Question: Issue No. 57 of HPC Bridge Views included an article about ultra-high performance concrete (UHPC) in Iowa. How were the beams cured for the Jakway Park Bridge in Buchanan County?
Answer: The pi-girders in the Jakway Park Bridge were made with a particular type of ultra-high performance concrete (UHPC) known as Ductal®. This is also sometimes known as reactive powder concrete. The beams were cured in two stages.
The first stage involved curing at ambient temperatures, although steam curing up to 115ºF (46ºC) would be allowed in a similar manner to curing precast, prestressed concrete beams. The pi-girders were covered with insulating tarps and kept at ambient temperature until match-cured cylinders indicated a compressive strength of 5100 psi (35 MPa). Then the forms were released but left in place. Curing at ambient temperatures continued until the compressive strength of match-cured cylinders attained 14,500 psi (100 MPa). At that point, the forms were removed and the strands were detensioned.
The second stage of curing then began with thermal treatment applied to the UHPC beams with moisture present. The goal was to have a thermal treatment of approximately 190ºF (88ºC) applied along with relative humidity of at least 95% for at least 48 hours. Thermal treatments have been shown to enhance not only the strength of the member but the durability as well. The beams were wrapped with insulating tarps and steam was released underneath the girders. The temperature was increased gradually over a period of approximately six hours. Once the second curing period was completed, the curing temperature was decreased gradually over a period of approximately six hours.
Measured compressive strengths for one set of beams were 5400 psi (37 MPa) at 28 hours, 14,900 psi (103 MPa) at 50 hours, and 32,400 psi (223 MPa) after the second stage curing. For more information on the curing and material properties of this type of UHPC, see the Federal Highway Administration Report “Material Property Characterization of Ultra-High Performance Concrete,” Report No. FHWA-HRT-06-103 and Michigan Technological University’s Report for Michigan DOT “Ultra-High Performance Concrete for Michigan Bridges, Material Performance-Phase 1,” Report No. MDOT RC-1525.
The answer to this question was originally published in the May 2009 issue of Bridges – an E-newsletter published by the Portland Cement Association.