## 1551Compressive Strength

Concrete properties such as modulus of elasticity, tensile or flexural strength, shear strength, stressstrain relationships, and bond strength are usually expressed in terms of uniaxial compressive strength. Compressive strength is the common basis for design for most structures, other than pavements, and is the common method of routine quality testing.

Maximum, practically achievable, compressive strengths of HPC have increased steadily over the years. Presently, 28-day strengths of up to 100 MPa are routinely obtainable. Under special treatment and with a special formula, a so-called RPC or ultrahigh-strength concrete has been developed. Its compressive strength can reach 200 MPa or even 800 MPa.

Testing variables have a considerable influence on the measured compressive strength. The major testing variables are: mold type, specimen size, end conditions, and rate of loading. The sensitivity of measured compressive strength to testing variables varies with the level of compressive strength.

Apart from increasing the compressive strength of HPC, the stress-strain behavior in compression should also be examined in detail. A number of investigations [34-36] have been undertaken to obtain the complete stress-strain curves in compression. Axial stress-strain curves for concretes with com-pressive strengths up to 200 MPa were obtained by different researchers. It is generally recognized that for HPC with high compressive strength, the shape of the ascending part of the curve becomes more linear and steeper, the strain at maximum stress is slightly high, and the slope of the descending part becomes steeper when compared to normal concrete, as shown in Figure 15.19. It implies that HPC may be more brittle than normal concrete.

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