ACI Flatwork Technician cheat sheet

Concrete mix proportions: cement, sand, coarse aggregate, water, and admixtures; typical water-cement ratio by weight for flatwork is 0.45 to 0.55.

Slump test (ASTM C143) measures workability; typical flatwork slump is 4 to 6 inches (100-150 mm) for hand placement, 6 to 8 inches for pumped concrete.

Air content for freeze-thaw durability: 5-7% for 3/4 inch aggregate, 6-8% for 1/2 inch aggregate, 7-9% for 3/8 inch aggregate.

Concrete weighs approximately 145-150 lb/ft³ (2320-2400 kg/m³); a cubic yard weighs about 4000 lb (1800 kg).

Rule of thumb: 1 cubic yard of concrete covers 81 ft² at 4 inches thick, 65 ft² at 5 inches, 54 ft² at 6 inches.

Curing: keep concrete moist and above 50°F (10°C) for at least 7 days (normal cement) or 3 days for high-early strength; curing compounds must comply with ASTM C309.

Subgrade preparation: compact to 95% Standard Proctor density, proof-roll to detect soft spots, and provide uniform support; minimum slab thickness for residential flatwork is 4 inches (100 mm).

Joint types: isolation joints separate flatwork from structures; contraction joints control random cracking and should be placed at 24 to 36 times slab thickness (e.g., 10-12 ft for 4-inch slab).

Concrete temperature limits: place between 40°F and 90°F (4°C and 32°C); hot weather may require ice or retarders, cold weather requires heaters or accelerators.

Finishing sequence: screed, bull float, wait for bleed water to disappear, then float, trowel, and broom or other texture.

Never finish concrete while bleed water is present; doing so traps water and reduces surface strength, causing dusting or scaling.

Bull floating (or darbying) immediately after screeding opens the surface and removes high spots; wait until concrete sets enough to support the float without leaving ridges.

Troweling: wait until concrete can support the trowel without excess mortar; each pass should be at a higher blade angle and lower speed; final trowel for hard, dense surface.

Broom finish: drag a broom across the surface perpendicular to the direction of traffic; for slip resistance, use a fine broom for indoor, coarse for outdoor.

Edging creates a rounded edge (typically 1/4 to 1/2 inch radius) to reduce chipping; jointing tool cuts contraction joints to 1/4 of slab depth.

Curing after finishing: apply curing compound or wet cure immediately after finishing; do not allow surface to dry out.

Hot weather finishing: use retarders, work at night, cool ingredients, and finish quickly; avoid overworking surface which brings bleed water to top.

Cold weather finishing: protect concrete from freezing for at least 24 hours; use accelerators, heated water, and insulating blankets; never add calcium chloride to air-entrained concrete.

Advanced finishing techniques: stamping, exposed aggregate, polished concrete, and overlays require precise timing and mix design adjustments.

Stamped concrete: apply release agent and stamp when concrete is plastic enough to hold imprint but firm enough to prevent stamp sinking; typically 1-2 hours after finishing.

Exposed aggregate: retarder applied after floating; surface washed to reveal aggregate when concrete has set enough to avoid dislodging stones; timing critical.

Polished concrete: grinding with progressively finer diamond abrasives (from 30 grit to 3000 grit) and applying densifier to harden surface; requires hard-troweled, dense surface.

Self-consolidating concrete (SCC) for flatwork: high slump flow (24-30 inches) and low yield stress; must use formwork tight enough to prevent leakage.

Joint layout for large slabs: contraction joints at 24-36 times thickness, but for irregular shapes, use isolation joints at re-entrant corners and keep panels square as possible.

Shrinkage-compensating concrete: used for large flatwork to reduce cracking; requires special curing and expansion joints.

Quality control: test fresh concrete for slump (ASTM C143), air content (ASTM C231), and temperature (ASTM C1064); hardened tests include compressive strength (ASTM C39) and flexural strength (ASTM C78).