Improvements in Laser and GPS Technology Result in More Precise Paving
Laser and GPS technology continues to improve, resulting in even more precise paving.
People have an amazing ability to adapt to and even become blasé about technological wonders. Things that we take for granted (flying, television, cell phones, the Internet) would seem like something out of science fiction to anyone 100 years ago. Now we can’t imagine what life was like without them. The same is true of lasers and orbiting satellites. Once relegated to speculative stories of ray guns and space ships, these inventions have become workaday tools that every construction site depends on. The unique properties of lasers make them perfect for distance measurement and provide a level datum to guide building and earthwork projects. It guides the movements of men and machines; establishes straight and true lies of construction; and levels and accurately grades, maximizing job efficiency.
Global Positioning System (GPS) satellites allow any person, tool, earthmoving equipment, or the business end of this equipment (dozer blades, excavator buckets, etc.) to determine its position is space with a high accuracy. Both are necessary on any construction site, but especially so when lying out and paving a roadway or highway. Each is involved in every stage of the construction process. And while each technology is decades old, there are continued refinements and improvements. As the technology advances, the use of thee tools continues to spread throughout the industry, finding wider and new applications.
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Road Surveying Tasks and Procedures
Construction surveying follows certain procedures to ensure the accuracy of measurement and quality of construction. Each roadway construction project includes the following tasks, each of which requires is own stage of surveying.
The first task for any construction survey job is to find, confirm or if necessary establish one or benchmarks to guide subsequent surveying efforts. A surveyor’s benchmark is a physical marker set into the ground at a precise northing, easting, and elevation. Anchored into the ground and protected by a pipe sleeve and cap cover, a benchmark rod established a regional datum, and fixed reference for all local surveying.
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Even advanced GPS and laser systems should reference a physical benchmark to establish the basis for determining location and elevation across the worksite, or along the proposed road alignment. The calibration of a GPS unit or establishing a basis for a laser leveling system is performed by taking a measurement off of the benchmark and comparing to its established elevation. As work progresses across the site, measuring and leveling systems can utilize temporary benchmarks established relative to the base location. If necessary, in the absence of established and recorded benchmarks, a known object such as a manhole lid or a building foundation can be used as relative benchmarks for localized surveying.
After establishing benchmarks, the initial task directly involved with roadway construction involves the earthwork needed to establish a subbase for road construction. The location and direction of this work is determined by the designed alignment of the proposed roadway. Road alignments consist of level straight lines, sloped straight lines, cross grades, intersections, circular turning curves, and vertical parabolic curve.
Measured in a series of 100-foot stations, the locations and elevations of this series of lines and curves (portrayed as horizontal plan views and vertical profiles) determine the relationship between the road construction and existing topography. A comparison of existing topography and proposed road grades will determine if excavation is required oradditional fill soil is needed. Once alignment and earthwork needs have been calculated, the surveyor can use the centerline of the proposed road to establish offset limits a some distance beyond the limits of the roadwork for the clearing and grubbing of vegetation and objects from the proposed road base.
Once the ground has been cleared, mass earthwork along the established line and grade of the proposed roadway can begin. Instead of traditional ground staking with marked wooden stakes to guide slope grading, laser/GPS systems utilize digital terrain models (DTM) to guide earthmoving equipment. Utilizing this information grade controls can be established that define the surface of the sub grades, subbase, and pavement surface of the roadway. Surfacing completes roadway construction with the installation of an asphalt or concrete surface pavement.
In addition to the roadway proper, concurrent survey is required for drainage systems and ancillary structures were needed. The locations and alignments of drainage features (ditches, curbs, culverts, etc.) are established to ensure that ponding of water does not occur. The locations of other miscellaneous construction features and ancillary structures such as bridge abutments are also surveyed. And lastly, after construction is complete, post-construction certification surveying is performed.
The basics of surveying with a laser remain unchanged, even with advanced technology. Both laser and traditional levels use horizontal line of sight to measure elevation at a particular point by measuring the sited height of a graduated measuring rod set at the point in question. While traditional levels use the human eye, aided by a telescopic sight, to perform the reading and laser level uses a beam of light, which is read by a sensor on the road. The level itself is set on a tripod whose legs are set to provide a stable platform which either is a manually adjusted traditional level with a bubble level or a self leveling laser.
A Caterpillar paver using grade and slope control
Types of Pavement
There are two types of pavement used to construct roadways: rigid concrete pavement or flexible asphalt pavement. Asphalt pavement is covered with a wearing surface of asphalt, usually Hot Mixed Asphalt (HMA). Asphalt cement is made from crude oil and used as a binder for a mix of stone, sand, and gravel to make bituminous concrete. These aggregates can make up to 95% of the asphalt material by weight. HMA is manufactured at a facility offsite and then transported to the roadway project by trucks. There, the HMA is dumped into hoppers and fed to paving machines. The pavers then place and spread the asphalt, evenly applying it to its subbase course. Once spread over the roadway, it is compacted in place by smooth drum rollers or pneumatic tire compactors. Once cooled, the final finished grade can be surveyed and re-rolled if necessary to achieve design elevations and slopes.
The asphalt surface is only the top layer of the roadway course. Starting with the natural subgrade a compacted subgrade consisting of native soils 6 to 12 inches thick. If subgrade soils are weak, they can be strengthened by the addition and mixing of lime, cement, or fly ash. On top of this is a subbase course of large gravel 4 to 12 inches thick, and then, a base course of asphalt concrete also 4 to 12 inches thick. Then comes a prime coat to allow adherence for the next layer, a binder course of asphalt concrete 2 to 4 inches thick. This is finally covered with a tack coat of asphalt to adhere the final 1- to 2-inch surface course. Each one of these layers needs to be placed and compacted in place as required by the construction specifications. And, to ensure proper construction, the surface of each layer needs to be certification surveyed to record each layers slope, cross grade, and thickness.
Unlike flexible asphalt cement pavement, concrete cement pavement is a rigid structure. As such it disperses vehicle loads over a wide area, and in doing so requires a less foundational support. So, concrete pavements lack the same thick subbase as asphalt pavement. In fact, the aggregate base under the pavement is more for leveling and drainage than structural support. However, the concrete will be reinforce with steel rebar against flexure caused by repetitions of axle loads form trucks and heavy vehicle. This reinforcement can consist of extensive rebar or wire mesh serving ore as reinforcement against shrinkage and temperature.
Pavement Construction Machinery
A fleet of equipment is required for constructing roadway pavement. Each of these will have unique capabilities, functions, and laser/GPS guidance needs. At minimum, a road pavement construction fleet should consist of:
Reclaimers are used to grind up and pulverize old pavement that needs to be removed and replaced or do double duty to stabilize subbase soils. The end product can be Recycled Asphalt Pavement (RAP), which can be fed into a hopper for grinding and reuse as new pavement. These machines don’t necessarily need laser or GPS guidance.
Milling Machines have a similar function as reclaimers. However, they scrape off just the top layer, with the goal of establishing a rough surface with grooves that allow for easy adherence in preparation for further pavement application. These grooves can effectively increase the resultant surface area by up to 30% over its flat dimensions. In doing so,they remove old, cracked asphalt pavement. Since this operation has to be very precise, milling machines are typically outfitted with automated grade controls. By using these controls, the cross grades and longitudinal slopes of the pavement can be restored with high precision. Laser and GPS guidance can be used on these machines. Sweepers remove dust and debris or clean up a surface after it has been milled.
Graders and Scrapers are utilized to spread and place aggregate subbase material at a proper depth and grade. Again, this operation can be enhanced by the use of lasers and GPS guidance systems.
Dump trucks to haul HMA to the project area. The most expensive and useful version is the live bottom (flo-boy) dump truck. It uses a conveyor belt system to discharge HMA in a smooth flow without having the material segregate. Similar in function to dump trucks, material transfer vehicles serve as intermediaries between dump trucks and pavers. They also provide surge volume to handle frequent load deliveries. Neither dump trucks nor material transfer vehicles require laser or GPS guidance.
The key to the paving operation is the asphalt pavers themselves. These are self-propelled machines that lay down asphalt pavement and level it with a floating screed. HMA is pushed to the rear of the machine by conveyor belts, spread by augers, and leveled with the screed. The screed is the most important element as it determines the grade, thickness, and slope of the placed pavement. Laser and GPS guidance can be critical to the operation of this element.
Initially, up to 85% of an asphalt layer’s compaction is achieved by initial placement with the screed. However, once in place and spread over the road surface, asphalt concrete requires additional compaction. This is done by either a steel drum roller, or by a pneumatic tire roller. By applying heavy loads for extended period of time as it slowly works its way up and down the pavement surface, compactors achieve a densification and binding of the laced asphalt. The applied shear forces result in compressive strain that reduces the initial thickness of the pavement layer. Steel wheel rollers (either static or vibratory) typically weigh up to 20 tons and apply their loads via two or three steel drums measuring up to 5 feet in diameter. Pneumatic tire rollers on the other hand utilize smooth tires with four to six tires per axle. Compaction is by controlling the tire pressure in addition to weight, with pressure varying between 60 psi and 120 psi. Like the pavers, compactors also benefit from laser or GPS guidance.
Laser Surveying Technology and Techniques
“Laser” is an acronym that stands for “light amplification by stimulated emission of radiation.” Ever since its invention in over a half century ago, the laser has found literally thousands of industrial, medical, scientific, and construction uses. Whether it is as a simple laser pointer used in classrooms and business presentations, bar code scanners, reading or burning CDs and DVDs, photochemistry, holographic display, a surgical tool, or powerful lasers used as weapons of war or to cut through metal, the laser has become a very versatile tool. One of its most useful applications is in the field of surveying and construction as a range finder and level. Most dramatically, lasers can be used to measure the distance from the Earth to the Moon. But it is as a tool for distance measurement and level guidance on the construction site where lasers have become indispensable in the earthwork engineering.
On the job, laser levels operate by the simple action of a self leveling, spinning floating mirror set in a protective housing that send rapidly rotating beams of laser light extending up to 300 yards. The rapid rotation, in effect, turns the one-dimension beam into a two-dimensional flat plane of laser light set at a given elevation. This plane of light intercepts a sensor mounted on a surveyors rod or piece of equipment which emits an audible beep or flashes a signal light when it is at the exact elevation as the laser beam. In addition to GPS locational systems, laser guidance allows for fine grading of pavement with small changes in grade. It allows for raising and lowering of paving equipment as it advances along the roadway. Overall accuracy is approximately 1 millimeter.
GPS System Operations and Utilization
If lasers are all about light, then GPS is all about synchronized timing. Built by the US military during the height of the Cold War, GPS is not one instrument, but it is a string of 24 sophisticated satellites located in geosynchronous orbit. Each satellite covers 1/24th of the globe (15 degrees of longitude) from pole to pole. Each emits a specific timed signal that is received by a ground antennae. The time lag between the signals gives the distance between the emitting satellites and the antennae. By triangulating the distance to several satellites in range, the exact location of the antennae can be calculated to within 1 foot.
Down here on Earth, GPS guided earthmoving equipment is equipped with an automated positioning reporting system (APRS). With an auxiliary ground system located over a pre surveyed reference point or the local benchmark itself, accuracies of 1 centimeter can be achieved by the APRS. This is referred to as the site’s controlled area network (CAN). Together with GPS, CAN provides the machines precise position. In doing so, the edge of the blade or screed can be placed at the precise location, elevation, and angle necessary to construct the pavement.
Armed with data from a digital terrain model (DTM, an AutoCAD derived three dimensional map of the surface), the APRS integrates the GPS signals to guide the scraper blade or pavement screed via an operator interface and a machine interface. The rest is a system of continuous communication between the GPS, the CAN, the APRS and the operator. The machine controls responding to instructions from the APRS consist of electronically controlled servo-type valves. The servos are further connected rotating a suspended armature connected by a linkage to rotary spools. The spools in control the pressure, direction, and amount of fluid in the hydraulic system that actually move the scraper blade or pavement screed.
A Caterpillar paving operation
Major Manufacturers of Laser and GPS Paving Systems
Caterpillar Inc., the world’s largest heavy equipment manufacturer and supplier, provides world-class equipment for the paving industry, including two-dimensional (2D)—grade and slope—guidance systems for asphalt pavers and mills. These systems can be upgraded to three dimensional (3D) by the Cat dealer. While 2D grade control systems areused for most work, 3D is used when design complexity or meeting an elevation or smoothness specification is critical. Cat earth moving equipment can be equipped with grade control systems for base preparation applications to ensure a relatively smooth and uniformly compacted base upon which to pave. Cat Grade and Slope is a 2D system integrated with the Cat paver and screed.
The integration helps the system achieve greater control. The control boxes utilize touchscreen displays that are easier to understand and use. They allow greater control and show more information to the screed operator. The 2D system can be upgraded by the Cat dealer for 3D paving applications. 3D paving is used when specifications require a higher degree of accuracy than what 2D paving can normally provide. 3D uses a different workflow than 2D paving, and there are some infrastructure investments necessary to implement it.
Trimble has a different take on equipment guidance systems. Instead, they utilize total stations for 3D. While 2D machine control (use of sonic sensors and lasers) is very prevalent and used on virtually all modern pavers, 3D machine control is being used most often in applications with very tight tolerances for elevation or smoothness such as airport runways and race tracks. 3D milling is gaining in popularity as it’s the best place to introduce technology and improve smoothness. 3D milling is used when eliminating or minimizing longitudinal waves is the priority. If the surface is milled with traditional methods, the paving contractor then must decide between 2D and 3D for the paver. 2D can be used to place the smoothest ride with the use of an averaging beam, however this won’t necessarily optimize the material usage of the job. To maximize material usage and smoothness, 3D should be implemented. Lastly is the use of their systems to achieve intelligent compaction.
These systems provide the roller operator with a tool to ensure consistent pass coverage of the surface while monitoring the surface temperature to ensure optimum compaction. The technology has gained in popularity as more owners and contractors have become informed and comfortable with the deployment of the technology. Advanced users are finding new and creative ways to achieve a deeper level of integration at the OEM level with the available technology on the market. 3D technology will result in an increase in production once operators have adjusted to the modified workflow. This trend will accelerate as IRI tolerances and volumes are tightened up by owners and agencies responsible for the final product. As such 3D tech has a bright future.
Volvo provides Intelligent Compaction Systems under the name Compact Assist. This system offers pass mapping and temperature mapping for paving equipment. An expanded version, Compact Assist with Density Direct, offers pass mapping, temperature mapping, and density mapping. Both systems include front and rear mounted infrared temperature sensors. They communicate the gathered data points to operators via a 10-inch touchscreen mounted within the cab. The following components are included in the system: GPS, display, accelerometer, infrared mat temperature sensors, base station (optional), and GPS rover (optional).
Intelligent Compaction as a whole is being used to help operators have better quality control. Pass mapping, for example, provides these capabilities. Development of Compact Assist required a series of “blind” tests to check the consistency of operators who didn’t have pass mapping and those who did. Two scenarios emerged for this testing for those operators who were working without the benefit of the technology: Stronger operators would have a tendency to over compact the asphalt to ensure it had been covered and less experienced operators would be fairly erratic—covering some areas just once while covering other areas eight to 10 times. What this means is when those core samples come back for payback on the job, those percentages are going to vary drastically. Pass mapping allows the operator to easily see how many times the compactor has rolled over a certain area of the mat. Each pass is given a different color designation on the display, so the operator can ensure uniform coverage with minimal overlap on passes. This saves time, money, and guesswork—helping ensure a much more consistent outcome. The temperature mapping provides the operator the latest temperature when the asphalt was worked, helping operators ensure they’re working the asphalt within the desired temperature zone.
Volvo Compact Assist with Density Direct is a quality control technology that provides operators with real-time density calculations of the surface area being compacted, to within 1.5% accuracy of core sampling. This accuracy makes it valuable for road building jobs that have a little time associated (anything over two weeks). The operator will use the first day of the job to gather data points on the machine that allow him to calibrate the machine to a particular setting, so he can much more easily gain consistency in density throughout the entirety of the paving job versus seeing variations in the density on the job from each pass. The display system shows the operator the density values on screen in two ways: with a color-coded pattern and the actual density values. It’s different than other systems because other systems are based on calculating a Compaction Measurement Value, which measures stiffness. Volvo’s, on the other hand, measures relative density.
A Classic Precision Paving Example for Classic Cars
An example of precision 3D paving is the construction of a racetrack for Corvette enthusiasts. The NCM Motorsports Park in Bowling Green, KY, is a test track for Corvettes, with the Corvette museum and a General Motors factory less than a mile away. A racetrack project requires smoothness, and the NCM park was no exception, with specifications prohibiting deviations of more than 3 mm (1/8 in.) around the track and a smooth joint in the center of the course surface.
“There could not be a cold longitudinal joint,” says Chris Higgins, engineering manager at Scotty’s Contracting & Stone. That meant two pavers worked in echelon, and the joint was compacted while still hot. “It’s really like there isn’t a joint,” says Higgins.
“The final day of surfacing on the track consisted of 13 hours of nonstop paving with pavers side by side to eliminate the joint in the track,” says Kenny Reynolds, paving manager at Scotty’s. “The transfer machines and pavers were refueled while they were all moving to eliminate the need for stopping.”
Cat AP1055D and AP1055E Asphalt Pavers laid down the mix. Four Cat CB64 Vibratory Asphalt Compactors handled breakdown, intermediate, and finish phases of compaction, achieving 92 to 93% density.
3D paving technology made preparation and paving of the track to exactly levels of accuracy possible. “During rough grading, the 3D AccuGrade dozers worked to within tolerances of tenths of a foot,” says J. D. Weis, general sales manager with SITECH Mid-South, which specializes in the sale of Trimble products. “Then machines equipped with 3D Total Station based Cat AccuGrade systems were used to complete the more accurate grading, down to hundredths of a foot. Now we’re coming back with the 3D paving systems and getting the tolerances down to thousandths of a foot.”
One of the benefits of the 3D paving system was its ability to use slope control on the joint match side of the trailing paver screed and have consistent height match. This accomplishment underscores the speed, accuracy, and inter-connectivity of their system.
Higgins says the technology, at all steps of the process, enabled enormous productivity gains. “We never would have finished on time without it. The biggest benefit of 3D paving is it precisely and accurately achieves the design, giving us the smoothest, most consistent surface possible.”
Ajax Paving, MDOT
With Trimble’s Paving Control System, Ajax Paving has completed work on a major reconstruction project for the Michigan Department of Transportation (MDOT). The reconstruction project consisted of a 7-mile stretch of I-96 from Newburgh Road in the city of Livonia to Telegraph Road in Redford Township. The whole project scope was 56 lane miles, 22 ramps, 37 bridges, 74 approaches on the bridges, and 10 retaining walls. One of several contractors, Ajax’s scope of work included 270,000 cubic yards of concrete paving.
“I used Trimble Business Center–HCE to build all of the models and keep track of my data going back and forth between machines and data collectors,” says Jeff Robinson, automation manager at Ajax Paving. “We had two of our batch plants set up on the job that produced 253 truckloads of material per day, which is more than 100,000 man hours in total.”
The Ajax team loaded the 3D road design created in Business Center–HCE onto the machine control box. Then, the Trimble PCS900 used automatic steering and grade control to keep the slipform paver on design. Robinson points out that because zero string lines are needed, concrete-paving operations can start sooner and the crew can work continuously. An accurate 3D design model and precise control of the paver minimizes material wasted, and the team can also perform fine grading work more quickly and with greater accuracy.
“It was well planned and executed because very rarely do we have a job that we are able to pave every day—and we did that for almost four months,” says Robinson. “We paved every single day, finishing the job in just 167 days—a full 17 days ahead of schedule.”
Robinson credits Trimble Paving Control for the significant time savings. For a project of this scope, in the past Ajax would have had to pound in stakes and run line approximately every 25 feet to check grade. Using the Trimble systems on the I-96 project, dirt contractors could cut to grade in the morning, place stone on the grade, and by that afternoon Ajax could trim the fine grade. The following day, Ajax could then place the concrete on the stone.
“If it wasn’t for the Trimble Paving Control System and going stringless, this job would likely have needed 12 full-time stringline setters just to set up for trimming,” says Robinson. “Now many of those guys can do other high-value things. It was a challenge at first, but it didn’t take long for management to come around; there’s no doubt they now can see the benefits.”