Prevent Callbacks for Pavement Slippage
BY John Ball
The theme this month incorporates quality control/quality assurance (QC/QA), which often employs tools to ensure the surface course that you pave is rolled to its best density. No matter how perfectly you match density specs, you can damage a new surface course’s chances for long life if you don’t adhere it to the base correctly.
One of the reasons a contractor might be called back to repair a job for the county or other agency is that the new pavement surface is slipping or shoving. Not only does a raveling pavement section look bad for our industry, but it also promises to let moisture into the layers of the pavement structure below.
To avoid pavement slippage, the contractor wants the crew to follow best practices from the start of the job to the finish. As with all projects, make sure you wear personal protective equipment (PPE) in the field. I also recommend milling crews use headsets to help with safe and efficient communication.
Mill Best Practices
Let’s say the project is a mill-and-fill. Before the milling crew even begins the job, members of the crew want to inspect the piece of equipment. Make sure the mirrors are in good condition and the backup camera is working for safe operation. Then check the teeth and cutting drum specifically for wear or missing tools. Make sure all the teeth are the same brand and type/material to ensure a consistent cut.
Look at the entire cutter drum housing. The door behind the cutter moves up and down, similar to an endgate on the paver, to keep the chunks and fines in rotation with the drum. If this door is worn or damaged, material will sneak out, causing more work for the sweeping team than necessary. Also double-check the endgates on the side of the housing to ensure excess material won’t bleed out from these areas. You want the frame of the tractor to be tight and in good repair.
Your first step in getting a consistent application of tack is to make sure the distributor truck is clean and well-maintained.
In addition to setting up the cold planer to achieve a surface with the specified grade and slope, the operator will set it up to move forward at a consistent, reasonable speed. We must go at a consistent speed to keep the rotation of the cutter consistent. Altering the rotation by gunning the engine or trying to race the clock will cause the cutter to rip and tear up the pavement instead of cutting a smooth and even groove.
The operator also wants to see a guide bar on the mill, just like the paver operator wants a guide bar on the paver. We must follow a straight line when milling down the lane so the grooves follow a straight line. This will make brooming easier.
Look to the watering system as well. You want to ensure the tips are all the same and working correctly for the speed at which you’ll operate. The watering system will be a mist to keep dust down without turning the dust into a coagulated paste that the milling machine can’t pick up and the brooms will have to fight. There are more tips for milling machine maintenance and milling success in the article “Easy Cold Mill Start-up Begins the Night Before” on TheAsphaltPro.com.
Once you begin milling, look back every now and then to double-check your work. The readings on the control panel may tell you you’re milling at a depth of 1.5 inches but placing a 4-foot level down and using a ruler or tape measure to check your depth will show whether you’re actually at 1.5 inches or at 2 inches—or worse. Make sure your controls are reading correctly.
Then, to mill the adjacent lane, overlap the joint 4 to 6 inches. Of course, you’ll use automation to match the depth you need, but, once again, make sure everything is reading correctly. Use the 4-foot level and tape measure to double-check the readings and to double-check your quality.
Sweep Best Practices
The crew will clean the milled surface completely to pick up all the residue. Dust and fines left in the grooves will prevent tack—or hot-mix asphalt (HMA)—from adhering to the surface. This results in a surface that could potentially shove and move under traffic loads.
As they did with the cold planer, the crewmembers will inspect equipment before they get started. First and foremost, check the safety lighting and backup alarms on the brooms. As we’ve discussed in AsphaltPro before, the lights on brooms could be your only saving grace if something else goes wrong and a cloud of dust obscures your machine from high-speed traffic or backing construction equipment. Make sure strobes and other lights work. Make it a priority. Read about other broom maintenance factors, including a pre-shift checklist and information on when to replace broom cores, in the article “How to Maintain and Use the Sweeper Right.”
Make it a priority to keep safety lighting functional.
To ensure a clean milled surface ready for tack, follow best sweeping practices.
Step 1. Start with a cold planer that can collect fully as it moves down the lane. Do not allow traffic to flow on that surface before you sweep it. Traffic will grind the fine particles into the grooves. You’ve got to pick up material right away.
Step 2. Perform the first sweep pass with a tractor broom to get the chunks. We push those particles into the next lane for the skid steer to scoop up. The milling crew will set over and pick up fines that are brushed onto the adjacent lane, but you want to get up as many as possible to help the cutter in its job. As discussed in the article linked above, you must watch your down-pressure, watch your speed, watch your broom angle, watch your rotation, and make sure the water system works.
Step 3. Perform the second sweep pass with a vacuum broom to ensure fines have been collected. The vacuum broom will have side sweepers and will go at a reasonable speed. Have a spare truck out there to dump the bucket into. This should be the final sweep and should give you a clean enough surface for tacking.
Your first step in getting a consistent application of tack is to make sure the distributor truck is clean and well-maintained.
If your project calls for a thin lift overlay or other surface course to be paved atop an unmilled surface, it is imperative that you clean the existing surface very well immediately before paving begins. Dirt, leaves, litter, grass and weeds, and fines and dust will prevent tack or HMA from adhering to the lower surface. It’s worth sending the broom down the lane to make sure your expensive materials will stay in place and the DOT won’t have to call you back to fix a slipping pavement later.
If your project must be delayed and paving doesn’t take place immediately after milling or immediately after cleaning, it is well worth the time and effort to send the broom down the lane one more time when the crew arrives for tacking and paving the next day.
Tack Best Practices
Whether you’ll be tacking a freshly milled or existing roadway prior to paving, this is another chance to guard against slippage problems later. Getting the tack coat down correctly is vital to the success of the surface to follow. Read the article “Why You Need to Tack Before Paving” for a detailed look at tack coats, their purpose as a glue between surfaces, how temperature affects them, and the use of emulsifiers with them, among other topics.
Once again, perform an inspection of your equipment before you begin. Maintain and set up your distributor truck/tack wagon for success, as seen in the article “How to Maintain Spray Bars.”
To ensure your tack coat works as the glue between surfaces, you’ll want to follow these best practices.
Step 1. Make sure you have enough gallons in your tank for the whole day. There’s no point in showing up with 200 gallons in the tank if the project will require 500. At some point in the shift, you’ll be wasting everyone’s time. Arrive early enough to the job to heat the material to its correct working temperature before you’re needed.
Also be aware of your limitations. Not too many distributor truck drivers or paving foremen are also chemists, so we shouldn’t be adding water to the tank to stretch a product. If you add too much water to an emulsion-based tack, you’ll know because it stays brown and doesn’t break. This adds to your delay before paving and dilutes the tack too much. It won’t work as a glue.
I really like trackless tacks, even though they’re a bit more expensive than other tacks. They break quickly and are ready for paving in four to six minutes. The heat of the HMA layer actives the tack and enables the glue-like performance, all while helping your crew keep a mess at bay. You don’t have to worry so much about straying haul trucks if you’re using a trackless tack.
Step 2. If you aren’t working with a spray paver, you want to spray the tack coat immediately ahead of the paving team to prevent the motoring public or other equipment from driving across the tack. If working with a slow-setting tack material, figure out the time it will take for the material to break, and spray accordingly. The idea is to make the window of opportunity for someone to track the material off your project—or to mess up your perfect application—as small as possible. You’ll work with the foreman on the job to assess how many haul trucks are delivering HMA, thus how many feet you need to tack in front of the paver to accommodate the material delivery schedule.
Step 3. Ensure proper coverage when spraying. You’ll ensure the spray bar is set 12 inches from the surface to be tacked, but also ensure the spray nozzles are correct and consistent. You want to see the spray emit from the nozzle in a fan-like pattern. Each nozzle is spaced so that each triangle shape of spray can overlap the triangle shape of spray next to it—this, combined with the height of the bar, effects the triple overlap. Depending on the application rate your department of transportation requires for your specific project, that overlap will be more or less severe to give a heavier or lighter application of tack on the surface.
The HMA course will slip and slide if there’s too much tack. It won’t adhere properly across the width of the lane if there’s too little tack. Either situation results in the callback this article is designed to help you avoid.
Step 4. Ask yourself if the timing is right to pave. If the tack didn’t break yet, driving the haul truck on it is asking for trouble. Placing the HMA course on wet tack is premature; the HMA won’t adhere to the existing surface if the tack isn’t ready to serve its purpose as glue. You’ve wasted the product.
Time and temperature matters. Figure out the time it takes for the material to break. This has everything to do with the weather and the temperature of the day.
Prevent Slippage Best Practices
To prevent the pavement from slipping a few months or a year after your job, get the basics right from the start. Pay attention to the weather, the products, the equipment and the best practices of each step to get a top quality job that lasts.
John Ball is the proprietor of Top Quality Paving & Training, Manchester, New Hampshire. He provides personal, on-site paving consulting services around the United States and into Canada. For more information, contact him at (603) 493-1458 or tqpaving@yahoo.com.