Gradation of Coarse Aggregate by Sieve Analysis

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AIM OF THE EXPERIMENT: To determine the gradation of coarse aggregate by sieve analysis.

CODE OF REFERENCE:

• IS 383 (1970): Specification of coarse aggregate and fine aggregate from natural sources for concrete [CED 2: Cement and Concrete]

APPARATUS USED:

• Weight pan.
• IS Sieve of sizes – 40 mm, 20 mm, 10 mm, 4.75 mm.
• Round pans to fit sieve
• Brushes
• Sieve Shaker

Fig 1: Sieve Shaker motor operated
COURTESY: MATEST

THEORY
Coarse aggregate is the one retained on 4.75 mm IS Sieve. When the aggregate contains sizes differently in suitable proportions, it is called graded aggregate. The workability of concrete is also improved with well graded aggregate. Single size aggregates make the concrete harsh and does not work well with trowel and between the cables, strands or sheathing. The coarse aggregate may be of the following types –

• Crushed ravel or stone which is obtained by crushing of hard stone or gravel.
• Uncrushed stone or gravel resulting from natural disintegration of rock.
• Partially crushed stone or gravel which is obtained by blending of above two types.

The graded coarse aggregate is defined by its nominal size i.e., 40 mm, 20 mm, 16 mm, 12.5 mm, etc. For example, a graded aggregate of nominal size 12.5 mm means an aggregate most of which passes the 12.5 mm IS Sieve. Because the aggregates are formed due to natural disintegration of rocks or by the artificial crushing of rock or gravel, they derive their many of the properties from the parent rocks. These properties are chemical, mineral composition, petrographic description, hardness, strength, pore structure, colour, etc. Other properties which are not possessed by parent rock or gravel are shape, size, texture, absorption, etc. Furthermore, sub-classifications are there like in shape, round, angular varieties are there, in unit weight classification normal weight, lightweight is there and many more. All these properties may have a considerable effect on the quality in fresh and hardened states.

Within these limits, the nominal maximum size of coarse aggregate may be as large as possible for low to moderately high strength concrete. The bigger the size of aggregate, lesser is the surface area and hence less amount of water is required for wetting the surface and less matrix or paste is required that lubricates the surface so as to reduce internal friction. For a given quantity of water and paste, bigger size of aggregate will give higher workability.Coarse aggregates are used in various tests like slump test, vee bee test, etc. in order to ascertain the suitability of coarse aggregate in making better quality of concrete.

RELEVANCE OF THE EXPERIMENT:

• This test is performed as it determines the paste requirement for workable concrete. Moreover, this paste requirement controls cost.
• Moisture content affects workability of concrete. This is important to control economy. Hence it is required to reduce the amount of paste consistent with the production of concrete that can be handled, compacted and finished while providing strength and durability.
• This is evaluated in order to provide strength and durability to concrete. It is also important to provide concrete thermal stability.
• Gradation test can also be done to predict the performance of a given soil sample. Moreover, Engineers calculate these consolidations to have better structural analysis.
• If this property is not evaluated then estimation of concrete for working will not be possible as a result economic crisis will emerge.

PROCEDURE

1. The sample should be oven dried at temperature 100⁰C to 110⁰C before weighing and sieving.
2. The dried sample of coarse aggregate is weighed.
3. The weighed sample is placed on the uppermost sieve. The sieve is arranged according to sizes.
4. The sieve with biggest opening is kept on top while the sieve with smallest opening is kept at bottom.
5. Below the sieve of smallest opening, pan is kept in order to collect the remaining samples which does not conform to the sieve sizes above.
6. A brush is used to clean the additional particles or impurities remaining on the sieve shaker.
7. After making all the arrangement the sieve shaker is turned on and it is operated continuously for around 10 – 15 minutes in order to obtain correct results.
8. After operating the shaker is turned off and the sieves are taken out for weighing the remaining samples on each of the respective sieves.
9. Cumulative weight passing through each sieve sizes are calculated as a percentage of the total sample weighed.

OBSERVATIONS AND RESULTS
The coarse aggregate both graded and single sized shall be supplied in nominal sizes as per specifications given as under:

Weight of sample taken = __________ kg.

Table 1

The minimum weight for different nominal sizes of aggregates may be taken as below:

Table 2

RESULT
The sample of aggregates does/does not conform to IS 383-1970.

DISCUSSION

• Past record investigate that aggregate gradation dictates the proportion of aggregate to cement paste in concrete can play a major variable that determines the overall durability of construction material.
• Aggregate gradation is defined as the particle size distribution of both the stone and sand present in the concrete matrix.
• This property of aggregate has been heavily undergoing research for over a century but the effects of which on concrete properties is still somehow misunderstood.
• This experiment proves to be a helpful tool to distinguish whether the cement paste is mortar or concrete.

PRECAUTIONS
• The hand gloves should be worn during the experiment so as to avoid any injury.
• Safety shoes and aprons at the time of test must be worn for protection.
• Cleaning and drying before the commencement of test is recommended.
• Careful sieving should be adopted.
• The electric balance should be checked before weighing.
• Sieves should be arranged in correct order.

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