In Hot Mix Asphalt (HMA) construction, one of the toughest things to achieve is final mat smoothness. As owners and agencies make improvements to specifications, ride quality or ridability becomes more and more important. Some states have developed smoothness specifications for new construction, and have tacked on incentives of five to ten percent for exceeding specification or five to ten percent penalties for failing to meet specification. We expect to see Caltrans adopt similar policies. Caltrans’ requirement of no more than 0.01 foot (1⁄8 inch) variation per twelve feet of pavement is already difficult to achieve. Most owners and agencies follow Caltrans specifications, so smoothness, along with compaction, has become a very important construction requirement.
One of the most common problems in achieving smoothness is the presence of surface waves, either short waves (ripples) or long waves. Short waves are generally 1 to 3 feet apart, and 1.5 to 2 feet is most common. Long waves are considerably further apart, and may correspond to the distance between truck loads of mix. An additional type of defect is a washboard effect, caused by improper operation of the vibratory roller. The distance between this type of wave is generally very small, typically less than three or four inches.
The primary cause of ripples is a fluctuating head of material in front of the paver screed. This variation in the amount of mix, carried back to the augers by slat conveyors and deposited in front of the screed, causes the screed to rise and fall. Too much mix, and then too little mix in front of the screed, causes the wavy surface as the screed reacts to the variable forces upon it. Another cause of ripples can be a screed in poor condition, one that has excessive play in its screed control connections. Ripples can also be formed in the mat by improper mounting, or sensitivity of the automatic grade control on the paver. The problem might also be related to a bouncing ski, or to the truck driver holding brakes while the truck is being pushed by the paver.
The only cure for ripples is prevention. The most important factor in ripple prevention is to keep the head of material as consistent as possible. In addition, mix stiffness, related to temperature and composition, should remain as constant as possible. The following tips will help to assure a smooth pavement.
- Try to maintain continuous, uninterrupted paver movement.
- If the paver has to stop, bring the paver to a stop smoothly and quickly. When restarting, accelerate the paver to its former speed smoothly and quickly.
- Operate the paver at a speed that matches the mix delivery and will reduce the number of starts and stops.
- Do not allow the paver hopper to run low or less than half full. This will affect the head of material.
- Do not allow trucks to back into the paver. Instead, they should stop short of the paver and allow it to move forward and push the truck.
- If a pick up machine is used, the slat conveyor should pick up all of the mix in the windrow without leaving any mix on the existing surface.
- While trucks are exchanged, the speed of the paver should not be slowed to allow for continuous movement of the paver. This affects the head of material, and therefore the smoothness.
- Hopper flow gates should be set at a height to permit slat conveyors and augers to operate as close to 100% of the time as possible. A good rule of thumb is that the head of material should be located near the center of the auger shaft.
- If the paver screed is operated manually, screed men should not turn the thickness controls, except to increase or decrease the thickness of the layer being placed. Remember that after the controls are turned, it takes five times the length of the tow arm before the screed completes the chanv