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Preventing Cracks in a Warehouse Slab

Hotline Problem

We are the architects for construction of a warehouse. We want to avoid cracks we have experienced in the past.

On our last job, we put in a 6-inch slab with joints sawed 12 or 13 feet apart. Two layers of wire mesh were used, top and bottom. We've had a lot of cracks between joints. How can we prevent them? One contractor suggested eliminating the bottom layer of mesh and adding an inch of concrete thickness. Does that make sense?

Troubleshooting Response

Not using the mesh properly is probably a large part of your problem. The 12-foot spacing of joints for 6-inch slab should have been good enough to control cracking without mesh.

In our conversation you indicated you had carried the mesh through the joint without cutting any longitudinal wires. Even if cracks occurred at the sawcut, the mesh would prevent the cracks from opening up enough to function as control joints. That's why the intermediate cracking.

It is preferable to stop the reinforcing at the joint. However, if the mesh is carried through the joint, every other longitudinal wire should be cut at the joint. This will allow the joint to open and relieve the tensile stress in the concrete between joints.

The contractor's suggestion to replace a layer with 1 inch of concrete makes sense, but it doesn't mean that 1 inch of concrete serves the same purpose as the wire mesh. The thickness of the slab required to carry the load bears little or no relationship to whether or not wire reinforcement is used. The upper layer (about 2 inches from the top) serves to hold together cracks that might occur between joints.

The lower layer serves little purpose. It does not provide structural reinforcement for carrying heavier loads, and it is the wrong place to hold shrinkage cracks together. If it equates to 1 inch of concrete, it is in cost, not in effectiveness. Since the lower layer of reinforcement isn't serving any good purpose, the cost of it may as well be applied to additional thickness of concrete, which really does increase the load carrying capacity of the slab.