What is UHPC (ultra-high performance concrete)?

UHPC, ultra-high performance concrete, is an extremely dense concrete with a compressive strength that is typically around or above 150 N/mm² (150 MPa): several times that of ordinary structural concrete. With fibres it is often called UHPFRC. It goes a big step further than high-strength concrete.

Mix design

UHPC gets its strength from the densest possible particle packing:

• Very low water-cement ratio: in the literature usually below 0.25, often around 0.15 to 0.20.
• High cement and silica fume content: the ultrafine silica fume fills the gaps between cement grains.
• Fine aggregate: usually quartz sand and quartz flour instead of gravel, with a small maximum grain size.
• High dose of superplasticizer: without it, a mix with so little water would be unworkable.
• Fibres: often short steel fibres, which mainly improve tensile strength and ductility.

Difference from high-strength concrete

High-strength concrete is ordinary concrete in a higher strength class that still uses gravel. UHPC is a different material: no coarse aggregate, much higher strength and, thanks to the fibres, tensile capacity even after cracking. That is why UHPC designs can sometimes use little or no conventional reinforcement.

Application

• Slender precast elements such as stairs, balconies, facade panels and thin shells.
• Bridge decks and footbridges with a low self-weight.
• Joints and connections between precast elements.
• Repair and strengthening, as a thin overlay on existing concrete.

Points to watch

• Specialist work: mix, mixing, placing and curing are critical. UHPC is mostly produced in a factory, not in a simple site mixer.
• Cost: much more expensive per cubic metre than normal concrete; the benefit comes from using less material and slimmer shapes.
• Design: design rules differ from ordinary reinforced concrete. Use a structural engineer with UHPC experience.

Related terms

• High-strength concrete
• DSP concrete
• Silica fume
• Superplasticizer
• Compressive strength
• Concrete strength class

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.