The particle shape of fine aggregates is very important for making concrete. Recent tests however have shown that a good quality concrete made from the sand having cubicle particles and sharp edges, gives higher tensile strength. Same way rounded sand particles gives more compression strength to the concrete. So to achieve both the effect it is recommended that the sand particles should be cubicle with grounded edges.
Where concrete of high strength and good durability is required, fine aggregates confirming to zone one to zone four may be used. The concrete mix design is very important. The fine aggregates grading becomes progressively finer from zone one to zone four, the ratio of fine aggregates to course aggregates should be progressively reduced. Actual ratio of fine to course aggregates for any particular mix will depends upon the actual grading, particle shape and surface texture of both fine and course aggregates. Use of zone four sand is not recommended for reinforced concrete purpose.
Experiments have shown that considerable variation in strength of mortar may occur owing to the form and variety of the sand particles. The strength of mortar may differ by about 50% of average. Sand particles should however be hard for more strength. It is not possible in river sand, that all particles should be of higher strength. This can be achieved only by making sand with the help of machines. In machine made sand we can use the raw material of higher strength.
From the above following are the requisites of sand :
• All the sand particles should have higher crushing strength.
• The surface texture of the particles should be smooth.
• The edges of the particles should be grounded.
• The ratio of fines below 600 microns in sand should not be less then 30%.
• There should not be any organic impurities
• Silt in sand should not present more then two percent. Total limit of 5 %.
• In crushed stone sand the permissible limit of fines below 150 microns may be up to 20 %. This does not affects the 5 % permissible limit