Concrete: Mixing, Placing, and Curing

What concrete is

Concrete is portland cement, water, sand, and stone. Cement and water react (hydration) to form a paste that binds the aggregate and keeps gaining strength for weeks; most of the gain happens in the first month, and it continues slowly for years ¹. Ready-mix concrete is ordered by strength (psi at 28 days), slump, air content, and maximum stone size.

Water

The single biggest control on strength and durability is the ratio of water to cement: less water, stronger and less permeable concrete. Alberta’s agriculture guide puts a number on it: adding as little as 20 liters of water per cubic meter cuts strength about 15 percent, so water shouldn’t be added at the site unless it was planned into the mix ². A cubic yard is about three-quarters of a cubic meter, so that’s roughly 4 gallons a yard.

If the concrete arrives too stiff to place, the fix is ordering a higher slump or a water-reducing admixture next time, not a hose in the drum.

Air

Water in concrete expands when it freezes. Air-entrained concrete contains billions of tiny bubbles, normally 5 to 8 percent of the volume for freeze-thaw exposure, which give the ice room and keep the surface from scaling and cracking ². Driveways, walks, steps, garage slabs, and anything exposed to Wisconsin winters and road salt should be air-entrained.

Slump

The slump test measures workability: fill a 12-inch cone with fresh concrete, lift it, and measure how far the pile sags. Most farm and residential work uses slumps of about 2 to 5 inches (50 to 125 mm) ²: stiff enough for slabs to hold their shape, wet enough to flow into wall forms with vibration.

Placing and finishing a slab

1. Prepare: compacted base, vapor retarder where required, forms set to grade, reinforcement or mesh supported off the ground, everything dampened so the base doesn’t suck water out of the concrete. Never pour on frozen ground ³.
2. Place concrete close to its final position; don’t drag it long distances, which separates the stone from the paste.
3. Screed it off level with a straightedge riding the forms.
4. Bull float to push down the stone and bring up paste.
5. Wait for the bleed water on the surface to disappear. Working water back into the surface makes a weak, dusty top.
6. Edge, joint, and trowel or broom. Control joints, cut or tooled to a quarter of the slab’s depth, tell the inevitable shrinkage cracks where to go.

Curing

Curing keeps the concrete moist so the cement keeps hydrating and the concrete keeps gaining strength ¹. Methods: sprinkling or fogging, wet burlap or curing blankets, plastic sheeting, or a sprayed curing compound ¹. Concrete that dries out in the first days, from sun, wind, or heat, ends up weaker and more likely to crack and dust. The rate of hardening also depends on temperature: cold concrete gains strength slowly ¹.

Cold weather

Wisconsin pours a lot of concrete in spring and fall. The National Ready Mixed Concrete Association’s rules ⁴:

• Protect concrete from freezing until it reaches at least 500 psi, about two days for most concrete kept at 50 °F.
• Concrete that freezes while still plastic can lose more than half its potential strength, and its durability suffers.
• Concrete that will see repeated freeze-thaw cycles should be protected until it reaches about 3,500 psi.

In practice: thaw and keep the ground unfrozen, order heated concrete and an accelerator, cover the work with insulated blankets right after finishing, and don’t strip forms early. And never place a footing on frozen soil ³.

What to take from this

Concrete’s strength comes from the cement paste, so keep water to what the mix was designed for: 20 extra liters per cubic meter costs about 15 percent. Air-entrain anything that freezes outdoors (5 to 8 percent), order a 2 to 5 inch slump, place without segregating, wait out the bleed water before finishing, and cut control joints. Keep it moist to cure, and in cold weather keep it from freezing until 500 psi, about two days at 50 °F.