Jon I. Mullarky and Leif Wathne, Federal Highway Administration
The use of unbonded caps on test cylinders is becoming an increasingly popular part of the procedure to determine the compressive strength of concrete. The ASTM Standard Practice for Use of Unbonded Caps in Determination of Compressive Strength of Hardened Concrete Cylinders (ASTM C 1231) previously in effect, cautioned that unbonded caps are not to be used for acceptance testing of concrete with compressive strengths above 7000 psi (48 MPa). The current version of the Standard Practice has a limit of 12,000 psi (85 MPa). The limitation of 7000 psi (48 MPa) had created a difficulty for many testing laboratories, including the Federal Highway Administration’s (FHWA) Mobile Concrete Laboratory (MCL), that routinely use unbonded pads in the testing of lower strength concretes and would like to use the same procedure for high strength concretes. The difficulty is further complicated by the lack of commercially available capping compounds that are suitable for use with high strength concretes.
Previous Research
According to published literature, there is little agreement about the best method for the end treatment of high strength concrete cylinders. Some research suggests that pad caps are the most suitable, since strength results are typically slightly higher and significantly less variable than results using sulfur caps.(1) Pistilli and Willems concluded that, for high strength concretes, use of pad caps resulted in slightly higher and less variable strengths than with sulfur caps.(2) However, their preferred end treatment was grinding, since sulfur caps were unsuitable at strength levels above 13,000 psi (90 MPa). Others believe that surface grinding is the most suitable method for high strength concretes with compressive strengths above 10,000 psi (69 MPa).(3) ASTM C 1231 implies that, for concretes with compressive strengths up to 12,000 psi (83 MPa), strengths measured on specimens with sulfur caps and ground ends are the same, since the qualification requirements allow for comparisons of pad cap strengths to either of the two methods.
One common thread in all of this literature, however, is that the most appropriate method for providing plane cylinder surfaces for high strength concrete is grinding.
FHWA Evaluation Program
The FHWA’s MCL is often called upon to support the implementation of high performance concrete programs by state highway agencies. Routinely the MCL tests high strength concrete. The staff of the MCL, with assistance from the Turner-Fairbank Highway Research Center, recently conducted a small-scale investigation to determine if grinding or the use of pad caps was appropriate for compressive testing of high strength concrete.
Twenty-eight 4×8-in. (102×203-mm) cylinders were cast from a single batch of high strength concrete. Mixture proportions for a silica fume concrete used in a previous, unrelated investigation were used for the test program. The concrete was known to reach a compressive strength of 12,000 psi (83 MPa) in 36 days. Test specimens were molded and cured in accordance with ASTM C 192.
All specimens were tested in accordance with ASTM C 39. Prior to testing, the cylinders were randomly divided into three groups of nine cylinders. One group was capped with a high strength sulfur capping material. Ends of the second group were ground to meet the requirements of ASTM C 39. The third group of cylinders was tested using 70 durometer neoprene pads.
Unbonded and ground specimens were paired with sulfur capped specimens and evaluated using the statistical method outlined in Section 9 of ASTM C 1231. There was no significant difference in compressive test results between the group receiving sulfur end treatments and the group tested with neoprene pad caps. There was, however, a significant difference between the average values for cylinders tested with pad and sulfur caps and the average values for cylinders prepared with ground ends. Grinding led to a strength reduction of approximately 15 percent compared to the reference specimens with sulfur caps and unbonded caps. Variability, as measured by the standard deviation, was approximately twice the variability of the reference specimens.
Conclusions
For the high strength mixture studied, the specimens tested with neoprene pads and sulfur caps had higher strengths and lower variability than companion specimens with ground ends. Consequently, the neoprene pads met the qualification guidelines of ASTM C 1231, and are appropriate for use in testing concrete with compressive strengths up to 12,000 psi (83 MPa). However, the difference between the results of the specimens with ground ends and the specimens with sulfur caps led to doubts about the assumption in ASTM 1231 that both methods provide a basis for comparison. The test results clearly indicate the importance of considering different end preparation methods and selecting the most appropriate one for use in testing throughout the project.
References
- Carrasquillo, P. M. and Carrasquillo, R. L., “Evaluation of the Use of Current Concrete Practice in the Production of High-Strength Concrete,” ACI Materials Journal, Vol. 85, No. 1, January-February, 1988, pp. 49-54.
- Pistilli, M. F. and Willems, T., “Evaluation of Cylinder Size and Capping Method in Compression Strength Testing of Concrete,” Cement, Concrete, and Aggregates, CCAGDP, Vol. 15, No. 1, Summer, 1993, pp. 59-69.
- Burg, R. G., “Compression Testing of High Strength Concrete,” HPC Bridge Views, Issue No. 6, November/December 1999, pp. 3.