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Parking Lot Poured Slab Cracking

Parking Lot Poured Slab Cracking

Parking Lot Poured Slab Cracking

We had a contractor pour a six inch slab 80 x 100' over two new underground storage tanks with 14' of 3/8" clean aggregate backfill, drop poured into excavation after aggressive hand compaction of aggregate under and around tanks.  No mechanical compaction of subgrade was performed until final 10-12".  Walk behind plate tamper was used by contractor and site was leveled.

Contractor told me that instead of highway mat, hex mesh or conventional rebar he would recommend fibermesh concrete with a 5000# PSI only; no reinforcement was needed.  He fine graded the site with approximately 1"+/- CA6 over the CA16 without filter fabric separation.  Traffic, including our delivery of gasoline (Tanker carrying approx. 7K gal.) was allowed over pavement in 4 days...avg. temp. 45 degrees F.  

Within 7-10 days cracking started.  I need help.

1. What specifications should he have used, or me?
2. Was he correct that the fibermesh in concrete mix was sufficiently strong enough for the depth of backfill shrinkage from traffic vibration?
3. No engineer was used to specify concrete, etc.
4. At 96 density proctor the 4% loss by self compaction of the subgrade; according to the contractor, could result in a 5.5 inch loss under the concrete?
5. It would help me convince the contractor to repair his problem if I had some references from knowledgeable sources or ASTM/ANSI/ACI, etc.  I have spent an inordinate amount of time trying to research this without a MAP!


RE: Parking Lot Poured Slab Cracking


this will sound very harsh but...

You seem to want to close the stable door well after the horse has bolted.  The time to ask all of those questions was before you signed a contract.  

From what you have told us, I do not like your chance of demonstrating to the contractor that the problem is "his", unless he has clearly contravened the contract.  Merely adopting some practice that perhaps should have been prohibited by a properly written contract specification does not make it 'his' problem.  

You say that no engineer was used to specify the concrete etc.  There is a lot that should have come under the 'etc'.  - including material to be used as backfill, compaction (limiting mechanical compaction to the top foot would seem to be just asking for problems), slab design, concrete mix, placement, curing, prevention of early loading.

It is much too late, of course, but even now I would strongly advise you to engage the consulting engineer that you should have gone to in the first place.

RE: Parking Lot Poured Slab Cracking

Austim is right on target (as usual!).  While there are certainly standards of care to which the contractor should subscribe, it is usually the contract and supporting documents that get followed.  I suspect the contractor is taking advantage of you, but is being allowed to do so by the contract.  Your item 4 implies the contractor is even perhaps a bit more technically adept than many (though he is not necessarily correct in this, he's on the right track).

I would second austim's comment to get a consulting engineer involved at this point.  He might not be able to correct the ills of the past, but can advise you as to appropriate corrective action and mitigation.

I believe this one follows the adage...pay me now or pay me later! The price tag goes up with time!

RE: Parking Lot Poured Slab Cracking

It is more expensive to repair than design!!!!!

your problems start from
"No mechanical compaction of subgrade was performed until final 10-12"."  Compaction must be done in 6 to 8" intervals.

Fill was not certified  no engineering plan

no mat reinforcement   Steel is cheap

heavy loads applied after "4 days...avg. temp. 45 degrees F."  at these low temps the concrete had not takes much of a cure.

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