Study on rate of strength gain of concrete mix prepared with locally available composite cements

Abstract

In building construction sector, there is a common complain that concrete made with Portland composite cement (PCC) does not gain target strength within specified period (28 days). There is another argument found that the quality of this cement productions are not properly controlled by the cement industry of Bangladesh. This study is conducted to find out the fact on strength gain characteristics of concrete made with Portland composite cements (PCC) that are locally available. For this, concrete have been made with Portland composite cement (PCC) of four different brands (brand-A, brand-B, brand-C and brand-D) and one ordinary Portland cement (brand-E) for the target strength of 2500 psi, 4000 psi and 6000 psi where mix design procedure of concrete of ordinary Portland cement has been followed. The compressive strength of concrete cylinders have been tested at 3 days, 7 days, 14 days, 28 days, 3 months, 6 months and 1 year of age of concrete. Concrete have been cured in two different ways, one set of concrete cylinders have been cured continuously until they are tested while the other set of concrete cylinders have been cured only 14 days under water. Compressive strength of concrete is taken as a measure to determine the rate of strength gain of concrete with age. Comparison of compressive strength gain of concrete made with Portland composite cement and ordinary Portland cement have been performed. Also the effect of curing on the strength development has been investigated. From the experimental observation, it is found that the early age strength of concrete made with Portland composite cement (PCC) is lower in magnitude than that of concrete made with ordinary Portland cement (OPC) due to the presence of fly ash in Portland composite cement (PCC) which is responsible for the pozzolanic reaction. The continued pozzolanic activity of fly ash contributes to increased strength gain at later ages at continued curing condition. It is also found that drying ambient conditions reduce the strength potential of concrete made with Portland composite cements (PCC) because the secondary (pozzolanic) reaction fails to contribute to the development of strength. It is found that concrete made with Portland composite cement (PCC) for the target strengths of 2500 psi, 4000 psi and 6000 psi gains 55 to 60 percent of its target strength at 7 days of age and 80 to 85 percent of its target strength at 28 days of age at continuous curing condition. At 14 days curing condition, it gains 75 to 80 percent of its target strength at 28 days of age. It requires 50 to 70 days, 80 to 100 days and 180 to 200 days to gain the target strengths of 2500 psi, 4000 psi, and 6000 psi respectively at continuous curing condition. At 14 days curing condition, it requires 90 days, 180 days and more than a year to gain the target strength of 2500 psi, 4000 psi and 6000 psi respectively.