What is UHPFRC?
UHPFRC as the name suggests is a mix of ultra high performance concrete with a percentage, by volume, of steel or synthetic fibres, normally between 2-3%. Ultra high performance concrete is characterised by a water to binder (W/B) content below 0.2, which is achieved by the addition of a superplasticiser (Morin et al. 2001) along with an increase in binder content (Resplendino 2011). Aggregate size is also very small when compared to conventional concrete, with the largest size being in the order of 0.5-1.0 mm. Particular attention is also made to the mechanical strength of the aggregate in order to avoid weak points in the concrete. Further additions such as microsilica, which is 5-10 times smaller than cement particles and has a specific surface area 50 times greater than cement (Resplendino 2011, Habel et al. 2006, Yu et al. 2014, Spasojevic 2008), helps to reduce the capillary porosity. It also helps to improve the interfacial transition zones between the binder and aggregates and between the binder and fibres. Cement particles can also be substituted with Ground Granulated Blast-furnace Slag (GGBS) to help produce a more sustainable and economical material. The resulting material is extremely compact with remarkable mechanical and durability performances.
The curing process consists of heating the material to a high temperature (90oC) at a humidity close to saturation for approximately 48 hours. This process can significantly increase the durability of the concrete, reduce dry shrinkage and creep. UHPFRC has a very long dormant phase prior to setting but when setting begins there is a quick increase in resistance (Resplendino 2011, Habel et al. 2006, Yu et al. 2014, Máca et al. 2014).
High strength concrete can easily achieve compressive strengths greater than 150 MPa but the failure mechanism is very brittle. In fact the failure in compression can be explosive with no evidence of a plastic domain. UHPFRC can achieve tensile strengths in the range of 8-11 MPa and above. The addition of fibres help to provide a non-brittle mechanism in bending and significantly increase the fracture toughness. This is achieved by the fact that when cracks form in the concrete the fibres allow stress to be transferred between the crack planes (Olesen 2001). As cracks start to propagate in the concrete, stresses are transferred from the concrete matrix to the steel fibres and produce an increase in stress with additional strain. This increase in stress with strain is known as pseudo strain hardening. As cracks begin to open the steel fibres act to bridge the gaps in the concrete until they yield or pull out of the matrix. This causes a reduction in stress with strain known as pseudo strain softening. (Naaman & Reinhardt 2003). Fig. 1 shows the tensile stress-strain relationship for UHPFRC.