ABSTRACT
The paper outlines the investigation undertaken to determine the causes for cracking observed in an eight year old concrete foundation for Boiler Feed Pump in a Super Thermal Power Station. The foundation block was exposed to thermal cycles of 65 C (max) during the operation of the plant and 20 C (min) during the shut down. Preliminary assessment showed that the cracking was not due to structural effects. Hence, the investigation was pursued to ascertain the characteristics of the concrete under thermal loading. As the aggregates used were limestone, it became the suspect. Experiments were designed to compare the performance of concrete made with limestone aggregates and quartzite aggregates, and exposed to cyclic thermal loading. The limestone concrete showed 6% reduction in compressive strength with respect to control at the end of 80 cycles of 30–90 C heating and cooling. Resonance frequency and ultrasonic pulse velocity tests indicated a net reduction of 7% and 8% respectively at the end of 70 cycles. Residual expansion tests on concrete prism specimen showed 0.15 percent cumulative residual expansion at 50 cycles. The experimental results indicated that thermal incompatibility of limestone aggregates could lead to early cracking, reduction in strength and other properties of concrete subjected to heating-cooling cycles. Based on the observations, recommendations were given towards minimising further damage to concrete foundation.
