Asphalt Paving: From Problem to Solution
By John Comfort III; MPA, BA
Arvada, CO 80003
This paper will focus on asphalt paving being done in Denver, Colorado for aftermarket repair and reconstruction of parking Investments. Asphalt paving can be broken down into these sub processes: diagnosis of the problem, alternatives to fix the problem, removal of failed or damaged asphalt, choosing an appropriate mix design for repair, transportation of mix from a plant to the work area, preparing the work area for asphalt paving, asphalt paving/installation, and compaction.
Diagnosing the Problem/Alternatives to Fix the Problem/ Removal of Damaged Asphalt
The first step of asphalt paving is through understanding that there is a problem in the first place. This article will not go into great detail about this step, but damaged asphalt can occur with holes in the surface (i.e. potholes), medium or large cracks in the surface, or a “grouping” of cracks that resemble that back of an alligator, also known as alligatoring. Another major problem with many asphalt areas has to do with the pooling of water on the surface of the asphalt, which can lead to damage as described above.
Once a problem has been diagnosed there are two primary asphalt paving options that are used to fix the problem. The first is mill and overlay, which is the process of grinding down a predetermined amount of asphalt, applying a sticky bonding material called tac, and paving the same amount of asphalt removed back on top of the new surface. The standard depth of mill and overlay is two inches, but depths can vary based on numerous variables. Mill and overlays are best when a surface is worn, but not so far gone as to appear like the surface of the moon (i.e. potholes everywhere). The other main type of asphalt paving is called remove and replace, or full depth replacement. This happens when, for instance, a pothole is so deep as to show the dirt or ground. In this case, the company cuts around the damaged area, fixes the dirt (or subgrade), and fills or paves in the hole to be flush with the surrounding surface.
Removing asphalt is a key component of the asphalt paving process that customers generally overlook. This can frequently take longer than the actual “paving process”. Also, when doing full depth replacement, new problems (such as broken water lines, underground water paths, and damaged subgrade) can be discovered that will increase job completion time and increase price.
Choosing Appropriate Mix Design/Transportation to Work Area
Picking the correct mix design is imperative in asphalt paving. Choosing the wrong mix design will unnecessarily increase job cost, can lead to early and unneeded damage, and can even cause traffic noise from tire friction to be louder. Asphalt mix is comprised of aggregate, heated oil, fines, and sand. In terms of picking a mix design for an aftermarket private parking lot, one generally has two options: ½”/20% RAP (recycled asphalt product) or ¾”/20% RAP. ½” involves smaller average aggregate than ¾” and tends to be slightly more expensive but better looking because it is more tightly consolidated. ¾” is slightly less expensive and is actually stronger in that it can stand up to greater pounds per square inch (psi) for longer periods of time. However, ¾” causes much louder road noise and is less aesthetically appealing (i.e. larger rocks that turn from deep black to grey faster over time). When it comes to paving a private aftermarket parking lot, ½” is plenty strong and much more appealing on the eye, and thus, is the mix type of choice for most parking lot asphalt paving projects in the Denver Metro area. ¾” is used in industrial applications to increase surface strength, generally in areas where heavy trucks constantly drive on the asphalt lot.
All asphalt paving projects are bound to the plant were the asphalt is made by two concepts: time and distance. It is a general rule of thumb that asphalt paving (i.e. from the truck to the work area) should happen when the asphalt temperature is between 300 degrees to 220 degrees F. Thus, it is important to choose the closest asphalt plant to the work area. Asphalt is manufactured between 350 and 270 degrees F, and while it is well insulated in the truck, asphalt paving companies carefully plan logistics to guarantee the mix is in the ground well before the temperature drops below 220 degrees F. Weather also plays a key role in asphalt paving. For example, in the Denver Metro area, asphalt paving companies will typically not pave if the air temperature is not 50 degrees F and rising.
Preparing the Work Area/Installing the Mix
After the asphalt has been removed from a work area and before the actual asphalt paving process can begin, the area needs to be prepped for paving. The preparation process differs when one is doing a mill and overlay compared to a full depth replace project. When doing a mill and overlay, there is still an asphalt surface underneath the removed surface, so access to the subgrade is not possible. In this case, an asphalt paving company will clean any millings or debris from the milled surface and then prime the existing/milled surface with a sticky film called tac. Tac is also applied to the edges of the surface. Tac bonds the old surface to the new surface and ensures that the new surface does not slip over or along the old surface during compaction or after the lot has been opened.
In the case of a full depth remove and replace, an asphalt paving company has more options to deal with regarding the existing surface, which is called base/dirt/subgrade. As with before, the asphalt paving company will apply tac to the edges of work area, however there is no need to apply tac to base or dirt. However, because the subgrade is exposed, this gives the asphalt paving company the ability to address any issues with the subgrade, which includes:, repairing any water issues, compacting the existing subgrade, removing damaged or wet subgrade, and/or installing a layer of road base or squeegee. These additional options with full depth remove and replace are what makes this the best alternative when one is working on a damaged parking lot with numerous potholes and alligatoring.
The installation of hot mix, or asphalt, is accomplished in one of two ways, and depends on the size of the work area and the type of asphalt paving being performed, full depth or overlay. Full depth remove and replace jobs vary in size and scope. Smaller jobs, spread over a large distance in which small holes have been cut, generally involve hot mix being dumped or moved from the truck into the hole and then compacted. Depending on the size of the hole, this is done in one lift or in multiple lifts. A lift is amount of asphalt placed in a work area (hole) that raises the surface of the ground. Best asphalt paving practices call for no more than one 4” lift at a time, which allows the asphalt to be properly compacted before the next lift is done. However, it is difficult to do lifts in small/pothole sized work areas, and thus, pothole asphalt paving generally involves placing asphalt in one lift into the hole. For larger remove and replace projects, a paver is used to equally spread the asphalt across the work area. When using a paver, lifts should have no more than a 4” depth. Overlay projects also use a paver to equally spread the mix. While overlays can happen at any height, a standard 2” overlay can be done in one lift.
The culmination of an asphalt paving project is compaction. This is the process that, when not done properly, leads to the majority of future asphalt failures. Compaction is performed with a steel drum and rubber tire, which apply large amounts of pressure in small areas to increase the density of the paved surface. Asphalt density is directly correlated to strength, durability, and longevity.
Compaction reduces the volume of air in the hot mix asphalt. The volume of air is quantified by the relative volume of air voids in the pavement to a theoretical maximum density. Density can be measured with a core sample or by portable density-measuring devices. A density value of 95% is considered good; any value below 92% is generally considered unacceptable.
There are multiple processes that need to be executed properly for an asphalt paving job to be considered a success. Failure to procure the right mix, monitor and maintain temperatures, prepare the work area, properly pave the asphalt, or compact the new asphalt can result in having to start from square one and do everything all over again.
“Asphalt Paving Practices”, http://www.asphaltisbest.com/wp-content/uploads/2014/03/Hourscht-MAPA-Training-temperature-and-Joints.pdf
“Asphalt Paving Operation”, https://engineering.purdue.edu/CEM/people/Personal/Halpin/Sim/Examples/pave.htm
“Pavement Interactive”, https://www.pavementinteractive.org/
“The Colorado Asphalt Pavement Association”, www.co-asphalt.com