Sunday, February 9, 2014
Prevention of cracks in Fresh Concrete - Crazing / Hair cracks
Identification
a) Craze cracks are fine cracks appearing in the hexagonal or octagonal pattern on the surface of the concrete.
b) They may be seen to some extent on nearly all surfaces, but they are most prevalent on trowelled surfaces.
c) They appear early and remain practically unchanged for the life of the structure.
d) They are especially noticeable when the concrete is drying after the surface has been cut.
e) The patterns are usually several inches in diameter but may be smaller.
Causes for crazing
a) Excessive floating and trowelling brings water, cement and fines (dust) from the aggregate to the surface to produce a surface skin which will crack.
b) Spreading dry cement on a surface that is too wet to trowel and sprinkling water on concrete too dry to trowel both produce a skin likely to suffer from crazing.
c) Rapid loss of moisture due to absorption of water by a dry subgrade or dry aggregate.
d) Rapid loss of moisture due to high rate of evaporation from concrete surface, owing to hot sun or dry winds.
e) Improper curing
Methods for preventing crazing
a) Dampen the subgrade before placing concrete.
b) Batch absorptive aggregates in a moist condition.
c) Delay trowelling until surface moisture has disappeared.
d) Do only as much trowelling as necessary to produce a good surface , do not over finish.
e) Never sprinkle dry cement or water on the surface during finishing operations.
f) Start curing as soon as possible.
g) Protect the concrete and working area with sun shades and wind breaks.
h) Organise the work so as to prevent unnecessary delays.
i) If possible, avoid placing concrete in the hot sun, especially when the temperature crosses 45 Degree C.
j) Avoid temperature stresses. Applying cool curing water to hot concrete surfaces produces a fine pattern cracking. For best results, the temperature differential between the concrete and the curing water should not exceed 14 Degree C.
Identification
a) Craze cracks are fine cracks appearing in the hexagonal or octagonal pattern on the surface of the concrete.
b) They may be seen to some extent on nearly all surfaces, but they are most prevalent on trowelled surfaces.
c) They appear early and remain practically unchanged for the life of the structure.
d) They are especially noticeable when the concrete is drying after the surface has been cut.
e) The patterns are usually several inches in diameter but may be smaller.
Causes for crazing
a) Excessive floating and trowelling brings water, cement and fines (dust) from the aggregate to the surface to produce a surface skin which will crack.
b) Spreading dry cement on a surface that is too wet to trowel and sprinkling water on concrete too dry to trowel both produce a skin likely to suffer from crazing.
c) Rapid loss of moisture due to absorption of water by a dry subgrade or dry aggregate.
d) Rapid loss of moisture due to high rate of evaporation from concrete surface, owing to hot sun or dry winds.
e) Improper curing
Methods for preventing crazing
a) Dampen the subgrade before placing concrete.
b) Batch absorptive aggregates in a moist condition.
c) Delay trowelling until surface moisture has disappeared.
d) Do only as much trowelling as necessary to produce a good surface , do not over finish.
e) Never sprinkle dry cement or water on the surface during finishing operations.
f) Start curing as soon as possible.
g) Protect the concrete and working area with sun shades and wind breaks.
h) Organise the work so as to prevent unnecessary delays.
i) If possible, avoid placing concrete in the hot sun, especially when the temperature crosses 45 Degree C.
j) Avoid temperature stresses. Applying cool curing water to hot concrete surfaces produces a fine pattern cracking. For best results, the temperature differential between the concrete and the curing water should not exceed 14 Degree C.
