Effect of coal bottom ash as partial replacement of sand on workability and strength properties of concrete

Large quantity (35 million tons) of coal bottom ash is produced by thermal power plants in India. The present method of disposal of coal bottom ash on open land is the main cause of an environment hazard for the surrounding community. As utilization of coal bottom ash can help in alleviating environmental problems, thus the present work was done to explore the possibility of its use as sand replacement in concrete manufacturing. Two types of control concrete mixtures to develop 28 d compressive strength of 38 MPa designated as concrete 'A' and 34 MPa designated as concrete 'B' were made with river sands having different fineness of modulus. In both grades of concrete mixtures, sand was replaced with coal bottom ash at 20, 30, 40, 50, 75 and 100% levels. Workability and bleeding of concrete decreased on use of coal bottom ash as fine aggregate. Test results show that compressive and splitting strength of concrete mixture 'A' did not changed significantly when sand having fineness modulus of 1.97 was replaced with coal bottom ash. However, in case of concrete mixture 'B', when coal bottom ash was used as replacement of sand having fineness modulus of 2.58, the compressive and splitting tensile strength of bottom ash concrete decreased at early curing age. After 90 d of curing age, compressive and splitting tensile strength of bottom ash concrete mixtures was almost comparable to that of control concrete mixture. Bottom ash concrete mixtures displayed lower modulus of elasticity and abrasion resistance as compared to control concrete mixture. Abrasion resistance of bottom ash concrete mixtures improved significantly with age. Pulse velocity through bottom ash concrete mixtures indicates that good quality concrete can be made with coal bottom ash as replacement of either type of sand.