Mineralogic and petrographic evaluation of aggregate quality – effect on compressive strength of concrete pavement

Abstract

ABSTRACT The causes of low compressive strength in newly placed slip-form concrete pavement were investigated using stereo and transmitted polarised light microscopy and scanning electron microscopy (SEM). The poor-quality, low-strength concrete is attributed to cracks and gaps that fully or partially surrounded unsuitable, weathered, and altered basic and intermediate volcanic gravel coarse and fine aggregate particles and lack of bond with the cementitious paste. Weathering and mineral alteration was more severe at the rims of affected coarse particles, causing thin weathering rinds of weak minerals at aggregate surfaces. SEM X-ray elemental mapping showed rock-forming elements such as sodium, magnesium, calcium, and iron were present in higher concentrations in the interior portion of the aggregates; but occurred in lower concentrations at the rims, suggesting that these elements leached out during weathering processes. This study shows the importance of periodically evaluating aggregates for deleterious materials and overall quality so that changes in the quality of the mined and processed aggregate can be detected before incorporation into concrete. Also, it is important that materials engineers who select or approve aggregates should have a petrographer evaluate new or changed concrete aggregate sources for unstable minerals and the nature of any surface coatings that can affect concrete-making properties.