ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/697527
C O N C R E T E B R I D G E T E C H N O L O G Y 36 | ASPIRE Summer 2016 commonly used name and definition in the United States is the one introduced by Graybeal 3 : "UHPC-class materials are cementitious-based composite materials with discontinuous fiber reinforcement, compressive strengths above 21.7 ksi (150 MPa), pre-and post-cracking tensile strengths above 0.72 ksi (5 MPa), and enhanced durability via their discontinuous pore structure." In comparison, conventional concrete is without fibers and typically has a compressive strength of 4 to 10 ksi. The ingredients of a UHPC mixture can vary. Early mixtures generally consisted of about 1200 lb/yd 3 (700 kg/m 3 ) of portland cement, 25% silica fume, 25% silica powder, and fine sand with maximum grain size of 0.03 in. (0.8 mm). A very low water-binder ratio of 0.16 to 0.20 was used. For flowability, a large quantity of high-range water- reducing admixture must be used. Steel fibers in the amount of about 2 to 2.5% by volume are used. The fibers are cut from very fine, 360 ksi (2500 MPa) wire. Other mixtures have been developed; for example, Tadros et al. 4 report on a mixture that uses local aggregates and has a cost that is about 10% of the cost of early UHPC mixtures. However, this mixture does not strictly meet the definition of UHPC because the compressive strength is only 18 ksi (124 MPa). Factors Inhibiting Widespread Use of UHPC The original prebagged UHPC product introduced to the U.S. market had tight tolerance specifications. The steel fibers had to be imported from abroad, which required a waiver of Buy America requirements for many projects. As a result, the unit cost was relatively high. In addition, the UHPC was expected to be mixed in high-energy mixers for 8 to 17 minutes, plus another 10 min. for loading the mixer and unloading the mixture into a ready-mix truck or other transportation devices. However, Graybeal 3 has reported that mixing of UHPC can be performed using conventional mixers, as long as high energy input is provided. Temperature of the mixture, due to increased mixing time, can be controlled through use of ice water. The steel fibers are now available from a manufacturer in the United States. Upon placement, the early development of UHPC called for curing for at least 48 hours at a high, 90°C (194°F), temperature. Some of the original mixtures were also required to be cured in high-pressure chambers. This is inconsistent with standard practice of 12- to 16-hour, overnight curing with maximum temperatures of 70°C (158°F). Loss of productivity and high materials costs could result in a premium of 400% or more of the cost of conventional concrete. This sharp increase cannot be offset by the anticipated reduction in total quantities. Wille et al. 5 have demonstrated that an optimized mixture can achieve the required strength without the originally required heat or pressure curing. An effort is urgently needed in the United States to publish American Association of Highway and Transportation Officials (AASHTO) specifications for design and construction with UHPC. Australia, France, Japan and most recently Switzerland have already published design recommendations and model code language. The Malaysian Experience Introduction of UHPC in Malaysia was started by a couple of engineers in 2006. The company DURA was co-founded by Dr. Yen Lei Voo after he completed his Ph.D. in Australia on the topic of UHPC. His advisor was Professor Stephen Foster, who had been championing UHPC in Australia. Interestingly, the use of UHPC in Australia has stagnated since the construction of its first bridge, the Shepherd Gully Creek Bridge, Ultra-high-performance concrete (UHPC) was first introduced as reactive powder concrete in the early 1990s by the French contractor Bouygues. 1 When introduced, it came in two classes, Class 200 MPa (29 ksi) and 800 MPa (116 ksi). Since then, much research has been performed by the Federal Highway Administration (FHWA) 2 and researchers in other countries around the world, including Australia, Austria, Canada, Croatia, France, Germany, Italy, Japan, Malaysia, the Netherlands, New Zealand, Slovenia, South Korea, Spain, Switzerland, and the United Kingdom. In the United States, several state depar tments of transportation have expressed interest in using UHPC in their bridge projects, supported by FHWA research as well as that done by their local universities. Most notably, Virginia has produced I-beams with UHPC and Iowa has built two bridges with UHPC beams and one with a UHPC deck. A significant interest has recently been directed at using UHPC in longitudinal joints between precast concrete beams. I t a p p e a r s t h a t t h e hig h c o s t o f UHPC has discouraged owners from implementing use of this outstanding material in applications beyond the initial demonstration projects, most of which had been subsidized by government technology implementation programs. The exception to this trend has been the significant success of the company DURA Technology (DURA) in Malaysia. Over 70 bridges have been built by DURA in that country since 2010. This article provides a summary of the steps taken by DURA to develop solutions with UHPC that are cost-effective on a first-cost basis. When the superior durability of UHPC is factored in, its value increases dramatically. What is UHPC? There is no universal definition of UHPC or even its name. It appears that a Taking Ultra-High-Performance Concrete to New Heights The Malaysian Experience by Dr. Maher K. Tadros, e.construct.USA LLC and Dr. Yen Lei Voo, DURA Technology