UHPFRC - Optimisation of mix proportions: concrete platform

Concrete commonly used for civil engineering construction has a compressive strength of between 30-50 MPa. High strength concretes with a compressive strength of up to 120 MPa, or even 300 MPa for special applications, have been developed but they still have a brittle failure behaviour and a poor tensile strength. Ultra high performance fibre reinforced concrete (UHPFRC) has been recently developed, with a high compressive strength (170–230 MPa) and much improved ductility and tensile properties (typically 25–60 MPa). These properties are achieved by using a high cementitious binder content, careful design of the particle packing for maximum homogeneity, elevated temperature curing and a high dosage of fine steel fibres. This paper describes a project focusing on improving and optimising the mix procedures for UHPFRC. Partial cement replacement with undensified silica fume and other cementitious materials, such as ground granulated blast furnace slag (GGBS), and pulverised fuel ash (PFA), combined with a polycarboxylate based superplasticiser has been studied to reduce the water required for workability and hence increase the compressive strength. Fresh and hardened concrete properties of UHPFRC were influenced by the excess paste volume; a 20% excess paste volume was considered to be the optimum. Flow table measurements indicated that 160 to 200mm in diameter was needed for UHPFRC to be easily compacted into moulds. Water-binder ratios as low as 0.15 are required for UHPFRC. The optimum silica fume dosage was found to be 10% by weight of binder. Up to 50% GGBS or 30% PFA can be used as a partial cement replacement without significant affecting the mechanical properties. The effect of steel fibre dosage on the compressive strength, flexural strength, fracture energy and flexural toughness has been investigated. The addition of steel fibres was found to enhance the compressive as well as the tensile strength of UHPFRC. However, the use of a fibre dosages in excess of 3.5% by volume did not improve the flexural behaviour significantly.