huangyhg

concrete slab cracks
if the contractor did not cure the slab, the potential (if not the actuality) exists that your slab is under-strength and will not perform. curing is part-and-parcel of any concrete work and should have been done.
i would recommend that you hire a testing lab to take core samples of the slab and test per american concrete institute (aci) requirements. a structural engineer can also assist in investigating the slab and the tests and make recommendations as to any repairs to the cracks or....most probable: remove the slab and replace as the lack of curing is highly undesireable and will most likely provide you with vast opportunities to spend money in the future for slab maintenance.
the cracks you are seeing result from drying shrinkage. to have them showing at the surface after only two days indicates you have several problems. first, it is likely the mix was overly wet, thus having a high water-cement ratio. this will lead to lower compressive strength and reduced durability. both will affect the long term performance of the slab.
the second concern is that with cracks showing so quickly, it is likely that many cracks will develop and they will ultimately be fairly wide cracks.
jae's advice to get a testing lab involved is good. i would take that one step further and make sure the testing laboratory has an engineer experienced in concrete materials and the evaluation of same. there are many things that can be learned from an appropriate investigation of the concrete, most of which are directed at making the contractor perform remediation or corrective action, up to and including removal of the slab and placement of new, properly placed and cured concrete.
if you are early enough in the game to withhold payment, this would be an appropriate step until the issues are satisfactorily resolved.
keep us informed of the progress.
i agree with the coments of jae and ron, i personally would be inclined to reject the slab as defective work. an epoxy topping may accomodate your imediate problem.
its extraordinary how often this type of query shows up in the various forums, plastic shrinkage - drying shrinkage. properties of concrete, a.m. neville should be required reading. probably there is little review conducted of procedures and execution in the field. almost entirely this type of query/problem seem to arise in slabs on grade where perhaps the structural element is seen by some as a 'non- structural' element with quality control left up to the trade. concrete is really the only structural element the we as structural engineers are responsible for the 'manufacturing' of on site. it's composed of a variety of constituients and goes from a 'liquid' state to a 'solid' state in a very short time.
we are planning to build a house in wa state in the future. just how does one prevent a slab from cracking? the plan is a slab on grade house with hydronic heating. we would like to stain and wax the slab instead of tile or carpet. this is why we are so interested in preventing any kind of cracking. thanks!

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