Hard Pressed

May 1, 2006

Compaction of soil or asphalt must be done right. Over-compaction can be as harmful as under-compaction. You can easily tell the results of under-compaction in structures: cracks in the foundations and walls, windows that won’t open and shut easily, doors that stick. They are careless results that can become expensive. With over-compaction, the danger is that the contractor will have to restart the project from the beginning—not at the beginning of compaction but at the beginning of excavation. The reason is that over-compaction can alter the structure of the ground, including sub-bases you have placed. The best approach is to do all compaction as specified, with no shortcuts and no extra passes “just to be sure.”

How can you prove you did the job right? For several years you have been able to measure the compaction performed by your rollers on asphalt and soil. One of the best improvements in recent years has been the availability of onboard devices that not only can tell you where you need to do some work but also can record what you have done. Manufacturers promote their own style, but what they do is similar. The devices measure the stiffness of the compacted material; they note changes and guide the operator to the desired or specified density. Your compaction equipment today can not only do the job right but prove it, too.

The Value of Intelligence in Compaction
We asked Steve Wilson, product development and support manager for Bomag Americas Inc., what words of wisdom he would offer concerning compaction. He has many years of good experience with compaction equipment of all kinds; his insight seemed interesting and helpful enough to pass along here. “Contractors specializing in soil or asphalt compaction could probably talk a lot about confidence and success, depending on their individual experiences,” says Wilson. “What they may say is that confidence comes in varying degrees. Until recently, a contractor’s confidence relied entirely on the expertise of a roller operator, in combination with density tests taken from random positions on a job site. After completion of several successful projects, a contractor’s confidence would justifiably grow. But even with the best work done by the best workers, there was simply no way to know for certain that optimum compaction had been achieved over the entire area of a project.

Photo: Sakai
Photo: Dynapac

“From time to time,” he continues, “projects failed due to improper compaction of a surface or sub-base. In the construction business, failure can mean having to redo small or large portions of a project, leading to wasted time and expense. It would be ideal for a person [in any profession] to possess a tool that could help guarantee success. Fortunately for contractors in the compaction business, there is such a tool in the form of rollers with Intelligent Compaction systems. This technology has begun to take off in the United States in the past several years, and for good reason, because it gives the contractor 100% confidence that the job was done correctly. A closer look at Intelligent Compaction will illustrate exactly why the system is so valuable.”

To fully appreciate the advantages, it’s important to first understand the fundamental differences that the technology presents in comparison with standard rollers. Work on a job site begins much the same way with or without Intelligent Compaction rollers. Both situations may involve making successive passes with a roller over a test strip or test lane of material. A nuclear or non-nuclear testing device is then used to determine if the specified density for that particular job has been achieved. But in terms of what the two roller types are trying to accomplish with this process, the similarities end there. When using a basic roller, the test procedure is intended to define the maximum number of passes needed to achieve proper compaction, with the assumption being that, if there is no deviation in the material and from that rolling pattern and operating speed, the same end result will be achieved throughout the entire project. With Intelligent Compaction, the initial control test is not performed to establish a set number of necessary passes to be made but rather is used to determine an energy measurement that correlates with the specified density to be achieved over the whole area. As it moves along the work surface, the roller measures vibratory energy reflected from the material being compacted. The energy numbers attained have a direct correlation with density readings from a proven testing device. Most of these systems include some form of visual display that shows density measurements as the roller moves over the material. All the roller operator has to do is achieve the same energy value from the material over the entire surface.

“A conventional roller isn’t able to give any quantifiable feedback, so the operator must rely on observations and experience to make compaction as uniform as possible,” observes Wilson. “And in many cases today, unseasoned roller operators are often forced into duty, making it more difficult to achieve consistent compaction. Once the job is complete, density tests are taken manually from randomly selected positions. Because the testing is random, it is impossible to guarantee uniform or proper compaction over the entire job site. Even if every tested position was compacted properly, there’s no way of knowing if soft spots exist in other locations. This is in sharp contrast to an Intelligent Compaction system, which monitors the compaction process consistently over 100% of the area that’s being compacted. Whether it’s a 40-year veteran or a first-time roller operator, no guessing is needed. The operator actually knows when proper compaction has been achieved in any given location.”

Photo: Bomag
Vectoring, or changing the angle of the energy delivered from the drum, allows the roller to adjust the output energy applied to the work surface.
Photo: Bomag

These systems have the ability to achieve consistent, uniform compaction because they are intuitive machines. In fact, the design of the system requires that it be built into the roller to function because it is the roller itself that monitors the vibrational energy, or stiffness, of the compacted material. Material stiffness equates directly to density. Based on the density readings that the machine receives, it will automatically adjust the output energy that the roller’s drum is putting into the work surface. The roller makes this adjustment through a process called vectoring, which involves changing the angle of the energy delivered from the drum. For softer areas, the drum will direct energy at a straight vertical angle. As the material stiffens, the roller directs the drum’s energy at a more horizontal angle, eventually becoming fully horizontal as the surface reaches optimum compaction. By delivering the appropriate levels of compactive effort into the material at all times, Intelligent Compaction not only ensures that the required density is achieved, but also prevents material from being over-compacted.

We have already noted that over-compaction can be just as problematic as under-compaction. Once the material has achieved maximum density, continuing to deliver additional force can be destructive and cause the material to lose density. The feedback from some Intelligent Compaction systems will let the operator know that enough work has been done on a given area. For other systems, the roller adjusts force automatically even if the operator isn’t paying attention. In addition to avoiding the damaging effects of over-compaction, Intelligent Compaction also prevents roller operators from wasting time in areas that no longer need attention. Knowing exactly when to stop allows the operator to move on to another part of the surface where further compaction is needed. For a contractor, this immediately translates into cost savings for labor and fuel, and it also prevents unnecessary wear and tear on the machine.

Intelligent Compaction rollers can expedite jobs even further because the system’s design allows it to physically produce more energy. Conventional rollers have a dual amplitude system and are essentially limited to two different force values that can be delivered. Only one amplitude is available on Intelligent Compaction systems, but it is a higher amplitude that produces more force. Unlike a conventional roller, it uses the vectoring process to regulate how much of that force is actually delivered to the material. The roller is not limited to one force value but rather can adjust to any value necessary. In some cases, rollers with Intelligent Compaction will produce up to 30% more centrifugal force than their counterparts without the technology. Putting 30% more energy into the material means achieving density about 30% faster. For example, a job that previously took 10 passes to achieve proper compaction could potentially be completed in only seven passes. Depending on the job and the material to be compacted, the higher energy output provided by an Intelligent Compaction roller could also allow for compaction of thicker material. Being able to place deeper lifts in fewer layers is another way for a contractor to save time and money. With fewer passes required to compact the work surface—or greater lift depths that can be placed and compacted—Intelligent Compaction rollers can achieve the end result much sooner, allowing the operator to move forward to other areas of a project.

Standing by Your Work
The day-to-day cost savings that Intelligent Compaction affords a contractor certainly help justify the cost of purchasing the more expensive system. Also important is what the technology provides to the contractor well after the last day of the job. The systems not only monitor the compaction process for the purpose of adjusting to changing densities of the material, but they also provide documentation of the work done over 100% of the project.

This documentation can be stored electronically or printed on a piece of paper. The printout shows the travel speed of the machine, the distance traveled, the force value produced by the machine, and the compacted material’s vibrational energy reading. But more than simply showing raw data, the information illustrates a progression of compaction results achieved throughout the project. This gives the contractor documented proof to qualify that the work was done correctly in the event that other problems are discovered at a later time. In the case of a failed road, excavation could determine the exact cause of the failure. If it’s found that a certain area did not receive proper compaction, the contractor could be held at fault if he can’t prove the job was done correctly. If a roller without an Intelligent Compaction system was used, the only available documentation is the random density testing from the site. And if the failed area wasn’t tested, the contractor could be held financially responsible for every aspect of the road’s repair.

The performance and accuracy of Intelligent Compaction has earned the technology high credibility in the construction business. Documented results from these systems are widely accepted by departments of transportation at the federal and state levels. In situations where a warranty issue comes into question or goes into litigation due to some sort of later failure on a project, contractors have the ammunition to show that their end of the job was done correctly. A contractor’s job will never be stress-free. With looming deadlines and unexpected setbacks, there will always be some reason for concern. But Intelligent Compaction can alleviate several of those worries and provide versatility in the process. An asphalt contractor could utilize the technology to test the density of an existing road or previously compacted base before placing and compacting asphalt, providing further insurance that a project will succeed. The technique also allows a contractor to work in a variety of different materials and applications, creating more opportunities for profit. “Getting good value out of a product isn’t always easy,” says Wilson. “But Intelligent Compaction systems take care of this on their own by providing the ability to produce improved, uniform results. And the subsequent profits, labor savings, and the insurance of documentation all create direct monetary benefits for the contractor. It’s these benefits and the end result of proper compaction being achieved over 100% of the job that make for a truly confident contractor. And with that full confidence firmly in place, success is sure to follow.”

Compacting Small Areas
It would be unfair to think of compaction as something that is only done by big rollers smoothly going along the street or highway. Every day there are thousands of compaction jobs done where the area to be compacted may be less than 10 square yards. Think of the ground preparation for sidewalks, garden paths, driveways, and golf course paths. Think of the compaction needed around newly installed lampposts or telegraph poles. Sometimes the area to be compacted is no bigger than that of a card table! Or it may be a trench you must fill and compact. And, of course, there are just the right machines to do it perfectly for you. They may be called rammers, tampers, plate compactors, reversible plates, vibratory plates, walk-behind rollers, and (slightly bigger) ride-on rollers. Bearing in mind that compaction is a science, you should be aware of the nature of soils and the best machines to compact them. Tampers (which can be impact or vibratory) work well on cohesive materials because they force out the water and air voids. For granular materials, tampers work well if they are the vibratory type, letting the particles in the soil rearrange themselves until they reach the required density.

Photo: Multiquip
Photo: Multiquip

We mentioned different types of soil. What you are most likely to meet are coarse aggregate, fine aggregate, and rockfill. The latter contains the bigger stuff, cobbles over 2.5 inches and boulders up to 4 inches. You’ll require heavy equipment to do that, machines with a high compact force. Fine aggregate looks and sounds the easiest, but it isn’t. The clay and silt in fine aggregate have great cohesion. Again, heavy plates or rollers will do a good job; the rollers could have smooth or padfoot drums. Coarse aggregate is sand and gravel, nothing bigger than 2.5 inches, and it is popular for sub-bases, backfills for trenches, and building foundations. Medium and heavy plates compact coarse aggregate well, as do single-drum rollers and vibratory tandem rollers. From talking to contractors, simple-to-compact coarse aggregate is the most common soil at construction sites, but don’t bank on it! Check your soil before you start pressing down.

Photo: Bomag

At this stage we should mention some excellent sources for information and advice on all aspects of compaction. It is a science, and those who have studied it well have many good ideas to impart. Check with these manufacturers for practical advice on the best equipment to use for your specific materials and projects: BOMAG, Vibromax, Hamm, Caterpillar, Multiquip, Stone Construction Equipment, Wacker, Ingersoll-Rand, Hypac, Allied Construction Products, and Ammann.

One of the leaders in smaller compaction equipment is Multiquip. Take its MVC-88 plate compactor. It weighs just over 200 pounds, and the size of the plate is 19.7 by 20.7 inches. How much compaction force can such a minor machine produce? 3,450 pounds. It can travel forward at 72 feet per minute and cope with 25% slopes. Multiquip’s MVC-80 Series vibratory plate compactors now reduce vibration by 50% contrasted with its predecessor models. With a new handle design, operators report less fatigue and better productivity. These plate compactors are excellent for both granular soils and asphalt; you can probably imagine many residential jobs where they would be well suited. You can lift it by using the lifting bale (without lifting strap or hook-up), or two people can load it onto the pickup. There are four models in the MVC-80 Series.

Wacker offers both vibratory asphalt plates and vibratory soil plates. For asphalt, there are nine models, with operating weights ranging from 142 to 194 pounds. Plate sizes vary from 20.5 by 14 inches to 23 by 19.5 inches; and the centrifugal force goes from 2,470 to 3,375 pounds. Wacker tells us that the compression depth for all models is 12 inches and the compressions area (square feet per hour) varies from 5,900 for the smallest VP 1135AW to 9,750 for larger models like the WP 1550W. The vibratory soil plate compactors (12 of them) range in operating weight from 137 to 250 pounds. Other aspects are similar to those of the asphalt compactors. Wacker also offers rammers and reversible plate compactors, which are especially suited to confined areas with granular and mixed soils. Reversible plates work on most soils but have been especially popular for semi-cohesive soils. They have a variable forward and reverse speed (up to 92 feet per minute).

On the vibratory plate compactors from Arrow-Master, the manufacturer emphasizes the reversible guide handle and the double formed leading edge (for better climbing). The P2000 is for both soil and asphalt compaction. Its operating weight is 210 pounds, and the overall plate size is 21.5 by 21.6 inches. The effective plate size is 244 square inches. This model delivers 2,700 pounds of centrifugal force and can travel forward at 60 feet per minute. The larger P5000 is for soil compaction only. It weighs 245 pounds, has the same overall and effective plate size as the P2000, but can travel more than twice as fast at 135 feet per minute. Its centrifugal force is 5,300 pounds.

There is another way to compact soil. Attachments, sometimes called compactor drivers, can be mounted on your already-available carriers. Allied Construction Products makes the Ho-Pac vibratory compactor/drivers. They use an eccentric, rotating weight that gives vibrations and impulse energy. Contractors who have found this method suitable for their applications say it is most effective. It has been used for trenches, slopes, and general excavation compaction. With its vibration, it’s probably most suitable for granular soils and can compact to densities more than 95% Proctor. (The Allied CP drivers are also used at landfills for compacting waste.) Check with the manufacturer to see how much carrier you need for different sizes.

Similar to the last-mentioned compaction tool are the RAV Series structure backfill, pipeline construction, and work in confined spaces such as shafts (where the combination of boom and attachments gets the job done quickly). The RAV-700P weighs 1,543 pounds and produces 18,450 pounds of centrifugal force; it works from excavators of 7 to 20 tons. The RAV-1000P weighs 2,205 pounds, has been most effective when mounted on 18- to 40-ton excavators, and gives 24,750 pounds of centrifugal force. There’s a much smaller model, too, the RAV-400P for use on 2- to 15-ton excavators. It weighs 882 pounds and produces 11,700 pounds of impact force. You can change these compaction attachments from the cab.

Whatever you must compact and prepare for the next stage of construction, the right equipment has been engineered and developed, as has the most helpful advice about the best way to approach the job. This is certainly one area of construction where our manufacturers have spent much time and money on the science of the work and the best techniques to achieve success.