Estereotipos

Sprayed seals, also known as chip seals, contain less aggregate and less bitumen than asphalt, and are consequently cheaper and more flexible. Hence they are better suited to marginal-quality natural gravel basecourses, which have relatively low stiffness, and to providing economical all-weather road surfaces in remote areas. In places like inland Australia and southern Africa, this also allows a cost trade-off, whereby low-cost flexible pavements on strong, dry subgrades and topped with a thin coating of bitumen can be used instead of asphalt on crushed rock basecourses. However, sprayed seals demand more skill from the plant operators to ensure good stone coverage and adhesion, and in the choice of bitumen viscosity (too stiff and binding is poor, too thin and penetration is excessive).

Primer

This method of seal construction (Figure 9.3) is usually carried out in two or more stages. The first step is the application of a low-viscosity bituminous primer to the finished pavement surface. ‘Finished’ in this context means compacted, shaped, swept and moistened just before spraying. The primer coats and binds loose particles, dustproofs and waterproofs the surface,

Figure 9.2 Functions and approximate thicknesses of asphalt layers in road

penetrates and fills the pores in the upper 20 mm or so of the basecourse, and provides a good bonding surface to which the subsequent seal coat can adhere. Thinned or ‘cut-back’ bitumen is the most commonly used primer these days, though coal tar was formerly a cheaper alternative.

The main pavement material properties affecting the success of priming are basecourse compatibility and its surface texture.

• Basecourse-primer compatibility depends on the surface chemistry of the gravel and of the bitumen, with the most common causes of seal debonding being excessive clay or salt at the top of the basecourse. The salt may be inherent with calcrete pavement materials (Figure 9.4), or introduced with saline compaction water (bore water), or drawn by capillary rise and evaporation from a shallow saline water table; in any case this is mainly a problem in arid areas.

• Surface texture of basecourse is related to its compacted density and hence to its porosity (or voids ratio). Well-graded and densely compacted crushed rock basecourses, which ‘ring’ when struck, have a tight surface texture and require only light applications of low-viscosity primer. At the opposite extreme are open-textured surfaces, which are highly absorbent. These are said to be ‘hungry’, needing heavy spraying with a high- viscosity primer to ensure that the upper surface of the basecourse is adequately bound. This condition results from the use of gap-graded or

Figure 9.3 Spreading cover aggregate (chippings) on sprayed hot bitumen, Hamilton,

New Zealand. The one-stone-thick layer was subsequently rolled before being re- opened to traffic.

fines-deficient bases, or natural gravels containing porous aggregates (usually duricrust nodules).

Primerseal

A primerseal is used instead of a primer where traffic is to be allowed onto the road for some time, say 3–12 months, between priming and final sealing. This consists of a medium-viscosity bitumen topped with a blinding layer of sand or crusher grit. The purpose of a primerseal is to allow time for weak areas in the pavement to become apparent and to be patched, or to await hotter or drier weather for final sealing. It may also be delayed until a specialist sealing contractor arrives, since more skill and better equipment are required for this stage of the sealing process.

Figure 9.4 Debonded primerseal, near Broken Hill, New South Wales. The cutback

bitumen has penetrated about 10mm into a calcrete gravel basecourse; seal detachment is probably due to salt in this material. (Photo: I.R.Wilson.)

Surface dressing

The final seal coat or surface dressing consists of a sprayed film of bitumen about 1 mm thick with monosized aggregate rolled into its surface, so that stones protrude above and below the seal (Figure 9.5). The bitumens used for sealing have to meet a higher specification than primers, remaining flexible and waterproof through a working life of 10 years or more. The covering aggregate is sprinkled evenly across the hot bitumen so that the stones are nearly touching each other; in time, loose particles are either broomed away or rearranged by traffic to create a stone mosaic surface. By locking aggregate chips together in this way, both the wear resistance and skid resistance of the seal are maximized. The aggregate mosaic generally covers only about 50% of the seal area when first spread, but this increases to about 70% with rolling and to 80% after a few months of traffic. Usually only one application of bitumen and cover aggregate is required, but ‘hungry’ seals may need an additional light surface enrichment coat. An extra application of finer aggregate is also sometimes necessary where the stone mosaic surface fails to develop properly due to stripping.

Resealing

With good maintenance and only moderate traffic growth, a natural gravel pavement may last 20–30 years, and be resealed two or three times during this period. Resealing is necessary because bitumen oxidizes and becomes brittle with age, especially in bright sunlight. This allows fine cracks to develop and moisture to enter the basecourse, causing it to deform. One of the chief aims of pavement maintenance is, therefore, to ensure that these cracks are plugged as soon as possible. Thicker bitumen coats oxidize more slowly, so that—other things being equal—a viscous grade of bitumen is likely to prove more durable. The resealing procedure involves gap-filling with successively finer aggregates, such that 10 mm aggregate is used to fill the spaces between the 20 mm stones of the original seal (Figure 9.5).

9.2 ASPHALT

Asphalt, asphaltic concrete (AC), bituminous concrete, hot mix, cold mix and plant mix are names for mixtures of coarse and fine aggregate, inert filler and a bituminous binder. For convenience here, all varieties are collectively referred to as asphalt, though terminology varies greatly between British, American and Australian usage. The main functions of asphalt in road-making are summarized in Figure 9.2 and below:

Figure 9.5 Design features and construction stages of a sprayed seal. Note the characteristics of a poor seal and compare with

course over unbound fine crushed rock (FCR) pavement layers. These are designed to be about twice the nominal particle diameter in thickness, so that 10 mm stone requires a layer thickness of 20–25 mm.

• Overlay The overlay is two to three times thicker and performs a load- sharing or load-alleviating role for unbound pavements approaching the end of their fatigue life. It also corrects surface irregularities created by pavement deformations.

• Basecourse Pavement course asphalt (base and sub-base) differs from wearing course in having a higher proportion of coarse aggregate, larger maximum particle sizes, less fines and less bitumen. Asphalt is also widely used as a patching and filling material for road surface maintenance. Cold mix, an open-graded mixture of coarse aggregate bound with slow-setting bitumen emulsion, is commonly used in this application.

Although the names and some of the functions are similar, the differences between asphaltic concrete (AC) and Portland cement concrete (PCC) are worth noting. AC design aims for high shear strength and flexibility, with a proportion of air-filled voids—usually about 4%—left to accommodate thermal deformation; PCC is designed for high compressive strength, rigidity and negligible porosity. Cement provides both the binder and filler in PCC, but in AC they are separate ingredients. The coarse and fine aggregates used are similar in both cases, though AC is more sensitive to their surface properties. The bituminous binders used in AC deteriorate with time, limiting its useful life to about 3–10 years; PCC would normally be expected to perform satisfactorily for at least 10 times as long.

Like PCC, asphalt can be produced in large central batching plants or on construction sites using mobile equipment, with the central batching offering better quality control but greater transport costs. A conspicuous difference between AC and PCC plants is the presence of a rotary drying kiln in the former (Figure 9.6). Aggregate used in AC has to be perfectly dry, dust-free and hot to ensure thorough bitumen coating of all particles, and the kiln performs all three tasks. Some of the dust loosened by drum rotation and drawn off with the hot exhaust air may be later blended back into the asphalt mix as -75 µm filler.

Whereas spray sealing is something of a skilled craft, asphalt-making and laying is more of a manufacturing process (and hence more amenable to quality control). Asphalt is spread and levelled in a similar manner to wet concrete, but is compacted by rollers like roadbase, rather than by vibratory compaction as with PCC. Further compaction is induced by traffic.