USE OF CRUSHED SAND IN CONCRETE
As per IS383 and MoRT&H specifications, the crushed sand is defined as fine aggregate produced from crushing hard stone. In construction terminology, it is called as crusher dust produced from secondary crushing from horizontal or vertical shaft impactors. These are controlled and have material passing less than 0.150mm (150microns IS sieve), not more than 20% and material passing 0.075mm (75micorns IS sieve) not more than 15%.
The benefit of using crushed stone sand instead of natural river sand for concrete:
Even though the Crushed sand and River Sand are natural sources, crushed sand is a by-product from crushers while producing coarse aggregate. Few of the Indian states are already put a ban on quarrying of natural sand from the river bed to control the depleting water table.
If we compare the cost of natural river sand against crushed sand, the rate is very high. These supply and prices are controlled by the market/seller. By using the crusher dust as crushed stone sand we will avoid buying river sand for concrete and pay for only transporting the crushed stone sand to our site like the coarse aggregates. The utility of crushed stone sand is realized now by only the affected, who do not get good quality natural river sand.
Remember that crushed stone sand is available as long as coarse aggregate is produced but natural river sand is becoming rarer. The crushed stone sand is a by-product while making coarse aggregate. Also, there can be no concrete without coarse aggregate but we can do concrete without natural river sand. Hence let us conserve natural river sand for other utilities than for concrete.
Controlling the quality of Crushed Sand:
To obtain the best quality of crusher sand the boulders shall be hand picked. Most of our crushing methods are mechanized and hence hand picking of boulders are impractical. But hand picking of boulders eliminate natural fines and hence reduce the clay content which is harmful to concrete. The modern crushers are provided with ‘Grizzly feeders’ are consistently giving us best results. These Grizzly feeders screen out all the natural fines from quarry and quarry blast pieces and hence improve the quality of crusher sand. Better gradation control and clay control, screen size and Grizzly opening size are vital. For best results, it is practiced to have a screen size of 5.0 or 4.75mm and Grizzly opening 12 to 22mm.
Crushed stone sand for concrete:
IS383 and clause 1008 of MoRT&H allows us to the use of crushed stone sand in concrete. These specifications have allowed some quantity of increased fines compared to natural river sand because the fines generated from crushing hard stones are free from clay. Thus the fines are not deleterious to concrete and improve the surface finishing of concrete members. We can use crushed stone sand (100%) as a fine aggregate with no problems for normal site concrete including slip-form and pumping.
Workability of concrete:
The crushed sand will have more water absorption than natural river sand, generally 1 to 1.5% more. But this water absorption is a permanent physical property of crushed sand and will not affect the durability of concrete. Due considerations and corrections at the concrete mix design stage, will help this additional water to give better workability of concrete during placing. This additional water is absorbed by crusher sand and will not be available for hydration of cement and thus does not reduce the strength of concrete.
The cohesiveness of concrete:
Use of crushed sand increases the cohesiveness of the mix compared to the natural river sand. This is due to the additional fines available. If required the percentage of fine aggregate in the total aggregate can be reduced by 5% compared to the proportion fixed for natural river sand. This is especially true in higher cement content mixes (above 450kG/cum) where the stickiness is notice
The durability of concrete:
Comparing with the concrete made using natural river sand, the durability of concrete is better with crushed sand. The fines in the crushed sand are from crushing hard and durable stone and hence pure, inert and do not contain clay. Hence they exhibit fewer shrinkage properties than the natural sand. Because of high cohesion and more workable, the concrete gets densified more and thus less permeable.
Estimating the percentage of material finer than 75 microns:
Wet sieve method shall be used for determining the materials passing 75microns. Since in dry sieving, all the fractions passing 75micron cannot fall through the small aperture by themselves. Wet sieving makes sure that the particles are loose and are able to pass through with the lubricating effort of water. However, due to the requirement of estimating the weight of dry material retained on 75 micron, the material has to be dried after sieving thus increasing the testing time. An assessment by volume using a measuring cylinder and differential settlement gives a quick assessment but requires a prior calibration table of weight method Vs volume method for accurate interpretation. There are no set limits for assessment of 75 microns passing by volume and hence the calibration shall be done. The value from the volume method will be used for reading the corresponding value by weight method and this, in turn, will be checked for acceptance in accordance with Table 1 of IS383 which specifies maximum limit of 15% for crushed fine aggregate.
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