The traditional approach to road pavement design can be traced back to the 1950s in the UK and relied heavily on empirical practice; it was acknowledged at that time that a pavement structure is difficult to model theoretically and incorporates materials with complex mechanical properties, write Transport Infrastructure Ireland's Tom Casey and Eddie Winterlich.

Detailed specification of prescribed mixtures

Due to limited computational methods, it was thus by necessity that a simple approach was developed. What evolved was an empirical based methodology – a mix of practical experience and structural theory combined with a means to control material properties through detailed specification of prescribed mixtures. A set of easily interpreted design charts incorporating a high level of conservatism was the focus of the design method.

Within Transport Infrastructure Ireland (National Roads Authority) both authors have participated on European standards development for pavement materials.

The widely practised design method currently used in Europe, is more reliant on basic engineering theoretical modelling incorporating detailed material structural properties, ie an analytical method.

In 2015 we embarked on a major change to our pavement engineering practice based on the principle that the design of any civil engineering structure requires a technical assessment of the mechanistic properties of its component parts together with an understanding of how it can fail in service.

Structural failure

Pavement structural failure is principally caused by traffic loading assessed on the number and the magnitude of wheel loads. For design purposes the concept of a standard wheel load, of 40 kN, is used.

The accumulation of expected loads over its design life is converted to an equivalent number of standard axles using a fourth-power law that derives from the AASHO road test conducted in the 1960s. Environmental conditions also play an important role as pavement materials' mechanical properties change with temperature and moisture conditions.

The two principal modes of failure are cracking of bound layers due to fatigue and rutting in the wheel tracks, which arises because of an accumulation of permanent (irrecoverable) strains. The stress conditions at the top of the subgrade are critical because the fundamental role of the pavement is to protect the soil from being overstressed and deforming.

Irish Analytical Pavement Design Method

TII and its technical consultants Arup conducted research into an analytical approach and determined that the US mechanistic-empirical (MEPD) method of design using theoretical analysis and complex software but also relying on an empirical element was an optimal method for the Irish Analytical Pavement Design Method (IAPDM).

Within IAPDM, the pavement is modelled as a multilayer structure with layers that principally exhibit either linear-elastic, plastic, linear or nonlinear-viscoelastic, behaviours etc dependent on their nature. 

ME Pavement Design utilises the theories of mechanics of materials to predict the pavement's response (stresses and strains) to load. The pavement's response is then correlated into damage, which is accumulated over the design life and results in predicted distresses over time.

Adoption of ME pavement design methodology to replace the existing empirical design methods requires the development of guidelines for practising engineers to enable uniform use of the IAPDM design methods.

Pavement design constitutes one of the essential aspects of any road, not only because it is directly related to its functionality and users’ safety, but also because of its cost-related implications.

The ability to incorporate new materials and production methods, as well as the new requirements and in particular environmental considerations, necessitate the development of innovative methods for the design of new road pavement or for the management of the already constructed ones.

Engineered solutions to renewal programme of existing pavements

Current methods only take into account a small part of the parameters that affect pavement throughout its useful life. Thus, some designs are accepted without considering other possibilities which could provide a more desirable response to external loads and better recoup the investment.

IAPDM is also applicable in the development of engineered solutions to the renewal programme of existing pavements, enabling a range of assessments to extend their useful life and to avoid unexpected failures.

Another unique feature of the development of this revised approach was the acknowledgement of the significant benefits that collaborations between academic researchers, specifiers and industry can deliver.

This approach raises the quality outcomes as it bridges any perceivable gap between theory, standards, and production methodologies. This reflects the opinion at a European level that “standardisation activities are an essential channel for the market adoption of research results and for the diffusion of innovations”.

The range of course presenters (the authors are presenting the following from 9-5pm on Wednesdays until March 9: CPD Certificate in the Design and Lifecycle of Road Pavements – Online Classroom) reflects this diversity of skills and competencies to ensure a well-structured and beneficial management of a valuable national asset.

The timing of this development dovetails with two current dominant issues of national importance, Brexit and climate change. The development of material specifications and design standards independent of our past reliance on UK systems is highly beneficial. 

Climate breakdown demands more than current pledges can deliver. The 2021 circularity gap report estimated that globally the built environment is responsible for circa 50 billion tonnes of raw material extraction each year.

The report also notes that construction creates an estimated third of the world's overall waste, and at least 40% of the world’s carbon dioxide emissions. It further recognises that though we need materials to enable our lifestyles; this produces emissions. Through smart strategies and reduced material consumption, global GHG emissions can be reduced by 39% and virgin resource use by 28%.

Circular economy

The report identifies material transformation to deliver the goal of a socially just and ecologically safe space, requiring intelligent resource management to stem consumption and cut emissions, so their impact falls within planetary boundaries.

The IAPDM methodology combined with a redefining of pavement material properties enables us to be fully compatible with the requirements of the circular economy.

The Ellen MacArthur foundation are leaders in the development of a circular economy. They identify that the core necessity as transforming every element of our take-make-waste system. This consists of how we manage resources, how we make and use products, and what we do with the materials afterwards. They identify that a circular economy is based on three principles, driven by design:

  • Eliminate waste and pollution
  • Circulate products and materials (at their highest value)
  • Regenerate nature

In their report on the need for business to adopt to a circular economy, Deloitte University Press succinctly states: “Used effectively, design and designers truly do have the power to transform nearly everything: concepts, brands, categories, markets, technologies, materials, logistics systems, experiences, industries, even governments.”

Whereas we doubt our pavement course will change a government we are confident that it will enable the collective industry to contribute to delivering high-quality assets at minimal cost to the consumer and the environment.

Authors: Tom Casey C.Eng FIEI; Eddie Winterlich C Eng MIEI

CPD Certificate in the Design and Lifecycle of Road Pavements – online classroom runs until March 9. Click here to book.