PACKING CONCEPT IN MODERN CONCRETE

Abstract

Concrete is the second largest material used after water in the world. The use of concrete in construction industry has increased immensely since its advent. The cost of concrete is primarily controlled by the amount of cement it contains. In order to reduce the paste content without jeopardizing the strength factor of concrete, packing theory is introduced. The packing concept in concrete is defined as the packing or arrangement of solid particles of the mix; mainly aggregate and binder in such a manner that minimizes the voids present in the mix. As small size aggregates fill the voids between large size aggregates and binder fills the voids between fine aggregate, the overall void ratio of the mix decreases. In the first domain of the project, to achieve the maximum packing, two ranges of material were taken; fine aggregate (0 - 2.36 mm) and coarse aggregate (2.36 - 6 mm). The properties of aggregate both coarse and fine were determined. The particle distribution curves of fine and coarse aggregates were developed and compared with the particle distribution envelope provided by the ASTM standards. The curves were then modelled according to Modified Andreasen & Andersen Model. The purpose was to optimize the particle distribution that would result in maximum packing of aggregate. Prisms (40 x 40 x 160 mm) were then casted having three water-cement ratios of 0.30, 0.35 and 0.40 for both SCM and SCC systems. The strength test results show that, for an optimum value of distribution modulus and lower water-cement ratio, highest strength is achieved. In the second domain of the project, two ranges of material were taken; fine aggregate (0 - 2.36 mm) and coarse aggregate (13.2 mm down). The specific gravity, rodded and un-rodded bulk densities and void ratios were determined for different combinations of coarse and fine aggregates. The maximum packing was achieved for the mix having equal parts of fine and coarse aggregate. The concrete cubes of 4 inches were then casted for different sand to coarse ratios i.e. 60:40, 50:50 and 40:60 keeping a constant water-cement ratio of 0.42 for the self-consolidating concrete (SCC) systems. The strength test results show that the packing mix that has the minimum amount of voids contains equal parts of coarse and fine aggregate and results in higher strength and durability than a normal concrete. The research work concludes that higher the packing density of SCC systems, lower is the cement paste demand in the mix. This reduction in the requirement of cement paste leads to higher strength, durability and sustainability.