Concrete Placement, Finishing, and Curing
Placement and the face that shows
The mix must suit tilt-up work: workable enough to consolidate around dense steel and inserts, while meeting the specified maximum aggregate size, slump, and air content. Place in lifts, consolidate with internal vibration without dragging the vibrator through the steel, and screed to the form elevations. The surface you finish becomes the face seen from outside the building, so troweling, brooming, or retardation must match the approved architectural sample.
On a cast-on-slab panel, curing and bond breaking are the same operation: the curing compound applied to the slab keeps the panel from bonding to it. Apply the specified coverage in the specified number of coats - commonly two, applied at right angles to each other - and protect the film until lift.
Strength, not the calendar
Lifting is permitted when the concrete has reached the strength stated by the engineer of record, verified by field-cured cylinders or an approved maturity method. Elapsed time is not a substitute. Lab-cured cylinders do not represent concrete curing in a panel on grade.
Traps
- Relying on lab-cured cylinders, or lifting because "it has been three days."
- Overfinishing the face, trapping bleed water and producing a weak surface layer.
- Skipping the second coat of curing compound and getting a panel that sticks.
- Placing in heat, wind, or freezing weather without the approved protection plan.
Worked example
If the specified lift strength is 3,000 psi and field-cured cylinders read 2,400 psi at 24 hours and 3,200 psi at 48 hours, the lift waits for the 48-hour break - and the crane schedule is built around the break, not the reverse.