How to Choose Good Quality Aggregates for Construction?
Aggregates play an important role in the properties of fresh and hardened concrete. The gradation of aggregates, its maximum size, unit weight, and moisture content affects the overall character and performance of a concrete mix.
Selection of good quality aggregates, not only provide a concrete with higher quality but also with high economy. That means if we select maximum allowable aggregate size for the mix, reduces the cement requirement. As cement is the most expensive ingredient in a concrete mix, it finally lowers the cost of concrete mix.
The use of less cement, means the water requirement is less given that the water-cement ratio is kept constant.
A lower water-cement ratio helps reduce the possibilities for shrinkage and other cracking issues that arise due to restrained volume change in concrete.
Enlisted below are the most important factors that needs to be considered while choosing good quality aggregates for concrete mix design.
1. Choose Aggregates with Good Quality Counts
Always make sure that good quality aggregates are purchased that is verified by regular test results in compliance with ASTM C 33, “Standard Specifications for Concrete Aggregates.” For this, you can check the history of performance of the local aggregate to know how well the material has performed.
Use aggregates:
- Clean, hard and strong
- Have durable particles
- Free from absorbed harmful chemicals, coatings of clay or contaminants
Avoid aggregates:
- Friable aggregates that have chances to split
- With soft and porous materials
- Presence of “chert” that can cause popouts
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| Presence of Chert Causing Popout |
The allowable percentage of deleterious substances for both fine and coarse aggregate are listed in ASTM C 33 Tables 1 and 3
2. Choose Aggregate with Good Gradation
Aggregates with optimized gradation helps to achieve a concrete mix with good workability and finishability.
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| Well-graded particle size distribution for this conventional concrete mix. |
The proportion between fine and coarse aggregate changes based on the unique characteristics of each aggregate, their placement method and the required finish.
- Fineness modulus (FM) of aggregates is an index of fineness of an aggregate.
- FM of fine aggregates must fall within 2.3 to 3.1 and must not change more than 0.2.
- Excessive fine materials (, possess high surface area that demand high water content that result in sticky mix.
- Excessive coarse materials produce harsh mix, which is difficult to place, consolidate and finish.
- The gradation of fine aggregates must fall within the limits as per ASTM C33-Section 6. Any deficiency in this requirement can be solved using air entrainment, adding cement, or any supplementary cementitious material (SCM).
- The gradation for coarse aggregates is stated in ASTM C 33 Table-2. To avoid excessive segregation of particles, rework the piles to blend all the sizes and then perform batching process.
3. Choose the Aggregate Size
Coarse aggregates are aggregate particles with a size greater than 4.75 mm. Usually, a size between 9.5 mm and 37.5 mm in diameter is employed.
Fine aggregates are usually sand or crushed stone which is less than 9.55 mm in diameter.
The most common size of aggregate used in construction is 20 mm and larger sizes like 40 mm create a mass of concrete.
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| ACI 318 requirements for the nominal maximum size of aggregates, Dmax, based on concrete dimensions, B, T, and reinforcement spacing, S. |
The maximum size of the coarse aggregate depends on the size, shape, and reinforcement. The size of coarse aggregate must not exceed:
- 3/4th of the clear spacing between the rebar or cover
- 1/3 the depth of the slab
- 1/5th the narrowest dimension of a member
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