Jerzy Zemajtis, CTLGroup
For long-span bridges, predictions of long-term deflections and prestress losses are important design considerations. The accuracy of these predictions can be improved when measured values of modulus of elasticity, creep, and shrinkage of the concrete to be used in the structure are determined. This article describes the ASTM test for concrete creep.
The creep test procedure is defined in ASTM C 512. In the procedure, timedependent strains are measured on concrete cylinders in compression and under laboratory-controlled conditions. The stress level is constant throughout the test and its magnitude is up to 40 percent of the concrete compressive strength. The 40 percent limit is due to the linear relationship between creep and stress in that range. The cylinders are usually loaded at a concrete age of 28 days; however, loading at other ages is also used depending on the application.
One of the assumptions of the method is that the total measured strain is the algebraic sum of elastic strain, drying shrinkage and autogenous shrinkage strains, and the creep strain. The corresponding drying shrinkage and autogenous shrinkage strains are measured on companion non-loaded cylinders that are stored under the same temperature and humidity conditions. It must be emphasized that the companion cylinders are not “shrinkage test cylinders.” When specifications call for a shrinkage test, AASHTO T 160 (ASTM C 157) is usually specified.
Another assumption is that the elastic strain is the initial strain developed on load application. The creep strain is obtained by subtracting the elastic strains and strains measured on companion non-loaded cylinders from the total strains measured on the loaded cylinders.
The concrete specimens used in creep testing are 6-in. (150-mm) diameter by 12-in. (300-mm) long cylinders. The standard provides tight tolerances for the diameter and the length of the test cylinders; thus, special care must be exercised during specimen fabrication and preparation for testing. The gage length is usually equal to 10 in. (250 mm) and cannot be less than 6 in. (150 mm). A minimum of two cylinders are placed into a creep frame and stressed to a desired level. In addition, two non-instrumented half-length cylinders are placed at each end of a stack to eliminate end effects on the strain measurements.
Typical frames are capable of stressing cylinders to about 2500 psi (17 MPa). This means that the compressive strength of these cylinders, if stressed up to 40 percent of their strength, cannot be more than 6200 psi (43 MPa). For concretes with strengths greater than this, high-strength frames must be used. These frames can accommodate concrete with compressive strengths as high as 23,000 psi (159 MPa).
The standard curing requirements for the creep specimens are as follows:
- Remove from molds at 20 to 48 h after casting
- Store in a moist condition at 73.4 ± 3.0°F (23.0 ± 1.7°C) until the age of 7 days (storage in water is not permitted)
- Store and test at 73.4 ± 2.0°F (23.0 ± 1.1°C) and at a relative humidity of 50 ± 4 percent until completion of the test
Other curing conditions and temperature regimes are also permissible, provided they are properly detailed in the report. Simulation of hot or cold weather or different humidity conditions can be accomplished by placing the creep frames in environmental rooms.
The measured strains, after subtraction of the initial loading strains and the shrinkage strains measured on the companion specimens, represent the sum of basic creep and drying creep. Basic creep occurs in concrete exposed to conditions that do not allow any moisture movement between the concrete and the environment (i.e. mass concrete structures). When basic creep is of primary interest, the specimens are sealed in moisture-proof jackets at the time of fabrication or stripping. They are sealed throughout the period of storage and testing. Drying creep is the additional creep that is caused by drying and is measured on unsealed specimens. Note that drying creep is not the same as shrinkage.
The test must be run for at least 90 days for useable results. Longer periods, such as one to three years, are often specified for major structures. The test results are often presented in the form of specific creep or creep coefficient. The specific creep or unit strain is defined as creep per unit stress, and the creep coefficient is the ratio of creep strain to initial strain.
Editor’s Note
This article is the fifth in a series that describes tests for use with HPC. Previous articles appeared in Issue Nos. 36, 37, 39, and 40.