Ronald G. Burg, Construction Technology Laboratories, Inc.
High performance concrete (HPC) used in the construction of a bridge or bridge element often includes high strength concrete (HSC). To date, concrete with design compressive strengths of up to 19,000 psi (131 MPa) have been used in building construction while concrete with design compressive strengths up to 14,700 psi (101 MPa) have been used in bridge construction.
An important aspect in the successful use of high strength concrete is implementing the required quality control measures. For concrete, the most common quality control parameter, and basis for acceptance, is compressive strength. The various AASHTO and ASTM standards that prescribe the methods to cast, cure, prepare, and test concrete specimens were developed based on concretes with compressive strengths in the range of 1500 to 6000 psi (10 to 41 MPa). In the past several years, there has been considerable work done to determine if these standards are suitable for HSC or if modifications are required. However, it often takes several years for existing standards to be revised based on recently completed work. Therefore, this article summarizes some of the important findings from recently completed and on-going work that can be implemented on a project that uses HSC.
Specimen Size
Because of the high loads required to break cylinders made with HSC, many laboratories and test agencies prefer to use 4×8-in. (102×203-mm) cylinders in lieu of 6×12-in. (152×305-mm) cylinders. With the smaller cylinder, 20,000 psi (138 MPa) concrete can be tested using a 300,000-lb (1.33-MN) machine, whereas a 6×12-in. (152×305-mm) cylinder requires at least a 600,000-lb (2.67-MN) machine. On average, 4×8-in. (102×203-mm) cylinders have measured compressive strengths approximately 2 percent higher than 6×12-in. (152×305-mm) cylinders when testing high strength concrete. Given the small magnitude of this difference, the smaller specimens can be used to judge the acceptance of concrete based on compressive strength. However, due to the greater variability in measured strength exhibited by the 4×8-in. (152×305-mm) specimens, it may be necessary to test more specimens to obtain a representative value of compressive strength.
Cylinder Molds
As concrete strength increases, the dimensional consistency of the test specimen, and thus the mold used to cast the specimen, becomes more critical. Although heavy gauge steel molds result in cast specimens that are closer to perfectly round, plastic single-use molds yield cast specimens that do not noticeably affect measured compressive strength. Plastic molds can be used with HSC. However, care must be taken to ensure that the bottom of the mold is not damaged while rodding the fresh concrete, and in no circumstances should single-use plastic molds be reused.
Cylinder Curing
Most HSC has a high cementitious material content and a low water- cementitious material ratio. Some of these types of concretes are prone to a condition known as self-desiccation whereby the interior of the concrete dries at a more rapid rate than the exterior. Because of this condition, some researchers have suggested that HSC cylinders should be cured underwater rather than in a moist room. However, work on concrete with compressive strengths as high as 18,000 psi (124 MPa) shows that underwater curing is not required for HSC. A conventional moist-curing room meeting the relevant requirements can be used.
Cylinder Ends
Properly prepared cylinder ends are paramount to obtaining representative compressive strength data. At concrete strengths below 10,000 psi (69 MPa), conventional methods of end preparation, capping with sulfur- based compounds, or the use of pad caps are suitable. At concrete strengths in excess of 10,000 psi (69 MPa), surface grinding of cylinder ends is the most suitable method. However, if appropriate capping techniques are used, in particular thin caps, it appears that some capping materials may be suitable for testing high strength concrete. Selection of a suitable capping compound cannot be based on the compressive strength of the capping compound. The most direct means to judge the adequacy of a particular capping compound is comparative testing against cylinders with surface-ground ends. Without conducting this type of comparative analysis, capping compounds should only be used on concrete with compressive strengths up to 10,000 psi (69 MPa) and concrete with compressive strengths above 10,000 psi (69 MPa) should be tested using surface-ground ends.
Summary
All relevant standards should be strictly followed when testing HSC, as any deviation from the prescribed methods will have a significant impact on measured compressive strength.
Further Information
For further information on compression testing, see: 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.