Rubblizing is the process of fracturing pavement of Portland Cement Concrete into angular pieces for direct overlay. Resonant Machines, the pioneer of Rubblization, has participated in over 25 different independent studies over the last two decades. Such analysis has shown the rubblized roads with an asphalt overlay have an average useful life of 22 years, while costing 60% less than the cost of tear out and replace of concrete and taking approximately 1/5th the time. RMI averages about one lane mile per day or approximately 7,000 square yards. Rubblization should be considered first when rehabilitating a concrete road.
Our rubblization process also has benefits to high traffic urban areas, with reduced user costs. These arteries are very sensitive to traffic disruption from both the public and to local businesses. Rubblization is the most effective way to rehabilitate these arteries with minimum impact on both the driving public and local commerce. In addition to RMI’s speed and efficiency, the low amplitude, high frequency method, reduces ground vibrations and noise impact of the surroundings.
The concrete slabs structural integrity must be severely degraded to prevent reflective cracking. The slab must be fractured all the way through and the bond between the concrete and any reinforcement must be destroyed, all without damage to the base profile or the base material. Once the slab’s integrity is compromised, heavy traffic loading will not cause slab reflection, resulting in cracks in the asphalt overlay Figure 1, provided the concrete is broken in such a way that it retains the maximum modulus, maximum structural coefficient, and the maximum ability to distribute surface loading over a wider base. Achieving the maximum modulus requires that the concrete be fractured on the shear plane, i.e., at a 45 degree angle. This angular fracturing pattern provides for a greater modulus than that of a vertical fracturing pattern, and spreads the load over a greater area resulting in a greater modulus than may be achieved by other fracturing methods.
The structural coefficient of a typical crushed stone base is .14, a stabilized base is .25, and that of a Resonant rubblized slab on a substantial base material may range from .25 to .28. Slabs supported only by a sand or unconsolidated base material will produce a structural coefficient of from .16 to .25.
The objective is to break the concrete in such a manner that the broken slab does not expand excessively, does not damage or invade the base in anyway and the broken pieces aren’t too large or displaced in relationship to each other. The breaking pattern must be like a “jigsaw” puzzle, all the broken pieces lie together in an interlocked, undisturbed fashion Figure 1. This results in a much wider distribution of traffic loads; the pieces “work or flex” together, distributing the loads over a much wider area . This can only be accomplished by breaking the concrete with a low (3/4″) amplitude, high frequency resonant impact hammer.
Drop hammers break concrete when impacting the slab by putting the bottom of the slab in tension, thereby displacing part of the broken slab into the base. This disrupts the interface (the interlock) of the broken pieces in relationship to each other, resulting in damage to the base by the invasion of the broken pieces, reducing the ability of the base to support and distribute the traffic loads. Most significantly, drop hammers break concrete in such a manner that there is little “jigsaw” puzzle or interlocking pattern to distribute loads. High impact broken concrete is inconsistent in the broken sizes, creating hard and soft spots. The result is that the load distribution of concrete broken by a high impact drop hammer is nearly non-existent, i.e., straight down Figure 2. This can cause rutting of the surface because the broken concrete pieces could not adequately distribute the loads, deforming the base, or more asphalt will have to be laid to achieve similar expected results. Drop hammers do not debond the reinforcing steel from the broken concrete. Failure to debond the steel will result in reflective cracking.
All of the data showing the benefits of Rubblization have been derived from concrete broken by Resonant hammers. The “fractures” throughout the slab of low amplitude, high frequency resonant broken concrete eliminate slab action and prevent reflective cracking. Because these fractures remain in a “jigsaw” type relationship to each other they provide substantial load distribution over the base, each piece distributing some of the load to the next Figure 2. Breaking of the slab in consistent sizes is important for homogenous flex of the slab to evenly distribute imposed loads over the base.
A low amplitude, high frequency resonant impact fractured slab does not drive broken pieces of concrete into the base material. The base of a resonant broken slab remains as smooth as the day before it was broken Figure 2. Low amplitude, high frequency resonant impacts dissipate all of the breaking energy by the time the fracture propagates through the slab, leaving the bottom of the slab smooth and the base material neither invaded nor disrupted. The low amplitude of the hammer does not drive the broken material into the base and does not damage cement treated bases or underground utilities.
Compare the surface and edges of concrete broken by a high impact, multi-head drop hammer Figure 3 and Figure 4, with that of concrete rubblized by a resonant hammer Figure 5 and Figure 6.
Economics: Rubblizing vs. Reconstruction
Rubblization is a form of “rehabilitation”, comparable to “new” construction. When a concrete slab has deteriorated to the point that either some form of rehabilitation or total reconstruction is necessary, the cost comparison is dramatic. A comparative analysis was made on a project in Arkansas. Total reconstruction cost was 3.4 times greater than rubblizing. Other states have experienced cost ratios from total reconstruction vs. rubblization in the range of 3.3:1 to 4:1. In addition, rubblizing takes but one-fifth of the time and is not nearly as disruptive to the public. The resonant hammer breaks from 6,000 to 10,000 square yards of concrete per day.
Rubblization of a properly selected project, and a properly designed procedure can produce a rehabilitated road that may be expected to last in excess of 22 years, approximating the life of reconstruction, at a fraction of the cost in both time and money. Other methods, from crack and seat to patching, have proven to be short-term remedial solutions that must be regularly reworked and maintained. Rubblized pavements tested with the Falling Weight Deflectometer method have shown that the inherent strength of the rubblized layer is between 1.5 and 3 times as effective in load distributing characteristics compared with a high quality dense graded crushed stone base.
This strength has been measured and has increased each succeeding year. One of the reasons for the improved deflection measurements each year is that the surface fines produced by the low amplitude, high frequency resonant hammer are continuously wedged deeper into the fracture cracks by the traffic loads and vibration. The edge drains have also further dried the base material, increasing the modulus i.e., producing smaller deflections.
Many urban areas have interstate and/or traffic diversion loops that are highly traveled by both local and through traffic. These arteries are very sensitive to traffic disruption from both the using public and from the perspective of the businesses affected by construction. Rubblizing has been the most effective way to rehabilitate these arteries with minimum impact on both the driving public and local commerce.
Work schedules at night or during limited hours can be designed to accommodate specific circumstances and still produce a new road in one-fifth of the time it takes to reconstruct. The Raleigh, North Carolina Beltline rehabilitation Project on I-440, a rubblization and widening project, earned the National Quality Initiative (NQI) Achievement Award.
In 2000, the state of Arkansas embarked on the largest rubblization commitment to date. Nearly three hundred (300) miles of four-lane interstate have been selected to be rubblized. These projects are currently underway and will be completed in four to five years. Both Rubblizing methods (Resonant and Multi-Head) were tested and evaluated. The Resonant method was selected as the only method to produce a true rubblization product, and the method to be used on the entire Arkansas project.
Very infrequently, a rubblizing project will encounter limited areas where the base material has deteriorated to the point that it cannot support substantial loads on the broken slab. The conditions are most often in low areas, in areas with substantial amounts of trapped water, in areas with a high water table further contributing to trapping water under the slab, and in areas where the base consists of a very wet clay with silt imbedded in the clay.
Indications that an area may be unsuitable or needs to be removed and replaced can be seen when the resonant breaker leaves tire ruts of two inches or more. These areas will not hold up and should be removed and replaced. However, if the resonant breaker is causing the slab to pump but does not produce significant tire rutting, the slab is supporting the weight of the 60,000-pound machine and should be sufficiently strong to support traffic loads as be edge drains begin to drain the base and sub-grade. The weight of the resonant breaker is sufficient to expose these conditions without the use of a rubber tired “proof” roller.
A heavy rubber tired “proof” roller has the potential of causing damage to the base and to the interlocked rubblized concrete. Two to three passes by a 10 ton, smooth drum vibratory roller are sufficient to settle the surface fines into the surface cracks, further increasing the modulus of the broken concrete, to settle the broken slab into any voids that may exist in the base, and to produce a uniformly smooth surface to pave on. One pass of a water truck over the surface before the final “vibratory” pass has proven to help prepare the surface for paving.
Several states have in their specification language the concept that the Rubblized concrete needs to be “compacted”. The slab is fractured, not pulverized, and while the surface can be smoothed and “settled”, no compaction in the general sense is possible or needed.
The second part of the Rubblizing system is to get the broken PCC slabs supporting base material dry, and keep it dry. This is accomplished by the installation of “edge drains”. Edge drains are designed various ways, but most commonly they are between eighteen (18) and twenty-four (24) inches deep, twelve (12) inches wide, and are lined with a felt or geo-fabric filter. Laid in the bottom of the trench is either a four (4) inch perforated PVC pipe or a geo-tech covered pipe which is covered with aggregate or pea gravel Figure 7. Approximately every one to three hundred yards, or located in low places, are lateral lines to take the drained water away from the road. The outside edge of each lane direction needs to be drained, as well as all low sides as in areas of super elevation.
The drains in place, the water that has been trapped in the base and sub-grade for years has a way out. The “difference in potential” of an edge drain compared to an earthen dam allows the water to drain away from under the slab resulting in a dry, firmer base, i.e., a higher modulus. Small vibrations from traffic moving over the road facilitate the movement of the previously trapped water. It is important that the base material not be invaded by broken concrete. Every place the base is invaded is a potential water trap that will not easily drain to the edge drain system. The “working” of the broken slab on these water pockets leads to the deterioration of both the base and surface overlay.
The top two (2) to three (3) inches of the Rubblized slab is a drainable layer. Properly designed edge drains extend up to that drainable layer so that water may drain, if and when it gets through a surface crack in the asphalt. The bottom six (6) to eight (8) inches of the Rubblized section is impervious, i.e., water that may come from above never gets to the base or sub-grade.
Beginning in 2002, the great majority of all Rubblizing projects have been overlaid with Super Pave. A properly designed overlay should, when combined with properly fractured concrete slab and well-designed edge drain system, provide a twenty-two (22) plus year highway. Overlay thickness is peculiar to each project considering average daily traffic (ADT), percentage of trucks, modulus of base and sub-grade layers, environment, and design criteria.
Rubblization eliminates inherent distress in hot mix asphalt (HMA) overlays such as:
- Reflective Cracking
- Loss of Bonding/Raveling
- Moisture Damage
- Rough Riding/Faulting
- ASR and other degenerative reactions
Design Procedures information sources are as follows:
- AASHTO Design Procedures (SN)
- AI – Manual Series (MS) – 17
- AI Computer Program (CP) – 4
- NAPA Information Series (IS) – 117
- Various State D.O.T.’s