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2.MECHANical and engineering ASPECTS of PAVEMENTS and Material Re-USe

2.1Structural layers in a pavement

This section aims at providing some recalls on pavement construction and design, which justifies the use of a typical road structure for WP3.

Road construction process and pavement structures are more or less the same all over the world with minor differences in types of materials depending on availability and climate.

Terminology is an important part of communication. Technical references often have there own terminology. Parallel with the important work in COST Action 333 a glossary of terms was produced, assembled mainly from existing technical references including many additional terms used in reports from different European groups. COST 333 (1999). For mechanical and engineering elements this Deliverable 9 report has the intension to use the glossary from COST 333. There are small differences in the terminology between Europe and USA regarding road construction. In the following short extract of the description, the American terminology for road construction is used. Where the USA terminology differs from the European terminology, both the USA terms and the European terms according to COST 333 are given in this part of the report, the European terms is shown by the notation E: …….

As stated by the report “A Review of Water Movements in Highway Environment” by Apul et al (2002), the two major types of pavements are flexible pavement (asphalt pavement) and rigid pavement (Portland Cement Concrete pavement) (E: concrete pavement). Component layers for each of these sections are similar with minor differences in types of material used and number of layers (see Figure 2.1). However, the same composition into functional layers (surface, base, subbase, subgrade layers,…) can be applied to both type.

The purpose of the surface layer is to provide a safe and smooth riding surface with maximal skid resistance, and minimal load and non-load associated fractures and deformations. It is also to protect the structure underneath, especially from rainfall. The surface layer can be asphaltic or PCC (E: concrete).
Those layers constructed of asphaltic concrete are referred to as the wearing surface layer (E: wearing course). In some cases (countries) the surface course is made of two layers, the wearing course and below, the bonding layer, which is a transition layer with the base course. Below the wearing course is the base course, which may sometimes be underlain by a sub-base layer. Base and sub-base layers may be designed to (1) prevent water pumping, (2) protect against frost action, (3) drain excess water, (4) prevent volume change of the subgrade, (5) increase structural or load-supporting capacity, and (6) expedite construction. Base and sub-base layers are constructed above the subgrade, which is the native soil In some cases (countries) there can be a transition layer with the natural soil, i.e. the capping layer. Desirable properties of subgrades include strength (resilient modulus), ease of drainage, ease of compaction, permanency of compaction, and permanency of strength.

Asphalt concrete pavements have an asphalt concrete surface and, depending on the traffic a bituminous base or a granular base. Asphalt concrete consists of asphalt aggregate (E. aggregate) (95 percent by mass) and asphalt binder (E: bituminous binder).

Relatively recently, polymers or other anti-stripping agents have been used to reduce rutting, fatigue distress, low temperature performance or moisture susceptibility. Typically the asphalt concrete mixture (E: asphalt mixture) properties that are adjusted for optimal performance are, air voids content, density, resistance to water stripping, resilient modulus, rutting resistance. To some extent, each of these properties affects the water movement in the asphalt concrete.

Figure 2.1. Structural layers of a typical flexible and rigid pavement – USA terminology. (Figure 1.2 Apul et al (2002))
Regarding PCC pavements (E: concrete pavement), components include a granular or stabilized base, a sub-base, a PCC slab (E: concrete slab) on the surface, and occasionally an overlay of asphalt concrete. The basic materials used in PCC (E: concrete) are Portland cement (15 percent by mass), water, coarse aggregate (crushed stone or gravel), and fine aggregate (usually sand). Sometimes, supplementary cementitious materials and chemical admixtures may be used to modify the properties of fresh or hardened concrete. Portland cement sets and hardens when its raw materials (containing lime, iron, silica, and alumina) react with water. The supplementary cementitious materials commonly used are also reactive materials, and include coal fly ash, blast furnace slag, and silica fume.

The base and the sub-base may be granular or stabilized.

The stabilized base (E: cement-bound base) or sub-base consists of a mixture of aggregate, cementitious materials (e.g. coal fly ash, silica fume, and ground granulated blast furnace slag), and water that is compacted to form a dense and strong layer. Granular base and sub-base layers do not have any reacting materials but instead consist of unbound aggregates only. The aggregates may be sand, gravel, crushed stone or quarry rock, slag or other hard, durable material of mineral origin.

Soils and oversize materials are used for embankments. Different types of soils ranging from granular (sand and gravel) to more finely sized (silt and clay) soils may be used for construction. Saturated clays, silty clay, and presence of organic matter are not desirable.

Comments from SAMARIS WP3 group

For the purpose of WP3 of SAMARIS, a typical road structure has been agreed. It is close to the one used for a typical road structure in COST 337 (2002), but it also takes into account elements from the Federal Highway Administration (FHWA) framework for evaluating use of recycled materials in the highway environment. FHWA-RD-00-140 (2001).

For an operational construction the whole road structure will depend on local conditions (e.g. topography, traffic load), and thus the real situation may differ from the one shown in Figure 2.2 which includes wearing course (application 1), road base (application 2), sub-base (application 3), subgrade (application 4) and lastly shoulders and landscaping (application 5) which also includes embankment). Figure 2.2 represents a general and complete diagram, indispensable to study all layers and to design the general assessment methodology of WP3. In practice, for some structures, some layers will not exist.





Figure 2.2: The typical road structure used in SAMARIS WP3
Figure 2.2 is also a simplified description, as. bonding layer is included in application 1 and capping layer in application 4. As an example, subgrade is normally made with native soil, there should not be any use of alternative material in that application. However, in certain cases and countries, some external material can be used as a substitute or an improver of native soil in order to make the subgrade fulfil its function. It is called capping layer. Therefore in WP3 Deliverable 4 and Milestone 7, the use of alternative materials in the subgrade is considered.

One of the purposes of the SAMARIS milestone M7 was to validate the functions each part of the road structure (called “application”) has to fulfil. For that, a questionnaire designed on the basis of a literature review, was sent to 15 countries in 2003. Four countries (Austria, Denmark, France, and Sweden) have provided an answer.

A conclusion of the M7 questionnaire interpretation was that a given pavement layer does not seem to be directly linked to a given function, but rather to a range of functions. Wearing course is correlated to “resistance to traffic stresses”, “traffic safety – skid resistance”, “comfort - evenness” and “stiffness” at first, but can also contribute to “resistance to vertical load – load bearing”, “load distribution to lower layers” and “prevention of water infiltration”. Road base is correlated to “resistance to vertical load – load bearing”, “resistance to traffic stresses and strains”, “load distribution to lower layers” and “stiffness”, but can also contribute to “anti-frost”. Sub-base is correlated to “resistance to vertical load – load bearing” and ”drainage”, but can also contribute to “stiffness”, “anti-frost” and “anti-capillary rise”. Subgrade is correlated to “resistance to vertical load – load bearing”, but can also contribute “load effect – physical stabilisation” Application “shoulder and landscaping” is correlated to “drainage” and “wind erosion” at first.