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The aggregates used in the production of concrete are inert granular materials such as gravel, crushed stone, sand, slag, recycled concrete, and geosynthetic aggregates. The aggregates may be natural, manufactured, or recycled.
The aggregates are broadly classified into two types based on the size, namely- fine aggregate and coarse aggregate.
In this article, we discuss the properties of the standard aggregates used in the production of concrete as per the American Standards (ASTM- C33/C33M).
The fine aggregate shall consist of natural sand, manufactured sand, or a combination thereof.
The fine aggregate shall not pass 45% from a single sieve and retained on the next consecutive sieve as shown in table-1. The fineness modulus of the fine aggregate shall be not less than 2.3 and more than 3.1. The fineness modulus shall not vary more than 0.20.
Table 1: Sieve Size and Passing Percentage of Aggregates.
|4.75-mm (No. 4)||95 to 100|
|2.36-mm (No. 8)||80 to 100|
|1.18-mm (No. 16)||50 to 85|
|600-µm (No. 30)||25 to 60|
|300-µm (No. 50)||5 to 30|
|150-µm (No. 100)||0 to 10|
2. Deleterious Substances
The range of deleterious substance permissible in the fine aggregate as per the ASTM is prescribed in the Table-2 below-
Table 2: Deleterious Substance in Fine Aggregate
If the organic impurities exceed the prescribed limits, the aggregates shall be subjected to the test for organic contaminants and producing a
color darker than the standard shall be rejected provided that discoloration is due to coal, lignite, or similar discrete particles.
The fine aggregate failing in the test is not prohibited, provided that, when tested for the effect of organic impurities on the strength of mortar, the relative strength at seven days, calculated in accordance with test method ASTM-C87, is not less than 95 %.
The fine aggregate shall be subjected to 5 cycles of the soundness test, which shall have a weighted average loss not higher than 10% and 15% when sodium sulfate and magnesium sulfate are used respectively.
If the sample of the fine aggregate tested fails, the concrete shall be made with the use of the same fine aggregate and tested for freezing and
thawing tests as per ASTM C666/C666M.
The coarse aggregate shall consist of gravel, crushed gravel, crushed stone, air-cooled blast furnace slag, or crushed hydraulic-cement concrete or a combination thereof, conforming to the specification as given below,
The coarse aggregates shall conform to the requirements as prescribed in
Table 3 for the size number specified above.
Table 3: Grading Requirement of Coarse Aggregate
2. Deleterious Substances
The limits for the deleterious substance in the coarse aggregates shall be within the range prescribed in the Table 4 below-
Table 4: Deleterious Substance in Coarse Aggregate
If the class is not specified, the requirements for Class 3S, 3M, or 1N shall apply in the severe, moderate, and negligible weathering regions, respectively as picturized in the figure below-
The coarse aggregate used in concrete shall be subject to wetting, extended exposure to the humid atmosphere or contact with the moist ground shall not contain any materials that are deleteriously reactive with the alkalies in the cement in an amount sufficient to cause excessive expansion of mortar or concrete.
Methods of Sampling and Testing
The sample and testing for the fine and coarse aggregates shall be in accordance with the following methods, except as otherwise provided in this specification.
- It is not prohibited to use the same test specimen for sieve analysis and the determination of material finer than the 75-µm (No.200) sieve.
- The use of separated sizes from the sieve analysis is acceptable for soundness or abrasion tests. However, additional test specimen preparation is required.
- For other test procedures and the evaluation of potential alkali reactivity, when required, use independent test specimens.
Sampling of Aggregate
The sampling for the aggregates shall be in accordance with Practice D75 and Practice D3665.
Testing of Aggregate
The types of tests on aggregate shall be as tabulated in the table below-
Table 5: Recommended Test for the Aggregates as per ASTM
|Name of Test on Aggregate||ASTM Reference|
|Grading and Fineness Modulus||ASTM C136 - Test Method for Sieve Analysis of Fine and Coarse Aggregates.|
|Amount of Material Finer than 75-µm (No. 200)Sieve||ASTM C117 - Test Method for Materials Finer than 75-µm (No. 200) Sieve in Mineral Aggregates by Washing.|
|Organic Impurities||ASTM C40 - Test Method for Organic Impurities in Fine Aggregates Concrete.|
|Effect of Organic Impurities on Strength||ASTM C87- Test Method for Effect of Organic Impurities in Fine Aggregate on Strength of Mortar.|
|Soundness||ASTM C88 - Test Method for Soundness of Aggregates by Use of Sodium Sulfate or Magnesium Sulfate.|
|Clay Lumps and Friable Particles||ASTM C142 - Test Method for Clay Lumps and Friable Particles in Aggregates.|
|Coal and Lignite||ASTM C123 - Test Method for Lightweight Particles in Aggregate.|
|Bulk Density of Slag||ASTM C29/C29M - Test Method for Bulk Density and Voids in Aggregate.|
|Abrasion of Coarse Aggregate||1. ASTM C131 -Test Method for Resistance to Degradation of SmallSize Coarse Aggregate by abrasion and Impact in the Los Angeles Machine.|
2. ASTM C535 - Test Method for Resistance to Degradation of LargeSize Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine.
|Reactive Aggregates||Appendix X1 of ASTM C33/C33M - Methods for Evaluating Potential for Deleterious Expansion due to Alkali Reactivity of an Aggregate.|
|Freezing and Thawing||ASTM C666/C666M - Test Method for Resistance of Concrete to Rapid Freezing and Thawing.|
|Chert||1. ASTM C123 - Test Method for Lightweight Particles in Aggregate (used to identify particles in a sample of coarse aggregate lighter than 2.40 specific gravity).|
2. Guide C295 - Guide for Petrographic Examination of Aggregates for Concrete (to identify which of the particles in the light fraction are chert).
The properties of concrete such as workability, durability, strength, weight, and shrinkage are governed by the properties (composition, shape, and size) of aggregate used in the concrete.
Generally, aggregate passing the seive of 20mm and retaining the seive of 12.5mm is preferred for the production of concrete.