Steve Wilson, Manager, Marketing Services
BOMAG Americas, Inc.
Little by little, the experienced roller operator is becoming a rarity on today's job sites. This isn't for a lack of demand. In fact, due to more sophisticated asphalt mix designs, increasing quality expectations and ever-shrinking schedules, compaction requirements are more complex than ever. The problem, from a contractor's perspective, is simply a shortage of seasoned equipment operators and apportioning these limited resources in a logical fashion. Equipment such as pavers and graders offer a much higher learning curve and therefore typically claim the most experienced available personnel. But since rollers are relatively simple to operate, all too often it's the inexperienced laborers that are presented with the compaction task - and expected to do a decent job in the process.
Achieving adequate density, on either soil or asphalt, is much more critical than many give it credit. Sure, anyone can be shown the fundamentals of operating a roller and how to conduct a particular rolling pattern, but knowing the signs of under-compaction, over-compaction, or when a material is too hot or too cold to compact are lessons only gained through experience. And if learned the hard way, it can mean the difference between the eventual success or failure of the finished driving surface.
But having an experienced roller operator doesn't necessarily guarantee success either. Typically, these operators perform compaction based on knowledge gained from previous applications. Change one aspect of a job compared to a previous application, such as new asphalt mix designs or varying soil moisture content, and the compaction requirements change with it. Though the experienced operator can usually identify and adapt to these problems much faster than the inexperienced operator, there is always the hazard of insufficient compaction. And with so much riding on proper roadbuilding techniques, it's difficult to take a chance with any step of the process.
Therefore, based on ever-increasing jobsite demands and the lack of qualified personnel to deal with them, roller manufacturers have stepped up to the plate to introduce equipment to take the guesswork out of compaction. Instead of relying on operators to know how to handle the differing compaction requirements from one jobsite to another, manufacturers are offering compaction measurement and control systems for their rollers - and almost entirely taking the human element and error out of the process with "intelligent compaction."
Simply stated, intelligent compaction systems automatically measure and control the energy output of a roller's drum. First introduced in the early 1990s, several manufacturers now offer such technologies for either their line of soil compactors, asphalt compactors or both. And in regards to those manufacturers that aren't currently offering intelligent compaction systems, most are in the process of developing one.
Though each manufacturer's system is different in its specific execution, all generally are designed to work in the same way - measuring and reacting to the changing stiffness of the material being compacted.
Be it a soil or asphalt roller, intelligent compaction systems utilize devices - in some cases a set of accelerometers - to measure both the horizontal and vertical reaction of the drum to the material it's compacting. At first, when the material is soft, most of the drum's energy is directed into the work surface and little reaction is read by the accelerometers. But as the material stiffens, essentially meaning it's becoming more dense or compacted, less energy is transmitted into the surface and more is reflected back into the equipment. The result is a reactive, or erratic, response by the drum, which is read by the accelerometers and immediately translated by the cab-mounted microprocessor.
This measurement capability alone is invaluable to the experienced operator, but intelligent compaction systems take it a step further by actually controlling the output of the drum. As the material being worked continues to stiffen and it approaches optimum density, the microprocessor calculates the accelerometer-gathered data and begins to redirect the energy of the drum to avoid over-compaction. It does this through a process called vectoring.
When first working on a soft material that is capable of accepting compactive energy, the drum projects this energy into the material at a straight vertical, or up-and-down, angle. But when approaching optimum density, intelligent compaction systems will sense the change in the drum reaction and begin to redirect, or vector, the energy of the drum. Instead of direct blows, the microprocessor will begin to manipulate the angle of drum's energy to produce more glancing blows. As the roller continues to work on this same material and density builds, the microprocessor continues to vector the output energy - eventually going from a fully vertical impact to a fully horizontal impact.
When reaching the point that the drum is basically skipping back and forth laterally, the material receives very little influence from the vibration, but additional points of density may be picked up without over-compacting. At no point during the vectoring process is the amount of force that the drum produces changing. It is simply redirected to best cater to the material's density potential.
This automatic stiffness measurement and control of drum output goes a long way toward taking the guesswork out of compaction, helping to ensure optimum density results while preventing material damage. But this technology does nothing to prevent operators from working asphalt that's either too hot or too cold. That's why several intelligent asphalt compaction systems include a mat temperature sensing capability.
When working with sensitive asphalt mix designs, the temperature of the material can be critical during compaction. Roll on a material when it's too hot and there's a possibility that it could be pushed or shoved in the process. Conversely, if the material is too cold, it will no longer effectively receive compactive effort and the operator is wasting time. The asphalt mat temperature sensing system helps prevent both situations.
Measuring the asphalt temperature directly beneath the roller, a microprocessor calculates this reading and delivers it to the operator immediately. If the material is too hot, the system indicates to move the roller further back from the paver, allowing the asphalt to cool to a more compaction-receptive temperature range. If too cold, the opposite approach is advised. Though it requires some interpretation on the operator's part, the asphalt mat temperature sensing system combined with other intelligent compaction features helps ensure mistake-free applications. All that's really left for the operator to do is drive.
Compacting soil, too, requires some operator interpretation and common sense before starting. If the soil is too wet or too dry to be properly compacted with a standard roller, an intelligent compaction roller also will not be able to achieve effective compaction on the soil until conditions improve. If there is any question, however, rolling a test strip will verify whether the soil's moisture level is appropriate for compaction.
Of course, maximizing compaction density, minimizing material damage and avoiding mistakes are huge benefits associated with intelligent compaction systems. Anytime a contractor has to go back and rework a particular area due to under-compaction, over-compaction or a myriad of other problems, the time loss alone is enough to justify considering an intelligent compaction system - not to mention the extra expense and potential reputation damage. But there are several other benefits associated with intelligent compaction that are further driving the demand for this technology.
One of the added benefits of intelligent compaction technology is the protection of a contractor's capital equipment investment. When over-compacting a surface, be it asphalt or soil, the operator is not only running the risk of damaging the material, but the roller as well. As mentioned earlier, drum vibration energy is directed into the work surface when the material is still soft and receptive to compaction. But as the material begins to stiffen, it is less and less capable of receiving compactive energy and this energy is soon reflected back into the roller. Over time, this practice is a prescription for a short equipment lifespan.
Utilizing an intelligent compaction system helps alleviate equipment concerns. Because the energy adjusts as the material achieves density, there is less potential for energy to be redirected back into the roller. This reduces required maintenance and encourages a longer service life. And given the high costs associated with new equipment, the more one can stretch roller longevity - the better.
Time is Money
Another benefit of intelligent compaction systems is increased efficiency. For example, consider the roller operator encountering a job armed only with experience. He or she assesses the jobsite and determines that the surface will need five passes with the roller for proper compaction. After completing the passes, an independent testing device is used, either nuclear or non-nuclear, and it's found that adequate density has been achieved. But were all of these passes necessary?
Going back to this same job with a roller featuring an intelligent compaction system, the operator may find that the surface only requires three or four passes for optimum compaction. Additionally, after qualifying results against an independent testing device, there is no need to conduct independent testing because the roller does it on the fly. Therefore, instead of wasting one or two passes and taking extra time to test the material, the job is completed with less effort. Decreased effort means less time required on the job, less fuel to burn, less maintenance, less wear-and-tear hours on the equipment and a better chance of meeting deadlines or receiving bonuses. All this, in addition to knowing that the best possible compaction results have been achieved, offers quite the argument for implementing an intelligent compaction system.
Knowing that the best possible compaction results have been achieved in the shortest period of time is good, but it's better if one can prove it. Today, all contractors must agree to some type of guarantee on their work. This is typically specified in the bid and it locks the contractor into a particular quality or performance standard. Because of this, contractors should document the results achieved on every job in order to offer some sort of recourse in the event that a problem arises within the given warranty period. If this documentation isn't available or sufficient enough to prove that the job was done right the first time, the contractor may be required to come back and fix the problem at his or her own expense. To combat this, most intelligent compaction systems offer a way to document job-site results through the use of onboard printers.
Printouts provide the ultimate documentation for both soil and asphalt compaction jobs because they provide a readout of the entire area covered with the roller. In comparison with engineering core samples or on-site measuring devices that prove definitive compaction results only where the test was administered, intelligent compaction system printouts indicate the density achieved for the entire length of the lane traveled and the entire width of the drum. Therefore, they are able to show what was achieved for 100-percent of the compacted area. Additionally, the printouts exhibit a progression of compaction, from one pass to the next, they indicate the speed and frequency of the roller during the application, and include a time/date stamp to determine the job's pace.
Other than providing the contractor with peace of mind, these printouts act as qualifiers for achieved results and help verify that the job was done right. This can be crucial if sometime down the road a project fails and the county, state or federal authority comes back to the contractor for answers.
A Bright Future for Intelligent Compaction
Even with the unquestionable benefits offered through intelligent compaction systems and the fact that several manufacturers are implementing these capabilities into their equipment lines, few contractors in the U.S. are currently taking advantage of this technology. In Europe, on the other hand, roughly 80- to 85-percent of all rollers are sold with some form of intelligent compaction. Of course, some of this has to do with the longer availability of the technology, but for the most part, it's simply due to a lack of knowledge about what's available and what intelligent compaction systems could mean for the contractor.
Fortunately, this message is starting to gain traction domestically. The Federal Highway Administration (FHWA) has recently shown a lot of interest in intelligent compaction. Working with manufacturers that currently offer this technology, the FHWA is learning more about its capabilities. Recognizing the advantages of being able to optimize material compaction, the agency is now considering recommending the use of intelligent compaction systems to state agencies, which would eventually filter down to the contractors.
But the real challenge is education about this technology - getting past the 'that's how we always did it' mentality and illustrating what this technology is and what it can mean to those that use it. Show people a better way to do the job and they'll be interested. But show them how to do the job better, while making more money in the process, and they'll be convinced.