Properties of Fresh Concrete:
Fresh Concrete can be easily molded into any designed shape in construction. It can be prepared on the spot and may give a wide range of properties from easily available raw materials.
We will discuss in details below the different properties of Fresh concrete.
- Plastic Shrinkage.
- Thermal Shrinkage.
- Thermal Expansion.
- Water Cement Ratio.
Workability, in the simplest language, is the ease with which freshly prepared concrete can be transported and placed for the job and compacted to a dense mass.
The fresh concrete which may be expected to give the best results must possess the property of workability. This is the most important property of fresh concrete.
Fresh concrete should be capable of spreading uniformly without inducing any segregation of the aggregates.
A workable concrete should have a right balance between the plasticity and mobility for a particular job in particular place.
Besides plasticity or consistency and mobility, the third most important aspect considered to define workability is the ease with which the freshly placed concrete can be compacted without developing any defects.
Since workability depends on a number of factors, no single test is thought to be sufficient to express this property of fresh concrete.
For quality construction, the following three tests are required to be carried out.
- Slump test.
- Compacting factor test.
- V-B Consistometer test.
Compaction Factor Test:
This test is developed in the UK by Glanville. In this test samples of concrete are weighed first when concrete is partially compacted, and then after the full compaction.
Then, the samples are compacted using specified techniques and apparatus.
The ratio between partially compacted weight to the fully compacted weight gives a measure of compaction factor. It is always less than 1.
The reliability of this test has been questioned by many peoples. But still, for very stiff and dry concrete, this test is often considered to be a good indicator of workability.
V-B Consistometer Test:
This test is developed by Bahrner in Sweden. In this test samples of fresh concrete is taken in a metallic cone and compacted by a specified vibrator to an ultimate shape of a flat cylindrical mass.
Total time taken for this process, in seconds, is recorded. This is called V-B time and gives a measure of compatibility of the concrete.
This test is useful in determining the workability of concrete mixes with aggregates size up to 20 mm.
It is expensive as compared to Slump Test and Compaction Factor Test and is used mostly in precast concrete works.
When concrete changes its state from fresh to hardened then this process is called setting. And the time required to complete this process is known as Setting Time of Cement.
The setting time for Portland cement is about 30 – 45 minutes.
The separation of concrete ingredients from each other is known as segregation. This can be caused due to excessive vibration in concrete mixer machine or falling concrete from more than 1-meter height.
This is the shrinkage that the fresh concrete undergoes untill it sets completely. It may also be called initial shrinkage.
This can be due to excessive loss of water from the concrete due to evaporation, bleeding, and soaking by formwork.
Excessive shrinkage at initial stages may develop cracks. Therefore, all precautions should be taken to avoid excessive loss of water.
This may be due to falling in temperature of concrete-mix from the time it laid to the time it sets completely.
Due to change in temperature, some shrinkage may be expected. Sometimes, It may be negligible on its own account.
In massive concrete works, when the upper layers are laid before the lower layers have completely set, there may arise a phenomenon of thermal expansions – in the lower layers.
This is because the heat of hydration gets accumulated in those layers and may attain magnitudes beyond acceptable limits.
Water Cement Ratio:
The compressive strength decreases, in general, with increasing water cement ratio and vice versa.
Hence, when minimum water is used just to ensure complete hydration of the cement, the resulting concrete will give maximum compressive strength on proper compaction.
I have written a detailed guide on Water Cement Ratio, don’t forget to read it for better understanding.
So, above are some properties of fresh concrete. While working on the site, you must take care of everything discussed above to ensure the best quality.
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