Compacting for the Long Haul

Aug. 28, 2014

How do you choose what shoes to wear? Everyday shoes work well for going to work or going out on the town on the weekend. Tennis shoes can be used for those same activities. But you really can’t wear everyday shoes when you work out at the gym. You can’t wear hiking boots at the gym.

The same might apply when choosing a compactor for road building compaction. One of the most important variables in road building”¦is compaction. Each layer of the roadway should support the weight placed on it. From the subgrade, to the base, to the paved base, to the final grading”¦each layer (or lift) needs to be built out of the proper material and be the right thickness and density. If just one layer is substandard, the road will fail. So the choice of compaction equipment is one that’s not taken lightly.

A number of factors help determine the machinery to be used. Your previous experience with these factors will also be significant in the selection process. You’ll consider the type of soil to be compacted, the method specification and/or the available equipment. Climatic and traction conditions, not to mention how well a machine will conform to the hauling and spreading operation, are all plugged into the equation. And, sometimes, how interested you are in standardizing the equipment in your fleet plays a role in your decision-making process.

One type of compactor will not do all things in all applications. Each type has its own range of operation and the kinds of materials on which it will be the most economical.

Four Basic Types Of Compactors
The experts at Caterpillar have broken it down for me into the four basic types of compaction machines.

First, there are “tamping foot soil compactors.” They have four steel wheels, or “drums,” that have tapered tamper feet. The wheels do not vibrate, so that means that they are static and work at high speed to give the energy that’s necessary for compaction. They’re large, and they generally are used on sites that are big enough to let them run at speeds fast enough for high production. They are typically for base construction tasks and embankments.

Next, we have “vibratory soil compactors.” These vary in size and have a single vibrating drum suspended in a weighted yoke. The vibration and the weight are the source of the energy that helps to rearrange and compact the soil particles and pack them into a more dense mass. They can have smooth drums (for granular applications) or padfoot drums (for cohesive and semi-cohesive soils). They are also used for base construction work and embankments and slightly more preferable to tamping foot soils compactors because they’re more versatile, easier to transport since they’re usually smaller, and they can achieve compaction targets more quickly on most soils because of the vibration.

Compactors of the third kind are “pneumatic tire rollers.” They have staggered tires that have a kneading action that works from the top down. The result is a smooth, sealed surface. They can be used on both soil and asphalt on usually small to medium size jobs, most often as a finishing roller, to smooth and seal the surface after a vibratory roller.

Finally we have the “tandem vibratory rollers.” They have two steel drums in which each one vibrates. These rollers are used on asphalt to bring the mat to a target density and smoothness. These models have different vibratory systems that can perform different combinations of frequency and amplitude for efficient compaction. Larger-size rollers are used on highway-class work, urban streets, and larger commercial jobs. Smaller rollers are for approaches, intersections, bike trails, utility work, and commercial jobs.

Jon Sjoblad heads up marketing and communications for Caterpillar Paving Products. He points out that Cat’s 815F Series 2 tamping foot soil compactor is designed for heavy-duty compaction. Its wheel design gives the operator four-wheel-width coverage in just two passes, with front and rear wheel tracking that eliminates gaps and overlaps.

For a vibratory soil compactor, Sjoblad says the Cat CS56B has an optimized drum yoke design, and the Cat pod-style vibratory system yields more amplitude.

As for pneumatic tire rollers and tandem vibratory rollers, Caterpillar features the CW14 for the former, and the CB54B for the latter. The CW14 is a standard nine-wheel pneumatic roller with an option for 11 wheels. And at 10 tons, the Cat CB54B does very well on different asphalt mix designs as well as other granular materials.

Earlier this month, Case Construction Equipment rolled out a couple of new high-frequency asphalt compactors. The Tier 4 Interim, double drum DV209 and DV210 give operators the ability to dial compaction performance to match the thickness and required density of each lift. Their operating weights are both above 21,000 pounds, and both have a drum width a little greater than 66 inches, making them just right for most midsize and large road projects.

Credit: Multiquip
The AR13HAR from Multiquip offers
hydrostatic drive and variable speed
control for forward and reverse speeds.

Choosing The Right One
Katie Pullen is a brand-marketing manager for Case Construction Equipment. She points to a number of variables when it comes to choosing the right roller for each compaction job. First to consider is whether you’re compacting soil or asphalt, followed by such other factors as the width of the area that needs to be compacted, the final density required, the grade of the area, the type of asphalt mix, soil type, and the materials that are underneath the lift being compacted.

For cohesive materials like clay, Pullen says a pad-foot drum is required, since the cohesive soils tend to stick together and slip over each other. The pad-foot drum is able to compact these types of soils by shearing them.

When compacting more granular materials such as sand and gravel, a smooth drum is usually employed. It lets the material lock together like a puzzle piece. And according to Pullen, having optimum moisture in the soil is critical here, because it allows for movement without friction.

The next thing to consider is drum size. Contractors are trying to achieve a required compaction density in the least amount of passes, and that doesn’t always mean that bigger is better. Case Construction Equipment has seen situations in which contractors have a large road job and automatically think an 84-inch drum would get the job done faster than a 66-inch drum. But by minimizing overlap, and avoiding over-compaction, the 66-inch configuration could be the best one for getting the job done.

Another factor Pullen advises to give careful consideration is the weight of the machine. The static weight plus the force generated by its eccentric motor creates centrifugal force, which can determine compaction depth. Deeper lifts will require a heavier machine that has a greater centrifugal force. The considerably thinner lifts of asphalt call for frequency over centrifugal force. The distribution of a compactor’s weight is also important. Some models in the Case fleet have the engine mounted in the rear, with the water tank mounted to the front. The near equal weight distribution results in the same compaction weight going down on the front and rear drum.

And speaking of water tanks, Jonathan Williamson, director of marketing for Multiquip Inc., tells me asphalt applications require that the equipment is outfitted with a water tank to lubricate the roll or plate as it passes over the material. This keeps the hot material from sticking to its surface. Williamson adds that gradability is another consideration, since compactors are also used for grades on highway on- and off-ramps. While building a road, if there’s a drain that has to be compacted around, then a rammer or smaller forward plate would be your only option. The outer edges of asphalt surfacing on roads are sometimes compacted with smaller forward plates because it’s so physically near the curbs and sidewalks.

For compacting cohesive soil types in confined areas, Williamson says Multiquip has its MQ MTX-series rammers series, which offers Honda GX100 and Subaru ER12 engines and that have working widths of 10.4 inches and 11.2 inches, impact force ranges of 3,064 ft.-lbs. to 3,505 ft.-lbs., and operating weights from 141 to 181 pounds. Smaller jobs with granular soils and asphalt can us Multiquip’s MQ MVC plates, which are equipped with Honda GX engines and working widths from 14 inches to 20 inches, producing centrifugal forces of 2,275 to 3,417 ft.-lbs. As for their heavies that can be used for compacting asphalt and granular and semicohesive soils, Multiquip offers the versatile MQ double-drum walk-behind and ride-on rollers. The AR13HAR is a 36-inch ride-on roller with a Honda GX630, vibrations per minute (VPM) of 4,000, centrifugal force of 3,100 ft.-lbs., and a 40-gallon water tank. The MQ MRH series of 26-inch walk-behind rollers have either Honda gas or Kubota diesel engine options and centrifugal forces of 2,428 ft.-lbs. or 5,300 ft.-lbs., and 3,300 VPM.

Williamson says most customers who order from Multiquip already know what they want because they’re experienced and familiar with their compaction equipment. But for those who are not so well versed, he says, “When we get a customer call who is not too familiar with MQ and asks for a recommendation, they usually give us a weight class that they’re looking for and then we give them a few pricing options around that general weight class. Those customers then make a purchasing decision based on their perceived value of the weight criteria they gave us, and the different pricing options.”

The experts at Sakai America Inc. say there are a number of situations that will require the use of different rollers. According to Josh Steele, marketing manager for Sakai, some of the most common are compacting on sandy, semicohesive soils that may require different methodology than cohesive clays and silts. There are times when you just can’t use a vibratory roller on a sandy base material. He also says the surrounding infrastructure has to be taken into consideration. For example, when paving around a historic downtown, utility lines, large residential areas or on bridge decks, the job will often be deemed as “no vibe,” meaning either a static roller or an oscillatory roller should be used. That will avoid damaging the surroundings as well as reducing the noise involved in rolling.

For compaction on a sandy base, Steele says, “Any of Sakai’s rollers will work in static mode; however, we also offer a static, three-wheel roller: the R2H-2. It has a compaction width of 83 inches and a fully ballasted weight of 31,625 pounds, with a PLI of 365. Our ND Series of Oscillatory rollers features drums that can switch from oscillation to vibration or static on the fly. This gives greater versatility without sacrificing performance by only operating one drum dynamically at a time. The SW652ND has 58-inch drums and weighs in at 16,865 pounds. The SW770ND has 67-inch drums and weighs 23,810 pounds. The SW850ND offers 79-inch drums and a weight of 29,480 pounds.

The Power Of Two (Or More)
Once asphalt is laid out by the paver screed, breakdown rollers remove the majority of the air voids from the pavement structure. This is why you should use the kind of compaction equipment that is of the proper type and size to uniformly cover the entire pavement surface. Choosing the breakdown compactor is often done on the basis of its rolling width and the optimizing uniform coverage of the paved panel. Picture this: If the laydown width is 12 feet, a compactor with a 78.7-inch drum width can cover the width of the panel in two passes side by side with sufficient overhang of the edges of the drum, and an overlap in the center.

Dale Starry, the compaction equipment product manager at Volvo Construction Equipment, says the company’s DD138HFA compactors are used as breakdown compactors. If the drums are set 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. You can also establish a compaction speed by controlling the spacing of the drum impacts. A high-frequency Volvo DD138HFA with 4,000 vibrations per minute, with a large drum diameter that requires a spacing of 10 impacts per foot vibration for smoothness, the rolling speed while vibrating is 400 feet per minute. In a five-pass rolling pattern, the DD138HFA has an average speed of about 67 feet per minute.

A paving project can be a bit of a juggling act. The timing is essential between material production and delivery, laydown and then compaction. One thing that Starry suggests is to have tandem breakdown compactors or two breakdown compactors working in formation. The two should be similar in size, weight, and vibration specifications so that you get equal and uniform compaction across the length and width of the pavement. A second compactor usually allows the entire train to move faster and achieve higher production, which can more than offset the added cost of a second breakdown compactor. Aesthetically speaking, there’s uniformity and consistency of density and in smoothness readings. And since they don’t have to run at extremely fast speeds to keep up with the paver, the new pavement is superior.

Specify Your Specification
Choosing the equipment means choosing the type of specification that is going to be used. According to the Caterpillar Compaction Manual, the contractor should determine what limits have been put on material placement, preparation, and compaction. The general types of specifications used to establish minimum standards of compaction for embankments, bases, or asphalt courses are: Method Only, Method and End Result, Suggested Method and End Result, and End Result.

The Method Only specification decides the type of equipment to be used, the number of passes, the roller speed, layer thickness, and other details. It does not give targeted results. This is typically the least-wanted specification because it forces the contractor to get certain equipment, and use it, even though other equipment might produce better results and save money.

The Method and End Result specification is even more restrictive. Here, a contractor would have to achieve 95% AASHTO, but use a minimum number of passes, on a specific lift thickness. It requires the contractor achieve an end result, by using equipment that may not necessarily be able to achieve that End Result.

A far more flexible arrangement is the Suggested Method and End Result specification. The experienced contractor has room to bring his experience and initiative to bear. At the same time, the less experienced contractor is given helpful guidelines.

With the End Result specification, only the final compaction conditions are specified. That means the contractor is free to choose for himself the equipment that will do the job as well as enhance the productivity of other equipment. Strict testing procedures are involved, but the development of compaction technology is allowed, which speeds up the compaction process and reduces cost.

Road Maintenance Compaction
The professionals over at Lycox Enterprises Inc. tell me they do a different kind of compaction business. Their equipment is more specialized for road maintenance. Udo Lyngby-Cox is the general manager at Lycox. He explains that road construction and maintenance both call for different compactors. In new road construction, you use self-propelled vibratory, sheep’s foot, and smooth-drum compactors and sometimes static pneumatic compactors.

Lyngby-Cox tells me, “In road maintenance you use 99% static pneumatic compactor. The advantage you get from our equipment is that you can connect our Walk ‘n’ Roll packer/roller to the rear of the motor grader and use the hydraulic system from the motor grader for the down pressure. That means you don’t need the static weight, and one man can do two jobs at the same time.”

Lycox has a new line of Walk ‘n’ Roll packer/rollers. They are the WR75 Series 3, the WR90 Series 3, and the WR105 Series 3. Depending on the model, they come with 10 to 14 easy access, 7.50- by 15-inch smooth roller tires mounted on individual 6,000-pound hubs, five to seven heavy-duty independent walking beams, heavy 12-inch by 6-inch single-box tubing mainframe with a high arch gooseneck and machine weights ranging from 2,750 pounds to 3,800 pounds. When one of these is attached to a motor grader, the Walk ‘n’ Roll allows the operator to blade and compact the road in one operation. Lyngby-Cox says the process can conserve water when it’s dry and seal out moisture when it’s wet. He calls his compactors budget saving tools that lead to better roads and less road maintenance.

Before All Is Said And Done”¦
Here are a few more things to consider from Case Construction Equipment’s Katie Pullen. Serviceability is one. Look for compaction equipment that has service items in areas that are easily accessible. If a machine goes down and you can’t service it quickly, your deadline would be put in jeopardy. Look for equipment that has scheduled maintenance. It’s important in terms of convenience and critical if the compactor needs to be serviced in the middle of a job. And then, Pullen says, look for ease of operation and comfort. While sitting in the cab, make sure you have visibility of the drums (sloped hoods to the rear are an asset on soil compactors). Check for seat maneuverability (sliding and rotation) in the cab for both forward and backward operation, and see if there are convenient intuitive controls to adjust amplitudes and frequencies on the fly. Construction equipment is general is becoming more and more operator and user friendly in terms of comfort.

Now of course there’s a whole science to the art of road building compaction, complete with mathematical formulas, frequencies, amplitudes, applications, depths, and physics. I’ll leave the specifics to the experts. There are hardened rules to follow. But keep in mind, the machines may overlap in the various applications, and you may even see some compactors working on materials that are being used outside of their intended purpose. That’s why contractors and operators are the most important variables. They must know the material, the requirements, and the applications of each type of compactor and then figure out how to approach the job in the most efficient way possible.

A Word About Intelligent Compaction
Intelligent compaction can be described as having three things: measurement, positioning, and data analysis. The ability to know in real time just how close you are to reaching compaction target requirements boosts efficiency and saves time and money.

One of the measurement technologies being used by a number of companies is compaction meter value, or CMV, to measure compaction. It uses an accelerometer that’s mounted on the vibrating drum and measures the ground’s response to being struck by the vibrating drum. The technology is most often used with smooth-drum vibratory soil compactors.

Caterpillar has developed an exclusive measurement technology it calls MDP, which stands for “machine drive power.” Jon Sjoblad says MDP measures compaction by measuring rolling resistance. In other words, it measures the amount of energy it takes to propel over the ground. The more compacted a material is, the less energy it will take to move the compactor over it. The technology can be used with smooth drums of padfoot drums, and it can take measurements with the vibratory system on, or off. MDP has a broader application range than accelerometer-based technology.

When it comes to asphalt applications with tandem vibratory rollers and pneumatic tire compactors, Cat Compaction Control on asphalt measures the temperature of the mat, as opposed to measuring compaction. If you add positioning capabilities, it can map those measurements as well as measure coverage and the number of passes the machine makes. With the operator keeping track of and in control of the data, there is a consistency and uniformity to the compaction that results in a smooth, high-quality, long-lasting road.

Combining compaction measurement technology with positioning technology and data analysis, you create a very efficient, cost-effective compaction tool.