Concrete vs. Asphalt (2)

Concrete is an excellent choice for an attractive, durable, and low-maintenance choice for your parking area. Two factors account for a quality product that will last: what's under the concrete and the weather conditions when it is poured.

Regardless of whether you choose concrete or asphalt for your parking lot, the subgrade and subbase will determine how well your parking area holds up over time. An uneven, poorly drained base will affect the surface and lead to more potholes, excessive cracks, and will significantly impact the lifespan of the product.

the Subgrade and SubBase

Subgrade is defined as the natural ground, graded and compacted, on which the concrete parking area is constructed. Because of the rigidity of concrete slabs, the pressures on the subgrade are very low. Thus, concrete slabs do not necessarily require strong support from the subgrade. It is important, however, that the subgrade support be reasonably uniform without abrupt horizontal changes from hard to soft, and that the upper portion of the subgrade be of uniform composition and density. Soft spots, frost-susceptible soils and hard/soft spots are among the most common problems for lack of support in the subgrade.

No special subbase material is required for most light duty concrete parking areas provided that you have properly prepared the subgrade. The subbase is a layer of sand or gravel placed on top of a prepared subgrade.


Inspection Checklist

1) Has all sod, topsoil and vegetation been removed?
Removing all organic matter will provide good support and prevent vegetation from pushing up through your parking lot, causing cracks and other failures.

2) Are there any pockets of peat, muck, soft soils or random fill?
These areas must be undercut, properly back-filled and compacted with soil similar to the existing suitable subgrade soils. A soils engineer might be consulted if these area are excessively large or deep.

3. Are there any wet areas in the subgrade?
If the soils under the wet area are firm and unyielding, but saturated with water, then the soils must be blended with adjacent soils, dried and re-compacted to create a uniform subgrade.

4. Are there any utility trenches within the area to be paved?
Utility trenches require proper backfill and compaction procedures with similar suitable subgrade soils.

5. Will the pavement be placed over areas next to the building or other structures that have been excavated and back-filled?
Just like any utility trenches, the backfill and compaction must be with similar suitable subgrade soils.

6. Has there been fill placed in any portion of the area to be paved?
Fill should be of similar quality and compaction as surrounding soil.

7. Are the subgrade soils uniform?
The areas between dissimilar soils should be excavated, blended and re-compacted.

8. Has a leveling course of sand or gravel been used?
Sand and/or aggregate base layers can help support construction traffic as well as long-term pavement support. However, if the subgrade soil is clay, a leveling course may trap water and additional under drainage may be required, removing that water to ditches or storm water systems.

9. Has the final grade been prepared for a uniform thickness of concrete?
If the final grade has not been properly prepared then it must be fine graded with the required slope for proper drainage and consistent concrete thickness.

Once all these items have been checked off, the contractor likely will do a test roll using a large vehicle to test for rutting in the base and overall grade. These final checks will ensure water will not pond on the parking area.


Curbs and Gutters

Even if you decide to use asphalt, curbs and gutters should be of concrete to provide a hard edge to the asphalt and prevent edge crumbling. The proper placement of curbs also help control access to the roadway, a key component of traffic control, providing a safe entry and exit for motorists. If you parking lot incorporates sidewalks or building entrances, be sure to consult local and state codes to meet all accessibility standards.

If the parking area will be concrete and curb and gutter is placed first, joint placement must match that of the concrete parking area to ensure that joints properly line up and do not cause uncontrolled cracking due to mismatched joints.


Weather Conditions Affect Concrete

Both hot and cold weather affect how concrete pavement should be constructed and treated.

The American Concrete Institute (ACI) defines cold weather as: "When air temperature has fallen to, or is expected to fall below, 40°F (4°C) during the protection period."

Hot weather, on the other hand, is defined by ACI as: "Any combination of the following conditions that tends to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration. This can include high air temperatures, low humidity, high winds or hot sunlight."

Your contractor is responsible for concrete placed during any weather conditions. Discuss with your concrete contractor how they might adjust their operations to account for any foreseen weather conditions.


Curing and Opening to Traffic

Immediately after the concrete has been placed, struck off, smoothed and textured, cover the surface with a white pigmented curing compound or by covering with a white polyethylene or waterproof paper for seven days.

Mid-April to mid-September: Use a sprayed on curing compound applied according to manufacturer's recommendations.

Mid-September to mid-April: Use waterproof covers and maintain curing for at least 7 days at temperatures above 40°F. If concrete sealers are to be used, allow a 30-day air drying period before applying the sealer.

Pavement under construction should be cordoned off with barricades while the concrete is poured and during the following curing period, which ranges from 2 to 7 days. Sawing equipment or other tools may be used by the contractor, but all vehicle traffic should remain off the newly placed concrete until fully cured and at the required strength.