Q.: Will it hurt to grind concrete the day after its placed? We're the general contractor for an office building project. We told the flatwork contractor to use a broom finish for a wide sidewalk around the building perimeter. He misunderstood and gave it a rake finish. We want to try to save the slab, if possible, by grinding.
A.: We posed the question to Leo Swan of Equipment Development Company, Inc. Leo says you can usually start grinding within 8 hours of placement. Watch for tearing of aggregate particles and stop grinding if this happens. A rake finish probably will have furrow depths of 3/8 inch or so; grooves in a broom finish generally don't exceed 1/32 inch. You may be able to grind just enough of the rake finish away to leave shallow grooves that will still give needed traction. However, aggregate will be exposed. If the resulting surface isn't acceptable to the owner, you could try texturing it with a drum-type planer with small carbide teeth.