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Tuesday, July 16, 2024

Concrete Sand- Properties, Types, Bulking & Quality Control

Table of Content for Concrete Sand-

  1. Introduction
  2. The functions of Sand in Concrete & Mortar
  3. Desirable Properties of Sand
  4. Types of Sand
  5. Bulking and Test for Bulking
  6. Importance of Testing of Bulking
  7. Moisture Correction
  8. Quality Control at Site- Site and Laboratory

Sand is the basic construction material required to produce concrete and mortar. When sand is mixed with cement and water, a mixture is produced which hardens on the setting. The sand particles are bind together by the cement gel produced on the hydration of cement.

The sand is naturally produced by weathering of the rocks. The rock is broken into small pieces due to physical or chemical processed and transported to places by means of water, wind or by humans.

The sand can also be produced by mechanical grinding of the rocks. This sand is called artificial sand or manufactured sand or simply M-sand.

The sand is also called fine aggregate as it passes through the 4.75mm IS sieve. Thus, it can be said that the sand particles are smaller than 4.75mm. The sand particles should also be greater than the 75 µ.

IS 383:1970 gives the specifications for the coarse and fine aggregates from natural sources.

The functions of Sand in Concrete & Mortar

Sand is used in mortar and concrete for the following purpose-

  1. The sand sub-divides the paste of binding material into thin films and allows it to adhere and spread.
  2. Sand fills up the gap between the building blocks and spreads the binding material.
  3. It adds to the density of the mortar.
  4. The sand prevents the shrinkage of the cementing material.
  5. It allows the carbon dioxide from the atmosphere to reach some depth and thereby improve setting power.
  6. The cost of cementing material per unit volume is reduced as this low-cost material increases the volume of mortar.
  7. The silica of sand contributes to the formation of silicates resulting in the hardened mass.

Desirable Properties of Sand

The properties of good sand are as follows-

Concrete Sand Image Courtesy- Polytechnic Hub
  1. Sand should be chemically inert. It should not react with the cement or admixture and form an undesirable compound.
  2. It should be free from organic or vegetable matter. The organic matter may decompose later and leave the voids in concrete thus reducing the strength of the concrete.
  3. Sand should be free from salt. The salts produce patches on the concrete surface and adversely affects the durability of the concrete.
  4. It should contain sharp, angular and coarse grains. Round and smooth particles result in lesser strength as they don’t interlock among selves.
  5. Sand should be well graded. Well-graded sand fills more voids as compared to the poorly-graded or uniform sand.
  6. It should be hard. The sand particles should not crush under the load.

Types of Sand

1. Sea Sand

The particle size is too fine and too uniform for good class work. Sea sand used in its natural state causes corrosion to steel reinforcement due to the presence of salts. The higher percentage of salts also increase the setting time of the concrete, retards hardening, and may cause efflorescence.

The sea sand may be used after thorough washing. It must also be tested for organic impurities.

2. Pit Sand

The sand obtained from the dry river beds is called pit sand. It is the best kind of sand available. It has angular and rough particles thus it provides more strength to the concrete as compared to the river sand.

3. River Sand

The river sand is good quality sand. It may be contaminated with mud, silt or mica. It should be checked for the silt content before use. If the silt content is found higher than the allowed limits, the sand is to be washed to reduce the silt.

4. Crushed Stone Sand

This sand is also called Artificial Sand Or Manufactured Sand or M-Sand. The M-sand contains a higher amount of dust and needs to be washed before using. The M-sand particles can be flaky and may result in harsh concrete.

Bulking of Sand

The increase in the volume of the sand due to available moisture is called Bulking of Sand. The sand has minimum volume when absolutely dry. The volume increases with the increase in the moisture content it keeps increasing until the sand becomes saturated. The volume is equal for both the absolute dry condition and absolute wet condition.

The volume of the sand increases by 10-20% (can also be as high as 30%) for the moisture content of 2-5%. The bulking of sand also depends on the particle size. Smaller the particle size, higher is the bulking.

Test for Bulking at Site

Partly fill a container with the sand sample and level the top surface without pressing the sand. Measure the depth of the sample and record it as ‘D’. Now mix the sand with sufficient quantity of the water and stir well. Allow the sand to settle. Measure the depth of the settled sand as ‘D1’.

The percentage bulking is ((D-D1)/D1)*100

The principle of this test is that the volume of absolute wet sand is as same as the absolute dry sand and the bulking is nil in this condition.

Importance of Testing Sand Bulking

The increase in the volume of the sand may result in the wrong mix of concrete when the raw materials are mixed by volume.

Consider a mix of 1 cement, 2 sand and 4 coarse aggregates. For the 25% bulking, the sand to be used in the mix is 2*1.25 i.e., 2.5 parts. This means for every 1 part of cement; 2.5 part of sand and 4 part of coarse aggregate is required.

The correction required in sand volume due to bulking can be avoided if weight batching is used. In weigh batching, the materials are mixed in the ratio of their weights and not volume. For example, to produce a mix of 1:2:4, 1kg of cement requires 2 kg sand and 4 kg coarse aggregate.

Moisture Correction of Sand

The sand used in concrete is almost never absolutely dry. The presence of moisture in the sand may change from time to time and place to place due to change in temperature, stacking, rain etc.

The moisture in the sand can give higher slump thus decreasing the durability of the concrete. To get the required slump, the moisture content of the sand needs to be determined so the water required for the mix can be reduced suitably.

Quality Control of Sand

A. QC at Site

  1. Upon visual examination, the sand for concrete should be coarse and finer for the plaster.
  2. Rub the sand on damp palms, if the hand becomes dirty it shows the presence of silt. Further, test on checking the percentage of silt should also be done.
  3. No organic impurities like vegetative matter, leaves, twigs or dead animal remains etc. should be present in the sand.
  4. Spread the sand sample into the thin layer, all the material that can be crushed by finger should not be present. Materials crushed by finger are usually clay particles.
  5. The sand should not have deleterious materials like coal, shale, and mica etc.
  6. It should be chemically inert i.e.; it should not react with cement.
  7. The colour of the sand varies from brown to white. The shining sand indicates a higher percentage of mica.
  8. Taste a pinch of sand, if it tastes salty it shows the presence of the salts. The sand having salts should be washed before using.

B. QC at Laboratory

The following tests are done on the sand in laboratory.

1. Sieve Analysis

The gradation of the sand is done by IS sieve set as per the IS2386:1963 (Part-I).

Particle Size of Sand for concrete and masonry
Particle Size of Sand.
Image Courtesy- Quora

The dry sand sample is weighed and sieved through the IS sieve set. The weight passing through each sieve is reported as per cent of the total weight of the sample. The sand is classified into zones.

The zone classification for % passing for the sand as per IS 383:1970 is as follows-

Zones for Concrete, Mortar and Plaster Sand
Zones of Sand

The sand mean particle size decreases from Zone-I to IV. The sand to be used for concrete should fall in Zone-II.

The Fineness Modulus (FM) is also determined in the sieve analysis. It is calculated as the sum of cumulative percentage retained on each sieve divided by 100. The higher value of FM indicates coarse particles.

The sand can be classified on the basis of FM as follows-

Classification of Concrete Sand
Classification of Sand on the basis of Fineness Modulus

2. Estimation of Deleterious Materials & Organic Impurities

As per IS2386:1963 (Part-II). The maximum % of deleterious materials in the sand as per IS 383:1970 is as follows-

Allowable Deleterious material for concrete sand
Max Allowed % of Deleterious Materials in Sand

The silt content is generally determined by volume. The IS code doesn’t specify the silt content determination by volume, in this case, CPWD Manual should be referred. The silt content of the sand as per CPWD Manual should not exceed 8% by volume.

3. Specific Gravity, Density, Void Ratio, Absorption & Bulking

As per IS2386:1963 (Part-III). The density of the sand ranges from the 1400-1650 kg/cum depending on specific gravity, presence of moisture and compaction.

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Happy Engineering!

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