Types of Bitumen
Penetration Grade Bitumens
Refinery bitumens are produced with a range of viscosities and are known as penetration grade bitumens. The term derives from the test that is used to characterise them according to hardness. The range of penetration grades for road bitumens is from 15 to 450, although the most commonly used are in the range 25 to 200.
The range is produced partly through careful control of the distillation process and partly by fluxing ‘residual’ bitumen with oils to the required degree of hardness. The specification for penetration grade bitumens is contained in BS EN 12591.1 Table 1 indicates a range of tests with which penetration grade bitumens for road purposes must comply.
These bitumens are specified by their penetration value (BS EN 1426) and softening point (BS EN 1427), which indicate hardness and equiviscosity temperature, respectively. However, they are designated only by their penetration, e.g. 40/60 pen bitumen has a penetration of 50 ± 10.
In addition there are limits for loss on heating (BS EN 13303), which ensures that there are no volatile components present whose loss during preparation and laying would cause hardening of the bitumen, and solubility (BS EN 12592), which ensures that there are only negligible amounts of impurity.
Refinery bitumen may be further processed by air blowing.
This consists of introducing air under pressure into a soft bitumen under controlled temperature conditions. The oxygen in the air reacts with certain compounds in the bitumen, resulting in the formation of compounds of higher molecular weight. Thus the asphaltenes content increases at the expense of the maltenes content, resulting in harder bitumens that are also less ductile and less temperature susceptible.
Although these bitumens are mostly used for industrial applications such as roofing and pipe coatings, low penetration grade road bitumens (paving grade bitumens) can also be produced by this process.
Penetration grade bitumen is thermoplastic, thus its viscosity varies with temperature. At ambient temperature it can be more or less solid and to enable it to be used for road construction it must be temporarily changed into a fluid state.
This may simply be achieved by raising the temperature. However, for surface dressing and some types of bituminous mixture it is necessary to have a fluid binder that can be applied and mixed at relatively low temperatures, but have an adequate hardness after laying.
Cutback bitumens are penetration grade bitumens that have their viscosity temporarily reduced by dilution in a volatile oil. After application the volatile oil evaporates and the bitumen reverts to its former viscosity. The curing time and viscosity of cutbacks can be varied according to the volatility of the diluting oil and the amount of diluent used. In the UK, cutbacks are manufactured using 70/100 or 160/220 pen bitumen diluted with kerosene. Three grades are produced to comply with a viscosity specification based on the standard tar viscometer (STV).
Table 2 from BS 3690 shows that cutbacks also must comply with solubility, distillation and recovered penetration requirements. The last two are to ensure that the diluent will evaporate at the required rate and that the residual bitumen will have an appropriate hardness for the performance requirements.
An emulsion is a two-phase system consisting of two immiscible liquids, one being dispersed as fine globules within the other. A bitumen emulsion consists of discrete globules of bitumen dispersed within a continuous phase of water, and is a means of enabling penetration grade bitumens to be mixed and laid.
Dispersal of the bitumen globules must be maintained electrochemically using an emulsifier which consists of a long hydrocarbon chain terminating with either a cationic or an anionic functional group. The hydrocarbon chain has an affinity for bitumen, whereas the ionic part has an affinity for water. Thus the emulsifier molecules are attracted to the bitumen globules with the hydrocarbon chain binding strongly to the bitumen, leaving the ionic part on the surface of the globule, as shown in Fig. 1.
Each droplet then carries a like surface charge depending on the charge of the ionic part of the emulsifier. Cationic emulsions are positively charged and anionic emulsions are negatively charged. The globules therefore repel each other, making the emulsion stable.
Cationic emulsions are preferred because they also aid adhesion, the positively charged bitumen globules being strongly attracted to the negatively charged aggregate surface. Emulsions must satisfy two conflicting requirements in that they need stability for storage but also may need to break quickly when put into use. The stability of an emulsion depends on a number of factors, as follows:
- The quantity and type of emulsifier present. Anionic emulsions require substantial water loss before they break, whereas cationic emulsions break by physicochemical action before much evaporation has taken place. The more free emulsifier ions there are in the continuous phase, the easier it is for the negatively charged aggregate surface to be satisfied without attracting the bitumen globules.
- Rate of water loss by evaporation. This in turn depends on ambient temperature, humidity and wind speed as well as rate and method of application.
- The quantity of bitumen. Increasing the bitumen content will increase the breaking rate.
- Size of bitumen globules. The smaller their size, the slower will be the breaking rate.
- Mechanical forces. The greater the mixing friction or, in the case of surface dressing, the rolling and traffic action, the quicker the emulsion will break.
The viscosity of emulsions is important because a large proportion of emulsions are applied by spray. The viscosity increases with bitumen content and is very sensitive for values greater than about 60%. The chemistry of the aqueous phase is also important, viscosity being increased by decreasing the acid content or increasing the emulsifier content. The viscosity for road emulsions is specified in BS 434 Part 1 and BS EN 13808.
Polymer-modified bitumens (PMBs) are penetration grade bitumens that have been modified in terms of their strength and rheological properties by the addition of small amounts (usually 2 – 8% by mass) of polymer.
The polymers tend to be either plastics or rubbers and alter the strength and viscoelastic properties of the penetration grade bitumen by increasing its elastic response, improving its cohesive and fracture strength and providing greater ductility.
Typical examples of rubbers (thermoplastic elastomers) used to modify bitumen include styrenic block copolymers such as styrene–butadiene–styrene (SBS), synthetic rubbers such as styrene–butadiene rubber (SBR), and both natural and recycled (crumb tyre) rubbers. Plastics (thermoplastic polymers) tend to include polyethylene (PE) and polypropylene (PP) as well as semi-crystalline polymers such as ethylene– vinyl acetate (EVA).
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