Presenting ‘Designing with statistical certainty’ at CAPSA 2020

Roads & Bridges | CAPSA 2019

Showcasing excellence, the Conference on Asphalt Pavements for Southern Africa (CAPSA) celebrated its 50th anniversary in 2019 and brought together local and international experts.

These included Ané Cromhout, a JG Afrika pavement engineer, who presented a paper, titled ‘The Impact of probability density function (PDFn) on indicators relating to pavement performance for flexible pavement structures’. It was co-authored by Paul Olivier, Managing Director of JG Afrika, together with JG Afrika pavement specialist Dr Emile Horak.

PDFns are used to describe the probability distribution of a continuous random variable and are an invaluable tool for optimum road design, quality control, and durability.

“PDFns minimise the “data-cleaning” process by removing outliers to efficiently detect localised pavement structural weak spots that would otherwisehave been hidden due to the law of averages,” explained Cromhout. “Application of the analysis process using PDFns is nearly limitless, enabling engineers to make better judgement calls and apply critical thinking to the way in which data is handled.”

Horak presented a paper, titled “A paradigm shift on permeability of thin asphalt surfacing”.

“Typically, the PDFn of density and voids in the mix (VIM) have been shown to follow the normally distributed bell-shaped curve,” said Horak. “The average and mean values may be inside the density or voids range specifications. However, there may be values beyond the known precipice of density and voids, which is noteworthy considering that experience shows that asphalt surfacing fails due to the outliers and not, as previously thought, the values of the averages.

“Statistical procedures should, therefore, be re-examined to reflect the PDFns of as-built results to provide a better appreciation of variability outside the specified ranges,” he continued.

He also noted that various aspects relating to the construction and mix design could contribute significantly towards vertical permeability. Factors such as micro-cracks due to rolling technique or cold compaction and the continued use of rolled-in-chips (RIC) are now known to be the main facilitators of water intrusion into asphalt layers.

Horizontal permeability in asphalt layers can be between 3 and 10 times higher than vertical permeability. For this reason, asphalt layers often display confusing signs of permeability in areas where the traditional indicators, such as VIM, may not support this.

“Furthermore, vertical and horizontal permeability are seldom measured separately in the laboratory,” he explained. “In South Africa, the field permeameter (Marvil) is also prone to measurement error.”

Horak also suggested that permeability potential be calculated via published correlation relationships with voids and other aggregate packing information. This should be used as a benchmark to determine permeability potential even before more detailed field or laboratory measurements of permeability are undertaken.

He also noted that it had recently become apparent that permeability potential of hot mix asphalt (HMA) could be monitored via binary aggregateprinciples that were correlated with Bailey ratios and porosity calculations from grading and aggregate packing. An improved understanding of the interconnectedness of voids can be established and used in a benchmark type quality assurance and control approach to determine whether an asphalt layer is prone to high permeability.

“Moreover, the actual indirect benchmarking of permeability can be undertaken using international studies based on a wide variety of published information that is linked to packing principles, VIM and pore size of interconnected voids,” he added. “It can discern between highly variable and less variable permeability of HMAs – even if only used to determine the PDFn of the derived permeability.”

Cromhout also participated in a workshop on the optimal design of asphalt that was held on the last day of CAPSA 2019. It continued the focus of a workshop that was held at CAPSA 2015 to transition South Africa towards performance-based testing and the finalisation of TRH8.

Specialised Road Technologies also exhibited its new digital video analysis technology at CAPSA 2019. Developed specifically for Airports Company South Africa, the technology is also being enhanced to create a road-asset register in a geographic information system platform for Bakwena Platinum Toll Concessionaire.