NCCER Concrete Finishing cheat sheet

Concrete Finishing

• Slump test (ASTM C143): measure drop of the concrete cone from the top of the mold to the displaced center; typical structural range is 1 to 4 in (25 to 100 mm) unless plasticizers or special placement require more.

• Air content by pressure meter (ASTM C231) for normal-weight concrete; volumetric method (ASTM C173) is used for lightweight or slag aggregate. Typical air entrainment for moderate freeze-thaw exposure is 5 to 7 percent.

• Finish timing is governed by bleed-water loss: wait until water sheen disappears before bull floating; floating while bleed water is present drives water into the surface and lowers the water-cement ratio locally, causing dusting and scaling.

• Screeding removes excess concrete and strikes off to grade; bull float opens the surface and knocks down ridges; darby or highway straightedge removes high spots and fills low spots; troweling follows when the surface is firm enough to support the machine.

• Broom finish is produced by dragging a broom or rake across a floated surface; the coarser the bristles and deeper the drag, the rougher the texture and the higher the wet coefficient of friction.

• Control joints (contraction joints) are sawn or tooled to a depth of at least one-quarter of slab thickness (1 in minimum for a 4 in slab) and spaced roughly 24 to 36 times slab thickness for unreinforced slabs.

• Curing maintains moisture and temperature for strength gain and durability; ACI 308 methods include ponding, wet burlap, plastic sheeting, and curing compound applied at the manufacturer's coverage rate, typically within 10 to 30 minutes of finishing.

• Cold-weather practice (ACI 306): concrete temperature must be maintained above 40 to 50 degrees F during protection; hot-weather practice (ACI 305): lower concrete temperature, place at cooler times, and use evaporation retarders when the evaporation rate exceeds 0.2 lb/sq ft/hr.