The strength of concrete.

Abstract

The quality of concrete is judged largely on the strength of that concrete. Equipment and methods are continually being modernized, testing methods are improved, and means of analyzing and interpreting test data are becoming more sophisticated, yet we still rely on the strength of the same 6 by 12-inch cylinders , made on the job and tested in compression at 28 days age, as we did 90 years ago. Interestingly, the 2008 edition of the ACI 318 Standard (ACI 318-08) now specifically addresses the use of 4 by 8-inch cylinders for evaluation and acceptance of concrete (ACI 318 Section 5.6.2.4). See discussion on strength specimens in Chapter 13, Section 13.5. 3.1. The Importance of Strength Obviously, the strength of any structure, or part of a structure, is important, the degree of importance depending on the location of the structural element under consideration. The first-floor columns in a high-rise building, for example, are more important structurally than a nonbearing wall. Loading is more critical, and a deficiency in strength can lead to expensive and difficult repairs or, at worst, a spectacular failure. Strength is usually the basis for acceptance or rejection of the concrete in the structure. The specifications or code designate the strength (nearly always compressive) required of the concrete in the several parts of the structure. In those cases in which strength specimens fail to reach the required value, further testing of the concrete in place is usually specified. This may involve drilling cores from the structure or testing with certain nondestructive instruments that measure the hardness of the concrete. Some specifications permit a small amount of noncompliance, provided it is not serious, and may penalize the contractor by deducting from the payments due for the faulty concrete. Statistical methods, now applied to the evaluation of tests as described in Chapter 26, lend a more realistic approach to the analysis of test results, enabling the engineer to recognize the normal variations in strength and to evaluate individual tests in their true perspective as they fit into the entire series of tests on the structure. Strength is necessary when computing a proposed mix for concrete, as the contemplated mix proportions are based on the expected strength-making properties of the constituents. 3.2. Strength Level Required The code and specifications state the strength that is required in the several parts of the structure. The required strength is a design consideration that is determined by the structural engineer and that must be attained and verified by properly evaluated test results as specified. Some designers specify concrete strengths of 5000 to 6000 psi, or even higher in certain structural elements. Specified strengths in the range of 15,000 to 20,000 psi have been produced for lower-floor columns in high-rise buildings. Very high strengths, understandably, require a very high level of quality control in their production and testing. Also, for economy in materials costs, the specified strength of very high-strength concrete is based on 56 or 90-day tests rather than on traditional 28-day test results. To give some idea of the strengths that might be required, Table 3.1 is included as information only. Remember that the plans and specifications govern. Note that the International Building Code (IBC) (Section 1905.1.1) and the ACI 318 Standard (Section 5.1.1) indicate a minimum specified compressive strength of 2500 psi for structural concrete. Simply stated, no structural concrete can be specified with a strength less than 2500 psi. Other properties of the concrete can be significant for concrete exposed to freeze-thaw conditions, sul-fate exposure and chloride exposure (effects of chlorides on the corrosion of the reinforcing steel). Strength, however, remains the basis for judgment of the quality of concrete. Although not necessarily