Materials

ConcreteNetwork.com
Walking in Ann Arbor in winter.
dnj_Brian

When thinking about the conditions a parking lot pavement needs to withstand, imagine a parking lot in Michigan in January. Conditions don't get much more severe than that--believe me, I grew up there and scraped snow and ice off my share. Concrete parking lots across much of the U.S., all the areas that have cold weather, even if it's not as extreme as in Michigan, are exposed to freezing and thawing, deicing chemicals, and heavy loads. Any of these can and will destroy concrete that isn't properly proportioned.

Your primary concern should be durability. Durability for a pavement means two things: a good strong surface layer and freeze-thaw resistance. Strength is also important mostly because it leads to more durable concrete. In general, in freeze-thaw areas with deicing salts, 4000 psi should be the minimum compressive strength for a parking lot. The real strength issue is the flexural strength rather than the compressive strength, since pavements need to resist bending and cracking more than crushing. Remember that flexural strength is usually about 15% of the compressive strength and the overall bending strength of the pavement is directly related to its thickness.

ConcreteNetwork.com
Lack of entrained air in this concrete led to severe surface spalling.

So to get the durable concrete we want for parking lots, there are a few things to remember:

  • Air entrained concrete is critical for keeping the surface in good shape. Freeze-thaw cycles, especially with deicing salts, will spall the surface of non-air-entrained concrete within a year. Make sure you get the air content noted in the table below.
  • As with any slab, you'll get less shrinkage (and therefore less cracking) with larger top size aggregate, however, keep it less than 1/3 the thickness of the slab. Well-graded mixes are more workable and also shrink less (and therefore crack and curl less).
  • If there is sulfate in the soil, use sulfate-resistant cement--Type II or Type V. Pozzolans (fly ash, slag) or blended cements can also help to mitigate the effects of sulfate.
  • Concrete slump should be 4 inches for screeded applications or 1 inch for slipforming.
Air content to get durable concrete
Total air content, %*
Maximum aggregate size, in. Severe Exposure Moderate Exposure
3/8 7.5 6
1/2 7 5.5
3/4 6 5
1 6 4.5
1 1/2 5.5 4.5
2 5 4

* This is total air, including both entrained air and entrapped air; tolerance is ±1.5%

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