Factors Affecting Concrete Workability and Key Testing Methods

Concrete is a composite material composed mainly of water, aggregate, and cement. Usually, there are reinforcements and additives added to the final product to give it the required physical characteristics. These materials combine to create a fluid mass that is simple to shape and mold. The remaining elements are combined with the cement over time to create a strong, multip

The majority of a concrete mixture is composed of fine and coarse particles. The most common materials used for this are crushed stone, sand, and natural gravel.

Concrete production is the process of mixing together various ingredients—water, aggregate, cement, and any additives—to produce concrete. Concrete production is time-sensitive. After the components are combined, laborers have to pour the concrete before it solidifies.

The majority of concrete produced in the modern era is produced in massive industrial facilities known as concrete plants, also frequently referred to as batch plants. In general, there are two primary types of concrete plants: central mix plants and ready mix plants. Whereas a central-mix plant mixes every ingredient including water, a ready-mix plant mixes every ingredient excluding water. Since hydration starts at the plant, a central mix plant must be located closer to the work site where the concrete will be utilized. This allows for more precise control over the quality of the concrete through better measurements of the amount of water injected.

Workability

It is the amount of useful work done internally to produce full compaction. Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) without reducing the concrete’s quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content, and age (level of hydration) and workability can be modified by adding chemical admixtures, like superplasticizers. Raising the water content or adding chemical admixtures increases concrete workability.

Factor affecting workability

Water Content

Water Content requires a water/cement ratio of about 0.25 for hydration and a water/cement ratio of 0.15 for filling the voids in the gel pores. Put another way, a water/cement ratio of roughly 0.38 would be necessary to both hydrate and fill the gel pores with cement particles. The amount of water in a given volume of concrete will greatly affect how workable it is. One of the key elements influencing workability is concrete’s fluidity, which increases with the amount of water per cubic meter of concrete. In order to maintain the same strength, more water can be added as long as an equivalent amount of cement is also added to maintain a steady water/cement ratio.

Abrams’ law (also called Abrams’ water-cement ratio law) The law states the strength of a concrete mix is inversely proportional to the mass ratio of water to cement. As the water content increases, the strength of the concrete decreases.

S= \left(\frac{A}{B^x}\right) where x is the water-cement ratio and A, B are constant.

Mix proportions: The ratio of cement to aggregate has a significant impact on workability. The concrete gets thinner as the aggregate/cement ratio increases. Lean concrete has limited aggregate mobility because there is less paste available per unit surface area of aggregate to provide lubricating. However, more paste is available to make the mix cohesive and fatty, improving workability, in the case of rich concrete with a lower aggregate/cement ratio.

• The higher the ⁄ ratio, the leaner the mix, and the leaner mix is less workable.
• The lower the ⁄ ratio, the richer the mix and the more workable.
• Flaky particles reduce workability because of higher surface area

As per IS 456 clause 5.4.1.2, the average 28-days compressive strength of at least three 150mm cubes prepared with water proposed to be used shall not be less than 90% of the average of strength of three similar concrete cubes prepared with distilled water.

Surface texture

Surface texture: The total surface area of rough-textured aggregate is greater than that of smooth-rounded aggregate of the same volume, which explains how surface texture affects workability. Based on previous conversations, it can be deduced that aggregate with a rough texture will have low workability, whereas aggregate with a smooth or glassy texture will have higher workability. Higher workability is also influenced by smooth aggregates’ decreased inter-particle frictional resistance.

Shape of aggregates

Aggregate shape: Aggregate shape has a significant impact on workability. When compared to spherical or cubical-shaped aggregates, concrete with angular, elongated, or flaky aggregate is extremely harsh. Because rounded aggregate will have less surface area and fewer voids than angular or flaky aggregate for a given volume or weight, it will be more workable. In addition, the frictional resistance is significantly decreased due to its round shape. This explains why concrete is more workable with river sand and gravel than with crushed sand and aggregate.

Grading of aggregates:

Grading of aggregates: This is one of the factors which will have maximum influence on workability. A well-graded aggregate is the one that has the least amount of voids in a given volume. Other factors are constant, when the total voids are less, excess paste is available to give a better lubricating effect. With an excess amount of paste, the mixture becomes cohesive and fatty which prevents segregation of particles. Aggregate particles will slide past each other with the least amount of compacting effort. The better the grading, the less is void content and the higher the workability.

The water-cement ratio needed for cement to complete its hydration process ranges from 0.22 to 0.25. More water in the mixture is required to make finishing and putting concrete easier (workability of concrete). Lowering a mixture’s water content may make it stiffer, which decreases workability and raises the possibility of placement issues. If concrete is easily mixed, poured, and completed, it is considered workable. There shouldn’t be any bleeding or segregation in a workable concrete.

Segregation

Segregation – segregation is said to occur when coarse aggregates depart from the finer aggregate. i.e. get the concentration of coarse aggregates at one place and finer aggregates at another. Segregation is caused when concrete is dropped from a considerable height or changes in direction while placing concrete. The danger of segregation can be reduced by the use of air entrainment. Segregation can be reduced by air entrainment in concrete.

Bleeding

Bleeding – Bleeding of concrete is said to occur when the excess water comes up at the surface of concrete. Bleeding is a function of (a) air velocity, (b) temperature, and (c) humidity. If the rate of bleeding is roughly equal to the rate of evaporation, then bleeding will not cause any problems. If the rate of bleeding is less than the rate of evaporation, then the surface becomes dry, because of which cracks appear on it. Bleeding can be reduced by using fine cement/use of pozzolans. Bleeding is lower with finer cement, cement having high C3A content, or when calcium chloride is added. Rich mixes are less prone to bleeding than lean ones. Air entrainment effectively reduces bleeding.

Efflorescence

Efflorescence – White substance coming at the top of the concrete surface. The presence of chloride in water is responsible for effloresces. So, seawater should not be used for RCC. But if for the entire life, the structure will be submerged in the sea then seawater can be used.

Test for Workability

Slump Test

Slump test ( Unit –mm) The mold for the slump test is the frustum of a cone, as shown in the figure. Concrete is filled in three layers and the cone is slowly lifted and the unsupported concrete will now slump. The decrease in the height of the center of the slumped concrete is called slump and is measured in ―mm. A slump test is used to check hour-to-hour workability on site.

Compaction factor test

The concrete is filled at the top hopper and allows falling in the lower cylinder to pass through the middle hopper. The actual density of concrete in the cylinder is measured and the compaction factor is calculated as follows

Vee – Bee test

Vee – Bee test (Unit-sec) is a lab test in which using vibration, the time is calculated for the change of state of concrete from cone to cylinder. This is a good test for dry concrete whose slump value can’t be determined.

Ball penetration test

Ball penetration test:– (Unit –mm) This test is the same as the slump test that can be used in the field. In this test, a metal hemisphere of 152mm diameter and 14kg weight is sank in the concrete under its own weight, and sinking depth is measured

Flow table test

This test is popular for concrete made with superplasticizer admixtures.

Table showing the degree of workability for different values of slum, compaction factor, and Vee-bee time.

Factors Affecting Concrete Workability and Key Testing Methods

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