Precast Solutions - July/August 2008 - (Page 6)

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: • Thermally treated specimens attained average compressive stresses greater than 200 MPa (29,000 psi) independent of onset of curing. • Air-cured specimens at 28 days had a compressive stress of 165 MPa (24,000 psi). • Thermally treated and air-cured specimens showed negligible chloride ion penetrability. • All specimens were freeze-thaw resistant even though UHPC does not include air entrainment. • Compressive strength, Poisson’s Ratio and modulus of elasticity did not vary after UHPC was thermally treated. (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 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. 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 USE OF UHPC BY THE FEDERAL HIGHWAY ADMINISTRATION Since 2001 when the Federal Highway Administration reinforcing bar cages, UHPC with steel or organic (polyvinyl alcohol) fibers can produce bridge decks not susceptible to chloride ingress (from deicing salts) and Table 2. Range of material characteristics of UHPC with steel fibers. Strength Compressive Flexural (equivalent) Youngs Modulus Direct Tension Mean Values 150-200 MPa 25-40 MPa 50-55 GPa 10-15 MPa Durability Courtesy of Lafarge North America Mean Values 100% <10 g/m2 1.2 <10-20m2 V10 <0.5 mm Freeze/thaw (after 300 cycles) Salt-scaling (loss of residue) Abrasion (relative volume loss Index) Oxygen Permeability Cl permeability (total load) Carbonation depth 6 PRECAST SOLUTIONS | JULY/AUGUST 2008

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Precast Solutions - July/August 2008 Contents What You Should Know About UHPC A Winning Hand Fresno Federal Expanding U.S. Wind Power Precast Concrete Pipe

Precast Solutions - July/August 2008

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