Layers of flexible pavement

A pavement is a set of superimposed layers of imported materials (selected, processed unbound and bound materials) that are placed on the natural soil for the creation of a road. A pavement is a complex structure that has to accomplish various functions different from each other. 

The main structural function of a pavement is to sustain traffic loads and distribute them to the subgrade. The stresses transferred to the surface of the subgrade should be such as to cause minimal deformation of the subgrade soil layer. Additionally, part of the upper layers of the pavement structure should be almost impervious to water, so that the subgrade, as well as the unbound layers, is protected from the detrimental effect of surface water. Finally, the pavement surface should be skid resistant, resistant to the polishing action of tyres and even. 

In general, the flexible pavement structure consists of two characteristic sets of layers with different mechanical properties and performance the unbound or hydraulically bound aggregate layers, seated on the subgrade, and the bound asphalt (bituminous) layers, seated on the previous set of layers. 

The above separation of the flexible pavement structure is based on the different mechanical performance of the layers and constitutes the base for the development of any flexible pavement design methodology. 

The asphalt layers are distinguished into surface or wearing course, binder course and asphalt base course, while the unbound or hydraulically bound aggregates are distinguished 

---

The AC is used in all layers of flexible pavements. However, its use should be limited or excluded from surface layers in roads with a speed limit above 70 km/h. There are more effective and, in some cases, more economic mixtures for use in surface layers than AC. 

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. 

Forty years after the development of Boussinesq s analysis, Westergaard (1926) developed his own analysis for the determination of stresses in a two-layer system, where the first layer consisted of a concrete slab. Even though this theory did not cover the case of flexible pavements, it triggered the development of analysis for flexible pavements with two layers. 

A three or more layer system (multi-layer system) simulates better the real situation of flexible pavements, since there are more than two distinct layers in a typical pavement. 

The sub-base layer provides a uniform and even surface to support the concrete slab and assist construction works. Its thickness certainly contributes to the strength of the pavement but not as much as the sub-base/base layer in flexible pavements. 

Hot asphalt surfacing overlay restores micro- and macro-texture and is the most suitable treatment for restoration of skid resistance of continuously reinforced pavement surface. All types of hot asphalts used in restoration of skid resistance of flexible pavements can be used, namely asphalt concrete for very thin layers, SMA and porous asphalt. 

Recycling may be applied to all layers of flexible and rigid pavements. This chapter emphasises on asphalt layer recycling and, to a lesser extent, on concrete slab recycling. Recycling of the unbound or hydraulically bound materials, although possible, is effectively restricted to pavement reconstruction works, which are not as common as maintenance and rehabilitation of asphalt layers. However, provided the unbound or cement-bound materials possess the required properties, the techniques and procedures followed are similar to those as in new pavement construction. 

The base course layer is positioned between the sub-base course (or subgrade, if there is no sub-base constructed) and the asphalt layers. Together with the asphalt layers, in a typical flexible pavement, it constitutes one of the main two structural elements of the pavement. The base course layer performs the following functions ... 

Generally speaking, the CSMs are required to have (a) some minimum compressive strength depending on the type of overlaying layers and hence type of pavement (flexible or rigid) and the importance of the layer in the pavement structure (base course or sub-base) and (b) freeze-thaw durability. 

In a pavement design methodology such as the AASHTO methodology (AASHTO 1993), the elastic modulus, E (ASTM C 469 2010), or alternatively, the unconfined compressive strength (7 days) (ASTM D 1633 2007) of the CTA base, needs to be determined. With either value, the structural coefficient (<72) is derived and the thickness of the corresponding layer as well as of all layers of a flexible pavement is determined (see Section 13.4.4.3). For a rigid pavement design, the elastic modulus of the sub-base is used (see Section 14.11.1). 

The asphalt layers in a flexible pavement consist of the asphalt base, the binder course and the surface layer or wearing course. 

The asphalt base is the first and most important layer of a flexible pavement, constructed over the unbound or hydraulically bound base. Together with the other overlying asphalt... 

By definition, the subgrade is considered as one layer. Thus, in the typical flexible pavement that consists of unbound layer (base/sub-base) and bitumen-bound layers all consisting of asphalts with the same mechanical properties, the number of distinct layers is three. 

Some of the most widely used computer software developed to calculate stresses, strains and displacements in flexible pavements with the use of the multi-layered elastic theory are CHEVRON, BISAR, ELSYM5, KENLAYER and WESLEA. 

During the 1980s and later on, various other computer programs of multi-layer system for flexible pavements were developed using 2D or 3D analysis of finite elements. 

The analytical methodologies enable to design flexible pavements consisting of, theoretically, any number of different asphalt and base/sub-base layers. Layer thickness determination may be carried out using any configuration of axle loading and at any environmental temperature some methodologies have the ability to examine partial bonded or un-bonded interfaces apart from the fully bonded interfaces. The determination of the thickness of the layers is carried out by the use of appropriate software developed. 

The design criteria or distress modes adopted by most analytical and semi-analytical methodologies for flexible pavement design are as follows (a) fatigue of treated layers, that is, the asphalt layer or the hydraulically bound layer not to crack under the influence of traffic and (b) deformation of the subgrade, that is, the subgrade to be able to sustain the traffic without excessive deformation. 

The methodology can be used to design flexible pavements composed of the following (a) asphalt layers and unbound layers (b) asphalt layers, exclusively (full-depth pavement) (c) asphalt layer and cold asphalt layers and (d) asphalt layer, cold asphalt layer and unbound layer. 

The thickness determination of a flexible pavement with asphalt concrete layers over untreated aggregate base/sub-base is obtained using the appropriate nomograph. A sample of those nomographs, for 15.5°C MAAT and for 300 mm thickness of base/sub-base, is given in Figure 13.3. 

The determination of the thickness of a full-depth asphalt concrete pavement is carried out from a similar to a flexible pavement with unbound layer nomographs. A sample of the nomographs used for 15.5°C MAAT is shown in Figure 13.4. 

Hence, the flexible pavement with cold base and unbound aggregate base/sub-base will consist of top 50 mm asphalt concrete, a layer of cold asphalt of thickness T A-base a base/ sub-base of thickness 150 or 300 mm, as selected. 

The methodology has introduced the concept of the foundation and of the pavement structure being either flexible or rigid. The design criteria used to develop the design charts are as follows for the foundation design, the deflection of the foundation surface and the minimum thickness of the upper foundation layer (Chaddock and Roberts 2006), and for the pavement design, the strain of the asphalt layer and the stress of the hydraulically bound layer (Nunn 2004). 

Flexible pavement, in this methodology, is considered the structure consisting of all layers above foundation. The upper layers of the flexible pavement are bound in bitumen and the lower (base) layers are bound in either bitumen or hydraulic binder. 

Figure 13.19 Asphalt layers thickness determination of flexible and flexible with HBM base pavements.

Figure 13.20 Flow chart of AUTh flexible pavement design methodology. (From Nikolaides, A.F., Flexible Pavements Pavement Design Methodology, Bituminous Mixtures, Antiskidding Layers, 1st Edition. Thessaloniki, Greece A.F. Nikolaides. ISBN 960-91849-1-X, 2005.)...

---