What is the workability of concrete?
The workability of concrete is the ability in its plastic phase, in which the fresh concrete can be transported and placed without segregation and bleeding.
The workability of the fresh concrete determines how easily and uniformly the fresh concrete can be mixed, laid, compacted, and finished.
Importance of workability of concrete
The workability of the fresh concrete is directly proportional to the water content. The strength of the concrete is inversely proportional to the water-cement ratio.
What are the different types of concrete workability ?
The workability of concrete can be classified into the following five degrees of workability of concrete:
- Very Low workability concrete. (measured by compaction factor test)
- Low workability concrete. (measured by slump test)
- Medium workability concrete. (measured by slump test)
- High workability concrete. (measured by slump test)
- Very high workability concrete. (measured by Flow test)
What are the factors affecting workability of Concrete?
The workability of concrete depends on the following properties of concrete.
- Water Content
- Use of Admixtures
- Mix Proportions
- Grading of Aggregate
- Shape of Aggregates
- Size of Aggregates
- Surface Texture of Aggregate
How water-content affect the workability of concrete?
The water content in a given volume of concrete has a significant influence on the workability of concrete. The higher the water content per cubic meter of concrete, the greater the workability of the concrete.
It should be noted that from a desirable point of view, increasing the water content is the last solution to be adopted to improve workability even in uncontrolled concrete.
For controlled concrete, the water content cannot be arbitrarily increased. In case all other steps to improve workability have failed, you may consider adding more water only as a last resort.
More water can be added, provided a correspondingly larger quantity of cement is added as well to keep the water/cement ratio constant so that the resistance remains the same.
How Admixtures influence the workability of concrete?
The workability of concrete can be improved by the use of air-entraining agents that reduces internal friction between the particles. They also act as fine artificial aggregates with a very smooth surface. You can see that the air bubbles act as a kind of ball bearing between the particles to slide with each other and give easy mobility to the particles.
What are the influences of Mix Proportions on the workability of concrete?
The aggregate-cement ratio is an important factor influencing workability.
In lean concrete, there is less paste available per unit area of aggregate, and therefore the mobility of the aggregate is restricted.
On the other hand, with greasy concrete with a lower ratio of aggregate to cement, more paste is available to make the mixture achieve better workability of the concrete.
What are the influences of Grading of Aggregate on the workability of concrete?
A well-graded aggregate is one that has the least amount of voids in a given volume. When the total voids are smaller, there is excess paste available for better lubricity. Well-graded aggregates slide past each other with the least amount of compaction.
How the shape of aggregates influence the workability of concrete?
The influence of surface texture on workability is due to the fact that the total area of the rough aggregate is greater than the area of a smooth and rounded aggregate of the same volume.
Not only that, the round shape greatly reduces frictional resistance. This is why sand and river gravel provide better workability for concrete than crushed sand and aggregate.
How the sizes of aggregates influence the workability of concrete?
The larger the aggregate, the smaller the surface area, and therefore less water is required to wet the surface and less matrix or paste is required to lubricate the surface to reduce internal friction. For a certain amount of water and paste, a larger size of aggregates leads to higher workability up to certain limits.
What are the influences of Surface Texture of Aggregate on the workability of concrete?
The influence of the surface texture on the workability is due to the fact that the total area of the rough textured aggregate is greater than the area of a smooth rounded aggregate of the same volume.
The smooth-textured aggregate will give better workability and the rough-textured aggregate will give poor workability.
Which tests are commonly used for the measurement of workability of concrete?
The workability of concrete is measured by the following types of workability tests.
1. Slump Test
2. Compacting Factor Test
3. Vee Bee Test
4. Flow Test
5. Kelly Ball Test
Slump Test of concrete:
The slump test is basically a measure of the consistency of a concrete mix.
The test is carried out with a steel frustum cone mold, the upper diameter is 100mm, the lower diameter is 200mm and the height is 300mm.
According to Indian standards, the maximum size of the aggregate should not exceed 38mm in the slump test.
The slump cone test value is only suitable for concrete with medium to high workability. It can be seen that the workability of the concrete increases as the value of the slump increases.
Compacting Factor test of concrete:
The test is based on the principle of measuring the amount of compaction achieved by a standard amount of work done by allowing the concrete to fall through a standard height.
This test is more accurate than the slump test, especially for concrete mixes with medium and low workability.
The workability of concrete by compaction factor test is more popular for determining the workability of concrete mix in laboratories.
The compaction factor is the ratio of the weight of the partially compacted concrete to the weight of the fully compacted concrete.
Vee Bee Test of concrete:
The Vee-Bee test for concrete is suitable for stiff concrete mixtures with low and very low workability. This is a good laboratory test to indirectly measure the workability of concrete.
In this test, the time required for complete deformation in seconds is the required measure of workability and is expressed as a number of Vee-Bee seconds.
Flow Test of concrete:
This is a laboratory test that gives an indication of the quality of the concrete in terms of consistency, cohesiveness, and tendency to segregate.
In this test, a standard of concrete is jolted. The spread or flow of the concrete is measured and the flow is related to the workability of the concrete.
Kelly Ball Test of concrete:
The test was developed by Kelly and is hence known as Kelly Ball Test. This is a popular field test for the workability of concrete.
The advantage of this test is that it can be performed on concrete in place and it is said that this test can be performed faster and with greater precision than the slump test.
The downsides are that it requires a large sample of concrete and it cannot be used when the concrete is placed in a thin section.
FAQ – Workability of concrete:
What is the factor affecting the workability of concrete?
Factors that influence the workability of concrete are
• Water Content
• Use of Admixtures
• Mix Proportions
• Grading of Aggregate
• Shape of Aggregates
• Size of Aggregates
• Surface Texture of Aggregate
What is high workability concrete?
High Workability Concrete is a flowing concrete that can be cast into place without the use of vibrators to construct a structure free of honeycombs and bug holes.
High Workability Concrete is Also known as self-compacting concrete.
What is the slump value for high workability concrete ?
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The slump of concrete for high workability is 100 mm-175 mm.
What is the slump value for low workability concrete?
The slump of concrete for low workability is 25 mm- 50 mm.
What is the compaction factor test?
The compaction factor test of concrete is one of the workability of concrete tests in the laboratory as well as in the field.
How do you increase the workability of fresh concrete?
The ways workability of concrete increases by-
• By increasing the Water Cement Ratio of Concrete| What is the Ideal Water Cement Ratio in Construction?.
• By increasing the mixing time.
• By increasing the mixing temperature.
• By increasing the size of aggregate.
• Using rounded and smooth aggregate instead of irregular shapes.
• Using non-porous and saturated aggregate.
• Addition of air-entraining mixtures.
What are the requirements of workability?
The workability of the fresh concrete must be thus and so that it can be properly mixed, transported, placed in the formwork, and properly compacted with minimum effort without the chances for bleeding and segregation.
Why does workability decrease with time?
Sometime after the concrete is mixed with water, the concrete’s workability begins to decline. This happens because of the amount of available water in concrete that is being utilized in the hydration of cement compounds for the sake of bonding.
Which shape of aggregate gives better workability?
The workability of concrete is more when smooth and rounded aggregate is used instead of irregular angular aggregate.
Which workability test is most suitable for concrete of very low workability?
The compaction factor test of concrete is more precise than the slump test of concrete in the case of concrete mixes of very low workability and is normally used when concrete is to be compacted by vibration.
How the workability of concrete is expressed?
The workability of concrete is expressed by mm. It is the height of the concrete falling after the removal of the slump cone.
What is the workability of cement?
The workability of cement is defined as the property of cement in its plastic phase, which can be easily used in mortar or concrete.
