Cracks, Joints and Heaving in Wichita Concrete

Why concrete cracks at all

Concrete shrinks as it hardens and dries, and it expands and contracts with temperature and moisture. It is strong in compression but weak in tension. NRMCA's joints guidance says that when the natural tendency to shrink is restrained, tensile stresses can exceed the concrete's strength and cause cracking, and that cracks cannot be prevented entirely, only controlled and minimized (NRMCA, CIP 6: Joints in Concrete Slabs on Grade).

Joints: planned cracks

NRMCA calls joints "pre-planned cracks" (NRMCA, CIP 6: Joints in Concrete Slabs on Grade). A contraction joint is a groove that creates a weak line so the slab cracks there, neatly, rather than at random. Its recommendations for flatwork:

• Maximum joint spacing of 24 to 36 times the slab thickness; about 10 feet for a 4-inch slab, and no more than 15 feet in any case.
• Panels square or nearly so, with length no more than 1.5 times width. Avoid L-shaped panels.
• Joint grooves at least one-quarter of the slab thickness deep.
• Isolation joints where driveways and patios meet sidewalks, garage slabs, stairs and other fixed points.

A crack running inside a joint groove is the joint doing its job. Many random cracks in older Wichita driveways trace back to joints spaced too far apart or cut too shallow.

Reading the cracks you see

NRMCA notes that most random cracks appearing at an early age, although unsightly, rarely affect the structural integrity or service life of concrete; D-cracks and alkali-aggregate reaction are the main exceptions (NRMCA, CIP 4: Cracking Concrete Surfaces).

Heaving and settling on Kansas clay

The Kansas Geological Survey describes expansive soils as a pervasive Kansas problem: they swell with water and shrink in dry periods, cracking foundations, driveways and sidewalks (Kansas Geological Survey, GeoRecord: geologic hazards and expansive soils). K-State Research and Extension notes the pressure is enough to move concrete sidewalks (K-State Research and Extension, Clay Soil: It Shrinks and Swells). On these soils, cracks are often not about the concrete at all but about the ground under it moving unevenly. See expansive clay soil under slabs.

What causes cracks in new work

NRMCA's cracking guidance lists the usual culprits: improper jointing, poor subgrade preparation, high-slump concrete or water added on the job, finishing problems, and inadequate or no curing (NRMCA, CIP 4: Cracking Concrete Surfaces). It recommends removing topsoil and soft spots, compacting the soil or granular fill under the slab, sloping the subgrade for drainage, and curing properly. Each of those is worth seeing on a written estimate.

Plastic shrinkage on windy days

NRMCA explains plastic shrinkage cracks happen when the surface loses water faster than bleed water replaces it, usually in hot, dry, windy conditions. They tend to be parallel, about 1 to 3 feet apart and relatively shallow (NRMCA, CIP 5: Plastic Shrinkage Cracking). Windbreaks, fogging and covering between finishing steps help. Wichita's steady breezes make this a real consideration on summer pours.

Maintenance that slows the damage

• Seal joints and cracks with a flexible sealant to keep water out of the subgrade.
• Keep roof water and sprinklers off slab edges.
• Fill gaps where slabs meet the foundation so water does not run down beside the house.
• Watch trip edges and mark them until repaired.

Repair, lift or replace?

A level crack in an otherwise sound slab is a sealing job. A panel that sank intact can often be lifted. Widespread cracking, broken panels and D-cracking usually call for replacement, with better base work, joint layout and drainage. Repair and resurfacing covers the middle ground.

See the estimate factors page for how each choice changes the scope.

Most cracking starts with the ground; see expansive clay soil under slabs.

Sources: NRMCA, CIP 4: Cracking Concrete Surfaces; NRMCA, CIP 5: Plastic Shrinkage Cracking; NRMCA, CIP 6: Joints in Concrete Slabs on Grade; Kansas Geological Survey, GeoRecord: geologic hazards and expansive soils; K-State Research and Extension, Clay Soil: It Shrinks and Swells.