Tensile strength of low, medium, and high-strength concretes at early ages

This paper presents an experimental study of early-age tensile strength (i.e., modulus of rupture) of low, medium, and high-strength concretes. In this study, low strength indicates a 28-day concrete compressive strength of 30 MPa (4300 psi), medium strength indicates a 28-day concrete compressive strength of 70 MPa (10,150), and high strength indicates a 28-day concrete compressive strength of 100 MPa (14,500 psi). Tests on modulus of rupture specimens were carried out at frequent intervals during the first 3 days after casting to observe the influence of concrete strength. The influence of three different curing conditions--temperature-matched curing, sealed curing, and air-dried curing--were investigated. It was found that the gain in modulus of rupture of temperature-matched cured concrete beams is higher than that of sealed and air-dried beams. After an initial retardation period, the 70 and 100 MPa (10,150 and 14,500 psi) concretes showed a higher rate of flexural strength gain than the 30 MPa (4300 psi) concrete. It was observed that the current ACI Code expression for modulus of rupture overestimates the modulus of rupture for very early-age concrete and underestimates the modulus of rupture for concrete strengths above 15 MPa (2175 psi). It was also found that the equation proposed by the ACI 363 Committee overestimates the modulus of rupture for all concrete strengths and curing conditions investigated. Relationships are proposed to predict the modulus of rupture as a function of the concrete compressive strength. These relationships are applicable at very early ages up to an age of 91 days and account for a wide range of concrete compressive strengths and curing conditions.