Bulking of Sand & Its measurement , and Soundness of Aggregate | Concrete Technology by Dr K Mohan

Lecture-6: Bulking of Sand & Its measurement , and Soundness of Aggregate | Concrete Technology by Dr K Mohan.

Dr K Mohan is an internationally renowned and an acclaimed cement scientist. He is a former Director General of National Council for Cement and Building Materials (NCCBM).
He holds a Ph.D. degree in Cement Chemistry from University of Aberdeen, Scotland, U.K.

Bulking of Sand:

• Bulking of sand is an increase in the volume of a given mass of sand caused by the presence of water (free moisture).
• The bulking of sand is caused by the films of water which push the particles apart.
• The extent of bulking depends upon the percentage of moisture present in the sand and its fineness.
• Bulking increases gradually with moisture content up to a certain point and then begins to decrease with further addition of water due to the merging of films. At this stage, the bulking is practically nil.
• Sand increase in volume, to the extent of 20-30 %, when they contain moisture between 2-8 %. This is because moisture in small proportions forms thin films around the sand grains.
• Fine sands bulk greater than coarse sand. When the moisture content is increased beyond 8-10 %, the bulking of sand effect almost disappears.
• Thus, the bulking effect is maximum when the moisture content in the sand is between 4-6 %. As the water content increases, this effect goes on decreasing, becoming negligible at 15-20 % moisture content.
• The typical graphs shown in Fig. give the variation of percent bulking with moisture content. Finer sand bulks considerably more and maximum bulking is obtained at a higher water content.

• Bulking may be to the extent of 40 % of the original dry volume of sand in the fine sands and 15 % in the case of coarse sands.
• If the sand is measured by volume and no allowance is made for bulking, the mix will be richer than that specified because for given mass, moist sand occupies a considerably larger volume than the same mass of dry sand.
• This results in a mix deficient in sand increasing the chances of segregation and honey – combing of concrete. The yield of concrete will also be reduced.
• It is necessary, in such a case, to increase the measured volume of the sand by the percentage bulking, in order that the amount of sand put into concrete is the amount intended for the nominal mix used (based on dry sand).
• If no allowance is made for the bulking of sand a nominal concrete mix 1:2:4, for example, will correspond to 1:1.74:4 for a bulking of 15 %.
• An increase in bulking from 15 % to 30 % will result in an increase in the concrete strength by as much as 14 %. If no allowance is made for bulking the concrete strength may vary by as much as 25 %.
• The percentage of bulking can be determined by following this method: -
• A sample container is taken and it is filled with 2/ 3 part of sand. The height of sand is measured (h1). Now container is filled up to 2/ 3 part by water and sand is added and stirred and sand is allowed to settle down. Now the height of sand in water is measured (h2).
• Percentage Bulking of sand = h1 – h2/ h2 x 100

Soundness of Aggregate
• Soundness refers to the ability of aggregate to resist excessive changes in volume as a result of changes in environmental conditions, e.g. freezing and thawing, variation in temperature, alternate wetting and drying under normal conditions and wetting and drying in salt water.
• The aggregate is said to be unsound when volume changes result in the deterioration of concrete. This may appear in the form of local scaling to extensive surface cracking or to disintegration over a considerable depth, and thus vary from an impaired appearance to a structurally dangerous situation.
• IS:2386 (Part V) – 1963 describes soundness test. The soundness test consists of alternate immersion of sample in a solution of sodium or magnesium sulphate and oven drying it under specified conditions. The accumulation and growth of salt crystals in the pores of the particles is thought to produce disruptive internal forces similar to the action of freezing of water or crystallisation of salt. Loss in weight, is measured for a specified number of cycles.
• According to IS: 383 – 1970, the average loss of weight after 10 cycles should not exceed 12 % and 18 % when tested with sodium sulphate and magnesium sulphate respectively.


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Bulking of Sand & Its measurementSoundness of AggregateBulking of Sand