The dictionary defines “recycling” as: “to treat or process (used or waste materials) so as to make suitable for reuse.” When the general public thinks of recycling, it is typically in terms of waste materials from household and commercial waste that are extracted, separated, processed, packaged, and resold as raw materials (ferrous and non-ferrous metals, office paper, cardboard, newsprint, glass of various colors, and different types of plastics). A typical recycling effort normally includes a strong program of composting organic waste (food scraps, garden waste, lawn clippings, and leaves). Lastly, there are materials recycled from construction and demolition debris left over from building projects that can be reused for other construction efforts (bricks, structural steel, electrical wire, plastic conduits, metal pipes, asphalt roof shingles, etc.).
Not many people think of recycling in terms of concrete or asphalt. But of the total amount paid for construction by the public sector in 2017 ($280.0 billion), $87.7 billion—or 31%—was spent on roads and highway construction (Source: www.statista.com). By comparison, the entire municipal solid waste recycling market in the US is projected to exceed $6.5 billion (Source: Harris Williams and Co. Ltd., 2013). These raw numbers alone indicate the huge potential of pavement recycling. This potential will expand with every advance in techniques, technology, and equipment, improving efficiency and increasing return on investment.
There are two primary types of pavements used in roads and highways: rigid concrete and flexible asphalt. Physically, their names say it all. Concrete is rigid and resists deformation under traffic impacts, while asphalt pavement is flexible and gives under applied truck and vehicle loads.
Rigid concrete pavement consists of a Portland cement concrete slab with embedded wires, rods, and bars of steel for reinforcement. Underlaying this surface slab is a subbase layer of large aggregate stones, which in turn are underlain by a compacted soil base. Concrete is very strong in compression but weak in tension (with a strength of 3,000 psi). When a heavy truck axle load travels down a concrete pavement, the pavement wants to bend upward in a U-shape. As a result, the upper half of the concrete pavement is compressed together and the bottom half is pulled apart by tension. Under tension, concrete tends to crack and fail.
So, before the concrete pavement is poured in place, steel rebar of various dimensions and spacing intervals (depending on the anticipated traffic loads) are preset in what will be the bottom portion of the pavement layer. When the concrete cures and hardens, the steel, which is very strong against tension loads, will act to resists the tension forces that concrete cannot. So, once it is set, the reinforced concrete pavement acts as a rigid bridge, horizontally distributing the applied axle loads instead of simply bending under the applied weight and directly transmitting the load vertically down into the underlying base of the pavement foundation.
But applied loads from trucks and vehicles are not the only forces impacting a rigid concrete pavement. Being rigid makes its susceptible to expansion and contraction resulting from changes in temperature. To allow some freedom of movement, concrete pavements are poured into segments separated by gaps and joints that allow for anticipated expansion and contraction. These joints are themselves reinforced by short, steel dowel bars set perpendicular to the joint alignments at regular intervals, holding the overall pavement together and preventing vertical differential movement between adjacent slabs.
The concrete itself is a mixture of cement, a natural binding agent (a mixture of lime, silica, alumina, calcium sulfate, fly ash, and other materials). The components of cement are pozzolans that undergo a chemical reaction that triggers a physical change when mixed with water. The resultant slurry chemically reacts with calcium hydroxide to form a hard material. In addition to the binding cement, the rest of the concrete mixture includes aggregates, sand, crushed rock, recycled concrete chunks, and water. The time to cure after pouring is typically 28 days. Usually more expensive than flexible asphalt pavement, concrete pavement lasts much longer (30 years versus five to 10 years for asphalt), resulting in lower maintenance costs and it is superior overall financially.
The other pavement option is flexible asphalt pavement or bituminous concrete pavement. Instead of cement, the binding agent is bituminous asphalt. Asphalt is an organic, carbon-based material. It is a petroleum product produced from the fractional distillation of crude oil and consists of polymers such as naphthene, hydrocarbons, and asphaltenes. The end result of this process is a black, sticky, tar-like emulsive that acts as a binding agent for the additional components of the asphalt (stone, sand, and gravel). In this way, it serves the same function as concrete cement. But asphalt only forms a small percent of the overall mass of the asphalt pavement, typically no more than 5%, the other 95% being the additive components.
Both flexible and fluid (especially at high temperatures), asphalt can be easily poured and spread over a roadway. This process usually requires the placement of multiple layers of asphalt, each held together by an adhesive coating sprayed on between each layer. The number of layers depends on the overall final thickness of the asphalt pavement. In addition to the multi-layer asphalt pavement surface, a flexible pavement also includes a final surface wearing course of asphalt, a binder course, a base course of bituminous concrete, subbase course of larger aggregate, and a subgrade of compacted-in-place soil.
Instead of a rigid bridge translating axle loads horizontally, asphalt pavement is flexible and transmits the loads directly downward into the pavements base layers. This is because flexible pavement is not reinforced by structural steel rebar and its binding agent is fluid and flexible in contrast to hard and rigid concrete. The applied loads create a deformation that passes down through the surface pavement to the underlying granular layers. This deformation is more V-shaped with the point of the deformation coinciding with the contact point of the tire in contact with the road surface. The overall pavement is designed to withstand these anticipated loads. Each layer, including the underlying compacted soil base, helps carry the load.
Though flexible and fluid at the high temperatures required for application, it also cures and hardens to create a firm surface. Unlike rigid concrete cement, which requires 28 days to cure, asphalt pavement can often be driven on later the same day it is placed. This is a considerable convenience to motorists and an important safety feature as it shortens the time that a roadway must be partially or completely shut down for construction or repair work. As stated above, asphalt is cheaper than concrete pavement but has a shorter operational lifetime. While this may not provide superior financial projection, its lower costs allow for better cash flow management, especially for organizations and contractors that are cash-strapped and unable to afford the high initial cost of concrete pavement. And its repairs requirements, though more frequent, are often relatively simple and localized, not requiring complete replacement of whole pavement slabs.
Hot and Cold—Pavement Recycling and Repavement Methods, Technology, and Equipment
Pavement recycling methods are defined by whether the pavement is not reheated during the recycling process (cold recycling) or actually is reheated (hot recycling), whether the recycling is done at the job site (in-place recycling) or at an off-site facility (central plant), by the depth of the material removed for recycling (surficial, partial depth, or full depth), or how the recycled pavement material is replaced (remixed or repaved). Hot repaving is almost always done onsite and remixing is almost exclusively done with hot mix. This leaves us with six major methods of bituminous pavement recycling.
Cold, in-place recycling at partial depth. This is the most common and often the most cost-effective method of pavement recycling. In accordance with American Association of State Highway and Transportation Officials (AASHTO) standards, this method involves seven distinct steps performed in sequence: milling, crushing, additives, placement, compaction, moisture seal, and surface course placement. In broad strokes, this operation first involves the in-place crushing of pavement material down to a depth of 2 to 5 inches and its removal for further processing. This task is referred to as “milling” and is performed by a specialized milling machine that pulverizes, scrapes, cuts, and crushes the surface material down to the required depth. This initial milling operation can result in chunks and particles of asphalt concrete of wildly varying dimensions.
Pavement processing in the form of additional crushing is performed to produce a consistent particle size and smooth gradation of the recycled material. This step is performed by a screening and crushing plant that received the material from the initial crushing process. This plant is small enough to be mobile and is typically a trailer-mounted unit. Additional crushing further reduces large particle sizes, while the screener sorts out material, allowing the right size material to pass through and returning the larger particles back to the crusher for further processing until they are small enough to be used. In order to smooth out the gradation of the crushed particle and ensure a higher quality material, the new aggregate can be fed into the crusher hopper and mixed with the recycled material.
The crushed material then receives additives and recycling agents as well as foamed asphalt and emulsified asphalt. The emulsified asphalt acts as a binding agent (though some processes have used recycled fly ash for coal burning operations, lime, and Portland cement). The mixing of additives can be performed with a high degree of efficiency and typically superior quality by separate pug mills mounted to trailers pulling them along as the work advances down the roadway or by a mixing unit within the milling machine itself (often at lower cost).
The mixed and crushed material is placed back over the top of the previously milled pavement as a smooth, consistent layer. The equipment used to perform this operation is typically a specialized asphalt paver utilizing windrow pickup of the material or by having the material placed directly into the pavement machine’s hopper. This equipment is a more sophisticated and capable version of a more traditional motor grader (which is still often used for secondary roads and parking lots with low traffic loads). The material is placed in controlled lifts to ensure that subsequent compaction efforts achieve maximum desired densities. Given that the maximum size of the recycled particles is usually between 1 1/2 to 2 inches, the individual placement lifts are typically no more than 2 inches thick.
Once each lift has been placed and spread, the recycled material is subject to compaction. It can be subjected to multiple cycles of compaction until it achieves the required in-place density and strength. This task is performed with either a vibratory smooth drum (steel wheel) roller or large pneumatic tire compactors. Depending on the type of additive included in the recycled pavement mix, compaction can be done immediately or after a short delay. Those recycled pavements utilizing lime and Portland cement as a binding additive need to be compacted immediately in order to seal off the surface and eliminate voids within the material since lime and Portland cement can react with any moisture that makes it into the pavement. The reaction can have adverse consequences for the subsequent strength of the material. An asphalt emulsifier additive needs time to break down somewhat before it is subject to heavy compaction. As with ordinary pavement construction, the surface of each compacted lift can be covered with a thin binding agent to ensure its bond to the next pavement layer placed and compacted above it.
A moisture sealant (often referred to as a “fog seal”) is applied to the surface of the final lift of the compacted pavement layer. Moisture intrusion can, over time, degrade the strength of the pavement and this sealant is intended to prevent this from happening. This degradation is referred to as “surface raveling.’’ Fog seal is required to achieve high-quality pavement and is a necessity for those recycled pavements that utilize lime and Portland cement as additives. The moisture sealant itself forms a membrane underneath which the pavement can properly cure and set in place.
Finally, the recycled pavement construction is finished with the construction of a surface course. This can be either recycled cold mix placed as described above or a layer of new bituminous concrete placed as hot mix asphalt (HMA) overlay or bituminous surface treatment (BST). BST, also referred to as “chip seal,” is typically used on low traffic, secondary roads. It is applied as a sprayed-on emulsifier that receives a thin layer of aggregate which is then roll compacted and embedded into the roadway surface. As with ordinary concrete construction, this surface is bonded to the recycled pavement with a tack coat to ensure strong adhesion between the two layers.
The remaining methods follow the same placement procedures as the ones described above, but use different methods for the actual mixing.
Cold Central Plant Recycling. Typically, in-place pavement recycling is preferred since most of the necessary construction materials are already there at the job site and the contractor does not have to pay the transportation costs incurred by hauling in material for an off-site processing plant. But in those cases where additional off-site materials are readily available and/or required, a central processing plant is necessary. The overall process itself is similar to that of cold in-place recycling except that the process occurs at an off-site location instead of at the road site. The two procedures are compatible and are often combined for a pavement recycling project.
Full Depth Reclamation (cold in-place recycling-full depth). This utilizes the same techniques of crushing and scraping, but to the complete depth of the existing pavement surface (6 to 12 inches). This process follows the same eight steps: crushing the existing roadway to the complete depth of the pavement, moisture conditioning of the recycled pavement material to achieve required density, compaction in place with a sheepsfoot roller or pneumatic tire compactor, shaping with a grader to achieve a smooth finish and consistent thickness and slope gradient, finish rolling with a static steel drum roller to produce a smooth surface by setting any loose or protruding aggregate, application of a moisture sealant, and the installation of a surface treatment (BST) or a surface course (HMA). This process (performed with or without the addition of stabilizing additives) can also remove portions of the underlying aggregates base materials. The material is processed and replaced in controlled layers to re-establish the original thickness of the asphalt pavement, and since every process results in some wastage, this approach almost always requires that addition of additional asphalt material from an off-site source in order to make up the difference.
Hot In-Place Recycling. This is a form of surface recycling that differs from cold mix recycling in that it requires the application of enough heat to make the asphalt fluid. This is referred to as “heater scarification” with the heat provided by propane radiant heaters. The surface is then scarified to a depth of 1 to 2 inches. Scarification is performed with rows of non-rotating teeth. Since the material has already been softened by the applied heat, applying rotational mechanical force to bust up the pavement is not necessary. This scarified layer is then mixed with recycling additives. The additive includes a rejuvenator agent that improves the asphalts viscosity and adherence ability. The mixing is performed by an auger system where the additives are fed into the mix. Once fully prepared, the hot mixed surface asphalt is spread with an asphalt paver (typically as a single lift) to create a smooth surface finish and then compacted with a static steel drum roller compactor. Though efficient and cost-effective for shallow repair work, hot in-place recycling is not as effective (given its depth limitations) in repairing major potholes, deep ruts, or pavement that is completely crumbling.
Variations on hot in-place recycling include remixing and repaving. Remixing requires the use of a pugmill or mixing drum to mix old asphalt with new material. The resultant mix is placed in a single layer and is roughly 3:1 old to new asphalt material. This new material has to be sourced from an offsite central processing plant. Hot mix repaving is a variation that includes the placement of a new lift of HMA on top of the loose layer of recycled material. Both are then compacted in place as a single lift.
A Word About Rigid Concrete Pavement Recycling. It is wrongly believed that only asphalt pavement can be recycled in a cost-effective manner. When its useful life is over, concrete pavement can be removed and completely recycled as manmade aggregate for the next application of concrete mix. The resultant material is referred to as reclaimed concrete material. It is used primarily as an aggregate substitute when making new reinforced concrete pavement, a subbase course for new roadway construction (both concrete and asphalt), fill material for embankments and foundations, and drainage material for French drains.
Breaking up the concrete is just the first step since rigid concrete payment includes significant amounts of steel rebar reinforcement and dowel bars. Concrete pavement is typically broken up into large but manageable sized chunks for hauling off to a central processing plant. There, the concrete is crushed further and the exposed steel magnetically removed. The crushed concrete is segregated by a series of screens with larger particles either sent back for further size reduction or kept separate for uses appropriate to their large size.
Costs and Benefits—the ROI of Recycling and Repaving
It seems intuitively obvious that pavement recycling would be a major cost saver for roadway construction. The exact benefits and costs depend on the location of the work being performed and its economic and environmental aspects as well as its engineering design characteristics. While making the financial choice of whether or not to use in-place pavement recycling is still more of an art based on experience, more and more data from the field is making it more of a choice based on a rational science.
Several state Departments of Transportation (DOT) have performed in-depth analyses of this issue. For example, the Wyoming DOT concludes:
“For example, consider $5 per ton and $120 per ton as average costs of aggregate and liquid asphalt, respectively. The cost of a 100% virgin mix with 6% asphalt comes out to be $11.90 per ton. If the contractor uses a half-lane milling machine and hauls the RAP back to the HMA plant, the total cost for RAP is $3.70 per ton, considering $1.70 per ton for machine and labor milling and $2.00 per ton for trucking costs. Hence, the savings, compared to using virgin aggregate material, is $8.20 per ton.[…] It should be noted that these savings are in initial cost.[…] All cost analysis tables were obtained from the 1997 FHWA report entitled Pavement Recycling Guidelines for State and Local Governments.” (Source: Andreen, Rocheville, and Ksaibati, “A Methodology for Cost/Benefit Analysis of Recycled Asphalt Pavement (RAP) in Various Highway Applications,” University of Wyoming, July 27, 2011.)
The report also includes typical cost savings with hot mix recycling for different regions within the US, as shown in the table.
Terex Bid-Well remains a leader in the field of pavement construction with the development of its new remote-control Terex Bid-Well 4800RC paver. This new paver offers standard paving widths ranging from 12 to 116 feet and comes standard with a wireless radio remote control that enables the operator to adjust critical machine functions from ground level while paving, offering more flexibility for quickly fine-tuning paver settings to meet changing conditions. The radio remote control panel allows the worker to carry along machine controls and operate the paver from ground level, 360 degrees around the paver. The initial display screens include a login page, so only authorized operators can control the unit, and a daily maintenance prestart-up checklist for the carriage, power unit, legs, travel bogies, and controllers. “Our wireless remote allows the operator to step down from the frame-mounted platform,” says Marty Bachey, sales manager for Terex Bid-Well. “This gives the operator the 360-degree view of the paving site, offers the ability for the operator to communicate with ground workers to make adjustments without stopping the paver, and improves operating safety.”
The 4800RC complements slip form pavers in the company’s fleet by quickly and efficiently paving urban development roads, shoulders, ramps, and slabs that flare. Its ability to pave both bridge and slab-on-grade projects significantly improves machine utilization. To deliver the highest surface finish quality on bridge deck and flatwork projects, the 4800RC features the patented Terex Bid-Well Rota-Vibe system that delivers up to 5,000 vibrations per minute (vpm) over the roller’s 11.5-inch length to effectively consolidate the top 2.5 inches of concrete. Innovative dual spud vibrators impart vibration right up to the edge of the slab on flatwork projects and consolidate up to 30 inches of concrete.
The 4800RC paver’s solid 48-inch deep truss frame offers the rigidity to prevent frame deflection at extended paving widths, maintaining consistent slab thickness and surface smoothness. Standard 18-foot truss end segments offer up to 15 feet of leg travel to each side of the machine for on-the-fly variable-width paving up to 30 feet. The machine’s standard universal power crown adjuster allows the contractor to mount the crown adjuster in-line for a straight machine or position it on adjacent hinge points to align the adjuster at the skew angle of the bridge deck. Standard powered legs on the 4800RC offer quick and easy leg elevation adjustments. Both crown and leg changes can be made from the operator’s platform or from the remote control unit.
BOMAG/Compaction provides the equipment essential to the first and last stages of any roadway construction effort—compaction. “If the subbase isn’t properly compacted to the right densities, it will shift, causing cracks or failures in the slab,” says Bert Erdmann, product manager, heavy compaction for BOMAG Americas. “This can result in costly call-backs for the contractor to repair or replace the concrete.” To ensure proper compaction, BOMAG provides grading and paving contractors several different compaction measuring technologies for a higher level of compaction control with its line of single drum rollers.
The most recent additions to their fleet are the 3.5-ton single drum rollers, the BW 124-5 series. The rollers come in both smooth and pad foot drum configurations to help contractors meet spec densities. These include the intuitive BOMAG ECONOMIZER compaction measurement system, now available as an option for the new BW 124-5 series, which alerts operators of compaction progress, reducing passes and saving time and money. It requires no calibration to reliably deliver real-time compaction progress. As the degree of compaction increases, more LED lights on the ECONOMIZER display illuminate to indicate when optimum compaction is achieved.
For larger concrete projects like urban development roads, county highways, and mainline paving, BOMAG offers the new BW 177-5 and BW 211-5 series rollers. The 7-ton BW 177-5 and 12-ton BW 211-5 rollers come in both smooth and padfoot drum configurations and offer rolling widths of 66.4 and 84 inches respectively. Like the BW 124-5 rollers, these larger series rollers can be equipped with the economical ECONOMIZER compaction measurement system. For the next level of compaction measurement, there is the BOMAG Terrameter system for the BW 177-5 and BW 211-5 series. Terrameter uses two accelerators to continuously measure the physical stiffness of the soil, and this stiffness value is displayed in the form of the EVIB and can be used to locate, document, and address soft spots and non-uniform areas.
For high-end compaction control for high-profile concrete paving applications, contractors can opt for the BOMAG VARIOCONTROL (BVC) Intelligent Compaction System. BVC automatically measures soil stiffness multiple times per second to optimize compaction forces in response to current soil conditions. With BVC technology, less experienced roller operators can achieve the same results as a 40-year-veteran. Drum compaction energy of a BVC roller is automatically adjusted by the specifically designed BOMAG exciter system. On loose material, the drum’s vibration angle is a true vertical, so full compaction energy is transmitted into the soil for maximum depth penetration. As soil stiffness increases, the drum compaction angle automatically changes from full vertical to ultimately purely horizontal when optimum compaction is achieved.
Hyundai Construction Equipment Americas offers a compaction-roller product line that includes three tandem-drum rollers—including the most recently introduced Hyundai HR26T-9 model—used primarily in asphalt paving and four single-drum roller models designed for soil and aggregate compaction. “The Hyundai compaction roller product line combines the most up-to-date compaction technologies with quiet, fuel-efficient engines, a safe and convenient operator environment, and easily accessible maintenance points for outstanding overall performance and value,” says Chad Parker, senior product specialist and sales trainer, Hyundai Construction Equipment Americas. “This product lineup gives solid options to users across the compaction market’s highest-volume segments.”
The Hyundai HR26T-9 model has an operating weight of 6,400 pounds (2,900 kg) and drum width of 47 inches (1,200 mm), with a working width of 49 inches (1,250 mm). A Tier 4 Interim-compliant Deutz D 2011 L2i diesel engine rated at 31 horsepower (23 kilowatts) powers the HR26T-9.
While the Hyundai tandem-drum roller models are primarily designed for asphalt paving applications, contractors also may use these compact machines for soil compaction. The tandem-drum rollers feature front and rear scrapers to keep the drums free of material buildup. They also provide 55-gallon (208.2 liter) water-spray systems that can sprinkle the pavement surface of the drums, with operator-adjustable sprinkling intervals. Other standard equipment on Hyundai tandem-drum roller models includes ROPS roll bar, hydrostatic drive, hydrostatic vibration system at both drums, automatic vibration mode (double/single vibration), spring-loaded brakes on both drums, manual emergency stop switch with touch-sensitive switch at the seat, spring-mounted driver’s compartment, four headlights, driver’s seat with armrests and safety belt, and lockable dashboard.
Hyundai’s four single-drum rollers—the HR70C, HR110C, HR120C, and HR140C—meet Tier 4 Final emissions regulations with Deutz and Cummins engines that range in horsepower from 73 to 132 (54 to 97 kW). The HR70C model features a Deutz TD2.9 L4 engine. The HR110C, HR120C, and HR140C models all feature a Cummins QSF 3.8 engine. The four single-drum models are designed primarily for soil compaction applications. The compaction drum is available as a smooth or pad-foot surface. A pad-foot kit is available for use with the smooth drum. Continuous tractive force adjustment automatically assures maximum traction. Two-stage vibration provides both surface and deep compaction. In the smooth-drum configuration, Hyundai’s single-drum roller models have operating weights ranging from 15,650 to 30,865 pounds (7,100 to 14,000 kg). Drum widths range from 67 to 83 inches (1,700 to 2,100 mm). Gradeability without vibration ranges from 45 to 50 degrees.
Since 1837, John Deere has been a world leader in providing advanced products and services for those whose work is linked to the land—those who cultivate, harvest, transform, enrich, and build upon the land to meet the world’s dramatically increasing need for food, fuel, shelter, and infrastructure. For roadway work, they provide a series of capable, technologically advanced road graders and associated control systems. North America’s largest Power-Angle-Tilt (PAT) crawler dozer now uses John Deere SmartGrade. Adding SmartGrade technology to the Dubuque, IA-manufactured 950K PAT improves its quality and accuracy through the complete integration of the Topcon 3D-MC2 Grade-Control System. The system is fully incorporated into the machine cabin, structures, and software while eliminating vulnerable external masts and cables. By removing the masts and leveraging position sensing, the operator can now run without limitation, using all of the machine functions, like blade pitch, circle side-shift, and circle rotate, without risking damage and all while staying on grade. In the cab, the grade system interface is built into the Grade Pro (GP) controls available in the Deere exclusive fingertip or dual joystick design.
The 950K PAT SmartGrade dozer incorporates an EPA Final Tier 4/EU Stage IV John Deere 9.0-liter engine with 280 hp. The efficiently designed hydrostatic powertrain gets approximately 15% more power to the ground versus a conventional torque-converter powertrain. “The 950K PAT dozer and our entire SmartGrade lineup have been very successful since their respective launches,” says Nathan Horstman, crawler dozers product marketing manager, John Deere Construction & Forestry. “It only made sense to put these two innovations together and provide customers with a game-changer in terms of dozer productivity and efficiency on the job site.” A key feature of the integrated machine control is Auto SmartGrade. This allows the operator to easily adjust the system when moving the machine from one soil type to another, unlike an after-market system, which often requires the GPS manager to make a trip to the machine to recalibrate the system. Auto SmartGrade automatically lifts the blade over heavy loads before track slippage occurs, then returns the blade to grade. SmartGrade also limits the number of passes required, reducing the pace of wear on the undercarriage. The John Deere SmartGrade dozer is nearly 7% more accurate across the entire speed range of the dozer when compared to conventional masted systems.