Influence of coarse aggregate on the permeation, durability and the microstructure characteristics of ordinary Portland cement concrete

Reinforced concrete structures in marine and similar environmental conditions have to withstand severe exposures. Due to this, the mix design procedures have to be revised to include durability as a primary criterion along with strength and workability. In line with this need, an investigation was carried out at Queen's University Belfast with the financial help of the Department of Environment, Transport and the Regions, UK. The main objective of the research was to understand the influence of mix parameters on workability, strength, durability and microstructure and to propose a new mix design procedure within which durability has a higher priority. The present paper reports part of this study, which investigates the effect of size and grading of the coarse aggregate on the durability and the microstructure characteristics of concrete. The durability tests carried out included air permeability test, freeze-thaw/salt scaling resistance test and an accelerated carbonation test. The microstructure study was carried out using a scanning electron microscope (SEM) and included image analysis of the interfacial transition zone (ITZ) around the coarse aggregate. The results indicated that as the proportion of larger size aggregate increases in the mix, the local porosity at the ITZ increases and the overall durability decreases. It has also been found that reducing the average aggregate size in a mix improves the pore structure and enhances the durability of the concrete.