This morning I walked by the big parking lot again. It’s an embarrassment. The asphalt was the perfect stuff for the job, but the way it was applied was atrocious. Less than four weeks after the job was done, there were huge puddles. The job had to be redone, but it is still not good. There’s another parking lot done a few weeks later, by the hospital, and that has shown no weeds peeping through, no puddles, and no rework. Putting it simply, there are three stages to asphalt paving: before the asphalt, putting the asphalt down, compacting the asphalt mat. So what makes one asphalt paving job good and another bad? The answer is simple and obvious. It’s not like killing bugs in the summer. You don’t just squirt some asphalt from a can and the problem’s solved.
One invention that is both simple and practical and can make an amazing difference to the very first step in any paving job is the AggreScreed from TerraTec Industries in Colorado. Earthmovers will appreciate this; it was invented by an earthmoving contractor. The AggreScreed dozer blade attachment adjusts to 24 feet in 2-foot increments and is attachable to virtually any dozer in the 125- to 250-horsepower range. It can lay gravel at any depth from zero to 15 inches and can even create a 3.5% crown in a single pass. There are plows fixed to either side of this blade to capture and keep materials in front of the blade to achieve proper depth. No, you don’t need staking with this accurate blade. The width is adjusted by positioning 24 bolts (takes about 30 minutes manually). The hydraulic system, controlled by a box in the dozer cab, moves the dozer attachment to your determined position. The blade weighs 6,000 pounds, attaches/detaches in 15 to 20 minutes, and can manage gravel to a depth of 15 inches.
“The AggreScreed AS1624 is very simple to not only mobilize but install for use,” says Colby Robertson of Robertson Contractors, Inc. “We attached the screed to our Caterpillar D6N dozer and enjoyed labor cost savings of 25% in our four-day trial period. It is very possible to lay 1,000 tons per hour accurately with this tool. The most impressive point in using the AggreScreed was our ability to use every single ton of material, enduring no overruns regarding material costs. We estimated a volume of 14,000 tons and we ended up with 14,080. This type of material control is what we strive for when laying aggregate base courses. The consistency of the “˜mat’ left after the AggreScreed is nothing short of amazing.”
So there’s one excellent idea for making those first steps in paving accurate and fast.
Finding good paving equipment is not a problem. The accuracy and reliability of today’s equipment is excellent and it’s interesting that different manufacturers offer similar advice to their customers about good practices in paving, regardless of the brand of equipment used.
Caterpillar publishes a booklet entitled Asphalt Paving by the Numbers. It’s a practical, no-nonsense set of guidelines for anybody who plans to pave with asphalt, whatever brand of equipment you are using. The first consideration noted by Caterpillar is safety. “Improper operation, lubrication, maintenance or repair of this product can be dangerous and could result in injury or death,” notes the booklet. “Do not operate or perform any lubrication, maintenance or repair on this product, until you have read and understood the operation, lubrication, maintenance, and repair information.” If that sounds too obvious, just remind yourself of the people you’ve met who always know how to run anything, whatever the new features, whatever the innovations in operation, and who imagine that operations manuals are simply thick mats for coffee cups.
The introduction to “Paving by the Numbers” reminds us that producing a smooth asphalt surface with a uniform finish is not automatically a simple operation. “A well-trained paver crew, which follows a set of established fundamental practices, will avoid many of the problems that affect mat quality,” advises Jon Sjoblad, an expert with worldwide experience at Caterpillar Paving Products.
Step 1 is lighting the screed heaters, with explicit instructions on how to do that and precautions to take, such as not allowing the screed plate to overheat. Cat pavers have an automatic shut-off but you may have to shut some heater systems off manually on other pavers. Fifteen minutes is the maximum recommended heating time. Then light one burner at a time; trying to light several simultaneously may trip the circuit breaker. In this article, I will not mention every sub-step but give you an overview of the instruction. You should acquire the booklet; it’s 44 brief, well-illustrated pages and easy to read.
The second step, after the screed is warmed up, will be centering the tow points, remembering that (with automatic grade and slope functions used) corrections to mat thickness are created by tow point movement. That’s why it is important to center the tow point for equal travel in both directions.
Step 3 sets the desired paving width, with the screed still raised. It’s good practice to balance the extender width so that forces against each extender are equal, so move the left and right extenders out an equal amount, 2 feet on each side if you need four feet of extender.
Step 4 sets the crown of the main screed.
Step 5 sets the height of the extenders (with good instructions on how to do that), and Step 6 sets the extender slope to the specifications of your job. If an extender slope is not required, the indicator on both sides is set at zero.
Step 7 has you positioning the starter boards under the screed. You’re almost ready now! But, at this stage, make sure the raise/lower switch for the screed is on the lower or “float” position on both consoles. At this stage you also remove any slack from the tow point by moving the machine forward until the tow arm roller contacts the tow point frame.
Paving success is in the details. Nulling the screed with manual depth control cranks is part of Step 8. Adjust so that the full weight of the screed is supported by the starting reference. Turn the depth crank to increase mat thickness; when resistance is felt, lock the depth crank in that position. (Do the same for both sides.) Screeds do differ, so always follow the manufacturer’s guidelines for the particular screed you are using.
The end gates float on the grade so Step 9 is positioning them for accuracy. You also adjust the guide tubes to move up or down over grade irregularities, usually with a space around 3″ for the spring-loaded skis.
Steps 10 through 12 tell you more about adjustment and positioning of the controls for auger height, feeder sensors, feeder systems, and the ratio control.
Filling the auger chamber requires care (Step 13). Use manual switches on the operator’s console to convey and auger material to the auger chamber and out to the ends of the augers. Use the manual auger switch to move material to the end of the screed. About half full is ideal; overfull will result in a bump when starting off the transverse joint. When the auger chamber is half full, put the conveyor and auger system switches on the operator’s console in the auto mode. And off we go! Make sure gear selector is on the PAVE mode, with the throttle set at high idle. Release the parking brake and the propel lever is moved all the way forward. Turn the speed control dial clockwise until the calculated paving speed is reached.
Still in Step 14, screed personnel should note the level of material at the outboard ends of the auger shafts to see if the auger shafts are one half covered. This Step 14 is important; it’s what to do as the paver starts its job. As the paver comes up to speed, the operator will look down at the center of the auger chamber with a goal of maintaining the mix level so that one half of the auger shaft is covered. If you use a screed with front mounted extenders, adjust the strike off until there is between six and eight inches of material in front of the exposed main screed. As paving stabilizes, check the auger speed (20-40 rpm is good) and avoid any on-and-off operation of the augers, because that can cause material segregation stripes in the may and a certain loss of smoothness. The screed personnel will now be checking constantly for transition marks and a uniform mat.
Speed is critical to successful paving. It should be a constant speed. “Usually the contractor can set up the paver to pave smoothly at any speed that matches the delivery of the mix to the job site,” reaffirms Jon Sjoblad. “The important thing is to keep the speed constant because, if the paving speed is changed drastically, the screed will either rise or fall and the smoothness of the asphalt mat will suffer. If paving speed changes, the demands on the feeder system change, so the feeder system controls will have to be adjusted to match the new material demands. For the best paving results, always follow the basic fundamentals of paver setup and keep the operation consistent.” Do the preparations right, every time. Then you won’t have those costly, profit-busting reworks.
Downtime can be expensive with an asphalt paver. If your paver breaks down on the job, you may have 10 to 20 trucks full of hot mix, a multi-million-dollar asphalt plant and 30 to 50 people, all sitting idle. Some contractors have recorded internal costs as high as $200 to $300 per minute when not continuously paving. That’s why preventive maintenance of your pavers is a business necessity.
The AggreScreed blade prepares an aggregate base.
Teamwork keeps the train going smooth.
Keeping Your Paver Fit for Top Performance
Just imagine that you have taken delivery on a new rubber-track 10-foot paver with the most popular front-mounted hydraulic extendable screed. You want to run it for 3,000 hours, or three years. Trouble free .The job of daily inspections, lubrications, and minor adjustments belongs to your paving crew. “There’s no better first line-of-defense on paver maintenance than the crew,” says Ben Burra, equipment superintendent with Arrow Road Construction, Mount Prospect, IL. Arrow owns and operates six highway pavers and four of them are Roadtec units. “Each of our crews stays with one paver through the season,” says Burra. “The crew gets to know its own paver very well (all of its tendencies and so forth) so we in the equipment department keep up good communications with our crews about how the pavers are performing.”
“Cleaning your paver daily is one of the most often overlooked maintenance chores,” comments Jeff Smith, vice president of product support for Roadtec. A new Roadtec paver comes with an environmental wash-down kit. It has a separate tank for environmentally friendly cleaning fluid, a hose reel and a spray wand. Or, the company offers an automatic cleaning system as an option. (A pause here for friendly nagging.) You should always consult your owner’s manual for a complete checklist for paver maintenance, whatever brand it is. On a tracked paver, it’s very important to keep the undercarriage clean and you should get down there and make sure there are no chunks of asphalt sticking to the undercarriage.
Daily maintenance on a Roadtec RP 195 paver should include the following:
- Check the engine oil and radiator.
- Conveyor idler bearings should get two shots of grease.
- Vibrator bearings should get two shots.
- Conveyor head shaft bearings should be purged with grease while hot and running.
- Auger hanger bearings should be purged while hot.
- Auger chain case bearings should be purged while hot and running.
- Check your hydraulic tank.
Weekly, you need to check slat chains, to make sure they’re not too loose. “You don’t want to see more than half a link showing at the bottom of your paver frame,” warns Smith. “You don’t want it to be really tight, or excessively loose. I recommend checking conveyor drive chains and auger drive chains once a week. A Roadtec paver has four drive chains, one for each conveyor and one for each of two augers, the left and right one. There should be no more than one inch of play in the conveyor drive chain.” On a typical paver there’s an auger chain on the left, and another on the right. Those chains are invisible, but you can just grab the end of the auger on each side and try to wiggle it. “Push it from full forward to full reverse, and if you get more than half an inch of play, it needs adjustment,” says Smith. It is especially important to change hydraulic filters after the first 50 hours of operation on a new machine. Doing so assures that any rough edges from manufacturing in pumps and motors are taken out of the system with the first filter change.
A Roadtec rubber-tracked paver has six hydraulic filters; they need to be changed at 250-hour intervals. Change the hydraulic oil, using manufacturer-recommended product, every 1,000 hours or once a season, whichever comes first. Change the engine oil and filters after the first 100 hours of operation. Check the water in the battery and inspect and verify all grease lines and functions. At 100 hours, it’s important to inspect and adjust both slat chains. The same goes for your track pressure. After the first 250 hours of operation on a new machine, change the fluid in three sets of planetaries: the auger, conveyor, and track drive planetaries. Use only 80W-90W oil. From there on, check the fluid level every 250 hours of operation and change the fluid every 1,000 hours of operation.
“Proper track adjustment ranks as one of the most-neglected aspects of asphalt paver maintenance,” says Smith. “There are cogs, or lugs, running down the center of each track, and those cogs need to be kept running straight. Check the cogs often for telltale wear on the sides. Asphalt build-ups around the bogey wheels, the front idler wheel and the drive wheel can throw a track out of alignment.” Burra recommends checking track alignment visually while the paver hopper is loaded and the machine is paving. That way you get a true reading on the track. To check it, say, after it has turned in the yard, empty, may give you a mistaken reading of track alignment. The track systems are equipped with grease fittings that need to be greased at intervals of 250 working hours. Clean any dirt and grease from the fittings and grease with high temperature grease. On Roadtec pavers, the track is automatically tensioned. A hydraulic cylinder maintains the proper pressure on the track, but you need to check that pressure with a hydraulic pressure gauge. Locate the proper port in the hydraulic system, apply the gauge, and check the pressure. That needs to be done four times a year.
The life of your engine greatly depends on good maintenance. That’s why changing the fuel and oil filters at proper intervals is very important. The engine fuel filter on an RP-195 paver should be replaced every 250 hours of operation. Refer to the engine maintenance manual for the proper filter. The Caterpillar engine in a Roadtec RP-195 paver is equipped with a fuel/water separation filter. Water in the fuel can cause the engine to run erratically and can cause an electronic injector unit to fail. The fuel/water separator also provides filtration, Roadtec says. To help extend the life of the secondary fuel filter, the element should be drained daily and changed every 250 hours of operation.
Your annual paver maintenance consists of a complete drain-flush and refill of the following fluids: radiator/hydraulic cooler; hydraulic tank; auger planetary, conveyor planetary, and track drive planetary; and pump drive gear box. Check all pump pressures, pressure check the planetaries, and inspect and replace inner and outer auger bearings if needed. Inspect the screed plates and auger flights. Consult your owner’s manual for more. A well-maintained asphalt paver will last for years. Toward that end, maintenance time and money are excellent investments, and you won’t regret making them.
The Role of Compaction
The production, laydown, and compaction of hot-mix asphalt or warm-mix asphalt are time sensitive and even moderate delays can interfere with your goal to produce the desired long-life pavements-whether that pavement is a street, highway, or parking-lot. “The paver screed is the first tool used to improve the load-carrying capacity of pavement materials during the laydown process,” asserts Dale Starry, global road competence manager at Volvo Construction Equipment. “Roller compaction is the next phase. Depending on the application, the material specifications, the type of paver, and other variables, one or more compactors are used to further increase the density of the pavement.” Traffic compaction is the final phase of the mat’s compaction, with high volumes of heavy traffic reducing pavement voids even when that pavement has been properly compacted during the construction phases of laydown and roller compaction.
The temperature at which pavement is laid by the paver finisher is important to successful compaction. “Stone mastic asphalt (SMA) mixes are produced with binders that are relatively stiff and aggregates that are often blocky and coarse-graded,” notes Starry. “SMA is produced regularly at temperatures higher than 350°F and laid before the mix cools below 300ºF. Superpave mixes that contain polymer-modified binders are produced and processed at similar high temperatures. Marshall-mix asphalt binders are made at somewhat lower temperatures to prevent binder damage through overheating. Those mixes are placed typically at temperatures between 250°F to 300°F. Some newer technology mixes, known as warm-mix asphalt, allow mixing temperature to be reduced as much as 100°F, for considerable energy savings during production.” Processing time for all the mixes mentioned is approximately the same. Uniformity of temperature during the paving process is of paramount importance to achieve uniform pavement air void content and bearing capacity.
“When following pavers with vibratory screeds, compactors need to increase material density by 20% on average,” says Starry. “This needs to be completed before the pavement mix cools below its limit of workability. Depending on mix properties and other variables, this low temperature limit can be as warm as 175°F or as cool as 120°F. It depends on the mix design and asphalt cement binder properties.”
What is known as breakdown rolling is when the majority of air voids are removed from the pavement structure, so it is important to choose the right type and size of compaction equipment for the breakdown rolling, to cover the whole pavement uniformly. You can select a breakdown compactor on the basis of its rolling width as compared with the pavement to be compacted. If the laydown width is 12 feet, say, a breakdown compactor with a drum width of just over 6 feet (78.7 inches) can cover the panel in two passes side-by-side with adequate overhang of drum edges and overlap in the center.
A breakdown compactor with a narrower drum width will reduce productivity, since three passes side-by-side will be needed, but selecting a compactor with a greater drum width will not necessarily increase productivity and may cause non-uniform panel density when excessive overlap in the rolling pattern occurs. Compactors that follow the breakdown compactor should also be selected so that the compactor train achieves the two objectives of density and smoothness, while maintaining the balance of productivity set by the paving train.
The breakdown compactor needs the mix from the paver to be low enough in viscosity for good aggregate particle rearrangement; that eliminates most of the voids. The compactor train tries to keep up, then, with the average speed of the paver. “Many paving projects employ a common practice that contractors should avoid,” comments Starry. “That bad practice is rolling too fast with the breakdown compactor, especially if you’re using a vibratory compactor.”
An unwanted result of such forced speed in compaction can be that density tests do not meet minimum requirements. “Some of today’s pavers can produce laydown rates of 100 feet a minute, or even faster, and it’s impossible for a single breakdown compactor to keep up with that speed. Even compactors with the highest vibration frequency are pressed to compact at a sufficient speed to keep up with a paver moving at that speed.”
Volvo DD138HFA compactors are often used as breakdown compactors. If the drums on the compactor are set to the first amplitude setting for compaction of a thin overlay, the rolling speed (using the automatic speed control system that provides 12 impacts per foot drum impact spacing) is 3.8 miles per hour. That means this compactor will move at 334 feet per minute when operated with the automatic (cruise control) system. Another way to control appropriate compaction speed is through the control of drum impact spacing. Using that same Volvo DD138HFA compactor, a high-frequency model with 4,000 vibrations per minute, and based on a large drum diameter that requires a spacing of 10 impacts per foot vibration for smoothness, rolling speed when vibrating is 400 feet per minute.
“Managing a paving project is a balancing act,” explains Starry. “It is necessary to equalize material production and delivery with laydown and compaction. Whenever one or more of these activities becomes unbalanced, problems occur. If sufficient mix can be produced and delivered to the paver to let it operate continuously at a speed of 100 feet per minute, what is a paving crew supposed to do to earn the available incentives for density and smoothness? The solution could be to have tandem breakdown compactors or two breakdown compactors working in echelon. These two compactors should have similar size, weight and vibration characteristics so that performance is equal and uniform across the width and length of the pavement. Adding a second breakdown compactor usually permits the entire paving train to move faster and achieve higher production, a productivity measure that can more than offset the added cost of owning and operating the second compactor. Added breakdown compaction also reduces the need for intermediate compaction and expedites the finish rolling process.”
Eliminating Material Segregation
At every stage when you are managing aggregate and asphalt, there is a chance that you will see material segregation, and there are places where you may not see the segregation. At the quarry, for instance, poor methods of stockpiling and inefficient loading can affect the product’s gradations adversely. At the HMA plant, methods can affect the quality of the material, especially perhaps in the storage and truck loading stages. On its way to your site, there will be some degree (no pun intended!) of thermal segregation and it may not be obvious. Thermally segregated material makes the material more difficult to compact and it will affect mat densities (which will affect mat life). The paving crew has most of the burden thrown at us by segregation, but there are ways to manage it.
Terex, through its Cedarapids asphalt equipment division, publishes an excellent article entitled “Segregation Busters,” and I will quote from it here but, as with Caterpillar’s booklet mentioned above, strongly recommend that you and your crews read and understand the whole thing. “First and foremost, the paving crews must be thoroughly educated in the correct paving techniques and how to properly use each piece of equipment on the site,” advises Terex-Cedarapids. “No paving equipment will end segregation unless the crew operates it in the correct manner.” One practical suggestion for the earliest stages of the paving work is to use the three-dump method for loading trucks. That means dumping two batches, at each truck end, and a final batch in the middle. That technique will reduce the runoff of larger particles to the sides and end of the truck.
Some other hints from Terex include regular cycling (where permitted by the specs) of the mix in the hopper wing; that will reduce large build-ups of segregated material.
Keep the hopper full, leaving as much surge as possible between truck exchanges to minimize cyclical segregation by allowing hot, uniform material from the next truck to blend with mix from the previous dump. A consistent flow of material to the spreading augers will prevent them from spinning too fast or too slow, either condition a cause of longitudinal segregation. Make the sure the conveying and spreading systems are timed to deliver enough (but not too much) material to the spread augers. Constant fine-tuning of the equipment (for the lead crown setting and strike off adjustment, for example) will help prevent segregation. And have the spread auger length. Using auger extensions and retaining plates helps control the flow of asphalt and prevents material from cascading to and segregating at the end gates.
The advice from Terex-Cedarapids ends with five keys to success, and they are all attainable and worth doing every time.
“No matter how segregation issues are addressed, through equipment selection, fine-tuning paver components, proper paving techniques, using a hopper insert, or a combination of these, remembering these five keys will prevent material and thermal segregation,” reads the educational article.
- Keep the material contained.
- Prevent dropping the material.
- Prevent dribbling the material.
- Move material in a smooth, uniform, and uninterrupted manner.
- And, above all else, don’t spill the material in front of the paver.
No, as mentioned earlier, good asphalt paving is not like squirting bug spray and hoping it hits the target. But doesn’t it turn out well when all the rules are followed, when the paving crews stay alert to changes in site conditions, and the helpful operating instructions of equipment manufacturers are followed?
