Chapter 9: Thin Functional Hot Mix Asphalt Overlay Projects

Troubleshooting

This section provides information to assist maintenance personnel with troubleshooting problems associated with placing any of the thin HMA overlays. Table 12 presents a troubleshooting guide that associates common problems to their potential causes, whereas Table 13 lists some commonly encountered problems and their recommended solutions.

An accessible version of Table 12 is available here.

Table 12: Troubleshooting Guide (3)
✓'s indicate causes related to paver. X’s indicate other problems to be investigated.
Cause
Cause	Problem
Fluctuating Head of Material	✓	✓				✓					✓
Feeder Screws Overloaded	✓	✓				✓			✓
Finisher Speed Too Fast	✓				✓
Too Much Lead Crown in Screed					✓
Too Little Lead Crown in Screed				✓
Overcorrecting Thickness Control Screws	✓										✓
Excessive Play in Screed Mechanical Connection	✓	✓					✓	✓				✓
Screed Riding on Lift Cylinders	✓	✓				✓		✓		✓	✓	✓
Screed Plates Worn Out or Warped			✓	✓	✓	✓
Screed Plates Not Tight	✓					✓		✓				✓
Cold Screed			✓	✓	✓	✓
Moldboard on Strikeoff Too Low					✓
Running Hopper Empty Between Loads		✓				✓
Feeder Gates Set Incorrectly		✓		✓	✓
Kicker Screws Worn Out or Mounted Incorrectly				✓
Incorrect Nulling of Screed												✓
Screed Starting Blocks Too Short												✓
Screed Extensions Installed Incorrectly					✓	✓
Vibrators Running Too Slow						✓				✓
Grade Control Mounted Incorrectly	✓	✓						✓			✓
Grade Control Hunting (Sensitivity Too High)	✓										✓
Grade Control Wand Bouncing on Reference	✓										✓
Grade Reference Inadequate	✓	✓
Sitting Long Period Between Loads		✓				✓
Improper Joint Overlap											✓
Improper Mat Thickness for Max. Aggregate Size			X			X		X		X
Trucks Bumping Finisher		X					X
Truck Holding Brakes		X					X
Improper Base Preparation	X	X				X				X			X	X		X	X
Improper Rolling Operation	X										X	X	X	X		X	X
Reversing or Turning Too Fast of Rollers		X												X		X	X
Parking Roller on Hot Mat		X														X	X
Improper Mix Design (Aggregate)	X		X			X			X				X	X	X		X
Improper Mix Design (Asphalt)	X		X			X			X				X	X	X		X
Mix Segregation	X	X	X			X			X
Moisture in Mix			X										X	X	X		X
Variation of Mix Temperature	X	X	X			X		X					X	X	X	X	X
Cold Mix Temperature			X	X	X	X		X		X	X	X					X

Note: Often, a problem can be caused by more than one item, therefore, it is important that each cause listed is eliminated to assure solving the problem.

Table 13: Common Problems and Related Solutions
Problem	Causes and Solutions
Surface Waves	CAUSES A fluctuating head of material in front of the paver screed causing it to rise and fall usually causes surface waves. Worn or badly set screeds can cause surface waves. A mix that is too stiff or that has cooled too much before compaction will cause surface waves. Long waves can be caused by adjusting the screed too often and not allowing an adjustment to fully take effect before changing it again. Dump trucks bumping the paver when delivering a load of mix can cause long waves. SOLUTIONS The solution for avoiding surface waves is to control the material amount, temperature, and screed correctly. Pave continuously with a pick up machine where possible.
Wash Boarding	CAUSES Wash boarding is caused by improper use of vibratory rollers, either in amplitude setting or in speed of roller. SOLUTIONS Use higher roller amplitudes for thicker layers and lower amplitudes for thinner layers. Slow down the roller.
Tearing	CAUSES Poor paver operation, or the mix being too cold and/or too stiff causes tear marks. SOLUTIONS Tear marks can be avoided by adjusting the degree of crown and ensuring the mix temperature is correct.
Non Uniform Texture- Segregation	CAUSES The mixture separating in the hopper or in transportation causes segregation. Poor paver set up. Low mix temperature or poor grading or mix design. Prone to occur in thin overlays. Weak base layer. SOLUTIONS Ensure thickness is at least twice that of largest stone size, mix design is correct, and the paver is properly set up. Ensure mix temperature is correct.
Screed Marks	CAUSES Transverse screed marks occur when the paver stops and starts and longitudinal screed marks occur when extensions are used on the screed. Poor paver set up or worn or dirty screeds. Low mix temperature or poor grading or mix design. SOLUTIONS Set paver and screed correctly. Use windrowing to ensure paver does not stop. Ensure the mix is within specification.
Surface Shadows	CAUSES Caused by overloading augers in the paver. May be caused by low mix temperature or poor grading or mix design. SOLUTIONS Adjust the distance between the screed and the tractor of the paver. Ensure that the level of mix is near the center of the auger shaft. The augers should NOT be totally covered with mix.
Roller Checking and Roller Marks	CAUSES Deflection under the roller (i.e., mix too hot) or mix design is poor. Too much asphalt in the mix, too much middle size sand in the gradation (1.18-600μm sieve). SOLUTIONS Wait until the mix cools further or adjust the mix design.
Bleeding and Fat Spots	CAUSES High mix temperature or poor grading or mix design. Too much asphalt in the mix or amount of fines too low in the grading. Mix design not taking the correct traffic level into account. Moisture in the mix or on the pavement. Extremely high applications of tack coat. Existing bleeding surface. SOLUTIONS Solve by ensuring aggregates are dry during the mixing process, that pavement is not bleeding, that pavement is dry, and that mix is correctly designed for traffic and aggregate.
Shoving	CAUSES Caused by excess asphalt in the mix. Improper roller operation such as sudden reversal. Rolling before the mat is stable enough. Roller going too fast. SOLUTIONS Ensure mix is at correct temperature. Ensure roller is not going too fast. Check and correct mix design if necessary. Consider use of modified binders.
De-lamination	CAUSES Insufficient tack coat. Mix is too cold during compaction. Existing surface being too cold for paving. Dirty surface on which an overlay is being placed. SOLUTIONS Ensure paving temperatures are correct. Ensure the surface is substantially free of debris.
Poor Joints	CAUSES Paver operating at different elevations when paving adjacent lanes. Poor joint practice, especially in compaction of thin layers. SOLUTIONS Make sure joints are correctly formed and compacted at the correct temperature.
Raveling	CAUSES Insufficient asphalt in the mix. Poor compaction. SOLUTIONS Ensure mix design conforms to the specification. Ensure compaction is carried out at correct temperatures.