By Mark Buncher Ph.D., P.E. and Dwight Walker, P.E.
Rubblization with an asphalt overlay is the recommended solution for rehabilitating old concrete pavements. Rubblization is the process of fracturing an existing Portland cement concrete pavement (PCCP) into small, interconnected pieces that serve as a base course for a new hot mix asphalt (HMA) overlay.
Since there are no hauling or disposal costs and all of the existing pavement materials remain in place, rubblization is a cost-effective rehabilitation method. The net effect of rubblizing is converting a deteriorating rigid pavement into a new flexible pavement with no reflective cracking.
Properly rubblizing PCCP results in the complete destruction of any slab action before applying the HMA overlay. For concrete pavements containing steel reinforcement, such as jointed reinforced concrete pavement (JRCP) and continuously reinforced concrete pavement (CRCP), the steel should be ruptured or the concrete-to-steel bond should be broken. This is necessary to eliminate any slab action that causes reflective cracking. Rubblization reduces the existing concrete pavement into a layer that resembles an aggregate base with a high degree of particle-to-particle interlock and is stiffer than most aggregate base layers.
In addition to rubblization, there are two other fractured slab techniques: crack-and-seat for PCC without steel reinforcement, and break-and-seat for PCC with steel reinforcement. These two techniques typically use a guillotine breaking hammer to fracture the slabs into approximately 24- to 30-inch pieces. This reduces the expansion and contraction that occurs with PCC slabs, and thus reduces the stresses and strains that occur in an asphalt overlay from any underlying slab action.
The Asphalt Institute recently completed the report, Development of Guidelines for Rubblization, for the Airfield Asphalt Pavement Technology Program. This report provides specific details on all aspects of rubblization. Although the study addressed airfields, much of it can be applied to rubblizing highways. The complete report can be found at www.aaptp.us.
Rubblization started as a means of breaking concrete for removal and replacement. According to Phil Kirk, former CEO of Resonant Machines, Inc. (RMI), a New York materials engineer, John Frizzel, observed a breaking machine working on a concrete removal job in New Jersey in 1986. He believed that the slab fracturing operation might leave a slab with sufficient modulus to distribute loads yet eliminate the slab movement that produces reflective cracking. The concept began a breakthrough in highway and airport rehabilitation.
Currently there are two major methods of rubblizing, the Resonant Pavement Breaker (RPB) and the Multi-Head Breaker (MHB). These two technologies share the rubblization market for highways and airfields.
Resonant Pavement Breaker
After Raymond Gurries invented the RPB in the early 1980s, Howard Gudgell, a mechanical engineer, modified it. He literally reinvented the RPB to make it more powerful and durable.
The modern RPB varies its settings to fit site conditions. According to the project’s needs, RMI varies the size of the breaking machine, its ground speed, the amplitude and the frequency of the blows.
The RPB generates intersecting diagonal cracks through the entire PCC thickness at approximately a 45-degree angle. The rubblized concrete pieces have a high degree of angular interlock, and they increase in size with depth. Pieces are typically largest at the bottom of the slab or below any reinforcing steel.
George Shinners, president of Antigo Construction, saw a guillotine breaker in a construction warehouse in Germany in the early 1980s. He brought it back to Wisconsin, knowing that WISDOT wanted to do more breaking and seating. WISDOT accepted the guillotine breaker as a way to get better production.
By the early 1990s, Antigo began manufacturing its own guillotine breakers and was operating 10 breakers across the United States. In 1995, Antigo developed the MHB by modifying its single-hammer guillotine design to produce a machine with 16 hammers in two rows. The MHB has now completed rubblization projects in 32 states and Canadian provinces.
The multi-head breaker operates as a single-pass machine with a breaking width of one vehicle-lane. The operating components of the MHB—hammer weights, number of hammers, hammer drop heights and travel speed—can be varied to produce a wide range of fracture energies to accommodate project conditions.
Poor drainage often causes weak subgrade and support conditions that can inhibit the rubblization process. The installation of longitudinal edge drains prior to rubblization is recommended for all rubblization projects, unless a well-functioning drain system exists or the subgrade consists of self-draining material such as sand. If trapped water beneath the slabs is not drained, it will inhibit the rubblization process.
According to the Asphalt Institute’s report, edge drains will ease the rubblization process and will improve the overall, long-term pavement performance. The drainage system should be installed and functioning at least two weeks prior to the actual rubblization process so the subgrade can dry out. Ideally, the drainage system should be installed prior to performing any evaluation of support conditions.
On newly constructed pavements, non-destructive, in-situ test data shows that the pavement structure will increase in modulus and produce lower final deflections for up to three years after construction because of the tightening of the aggregate interlock and the subgrade drying out. The same principal applies with a pavement system prior to rubblization.
Before beginning to break the concrete, an evaluation of the project conditions should be made. The combination of thin concrete slabs, weak subbase and soft subgrade can produce marginal stability and difficulty in effectively rubblizing PCC. Typical problems that can occur on these marginally stable pavements are large unbroken PCC pieces, rutting and “punch-throughs,” and shifting and rotation of large PCC pieces by the hot mix asphalt haul trucks and pavers. If marginal conditions are found, the rubblization equipment operating characteristics (breaker travel speed, breaking effort, etc.) can be adjusted to achieve better results.
The intent of rubblization is to eliminate slab action and to convert the slabs into a stable, unbound base layer. Limiting particle sizes and requiring that steel be debonded are the prevailing specification criteria to ensure there is no slab action.
Factors affecting particle size include the PCCP thickness, material properties and reinforcing steel. Other factors include subgrade support and type of equipment. Both highway and airfield specifications typically establish different particle size requirements depending on the presence of reinforcing steel, depth within the fractured slab, and location relative to the pavement edge.
For highway pavements, the concrete should be broken from sand size to 8- to 9-inch pieces. No individual pieces should exceed 9 inches in any dimension. The majority of the pieces should be 1 to 3 inches in size. For airfield pavements, the concrete in the upper half of the slab should be broken such that no particles exceed 6 inches in any dimension and no pieces are larger than two times the slab thickness in any dimension for the bottom half of the slab or below any reinforcing steel.
Because test pits require significant work, some agencies do not specify a defined test procedure. Although the rubblization process is somewhat self-regulating for the majority of operations, test pits are still highly recommended.
Thickness of Asphalt Overlays
Consultants and state DOT engineers agree that an increase in the thickness of the asphalt overlay will provide a significant increase in pavement life. They agree that increased HMA thickness decreases the maximum deflections and also decreases the possibility of reflective cracking in the overlay. For designing a rubblized pavement, the HMA overlay pavement thickness is determined in the same way that a new asphalt pavement would be, treating the rubblized layer like a high quality base or stabilized base.
When utilizing a layer-elastic design procedure, the thickness of the asphalt overlay depends on the assumed modulus of the rubblized concrete and the volume and weight of the traffic traveling the pavement. A literature review by the Asphalt Institute found 17 rubblized sections around the country where the modulus of the fractured PCC was measured after rubblization. The average modulus was 205 ksi, with the range being 100 to 430 ksi. This compares favorably to crushed stone, which FAA airfield design procedures treat as having a modulus of 50 to 60 ksi. Generally, the thicker slabs and those with reinforcing steel had higher values.
Rubblization has been successful in rehabilitating both highways and airfields. Airfield pavements, however, can be significantly different from typical highway pavements. One of the key differences is the thickness of the pavements. Highway PCCP thickness typically ranges from 8 to 12 inches, while airfields can be as thin as 6 inches or as thick as 25 inches. Both RPB and MHB equipment have successfully rubblized airfield pavements exceeding 24 inches in thickness.
From Arabian Airports to Russian Roads to I-295 in Maine “I think we’ll see a substantial growth in the rubblizing market in the next 10 years,” says Matt Shinners, vice president of Antigo Construction. Not only is interest growing in rubblization in the United States, but there is increasing interest in rubblization abroad. “We’ve worked in Afghanistan, England and Ireland,” says Shinners, “and we’re getting ready to do an airport job in Saudi Arabia. The Saudi job is the Jetta Airport on the Red Sea. It has three runways and taxiways. We’re going to do two of the three runways—one at a time. The total job will be 1.3 million square meters.”
RMI’s Tom Schneider says that Russian engineers appreciate the value of rubblization. “So far we’ve identified 2 million square yards of projects that need to be rubblized. And the domestic market is still growing,” adds Schneider. “Soon we will be doing our first rubblizing project on I-295 in Maine. We will be doing 250,000 square yards on two southbound lanes—a total of 19 miles, south of Augusta and north of Portland. The concrete there is 9 inches thick, but the Maine DOT is milling off 2 inches so we will be rubblizing 6 or 7 inches of concrete. We’re going to use our lighter machines on this project. We’ll only be on the project 14 days.”
Overall, rubblization of airfields and highways is anticipated to increase significantly in the future.
|Mark Buncher is the Asphalt Institute’s Director of Engineering and Dwight Walker is the editor of Asphalt Magazine.|