07-01-2022, 09:05 AM
Crazing
Cracking of concrete can occur both in plastic as well as in hardened stage. After several weeks or even earlier hardened concrete displays hair line, discontinuous, closely spaced surface cracking. This is called 'Crazing'.
The use of smooth and impermeable formwork surface (steel, plastic) or over trowelling of rich concrete mixes often results in concentration of cement paste on the exposed surface. Due to inadequate curing and smooth finishing high humidity gradients between the surface and the bulk of concrete is created. The cement paste which is exposed to drying conditions cracks easily. This causes drying shrinkage resulting in crazing.
PREVENTIVE MEASURES :
The measures which can be adopted at site to reduce crazing are given below:
Avoid rich concrete mixes with high cement contents.
Adopt the lowest possible cement and I or water content, consistent with workability, strength and durability. Use of admixtures like plasticizers or
super plasticizers which can greatly help in reducing cement and I or water content of the concrete mix.
Avoid non-cohesive and under sanded concrete mixes.
Curing should commence as early as possible to avoid rapid drying of concrete surface due to high ambient temperatures, low humidity and high
windy conditions. Curing process should be continuous avoiding wetting and drying cycles.
Trowelling for smooth finish must be avoided on the top surface of the slab.
Excessive trowelling causes crazing on the smooth surface.
Remove the surface skin either by brooming or by any other method.
Rougher surfaces resist crazing better than smooth trowelled surfaces.
Limit the maximum size of aggregate.
Avoid smooth and non absorbent formwork. The surfaces being smooth and non absorbent cause crazing. Smooth steel plate or sealed plywood
shutters can cause crazing. Normal plywood formwork is preferred.
For more details download ....
Cracking of concrete can occur both in plastic as well as in hardened stage. After several weeks or even earlier hardened concrete displays hair line, discontinuous, closely spaced surface cracking. This is called 'Crazing'.
The use of smooth and impermeable formwork surface (steel, plastic) or over trowelling of rich concrete mixes often results in concentration of cement paste on the exposed surface. Due to inadequate curing and smooth finishing high humidity gradients between the surface and the bulk of concrete is created. The cement paste which is exposed to drying conditions cracks easily. This causes drying shrinkage resulting in crazing.
PREVENTIVE MEASURES :
The measures which can be adopted at site to reduce crazing are given below:
Avoid rich concrete mixes with high cement contents.
Adopt the lowest possible cement and I or water content, consistent with workability, strength and durability. Use of admixtures like plasticizers or
super plasticizers which can greatly help in reducing cement and I or water content of the concrete mix.
Avoid non-cohesive and under sanded concrete mixes.
Curing should commence as early as possible to avoid rapid drying of concrete surface due to high ambient temperatures, low humidity and high
windy conditions. Curing process should be continuous avoiding wetting and drying cycles.
Trowelling for smooth finish must be avoided on the top surface of the slab.
Excessive trowelling causes crazing on the smooth surface.
Remove the surface skin either by brooming or by any other method.
Rougher surfaces resist crazing better than smooth trowelled surfaces.
Limit the maximum size of aggregate.
Avoid smooth and non absorbent formwork. The surfaces being smooth and non absorbent cause crazing. Smooth steel plate or sealed plywood
shutters can cause crazing. Normal plywood formwork is preferred.
For more details download ....
