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ULTRA-HIGH PERFORMANCE CONCRETE (UHPC)

Ultra-High Performance Concrete (UHPC) is a solution developed to address some of the main design, service life and life cycle costing issues associated with the use of concrete.

Ultra High Performance Fiber Reinforced Concrete is a steel fiber reinforced concrete consisting of an optimized gradation of fine powders and a very low water to cementitious materials ratio. It exhibits enhanced strength, durability and ductility properties when compared to normal concrete or high performance concrete.

UHPC is steel fiber-reinforced reactive powder concrete that typically displays twice the compressive strength of any High Performance Concrete (HPC) used in United States bridge construction. The French firm Bouygues SA developed the reactive powder concrete originally, which is engineered to be a highly compacted concrete with a small, disconnected pore structure that helps to minimize many of the limitations of typical HPCs. These advancements are achieved through a combination of finely ground powders and the elimination of coarse aggregates. The addition of small steel fibers to the mix is responsible for much of the tensile strength and toughness of the material.

These fibers eliminate the need for mild reinforcing steel in the girders and knit the material together after cracking has occurred. The placement and curing of UHPC can be performed using procedures similar to those already established for use with some HPCs. The fluid mix is virtually self-placing and requires no internal vibration. If required, external form vibration causes the mix to flow smoothly into place.

Following an initial set of 24 hours, the curing process requires at least an additional 48 hours, including a vapor bath at a constant 88 °C (190 °F). Hence it is available for loading within a period of 3 days as compared to almost 30 days in the case of normal concrete.

Table-1 enlists the various constituents of the UHPC used for the girders, which are more or less the same as other concretes except for the proportions. Glenium 3000 NS was the superplasticizer recommended by UHPC premix manufacturer Lafarge, while Rheocrete CNI was recommended as the accelerator. The steel fibers were 0.2 mm in diameter and 13 mm. in length and were added at a ratio of 2% by volume.

It can be seen that the amount of water is very small, in comparison to the large amounts cementitious materials and superplasticizers. The ratio of water and accelerator to cementitious material (i.e., cement and silica fume) is 0.15.

Numerous other countries already employ UHPC in different structural applications. Canada and South Korea have used UHPC for pedestrian bridges: the Sherbrooke Bridge in Quebec, Canada, built in 1997, a pedestrian bridge of 190 ft. with a deck thickness of only 1.25 in.; and the footbridge of Peace in Seoul, Korea, built in 2002. The span of this bridge is 400 ft. and its deck thickness is 1.25 in. Portugal has employed it for seawall anchors, Australia has committed to its use in a vehicular bridge, and France has used it in building power plants. In all of these cases, the material was chosen for its ability to stand up to high stress, both environmental and load-related. The increasing deployment of UHPC worldwide and results of FHWA’s testing the product bode well for its future use.


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