Why the well graded aggregates are preferred in concrete?

Proper classification of aggregates improves workability, uniformity, homogeneity and finishing qualities of the concrete mix. A good leveling system will help ensure strength, durability, etc. of concrete. Proper gradation of coarse-grated aggregates is one of the most important factors in the production of workable concrete.

Proper gradation ensures that a sample of aggregates contains all standard fractions of aggregate in the required ratio, so that the sample contains minimal voids. A well-graded aggregate sample that contains minimal voids will require a minimum of paste to fill the voids in the aggregates. Minimal paste means less cement and less water; leading to greater economy, greater strength, lower shrinkage 26% greater durability. Workability is improved when there is an excess of paste above that required to fill the voids in the sand, and also an excess of mortar (sand plus cement) above that required to fill the voids in the coarse-grained aggregate because the fine material lubricates the larger particles.

Cement paste or “matrix” that binds coarse-grated aggregates is weaker than aggregates. It is this matrix that is vulnerable to all the ills of concrete. It is more permeable and susceptible to deterioration from attack by aggressive chemicals. Therefore, the smaller the amount of such a weak link in the concrete, the better the concrete.

This objective can be achieved by having well-graded aggregates. It consists of particles of all sizes. The voids are almost filled with smaller aggregates that give the constant unit weight of the compacted mass. It is the most preferred classification of a global sample.

This sample has high stability and less permeability. The classification or size distribution of the aggregate is an important characteristic because it determines the paste requirement for workable concrete. This paste requirement is the factor that controls the cost, since cement is the most expensive component. Therefore, it is desirable to minimize the amount of paste consistent with the production of concrete that can be handled, compacted and finished, while providing the necessary strength and durability.

The required amount of cement paste depends on the amount of empty space to be filled and the total area to be covered. When the particles are of uniform size, the spacing is the largest, but when using a range of sizes, the voids are filled and the paste requirement is reduced. The more these voids are filled, the less workable concrete becomes, therefore, a compromise between workability and economy is necessary. A smooth surface can improve workability, but a rougher surface results in a stronger bond between the paste and the aggregate, creating greater strength.

However, such aggregates rated at maximum density give a hard concrete that is very difficult in ordinary concreting. Other things that remain the same, it can be said that concrete made of aggregate leveling having less surface area will require less water, which consequently will be the strongest. Aggregate classification is the particle size distribution of a sample of aggregates based on sieve analysis and sedimentation analysis. The density of the aggregates is necessary in the dosing of the mixture to establish weight-volume ratios.

It has also been discovered that the surface área of aggregates can vary widely without causing a very appreciable difference in the strength of the concrete and that the water required to produce a given consistency depends more on other characteristics of the aggregate than on the surface área. Screen analysis is a process in which aggregates are allowed to pass through a set of screens arranged in descending order. Aggregate classification: It is a measure of how well distributed the particle sizes are in an aggregate. Although aggregate is considered inert filler, it is a necessary component that defines the thermal and elastic properties of concrete and dimensional stability.

The particle size distribution of an aggregate mass should be such that the smaller particles fill the voids between the larger particles. A dense gradation refers to a sample having approximately equal amounts of aggregates of various sizes. In this sample, small size aggregates are present only in a small percentage, which results in more air voids due to the absence of fillers. Concrete is more workable when a smooth, rounded aggregate is used instead of a rough, angular, or elongated aggregate.

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