Technical Questions

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Increasing the load capacity of a slab-on-ground using an overlay

Q. Our client is repurposing a warehouse from light storage to heavy storage, but the current slab-on-ground is inadequate for the forklift truck traffic required in the new application. We’re planning to enhance the slab using a topping course with a vapor retarder (membrane) between the two concrete layers. I anticipate that some random cracking may occur. Do you see any other areas of concern?


A. As stated in Section 6.1 of ACI 224R, overlays “are commonly used for rehabilitation of deteriorated bridge decks, strengthening or renovating pavements, warehouse floors, walkways, and other concrete flatwork; and in new two-course construction.” Depending on the application, overlays can be unbonded, partially bonded, or bonded to the original concrete.

If a vapor retarder or other type of membrane is used between the base slab and the overlay, the topping course will be unbonded. Unbonded overlays are recommended when the original slab has sufficient thickness but exhibits severe deterioration. An unbonded overlay must be designed to be sufficiently thick to withstand the applied loads. ACI 302.1R recommends a minimum thickness of 4 in. (100 mm) for unbonded overlays but, depending on the use, the overlay may need to be substantially thicker. The shrinkage of the overlay concrete should be minimized as curling/warping can be problematic for slabs placed on existing slabs.

Joints in overlay slabs should be located over all joints in the existing slab; and for unbonded overlays, additional joints may be recommended depending on the overlay thickness. Also, as forklift traffic is anticipated for your project, attention should be given to achieving joint stability. If necessary, depending on the concrete shrinkage potential, the use of dowels or continuous reinforcement can improve the joint stability.

As noted previously, the use of a vapor retarder will result in an unbonded topping slab. However, if the goal is to increase structural capacity of the existing slab, a bonded overlay may be more efficient, as it will respond monolithically with the underlying concrete. Bonded toppings can be substantially thinner than unbonded toppings and often do not result in a change in the floor surface elevation.

A condition evaluation of the concrete slab is required prior to making a decision on the type of an overlay to be installed. Any active cracks need to be stabilized to avoid being reflected through the topping. In the case you describe, deterioration of the existing slab apparently isn’t an issue, so a bonded concrete overlay is probably the best solution.


References: ACI 325.13R-06; ACI 302.1R-15; ACI 360R-10; ACI 224R-01; ACI 546.3R-14

Topics in Concrete: Repair; Slab; Troubleshooting

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