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how to thicken an existing concrete slab on grade
i am looking for help on how to effectively and efficiently thicken an existing slab on grade that was poured too thin.
a little background: this is an industrial facility that we specified on our drawings a slab on grade thickness of 6". the facility has forklift traffic. the slab started cracking very soon after installation. after the slab started cracking, a testing firm came in and tested the slab. in their report, they found that the slab had evidence of curling. after taking core drilled samples, it was found that the slab was poured at more like 4.75" rather than 6" and thus deflection and cracking was occuring.
i am now in the process of coming up with a solution to fix the cracks and the curling, but i am stumped on how to make the deflection issue go away, if the slab is too thin, how do i make it thicker?
the owner would prefer not to go on top with more concrete or completely rip out the slab entirely, is there another option out there?
thanks for your help!
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if the slab was placed on crushed aggregate could you use pressure grouting like preplaced aggregate concreteing? that way the finished floor elevation would remain the same but the "apparent" thickness would increase.
the best way to test something is to squeeze it, slowly, until it breaks!
perhaps the repair could be for the traffic aisles only. i suggest a cement treated base beneath the replacement slab and stiff wwr, (6"x6"- w7.5xw7.5), on continuous chairs in the center of the new slab. use large stiff plates beneath shelving posts on the thinner slab.
add control joints at selected locations by saw cutting may be the way to release the undesirable stresses and flaten the slab.
you say that the slab started experiencing problems "very soon after installation". was this before the forklift loads were applied? you may have a more serious problem than just pouring the slab too thin. even if the slab was poured exactly to specified thickness, you may have gotten the same cracking, curling and deflection for other reasons (sub-base prepartion, slab reinforcing and jointing, concrete mix design, concrete curing, etc.). have you eliminated all other potential causes?
taro made the points.
the "curling" and deflection may both be the same thing, and may indicate problems with the material under the slab. the cracking is most likely due to restraint shrinkage. you didn't mention the joints. how are they performing? using only the information you have provided, i would not be confident that there is a good solution for your problem without ripping out the slab and starting over.
preplaced aggregate grouting or prepakt concrete {same thing }is only possible when the gravel is clean and dust free so that the grout binds well to the gravel.this will probably not be the case here. pressure grouting can support or lift the slab and permeation grouting stiffen the base material. in this situation the effect might or might not work. it is also expensive .grout per cub meter is about 4 to 5 times the price of concrete.it also needs expensive equipment and personnel.
intrusion prepakt /marineconcrete.com

i assume since curling has taken place, the slab has lost bearing with the subgrade around the edges of the panels and as a result, flexural cracks have developed over the surface of the slab.
i attended a presentation on concrete pavements not so long ago which dealt with curling. basically bearing can be restored by injecting grout into the voids that are developed between to the slab and the subgrade.
another approach could be to wet the slab so it hogs up level so new control joints can be cut around the curled regions of the slab.
it shouldn't take long for this to occur. i have heard of a professor who was study the deflections of reinforced concrete slabs under long-term load conditions when the roof of his laboratory was taken away in a violent storm and flooded the labs.
he noticed with his recordings that after being exposed to water for about two days, the deflections of his slab actually reduced.
i doubt the culing or cracking would be from the slab being 4 3/4 inches instead of 6". most likely it would be from any of the following:
to much water in the concrete mix or to high w/c ratio in mix
inadequate slab reinforcement (hopefully you did not specify a light mesh)
improper placement of the slab reinforcement.
improper curing of the concrete. did they cover and use a curing agent on the slab to prevent curling?
poor joint detail. for a slab having forklift traffic, the slab should have been placed in alternate pours in a checkerboard fashion with keyed and/or doweled joints.
if you specified saw cut control joints, did they cut them within 24 hours of placing the slab.
poor subgrade
a little more information regarding the location and type of cracks as well as the slab design would required to help you out here.

simon616. just wondering how the too thin placement was not noted during concreting. any supervision on your part of independent agency? secondly, regardless of the curling and cracking, shouldn't the contractor be on the hook to remediate the slab to meet the project requirements? he had a duty to pour a 6 inch slab; didn't do it so he did not live up to the requirements of the contract. repairs and making good should rest with him. eh?

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