Optimization of Gradation and Fineness Modulus of Naturally Fine Sands for Improved Performance as Fine Aggregate in Concrete

Sand is an accumulation of grains of mineral matter derived from the disintegration of rocks. It is distinguished from gravel only by the size of the grains or particles. Sand deposits of varying gradation are available in different regions of the world. The prime use of sand, as fine aggregate is in the manufacture of concrete. As fine aggregate comprises up to 30% of the total volume of concrete, consequently gradation and fineness modulus of sand are among principal factors affecting the performance of fresh and hardened concrete. For the purpose of this study, eight sand deposits were selected from various regions of Pakistan. The grain size distribution was compared with the ASTM gradation limits recommended for concrete. It was revealed that only two sands satisfy the ASTM gradation limits while all the others do not comply being on the finer side. For betterment of the construction industry, a sand optimization technique was developed. The optimization phase consisted of sieving the crusher fines and mixing different fractions of crusher fines with the natural sand in different combinations to get desired sand gradation. The optimized gradation curves matched with the ASTM limits and the optimized fineness modulus reached within the ASTM limits. The concrete made with optimized sands showed up to 39% increase in compressive strength. The composition of mineral constituents was also evaluated through x-ray diffraction and x-ray fluorescence, which indicated that the fine aggregate having higher silica content resulted in concrete with higher strength as compared to the one with higher calcium content. It was concluded that fine graded sands could be used in concreting after optimization that will reduce the huge transportation costs of using coarse sand from far-flung areas.