Microstructure of four-graded roller compacted concrete

This paper presents an experimental study on microstructure of four-graded RCC. Two temperatures of 20 degrees C and 40 degrees C were picked to represent interior and surface layers of four-graded RCC. The most used fly ash contents of 50% and 70% were selected, and two ages of 28 days and 56 days were tested. Three levels of paste, mortar and concrete were used to simulate different microstructural regions in four graded RCC. 3D digital microscope experiment, micro-hardness test and back-scattered electron image analysis were carried out sequentially. It was found that four-graded RCC microstructure can be divided into three characteristic parts: bulk paste, interface transition zone around fine aggregate (1TZ-f) and interface transition zone around coarse aggregate (ITZ-c). By comparing the average wear width and depth, the strength and bonding ability of bulk paste was the highest, followed by ITZ-f and ITZ-c. Compared to the surface layer, the micro-mechanical strength of bulk paste and ITZ-f was higher in the interior layer, but that of ITZ-c was opposite. At early age, 50% fly ash content was more conducive to microstructure than 70%. The micro-hardness value of ITZ-c presented a decreasing trend with age and that of ITZ-f did not grow. In terms of product structure, Ca(OH)(2) content in ITZ-c gradually decreased from aggregate surface to paste part, and Ca(OH)(2) in ITZ-f had been partially converted to C-S-H gel. The proportions of hydration product and porosity were highest in ITZ-c, followed by 1TZ-f, with bulk paste having the lowest. Microstructure of four-graded RCC was significantly affected by the combination of aggregate size and free water content on aggregate surface. (C) 2018 Elsevier Ltd. All rights reserved.