TECHNICAL NOTES

Technote 11: Foamed Bitumen

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Introduction

General

Bitumen must be made mobile (or liquid) to be used in any pavingapplication. That is, the viscosity of the bitumen must be decreased. Bitumen is semi-solid at room temperature and to make it mobile requires the following processes separately or in combination:

1. Application of heat
2. Addition of solvent
3. Emulsion formation
4. Foam

In the bitumen foam process, hot bitumen is permitted to expand by the controlled addition of small amounts of water or steam to form a bitumen foam. This process was patented in the USA by Professor Cysani of Iowa University in the mid-1950s. Mobil acquired patent rights in 1968.

The foam process was first used to mix sands with bitumen to create stabilized pavements. The steam process proved cumbersome and inaccurate for the even distribution of the bitumen in the finished product, however, due to the difficulty of controlling the degree of expansion of the foam and its rate of collapse.

Mobil Oil Australia solved many of these difficulties by developing a cold water system that injected cold water directly into the bitumen. This process requires control of only two variables - water and bitumen. At first, foam was made in a single chamber and metered through a series of nozzles to be mixed in a pugmill with damp aggregates. The system was revised so that hot bitumen is the metered material and the foam is made in individual expansion chambers in each nozzle.

The current system has been further developed to allow the use of:

• computer control to meter all components precisely
• surface active additives to control foam expansion and life
• different jet/nozzle types for different applications

ranges suitable for producing basecourse mixes for rural and heavy duty pavements, patching mixes, and as straight binder for the spray sealing (or chip sealing) of roads. The advantages of foam systems will be discussed at length later in this module.

Physical Characteristics

When water and hot bitumen are mixed, the water expands as it changes from a liquid to a gas phase (steam). The huge volume increase associated with this phase change effectively aerates the bitumen and can cause it to expand to approximately one thousand times its original volume. The absolute degree of volume expansion of the bitumen is dependent on the bitumen chemistry and grade, its temperature, the amount of water, and the presence of any surface active chemicals in the system.

Foam composition by mass is typically 97 percent bitumen, 2 percent water and 1 percent additive; however, by volume this material will contain about 95 percent air. Thus, the viscosity of bitumen foam is dramatically lower than the viscosity of bitumen at the same temperature. Foam may be characterized by the degree of volume expansion and the time taken for the foam to collapse to half its maximum expansion (half life). Both properties are important. The degree of expansion will determine the for coverage or "wetting" of the aggregate.

The presence of additives allows foam characteristics to be customized for a specific application. For example, the preferred level of foam expansion for spray sealing applications is 8 or 9; it is 12 or 14 for enriching asphaltic pavements, and 16 to 17 for stabilization or cold mix operations.

To determine the level of additives required for a specific application a full scale laboratory evaluation-is mandatory before any practical work is carried out.

Figure 4 shows a typical graph that plots volume expansion, water content, additive level, and half life to show the dependence of these properties on each other.

General Advantages of Foam

Bitumen foam is a useful method of adding bitumen to damp or cold aggregates. The aggregate in conventional hot mixed systems must be heated and partially dried before mixing with bitumen. The finished product may not be allowed to cool before application and can be stored only for short periods of time in specially constructed hot storage bins.

The addition of special adhesion additives to the bitumen foam displaces water from the surface of electronegative aggregates and creates a strong adhesive bond. The low viscosity of the foam ensures wetting and film formation. For cold mixes, the foamed bitumen appears to create thin films of water between the coated particles. This prevents the bitumen from coalescing into a 3D matrix that would keep the mix from curing. However, on compaction these films are squeezed out and the mix is cured into a rigid structure capable of accepting light traffic. In well drained areas and under favorable weather conditions, the mix may take several days to reach a fully cured state. This contrasts with the use of bitumen emulsions for the same application where the higher initial water levels in these mixes may take many months, even years, to cure fully. From a practical viewpoint, cold mixes prepared by the foam process may be stockpiled for a significant period (up to several weeks) before being successfully compacted into the final pavement.

Kerosene (or other hydrocarbon solvents) is not required as cutter stock. Therefore, foamed bitumen can be considered an environmentally acceptable alternative to the use of bitumen cutbacks.

• Foam is another way of applying bitumen.
• Foam may be mixed with aggregate or sprayed for sealing applications.
• Foam mixes may be made in situ for recycling old road bases or may be plant made mixes.
• Foam mixes made in a plant may be stockpiled for up to one month.
• Foam mixes can be made with recycled or virgin aggregates.
• Foam mixes can replace hot mix as base courses in multi-layer pavements.
• Foam mix combined with an emulsion system can be used for all layers of a pavement mix design, overcoming curing problems.
• Foam mixes are more economical than hot mix and can make use of cheaper equipment.
• Foam systems overcome fume problems.
• Foam spray systems cure faster than emulsions or cutbacks.
• Bitumen foam is environmentally friendly.

Applications of Bitumen Foam

The advantages of bitumen foam may be utilized in a number of applications

1. Stabilization of granular pavements
2. Cold Recycling
3. Cold Mixes
4. Chip Seal