Comparative Study of Different Concrete Grades in RCC Buildings

The grade of concrete is one of the first decisions made in the design of a reinforced concrete building, and it influences member sizes, reinforcement quantity, stiffness, durability, and cost. IS 456:2000 prescribes a minimum grade of M20 for reinforced concrete in mild exposure and higher grades for more severe exposure conditions. In practice, many designers continue to use M20 or M25 throughout a building without examining whether a higher grade could give a more economical or better-performing structure. The present study compares the structural behaviour and material requirement of an RCC building designed with different concrete grades. A G+9 storey residential RCC framed building with a plan of 24 m by 18 m is modelled in STAAD.Pro. Four versions of the model are prepared with concrete grades M20, M25, M30, and M40, keeping the geometry, loading, and reinforcement grade of Fe500 the same. The modulus of elasticity of each grade is taken as 5000 times the square root of the characteristic compressive strength, as given in IS 456:2000. Dead and live loads are applied according to IS 875 (Parts 1 and 2), and seismic loads are applied for Zone III according to IS 1893 (Part 1):2016. The four models are analysed under limit state load combinations, and the results are compared in terms of storey displacement, storey drift, fundamental time period, base shear, axial force and bending moment in critical columns and beams, and the reinforcement required. The concrete frame design module is used to design all beams and columns according to IS 456:2000, and in a second stage the column sizes are reduced where the higher grades permit, so that the saving in concrete volume and floor area can be assessed. The results show that increasing the grade of concrete increases the stiffness of the frame, which reduces lateral displacement and drift and slightly shortens the natural period. Because the seismic weight is nearly unchanged, base shear increases only marginally with the grade. Column reinforcement reduces considerably as the grade increases, while beam reinforcement changes much less because flexural capacity depends mainly on the steel. When column sizes are optimised, the M30 and M40 models give smaller columns and a lower total material cost than the M20 model. The study concludes that the choice of concrete grade should be made on the basis of structural performance and overall cost rather than the unit price of concrete alone. For multi-storey buildings, higher grades are particularly beneficial in the columns of the lower storeys, and a combination of grades over the height of the building may offer the best balance between economy, durability, and constructability.