It raises the water content at the surface, weakening it. The result is dusting, scaling and crazing.
Placing & Finishing Concrete.
How concrete is placed and finished decides how it looks and how long it lasts. Prepare well, place evenly and finish at the right time.
At a glance:
75–125 mm : Typical slump for flatwork
24–30× : Slab thickness = max joint spacing
¼ depth : Control joint cut depth
Prepare the Site.
Concrete is only as good as what's underneath it. Compact the subgrade and base evenly. Remove soft spots, organics and standing water. Set forms to line and grade, and brace them to handle the weight.
Place reinforcing on chairs so it sits in the right position within the slab, not on the ground.
Place It Evenly.
Place concrete as close to its final position as possible. Don't drag it long distances with rakes or vibrate it sideways, as this separates the stone from the paste. Consolidate walls and footings to remove trapped air.
Finish at the Right Time.
Strike off and bull float right after placing. Then wait. Water will rise to the surface (bleed water). Do not finish while it is present.
Once the bleed water has gone and the surface can hold your weight with a light impression, start final finishing. Working water back into the surface weakens it and leads to dusting and scaling.
Cut Control Joints.
Concrete shrinks as it dries, and it will crack. Control joints decide where. Cut or tool them to a quarter of the slab depth, spaced no more than 24 to 30 times the slab thickness. A 100 mm slab needs joints every 2.4 to 3m. Keep panels close to square.
Frequently Asked Questions
Why shouldn't I sprinkle water on the surface to finish it?
As soon as the concrete is firm enough that the saw doesn't ravel the edges. Usually within 6 to 18 hours, depending on temperature.
Should air-entrained concrete be steel-trowelled?
Avoid hard steel trowelling on exterior air-entrained slabs. It can seal the surface and lead to delamination. A broom finish is standard for exterior work.
Interior slabs on grade usually do, especially under floor finishes. Check your building plans.
Sealing the surface before trapped air and bleed water escape. Avoid early trowelling, especially in wind or sun.
Disclaimer: These guides are general information. Always follow your engineer's specification and local building requirements.
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