12. Mat Problems

12.1.1 Description

An asphalt surface can have two types of waves: short and long. Short waves, also sometimes called ripples or auger shadows, are generally 1 to 3 ft (0.3 to 0.9 m) apart, with 1-1/2 to 2 ft (0.45 to 0.60 m) being the most common separation. Long waves are considerably farther apart. The distance between them may correspond to the distance between truckloads of mix. Long waves may also be associated with the reversal points of the compaction equipment, particularly on thick-lift construction or when the mixture being placed is tender and moving longitudinally under the compaction equipment.

An additional type of defect in the pavement surface is a roughness or washboard effect caused by improper operation of a vibratory roller. The distance between these waves is generally very small, typically less than 3 or 4 inches (75 or 100 mm).

Transverse bumps may also appear in new asphalt overlays on top of crack sealants. This is caused by the sliding or shoving of the new mix during the first pass of the breakdown roller.

12.1.2 Causes

A major cause of short waves or ripples is a fluctuating head of material in front of the paver screed. The variation in the amount of mix being carried back to the augers by the slat conveyors and deposited in front of the screed causes the screed to rise and fall as the force pushing against it changes. Too much mix (at the top of the augers) and then too little mix (at the bottom of the augers) being carried in the auger chamber in front of the screed causes the wavy surface as the screed reacts to this variation in force. The fluctuating head of material causes the screed to rotate around its pivot point and “hunt” for an angle of attack. As the angle of attack of the screed changes, the thickness of the mat being placed also changes, and the smoothness of the new layer is directly affected (see Section 7.3.3).

A major cause of short waves or ripples is a fluctuating head of material in front of the paver screed.

Another cause of short waves is a screed that is in poor mechanical condition—one with excessive play in the screed control connections (see Section 7.3). Short waves can also be formed in the mat by improper mounting or sensitivity of the automatic grade control on the paver or by use of an inadequate grade reference device. The problem may also be related to a mobile reference (floating beam) that is bouncing or to the truck driver holding the brakes while the truck is being pushed by the paver (see Section 6.6.1).

Short waves can also be related to the mix design, particularly with a mix that varies in stiffness because of changes in the mix temperature or composition (see Chapter 3). As the stiffness of the mix varies, the forces of the mix pushing on the screed vary as well, causing the screed to rise and fall and resulting in a mat with short waves. Finally, if the mix design is improper in aggregate gradation, asphalt content, mix temperature, or moisture content (the mix is tender), the rollers may shove and displace the mix during the compaction process. However, short waves are typically placed in the mat by the paver because of either its operation or changes in mix stiffness, rather than by the operation of the compaction equipment. Long waves are caused by some of the same variables that result in short waves. Fluctuation in the amount of material in front of the screed and variation in mix stiffness cause the screed to react to the change in the force exerted on it.

If the distance between the wave peaks corresponds to the length of pavement between truckloads of mix, however, the waves may have been caused by incorrectly set hopper flow gates on the paver (see Section 7.2.2) or by the paver hopper and slat conveyors being emptied between loads of mix (see Section 7.2.3). Poor mechanical condition and improper operation of the screed (continually changing the manual thickness control cranks, for example; see Section 7.3.4), as well as incorrectly mounted automatic grade controls (see Section 7.4.2), can cause a long-wave problem. If a stringline is being used as a grade reference, a sag in that line between support posts can also be a cause of long waves (see Section 7.4.2.7). Another factor contributing to long-wave roughness is improper delivery of the mix to the paver, particularly if the haul truck bumps into the paver or if the truck driver holds the brakes while the truck is being pushed by the paver (see Section 6.6.1). One additional factor can be the condition of the underlying surface: the long waves may reflect the waves in the base material.

Long waves may also be found at those points where the compaction equipment reverses direction. This problem is most prevalent when the asphalt layer being placed is more than about 4 inches (100 mm) thick. The problem may be exacerbated when the maximum-size aggregate used in the mix is relatively small compared with the lift thickness. The waves are caused by a bow wave that forms in front of the roller when the mix is tender.

Long waves can be caused by truckload-to-truckload segregation of the mix (see Section 6.4) and by changes in mix temperature (see Section 3.2.5). Both of these deficiencies cause the forces on the screed to vary, resulting in a wavy surface. The compaction equipment can also create a wavy mat if the roller operator turns or reverses the machine too abruptly.

Roughness or washboarding is normally caused by improper operation of a vibratory roller (see Section 8.5). This type of equipment should be operated at a high frequency and corresponding speed to achieve a minimum of 10 IPF; it is, however, generally good practice for rollers not to exceed 2-1/2 to 3 mph (typically walking speed) to ensure sufficient dwell time. The amplitude should be set in relation to the thickness of the layer being compacted—usually a higher amplitude setting for a thicker layer of mix and a lower amplitude setting for a thinner lift. The washboard effect can be worse if the roller is operated at a high speed, particularly if the frequency setting is less than 2,400 VPM.

Transverse bump formation is the result of the breakdown roller creating a bow wave or shoving of the overlay asphalt during the first pass. Heat from the overlay is transferred down into the substrate pavement and crack sealant. The adhesive nature of the crack sealant produces a resistant force greater than the surrounding pavement friction. As the bow wave in front of the breakdown roller passes over the higher-friction adhesive sealant, a reduction in speed of the bow wave occurs and the breakdown roller passes over the bow wave, creating a bump slightly offset from the crack below in the direction of the paving machine.

12.1.3 Solutions

Short waves (ripples) can be eliminated only by preventing their formation. The most important factor in preventing short waves is to keep the amount of mix (head of material) in front of the screed as consistent as possible. In addition, the stiffness of the mix, which is related to both its temperature and its composition, should be maintained as constant as possible. The amount of mix is controlled by properly setting the hopper flow gates and by keeping the slat conveyors and augers operating as much of the time as possible (close to 100 percent) while the machine is moving forward. Mix stiffness is controlled at the asphalt plant by keeping the mix temperature, aggregate gradation, and fluids content (asphalt content plus moisture content) as constant as possible. Any factors that cause either the volume or stiffness of the mix at the screed to change can cause short waves or ripples in the mat.

Short waves (ripples) can be eliminated only by preventing their formation. The most important factor in preventing short waves is to keep the amount of mix (head of material) in front of the screed as consistent as possible.

Surface waves caused by problems with automatic grade controls can be detected by shutting off the grade controls and determining whether the waves continue to form. If the grade controls are at fault, the operation and maintenance manual supplied with those controls should be consulted to determine the proper corrective action. Sags in a stringline reference can be found by sighting down the line as the grade sensor wand passes along the string. Short or long waves caused by the mechanical condition or operation of the paver screed can usually be detected by careful observation of the paver during mix laydown. The long waves formed by incorrect operation of the haul truck or compaction equipment can also be detected easily by observing those operations.

If washboarding is caused by incorrect operation of a vibratory roller, a change should be made in one or more of the following: the vibratory amplitude setting, the vibratory frequency, and the speed of the roller.

Methods to prevent the transverse bumps from crack sealant on the existing surface include the use of asphalt overlay mixtures with high frictional properties such as open-graded mixtures, stone mastic asphalt, or dense-graded mixtures with highly angular and fractured aggregate. Breakdown rolling with the nondriven front drum moving forward tends to push the mixture instead of pulling the mixture under the drive drum. This push creates a larger bow wave in the mixture, often resulting in transverse bumps. Use of a stiffer tack coat and/or allowing the tack coat to completely set to improve the adhesive bond between the overlay and the substrate has also resulted in less overlay shoving and less bump formation.

12.1.4 Effects on Performance

Long-term pavement performance is affected by surface waves, both short and long, in two ways. First, the waves reduce the smoothness of the pavement, which lowers the pavement condition rating or the present serviceability index of the roadway. The structural performance of the pavement will be changed, however, only if the waves are severe enough to increase the dynamic or impact loading of the pavement under aircraft loading or heavy truck traffic. Aircraft are, however, susceptible to excessive vibrations and subsequent component fatigue, particularly in response to long surface waves. Second, short waves and the factors that cause them can affect pavement density levels. A tender mix is generally more difficult to compact properly than is a stable mix; the result may be a decrease in density and a corresponding increase in air void content—leading to a reduction in pavement service life.

Washboarding is basically roughness built into the pavement surface during the compaction operation. Because it affects the degree of density obtained during the compaction process, this type of defect can significantly reduce the long-term durability of the pavement layer. In addition, washboarding contributes to a rough ride for the vehicles using the pavement.

Transverse bumps also contribute to loss in ride quality.