Concrete Parking LotConcrete parking areas are an excellent choice for a durable, low-maintenance, and aesthetically pleasing parking area for retail developments, grocery stores, industrial sites, and commercial businesses.

Suggested Specifications for Concrete Parking Areas

Concrete parking lot pavements should be plain, non-reinforced concrete built in accordance with the specifications.

The goal is to place concrete parking areas on properly prepared subgrades that are uniform in soil composition and compaction. A properly prepared subgrade will provide uniform support to the concrete slab and minimize, if not eliminate, subgrade related cracking.

Background Information
Subgrade is defined 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 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.

No special sub-base material is required with concrete parking areas provided that you have properly prepared the subgrade. Sub-base is defined as a thin layer of sand or gravel placed on top of a prepared subgrade.

Potential Problems
The major causes of non-uniform support from the subgrade are hard and soft spots, back-filling and frost susceptible soils.

See the Parking Area Subgrade Checklist to determine if potential subgrade problems exist:

Checklist For Pre-Job Site Inspection

1. Has the sod, topsoil and vegetation been removed?
All organic material must be removed before paving operations are performed.

2. Walk the site. Are there any pockets of peat, muck, soft soils or random fill?
All pockets of peat, muck, soft soils, or random fill must be undercut, properly back-filled and compacted with soil similar to the existing suitable subgrade soils (i.e. clay with similar clay, etc.) If these areas are excessively large or deep, then a soils engineer should be consulted.

3. Are there any wet areas in the subgrade?
When there are wet areas in the subgrade the water must be removed and a determination made to see if there are any soft spots. If there are soft spots then refer to solution No.2 above. 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?
If there are utility trenches in the area to be paved, determine if proper backfill and compaction procedures have been used. If the trench areas are either soft and yielding or back-filled with soil that is not similar to the existing suitable subgrade soils then further corrective action may be necessary.

5. Will the pavement be placed over areas next to the building or other structures that have been excavated and back-filled?
If the pavement is to be placed over areas adjacent to a building or other structure that has been excavated and back-filled then the same solution is recommended as for utility trenches. See No.4.

6. Has there been fill placed in any portion of the area to be paved?
If there has been more than one (1) foot of fill placed in any areas of the subgrade then the same solution is recommended as for utility trenches. See the above solution No.4. If there is less than one (1) foot of fill, then special attention should be given to those areas during the test roll. If the fill material is not similar to the existing suitable subgrade soils, then refer to solution No.7 below.

7. Are the subgrade soils uniform (i.e. all clay, all sand)?
The areas between dissimilar soils should be excavated to a six (6) inch depth, blended and re-compacted to create a transition of approximately ten (10) feet from one soil type to the other.

As a preliminary check, you should now test roll the grade with a vehicle that has a minimum 5000 pound axle load.
8. Is there noticeable rutting of the subgrade during the test rolling?
In clay soils the areas of noticeable rutting must be excavated (disked), dried out, and re-compacted. In granular soils (sand) minor rutting should be anticipated and is normal. When there is deep rutting you must excavate (disk), dry out and re-compact the soil.

Before placing the concrete: 
9. Has the final grade been prepared so that you may place a uniform thickness of concrete?
If the final grade has not been properly prepared then it must be fine graded to accomodate placement of a uniform thickness of concrete.

A second test rolling using a loaded dump truck should now be performed.
10. Is there any noticeable rutting or displacement of the subgrade?
Areas of noticeable rutting must be excavated, dried out, and re-compacted to create a stable and uniform subgrade.
11. Has a leveling course of sand or gravel been used?
If the subgrade soil is clay, a leveling course may trap water and additional under drainage may be required. The owner or his representative should be notified of this condition.

General Recommendation:Whenever one or more of the items in the checklist have not been met, the paving contractor shall notify the owner in writing of these conditions prior to concrete placement.

Disclaimer:This information is intended for the use of experienced personnel who are competent to evaluate the significance and limitations of its contents and who will accept responsibility for the application of the material it contains.

Pavement Thickness
Slab depth for parking lots should be as shown on the plans.
Recommended parking area slab depths, based on use, are as follows:

  • Light Use: 4" concrete - Parking for passenger cars, light trucks and occasional use by heavier trucks.
  • Medium Use: 5" concrete - Driveways and parking areas for light to medium trucks plus occasional use by heavier trucks.
  • Heavy Use: 6" or thicker concrete - Driveways and parking areas for heavier commercial and industrial trucks. (Pavements for heavy industrial trucks should be designed on an individual basis.)

Concrete Quality
In general, concrete should be mixed and delivered according to the requirements of ASTM C94, "Standard Specifications for Ready Mixed Concrete". Concrete specifications are as follows:

  • Minimum compressive strength of 4000 psi at 28 days.
  • Air content of 6½ ± 1½%
  • Maximum five (5) inch slump. When mixes containing high range water reducers (superplasticizers) or combination of type A and type E admixtures (flowing concrete) are used, the slump as placed should not exceed eight (8) inches.
  • Aggregates meeting ASTM C33 for severe exposure conditions.

Subgrade Preparation
The subgrade under all pavement should be brought to the required lines and grades and compacted to a uniform density. Any pockets of soft material that cannot be compacted should be removed and replaced with suitable material. Ready mixed concrete trucks and other equipment can operate on the prepared subgrade providing the subgrade is kept smooth and compacted prior to placing concrete. At the time of concrete placement, the subgrade should be in a moist, but not muddy condition. Adding granular sub-base material is not required.

Concrete Placing, Finishing and Texturing
A uniform thickness of concrete (according to the design) should be placed on the prepared subgrade, adequately consolidated, and struck off to the proper elevation. All concrete should be placed continuously to prevent the formation of "cold joints". Wherever placment operations stop, a bulkhead will be installed to form a straight joint. Concrete should not be placed on a frozen subgrade. The sequence of finishing operations should be as follows: strike off and consolidation, floating (if necessary), edging, and texturing.

No finishing operations should be performed when there is excess moisture or bleed water on the surface. In general, adding water to the surface of the concrete to assist in finishing operations is not recommended.

Check the pavement surface with a ten foot straight edge.

A uniformly textured surface should be provided.

Curbs should be constructed at the location shown on the plans. Curb and gutter can be constructed separately from the slab, or the curb may be cast integrally with the slab. The curbs should be textured to match the pavements.

Immediately after texturing, cover the surface with a white pigmented curing compound complying with ASTM Specification C309 (at the rate of one gallon per 200 square feet) or by 7-day coverage with white polyethylene or waterproof paper.

Mid-April to mid-September: Use a sprayed on curing compound applied according to the manufacturers 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.

The pavement should be jointed to control cracking. The joint lay out, compatible with the contractors paving method and equipment, must be submitted to the architect or engineer for approval prior to construction.
Longitudinal and transverse spacing should be at regular intervals. Individual spacing may vary slightly to meet catch basin and manhole castings. (The suggested transverse joint spacing is: 10 feet for 4" slabs, 12 feet for 5" slabs, and 15 feet for slabs 6". The suggested longitudinal joint spacing is 12 feet.)
Contraction joints may be made by sawing, tooling, or installing an approved insert to a minimum depth of 25% of the slab thickness. Sawed joints shall be cut as soon as possible without ravelling the concrete.
Joints should be continuous across the slab and must extend completely through the curb.
Install full depth expansion (isolation) joints, ½" thick, to isolate fixed objects abutting or within the paved area. These fixed objects include existing pavements, sidewalks, castings and structures.
Joints do not require sealing.

Opening to Traffic
Pavement under construction should be protected with barricades and all traffic (with the exception of joint sawing equipment) for seven (7) days.

Pavement Protection
The contractor is responsible for concrete placed during rain or cold weather and any concrete damaged by rain or low temperature should be removed and replaced at the contractor's expense.
When concrete is placed late in the year the contractor should submit a plan of his procedures for protection to the architect or engineer for approval prior to construction.

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