|PHASED CONSTRUCTION - ASPHALT PAVEMENT SYSTEMS|
The phasing of residential asphalt construction is a common practice that is occasionally practiced on retail, commercial and industrial projects. Phased asphalt construction usually means that the base stone and base course of asphalt are placed early in the construction process, while the final asphalt surface is placed near project completion. For residential projects, this can mean that the surface asphalt is placed several months or more after the base stone and base asphalt are installed. For retail or commercial construction, the time between the installation of base and final asphalt is usually much shorter.
What are the implications of the phasing of asphalt? Is this a good idea?
Pavement Design - A Basic Primer
All pavement systems are designed on the basis of three factors: (1) the strength of the subgrade soil, (2) the traffic loading conditions, and (3) the strength of the materials that are combined to create the pavement system. The strength of the soil is usually evaluated empirically, most commonly based on a test known as the California Bearing Ratio (CBR) test. The traffic loading conditions are a function of the in-service use of the site. For residential construction, most jurisdictions will publish an average traffic loading factor per residential unit. For retail or industrial use, the traffic loading conditions are normally determined by a traffic study or provided by the end user of the facility. The strength of the asphalt, usually expressed in terms of a "structural number", is a function of the materials that make up the asphalt system. Commonly, the local department of transportation will publish a structural number factor for the various types of stone, base asphalt and surface asphalt used in an area.
Most pavement systems are supported by native soils, either as they exist in place or after being remolded as compacted engineered fill. Usually, these soils are not subject to substantial modification (there have been earlier Lessons Learned on cement and lime stabilization to modify native soils). Traffic loading conditions, while sometimes subject to much discussion, are also "fixed" in that they are a function of the use of the site. The structural number, then, is the only factor that can be manipulated, generally, by the designer.
Phased Construction - Performance Implications
The idea of phasing asphalt systems probably got its start in residential development. In residential development, it is desirable to create access to the overall development as quickly as possible. This not only makes it easier for trades to access individual housing sites, but also makes it possible for brokers to show a property. And, of course, phasing the construction allows the final surface course of the pavement system to have a nice, smooth, clean finish, especially following those inevitable pavement cuts that are required to install utilities.
The down side of phased construction comes about as a result of the disparity between the construction traffic loading conditions and the structural number. For residential construction, the construction phase actually represents the greatest loading on the asphalt. Front end loaders, brick and concrete trucks, tractor trailers hauling framing, and other heavy vehicles use the pavement during construction, and the pavement does not have its full strength. The final surface course of asphalt, generally represents about 30% of the ultimate structural number of the pavement system, so by phasing construction, the greatest loading occurs under the lowest strength condition. It is this phasing of asphalt, in residential development in particular, that often results in the high percentage of failures during the construction phase, and even soon after the final asphalt surface is placed.
Is There A Solution?
The simple answer is: Yes and No (This is the most commonly used answer in the P.E. exam). It would be a simple matter to increase the structural number of the intermediate system to be consistent with the construction loading conditions. Unfortunately, this would also greatly increase cost. So, here are some alternative solutions for your consideration: (1) provide different pavement sections according to the most likely loading conditions, i.e., increase the entrance roadway thickness to compensate for the heavier loading, (2) in low lying areas, where water may be an issue, thicken the pavement system, and (3) if there is a "silver bullet" for minimizing failures while at the same time keeping construction cost as low as possible, it may be through the use of an inexpensive geotextile. Geotextiles do three things: reinforce, separate and drain. More importantly, where an intermediate pavement system begins to fail, a geotextile would have the tendency to control the extent of failure. Unfortunately, the geotextile could interfere with later excavations for utility lines, so this would need to be considered in the overall planning.
Phased asphalt construction is a concept that creates a cleaner, smoother more attractive final pavement surface. Unfortunately, under some circumstances, an intermediate pavement is simply not strong enough to support the construction traffic loading. Presented above are some ideas to keep in mind as you plan your next development, where phasing the asphalt pavement system is a consideration.
We hope these Lessons Learned will be beneficial to you in the planning of your next project.
ENGINEERING CONSULTING SERVICES, LTD.