EP0929719B1 - Three-layered road structure - Google Patents

Description

The present invention relates to a new pavement structure, the load-bearing part surmounting a floor comprises three successive layers of bituminous materials bonded together.

Pavement structures have long been constructed with natural or crushed aggregates, but first without incorporating binders intended to increase their cohesion. Today, the two techniques, with or without incorporation of binders, are used independently or combined, (see e.g. EP-A-0 069 015).

With such a technology using structures made of non-material the main rule for their conception is the progressiveness of the stiffness modules. The less efficient materials are put under-layer, in contact with the ground with possible interposition of a layer of shape; these are generally materials of low lift. Are coming then more elaborate materials and finally the wearing course which alone is carried out with a bituminous binder.

An example of a conventional structure in untreated materials will be indicated in table 1 above which illustrates the rule of progressiveness of the modules. LAYERS THICKNESS MATERIALS ELASTIC MODULE Rolling 6 cm coated 5400 MPa Based 20 cm Serious Reconstituted Humidified 0/20 360 MPa Foundation 25 cm Untreated severe 0 / 31.5 120 MPa Ground - ground 20 MPa

With the increase in traffic, material structures have appeared treaties. These materials are gravels or sands treated either with cement, or with a related hydraulic binder, either bitumen.

These structures are designed to work in bending and calculated in result.

In such a case, the progressiveness of the modules between the layers of foundation and base may still exist but is not the general rule, on the contrary.

• des structures (famille F1 du tableau II ci-après) ayant des couches de base et de fondation de même rigidité. Ce sont en général les mêmes matériaux ou des matériaux semblables qui composent les deux couches, et
• des structures (famille F2 du tableau II ci-après) dont la couche de base est nettement moins rigide que la couche de fondation. Dans ce cas, la couche de base joue un rôle contre la remontée des fissures transversales de retrait qui se développent inévitablement dans les couches en MTLH (Matériau Traité aux Liants Hydrauliques).

In practice, we distinguish:

• structures (family F1 of table II below) having base and foundation layers of the same rigidity. These are usually the same or similar materials that make up the two layers, and
• structures (family F2 of table II below) whose base layer is much less rigid than the foundation layer. In this case, the base layer plays a role against the rise of transverse shrinkage cracks which inevitably develop in layers in MTLH (Material Treated with Hydraulic Binders).

From a structural point of view, the base and foundation layers are the thickest layers and ensure the durability of the structure. The surface layer, on the other hand, provides the user with security and comfort. Type of structure Semi-rigid Concrete Thick bituminous Mixed reverse Surface layer coated Cement concrete Coated Coated Coated Basecoat MTLH Cement concrete Severe bitumen Severe bitumen HRM Foundation layer MTLH Lean concrete Brave bitumen MTLH MTLH Operation F1 F1 F1 F2 F2 GRH: Serious Reconstituted Humidified F1: Structure with E _CB ≥ E _CF F2: Structure with E _CB ≤ E _CF

a) par fatigue due à des flexions répétées de la couche traitée la plus sollicitée, en général la couche de fondation, et/ou

b) par déformation permanente qui résulte d'une pression verticale excessive sur les matériaux non traités, en particulier sur le sol.

Damage to pavement structures can occur in two separate processes:

a) by fatigue due to repeated bending of the most stressed treated layer, in general the foundation layer, and / or

b) by permanent deformation which results from excessive vertical pressure on the untreated materials, in particular on the ground.

To avoid its rupture, the dimensioning of a pavement structure therefore consists in limiting these stresses below the admissible values which depend on the performance of the material and the traffic it will have to undergo, in increasing the thickness of the constituent layers of said structure.

To replace a conventional two-layer system, as it was known in the prior art, consisting of a base layer and a foundation layer, the object of the present invention proposes a new pavement structure of which the bearing part is constituted by a system special three-layer.

It generally comprises two thin outer layers made of high performance materials, and a middle layer of more modest but inexpensive performance. The adaptation of such traffic structure is achieved by increasing the bending moment admissible by increasing the leverage of the system, i.e. by increasing the thickness of the central layer, that is to say the one using to the least expensive material.

• une couche inférieure qui repose sur le sol avec interposition éventuelle d'une couche de forme, d'épaisseur H_i telle que : 4 cm ≤ Hi ≤ 10 cm et de module élastique : E_i
• une couche médiane d'épaisseur H_m telle que : 4 cm ≤ Hm ≤ 20 cm et de module élastique E_m tel que : 2 000 MPa < Em < 8 000 MPa
• une couche supérieure d'épaisseur H_s telle que : 4 cm ≤ Hs ≤ 10 cm et de module élastique E_s, lesdits modules élastiques desdites couches répondant aux relations d'inégalité suivantes : 2 < Ei Em < 10   et   2 < Es Em < 10

Thus, the present invention relates to a pavement structure, the bearing part of which, surmounting a ground, comprises three successive layers of bituminous materials bonded together, namely:

• a lower layer which rests on the ground with possible interposition of a form layer, of thickness H _i such that: 4 cm ≤ H i ≤ 10 cm and of elastic modulus: E _i
• a middle layer of thickness H _m such that: 4 cm ≤ H m ≤ 20 cm and of elastic modulus E _m such that: 2,000 MPa <E m <8000 MPa
• an upper layer of thickness H _s such that: 4 cm ≤ H s ≤ 10 cm and of elastic modulus E _s , said elastic moduli of said layers responding to the following inequality relations: 2 < E i E m <10 and 2 < E s E m <10

It should be noted that throughout the text of the description and claims, bituminous materials will be understood very general all types of road materials, natural, artificial or recycling, in particular gravels and / or sands treated with binders hydrocarbons or possibly hydraulic binders as well as mixtures of such binders. As examples of bituminous binders, we will mention pure or modified bitumens, in the form of an emulsion or bitumen foam or in fluidized form, with or without additives.

It should also be noted that all the elastic modules are understood to be being determined under average conditions of use, i.e. ordinary temperatures and traffic speeds.

Other characteristics and advantages of the present invention will appear on the reading made below with reference to the detailed study of the operation of such a pavement structure, object of the present invention.

According to an additional characteristic of the present invention, the layers lower and upper have elastic modules at least substantially identical, and advantageously equal to each other.

According to another characteristic of the present invention, the thickness H _i and the thickness H _s of the lower and upper layers are at least substantially identical and advantageously equal to each other.

According to a particularly advantageous characteristic of the present invention, the lower and upper layers of said structure pavement are made of gravel and / or sand treated with a binder hydrocarbon.

For the realization of such lower and upper layers, use will be made of preferably a high modulus bituminous concrete, for example, a mix of type of that sold by the Applicant under the designation BBTHM®, that is to say a Very High Modulus Bituminous Concrete.

According to another advantageous characteristic of the present invention, the middle layer of said structure will be made of gravel and / or sand treated with a hydrocarbon binder, a hydraulic binder or a mixture of such binders. As an example of a binder, mention will be made of GRAVE-MOUSSE® marketed by the Applicant.

If it is important to specify that the three successive layers of materials bituminous must be bonded together, it should be noted that this adhesion between adjacent layers can be obtained by any means. It is as well as the successive implementation of the different hot layers as well that the particular composition of the latter can lead to a bonding satisfactory, without the need to resort to the application of additional layers of other materials.

Advantageously, the bonding between said layers can be carried out by interposition of a bonding layer or, if necessary, a layer impregnation and a bonding layer.

The bearing part of the pavement structure, according to the invention, comprises a lower layer which can rest either directly on the ground or with interposition of a form layer intended to standardize the lift of the structure. Such a form layer is conventionally produced in a material treated or not, and can reach thicknesses of the order of 30 cm.

Such a pavement structure in accordance with the present invention can finally, if necessary, topped with a wearing course, the precise nature of which will be chosen by the skilled person according to the traffic to which said pavement is intended to be submitted. It could for example be a conventional surface coating with a thickness of approximately 2.5 cm.

However, it is important to note that for certain specific applications, and due to the nature and formulation of the top layer, it is not no need to overcome the structure according to the invention by a wearing course.

Study of the functioning of a pavement structure in accordance with the present invention was first carried out theoretically in modeling, then numerical calculations.

The model used is a multilayer system in linear elasticity subjected to a load corresponding to two twin wheels of 6.5 T. This scenario corresponds to the French standard axle of 13 T.

• les couches inférieure et supérieure sont de faibles épaisseurs : HS = Hi = 6 cm
• l'épaisseur de la couche médiane est variable en général, telle que : 8 cm ≤ Hm ≤ 20 cm
• le contraste de module : 2<Es Em < 10 2< Ei Em < 10
• les matériaux constituants la couche supérieure et la couche inférieure ont des caractéristiques voisines. Pour simplifier l'étude ci-après, les deux matériaux seront supposés identiques : d'où E_i = E_s.

The parameters for modeling a three-layer structure on a support of infinite thickness are indicated below.

• the lower and upper layers are thin: H S = H i = 6 cm
• the thickness of the middle layer is generally variable, such as: 8 cm ≤ Hm ≤ 20 cm
• the module contrast: 2 < E s E m <10 2 < E i E m <10
• the materials constituting the upper layer and the lower layer have similar characteristics. To simplify the study below, the two materials will be assumed to be identical: hence E _i = E _s .

In addition, we will take for this material the characteristics of BBTHM®, a high-modulus asphalt from the Jean Lefebvre Company.

• Lorsque la structure est constituée d'un même matériau (E_s/E_m = 1) les diagrammes sont sensiblement linéaires. La flexion se traduit par de la compression en surface et de la traction en partie inférieure.
• Lorsque le contraste de module est élevé (E_s/E_m = 35), par exemple en intercalant une grave non traitée entre les deux couches d'enrobé à module élevé (BBTHM®), les deux couches, haute et basse, fléchissent indépendamment l'une de l'autre ce qui a pour conséquence :
• l'apparition de traction importante à la base de la couche supérieure et l'augmentation sensible de la contrainte de flexion à la base de couche inférieure.
• La réduction du rôle joué par la couche médiane et en particulier de son épaisseur. L'augmentation de l'épaisseur de cette couche ne permet pas de réduire sensiblement la contrainte de traction à la base de la couche supérieure.
• Enfin dans le cas intermédiaire (E_s/E_m = 4), le diagramme des contraintes se présente sous forme de lignes brisées comme précédemment mais avec trois différences fondamentales que sont :
  • la disparition de la contrainte de traction à la base de la couche supérieure (donc pas de risque d'endommagement),
  • la faible augmentation de la contrainte de traction à la base de la couche inférieure par rapport à la structure monolithique,
  • conservation d'une forte dépendance entre les couches ce qui permet de mobiliser un moment de flexion plus important en augmentant l'épaisseur de la couche médiane (l'effet de bras de levier persiste).

Figure 1 attached shows that the diagrams of stresses and deformations along a vertical axis to the right of the load are differentiated according to the ratio E _s / E _m .

• When the structure consists of the same material (E _s / E _m = 1) the diagrams are substantially linear. The bending results in compression on the surface and traction in the lower part.
• When the modulus contrast is high (E _s / E _m = 35), for example by inserting an untreated bass between the two layers of high modulus asphalt (BBTHM®), the two layers, high and low, flex independently one of the other which results in:
• the appearance of significant traction at the base of the upper layer and the appreciable increase in the bending stress at the base of the lower layer.
• The reduction of the role played by the middle layer and in particular of its thickness. The increase in the thickness of this layer does not make it possible to substantially reduce the tensile stress at the base of the upper layer.
• Finally in the intermediate case (E _s / E _m = 4), the stress diagram is in the form of broken lines as before but with three fundamental differences that are:
  • the disappearance of the tensile stress at the base of the upper layer (therefore no risk of damage),
  • the slight increase in the tensile stress at the base of the lower layer compared to the monolithic structure,
  • preservation of a strong dependence between the layers which makes it possible to mobilize a greater bending moment by increasing the thickness of the middle layer (the leverage effect persists).

The determining values to be examined in such a structure are the deformations at each interface.

They make it possible to evaluate the mode of stress (traction or compression) and to assess whether it is a critical level where phenomena damage could arise.

The graphs in Figures 2 to 4 appended show how these deformations vary as a function of the ratio E _s / E _m and the thickness H _m .

The deformation at the base of the upper layer is illustrated in Figure 2.

The base of this layer is extended for the high E _s / E _m ratios.

For a value of Es = 500 MPa (case of an untreated bass with an inverse structure for example) the deformation is greater than 100.10 ^-6 which makes it a critical point for the dimensioning.

In addition, it should be noted that the thickness H _m of the middle layer has little influence on the level of deformation. In fact, the only way to reduce this value is to increase the thickness of the top layer. Such a solution is expensive, since it is the most efficient material.

For the low E _s / E _m ratios, we find the behavior of conventional structures, characterized by upper layers working entirely in compression.

For the intermediate ratios of E _s / E _m , for example for values E _m between 2000 and 5000 MPa, the deformation is positive, but remains sufficiently weak so that there is no risk of damage.

Deformations in the lower layer, at the level of the upper fiber are illustrated in Figure 3.

This fiber is in slight compression for low values of E _m (500 to 1,500 MPa) and in moderate traction beyond.

Deformations in the lower layer, at the level of the lower fiber are illustrated in Figure 4.

This fiber is in tension whatever the E _s / E _m ratio. This deformation is more sensitive to the thickness of the middle layer than to its rigidity.

A quadrupling of the E _m module (5,000 to 2,000 MPa for example) can be compensated by an increase in thickness (H _m ) of 2 cm (8 cm to 10 cm).

As the layers are glued together without risk of slipping, the deformations in the middle layer are those of the adjacent layers.

E_m > 2 000 MPa pour ce qui concerne la fibre supérieure

E_m < 8 000 MPa pour ce qui concerne la fibre inférieure.

The favorable ranges of the module E _m which lead to deformations by compression or by weak traction are:

E _m > 2,000 MPa for the upper fiber

E _m <8000 MPa for the lower fiber.

In conclusion, it emerges from the previous analysis that the range of interest for the module (E _m ) is between 2,000 and 8,000 MPa or more preferably, between 4,000 and 8,000 MPa.

• les couches supérieure et médiane sont peu sollicitées en traction et auront une forte durabilité, et
• la couche inférieure est la couche la plus sollicitée et son niveau de déformation peut être ajustée à une valeur admissible dépendant du trafic, en faisant varier l'épaisseur de la couche médiane.

Indeed, under these conditions:

• the upper and middle layers are not very stressed in tension and will have a high durability, and
• the lower layer is the most stressed layer and its level of deformation can be adjusted to an admissible value depending on the traffic, by varying the thickness of the middle layer.

The dimensioning of the three-layer structure can be determined as follows.

• fatigue de la couche inférieure du tricouche (figure 5a) :
• Au cours de cette première phase, seule la couche inférieure est sollicitée en traction. Le nombre de sollicitations que peut supporter la structure au cours de cette phase peut être calculé en appliquant la méthode de calcul habituelle qui sera détaillée ultérieurement.
• fatigue de la couche médiane (figure 5b) :
• Après rupture de la première couche, la structure fonctionne différemment. La couche médiane travaille cette fois-ci en traction et ce mode de fonctionnement durera jusqu'à la rupture de cette couche.
• fatigue de la couche supérieure (figure 5c) :
• Après rupture des deux premières couches, la couche supérieure va être à son tour sollicitée en traction et donc subir un processus d'endommagement.

Analysis of the operating mode of the three-layer structure makes it possible to predict its failure mode. The process of ruining such a structure has three phases which have been illustrated in Figure 5:

• fatigue of the lower layer of the three-layer (Figure 5a):
• During this first phase, only the lower layer is stressed in tension. The number of stresses that the structure can withstand during this phase can be calculated by applying the usual calculation method which will be detailed later.
• fatigue of the middle layer (Figure 5b):
• After breaking the first layer, the structure works differently. The middle layer works this time in traction and this mode of operation will last until the rupture of this layer.
• fatigue of the upper layer (Figure 5c):
• After rupture of the first two layers, the upper layer will in turn be stressed in tension and therefore undergo a damage process.

In fact, given the original mode of operation of this structure, its lifespan is the sum of three elementary durations.

a) une structure avec le même matériau en couche de base et de fondation (ou modèle tricouche avec E_s = E_m = E_i). Dans ce cas, la structure est monolithique (voir figure 6a), et le processus de ruine ne comporte qu'une phase qui est l'endommagement de la couche de fondation. En effet, comme les couches sont collées entre elles et qu'elles ont des rigidités au moins sensiblement voisines, toute fissure apparue à la base de la structure se propage très rapidement vers la surface. Cette phase de propagation est négligeable devant la phase d'amorçage de la fissure. Dans le cas de la structure tricouche, la présence d'une couche médiane nettement moins rigide, bloque la propagation de la fissure et une nouvelle phase d'amorçage est nécessaire pour que le processus de ruine puisse progresser.

b) une structure inverse (ou modèle tricouche avec E_m ≤ E_i et E_s). Dans un tel cas de figure, il y a endommagement simultané de la couche inférieure et de la couche supérieure (voir figure 6b) et il n'y a pas sommation des durées de vie des couches.

This particularity is never encountered for conventional structures. As an example, we can examine the following two extreme cases:

a) a structure with the same material in base and foundation layer (or three-layer model with E _s = E _m = E _i ). In this case, the structure is monolithic (see Figure 6a), and the ruin process involves only one phase which is the damage to the foundation layer. Indeed, as the layers are bonded together and they have at least substantially similar rigidities, any crack which appears at the base of the structure propagates very quickly towards the surface. This propagation phase is negligible compared to the crack initiation phase. In the case of the three-layer structure, the presence of a clearly less rigid middle layer, blocks the propagation of the crack and a new phase of priming is necessary so that the process of failure can progress.

b) an inverse structure (or three-layer model with E _m ≤ E _i and E _s ). In such a case, there is simultaneous damage of the lower layer and the upper layer (see Figure 6b) and there is no summation of the lifetimes of the layers.

To illustrate the improvement in service life of the three-layer structure according to the invention, the calculation proposed to illustrate the approach focused on the following structure: 6 cm of BBTHM® + 8 cm of GM + 6 cm of BBTHM® on a PF2 platform.

GM designating a grave treated with bitumen foam, in particular the Grave-Mousse®.

The modeling of the three phases is presented in Table III below: Layer 1st phase 2nd phase 3rd phase higher 17,800 17,800 17,800 median 5,500 5,500 1,100 lower 17,800 3,500 (*) 3,500

• 1ère phase : 85,5 10^-6 à la base de la couche inférieure
• 2ème phase : 112 10^-6 à la base de la couche médiane
• 3ème phase : 87,6 10^-6 à la base de la couche supérieure.

The calculated critical strains are as follows:

• 1st phase: 85.5 10 ^-6 at the base of the lower layer
• 2nd phase: 112 10 ^-6 at the base of the middle layer
• 3rd phase: 87.6 10 ^-6 at the base of the upper layer.

The fatigue characteristics necessary for the calculations are given in Table IV below: Epsilon 6 Slope SN SH BBTHM® 132 10 ^-6 0,175 0.29 1 cm Grave-Mousse® 109 10 ^-6 0.126 0.6 1 cm

ε_t :

déformation à la base des couches bitumineuses

ε₆ :

déformation qui aboutit à une durée de vie équivalente à 10⁶ cycles

N :

nombre de cycles

K_c :

coefficient de calage

K_c =

1 pour le BBTHM®

K_c =

1,3 pour la Grave-Mousse®

K_r :

est un coefficient qui ajuste la valeur de la déformation admissible au risque de calcul retenu en fonction des facteurs de dispersion sur l'épaisseur (écart-type Sh) et sur les résultats des essais de fatigue (écart-type SN) Kr = 10-ubδ

u :

variable centrée réduite associée au risque r

b :

pente de la loi de fatigue du matériau (loi bi-logarithmique)

δ :

écart-type de la distribution de log N à la rupture δ = [SN2 + (c2/b2) Sh2]0.5

c :

coefficient reliant la variation de déformation à la variation aléatoire d'épaisseur de la chaussée, Δh, (logε = logε₀ - c Δh).
Pour les strutures courantes, c est de l'ordre de 0,02 cm^-1.

K_s :

le coefficient de plate-forme. Il vaut 1 dans le cas qui nous concerne.

The lifespan of each layer is calculated from the formula below, in accordance with the French technical guide for the design and sizing of pavement structures. ε t = ε 6 b NOT 10 6 . K vs . K r . K s with

ε _t :

deformation at the base of the bituminous layers

ε ₆ :

deformation which results in a lifetime equivalent to 10 ⁶ cycles

NOT :

number of cycles

K _c :

setting coefficient

K _c =

1 for BBTHM®

K _c =

1.3 for Grave-Mousse®

K _r :

is a coefficient which adjusts the value of the admissible deformation to the selected calculation risk according to the dispersion factors on the thickness (standard deviation Sh) and on the results of the fatigue tests (standard deviation SN) K r = 10 -ubδ

u:

reduced centered variable associated with risk r

b:

slope of the law of fatigue of the material (bi-logarithmic law)

δ:

standard deviation of the distribution of log N at break δ = [SN 2 + (c 2 / b 2 ) Sh 2 ] 0.5

vs :

coefficient relating the variation in deformation to the random variation in thickness of the roadway, Δh, (logε = logε ₀ - c Δh).
For common structures, this is of the order of 0.02 cm ^-1 .

K _s :

the platform coefficient. It is worth 1 in the case which concerns us.

For the following calculations, we will take U = 2.05 which corresponds to a 2% risk of breakage (risk usually retained for pavements with heavy traffic).

To calculate the service life of the three-layer structure, the density of probability of rupture of each of the three layers represented at Figure 7 attached.

The probability of failure of the three-layer which is the product of the probabilities of rupture of each of the layers is given in Figure 8 attached.

We observe that for a risk of rupture of 2%, the expected lifespan is such that: log (N) = 7.04 is N = 11.10 6

It should be noted that the thickness of a monolithic structure (consisting of BBTHM® only) equivalent in lifespan is 25 cm and 38 cm for a classic gravel bitumen structure, value which is compare to the 20 cm of the three-layer structure.

• la première est une structure tricouche constituée de 20 cm de matériaux (6 cm de BBTHM® + 8 cm de Grave-Mousse® + 6 cm de BBTHM®).
• la seconde est une structure monolithique constituée par 25 cm de BBTHM®. Il est à noter que ce type de matériau est aujourd'hui le matériau le plus performant permettant les structures les moins épaisses.
• la troisième est aussi une structure monolithique constituée par 38cm de grave bitume de classe 2 (GB2).

Tricouche BBTHM GB2 BBTHM®
ou
GB2 Epaisseur (cm) 2X6 25 38 Masse surfacique (kg/m²) : 284 591 816 - des granulats 16 34 34 - du bitume 300 625 850 - TOTALE Grave-Mousse® Epaisseur (cm) 8 - - Masse surfacique (kg/m²) : 170 - - - des granulats 6 - - - du bitume 176 - - - TOTALE Total Epaisseur (cm) 20 +25% +90% Masse surfacique (kg/m²): 454 +30% +80% - des granulats 22 + 55 % + 55 % - du bitume 476 + 31% + 79 % - TOTALE The mechanical performance of the three-layer structure according to the invention allows significant savings in materials. Table V below gives the material balance of three equivalent structures in terms of life built on identical supports:

• the first is a three-layer structure made up of 20 cm of materials (6 cm of BBTHM® + 8 cm of Grave-Mousse® + 6 cm of BBTHM®).
• the second is a monolithic structure consisting of 25 cm of BBTHM®. It should be noted that this type of material is today the most efficient material allowing thinner structures.
• the third is also a monolithic structure consisting of 38cm of class 2 bitumen (GB2).

three-layer BBTHM GB2 BBTHM®
or
GB2 Thickness (cm) 2X6 25 38 Mass areal (kg / m ² ): 284 591 816 - aggregates 16 34 34 - bitumen 300 625 850 - TOTAL Grave-Mousse® Thickness (cm) 8 - - Mass areal (kg / m ² ): 170 - - - aggregates 6 - - - bitumen 176 - - - TOTAL Total Thickness (cm) 20 + 25% + 90% Mass areal (kg / m ² ): 454 + 30% + 80% - aggregates 22 + 55% + 55% - bitumen 476 + 31% + 79% - TOTAL

• une économie très importante du matériau le plus cher, c'est-à-dire plus de la moitié de bitume ;
• une économie de granulats (30 %) qui génère de plus des économies de transport et de mise en oeuvre ;
• une réduction d'épaisseur qui, dans le cas de décaissements en traverse d'agglomération, conduit à des économies de terrassement.

This material balance highlights the following advantages compared to the most efficient conventional structure:

• a very significant saving of the most expensive material, that is to say more than half of bitumen;
• savings in aggregates (30%) which also generates savings in transport and implementation;
• a reduction in thickness which, in the case of disbursements across the agglomeration, leads to savings in earthworks.

An example of a three-layer structure was tested on the Tatigue merry-go-round. Central Laboratory of Bridges and Roads (LCPC) in Nantes, with the aim of define the actual operating mode and check its fatigue resistance.

The pavement structure defined in figure 9 was built with material classic and according to the usual methods used in road engineering. In In this sense, the test board is well representative of the technique to be studied.

a) Le BBTHM® Il s'agit d'un enrobé à module élevé de granularité 0/10 comportant 90 % de granulats concassés issus de roches massives et 10 % de sable naturel roulé. Le liant est un bitume dur de grade 10/20 dosé à 5,8 ppc.Cet enrobé est conforme à la norme NPF 98 128 définissant les enrobés à module élevé (EME de classe 2).Les principales caractéristiques mécaniques de cet enrobé sont :

• module complexe à 15°C et 10 Hz:
     /E*/ = 17 800 MPa
• résistance à la fatigue (essai à déformation imposée)
     déformation admissible à 10⁶ cycles
     (15°C - 10Hz):ε₆= 132 10^-6
     pente de la droite de fatigue : b = -0,175

b) La Grave-Mousse®

In addition to the Humidified Reconstituted Grave (GRH) used as a form layer, the three-layer structure comprises two types of material.

a) BBTHM® This is a high-modulus asphalt with a granularity of 0/10, comprising 90% of crushed aggregates from massive rocks and 10% of natural rolled sand. The binder is a hard bitumen grade 10/20, dosed at 5.8 pp. This asphalt conforms to standard NPF 98 128 defining asphalt with high modulus (EME class 2). The main mechanical characteristics of this asphalt are:

• complex module at 15 ° C and 10 Hz:
  / E * / = 17,800 MPa
• fatigue resistance (imposed strain test)
  deformation admissible at 10 ⁶ cycles
  (15 ° C - 10Hz): ε ₆ = 132 10 ^-6
  slope of the fatigue line: b = -0.175

b) La Grave-Mousse®

Grave-Mousse® is a seat material manufactured by incorporation hot bitumen in the form of foam in aggregates not heated.

The granularity is that of a Humidified Reconstituted Grave (GRH) with a solid curve which has good characteristics of short-term stability. The formula used, with a granularity of 0/10, is entirely composed of crushed aggregates from rocks massive. The bitumen content is 3.5 ppc. It is a bitumen chosen for its foamability and its average hardness of grade 70/100.

• module complexe à 15°C et 10 Hz:
     /E*/ = 5 500 MPa
• résistance à la fatigue (15°C et 10 Hz)
     ε₆= 109 10^-6
     pente de la droite de fatigue : b= - 0,126

The main mechanical characteristics of this material are:

• complex module at 15 ° C and 10 Hz:
  / E * / = 5,500 MPa
• fatigue resistance (15 ° C and 10 Hz)
  ε ₆ = 109 10 ^-6
  slope of the fatigue line: b = - 0.126

Fatigue test procedure:

The fatigue test took place in four progressive loading phases (Figure 10):

• The first is a preloading phase. Its purpose is to recreate the usual conditions for maturing of materials and in particular that of Grave-Mousse® which seems to be accelerated by the effect of traffic. This phase, carried out in November 1994, included around 70,000 twin wheel weights weighted at 4.5 T.
• The second phase, which constitutes the main part of the fatigue test, started in August 1995 after completion of the experiments in progress on two other rings of the arena. The estimated number of passages for this phase was 1.5 million. In fact, it included 2.1 million twin wheel arches weighted at 6.5 T, a load which corresponds to an axle of 13 T, the French reference axle. The second phase was completed in November 1995.
• The third phase, extended by the fourth, aimed to assess the limit resistance of the new structure which, at the previous stage, showed no outward signs of fatigue. During these two phases, the twin wheels were ballasted at 7.5 T for 100,000 passages then at 8.5 T (standard axle with overload at 17 T) for 500,000 other passages. The test was halted after the structures had undergone the equivalent of 4.7 million 13 T axle passes. During this period, the water and thermal conditions changed. They are specified in Figures 11 (level of the water table) and 12 (temperatures in the structure).

This number of equivalent axles was determined as follows:

Throughout the fatigue test, the structure was examined by measurements carried out on the surface (deflection, rut depth, visual surveys) or in the body of the structure (strain gauges, ovalization).

Deflection measurements are shown in Figure 11. They are measured under the twinning in service at the time of the test, the mass of which varies from 4.5 T at the start at 8.5 T at the end of the experiment.

These raw values have been corrected proportionally to the load to bring them back to the load of the standard French twinning (6.5 T).

It should be noted that the first two values measured under a load of 4.5 T were added before the implementation of the second layer of BBTHM®.

It is clear from this graph that the deflection varies with the temperature measured in the structure.

A detailed analysis of the phenomenon shows that this variation is linear (Figure 12) and that to a variation of temperature of 1 ° C corresponds a variation of the deflection of 2/100 ^e mm.

Corrected for load and temperature variations, the deflection does not change not significantly except at the end of the experiment where the slight observed increase can be attributed to the high level of the water table water. These findings suggest that the structure was no longer damaged during the fatigue test.

Measuring the rut depth or rutting in the web bearing is measured on the transversoprofilograph.

The graph in Figure 13 shows the evolution of this parameter over the loads.

The initial rutting after implementation of the third layer is 5 mm; it more corresponds to a cross section defect generated during compaction than an evolution under traffic. Then rutting increases linearly with the number of loads, but very slowly at a rate of 1 mm per million passages of the 6.5 T twinning

The behavior of the structure with regard to rutting is therefore very satisfactory, both from the point of view of bituminous layer creep and level of permanent deformation of the supporting soil that the structure protects effectively.

It should be noted that during the experiment, no degradation was not observed on the surface.

During an ovalization test, the deformations of a coring hole are measured at the passage of the load. The exploitation of the results makes it possible to find the deformations in the structure before coring, so in normal running.

Two tests were carried out, one in the tread having undergone possible fatigue damage (1.645 million passages), the other out of this band. This last test is representative of the operation of the structure before any damage.

During the test on the undamaged structure, the diagram of deformations thus determined (Figure 14) shows relatively profiles continuous according to that calculated using the numerical model.

During the test in a circulated area, the diagrams obtained in this zone (figure 15) show a sudden variation in the lower half of the middle layer. This type of profile suggests a sliding plan at this level (a cut made later in the structure confirmed this hypothesis).

The modeling of this phenomenon by a simple sliding interface leads excessive deformation. Good agreement was obtained in modeling this degraded area with a thin layer 1.5 cm thick and low stiffness of 500 MPa. This rigidity corresponds substantially to that of a serious untreated. The adjustments thus obtained are presented in figure 16.

During the destructive tests, coring and cutting were carried out in the structure at the end of phase 2.

Core drilling confirmed the presence of a fracture surface at the part bottom of the middle layer to the right of the tread. However, the cores taken from outside the tread were intact.

The cross-sections confirmed this diagnosis. They made it possible to visualize this rupture surface and show that it was limited to the strip of rolling.

This observation indicates that it is indeed a fatigue rupture of the Grave Mousse® and not a pre-existing defect.

The fracture surface being horizontal, it does not result from fatigue by bending which would have led to vertical cracking. She would rather be there consequence of shear fatigue. The computation of the stress of shear in the middle layer confirms this hypothesis.

The graphs in Figure 17 show that the shear stress under the twinning wheels reaches a value of 0.2 MPa. To this value corresponds a deformation of the order of 70.10 ^-6 (for a module of 4000 MPa and a fish coefficient of 0.35) which is quite significant for this material whose admissible value for a million cycles is :
ε ₆ = 109.10 ^-6

In addition, this maximum is relatively constant over the entire thickness of the middle layer while the vertical stress decreases strongly with the depth which is favorable to the development of the cracking plane in lower part of this layer.

This three-layer structure presents an unusual habit of failure which differs from the failure mode described above. In fact, it is one of the possible modes of a more general process presented in figure 18.

In conclusion, this experiment has shown that it is possible to minimize the cost of a pavement structure by a judicious choice of materials according to their mechanical characteristics.

The theoretical approach made it possible to define the ratio of the rigidities of the materials so that the functioning of the structure is optimal (F _s = E _i and 10> E _s / E _m > 2).

The LCPC fatigue roundabout experiment in Nantes made it possible to check the operation of such a structure constructed in real life greatness by usual means. The functioning of the structure was found to be in line with modeling predictions and resulted in a excellent resistance to traffic.

At the end of the experiment, the test board had no degradation despite the 2.7 M of passages including 600,000 overloaded by compared to the French legal axle. Total equivalent traffic corresponds to approximately 5 million 13 T axles.

Claims (10)

1. A road structure whose bearing part on top of the ground comprises three successive layers of bituminous materials bonded with one another, namely:

   a base layer which rests on the ground with optional insertion of a forming layer of thickness H_i such that: 4 cm ≤ Hi = 10 cm    and of elastic modulus E_i

   a median layer of thickness H_m such that: 4 cm ≤ Hm ≤ 20 cm and of elastic modulus E_m such that: 2 000 MPa < Em < 8 000 MPa

   a top layer of thickness H_s such that: 4 cm ≤ Hs ≤ 10cm    and of elastic modulus E_s, said elastic moduli of said layers complying with the following inequality relationships: 2 < Ei/Em < 10 and 2 < Es/Em < 10
2. A road structure according to claim 1, characterized in that the base and top layers have at least substantially identical elastic moduli.
3. A road structure according to claim 2, characterized in that the base and top layers have identical elastic moduli.
4. A road structure according to any one of claims 1 to 3; characterized in that the thicknesses of the base and top layers H_i and H_s are at least substantially identical.
5. A road structure according to claim 4, characterized in that the thicknesses of the base and top layers H_i and H_s are equal.
6. A road structure according to any one of claims 1 to 5, characterized in that said base and top layers are constructed of sand and/or gravel treated with a hydrocarbon binder.
7. A road structure according to claim 6, characterized that the base and top layers are constructed from high modulus asphaltic concrete.
8. A road structure according to any one of claims 1 to 7, characterized in that the median layer is of sand and/or gravel treated with a hydrocarbon binder, a hydraulic binder or a mixture of such binders.
9. A road structure according to any one of claims 1 to 8, characterized in that said layers are bonded to each other by interposing a bonding layer or an impregnation layer and a bonding layer.
10. A road structure according to any one of claims 1 to 9, characterized in that said structure is topped with a wear layer.

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