Effects of Aggregate Type, Water-to-Cementitious Material Ratio, and Age on Mechanical and Fracture Properties of Concrete

Abstract

The effects of age and aggregate type on the behavior of normal, medium, and high-strength concrete, and the relationships between compressive strength, flexural strength, and fracture properties (fracture energy and characteristic length) are studied. The concrete mixes contain either basalt or crushed limestone aggregate with a maximum size of 19 mm (3/4 in.) and an aggregate volume factor (ACI 211.1- 91) of 0.67. Mixes are tested at ages of 7, 28, 56, 90, and 180 days. Water-tocementitious material (w/cm) ratios range between 0.25 and 0.46. In the study, compressive strengths range from 20 MPa (2,920 psi) (7 day normal-strength limestone concrete) to 99 MPa (14,320 psi) (180 day high-strength basalt concrete). High-strength concrete containing basalt attains a higher compressive strength than high-strength concrete containing limestone, even at a slightly higher w/cm ratio. Medium-strength concrete containing limestone exhibits slightly higher compressive strength than concrete containing basalt. Compressive strengths for normal-strength concrete are similar for limestone and basalt. The w/cm ratio is the primary controlling factor for determining compressive strength. Higher strength concretes gain a greater portion of their long-term compressive strength at an earlier age than lower strength concretes. The flexural strengths range from 4 MPa (550 psi) to 14 MPa (1,960 psi). High-strength concrete containing basalt yields significantly higher flexural strengths than high-strength concrete containing limestone at the same age. The limiting factor appears to be the tensile strength of the aggregate. For the normal and mediumstrength concretes, aggregate does not significantly affect the flexural strength. The w/cm ratio and aggregate strength are the primary controlling factors for determining flexural strength. Flexural strength generally increases with increasing age. The fracture energies range from 27.7 N/m (0.158 lb/in.) to 202 N/m (1.152 lb/in.). Concrete containing basalt yields significantly higher fracture energies than concrete containing limestone at all w/cm ratios and ages. This is due to less aggregate fracture and a more irregular fracture surface in basalt concrete, causing greater energy dissipation. Compressive strength, w/cm ratio, and age seem to have no effect on fracture energy, which is principally governed by coarse aggregate type. The characteristic length is higher for concrete containing basalt than for concrete containing limestone. Characteristic length decreases with an increase in compressive strength. The peak bending stress in a fracture test is linearly related to flexural strength.