Workability Of Concrete
The workability of concrete is the ease, with which concrete mixes can be mixed, transported, placed in formwork, and compacted as completely as possible while using the lowest possible water/cement ratio.
In the workable concrete mix, the function of water is to lubricate the concrete so that the concrete can be compacted with minimum effort at the site of work.
Care should be taken that the concrete may not experience segregation, loss of homogeneity, compaction with a minimum amount of effort, and finish easily; the presence of a certain quantity of water is of vital importance.
Workability of concrete is also the ability of fresh concrete to fill formwork properly with a sufficient amount of vibration and without reducing the concrete’s quality.
The fresh concrete workability mainly depends on water content, cement content, aggregate particle size distribution, and amount of hydration and can be modified by adding chemical admixtures, like superplasticizer
Factors Affecting Workability of Concrete
The following are the factors affecting concrete workability,
1. Water content or Water Cement Ratio
As we increase the water-cement ratio workability increases simultaneously. But, the excessive water content may also result in the bleeding of concrete mix.
Moreover, high water content provides loose cement slurry which is capable of escaping through the formwork joint.
As we increase water content, the strength of concrete gets reduced. To avoid this and to keep the water-cement ratio constant add a sufficient amount of cement also while increasing water content in the concrete mix so that the strength remains the same.
Sometimes when concrete becomes too loose or has high workability. Cement can be added to it to make concrete good workability.
2. Mix Proportions
When concrete mix contents a higher amount of aggregate or aggregate/cement ratio is greater, less cement slurry is available to make concrete lubricant or workable.
On another side, if the concrete is having a lower aggregate/cement ratio, more paste is available to make the mix cohesive and fatty to give better workability
3. Size of Aggregate & Surface Texture
The smaller the size of aggregate particles higher will the surface area of aggregate and which requires a greater amount of cement paste to lubricate them.
So, a smaller size aggregate needs a more water-cement ratio to get good workability.
On other hand, a bigger size aggregate requires less surface area, and hence less amount of water is required for wetting the surface and less amount of cement paste is required to reduce internal friction between them.
As the aggregate size is high low water requires lubricating it, hence the extra water is available for workability.
In the case of porous aggregate, more water is required to lubricate them compared to nonporous aggregate for achieving the same degree of workability.
4. Shape of Aggregates
It is a fact that concrete mix with rounded aggregate has greater workability with less water-cement ratio compared to angular, elongated, or flaky aggregate. This shape of aggregate makes concrete harsh and difficult to handle.
The rounded aggregate offers better workability due to the fact that for the given volume or weight it has less surface area and fewer voids than the angular or flaky aggregate.
Moreover, because of their rounded shape, the frictional resistance is also greatly reduced.
This is a major reason for river sand and gravel cost more than crushed.
5. Grading of Aggregates
Graded aggregates are those aggregate which has the least amount of voids in a given volume and that offer higher workability to concrete.
As the number of voids is reduced, the excess paste is available to lubricate the particles. With this excess amount of paste concrete mix becomes cohesive and fatty which prevents segregation of particles.
6. Use of Admixtures
Concrete workability can be increased by adding chemical admixtures. As a chemical admixture air-entraining agent is used, which acts as ball-bearing between particles and increases workability, and mobility and decreases bleeding, and segregation.
For a better lubricating effect and more workability, pozzolanic materials can be used.
7. Weather Conditions
The weather condition has a significant effect on the workability of concrete. As the temperature increase, the evaporation process becomes rapid, and concrete loose water content quickly.
As the water content in concrete reduces, workability also gets reduced. In an area, if the velocity of wind is high water in the concrete evaporates quickly and that results in lower workability of concrete.
Workability of Concrete Types
The workability of concrete can be classified into three types as follows,
- Unworkable Concrete
- Medium Workable Concrete
- Highly Workable Concrete
1. Unworkable Concrete
Unworkable concrete in which the concrete has a very little amount of water. It is also known as Harsh Concrete, in which the hand mixing of concrete is likely very difficult.
Segregation of Aggregates is high in such concretes, leading to a difficulty to maintain the homogeneity of such concrete mix.
2. Medium Workable Concrete
Medium workable concrete is considered good in the sense of all properties a concrete should possess. The same is the reason it is used in most construction works.
It can be easily mixed, transport, place, and compacted without much segregation and loss of homogeneity.
3. Highly Workable Concrete
As the name suggests Highly workable concrete will be very easy to mix, transport, place, and compact. In the circumstances where effective compaction of concrete is not possible, this type of concrete is well-suited.
With high workability, the problem is such that there are high chances of segregation of concrete and loss of homogeneity.
Desirable Workability of Concrete for Construction
The desirable workability of concrete depends on two factors which are as mentioned below,
Workability Depending on Section size, amount, and spacing of Reinforcement
For a narrow, complicated section with several narrow corners and inaccessible parts, highly workable concrete is recommended to obtain full compaction of concrete with sufficient effort.
The section is crowded with steel reinforcement and if the spacing between the bars is relatively small, then compaction is difficult. For such cases also highly workable concrete is recommended by engineers.
With no limitations of the critical section or heavily reinforcement concrete, we can get a wide range of workability for concrete casting in construction.
Method of Compaction
The second factor responsible for selecting the type of workability is the method of compaction. Manual compaction of concrete requires more workability because the hand compaction method is not much uniform and effective.
Vibrators or machines provide high workability rang because of the efficiency and effectiveness offered by such machines.
Relationship Between Strength And Workability Of Concrete
The Strength of Concrete defines the nature of concrete hence it is the most important property for us. However, its dependency on other properties of concrete forces us to relate them with strength.
Strength is affected by density ratio or compaction and in turn, compaction depends on sufficient workability. Fresh concrete should have specific workability in order to obtain maximum density with a reasonable amount of work.
However excessive workability can affect compressive strength or even lessen it. The compressive strength of concrete can decrease with an increase in the w/c ratio, which in turn increases workability.
Hence we could conclude that the strength of concrete is inversely proportional to the workability. So in order to obtain specific strength of concrete too much workability should be avoided.
Workability Concrete Test
workability of concrete is measured by
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