The Incredible Possibilities of Using Ultra-High-Performance Concrete
Ultra-high-performance concrete (UHPC) isn’t necessarily a brand new product to the industry, but recent advances will make it more accessible, and that’s an exciting prospect for architects and engineers alike. Originally invented in France, UHPC was introduced first in the early 1990s in two classes: Class 200 MPa (29 ksi) and 800 MPa (116 ksi). Despite its amazing properties, the high cost of the material limited the applications mostly to I-beams and decks in state bridge technology projects.
Pre-bagged UHPC exceeds $2000 per cubic yard. The addition of metal fibers increases the price to a total of nearly $3000 per cubic yard, depending on the amount and source of the wire. It’s no wonder the use has been limited to longitudinal joints and low volume to minimize the effect on the cost of a project. Architects and engineers could only imagine the impact this material would make on modern structural design across the industry. Not anymore.
A three-year program has begun that will culminate in guidelines for precasters, designers, and owners. Cost-effective high strength concrete mixtures having the same qualities as Reactive Powder Concrete (RPC) and UHPC materials have been produced in tests conducted between state highway agencies and local universities. At the same time, the Precast/Prestressed Concrete Institute (PCI) partnered with six major concrete companies to develop their own mixture. The results use the latest technology to create precast, pretensioned concrete beams in lengths up to 250 feet for office and residential buildings, parking structures, and bridges.
The new high strength mixture proportions result in a total materials cost of about $600 to $800 per cubic yard and finally make it a competitive product. Trial designs have also shown potential savings in about 50 percent of the concrete volume, affecting weight, shipping costs, erection, foundation, temporary supports, and more. If only 50 percent of the amount is used with UHPC, the price per cubic yard can double without exceeding the conventional concrete cost. The savings buffer would be more than enough to cover the cost of production and justify the risk of using a relatively new material.
How UHPC Works
This cement-based composite material contains steel fiber reinforcement (2% by volume) to enhance high compressive and tensile strengths and durability due to a discontinuous pore structure. There are different standards for ksi, but the PCI research project specifies 10 ksi at the time of application and 18 ksi after 28 days. Not only do the 360 ksi brass-coated steel fibers increase tensile strength far beyond that of conventional concrete, but the ability to flex is above 2 ksi at peak value eliminating the need for shear reinforcing bars.
A conservative estimate assumes that the fibers contribute about 1000 psi of shear strength resistance, while concrete resistance is roughly 100 to 400 psi. Even after adding conventional steel reinforcement, it’s capped at about 800 psi. It should be noted that long-term camber is not much different from the initial camber, but live load deflection can be a critical design parameter and must be carefully assessed.
The ingredients in UHPC vary but consist of Portland cement, silica fume, supplementary materials such as fly ash, silica powder or slag, and fine sand grains no larger than 0.03 inches. The components are proportioned to produce the highest particle density. A low water to cementitious material ratio of 0.16 to 0.20 remains highly flowable with the aid of special admixtures.
Structural engineers and architects are being challenged to step forward with creative solutions to overcome some hurdles in commercial production. Several countries around the world have already published recommendations, which will be helpful as U.S. codes and standards are updated to allow for this game-changing material.
Examples of High Strength Concrete Use
Piles. UHPC has a higher compressive capacity and better resistance to pile driving effects. The result is a reduction in weight by more than half while maintaining flexibility. Conventional concrete spirals may not be necessary with the use of high tensile strength fibers. The material toughness gives it the ability to absorb more energy.
Decked Bridge I-Beams. A conventional bridge system may have a deck spanning up to 180 feet. A UHPC-decked I-beam system can span up to 265 feet while using a fraction of the total concrete volume. It is possible to increase the span, reduce the weight, and eliminate shear reinforcement.
Parking Structure Inverted Tee Beams. A commercial garage with beams spanning 60 feet and supporting double tees in perpendicular directions has the potential to be one foot shallower and 65 percent lighter. Again, it is possible to eliminate rebar and count on the fibers for shear and torsion support.
UHPC is expected to reach a new level of applications in the next five to ten years. It will become a viable economical alternative with a lesser life cycle cost due to extreme durability. The new guidelines being developed will include bridge and building codes, as well as standards with examples for architects and engineers to reference. It will enable the design of more massive column-free expanses and take the industry well beyond the current structural limits.
1LaFarge Cement Company currently markets a trade named product called Ductal©
REFERENCES:
- Tadros, Maher K., Sevenker, Adam, and Berry, Rick, "Ultra-High-Performance Concrete." STRUCTURE magazine, April, 2019, Ultra-High-Performance Concrete, https://www.structuremag.org/?p=14370
Photo Credit: Pixabay.com
