What Makes a Driveway More Than Just a Black Top?
In response to numerous requests from property owners, as well as contractors, for our help in establishing a guideline for paving specifications on residential driveways and small to medium commercial jobs, we would recommend the following:
As with almost any construction project, the foundation is the starting point and it is also one of the most important factors when considering pavement design. The aggregate base acts as the foundation for the pavement. The main purpose of the base is to provide a frost barrier that will prevent heaving in the winter, and to provide a stable surface that will support the design load of the pavement.
If you are working with a stabilized sub grade that already has a sub base installed, or if the native soil of the site is gravelly in consistency, such as “bank run gravel” and this sub base is well compacted, a 2 to 4 inch layer of processed stone base or recycled aggregate meeting Connecticut Department of Transportation (Conn. D.O.T.) specifications should suffice. As an aside, if the aggregate base meets Conn. D.O.T specifications it would most likely meet New York State D.O.T. specifications for “Item # 4” aggregate base.
If you are working directly on the sub soil that is already at sub grade elevation, and it is a stable, free of organics surface, we would suggest an aggregate base of not less than 8 to 10 inches to provide an adequate frost barrier and to have enough strength to support the pavement. Soft sub soil conditions may require more sub soil remediation prior to installing the aggregate base to ensure proper stability.
The base material should be placed in thicknesses not exceed 6 inches in anyone layer to allow for adequate compaction. If the total required thickness exceeds 6 inches the base should be installed in multiple layers called “lifts.” While placing the base material, compaction from a mechanical source such as a vibratory roller will be required. A vibratory drum roller, as opposed to a static drum roller, will give the greatest compaction density because the vibrating action of the roller will allow the base material to compact from the bottom up. The vibratory roller should be used where ever possible and a vibratory plate compactor should be used in areas not accessible by the roller. After the compaction process is completed we suggest driving a fully loaded dump truck, when possible, across the area to observe if there is any deflection in the base. On occasion when thick layers of base are installed, such as 6 inches or more, additional time for settlement may be needed. Sometimes there is no substitute for a little “Mother Nature” such as a good rain fall then some time to dry.
It is also important to remember that the finished pavement may not surface drain correctly if the pitch is less than 2% or ¼ inch per foot. This also happens to be the same amount of pitch that is found in a home drainage or sewer system as mandated by most building codes. What that means is that the slope of the surface should have approximately 2 ½ inches of pitch for every 10 feet of run, because 10 feet equals 120 inches, and 2% of 120 is about 2 ½. This is the minimum standard used in most design criteria that ensures 100% surface drainage and overcomes most small unnoticeable imperfections that occur in the paving process. Sometimes Architects or Engineers will design a specification that is less than 2%, or at times site conditions just won’t allow for enough pitch. Given ideal work conditions, an experienced crew can manage with less pitch if and when no other solution is possible. But the possible negative effect of this may be that when there is some change to the finished surface profile of the pavement, whether created by traffic or normal wear and tear, the surface will no longer drain the way it did when it was completed. So given the multitude of factors that are commonly found in residential work it is best to have a little more pitch than a little less.
Pavement Thickness and Installation
As a rule of thumb, to achieve a compaction density of 90% to 95%, a pavement designed for use in residential driveways and light commercial applications, the finished compacted thickness should not exceed 5 times the largest stone size in the mix. For example, a State of Conn. D.O.T. Class 2, or a variation there of, would be used as the finish surface in most driveways. Class 2 has a maximum aggregate size of ½ inch. This would mean a maximum compacted thickness of 2 ½ inches can be used. This of course takes into consideration the typical size and weight of most rollers used by contractors doing these types of jobs. The difficult layouts of most driveways that may have adjoining masonry and ever changing pitch detail provides constraints in small projects that are normally not found in large scale projects such as road work. To provide proper compaction under those difficult conditions the types of rollers generally being used for residential and small commercial work is a 3 to 5 ton vibratory roller with 48 inch drums that has a static pound per square rating of 60 psi +/-, along with a smaller, lighter roller and a vibratory plate compactor to remove imperfections and finish roll.
On a small residential driveway that is not large enough to allow a service truck to enter, such as an oil delivery truck or garbage truck, then a 2 ½ inch thick single course of pavement will probably be sufficient providing the aggregate base requirements are met as stated in the previous section.
On large driveways or small commercial jobs where the traffic will be of mixed use including service vehicles such as parcel delivery trucks, oil delivery trucks, garbage trucks, lawn maintenance trucks, etc. a full depth pavement design of not less then 3 ½ inches, that should be placed in 2 lifts such as a 2 inch binder course with a 1 ½ inch top course, should be considered.
When ever possible a paving machine should be used, however installing by hand where a paver is not practical is acceptable. In any event, an experienced crew that has been together for some period of time using up to date equipment that they have been trained and certified on will produce the best results for the client.
This is meant only as a guide taken from such sources as the ABCs of Driveways from the Ontario Hot Mix Producers Association, the Asphalt Institute, and Roccie’s Asphalt Paving with their 50+ years of asphalt experience. This should not be used as a pre- engineered specification for a one size fits all application. A good, legitimate and experienced contractor can and should be able to determine if any of these suggestions for a given project will be sufficient or if any modification is needed. We can make no warrantee or guarantee, offered or implied, for the statements above. Seek the help of a professional!