One pavement rehabilitation option that has been gaining popularity in the U.S. recently is unbonded concrete overlays of existing concrete or composite pavements (UCOCP). While thicker (>7”) unbonded concrete overlays have performed very well in many states, current economic restrictions, as well as an interest in using less materials (sustainability), are guiding agencies toward optimizing concrete overlays. This can be done by making the best use of the existing pavement structure and designing a cost effective interlayer and concrete overlay. One area of deficiency in the application of unbonded concrete overlays is the lack of a rational design procedure that addresses all components of the structure and their interaction. While several design procedures have been formulated by local agencies and the concrete paving industry, few are based on detailed research and actual long term field performance. The increasing number of UCOCP projects, as well as test tracks like the Minnesota Road Research (MnROAD) facility, are beginning to provide detailed performance data that can be used to develop a much improved and unified mechanistic-empirical based design procedure for unbonded concrete overlays. Overall guidance on the economics and design of such features as pavement widening, super elevations, and safety features (raising guardrails) are covered in publications like the National Concrete Pavement Technology Center’s (NCPTC) “Guide to Concrete Overlays” (Harrington,et al. 2008). Detailed guidance on using existing unbonded concrete overlay design procedures will soon be provided by the NCPTC publication “Design of Concrete Overlays Using Existing Methodologies” (Fall 2011). A review of the existing design procedures included in that document show some areas in need of improvement and expansion. The 1993 AASHTO Pavement Design Guide is based on performance equations developed from 2 years of testing on the AASHO Road test in the late 1950’s. No overlay sections were included in those experiments. Guidance is supplied principally on successful empirical experience and good general pavement design practices. Certainly the effects of various types of interlayers on long term performance are not considered in the design procedure. The use of the Remaining Life method in assessing the existing structural capacity of the pavement before overlay is acknowledged in the AASHTO guide as being deficient. This method typically leads to conservative (thick) designs, and does not take into account the recommended practice of using smaller panel sizes as slab thickness decreases. The MEPDG (Mechanistic Empirical Pavement Design Guide) is an improvement over the 1993 AASHTO design method, but has limitations as well when designing unbonded concrete overlays. While it can accommodate designs with smaller panel sizes, the minimum overlay thickness is 7 inches. Similar to the AASHTO method, the MEPDG does not consider friction or bond between the overlay and the interlayer. This again can lead to conservative designs. Characterization of the behavior and performance of the interlayer is critical in the design of unbonded concrete overlays. Many different materials have been used for the separation layer. The most common practice is to use an asphalt based material. This material is placed prior to the concrete overlay on existing concrete pavements, or for composite pavements (asphalt overlay on old concrete) the existing asphalt is often milled to a minimum specified depth. Questions arise over whether the separator layer needs to be drainable, or what condition an old asphalt layer must be in to provide the long term functions of a separator layer. Several states, including Missouri, Kansas, Iowa, and Minnesota have recently used non-woven geotextiles for the separation layer. This is an adaptation of the practice in Germany of using geotextile between cement treated base layers and concrete pa

The primary purpose of this project is to create a unified national design guide for unbonded concrete overlays of existing concrete and composite pavements. This consists of the following distinct objectives: 1. Study and understand the field performance history of UCOCP as demonstrated by various test sections and in-service pavements. 2. Determine suitable separator layer (interlayer) materials and properties to insure long term performance of UCOCP systems. 3. Develop a design procedure for unbonded concrete overlays of existing concrete and composite pavements utilizing existing validated performance models, as well as new analytical models derived to address deficient or missing design parameters in existing methods. The design guide will be based on mechanistic-empirical principles, including the effects of various concrete overlay materials, separator layer (interlayer) types, panel thickness (4” to 10”?) and panel size, joint load transfer mechanisms, traffic loads, and climates (nation-wide) in which they must perform.

This research project would be carried out in 5 tasks: 1. Literature review and summary of existing UCOCP design procedures, and survey of performance of experimental and in-service UCOCP projects. 2. Develop separator layer (interlayer) design parameters and performance model(s) for various materials based on field testing results and limited laboratory testing (if needed). 3. Develop new, or improve existing UCOCP pavement response and performance prediction models that incorporate slab thickness (4” to 10”?) and panel size, joint load transfer mechanisms, axle load configuration, condition of the existing pavement, climate (nation-wide), and performance of a separator layer over time. Existing national climate models should be adopted and updated as needed. 4. Develop a unified national mechanistic-empirical design procedure for unbonded concrete overlays of existing concrete and composite pavements. The design procedure should consist of a self-contained spreadsheet or software program, but should be formulated to easily be adopted into DARWIN ME in the future. The procedure must accommodate a variety of climate conditions, axle load configurations, underlying pavement conditions, and time dependent performance of the separator layer. 5. Create a user’s guide on how to use the new procedure.

This study will provide a unified national design procedure that can be incorporated into local or national design guides for highway pavements. States and Agencies will achieve improved performance and lower life cycle costs from unbonded concrete overlays. This study will address many of the deficiencies in existing design procedures, particularly those associated with the selection of interlayers. Participating states to commit $20,000 each year for 3 years.