Bitumen film thickness

The theoretical average bitumen film thickness, according to Roberts et al. (1996), can be calculated using the following equation  

For the determination of the surface area of the total aggregate, SA, the procedure provided by the Asphalt Institute may be used (Asphalt Institute MS-2). The procedure utilises the aggregate grading and the surface area factors per sieve size (see Table 5.15). The calculations consist of multiplying the total percentage passing through a sieve by the surface 

Surface area factor is 0.41 m /kg for any material retained above the 4.75 mm (No. 4) sieve. 

It must be noted that the surface area factors shown in Table 5.10 are applicable only to the listed sieves, which should be used during sieving. 

The surface area factors have been determined assuming that the aggregate specific gravity is 2650. When the aggregate used has a significantly different specific gravity, the result of the surface area of the total aggregate mix should be multiplied by a correction factor a (= 2650/SGJ. 

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Based on the bitumen film thickness and average globule diameter estimates given earlier, one might estimate the density of the bitumen globules to be about 0.5 X 103 kg m-3 however, because of the additional entrained solids content, the density of the bitumen globules are actually typically on the order of 0.90 X 103 kg m-3 under process conditions. 

The aim of the design method is to determine the target binder content for DGCAs to have a satisfactory and lasting performance. It utilises the Marshall compaction and testing apparatuses and uses volumetric, Marshall and permanent deformation properties, as well as bitumen film thickness. 

Apart from the dry and soaked stability, the total void content after curing, the water absorption after soaking and the bitumen film thickness are determined. The water absorption, together with the thickness of the bitumen film, ensures that premature ageing of the bitumen will not occur. 

The target bitumen content of the mixture is determined from the diagrams plotted and considering the requirements of Table 6.2. The aim is to determine the target bitumen content in order for the target mix to have as few total voids and as low a percentage of absorbed water as possible, as well as soaked stability, retained stability and bitumen film thickness values as high as possible. 

Figure 6.1 Typical diagrams derived from modified Marshall mix design for dense-graded cold mixtures, (a) Soaked stability vs. bitumen content, (b) Retained stability vs. bitumen content, (c) Total voids vs. bitumen content, (d) Absorbed water vs. bitumen content, (e) Bitumen film thickness vs. bitumen content, (f) Dry bulk density vs. bitumen content.

The criteria used to determine the target bitumen content are soaked stability, retained stability, total void content, absorbed water and bitumen film thickness. When DGCA is going to be used in pavements with medium to high traffic (ESAL > 3000 per day), the target binder content should satisfy the creep coefficient requirement. Equipment... 

The results of soaked Marshall stability, retained stability, dry bulk density, total void content, the absorbed water after immersing specimens into water for 48 h and bitumen film thickness are calculated and reported for each specimen. The average values per binder content are plotted as in diagrams shown in Figure 6.1. To facilitate the calculations. Table 6.B.2 and relations given below may be used. 

Table VII shows three asphalt coatings used In thicknesses comparable to Industrial paints. Film thicknesses are from 0.7 to 3 mils. These coatings find wide use In Industry for the protection of structural steel, for the protection of metal parts In storage (removable with solvents), and as pipe and tank coatings. The aluminum pigmented coating has two main advantages aluminum appearance Is more desirable than black, and the pigment protects the bitumen from the actinic rays, thereby extending the life of the coatings.

Figure 16 (curve 2), depicts the experimentally obtained thickness for toluene-diluted bitumen emulsion films. At an industry-relevant diluent ratio of about 1 1 toluene to bitumen, the film thickness values scattered within a large range from about 50 to 60 nm. These films required long... 

Figure 16 Film thickness for bitumen in heptane (1) and bitumen in toluene (2) solutions for various diluent-to-bitumen ratios.

The thickness measurements for the heptane-diluted bitumen films are presented in Fig. 16 (curve 1). Below the onset of asphaltene precipitation at a heptane itumen ratio of about 1 1, the film drained to an equilibrium gray film of about 27 nm thickness. Above the precipitation onset at a heptane/bitumen ratio of 2 1, the black film reached a thickness of about 28 nm. The film thickness then decreased with increasing diluent bitumen ratio to about lOnm at a ratio of 20 1. The thickness then remained constant, indicating that a bilayer film was probably reached. At lower diluent ratios (< 20 1), the greater thickness of heptane itumen films may be caused by the presence of unprecipitated asphaltenes. 

Fine solids are frequently mentioned as being responsible for W/0 emulsions, especially in systems involving various crude oils. The solids fraction of bitumen consists of fine submicrometer clay particles that have been rendered asphaltene-like due to the adsorption of highly aromatic, polar material on the particle siufaces (45). To determine if this solid fraction played a role in the film stability, a solids-free bitumen was prepared where all solid material larger than lOOnm was removed from the sample. Films of toluene- and heptane-diluted solids-free bitumen showed little or no change in both the drainage patterns and the film thickness, indicating that the fine solids had little or no effect on the behavioiu or stability of water/diluted bitu-men/water films. This is consistent with the observation described in Sec. II, where removal of fine solids from diluted bitumen had no effect on subsequently formed water in diluted bitumen emulsion. 

These results indieate that the resin fraction determined the properties of heptane-diluted bitumen films at high diluent ratios. Since the thickness remained constant with frir-ther dilution, the film probably had a bilayer structure. The relative instability of heptane-diluted bitumen, deasphalted bitumen, and resin-I films agree well with the emulsion experiments of Yan (46) where deasphalted bitumen led to poorly stabilized water-in-bitumen emulsions. 

Fig. 22 in frames 1-5. It was observed by Angle et al. that, generally, for a stable bitumen film, a typical drainage time to a common film was 25 min and to arrive at a Newton film was approximately 30 min in a 300-pm film diameter. Film thicknesses were measured by interferometry, using a capillary balance technique for plane parallel films (A.D. Nikolov and D.T. Wasan, personal communication, 1998) (253). 

CW Angle, AD Nikolov, DT Wasan, HA Hamza. Demulsification of water-in-bitumen emulsions Role of emulsion film thickness stability. Twelth International Symposium on Surfactant in Solution Conference, Stockholm, 1998, Paper B05. 

Column R The thickness of the bitumen film is determined according to Section 5.4.2.8. Note The values in all other columns are taken from laboratory measurements. 

Exterior surface corrosion or rusting of pipes occurs by the formation of iron oxides. Painting to an appropriate specification will significantly extend the period to the onset of corrosion, but the durability of the paint finish is largely dependent on the quality of the surface preparation as well as the thickness of the coated film. Improperly installed insulation can provide ideal conditions for corrosion and should be weatherproofed or otherwise protected from moisture and spills to avoid contact of the wet material on equipment surfaces. Application of an impervious coating such as bitumen to the exterior of the pipes is beneficial in some circumstances. Hypalon and neoprene rubber-based anticorrosive coatings admixed with chlorinated rubber are finding use in many installations. 

Figure 15 Filrn-thimung process for bitumen in toluene (a) leads to a relatively thick gray film. For bitumen in heptane solution, thinning leads to formation of black spots, which eventually coalesce forming a thin black film (b).

While the asphaltene and resin fractions alone provide a partially stable film, the combination of resin and asphaltene produce extremely stable films. However, additional information is needed to confirm our assumptions that the films have a multilayer structure at lower diluent ratios and a bilayer structure at high diluent ratios. We hope that future measiuements of the disjoining pressiue-thickness isotherms will provide this confirmation as well as identify the surface forces that stabilize the water/diluted-bitumen/water films. 

Thin-film studies confirmed observations from other techniques. Among others, they revealed that the film separating two water droplets in heptane-diluted bitumen is about half the thickness when toluene is used as a solvent. Also, the film lifetime is considerably shorter in a paraffin-based system. Demulsifiers that are used in industry to lower the water content in the feed to refineries compete for the water-oil interface with a substance or substances that produce the protective steric layers. 

In another view. Void and Void (214) suggest that holes are formed in the interfacial film and this allows the droplets to merge. Ivanov and Dimitrov (234) indicated that holes are due to surfactant depletion at the interface. However, extensive studies conducted to understand the mechanism of destabilization of the thin/thick films formed between two droplets, or between droplet and bulk phase, indicate that the process is much more complex and may involve more than one mechanism. These are not all fully understood as yet for crude oil and bitumen systems. 

Bleeding or flushing of bitumen is the upward movement of bitumen and its appearance in the surface of the pavement. This results in the formation of a film of bitumen on the surface of variable thickness. Figure 15.18 shows a typical bleeding. 

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