During Construction using concrete, cement paste, mortar certain time is required for mixing, transportation, placing, compacting and finishing. During this time concrete, mortar, cement paste should be in plastic condition and should not harden and it is mandatory that one should use the concrete before it starts to loose its plasticity. So How should someone know how long is concrete workable? and How much travel time is permitted for concrete?
Well its Setting time that tells us how long concrete is going to be workable. Now the question is What is Setting Time of Cement?
The question was What is Setting Time?
Setting time of cement is the time required to stiffening of cement paste to a defined consistency. Setting time is then classified in two types Initial setting time and Final setting time based on plasticity.
Time elapsed between water added and time at which cement paste just starts loosing its Plasticity is called as Initial Setting Time of Cement. Normally for OPC Initial Setting Time is 30 minutes but should not be more than 60 minutes.
Basically the time interval for which the cement products remain in the plastic condition is known as the initial setting time.
Time elapsed between water added and time at which cement paste completely loose its Plasticity is called as Final Setting Time. Normally for OPC Final Setting Time is 10 hrs and not more than that.
Basically it is time taken by the cement paste or concrete to harden sufficiently or completely loose its plasticity.
Setting time of Cement plays a vital role in any kind of construction work. Concrete should not set too early nor too late. Concrete set before placing results in rigidity before placing or low workability. In the converse scenario, late setting will result in inadequate strength at desired age.
Initial setting time is time span between time of addition of water and the time when cement paste starts to loose its plasticity. That is why Initial setting time test is important for transportation, placing, compacting and curing of cement concrete.
Final Setting time is the time span between water added and the time when concrete completely loose its plasticity. Final setting time of cement helps in the safe removal of scaffolding.
To determine Initial and Final setting time of Cement.
A device used to determine Standard Consistency and Setting Times consists of rod weighing 300 grams, having a needle at one end and supported in the frame with Graduated scale for measuring the Penetration. Also having a mould to put the cement.
Vicat mould is having internal diameter 80 mm and 40 mm height. With non-porous base plate is provided at the base of the mould. Mould is having suitable clamping ring.
To take the weight of Sample of Cement. It should at least be with least count of 1 gram.
It is a timer used to record the time. Stop watch should be used to note down the reading of Consistency Test.
Sr. No. | Setting Time | Penetration (mm) |
---|---|---|
1 | ||
2 | ||
3 | ||
4 |
Initial Setting Time = T2 – T1
Final Setting Time = T3 – T1
where,
T1= Time of addition of water to Cement
T2= Time when the needle released fails to penetrate the block beyond 5mm from the bottom of the mould.
T3= The time at which the needle makes an impression on the surface of the test block.
Initial Setting Time of given Cement is : __________
Final Setting Time of given Cement is : __________
Type of Cement | Initial Setting Time (Min) Min | Final Setting Time (Min) Max |
---|---|---|
OPC 33 | 30 | 600 |
OPC 43 | 30 | 600 |
OPC 53 | 30 | 600 |
Sulphate Resistant Cement | 30 | 600 |
Super Sulphated Cement | 30 | 600 |
Low Heat Cement | 60 | 600 |
Time elapsed between water added and time at which cement paste just starts loosing its Plasticity is called as Initial Setting Time of Cement. Normally for OPC Initial Setting Time is 30 minutes but should not be more than 60 minutes.
Time elapsed between water added and time at which cement paste completely loose its Plasticity is called as Final Setting Time. Normally for OPC Final Setting Time is 10 hrs and not more than that.
To reduce setting time gypsum is added during manufacture.
For further reduction of setting time one can always use Retarding Admixtures.
Gypsum is added to reduce the setting time of cement by slowing down the rate of hydration of cement.
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