Concrete materials

Air entrainment is the single most misunderstood item on a residential concrete order. Air-entrained concrete contains millions of tiny bubbles that give freezing water somewhere to expand, which is what stops the surface from flaking apart. NRMCA's recommended total air content for concrete with 3/4-inch or 1-inch coarse aggregate is 6 percent. If you live where it freezes and your quote doesn't say "air-entrained," ask why.

Slump is how wet the mix is. Wetter concrete is easier to place and worse in almost every other way. NRMCA recommends placing most slab concrete at a 3 to 5 inch slump and warns that adding water at the jobsite increases segregation and bleeding and can leave a weak mortar layer at the surface. Water added to a truck to make finishing easier is a cost you pay later in scaling.

Reinforcement does not do what most homeowners think. NRMCA: wire mesh and reinforcement in slabs cannot prevent cracking. Placed at the correct depth, reinforcement reduces crack width and transfers load. It also needs at least 2 inches of concrete cover so salt and moisture don't reach the steel and rust it, which itself cracks concrete.

Synthetic fibers, typically dosed at 1 to 2 lbs per cubic yard, reduce plastic shrinkage and plastic settlement cracking and add toughness. NRMCA's own "do not use for" list is worth quoting to a salesperson: not for controlling cracking from external forces, not for higher structural strength, not as a replacement for structural steel, not to decrease slab thickness, and not to increase control joint spacing.

How to choose

Get psi and air content on the quote

"4,000 psi, air-entrained, 6% total air" is a one-line addition that gives you something enforceable. Verbal assurances do not survive a dispute.

Ask to see the delivery ticket

The ready-mix ticket states the mix ordered. It is the cheapest verification available and you can ask for a copy on pour day.

Watch for water being added on site

If the crew hoses down the chute repeatedly to make the mix flow, note it. NRMCA ties excess jobsite water to weak surfaces and scaling.

Don't pay extra for fibers you were sold as crack-proofing

Fibers are useful for early-age cracking. They are not a substitute for joints, thickness, or steel — NRMCA says so directly.

Base material is a material

Compacted granular fill under the slab does more for longevity than most upgrades. NRMCA lists improper subgrade preparation among the main causes of cracking.

Sealers are consumables, not upgrades

NRMCA suggests a breathable silane or siloxane-based concrete sealer or water repellent for slabs in harsh winter environments, applied when the slab is reasonably dry — it names late summer after a few dry days as an ideal time.