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FEATURE MATERIALS The Route 31 bridge in Lyons, N.Y., is the first in the U.S. crossing to use field-cast UHPC connections. Eight simple span deck bulb-tee girders were joined with non-contact lap splice deck-level connections. COMPOSITION UHPC is an advanced cementitious compos- ite material first developed in the 1990s and commercially available since 2000. It is typically acquired from a supplier in three separate components: pre-bagged cementi- tious powder, steel fiber reinforcement, and chemical admixtures. Water is then added at the construction site, and the UHPC is mixed and placed into the formwork using standard construction equipment. UHPC has an optimized gradation of gran- ular constituents; water-to-cementitious ma- terials ratio less than 0.25; and, a high percentage of discontinuous internal fiber reinforcement. Compared to more conven- tional concrete materials, UHPC exhibits su- perior properties such as exceptional durability, high compressive strength, usable tensile strength, and long-term stability. It generally contains high cementitious mate- rial contents, low water-to-cementitious ma- terial ratios, compressive strengths above 20,000 psi, and sustained tensile strength resulting from internal fiber reinforcement. Buy America provisions are relevant to the steel fiber reinforcement used in UHPC. States planning to use UHPC in projects 40 | SEPTEMBER 2012 should work with their FHWA division office early in the design process to determine the availability of a domestic manufacturer, and if necessary, submit a Buy America waiver request for FHWA's consideration. PRACTICE Field-cast connection design is critical to the overall performance of the structural system. Engineers must consider both service and ul- timate limit states, plus construction practi- cality and the system's long-term durability. "Construction of Field-Cast UHPC" exam- ines the steps involved in constructing con- nections, including design, prefabricated component preparation, formwork, mixing and placing, initial and final curing, surface profiling, and material testing. Critical steps in completing connections between prefab- ricated components include the preparation of the bonding surfaces where the precast component meets the field-cast UHPC. While the material can bond exceptionally well to conventional concrete, the bond strength is highly dependent on the surface of the precast concrete. As with other ce- mentitious grouts, UHPC is not likely to form a strong bond with smooth, dry, pre- cast concrete. An enhanced bond can be ob- tained by using concrete that has an ex- posed aggregate surface finish. The Tech Note provides guidance on mix- ing and placing (note page 39 sidebar). UHPC is sensitive to mixing deviations, so the supplier's specifications for mix propor- tions and timings must be followed. Mixing UHPC requires significant energy in order to disperse the liquids uniformly within the powder matrix. Both portable concrete pan mixers and conventional mixer trucks have been used on projects to mix UHPC. The vol- ume of UHPC that can be mixed is approxi- mately half that of conventional concrete that could be processed in the same mixer. Powered buggies or manual wheelbarrows have typically been used to place field-cast UPHC connections, with connection spaces filled consecutively. UHPC should always be placed into connection spaces so that succes- sive placements are poured into concrete that was recently cast and remains fluid. Although UHPC tends to exhibit long dwell times before it begins to set, once setting begins, strength gain occurs rapidly. The initial setting behav- ior is dependent on the UHPC temperature. While cooler temperatures are beneficial for WWW.CONCRETEPRODUCTS.COM