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Revista ingeniería de construcción

On-line version ISSN 0718-5073

Rev. ing. constr. vol.26 no.2 Santiago Aug. 2011

http://dx.doi.org/10.4067/S0718-50732011000200006 

Revista Ingeniería de Construcción Vol. 26 N°2, Agosto de 2011 www.ing.puc.cl/ric PAG. 224 - 239.

 

Asphalt toughness effect on bituminous mixture fatigue behavior

Efecto de la tenacidad del asfalto en la resistencia a fatiga de las mezclas asfálticas

 

Alfredo H. Noguera*1, Rodrigo Miró**

* Universidad Nacional Autónomo de México UNAM. MÉXICO.
** Universidad Politécnica de Cataluña. ESPAÑA.

Dirección para Correspondencia


Abstract

One of the most important parameters to define bituminous mixture behavior in service is the toughness that the bitumen provides when binding the aggregate particles; that is; the dissipated energy during the materials fracture process. The greater the dissipated energy in fracture, the better the bituminous mixture quality is. Hence, a relationship will have to exist if the toughness is removed during one load cycle (direct tension) or during many cycles along time (fatigue). The purpose of this study is to determinate relationships between the toughness of different bitumens and the fatigue behavior of their corresponding bituminous mixtures, for which, the toughness has been obtained by means of direct tension test and has been compared with the value of the fatigue law and dissipated energy obtained by fatigue bending test. Results showed that, this study expect to give important means for further research to predict fatigue life from a static test.

Keywords: Bitumen, aggregates, bituminous mixture, tenacity, fatigue


 

1. Introduction

Bituminous mixture is a complex multiphase material constitued by asphalt, mineral aggregate graduate and air, so a great variety of asphalt mixtures can be obtained from the mixture of these components. One of the most important characteristics to analyze in the mixture behavior is variability among the resistance of mineral skeleton and asphalt (Wagoner et al., 2007; A. N. Kvasnak et al., 2007; Kowalski et al., 2008; Kringos et al., 2008).

This resistance, also called the toughness of the mixture, can be analyzed from the point of view of dissipated energy, ie, a mixture with greater capacity to dissipate energy will be able to withstand traffic loads without disintegrating or breaking. The fracture mechanics has accepted this type of failure analysis in various materials, including asphalt.

This energy depends on the thermodynamic work of toughness, fracture mechanics and rheology (E. H. Fini et al., 2007; Arago et al., 2011).

According to modern views regarding this subject, during the action of tension force a work develops and it is stored as strain energy. When this energy reaches the magnitude of the density of fracture energy Gf material surface, the fracture occurs and the piece splits in two parts, releasing energy in the form of fracture, in such a way that each part immediately relaxes its tensions. The fracture energy Gf has proven to be a good design parameter that can be used in fracture mechanics to analyze the crack propagation of a pavement (Shen et. al., 2007; Kim et al., 2009; E. Denneman, 2010).

Associated with toughness there is another phenomenon of energy dissipation of bituminous mixture and it is fatigue of aggregate-asphalt system, wherein the compound gradually loses its available energy due to short duration loads every time that a vehicle passes (Adhikari et al., 2009). In the current SUPERPAVE specification, it is considered that fatigue does not occur if asphalt meets a minimum rheological value of G*sen 6. This value represents a decrease of complex modulus and is related to total dissipated energy per load cycle (D'Angelo et al., 2007). However, several studies have shown that there is no correlation between the rheological values and mixing behavior in service.

In order to contribute to a better understanding of the internal processes of asphalt-aggregate system and its correlation with its fatigue, this study has been developed using the essay Barcelona Direct Traction (BDT) to determine toughness of different asphalt (Pérez Jiménez et al., 1997) and testing of 3-point flexural (NLT-350/90), to determine the fatigue of a mixture made from these asphalts.

1.2 Research significance

Even if numerous studies have proposed various methods or tests to determine fatigue in asphalt mixtures, such as cyclic compression tests and dynamic flexural tests, even today these tests are complex and they require a lot of work for test tubes production, and also trial time is long. Therefore, the ideal would be to establish a relationship with fatigue from tests on asphalt or mastic.

This paper aims to establish a relationship between the toughness that offer different asphalts and fatigue behavior of the corresponding bituminous mixture. Knowing this relationship, one could have an idea of the fatigue behavior of the mixture from the toughness tests of asphalt, which are monotonic and therefore easier and faster to make, and be empowered to take account of this property in the design stage.

2. Materials

Asphalt used in experimental trials are: two penetration asphalts, B-60/70 and B-13/22; two asphalts modified with polymers, BM-3c and BM-3b, collected in the General Technical Specifications (PG-3). It has also been used a modified asphalt with crumb rubber BM-PN. Asphalts properties are the following:

Table 1. Characterization of asphalts used

In the experimental phase of research, 3 types of aggregates are used in the preparation of the test tubes to be used; a granitic type to evaluate toughness, a mixture of silica and limestone aggregates to assess fatigue.

3. Methodology

The experimental phase is conducted in two stages. The first is to determine the toughness of different asphalts using the BTD test, and the second to determine the fatigue life of a mixture, made with the same asphalts through dynamic flexural testing.

BTD test procedure consists of subjecting a cylindrical test tube with a notch at the base to a tensile stress parallel to this base and perpendicular to the notch of the tube.

Recess bases are placed into a cylindrical mold of 101.6 mm in diameter, to be used with Marshall compactor, applying 50 strokes only at the top side.

The test is performed at a constant displacement rate of 10mm/min and within a chamber heated in order to maintain the same temperature during the trial. The notch is opened as cracking of the test tube is produced.

To prepare the test tube it is used a standard mixture in two sizes, according to the sieves UNE:

• 80 % of agregate passes through a sieve of 5mm and is retained by the one of 2.5mm.
• 20 % of agregate passes through the sieve of 2.5mm and is retained by the one of 0.63mm.
• 4.5% asphalt content.

The purpose of using a standard mixture, without fines or filler, is to leave as only variable the type and nature of asphalt, so that resistance will depend exclusively on the properties of the asphalt used. Test temperature was 20 ° C (Table 2).

Similarly, to assess fatigue by testing flexural to displacement control, the same asphalt have been used in order to establish possible relationships with toughness.

The mixture produced is of type S-12 (semi-dense 12 mm maximum size), composed by two types of aggregates, a limestone one and the other siliceous with a ratio of asphalt aggregate of 4.5%. The test was carried out at a temperature of 20 ° C and with a frequency of 10 Hz. Shift range set for conventional asphalt varies from 160 to 220 fJm; in modified asphalts amplitude applied is higher, ranging from 220 to 360 |Jm (Table 3).

As it can be seen, two types of grain sizes were used, because with BTD test it can be studied the toughness of individual asphalts, while the other (type S-12) corresponds to a real type of asphalt mixture, usually used in asphalt construction.

Table 2. BTD Tests Matrix

Table 3. Matrix fatigue tests

4. Results Analysis

4.1 Toughness

In the direct tensile test it is recorded for each tested asphalt the displacement caused by variation of tensile strength. Test was carried out at a temperature of +20° C.

Figure 1 shows the characteristic curve of asphalt under direct tension. This curve shows an initial pre-peak steep that reaches a maximum tensile strength and falls with a post-peak slope until force is reduced to zero.

Upon analyzing the curve we can observe some relevant parameters such as maximum strength, secant modulus, fracture displacement, the slope post-peak and total energy.

Figure 1. Characteristic curve of pavement subjected to direct tension

- Maximum strength (Smax)

It is defined as the maximum force that can withstand the asphalt without suffering permanent deformation (elastic range). The maximum force that pavement is capable of supporting is related to temperature. At low temperatures the asphalt supports a greater tensile strength than intermediate or high temperatures.

- Secant modulus (α)

It is obtained by dividing the maximum force by the displacement produced by such maximum force. The secant modulus is also known as elastic modulus.

- Fracture displacement (δ)

At the asphalt performance curve we can distinguish three types of movements that can be considered of braking type; the first one corresponds to the displacement (δ Smáx) that occurs when reached the maximum tensile force (Smax); the second one corresponds to displacement (δ 50% max) reached when the load has been reduced (50% Smax) at the post-peak curve and the third one corresponds to total displacement (δ max) when the tensile strength is reduced to zero.

Here the three failure criteria described above are represented; the first criterion (Fmax-δFmax) does not reflect toughness of the mix because it does not consider the full range of movement that the asphalt offers. The third criterion (δmax), is a difficult one to determine because, depending on the analyzed type of asphalt, especially modified asphalts, the curve can become very stretched and will not arrive at zero traction. In contrast, the second criterion (50% Smaxδ50% max), is considered more appropriate to represent toughness, because it represents the displacement that occurs in the post-peak curve, considering the elastic range and the plastic of the material. This shift increases the higher the temperature is.

- Slope post-peak

Analyzing the slope of decline curves, we see that at low temperatures, the slope is almost vertical since the break occurs in a fragile way. At intermediate temperatures, the rupture is more ductile and post-peak slope tends to be of higher rupture displacement, and at high temperatures the slope becomes almost flat.

- Total energy or tenacity

Total energy is obtained from the area under the force-displacement curve divided by the fracture area.

Below are the force-displacement curves obtained from the experimental work of the group of asphalt at a temperature of +20 °C (Figure 2).

Figure 2. Variation of toughness of the asphalts group at 20 °C

Table 4. Energy values obtained

These are the values that are used later in order to establish correlations with fatigue behavior.

4.2 Fatigue

Below are the laws of fatigue obtained for the asphalts group through dynamic flexural test (Figure 3).

Table 5. Fatigue laws obtained from the asphalts considered

Figure 3. Fatigue Laws in dynamic flexion traction

Another way to evaluate the fatigue behavior of the mixture is through the ratio of energy dissipated upon fatigue (Disipated energy ratio, DER), obtained from the cumulative sum of energy, cycle to cycle, until failure moment considered in the test tube.

(1)

DER = ratio of energy dissipated
σ= Applied stress
ε = Applied deformation
Φ = phase angle

By plotting values of stress and strain of each cycle the energy loops are obtained. In conventional asphalts they have a defined form, almost elliptical, and as the test tube fatigues, loops bend and deform (Figure 4).

Similarly, the loops of energy for the modified asphalts can be graphed (Figure 5), perceiving that the loops are kept in the same strain range and above it, distortion occurs; this does not occur in conventional asphalt where loops are present at different levels of deformation.

Figure 4. Loops of energy dissipated in conventional asphalt

Figure 5. Loops od dissipated energy in modified asphalt

From the accumulated energy to the breaking cycle and the number of cycles of failure for each test tube assayed, it is possible to draw a power law (Figure 6), similar to the law of fatigue presented above. In this case it represents the total energy dissipated (DER) versus number of cycles that the mix can withstand.

Figure 6. Energy Act of asphalts considered

4.3 Correlations between toughness and fatigue

Based on tests performed at the asphalts group, it deels on seeing whether a relationship exists between some of the parameters that define the behavior to direct tension (static test) and some of the ones that define the fatigue behavior (test). The possible relationship between the two tests would give an idea of the response to fatigue of a mixture based on the results obtained in direct tensile test, much faster and easier, and to take into account this property in the design phase.

4.3.1 Relationship between fracture displacement of 50% of the maximum tensile strength and strain obtained in cycle 1 of the law of fatigue

First, the relationship between displacement at break at 50% of maximum force to direct tension and strain in fatigue cycle 1 has been studied (Figure 7). The displacement of fracture adopted is the one that occurs when the initial charge has been reduced to a 50% and it is considered representative of the toughness (slope of the decay curve) and easier to determine than the displacement of total failure, that sometimes cannot be obtained for modified asphalts. Flexural deformation in loading cycle 1 was obtained from the fatigue law substituting the value N = 1.

Figure 7. Relationship between displacement at break and strain in the fatigue cycle 1

The above figure shows that BM-modified asphalts BM-3c and BM-3b have a higher direct tensile fracture displacement and also a greater fatigue deformation. Asphalt BM-PN, as in previous cases, is among the modified asphalt and the penetration ones. Then the asphalt penetration B-13/22 and B-60/70 can be find.

4.3.2 Relationship between fracture displacement at 50% of the maximum tension load and fatigue critical strain

Aside from considering the strain to fatigue in the first charge cycle, it was considered also the critical breaking strain of the mixture. The critical breaking strain corresponds to a strain value from which the crack of the test tube propagates in an unstoppable way until total failure; this value corresponds to the deformation of the evolution curve of deformation and its change into abrupt slope (Figure 8).

Figure 8. Fatigue critical strain

The following figure shows the relationship between these two parameters. It shows that the modified asphalts BM-3c and BM-3b have a higher tensile fracture direct displacement and also a more critical fatigue deformation in the mix made with them. Asphalt BM-PN, as in previous cases, is among the modified asphalt and also among the penetration ones. Then, the asphalt penetration B-60/70 and B-13/22 are placed respectively.

Figure 9. Relationship between fracture displacement and critical fatigue deformation

4.3.3 Relationship between fracture displacement at a 50% of maximum load in tension and the slope of fatigue law upon flexural test

Another possible relation between the two tests (direct tension and flexural) can be established between the displacement at fracture with direct tension and the slope of fatigue law for each of the asphalts, at a temperature of 20 ° C (Figure 10 ).

Figure 10. Relation between tensile fracture displacement and the slope of fatigue law

4.3.4 Relationship between secant modulus of asphalt obtained from direct tensile test and dynamic modulus of the mixture obtained from flexural test

Finally, it is possible to establish a relationship among the direct tensile secant modulus and flexural dynamic module (Figure 11). In this case, the modified asphalts with fewer modules (drying and dynamic), are at the bottom of the trend line, followed in ascending order by B-60/70 conventional asphalt, asphalt BM-PN and at the end by B-13/22 asphalt, since the latter ones having higher hardness present larger modules, both of direct and flexural tension.

Figure 11. Relationship between tensile secant modulus and flexural dynamic module

5. Conclutions

Correlations obtained show that asphalt toughness and mix fatigue are related, and therefore one has effect on the other one.

This possible relationship between some of the parameters that define behavior in direct tension (static test) and some that define fatigue behavior (test), would allow to have an idea of the response to fatigue of a mixture from the results obtained in direct tensile test, in a much faster and easier way, and to be in conditions of taking into account this property during the design phase.

First, are the deplacement values related to 50% of the direct tensile maximum load and strain at fatigue in the first charge. The results obtained shows that modified asphalts BM-3c and BM-3b have a greater shift to direct tension, and also a greater strain to fatigue. Asphalt BM-PN is among the modified asphalts and penetration ones. Then, the penetration asphalts B-60/70 and B-13/22 are respectively placed.

Second, the energies obtained were related to direct tension and fatigue, the results show that although it is difficult to establish a quantitative relationship between the energy value of the two trials, both are related. One more possible relationship obtained from the two trials, flexural and direct tension, is between the displacement at fracture of direct tension and the slope of the law of fatigue for each of the asphalts. Modified asphalts have an increased traction displacement and a lesser steep in their fatigue laws than the ones of conventional asphalts, with a lower tensile deformation but with a steeper slope of the law of fatigue.

Similarly, another possible relationship is established between the displacement at fracture to direct tension and the slope of the energy act. In this case, the relationship is inverse to the previous one. Conventional asphalts, with a smaller displacement of modified asphalts, have a lower slope on their energy act.

Secant modules to direct tension and dynamic flexural strength are also related: the greater or smaller is one, greater or smaller is the other one. Polymer-modified asphalts are those with smaller modules, followed by B-60/70 conventional asphalt, asphalt BM-PN and B-13/22 asphalt, that being harder, present greater modules both of direct and flexural tension.

6. References

Adhikari Sanjeev, Shen Shihui y You Zhanping (2009), Evaluation of fatigue models of hot-mix asphalt through laboratory testing. In Transportation Research Record: Journal of the Transportation Research Board, No. 2127, TRB, National Research Council, Washington, D.C., 36-42.

Aragao Francisco Thiago, Kim Yong-Rak, Lee Junghun y Allen David H. (2011), Micromechanical model for heterogeneous asphalt concrete mixtures subjected to fracture failure. Jounal of materials in civil enginering, 23, 30.

Centro de Estudios y Experimentación de Obras Públicas (CEDEX) (1990), Ensayo de Fatiga en Flexotracción Dinámica de Mezclas Bituminosas.

D'Angelo John, Kluttz Robert, Dongre Raj, Stephens Keith y Zanzotto Ludo (2007), Revision of the SUPERPAVE high temperature binder specifications: The multiple stress creep recovery test. Journal of the Association of Asphalt Paving Technologists, Vol. 76, 123-162.

Denneman E. (2010), Method to determine full work of fracture from disk shaped compact tension test on hot mix asphalt. Proceedings of the 29th Southem African Transport Conference (SATC 2010), Pretoria, South Africa. ISBN: 978-1-920017-47-7

Fini E. H., Al-Qadi Imad y Masson Jean Francois (2007), A new blister test to measure bond strenght of asphaltic materials. Journal of the Association of Asphalt Paving Technologists, Vol. 76, 275-302.

Kim Hyunwook y Buttlar William G. (2009), Discrete fracture modeling of asphalt concrete. International Journal of Solids and Structures, 46, 2593-2604.

Kowalski Karol J., McDaniel Rebecca S. y Olek Jan (2008), Development of a laboratory procedure to evaluate the influence of aggregate type and mixture proportions on the frictional characteristics of flexible pavements. Journal of the Association of Asphalt Paving Technologists, Vol. 77, 35-70.

Kringos N., Scarpas A. y De Bondt A. (2008), Determination of moisture susceptibility of mastic-stone bond strength and comparación to thermodynamical properties. Journal of the Association of Asphalt Paving Technologists, Vol. 77, 435-478.

Kvasnak A. Andrea y Williams R. Christopher (2007), Evaluation of interaction effects between asphalt binder and fillers using a moisture susceptibility test. Journal of the Association of Asphalt Paving Technologists, Vol. 76, 163-200.

Pérez Jiménez F., Miró Recasens R. y Fonseca Rodríguez C. (1997), Essai BTD pour la Determination de la Ténacitè et Resístanse à la Fissuration des Mélanges Bitumineux. Mechanical Test for Bituminous Materials, RILEM, 391-396.

Shen Shihui y Carpenter Samuel. (2007), Development o fan asphalt fatigue model based on energy principles. Journal of the Association of Asphalt Paving Technologists, Vol. 76, 525-574.

Wagoner Michael P., Buttlar William G., Paulino Glaucio H., Blankenship Philip. (2005), Investigation of the fracture resistance of hot-mix asphalt concrete using a disk-shaped compact tension test. In Transportation Research Record: Journal of the Transportation Research Board, No. 1929, TRB, National Research Council, Washington, D.C., 183-192.


E-mail: JHernandezN@iingen.unam.mx

Fecha de recepción: 27/ 10/ 2010, Fecha de aceptación: 20/ 05/ 2011.

Adhikari Sanjeev, Shen Shihui y You Zhanping (2009), Evaluation of fatigue models of hot-mix asphalt through laboratory testing. In Transportation Research Record: Journal of the Transportation Research Board, No. 2127, TRB, National Research Council, Washington, D.C., 36-42.        [ Links ]

Aragao Francisco Thiago, Kim Yong-Rak, Lee Junghun y Allen David H. (2011), Micromechanical model for heterogeneous asphalt concrete mixtures subjected to fracture failure. Journal of materials in civil enginering, 23, 30.         [ Links ]

Centro de Estudios y Experimentación de Obras Públicas (CEDEX) (1990), Ensayo de Fatiga en Flexotracción Dinámica de Mezclas Bituminosas.        [ Links ]

D'Angelo John, Kluttz Robert, Dongre Raj, Stephens Keith y Zanzotto Ludo (2007), Revision of the SUPERPAVE high temperature binder specifications: The multiple stress creep recovery test. Journal of the Association of Asphalt Paving Technologists, Vol. 76, 123-162.        [ Links ]

Denneman E. (2010), Method to determine full work of fracture from disk shaped compact tension test on hot mix asphalt. Proceedings of the 29th Southem African Transport Conference (SATC 2010), Pretoria, South Africa. ISBN: 978-1-920017-47-7        [ Links ]

Fini E. H., Al-Qadi Imad y Masson Jean Francois (2007), A new blister test to measure bond strenght of asphaltic materials. Journal of the Association of Asphalt Paving Technologists, Vol. 76, 275-302.        [ Links ]

Kim Hyunwook y Buttlar William G. (2009), Discrete fracture modeling of asphalt concrete. International Journal of Solids and Structures, 46, 2593-2604.        [ Links ]

Kowalski Karol J., McDaniel Rebecca S. y Olek Jan (2008), Development of a laboratory procedure to evaluate the influence of aggregate type and mixture proportions on the frictional characteristics of flexible pavements. Journal of the Association of Asphalt Paving Technologists, Vol. 77, 35-70.        [ Links ]

Kringos N., Scarpas A. y De Bondt A. (2008), Determination of moisture susceptibility of mastic-stone bond strength and comparación to thermodynamical properties. Journal of the Association of Asphalt Paving Technologists, Vol. 77, 435-478.        [ Links ]

Kvasnak A. Andrea y Williams R. Christopher (2007), Evaluation of interaction effects between asphalt binder and fillers using a moisture susceptibility test. Journal of the Association of Asphalt Paving Technologists, Vol. 76, 163-200.        [ Links ]

Pérez Jiménez F., Miró Recasens R. y Fonseca Rodríguez C. (1997), Essai BTD pour la Determination de la Ténacitè et Resístanse à la Fissuration des Mélanges Bitumineux. Mechanical Test for Bituminous Materials, RILEM, 391-396.        [ Links ]

Shen Shihui y Carpenter Samuel. (2007), Development o fan asphalt fatigue model based on energy principles. Journal of the Association of Asphalt Paving Technologists, Vol. 76, 525-574.        [ Links ]

Wagoner Michael P., Buttlar William G., Paulino Glaucio H., Blankenship Philip. (2005), Investigation of the fracture resistance of hot-mix asphalt concrete using a disk-shaped compact tension test. In Transportation Research Record: Journal of the Transportation Research Board, No. 1929, TRB, National Research Council, Washington, D.C., 183-192.        [ Links ]

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