GRANULAR SKELETON TOPOLOGY - CONTRIBUTION TO THE COMPRESSIVE STRENGTH OF CONCRETE

It is well known that the compressive strength of mature concrete depends primarily on the porosity of the matrix. But other factors may also influence this property. The aim of this paper is to evaluate the main secondary parameter affecting concrete strength and related to the topology of the granular skeleton (i.e. size distribution, shape and spatial arrangement of grains). A brief analysis of the compressive failure of concrete is presented,focusing on the basic cell of the phenomenon: a sandwich of two pieces of coarse aggregate, separated by a layer of paste. The thickness of this layer when the pieces of aggregate are of the maximum nominal size is called the 'maximum paste thickness' (MPT). For any concrete mix, a calculation of the MPT is proposed, using the aggregate proportion by volume g, the packing density of the aggregate g* and the maximum size of aggregate D. g* is mainly governed by the grading span of the aggregate, and can be evaluated by using Caquot's law. The significance of the MPT as the secondary factor influencing the compressive strength of concrete is then evaluated by using five sets of data taken from the literature. By plotting the MPT against the ratio of actual strength to theoretical strength (predicted by Feret's law), a fair correlation is found. The MPT incorporates two other parameters, namely the paste volume and the maximum size of aggregate, which are already known to influence the strength of concrete. When this new term is added to the original Feret's law, the mean error of the prediction is halved. A further validation of the law shows that the influence of the MPT applies only to gravels and not to crushed limestone aggregates. This explains why conflicting results have been published on the optimal maximum aggregate size in high-strength concrete mix design (which appears to be,for rounded aggregates, below 20 mm). Finally, microstructural considerations are presented in order to explain why, for certain mixes, the compressive strength decreases when the MPT increases. This finding sheds new light on the influence of aggregate segregation, wall effect and size of moulded specimens on the compressive strength of concrete.