Workability is the amount of work to produce full compaction when concrete is wet.
It depends on the water-cement ratio in concrete.
It is essential to determine whether the concrete is suitable for pumping,self-levelling.
4 Important facts of workability
- To achieve the required degree of workmanship if more water is added then it results in the concrete of weak durability and low strength.
- If the mixture of concrete is too wet the proportions of coarse and fine aggregate can be altered and it will not affect the strength of concrete and degree of workability of concrete can be achieved.
- If the mixture of concrete is very dry then in the proportion of original mix by adding a small quantity of water-cement paste will not affect the concrete strength and degree of workability of concrete can be obtained.
- It is also affected by the maximum size of the coarse aggregates to be used in the mixture.
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What are the Workability Tests?
For measuring the workability of concrete mixture the following test is performed.
- Slump test
- Compaction factor test
- Vee-Bee test
- Flow test
1.Slump Test
The slump test can be performed either in the laboratory or at the site of work and it is the most regularly practised method of estimating the consistency of concrete.
Slump cone of the top diameter of 10 cm and bottom diameter 20 cm and a height of 30 cm is practised for performing this test.
There will be three layers of concrete.
Using a standard rod of 16 mm diameter and 60 cm in length, each layer is tapped for 25 times.
The height of concrete fallen under gravity measured is called Slump.
Advantages of slump test:
i. It helps to detect the variation in water content of consecutive batches of concrete of the identical mix.
ii. The apparatus is cheap, portable and convenient to be used at the site.
Disadvantages of Slump test:
i. For very dry concrete or every wet or and stiff mix this test is not suitable.
ii. All factors contributing to workability this test could not able to measure.
Recommended slumps for various concrete works:
1. The slump of 25 mm to 40 mm is recommended for Concrete used in road construction.
2.The slump of 50 mm to 100 mm for Beams and slabs:
3.The sump of 75 mm to 125 mm for Columns:
4.The sump of 80 mm to 150 mm for Normal RCC work:
5.The sump of 25 mm to 50 mm for Mass concrete works.
2.Compaction factor Test
The degree of workability of concrete in the compaction factor test is estimated in terms of internal energy needed to compact the concrete.
The compacting factor test can be used in both laboratories and infield.
For concrete of low workability, this compaction factor test gives reasonable results.
The degree of compacting factor or compaction is estimated by the density ratio.
Compaction factor = Truly achieved density in the test / same concrete density after fully compacted.
Given below a few compaction factor results :
1. For flowing, concrete with high workable has a compaction factor of 0.95.
2. For plastic concrete with medium, workable have a compaction factor of 0.92.
3. Stiff plastic concrete with low workable has a compaction factor of 0.85.
4. Stiff concrete with very low workable has a compaction factor of 0.75.
3.Vee-Bee Test :
This test is preferred for finding workability of stiff concrete mix with very low workable.
Given below Vee-Bee tests results :
1. Vee-Bee time of 5 to 3 seconds represents stiff plastic concrete are medium workable.
2. Vee-Bee time of 10 to 15 seconds represents stiff concrete of low workable.
3. Vee-Bee time of 18 to 10 seconds represents very stiff concrete are very low workable.
Vee-Bee Consistometer
Indirectly the workability of concrete can be measured in the laboratory using this meter.
This examination consists of a metal pot, vibrating table, standard iron rod, sheet metal concrete.
The required time in seconds for changing the shape of concrete from slump concrete shape to cylindrical shape is termed as Vee-Bee degree.
For extremely dry concrete and for whose slump value cannot be determined by slump test, this
vee-bee consistometre test is really suitable.
4.Flow Test
An indication of the quality of concrete concerning cohesiveness, consistency and the proneness to segregation is given by performing flow test in the laboratory.
The flow test is related to workability. The flow or spread of the concrete is measured and
Flow percent = 100 x (spread dia in cm – 25)/25.
Flow per cent value ranges from 0% to 150%.
Among the above tests, the essential characteristics of the concrete are determined by the compaction factor test, which relates very close to the workability requirements of concrete
It is one of the best tests to determine the workability of concrete.
What is Concrete explain?
Concrete is a composite outcome produced by mixing fine aggregate, cement, coarse aggregate and water in an appropriate proportion.
home construction the following types of concrete used
1.Plain cement concrete (PCC)
2.Reinforcement cement concrete (RCC)
Plain cement concrete (PCC)
Ingredients :
PCC consists of sand, Cement, Coarse aggregate and water are the primary ingredients of plain cement concrete.
Example: Cement concrete, M10 grade -1:3:6 (1 part cement,3 parts sand,6 parts the aggregate)
Reinforced cement concrete (RCC)
In the tension Concrete is weak. So steel is used as a reinforcing material, as it is strong in tension and have the same coefficient of linear expansion as that of concrete.
Ingredients:
1.Cement,
2.Coarse aggregates and fine aggregates.
3.Water
4.Admixtures
5. Reinforcement
Mixing the materials of concrete :
The process of folding, rolling and levelling of particles is known as the mixing of concrete.
The mixing of concrete can be done either with Machine or Hand mixing.
Water-Cement Ratio in concrete:
Water cement ratio in a concrete mix is the ratio of cement and water by weight.
To have a decent degree of workability, the minimum amount of water should be utilised.
cement concrete important water properties:
1. Organic solids content should not more than 0.02%
2.Content of inorganic solids not more than 0.3%
3.Turbidity nor more than 2000 m
4.Sulphate’s content not less than 0.05%
5.Acid not more than 10,000 pm
6. Sulphate’s alkali chlorides content not more than 10%.
7.pH should be between 4.5 to 8.5.
Strength of Concrete, Water-cement ratio relation :
The strength of concrete is inversely proportional to water-cement ratio, lower the water-cement ratio, greater the strength of concrete and vice-versa.
This relationship can be shown graphically.
The curves bend down words when the water-cement ratio grows to 0.4, which symbolises that the concrete is not workable and such concrete should not be practised in construction. If used honeycombs will be obtained.
Water quantity in concrete thumb rule
1.Weight of concrete = 28% of the weight of cement + 4% of the weight of total aggregate
2.Weight of concrete = 30% of the weight of cement + 45% of the weight of total aggregate
Bleeding of Concrete
At the surface of concrete mix if excess water comes producing small pores resulting in the mass of concrete is termed as bleeding of concrete.
Segregation
When coarse aggregate is separated from the finer materials segregation is caused which results in large voids, less strength and less durability.
Conclusion:
Above mentioned tests are utilised for determining the workability of concrete.
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