Abstract: Concrete, one of the maximum consumed and extensively used, as a principal construction material in the world with a vital role in the construction field due to its abundant advantages such as strength, durability, easy of fabrication, non- combustibility properties, it possesses over other construction materials. During recent years due to its added advantages over standard concrete, the use of high strength concrete for construction has become common, especially for multistoried structures, bridges and high rise buildings etc. As the cost of concrete is high and in of view of economy, now a day's fly ash is being used in construction replacing by mass of cement. The characteristic compressive strength of concrete is being used in design of structures, use of this value is a problem since the control specimen sizes and shapes are varying from country to country. In India, the characteristic compressive strength is usually measured based on 150 mm cubes, whereas the ACI code of practice specifies the design compressive strength based on the standard 150 x300 mm cylinder in the design of reinforced structures. In this context the size and shape of concrete is an important parameter for the compressive strength, which necessitates the study of effect of size and shape of the specimen. Hence in this study different sizes and shape of specimens are prepared cured and tested in compression at an age of 7, 28, 56 and 91 days.

Keywords: Compressive strength, fly ash, high strength concrete, cement.