Slab Cracks
Slab Cracks
(OP)
Hello,
We are buidling a new home which is pretty much complete except for final inspections. In doing a walk-through we noticed 4 cracks in the concrete on the covered veranda at the back of the house (veranda is about 40 ft by 8 ft). The cracks are running the 8 ft. length. The covered veranda was poured together with the rest of the foundation, and it is a structurally engineered pier and beam slab. At first it looked like someone dribbled some black paint on the concrete, but in further inspection they are small cracks. The cracks are about 3 - 4 ft long and there are 4 such cracks spaced across the veranda. The builder says that it's the nature of concrete to crack some, and the cracks have to reach a certain width before there is a concern. I could hardly feel anything when I rubbed my fingernail across the crack so the cracks aren't wide. If they are just surface cracks I'm not concerned, but how do I tell if they will develop into something more signinficant?
I am thinking about calling the structural engineer who designed and inspected the slab, but I know he will charge to look at it.
Thanks,
We are buidling a new home which is pretty much complete except for final inspections. In doing a walk-through we noticed 4 cracks in the concrete on the covered veranda at the back of the house (veranda is about 40 ft by 8 ft). The cracks are running the 8 ft. length. The covered veranda was poured together with the rest of the foundation, and it is a structurally engineered pier and beam slab. At first it looked like someone dribbled some black paint on the concrete, but in further inspection they are small cracks. The cracks are about 3 - 4 ft long and there are 4 such cracks spaced across the veranda. The builder says that it's the nature of concrete to crack some, and the cracks have to reach a certain width before there is a concern. I could hardly feel anything when I rubbed my fingernail across the crack so the cracks aren't wide. If they are just surface cracks I'm not concerned, but how do I tell if they will develop into something more signinficant?
I am thinking about calling the structural engineer who designed and inspected the slab, but I know he will charge to look at it.
Thanks,





RE: Slab Cracks
RE: Slab Cracks
The contractor is right (partly). It is in the "nature" of concrete to crack. It is due to shrinkage. The contractor can control the location of these cracks by putting in control joints. You will have to decide whether your designer or your contractor was responsible for locating the joints.
RE: Slab Cracks
Maybe you can persuade the contractor that it would be fair compensation from him for the cracks being there, although - as stated above - it is very common for this to occur on pours of the dimensions you describe.
RE: Slab Cracks
RE: Slab Cracks
RE: Slab Cracks
Finally, proper control of thickness, placement, finishing and curing of concrete will reduce random cracking to a very low probability.
RE: Slab Cracks
RE: Slab Cracks
I see the potential that the cracking may either be associated with the structural design and/or the normal shrinkage and cracking of a long, narrow slab. In my opinion, admittably without the benefit of seeing the drawings, the slab should have been jointed at 8' to 13'+ centers. The cut or hand tooled joints could be filled after the concrete is 60+ days old (to allow the concrete mass to undergo the majority of normal shrinkage).
I see both sides of the discussion of Ron & ishvaaag. The points being made by both posters must be understood by the client, contractor and the designers. It may well be that all involved need a little 'education'.
A more exact description of the slab cracking and the structure/slab design as a whole would be very helpful. If the cracks can be 'barely felt' then Ron may be over reacting but, in most cases of my experience, the observed concrete cracking is excessive and really should not be tolerated. I believe part of our (USA) problem is we try to do too much with too little. By that I mean that our slabs are probably too thin.
As I have noticed in some of ishvaaag's posts, he is familiar with thicker, more heavily reinforced slabs. I have to admit that if I could increase the average slab thickness by even 1 to 3 inches, the observed cracking would be a lot less.
I would like to hear from some others regarding this subject. I know that IJR can present a question or case to draw out some more on this subject.
RE: Slab Cracks
what I'm tring to say is. Compacted base to maximum. Grade base so thickness is uniform. Control slump doing placement to 3-4 slump. Dryer better. No cold joints. No over working of finish. Cover immediate for curing. Cut joints deep and completely within first eight hours. Recover, keep wet for seven days. Also you can pour a weaker mix at a dryer slump and get better results due to cement notsetting as quick.. My slab is seven months old with no cracks.
RE: Slab Cracks
But it is not being done (specially in the materials area) because it is more costly.
In any case I think anyone will see pertinent that shrinkage stresses out of restrain can't build past the limit tensile strength at the same age, and simply this target is not met by the ORDINARY concretes and placements, at least if you consider 2D elements such walls and slabs.
This I have seen with my eyes and no words are going to convince me of the contrary.
RE: Slab Cracks
Most contractors with any kind of experience would have put them in.
PS. Do the cracks go through the rest of the slab(ie In the house?
RE: Slab Cracks
RE: Slab Cracks
I am the original poster of this thread and appreciate all the replies. The cracks have gotten just a little bigger, but are still not very wide so I am hoping they are just surface cracks.
I have another related question though. Isn't a big pour with no expansion joints what they do for a garage floor? I usually don't see any expansion joints in a pour like that? My garage also has one or two small surface cracks, and I have seen the same in my brothers garage (older home).
Thanks,
Gerald
RE: Slab Cracks
Most garages are about 400 sf, which is the upper limit for placement without control joints. You are likely seeing typical shrinkage cracks in your floor due to all of the things previously discussed in this thread. See also my FAQ in the Concrete Engineering forum.
Since concrete rarely gets any larger than at time of placement, there is rarely a true need for "expansion" joints. Usually only necessary in long, narrow strip placements (pavements).
The joints that are necessary for good concrete performance are control joints (spaced at 24 to 36 times the thickness) and construction joints (where construction starts and stops).
Ron