What is foamed bitumen

The use of foamed bitumen for the manufacture of stabilised base materials has increased in popularity due to economic and practical considerations. Although the technology of foamed bitumen is relatively old, dating back to the late part of the 19 th century, sophisticated machinery in the developed world, as well as the increased use of labour intensive construction methods in developing countries, has increased the viability of this method of construction and/or rehabilitation.

The use of foamed bitumen dates back to 1889, when bitumen was first used in full-depth repairs in Nebraska to improve the bearing capacity of the base material. In 1928 August Jacobi produced and patented the first hot bitumen foaming system, and in 1957 Prof. Laids Csanyi of Iowa State University demonstrated the benefit of the addition of foamed bitumen to marginal quality aggregates. The addition of cold water to the preheated bituminous binder results in the formation of the bituminous product known as foamed bitumen.

Advantages of foamed bitumen treatment

Foamed bitumen has numerous advantages over other cold mixes and Hot Mix Asphalt (HMA) with respect to its use as a stabilization agent. These advantages are applicable to all forms of pavement construction. These advantages include:

Conservation of energy: The foamed bitumen is applied in a heated state to cold, damp aggregate, thus negating the necessity to preheat the aggregate which consumes the majority of heat needed in conventional processes.

Suitability of aggregate: Although a limited number of investigations have been conducted in this regard, it can be stated with fair certainty, based on the aggregates already tested and the knowledge of foamed bitumen behavior, that foamed bitumen will be more suited to a variety of aggregates than bitumen emulsions which are compatible only to specific ionically charged aggregates.

Lower binder contents: The foamed bitumen process partially coats the larger aggregate within the mix. This fact ensures that the binder is used efficiently in binding together the small particles and forming a well structured foamed mortar that binds in the larger aggregate.

A reduction in compaction moisture content difficulties: The foamed bitumen causes a significantly smaller increase in the moisture content of the recycled material that has been stabilized with the foamed binder, in comparison to the same material that has been stabilized with emulsion. Thus where materials are at optimum moisture content prior to recycling, foamed bitumen is a more suitable stabilisation agent.

Early strength characteristics: Although the early strength of foamed mixes is less than HMA, foamed mixes are significantly stronger at early age than bitumen emulsion, and can be trafficked immediately after placement, providing the stresses are not excessive.

Stockpiling ability: Foamed bitumen mixes can be stockpiled for future use without any significant strength reduction if the mix is kept at optimum moisture content.

Labor intensive advantages: Due to the fact that the foamed bitumen mix is a cold mix and that the finer particles bind to the foamed bitumen bubbles, the material retains an unmodified aggregate characteristic and can be placed using labor intensive methods.

Cold in-situ recycling and foamed bitumen treatment

In most rehabilitation projects in which foamed bitumen treatment is utilised, application of the foamed binder to the existing pavement materials is accomplished through cold in-situ recycling. Cold in-situ recycling has numerous benefits over more traditional rehabilitation methods. The most prominent of these include the following:

Environmental: Use is made of the existing pavement, thus decreasing the number of spoil sites necessary and minimizing the volume of additional material that needs to be imported from quarries.

Recycled layer quality: Modern recycling machines ensure a consistent and high quality mix.

Structural integrity: Cold in-situ recycling produces thick, bound layers that do not contain weaknesses at interfaces due to their homogeneous nature.

Subgrade disturbance: Cold in-situ recycling causes significantly fewer disturbances to poor quality subgrades than conventional rehabilitation methods. It is a single pass process, with tracks or high flotation tires passing only once over the exposed subgrade.

Shorter construction time: Modern recycling machines are capable of high production rates and thus shorter construction times. This reduces traffic disruption time and is thus a benefit to the pavement user.

Traffic safety: This process results in significantly increased levels of safety to workers and road users.

Cost effectiveness: All the factors mentioned above, make cold in-situ recycling a cost-effective method of pavement rehabilitation.

The interim guidelines

The interim guidelines include criteria for the identification of suitable projects which includes discussion on project selection, risk assessment, the recycling process, traffic and the environmental benefits of using this approach. The mix design chapter discusses mix components and procedures and mechanical tests for the selection of the optimum foamed binder content.

The structural design chapter discusses the typical behaviour of pavements with foamed bitumen treated layers, and the mechanistic-empirical approach for designing the pavement structure is presented. The chapter includes design catalogues for new construction and design charts for deep in-situ recycling. The guideline also includes a discussion on the critical aspects on construction of foamed bitumen treated layers and provides recommendations for quality control. Laboratory test methods for the proposed mix design procedure and the input value and transfer functions neccessary to perform the mechanistic-empirical pavement structure design are contained in the guideline.

For more information regarding foamed treatment of materials please vist the CSIR website

For more information concerning the interim guidelines or the purchase thereof please contact the Asphalt Academy