ABSTRACT

Since last few decades the consumption of construction material is increased at large scale world-wide, this is due to migration of people from villages and towns to settle in big cities, thus, requires accommodation. Beside this, old structures are demolished, new buildings are being erected rapidly, hence demolished and construction waste is generated, which poses the serious problems of environmental issues and need of space for dumping. A better way of dealing with this waste is by utilizing it in new concrete. It will not only reduce the waste but also saves environment and natural deposits of the aggregates. This research work uses old concrete as partial replacement of coarse aggregates in new concrete. Large blocks of demolished concrete are collected and processed manually by hammering down to a size approximately equal to 1-inch. Fundamental properties, such as water absorption, specific gravity, unit weight and density of this material are evaluated. These properties show increased water absorption and reduced specific gravity, unit weight and density of recyclable aggregates in comparison to natural coarse aggregates. Total of 120 reinforced concrete beams are cast in this experimental investigation; 60 beams with 50% replacement of natural coarse aggregates with coarse aggregates from old demolished and same are cast with all natural coarse aggregates. Two mixes i.e 1:2:4 (normal mix) and 1:1.5:3 (rich mix) with water cement ratio of 0.5 are used. Beams are reinforced with 2#4 deformed bars of 60 grade in tension and compression zones with #3 bars of same strength as stirrups at 6" c/c spacing along the length of the beam. For all beams, the dimensions are kept same equal to 36"x6"x6". After casting, beams were cured for 28-day by fully immersing in water. Then the beams are exposed to fire for 0, 6, 12, 18 and 24 hours in purpose made oven at 1000 degree centigrade.
Analysis of the results shows decrease in flexural strength and increase in deflection of all the beams for all fire durations. With increase in duration of fire, increased reduction in flexural strength is observed. Minimum and maximum reduction in flexural strength of recycled reinforced concrete beams cast with normal mix is recorded equal to 20.37% and 59.15% for 6-hour and 24-hour fire duration respectively. The same for rich mix concrete beams is observed equal to 22.33% and 68%. The rich mix reinforced concrete beams with recyclable aggregates observed more reduction in flexural strength due to more cement content in the beams. Similar trend of higher deflection is observed in the beams with increase of 130% and 128% for normal and rich mix beams at maximum fire duration. Regression analysis is done to develop numerical equation for predicting the flexural strength of proposed reinforced concrete beams. For the purpose NCSS and Microsoft Excel are used. It is observed that the predicted equations from both software's are almost same with minor difference in few coefficients. These equations are then used to compute the flexural strength. The obtained results are compared with the experimental observations. It is found that both sets of the results are in good agreement.

Keywords: Green Concrete, Demolishing and Construction Waste, Recycled Course Concrete Aggregates, Reinforced Concrete Beams, Fire Effect on Concrete, Fire Resistance, Flexural Strength, Deflection, Regression analysis.