Concrete Slab Foundations: A Brief History

As a residential foundation, concrete slabs really took off during the building boom that followed World War II

. With war veterans returning to their families (or starting families of their own) in communities across the country, the pressure was on to complete houses quickly. Builders discovered that it was cheaper and faster to pour a concrete slab foundation than to frame a floor with posts, beams, joists and sheathing.

Some slabs were poured inside short masonry perimeter walls (stem walls) that supported the building's wood-framed walls. Other slabs were formed with a thickened perimeter that could act like a footing, eliminating the need for stem walls. (See drawings) The latter type of slab is often referred to as a monolithic slab, because it's usually completed in a single continuous pour.

These two construction methods are still used today. Early on, slabs were usually no more than 4 in. thick, and no reinforcement was used. It didn't take long for builders to discover that without proper reinforcement, a slab was likely to crack and even buckle or shift, creating problems that were expensive to repair.

Today building codes mandate numerous requirements for concrete slabs, including minimum levels of reinforcement. Steel reinforcing bars (rebar) are typically used in combination with steel mesh, which is often referred to as welded wire mesh. However, even with proper reinforcement, slabs can be damaged, sometimes extensively. Highly expansive, clay-rich soils can be found in many parts of the country. These soils can exert sufficient pressure to crack and displace slabs as well as foundation walls. If the soil under a slab settles so that a section of the slab isn't supported, the slab can crack and settle. In cold climates, frost heaving can damage slabs if the soil beneath them freezes.

If there are problems with a concrete slab, the first step is to determine if the problems are aesthetic or structural. Since aesthetic problems concern appearance rather than structural performance, they can often be solved by the homeowner or an experienced mason or remodeling contractor. However, if the problems affect the slab's structural performance, it's best to contact a concrete or foundation repair specialist.

by: John Smith

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