Cracks in a concrete slab foundation on Texas clay

A crack in a concrete slab foundation in Texas is usually a shrinkage crack that formed as the concrete dried, and on its own it rarely matters structurally. It starts to matter when the slab around it has changed shape, which on the expansive clays under much of the state is a seasonal process that a crack can record but cannot measure.

Measuring it takes a floor elevation survey and a reading of the house above. The scope 254 Engineering describes for that evaluation is on the foundation inspection page.

How Texas slabs are built

Many Texas houses stand on a slab on grade: a concrete floor cast on the ground with thickened beams under its perimeter and across its interior. The slab is reinforced in one of two ways. A conventionally reinforced slab carries steel bar. A post tensioned slab carries steel tendons stressed after the concrete hardens, and the Post-Tensioning Institute's standard PTI DC10.5 governs the design of post tensioned slabs on expansive and stable soils.

The code behind both is the International Residential Code. Section 214.212 of the Local Government Code adopts the IRC as it existed on May 1, 2012 as the municipal residential building code in Texas, applies it to construction, alteration and repair of residential structures in a municipality, and lets each city adopt local amendments.

The code treats clay as a design condition. TDLR's technical bulletin on foundations on expansive soils summarizes the IRC test, which includes a plasticity index of 15 or greater, and the requirement that foundations on such soils be designed for them under the building code.

Why the clay moves the slab

The USDA's fact sheet on Houston Black, the soil recommended as the Texas state soil, describes it as a classic Vertisol that shrinks and swells markedly with changes in moisture content. It notes that water enters these soils rapidly when they are dry and cracked and very slowly when they are moist, and that their very high shrink swell potential is a common limitation for building sites.

A slab on that soil moves with it as the moisture beneath it changes. The soil differs by region, from the Blackland Prairie behind Dallas Fort Worth to the coastal clays of Greater Houston, and the region pages describe each.

Shrinkage cracks are expected

The National Ready Mixed Concrete Association's guidance on cracking states that all concrete has a tendency to crack and that crack free concrete is not possible. It attributes most cracking to design and construction practice, recommends contraction joints spaced at 24 to 36 times the slab thickness to control where the concrete cracks, and names rapid moisture loss from fresh concrete in dry weather as a cause of plastic shrinkage cracking.

Its conclusion is the one a worried owner needs: most early random cracks, though unsightly, rarely affect the structural integrity or service life of the concrete.

A fine crack with no offset, found when carpet comes up or visible on a garage floor, is most often this kind.

Slab cracks that point to movement

• A vertical offset across the crack, so one side sits higher than the other.
• A crack that runs through the perimeter beam and is visible on the outside face of the foundation.
• Floor tile that cracks along a line, especially a line that continues across rooms.
• A crack that lines up with distress above it: doors that bind, cracks at the corners of openings, gaps where walls meet the ceiling.
• A crack that is wider at the end of a dry season than at the end of a wet one, year after year.

Water under the slab is part of the same picture. The ASCE Texas Section guidelines list leaking plumbing among the factors that contribute to foundation movement, and section 7.6 calls for water and sanitary lines to be tested for leaks where a repair includes jacking or lifting.

How to read a crack in general, and how to keep a dated record of one, is set out in a crack in the foundation.

Cracks found under flooring

Many slab cracks are first seen during a flooring job. That is the moment to photograph and measure them, with a ruler in frame and the location noted on a sketch of the room, before new flooring covers them for another decade.

Flooring also affects measurement. The ASCE Texas guidelines say floor elevation surveys should be adjusted for the thickness of floor coverings, because a change from tile to carpet can read as movement on a survey that ignores it.

Post tensioned slabs: locate the tendons before anyone cuts

A post tensioned slab is not a slab to saw, core or drill on a guess. An article on existing post tensioned foundations in STRUCTURE magazine advises that tendons be located in the areas next to any planned core, warns that removing concrete that is actively resisting a loaded anchor can cause cracking or blowouts, and says an opening near live anchors will typically require engineering analysis. It describes the anchor zone as roughly 24 to 36 inches from the anchor face.

That matters most when a plumber needs to reach a line under the slab, or a remodel adds a floor drain or moves a wall. The time to answer the question is before the saw arrives. Where a repair or alteration needs defined work, a repair specification sets it out so the contractor builds to a plan.

What an engineer measures on a slab

Under the ASCE Texas guidelines a Level B evaluation records relative elevations across the slab, draws them to scale, and reports deflection and tilt against tolerances. Deflection is measured as the largest deviation of a point from a straight line between two others, divided by the distance between them. The guidelines use the residential code's L/360 limit as an analogy and say a profile under it with no visible distress is unlikely to have deflected materially.

They add cautions. Over distances shorter than about 20 feet, unevenness built into the original pour can dominate the reading. A single survey may or may not be enough, and a comparison with an earlier survey, adjusted to a common reference point, says more.

On repair, the guidelines set realistic expectations. Section 7.4.3.1 says it is unlikely that elevation adjustments will result in a level foundation, and section 7.4.3.3 specifies epoxy injection for slab cracks of about 1/32 inch and larger where stiffness has to be restored as part of the repair.

For a slab crack that shows any of the movement signs above, the measured evaluation comes before any repair decision. The foundation inspection page sets out what it covers, and the published price list states its fee.