Morphological characteristics of roller-compacted concrete mixes containing reclaimed asphalt pavement aggregates

In this paper, the morphological characteristics of roller-compacted concrete (RCC) mixes containing reclaimed asphalt pavement (RAP) aggregates was studied using a scanning electron microscope (SEM). The SEM investigation was performed on the fractured RCC specimens containing 50% RAP (denoted as 50RAP mix). SEM investigation validated that the asphalt-cohesion failure in the case of RAP-concrete mixes will be predominant in reducing the mechanical strength even though additional calcium silicate hydrate (C-S-H) gels are provided by the fly ash particles. Meanwhile, the morphology of the 50RAP mix is almost similar to that of the conventional concrete, except that it consists of two types of interfacial transition zone (ITZ): a normal ITZ occurring between the natural aggregate and the mortar while another ITZ occurring between the RAP and the mortar. Hydrated C-S-H phases with fibrous, needle-like, reticulated, small discs, and tapered fibers morphologies were also observed in the SEM micrographs of the 50RAP mix. The highly porous ITZ of the RAP-concrete relative to the ITZ of the conventional-concrete has a negative impact on the strength of the RAP-concrete but the softening of asphalt at higher temperature clogs the empty pores and lowers the porosity of the concrete. The presence of needle-like ettringites were also identified in the mix containing RAP which may also be one reason for causing expansion in the RAP-concrete. In a nutshell, the utilization of RAP in RCC should be limited to paving applications only rather than in structural concrete. © 2020, Associated Cement Companies Ltd.. All rights reserved.