The AASHTO Standard Specifications for Highway Bridges – Division II and the AASHTO LRFD Bridge Construction Specifications include provisions related to accelerated curing of concrete through the use of elevated temperatures. This article contains a review of some of the provisions and presents proposed changes that are more appropriate for use with high performance concrete.
Curing Temperatures
The current specifications state that the initial application of steam or heat shall be from 2 to 4 hours after final concrete placement to allow initial set of the concrete to take place. If retarders are used, the waiting period is increased to 4 to 6 hours. Since today’s concretes contain a wider variety of constituent materials than in the past, the waiting period of 2 to 4 hours or 4 to 6 hours may not be appropriate. The actual measurement of time of set using AASHTO T 197 (ASTM C 403) entitled “Time of Setting of Concrete Mixtures by Penetration Resistance” for the specific concrete is a more precise approach and should be used for all concretes.
The current specifications have requirements for the rate of temperature rise, maximum temperature, and rate of temperature decrease. These temperatures are defined in terms of the temperature of the curing enclosure. Since high strength concretes generate significantly more heat than conventional strength concretes, it is important that concrete temperatures be monitored rather than enclosure temperatures.
Finally, the specifications do not require immediate transfer of the prestressing force if the ambient temperature is maintained above 60°F (16°C). For precast, prestressed concrete members, the transfer of the stressing force to the concrete should be accomplished immediately after accelerated curing to minimize the likelihood of vertical cracking in the members from thermal contraction. Cracking is more likely in deep members particularly when high strength concrete is used.
Cylinder Curing
When a precast concrete member is steam or radiant-heat cured, the compressive strength test cylinders are required to be cured under conditions similar to the member. Traditionally, this has been interpreted to mean that cylinders placed under the same covers as the member are acceptable. However, for high strength concrete, the heat generated within the member can result in higher temperatures in the member than in the cylinders.* This is particularly true when steam or radiant-heat curing is not used. Since the concrete strength at any point in time is related to the maturity of the concrete, different temperatures result in different compressive strengths. Consequently, match curing is essential for high strength concretes if realistic values for strength are to be measured.
For specified concrete compressive strengths greater than 6000 psi (41 MPa), test cylinders should be match cured in chambers in which the temperature of the chamber is correlated with the temperature in the member prior to release of the prestressing strands.* Temperature sensors for the match curing system should be placed at the most critical locations for release of the prestressing force and for design. After release of the prestressing strands, cylinders should be stored in a similar temperature and humidity environment as the member.
Test Age
For a discussion of the benefits of specifying compressive strengths at 56 days, see HPC Bridge Views, Issue No. 5, September/October 1999.
Editor’s Note
This article is the second in a series that addresses specification changes that are needed to facilitate the implementation of HPC. The proposed revisions are based on work performed as part of FHWA Project No. DTFH61-00-C-00009.
*See HPC Bridge Views, Issue No. 2, March/April