Concrete Driveways Do's & Don'ts

The following points should be clearly understood and agreed to by all parties, including: ready mixed supplier, finisher, home builder and owner.

MIX DESIGN

  • Ready Mixed Concrete Association (RMCA) recommends 4,000 psi, air entrained concrete for outside paving. Slump (stiffness of the mix) may exceed 3" (three inches) only if water reducers are used and may exceed 5" (five inches) only if high-range water reducers are used.

UNDERSTAND WATER TO CEMENT RATIO

  • Do not add excessive water at the jobsite. One water addition may be made at jobsite provided it does not exceed the approved mix design.

SUBGRADE PREPARATION

  • Uniformity is the key to proper subgrade compaction and is more important than subgrade strength.
  • "Roll test" compacted subgrade with a dump truck. Wheel penetration into compacted subgrade should be uniform. Soft spots must be removed and replaced with good material.
  • Never place concrete on bone dry subgrade. Spray subgrade so that it is damp and does not suck water from the concrete. Do not place concrete directly on polyethylene vapour barriers. This may cause cracking.

PLACING

  • Deposit the concrete as close to its final location as possible -- the less you move it the better.
  • If the slab is on a slope, start at the low end and work uphill or, use 3" maximum slump concrete.
  • Do not use garden tools to move concrete or let concrete drop from a chute or bucket more than 3 feet. This may cause aggregate segregation.
  • The proper tools to move concrete horizontally are: square nose shovels, come-alongs, or concrete rakes.

FINISHING

  • Screed (strike off) the concrete to proper elevation. Bull float before bleed water accumulates. (Do not use float excessively or bleed water may be sealed in.)
  • Never perform any finishing operation with bleed water present. This will cause scaling.
  • Do not add water to the surface while finishing the surface. This will cause scaling or crazing.
  • Do not "hot shot" (addition of raw cement to the surface). This may cause scaling.
  • Brooming must await the evaporation of bleed water.
  • If weather is excessively hot, windy, or dry, the concrete should be protected by covers. This prevents rapid evaporation before finishing begins and avoids plastic shrinkage cracks.
  • Do not steel trowel exterior concrete, such as driveways or pavements.

JOINTS

  • Control joints should be placed at a maximum spacing of thirty (30) times the slab thickness. (EXAMPLE: A 4" slab = 8 ft. joint spacing.)
  • Control joints may be made after finishing by jointers (groovers) or they may be sawed as soon as the concrete is hard enough that no raveling occurs.
  • Control joints should be at least one-fourth (1/4) the slab depth. (4" slab = 1" deep joint)
  • Isolation joints must be placed where new concrete meets a fixed object (footing, wall, sidewalk, porch, etc.) Isolation joint material (usually 1/2" thick) must be placed before placing concrete.

CURING

  • Concrete should be cured by using a quality curing compound, cure & seal compound, wet burlap, or ponding.
  • Uncured concrete may develop only 50% of its ultimate strength.
  • Protect concrete against freezing temperatures for at least three days.

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