The following letters were received concerning the article entitled “Capping Cylinders for Testing High Strength Concrete,” in Issue No. 14.
Nicholas J. Carino, National Institute of Standards and Technology (NIST)
In 1994, NIST and FHWA published the results of a study on the effects of testing variables on the measured strength of concrete cylinders.(1) Two concrete mixtures were used: an ordinary strength mixture of about 6500 psi (45 MPa) and a high strength mixture of about 13,000 psi (90 MPa). End preparation consisted of sulfur capping and grinding. An industrial grinder manufactured by the Blanchard Machine Co. was used to grind the ends of the cylinders. A total of 48 cylinders were tested with each end condition. The average strength of the ground cylinders was about 2 percent higher than those with sulfur caps. However, for the 13,000 psi (90 MPa) concrete, the measured strength for some of the ground cylinders was as much as 6 percent higher than for the capped cylinders.
In a study(2) by the National Ready Mixed Concrete Association (NRMCA), the strengths of cylinders with ground ends were compared with the strengths of cylinders with two different types of sulfur caps and cement paste caps. The Blanchard grinder was also used in this study. Nominal compressive strengths were 7,000, 11,000, and 17,000 psi (48, 76, and 117 MPa). Measured strengths as a percentage of the strength of cylinders with ground ends are given in Table 1. The NRMCA study demonstrated that sulfur capping compound could be used successfully to test high strength concrete if the caps are 1/8 in. (3 mm) thick and the sulfur compound is allowed to harden for seven days before testing. In several cases, the strength of the cylinders with sulfur caps exceeded the strength of the ground cylinders. The difference, however, was less than three percent.
The 15 percent lower strength of the ground cylinders obtained in the FHWA study appears to be unusual. A comparativ study should be performed with cylinders prepared with the Blanchard grinder and those prepared with the grinder used in the FHWA work. There are obviously differences between the two grinding operations and we need to understand the nature of the differences.
Richard D. Gaynor, Formerly NRMCA and Chairman ASTM Task Group C 09.61
The most recent version of ASTM C 1231-00, entitled “Standard Practice for Use of Unbonded Caps in Determination of Compressive Strength of Hardened Concrete Cylinders,” includes the requirement that qualification tests are required for concrete strengths from 7,000 psi (50 MPa) to 12,000 psi (80 MPa). Use of unbonded caps is not permitted at strengths above 12,000 psi (80 MPa). The FHWA tests are consistent with this latest change and, hopefully, other users will be encouraged to make qualification tests at even higher strength
levels.
In his letter, Dr. Carino refers to the testing at NRMCA. The 2-in. (50-mm) cubes made of the sulfur capping materials were tested at ages from 2 hours to 28 days. Both materials showed appreciable strength gain between 6 hours and 7 days as shown in Table 2. The 7000 psi (50 MPa) concretes were capped with sulfur at least 2 hours before testing as permitted in ASTM C 617-94. As shown in Table 1, concrete strengths were reduced 2 percent with thin caps and 5 to 7 percent with the 1/4-in. (6-mm) thick caps. However, when 11,000 and 17,000 psi (76 and 117 MPa) cylinders were capped 7 days before they were tested, the thin caps provided strengths equal to the ground specimens. With the 1/4-in. (6-mm) thick caps, the results were satisfactory at 11,000 psi (76 MPa) but not at 17,000 psi (117 MPa).
The current ASTM C 617-98 has ightened the requirements for sulfur mortars. When sulfur mortar is used for cylinders stronger than 5000 psi (35 MPa) the cylinders are to be capped at least 16 hours before testing. For concrete strengths greater than 7000 psi (50 MPa), the manufacturer or user of the capping materials must provide qualification test data indicating that test results using the capping material indicate compliance with requirements. It would be helpful to know how much qualification testing has been done and whether the 16-hour requirement has been adequate to obtain the required concrete strength performance.
Ronald G. Burg, Construction Technology Laboratories, Inc.
The authors’ findings that high strength concrete (HSC) test specimens had higher and less variable measured compressive strengths when tested with neoprene pads or sulfur caps as compared to ground ends is contrary to what is reported in most of the published literature. PCI Committee Report 363.2R-98, Guide to Quality Control and Testing of High-Strength Concrete, states “the problems associated with capping can be eliminated by grinding the ends of test cylinders with equipment made for that purpose” and goes on to state “cylinders with ends prepared by grinding have less variable test results and a higher average strength for concrete stronger than 70 Pa (10,000 psi).” The writer’s own experience with testing HSC suggests that both grinding and capping of HSC test specimens can produce statistically equivalent measured compressive strengths when the appropriate capping compound is used and particular care is taken in preparing the ends of the specimens for test.
In an inter-laboratory study, that included concretes with nominal strengths of 9,000, 14,000, and 18,000 psi (62, 97, and 124 MPa), measured compressive strengths of specimens with capped ends were 100.1, 97.0, and 99.1 percent of the measured strengths of specimens with ground ends for each respective strength level.(3) This study also found that capping compound with the higher 2-in (50-mm) cube strength resulted in measured concrete strengths that were significantly less than those of companion specimens with ground ends. Clearly, the issue of quality control testing for HSC is an important one for which additional work is needed. The differing results obtained by this writer and Mullarky and Wathne emphasizes that we don’t fully understand the complexities of a seemingly simple test upon which many important decisions are based. I encourage more work in this area so that the industry can develop technically sound testing standards for high strength concrete.
Peter G. Snow, Burns Concrete, Inc.
For the LDS Conference Center in Salt Lake City, the engineer required a modulus of elasticity of 7 million psi (48 GPa). To achieve this value, a concrete with a compressive strength of 17,000 to 18,000 psi (117 to 124 MPa) was required. This raised the question about which capping system to use. Comparison testing prior to construction using 4×8 in. (100×200 mm) cylinders indicated a standard deviation for ground cylinders of 650 psi (4.5 MPa) whereas the standard deviation with neoprene caps of 70 durometer hardness was less than 300 psi (2.1 MPa). These results were developed for multiple batches of concrete as opposed to the single batch utilized in the FHWA study. Based on the data, pad caps were selected for the capping system.
Authors’ Response
The results of this small-scale investigation were surprising to the authors. Consistent with most literature, the authors expected the ground cylinders to have a higher strength and lower variability than the specimens with either of the other two end conditions. The opposite occurred. Comments suggest that the lower strengths may be related to how the cylinders were ground, and that end grinding is an issue that deserves more attention, particularly in the context of high strength concrete. For this study, cylinders were ground using Humboldt’s Endgrinder IV Model H2965—a machine made specifically for the purpose of grinding the ends of test cylinders. Every cylinder was checked for both planeness and perpendicularity prior to testing and met the requirements of ASTM C 39. Sulfur caps were made 24 hours prior to test with Forney’s HI-CAP having a nominal strength of 9000 psi (62 MPa).
It should be noted that the neoprene caps met the qualification guidelines of ASTM C 1231 only because the guidelines ignore strength differences between the two capping methods when the strengths of the neoprene capped specimens are greater than those of the ground (reference) specimens. A better understanding of the impact of different grinding methods is needed before allowing ground specimens to be used as qualification reference specimens. A substantial amount of qualification and verification testing is required to use sulfur mortar or neoprene caps for high strength concrete, whereas end grinding requires none.
This study, as well as others mentioned in the discussion, suggests that sulfur caps and neoprene caps improved the precision of the test when compared to ground specimens. The implications of lower variability on HPC mixture design and quality control should not be ignored.
The authors agree with Mr. Burg’s assessment that the complexities of a seemingly simple test are not fully understood and we encourage further work to establish technically sound testing standards for high strength concrete. A comparative study of the effects of end grinding is currently being discussed between FHWA, NIST, and Virginia Transportation Research Council.
Editor’s Comment
The original intent of asking the authors to write an article was to answer the question—”Can unbonded neoprene caps be used to test high strength concrete?” The authors’ results seem to indicate that the answer is Yes. However, the qualification procedure of ASTM C 1231 is based on the assumption that cylinders tested with ground or capped ends provide a “true” measure of the cylinder strengths. As indicated by the test results in the original article, this may not always be the case, even though all appropriate procedures were apparently followed.
The ASTM qualification procedure is silent on what to do when the strengths of the cylinders with unbonded caps are substantially higher than the strengths of cylinders with ground or capped ends. Since the industry is now capable of producing concrete strengths well in excess of the 12,000 psi (85 MPa) used in the authors’ tests, there is a need for both unbonded caps and capping materials that can be used for concrete cylinder strengths above 12,000 psi (85 MPa). At the same time, a national research program is needed to answer the questions raised in this discussion so that the cylinder test can continue to be used with confidence for high strength concrete.
References
- Carino, N. J., Guthrie, W. F., and Lagergren, E. S., “Effects of Testing Variables on the Measured Compressive Strength of High-Strength Concrete,” NISTIR 5405, NIST, October 1994.
- Lobo, C. L., Mullings, G. M., and Gaynor, R. D., “Effect of Capping Materials and Procedures on the Measured Compressive Strength of High-Strength Concrete,” Cement, Concrete, and Aggregates, Vol. 16, No. 2, Dec. 1994, pp. 173-180.
- Burg, R. G., Caldarone, M. A., Detwiler, G., Jansen, D. C., and Willems, T. J., “Compressive Testing of HSC: Latest Technology,” Concrete International, Vol. 21, No. 8, August 1999, pp. 67-76.
More Information
More information about the use of neoprene caps is contained in the following references:
Carrasquillo, P. M. and Carrasquillo R. L., “Effect of Using Unbonded Capping Systems on the Compressive Strength of Concrete Cylinders” ACI Materials Journal, Vol. 85, No. 3, May-June, 1988.
Richardson D. N., “Testing Variables Effects on the Comparison of Neoprene Pad and Sulfur Mortar-Capped Concrete Test Cylinders,” ACI Materials Journal, Vol. 87, No. 5, September-October, 1990.