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Ultra high-performance concrete offers untapped potential for the U.S. precast market.
By Theresa (Tess) M. Ahlborn, Ph.d., P.E.
What do some structural researchers and transportation engineers view as the most important material technological breakthrough with greatest potential to transform product design and service life in the U.S. precast concrete industry? What new concrete technology has the potential to effect a major change in the way engineers and specifiers conceive of and design precast systems? The latest available research from both academia and transportation agencies points to ultra high-performance concrete (UHPC) as the most promising catalyst for new precast concrete product designs.
Relatively new to the United States, UHPC belongs to a family of concretes with remarkable compressive strengths that exceed 150 megapascals (MPa), or 22,000 pounds per square inch (psi) – more than four times the strength of conventional concrete. To date, the only large-scale implementation of UHPC in bridge superstructures has been initiated by the U.S. Federal Highway Administration. In other countries including Canada, Japan, France, Australia, New Zealand, South Korea and Germany, numerous exciting and innovative precast concrete systems have taken advantage of the unique material characteristics of UHPC.
What has prevented U.S. specifying engineers from taking advantage of untapped potential of UHPC designs? For the answers, let’s take a look at the material properties of UHPC, current research, production techniques, factors that limit full adoption of this new material technology in the United States, and the outlook for UHPC.
What is ultra high-performance concrete?
UHPC is a family of concretes offering a combination of material and performance characteristics that create products with:
UHPC mixtures contain portland cement, silica fume, quartz flour, fine silica sand, high-range water reducer, water, and steel or organic fibers. Materials are usually supplied to the precast concrete producer in three components: premix (blended, in-bulk bags); superplasticizer; and fibers. The microstructural properties of UHPC’s mineral matrix produce a highly compact, dense material within a low porosity matrix.
Research results for UHPC curing protocols similar to those of producers
Precasters and engineers need information on mechanical behavior variations for structural elements that are thermally treated at different concrete ages. Producers and specifiers also need data that are more representative of actual conditions and production techniques found in existing precast plants. Michigan Technological University (Michigan Tech) has conducted research into the effects of curing regimes and specimen age on the strength and durability properties of fiber-reinforced UHPC.
At Michigan Tech’s Cement & Concrete Research Facilities in Houghton, four different curing regimes were tested to determine the effects of curing on UHPC’s material characteristics, including compressive strength, freeze-thaw resistance, flexural cracking strength and chloride penetration resistance. The four curing regimes tested are:
1. 48-hour thermal steam treatment at 90 degrees C (194 F) and 100 percent relative humidity
2. Ambient air curing
3. Delayed (10-day delay before curing) thermal steam treatment
4. Double-delayed (24-day delay before curing) thermal steam treatmentThese curing regimes are representative of how precasters may manufacture products, choosing to cast products over several days and thermally treat together, or not thermally treat at all.
All UHPC test specimens were demolded at 70 hours and stored at room temperature until one of the four curing regimes was applied. The following standards were used with minor testing modifications for UHPC: ASTM C39 for compressive strength; ASTM C469 for modulus of elasticity; ASTM C1018 for flexural cracking strength; ASTM C666 for freeze-thaw resistance; and ASTM C1202 for chloride ion penetration. Here are some results of the study:
The significant impact of these findings for the precast manufacturer is that precasters may be able to produce several UHPC elements over a short time period and then cure them at a later time, providing more flexibility in casting and curing sequences. Further research is being conducted to determine how creep and shrinkage behavior under variable curing regimes may affect the final product. Testing is also simulating production sequencing typical of U.S. precast operations.
Use of UHPC by the Federal Highway Administration
Since 2001 when the Federal Highway Administration (FHWA) began its UHPC research program, two bridge superstructures have been built with UHPC I-beams and more are under construction or in design. Benjamin A. Graybeal, Ph.D., P.E., research structural engineer with the FHWA, is an expert on UHPC research and its use in transportation infrastructure. “The FHWA has made great strides in the past five years in beginning to use UHPC in the U.S. highway system,” explains Graybeal, “however, we still have a long way to go. Currently, each project using UHPC is still treated as a special case, because this material is a relatively new concrete technology, particularly in the U.S. As American precast concrete producers become more familiar with UHPC and the nuances of its production, I believe we will see an increased use of this material, particularly in bridge applications.”
In terms of widespread application of UHPC, the most likely significant market segment lies in precast bridge deck elements. More than 150,000 bridges in the United States are deemed structurally deficient or functionally obsolete. Because UHPC has enhanced mechanical properties and exceptional durability, using it to create prefabricated, lightweight, long-lasting bridge decks is something in which Graybeal sees great promise. “And obviously,” continues Graybeal, “using UHPC for prestressed bridge girders is a possibility that we have already begun to realize with the Mars Hill Bridge in Wapello County, Iowa, and the Cat Point Creek Bridge (presently under construction and scheduled to open to traffic this fall) in Richmond County, Virginia.”
By reducing or eliminating the need for steel reinforcing bar cages, UHPC with steel or organic (polyvinyl alcohol) fibers can produce bridge decks not susceptible to chloride ingress (from deicing salts) and the subsequent corrosion-induced deterioration. UHPC optimized designs and material properties in bridge decks can mean longer service life and lower maintenance costs for America’s transportation infrastructure.
Factors affecting a broader use of UHPC in the U.S.
Implementation of UHPC in the United States is progressing slowly for three reasons:
1. Lack of U.S. design codes for UHPC
2. Risk perception and lack of familiarity with UHPC
3. Initial costThe greatest challenge limiting the use of UHPC by American specifiers and precast producers is that current design codes are not readily adaptable to this class of concrete possessing strengths many times that of conventional concrete. Continued research is needed on the advanced properties of UHPC to provide a valid database for structural design.
Even at the federal level where UHPC research has been conducted, officials perceive the risks associated with a greatly expanded use of a product with a limited history of performance. State highway engineers, in particular, are hesitant to use new technology without a significant history of proven performance in large part because of their responsibility for public transportation safety. This understandable aversion to the risk of specifying and manufacturing products with the relatively new UHPCs also drives up its cost. Whenever a new technology is perceived as risky, whether due to lack of knowledge, producer comfort level or history of use, market forces in any industry will increase the price of using that technology.
How can specifiers adapt existing standards to work with UHPC?
While other countries like Canada, Germany and Japan have already begun to take advantage of the unique material characteristics of UHPC, U.S. engineers, designers and specifiers are interested in the applicability of current AASHTO (American Association of State Highway and Transportation Officials) and ASTM (American Society for Testing and Materials) testing standards for UHPC.
According to Graybeal, specifying engineers can “apply the current ASTM and AASHTO standards in a general sense” to UHPC projects. Since these standards were developed for conventional concretes, however, using them on UHPC can sometimes result in nonsensical results.
Because UHPC is so new, Graybeal explains, the “body of ASTM/AASHTO standards has not yet been developed to adequately cover UHPC.” For the specifying engineer, this becomes a situation of understanding the limits of applicability of the existing standards when specifying UHPC on a project and then choosing the standards accordingly. As an example, ASTM C39 works for specifying the compressive strength of UHPC, but research has demonstrated that the test can be run at a faster rate and using 3-inch diameter cylinders instead of conducting a much slower test on 4-inch or 6-inch cylinders as called for in the specification.
Another example how specifiers need to be discerning in applying current standards to UHPC is that of using ASTM C78 to test for flexural strength. ASTM C78 provides nonsensical results for UHPC’s flexural tensile strength, because UHPC is fiber-reinforced and exhibits flexural post-cracking tensile strength greater than its flexural initial-cracking tensile strength. A final example is ASTM C666, which would seem to indicate that applying cycles of freezing and thawing to UHPC can increase its freeze-thaw resistance. In reality, the test method is causing the apparent increased resistance through continued hydration and mass gain. UHPC exhibits excellent freeze-thaw resistance, however, with durability factors that easily surpass those of conventional concrete.
UHPC interim design codes can be developed for transportation infrastructures as research continues. Engineers and specifiers will need to understand optimal designs using this innovative material in a host of potential precast applications such as superstructures, substructures, sound barrier walls, crash and blast protection systems, precast pavements and hybrid systems. While design codes can take several years to be approved, interim recommendations are anticipated within the next few years.
Green innovation and the future of UHPC
While the current LEED (Leadership in Energy and Environmental Design) rating system lacks credits for ranking the numerous green benefits of precast concrete, points can often be achieved through the “Innovation in Design” category (see pages 4-9, Precast Solutions, May-June 2008 issue). UHPC can further promote green benefits with CO2 emission reductions. In addition, UHPC presents the opportunity for potentially lower embodied energy due to significant material reduction (up to 40 percent less weight) in optimized members. Long-term service life and expected minimal maintenance are material characteristics that lend themselves to reduced life cycle costs for structures. This enhanced durability needs to be factored into sustainable solutions to offset the currently higher costs of UHPC.
UHPC is a leap forward in concrete technology for durable highway structures. These concretes offer dramatic improvements in physical and mechanical properties over conventional concretes and are virtually impermeable to the harsh environmental factors that bridges throughout the United States are exposed to every day. UHPC has important advantages over conventional and high-performance concretes, especially in the production of smaller and lighter sections and the potential elimination of passive mild steel reinforcement. While UHPC’s greatest potential currently appears to be in transportation infrastructure, it is too early to predict what new solutions to design challenges will emerge as engineers and specifiers learn more about this new technology.