Thoughts on ultra-high performance fibre-reinforced concrete hereafter referred to as “UHPFRC”​

Elbe Pedestrian Bridge, Czechia - HeidelbergCement Group

Many articles have been written over the years regarding UHPFRC, to my knowledge, it’s been around under certain guises since the late 1980s. Proprietary designed materials from the likes of BCV® (Vinci Group, France), CERACEM® (SIKA, France), CRC® (CRC Technology, Denmark), DENSIT® (DENSIT® ApS, Denmark), DUCTAL® (LaFarge and Bouygues Group, France), EFFIX® (Italcementi Group, France), and SUQCEM® (Kajima, Japan). Some of these products seem to now not exist or have been recycled by new companies using the tradenames.

AFNOR released NF P18-470 in 2016 as when this document was published, there were no international or European projects addressing the same topic. These standards tried to address UHPFRC with its intended use in precast structures and precast structure elements, structures and structure members cast in place, parts of structures fixed by casting in place. The contents were intended to harmonise with the design standards in Eurocode 2.

As of writing, to my knowledge it is the only attempt I have seen to address these types of products with EN 206, Eurocode 2 is not enough for designing structures incorporating them and EN 13670 is not suitable for the execution of structures in UHPFRC on its own.

UHPFRC is generally characterised by having high compressive strength (120-200 MPa), high tensile strength (4-20 MPa) and excellent durability.

An absence of coarse aggregate, meaning above 4mm is often the weak link in high strength concrete. The use of silica fume, which provides ultimate microparticle packing in the concrete matrix, greatly reduces permeability to water and corrosive agents in combination with using a very low water-cement ratio typically <0.20. Steel fibres are included which provides UHPFRC with ductility and tensile strength after cracking. Polypropylene microfibres are added to improve the resistance to early plastic shrinkage and to improve the fire rating in the final state.

The advantages for the client can include lighter construction, resulting in savings to supporting structure and foundations. The reduced weight on columns allows their thickness to be reduced, which in turn results in an increased net lettable area and reduced cost. In reducing the overall mass has major benefits in reducing earthquake loads thus resulting in savings in lateral load resisting systems and foundations. A faster construction programme can be maintained allowing the building to be sold or leased as early as possible, thus minimising interest costs and exposure to inflation.

Even with the higher embodied carbon of the concrete as design, considerable reductions in embodied carbon can be achieved from considerably less material volume and transportation costs. Important considerations of the UHPFRC mix are in its use of silica fume and GGBFS, both of which are by-product materials of other industries and thus have a very low carbon impact. Important considerations to also bear in mind is the excellent durability. End of life the product would be recyclable with its components re-usable in lower grade structural applications. Also, consider the thermal mass of the concrete, this can be utilised to provide climate stability and thus reduce environmental energy demands.

There are severe limitations in developing a technology to produce UHPFRC with locally available raw materials, as in the case from your local Ready-Mix Concrete Supplier. However, with careful constraints, it is feasible to design a concrete having a compressive strength greater than 150 MPa. Utilising the correct grade of steel fibres, a tensile strength of above 20 MPa and fracture energy more than 10 kJ/m². Achieving a low water binder ratio (usually less than 0.20) will give low porosity and very high durability against all environmental exposures.

Some food for thought.