Ultra High Strength Concrete (UHSC) may be defined as concrete with specified characteristic cube strength of 50N/mm2 and above. Strength levels of 50 N/mm2 and even higher are being used for both precast and in-situ works. The main applications for UHSC in-situ concrete construction are in offshore structures, columns for tall buildings, long-span bridges and other highway structures. The main advantage is the reduction in size of compression elements and/or the amount of longitudinal reinforcement required.
The methods and technology for producing high strength concrete are not substantially different from those required for normal strength concrete. The target water/cement ratio will be in the range of 0.30–0.35 or even lower.
Superplasticisers / high range water reducers are used to achieve maximum water reduction. Silica fume (microsilica) or metakaoline can be used to enhance the strength at high levels.
UHSC is required in engineering projects that have concrete components which must resist high compressive loads. UHSC is typically used in the erection of high-rise structures. It has been used in components such as columns (especially on lower floors where the loads will be greatest), shear walls, and foundations. UHSC are also occasionally used in bridge applications as well.
In high-rise structures, high-strength concrete can be used. Not only has this special concrete made such projects feasible due to load capacity, it has also allowed the reduction of column and beam dimensions. Lower dead load results in reducing the loads associated with foundation design. Also, owners are also benefited economically since the amount of rentable floor space, primarily on the lower floors, increases as the space occupied by the columns decrease.
UHSC is occasionally used in the construction of highway bridges. UHSC permits reinforced or prestressed concrete girders to span greater lengths than normal strength concrete girders. Also, the greater individual girder capacities may enable a decrease in the number of girders required.
UHSC resists loads that cannot be resisted by normal-strength concrete. Not only does high strength concrete allow for more applications, it also increases the strength per unit cost, per unit weight, and per unit volume as well. These concrete mixes typically have an increased modulus of elasticity, which increases stability and reduces deflections.