Question

Compare the role of aggregates in concrete and bituminuous materials

Answer 1

*Aggregate in concrete is a structural filler, but its role is more important than what that simple statement implies. Aggregate occupies most of the volume of the concrete. It is the stuff that the cement paste coats and binds together. The composition, shape, and size of the aggregate all have significant impact on the workability, durability, strength, weight, and shrinkage of the concrete. Aggregate can also influence the appearance of the cast surface, which is an especially important consideration in concrete countertop mixes.

When selecting the most appropriate aggregate for a particular concrete mix, here are the key factors to consider:

Material:

Most natural stones and crushed rock are appropriate for use in concrete. Commonly used stones are quartz, basalt, granite, marble, and limestone. If a concrete countertop is going to be ground with diamond tooling, the aggregate will show, so aesthetics also affect the choice of aggregates.

Problems arise with soft, reactive or weak stone or rock. Lightweight aggregates, a topic for another discussion, are also used in concrete.

Size:

Aggregate size and gradation are the most important factors when selecting aggregate. Aggregate can be large or small, from fist-sized rocks to fine sand. Aggregates larger than ¼ inch are classified as coarse aggregate, while anything smaller than ¼ inch is termed fine aggregate. As a general rule, the largest aggregate should be no greater in diameter than one-third the depth of the slab, or one-fifth the smallest dimension of the form. For example, the largest piece of aggregate allowed for a 1 ½-inch-thick countertop slab is ½ inch. Generally coarse aggregate is blended with finer aggregates (such as sand) to fill in the spaces left between the large pieces and to “lock” the larger pieces together. This reduces the amount of cement paste required and decreases the amount of shrinkage that could occur.

Shape:

Aggregate shape influences strength, but has more of an immediate impact on the workability of the plastic concrete. Rough, angular particles pack tighter, have more surface area, and have greater interparticle friction than smooth, rounded particles, which reduces workability. Angular particles also require a bit more cement paste to coat them than rounded particles. Therefore, mixes containing them will require a slightly higher cementitious content.

Gradation:

In general, coarse aggregates tend to be about 10 times larger than the fine aggregates in concrete, but the range of sizes could be greater than that in certain circumstances. As shown in the figure, there are three typical range categories:

• Well-graded aggregate has a gradation of particle sizes that fairly evenly spans the size from the finest to the coarsest. A slice of a core of well-graded aggregate concrete shows a packed field of many different particle sizes.
• Poorly graded aggregate is characterized by small variations in size. This means that the particles pack together, leaving relatively large voids in the concrete.
• Gap-graded aggregate consists of coarse aggregate particles that are similar in size but significantly different in size from the fine aggregate. A core slice of gap-graded concrete shows a field of fine aggregate interspersed with slightly isolated, large aggregate pieces embedded in the fine aggregate.

• 1.Almost 94 to 95% of hot mix asphalt (HMA) are aggregates. These have a nominal size ranging from 4 mm to 25 mm. The gradation of aggregates varies for different layers, depending on it's function. Aggregates as large as 40 mm are also used in the lower layers of a pavement.

  2.In cement concrete, the voids between the coarse aggregate and cement needs to be filled by fine aggregate in order to obtain maximum density and least number of voids, to maximize strength.

  3.In asphaltic or bituminous concrete, a minimum percentage of voids is required, so that the binder, which is bitumen, can be accommodated in these voids when a wheel load is applied. The load transfer due to grain to grain contact of the aggregates causes the thin film/coating of bitumen to displace and if the voids were absent then the binder (bitumen) would simply be ejected out, leading to a pavement distress, called bleeding.

  4.Refer to MORTH specifications for more details regarding gradation of aggregates. For DBM there are two gradings.