Like many other highway agencies, Alabama has lots of miles of deteriorated Portland cement concrete (PCC) pavements that are reaching the end of their service lives. Rehabilitating these pavements is a challenge.
Removing the old concrete and replacing it with new PCC pavement is an expensive option. Removing the concrete and constructing a new hot mix asphalt (HMA) pavement is normally less expensive, but it still incurs the cost of removing and wasting the old pavement. Overlaying the existing concrete with HMA is a relatively inexpensive and commonly used alternative. However, with this option, reflective cracking can be a problem.
Many highway agencies are finding that rubblization of the deteriorated concrete and overlaying with HMA is the most effective treatment. Rubblization is the process of fracturing the worn-out concrete into small, typically 2- to 6-in., pieces and converting it into an interlocked but flexible base material. The rubblizing process breaks the bonds between any reinforcing steel and the concrete, eliminates slab movement and reduces the possibility of cracks and joints reflecting through the asphalt overlay.
Alabama and Arkansas Experience
Alabama and Arkansas are two states that have used rubblization extensively in the last few years. Alabama has rebuilt 17 projects and Arkansas has rubblized 42 jobs. All are reported to be performing well. Reflective cracking has not occurred and rutting has not been an issue. Both states have seen a great improvement in the overall ride quality of the rubblized sections.
Alabama hosted a workshop on their experience with rubblization in December 2005. Participants were able to hear the details of a section of I-65 south of Montgomery and to visit the site and see a demonstration of the technique.
Providing pavement drainage is critical to the success of rubblized pavements. Edge drains are installed or replaced prior to fracturing the concrete. Any existing asphalt overlay or patches are removed. The concrete is fractured and rolled to achieve an interlocked, unbound base layer. The typical production rate when rubblizing is up to one lane-mile per day per machine. Then the HMA overlay is applied.
The overlay thickness is determined in accordance with normal structural design procedures. The broken concrete is typically assigned a layer coefficient value of 0.20. The value can range from 0.14 to 0.30. The minimum overlay thickness is normally considered to be 4 or 5 in.
The Alabama test site on I-65 was originally constructed in 1968. It was a 9-in. thick jointed, plain concrete pavement built over 6 in. of cement-treated subbase. The original traffic volume was 3,550 vehicles per day with 14 percent trucks. By 2004, the traffic had grown to 29,120 vehicles per day.
The HMA overlay thickness was about 10.5 in., including an open-graded friction course wearing surface.
With the success of recent projects, rubblization has proven to be a cost-effective means of rehabilitating concrete pavements, saving both time and money.
(This article was reprinted with permission from the Spring 2006 edition of “Asphalt” and www.asphaltmagazine.com.)