Concrete pavements for car parking are constructed as slab-on-grade structures which are not subjected to heavy traffic and movement as compared to highway concrete pavements. They provide access to both heavy and light delivery vehicles. Concrete pavements as car parking slots offer superior performance when compared with other pavement types.
The main features and design considerations of concrete car parking slots are briefly described in the article.
- Features of Concrete Pavements as Car Parking Slots
- Design Consideration for Concrete Pavements as Parking lots
- Benefits of Concrete Pavements as Car Parking Slots
- Construction Steps
Features of Concrete Pavements as Car Parking Slots
Concrete pavements form a hard surface which protects the soil underneath the pavement from high design loads. When compared with asphalt pavements, concrete pavement as parking slots offer high stiffness. Also, the load coming over concrete pavement is distributed over a large area of subgrade, thus avoiding chances of settlement.
The size of concrete pavement for car parking can vary from small to large depending on whether the slot is for small multiple housing projects or for large shopping centers. These parking pavements are less deformed and demand less maintenance compared to other parking slot types. This pavement type is easily installed and also offers long term strength and durability.
Design Consideration for Concrete Pavements as Parking lots
An economical and straight forward design is followed for constructing concrete pavements as a car parking facility. ACI 330R-80- (Guide for Design and Construction of Concrete Parking Lots) follows the design of concrete parking lot as per Portland cement association’s thickness design method. Knowing the value of subgrade reaction (k), expected vehicle loads and number of load repetitions for 20-year pavement life, the design thickness can be determined from the nomograph given in ACI 330-01.
The thickness method is an empirical method ( AASHTO Design Method) used to design highway pavements. As concrete pavements used for parking lots are not subjected to high dynamic forces and loads as in highway pavement, the design load conditions are reduced accordingly and designed.
Some of the considerations for the design of concrete parking lots are:
- Concrete parking lots are not just concrete pavement. It must incorporate slabs, joints, drainage facilities, and light poles. All the elements must go together.
- Concrete parking lots are constructed directly on the existing soil grade instead of a subgrade that is compressed. The only requirement is to place on a subgrade that is compacted and leveled in order to prevent cracks and sinking.
- A thickness of 4 inches is suitable for concrete parking lots that carry light vehicles. For carrying bigger and heavier vehicles, the thickness of the concrete pavement can be 5 to 6 inches. The thickness varies based on the number of load repetitions, the weight of vehicles and the condition of subgrade.
- The slope is provided for concrete pavements to facilitate proper drainage. The minimum value of which is 1/8 inch per foot, 1/4 inch per foot and maximum can be 6%.
- Entrances of parking lots can have a slope of not greater than 8%.
- Joints are properly planned and provided for concrete pavements to avoid future cracks and repairs.
Benefits of Concrete Pavements as Car Parking Slots
The main benefits of concrete pavements as car parking lots are:
- Simple Maintentance requirements for concrete pavement.
- Good drainage property even on flat slopes.
- Good resistance to deformation.
- Both the parking-stall and the traffic lane can be incorporated.
- Concrete pavement is less affected by the leaking oil and petroleum products from the vehicle.
- Concrete pavement is less susceptible to rutting and potholes under heavy load.
- Concrete parking lots can be given any color.
- Concrete parking is less heated up and less toxic. It can be constructed by using recycled construction materials like slags, recycled concrete aggregates, etc which hence make this a green and economic construction.
Also Read: Types of Concrete Pavements
The construction of concrete pavements for car parking is carried out in compliance with certain plans and specifications that meet the owner’s desire. The main construction features are:
- Proper planning
- Proper Selection of Equipment
- Realistic delivery of Concrete
- Determination of the Construction Sequence
The above criteria are accomplished by having a project team of architect, engineers, contractors and sub-contractors, concrete suppliers and pavement test agencies.
The basic concrete parking pavement construction steps as per ACI 330R-08 are:
- Preparation of Subgrade
- Construction Layout
- Equipment for Pavement Construction
- Placing and Finishing
- Pavement Markings
1. Preparation of Subgrade
Concrete parking lot construction requires a uniform and well-compacted subgrade with proper elevation. This is the main factor that governs the quality of the concrete pavement. Consistent load support and uniformity in the thickness throughout the pavement are obtained by having proper subgrade and elevation.
Earthwork operations like excavations and filling must be carried accordingly in order to get well-packed subgrade. Loose and weak materials must be removed and replaced to have a quality subgrade. Various cementitious materials can be incorporated to get quality soil. The subgrade must not be muddly, frozen and disturbed. It must be well compacted, firm and uniform.
Proper backfilling must be conducted when the utility trenches and other excavations in the area are paved. These backfills are compacted with mechanical tampers ( 150mm lifts )to finish grade more than the normal subgrade preparations.
The final grading is checked to find whether the subgrade surface is at proper elevations. The tolerances for this is between 6mm and 13mm. The pavement’s performance deviates if there is any deviation from this value
2. Construction Layout
The construction layout for concrete parking lot construction must be developed and agreed by both the engineer and the contractor. The layout shows the location of joints, lighting fixtures, other objects and the paving sequence. This layout is the reference for concrete delivery trucks, drainage construction and movement and also for the efficient use of paving equipment.
Paving is performed in lanes. The width of each paving lane is the multiple of joint spacing. This width is mainly dependent on the equipment and the construction method chosen by the contractor.
3. Paving Equipment
Different forms of adequate cross-section and strength can be employed for pavement construction. The forms are set up above a subgrade that is properly compacted and cut to grade. The forms can be wood, steel or any accepted material.
Enough numbers of pins or stakes are used to keep the forms in place and prevent lateral movement during concreting. All the forms must be cleaned and greased so that final pavement edges are neat. Before placing the concrete, the form grade and lines must be rechecked.
After placing the concrete, the surface can be leveled by using a straight edge. Compaction can be performed by a mechanical paving machine or laser screed or vibrating screed or by a hand-held vibrator.
Instead of using fixed forms, the slipforming method can be followed. This method spreads, consolidates and finishes the concrete in one go. Slipforming is performed in forward motion as possible.
Also Read: Slipform Construction Technique
5. Placing and Finishing
- The concrete is placed over a subgrade which is dry and free from surface water.
- The concrete is placed uniformly ahead of the paving equipment until the final position. This hence requires minimum rehandling.
- Consolidation must be carried out along the faces of the forms and excess is struck off to the required elevation and cross-section.
- When the slipform method is employed, the concrete must be of required consistency so as to prevent edge slump.
- Once the strike off is performed, a scraping straightedge or a bull float is used to level the surface.
- The surface is finished to remove maximum irregularities.
- Isolation joints, tool joints, and the edges must be rounded to the radius as per the specifications.
After finishing the concrete pavement, the surface has to be cured by any curing compound accepted by ASTM C309 or C1315. The application is performed immediately after the evaporation of free water from the surface. The rate of application is as recommended by the manufacturer. ACI 308R -01 have mentioned many curing materials and methods for concrete pavements.
These pavements have contraction joint and isolation joints. Contraction joints can be made either by inserts or by sawing or tooling. The depth of contraction joints made by a conventional saw can have a minimum value of one fourth the slab thickness. Contraction joints can be made after 4 to 12 hours after completing the finishing works of the pavement constructed by conventional methods.
In order to separate the light standards, drainage, building foundation, pavement approaches, and existing islands, isolation joints are constructed. The isolation joint material must be continuous from form to form. These joints must extend from the top surface of the slab to the subgrade.
7. Pavement Markings
Pavement markings have to be applied on a clean and dirt free concrete pavement surface. The markings are applied as per the recommendation from the manufacturer. The marking material must be compatible with the curing compound.
Once the construction is completed, the parking lot must not be opened for normal traffic for a period of 3 days. For heavy traffic, the slab is only opened after 7 days. Field-cured cylinders can be tested to determine whether the concrete pavement has achieved the required strength.