PAVEMENT DESIGN GUIDE

INTRODUCTION The pavement design guide refers to flexible pavements and it consists of three parts: Part 1 gives general information about the pavement components, pavement behaviour in relation to traffic and a glossary of terms used in the pavement design. Part 2 describes in detail the pavement design methodology developed. It starts with the traffic, the subgrade and the climatic conditions assessment followed by the requirement for providing or not of a capping layer and the design of the foundation layer, the bituminous or main structural layer and the bituminous wearing course, which must have good antiskidding surface. Part 3 details the materials to be used in pavement construction, starting from the materials for the foundation layer, followed by the materials for the bituminous or main structural layer and the wearing course. Part 3 also includes some proposals on the modification of the standards PTP A260 and A265, essential, in the opinion of the team of experts, for the improvement of the asphaltic concrete behaviour. The design guide was developed by a committee of experts on behalf of Egnatia Odos A.E. (EOAE). Although it is tailor made for the Egnatia road it may also be used for designing any other major road in the Greek network providing that there is a written permission by EOAE and a consultation with the leader of the team of experts Dr A. Nikolaides, Assoc. Professor, Aristotle’s University of Thessaloniki.

Part I

Pavement Design Guide

2 General Information

PART 1:

GENERAL INFORMATION

1. Background Current Greek standards in pavement design lack of in-depth analysis and justification on the determination of the layer thickness of a pavement. Flexible pavements are designed primarily on personal practical experience with the help of an official circular1 issued back in 1961. Significant developments have taken place, ever since, in other countries in connection to the pavement design. The traffic volume and its composition has changed dramatically ever since, new improved materials and techniques have been developed and by no means the recommendations of this circular can be applied to highways or principal state roads. Egnatia Odos A.E. have noted in the OSMEO that the methodology for pavement design is under review. In order to facilitate the preparation of the tender documents currently uses a very simplified method from which the thickness of each layer of a flexible pavement can be determined from a table, for a given traffic category and assumed bearing capacity of the formation layer (subgrade). It is not capable to design analytically a pavement for n-number of years based on cumulative number of traffic loading. The design guide developed and described here provides the facility, for the fist time in this country, to design analytically a flexible pavement taking into consideration all major factors involved and affecting the future performance of the pavement. It is based on the structural design philosophy established, which requires an understanding of the behaviour of the materials under load and an appropriate theoretical analysis of the design problem. The design of a rigid (concrete) pavement is not covered by this design guide because the design philosophy is completely different than that of a flexible pavement. In Greece, no concrete pavement has yet been constructed in the national and regional road network. Consequently, in this study it was judged that a proposal to introduce concrete pavements presented the following main problems and difficulties: a) complete absence of construction experience, b) absence of suitable, particularly expensive, equipment, c) requirement for proper construction, in particular at the joints, otherwise the advantage of infrequent maintenance disappears and d) increased cost of maintenance and reconstruction in comparison with flexible pavements. It is obvious that the introduction of rigid pavements, in the case of Egnatia road, the construction of which is underway, would constitute a perilous experiment.

1

Ministry of Transportation & Public Works, General Secretariat of Public Works, Division C2, Department A, Circular 25460/3357 ./21.7.1961, Applied types of flexible pavements ç/ì/åååå

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2. Pavement structural components The typical cross section of a flexible pavement is as shown in Figure 1.

Antiskidding surface

Wearing course

Binder course Asphaltic base Base Formation Sub-base Capping layer

Figure 1

Subgrade or pavement foundation layer

Typical cross section of a flexible pavement

The wearing course, the binder or basecourse and the asphaltic base are all bound with bituminous binder. The base and sub-base layers are normally unbound and in some cases weakly bound with cement granular or soil material. The wearing course is the upper most bituminous layer of the pavement. Its function is to provide an even rolling surface with good antiskidding properties. The antiskidding properties are provided either by the gradation and the hardness and durability of the aggregates of the mixture that the wearing course is constructed of, or by a specially designed antiskidding mixture (layer). Apart from this, the wearing course should not deform under traffic and it should resist crack propagation. It is desirable to contribute to the strength of the pavement as well as to be almost impermeable to water and to reduce the noise generated by the moving wheel. If the first two properties mentioned in the last sentence do not coexist, they should be provided by the underlying asphaltic layers. The binder course or basecourse provides the platform on which the wearing course is laid. Together with the underlying asphaltic layer it contributes to the strength of the pavement and forms the main structural layer of the flexible pavement. In case the wearing course is water permeable, the binder course must always be of a dense graded mixture (asphaltic concrete). Additionally, the binder course should not deform easily under traffic, should resist crack propagation and it should have good fatigue life. The asphaltic base together with the binder course is the main structural layer of the flexible pavement required to distribute the applied traffic loading so that the underlying materials are not overstressed. It should sustain the stresses generated within itself without excessive or rapid deterioration. It is the layer which contributes the most to the overall stiffness of the pavement and to the pavement’s resistance to fatigue cracking. Due to its asphaltic nature and its greater thickness compared to other asphaltic layers, it contributes ç/ì/åååå

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substantially to the permanent deformation of the pavement. It is therefore necessary, that the asphaltic base should have good dynamic and static stiffness modulus and good fatigue performance. The base/sub-base layer is the unbound layer (sometimes cement bound) which may consist of two sub-layers: the base and the sub-base layer. Its function is to reduce further the vertical stresses induced by the traffic loading to the subgrade. It provides a good platform for laying and compacting the asphaltic base. During construction, it also provides a good platform to the construction vehicles. Indirectly, it also acts as a frost protection layer to the frost susceptible subgrade. The capping layer is a subgrade improvement layer which protects weak subgrade from damage during the period of construction as well as throughout the pavement’s service life. It may be constructed with relatively cheap suitable imported material or it may be a treated in-place material. It is placed between the native subgrade and the sub-base layer, as a replacement of the native subgrade (i.e. say 600mm of capping layer is required, remove 600mm of subgrade and replace it with better quality material). The base/sub-base layer together with the capping layer (when used) form the foundation of the pavement. 3. Pavement non structural components Apart from the above mentioned pavement components there is also the drainage layer and, in some cases, the frost protection layer. Either layer or both together, when placed, they are not considered to contribute substantially to the strength of the pavement and therefore they are characterized as non structural layers. The drainage layer is provided to protect the subgrade, the capping layer and the base/subbase layer, both during construction and during the service life of the pavement, from incoming surface rain water through the overlying layers. When the water table is high and the subgrade is moisture sensitive the drainage layer is also beneficial. The drainage layer is placed between the subgrade or capping layer and the sub-base layer, and is connected to the french or fin drain. The thickness of the drainage layer is usually 200mm, and never less than 150mm. The drainage layer placed at the above mentioned position does not act, by any means, as drainage layer for lowering the water table. In case that lowering the water table is required and if drainage layer is selected to resolve this problem, the position of the drainage layer should be lower into the subgrade. In order to stop pore clogging by fines from other adjacent layers, geosynthetic materials acting as separators may be used when those layers are constructed of fine soil or fine capping material. The frost protection layer is constructed, if needed, in order to protect the frost susceptible subgrade. This layer is an extension of the sub-base layer, and its thickness should be such that the total thickness of the pavement equals or is greater than the depth of frost penetration.

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4. Pavement performance The pavements, under the influence of the traffic loading, deteriorate gradually. Each vehicle’s axle causes a certain minute damage which accumulates and shows at the end of the pavement’s service life as a rutting or/and cracking. This is considered as structural failure and it is apparent at the end or near the end of the pavement’s design life. The rate of pavement’s deterioration changes with time. It is very small at the beginning, it gradually increases as time passes and it increases noticeably as the pavement reaches its critical condition. Critical condition may be defined as when the rut depth is 10mm or/and one longitudinal crack appears. Apart from the above, the surface of the pavement may lose its skid resistance or it may show some disintegration (raveling or small potholes) or corrugations and shoving. This is also a failure but is characterized as non-structural failure (wearing course failure). Loss of skidding resistance is associated with the type of aggregate and traffic volume and it starts from day one, almost at the same loss rate. The other type of failures are associated with the bituminous mixture and, if they are to appear, they appear at a very early stage after construction. During a pavement design the main concern is to design the pavement in such a way as to withstand all the traffic loading anticipated throughout the design life selected, i.e. provide the necessary thickness so as structural failure occurs at the end of the design period. During construction stage the main concern is that the prescribed materials and mixtures are properly placed and used in order not to impose to the pavement so premature failures. The loss of surface skidding resistance with time, as well as the other non structural type of failures, should always be corrected every time they are apparent. Generally, to monitor the performance of a pavement, structurally or otherwise, the responsible highway authority should use a number of assessment machines or methods. The description of these machines and methods is outside the scope of this work.

5. Glossary of terms Antiskidding layer :

The upper layer of the pavement which comes into contact with the vehicles’ tires specially designed to act as antiskidding layer (the term antiskidding layer is used in Greece in contrast to our wearing course which does not have good antiskidding properties) Asphaltic base : Main structural bituminous layer of the pavement, placed above the base Base : A layer above subbase which provides a platform upon which the asphaltic base may be laid Binder course : A platform layer upon which the wearing course may be laid Bituminous or asphaltic layer : A layer bound with bituminous (or asphaltic) binder Capping : A subgrade improvement layer Deformation : The irreversible movement/compression of pavement layers leading to rutting and settlement ç/ì/åååå

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Design period (or life): Equivalent wear factor :

The number of years for which the pavement is designed Factor representing pavement damage due to a standard axle loading Fatigue : The formation of cracks in the pavement materials under repeated loading Flexible pavement : A pavement consisting of bound with bituminous binder layers and unbound (or weakly cement bound) layers Formation level : Level upon which sub-base is placed Foundation or formation layer : A platform layer, ≥ 600mm in depth, upon which the pavement may be constructed Pavement : All layers above formation level Reconstruction : The replacement of almost all bound and, in some cases, the unbound layers of an existing pavement with new (or recycled) materials Standard axle : A single axle with two sets of dual tires and an 80 kN total force Stiffness modulus : The equivalent of elastic modulus but for materials whose stiffness varies with temperature and time of loading Sub-base : A layer in direct contact with the subgrade which provides a platform upon which the base is constructed (sometimes may not exist as a separate layer) Subgrade : Soil underlying a pavement, may be natural soil or brought to site Vehicle category : Description of vehicle by general weigh/damaging power, e.g. trucks or heavy trucks, etc. Vehicle type : Description of vehicle by type and number of axles, e.g. with 2 axles, or 3 axles, or 3 axle articulated, etc. Wearing course : The upper most layer of the pavement

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