Preventing Concrete cracks

1. Make sure the subgrade is compacted
2. Use a Low Water-to-Cement Ratio-- How to Calculate Water to Cement Ratio's
3. Properly Curing Concrete Slabs
4. A case for allowing the time in the schedule to water cure

1.Make Sure The Subgrade is Compacted

Excavations beneath the slab that are not to be filled with concrete (usually plumbing or other mechanical trenches) should be brought back to grade in compacted lifts. This means a 24" trench would be backfilled 6" at a time, each "lift" being mechanically vibrated so it is left compact.

If these excavations are not compacted when backfilled, this loose soil will settle over time leaving the concrete over that area with no earth under it. This becomes a prime place for concrete to settle. Since the soil next to these excavations is native soil, the uncompacted trench can literally become a thoroughfare for water.
Most rental yards have compacting equipment and it is worth the investment to use it.
Excavations from the house to the street for utilities should also be backfilled and compacted in the same manner so areas under the driveway concrete don't sink. Uncompacted areas under lawns can be identified by areas of sunken grass-so it is good idea to compact trenches even under areas not receiving concrete.

2.Use a Low Water to Cement Ratio: How to Calculate Water to Cement Ratios

A low water to cement ratio is the number one issue effecting concrete quality.
Low water cement ratio impacts all of the desired properties of concrete listed in the desired properties of concrete section.
Use a maximum .50 water to cement ratio when concrete is exposed to freezing and thawing in a moist condition or to deicing chemicals per the 1997 Uniform Building Code. (Table 19-A-2)
Use a maximum .45 water to cement ratio for concrete with severe or very severe sulfate conditions per the 1997 Uniform Building Code (Table 19-A-4)
Water permeability increases exponentially when concrete has a water cement ratio greater than .50.
Durability increases the less permeable the concrete mix is.
Strength improves with lower water cement ratios. A .45 water cement ratio most likely will hit 4500 psi (pounds per square inch) or greater. A .50 water cement ratio will likely reach 4000 psi or greater.
For complete Uniform Building Code information regarding concrete construction, review with your architect, your ready mix supplier, or at your local library.
The water to cement ratio is calculated by dividing the water in one cubic yard of the mix ( in pounds) by the cement in the mix (in pounds). So if one cubic yard of the mix has 235 pounds of water and 470 pounds of cement- the mix is a .50 water to cement ratio.

3.Properly Curing Concrete Slabs

Why cure concrete. Curing serves two main purposes.

• It retains moisture in the slab so that the concrete continues to gain strength
• It delays drying shrinkage until the concrete is strong enough to resist shrinkage cracking.

Properly curing concrete improves strength, durability, water tightness, and wear resistance.
How to cure concrete.

• Water cure:
  The concrete is flooded, ponded, or mist sprayed. This is the most effective curing method for preventing mix water evaporation.
• Water retaining methods:
  Use coverings such as sand, canvas, burlap, or straw that are kept continuously wet. The material used must be kept damp during the curing period.
• Waterproof paper or plastic film seal:
  Are applied as soon as the concrete is hard enough to resist surface damage. Plastic films may cause discoloration of the concrete-do not apply to concrete where appearance is important.
• Chemical Membranes:
  The chemical application should be made as soon as the concrete is finished. Note that curing compounds can effect adherence of resilient flooring, your flooring contractor and/or chemical membrane manufacturer should be consulted.

All the desirable properties of concrete are improved by proper curing!

4.Allow Proper Time to Water Cure

After concrete is placed, the concrete increases in strength very quickly for a period of 3-7 days. Concrete which is moist cured for 7 days is about 50% stronger than uncured concrete.

Water curing can be done after the slab pour by building dams with soil around the house and flooding the slab. The enclosed area is continually flooded with water. Ideally, the slab could be water cured for 7 days.Some builders on a tight schedule water cure for 3 days as this achieves approximately 80% of the benefit of water curing for 7 days.
Consider planning your job to pour at the end of the week, build berms, then flood over the weekend. You get the benefit of water curing without losing too much time in the schedule.