Pavement engineering is a constantly evolving discipline as technology developments in construction equipment continue as well as in the field of material and recycling technology in conjunction with calibration of mechanistic-empirical pavement design methods to field performance. Sustainable pavement construction is no longer considered an option as there is a real need to reduce waste and carbon emissions with the aim of delivering economic and environmental benefits. The use of foamed bitumen with stabilisation geogrids in the pavement structure is one such option.
Foamed bitumen (FB) is a cold recycling technology originally developed by Iowa State University f in the 1950s or cold asphalt applications which was later refined by Mobil Oil in which cold water is injected under pressure into hot bitumen to produce foam[1] .
FB mixes can exhibit characteristics of either unbound modified materials or bound materials. The non-continuous” binding of the individual aggregate particles makes FB mixes different from all other pavement materials. The dispersed bitumen changes the shear properties of the material by significantly increasing the cohesion value whilst effecting little change to the internal angle of friction[2]. The transition between bound to unbound behaviour occurs approximately around 3% bitumen content, with higher bitumen mixes behaving more like traditional asphalt. For example, FB mixes in South Africa contain 2.0–2.5% bitumen whilst the bitumen content of mixes produced in Australia typically is in the order of 3.5%[3],[4].
The FB material exhibits void sizes similar of that of entrained air in concrete mixes providing a higher capacity to store and dissipate moisture. This characteristic makes FB to exhibit resilience characteristics after a flood event where conventional asphalt would generally suffer from moisture damage.
The foamed bitumen process is largely being used for the insitu rehabilitation of existing granular pavements using equipment developed by Wirtgen[5]. Secondary binders are generally used in insitu applications and the type and dosage rates will depend on the material properties of the material to be either modified or stabilised.
For FB mixes produced in a conventional batch plant, the mix may not need to incorporate a secondary binder if the FB has been designed to perform as an unbound material. The FB material can also be stored for up to 9-12 months offering the ability to supply FB in short quantities which represents a good option where local demand does not warrant the need to procure hot mix asphalt.
There is great synergy between mechanical stabilisation using geogrids and FB technology as TriAx geogrid and the FB aggregate are chemically and mechanically compatible under an applied load due to the high rib profiles and radial stiffness ratios. This mechanism results in confinement and lateral restraint of the granular particles. Experience to date shows that reinforced asphalt is a cost-effective solution for the maintenance and management of thin asphalt over cemented subbases or concrete pavements. Asphalt geogrids have been used in NSW for more than 25 years[6]
Source: Wirtgen Cold Recycling Technology (2010)
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[1] The Shell Handbook (2003) Fifth Edition, Page 407.
[2] Asphalt Academy (2009) A Guideline for the Design and Construction of Bitumen Emulsion and Foamed Bitumen Stabilised Materials
[3] Austroads (2018) Design and Performance of Foamed Bitumen Stabilised Pavements. Technical Report AP-T336-18.
[4] Transport Roads & Maritime Services (2015) Foamed Bitumen Stabilisation. Pavement Technical Direction PTD 2015/001.
[5] Wirtgen Cold Recycling Technology Manual (2010). Fifth Edition.
[6] RTA NSW (1998). Geogrid Guide. Document No. TP-GDL-017. Pavements Branch. RTA Technology.
