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Aggregates form about 75% of concrete by volume. In terms of weight it may form up to 85% of concrete. Therefore attention has to be given on the selection and quality control of aggregates.
Although aggregates are inert in nature it has a strong effect on the concrete's thermal and elastic properties. And although it only acts as a filler, it affects the dimensional stability of concrete.
The compressive strength of aggregates is an important factor in the selection of aggregates. Most concrete aggregates have compressive strengths much higher than other ingredients of concrete.
Physical and mineralogical properties of aggregate have to be tested before being used in concrete mixes. Examples of such shape and texture, size gradation, moisture content, specific gravity, reactivity, soundness and bulk unit weight. These properties in addition to the water-cement ratio affects the strength, workability, and durability of concrete.
Meanwhile, the shape and texture of aggregate affects the workability of fresh concrete. Workability is better when aggregates are smooth and rounded compared to those with rough angular or elongated shapes. Most natural sands and gravel from river beds are smooth and rounded. They can be considered good choice of aggregates.
On the other hand, crushed stones are more angular or elongated. They have a higher surface-to-volume ratio. As a result, it allows better bonding but require more cement paste to get better workability.
The surface texture of aggregates also have an effect on workability. Aggregates with smooth surface will give better workability. Aggregates with rough surface provides a better bond between the cement paste and aggregates contributing to higher strength.
Aggregate grading or size distribution affects the requirements of cement paste for workability. And this affects the cost since cement costs more than other ingredients. Therefore the common goal would be to minimise the amount of cement paste and yet maintain sufficient workability that allows easy handling, compaction and finishing and yet achieving the required strength and durability.
The required amount of cement paste depends on the amount of voids that need to be filled and the total surface area to be covered. Aggregates of uniform size provides the greatest spacing through the available voids and thus provide high workability but the cost is higher. If we were to use a range of sizes, voids are filled in a higher degree, so the requirement for cement paste is lower resulting in lower cost but the workability is lower. So, in the selection of aggregates, the trick is to strike a balance between workability and cost.
The moisture content is an important factor as it affects the water-cement ratio. Aggregates normally contain moisture to a certain degree depending on the porosity of the particles as well as the moisture condition of the storage area. The moisture content may range from less than one percent in gravel to up to 40 percent in very porous sandstone and expanded shale.
Aggregates can be found in four different moisture states:
Oven-dry (OD),
Air-dry (AD),
Saturated-Surface Dry (SSD) and
Wet.
Of these four states, only OD and SSD correspond to a specific moisture state and can be used as reference states for calculating moisture content. In order to calculate the quantity of water that aggregate will either add or subtract to the paste, the following three quantities must be calculated:
absorption capacity,
effective absorption, and
surface moisture.
Most stockpiled coarse aggregates are in the AD state with an absorption of less than one percent, but most fine aggregates are often in the wet state with surface moisture up to five percent. This surface moisture on the fine aggregate creates a thick film over the surface of the particles pushing them apart and increasing the apparent volume. This is commonly known as bulking and can cause significant errors in proportioning volume.
The density of the aggregates is required in mixture proportioning to establish weight-volume relationships. Specific gravity is easily calculated by determining the densities by the displacement of water. All aggregates contain some porosity, and the specific gravity value depends on whether these pores are included in the measurement.
There are two terms that are used to distinguish this measurement;
Absolute Specific Gravity (ASG) - refers to the solid material excluding the pores,
Bulk Specific Gravity (BSG) - includes the volume of the pores.
For the purpose of mixture proportioning, it is important to know the space occupied by the aggregate particles, including the pores within the particles.
The BSG of an aggregate is not directly related to its performance in concrete, although, the specification of BSG is often done to meet minimum density requirements.
For mixture proportioning, the bulk unit weight is required. The bulk density measures the volume that the graded aggregate will occupy in concrete, including the solid aggregate particles and the voids between them.
Since the weight of the aggregate is dependent on the moisture content of the aggregate, a constant moisture content is required. This is achieved by using OD aggregate. Additionally, the bulk density is required for the volume method of mixture proportioning.
Although aggregates are inert and act as filler in concrete, the different properties of aggregate have a large impact on the strength, durability, workability, and economy of concrete. Although these different properties of aggregate allow flexibility to meet their design and construction requirements, they have to be monitored to ensure compliance to be able to achieve the required properties be they of fresh or hardened concrete.