Design Features

Joint Spacing and Layout

An important design concern for thin concrete overlays is the joint spacing and layout. As mentioned previously, thin concrete overlays typically use joint spacings and layouts that are much shorter than conventional joint spacings, with the following considerations:

• To reduce curling stresses in the overlay and shear stresses at the concrete/asphalt interface, the bonded overlay panels of asphalt pavements are typically 6 ft by 6 ft (1.8 m by 1.8 m) or less in dimensions
• The jointing pattern of the bonded concrete overlay of existing concrete pavement must match the jointing pattern of the existing pavement to avoid reflection cracking.
• Another important issue with the use of short joint spacing is the joint layout in relation to the location of traffic loads. For example, the Minnesota Road Research facility (MnROAD) observed that 4 ft by 4 ft (1.2 m by 1.2 m) panels failed before 6 ft by 6 ft (1.8 m by 8 m) panels because the use of 4 ft by 4 ft (1.2 m by 1.2 m) panels places a longitudinal joint directly in the wheelpath of the trucking lane (Burnham 2008).
  
  

Interlayer

An interlayer (or separation layer) is used in unbonded overlays of existing concrete pavements to separate the overlay from the existing pavement. The interlayer is commonly referred as the “stress-relief” or “crack arresting” layer; that is, the interlayer is a region that allows the overlay and existing pavement to move independently in the horizontal direction. Thus, distresses in the existing pavement are less likely to propagate into the overlay. Interlayers are typically constructed using roughly 1 to 2 inches (25 to 51 mm) of asphalt, however some agencies have experience using geosynthetic fabric as an alternative interlayer.

Joint Load Transfer

Dowel bars are not used in bonded overlays. For thin unbonded overlays the presence of dowels in thin overlays creates constructability issues (e.g., paver clearance over dowel baskets), and the low concrete cover may lead to spalling. Fibers may be added to improve load transfer between panels, and may also be important to prevent erosion of the interlayer and base material and later distress issues (Hansen and Liu 2013). The use of tie bars for longitudinal reinforcement is recommended when heavy traffic loads are anticipated.

Both Colorado and Iowa have documented their experience with the use of tie bars in both thin and ultra- thin overlay projects (Rasmussen and Rozycki 2004).

Shoulder

Shoulder type selection is often influenced by existing conditions and constructability considerations. For example, the presence and type of widening units in the existing pavement can influence shoulder selection. Also, lack of clearance or right-of-way can prevent the construction of concrete shoulders. Regardless of the selection, available design procedures can accommodate and compare different options for the shoulder. Whereas some methods, such as the CDOT method, assume concrete shoulders with tied longitudinal joints by default, the AASHTO M-E method allows for the selection of tied shoulders with variable load transfer efficiencies.

Drainage

Prior to the design process, the drainage capability of the existing pavement should be evaluated to determine if steps should be taken to provide adequate drainage. This may include cleaning existing measures (e.g., underdrains, outlets) to improve drainage during the overlay preparation process. For unbonded overlays, an additional drainage concern is the drainage capacity of the interlayer. In dry climates, the interlayer may not require attention in terms of drainage; however, when moisture penetration is a concern, interlayer design should address the need for moisture to exit the interlayer. One solution is to use fabrics with higher levels of permeability and transmissivity.