Does Aggregate Add Strength to Concrete?

Aggregates are a major component of concrete, making up between 60-80% of its volume and 70-85% of its mass. Not only do they play a key role in the strength, thermal and elastic properties, dimensional stability and volume stability of concrete, but they also have an impact on its workability. Rough, angular particles are more tightly packed, have more surface area and greater inter-particle friction than smooth, rounded particles. This reduces workability, as more cement paste is needed to coat them.

The absorption and surface moisture of aggregates is also important for achieving the desired strength. The shape of the aggregate influences strength, but has a more immediate effect on workability. Cement is more likely to be affected by shrinkage. Proper storage and reprocessing of stockpiles can counteract excessive segregation.

Classification of coarse-grained aggregate can minimize the possibility of segregation, especially for increased workability and improved concrete compatibility. Aggregate can also influence the appearance of the cast surface, which is an especially important consideration in precast concrete countertop mixes. All aggregates contain some moisture depending on the porosity of the particles and the moisture state of the storage area. These different aggregate properties allow designers and contractors the greatest flexibility to meet their design and construction requirements. For a given weight, the larger the maximum size of the aggregate, the smaller the surface area of the coarse-grated aggregates and vice versa. To achieve adequate workability, the volume of the cement paste must be high enough to encapsulate all aggregate particles and provide some workability while the concrete is fresh.

Other physical and mineralogical properties of the aggregate must be known before mixing the concrete to obtain a desirable mix. Aggregates must be solid, clean, hard, durable, and free of excessive fines or contaminants that could affect cement hydration or alter the paste-aggregate bond. Most stacked coarse-grained aggregates are in the AD state with an uptake of less than one percent, but most fine aggregates are often in the wet state with a surface moisture of up to five percent. Although well packed, the size contrast does not provide the same uniformity as a well-graded aggregate. The SI limits for tests such as aggregate crush value, added impact value and abrasion value must be taken into account when selecting and dosing concrete aggregate. A track record of good performance from local aggregates can also provide an indication of how it will perform in service. In summary, these are some of the most important factors to consider when selecting and dosing concrete aggregate.

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