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SPRAYED SEALING PRACTICE IN AUSTRALIAWalter Holtrop, Australian Asphalt Pavement Association,Australia
This paper provides an overview of current sealing practice in Australia, including priming andprimersealing, types of sprayed seals and their selection, the current national seal designprocedure, and plant and field procedures commonly used.
Asphalt is the preferred treatment in urban areas, on heavily trafficked urban freeways andarterial roads, and areas of high traffic stresses. Sprayed sealing is the surfacing treatmentcommonly used in rural areas, and is the most economic type of surfacing for the rural roadnetwork. It is also used for specific applications, such as strain alleviating membranes tominimise crack reflection, on all classes of roads.
The main type of sprayed seal is a single layer of binder covered with a single layer ofaggregate (single/single seal) used on both new and resurfacing work. On new work thepavement material is usually locally available gravel, often of marginal quality, with better qualitycrushed rock material used on the more heavily trafficked roads.
Sprayed seal design as referred to in this paper is the design of rates of application of binderand aggregate spread rates. The continued success of sprayed seals as a surfacing requirescare in choosing an appropriate treatment for the conditions, a high standard of preparation ofpavements and attention to detail. To successfully select and design a sprayed seal requires amix of engineering and practical know how.
The technique of sprayed sealing was adopted in Australia because of its relatively low cost andspeed of construction compared to other forms of pavement surfacing, and has been themainstay of road authorities in Australia to provide a safe, all-weather rural road network. Majorfactors that contributed to improvements in the performance of seals in the early days were: Adoption of the design procedure developed by F M Hanson, and national continuous
improvement thereof over many years by state road authorities.
Development of major plant items and practical, proven, field procedures specifically forsprayed sealing works in Australian conditions.
Introduction of hot bulk bitumen, and the use of cutter oil to field produce cutbackbitumen mixtures, on site at the time of use and most appropriate for the prevailingweather conditions.
In 2005, ARRB Group, at the direction of Austroads (the national body which representsthe interests of the state road authorities) arranged a national Sprayed SealingWorkshop for practitioners. Attendees were from road authorities and sealingcontractors, including senior management and technical specialists. The following areconsidered most important of the many issues that were discussed.
Loss of skilled practitioners, and the difficulty in replacing them in the short term. Rapid increase in the number of large heavy vehicles and their effect on seal
performance. Potential embedment of aggregate into new granular bases, and asphalt patches
on existing roads to be resealed, and how to determine the allowance required tobe made for this in the seal design procedure.
Selecting the cheapest type of treatment available.
Unrealistic performance/service life expectations when resealingcracked/distressed pavements.
Developing a formal design method for primerseals.
There are major climatic variations throughout Australia from semi-tropical to extreme hottemperature conditions, and some areas of alpine conditions (see Figure 1). Rainfall patternsrange from high rainfall in the north to extremely low rainfall in the central desert regions.
Figure 1: Variations in Maximum Temperature
Traffic volumes used in seal design are generally provided as Average Annual Daily Traffic(AADT), which is the total traffic carried by the road. Traffic can vary from very low, < 100 AADTfor local roads in rural areas to as high as 40 000 AADT on freeways and major routes.Commercial vehicles in general make up about between 5 and 10% of the total traffic, but thiscan vary from almost nil to as high as 35% on the major freight carrying roads, and over 50% onquarry and mining access roads. The rapid increase in large heavy vehicles over the last fewyears (classed as having seven or more axles) is placing increased demand on pavementmaterials, sprayed seals and the design and construction processes. This increase in largeheavy vehicles is expected to continue.
Some facts about Australia
The following information is collected from various sources such as Austroads Road Facts,Australian Census, AAPA industry statistics. This is provided for background information only,and not as accurate statistics:
Approximately 21 million, the majority of whom live within 100 km of the coast
7.7 million square km (roughly 3000km x 2800km)
(iii) Road network
approx 800 000 km total length
approx 500 000 km gravel surface, earth or unformed
approx 307 000 km surfaced with seals or asphalt, and minor lengths of slurry andconcrete.
(iv) Sprayed seals
Estimated 270 000km (approximately 90% of the surfaced length)
Average annual use varies from 750 - 800 000 tons, with approximately half of this used onsealing works
(vi) Network value
In excess of $100 billion (Australian)
(vii) Annual expenditure on sealing
Approx $450 million (Australian)
(viii) Average seal life
Based on typical intervention levels, for single/single seals from about seven years for smallaggregate to 12 15 years for larger aggregate.
(ix) Sprayers operating
An average of 200 calibrated and operating sprayers are listed on the AAPA web page. Sealingcontractors own most of these, but a number of road authorities and councils also own sprayers.
Bituminous based materials used are covered in various Australian Standards and Austroadsspecifications. Common materials used in sprayed sealing are:
Class 170 (approximately equivalent to 85/100 penetration).
(ii) Cutback bitumen
Cutback bitumens are used for:
sealing, generally C170 mixed with cutter as required
priming/primersealing, Australian Standard grades, proprietary grades and fieldproduced equivalent to the standard grades of cutback.
(iii) Cutter oil
A light solvent such as lighting kerosene or aviation turbine fuel
(iv) Bitumen emulsionGenerally Australian Standard grades of cationic emulsions, with specialty grades developed forpriming.
(v) Aggregate precoating materials
Oil or bitumen based, specialty grade of bitumen emulsion.
(vi) Adhesion agents
Amines to promote wetting and adhesion in damp conditions, and for aggregates with pooraffinity to bitumen.
(vii) Polymer modified binders (PMB)
Manufactured based on SBS, PBD and crumbed rubber polymers, generally used as hotbinders but also available as emulsion.
(viii) Multigrade bitumen
C500/170 for sprayed sealing, designed to behave like C500 in hot conditions and C170 incooler conditions.
Single sized aggregates are preferred because this provides maximum tyre contact and macrotexture for surface drainage. Common aggregate sizes used are 7, 10 and 14 mm, with some16 and 20 mm.Australia is fortunate in that there are sufficient different aggregates available able to meet thespecified requirements for the general range of traffic and climatic conditions. The aim is toselect an appropriate and economic aggregate for the conditions. As deposits diminish, and itbecomes more difficult to open new quarries due to environmental restrictions, it is expectedthat greater value will be placed on aggregates of better quality required for heavily traffickedroads and to meet skid resistance requirements.
Aggregates are commonly manufactured by crushing and screening of suitable rock deposits.Aggregate specifications are reasonably uniform across Australia and appropriate properties arespecified to cover the range of traffic conditions, loads and weather conditions, including specificproperties, for example, such as may be required for wearing qualities and polished stone valuefor skid resistance.Quarries are required to separate production into lot sizes, and test and report on grading,median size, flakiness index and Average Least Dimension (ALD).TYPES OF SPRAYED TREATMENTSSprayed treatments are broadly separated into two main types:(i) Initial treatment on new pavements: prime and seal
primerseal (small aggregate), followed by a final seal (usually larger aggregate) one or twoyears later
prime and small aggregate seal, followed by a final seal one or two years later
a final seal is a part of the selection and design process for the overall initial treatment.
New works are generally constructed using unbound granular materials such as local gravels onlow traffic roads, with crushed rock on the more heavily trafficked roads. When rehabilitatingthese roads they may be overlaid with another layer of granular material, or they may bestabilised using cement. Bitumen stabilisation is more expensive and therefore not often used.Where practical, pavements are primed and sealed, but to minimise disruption to traffic thepredominant treatment is now a primerseal, followed say 12 to 18 months later with a finalsurfacing.The prolonged drought conditions are affecting road construction, particularly in rural areas, andthis aspect has to be considered when selecting a suitable initial sprayed seal surfacingtreatment.Cheaper local marginal materials used in rural locations are