A STUDY OF PROPERTIES OF SELF - COMPACTING CONCRETE

Abstract

Self-compacting concrete (SCC) is a concrete that compacts fully under its own weight. It was primarily invented in Japan in the late 1980's. Over the past several years, self-compacting concrete (SCC) has gained popularity due to its commercial benefits in terms of ease ofplacement, no requirement of external vibration and improved durability. Mix compositions and fresh properties of self-compacting concrete are clearly different from that of ordinary concrete. The higher content of powder materials (cement, flyash, etc.) and lower content of coarse aggregates change the granular skeleton and microstructure of the selfcompacting concrete. The hardened properties of the self-compacting concrete are bound to be influenced by these factors. Before applying the present design methods to structural design of SCC members, the performance of the selfcompacting concrete is required to be thoroughly investigated. A critical review of the existing literature has shown that most of H^investigations have been focussed on only a few aspects of self-compacting concrete instead of an overall study of the material. Moreover, the properties of the self-compacting concrete are very sensitive to the properties of its constituent materials. The available information on the properties of the fiber reinforced self-compacting is very limited. Therefore, the present investigation was planned to carry out a comparative study of the various properties of ordinary concrete, self-compacting concrete and fiber reinforced self-compacting concrete. An ordinary concrete, a self-compacting concrete and a fiber reinforced selfcompacting concrete having similar compressive strengths at 90 days age were u developed using the locally available materials. Trial mixes were prepared to develop the three types of concrete. Around corrugated short steel fiber, 25 mm long with aspect ratio of 31.25, was used as the reinforcing fiber in the fiber reinforced self-compacting concrete. Compressive strength tests were carried out on ordinary concrete and self-compacting concrete cube specimens at various ages and for three different curing conditions (Normal water curing, Accelerated curing by boiling water method and Accelerated curing followed by normal water curing). Compressive strength tests on fiber reinforced self-compacting concrete cube specimens were carried out after 7, 28, 60, 90, 120 and 150 days normal water curing. The cylinder compressive strength of the three types of concrete were also evaluated after 7, 28, 60, 90, 120 and 150 days normal water curing. Specimens of the three types of concrete were tested after 7, 28, 60 and 90 days normal water curing to determine the complete stress-strain characteristics, modulus of elasticity, poisson's ratio, split tensile strength and flexural tensile strength. Beams of the three types of concrete with the same longitudinal reinforcement, without stirrups were tested in shear for three different shear span to effective depth ratios (1.0, 1.5 and 2.0) under third point loading at 28 and 90 days age. The response of the beams loaded upto failure, deflection of the beams with varying load, crack development with increasing load, first shear crack strength, number of micro cracks, height and spacing of micro cracks and ultimate shear strength were observed. Specimens of the three types of concrete were tested for water permeability after 7, 28 and 90 days normal water curing. SEM images of ordinary concrete and self-compacting concrete were studied to identify the changes in the microstructure of the selfcompacting concrete compared to the ordinary concrete. in The physical properties of the basic constituent materials, viz., cement, fly ash, micro-silica, fine aggregates and coarse aggregates were obtained as per the relevant Indian Standard Specifications. Some of the major conclusions which emerged from an analysis ofthe test data are summarizedbelow. For the given materials and mix compositions, it was difficult to develop self-compacting concretes with a coarse aggregate content higher than 45% or lower than 15% ofthe total aggregate content. The early age strength of SCC was reduced due to the high content of flyash. The increase in the compressive strength of ordinary concrete from 28 days to 90 days was only 5.9% whereas the increase in the compressive strength of self-compacting concrete during the same period was 25.3%. As a result, though the SCC had a lower strength at early ages, it gained a strength at later ages comparable to ordinary concrete. Both ordinary concrete and self-compacting concrete suffered retrogression of strength when subjected to a higher temperature during the early period of hydration. A maximum increase of 10.5 percent in the compressive strength of selfcompacting concrete was obtained due to the addition of 1% volume fraction of a short steel fiber (25 mm long with 0.8 mm diameter). The split tensile strength of self-compacting concrete was higher than that of ordinary concrete (about 9.8%) with a similar compressive strength at 90 days age. The split tensile strength of the fiber reinforced self-compacting concrete was higher than that of the plain self-compacting concrete by 31.9% to 36.7% at various ages. Similarly, the flexural tensile strength of the SCC was higher than that of IV the ordinary concrete with similar compressive strength by 7.4%. The addition of 1% steel fibers increased the flexural tensile strength of SCC by about 40%. Thus even short steel fibers had a significant influence on the split tensile strength and flexural tensile strength of self-compacting concrete. A much lower and uniformly distributed porosity exists within the interfacial transition zone (ITZ) of self-compacting concrete due to the microfiller effects of the flyash and microsilica particles. As a result, though the SCC mix had a comparatively lower coarse aggregate content, the modulus of elasticity was almost similar to that of the ordinary concrete with a comparable ultimate compressive strength. The ultimate shear strength of SCC beams at 90 days age were 9.2%, 11.0% and 12.6% higher than that of ordinary concrete beams for a/d ratios of 1.0, 1.5 and 2.0 respectively. This was due to the denser cement matrix of SCC which enabled a better load transfer. At 28 days age, the increase in the ultimate shear strength of SCC beams due to the addition of steel fibers in the mix were 50%, 38.46% and 18.8% for a/d ratios of 1.0, 1.5 and 2.0 respectively; whereas at 90 days age the increase in the ultimate shear strength of the SCC beams were 35.5%, 28.37% and 20.17% for the three a/d ratios respectively. The coefficient of water permeability of SCC was much lower than that of ordinary concrete. The coefficient of water permeability of SCC was 61.76% lower than that of ordinary concrete at 90 days age when both the concretes gained similar compressive strengths. The absence of micro cracks in the ITZ of SCC makes it less permeable than ordinary concrete. The effect of steel fiber addition on the coefficient ofwater permeability of SCC was marginal.