Review on Applications of Lignin in Pavement Engineering: A Recent Survey

Lignin is the second-largest plant polymer on Earth after cellulose. About 98% of lignin produced in the papermaking and pulping industry is used for combustion heating or power generation. Less than 2% of lignin is used in more valuable fields, mainly in the formulation of dispersants, adhesives, and surfactants. Asphalt is one of the most important materials in pavement engineering. It is a dark brown complex mixture composed of hydrocarbons with different molecular weights and their non-metallic derivatives. Because the chemical structure of lignin is similar to that of asphalt, it is a carbon-based hydrocarbon material. More researchers studied the application of lignin in pavement engineering. In this paper, the structure, application, and extraction technology of lignin were summarized. This is a review article describing the different applications of lignin in pavement engineering and exploring the prospects of the application. There are three main types of pavement materials that can be used for lignin in pavement engineering, which are asphalt, asphalt mixture, and roadbed soil. In asphalt, lignin can be used as a modifier, extender, emulsifier, antioxidant, and coupling agent. In asphalt mixtures, lignin can be used as an additive. In road base soils, lignin can be used as a soil stabilizer. Furthermore, the article analyzed the application effects of lignin from the life cycle assessment. The conclusions suggest that lignin-modified asphalt exhibits more viscosity and hardness, and its high-temperature resistance and rutting resistance can be significantly improved compared with conventional asphalt. In addition, some lignin-modified asphalt binders exhibit reduced low-temperature crack resistance and fatigue resistance, which can be adjusted and selected according to the climate change in different regions. The performance of lignin as an asphalt mixture additive and asphalt extender has been proved to be feasible. Lignin can also produce good mechanical properties as well as environmental benefits as a soil stabilizer. In summary, lignin plays an important role in asphalt pavement and roadbed soil, and it is likely to be a development trend in the future due to its environmental friendliness and low cost. More research is needed to generalize the application of lignin in pavement engineering.

Recycled Polyethylene Modified Asphalt Binders and Mixtures: Performance Characteristics and Environmental Impact

This study addresses the effects of recycled polyethylene (RPE) on the performances of both asphalt binders and asphalt mixtures. Whether using RPE in an asphalt mixture might leach harmful chemicals into rainwater or melted snow was also determined. Two processes, wet and dry, were used to formulate the RPE modified asphalt binders and mixtures. In the wet process, RPE was added to asphalt binder. In the dry process, it was added to heated aggregates. RPE from two sources and PG 64-22 virgin asphalt binders from two sources were used in this study. In conclusion, RPE improved the rutting resistance of the asphalt binders and asphalt mixtures. However, it had adverse effects on their resistance to intermediate-temperature and non-load associated cracking. The dry process could produce a mixture with a higher RPE dosage compared with the wet process using one virgin asphalt binder but not the other; thus, the virgin asphalt binder source was a significant factor for the dry process. Based on an embryotoxicity test, it was found that RPE can be used by the asphalt paving industry without creating any significant environmental risks.

Meso- and Macro-Mechanical Analysis of the Frost-Heaving Effect of Void Water on Asphalt Pavement

In cold regions, many types of structural damages are caused by the frost heaving of asphalt pavements. Hence, it is important to quantitatively determine the frost-heaving effect of asphalt pavement using a mechanical method to control frost-heaving damage. In this study, first, the internal voids of the asphalt mixture were regarded as a single void, and the water phase transition generating the freezing water in the voids was simulated using a simplified hollow sphere model to create a uniform internal pressure. Second, the prediction equation of the equivalent linear expansion coefficient was proposed by taking the phase transition of water in the saturated asphalt mixture voids into account. A step function was used during the phase transition of water to determine the sudden change in the equivalent linear expansion coefficient, heat capacity, density, and thermal conductivity. Finally, the typical cooling conditions were simulated with the water phase transition and the nonwater phase transition. The experimental results showed that the proposed model could accurately simulate the effect of frost heaving. Higher stress and strain were generated on the surface and in the interior of the pavement, and the positions of maximum stress and strain occurred on the pavement surface under the frost-heaving conditions. The compressive strength of the asphalt mixture in a uniaxial compression test is about 4.5–6 MPa with a single freeze–thaw cycle. Furthermore, when frost heaving occurs on the asphalt pavement between 5.8 and 6.5 MPa, the numerical simulation method can be used to calculate the internal stress of the structure, which found that the compressive stress under the frost-heaving condition was the same magnitude as the compressive strength under the freeze–thaw testing condition.