash which is 5 percent more than control concrete. From the figures it is observed that as the fly ash, condensed silica fume and recycled aggregate percentage increases, the split tensile strength is gradually decreasing and it follows the same trend of flexural strength. As discussed earlier the optimum percentage of mineral admixture is obtained as 20% fly ash with 5% CSF without recycled aggregate. Table 4: Average 28 day flexural, split tensile strengths in MPa for all the typical combinations and the percentage increase/ decrease over control concrete MIX Split Tensile Strength (MPa) Flexural Strength (MPa) (0,0,0) 4.3 0% 9.2 0% (20,5,0) 4.4 3% 9.2 0% (20,10,0) 4.5 5% 8.4 -9% (20,15,0) 4.4 3% 7.5 -19% (30,5,0) 3.5 -17% 7.3 -21% (30,10,0) 3.4 -21% 7.0 -24% (30,15,0) 3.4 -21% 6.8 -26% (40,5,0) 3.1 -27% 6.3 -31% The split tensile strength of recycled aggregate triple blended concrete mix having 20% fly ash, 10% condensed silica fume and recycled aggregate with 50% replacement is less by 4 percent compared to control concrete, which is negligible and further these strengths can be improved by adding fibres. 5.5 Optimum Mix for Triple Blended High Strength Recycled Aggregate Concrete The strength of triple blended concrete with fly ash percentage of 20%, condensed silica fume percentage of 5% and 25% recycled aggregate considered the strength is on par with control concrete. On the overall, strength loss with the higher percentages of fly ash is compensated by silica fume. With the fly ash percentage of 20% and with the increase of silica fume percentage up to an optimum of 10% along with 50% recycled aggregate the strength reduction is negligible. Thus an optimum high strength concrete mix possessing optimum strength properties can be obtained resorting to triple blended recycled aggregate concrete. 6.0 CONCLUSIONS Based on the present experimental investigation the following main conclusions are drawn. 1. Higher dosages of superplasticizer are required for high strength concrete mixes particularly when mineral admixtures and recycled aggregates were employed to maintain workability.
  7. 7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME 274 2. Twenty percent fly ash generates marginal increase in strength beyond which decreases with higher percentages of fly ash. 3. The use of 50% of RCA as partial replacement of natural aggregate reduced the strength of triple blended high strength concrete marginally. But it would help in consuming the construction and demolition waste to some extent apart from consuming the industrial wastes, thereby achieving the sustainability. 4. It is recommended to use 20 percent fly ash and 10 percent silica fume as partial replacement of cement and 50 percent recycled aggregates as replacement of natural aggregate for the optimum strength properties. 5. It is the right time to seriously think of reusing demolished concrete for the production of recycled concrete in our country. Recycling would not only conserve the resources but would also promote safe and economic use of such concrete which is the need of the hour for a country like India. ACKNOWLEDGEMENTS The authors express their sincere regard and gratitude to the management of ACE Engineering College, Hyderabad, for the facilities provided for the experimentation work in connection with the present paper. Our special thanks to Professor & Head Dr. P.J.Rao, for his constant encouragement and help. REFERENCES 1. Malhotra, V.M., 1980, “Strength and durability characteristics of concrete incorporating a pelletized blast furnace slag fly ash, silica fume, slag and other mineral by-products in concrete”, SP-79. V.2’American Concrete Institute, Detroit pp. 891-922. 2. IS: 1344-1968 : Indian standard specifications for pozzolonas”- Bureau of Indian Standards. 3. IS: 7869 (Part 2) – 1981 : Indian standard specifications for admixtures in concrete. 4. Folagbade, O. S. (2012). Compressive strength development of concretes containing ternary blended cements. Indian Concrete Journal, 86(10), 9-16. 5. Knaack, A. M., & Kurama, Y. C. (2013). Design of Concrete Mixtures with Recycled Concrete Aggregates. ACI Materials Journal, 110(5), 483-493. 6. Limbachiya, M., Leelawat, T., & Dhir, R. (2000). Use of recycled concrete aggregate in high- strength concrete. Materials and Structures, 33(9), 574-580. 7. Tam, V. W. Y., Tam, C., & Wang, Y. (2007). Optimization on proportion for recycled aggregate in concrete using two-stage mixing approach. Construction and Building Materials, 21(10), 1928-1939. 8. Thomas, M. D. A., Shehata, M. H., Shashiprakash, S. G., Hopkins, D. S., & Cail, K. (1999). Use of ternary cementitious systems containing silica fume and fly ash in concrete. Cement and Concrete Research, 29(8), 1207-1214. 9. M C Limbachiya, A. Koulouris, J J Roberts and A N Fried, “Performance of Recycle Aggregate Concrete”, Kingston University, UK, 2004. 10. Khaldoun R, “Mechanical properties of concrete with recycled coarse aggregate”, Building and Environment journal, volume 42, 2007, 407–415. 11. S. K. Singh and P. C. Sharma, “Use of recycled aggregates in concrete- A Paradigm Shift”, National building materials journal, 2007. 12. Salem Ahmed Abukersh, “High quality recycled aggregate concrete”, Ph. D thesis, School of Engineering and the Built environment, Edinburgh Napier University, UK, 2009.

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