A parking lot is constructed of 3 basic components:
1) Sub-grade:
This is the soil on which the pavement is constructed. Every soil type has a unique maximum strength and optimal moisture content. The clay soils, which are very strong at low moisture but very plastic or flexible when over-saturated. The contractor must achieve proper density in the sub-grade before installing crushed stone layer.
2) Crushed stone: This material is purchased at a state-certified quarry and has a guaranteed strength coefficient at proper density. The crushed stone is trucked to the job site, spread with a grader, and compacted with various rollers. It is then wet and re-compacted until proper density is achieved. This usually takes several days because the water has to evaporate from the surface in order to form a tight bond between stone particles. There are specific engineering tests that can be done to verify density; however a less expensive and quicker test is to proof roll the surface with a loaded truck. During the proof roll, the truck slowly rolls over the stone layer while an inspector carefully watches the surface; there should be no visible deflection or movement of the surface. A properly-trained inspector can detect the slightest deflection.
3) Asphalt: This is by far the most expensive part of the construction process because the asphaltic cement that holds the rocks together in an asphalt mix is a petroleum product. The hot asphalt mix (350° F) is applied over the stone layer with a special asphalt spreader and then immediately rolled with a series of special rollers to achieve maximum density. Since the asphalt is applied hot, this work must be done during warm weather to avoid seams and premature wear. Problems can also occur if the right equipment is not used; the material must be transported efficiently to the job and spread and compacted properly in order for the pavement to perform as designed.
Pavement Design & Construction
Most parking lot paving projects are constructed of crushed stone (also known as crushed aggregate base course) and asphalt. An architect or engineer will vary the thickness of the crushed stone and asphalt layers depending on:
1. The traffic expected on the pavement
The type and extent of truck traffic a pavement receives is the most critical factor in the design process. It is almost irrelevant how many cars use a pavement because one loaded dumpster truck can create as much stress on a pavement as 30,000 cars!
2. The strength of the ground it will be built on
The strength of the ground or sub-grade is measured to determine the California Bearing Ratio (CBR), the most commonly-used rating for measuring relative strength of soils. The stronger the sub-grade, the longer a pavement system will last.
3. The length of service the owner is expecting from a parking lot paving project
The design life the owner wants initially and what they decide after seeing various prices are usually very different! It is impossible to visually inspect a pavement and tell how long it will last; several investigative tests are required.
Property Manager hint: Provide your estimator with the average daily use of the pavement by buses, tractor-trailer trucks, dumpster trucks or other heavy vehicles
Generally speaking, the thicker the section of stone and asphalt, the longer the pavement will last and the more expensive it will be to construct. Since cost is usually the most important consideration, most pavements are designed by default, meaning the owner constructs the least thickness he can, rather than opting to pay for a long life expectancy. However, regardless of what the expected service life of a pavement is, it will start to deteriorate within one year of construction.