By Phil Blankenship, P.E.
Composite materials are all around us—from the shoes (rubber, leather, and cloth) we wear to the asphalt and concrete roads on which we drive. For the purpose of this article, a composite pavement is one that is primarily composed of Portland cement concrete (PCC) with some thickness of hot mix asphalt (HMA) on top of the concrete. One concern with composite pavements is reflective cracking.
Reflective cracking is a result of horizontal and vertical movements at the joints and cracks in the underlying PCC. Temperature changes cause the slabs to contract and expand, causing horizontal movements at the joints. Traffic loadings also may produce differential vertical movements at the PCC joints and cracks, causing shear and bending stress in the HMA overlay. The outcome is a pattern of cracking in the asphalt surface that reflects the joints and cracks in the underlying PCC.
Reflective cracks allow water to penetrate the surface and migrate into the aggregate base and subgrade, weakening those layers. Reflective cracks are more than an aesthetic concern; they eventually weaken the overall pavement structure and reduce ride quality.
The key to delaying or eliminating reflective cracking is to reduce or eliminate the stresses and strains produced in the asphalt overlays. There are several treatment strategies for providing crack relief for composite pavements. These include:
- Removing all the HMA, then using fractured slab technology (rubblization, break-and-seat, crack-and-seat) to stabilize the PCC before placing the HMA overlay. (This is the only way to eliminate the underlying slab action that causes reflective cracking.)
- Placing an HMA overlay, then Saw-Cut and Sealing the overlay.
- Reinforcing the existing pavement using grids plus an HMA overlay.
- Absorbing high strain (or movement) and waterproofing with fabrics and asphalt interlayers plus an HMA overlay. (Fabrics and asphalt interlayers also waterproof the underlying layers.)
While rubblization and other slab-fracturing technologies have proven to be cost effective, they may not be feasible in all situations. If the composite pavement has a substantial HMA thickness built up over the years with overlays, the agency may not want to mill and remove their investment in structural capacity.
With any fractured slab technology, all HMA must be removed for effective PCC breakage. Other reasons that fractured slab technology may not be preferred include grade-matching concerns, policy issues or unsuitable underlying support conditions. In some cases, agencies may just want more options.
When fractured slab technology is not an agency choice, interlayers provide a practical option. Interlayers substantially reduce reflective cracking but do not completely eliminate it. Interlayers can also speed construction by not having to expose the underlying concrete pavement.
There are many interlayer options that vary in performance and cost. The remainder of this article will focus on an example of a highly strain absorbing, reflective crack relief interlayer (RCRI).
Near the Asphalt Institute’s headquarters in Lexington, Kentucky, a five-mile reflective crack relief interlayer was placed on I-75/I-64. This section of highway is one of Kentucky’s more heavily loaded pavements with nearly 40-million equivalent single axle loads (ESALs).
After milling part of the existing worn out overlay and leaving the old concrete layer untouched, the RCRI was placed by ATS Construction. This crack relief layer was a 4.75-mm nominal size fine-graded mixture with 8 percent polymer-modified asphalt cement. It was compacted to 96 percent solid density to aid in retarding cracks and blocking water intrusion into the base. The mixture was designed with low air voids and the polymer was specifically suited for crack absorption. SemMaterials, L.P. supplied the interlayer asphalt, design, and field support for ATS Construction. SemMaterials’ brand is called the Strata® Reflective Crack Relief Interlayer.
One unique criterion for this mixture was that it had to meet performance based specifications. The Kentucky Transportation Cabinet’s (KYTC) specifications included a high-strain fatigue test that has been found to simulate movement from the existing cracks and concrete joints, as well as a stability test to demonstrate that the mixture can withstand construction traffic.
The RCRI was covered with three inches of Superpave 19-mm base and 1.25 inches of Superpave 9.5-mm surface. The asphalt binder for the base and surface courses was polymer-modified PG 76-22. These standard KYTC mixtures were chosen to protect the interlayer and resist rutting on this highly loaded pavement.
According to Steve Farmer, KYTC District 7 Operations Engineer, the asphalt RCRI was selected to retard reflective cracks from the old concrete pavement and extend the overlay life of this busy section of highway.
KYTC has placed four RCRI projects since 2000 by different contractors and suppliers. Nationwide, 20 Departments of Transportation have placed this type of interlayer over nearly 400 miles of highway. More than 1700 lane-miles have also been placed throughout China.
Implementing the Technology
The methodology for addressing reflective cracking of composite pavements has been often described and is generally well known but has not been fully implemented. Many agencies have tried various layer combinations and materials over time. Because reflective cracking is not immediate, many agencies struggle to understand this complex pavement problem. It may take two to four years for 100 percent of the reflective cracks to reappear, and then another two to four years for the resulting water damage to force pavement repair or replacement.
Final product solution cannot be found in just one trial. Agencies should encourage research and product trials. Trials should lead to product improvements. Agencies must be willing to take the “risk” and try the new products on their projects. Equally important, agencies must monitor the projects once they are placed.
Cracking data from many projects shows that the RCRI slows the rate that cracks reappear by about 70 percent when compared to a standard overlay with no interlayer.Cracks over the interlayer reappear at a rate of 6 percent a year while cracks in a typical overlay reappear on an average of 20 percent per year. Reflective cracks rarely penetrate through the interlayer so the interlayer can continue to maintain the ride quality and protect the base from moisture.
RCRI technology again demonstrates that innovative asphalt pavements with proper engineering can improve some of the toughest problems such as reflective cracking.
|Phil Blankenship is the Senior Research Engineer in the Asphalt Institute’s Division of Research and Laboratory Services.