Cold bituminous mixtures

Bituminous mixtures for road pavements consist mainly of mineral aggregate and bitumen. The mineral aggregate can be divided into three fractions according to particle size, i.e. stone, sand and dust. Bitumen is a product that is obtained by refining crude oil and the standard product is called penetration grade bitumen. It is sometimes also referred to as asphalt-cement. The bitumen forms a mastic with the finest particles of the mineral aggregate, which glues the larger particles of the mineral aggregate together. The proportion of bitumen in bituminous mixtures varies depending on the type of mixture and application, but roughly ranges from 1% (added in the form of bitumen emulsion as compaction aid) up to 10% (guss-asphalt) by mass.

Penetration grade bitumen at ambient temperature has a high viscosity. In order to facilitate mixing with the mineral aggregate, the viscosity of the bitumen needs to be reduced. The following processes may be used to achieve this:

• Raising the temperature; • Emulsifying;

• Foaming; and

• Adding selected petroleum solvents (cutting back).

In the first process, the temperature of the bitumen needs to be raised to approximately 155 – 175 ºC to reduce the viscosity sufficiently to facilitate proper mixing with the mineral aggregate. At the same time, the mineral aggregate itself needs to be heated to the same temperature to prevent an immediate reduction in the viscosity of the bitumen on first contact with the mineral aggregate, especially with the larger particles. The bituminous mixture obtained by this mixing process is called hot-mix asphalt, asphalt-concrete or in short, asphalt. Hot-mix asphalt mixtures need to be placed and compacted at sufficiently high temperatures, normally in excess of 110 ºC, before the viscosity of the bitumen, and thus of the mix, increases again as a result of lowering temperatures. At in-service temperatures the viscosity of the bitumen is high enough to provide sufficient stiffness to the asphalt mix.

The three other processes listed above differ from hot-mix asphalt in that the mineral aggregate does not need to be heated and can be mixed with the bitumen at ambient temperatures (depending on the type of cut-back bitumen it may sometimes be necessary to warm the aggregate). The technology is therefore referred to as cold- mix technology and the end-product is known as cold-mix asphalt or cold-mixes. These mixes can be placed and compacted at ambient temperatures.

There are certain other technologies, inter alia partially heating the mineral aggregate (half-warm mixes) or using certain additives to reduce the viscosity at

mixing temperature (Fisher-Tropsch waxes), but these fall outside the scope of this study.

Figure 1: Mixing technologies for penetration grade bitumen

Emulsifying bitumen is generally done using a colloid mill. The bitumen temperature is raised sufficiently high, 140 – 180 ºC after which the bitumen is mixed with an aqueous solution and forced through a small gap in the colloid mill. In this process the bitumen is sheared into small droplets, approximately 5 – 10 micron in diameter, which are suspended in the continuous water phase. The bitumen is the discontinuous phase in the emulsion. The surfactants in the aqueous solution keep the droplets in suspension and give the emulsion its stability. Bitumen emulsions can be anionic or cationic, which refers to the particle charge of the bitumen droplets in the emulsion and depends on the type of emulsifier (surfactant) used. The residual bitumen content in emulsions is generally between 60 % and 65 %.

Bitumen emulsion can be mixed with the mineral aggregate at ambient temperatures. The mineral aggregate may be moist and there is no need to heat the aggregate. Once mixed with the aggregate, the emulsion starts to “break” and the emulsion water to evaporate. Breaking of the emulsion, also referred to as setting, is the process in which the state of the bitumen changes from being dispersed in the water phase to forming an adhesive layer on the surface of the aggregate. The rate of breaking depends on the type of emulsion, which can be from rapid-set, e.g. for spray applications to slow-set, e.g. for pre-mix applications.

Foaming of bitumen is the process in which cold water (foamant water) is injected into hot bitumen (approximately 150 – 170 ºC). The transition of the foamant water from a liquid to a gaseous form on contact with the hot bitumen results in expansion of the bitumen into foamed bitumen. This state of expansion is only temporary as the water vapour escapes from the foamed bitumen causing it to collapse. The expansion ratio and the half-life time characterise respectively the maximum expansion and the rate of collapse of foamed bitumen. During the short period of time during which the foamed bitumen is in an expanded state, its viscosity is sufficiently low for the bitumen to be mixed with the mineral aggregate. This mixing can be done at ambient temperatures and, as for bitumen emulsion mixes, the aggregate may be moist.

Penetration grade bitumen

Hot bitumen Cut-back

bitumen Bitumen

emulsion Foamed bitumen

The process of adding selected petroleum solvents, such as kerosine or paraffin, is also referred to as “cutting-back” the bitumen. Cut-back bitumen contain solvents with a low boiling point that will evaporate from the mix with time. The evaporation of the solvent increases the viscosity of the bitumen and thus the stiffness of the cold- mix asphalt. During placing and compaction the solvent should not have evaporated yet and because the evaporation process takes time, the increase in mix stiffness is only slow after compaction and opening to traffic. Cut-back asphalt mixes are nowadays mainly used for patching application in small quantities. There are certain disadvantages in terms of health, safety and environment (HSE) associated with the use of cut-back asphalt. Cut-back asphalts fall outside the scope of this study.