SECTION 2

UHPC Background

UHPC is a concrete mixture consisting of formulations that often combine portland cement, fine sand, silica fume, high-range water-reducing admixture, fibers (usually steel), water, and various chemical admixtures. The two main advantages of UHPC over conventional concrete are increased strength and durability. The higher cement content of UHPC (approximately 30% compared with 15% to 20% in conventional concrete) and the use of chemical admixtures and steel fibers contribute to these improved properties. UHPC is stronger than conventional concrete, with a compressive strength of at least 21.7 kips per square inch (ksi) compared with 3–6 ksi in conventional concrete and a tensile strength greater than 0.72 ksi compared with around 0.5 ksi in conventional concrete. UHPC also has a discontinuous pore structure and a slightly lower porosity, leading to lower permeability and significantly reduced water infiltration. Reducing water infiltration protects the steel fibers from corrosion and reduces cracking caused by the freeze–thaw cycle concrete is exposed to in cold-weather climates.

Early interviews and FHWA data on UHPC use showed that most UHPC applications in the United States have been for deck-level connections. The FHWA database of UHPC deployments in the United States indicated that UHPC had been used in approximately 200 projects across the country through 2018 (FHWA 2019b). For reference, about 5,800 bridges were constructed each year in the United States between 2006 and 2012 (FHWA 2013). Of the 200 UHPC deployments, 185 included connections and 179 were for deck-level connections only. The other UHPC uses were beam and joint repairs, beams and girders, and roadbed overlays (Figure 2-1). Although these other UHPC uses, particularly overlays and repairs, have become more common in recent years, the vast majority of UHPC use in the United States as of 2018 has been for deck-level bridge connections.

Desk research and stakeholder interviews indicated that UHPC-C adoption is tied largely to the use of PBEs and accelerated bridge construction (ABC), a method of building bridges from PBEs or prefabricated bridge units that include a precast deck and supporting members, such as steel or concrete girders connected on site, and is quicker and reduces traffic delays. UHPC has also been used on precast deck projects even when ABC methods are not used.

The benefits of UHPC have the potential to increase the life span of bridges and decrease maintenance costs but are countered by substantially higher materials costs, though lower-cost mixes are being developed.¹ The combined benefits and costs make UHPC particularly beneficial

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¹ The U.S. UHPC market was previously dominated by a single proprietary mix, Ductal, produced by LafargeHolcim. After the patent for Ductal expired, more companies entered the U.S. UHPC market, and universities and state departments of transportation began researching generic mixes. As of 2021, four commercial UHPC suppliers and a growing number of research projects were dedicated to creating generic UHPC mixes.

for bridge connections (Figure 2-2), which require high performance across a small amount of material. Recently, UHPC link slabs have also been used to eliminate deck joints in new construction and rehabilitation projects.

Several owner agencies indicated they maintained a general convention, if not a standard, for using UHPC-C when adopting PBEs for bridge decks. However, no interviewees said their state had broadly adopted PBEs and ABC methods. The states generally saved those methods for high-traffic areas where construction disruptions would be particularly burdensome.