Coal Fly Ash Based Prime Coats for Enhanced Interlayer Adhesion in Flexible Pavements

Shaban Ismael Albrka Ali (1) Department of Civil and Construction Engineering, College of Engineering, A’Sharqiyah University, Ibra 400, Oman. 2) National Project for Disaster and Crisis Management, Libyan Authority for Scientific Research, Al-Nasser Street, Tripoli)
Ahmed Suliman B. Ali (3) School of Civil Engineering, College of Engineering, Universiti Teknologi MARA, Shah Alam 40450, Selangor, Malaysia. 4) Libyan Authority for Scientific Research, Tripoli 00218)
Mohammad Al-Zu'bi (Department of Civil Engineering, Munib and Angela Masri Faculty of Engineering, Aqaba University of Technology, Aqaba 11947)
Ashraf Abdalla M. Radwan (School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai 81310, Johor)
Allam Musbah Al Allam (Libyan Centre for Engineering Research and Information Technology, Bani-Walid)
Naeem Aziz Memon (School of Civil Engineering, College of Engineering, Universiti Teknologi MARA, Shah Alam 40450, Selangor)
Munder Bilema (School of Civil Engineering, Tuanku Syed Sirajuddin Engineering Campus, Universiti Sains Malaysia, Nibong Tebal 14300, Penang)

Civil Engineering Journal
Vol. 12 No. 8 (2026): August

The establishment of strong interlayer bonding between the base course and asphalt surface is critical for the durability of flexible pavements. Inadequate adhesion at this junction commonly causes slippage, fissures, and early deterioration, consequently diminishing pavement longevity and escalating maintenance expenses. Conventional prime coats, such as cutback and emulsified asphalts, possess drawbacks, including ecological issues and variable efficacy in adverse site conditions. This study investigates the Prime Coal Fly Ash Coat (PCFAC), which is developed utilizing Coal Fly Ash (CFA), an abundant industrial by-product, as an eco-friendly substitute for prime coats. Experimental work involved preparing granular base layers in accordance with laboratory specifications, followed by the application of PCFAC mixtures at different CFA-to-water ratios and distribution rates. Specimens were cured under laboratory conditions for 24 hours following base preparation and an additional 24 hours subsequent to PCFAC application before testing. Findings indicated that higher CFA content enhanced mixture stability and adhesion, with PCFAC-2.5 demonstrating optimal performance. The optimum distribution rate of 1.75 kg/m² was determined by comparing penetration depth, pull-off strength, and direct shear strength, balancing sufficient infiltration with strong adhesion and effective workability. At this rate, the PCFAC attained a pull-off tensile strength of 322 kPa and a direct shear strength of 3.5 MPa, signifying the highest values among the investigated PCFAC formulations. The findings highlight PCFAC’s capability to enhance interlayer bonding while valorizing industrial waste, thereby contributing to sustainable pavement practices. The manuscript further examines the limitations of the study, such as the adaptation of current testing standards, and proposes recommendations for further research and field validation.