Concrete guides

The finishing sequence is where most surface defects are created. NRMCA's rule: initial screeding must be promptly followed by bull floating, and subsequent finishing must not be performed while bleed water is on the surface or before bleeding has finished. Troweling water back into the surface makes a weak layer that scales later. For exterior slabs, a broom finish is what you want — better traction and less over-finishing than a steel trowel.

Joint timing is a clock, not a preference. NRMCA: conventional saw-cut joints should be run within 4 to 12 hours after the concrete has been finished; early-entry dry-cut joints generally 1 to 4 hours after finishing. Cut too early and the edges ravel; cut too late and the slab cracks on its own first. Joints need a groove depth of at least 1/4 the slab thickness and no less than 1 inch.

Isolation joints are the other half of the plan and get skipped constantly. They separate the slab from walls, footings, columns, steps, and adjoining slabs, so the two can move independently. NRMCA specifically calls for isolation joints at junctions of driveways with walks, curbs, or other obstructions.

Then curing. Start immediately after finishing, continue 3 to 7 days, and use either water methods (fogging, ponding, wet burlap kept continuously moist) or retention methods (plastic sheeting at least 4 mils conforming to ASTM C171, or liquid membrane curing compound conforming to ASTM C309 or C1315). NRMCA warns that alternate wetting and drying is not an acceptable curing practice.

How to choose

Plan joints before the pour

NRMCA's first rule for jointing is to plan the exact location of all joints, including the timing of sawing, before construction begins.

Keep panels square

Panel length should not exceed 1.5 times the width, and L-shaped panels should be avoided. Odd shapes crack across the inside corner.

Be present for bull floating and the joint cut

These two moments, hours apart, determine most of what you will live with. You do not need to supervise — just observe and photograph.

Ask which curing method they'll use and for how long

"White-pigmented curing compound, two coats at right angles" or "wet burlap kept continuously moist for at least 3 days" are real answers.

Don't let anyone finish while bleed water is standing

This is the single most common cause of a surface that fails in year two.

Confirm the slab drains

The slab and the subgrade should be sloped for drainage. Water sitting on concrete is the start of most durability problems.