Compressive strength or Performance?
Today, with standards and guidelines for UHPC in several countries around the globe, UHPC is still too often classified as a concrete with a compressive strength of >150 MPa or, depending on the standard or guideline, >120 MPa and a tensile strength greater than 5 MPa. When the strength is the most important mechanical property, we should perhaps use the classification Ultra High Strength Concrete.
Pouring of large vault door with Ultra High Performance Concrete
In general, people assume that any concrete with a compressive strength above 150 MPa is an UHPC and that all UHPC’s have the same ultra high performance (properties wet and cured concrete). However, even when the same compressive strength is reached there can be large differences between the properties of the UHPC products and/or their suppliers. This is extremely confusing since many still believe that it is all the same grey stuff (sometimes white but that is even more complicated). Also, in project tenders confusing or even wrong descriptions are used. For example, in the tender description for the façade panels of an embassy 3 acceptable manufacturers and products were described but no properties were mentioned. As a basis of design manufacturer, they use two products of a well-known supplier, however, both products have a compressive strength in the range of 110 – 130 MPa while the other two suppliers had different properties of their material. On another page they refer to another product, thus product 3, of the same supplier with properties which were completely different, this was clearly UHPC, and including metallic fibers. Further, they demanded, related to potential bomb attacks, a limiting deflection so the windows behind cannot be impinged and that the UHPC panels stay intact rather than breaking into pieces significant enough in size to become hazardous projectiles. UHPC can be used extremely well to resist high impact and even ultra-high impact of explosives as already demonstrated by Bache with CRC panels, but this demands other performance and another fixation of the panels than described in the tender documents.
Very slender precast Hybrid Reinforced Ultra High Performance Concrete bridge panels
Ultra thin Hybrid Reinforced Ultra High Performance Concrete overlay on concrete viaduct
It is thus too often the case that there is some misunderstanding and confusion and that the UHPC specified or the UHPC product made, cannot fulfill the task, or function it was designed for according the high – or ultra high standards. Unfortunately, there are some recent examples of this misunderstanding between supplier and client and/or not delivering according to the standards on large projects. Or, what as often happened, that the UHPC supplied is performing too well and thus actually too expensive and unnecessary for the specific application.