Slab Cracks 1

[berean315]

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,

 

[ishvaaag]

Really Shrinkage is a natural phenomenon always present in the setting and aging of concrete. It seems your problem is not very severe, so it maybe at least the ordinary precautions were taken. In short, if the problem does not worsens to your total inaceptability maybe is better not to do anything about. If such is the case now, one has always to think if one paid for a completely uncracked slab (something almost no one in his/her senses would warrant), I mean, either the slab is seeverely jointed and exceptionally cared for or at least the hairline cracks appear. You would love to see the 3 mm cracks anywhere in some supermarket here (of a french multinational) of same origin, this even with some joints made, and nobody paying attention.

 

[Ron]

If you already have cracks that are prominent and you are at the "walkthrough" stage, they will increase in size due to long term shrinkage.

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.

 

[rustbuster1]

Now that they are present you should address the problem of minor cracks by recognizing them as seep paths for wetness into the slab bulk. This is one cause of crack growth over time. This conrete may be covered, but it probably will still get wet, so you should obtain a clear epoxy sealer and seal the slab and the cracks - I'd wait another week or two and for a cooler day to do it.

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.

 

[ishvaaag]

I sincerely see no right to compensation for normal hairline cracks on normal concrete. The fact of the concrete shrinking upon set is as natural as the variations of volume of water when heating or congealing from 4 ºC deg. It is natural. Only in very specific contractual provisions including of course special cost provisions paying for special concretes (more likely mortars) would I see this being rightful cause of reclamation. This means not I don't feel sympathy for people wanting the best in construction, we are designers and consultants working precisely for that, but in this we must not deny the true nature of the materials, and in doing it, the due service to truth.

 

[ishvaaag]

Only to add that in +400000 sqm built mostly concrete structures including significant amounts of wall and slab on the ground I have seen hairline and more than even hairline cracks upon which the architect's team was having variegate control (sometimes owner-builder) and never in my experience a reclamation on shrinkage/creep caused cracks developed, what clearly points that as long the designers and builders perform reasonably well, the general consensus in the area in which I practice is that these cracks are understood -as are- unavoidable and tolerable when all design and construction practices have been put unto play with enough care and in good faith understanding.

 

[Ron]

ishvaag...I disagree that cracks are unavoidable. Most are clearly avoidable by proper jointing methods. Further, there are several means by which shrinkage can be reduced, thus reducing the incidence of cracking and reducing the need for closely spaced joints.

Finally, proper control of thickness, placement, finishing and curing of concrete will reduce random cracking to a very low probability.

 

[ishvaaag]

Even having seen your ample authority in the posts you have made, I have to say that all my experience indicates that all ORDINARY mass and reinforced concrete shows at least hairline cracks that can be seen in the sun with normally sharp sight. Of course SPECIAL mortar and concrete mixes exist that aim to neutralize the shrinkage effects, plus of course many sound measures in how to reinforce and include joints having the same target. In my experience (which of course has not extended to the most expensive buildings) these special mixes with such target and for non exposed architectural concrete are NEVER used. The most popular use of these things here is non-shrinking mortars used for infills of plate bearings, where good contact is wanted ensured. The use of proper meshes (usually thin and of small module), not using thick bars near the surface, doubly reinforcing slabs on the ground and establishing joints quite close I have seen of course to diminish the incidence, number and width of cracks, but I think we must not deny a natural property of a material. It is like the tensile or shear strength of concrete, it has a limit. Pass it and you can reinforce as much as you want, you will have disgregate concrete.

 

[emmgjld]

I find the description of the cracks to be interesting, 4 cracks running to the narrow dimension, do not extend from edge to edge and are present in a slab which was placed with the 'pier and beam' structure.

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.

 

[bb29510]

I feel cracks are totally advoidable. I feel that cracks are not a subject of material but a subject of method. Controlling slumps on a compacted base material will work wonders. Also immediate curing of slab up to seven days will help. Alot can be done to improved the chance of cracking to a minimum, but no one item will cure cracking.
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.

 

[ishvaaag]

Brownbag, much can be done no doubt.

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.

 

[doall]

I would just like to say that as a residential concrete contractor, I would never pour a slab without some kind of controll joint for the simple reason that it is mutch easer to explain why you put control joints in, then why your slab cracked! For me this is cheap insurence! The slab refered to above would look great with 4 joints 8' apart.
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?

 

[ckj]

Many factors , most of which have been mentioned, will cause concrete cracking. All of these can be addressed either as part of the design, prepartation, placement or curing. Most cracks are shrinkage cracks and the two principle measure to prevent them ar the use or reinforcement and control joints. Reinforcement reduces the number of visible cracks by holding them together. Instead of having a few large cracks, proper reinforcement will produce many small micro cracks which will for all intents and purposes act as a crack free slab. However,it is un economicl to put in sufficient reinforcement to prevent all vissible cracks in large slabs. In these situations the slabs should be broken into smaller slabs by the use of strategically placed shrinkage control (contraction) joints. I agree with doall in that a long narrow slab such as yours should have had 3 to 4 joints (depending on the type of joint used).

 

[berean315]

Hello again,

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

 

[Ron]

Berean315...
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