Coarse Aggregate | Coarse Aggregate Size | Coarse Aggregate Sieve Analysis
Coarse aggregate is a building and construction ingredient made of stones or rocks extracted from ground deposits. Some common ground deposits include crushed stone, river gravel, rock from quarries, and even previously used concrete.
It consists mainly of quarry stones, gravel, uncrushed stones, and coarse aggregates greater than five millimeters.
Coarse Aggregate Sieve Analysis
Sieve analysis helps to determine the particle size distribution of the coarse and fine aggregates. The sieve analysis of coarse aggregate determines the gradation (the distribution of aggregate particles, by size, within a given sample) to determine compliance with design, production control requirements, and verification specifications.cement paste consumption
Standard Method for Sieve Analysis of Fine and Coarse Aggregates
Characteristics of Coarse Aggregates ( Per Shapes)
- Flaky aggregates, in which thickness is much lesser than the length or width
- Elongated aggregates possess well-defined edges formed at the intersection of roughly plainer faces.
- Angular aggregates are partly shaped and have well-defined edges.
- Rounded aggregates are fully worn out by water and have no edges and are completely shaped.
Extreme angularity and elongation increased the amount of cement required to give strength, difficulty finishing, and the effort required to pump. It also increased the amount of required mixing water. Properly graded angular particles produced a dense concrete and Flaky are elongated aggregates lead to failure of concrete structures. It reduces strength under repeated loading and vibrations.
An aggregate is classified as being flaky if it has a thickness, smallest dimension of less than 0.6 of its means of size.
The flakiness index often an aggregate sample is found by separating the flaky particles and expressing their mass as a percentage of the mass of the sample tested
An aggregate is classified as being elongated if it has a length greatest dimension of more than 1.8 of its means of size. The elongation index often an aggregate sample is found by separating the elongated particles and expressing their mass as a percentage of the mouth of the sample tested. It does not apply to a size smaller than 6.3 millimeters.
Flakiness and Elongation permissible limits for coarse aggregates used for all various construction works.
Mechanical Properties of Coarse Aggregate
Abrasion value is the percentage value due to relative rubbing action between the aggregate and steel balls used as an abrasive charge.
It is a relative measure of resistance to crushing under a gradually applied compressive load.
It means resistance to sudden impact or shock.
Aggregate Gradation and Proportion
Gradation means particle size distribution of aggregates. It is determined by sieve analysis. If the concrete contains all fractions of aggregates, such a mix will have minimal voids. Consequently, the mix with fewer voids will require minimum cement paste and minimal paste means less cement and less water.
Gradation of aggregates is one of the most influential aggregate characteristics in determining how it will perform. Gradation helps determine durability, porosity, workability, cement and water requirements, strength, and shrinkage.
If gradation is improper, more cement-based will be required, increasing the cost of concrete. It will also make concrete less dense.
The increase in voids ratio leads to more cement paste consumption, and hence the concrete becomes vulnerable to deterioration due to chemical attacks and becomes more permeable.
Gradation of aggregates directly influences:
- Cost-effectiveness due to lesser consumption of cement
- Higher-strength due to increasing density.
- Workability of concrete as it eases concrete placing increase makes concrete more durable
- Leads to lesser shrinkage cracks
Fine Aggregates: Fine aggregates for concrete work shall preferably fall to zone 1 -3 category. Based upon sand zoning concrete mix should be properly designed for proportioning.
Coarse Aggregate: Coarse aggregate size should be less than one-quarter of the structural member’s minimum thickness.
Coarse aggregate size should be such that the concrete can be placed without difficulty, surround all the reinforcement and fill the corners within formwork. For most concrete work, 20 millimeters aggregate is suitable.
Determining the Proportion of Coarse Aggregate
Based upon gradation done proportion of aggregates is determined. Generally, two sizes of downgraded coarse aggregates are utilized in construction; that is 20 millimeters and 10 millimeters.
The factors affecting the selection of coarse Aggregate is;
- The shape of the Aggregate like rounded, irregular or flaky
- Maximum coarse aggregate size,
- Zone particle sizes,
- Degree of workability required and
- Amount of permissible waters cement ratio.
While mixing aggregates, the coarse and fine Aggregate shall be batch separately. On an average concrete contains 20 to 30% fine aggregates and 50 to 60% coarse aggregate.
Different size of aggregates reduces the gap in concrete and makes concrete denser. Poorly graded concrete contains particles of the same size and leaving large voids in concrete(gaps).
Graded aggregates have medium, that is intermediate size particles missing and leading to consumption of cement.
Coarse Aggregate Specific Gravity
Coarse Aggregate Specific Gravity is calculated by dividing the oven-dry Aggregate’s weight by the absolute volume of the same Aggregate, excluding the void content in the aggregate particles. The specific gravity of aggregates normally ranges from about 2.5 to 3.0. It is the typical data that is validated to be used in concrete mix proportioning calculation.
Aggregate specific gravity is required to evaluate weight-to-volume correlation and calculate other volume-related quantities like voids in mineral aggregate and voids filled by asphalt. Absorption can be used as an indicator of aggregate durability and the volume of asphalt binder it is likely to absorb.
Coarse Aggregate Density
The density of coarse Aggregate is the aggregate mass per unit volume of coarse Aggregate. The density is expressed in kg / m3 or lb / ft3 and indicates the Aggregate’s compactness. Most aggregates have a relative density between 2.4-2.9 with a corresponding particle density (Mass) of 2400-2900 kg / m3 (150-181 lb / ft3).
Storage of Aggregates.
Aggregates should always be stored on hard and firm ground. The ground should have a sufficient slope so that adequate water can be drained.
Aggregates should be segregated based on their type of grade and source. This is done by constructing separate storage bins. Provision should be made to prevent intrusion of foreign matter. In case of aggregate becoming wet, they shall not be used for at least 24 hours, and in hot areas always provide shelter to control the temperature for aggregates.