Review maps which bitumen modifiers could help Bangladesh’s roads survive heat and monsoon

Bangladesh’s roads are under a double assault. Pavements must endure heavy and often overloaded freight traffic while also coping with rising temperatures, flooding, and increasingly erratic monsoon rainfall. A new systematic review, published in Discover Civil Engineering, brings together nearly twenty-five years of global research on bitumen modification to ask which additives could make the country’s asphalt binders more durable under these tropical and subtropical conditions, and why laboratory results so often disagree with one another.

The review, led by Nasif Hossain Imon and M. Neaz Murshed, screened peer-reviewed journal articles and conference papers published between 2000 and 2025 from Scopus, Web of Science, and Google Scholar. The authors emphasize that no laboratory experiments, numerical simulations, or field trials were conducted as part of the work. Instead, they performed a qualitative thematic synthesis, grouping studies by modifier category and performance focus, and deliberately avoided statistical meta-analysis because of substantial heterogeneity in materials, dosages, testing protocols, and climatic exposure across the source studies.

The clearest pattern to emerge concerns rutting, the permanent deformation that develops under repeated wheel loads at high pavement temperatures. Across nearly all modifier categories, including styrene-butadiene-styrene (SBS) polymers, crumb rubber, waste plastics, and nano-clay, the literature most consistently reports increased stiffness, higher softening point, and enhanced resistance to permanent deformation. The review characterizes rutting improvement as the most robust and consistently reported outcome, whereas fatigue performance remains the most condition-dependent parameter, with some studies finding improved fatigue resistance and others documenting stiffness-induced brittleness, particularly at higher dosages or after aging.

The mechanisms behind these gains differ by modifier. SBS, typically applied at 3 to 6 percent by weight of bitumen, forms a three-dimensional elastic network in which polystyrene blocks act as physical cross-links and polybutadiene segments supply flexibility, improving elastic recovery and resistance to deformation. Crumb rubber, reported at roughly 5 to 20 percent depending on the process, swells in the aromatic fractions of the binder and partially devulcanizes, raising viscosity and energy dissipation. Nano-clay, generally used at 1 to 5 percent, creates layered platelets that lengthen diffusion pathways for oxygen and moisture, slowing aging while adding stiffness. Bio-oils act differently, restoring light fractions lost to oxidation in aged binders and improving flexibility, though excessive amounts can over-soften the binder.

Dosage control emerges as a decisive theme. The review reports that performance improvements tend to plateau beyond the upper bound of typical dosage ranges, with excessive amounts often producing instability, brittleness, or phase separation. For Bangladesh-relevant and climatically comparable studies, the authors list reported optimal dosages including PET at 6 to 10 percent, crumb rubber at about 15 percent, crumb rubber-plastic blends at 10 to 15 percent, LDPE and HDPE plastics at 6 to 10 percent, SBS at about 5 percent, fly ash at about 4 percent, wax at about 3 percent, and jute fiber at about 0.5 percent. All values reflect individual study conditions and are presented for comparative reference rather than as design recommendations.

Conflicting findings receive particular attention. Crumb rubber is widely reported to enhance fatigue resistance through improved elasticity, yet some studies note increased brittleness, which the review attributes to variations in particle size, digestion temperature, and degree of devulcanization; insufficient swelling or excessive thermal exposure during mixing can leave a stiffer, more brittle binder. Plastic waste modification reliably improves rutting resistance by raising stiffness and softening point, but thermoplastics such as polyethylene and polypropylene may reduce flexibility under cyclic loading, making fatigue performance sensitive to dosage and compatibility. Climate itself matters: in hot conditions stiffness helps resist rutting, while in cooler or moisture-prone settings the same stiffness can promote thermal cracking and stripping.

The review also flags environmental and practical risks that could complicate adoption in tropical countries. Plastic-modified asphalt raises concerns about potential microplastic release during long-term service, abrasion, or milling, with the extent of leaching still insufficiently quantified. Crumb rubber binders face accelerated aging under prolonged heat, odor emissions during production and placement, and variability in rubber quality from heterogeneous sources. Storage stability and phase separation during high-temperature storage, higher mixing temperatures, increased energy demand, and the need for specialized equipment are cited as barriers, particularly in developing-country contexts where institutional capacity may be limited.

Translating these patterns into Bangladesh’s context, the authors propose a decision-oriented framework built on four dimensions: climate exposure, traffic loading class, budget constraints, and sustainability priority. They note that pavement surface temperatures in Bangladesh often exceed 60 degrees Celsius, combined with intense monsoon rainfall and frequent axle overloading on major freight corridors. Within this conceptual prioritization, SBS and crumb rubber are suggested as candidates for heavily trafficked national highways, crumb rubber or plastic blends for regional roads where cost matters more, and plastic or bio-oil modifiers for low-volume rural roads, subject to pilot-scale validation. The authors stress that this hierarchy is interpretive and feasibility-oriented, and does not imply verified field performance outcomes.

The review is careful about the limits of its evidence. It identifies candidate corridors such as the Dhaka-Chattogram Highway and climate-vulnerable coastal districts like Barisal, Khulna, and Satkhira as indicative planning contexts discussed in prior literature, not as sites with documented performance results. No life-cycle assessment or life-cycle cost analysis was conducted, so environmental and economic benefits discussed in the literature are treated as qualitative expectations rather than measured outcomes. The authors also state that no generalized quantitative estimates of durability enhancement or economic benefit can be derived from the available evidence.

The recommendations point toward regulatory and research steps before any large-scale deployment. The authors call for modifier-specific provisions under the Roads and Highways Department Standard Test Manual and the LGED Rural Road Design Guide, aligned with AASHTO M320 performance grading and BNBC 2020 material criteria, alongside nationally harmonized testing protocols covering moisture susceptibility, aging, and storage stability under tropical-monsoon conditions. Future work, they argue, should prioritize field-scale evaluation of waste-modified binders under local traffic and environmental conditions, detailed rheological and microstructural characterization of locally sourced materials using techniques such as dynamic shear rheometry, infrared spectroscopy, and electron microscopy, and life-cycle assessments to quantify emissions and costs. Until such validation is complete, the review positions modified bitumen as a promising but still exploratory pathway for climate-resilient pavements in Bangladesh and comparable tropical developing countries.

Subject of Research: Bitumen modification strategies for climate-resilient road infrastructure in Bangladesh

Article Title: A review of innovative approaches to enhance bitumen properties for sustainable and climate resilient road infrastructure in Bangladesh

Article References: Imon, N. H., & Murshed, M. N. (2026). A review of innovative approaches to enhance bitumen properties for sustainable and climate resilient road infrastructure in Bangladesh. Discover Civil Engineering, 3(1), Article 273. https://doi.org/10.1007/s44290-026-00656-y

Image Credits: AI Generated

DOI: 10.1007/s44290-026-00656-y

Keywords: bitumen modification, Bangladesh, road infrastructure, crumb rubber, SBS polymer, plastic waste, bio-oil, rutting resistance, fatigue performance, tropical climate, systematic review, pavement durability