Question:
What curing is necessary for HPC precast, prestressed concrete beams after the strands are detensioned?
Answer:
Curing involves actions taken to maintain proper moisture and temperature conditions in freshly placed concrete. This allows cement hydration and pozzolanic reactions to occur so that the specified hardened concrete properties such as strength and durability can be developed. Accelerated initial curing of precast, prestressed concrete beams is usually necessary to obtain the specified release strength so that the strands can be detensioned within a reasonable time. The accelerated initial curing is achieved with an elevated temperature provided either by externally applied heat such as steam or from the internal heat of hydration. After this initial curing period, strength gain is much slower.
High performance concrete (HPC) differs from conventional concrete because HPC has a lower water-cementitious materials ratio and contains one or more supplementary cementitious materials (SCM). Therefore, HPC has a much lower volume of water-filled space per unit volume of cementitious materials. During the hydration process, the water-filled spaces or capillary pores are quickly filled with hydration products and discontinuity between the pores occurs. The cement and SCM particles are much closer to each other in HPC and the discontinuity of capillary pores can occur much faster because smaller pores need less hydration products to fill them. Accordingly, as the capillary pores become discontinuous, the permeability of the concrete is reduced and water migration within the concrete decreases significantly. This stage of hydration is achieved early by using elevated temperatures with retention of moisture. Because of the difficulty of water migration through the concrete, additional moist curing beyond this point provides minimal benefit except for possibly improving the durability of the surface concrete.
Experience with conventional strength, precast, prestressed concrete beams has shown that initial accelerated curing alone is adequate to provide both the required release strength and the specified design strength. Data recently collected from completed projects that used HPC for the precast, prestressed concrete beams also verify that adequate release strength and specified design strength can be achieved using the initial accelerated curing method followed by no further curing after the release strengths are obtained. The data also confirm that chloride permeability values below the specified values can be achieved using only the initial accelerated curing.
It is concluded that adequate curing of precast, prestressed concrete beams for strength and durability is provided by initial accelerated curing that is terminated after the release strength is obtained. Additional moist curing beyond the initial accelerated curing period is not necessary and provides only marginal benefits.
References
Related information about the effect of curing temperatures on strength development and data showing that match curing provides the most accurate strength verification method can be found in HPC Bridge Views, Issue No. 2, March/April 1999.
Additional information is available in the following references:
Wang, C., Dilger, W. H., and Langley, W. S., “Curing of High Performance Concrete – An Overview,” Symposium Proceedings, PCI/FHWA International Symposium on High Performance Concrete, New Orleans, Louisiana, Precast/Prestressed Concrete Institute, Chicago, IL, October 1997, pp. 283-293.
Meeks, K. W. and Carino, N. J., “Curing of High-Performance Concrete: Report of the State-of-the-Art,” NISTIR 6295, NIST, March 1999.
High Performance Concrete, Compact Disc, Federal Highway Administration, Version 3.0, February 2003.
Answer contributed by Jerry L. Potter with consultation by Jon Mullarky and Lou Triandafilou, all with FHWA. Jerry Potter may be contacted at [email protected] or 202-366-4596.