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Portable pan mixer yields UHPC for field- cast connections on the Keg Creek Bridge in Council Bluffs, Iowa. Tech Note spotlight: Mixing and Placing As with any pre-bagged cementitious com- posite, UHPC must be mixed according to the supplier's specifications, which fre- quently provide information on the con- stituent volumes to be mixed and the order of constituent addition to a partic- ular mixer. UHPC is sensitive to mixing de- viations, so mix proportions and timings must be followed. The addition of water or chemical admixtures beyond those specified can cause detrimental impacts on the early and long-term performance of the field-cast UHPC. The weather conditions surrounding the UHPC prior to and during mixing can impact the fresh properties of the con- crete. The temperature of the UHPC will increase during mixing, and some mix water will be lost due to evaporation. Mixing UHPC under cool temperatures WWW.CONCRETEPRODUCTS.COM and away from direct exposure to sun and wind is advantageous. Maintaining a reduced temperature on the pre-bagged UHPC powders and the mix water is advantageous. The fluidity of UHPC can become reduced, and the likeli- hood of surface dehydration can increase if the mix temperature at the conclusion of mixer cycle exceeds 80°F (26.7°C). Cubed ice has been demonstrated to be a viable replacement for some or all of the mix water under warm conditions. UHPC can be mixed in most concrete or grout mixers. UHPC mixing requires signif- icant energy input in order to disperse the liquids uniformly within the powder ma- trix. Higher energy mixers will complete the mixing process more quickly. Both tow-behind pan mixers and conventional concrete ready-mix trucks have been used to mix UHPC. As a rule-of-thumb, the vol- ume of UHPC that can be mixed in one of these mixers is approximately half of the volume of conventional concrete that could be mixed in the same mixer. The choice of mixer should be deter- mined based on the rate of UHPC deliv- ery required at the construction site. Frequently, a field-cast connection be- tween bridge components may require more UHPC than can be quickly deliv- ered. As such, the connection needs to be divided into smaller sections so that the placement rate allows the UHPC to quickly fill the divided connection space. This requirement is based on the fact that, like conventional concrete, UHPC is susceptible to surface dehydration. — from Construction of Field-Cast Ultra- High Performance Concrete Connections SEPTEMBER 2012 | 39