WO2017168214A1

WO2017168214A1 - Test method for determining the fatigue resistance of asphalt mixtures by means of stress sweep and method for obtaining samples

Info

Publication number: WO2017168214A1
Application number: PCT/IB2016/051862
Authority: WO
Inventors: Gonzalo Alfonso VALDÉS VIDAL; Alejandra Tatiana CALABI FLOODY
Original assignee: Universidad De La Frontera
Priority date: 2016-03-31
Filing date: 2016-03-31
Publication date: 2017-10-05

Abstract

The present invention relates to a method for determining the fatigue resistance of an asphalt mixture by optimally reproducing the stress state of a road surface, said determination being performed by obtaining a fatigue law for the asphalt mixture. The method comprises performing a stress sweep during uniaxial cyclical tension and compression testing on a cylindrical sample, a perimetral cut being made in the centre of the sample to create a fault plane. The invention also relates to a method for obtaining the sample.

Description

METHOD OF TESTING TO DETERMINE THE RESISTANCE TO FATIGUE OF ASPHALTIC MIXES BY TENSION SWEPTING AND PROCEDURE TO OBTAIN PROBES FIELD OF APPLICATION

The present invention relates to the civil works industry, in particular to the field of pavement engineering, in the area of mechanical characterization of asphalt mixtures for their design and use in the construction of flexible structural pavements of roads and highways.

STATE OF ART

Fatigue cracking of asphalt pavements generally occurs in pavement areas subject to the impact of numerous and repeated loads for traffic, mainly in the roadway area, in which the asphalt mix suffers fatigue, damage, or gradual loss of its structural properties, product of repeated cyclic stress loads. Generally this type of failure is characterized by having a geometric pattern described by numerous interconnected cracks, forming pieces of acute angles, also known as "crocodile skin" as shown in Figure 1.

Asphalt mixtures are subject to cyclic tensions of tension and compression caused by vehicular traffic, which manifest themselves in both the surface area and the base of the asphalt layers that make up the pavement structure as can be seen in Figure 2. Said figure represents the stress state of an asphalt layer (11) due to the support of a wheel (10) of a vehicle, as a result of the applied load P, a portion of the pavement in contact with the wheel is subjected to stresses of compression, while other areas of the pavement at the periphery of the contact area are subjected to tensile stresses. It is also noted that at the base of the asphalt layer (11) something similar occurs but with the compression and traction zones exchanged. Then, with the passage of the vehicle the pavement is subjected to alternating tension-compression efforts. It is for this regime of cyclic loads that the resistance to fatigue is the most important property of the asphalt mixtures, and that it is used in the current analytical methods of design and calculation of thicknesses of pavement structures to predict the behavior and the properties of the pavements during its useful life.

In the determination of parameters related to the fatigue resistance of a material, the fatigue law has special importance. Hence, each standard fatigue testing procedure is used to determine the fatigue laws of a material. Currently, with standardized methods to determine fatigue resistance, a specimen is subjected to cyclic tension-compression loads with constant amplitude, which requires expensive equipment, numerous long-term tests, and the fabrication of numerous prismatic specimens. difficult to obtain, to characterize the fatigue behavior of asphalt mixtures. To appreciate the magnitude of the problem, consider that in some cases between 10 and 18 specimens are used for each fatigue law, and that a test takes 4 to 12 hours, depending on the type of test and the amount of load or deformation submitted. In practice, to characterize an asphalt mixture using standard methods, a two-month test is needed for two weeks. In addition, product of the traditional criteria of fatigue based on displacement or controlled deformation, the failure of the material is presumed when its rigidity has diminished by half, although this criterion is questioned in the literature for more ductile mixtures or that use binders modified. As a result of this, the fatigue resistance property of asphalt mixtures is not adequately evaluated in the current asphalt mixtures tests, although it is considered one of the most important deteriorations of flexible pavements.

In the state of the art there are documents related to the determination of the resistance to fatigue of materials such as the patent US 8,627,713, which describes a method to evaluate the fatigue resistance of a polymer sample, particularly related to the field of flexible pipes for transport of pressurized and / or corrosive fluids. The document mentions the use of tubular specimens with axial symmetry, and with a perimetral notch in its central zone to produce a state of tension similar to that which the pipes must support.

On the other hand, document ES 2.386.224 describes a method of fatigue testing of material in bituminous mixtures, such as for example asphalt. In said method it is proposed to position a specimen in the form of a rectangular prism, which contains a slit in its central portion in order to induce rupture on said section. The document calls the method as Deformation Sweep Test (EBADE) and mentions that a plurality of tension-compression cycles are exerted on the specimen, where the magnitude of the deformation remains controlled and is increasing between cycles. The main drawback is related to the prismatic shape of the specimens, for the longer time they require to obtain them compared to, for example, a cylindrical specimen; and because said geometry does not ensure that the stresses in the specimen have the same orientation as the stresses in a pavement, with respect to the sense of compaction. There is also the existing questioning for tests with controlled deformation relative to the criterion of unrepresentative failure for more ductile mixtures or that use modified binders.

The document Tangella et al. 1990 presents a comparison between the controlled tension test methods and the controlled deformation test, this analysis is summarized in Table 1.

Table 1: Comparative table of test variables for controlled stress test and controlled strain test (Tangella et al., 1990)

TECHNICAL PROBLEM TO RESOLVE

The objective of the present invention is to determine the fatigue strength of asphalt mixtures in such a way that, compared to current methods of the prior art: a shorter execution time is required, a smaller number of test pieces to be tested is required to validate the results, the required specimens are easier to obtain, and that allows to determine the mechanical properties of the asphalt mixtures more precisely in relation to the real tensional state to which they are subjected in the pavement structure.

It also seeks to reduce costs compared to the methods currently regulated to determine the fatigue resistance of asphalt mixtures; first, by using less time in the manufacture of the specimens; second, using less expensive equipment; and third, by reducing the total time to obtain a fatigue law for each asphalt mixture.

GENERAL DESCRIPTION

The present invention provides a method for obtaining a test piece and an assay method that employs said test piece to determine the fatigue resistance of a material, especially an asphalt mixture, more specifically the invention corresponds to a new uniaxial test method of Tension sweep for the analysis of fatigue in asphalt mixtures, called DUSST for its acronym in English.

The proposed procedure teaches to obtain a solid cylindrical specimen extracted perpendicular to the compaction of the samples, whether these samples are manufactured by standardized design procedures such as Marshall and rotary compactor, or from witnesses extracted on roads. The extraction of the specimen takes place in the form of a solid cylindrical core which is generated parallel flat ends and a notch perimetrally in the central area of the cylinder to induce the plane of failure.

The test method comprises the steps of: providing a plurality of specimens obtained according to the method taught; apply to each test tube a plurality of series of tension-compression cycles, wherein a maximum applied voltage remains constant during each series of cycles of said plurality of series of cycles, and the maximum applied voltage progressively increases between successive series of cycles until the complete failure with rupture of each specimen; register, while applying the series of tension-compression cycles to each specimen, the values of the maximum applied tension and the evolution of the axial deformation of each specimen, in the tension-compression cycles; determine and record, while applying the series of tensile-compression cycles to each specimen, the evolution of the dynamic modulus of the asphalt mixture and the evolution of the energy dissipated by the specimen, in the tension-compression cycles; determining a level of deformation under which the asphalt mixture does not suffer from fatigue damage and a level of deformation upon which the asphalt mixture is fatigued after a predetermined amount of load applications; and determine a fatigue law for the asphalt mixture.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 presents an image that exemplifies fatigue cracking of an asphalt pavement.

Figure 2 contains a schematic drawing of the stress state of the asphalt layers in a pavement.

Figure 3a illustrates a sample, manufactured or witness, from which a cylindrical core for the preparation of a test specimen according to the present invention is extracted.

Figure 3b illustrates a test specimen according to the present invention, with a diametral notch in its central zone.

Figure 4 illustrates the assembly of the specimen for the test according to the present invention.

Figure 5 presents a graph that exemplifies a voltage signal imposed in the test method of the present invention.

Figure 6 presents a granulometry graph of aggregates for the asphalt mixtures used in the examples.

Figure 7 presents a graph of the evolution of the deformation during the execution of the test method of the present invention, for three asphalt mixtures.

Figure 8 presents a graph of the fatigue laws obtained from the test method of the present invention, for three asphalt mixtures.

Figure 9 presents a graph of the evolution of the dynamic module during the execution of the test method of the present invention, for three asphalt mixtures.

Figure 10 presents a graph of the evolution of the energy dissipated by the asphalt during the execution of the test method of the present invention, for three asphalt mixtures. DETAILED DESCRIPTION OF THE INVENTION

The present invention corresponds to a uniaxial stress scanning method for fatigue analysis in asphalt mixtures, the methodology consists of performing a stress sweep during a uniaxial tension-compression cyclic test on a cylindrical specimen in which it has been made in the central area a perimeter notch, reducing its section in that area with the purpose of inducing the fault plane.

By the present invention, with reference to Figures 3a and 3b, the cylindrical shaped specimens (3) are obtained from the extraction of a cylindrical core (2) with a diameter of between 3 cm and 7 cm, preferably 5.08 cm (2 inches), from samples (1) manufactured by standard manufacturing methods, such as the Marshall method or the rotary compactor method (EN 12697-10), among others, as well as samples or witnesses extracted from the structure of a pavement (Road Manual, volume 8, method 8.502.3). The axial direction of said cylindrical core (2), arrow E in Figure 3a, is perpendicular to the sense of compaction of the samples or witnesses (arrow C) from which they are obtained.

Subsequently, the extracted cylindrical core (2) is worked to generate a specimen (3), Figure 3b. The facing is made of its ends, for example in a lathe, to confer flat faces (5, 6) perpendicular to the axis of the cylindrical core (2). Then, a perimeter notch (7) between 2 mm and 10 mm deep, and preferably 5 mm deep, is made in the central area of the specimen. In the case of using a saw to make the notch (7) the width of the notch is defined by the thickness of the saw, with thicknesses of between 3 and 7 mm being recommended.

The objective of the perimeter notch is to produce a stress concentration and to induce the fault plane in said zone. The height of the test pieces between flat faces (5, 6) can vary from 60 mm to 120 mm, and will depend on the diameter of the original sample or control. The cylindrical samples manufactured in the laboratory for the design of mixtures, like the witnesses of pavements, are 100 and 150 mm in diameter. Therefore, this diameter is what limits the length of the DUSST specimens.

For the execution of the test, with reference to figure 4, the test piece (3) is fixed between two metal plates (20), which must be parallel between their faces to guarantee the correct application of the forces. Then the specimen is fixed to a press (not shown in the figures) by means of some fixing means that prevents movements, or games, in the example of figure 4 it has been fixed by means of two accessories (22) type female where it is inserted the plate (20) and fixed with two screws at each end. Later in the In the test pieces or at least two sensors are placed that are used to record the average of both measurements and that allow to measure the deformations of the test piece during the execution of the test. On the other hand, it can be used in any type of press that can perform dynamic traction-compression movements.

The test is carried out on the specimen (3) applying uniaxially a routine composed of a plurality of series of tension-compression cycles, where the applied maximum tension remains constant during each series of cycles of said plurality of series of cycles, and progressively increases between the successive series of tension-compression cycles of said plurality of series of tension-compression cycles. As illustrated in Figure 5, the excitation signal of said series of tension-compression cycles is a sinusoidal signal whose frequency is kept constant throughout the test. The frequency can vary from 2 to 30 Hz, depending on the capacity of the press.

The temperature at which the test is carried out is kept constant during the test of each test piece, and it can be varied between the tests of different test pieces of said plurality of test pieces. The test can be carried out in the range between -30 ° C and 60 ° C.

While the series of tension-compression cycles are applied, the values of the maximum tension applied in the series of tension-compression cycles are recorded, and the deformation of said specimens in the tension-compression cycles until the failure of each test tube.

The tension applied in the series of tension-compression cycles that progressively increases between the successive series of tension-compression cycles until the failure of the specimen occurs. In one embodiment the increments between successive series have the same magnitude, however, alternatively variable increments can be used, or, alternatively, by an increment function based on the initial tension. Alternatively, rest periods can be left between series of cycles.

During the application of the plurality of series of tension-compression cycles, the evolution of the rigidity or dynamic module of the material and the energy dissipated by the specimen are recorded.

In the present invention has been defined as a criterion of failure of the material, the value of the average deformation between cycles 100 and 500 of the series of cycles in which the material fails completely (complete failure is understood for this methodology when the deformation reaches or it exceeds twice the average value of the deformation of the first cycles of the test, in the same series of cycles, said first cycles of the test being comprised between cycles 100 and 500). Additionally, an initial deformation is registered, corresponding to the level of deformation under which the asphaltic mixture does not suffer from fatigue damage, and which is defined as the average deformation of the series of tension-compression cycles prior to the first series of cycles in the that the difference between the average of deformation between cycles 100 and 500 of said series and the average of deformation of the last 400 cycles of said series exceeds 10% of the average value of deformation registered between cycles 100 and 500 of said first series of cycles.

A dynamic module is also recorded, defined as the average value of the modules calculated from the first 100 to 500 cycles of the first series of tension-compression cycles. Where, the dynamic modulus is calculated as the value of the registered voltage divided by the recorded deformation.

Results of the trial

The main results obtained from this test method are the levels of relevant deformations recorded, these being: the level of deformation under which the asphaltic mixture does not suffer fatigue damage, associated with a large number of load repetitions (established in 5x10 ⁷ repetitions); and the level of deformation over which the mixture fatigates quickly, after a small amount of loading applications (established in 5x10 ³ repetitions).

With these relevant levels of deformation, the parameters related to the fatigue resistance of said material can be determined, such as the fatigue law that allows later to evaluate the durability or perform a thickness sizing of a pavement structure.

More specifically, the present invention discloses a method for obtaining a test piece for tests to determine the fatigue resistance of an asphalt mixture, in order to better reproduce the stress state of a pavement, comprising the steps of: having a cylindrical sample of the asphaltic mixture obtained by a standardized compaction technique or by the extraction of a road pavement marker, where the cylindrical sample has an axis of symmetry parallel to the compaction direction of the asphalt mixture; extract from the cylindrical sample a solid cylindrical core in a direction perpendicular to the axis of symmetry of the cylindrical sample and to the compaction direction of the asphalt mixture; facing both ends of the solid cylindrical core, to define parallel flat ends; and to realize a notch of perimetral way in the central zone of the cylinder. Where the cylindrical sample has a diameter of 100 to 150 mm, and in the step of facing the solid cylindrical core the procedure includes establishing the length of the specimen in 60 mm, for a cylindrical sample of diameter 100 mm, or including establishing the length of the test piece from 60 to 120 mm, for a cylindrical sample with a diameter of 150 mm.

In addition, the solid cylindrical core has a diameter of 3 to 7 cm, and preferably has a diameter of 5.08 cm (2 inches). Where, the notch has a depth of 2 to 10 mm, and is rectangular.

On the other hand, the method to determine the resistance to fatigue of an asphalt mixture, reproducing better the state of tensions of a pavement, where said determination is made by obtaining a fatigue law for the asphalt mixture, comprising the Steps of:

providing a plurality of solid cylindrical specimens with a perimeter notch in the central zone of the cylinder and where the axis of each specimen is perpendicular to the compaction direction of the asphalt mixture;

- apply to each test tube a plurality of series of tension-compression cycles, wherein a maximum applied voltage remains constant during each series of cycles of said plurality of series of cycles, and the maximum applied voltage increases progressively between successive series of cycles until the complete failure with rupture of each test tube;

- register, while applying the series of tension-compression cycles to each specimen, the values of the maximum applied tension and the evolution of the axial deformation of each specimen, in the tension-compression cycles; determine and record, while applying the series of tension-compression cycles to each specimen, the evolution of the dynamic modulus of the asphalt mixture and the evolution of the energy dissipated by the specimen, in the tension-compression cycles;

determining a level of deformation under which the asphalt mixture does not suffer from fatigue damage and a level of deformation upon which the asphalt mixture is fatigued after a predetermined amount of load applications; Y

- determine the fatigue law for the asphalt mixture.

Where, the series of tension-compression cycles are applied following a sinusoidal excitation signal; and the application of the plurality series of tension-compression cycles is carried out at a constant temperature for each specimen and can vary between different test tubes; and optionally comprises rest periods between cycles, during which the specimen is not subjected to stresses.

Where, before the complete failure with rupture of each test tube, a fault deformation is registered that meets the criterion of failure of the material that defines it as the average deformation between cycles 100 and 500 of the series of cycles in which the The test piece records a level of deformation that reaches or exceeds twice the average value of the deformation of the first test cycles, in the same series of cycles, said first test cycles being comprised between cycles 100 and 500.

Where, the level of deformation under which the asphaltic mixture does not suffer from fatigue damage, established for this procedure within 50 million cycles, corresponds to the average deformation of a series of tension-compression cycles prior to a first series of cycles, said first series of cycles is such that the difference between the average of deformation between cycles 100 and 500 and the average of deformation of the last 400 cycles exceeds 10% of the average value of deformation registered between cycles 100 and 500 of said series.

Furthermore, in order to determine the level of deformation on which the mixture is fatigued after a predetermined amount of loading applications, said predetermined amount of loading applications is established for this procedure in 5000 cycles, at the end of which at least one test piece fulfills the criterion of failure of the material.

Finally, to determine the fatigue law for the asphalt mixture, an interpolation is made between the level of deformation on which the mixture is fatigued after a predetermined amount of load applications, associated with the 5000 cycles, and the level of deformation under which the asphalt mixture does not suffer damage by fatigue, associated with 50 million cycles.

APPLICATION EXAMPLE

To present this new test method has included the application of the method of the present invention to the analysis of fatigue behavior of three asphalt mixtures, in which the aggregates used have a granulometry as shown in the graph of Figure 6 ( mix IV-A-12 according to Chilean specification), varying the type of asphalt binder. The asphalt cements used corresponded to a conventional binder, CA-24 according to Chilean specifications; an Asphalt Cement Modified with Polymers SBS, CA-MP; and a High Modulus Asphalt Cement, CA-AM, which is characterized by its low penetration and by being used for the manufacture of high modulus mixtures. The The aggregates used in the experimental tests corresponded to aggregates of fluvial origin that meet the Chilean specifications for use in asphalt mixtures. The three mixtures evaluated with the test method of the present invention were manufactured with 5.2% asphalt content.

The test method of the present invention was carried out by applying series of cycles of different voltage amplitudes, with a duration of 5,000 cycles each. It started with a voltage amplitude of 500 kPa (± 250 kPa) and the amplitude was increased by 100 kPa (± 50 kPa) for each series of cycles until the material failed. The frequency of the application of the tension was 10 Hz during the whole test and the test temperature was 20 ° C.

50.8 mm diameter (2 ") specimens with a perimeter notch 5 mm deep and 4 mm wide, extracted from 100 mm diameter cylindrical samples manufactured by the Marshall method, were used. mm.

During the test, the evolution of the deformation is registered for each series of 5,000 tension-compression cycles until the failure of the material. With the acquired data for each test you can obtain the curves of:

Evolution of deformation

Fatigue laws

Evolution of the module

Evolution of the dissipated energy.

Evolution of deformation

The results of the evolution of the deformation for the three mixtures evaluated to fatigue by means of the test method of the present invention can be seen in Figure 7. In these curves the evolution of the damage that is happening in the different mixtures to As tension levels are increased for each series of cycles of 5,000 repetitions each. This damage occurs in the evolution of the deformation for each cycle of stresses applied, registering a gradual increase in the slope of the deformation, until the failure of the test piece occurs. Figure 7 shows the sensitivity of the test method of the present invention against mixtures of different characteristics. For the more rigid mixture, manufactured with the CA-AM binder, low levels of initial deformation and failure deformation are observed, registering less evolution in the slopes of deformation for each series of stress cycles. In turn, the mixture manufactured with the conventional binder CA-24, presented a more ductile behavior, which is observed in the deformations, initial and failure, higher than those of the more rigid mix. Finally, and as expected, the mixture made with the improved polymer-modified binder, CA-MP, showed a better performance, recording the initial deformations and failure higher than those of the previous mixtures, together with a greater number of cycles and higher voltage applied from the voltage series to the fault.

The two levels of deformation (initial and failure) determined by the test method of the present invention are those associated with the fatigue law of an asphalt mixture for a pavement structure. The first one is the initial deformation, determined as the value corresponding to the average deformation of the previous step to that in which the difference of the average of deformation between cycles 100 and 500, and cycles 4,600 and 5,000, is less than 10%, and has been associated with that level of deformation below which there would be no fatigue damage, also called in the literature as a limit of fatigue (endurance limit). On the other hand, the level of deformation associated with the failure of the material is determined, on which the mixture has a rapid propagation of the fatigue damage. For this last level it has been associated to that average value of the deformation between cycles 100 and 500 of the series of cycles in which the material fails. From these two levels, which are observed for each mixture evaluated in Figure 7, a fatigue law can be determined quickly and easily that characterizes the asphalt mixture and allows the evaluation of its durability and design in a structure of pavement. The fatigue laws obtained for the mixtures evaluated are presented in Figure 8.

Evolution of the dynamic module

The evolution of the dynamic module during the execution of the test of the present invention is observed in Figure 9, in which it can be noted that the most rigid mixture, manufactured with the CA-AM binder, has the highest initial modulus value. In this case, the material behaves elastically to a high level of stress, but its degradation occurs more abruptly than the other mixtures evaluated. On the contrary, the modulus of the mixture of the most tenacious binder, CA-MP, presents a continuous and progressive deterioration as the steps or series of voltage cycles are increased, but without the occurrence of a sudden failure. The mixture with the conventional binder CA-24 registers an intermediate behavior, showing a continuous damage during the application of the voltage cycles.

Evolution of the dissipated energy

The energy that is dissipated in each cycle of the method of the present invention is calculated as the area of the hysteresis loop formed by the registers of the voltage values and deformation. As the amplitude of the imposed tension grows, the recorded deformation grows in turn, increasing the area of the loop, and therefore, the dissipated energy. The evolution of the energy dissipated in each cycle and the value of the accumulated dissipated energy until the failure of the mixture, (EDa), is shown in Figure 10. The values obtained from accumulated dissipated energy are 9,790,634 J / m3 ( CA-MP mixture), 2,817,392 J / m3 (mixture CM-24), and 543.1 19 J / m3 (CM-AM mixture). The mixture that shows the best behavior is the one manufactured with the polymer modified binder, CA-MP, dissipating around 18 times more energy in the whole fatigue process than the more rigid mixture with the CA-AM binder and almost 3.5 times more than the mixture made with the conventional binder CA-24. The parameter EDa, defined in the test method of the present invention, is considered a parameter that can characterize the fatigue behavior of the asphalt mixtures.

ADVANTAGE

The advantages of the method taught in comparison with the known methods are summarized below.

Requires a shorter test run time per test piece (up to 3 hours per test piece). It requires a smaller number of test pieces (3 against 10 to 18).

The cylindrical specimens are easily extracted from samples manufactured by a Marshall process, a rotary compactor or a control.

The test pieces are subjected to cyclic forces in a direction perpendicular to the direction of compaction (product of their extraction form), that is, in the same sense in which the tensional state is generated in the pavement.

By using a plurality of cycles with voltage of increasing magnitude the number of total cycles before the failure is decreased, with a direct effect on the decrease of the time of each test.

The criterion of failure adopted in the procedure is always in the zone of rupture where the cracks progress rapidly until the total failure, independent of the ductility or rigidity of the mixture. Sustained in the results of tests presented in Figure 7. Reproduces the actual failure sequence, which implies that the stresses to which the pavement structure is exposed originate deformations that, depending on the type of structure (rigid-flexible), will be greater or smaller, resisting accordingly, more or less cycles before fatigue. That is, as what is controlled in this essay is the tension in the load cycles, it can be observed that the deformation (which is the one that induces the failure) that is provoked in the different mixtures tested is also different, contrary to the current most used procedures that impose a deformation in the mixtures to be evaluated.

Claims

CLAIMS 1. A method to determine the fatigue resistance of an asphalt mixture, reproducing better the state of tensions of a pavement, said determination by obtaining a fatigue law for the asphalt mixture, CHARACTERIZED because it comprises the steps of:

a) providing a plurality of solid cylindrical specimens with a perimeter notch in the central zone of the cylinder and where the axis of each specimen is perpendicular to the direction of compaction of the asphalt mixture;

b) apply to each test tube a plurality of series of tension-compression cycles, wherein a maximum applied voltage remains constant during each series of cycles of said plurality of series of cycles, and the maximum applied voltage increases progressively between successive series of cycles until cause a complete failure with rupture of each test tube;

c) record, while applying the series of tension-compression cycles to each specimen, the values of the maximum applied tension and the evolution of the axial deformation of each specimen, in the tension-compression cycles; d) determine and record, while applying the series of tension-compression cycles to each specimen, the evolution of the dynamic modulus of the asphalt mixture and the evolution of the energy dissipated by the specimen, in the tension-compression cycles;

e) determining a level of deformation under which the asphalt mixture does not suffer from fatigue damage and a level of deformation on which the asphalt mixture is fatigued after a predetermined amount of loading applications; Y

f) determine a fatigue law for the asphalt mixture.

2. The method of claim 1, CHARACTERIZED because the series of tension-compression cycles are applied following a sinusoidal excitation signal.

3. The method of claim 1, characterized in that the application of the plurality of series of tension-compression cycles is carried out at a constant temperature for each specimen and can vary between different specimens.

4. The method of claim 1, CHARACTERIZED because before the complete failure with rupture of each specimen occurs a failure deformation that meets a criterion of failure of the material that defines it as the average deformation between cycles 100 and 500 of the series of cycles in which the specimen registers a level of deformation that reaches or exceeds twice the average value of the deformation of the first test cycles, in the same series of cycles, said first test cycles being comprised between cycles 100 and 500.

5. The method of claim 1, CHARACTERIZED because the level of deformation under which the asphalt mixture does not suffer from fatigue damage, within 50 million cycles, is established as the average deformation of a series of tension-compression cycles. prior to a first series of cycles, said first series of cycles is such that the difference between the average of deformation between the cycles 100 and 500 and the average of deformation of the last 400 cycles exceeds 10% of the average value of deformation registered between cycles 100 and 500 of said series.

6. The method of claim 4, CHARACTERIZED because in order to determine the level of deformation on which the mixture is fatigued after a predetermined amount of loading applications, said predetermined amount of loading applications comprises an amount of 5000 cycles, after which at least one test piece meets the criterion of failure of the material.

7. The method according to claims 4 and 5, characterized in that to determine the fatigue law for the asphalt mixture an interpolation is made between the level of deformation on which the mixture is fatigued after a predetermined amount load applications, associated at 5000 cycles, and the level of deformation under which the asphaltic mixture does not suffer from fatigue, associated with 50 million cycles.

8. The method of claim 1, CHARACTERIZED because it comprises rest periods between cycles, during which the specimen is not subjected to stresses.

9. Procedure for obtaining a specimen for tests to determine the fatigue resistance of an asphalt mixture, in order to better reproduce the stress state of a pavement, comprising the steps of:

to. having a cylindrical sample of the asphalt mixture obtained by a standardized compaction technique or by extracting a road pavement marker, where the cylindrical sample has an axis of symmetry parallel to the direction of compaction of the asphalt mixture;

CHARACTERIZED because it also includes the steps of:

b. extract from the cylindrical sample a solid cylindrical core in a direction perpendicular to the axis of symmetry of the cylindrical sample and to the direction of compaction of the asphalt mixture; c. facing both ends of the solid cylindrical core, to define parallel flat ends; Y

d. Perform a notch perimetrally in the central area of the cylinder.

10. The method of obtaining a test piece according to claim 9, characterized in that the cylindrical sample has a diameter of 100 to 150 mm.

The method for obtaining a specimen according to claim 10, characterized in that in the step of facing the solid cylindrical core includes establishing the length of the specimen in 60 mm, for a cylindrical sample of diameter 100 mm.

12. The method for obtaining a test piece according to claim 10, characterized in that in the step of facing the solid cylindrical core includes establishing the length of the specimen from 60 to 120 mm, for a cylindrical sample with a diameter of 150 mm.

13. The method for obtaining a test piece according to claim 9, characterized in that the solid cylindrical core has a diameter of 3 to 7 cm.

14. The method of obtaining a test piece according to claim 13, characterized in that the solid cylindrical core has a diameter of 5.08 cm (2 inches).

15. The method of obtaining a test piece according to claim 9, characterized in that the notch has a depth of 2 to 10 mm.

16. The process for obtaining a test piece according to claim 9, characterized in that the notch is rectangular.