Why Crazing in plaster occurs

Dnyan Deshmukh

Staff member
Crazing in Plaster
Crazing in plaster can happen due to many factors.
craze cracks are fine cracks appearing in hexagonal or octagonal pattern on the surface of plaster.
They appear early and remain unchanged for the life of plaster.

Causes of Crazing
It occurs due to excess fin content in the sand (silt or particles finer than 75 micron)

It also occurs due to highly fluid plaster material, which is applied on the surface.

It occurs due to dry base on which plaster is applied - when base absorb the water and fines accumulate on the surface it leads to crazing.

It occurs when dry cement is sprinkled on surface to finish too wet mortar.

Prevention of Crazing
Ensure sand used does not contains silt or finer particles more than specified limit (In general in case of rives sand, when silt is checked by volume it should not be more than 8% and in case of manufactured sand, when dry sieved particle passing through 75 micron should not be more than 5%)

Ensure the addition of water in mortar done by skilled mason and not by unskilled labour to ensure required workability in terms of handling and application.

Do wet the base surface on which plaster is to be carried out.

Do not allow mason to sprinkle dry cement on plaster, if he did mistake in adding water he should bare the losses.

This is what i learnt by observing the plaster activity.
 

Dnyan Deshmukh

Staff member
We took all precautions during the same still it occurs. Does weather effect matter ? If it then how can determine
Yes the weather condition in terms of temperatures and windy condition do affect it.

How cracking happens.

when a water from the surface of plaster starts evaporating due to high temperatures at surface and windy conditions, it leads to cracking.

The wind cause the hot air circulation at the surface and the constant evaporation of water from plaster due to high temperatures.

To prevent this, we can avoid doing plaster in high temperatures.

Another cause is, if you are using admixture to modify the properties of plaster (like crystalline water proofing), ensure the plaster does not take longer to set.

All those crazing and shrinkage occurs when mortar is not set, once the mortar hardens, it does not observed any more after that.

So basically it happens when mortar is fresh and not set.

You can apply your own theories on how to prevent crazing by observing the plastering activity and finding the causes if there are any other than listed here.