A comparative life cycle assessment study with uncertainty analysis of cement treated base (CTB) pavement layers containing recycled asphalt pavement (RAP) materials

Abstract

Cement Treated Base (CTB) is a mixture of aggregates, Portland cement and water that hardens through curing to create a strong and durable material that is widely used as a base course in road pavement construction. This layer can be used in either flexible or rigid pavements, and the range of possible materials used in CTB has been recently expanded to include Reclaimed Asphalt Pavement (RAP). Despite the considerable use of CTB in road pavements, there is only limited information regarding its environmental performance, especially when RAP is added. This paper presents a comparative assessment of the environmental performance of sixteen CTB mixtures, with and without RAP, with different cement percentages, different production methods and different recycling procedures. The thickness of the pavement layers required for a given purpose was calculated for each CTB mixture using a pavement design tool (KENPAVE®) and data obtained from laboratory tests (both primary and secondary data). The environmental sustainability assessment used the Life Cycle Assessment (LCA) methodology combined with uncertainty analysis. The functional unit (FU) consisted of a road pavement structure corresponding to a 1 km stretch of 22 m wide major urban road including a CTB layer that would enable a specified volume of traffic to drive safely over a 20-year lifespan. A cradle-to-gate system boundary was adopted. The characterisation modelling to quantify the potential environmental impacts of each pavement structure was carried out using the CML v. 4.4 2015 impact assessment method at midpoint level. The analysis shows that having higher percentages of cement in the CTB mixture allows a thinner base-course layer, thereby compensating for the increased environmental burdens related to the production of cement and transport. The uncertainty analysis shows that including RAP in the mixture leads to greater spread in the LCA results. Further, the results of a real case study show, regardless of the CTB composition, that mixed-in-place production substantially reduces the environmental impacts compared to central-plant-mixed production. Overall, this research increases the knowledge on the environmental performance of CTB layers containing high percentages of recycled materials and produced using alternative construction methods.