Aggregates In Concrete - Standard Properties
Aggregates are normally considered as an inert filler inside a concrete mix. But the aggregates in concrete can significantly affect the properties of both fresh and hardened concrete. The character & performance of the concrete mix solely depends on the gradation, maximum size, unit weight, and moisture content of any aggregate.
Proper selection of maximum permissible aggregate size in concrete can reduce the cost considerably. The requirements of cement in a concrete mix is lowered with the use of greater coarse aggregate. Least cement (within acceptable limits for stability) will require less water when the water-cement (w/c) ratio remains constant. The lesser water content will minimize the scope for shrinkage & cracking which are correlated with restrained volume change.
Given below, some vital conditions for selecting and proportioning concrete aggregate :
Typical Aggregate Proportions
Aggregates contain much as 60% to 80% of a standard concrete mix. For this reason, they should be chosen cautiously to be long-lasting, blended for maximum efficiency, and perfectly controlled to retain sound concrete strength, workability, finishing ability, and longevity.
The proportion of the materials for a good concrete mix should be as follow :-
- Cement : 7% - 15%
- Aggregate : 60% - 80%
- Water : 14% - 18%
- Air : 2% - 8%
Properties of Good Quality Aggregate
It should be clean, solid, strong with durable particles, and does not contain absorbed damaging chemicals, coatings of clay, or other contaminants which can influence the hydration of cement as well as weaken the paste-aggregate bond. The following types of aggregates should not be used:
- Brittle or vulnerable to being split.
- Containing major amounts of soft and porous materials.
- Specific types of "chert," as they contain very poor resistance to weathering and result in developing surface defects known as popouts
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When the aggregate gradations are superior on the basis of aggregate availability and project requirements, an inexpensive concrete is formed with proper workability and finishing ability. The ratios among coarse and fine aggregates are altered depending on the unique characteristics of each aggregate, the placement method, and the finish required.
The partition line between fine and coarse aggregate should be the 3/8-inch sieve. The fineness modulus (FM) stands for an index of the fineness of an aggregate. To calculate FM, the cumulative percentages are included with mass retained on each of a specified series of sieves and the sum is divided by 100. The FM for fine aggregate should remain between the range of 2.3 to 3.1. The FM should not change in excess of 0.2; if not, mix adjustments are required. Too many fine materials will require greater water demand and lead to a sticky mix. Too much coarse material will form harsh mixes that can't be arranged, solidified, and finished smoothly & properly.
As soon as a gradation is chosen for a project, the gradation should be retained within tight ranges to attain superior batch-to-batch consistency. To achieve this, the aggregated should be accumulated properly and reworked to resist too much segregation.
The followings are the maximum sizes of coarse aggregate to be utilized in a mix based on the size, shape, and reinforcing of a member :-
- 3/4 of the clear spacing among rebar or rebar and forms (cover distance)
- 1/3 the depth of the slab
- 1/5 the narrowest dimension of a member
Often aggregates are examined with combined grading of fine and coarse materials proportioned as intended in the proposed concrete mix. It creates an estimate of how the mix will function in concrete. Every region contains its own deficiencies in aggregates, but as soon as combined aggregate gradation is plotted (percentage retained vs. sieve size), these deficiencies are easily recognized and resolved. Substitute aggregate sources or supplementary aggregate blending should be used to process the elusive "ideal" gradation that maintains the best workability, pumpability, reduced shrinkage, and economy.