Cracking resistance of thin-bonded overlays using fracture test, numerical simulations and early field performance Sarfraz Ahmed a *, Eshan V. Dave b1 , William G. Buttlar c2 and Marvin K. Exline d3 a National University of Sciences & Technology, College of Civil Engineering, MCE, Risalpur Cantt - 24080, Pakistan; b Department of Civil Engineering, University of Minnesota Duluth, 254 Swenson Civil Engineering Building, 1405 University Drive, Duluth, MN 55812, USA; c Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 1212 Newmark Lab, MC-250, 205 N. Mathews Avenue, Urbana, IL 61801, USA; d Road Science LLC, 8400 S. State Road 46, Terre Haute, IN 47802, USA (Received 3 July 2011; final version received 9 July 2012) Thin-bonded bituminous overlays are becoming an increasingly popular pavement maintenance treatment, which can be used to restore smoothness, seal and renew the pavement surface and increase skid resistance. Thin-bonded overlays (TBOs) are constructed using a specialised type of paving equipment called a ‘spray paver’. A spray paver combines the operation of applying a tack coat and laying down asphalt concrete in a single pass. This allows for the application of a high rate of polymer-modified asphalt emulsion tack coat. Due to reduced thickness, cracking distress is more of a concern in this type of system. This paper describes a new approach for the evaluation of the cracking performance of TBO systems through fracture mechanics-based testing of laboratory and field specimens. Computer simulations and early field performance data are also used in the evaluation. This study is conducted in the context of three field projects which encompass seven different pavement test sections. The test sections allowed a number of variables to be studied, including type and application rate of tack coat emulsion and type of hot-mix asphalt gradation structure (gap graded vs. dense graded). Comparisons are also made between overlays constructed using spray paver and conventional paving process. All seven sections were computationally simulated to evaluate their performance in the context of thermal and reflective cracking potential. Fairly good agreement is observed between laboratory tests, computer simulations and field performance data. The results indicate that good thermal and reflective cracking resistance are expected from TBOs. Furthermore, it was observed that the cracking performance of TBOs depends on the type of gradation for the overlay mixture and the tack coat emulsion type and its application rate. Keywords: thin-bonded overlay; cracking; compact tension test; fracture energy; field performance; numerical simulations 1. Introduction In recent years, spray paver-applied-bonded wearing courses have become an increasingly popular pavement maintenance and rehabilitation treatment option [1–3]. Bonded wearing courses or thin-bonded overlays (TBOs) are constructed using specially built paver, which is commonly referred to as ‘spray paver’. A single-pass asphalt paving process is utilised, whereby the spray paver applies the tack coat emulsion and places the hot-mix asphalt (HMA) in one pass. This construction process enables the use of polymer- modified asphalt emulsions (PMAE) at high application rates and eliminates its loss through ‘tracking’ by construction equipment. Simultaneous application of tack coat and HMA facilitates interaction of the loose HMA with uncured emulsion, which provides additional binder coating of the individual mix particles with unaged polymer-modified binder residue from tack coat emulsion. Furthermore, the elevated temperature of the paver screed (350–3758C) promotes upwards migration of the tack coat emulsion [4]. Permeation of emulsion into the overlay makes these TBOs non-homogeneous and graded in nature [4]. Spray paver-applied overlays that utilise gap-graded mixes were developed in France in the mid-1980s, and were first introduced in USA in the early 1990s as ultra-thin-bonded wearing course or Novachip [5,6]. In this study, overlays constructed using a range of mixes through the spray paver with thicknesses ranging from 19 to 50 mm are referred to as TBOs. Early performance of TBOs with regard to ride quality and skid resistance has been previously reported [7]. Based on field performance data, previous researchers have reported good skid resistance with reduced hydroplaning risk, reduced ravelling, high-rut resistance, reduced pave- ment noise, lower back spray splash and decreased rate of deterioration by weathering, oxidation and traffic for TBOs [8–12]. Very limited information is available in the context on cracking performance of TBOs. Laboratory fracture evaluation of TBO systems has been described by Ahmed et al. [4,13]; however, no field cracking performance has been reported in the literature. Pavements with a loss of desirable surface character- istics are common candidates for use of TBOs as a maintenance or light rehabilitation option. Since TBOs are not able to provide a significant increase in structural load q 2013 Taylor & Francis *Corresponding author. Email: Sarfraz70@gmail.com International Journal of Pavement Engineering, 2013 Vol. 14, No. 6, 540–552, http://dx.doi.org/10.1080/10298436.2012.711474