| Summary: | Reinforcement spacers (i.e. bar supports, chairs) are crucial elements of reinforced concrete, but their influence on the microstructure and long-term durability is not clear. This study investigates the effect caused by plastic and cementitious spacers, and steel wire chairs combined with different aggregate sizes, curing and conditioning regimes on the transport properties, microstructure and chloride-induced corrosion of concrete structures. Concrete cylindrical samples were prepared with 25 and 50 mm high plastic, steel and cementitious spacers. Samples were then cured, conditioned and tested for oxygen diffusivity, oxygen permeability, water sorptivity and chloride diffusivity. Selected samples were pressure impregnated with fluorescent epoxy to study the extent and spatial distribution of epoxy intrusion. The interfacial zone between the spacer and concrete was examined using field-emission scanning electron microscope in the backscattered electron (BSE) mode. The ingress of chloride, particularly near the interface between spacers and concrete matrix was studied using micro X-ray fluorescence (μXRF). The effect of plastic and cementitious spacers on chloride- induced corrosion via capillary rise and cyclic wetting/drying was investigated using small reinforced concrete beams. The feasibility of improving the bond between spacer and concrete by increasing surface roughness of plastic spacers was also investigated. Results show that concrete samples containing plastic spacers consistently gave the least resistance to transport and the highest epoxy penetration followed by samples with cementitious spacers, and then steel spacers. The control samples (samples without spacers) had the highest resistance to transport in all cases. The epoxy penetration occurred mainly through the spacer-concrete interface. The microstructure of the spacer-concrete interface showed significantly lower cement content and higher porosity compared to 'bulk paste' farther away form the interface. Higher penetration of chloride ions was detected along spacer-concrete interface compared to the control sample or the bulk paste farther away. It is evident that spacers initiate early corrosion and this may reduce the service-life of reinforced concrete structures. The implications of these findings on durability of concrete structures are discussed. Several recommendations to improve the bond at the interface between spacer and concrete are presented.
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