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Enhancing DARWin ME to Design Pavements with thin Asphalt Surface Layers


Many states in the United States (US), which have good support layers and low to moderate traffic levels, often design pavements with thin asphalt wearing courses of 2-inch thickness or less and surface treatments as a riding surface. These wearing surfaces are used to provide a smooth ride and to water proof the structural base layer. These thin surfacings are also popular in Full Depth Reclamation projects where the existing layers are stabilized and new base materials are added followed by a thin surface. The vast majority of the road network in many states includes this type of pavement structures. The new American Association of State Highway and Transportation Officials (AASHTO) DARWin ME pavement Design guide is currently not recommended to be used when designing these thin surfaced pavements. The guide was developed for pavements which require thicker structures. However, given the number of miles of low-volume roads built and maintained each year, this is a major limitation affecting many Departments of Transportation (DOTs) and localities who have limited budgets and who would like to generate cost-effective pavement designs. One major concern about the new design guide is its inability to give sufficient structural benefit to granular base layers on good foundations (which include high stiffness subgrades) which are found in many southern and western states in the US or when bases are placed on stabilized subbase layers. When these good support conditions exist, sufficient performance data exist to more realistically account for the improved stiffness and load bearing capabilities of these confined and well-supported base layers.


This project will identify best practices in the structural design of pavements where the final surface layer is thin, defined as two inches of hot-mix asphalt pavement or less. Pavements where the final surface is either a one- or two-course surface treatment (i.e., chip seals, slurry seals, etc.) will also be included in this work.


There are millions of miles of thin surfaced roadways in the United States which are performing very well. The lessons learned from the design and performance of these roadways needs to be captured and incorporated into the DARWin ME framework because it does not adequately support the design of thin pavement structures. The need to have a pavement design method for thin surfacings is urgent in many states. The goal of this project will be to provide agencies with a design tool for thin surface pavements which will allow them to arrive at cost effective designs (saving money).


Task 1. A literature search is required on US and International design standards for thin pavements. This will include a review and summary of design practices of several DOTs as well as those used in South Africa, Australia and New Zealand (as well as other countries and agencies).

Task 2. Assemble and analyze the non-destructive test data and pavement performance data (including APT data) on thin pavement sections. Recommend realistic material properties for stiff subgrades, for subbase layers that have been treated with a stabilizer, and for granular base layers sitting on very good support layers. Identify pavement performance models for rutting, cracking and roughness applicable to this type of pavements.

Task 3. Recommend models and the appropriate calibration factors for incorporation into the DARWin ME Pavement Design Guide for the computation of layer rutting (once the base becomes wet), roughness, and shear failures. All these distresses should be included in mechanistic models using the current framework in DARWin ME. The new models should identify the terminal conditions that are relevant to this type of pavements. High deflections and higher rut depths may be acceptable for this type of pavements.

Task 4 In addition, develop recommendations on a secondary design check module for thin pavements where evaluations are made of the heaviest truck loads anticipated over the designed pavement to guard against “single shot” shear failures of base or subgrade layers.

Sponsoring Committee:AKP30, Design and Rehabilitation of Asphalt Pavements
Research Period:24 - 36 months
Research Priority:Medium
RNS Developer:Trenton Clark
Date Posted:01/27/2014
Date Modified:01/28/2014
Index Terms:Pavement design, Wearing course (Pavements), Subbase (Pavements), Full-depth reclamation, Pavement layers, DARWin-ME (Computer program), Thickness, Mechanistic-Empirical Pavement Design Guide,
Cosponsoring Committees: 

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