MICROCRACKING AND ENGINEERING PROPERTIES OF HIGH-STRENGTH CONCRETE

The main purpose of this investigation was to establish the differences in mechanical properties between high-strength and normal-strength concretes, and to explain those differences in terms of differences in observed internal microcracking in concrete at different stages of loading. Concretes made using gravel and crushed limestone coarse aggregates at each of three different strength levels ranging from 4,000 psi (27.58 MPa) to 11,000 psi (75.86 MPa) were studied.; First, the results of the microcracking study are presented. An X-ray technique was used to study internal microcracking. Two classification systems were used to analyze the microcracks: bond-mortar crack analysis and simple-combined crack analysis. Internal microcracks were observed at four strain levels. The observed microcracking mechanism is related to the failure mode in uniaxial compression.; Second, the results of the study of the mechanical properties are presented. Data on compressive strength, strength gain with age, specimen size effect, static modulus of elasticity, Poisson's ratio, modulus of rupture, tensile splitting strength, unit weight, and drying effect on compressive and flexural strength of normal- and high-strength concretes are presented. Results from a study on the stress-strain (longitudinal and lateral strains) behavior of gravel and limestone concretes at different strength levels tested in uniaxial compression at a constant stroke rate are included. Comments on the effects of different rates of loading on the compressive strength and strain at maximum stress are given.; Third, a criterion for definition of failure in uniaxial compression for the concretes tested is presented. Failure is considered to occur at the discontinuity point defined as that point when a self-propagating microcracking mechanism is developed eventually causing disruptive failure with time. The predicted stress and strain ratios at discontinuity based on the microcracking study are compared to those at which sudden changes occur in the Poisson's ratio and volume change curves.