Climate change and performance specifications for pavement materials

In spite of challenges associated with climate change, the roads industry has adopted a performance specification approach to sustainability and resilience of pavement materials. The article will address adaptation measures related to climate change in sealed and unsealed roads. Historically, the impact on unsealed roads is known to be more than in sealed roads although quantification is subject to measures put in place to reduce the extent of the effects of climate change based on the expected performance of pavement materials. 

The direct or indirect contribution of human activity and including natural climatic variability to change of the global atmosphere over analogous timelines is known as climate change. In the context of roads, these changes affect the durability of pavement materials. Stringent measures must be implemented to ensure adaptation and in cases where specifications cannot be met, sustainable roads can be achieved through a more fit for purpose materials methodology. A few years after the first climate change convention saw the introduction of asphalt binder specifications through the Superpave program under the Strategic Highway Research Program. Such progress became the benchmark for the adoption of material related specifications in the road construction industry which led to several other countries customizing specifications suitable to their own needs over the years. 

Performance-related specifications for sealed roads mainly depend on expected changes in layer properties and thickness, traffic loading, moisture and climate loads. The contribution of climate change is unique to the type of pavement and specific pavement materials based on their participation in the integrity of the pavement. Bitumen, for instance, is used to bind the stone aggregate in addition to other mix design components. Its participation in the resistance to effects of climate depends on its ability to meet the specification requirements which consider future changes in temperature. At high temperatures softening of the binder reduces the load-carrying capacity of the pavement whereas more brittle fracture of bitumen occurs at low temperatures. Laboratory testing can inform more on immediate and extended susceptibility to aging and moisture ingression which enables a choice of binders that perform better on exposure to these conditions. The advent of superior performing binders as a result of modification offers more variety to best suit the needs of sealed road projects. 

Expected climate model data have been used as inputs to the Mechanistic-Empirical Pavement Design Guide. The exercise was done to simulate pavement performance and deterioration over time yielding insight on rutting differences that required attention from a material perspective. TRACC EXPERT is another collaborative tool utilized for the development of road techniques adapted to climate change through sustainability performance considerations of user preferences and job site-specific conditions. These available resources, if and when used correctly increase the probability of suitable selections of superior performing pavement materials that take into account climate change. 

Impacts of climate change in unsealed roads are more prevalent as a more fit for purpose approach tends to supersede specifications. The dominant factors in the performance of unsealed roads are weather and associated deterioration mechanisms, surfacing properties and construction quality, traffic loading and effectiveness of maintenance techniques. Marginal materials more often become what is available to work with, hence it is difficult to have a streamlined approach to adaptation. In Australia, a funnelled methodology to sustainable roads involves material, moisture and traffic characteristics feeding into fit for purpose material options. The lowest score is considered as the most fit-for-purpose option. To achieve the required performance; material risk, material options and life cycle cost assessments must be conducted. The outcomes of the process contribute to risk reduction through material modification, material stabilization and moisture management. 

Hazards likely to affect unpaved roads due to increase in temperatures as a result of climate change include rapid drying out of the road, clayey material cracking, development of roughness and quicker generation of dust and lose materials. In extreme cases of un-engineered earth, roads expected changes will be in terms of an increase in precipitation and groundwater level fluctuation coinciding with temperature changes and increased windiness. Superior performing materials that meet adaptation needs will not work in isolation as the very fundamentals of road design factoring in geometry, grading and costs will be tested for robustness in good constructions practices in addition to the preparation of the unknown. Abrupt weather events could also exacerbate the current state of the unsealed roads which usually happens without prior notification or warning. Climate blind specifications are bound to increase the burden of costs on road agencies and governments in the long run thus investment in expertise and participation of various stakeholders is paramount to improved adaptation. 

The use of historical climate data has proven inadequate for the prediction of climatic conditions. Pavement design seeks to augment functionality, safety and longevity. Choice of materials, however, is often selected on the supposition of a static climate which is counter-productive to the expected changes in weather as a result of climate change. Pavement deterioration is accelerated owing to an increase in global temperatures as a result of global warming. Life cycle cost assessments for both sealed and unsealed roads focusing on base scenarios and all other possible options would complement adaptation and identification of fit-for-purpose options. Preservation of the expected lifetime of the road should factor in expected changes in climate whilst choosing materials of appropriate quality and in cases where this is difficult to achieve rather a fit for purpose material use approach be taken. 

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