By John Davis
The best time to maintain a road is before it begins to show distresses, declared Wayne Jones, Asphalt Institute Senior Regional Engineer, at his recent “Pavement Evaluation and Repair” presentation at the National Pavement Exposition in Nashville.
When evaluating and repairing pavements, the pavement professional’s job is to extend pavement performance as much as possible, said Jones. “The contractor, materials engineer and design engineer have a common goal to extend pavement performance. Poor design, poor materials and faulty construction cause pavements to fail.”
According to Jones, a good design is the foundation for a long-lasting pavement, but the design has to be maintained.
“After the design engineer does his job, if the design isn’t maintained the pavement will deteriorate, no matter how well it’s designed,” said Jones. “Clogged ditches, outlets and drainage lines trap water in pavement and accelerate deterioration. Cracking, raveling, rutting and potholes accelerate the deterioration.”
One of the primary design flaws is the failure to realize the actual loads on the pavement. Examples are the dumpster pads in shopping centers and local roads where exponential growth was not anticipated. The dumpster area should be designed with thicker asphalt to accommodate the heavy garbage trucks that lift the dumpsters.
Many local roads have been designed with minimum roadway thicknesses and cannot sustain heavy traffic. Large residential tracts and shopping areas that were not anticipated crop up along local roads, bringing heavy truck traffic that is not sustainable.
Construction flaws can cause a well-designed road to deteriorate quickly. The single, biggest flaw is inadequate compaction according to Jones. “It leaves voids in the pavement and allows water to penetrate.”
“Most roller operators want to do right, but it is hard for them to always get proper coverage for good compaction. Setting up a good rolling pattern and then constantly monitoring the pattern will assure proper compaction,” adds Jones.
Pavement separation at longitudinal joints is a common distress caused by improper compaction. The proper construction and compaction of longitudinal joints will greatly extend the life of the pavement.
“We need to accurately evaluate pavement distresses so we understand how to fix them. Also, we need to know the extent of the distresses. Are they bottom-up cracks that go all the way through the structure, or are they top-down cracks in the surface that extend only into the intermediate course? Was the pavement failure due to age, materials or traffic load,” asks Jones.
If the failure was age-related, it can be handled with a surface treatment or thin overlay. If the failure was due to soft sub-grade materials, then the sub-grade materials must be stabilized or replaced. If the failure was due to lack of structural strength, then a structural overlay or rehabilitation should be considered.
A good way to evaluate pavement distress is to look at the Pavement Condition Index (PCI). This index describes the condition of an asphalt pavement at the time it is taken. The PCI can be as high as 95 if the pavement is in excellent condition. If the pavement is in poor or very poor condition, the PCI can be as low as 15 or 20 or even less.
Most agencies set trigger points when preventative maintenance treatments should be applied to prolong the need for expensive rehabilitation. If a pavement is not maintained, then at some point it reaches a minimum PCI and will have to be rehabilitated or reconstructed.
PCIs can be calculated for each type of distress or as a composite of all the distresses present in the pavement. Agencies often use the individual indexes to identify appropriate maintenance rehabilitation treatments.
Failure due to truck traffic
Heavy trucks on poorly designed pavement will cause it to fail. Unless designed properly, pavements will fail early and continue to be a constant problem.
Jones encourages consulting engineers to go to the owners and architects and explain to them why it is more cost effective to initially design and build the pavement properly to avoid all of the future costs.
Pavement intersections also need special attention. Trucks stopping and turning at intersections apply the highest stresses that can be applied to a pavement. Strengthening the design of the pavement at the intersections is the common sense approach to extending the pavement’s life.