Rehabilitating Concrete Pavements with Asphalt

When Portland cement concrete (PCC) pavements exhibit extensive distress such as joint faulting, slab cracking, spalling, or other issues that may affect ride quality, rehabilitation becomes imperative to restore service levels. However, this often requires lengthy lane closures, disrupting traffic flow, increasing user delay costs, and requiring substantial highway resources to ensure safety for both construction workers and the traveling public. 

Using asphalt overlays to rehabilitate PCC pavements is a common practice owing to its cost-effectiveness and reduced construction time compared to alternatives like concrete pavement restoration (CPR) or concrete overlays. According to the Federal Highway Administration (FHWA), nearly two-thirds of concrete pavements in the U.S. have been overlaid with asphalt¹. This process involves placing one or more layers of asphalt mix directly onto the existing PCC pavement or over a fractured PCC layer to improve functional or structural conditions.  

However, the long-term functional performance of these overlays can be compromised by reflection cracking, which stems from concentrated stresses at cracks and joints in the PCC due to traffic loading and environmental factors. Several methods can mitigate this distress, including slab fracturing techniques or using special materials such as stress-relieving interlayers and special crack-resistant asphalt mixes. Significant advances have been made over the past three decades in refining these processes and materials.  

NCAT recently updated NAPA’s IS-117 Guidelines for Use of Asphalt Overlays to Rehabilitate PCC Pavements (originally published in 1995) to provide insights into the state of practice and offer guidance on slab fracturing, structural design, and overlay mix design. 

Slab fracturing techniques aim to reduce stress concentration by breaking the slab into smaller segments to minimize movements that occur at joints or cracks of the concrete layer. Three commonly used techniques are crack and seat (C&S), break and seat (B&S), and rubblization. While the principles are similar, the methods differ in fragment sizes and equipment used. 

Left to right: Guillotine-style breaker, MHB badger breaker, and three rollers. Photos provided by Antigo Construction, Inc.

C&S and B&S involve fracturing slabs into shorter lengths with a guillotine or impact hammer and seating them onto the base using pneumatic rollers. C&S is suitable for jointed plain concrete pavements (JPCP), while B&S is used for jointed reinforced concrete pavements (JRCP). The difference between the two lies in the level of effort required to fracture the PCC layer. C&S aims to produce slab fragments less than 30 inches in size with tight cracks that permit load transfer with minimal structural integrity loss. B&S requires more effort to break or debond the steel reinforcement in the JRCP, producing fragments 12 to 18 inches in size. In both cases, the fractured slabs must be properly seated using pneumatic rollers to prevent voids from forming under the fractured slabs, which could result in movement and premature reflection cracking of the overlay. 

The rubblization process can be used on all types of PCC pavement. It involves breaking the slabs into smaller pieces, typically 3 to 8 inches, using specialized equipment such as multi-head breakers (MHB) or resonant pavement breakers (RPB) to produce a strong aggregate base. The MHB can rubblize a full lane width of pavement in a single pass using a low-frequency, high-amplitude mode. Conversely, the RPB uses high-frequency, low-amplitude resonant energy to fracture the concrete by creating high tension at the top of the layer. The rubblization process usually begins at the outside edge of a pavement, continuing toward the centerline. Vibratory rollers are often used to settle the rubblized layer and provide a smooth surface for the asphalt overlay. 

Following the fracturing and seating process, an asphalt overlay is applied to address structural and functional deficiencies. Structural overlay design can be performed using existing methodologies with minor adjustments. The AASHTO 1993 and mechanistic-empirical (ME) pavement design methods are the most commonly used. The AASHTO 1993 method utilizes the structural number concept to determine the overlay thickness and requires the layer coefficient of the fractured PCC layer as an input. This can be estimated using the AASHTO guide or by calculating the effective slab modulus based on non-destructive deflection testing using a Falling Weight Deflectometer (FWD) coupled with backcalculation procedures. Since FWD testing can only be done after the first asphalt layer is placed, this alternative can be used when field data from similar projects are available. For the ME design method, the elastic modulus of the fractured PCC layer is a critical input that can be assigned based on recommended ranges provided in the ME Pavement Design Guide or backcalculated from similar projects. 

Specialty mixes can further mitigate reflection cracking in the overlay. For example, gap-graded mixtures, which have a high coarse aggregate content and are rich in binder, provide a combination of stone-on-stone contact and added flexibility to minimize rutting and reflection cracking. Stone matrix asphalt (SMA) mixes have been widely used in the U.S. for this purpose since the early 1990s, with state agencies like Georgia and Wisconsin leading the effort. Similarly, asphalt-rubber gap-graded (ARGG) mixes have been used for decades to minimize reflection cracking in Arizona and California.  

High polymer modification can also enhance the elastic properties of asphalt binder, resulting in improved cracking resistance and rutting performance. These mixtures are used in Florida, Oklahoma, and Virginia to mitigate reflection cracking and other distresses. 

Special interlayer mixes have recently been developed to reduce reflection cracking. The Texas DOT developed a crack attenuating mix (CAM), while the New Jersey DOT developed a binder-rich intermediate course (BRIC). Both are fine-graded mixtures with a high binder content typically placed as an interlayer between the existing pavement and the asphalt surface layer. These mixes use high-quality aggregates and polymer-modified asphalt binders, and their performance is evaluated during the mix design process to ensure they meet specified rutting and cracking criteria. With the advancement of balanced mix design (BMD), more products are expected to arise thanks to its focus on performance and innovation potential rather than traditional volumetric requirements.   

Interface materials such as geosynthetics are another option to reduce the reflection cracking potential in composite pavements. The material acts as reinforcement, developing tensile forces near existing cracks and joints and reducing strains in the asphalt overlay. Geosynthetics also relieve stress by absorbing some of the horizontal movement in the old pavement and acting as a hydraulic barrier, reducing the impact of water intrusion.  

All the discussed options can help agencies cost-effectively and promptly rehabilitate their deteriorated PCC pavements. The updated IS-117 guidelines can be found at https://go.asphaltpavement.org/is-117.

Contact Adriana Vargas for more information about this research.

1. Federal Highway Administration (FHWA), 2020. Office of Highway Policy Information: Highway Statistics. https://www.fhwa.dot.gov/policyinformation/statistics/2020/hm12.cfm. Accessed August 8, 2022.