ASPECTS OF CONCRETE STRENGTH AND DURABILITY

Research is under way to study the strength and durability of concrete in Florida. Results of the first phase of this research are presented. Twenty-two concrete mixtures were prepared and tested for compressive strength, water permeability, chloride permeability (AASHTO T277), and corrosion resistance. Four groups of concrete mixtures that covered a wide range of materials and mixture proportions were included. Water/cementitious ratios used were 0.45, 0.38, and 0.33, corresponding to cementitious contents of 564, 658, and 752 lbs/cu yd, respectively. Different combinations of fly ash and silica fume were used. Fly ash content ranged between 10 and 50% by weight of the total cementitious material. Silica fume was included in proportions between 5 and 15%. Effects of fly ash and silica fume on strength and permeability of concrete and on corrosion of steel in concrete are discussed. Correlation is established between results of the Florida water permeability test and results of the AASHTO T277 chloride permeability test. It is also shown that concrete mixtures with equal compressive strengths do not necessarily produce equal levels of permeability, especially when fly ash and silica fume are included in the mixture. This lack of correlation continues until the strength reaches about 8,000 psi. At and beyond 8,000 psi, a well-defined trend is observed. With increased concrete strength, the permeability becomes consistently low to very low according to both AASHTO T277 and water permeability classifications. Findings from this phase of the research affirm the need to develop specifications for concrete durability based on requirements for both compressive strength and permeability of concrete.