Enhancing the spacer-concrete interface for durable structures
Creators: name: Chen, Xi
Publisher: name: Imperial College London
Year: 2022

description: Cementitious spacers are commonly used in concrete construction. A wide variety of spacers is available. However, the quality of these spacers and their potential impact on the long-term durability of concrete structures are not well understood. Available research on cementitious spacers have shown that the spacer-concrete interface is more porous and microcracked than the bulk concrete. This accelerates penetration of external aggressive species that could be detrimental to concrete durability. Yet very little fundamental research has been carried out on improving the spacer-concrete interface. This study aims at investigating the performance of cementitious spacers and their effects on concrete microstructure, mass transport properties and durability. Additionally, the study aims to develop novel methods to enhance mechanical and/or chemical bonding between spacers and concrete, and therefore improve the long-term durability of concrete structures. Cementitious spacers from top European manufacturers were examined for their dimensional, mechanical and durability properties, and their effects on concrete mass transport properties. Great variations in performance were observed between different spacers, especially in terms of porosity, water absorption and carbonation resistance. Some spacers were already carbonated at the time of delivery prior to usage. The incorporation of these spacers in concrete increased mass transport (e.g. oxygen diffusivity, oxygen permeability and water sorptivity) to various levels. Oxygen permeability was found to be most severely affected. This is attributed to the weak spacer-concrete interface that is highly porous and microcracked. New methods were developed to improve the spacer-concrete interface by enhancing their mechanical and chemical bonding. These include producing novel spacers with rough surface texture, with exposed fibres and/or reactive cementitious particles. Fibre-reinforced mortars (with three fibre types, two fibre lengths and two fibre volume fractions) and slag-based mortars (with two water/binder ratios and three slag replacement ratios) were investigated as alternative materials for producing cementitious spacers. Compressive strength and mass transport properties of the spacers were tested at different curing ages. Composite cylindrical samples containing half spacer and half concrete were prepared and tested for bond strength, mass transport properties, fluorescent epoxy impregnation, carbonation resistance and microstructure. These were compared against samples with conventional spacers. Results showed that the developed cementitious spacers achieved good compressive strength and low mass transport properties. Fibre-reinforced cementitious spacers produced with the new method decreased crack initiation and propagation at the spacer-concrete interface, and this led to lower mass transport properties, especially oxygen permeability. Cementitious spacers containing high slag replacement and low water/binder ratio also improved the spacer-concrete bonding. This was evident from the increased bond strength, decreased mass transport properties and denser microstructure at the spacer-concrete interface.