cracks in RCC Slabs 3

[irulandi]

i have constructing one hospital 11 storied building at mumbai.

i am facing cracks problems at concrete

in every pouring after five hours the concrete surface is showing cracks ( specifically in slabs )

Width of cracks maximum 6MM
Depth of cracks 20 to 30MM

The thickness of RCC slab is 150MM
Reinforcemet used per Cum : 140 Kg
Area covered per pouring time : 1000 Sqm.

Please give the information to avoid such type of problems.

 

[ishvaaag]

Drying shrinkage, given the timing. Tooo much area at once, no doubt, this makes over 30x30 m. Since you have the forms in place, lay the concrete in lesser chunks, maybe in checkerboard or leaving bands for later concreting. If you pour the concrete in such lesser size pieces you will see far less cracks.

The built parts, soffits and too much rebar or stressing against the indeformable meshes will be restraining the drying concrete as to produce the visible cracks. Having a not too thick slab also may be contributing. Differential loss of water top and bottom also help to crack upon restriction. A partly permeable soffit may help. Heat and wind may compound with concrete set to produce the big cracks.

I recommend then you to read about concreting in hot climates. Low growth of strength cement, doubly-reinforcing the slab, perfecting the mix, maybe some fy ash addition may help. Also proper curing under humid textiles (uncommon), but aspersion seems not be as efficient since more able to wash early age non completely set cement.

 

[Ron]

From your description of the crack size and timing, you are experiencing plastic shrinkage cracking. This is caused by the evaporation rate of the water at the surface of the concrete being greater than the "bleed rate". The surface of the concrete starts to support tensile shrinkage stresses that quickly overcome the limited cross section (usually less than 25mm thick) of "hardened" concrete overlying concrete still in its plastic state.

This typically occurs in hot weather conditions, when you have wind (or other evaporation enhancement). In order to prevent this, you will need to provide protection from evaporation, either through the erection of wind breaks and/or a fogging system to maintain high humidity at the concrete surface.

I would agree with ISHVAAAG's comment about reducing the size of the placement. This allows you better control over the initial curing conditions. "Checkerboarding" will not generally help this...that's a method that was thought at one time to be beneficial to control longer term drying shrinkage cracking, but doesn't really work very well for that purpose since little time elapses between placements.

You might also be experiencing some "rebar slump" where you get segregation of the aggregate right over the rebar, causing a depression and cracking directly over the rebar. Check your crack pattern...if it is regular, you might want to look at the rebar slump as a contributor (this can be corrected with a change in the mix design). If the pattern is irregular, particularly with short, discontinuous cracking, then true plastic shrinkage is likely the cause.

 

[dicksewerrat]

You may also want to add some fibers to the mix. This along with using a smaller pour size and checherboard it. protect the slab with curing compounds and or plastic.