Why Concrete Cracks and What to Do About It

What Causes Concrete to Crack?

Four mechanisms account for almost every crack a homeowner or property manager sees: plastic shrinkage, drying shrinkage, thermal movement, and subgrade failure. Each produces a recognisable pattern and demands a different response.

Plastic shrinkage cracks

Plastic shrinkage cracks form within the first 24 hours, while the concrete is still workable. When surface water evaporates faster than bleed water rises from below, the surface skin tightens and tears. Wind above 10 mph, air temperature above 85 degrees Fahrenheit, or low humidity all accelerate evaporation past the safe threshold of 0.20 pounds of water per square foot per hour. The resulting cracks are typically 1 to 3 feet long, shallow at under 1 inch, and run diagonally across the slab. A curing compound applied immediately after finishing, or a plastic sheet cover, prevents most plastic shrinkage cracking.

Drying shrinkage cracks

Drying shrinkage occurs over weeks to months as the concrete loses internal moisture and the paste contracts. A typical mix shrinks 0.04 to 0.08 inches per 10 linear feet. On a 20-foot driveway with no control joints, that shrinkage concentrates into one or two random cracks near the centre. Control joints cut at one-quarter of the slab depth, spaced no more than 2.5 times the slab thickness in feet, give the shrinkage a planned location to crack. A 4-inch slab gets joints every 10 feet, cut 1 inch deep.

Thermal movement cracks

Concrete expands roughly 0.000006 inches per inch per degree Fahrenheit. Over a 30-foot slab with a 50-degree temperature swing, that equals about 0.11 inches of movement. Without expansion joints at structure interfaces, such as where a driveway meets a garage wall, the slab pushes against a fixed object and cracks. Expansion joints filled with a compressible backer rod and polyurethane sealant absorb that movement. Thermal cracks typically run straight and are wider at one end than the other.

Subgrade failure and settlement cracks

Settlement cracks form when the ground beneath the slab drops unevenly. The slab then spans a void and deflects under load. A 4-inch unreinforced slab can bridge a gap of roughly 12 to 18 inches before cracking under a passenger vehicle. Voids form from organic fill decomposing, poor compaction, or water washing out fine particles under the slab. Settlement cracks are wider at the surface, often show vertical offset at the crack edges, and do not close under pressure. Rebar or fibre reinforcement does not prevent settlement; it keeps the pieces from separating further after the crack forms.

How Does the Water-to-Cement Ratio Affect Cracking?

The water-to-cement ratio, the weight of water divided by the weight of cement in the mix, directly controls both strength and shrinkage. A ratio of 0.45 to 0.50 produces 4,000 PSI concrete with controlled shrinkage. A ratio of 0.65, common when finishers or drivers add water to a stiffening truck, drops compressive strength to roughly 3,000 PSI and increases total shrinkage by 20 to 30 percent. A worked example: a standard 3,500 PSI mix calls for 28 pounds of water per 60-pound bag of cement. Adding one extra gallon, 8.3 pounds, pushes the ratio from 0.47 to 0.61 and produces measurable strength loss. Never add water to stiffen concrete once the truck arrives on site. Slump should be confirmed at 4 to 5 inches before the pour begins.

How Do Control Joints Prevent Random Cracking?

Control joints work by creating a deliberate plane of weakness that guides shrinkage cracks to a predictable location. The joint must be cut to at least one-quarter of the slab depth and completed within 4 to 12 hours of finishing, before shrinkage forces build. On a 4-inch residential slab, the joint is cut 1 inch deep. Spacing follows a simple rule: joint spacing in feet equals 2 to 2.5 times the slab thickness in inches. A 4-inch slab gets joints every 8 to 10 feet. A 6-inch commercial slab gets joints every 12 to 15 feet. Bay shapes should be square or nearly square. A panel that is twice as long as it is wide will crack across the long dimension regardless of joint placement.

What Do Common Crack Patterns Tell You?

Crack pattern identifies the cause before any repair begins. Hairline cracks under 0.02 inches wide, running parallel to control joints, are normal drying shrinkage and require only sealant if the slab is exposed to moisture. Map cracking, a fine network across the surface also called crazing, comes from a weak surface layer caused by overworking or adding water during finishing, and does not threaten structural integrity. A single crack running the full width of a panel with vertical offset at the edges is a settlement crack and requires investigation of the subgrade before any surface repair. Wide cracks above 0.04 inches that grow over months indicate ongoing movement and need an engineer's assessment, not a bag of crack filler.

When Is Crack Repair a DIY Job and When Is It Not?

Hairline cracks up to 0.02 inches wide with no vertical offset and no movement are suitable for DIY repair with a polyurethane or epoxy crack filler. Clean the crack to bare concrete, blow out dust, inject or pour filler, and allow 24 hours cure before traffic. That repair stops water infiltration. Any crack wider than 0.04 inches, any crack with vertical offset, and any crack that has widened by more than 0.01 inches over 3 months needs a contractor assessment before repair. Filling an active crack without addressing the cause produces a failed repair in one to two freeze-thaw cycles. A slab with more than 3 cracks per 100 square feet is a candidate for full replacement rather than patchwork repair.

How Do New Orleans Area Conditions Change the Risk of Cracking?

The south shore of Lake Pontchartrain and the west bank sit on soft alluvial clay and organic silt over a high water table, typically within 18 to 36 inches of the surface in many areas. Organic material in the fill continues to consolidate under load for years after a pour. A slab poured over uncompacted fill in Jefferson or Orleans Parish without a 4-inch compacted limestone base is operating without adequate support from the first day. Field experience on this soil shows settlement cracks developing within 12 to 24 months where subgrade preparation was skipped. The standard mitigation on this soil is 4 to 6 inches of compacted crushed limestone, a 6-mil poly vapour barrier, and rebar on 18-inch centres, rather than the fibre-only approach common in drier climates with stable subgrades. Confirm local subgrade requirements with a geotechnical professional before pouring slabs larger than 400 square feet in Orleans, Jefferson, or St. Bernard Parish.