Michael A. Caldarone and Ronald G. Burg, CTLGroup
With increasing compressive strength, the modulus of elasticity or the slope of the elastic stress-strain relationship increases, and the magnitude of inelastic post-peak strain capacity decreases. Stated differently, as strength increases, concrete becomes increasingly brittle and more sensitive to testing-related variables. Some of the most influential variables include specimen geometry and size, age, moisture content, moisture distribution, end preparation, loading rate, and testing machine parameters. In this article, end preparation will be discussed.
Bonded Caps or Ground Ends
When testing high strength concrete, end preparation is one of the most important variables influencing the compressive strength results. Tolerances for perpendicularity and end planeness are provided in AASHTO T 22. Carino et al.(1) investigated the significance of sulfur capping and grinding using concrete with strength levels of 6500 psi (45 MPa) and 13,000 psi (90 MPa). No strength difference due to the method of end preparation was observed for the lower strength concrete, but for the higher strength concrete, grinding resulted in as much as 6% greater measured strength.
Capping Material Thickness
The appropriateness of capping compounds depends to a large extent on the cap thickness provided. Lessard et al.(2) found a commercially available “high strength” capping compound to be satisfactory when used for testing concrete with strengths up to approximately 17,000 psi (120 MPa), provided the capping layer is less than 0.12 in. (3 mm) thick. For concrete compressive strengths greater than 7000 psi (50 MPa), AASHTO T 231 (ASTM C617) specifies a maximum average cap thickness of 0.125 in. (3 mm) and a maximum thickness for any part of the cap of 0.20 in. (5 mm).
Capping Material Strength
Certain capping materials appear to be suitable for testing high strength concrete; however, the compressive strength of the capping compound alone should not form the sole basis of selection. According to AASHTO T 231, sulfur mortar used with concrete compressive strengths greater than 7000 psi (50 MPa) must be prequalified by the manufacturer for testing at the higher strength levels. In a study by Burg et al.,(3) the performance of a conventional strength capping compound and a “high strength” capping compound were evaluated with respect to their suitability for use in testing high strength concrete. Both capping compounds were sulfur-based and commercially available. As anticipated, the compressive strength of the high strength capping compound was significantly higher than the conventional capping compound; however, the modulus of elasticity was lower and Poisson’s ratio higher for the high strength capping compound. High strength concrete tested with caps made with high strength capping compound had measured compressive strengths lower than the same concretes tested with caps made with the conventional strength capping compound. For the three nominal concrete strengths of 9000, 14,000 and 18,000 psi (62, 97, and 124 MPa), the differences in measured concrete compressive strength were statistically significant and suggested that compressive strength of capping compound is not a reliable indicator for suitability for use in testing high strength concrete. The most suitable means of judging the adequacy of a particular capping compound when testing high strength concrete is by performing comparative testing with cylinders having surface ground ends.
Bonded Caps or Unbonded Caps
Pistilli and Willems(4) compared traditional sulfur caps with unbonded neoprene pads in compressive strength testing of concrete with strengths ranging from 3000 to 18,000 psi (20 to 125 MPa) and compared sulfur caps with specimens having ground and lapped surfaces within the range of 13,000 to 20,000 psi (90 to 138 MPa). Significantly lower within-test variability occurred with neoprene pads compared to the sulfur caps for strengths above 8000 psi (55 MPa). The ratio of 4 x 8 in. to 6 x 12-in. (100 x 200 mm to 150 x 300 mm) cylinder strengths ranged from 0.96 to 1.06. The strength differences due to cylinder size did not appear to be of practical significance for concretes with actual measured strengths ranging from 4000 to 9000 psi (28 to 62 MPa). Grinding the ends of cylinders with measured strengths ranging from 12,000 to 20,000 psi (83 to 138 MPa) showed promise as an improved test procedure for end preparation. Provided the finished surfaces are smooth, neoprene pads could be a satisfactory alternative for concretes with strengths within the range of 13,000 to 20,000 psi (90 to 138 MPa). Currently, ASTM C1231 does not permit the use of unbonded caps for acceptance testing of concrete with a compressive strength above 12,000 psi (80 MPa). For higher strength concrete, the alternative is to grind the ends plane or cap with a suitable sulfur-based capping compound. Also, AASHTO T 231 and ASTM C1231 require qualification tests of bonded and unbonded capping systems, respectively, for use with concrete compressive strengths greater than 7000 psi (50 MPa).
Summary of Existing Standards
For concrete compressive strengths less than 7000 psi (50 MPa), bonded caps, unbonded caps, or ground ends may be used.
For concrete compressive strengths from 7000 to 12,000 psi (50 to 80 MPa), bonded caps, unbonded caps, or ground ends may be used provided that the bonded capping material and unbonded caps have been qualified per the appropriate standards. For concrete compressive strengths above 12,000 psi (80 MPa), bonded caps or ground ends may be used provided that the bonded capping material has been qualified per the appropriate standard.
In all cases, testing of high strength concrete must be performed in strict compliance with the appropriate AASHTO or ASTM procedures.
References
1. Carino, N. J., Guthrie, W. F., and Lagergren, E. S., “Effects of Testing Variables on the Measured Compressive Strength of High Strength (90 MPa) Concrete,” NISTIR Publication No. 5405, National Institute of Standards and Technology, Gaithersburg, Maryland, Oct., 1994, 141 pp.
2. Lessard, M., Chaallal, O., and Aïtcin, P.C., “Testing High Strength Concrete Compressive Strength,” ACI Materials Journal, Vol. 90, No. 4, July-August 1993, pp. 303-308.
3. Burg, R. G., Caldarone, M. A., Detwiler, G., Jansen, D. C., and Willems, T. J., “Compression Testing of HSC: Latest Technology,” Concrete International, Vol. 21, No. 8, August 1999, pp. 67-76.
4. Pistilli, M. F. and Willems, T., “Evaluation of Cylinder Size and Capping Method in Compression Strength Testing of Concrete,” ASTM Journal of Cement, Concrete and Aggregates, Vol. 15, Issue 1, July 1992, pp. 59-69.
Further Information
For further information about testing high strength concrete cylinders, please contact the first author at [email protected].
Portions of this article are excerpted from “High-Strength Concrete” by Michael A Calderone, published by Taylor & Francis, 2009.