Lignin fibers typically influence the mixture performance of stone matrix asphalt (SMA), such as strength, stability, durability, noise level, rutting resistance, fatigue life, and water sensitivity. However, limited studies were conducted to analyze the influence of fibers on the percent voids in mineral aggregate in bituminous mixture (VMA) during the mixture design. This study analyzed the effect of different fibers and fiber contents on the VMA in SMA mixture design. A surface-dry condition method test and Marshall Stability test were applied on the SMA mixture with four different fibers (i.e., flocculent lignin fiber, mineral fiber, polyester fiber, blended fiber). The test results indicated that the bulk specific gravity of SMA mixtures and asphalt saturation decreased with the increasing fiber content, whilst the percent air voids in bituminous mixtures (VV), Marshall Stability and VMA increased. Mineral fiber had the most obvious impact on the bulk specific gravity of bituminous mixtures, while flocculent lignin fiber had a minimal impact. The mixture with mineral fiber and polyester fiber had significant effects on the volumetric properties, and, consequently, exhibited better VMA over the conventional SMA mixture with lignin fiber. Modified fiber content range was also provided, which will widen the utilization of mineral fiber and polyester fiber in the applications of SMA mixtures. The mixture evaluation suggested no statistically significant difference between lignin fiber and polyester fiber on the stability. The mineral fiber required a much larger fiber content to improve the mixture performance than other fibers. Overall, the results can be a reference to guide SMA mixture design.

Steel slag (SS) is industrial waste, and there is a large amount of SS to be treated in China. Its disposal generates severe environmental pollution. One of the best ways to use SS is as a road base material. This paper reviews the possibility of using SS in semi-rigid base and evaluates the performance of SS base course. The interaction between three stabilizers (cement, lime–fly ash, and cement–fly ash) and SS is analyzed, and the influence of modifier content on the performance of base course is evaluated. The potential laws between SS, curing time, and unconfined compressive strength, as well as drying shrinkage and temperature shrinkage, are discussed and their effects on the performance of the base course are revealed. The finite element method, discrete element method, and molecular dynamics can be used to analyze the freeze-thaw, rutting resistance, and crack development of SS base. In addition, compared with traditional macadam base, the CO₂ emissions for the use of SS base are slightly more, one of the disadvantages of its use in production, transportation, and compaction. However, considering the overall mechanical, economic, and environmental benefits, it is recommended to use SS in semi-rigid base course. The future research scope of SS as base material is suggested.

The differential settlement between the newly built embankment and the old one is becoming a serious problem in the extension construction of roadways in some areas. To solve this problem, cement fly-ash gravel (CFG) piles are used to control settlements because of their simple construction. Sheet piles, which are mainly used in railway subgrade construction, can provide greater load performance and reduce settlement. The main objectives of this study were (1) to compare the settlement-controlling performance of sheet piles and CFG piles and (2) to evaluate the application of sheet piles to highway extension engineering. To compare the treatment effect, a case study was conducted. The settlements occurring at the instrumented locations were periodically monitored through the embedded settlement plates and horizontal inclinometers. The settlements in the construction processes were continuously observed. It was found that the settlement rate in the sheet-pile area lessened after surface construction, and the settlement was much smaller than that in the CFG-pile area, even though the initial settlement in the sheet-pile area was larger than that in the CFG-pile area before surface construction. The results suggest that the sheet pile has a significant effect on total and differential settlement control in extension construction. The insights in differential settlement indicate that the sheet pile could improve the subgrade bearing capacity and strengthen the stability of the embankment. Sheet piles could be one of the favorable ground-improvement methods to treat the settlement in highway extension constructions when the construction time and deformation are strictly restricted.