Bonding in cementitious materials with asphalt-coated particles: Part I – The interfacial transition zone

Reclaimed asphalt pavement (RAP), when used as a coarse aggregate, has been shown to reduce bulk concrete strength and modulus. Part I of this study quantifies and compares the interfacial transition zone (ITZ) for mortar with RAP aggregates relative to dolomite aggregates through image analysis of backscattered electron micrographs. The ITZ with RAP aggregates was larger and more porous with less calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) at the asphalt interface compared with dolomite aggregates. The CH morphology was not significantly affected, although the presence of the asphalt layer may be affecting the CH growth. The addition of silica fume reduced the porosity and size of CH particles in the ITZ with RAP, but not sufficiently to be similar to the ITZ of the dolomite mortar. The microstructural changes caused by RAP aggregates, primarily the larger, more porous ITZ, provide strong evidence for the observed reduction in concrete strength and modulus.