John J. Myers and Ramon L. Carrasquillo, The University of Texas at Austin
Temperature development during concrete hydration and curing conditions 180dramatically impact both mechanical and material pro p e rties of high strength/high performance concretes. During the fabrication of precast, prestressed beams for two HPC bridges in Texas, the temperature development during hydration was monitored to investigate the effect of concrete temperature and curing conditions on concrete compressive strength.* A commercially available match-curing system was utilized during the production of the precast, prestreessed beams to more closely investigate the concrete properties within the members and to evaluate the use of match-curing technology as a quality control (QC) tool in the precast industry.
Figure 1 illustrates concrete temperature s during hydration for a 54-in. (1.37-m) deep U-beam used in the Louetta Road Overpass. The results are re p resentative of all the U-beam casting dates with minor variations due to ambient conditions during casting. The U-beams have end blocks that range in thickness from 18 to 48 in. (455 to 1220 mm).
Temperature profiles for four locations in the member are shown in Fig. 1. The bracketed value following each monitoring location indicates the height of the therm o-couple from the base of the member in inches. In addition, the temperature of a s t a n d a rd QC member- c u red cylinder is presented. Member- cured cylinders are cure d adjacent to the precasting bed prior to release of the prestressing strands. To date, m e m b e r- c u red cylinders are used as the standard QC specimen by the Texas Department of Transportation to determine when the prestressing strands may be re l e a s e d .
Figure 2 illustrates the compressive strength of 4×8-in. (102×203-mm) cylinders at release (24 hours) and 56 days for different curing conditions. The first and second pairs of data are for specimens that were moist cured per ASTM and member cured, respectiely. The other four pairs of data re p resent match-cured specimens corresponding to diff e rent locations in the U-beams. The match-cured specimens were s t o red with the beams following initial cur¬ing. Clearly, the curing condition that attained the highest early temperature dis- played the highest release strength, but the lowest 56-day strength.
Both the ASTM moist-cured and member-cured cylinders underestimated the strength of the concrete within the member at release, but overestimated the concrete strength within the member at later ages. For the data shown in Fig. 2, the member-cured cylinders underestimated the strendth of the concrete in the member at telease by as much a 8 persent compter to the match-cured specimens. The ASTM moist-cured specimens overestimated the concrete strength at 56 days by as much as 17 percent. Therefore, the strength of the member is typically underestimated at release, but overestimated at later ages if ASTM moist-cured or member-cured cylinders are used for strength verification.
Further Information
Further information about the Texas HPC bridges is available in Proceedings of the PCI/FHWA International Symposium on High Performance Concrete (1997) available from PCI and in the following two reports:
Myers, J. J. and Carrasquillo, R. L., Production and Quality Control of High Performance Concrete in Texas Bridge Structures, Center for Transportation Research, The University of Texas at Austin, Preliminary Research Report 580/589-1, to be published.
Gross, S. P. and Burns, N. H., Field Performance of Prestressed High Performance Concrete Highway Bridges in Texas, Center for Transportation Research, The University of Texas at Austin, Preliminary Research Report 580/589-2, to be published.
*The two bridge projects were co-sponsored by the Texas Department of Transportation and the Federal Highway Administration .