Construction of a parking lot can either make or break the design. If not installed properly the design considerations put into the parking lot could be worthless, and it may not perform up to standards. The construction of the lot starts with the coordination of the other contractors on site, and planning a construction schedule. Below you can find information on the preparation of the subgrade, and two different ways of placing the concrete.
An essential part to the construction of concrete pavement is a uniform subgrade that is well prepared. Providing a consistent support will determine that the pavement will be the specified thickness. The subgrade should be able to support the pavement and all construction traffic and equipment (ACI 330R).
The subgrade should be filled with suitable material, and should extend to at least one foot beyond where the parking lot is going to be. It shouldn't be uncompacted, muddy, frozen, or disturbed when the paving process begins. The preparation of the subgrade should be done far enough in advance to eliminate interruptions in the paving process. When completed the subgrade should have a "moist, dense, firm, and uniformly smooth surface when concrete is placed on it"(ACI330R-12).
Excavations, utillity trenches, and other holes in the area that are going to be paved should be filled to the finish grade and compacted before preparation of the normal subgrade. Materials used for the backfill should be compacted with mechanical tampers in 6 in. lifts (ACI 330R). "Controlled low-strength material - a mixture of granular and cementitious materials and water is recommended for use in lieu of compacted backfill"(ACI330R-12). This mixture is further described in ACI 229R. Any signs of a subsidence of a backfill should be taken care of before paving. The hole should be excavated and refilled before paving begins(ACI330R).
A final fine grading should be checked with a template to make sure that the surface is at the specified location. Suggested tolerances for fine grading are no more than 1/4 in. above or 1/2 in below the design grade. Any difference greater than these can affect pavement performance due to the thin pavement designs that are sometimes used in parking lots. Variations in thickness of thin pavements can greatly affect the load-bearing capacity(ACI330R).
Construction using Fixed Forms
Forms are made from steel sheets and serve as a mould for the concrete slab and as rails for movement of pavers. These forms are placed directly in contact with the subgrade, and once they are locked in place, accurate leveling and setting are ensured by local tamping or with sand. Where the forms are to be placed, base-plates are used. The vertical and horizontal stiffness should be that no deflection can harm the surface of the fresh concrete when the fully loaded pavers pass over it (Villemagne 120).
The alignment of the forms should be almost exact. There shouldn't be a shift vertically or laterally by more than 1cm with respect to the designed alignment. Once the forms are placed, they should be coated on the inner side with an anti-adhesive before any concrete is placed, but before paving is to begin, a template between the forms can be passed to check whether the subgrade surface is at a distance from the upper slab which is equal to the thickness design. If it isn't, the surface of the subgrade is cleared and further work is done to fix the problem (Villemagne 120).
The concrete is usually laid in a single pass, and vibration is done by means of vibrating beams or poker vibrators that are mounted on a chasses. This is an important step, and the paver must be able to distribute, level, and compact the concrete satisfactorily. The concrete should be internally vibrated by the pokers over the length of the forms, and should not run in one spot too long to prevent segregation. After compaction, the concrete surface is first leveled cross wise, followed by longitudinal finishing (Villemagne120).
Fixed Forms (Source: ACPA)
Construction using Slip-Form Pavers
Slipform pavers are designed to spread, consolidate and finish the concrete in a single pass (ACI330R). Concrete is poured, either directly by a feeder on one side of the paver, or by a belt in which takes up the concrete poured inside the hoppers situated ahead of the paver. It is important that this feeder does not overload the slipform paver. The discharge of the concrete is done as the paver advances slowly.
The second part of the slipform paver takes care of the compaction by internal vibration. Vibration brings the concrete to maximum consolidation. The internal vibration of a slipform paver is accomplished by means of an unbalanced set of pokers that are kept in rotation by an electric or hydraulic motor, and the spacing between them should not exceed 50 cm. It is important to notice when a poker vibrator wears down. These worn down vibrators can act as a shield to vibrations of the nearby pokers and can leave a trail of poorly consolidated concrete behind it. This is sometimes not able to be seen on the surface of the concrete but will give rise to an area that will wear under traffic sooner than the consolidated portions. A quick check to see if the vibrators are working is to see if air bubbles rise to the surface (Villemagne 126).
The third section of the slipform paver is the casting section. It is shaped by one of two methods. The first is by an extrusion plate, which is an upper plate that is pulled along the surface of the concrete. The other method is by trimming. In this method, any excess concrete is trimmed and smoothened by two oscillating floats after it is passed under a vibrating beam which regulates the quantity of concrete released under the paver (Villemagne 127).
ACI 330R highlights important considerations that need to be considered when using this type of delivery system. The slipform paver should be operated with a nearly continuous forward movement. Delivery and spreading of concrete should provide uniform progress without stopping and starting of the paver.
Slipform Paver (Source: Gomaco.com)
Finishing and Texturing
Once the strike-off is performed, the surface should be leveled with a scraping straight edge or bullfloat. It should not be finished anymore than is necessary to remove irregularities. Once the concrete has set enough to maintain a texture, and there is no bleed water remaining on the surface, it can be dragged with a short length of damp burlap or other material such as a synthetic turf carpeting. These materials are sometimes attached to paving machines. Another way of texturing is to broom the surface to develop a skid-resistant surface and a uniform appearance (ACI330R-13).
Once the finishing and placing operations are completed and the surface water has disappeared the concrete should be protected from excessive evaporation by spreading a special product called a curing compound. This will form an impervious surface film. Usually, the product is sprinkled immediately after carrying out transverse striations with the sprinkling apparatus being mounted on the back of the striating machine. The liquid used is usually white and sprayed as a thin mist in a uniform manner, including the slab sides (Villemagne 151). The compound must meet ASTM C 309 or ASTM C 1315 and can be applied by a pressure sprayer that doesn't exceed 100 sq. ft/ gallon. Precaution should be taken on windy days to ensure the compound doesn't get blown away, and two applications at a 90 degree offset might be required under these conditions (ACI 330R-13)
Influence of Atmospheric Conditions
Steps should be taken while concreting is being performed when atmospheric conditions could affect the quality of the concrete. To prevent against these possible conditions, a close watch of the weather forecast is important. It is also advised to have a temperature and humidity reading in the vicinity of the job. Three different atmospheric conditions are described below, as well as the steps that should be taken.
Hot Weather Concreting
During hot weather, the transport, placing, and finishing of concrete should be done as fast as practically possible(ACI 330R). In the placing of concrete is done during hot weather, special attention should be paid to the risk of surface drying and the development of cracks. The higher the initial temperature of the concrete is, the more open the cracks at the joints due to thermal shrinkage will be. This could result in a low load transfer, therefore steps should be taken to lower the temperature of the concrete to ensure proper durability of the structure (Villemagne152).
The risks of early cracks developing is high, and concrete placed in the morning has a phase of maximum liberation of heat of setting that is the same time as the peak afternoon temperatures. The final phase is at night, which greatly risks cracking the following morning. As a result of this, it is advised to begin concreting in hot weather in the afternoon, so the maximum liberation of heat of setting can compensate for the fall in temperature at the end of the night (Villemagne 152).
Other possible considerations would be to cool the components of the concrete. This may include supplying the cement at the lowest possible temperature. Also, spraying of the aggregates may be done so that the water evaporation would ensure their cooling. A third possibility that is usually efficient, is to cool the mixing water of the concrete ( Villemagne153)
Cold Weather Concreting
The characteristic feature of concreting in cold weather is the delay in setting and hardening time due to the slow-down of cement hydration. If the temperature is likely to fall below freezing during the first twenty four hours, precautions should be taken. If the temperature is less than 41 degrees and a freeze appears likely the following night, all work of placing the concrete should be stopped (Villemagne 153). Curing in cold weather should provided adequate protection from freezing while sustaining moisture for the time that is necessary to achieve the desired concrete properties. Ways to do this include using curing blankets or polyethylene sheets that sandwich hay or straw(ACI330R)
A drizzle or fine light rain can be favorable to concrete because it keeps the concrete moist, but heavy or pouring rain can introduce many disadvantages. These include erasure of the striations in fresh concrete, dissolution of the curing compound, and a tendency for slab edges to collapse. If a heavy shower occurs nothing can be done immediately besides covering the portion of fresh concrete with polyethylene sheets. The problem with this is that the weight and contact with the concrete can erase the striations. roughness has to be restored later on by grooving or shot-blasting of hardened concrete, or sometimes even surface dressing. One step that must be done all the time, is that the curing compound must be renewed that has been completely or partially removed. Ways to prevent slab edges from deforming include by placing light formworks over a sufficient length as soon as the shower starts (Villemagne 153).
"Guide for Design and Construction of Concrete Parking Lots." ACI 330R. American Concrete Institute, 2001.
Villemagne, Maurice and Yves Charonnat, eds. Cement Concrete Pavements. Paris: A.A. Balkema Publishers, 1996.