ENGINEERING PROI~ERTIES OF RECYCLED CONCRETE AND USED STEEL REINFORCEMENT BARS

Abstract

This research was informed by the need to turn construction and demolition waste materials into resources so as to reduce demand for natural resources as well as reduce overall pollution and energy- consumption while creating jobs at same time. The aim was therefore to determine the engineering properties of recycled aggregate concrete and recyclable steel with a view to ascertaining whether they can be used for normal concrete works. The research was carried out by means of laboratory experiments. Two groups of experiments were conducted. The first group was performed OJ) three types of'.concrete produced using three different aggregates namely, original, washed recycled and unwashed recycled aggregates. For each type of these aggregate concrete types, experiments were conducted for grain size analysis, aggregate crushing value, workability, water absorption capacity, compressive strength, bending strength and modulus of elasticity. The second group of experiments pertained to reinforcing steel. For used steel of known and unknown origin, tests were conducted for tensile and yield strengths as well as percentage elongation and percentage reduction in area. From the results of this work, for washed recycled aggregate concrete, as the water-cement ratio increased by 0.05, water absorption capacity reduced by about 10% while unwashed recycled aggregate concrete has water absorption capacity ranging from 1.63% to 4.00%. The average densities of original aggregate concrete ranged from 2365kg/m3 to 2449kg/m3 , for washed recycled aggregate concrete, it ranged from 2375kg/m3 to 2376kg/m3 and for unwashed recycled aggregate concrete, it ranged from 2216kg/m3 to 2262kg/m3 to for the same water-cement ratio considered. The average compressive strength of original aggregate concrete ranged from 14.20 N/mm2 to 43.30 Nzmnr' and. that of washed recycled aggregate concrete ranged from 10.76 N/m~2 to 31.41N/mm2 , 2 while that of unwashed recycled aggregate concrete ranged from 7.42N/mm to 19.70N/mm2 for the same water-cement ratio all considered at 28 days. The tensile and yield strengths of used steel of both known and unknown origin was 5% to 22% and 4% to 48% higher than the tensile and yield strength of the corresponding new steel of both known and unknown origin respectively while the percentage elongation and percentage reduction in area of used steel are 13% and 15% lower than the percentage elongation and percentage reduction in area of the corresponding new steel respectively, and finally

the modulus of elasticity for used steel of both known and unknown origin ranged from 202 to 208kN/mm2 . This work has established that the strength and ductility properties of used steel of known origin satisfy the minimum strength and ductility requirements as specified by the relevant codes, provided they are not used in structural members expected to take part in plastic action because of their reduced ductility. In addition, the research showed that not all ribbed steel bars in the market are indeed high yield since the tested ribbed steel bars of unknown origin gave strength and ductility values similar only to code requirements for mild steel. Lastly, this work has provided experimental evidence that provided the used steel of known origin is appropriately cleaned before reuse, the performance characteristics compare well with those of new steel of known origin, given the same types and equal cross sectional areas. Thus, if recycled aggregate is properly washed and used steel of known origin appropriately treated, they can be recycled for concrete works.