Identifying Plastic cracks
a) Plastic cracks occur while the concrete is still in a plastic state, before the cement has set. They develop within 1 or 2 hours after the concrete has been placed, starting suddenly about the time the “water sheen” disappears from the surface.
b) They may vary in width from fine hair lines to as wide as 1/8 inch (3 mm) and from an inch to several feet in length. They are usually atleast 2 inches deep and may extend through a thin slab.
c) They have no definite pattern. But, generally , they may be originated perpendicular to the long axis of the slab.
d) They do not have the appearance of a clean break and never break through aggregate particles, although sometimes they follow the edges of aggregate particles or reinforced bars.
e) They are especially apt to occur in slabs, in the top of walls and beams, and in footing on sandy soil.
f) They are not progressive . Once the crack has formed, the stress in concrete is relieved and no further cracking develops from this cause.
Causes for cracking
a) The cause of plastic shrinkage cracks is the rapid evaporation of water from the surface. As the concrete is vibrated after placing , water rises to the surface of the concrete.(bleeding) . Under most weather conditions , some of the surface water evaporates. As long as the rate of bleeding exceeds the rate of evaporation there is a continuous layer of water at the surface as seen by the appearance of a “water sheen” on the surface. If evaporation exceeds bleeding, the sheen disappears and top surface of the concrete is placed intention resulting in formation of cracks.
b) Other plastic settlement cracks result from settlement or other movement of the forms or subgrade or aggregate particles, movement of reinforcing steel or embedded items, sagging or slipping of concrete on slopes and premature stiffening.
Methods for preventing Plastic cracks.
a) Dampen the subgrade, forms and (dry) aggregates to prevent absorption of water from concrete.
b) Start curing as soon as possible after concreting.
c) Protect the concrete with temporary covering during any appreciable delay between placing and finishing.
d) Use reinforcement steel perpendicular to anticipated crack directions near corners of openings to prevent shrinkage cracks around openings in slabs.
e) Settlement cracks may occur at or near the soffit of a beam when the beam and slab are placed monolithically, especially if high slump concrete is used. They may also develop at the top of a column or wall at the junction with a slab. The remedy of this case is to wait for about 2 hours after placing the beam, wall or column before placing concrete in the slab, using a low slump.
f) Shrinkage may result from segregation and bleeding , causing settlement cracks in the top of the slabs, walls and beams. Well graded aggregates, proper mix proportions, with careful handling and placing will minimize these cracks.
g) Delaying the finishing as long as possible , some times permits the finisher to close the cracks.
a) Plastic cracks occur while the concrete is still in a plastic state, before the cement has set. They develop within 1 or 2 hours after the concrete has been placed, starting suddenly about the time the “water sheen” disappears from the surface.
b) They may vary in width from fine hair lines to as wide as 1/8 inch (3 mm) and from an inch to several feet in length. They are usually atleast 2 inches deep and may extend through a thin slab.
c) They have no definite pattern. But, generally , they may be originated perpendicular to the long axis of the slab.
d) They do not have the appearance of a clean break and never break through aggregate particles, although sometimes they follow the edges of aggregate particles or reinforced bars.
e) They are especially apt to occur in slabs, in the top of walls and beams, and in footing on sandy soil.
f) They are not progressive . Once the crack has formed, the stress in concrete is relieved and no further cracking develops from this cause.
Causes for cracking
a) The cause of plastic shrinkage cracks is the rapid evaporation of water from the surface. As the concrete is vibrated after placing , water rises to the surface of the concrete.(bleeding) . Under most weather conditions , some of the surface water evaporates. As long as the rate of bleeding exceeds the rate of evaporation there is a continuous layer of water at the surface as seen by the appearance of a “water sheen” on the surface. If evaporation exceeds bleeding, the sheen disappears and top surface of the concrete is placed intention resulting in formation of cracks.
b) Other plastic settlement cracks result from settlement or other movement of the forms or subgrade or aggregate particles, movement of reinforcing steel or embedded items, sagging or slipping of concrete on slopes and premature stiffening.
Methods for preventing Plastic cracks.
a) Dampen the subgrade, forms and (dry) aggregates to prevent absorption of water from concrete.
b) Start curing as soon as possible after concreting.
c) Protect the concrete with temporary covering during any appreciable delay between placing and finishing.
d) Use reinforcement steel perpendicular to anticipated crack directions near corners of openings to prevent shrinkage cracks around openings in slabs.
e) Settlement cracks may occur at or near the soffit of a beam when the beam and slab are placed monolithically, especially if high slump concrete is used. They may also develop at the top of a column or wall at the junction with a slab. The remedy of this case is to wait for about 2 hours after placing the beam, wall or column before placing concrete in the slab, using a low slump.
f) Shrinkage may result from segregation and bleeding , causing settlement cracks in the top of the slabs, walls and beams. Well graded aggregates, proper mix proportions, with careful handling and placing will minimize these cracks.
g) Delaying the finishing as long as possible , some times permits the finisher to close the cracks.
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