Q: What is concrete?
A: Concrete is a composite material made from coarse aggregate (rocks), fine aggregate (sand), portland cement, and water. When mixed, the portland cement and water combine together in a chemical reaction to create the glue that holds together the aggregates. Concrete starts as a semi-liquid that can be placed or formed. It hardens over time to create a solid that is similar to rock.
Q: What is ultra-high performance concrete?
A: Ultra-high performance concrete (UHPC) is a class of concrete that combines together many of the recent advances in concrete science to create a material with far superior properties. In the fresh state the mix is usually designed to be more fluid than regular concrete thus making it easier to place into the formwork. In the hardened state it is much stronger than regular concrete. It also contains a large amount of randomly distributed steel fibers that help the UHPC maintain its load-carrying capacity even after overloads or in aggressive environments. Finally, the UHPC is much less permeable than regular concrete and thus the UHPC is far less likely to degrade due to freeze-thaw action or corrosion of embedded reinforcing steel.
Q: What are the steel fibers used in ultra-high performance concrete?
A: Imagine that you added a hundred pounds of needles to every ton of concrete. These needles would be randomly distributed throughout the concrete and would help to reinforce it on a micro scale. In practice, about 2 percent of the volume of a UHPC product is steel fibers. These fibers are usually small diameter steel wires that are cut to be less than 1 inch long. The fibers stitch together the UHPC so that, even if the UHPC experiences a large load that causes it to crack, the fibers will allow the UHPC to continue to resist the load.
Q: How does UHPC differ from the concrete normally used in bridges?
A: The normal concretes used today in highway bridges are designed to offer better performance than “sidewalk” concrete, but are prone to cracking and degradation primarily due to corrosion of the embedded reinforcing bars. UHPC offers much superior durability. Through its enhanced strength, it also allows for optimization of the design of the structure. UHPC structures can be easier to construct and can create opportunities to reduce the normal reliance on large volumes of field-cast concrete.
Q: Why is the Federal Highway Administration involved with UHPC?
A: FHWA plays a leading role in bringing innovative solutions to the highway infrastructure sector. By and large, State Departments of Transportation own and operate the infrastructure, but their perspective may not extend to national and international best practices and innovative ideas. FHWA recognized the potential in UHPC in the early 2000s and has, through their research arm at the Turner-Fairbank Highway Research Center, focused on developing deployable solutions that make use of the best that this technology has to offer.
Q: Is UHPC already being used in the highway infrastructure?
A: The first US deployment of UHPC in the highway infrastructure occurred in 2006. Broader deployment started in 2012 and today this technology is carrying loads in more than 300 bridges across the US with dozens more constructed each year. A few of these bridges use UHPC in the main girders that carry the traffic. Many of these bridges use UHPC in field-cast connections between prefabricated bridge components. In short, the UHPC stitches together the factory-constructed parts of the bridge in a way that makes the connections easy to install and at least as robust as the connected pieces. In recent years, there has been growing usage of UHPC as a repair material for rehabilitating deterioration in existing bridges to return them to good service.
Q: If UHPC is so great, why don’t we use it everywhere?
A: The simple answer is cost. Normal concretes cost a few pennies per pound, while the price of UHPC is more like a half dollar per pound. In short, the constituents in UHPC cost more. For this reason, it is important to use the UHPC in critical locations where its benefits reduce short and long term costs elsewhere. Moving forward, it is expected that the cost of UHPC will reduce as the technology matures.
Q: How might UHPC be used in the future to address infrastructure needs?
A: The technological advancements inherent in UHPC both will afford direct solutions to today’s infrastructure challenges and will be integrated into the normal concretes of tomorrow. One of the biggest opportunities for UHPC will be to rehabilitate and harden the highway infrastructure against the insidious effects of traffic loads, an unforgiving environment, and time. UHPC has the potential to allow for the long-term rehabilitation of road and bridge surfaces without requiring full reconstruction. It can also allow for the repair of substandard infrastructure in a way that meets current design requirements while minimally impacting the operation of the facility. Most importantly, it can open the door to longer service lives with reduced need for repair and thus reduced impact on the traveling public.