What is Ultra-High-Performance Concrete?
Ultra-high-performance concrete (UHPC) is an advanced concrete with high compressive and tensile strength with superior durability. ACI 239 defines UHPC as “concrete that has a minimum specified compressive strength of 22,000 psi (150 MPa) with specified durability, tensile ductility, and toughness requirements; fibers are generally included to achieve specified requirements” (ACI 239R-18, 2018).
UHPC was introduced by Richard and Cheyrezy in 1994 as a reactive powder concrete (RPC) which achieved 29,000 psi (200 MPa) of compressive strength. The fundamental principles of the development of RPC were:
· Enhancement in homogeneity: A reduction of the size of the coarse aggregate enhances homogeneity of concrete because the size of cracks that occur at the paste/aggregate interface depends on the diameter of the aggregates used. For RPC, the size of the coarse aggregates was reduced by a factor of approximately 50 (i.e., 1-inch aggregate was reduced to #30 sand).
· Reduction in porosity and improvement in microstructure: An increase in packing density improves the mechanical properties of the paste and reduces porosity.
· Ductility enhancement: Tensile ductility and toughness are enhanced by microfibers.
UHPC Mixture Design Compared to Conventional Concrete
There are many proprietary UHPC mixtures available in the market, and many agencies such as Federal Highway Administration (FHWA) and Department of Transportation (DOT) have developed nonproprietary UHPC mixtures using locally available materials. Figure 1 shows a common proportion of constituent materials of UHPC compared to conventional concrete(CC). UHPC consists of a larger volume of cementitious materials including cement, silica fume, and other SCMs, less water which leads to a very low ratio of water to cementitious materials (w/cm), more fine aggregates, high-range water reducer (HRWR), and approximately 2% volume of steel fibers which has a 0.5 in long generally.
Properties and Durability of UHPC
ASTM C1856 “Standard Practice for Fabricating and Testing Specimens of Ultra-High-Performance Concrete” modifies the existing standard test methods for testing properties of UHPC. Table 1 shows the test methods and the expected range of fresh and hardened properties of UHPC.
· Fresh Properties: UHPC is a flowable concrete like self-consolidating concrete (SCC). Thus, it measures a static flow spread value per ASTM C1437instead of a slump. The density of UHPC (145–155 lb/ft3) is similar to CC but slightly heavier due to the presence of steel fibers. The setting time varies from 7 to 24 hours depending on the mixture design. If a UHPC mixture is designed with a low w/cm and a large amount of HRWR, the mixture will have a delayed setting time. However, setting time can be adjusted by w/cm, HRWR content, or accelerator admixture.
· Hardened Properties: The compressive strength of UHPC is generally 8,000–12,000 psi at 24 hours and 18,000–22,000 psi at 28 days. Some UHPC mixtures are designed to have steam-curing for 48 hours at an early age (i.e., 2 days) which improves the microstructure of UHPC. As a result, it achieves a high compressive strength (i.e., 30,000psi) and removes shrinkage development after steam-curing. A nonproprietary UHPC mixture developed by Texas DOT’s study achieved 16,000 psi at 24 hours for prestress application (Hong et al. 2024). A high modulus of elasticity (MOE) is beneficial for a structure member requiring high stiffness that reduces deflection and displacement. A high tensile strength due to steel fibers improves the flexural and shear behavior of a UHPC girder. In addition, the high shear capacity due to the tensile strength can lead to a thinner web of a girder (i.e., 4 in. web thickness of UHPC pi-shaped girder compared to 7-8 in. web thickness of CC I-shaped girder). It was reported that the self-weight of UHPC girder is 40% less than that of CC. It makes a UHPC girder achieve a longer span (328 ft) (Voo et al., 2014).
· Durability: The biggest advantage of UHPC is its superior durability. UHPC is free from alkali-silica reaction (ASR) and delayed ettringite formation (DEF) due to the absence of available water in its paste and a denser matrix. It is well known that UHPC has a high resistance to freeze-thaw, scaling, and abrasion. In addition, UHPC has a longer service life (> 150 years) compared to CC (~75 years) due to its very low penetrability. Thus, UHPC would be a great choice for a structure near the shoreline and the watertightness required.
Cost
As discussed above, the use of UHPC in the construction industry gives a lot of advantages, but an expensive material cost is an obstacle to being used widely. Generally, the cost of nonproprietary and proprietary UHPC mixtures is 5 and 10 times more expensive than CC, respectively. Even though UHPC’s material cost is expensive, it can be a cost-effective solution as below:
· Structural Elements: First, a UHPC member can be designed with a thinner geometry due to its advanced mechanical properties. As a result, a lesser volume of concrete is needed, thus, less self-weight for a structural element. It makes a UHPC member achievable for a longer span. It leads to a reduced number of girders or columns for a structure. Second, reinforcements can be reduced or eliminated. Texas DOT study shows a comparable flexural and shear capacity without shear reinforcements for a pretensioned UHPC girder (Hueste et al.2023). This reduces costs related to reinforcement such as labor, fabrication, and materials. Lastly, the life cycle cost of UHPC is cheaper than CC due to a longer life span with less maintenance cost (i.e., a CC structure with a75-year life span needs demolition and reconstruction for 150 years in service. However, a UHPC structure can last for 150 years without reconstruction).
· Deck Overlay: UHPC has a high bonding strength with existing concrete. In addition, development length of reinforcement is recommended for UHPC which is pretty short compared to CC (FHWA-HRT-19-011,Design and Construction of Field-Cast UHPC Connections). This characteristic along with the high durability makes UHPC an attractive deck overlay or repair material. Even though the material cost of UHPC is expensive, when a thin thickness of UHPC overlay is considered, it is a cost-effective solution compared to the common overlay solutions and deck replacement as shown in Table 2.
Applications
· Bridge: There are more than 400UHPC bridge projects in the US including prestressed bridge girder, deck, deck overlay, connections, steel girder end repair, and substructures. Figure 2shows a map marked with UHPC bridges in the US constructed by 2021.
· Retrofit and Repair: Structures that were designed before the development of the modern design provisions have insufficient capacity such as seismic resistance. In addition, deteriorated structures need to be repaired to enhance strength and durability. UHPC has been adopted for retrofitting and repairing materials. Figure 3 shows a links lab application for a bridge deck in New York State.
· Other applications: UHPC has been used for architectural structures such as façade elements and canopies. In addition, it was used for water treatment facilities with a thinner wall thickness by utilizing its low permeability.
Twining Concrete Insight Services for UHPC
Twining Concrete Insight provides the following services to clients with AASHTO laboratory accreditation for UHPC (ASTM C1856)
· Laboratory trial batch and material performance evaluation for proprietary and nonproprietary UHPC.
· Nonproprietary UHPC mixture development using local materials.
· UHPC Mixture optimization for cost-effectiveness.
· Robustness evaluation of the combination of cementitious materials and chemical admixtures.
· Mixing procedure optimization for existing facilities of precast and concrete batch plants.
· Consulting and training personnel for mixing, transporting, placing, and curing of UHPC.
· Structural behavior improvement by enhancing orientation and dispersion of steel fibers.
About the Author
Hyeonki Hong, PhD, Concrete Engineer
Dr. Hyeonki Hong has expertise in both concrete materials and structural design. As a concrete engineer, he performs the evaluation of concrete materials including cementitious materials and aggregates, the performance evaluation of various types of concrete including UHPC, and the development of a thermal control plan for mass concrete. Before joining Twining, as a structural engineer, he has designed both concrete and steel structures such as blast-resistant buildings, chimneys, equipment support structures, and shelters in many countries.
