AustStab Conference 2023: Sustainable Pavements for Future Generations

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Introduction

AustStab is the Australian Pavement Recycling and Stabilisation Association. It is a not-for-profit industry association to help provide a unified approach to the recycling and stabilisation of road pavements and to develop national standards of research and performance.

AustStab held the 2nd Australian Pavement Recycling and Stabilisation Conference on 22 August 23. The theme was Sustainable Pavements for Future Generations. It was well attended by the industry and proved to be a very collegiate and informative affair.  The full list of conference presentations and speaker biographies are here. In this article we will summarise some of the more notable presentations.

Key Points / Summary

Australia’s roads are not resilient and not sustainable. A rapid pivot in approach is required. Experts agree that pavement recycling and stabilisation is the way forward, with several key advantages over conventional unbound granular pavements:

  • Lower cost of ownership (low construction cost, longer life)
  • Resilient (to flooding [climate change] and high wheel loads)
  • Truly sustainable (high recycled content, low carbon, no landfill)

There are various stabilisation approaches available (goals, methods, binders). The optimal approach is project/context specific.

The first keynote presentation was delivered by Michael Caltabiano, CEO of the National Transport Research Organisation (NTRO). The Australian Road Research Board (ARRB) is transitioning into the NTRO – expanding its work across all modes of transport – road, rail, ports and airports.

Michael presented the Flood Recovery Response Case Study, where the Victorian Department of Transport and Planning engaged NTRO to conduct asset performance assessments using NTRO’s state-of-the-art iPAVe technology. NTRO completed network wide surveys in 2021 & 2022, assessing road roughness, rutting, cracking, strength/deflection and profile. Many millions of dollars were spent after the 2021 iPAVE survey, resulting in the percentage of ‘good’ or better roads had increasing from 2% to 19% in 2022. In October 22, the Victorian road network was affected by floods.

Flood event Victoria – October 2022

The 2023 iPAVe surveys revealed a worrying degradation of the flood impacted roads, beyond the obvious visible problems (potholes etc). By 2023, there were no ‘good’ or better roads.  Moreover, the percentage of roads classed as ‘fair’ or better dropped from 70% in 2022 to just 9% in 2023.  Clearly, despite significant recent investment, the network was not resilient.

NTRO iPAVE Surveys of Victorian flood affected roads 2021-2023

Michael presented his vision for achieving 100% recycled roads, which hinged on making better, data driven decisions for our road assets. He explained that pavement recycling and stabilisation technology will be central to delivering the sustainable and resilient transport infrastructure required by Australia. He noted that conventional unbound granular pavements are not resilient, as evidenced by recent major failures of new pavements in high load applications. With the predicted trends of higher wheel loads and increasing frequency & severity of flood events, he encouraged the industry to emphasize the technical benefits of stabilisation to the roads market.

Keynote speaker Charles Webster is the Alliance Program Manager for the North Eastern Maintenance Alliance (NEMA). NEMA manages the VicRoads (Department of Transport and Planning) network in the Hume region, one of five regions overseen by Regional Roads Victoria.

In October 2022 Victoria experienced extensive flooding, particularly across the Northern and North East regions of Victoria. The flood required a significant initial response to manage the large amount of road closures that needed to be implemented. Close coordination occurred with emergency services during the acute stages of the flooding as well as mobilising a large workforce at short notice to deal with the response. After the initial response, the team focused on recovery and rectifying the damage that occurred across the 4,000km network.

Victorian Flooding 2022 – Pavement failures

Rectification works included 254,000m2 of granular stabilisation digouts, using 2% cement insitu stabilised to 200mm depth. The process was successful. However, a comparison with recent NSW “Heavy Patch Program” projects suggested that a ‘heavier’ stabilisation treatment would yield a much longer pavement life at a marginal increase in construction cost.

Light (VIC) vs Heavy (NSW) patching comparison

Charles emphasised that resilient pavements were possible, but relied on getting the basics right. The primary defence (against water) was good maintenance – surface seals, drains and mowing. Stabilised pavements are much more resistant to water ingress than granular pavements and provide an effective secondary defence.

Resilient Pavements – Getting the basics right

Kenny Festing from Apexx Engineering Consultants presented the results of his Master of Pavement Technology research thesis on the topic: “Can the use of stabilised material in pavement structures be categorized into a holistic pavement selection framework for use in the Australian context?”. The context for the topic and benefits to the road pavement industry are:

  • Pavement stabilisation will continue to increase in popularity as road asset owners seek more economical ways of maintaining networks.
  • Circular economy an increasing driver – less quarried material, more beneficial reuse.
  • Traffic volumes continuing to increase – mitigating impacts to road users a significant benefit of stabilising.

Kenny began with a series of ‘Stabilisation 101’ slides; briefly covering stabilisation definition, standards, techniques, pavement layers, stabilisation categories, binders and structures.

Next, he summarised the 2001 and 2022 survey results on stabilisation methods and performance of local government roads in Australia. In 2001 it was found that there was general doubt when specifying stabilised pavement materials, leading to poor outcomes. The 2022 survey confirmed that selection processes varied significantly:

  • ‘Pavement Experts’ undertaking detailed design using Austroads specification supplemented with industry publications.
  • ‘General Practitioners’ adopting historical designs or relying on local knowledge, often resulting in suboptimal outcomes.

So, the aim was to develop a methodology that was usable by both experts and practitioners. A 3 tiered selection framework was modelled.

Pavement Stabilisation Framework – Analytical Process
Pavement Stabilisation Framework – Process Overview
Pavement Layers, Stabilisation Categories and Stabilising Binders Selection Flowchart

To illustrate the methodology, he presented a worked example of a local council that has an existing flood affected rural road that is exhibiting signs of fatigue cracking in the wheel paths and is beyond maintenance activities of pothole repairs and patching.

Key points / Summary

  • Stabilisation has been documented as an effective treatment for rehabilitation of road pavements since the 1950s. Its popularity is increasing, driven by the circular economy.
  • Stabilisation offers a cost-effective solution to rehabilitating pavements whilst reusing as much of the original material as possible.
  • Austroads and industry bodies publish a wealth of technical information relating to the design of stabilised pavements. Australian literature is some of the most comprehensive and detailed in the world. However, this information is technical/detailed and the preliminary processes for selecting a stabilised pavement profile are scarce.
  • Stabilisation is often (erroneously) conceived to be ‘too expensive’ or ‘too difficult’ leading to pavement designers ignoring it in favour of less suitable options. The process can be simplified to improve user confidence and ‘demystify’ the selection.
  • The selection tools developed are not intended to replace the detailed technical guidance available – merely supplement to improve the uptake of stabilisation as a practice.

National Asset Centre of Excellence (NACOE) Project O24 aims to demonstrate recycled host material blends can meet the mix design requirements for foamed bitumen and cementitious stabilisation. A project briefing was provided by Dr James Grenfell, National Transport Research Organisation (NTRO) and Meera Creagh, Queensland Department of Transport and Main Roads (TMR).

TMR’s vision is to become a zero waste organisation and transport industry sustainability leader through circular economy practices. Meera outlined TMR’s Waste 2 Resource Strategy, which seeks to:

  • Minimise disposal to landfill
  • Achieve resource efficiency through circular economy practices
  • Facilitate market growth
  • Reduce greenhouse gas emissions from waste generation and resource use

Stabilisation forms a significant proportion of works on Queensland’s rural and urban networks. TMR aims to improve the use of recycled materials in stabilisation works.

The project selected 3 blends types of recycled materials for laboratory investigations:

  • Recycled materials blend: 100%RCC
  • Recycled maximum blend: 40%RCC, 20%RAP, 20%CB, and 20%RCG
  • Recycled minimum blend: 70%RCC, 10%RAP, 10%CB, and 10%RCG

Where:

  • RCC: recycled crushed concrete
  • RAP: reclaimed asphalt pavement
  • CB: crushed brick
  • RCG: recycled crushed glass

Two types of binders were investigated: cementitious stabilisation and foamed bitumen stabilisation.

Binder was applied in sufficient quantity to achieve Lightly-Bound (LB) materials (28-day UCS of 1.0 to 2.0 MPa). Laboratory tests were carried out as follows:

NACOE Project O24 Laboratory Testing Program

Tests confirmed that foamed bitumen stabilisation and cementitious stabilisation are feasible and viable methods to improve the engineering properties of recycled material blends.

The project, now in year 3, will next proceed to field trial on a moderately loaded section of the Brisbane Valley Highway. The field trial will use a 100% RCC lightly bound application because:

  • 100% RCC is a commercially produced blend
  • South East Queensland uses a lot of lightly bound materials
  • Cementitious vs bitumen costs (ie. Cementitious stabilisation costs are significantly lower).

This keynote presentation was delivered by Tony Aloisio, former Director of ecologiQ, Major Road Projects Victoria. After a series of technical presentations by other speakers, Tony took a big picture view. His themes were:

  • Evolution
  • Circularity
  • Visibility

Tony explained that the transition to a circular economy will not be a revolution; the process is an evolution on a compressed timeline. The technology already exists. As a society, we need to accelerate the transition. Pavement recycling and stabilisation is a truly circular solution. In the context of funding scarcity, materials scarcity, deteriorating road networks and the backdrop of climate change, there is a narrow window of opportunity right now to rapidly transition from legacy road construction methods to sustainable methods. The flood events brought by the recent ‘triple-dip’ La Niña has made road resilience a hot topic. Now is the time to act, whilst the country is acutely interested.

Tony noted that industry insiders have long been aware of the sustainability benefits of stabilisation, but the industry has not done a great job to date of communicating these benefits to decision makers and the broader community. He urged the audience to ramp up efforts to make the circular pavement solutions offered by stabilisation more visible to decision makers and influencers.

Visibly Circular Pavement Solutions

General Observations & Renolith Context

The various case studies demonstrated the efficacy of a variety of binders in certain scenarios:

  • Lime
  • Cement/cementitious blends
  • Bituminous

Lime is very good at subgrade stabilisation in high plasticity soils, but cannot achieve high strengths and is not well suited to low plasticity and granular materials.

Cement/cementitious blends are viable and cost-effective in a broad range of materials and applications. The key limitation is that cementitiously stabilised materials are prone to shrinkage cracking at higher strengths / binder contents.

Bituminous binders can be effective in granular, low plasticity materials, making them ideal for certain applications such as rehabilitating Australia’s airport pavements. Initial and lifecycle costs for foam treated base (FTB) and emulsion treated base (ETB) are better than conventional granular base, but typically not as appealing as cementitious stabilisation.

Renolith is compatible with lime, but is most often used in conjunction with cement/cementitious blends. Renolith is nanotechnology (admixture) that prevents shrinkage cracking and significantly improves the performance of stabilised materials. This allows a higher binder content (cement, SCMs, blends) and higher strength, resulting in thin, low-cost, resilient pavements.

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