ISO/TR 18228-10:2024

Technical
Report
ISO/TR 18228-10
First edition
Design using geosynthetics —
2024-05
Part 10:
Asphalt pavements
Conception utilisant des géosynthétiques —
Partie 10: Chaussées bitumineuses
Reference number
© ISO 2024
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Design considerations . 3
4.1 General .3
4.2 General design considerations .3
4.3 Geosynthetics used as an asphalt interlayer: product types and system functions .4
4.3.1 General .4
4.3.2 Asphalt interlayer functions .4
4.3.3 Geosynthetics used as an asphalt interlayer: product categories .6
4.4 Crack categories .8
4.4.1 General .8
4.4.2 Cracks with horizontal movements .9
4.4.3 Cracks with vertical movements.9
4.4.4 Cracks from horizontal and vertical movement .9
4.4.5 Structure-related cracks .10
4.5 Site investigation .10
4.6 Examples of system selection.10
4.7 Design models . 12
5 Installations .13
5.1 General . 13
5.2 Site preparation . 13
5.3 Bond coat application . 13
5.4 Installation of a geosynthetic used as an asphalt interlayer .14
5.5 Overlay application .14
6 Performance of interlayer systems .15
6.1 General . 15
6.2 Field assessment . 15
6.3 Laboratory tests .16
7 End-of-life and recycling . 17
7.1 General .17
7.2 Milling .17
7.3 Recycling .18
Annex A (informative) Overview of design software with geosynthetics . 19
Annex B (informative) Installation checklist .20
Annex C (informative) Specialized installation companies .22
Bibliography .23

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 221, Geosynthetics.
A list of all parts in the ISO/TR 18228 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
The ISO/TR 18228 series provides guidance for designs using geosynthetics for soils and below ground
structures in contact with natural soils, fills and asphalt. The series contains parts which cover designs
using geosynthetics, including guidance for characterization of the materials to be used and other factors
affecting the design and performance of the systems which are particular to each part, with ISO/TR 18228-1
providing general guidance relevant to the subsequent parts of the series.
The series is generally written in a limit state format and guidelines are provided in terms of partial material
factors and load factors for various applications and design lives, where appropriate.
This document includes information relating to the asphalt pavements. Details of design methodology
adopted in a number of regions are provided.
For more than 30 years roads have been built, maintained and operated using different types of geosynthetics
used as asphalt interlayers incorporated within asphalt pavements. Amongst other benefits, these products
are successful in mitigating reflective cracking in pavements, improving pavement performance, extending
pavement service life, resulting in a reduced total cost of ownership and a reduced carbon footprint.
Many of these products are related to geosynthetics used in geotechnical engineering and these products
have been adapted and adjusted for use as asphalt interlayers. A geosynthetic used as an asphalt interlayer
is special in the sense that it is used mostly between an existing pavement and a new asphalt layer. The
method of function of these products cannot be directly compared to, for example, concrete reinforcement
nor to soil stabilization mechanics. Moreover, geosynthetics used as asphalt interlayers are one part of a
system together with the tack or bond coat bonded in between two courses.

v
Technical Report ISO/TR 18228-10:2024(en)
Design using geosynthetics —
Part 10:
Asphalt pavements
1 Scope
This document provides general considerations to support the design guidance to geotechnical and civil
engineers involved in the design of structures in which a geotextile is used to fulfil the function of an asphalt
interlayer. The key potential failure mechanisms and design aspects to be considered are described, and
guidance is proposed to select engineering properties.
The state of the art is however limited and does not commend any particular design method. This document
can be used as a basis for further research on, for example, system selection, design, performance testing,
creation of local guidelines.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
bond coat
tack coat
bituminous binder used to promote the adhesion between layers in the construction and maintenance of
roads and paved areas
Note 1 to entry: In some countries a bond coat refers to a polymer modified bitumen while a tack coat refers to a
regular bitumen. In this document bond coat is used synonymously also for tack coat.
3.2
flexible pavement
layers of asphalt or bituminous concrete layers overlying a base of granular material on a prepared subgrade
3.3
installation aid
product attached to a paving grid in order to support the installation process in different ways without
providing any additional function (B, STR and R)
Note 1 to entry: An installation aid could consist of a light non-woven fabric, additional fibres in the apertures of the
grid or a thin synthetic foil. An installation aid can improve contact of the product to the base thus achieving better
adhesion during the paving process. A thin synthetic foil attached to a grid is sometimes used to decrease adhesion
experienced during unrolling.
3.4
interlayer barrier
B
function provided by paving geotextiles saturated by bitumen, which act, in conjunction with a bitumen
layer, as a barrier to the ingress of water and gasses, and thus prevent or delay the deterioration of the
pavement
3.5
interlayer system
geosynthetic products bonded in between two pavement layers for asphalt pavement application
Note 1 to entry: These can be paving geotextile, paving grid or paving geocomposite.
3.6
paving geocomposite
product that combines a paving geotextile and a paving grid
3.7
paving geotextile
geotextile fabric adequately saturated with bitumen providing a stress relief function (STR) and acting as an
interlayer barrier (B)
3.8
paving grid
product that has tensile elements which provides a reinforcement function (R) only
3.9
reflective cracking
vertical cracking through a pavement structure caused by stresses generated in the pavement foundations
resulting from movements that propagate upwards or downwards through the pavement structure
3.10
reinforcement
R
function which is provided by tensile elements of a geosynthetic used as an asphalt interlayer to delay or
prevent reflective cracking by the absorption of tensile forces
Note 1 to entry: Use of the stress-strain behaviour of a paving grid can improve the long-term mechanical properties
of asphalt.
3.11
rigid pavement
hydraulically bound pavement on a granular subbase
3.12
semi-rigid pavement
intermediate state between flexible and rigid pavements
Note 1 to entry: Lean cement concrete, roller compacted concrete, soil cement and lime-pozzolanic concrete
construction are examples of semi-rigid pavements.
3.13
stress relief
STR
function provided by an adequately bitumen-saturated interlayer (e.g. paving geotextile or paving
geocomposite) which allows for slight differential movements between the two layers and thus provides
stress relief to delay or prevent reflective cracking

4 Design considerations
4.1 General
This clause provides an overview of the different types of geosynthetics used as asphalt interlayers, their
functions, and their relation to the pavement design. These are crucial for differentiation of the products and
their functions in the system, particularly for specification purposes. The corresponding system functions,
related to each product type, are also described.
NOTE These functions do not occur in practice in their pure form and often overlap.
4.2 General design considerations
When working with a geosynthetic used as an asphalt interlayer system, the selection of the system
components is crucial for a successful project outcome. Detailed information about the trafficked area is
required and the expected performance of the chosen interlayer is usually clearly identified. For an economic
comparison it is often reasonable to estimate the expected performance of a system with and without an
interlayer. A comparison is normally be carried out over a period of time that demonstrates the efficiency of
the interlayer.
A geosynthetic used as an asphalt interlayer can have very different characteristics depending on the
method of manufacturing and the type(s) of raw material(s) used. Therefore, asphalt interlayers are not
simply interchangeable.
The objectives of using a geosynthetic as an asphalt interlayer include:
— extension of maintenance and rehabilitation intervals;
— extension of service life; and
— reduction of whole-of-life costs.
A geosynthetic used as an asphalt interlayer in a system can provide the following positive effects by their
different functions, or as a secondary effect of the function.
— Mitigation of reflective cracking through a reinforcing function and/or a stress relief function.
— Mitigation of water and gas ingress into the bound and unbound layers through their interlayer barrier
function.
— Enhanced uniform layer adhesion.
— Optimizing the performance of bound layers above and below the interlayer through their functions.
— Structural improvement through the reinforcement and/or stress relief function.
Increased stiffness, structural improvement and mitigation of water ingress can improve the fatigue
behaviour of the bound layers above the asphalt interlayer which again leads to the mitigation of reflective
cracking (including top down cracking).
In an interlayer system, it is essential that the properties of each element be adapted to the specific objective
of the measure to achieve the expected performance. Due to the temperature dependent visco-elastic
properties of the asphalt and further elements of the system, the properties of the entire system alter with
changes in temperature and loading. Therefore, any characteristics used in design normally take these
variations into account.
4.3 Geosynthetics used as an asphalt interlayer: product types and system functions
4.3.1 General
Geosynthetics used as asphalt interlayers are one part of a system which are combined to provide positive
outcomes. These systems usually consist of:
— the bound layer (flexible, rigid or semi-rigid) with an adequately prepared surface (milled or not-milled);
— a bituminous adhesive layer (type, quality and quantity according to each specific product);
— the geosynthetic used as an asphalt interlayer; and
— one or more layers of asphalt, a slurry seal overlaid with asphalt layers or a chip-seal (used, for example,
when using a paving geotextile under a surface dressing).
In combination with the system, each product provides different valuable functions to the construction.
4.3.2 Asphalt interlayer functions
4.3.2.1 Reinforcing function (R)
A new asphalt layer can be reinforced to mitigate the effects of reflective cracking using a paving grid or
paving geocomposite. Therefore, the product is normally anchored in the system in such a way that it is
able to absorb tensile forces. By absorbing tensile forces, the reinforcement can mitigate reflective cracking.
Further, if applied in the tension zone of the bound structure, it can increase the stiffness and strength of
the structure and lead to structural improvement of the asphalt layers. Both effects result in an improved
fatigue resistance in comparison to an unreinforced structure with the same thickness.
In general, there are two different mechanisms of load transfer from the asphalt to the grid which provide
reinforcement through (see Figure 1):
a) Adhesive shear bonding: The load transfer from the asphalt into the paving grid is achieved by the
adhesion and friction between the asphalt and the surface of the grid.
b) Integral ribs and load transferring junctions by structural horizontal interlock: The load transfer from
the asphalt into the paving grid is achieved by the anchorage of the paving grid in the asphalt matrix.
This type of load transfer occurs in combination with adhesive shear bonding.
Both mechanisms of load transfer result in a strengthening or stiffening of the asphalt layers. To dissipate
the forces, the interlayer normally has a certain anchorage length outside the tension zone. A minimum
length of 0,5 m to each side of a crack is usually sufficient, though this does ultimately depend on the specific
system and product.
a) Schematic illustration of load transfer “asphalt b) Schematic illustration of load transfer “asphalt
– grid” through adhesion – grid” through horizontal interlock
Figure 1 — Illustration of load transfer through adhesion and interlock

4.3.2.2 Interlayer barrier functions (B)
In asphalt road construction, an interlayer barrier is a layer which prevents the ingress of liquids (e.g.
water) and gases (e.g. oxygen) into the bound and unbound layers of the structure below the barrier. This
can be provided by a paving geotextile in conjunction with bitumen. When a paving geotextile is used, it
is important to have a certain bitumen retention capacity. Furthermore, an adequate quantity of bitumen
is needed to create a barrier with a sufficiently low permeability to mitigate aging and cracking of the
surrounding pavement. Refer to Table 4 for suggested quantities to provide interlayer barrier function (see
Figure 2).
Figure 2 — Schematic representation of interlayer barrier function
The interlayer barrier can provide the following benefits:
— maintaining the structural stiffness and the bearing capacity of bound and unbound layers over a longer
period of time;
— improvement of frost resistance and reduction of frost damage caused by freezing water in the structure,
bound and unbound layers (e.g. bursting effect, formation of ice lenses, drenching during defrosting
period);
— reduction in aging of the bitumen in asphalt layers surrounding the interlayer barrier caused by oxidation;
— slowdown of formation of embrittlement cracks below the interlayer barrier caused by oxidation.
4.3.2.3 Stress relief function (STR)
In asphalt road construction, stress relief function (see Figure 3) dissipates tensile strain, typically provided
by the visco-elastic characteristics of bitumen. Bitumen exhibits a visco-elastic behaviour in the service
temperature range, which means that when a load is applied to a bitumen film, there are three different
deformation reactions: elastic deformation (reversible), delayed elastic deformation (reversible), and
viscous deformation (irreversible). For a visco-elastic material it is typical that temperature and loading
speed determine the elastic or viscous behaviour.
A paving geotextile will store and hold the applied bitumen in place over the long-term. Moreover, the paving
geotextile assures a consistent and even layer thickness.

Key
1 crack
Figure 3 — Schematic representation of stress relief function of an interlayer
Stress in the bitumen layer can be caused by, for example, traffic loading and deformation of the base course.
These stresses can result in flow due to the visco-elastic properties of the bitumen used.
Tensile strain from movements of the underlying structure are absorbed to a large extent within the
bitumen layer by viscous flow reaction. Movements of the underlying structure are not propagated to the
new overlying layers of asphalt. Strain in the asphalt is reduced in order to avoid the local overstressing of
the asphalt. At the same time, the adhesion of the bitumen ensures that the bitumen layer has a good bond to
the underlying structure and the overlying layer of asphalt.
4.3.3 Geosynthetics used as an asphalt interlayer: product categories
4.3.3.1 Paving geotextile
A paving geotextile (see Figure 4) can be used in an asphalt pavement construction to provide, in conjunction
with an adequate quantity of bitumen, an interlayer barrier (B) and stress relief (STR) function. It can be used
as a single element or as part of a paving geocomposite. In order to provide an adequate level of performance
and lifetime in accordance with its functions, a typical nominal fabric weight is ≥130 g/m (according to
ISO 9864) with a bitumen retention of ≥ 0,9 kg/m (e.g. according ASTM D6140 or EN 15381, Annex C). The
paving geotextile in conjunction with the bitumen additionally provides a uniform layer thickness and an
enhanced uniform layer bonding.
Paving geotextiles do not increase the load-bearing capacity of the existing road structures but can maintain
the existing load-bearing performance by their ability to limit water penetration and frost damage due to
the barrier function. A paving geotextile alone does not provide a reinforcing function.

Figure 4 — An example of a paving geotextile
4.3.3.2 Paving grids
A paving grid (see Figure 5) is used in an asphalt pavement construction to provide a reinforcing function
(R). It can be used as single grid element or combined with an installation aid. The load transfer from the
asphalt into the paving grid is achieved by the anchorage of the paving grid in the asphalt matrix (see 4.3.2.1).
Paving grids provide a reinforcing function (4.3.2.1) by the absorption of tensile stress. The reinforcing
effect of the grids depends on the mechanical properties of the product, the position of the product in the
entire road pavement and its anchorage length.
Paving grids in asphalt systems can preserve the effective thickness of the asphalt by mitigating reflective
cracking from the underlying layer into the newly paved asphalt layers. Further, if applied in the tension
zone of the bound layers, they can, when loaded e.g. during trafficking, increase the stiffness and lead to
structural improvement of the bound layers above the interlayer.
A paving grid alone does not provide a stress relief (STR) or interlayer barrier (B) function.
a) A paving grid only consisting of reinforcement b) A paving grid consisting of a reinforcement
elements grid and a low weight geotextile acting as installa-
tion aid
Figure 5 — Examples of paving grids
4.3.3.3 Paving geocomposite
A paving geocomposite (see Figure 6) is used in asphalt pavement construction to provide, together with an
adequate quantity of bitumen, an interlayer barrier (B), stress relief (STR) and reinforcing (R) function. It is
composed of a paving grid attached to a paving geotextile.
A good bond between the bound layers is required in order to get a proper mobilization of the tensile forces
in the reinforcement.
Figure 6 — An example of a paving geocomposite
4.4 Crack categories
4.4.1 General
The following subclauses detail examples of crack origins and damages often seen through visual
assessment. These examples are not exhaustive. Cracks are mainly caused by different loads applied on the
road construction by:
— traffic: this can be because of vertical loads due to the axle loads, and/or tangential loads due to speeding/
braking or steering;
— temperature changes: contraction/expansion of materials due to temperature changes, and/or expansion
by freezing of water; and/or
— displacements in foundation and/or underground: this can be due to a variety of reasons, e.g. due to
contraction of bound material; settlements; post compaction; shear; contraction or expansion due to a
change in moisture content. These displacements can occur as rapid permanent deflection or as a result
of longer term repeated cyclic movement.

4.4.2 Cracks with horizontal movements
Cracks that are mainly caused by horizontal stresses or shear caused by temperature changes (seasonal as
well as daily; see Figure 7) occur in the following forms:
— cracks in longitudinal and lateral directions in asphalt paving with insufficient binder content, a too
hard binder, an aged binder on a load-bearing and deformation-resistant underlayer;
— cracks in longitudinal and lateral directions in cracked hydraulically bound and unbound subgrades,
subbases and concrete layers; and
— opening of longitudinal and traverse joints and of working seams.
Figure 7 — Temperature induced horizontal movement
4.4.3 Cracks with vertical movements
Cracks resulting from limited vertical bending or shear are often caused by traffic (see Figure 8) in the
following way:
— reflection of joints and cracks in underlying concrete layers and subbases with hydraulic binders into the
overlaying asphalt (can include limited bending of slabs); and
— reflection of single cracks and net-like cracks in surface layers of asphalt pavements, whose carrying
capacity is significantly reduced (e.g. by embrittlement of the binder or substantial traffic-based
deflections due to low load-bearing capacity of the subsoil).
Maximum vertical bending is achieved when the axle is immediately above the joint/crack while maximum
shear is achieved when the wheel is adjacent to the joint/crack.
Figure 8 — Traffic-induced vertical movement
4.4.4 Cracks from horizontal and vertical movement
Cracks from the combined effect of horizontal and vertical movements occur in the following forms.
— Net-like cracks in a thin asphalt paving.

NOTE Possible causes include insufficiently frost-proofed overall structure (alligator cracks), damaging water
penetration and self-cementing road bases.
— Crack accumulation in the wheel path of the asphalt pavement due to insufficient load-bearing capacity
of the layers underneath such as binder-poor bound layers or due to a too thin bound pavement for the
level of traffic loading.
— Cracks caused by temperature changes in rigid pavements.
4.4.5 Structure-related cracks
Structure-related cracks that are mainly caused by horizontal movements – as well as by vertical movements
at the same time – occur in the following forms.
— Longitudinal cracks due to road widening. The cracks are caused by changes of structural construction
(e.g. outer edge of old rough-stone pitching), changes in the thickness of the frost-proof paving, and/or
changes in the stiffness/load-bearing capacity between the old consolidated pavement construction and
the widened section or adding of another lane of differing material such as adjacent rigid and flexible
construction.
— Longitudinal cracks that occur when paving over concrete edging strips, tram rails and excavation of
all types.
4.5 Site investigation
The existing site conditions that are usually considered when using a geosynthetic as an asphalt interlayer
include:
— climate;
— type of the road (category, speed regime, number of lanes);
— current and expected traffic loads;
— type and condition of the existing unbound layers;
— type and condition of the bound flexible, rigid or semi-rigid layers;
— load bearing capacity of the traffic area;
— conditions lateral of the road (drainage, trees, etc.); and
— type and composition of the proposed new overlay.
Identifying the source of damage in the existing pavement structure is crucial for planning a successful
asphalt interlayer solution. There are different approaches to identifying and recording damage such as
visual inspection, destructive testing and non-destructive testing.
With a visual inspection the general condition of a pavement, the location and crack type as categorized
in 4.4, and deformations (e.g. rutting) can be identified. A visual inspection can only provide limited data
on the underlying layers. In addition to this, destructive testing is essential (e.g. core drilling) for getting
more information about the pavement structure (e.g. complete pavement thickness, pavement composition,
condition of the asphalt layers/concrete layer/cement stabilization, etc.), degree of layer bonding and crack
direction (top-down/bottom-up). Complementary additional non-destructive testing might be carried out
with, for example, ground penetrating radar (GPR). Falling weight deflectometer (FWD) is normally used
where differential vertical movement at a crack/joint is suspected.
4.6 Examples of system selection
Interlayer systems can be selected based on the mode of failure of the existing pavement and taking into
account the three functions (B, R, STR) of the solutions to be considered. In many cases there are a number
of reasons for cracking, so that one system is not be able to address all types of failures.

Tables 1 to 3 can help to select a suitable system depending on different requirements. In every case, it
is crucial that a system selection for the individual project is carried out carefully. The table shows
which properties are important for the specific case; a 0 means neutral while the number of + shows the
effectiveness of this function against the specific failure type.
Once the required function or functions for each area to be treated has been selected and a suitable interlayer
system found, a check is to be carried out to confirm that the chosen product is suitable for the individual
project conditions. The following conditions can limit the application of individual product types.
— Installation on a milled underlying layer: check if the geosynthetic used as an asphalt interlayer can be
installed on the milled surface and check the acceptable surface roughness.
— Installation in an environment with high pH-value (e.g. installation directly on concrete slabs or cement-
bound layers): check if the geosynthetic used as an asphalt interlayer is durable under these conditions.
— Minimum required asphalt thickness in the first layer above the interlayer: check minimum required
asphalt thickness for individual product.
Table 1 — Horizontal movements
Shrinkage through hard- Thermal expansions and Moisture induced swell-
ening / ageing (in asphalt) contraction ing (and shrinkage) of
underlying layers
Example of possible origin Bitumen embrittled due to Different behaviour of Cohesive underlying layer
oxidation through sur- materials, e.g. asphalt on swelling (and shrinking)
rounding conditions (e.g. in- concrete slabs or “new” as- from change of water
gress of water and oxygen, phalt on aged and hardened content due to changing
UV, etc.) asphalt layer(s) weather conditions and/or
vegetation
Crack width ≤ 6 mm > 6 mm ≤ 6 mm > 6 mm ≤ 6 mm > 6 mm
a a
R +++ ++ +++ ++ +++ ++
a a
STR/B ++ + ++ + +++ +++
a a
R/STR/B +++ ++ +++ ++ +++ +++
Key
+ improves performance
++ better performance
+++ best performance
a
In case failure is caused by water ingress or a changing water content, an intact drainage system is a significant factor for the
success of the rehabilitation.

Table 2 — Vertical movements
Shrinkage through hardening / ageing Moisture induced swelling (and shrink-
(in asphalt) age) of underlying layers
Example of possible Rocking concrete slabs paved with as- “Hard” change of bearing capacity in un-
origin phalt due to eroded foundation through derlying construction, e.g. caused by use
ingress of water and/or insufficient of different construction materials in joint
preparation of the unbound foundation region and/or material migration due to
ingress of water
Settlement ≤ 0,1 mm > 0,1 mm ≤ 0,1 mm > 0,1 mm
a a,b a a,b
R ++ + ++ +
a a
STR/B + 0 + 0
a a,b a a,b
R/STR/B ++ + ++ +
Key
+ improves performance
++ better performance
0 no improvement
a
In case failure is caused by water ingress or a changing water content, an intact drainage system is a significant factor for the
success of the rehabilitation.
b
Limited range of products due to different raw materials (check with the manufacturer).
Table 3 — Combined horizontal and vertical movements
Traffic induced Settlement Edge settlement Frost heave; Thermal expan-
bending Swelling of sion
unbound layer
through ingress
of water
Example of possi- Traffic; high axel Change of bearing Sliding of the Frost bursting or Expansion of un-
ble origin loads capacity in under- structure, in- expansion of ice derlying concrete
lying construc- duced by traffic lenses or expan- slabs with insuf-
tion, e.g. caused loads at the edge sive soils through ficient expansion
by different soil ingress of water joints
conditions or
ingress of water
c c c
Crack width ≤ 6 mm > 6 mm
c c c c c
Settlement
a,b b
R ++ + ++ ++ 0
a,b b
STR/B + 0 ++ + 0
a,b b
R/STR/B ++ + +++ ++ 0
Key
+ improves performance
++ better performance
+++ best performance
0 no improvement
a
In case failure is caused by water ingress or a changing water content, an intact drainage system is a significant factor for the
success of the rehabilitation.
b
Limited range of products due to different raw materials (check with the manufacturer).
c
Individual project related engineering judgement necessary (check with the manufacturer).
4.7 Design models
The goal of designers is to be able to incorporate analysis of the crack driving mechanisms presenting
themselves by simulating their progression through the overlay. As there is no single calculation method

for the design of roads accepted by all agencies or all countries, there are also no single design methods
for pavements incorporating anti-cracking interlayers. The design approach cannot be generalized and
is usually associated with the particular interlayer product being proposed. There are, however, several
[4]
specific calculation methods with all their advantages and disadvantages. The disadvantage of these
calculation methods is that they are limited to the use of a certain product or to a certain failure mode.
Recent studies deal with modelling the behaviour of a geosynthetic used as an asphalt interlayer in asphalt
layers. Current ME, 2D and 3D Finite Element design models (FEM) show good correlation with results
of laboratory tests and the results gained from use in on-site practice. However, because of the complex
interrelation and the many influencing factors, it is crucial that the design models are confirmed through
long term, real life performance measurements before they can be considered reliable. Therefore, the design
of asphalt interlayers still relies strongly on practical experience.
Some existing design methods are presented in Table A.1. Some of them are proprietary to some specific
product(s). Design methods often use reference designs based on laboratory and in-situ verification of final
performance.
5 Installations
5.1 General
Asphalt road paving with a geosynthetic used as an asphalt interlayer requires particular care and
experience. It is essential that specific project conditions are reviewed with a technical representative of
the interlayer manufacturer, who can provide expert assistance during installation. The system components
of the old roadway (including site conditions like dewatering and drainage, presence of existing cracks,
method of milling, etc.), the geosynthetic used as an asphalt interlayer, and the paving of any surface layers
is normally considered and coordinated. Therefore, it is crucial that the installation be carried out only by
qualified companies (see Annex C) or under the supervision of the manufacturer/supplier of the asphalt
interlayer. It is essential that the installation guidelines of the manufacturer are always considered and, if
necessary, adapted to the specific site conditions as the guidelines cannot cover all potential scenarios. A
non-conforming installation of a geosynthetic used as an asphalt interlayer can lead to premature failure of
the pavement. Annex B provides a checklist that can be used for quality control purposes and documentation
of the site condition. During installation, it is essential that health and safety instructions are considered.
5.2 Site preparation
The receiving surface has to be clean, dust-free, dry and free of loose particles. All potholes are to be filled
with bituminous material. It is crucial that cracks are treated according to the local guidelines. It is good
practice to treat cracks with a width of 3 mm to 20 mm, with an appropriate flexible sealant. Any unevenness
and/or vertical movements in the pavement structure are to be eliminated (e.g. by stabilizing concrete slabs,
relaxing unstable concrete slabs/levelling course). Depending on the product of the chosen solution, the
installation of the geosynthetic is to be carried out on a fine milled surface, of up to 10 mm peak to trough
regularity.
5.3 Bond coat application
A geosynthetic used as an asphalt interlayer is requiring a bond coat for installation. It is vital to understand
its importance as follows.
a) A bond coat is required to achieve a proper bonding between the asphalt layers – independent of using
an asphalt interlayer or not.
b) When using a geosynthetic used as an asphalt interlayer, the bond coat holds the asphalt interlayer in
position (i.e. no wrinkling or creasing) during the asphalt paving process.
c) If a certain quantity of bond coat is sprayed, it forms a seal/barrier between the underlying surface and
the overlaying asphalt (see 4.3.3.1).
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