Prevent Pavement Slipping" has the familiar ring of "prevent forest fires" or "prevent drunk driving." The thought of pavement menacingly slipping up on you brings a chuckle. However, for those responsible for the condition of roadways, pavement slipping is no laughing matter. The only effective repair is complete removal and replacement of the slip area. Therefore, prevention is always the best solution.
Pavement tears or slips if the tension caused by the traffic is stronger than the sum of the pavement's internal strength and the pavement's bond to the underlying layer. Analyzing these two components will assist in predicting and eliminating pavement slipping.
Heavy vehicles create much greater stress on the pavement, than light vehicles. A loaded tri-axle dump truck does the same damage to a pavement as 9,800 passenger cars. Therefore, heavy truck traffic is the most important factor in analyzing pavement deficiencies. A rolling vehicle at a constant speed applies most of its stress vertically to the pavement. Slippage is most likely where vehicles brake, accelerate or turn sharply. These actions try to pull the pavement apart by applying lateral tension to the surface.
Asphalt is very strong in compression, but relatively weak in tension. Asphalt shares this common characteristic with concrete.
Concrete is approximately 10 times stronger under a compressive load, than under a tension load. Under tension, asphalt cement, the glue that holds the pavement together, takes the majority of the stress. Since asphalt is a flexible pavement, it will strain slightly and may eventually crack as the traffic applies this stress thousands of times. If there is any weakness in the pavement, slippage can occur.
Slippage cracks have a typical U-shape that will indicate whether accelerating, braking or turning is the cause of the problem, because the pavement is literally pulled apart. As a heavy vehicle approaches an intersection and brakes hard, the top of the U-shaped crack will face with the travel lane. Conversely, if vehicles are accelerating, the U will face the opposite direction. If the vehicles are turning sharply, the U will pull to the outside of the turn. The top of the U-shape will always be in the direction the force is applied.
When planning a construction project, it's prudent to analyze the traffic flow to predict where high stress areas are likely to occur. The location of stoplights, turnouts and sharp bends in the pavement are areas that require consideration.
The two components that are available to resist the damage are the strength of the asphalt layer and the bond to the underlying surface. The ability of the pavement layer to resist this stress is mostly determined by the thickness of the layer. A 2-inch-thick asphalt layer will effectively have twice the tear resistance of a 1-inch layer.
The aggregate blend of the mixture has a small effect on its internal strength. However, stiffness of the asphalt cement and the presence of modifiers have a larger impact. In areas with very heavy lateral stresses, such as truck turning and racetrack corners, special polymer modified asphalts may be necessary to toughen up the mix to resist surface deformation. Modified asphalts can be difficult to obtain in small quantities and will increase the cost of the material from 25 to 50 percent.
Slippage problems can be prevalent on very thin overlays of ' inch to 1' inch without appropriate safeguards. The condition of the old pavement, the cleaning, or the application of tack are generally the causes of the problem.
A good tack, such as RS-1, RS-2 or AC-5, applied at the manufacturer's recommended rate and temperature is important for prevention. Uniform distribution of the tack is very important. Often distributors will have spray nozzles that do not spray properly, therefore, the tack is not uniformly distributed.
In some parts of the world, a rubber-tire roller is actually used behind the distributor to insure the tack is uniformly distributed in the construction area. A better solution, however, is a properly maintained distributor, run by a competent operator.