Aggregates for Concrete
Aggregates must conform to certain requirements and should consist of clean, hard, strong, and durable particles free of chemicals, coatings of clay, or other fine materials that may affect the hydration and bond of the cement paste. The characteristics of the aggregates influence the properties of the concrete.
Weak, friable, or laminated aggregate particles are undesirable. Aggregates containing natural shale or shale like particles, soft and porous particles, and certain types of chert should be especially avoided since they have poor resistance to weathering.
Characteristics of Aggregates
Resistance to Freeze Thaw:
(Important in structures subjected to weathering) – The freeze thaw resistance of an aggregate is related to its porosity absorption, and pore structure. Specifications require that resistance to weathering be demonstrated by the magnesium sulfate test.
(Important in pavements, loading plat-forms, floors, etc.) Abrasion resistance is the ability to withstand loads without excessive wear or deterioration of the aggregate.
(Important to strength and durability of all types of structures) Aggregates must not be reactive with cement alkalies. This reaction may cause abnormal expansion and mąp-cracking of concrete.
Particle Shape and Surface Texture:
(Important to the workability of fresh concrete) Rough textured or flat and elongated particles, due to their high surface area, require more water to produce workable concrete than do rounded or cubical aggregates.
(Important to the workability of fresh concrete) The grading or particle size distribution of an aggregate is determined by sieve analysis.
Specific Gravity (Density):
The specific gravity of an aggregate is the ratio of its weight to the weight of an equal volume of water at a given temperature. Most normal weight aggregates have a specific gravity ranging from 2.4 to 2.9. It is not a measure of aggregate quality. It is used for certain computations in a mix design.
Absorption and Surface Moisture:
The moisture conditions of aggregates are designated as:
Oven-Dry: Fully absorbent.
Air-Dry: Dry at the surface but containing some interior moisture, thus somewhat absorbent.
Saturated Surface-Dry: Neither absorbing water from, nor contributing water to the concrete mix.
Wet with free moisture: Containing an excess of moisture on the surface.
Batch weights of materials must be adjusted for moisture conditions of the aggregates.
Dry-rodded unit weight:
Dry-rodded unit weight is the mass (weight) of one cubic meter (foot) of dry coarse aggregate that is compacted, by rodding in three equal layers, in a standard container. For any one aggregate the dry-rodded unit weight varies with the size and gradation.
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