Al-Qadisiyah Journal for Engineering Sciences

Influencing interface morphology and modified asphalt binders on interlayer bonding performance

Document Type : Research Paper

Authors

Samer Muayad Alsadik 1
Hasan M. Al-Mosawe 1
Nick Thom 2

1 Department of Civil Engineering, College of Engineering, Al-Nahrain University, Baghdad, Iraq.

2 Department of Civil Engineering, College of Engineering, University of Nottingham, Nottingham, UK.

https://doi.org/10.30772/qjes.2026.163700.1677
Abstract
Interlayer bonding strength is a critical factor influencing the durability and performance of asphalt pavement systems. This study investigates the combined effects of tack coat type, application rate, and substrate surface texture on the interfacial shear strength (ISS) between asphalt layers. Four surface conditions, new wear, binder, aged-worn, and milled, were simulated to represent varying field scenarios. The interface morphology was quantitatively analysed using both the sand patch method and 3D laser scanning. Seven tack coat formulations were evaluated, including a conventional tack coat, hard-grade cutback, and a novel hybrid material modified with Styrene-Butadiene-Styrene (SBS) and polylactic acid (PLA) labels MS4-70 and MSP5-70, respectively. The Tack coats were applied at three residual rates (0.1, 0.23, and 0.35 L/m2 ). Direct shear testing was conducted to assess ISS under various conditions. The results reveal that surface roughness and tack coat type significantly impact bonding performance. Milled surfaces showed the highest texture and yielded superior shear performance due to enhanced mechanical interlock and bonding area. Conversely, the wear surface, characterised by a dense gradation and smooth profile, exhibited the lowest ISS values. The hybrid MSP5-70 tack coat represents a new generation of modified bonding that consistently produces the highest ISS values compared to conventional tack coats (CRS-1, CSS-1h, and RC70). These findings underscore the importance of tailored tack coat formulations and texture-specific application strategies for enhancing pavement structural integrity.

Highlights

  • 3D scanning revealed links between interface morphology and bond strength.

  • A novel hybrid MSP5-70 tack coat delivered the highest shear strength.

  • Milled surfaces maximized mechanical interlock and interlayer bonding.

  • Hybrid modification increased interlayer shear strength by up to 67%.

  • Surface texture and tack coat rate jointly controlled bonding performance.

Keywords

Tack coat
Shear bond testing
Interlayer shear strength
Surface texture
Roughness
Morphology
Crossmark
Subjects
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Al-Qadisiyah Journal for Engineering Sciences
Volume 19, Issue 3
Summer 2026
Pages 285-293

  • Receive Date 20 February 2025
  • Revise Date 16 April 2025
  • Accept Date 30 July 2026

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