Asphalt layers

Asphalt Modified asphalt, Layer must be thick... 

Powder Core Thread Inner Paper Wrap Asphalt Layer Outer Paper Wrap... 

Infiltration through joints, cracks and deformities in the asphalt layer... 

It is mentioned that nuclear devices may also determine the compacted density of asphalt layers. In this particular case, the method of determination is specified by ASTM D 2950 (2011). 

The increase of asphalt s stiffness modulus improves the load spreading ability of the asphalt layer hence, lower stresses are transferred to the subgrade. This could be interpreted as the ability to decrease the asphalt layer s thickness, and hence the pavement s thickness, for a given subgrade strength and under the given traffic conditions. 

The asphalts (bituminous mixtures) produced with elastomer-modified bitumen have, above all, improved elastic properties. As a result, the asphalt layer with elastomer-modified bitumen has better permanent deformation and fatigue performance when compared to the asphalt layer with conventional bitumen. 

In an evaluation study on asphalts with modified bitumens, it was found that asphalt layers with thermoplastic polymer-modified bitumen, when laid over cracked and uncracked surfaces, have more intensive cracking within a very short period, in contrast to the asphalt layers where elastomer-modified bitumen was used (Anderson et al. 1999). 

Because of the abovementioned properties and results, the use of thermoplastic polymer-modified bitumen is recommended in areas with high ambient temperatures where the asphalt layers are subjected to high traffic loads but not over cracked pavement surfaces. 

As can be seen in Table 3.24, the energy consumption for the construction of a concrete layer is higher than that of the asphalt layer with virgin materials. Similarly, when 20% reclaimed material is used, the energy consumed for an asphalt base layer is less than that for an asphalt base layer with virgin materials. The processes that use unheated aggregate and cold applied binders utilise the least amount of energy per tonne. 

For the construction of a concrete layer, the highest energy demand is required for the manufacture of cement, while for the construction of an asphalt layer, most of the energy is required for the manufacture of asphalt cement and heating during the hot mix production process (Chappat and Bilal 2003). 

HiMAs, known as enrobe a module eleve (EME), were introduced in France in the early 1980s as a measure to reduce the usage of oil-derived products by reducing asphalt layers thickness. The high stiffness of this material enabled the base course material to be reduced by up to 40% when compared with conventional French asphalt, grave bitumen (Nunn and Smith 1994). 

In the resilient modulus test, the cylindrical specimens have a diameter of 101.6 3.8 mm or 152.4 9 mm and thickness ranging from 38.1 to 63.5 mm. The specimens are compacted in the laboratory with a gyratory compactor, or a Marshall compactor, or derived from field coring compacted asphalt layer(s). 

By conducting the static creep test, it is possible to estimate the oncoming permanent deformation (rutting) of the asphalt layers after n years in service. 

Layers of pavements under the effect of axial loads are subjected to repeated tension. The magnitude of the tensile strain developed in pavements under a certain load mainly depends on the stiffness of the layers. For flexible pavements and after detailed measurements in situ, it was found (Pell 1967) that the magnitude of the tensile strain ranges between 30 and 200 microstrain, for a standard axle load (8 tonnes). This tensile strain generates conditions for fatigue of asphalt layers to occur, which appears as cracking. 

Pavement cracking, owing to the fatigue of asphalt layers, is one of the main modes of failure. 

Himeno et al. (1987) applied the dissipated energy concept to the failure of an asphalt layer in a pavement. Rowe (1993, 1996) showed that the dissipated energy can be used to predict life to crack initiation with good accuracy. 

The fatigue line could also be used to determine the permissible maximum tensile strain to be induced at the asphalt layer to achieve the desired service life of the pavement. This... 

Finally, pavers that can simultaneously lay two asphalt layers (normally wearing and binder course) in a single pass have been developed. This results in better interlocking of the courses, elimination of tack coating and a reduction of the paving time. The method is known as compact asphalt laying method. An example of a paver is shown in Figure 8.11. 

Fixed datum can be a tensioned reference wire placed on one side and near the paver, the adjacent mat or even the kerb, if it is not misplaced. The tensioned reference wire is typically used in new roads when paving the first asphalt layer. 

Apart from the levelling control system, there is also the slope control system that is integrated with the levelling control system in modern pavers. The slope control system is particularly useful when laying base asphalt layers, thick asphalt layers or asphalt layers in general, on curved sections. 

The compaction of asphalt layers is possibly the most critical stage of asphalt works. It is needed to achieve proper and uniform compaction, which in turn ensures a better long-lasting performance. 

Effective compaction is related to the type of compaction equipment used. Asphalt layer compaction equipment consists of self-propelled vehicles, known as rollers, which compact the asphalt layer by the effect of their self-weight or by additionally imposing dynamic loading. The desired compaction is achieved by applying a certain number of passes of the rollers over the asphalt layer, known as compaction effort. 

The surface of the cylinders or tyres that come in contact with the asphalt layer should be clean and in good condition. Dirt or oil on the surface of the cylinders or of the tyres will cause premature failures, while rough and worn surfaces may leave roller marks on the surface. 

The static weight of steel-wheel rollers varies from 3 to 13 tonnes depending on the model and ballast used. The typical static weight for compacting asphalt layers of thickness 4.0 to 6 cm is 8-12 tonnes. The diameter of drums or steel wheels and their width vary depending on the model used. The larger the drum diameter, the smaller the stress applied to the compacted surface, for the same static weight. 

Any type of roller can be used for the compaction of a hot asphalt layer. In general, lighter steel drum rollers or pneumatic rollers are used in mats of thickness less than 4 cm and heavier rollers are used in thicker mats. 

Vibrating rollers should only be used to compact asphalt layers of thickness greater than 5 cm, unless the vibrating roller is capable of altering the frequency and the amplitude, and experience has shown that satisfactory compaction can be obtained for the type of asphalt used. 

Pneumatic-tyre rollers are very useful for compacting thin and very thin asphalt layers or for primary compaction of overlays in general, or levelling courses over uneven surfaces. Pneumatic rollers have been found to eliminate the hairline cracks that appear on the finished surface when compaction is carried out at lower than permitted temperatures. 

To achieve proper and effective compaction of asphalt layers, the following points are recommended ... 

The method to be employed for determining the degree of compaction for acceptance of the compacted asphalt layer should always be stated in the contract document. 

The thickness of the asphalt layer may also be determined by a non-destructive method using short-pulse radar, according to ASTM D 4748. 

CEN EN 12697-36 also describes a non-destructive method for measuring the thickness of the asphalt layer by using an electromagnetic apparatus (eddy current principle) and an antipole fixed on the road prior to laying the asphalt. 

The final thickness of the asphalt layer should not deviate more than the permissible tolerance, specified in the contract document. 

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