A Recipe for Rubblization

By: Chuck Rathmann

Rubblization is a technology that is being used more and more by highway departments around the U.S. The Wisconsin Department of Transportation (WISDOT) specification for rubblization differs somewhat from many other states, as illustrated by a two-thirds of a mile rubblization project on Highway 67 in the village of Williams Bay, Wis.

Construction of the project started in late July 1999 and was completed in late August.

When WISDOT originally planned resurfacing work on Highway 67, the section in Williams Bay was not part of the project, according to WISDOT Project Manager Andy Smieja. "The village asked us if this section could be included. We originally thought the concrete could be milled down and blacktopped like a section in the nearby village of Walworth, but when we saw how badly the concrete was faulted, we knew that a different approach would be necessary."

"Of the methods of overlaying concrete pavement with asphalt, the major problem is reflective cracking," Crispell-Snyder Transportation Manager Gene Scharfenorth said. Elkhorn, Wis.-based Crispell-Snyder was retained to provide construction inspection services for the Highway 67 project. "If you just overlay asphalt on existing concrete, in a short time, reflective cracks will appear along the fault lines in the concrete. By breaking the concrete up into small pieces of specific sizes, you get back to a good granular base, eliminating reflective cracking."

Two of the technologies currently available to eliminate the reflective cracking in an asphalt overlay on top of concrete are rubblization and crack-and-seat.

"Crack-and-seat uses a series of large drop hammers to crack the pavement every 2 to 3 ft, and then go over it with a roller to seat it," Scharfenorth said. "It really amounts to dropping a large weight on a concentrated area. This can be a cause for concern in situations where utilities like sewer and water lines are buried not far from the surface. Crack and seat is preferable for more rural areas. Rubblization uses either a series of drop hammers or a sonic rubblizer. Because the hammer drops are distributed over a wider area, there is reduced potential for damage to utilities."

Another challenge in urban rubblization is to get the asphalt to vertically align with the existing curb and gutter. "We were concerned because when you rubblize, you need a thicker layer of asphalt. We could easily bury the curb and gutter. So before rubblization, we milled off an inch of concrete," Smieja said.

Wisconsin started using rubblization five years ago–a bit later than its neighbor, Michigan. It was Michigan’s spec that spread to many other states.

"Michigan’s spec requires that rubblized concrete must result in pieces of less than 6 in. in diameter top to bottom," said Matt Shinners of Antigo Construction. Antigo Construction was the rubblization subcontractor on the Williams Bay project. The firm works on rubblization jobs all over the U.S., and its sister company, Badger State Highway Equipment, manufactures the MHB Badger Breaker machine used on these projects. "This spec wasn’t realistic, and contractors in the field weren’t following it. The Wisconsin DOT worked with us on a unique specification."

WISDOT requires that the top layer of rubblized concrete be comprised of stones roughly 2 in. in diameter, a middle layer to be comprised of stones roughly 3 in. in diameter and the bottom 4 in. to consist of stones no larger than 9 in. in diameter.

"On the site, we dug a test hole with a backhoe to test the size of the resulting particles," Crispell-Snyder Senior Project Manager Scott Bever said. "According to the specification, the largest stones are to be no larger than 12 in. in diameter. We did leave some stones that were larger than 12 in. We couldn’t break them down any further without pounding the rubblized concrete particles into the soil below. This would result in a depression in the roadway."

Other states are experimenting with various other specifications. Illinois is testing two methods at Rantoul Airport. One method uses 3-in. stones on top, stones 9 in. or less in the middle and stones 18 in. or less, but most over 12 in., on the bottom, according to Shinners. Another consists of simply breaking concrete into pieces from top to bottom and overlaying it with asphalt.

"This method will probably have some reflective cracking, but not as much as if the concrete were simply paved over," Shinners said.

In situations where the subgrade beneath the roadway cannot support the pressure exerted by the drop hammers, the crushed concrete can be pushed into the ground, so even thicker asphalt must be used. Poor subgrade also can make it difficult or impossible to rubblize concrete.

According to Scharfenorth, problems with poor subgrade can make it necessary to dig out the pavement and gravel to assess the problem, and then bring in new aggregate to rebuild the base course.

"We did have a couple of areas of the Williams Bay project that presented a problem with poor subgrade," Bever said. "It never got bad enough that we had to remove the existing concrete and aggregate. We just sped the machine up and didn’t pound on it too much."

Speeding up the machine results in larger 6- to 8-in. pieces of concrete. The Badger Breaker’s design allows for independent control of 12 1,000- to 1,200-lb hammers. The impact of the hammers can be reduced individually, and their rate can be varied between 30 and 35 impacts per minute.

The machine also was sped up around manholes. "The slabs around the manholes had deteriorated," Shinners said. "The pieces rocked around a lot and we didn’t get much breaking. We didn’t worry about impact to underground utilities. We generally don’t concern ourselves unless there are box culverts. In that case, we just skip those areas or reduce our impacts."

The machine also was slowed down in places, according to Bever. "The quality of the crushed stone was pretty consistent," he said. "In some places, the pieces were a little big, so they slowed down the machine. Speed was varied based on quality of the concrete and the resulting diameter of the aggregate."

After being rubblized, the crushed concrete is comprised of particles larger than the requisite 2 in. in diameter, according to Bever.

"After being rubblized, the concrete is seated with one or two passes with a vibratory grid roller. The grid is comprised of special grooves that crushes the top layer of concrete," Bever said. "Then a rubber roller is used to compact any uneven areas left by the grid roller. The grid roller tends to bridge some areas, and the rubber roller gets in those grooves. Finally, a steel roller is used to eliminate any unevenness."

In situations where particles are larger, the grid roller is not as effective in reducing aggregate size.

"When you speed the machine up, you get particles in the 6- to 8-in. range," Shinners said. "The grid roller will not break them up further, but it will break off any edges and spawls and make sure stones are well-seated."

Other states treat compaction of rubblized concrete differently. Pennsylvania uses a large proof roller, and New York has a standard compaction specification that is used on all projects, according to Shinners.

Because the project was open to traffic throughout construction, timely paving after compaction was key. "We had to make sure the rubblized concrete was covered with asphalt the same day," Bever said. "This was necessary to keep people from driving on it. Rubblized concrete is easily disturbed by vehicle traffic."

While WISDOT has only been rubblizing concrete for five years, projects in other states have lasted 10-15 years between resurfacing projects, according to WISDOT Pavement Engineer Todd Peschke. Because WISDOT designs all its projects taking into account truck traffic (single axle 18 kips, 1,000-lb loading), the fact that almost 10% of the traffic on Highway 67 consists of trucks should not be a problem.

One factor that can reduce the life span of rubblization projects is moist subsoils. "When you break the concrete, you create a free-flowing system with respect to moisture," Peschke said. "The freeze-thaw cycle can cause heaving of the pavement."

Because of storm sewers serving the area of the Williams Bay project, moisture should not be a concern. With regard to weight bearing, however, rubblized concrete should perform better than a regular aggregate base course due to the interlocking of the concrete pieces, according to Peschke.