Improving the spacer-concrete interface for durable structures
Creators: name: Muslim, Fadhilah
Publisher: name: Imperial College London
Year: 2018

description: Spacers are important devices in reinforced concrete that are used to support reinforcing steel during concreting in order to achieve the required cover. They are placed at every meter length or less of steel reinforcement and left permanently in the structure. However, very few studies have been carried out on the effect of spacers on the performance of concrete. This work aims to develop a better understanding of the influence of spacers on the microstructure and long-term durability of concrete structures. The work also aims to develop methods to enhance bonding and mechanical interlocking of spacers with concrete. Extensive experimental testing was performed. Concrete samples containing a range of commercially available spacers (plastic or cementitious) were prepared with different binder types including Portland cement CEM I and CEM I blended with silica fume, fly ash or ground granulated blast-furnace slag. Samples containing horizontal or vertical spacers were prepared, cured in a fog room (100% RH, 21°C) for various periods before bond strength measurement. Samples were also conditioned at 75% RH, 21°C or in a 50°C oven for microstructure analysis and transport testing such as oxygen diffusion, oxygen permeation, water absorption, electrical conduction, chloride penetration, and fluorescent epoxy impregnation. Finally, a large number of prefabricated modified cementitious spacers were prepared with a range of mix compositions and surface textures and then cast into concrete for splitting tensile strength and transport testing. Results showed that the inclusion of spacer in reinforced concrete produces a weak, porous and micro-cracked interface regardless of whether the spacer was cast for horizontal or vertical reinforcement. This lowers the resistance of concrete to ingress of aggressive agents. Cross-sections of fluorescence epoxy impregnated samples images under UV light showed a preferential transport along the spacer-concrete interface. Higher penetration of chloride ions was also detected along the interface compared to the bulk paste farther away from the spacer and to the control sample without a spacer. The extent of the effect of spacer depends on the spacer type and binders used, and drying regime. Modifying the spacer surface textures has a clear effect on increasing their interlocking with concrete, and this leads to a significant improvement in the bond strength of the spacer-concrete interface by up to 133%. This was found to decrease the tendency for cracking at the spacer-concrete interface due to differential volume changes, thus leading to a significant decrease in pressure-induced flow. The effect of surface texture on the overall strength of spacer-concrete interface and transport properties is dependent on the roughness, shape, and orientation of the spacer surface texture. The implications of the findings from this study on the durability of concrete structures are discussed. Finally, recommendations on other possible methods to improve both bond strength and durability of the spacer-concrete interface are presented.