Workability Of Concrete
The term workability is used to describe the ease or difficulty with which the concrete is handled , transported and placed between the forms with minimum loss of homogeneity.Also defined as the amount of useful internal work necessary to produce full compaction.
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| Workability Of Concrete |
IMPORTANT FACTS OF WORKABILITY OF CONCRETE
1. If more water is added to attain the required degree of workmanship , it results into concrete of low strength and poor durability.
2. If the strength of concrete is not to be affected , the degree of workability can be obtained:
a. by slightly changing the proportions of fine and coarse aggregates , in case the concrete mixture is too dry.
b. by adding a small quantity of water cement paste in the proportions of original mix , in case the concrete mixture is too dry.
3. A concrete mixture for one work may prove to be too stiff or too wet for another work. For instance , the stiff concrete mixture will be required for thin sections containing reinforcing bars.
4. The workability of concrete workability of concrete is affected may by water content , water-cement ratio and aggregate-cement ratio.
5. The workability of concrete is also affected by the maximum size of the coarse aggregates to be used in the mixture.
TESTS OF WORKABILITY
1. SLUMP TEST
The standard slump cone , as shown in fig is placed on the ground. The operator holds the cone firmly by standing on the foot pieces. The cone is filled with about one - fourth portion and then rammed with a rod which is provided with bullet 16 mm and its length is 600 mm . The strokes to be given for ramming vary from 20 to 30 . The remaining portion of the cone is filled in with similar layers and then the top of concrete surface is struck off. so that the cone is completely full of concrete.
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| Slump Test Of Concrete |
The cone is then gradually raised vertically and removed. The concrete is allowed to subside and then concrete is obtained by deduction height of concrete after subsidence from 300 mm. The larger the slump, the softer or wetter the consistency is.
Table shows the recommended slump of concrete for various types of concrete.
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| Slump Test Of Concrete |
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| Slump Test Of Concrete |
ADVANTAGES OF SLUMP TEST
1. It grants the facility to measure accurately and almost instantaneously detect the difference in water content of successive batches of the concrete of the same identical mix.
2. The apparatus is cheap, portable and convenient to be used at site.
3. It can provide qualitative information about cohesion.
4. It is the most under appreciated test method in concrete technology. Its economy in time, equipment and labour on the one hand and the large amount of information it can produce on the other hand results in a huge benefit-cost ratio.
LIMITATIONS OF SLUMP TEST
1. As such, there is no direct relationship between the workability and the value of slump.
2. It is not suitable for a concrete in which maximum size of the aggregate exceeds 40 mm.
3. There are chances of many shapes of slump to occur and it is difficult to decide which is the correct value. Fig shows three forms of slumps that may occur.They are known as true slump, shear slump and collapse slump.The shear slump is obtained when the specimen collapses or shears off laterally.The collapse slump is obtained in the case of very wet concrete mixes.If shear and collapses slumps are obtained , the concrete is considered unsatisfactory for placing.
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| Slump Test Of Concrete |
4. The slump occurs only in case of plastic mixes . It does not occur in case of dry mixes.
COMPACTION FACTOR TEST
In this teat, the workability of concrete is measures in terms of internal energy required to compact the concrete fully.The concrete is allowed to fall in a lower cylindrical mould of 150 mm diameter and 300 mm height through two vertically placed conical hopper. The surplus concrete appearing above the top of the mould is removed with a trowel. The weight of the concrete in the mould is then found out.The theoretical weight of concrete without any air voids is worked out from the study of the concrete mix. The compaction factor is calculated as follows:
Compaction factor = observed weight of concrete/theoretical weight of concrete.
The relation between compaction factor and workablity is mentioned in table
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| Compaction Factor Test |
VEE-BEE TEST
The Vee-bee test is preferred for finding out the workability of stiff concrete mix having very low workability.
Hope you like the article regarding workability of concrete and various tests used to measure the workability. If you want to know more please comment. Thankyou.
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