Strain, Cracking, and Failure Described by an Ultimate Modulus
E. K. Schrader
Appears on pages(s):
cracking (fracturing); creep properties; failure; modulus of elasticity; strains; strength; tension; Structural Research
For years, the concrete industry has used ultimate compressive strength and elastic modulus as principal design and analysis tools. This can be very misleading when cracking and failure are evaluated. With modern concrete that include roller-compacted concrete (RCC) and lower strength mass applications, cracking that is serious may not occur until the concrete is strained well beyond the elastic region. Two things are needed to resolve this problem. First, a new property called the "ultimate modulus" should be determined, along with the elastic modulus. If these values are nearly the same, the concrete is brittle and may have a low strain capacity, even if it has a high strength. If the ultimate modulus is much lower than the elastic modulus, the material is "tough" and may have a high strain capacity despite a low strength. Examples are given in which deliberately designing a lower strength concrete has resulted in a much higher strain capacity. In one case with RCC, a mixture with five times less strength resulted in a tensile strain capacity (and resistance to thermal cracking) that was three times greater. Second, there should be a better understanding of the relationships between strain capacity, strength, and modulus (ultimate and elastic) in compression as compared to those material properties in tension. With the broader range of concrete mixtures possible in today's concretes (RCC being an example), the ratio between split cylinder tensile strength and compressive strength may be twice as high for a lower strength mixture than it is for a higher strength mixture. Somewhat offsetting this is the fact that the conversion factors from split tensile strength or flexural strength to direct tensile strength are substantially smaller for low strength concretes and greater (exponentially) for high-strength concretes. When only concretes in the compressive strength range of about 20 to 50 MPa are considered, the adjustment factor happens to be about one, so this phenomenon has not been obvious or very important in the past.