Placing asphalt is only a single battle in the war to reach project specifications. What comes after paving — breakdown, intermediate and finish compaction — is where bonus checks are often earned (or not).
How can you ensure your crew can avoid and, if necessary, overcome compaction problems? The following excerpt from The Cat Paving Products “Guide to Asphalt Compaction” highlights a few common compaction challenges, along with solutions your crew can implement today.
Asphalt pick-up on dry drum surfaces
The most common cause of downtime on double drum, asphalt compactors is a malfunctioning drum spray system.
If any part of the steel drum does not have a water film on the surface, hot asphalt is likely to stick to the drum. The stickier the asphalt mix, the more severe the problem. A small amount of asphalt pick-up on the drum quickly becomes a large problem. With each drum rotation, the amount of pick-up increases and the mat will begin to show divots.
When asphalt begins to stick to a drum surface, operation of that compactor must be terminated until the drum is completely clean and any problem with the drum spray system is repaired. Continuing to operate the compactor will result in severe mat damage that will require extensive handwork to fill in and level divots left in the mat.
The primary cause of dry areas on the drum surface is a plugged spray nozzle. Good maintenance and clean water supply are the keys to preventing plugged spray nozzles.
- Use clean water. Whenever possible, fill water spray reservoirs with water from approved sources. If you have to use pond water, for example, increase the frequency of maintenance steps.
- Change main spray system filters. Follow the filter change interval shown in the machine’s Operation and Maintenance Manual. When the main spray system filter is plugged, water bypasses the filter and unfiltered water goes to the spray bars. Unfiltered water is more likely to cause spray nozzles to plug. Always have a spare filter stored on the compactor or in the maintenance vehicle.
- Maintain inlet filters. Most water reservoirs have an inlet filter inside the reservoir fill port. The inlet filter is the first stage of water filtration. Do not discard the inlet filter. Place the water supply hose inside the inlet filter.
- Clean spray nozzles. The spray nozzles have internal brass or plastic screens. Nozzle screens should be examined daily for contamination. Clean nozzle screens thoroughly as needed. If you use contaminated water, increase the frequency of nozzle maintenance. If only one side of the nozzle is plugged, the spray pattern will be smaller and can cause the dry strip on the drum and start the asphalt pickup.
- Maintain water distribution mats. The drum will have some type of water distribution mat to help spread the water film evenly on the drum surface. As the distribution mats wear, you may have to adjust them to maintain good drum contact. Replace distribution mats according to wear indicators.
- Understand spray system capabilities. Most water spray systems offer full-time or intermittent spray. Never sacrifice water coverage in an effort to conserve water. It is better to stop more frequently for water refills than to stop for drum cleaning.
- Protect the water spray system during cold weather. An optional water spray system antifreeze kit is available. It includes a separate reservoir for antifreeze. At the end of the shift, the operator can circulate antifreeze through the system to prevent overnight freezing.
Asphalt pick-up on rubber tires
Asphalt also can stick to rubber tires. The severity of asphalt sticking to rubber tires depends primarily on the stickiness of the asphalt. Tire pick-up is also affected by the difference in temperature between the surface of the asphalt layer and the rubber tires.
When asphalt begins to stick to the rubber tires of a pneumatic compactor, the operator must immediately stop and correct the problem.
- Use a bio-degradeable release agent to clean the affected tires. Apply more release agent to the tires before resuming the compaction process.
- Be sure the distribution mats and tire scrapers are properly positioned and in good working condition.
- Move the pneumatic compactor on the asphalt layer in an area where the surface temperature is relatively low.
- Heat the tires by operating on the warm mat before moving ahead to a higher temperature zone.
Heating the rubber tires and keeping them at the correct temperature are very important. Wheel covers help keep heat confined around the front and rear axles. Caterpillar recommends using wheel covers on pneumatic compactors for all asphalt compaction applications. Covers are especially important when compacting asphalt that contains modified asphalt cement. If wheel covers are not installed, the tires are exposed to the ambient conditions and can lose heat rapidly.
Release agents are sometimes used to help prevent hot asphalt from sticking to rubber tires. Always confirm with the public works department what release agents are permitted.
Most compactors have a tire spray system that is often filled with water and an additive. Common additives include detergents, water softeners or purpose-designed additives that increase the film thickness of the water sprayed on the tires.
In some areas, natural vegetable oil is substituted for water in the spray system reservoir. Do not use petroleum distillates as they are harmful to asphalt and the environment.
To prepare the pneumatic compactor for placement in the desired position behind the paver, plan your approach to heating the tires. Apply a release agent (if needed), and keep the tires hot.
- Before the start of the paving and compaction process, operate the pneumatic compactor on a paved surface behind the starting point. Operate at high speed to build up heat in the rubber tires by flexing them.
- When using a release agent, wet the tires thoroughly just as the compactor is ready to start its first pattern.
- Check the temperature of the asphalt layer and guide the pneumatic compactor operator into the correct temperature zone.
- Be alert for any sign of excessive asphalt pick-up on the tires. Especially watch for asphalt clumps falling off the tires when the compactor stops and reverses.
- If you see excessive asphalt pick-up, immediately clean the tires. Move the compactor back to a cooler temperature zone. Gradually move the compactor forward, allowing the tires to heat up prior to reaching the desired temperature zone.
- Once the tires are heated, keep them heated. If there is an interruption to the paving and compaction process, do not park the pneumatic compactor. Move the compactor to a place on the asphalt layer where it can continue to roll in order to keep the rubber tires heated.
Deep pneumatic tire marks
Using a pneumatic compactor on hot asphalt layers, especially layers 3 in thick or thicker can result in deep tire marks that are difficult to clean up, particularly behind vibratory screeds.
Ordinarily, a pneumatic compactor is used during the intermediate phase of compaction on an asphalt layer that is already close to final target density. The tire marks it leaves in the mat are normally shallow and can be smoothed out by the finish-phase compactor.
However, if the compactor is used during the initial compaction phase or if the pneumatic compactor rolls an area where the mat is thicker and hotter than normal, the rubber tires can leave deep marks that do not clean up easily during finish compaction.
Using a pneumatic compactor during the initial phase is usually done when compacting a base or binder layer that is going to have another layer laid on top of it. In that instance, the tire marks and loss of smoothness are less of an issue.
Using a pneumatic compactor during the initial phase on the final layer (wearing layer) of asphalt is not common because the final layer is often measured for smoothness. A pneumatic compactor is normally in the intermediate position when compacting the final asphalt layer.
If deep tire marks appear during the compaction of the final layer:
- Move the pneumatic compactor farther behind the paver where the asphalt layer is cooler, or …
- Decrease tire pressure to flatten the tires somewhat and reduce the tire contact pressure.
Vibratory drum impact marks
When too much vibratory compaction energy is applied to an asphalt layer, impact marks that do not clean up during the finish phase may appear on the surface of the asphalt layer.
Bouncing often leads to these impact marks. Bouncing sometimes occurs when a crew tries to reduce the thickness of the asphalt layer by making numerous, slow vibratory passes. The compactor can only reduce the thickness of the layer so much. When the layer becomes dense, the drums will begin to bounce and leave impact marks.
If you begin to feel drum bouncing, or if you begin to see impact marks in the surface of the asphalt layer, you should adjust one or more of the following variables:
- Check the working speed to make sure you are operating in the range that produces 26 to 46 impacts per meter (8-14 impacts per foot).
- Switch to a lower amplitude setting.
- If available on the machine, switch to a higher frequency.
- Operate with one drum vibrating and one drum static.
- Operate in the static mode.
Eliminating bouncing, and overcoming other problems, are keys to successful compaction — and healthy bonus checks that boost your bottom line.
The Cat Paving Products “Guide to Asphalt Compaction” is available through participating Cat dealers.