Quantitative analysis of the microstructure of interfaces in steel reinforced concrete

This article reports the results of a backscattered electron imaging study of the microstructure of the steel– and aggregate–cement paste interfaces in concrete containing 9 mm ribbed reinforcing bars. The water to cement (w/c) ratio, hydration age, steel orientation, and surface finish were varied. For vertically cast bars, there was more calcium hydroxide (CH) and porosity and less unreacted cement at both the steel– and aggregate–cement paste interfaces when compared to the bulk cement paste. As the hydration age increased, the porosity near the interfaces decreased, and the CH increased with more CH close to the steel than to the aggregate. Horizontal bars had more porosity and less CH under them than above. An increase in the w/c ratio produced interfaces of higher porosity and lower levels of CH. Wire-brush cleaned bars had higher levels of CH at the steel–cement paste interface at 365 days when compared to uncleaned bars.