Ultra High Performance Concrete, or UHPC, is one of the latest advances in concrete technology and it addresses the shortcomings of many concretes today: It provides high strength to weight ratio, high tensile strength, high ductility, and volume stability. It has a compressive strength 2 to 3 times greater than conventional concrete and a flexural strength 2 to 6 times greater
These mechanical properties of UHPC make it ideal for prestressing applications. In addition to achieving high compressive strengths in excess of 25,000 psi (sometimes greater than 30,000 psi), UHPC is also nearly impermeable.
UHPC consists of fibre and steel reinforced cement; the cement mixture is composed of Portland Cement combined with silica fume and fine steel filings.
UHPC was developed in Europe in the 1980s for specialised applications that demand superior strength and corrosion resistance , such as for marine applications or seismic structures. More recently, the use of UHPC has expanded to applications requiring its high strength in narrow profiles, such as bridge spans and building facades in which the material’s strength, wear resistance, lighter weight and lower life cycle costs have been the driving determinates.
Why are all these countries taking advantage of UHPC? It’s an material that has a projected 100-year life span with exceptional strength, durability, flexibility and longevity.
UHPC has a compressive strength 10 times that of traditional concrete. Compressive strength is the ability of a material to resist bending under a load (or in compression). Normal concrete used in bridges has a compressive strength of 3,000 to 5,000 psi. UHPC has a compressive strength of 18,000 to 35,000 psi. Another measure of strength is tensile strength or tension. This is how strong a material is when you pull it. While traditional concrete has a tensile strength of 400 – 700 psi, UHPC has a tensile strength of about 1400 psi.
While UHPC’s strength is impressive, it’s durability further exceeds expectations. Durability is measured by how the material performs under extreme conditions. Freeze/thaw resistance, chloride resistance (like in road salts) and abrasion resistance are all part of durability. UHPC has properties similar to hard rock.
Freeze/thaw resistance is tested by subjecting concrete prisms to freezing and thawing while submerged in a water bath. UHPC exhibited 100% of its material properties after 600 freeze/thaw cyles.
Chloride permeability is measured by ponding a 3-percent sodium chloride solution on the surface of the concrete for 90 days. After 90 days, the level of migration of chloride ions into the concrete is determined. UHPC showed extremely low chloride migration when tested, less than 10% the permeability of normal concrete.
Abrasion resistance is determined by measuring the amount of concrete abraded off a surface by a rotating cutter in a given time period. UHPC demonstrates excellent abrasion resistance, nearly twice as resistant as normal concrete.
UHPC in architecture
UHPC can be cast into large panels create visually arresting building facades. A number of different finishes can be incorporated into a single panel, creating a look which would have been very difficult and expensive to install as individual parts. UHPC precisely replicates the pattern and texture of a mould surface, making it possible to create the most detailed designs, or to create natural looking finishes, such as stone, at a fraction of the cost and weight. Decorative 3D shapes and screens can provide both shade and aesthetic value. Architects can also cast three dimensional UPHC parts as permanent formwork for larger structures.