Study of rock–mortar interfaces. Part I: surface roughness of rock aggregates and microstructural characteristics of interface

Abstract

This two-part paper reports the results of an extensive study on the interface between cylindrical rock aggregates and mortar matrix with particular emphasis on the influence of the surface roughness of the rock and the density of the matrix on the thickness of the interface and on its strength. In Part I of the paper the surface roughness of three rock aggregates commonly used in concrete (sandstone, limestone and granite) is measured by means of the Talysurf apparatus on 100 mm long cored cylindrical specimens of various diameters. The roughness is expressed through a roughness index Ra. It is found that the sandstone aggregate has the highest surface roughness, followed by limestone and granite in that order. Investigations are also conducted, using scanning electron microscopy, on how the microstructure of the interface and its thickness are affected by the addition of silica fume to the mortar. A chemical analysis of the constituents in, and in the vicinity of, the interface was also undertaken with a view to identifying the interfacial zone which is distinctly free from sand particles. It was found that the relative thickness of the interfacial zone reduces when silica fume is added to the mortar matrix. For all mortar and rock types the bond was found to be mechanical rather than chemical in nature. Part II of the paper studies experimentally and theoretically the influence of the surface roughness of the rock aggregate and density of the mortar matrix on the strength of the rock–mortar interface.