7.3 Placing, Finishing, Curing, and Testing (slump, cylinders)

Quality concrete depends as much on placing, finishing, curing, and testing as on mix design. The NASCLA exam references ACI 304 (placing), ASTM C143 (slump), ASTM C31 (field cylinders), and ASTM C39 (compressive testing). Field quality control questions are common, so know the test methods and acceptance criteria by reference.

Placing rules:

• Deposit concrete as near its final position as possible; do not let it free-fall more than about 4-5 ft without a chute or tremie, or aggregate segregates.
• Consolidate with internal vibrators to remove voids/honeycomb — but over-vibration causes segregation and bleeding (trap).
• Do not place on frozen subgrade or in standing water.

Trap: dropping concrete from a great height into a deep wall form separates the coarse aggregate from the paste.

Slump (ASTM C143) measures consistency/workability using a 12-in cone filled in 3 layers, each rodded 25 times, then lifted; the slump is how far the concrete settles.

High slump usually means too much water -> low strength. Trap: a 6-in slump on a footing spec'd for 3 in signals an over-watered, weakened batch — reject it.

Finishing sequence (slabs):

1. Screeding/strikeoff — level to grade.
2. Bull floating/darbying — embed aggregate, level ridges. Do this before bleed water appears.
3. Wait for bleed water to evaporate — critical trap.
4. Floating — densify surface.
5. Troweling — hard, smooth finish (steel trowel).
6. Broom finish — texture for slip resistance (exterior walks/ramps).

Trap: finishing while bleed water is present traps it under the surface, causing scaling, dusting, and a weak crust.

Curing maintains moisture and temperature so cement can hydrate. Maintain moisture for at least 7 days for normal cement (ACI 308); concrete gains roughly 70% of design strength in 7 days and 100% (f'c) at 28 days.

Methods: water ponding/fogging, wet burlap, curing compounds (membrane), plastic sheeting. Trap: premature drying stops hydration and permanently lowers strength — strength is not recovered by re-wetting later.

Strength testing (ASTM C31/C39): Cast at least a set of cylinders per ACI (commonly one set per 150 CY or per 5,000 ft^2 of slab, per ACI 318). Standard cylinders are 6x12 in or 4x8 in, moist-cured, and broken in compression.

Worked numeric: A 6-in-diameter cylinder fails at 84,800 lb. Area = pi x r^2 = 3.1416 x 3^2 = 28.27 in^2. Strength = 84,800 / 28.27 = 3,000 psi. Acceptance (ACI 318): the average of any 3 consecutive tests >= f'c, and no single test more than 500 psi below f'c.

Placing and Consolidation

Place concrete as close to its final position as possible and consolidate with a vibrator to remove air voids — but over-vibration causes segregation (heavy aggregate sinks, paste rises). Avoid dropping concrete more than ~3–5 ft freely, which segregates it; use a tremie or chute. Do not let the truck exceed the 90-minute (or 300-revolution) delivery limit before discharge per ASTM C94. Never retemper with extra water to restore lost slump.

Finishing and Bleed Water

The finishing sequence: screed (strike off) → bull float → wait for bleed water to evaporate → edge/joint → float → trowel. The cardinal rule: never finish or trowel while bleed water is on the surface — sealing the water in causes scaling, dusting, and blistering. Broom finish gives slip resistance; steel-trowel gives a hard smooth surface. Air-entrained concrete should not be hard-troweled (it can blister).

Curing and Testing

Curing keeps concrete moist and warm so cement hydrates — minimum 7 days at ≥50°F for normal concrete (or until 70% of f'c). Methods: water/wet burlap, ponding, plastic sheet, or curing compound. Field tests: slump (ASTM C143) for workability; air content; temperature; and compressive strength cylinders (ASTM C39) broken at 7 and 28 days — a set of cylinders is typically taken per 150 yd³ or per day. Cure cylinders the same as the structure for valid results.

Common Exam Traps

• Trap: Finish while bleed water is present. Wait until it evaporates.
• Trap: Add water to restore slump (retempering) — ruins strength.
• Trap: Hard-trowel air-entrained exterior concrete — causes blistering.
• Trap: Test strength at 7 days as final — 28 days is the acceptance age.

Reading a Slump and Strength Result

Slump measures workability: a higher slump = wetter, more workable mix, but excess slump from added water signals lost strength. A spec might call 4 in ± 1 in; a 6-in slump fails and the load is rejected. Strength acceptance (ACI 318) generally requires the average of three consecutive tests ≥ f'c and no single test more than 500 psi below f'c. If cylinders fail, options escalate from re-test of retained cylinders to core drilling the structure to investigate the in-place strength.

Curing Temperature and Maturity

Strength gain depends on time and temperature together. Hydration nearly stops below ~40°F, so cold concrete gains strength slowly and must be kept warm; conversely, very hot curing gives quick early strength but lower ultimate strength. The maturity method estimates in-place strength from a time-temperature history, letting a contractor safely strip forms or open a slab sooner than the fixed 7/28-day rule. Whatever method, field-cured cylinders kept beside the structure best represent what the actual element achieved.