Concrete Slab Cracks

[kxa]

I have a situation where the 6” garage slab poured a year ago which till now, no cars ever parked but has numerous cracks. In fact, it seams like more and more cracks are showing up every day. The cracks which I don’t know how deep they are run every which way. My guess is that the concrete must have had too much water. The contractor is saying that there is nothing wrong and that having cars parked on the slab won’t cause any problem. He says that the cracks are superficial and a thin layer of epoxy coating made for garage floor will take care of it.

Does any body have experience with a similar situation? Thanks.

 

[frv]

There are forensic engineering firms that only go out and look at cracks (maybe that's a bit of a stretch). It's kind of difficult to assess the cause of the cracks without more info and, quite frankly, knowledge.

So here's my wild guess (emphasis on wild and I only suggest this because you say that more and more cracks appear every day).. is it possible that the concrete is undergoing ASR?

 

[BigH]

Do a check of the weather conditions at the time of the pour. Sounds like they might be, in some part, due to plastic shrinkage cracking (where the wind and temperatures cause the surface of the concrete to dry out faster than the strength gain in hydration) - but these usually show up in less that 12 hours after placing - and more like 4 to 5 hours. But, wondering why, you are seeing "new" cracks regularly appearing? Do you have a pattern yet?

 

[hokie66]

No one can really tell you why the cracks formed without seeing the slab and knowing how it was built, including details of support, reinforcement, mix design, weather, water/cement ratio, curing, and I could go on.

Is it a slab on ground or a suspended slab?

The most likely cause is restraint cracking due to drying shrinkage, just because that is the most common type of concrete cracking.

 

[Einsteim]

What are the dimensions of the slab? Are there control joints cut or formed into the slab?

 

[renko]

Have you considered ASR (Alkali Silica Reaction). An unfortunate, but sometimes inherent, chemical instability in concrete which often shows up as random cracking after a period of time.

 

[vincentpa]

To really know what went wrong, you need to do as hokie66 said and conduct a geotechnical investigation of the site. You will also need to drill cores and have them petrographically analyzed. It probably isn't shrinkage cracking if it happened a year after the concrete was poured. ASR is a good bet. Without the cores and the geotechnical investigation, you will be guessing. It could be related to expansive soils or settlement. Cracking can be related to many different things and many causes of the cracking produce the same crack pattern. If you really want to know concrete slabs, I suggest reading PCA "Slabs on Ground" and ACI 360.1 and 360.2. Then the contractor can't tell you what's wrong. You need to know about slabs to do this kind of work or else the contractors and lawyers will push you around with BS.

 

[civilperson]

In the final analysis, concrete cracks. (Some cracks are not visible without a magnifying lens.)

 

[JKW05]

Be careful of using an epoxy coating. If a lot of care is not used in prepping the slab, the coating may start blistering or peeling in the near future. It also may need re-coated in 5-10 years.

 

[vincentpa]

Putting a brittle epoxy coating won't do anything if the causes of the slab cracking haven't been fixed. The epoxy will just develop reflective cracking once when the slab continues to move. If it is ASR and the reaction has stopped, epoxy will work. Find out the cause first.

 

[archeng59]

hairline cracks? Or do the cracks vary in width from hairline to wide enough to put your finger into the crack? Is the slab at different elevations on each side of some or all of the cracks indicating differential movement of the slab sections? Like civilperson said, concrete cracks. Hairline cracks would not bother me, but other types of cracks could be related to other problems. If you can't assess the issue, have a licensed structural engineer investigate the problem.

 

[tedamiao]

Is that possible you have expansive soil on site?