Products and Applications
Depending on the composition and characteristics and the method of production, different bituminous mixtures could be available for road and airfield pavements applications.
Fig. 5.6 Structure of bituminous mixtures: (a) asphalt concrete; (b) stone mastic asphalt
In terms of composition and characteristics, taking into account the European experience, the most common bituminous mixtures used in pavements are:
– Asphalt concrete (EN 13108-1) [S-45].
– Asphalt concrete for very thin layers (EN 13108-2) [S-46].
– Soft asphalt (EN 13108-3) [S-47].
– Hot rolled asphalt (EN 13108-4) [S-48].
– Stone mastic asphalt (EN 13108-5) [S-49].
– Mastic asphalt (EN 13108-6) [S-50].
– Porous asphalt (EN 13108-7) [S-51].
– Reclaimed asphalt (EN 13108-8) [S-52].
The asphalt concrete specified in EN 13108-1 [S-45] and EN 13108-2 [S-46] is a dense-graded material with a large scope of application in pavement construction (Fig. 5.6a). The designation of the asphalt concrete is “AC D surf/base/bin binder” where “AC” is asphalt concrete; “D” is the upper sieve size of the mixtures of aggregates; “surf” is the surface course; “base” is the base course; “bin” is the binder course; and “binder” is the designation of binder used. The asphalt concrete “AC 20 base 35/50” is an asphalt concrete with maximum aggregate size 20 mm for base course with penetration bitumen 35/50.
A stone mastic asphalt (SMA) is a gap-graded asphalt concrete mixture with bitumen as a binder, composed of a coarse crushed aggregate skeleton bound with a mastic mortar (Fig. 5.6b). The designation of the stone mastic asphalt is “SMA D binder” where “SMA” is stone mastic asphalt; “D” is the upper sieve size of the mixtures of aggregate; and “binder” is the designation of binder used. The “SMA 11 70/100” is a stone mastic asphalt with maximum aggregate size 11 mm and penetration bitumen 70/100. Due to a high coarse aggregate content that interlocks to form a resistant stone skeleton, SMA provides a high permanent deformation resistance recommended for heavily trafficked roads. In fact, the coarse aggregates provides a more stone-on-stone contact than with conventional dense-graded bituminous mixtures. The presence of fibres in the mastic of bitumen and filler is fundamental in order to provide an adequate stability of the bitumen and to avoid
the drainage of the binder, during the transport and the placement of SMA in pavement construction.
Porous asphalt is a bituminous material with bitumen as a binder prepared so as to have a very high content of interconnected voids which allow passage of water and air in order to provide the compacted mixture with drain and noise reducing characteristics. This type of bituminous mixtures consists of an open-graded coarse aggregate, bonded together by modified bituminous binders. The designation of the porous asphalt is “PA D binder” where “PA” is porous asphalt; “D” is the upper sieve size of the mixtures of aggregates; and “binder” is the designation of binder used. The “PA 11 70/100” is a porous asphalt with maximum aggregate size 11 mm and penetration bitumen 70/100. The surface texture characteristics of porous asphalt provide a higher skid resistance favourable to traffic safety in comparison to the surface characteristics of the bituminous mixture (Fig. 5.7) and also a noise reduction.
Reclaimed asphalt is obtained by milling of asphalt road layers and by crushing of slabs ripped up from asphalt pavements or lumps from asphalt slabs and asphalt from reject and surplus production. Recycling bituminous mixtures results in a reusable mixture of aggregate and asphalt binder known as reclaimed asphalt pavement (RAP). The feedstock of reclaimed asphalt defined the quantity of material to be used as a constituent material for the manufacturing of hot asphalt mixtures. The particle size of reclaimed asphalt is the maximum size of the pieces of asphalt in the reclaimed material, expressed as a sieve size (U). Reclaimed asphalt shall be designated by the abbreviation RA, preceded by the asphalt particle size designation U and followed by the aggregate size designation d/D mm: U RA d/D. The reclaimed asphalt 40 RA 0/8 mm is a reclaimed asphalt, of which the aggregate has an upper sieve size of 8 mm and the asphalt particles have a maximum size of 40 mm.
According the temperature range of production, bituminous mixtures are classified in the following types :
– Cold mixes—produced with unheated aggregate and bitumen emulsion or foamed bitumen.
– Half warm asphalt—produced with heated aggregate at a mixing temperature (of the mix) between approximately 70 °C and roughly 100 °C.
– Warm mix asphalt (WMA)—produced and mixed at temperatures roughly between 100 and 140 °C.
– Hot mix asphalt (HMA)—produced and mixed at temperatures roughly between 120 and 190 °C, depending on the bitumen used.
The most common bituminous mixtures in pavement construction and rehabilitation are HMA that are produced in hot asphalt mixing plants by mixing the aggregates with the bitumen at elevated temperatures. The mixing temperature is sufficiently high in order to obtain the bitumen liquid enough for a proper mixing with and coating the aggregates. However, the temperature should not to be too high as to avoid excessive ageing of the asphalt. A HMA is laid and compacted when it is still sufficiently hot so as to have proper workability. HMA are often used in surface and binder courses in road and airfield pavements.
WMA technologies operate above 100 °C. Various techniques are used to reduce the effective viscosity of the binder enabling full coating and subsequent compactability at lower temperatures [42, 43]. The most common techniques are organic additives, chemical additives and foaming techniques. Organic and chemical additives typically may reduce the mix and compaction temperatures of between 20 and 30 °C. The type of additive must be selected carefully. Advantages and limitations of the use of additives are mentioned in the literature for each process. WMA comprises a great number of bituminous mixtures: asphalt concrete, SMA, porous asphalt, reclaimed asphalt, etc. These bituminous mixtures can be fabricated, laid and compacted by using WMA technologies instead of traditional HMA .
WMA has a significant number of advantages comparing to HMA, basically associated with energy saving which lead to a major reduction of emission of greenhouse effect gases, such as carbon dioxide, and pollutants .
HMA and WMA are prepared at central mixing plants. In these facilities, aggregates are mixed with the bitumen, eventually additives, to produce the bituminous mixture. Central mixing plants can be portable or stationary (permanent in one location and with greater production capacity). There are two types of central mixing plants: drum mix plants (continuous) and batch plants. Typically, the batch central mixing plant is composed by an area for the aggregates storage (cold feed bins). After the transportation and weighing of the aggregates, the next step is developed in the dryer where the aggregates are blended, heated and dried. In sequence, the hot aggregates are screening, weighing and storing in hot bins, in units mounted at the top of the central mixing plant. Finally, the mix of the aggregates with the bitumen is performed in the mixing unit. The final bituminous mixture is discharged into the trucks used to the transport to the site. In the case of a drum mix plant (Fig. 5.8a), all the process of aggregates heating and mixture with the bitumen is performed in the drum mixer (Fig. 5.8b).
Fig. 5.8 Drum mix plant: (a) general view; (b) inside part of the drum mixer
Fig. 5.9 Pavement construction: (a) asphalt paver; (b) rollers operating the bituminous mixture compaction
In the major situations, bituminous mixtures are placed in situ by asphalt pavers that spread the material in a uniform layer with a certain thickness (Fig. 5.9a). Previously, the existing underlying surface should be clean and a prime coat (granular layer) or a tack coat (bituminous layer) is applied. If the surface presents irregularities, an additional levelling is necessary with a bituminous mixture. In the case of thick courses, the bituminous mixtures should be placed in two or more layers. The placement should be performed always with favourable weather conditions (temperature and humidity).
When the spreading and finishing of layers are completed, the compaction is the next phase with the bituminous mixture still hot and workable. The compaction is carried out by pneumatic tyred rollers (asphalt concrete for instance) or steel wheel rollers (porous asphalt, for instance) or by the combination of both types of rollers. In each case, depending on the type of bituminous mixtures, the most adequate method to compaction should be selected (Fig. 5.9b).
The quality control is essential in order to ensure the desirable characteristics of the bituminous mixtures during the construction (e. g. temperature) and after the conclusion of works (e. g. density, void content, roughness, skid resistance and structural resistance).