Fat Spots

Fat spots in a SMA mat

Fat spot core on the same job

Description: Isolated areas in the mat where excess asphalt binder is visible on the surface.

Problem:  A few scattered fat spots may not significantly affect mat durability or pavement structure.  A large number of fat spots may affect:

• Pavement performance.  Air voids in fat spot locations may be excessively low leading to potential shoving and rutting. 

• Skid resistance.  Fat spots have a smooth surface texture and will decrease skid resistance, especially when wet.

Possible Causes: Causes can be related to HMA moisture, petroleum/diesel contamination or construction:

• Excessive moisture in the HMA.  Aggregate with significant moisture can cause fat spots because the moisture vapor will force asphalt binder to the mat surface behind the paver as it escapes from the mix.

• Petroleum or diesel contamination.  This can occur when petroleum products are spilled onto the underlying surface in isolated areas before paving or when these products contaminate the mix.

• Construction.  Mixes that use heavily modified asphalt binders, such as SMA, tend to generate accumulations of asphalt binder on construction equipment like pavers and MTVs.  As binder accumulates, it eventually breaks free of the equipment or drips onto the mat below causing a fat spot. 

Solutions: Solutions differ depending upon the cause:

• Excessive moisture in the HMA.  Ensure all aggregate is dry when combined with asphalt binder during the HMA process.  Moisture content should be as low as possible but generally not more than 0.5 percent.

• Petroleum or diesel contamination.  Be especially careful when using petroleum or diesel products around a paving site.  These products are banned by most agencies as a truck bed release agent because of their destructive effect on HMA mats.

• Construction.  When paving with modified binders, be particularly careful that excess asphalt binder does not accumulate on paving equipment and drip onto the newly placed mat.

Joint Problems

Poor longitudinal joint, already patched

Poor longitudinal joint, not patched

Description: Bumps, elevation differences, a lack of mix or low density near or in a transverse or longitudinal joint.

Problem: Typically, poor transverse joints result in a slight bump, which can affect roughness.  Poor longitudinal joints are generally lower in density, may fail prematurely from raveling or cracking, and may allow water infiltration into the base and subgrade.

Possible Causes: Poor construction, inadequate compaction or poor startup procedures.

Solutions: Use acceptable joint construction practices:

• Transverse joints.  Roll transverse joints parallel to the joint.  The beginning of a paving day almost always involves a transverse joint.  When starting a paver up, place the paver screed on blocks on the cold side of the joint.  The combination of the previously placed mat thickness (if any) and block thickness should equal the expected uncompacted thickness of the new mat.  This will allow the correct amount of mix to be placed at the joint interface.

• Longitudinal joints.  See the in-depth page on longitudinal joint construction.

Microcracking

Roller checking

Roller checking up close

Cracks from rolling a cold mat

Description: Short transverse cracks usually 25 - 75 mm (1 - 3 inches) long and 25 - 75 mm (1 - 3 inches) apart that appear on the mat surface during compaction. 

Problem:  By itself, microcracking is only a small detriment to pavement performance; the cracks are usually quite shallow and small.  However, microcracking can indicate mix tenderness, which can eventually lead to inadequate compaction either because (1) the mixture shoves rather than compacts under roller loads or (2) in an effort to avoid checking rollers are left off the mat so long that they are not able to complete compaction before cessation temperature is reached.  Therefore, microcracking can be an indirect indication of inadequate compaction.  Inadequately compacted areas have higher air voids and are susceptible to decreased stiffness, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture damage  (Hughes, 1984; Hughes, 1989).  Microcracking can also be an indication of compaction of a cool mat.  If a mat is too cool, the applied compactive effort may cause the surface to fracture.

Possible Causes: There are two principal causes of microcracking:

• Roller checking.  Mat deflection during compaction that is great enough to fracture the mat surface.  It can be caused by two principal means:

  • Excessive pavement deflection during compaction.  If underlying layers of HMA, base or the subgrade deflect excessively under roller loads, the mat will also deflect placing portions of the mat surface in tension.  If this tension is enough to overcome HMA cohesive forces, tiny fissures or cracks will develop on the surface.

  • Mix tenderness.  Tender mixes tend to shove when compacted, which causes areas of localized tension in the mat surface.  If this tension is enough to overcome HMA cohesive forces, tiny fissures or cracks will develop on the surface.   

• Compacting an excessively cool mat.  If rollers are operated on a cool mat, the surface can be cool enough and therefore stiff enough to fracture under the compactive effort.

Solutions: Solutions differ depending upon the cause:

• Excessive pavement deflection during compaction.  Properly prepare the surface on which the new mat is to be placed.

• Mix tenderness.  Eliminate the causes for tender mixes or develop a compaction sequence and pattern to accommodate the tender mix.  Compaction sequence and patterns will depend upon the type of mix tenderness (high temperature, mid-range temperature).

• Compacting an excessively cool mat.  Do not continue to compact a cool mat that shows signs of cracking under roller loads.  In extreme cases, if the mat below the surface continues to compact and the compaction must be done to achieve adequate mat density, then the trade off between microcracks (if compaction is continued) and a low-density mat (if compaction is stopped) must be carefully weighed.

Non-Uniform Texture

Description: Differences in appearance within the mat surface.  Extreme differences in mat surface appearance often take on the look of tearing or streaking and should be investigated as such.  Typically, mat areas that look rougher and more coarse than other areas are of the greatest concern.  HMA mixtures with larger nominal maximum aggregate sizes will naturally appear more coarse on the mat surface than mixtures with smaller nominal maximum aggregate sizes.

Problem: In and of itself, a rough surface texture is not necessarily detrimental to pavement performance.  Rough surface areas are, however, undesirable in parking lots and other areas of high pedestrian traffic.  Furthermore, rough areas can be an indication of low density, aggregate segregation or temperature differentials, which can all substantially reduce pavement life. 

Possible Causes: There are many potential causes for non-uniform mat texture.  Many times, the same mechanisms can cause mat tearing and non-uniform texture.  Therefore, an investigation of mat tearing causes is also warranted.  The following lists causes that are separate from mat tearing causes: 

• Handwork.  Handwork behind the paver screed will generally deposit HMA material on top of the mat either in thin layers or individual aggregate particles.  Even after compaction, areas of handwork are often noticeable.  Although minimal handwork is not detrimental to pavement performance, it can be unsightly and therefore detrimental to pavement appearance.

• Low mix delivery temperature.  HMA delivered at low temperatures may not allow adequate time for compaction.  If it is cool enough the compaction equipment (including the paver screed) may not be able to achieve a smooth surface texture.

• Soft base course or subbase.  If the base course or subbase deforms under the paver screed of compaction equipment, the surface texture may be different from the surrounding mat.

• Segregation.  Segregation, especially coarse segregation, is often visibly noticeable as a rough mat texture.

• Temperature differentials. Temperature differentials are often visibly noticeable as a rough mat texture.

Solutions: Solutions differ depending upon the cause.  Typical solutions involve minimizing handwork, delivering the HMA at a high enough temperature to allow for proper compaction, properly preparing base courses and subgrades and taking steps to eliminate segregation and temperature differentials.
 

Roller Marks

Roller mark caused by a quick turn

Description: Indentations that remain in the mat surface after rolling operations are complete.  Most roller marks are removed from the mat surface by finish rolling.

Problem:  Roller marks may cause an increase in roughness, but by themselves they are not particularly damaging to pavement performance.  Roller marks are, however, an indication of inadequate compaction and/or a tender mix. 

Possible Causes: Causes can be related to compaction, tender mixes or roller operation:

• Inadequate compaction.  A mat that is not compacted adequately before it cools to cessation temperature may still contain roller marks.

• Tender mix.  A tender mix will generally not be able to support the finish roller's weight until it has cooled enough to adequately stiffen the mix.  By this time, the mix may not be workable enough for the finish roller to remove the breakdown and intermediate roller marks.

• Improper roller operation.  Rollers left stationary on a hot mat (see picture at right above) will tend to sink into the mat with the resulting indentations being difficult to remove.  Rollers that are turned quickly may leave a mark as the outside of the leading drum or wheel digs into the fresh mat.  Excessively quick stopping can also leave a permanent mark.

Solutions: Solutions differ depending upon the cause:

• Inadequate compaction.  Ensure all compaction is done before the mat cools to cessation temperature.

• Tender mix.  Eliminate the causes for tender mixes or develop a compaction sequence and pattern to accommodate the tender mix.  Compaction sequence and patterns will depend upon the type of mix tenderness (high temperature, mid-range temperature).

• Improper roller operation.  Do not let rollers sit stationary on hot mats.  Operate rollers at appropriate speeds and avoid sharp turns.

Shoving

Description: Displacement of the HMA mixture in a longitudinal direction while compacting.  Shoving under traffic loading is covered in Module 9.7, Flexible Pavement Distress.

Problem:  Mats that shove under compaction loading are generally unstable and are likely to continue to deform under traffic loading leading to roughness problems.  If a mat is not adequately compacted because it is shoving, the resulting low density can cause the mat to be susceptible to decreased stiffness, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture damage  (Hughes, 1984; Hughes, 1989). 

Possible Causes: Tender mixes.

Solutions: Eliminate the causes for tender mixes or develop a compaction sequence and pattern to accommodate the tender mix.  Compaction sequence and patterns will depend upon the type of mix tenderness (high temperature, mid-range temperature).
 

Surface Waves

Washboarding

Description: Surface waves can appear in several different forms:

• Washboarding. Small waves generally about 75 - 100 mm (3 - 4 inches) apart. 

• Short waves (also called ripples or auger shadows).  Generally about 0.3 - 0.9 m (1 - 3 ft) apart with 0.45 - 0.60 m (1.5 - 2 ft) being the most common separation.

• Long waves.  Generally farther apart than short waves.  Can be in excess of 30 m (100 ft) apart.

Problem: All surface waves will cause an increase in roughness.  The pavement's structural capacity is only affected if the waves are severe enough to cause vehicles to bounce significantly as the traverse the waves.  This would increase impact loading and thus increase the overall loading to which the pavement is subjected.

Possible Causes: Causes can be roughly categorized by the type of surface wave:

• Washboarding.  Improper operation of a vibratory roller.  Generally, excessive roller speeds, low vibratory frequencies or excessive vibratory amplitudes for a given pavement thickness can lead to washboarding.

• Short waves.  Causes can are numerous but most relate to items that can cause frequent or periodic changes in the screed angle of attack, and corresponding changes in mat thickness.  Likely causes include:

  • Fluctuation of material head in front of the paver screed.  As the material head changes, the screed angle and elevation will change.  This fluctuating head can be caused by excessive auger starts and stops or an improperly set material feed rate.  Fluctuations may cause either short waves or non-uniform texture.

  • Screed in poor mechanical condition.  Screeds with excessive play in their controls may fluctuate slightly around the equilibrium screed angle.

  • Unresponsive screed.  Extremely high paver speeds, in excess of 25 m (83 ft) per minute for thin lifts or more than 15 m (50 ft) per minute for thick lifts (more than 62.5 mm (2.5 inches) thick), can cause to screed to be relatively unresponsive.

  • Improper mounting, operation or sensitivity of the automatic screed control system.  If the screed control system is improperly mounted, operated, or it is too sensitive it could supply a fluctuating grade reference causing the screed to fluctuate in response.

  • Truck driver applying the breaks too hard while being pushed by the paver.  If break application is too much the paver tracks or wheels may slip, which will cause a change in material head in front of the screed.

  • Improper mix design.  Mixtures whose stiffness is extremely sensitive to temperature of composition may compact differentially.  Tender mixes may shove and displace as they are compacted.

• Long waves.  Causes can be numerous and involve truck, paver and roller operation.  Likely causes include:

  • Fluctuation of material head in front of the paver screed.  If the problem as described under "short waves" occurs over a longer time interval it can result in long waves.

  • Screed in poor mechanical condition.  Screeds with excessive play in their controls may fluctuate slightly around the equilibrium screed angle.

  • Improper mounting or operation of the automatic screed control system.  If the grade control system is improperly mounted or operated it could supply a slowly changing grade reference causing the screed angle to slowly change in response.  If a stringline is used, sagging between support posts can cause long waves.

  • Improper mix design.  Mixtures whose stiffness is extremely sensitive to temperature of composition may compact differentially.  Tender mixes may shove and displace as they are compacted.

  • Emptying the paver hopper between truck loads.  This can be caused by improperly adjusted hopper flow gates or poor operating practices.  As the material is emptied and there is none to replace it in front of the screed, the material head in front of the screed will decrease. 

  • Compaction equipment operation.  Direction reversals, especially on lifts that are quite thick in relation to nominal maximum aggregate size, can leave bow waves in place at the point of direction reversal.  Abrupt roller turns can also cause mix shoving. 

  • Segregation. 

  • Temperature differentials.

Solutions: Surface waves can only be eliminated by preventing their formation.

• Washboarding.  Decreasing vibratory amplitude, increasing frequency or slowing roller speed will likely eliminate washboarding.

• Short waves.  Because there are so many different causes, solutions vary widely.  Most importantly, the amount of mix in front of the screed should be kept constant, which implies that slat conveyors and augers should be run at a near constant rate and as close to 100% of the time as possible.  Material flow to should be controlled by the hopper flow gates as much as possible.  Screed control problems can be identified by turning off the grade control device and determining whether or not the short waves continue.  Most other solutions involve strict and careful compliance with established best practices.

• Long waves.  Same solutions as short waves.

Tearing (Streaks)

Description: Mat tearing, or streaks in the mat, can be caused by different phenomena:

• Parts of the mat surface are dragged along by the screed.  Under normal operation, the screed passes cleanly over the mat, however, if the screed is applying excess downward force or the mat is too thin for the given nominal maximum aggregate size, it can begin to drag aggregate particles along the surface of the mat leave in longitudinal streaks behind where the dragging occurred.

• Inadequate mix delivered behind the gearbox.  This type of streak is usually 150 - 200 mm (6 - 8 inches) wide and runs longitudinally down the middle of the mat.

• Other streaks.  Other streaks can occur for a variety of reasons such as screed wear, screed crown, screed extensions, segregation and temperature differentials.

Problem: Torn mat areas are unsightly and can reduce long-term pavement performance.  Mat tearing usually results in a corresponding decrease in density relative to the mat as a whole.  Therefore, torn or streaked areas may have higher air voids and are susceptible to decreased stiffness, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture damage  (Hughes, 1984; Hughes, 1989). 

Possible Causes: Causes depend upon the type of mat tearing or streaking:

• Parts of the mat surface are dragged along by the screed.  Warped or worn screed plates will increase the likelihood of dragging aggregate particles along the mat surface because they generate more friction (warped screed plates) and have more potential defects that can grab aggregate particles (worn screed plates).  Excessive paver speed can magnify small aggregate particle disturbances to the point where the result becomes a visible streak.  An excessively thin mat in relation to nominal maximum aggregate size may result in the paver screed being supported by the larger aggregate particles.  Aggregate particles caught between the screed and the surface being paved may roll or be dragged forward.

• Inadequate mix delivered to a particular area in front of the screed.  Inadequate material flow to the location behind the auger gear box may be caused by improper flow gate settings, or improperly set, missing, or worn reverse augers located adjacent to the gearbox.  Reverse augers are a small portion of each auger located adjacent to the gear box where the augers are pitched the opposite way so as to cause mix to flow inwards (the rest of the auger is pitched to promote outward material flow) and fill in behind the gear box.

• Other streaks.  Other forms of mat streaking can be caused by:

  • Improper screed crown settings.  A wide centerline streak can be caused by a lack of lead crown in the screed.  Wide streaks on both outside mat edges can be caused by an excess of lead crown. 

  • Cold screed plate. Without preheating (usually with screed heaters), a screed plate will tend to tear the mat at the beginning of paving operations.  As the hot HMA heats the screed, this type of tearing will generally disappear.

  • Improper screed extension use or installation.  If screed extensions do not closely match the screed's crown and elevation they could create a small streak at the point of transition from screed to extension. 

Solutions: Solutions depend upon the type of tearing or streaking:

• Parts of the mat surface are dragged along by the screed.  Ensure the screed plate is properly maintained and replace worn or defective screed plates.  Decrease paver speed.  Adhere to nominal maximum aggregate size vs. lift thickness requirements.

• Inadequate mix delivered to a particular area in front of the screed.  Install reverse augers or paddles near the gearbox if they are not already installed.  If they are installed, check them for wear and proper operation.  Some pavers are more prone to this type of problem than others.

• Other streaks.  Properly adjust screed crown settings.  Typically, the lead crown should be set slightly higher than the tail crown by about 3 mm (0.125 inch).  Heat the screed before paving operations.  Be careful when using screed extensions.  Extensions that just lengthen the strike-off plate are especially prone to causing rough mat streaks.  Mix that passes under these kinds of extensions is struck off at the correct elevation but does not pass under the screed plate and therefore does not receive the compactive effort of the screed.

Transverse Screed Marks

Screed mark likely caused by the beginning of a day's paving

Screed mark and subsequent rough surface texture at a day's paving start

Description: Transverse indentations in the mat surface across the width of the mat.  These indentations occur when the paver stops between truckloads of HMA and the screed rests on the hot mat.

Problem: Generally, transverse screed marks will not affect a pavement's structure but they will affect roughness.

Possible Causes: Causes can be related to either the paver or the HMA mixture:

• Paver related.  If the screed is set up incorrectly and rides more on its back end, this back end can cause a transverse mark when the paver is stopped because the weight of the screed is being supported by a smaller area than if the screed were set up properly.  Haul trucks bumping the paver can also cause transverse screed marks.

• Mix related.  If a HMA mix is particularly tender, a heavy screed (e.g., one with hydraulic or rigid extensions) may leave a mark when left stationary on the mat.

Solutions: Solutions differ depending upon the cause:

• Paver related.  Repair and set up the screed correctly and ensure proper haul truck-paver interaction. 

• Mix related.  Eliminate the causes for tender mixes.  Eliminate the need to stop the paver by using windrow paving and/or a MTV.