DEVELOPMENT OF AN AUTONOMOUS SETUP FOR EVALUATING SELF
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HEALING CAPABILITY OF ASPHALT MIXTURES
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Jian Qiu a,b*
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a
Faculty of Civil Engineering & Geosciences
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Delft University of Technology
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Delft 2600GA, the Netherlands
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E-mail: j.qiu@tudelft.nl8
Tel.:+31 15 27 827639
Fax: +31 15 27 8344310
bKey Laboratory of Silicate Materials Science and Engineering of Ministry of Education
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Wuhan University of Technology
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Wuhan 430070, China13
E-mail: qiuj@whut.edu.cn14
Tel.:+86 27 8716259515
Fax: +86 27 8787389216
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A.A.A. Molenaar18
Faculty of Civil Engineering & Geosciences
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Delft University of Technology
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Delft 2600GA, the Netherlands
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E-mail: a.a.a.molenaar@tudelft.nl22
Tel.:+31 15 27 8481223
Fax: +31 15 27 8344324
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M.F.C. van de Ven26
Faculty of Civil Engineering & Geosciences
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Delft University of Technology
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Delft 2600GA, the Netherlands
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E-mail: m.f.c.vandeven@tudelft.nl30
Tel.:+31 15 27 8229831
Fax: +31 15 27 8344332
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Shaopeng Wu34
Key Laboratory of Silicate Materials Science and Engineering Ministry of Education
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Wuhan University of Technology
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Wuhan 430070, China37
E-mail: wusp@whut.edu.cn38
Tel.:+86 27 8716259539
Fax: +86 27 8787389240
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Submitted for consideration of presentation and publication at the
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2012 Annual Meeting of the Transportation Research Board
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*Corresponding author: J. Qiu
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Submission date: 28-07-2011
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Word account: 4208 + (2Tables+11Figures)*250 = 7458
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DEVELOPMENT OF AN AUTONOMOUS SETUP FOR EVALUATING SELF
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HEALING CAPABILITY OF ASPHALT MIXTURES
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ABSTRACT
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It is a well known fact that asphalt mixtures have self healing capabilities. Yet most of the self
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healing investigations are carried out using complex and time consuming fatigue tests. In
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order to investigate the self healing capability in a simple and efficient manner, a beam on
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elastic foundation test setup (BOEF) is proposed in this paper. With a notched asphalt
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concrete beam fully glued on a low modulus rubber foundation, the BOEF setup is able to
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control the self healing process autonomously including crack closure and healing at different
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rest periods and temperatures. A load-crack opening displacement (COD) curve was used for
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characterizing the self healing capability of the monotonic response with a
loading-healing-13
reloading procedure. In addition, small numbers of dynamic loads were also applied to the
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BOEF setup under different cracking and healing conditions. An apparent modulus of the
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BOEF asphalt beam was obtained based on the dynamic response of the COD. The results
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indicate that the self healing capability which is obtained in the monotonic tests is dependent
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on the healing time and healing temperature. The healing temperature has the largest effect of
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the two. Most of the apparent modulus of the BOEF asphalt beam recovers at the beginning of
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the self healing process. The influences of healing time and temperature are limited. As a
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result, the BOEF setup is proven to be a potential tool for the self healing investigation of
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asphalt mixtures.22
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Key words:24
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Self healing, crack growth, beam on elastic foundation, asphalt mixture
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DEVELOPMENT OF AN AUTONOMOUS SETUP FOR EVALUATING SELF
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HEALING CAPABILITY OF ASPHALT MIXTURES
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INTRODUCTION
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Self healing of bituminous materials has been investigated for over 40 years. Bituminous
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materials are believed to heal themselves during rest periods and hot summers. Many
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contributions have been made to investigate this phenomenon by means of different
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approaches and at different levels, ranging from molecular to pavement level [1-17]. For
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details of these approaches the reader is referred to the literature review made by Qiu [18].
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Most of the time, the self healing capability of bituminous materials is investigated
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during the fatigue test, which is a complex and time consuming process [19, 20]. Limited
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insight can be gained due to the complexity of the fatigue test itself. Hence, there is a need
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for developing a more efficient test method to evaluate the self healing capability of
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bituminous materials.
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A test setup called the Beam on Elastic Foundation setup (BOEF) has been used to
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evaluate the crack propagation through asphalt mixes for over 40 years [21-23]. This setup, in
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which an asphalt beam is supported by a rubber foundation, is designed to simulate a real
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flexible pavement structure. Because of the uniqueness of the rubber foundation,
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complications like permanent deformation from the simple supported beam bending setup can
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be neglected [21]. In addition, the rubber foundation can provide confinement to close the
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crack after the load is removed, which coincides with the first necessary conditions for
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healing named as crack closure [5, 9]. Hence, the BOEF test setup seems promising for self
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healing investigations instead of a time consuming and complex fatigue test.
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In this paper, the usefulness of the BOEF setup for evaluating self healing capability
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of asphalt mixtures is explored.
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EXPERIMENTAL
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Figure 1 shows the developed BOEF setup [24].
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A Dutch DAC 0/8 dense asphalt mixture was selected. The mixture has a bitumen
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content of 6.5%. A 70/100 penetration grade bitumen was used as a binder. The asphalt beam
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was produced with a target air voids content of 4%. The size of the beams is 35mm in width,
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70mm in height and 400mm in length with a small notch at the middle of the beam. This
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notch is 3.5mm wide and has a height of 15mm.
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A neoprene rubber was selected as elastic foundation. Young’s modulus of the rubber
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is 6.5MPa. Another piece of rubber with a Young’s modulus of 15MPa was used as a loading
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pad to introduce the load on top of the beam.
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In order to ensure full contact between the asphalt beam and the rubber foundation,
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the asphalt beam was fully glued on the rubber with Rengel SW 404 glue, except 10mm away
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from each side of the notch. The rubber-steel interface was also fully glued.
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A remote controlled Canon G11 camera was used for monitoring the crack
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propagation process in the asphalt beam. The crack opening displacement equipment, COD,
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was located right at the crack tip. The displacement was measured over a distance of 20mm
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using a linear variable differential transformer (LVDT).
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Figure 2 illustrates the test procedure. Both monotonic loads and dynamic loads were
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applied in the test program.
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First a load was applied at a constant COD speed of 0.001mm/s until the target COD
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level was reached. Three target COD levels were selected: 0.2mm, 0.6mm and 0.9mm,
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respectively. After the target COD level was reached, the specimen was unloaded with a COD
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closing speed of -0.001mm/s till the load level had returned to 0. At that moment, the external
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load was removed. A rest period of 1 hour was first applied at 5oC. Then healing periods of 3
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hours and 24 hours and healing temperatures of 5oC and 40oC were applied, respectively.
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After the healing period, the specimens were reloaded at a temperature of 5oC using a COD
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speed of 0.001mm/s till a COD level of 1.5mm. For specimens healed at a temperature of
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40oC, a conditioning time of at least 2 hours at 5oC was applied.
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FIGURE 1 Schematic of the BOEF setup (up) and the real TUDelft BOEF setup (down)
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FIGURE 2 Illustration of BOEF test procedure
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Apart from the monotonic loading described above, series of force controlled
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dynamic loads were also applied between the loading-healing-reloading procedures as it is
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shown in Figure 2. The terms Dyna0, Dyna1, Dyna2 and Dyna3 were used for dynamic
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loading before monotonic loading, after loading (before healing), after healing and after
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reloading, respectively. A load amplitude of 300N, a frequency of 5Hz and a duration of 5s
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COD Crack length with white paint and camera
LVDT 3.5 20 10 15 600 400 10 70 35 100 10 120 20 Load All in mm COD Crack length with white paint and camera
LVDT 3.5 20 10 15 600 400 10 70 35 100 10 120 20 Load All in mm COD=crack opening displacement VD=vertical displacement SD=side displacement Asphalt beam Rubber Steel COD Crack length with white paint and camera
LVDT 3.5 20 10 15 600 400 10 70 35 100 10 120 20 Load All in mm COD Crack length with white paint and camera
LVDT 3.5 20 10 15 600 400 10 70 35 100 10 120 20 Load All in mm COD=crack opening displacement VD=vertical displacement SD=side displacement COD Crack length with white paint and camera
LVDT 3.5 20 10 15 600 400 10 70 35 100 10 120 20 Load All in mm COD Crack length with white paint and camera
LVDT 3.5 20 10 15 600 400 10 70 35 100 10 120 20 Load All in mm COD=crack opening displacement VD=vertical displacement SD=side displacement Asphalt beam Rubber Steel Asphalt beam Rubber Steel
Time
Healing Healing periods: 3h, 24h Healing temperatures: 5oC, 40oCDyna0 Dyna1 Dyna2 Dyna3
Loadi
ng
ReloadingLoad
COD
-0.001mm/s
were applied at a temperature of 5oC. And the dynamic response of the COD was obtained.
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For specimens healed at a temperature of 5oC, the dynamic loads were also applied during the
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self healing periods. An apparent modulus was calculated for analysing the dynamic data as
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given in Equation 1.4
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(1)6
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MCOD is the apparent modulus of the BOEF beams, N/mm; Ldyna is the maximum
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dynamic load applied, 300 N; Cdyna is the measured maximum of the dynamic response of the
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COD, mm. It should be noted that the MCOD is representing an apparent stiffness of the
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notched BOEF beam, which is a system value and not a material modulus.
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RESULTS AND DISCUSSIONS
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Damage decomposition
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Figure 3 shows the load-COD (abbreviation LC) curve from the monotonic loading phase of
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the BOEF test. Because of the influence of the rubber foundation, the LC curve does not show
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abrupt failure like observed in loading tests on simply supported beams [25].
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Finite element modelling was used to simulate the LC curve. A 3D FEM model is
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developed using FEM code ABAQUS. In this modelling, the asphalt beam was assumed to
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behave visco-elastic. Damage development was not taken into account. The material
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parameters used in the simulation is listed in Table 1 [26]. Due to complications of keeping a
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constant COD by applying a vertical load in the FEM simulation, the simulation was carried
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out with the experimental vertical load as input. The COD, which was measured over the
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notch with a measuring distance of 20mm, was obtained from the model as output.
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TABLE 1 Material parameters for FEM analysis
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Elastic material parameters
Rubber pad Rubber foundation Glue Steel
Young’s Modulus [MPa] 15 6.5 4000 200000
Poisson’s ratio [-] 0.49 0.49 0.15 0.15
Visco-elastic material parameters of asphalt mixtures using Generalized Maxwell Model E0 [MPa] n Term1 Term2 Term3 Term4 Term5 Term6
i
4.52E-01 2.85E-01 2.00E-01 5.71E-02 5.09E-03 6.92E-04 19956
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[s] 3.05E-03 7.89E-02 9.72E-01 1.15E+01 1.70E+02 1.00E+03
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FIGURE 3 Comparison of LC curve with and without damage
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0 1 2 3 4 5 - 0.20 0.40 0.60 0.80 1.00 COD [mm] L oad [ k N ] ExperimentSimulation with visco-elastic property
dyna COD dyna L M C
When comparing the experiment LC curve with the simulated curve in which no
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damage was taken into account, the damage can easily be identified with the slope decrease of
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the LC curve and an increase of the COD for a certain load level.
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In addition, the visible crack was also monitored using a digital camera and those
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results are shown in Figure 4. The crack length refers to the visible crack development in the
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vertical direction and the crack width refers to the visible crack opening in the horizontal
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direction along the notch. It can be seen that the crack is visible after a COD level of 0.2mm
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and develops further. The development of the crack length tends to slow down after a COD
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level of 1mm because at that moment the crack is entering the compression zone [24].
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FIGURE 4 Development of the visible crack
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FIGURE 5 Decomposition of the COD development
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In order to better understand the damage development during loading, the total COD
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development is decomposed and the results are shown in Figure 5. The development of the
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COD can be decomposed into three zones.
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o The visco-elastic zone corresponds to the COD development without damage,
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which is calculated by means of the finite element simulation.
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o The macro crack zone denotes the visible crack width at the notch that was
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monitored from the photos.
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o The micro crack zone is a transit zone of the visco-elastic zone and the macro
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crack zone; it refers to the nonlinear effects of the micro cracks but these are not
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yet visible.
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It can be seen that macro cracks initiate at a COD level of 0.2mm. At that level
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already a significant amount of micro cracks has developed. At COD levels of 0.6mm and
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-0.2 0.4 0.6 0.8 1.0 - 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 COD[mm] H o ri z ont al D is p la c e m e nt [ m m ] COD-experiment COD-visco-elastic COD-without visible crackMicro crack zone
Visco-elastic zone Macro crack zone 0 5 10 15 20 25 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 COD [mm] C rac k length [ m m ] 0 0.2 0.4 0.6 0.8 1 1.2 C rac k w idt h [ m m ]
Visible crack length-Vertical Visible crack width at notch-Horizontal
0.9mm, macro cracks are dominant. Due to the clear difference in damage state at different
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COD levels, the healing phenomenon should also be studied at different damage levels.
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Autonomous crack closure
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Figure 6 shows the COD recovery curve during the unloading process. It should be noted that
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unloading was carried out with a COD speed of -0.001mm/s. The external load facility was
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removed when the load became zero. After that, the setup is subjected to an autonomous
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(internal) process which means that the setup is left alone and that all the recovery which is
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observed after removal of the load is due to recovery of the BOEF test setup (beam and
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rubber foundation). A nonlinear recovery of the COD can still be observed after removing the
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external load. This may be caused by the visco-elastic relaxation of the internal stress of the
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asphalt concrete beam and the closing of the crack due to the confinement of the rubber
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foundation. Most of the recovery occurs at the beginning of the process and is finished
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approximately one hour after the load was removed. The residual COD can be explained by
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the plastic deformation of the asphalt concrete beam or the mismatch of the crack faces [24].
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FIGURE 6 Recovery of the COD after unloading
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(a) (b) (c) (d)46
FIGURE 7 Example of crack formation and closure (0.9-5c-3h): (a) before loading; (b)
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loading till COD of 0.9mm; (c) unloading; (d) reloading till COD of 1.5mm
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The closure of the crack after a COD level of 0.9mm was monitored and an example
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is shown in Figure 7. It can be observed that the visible crack developed during the loading
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process fully closed after unloading. This confirms that the residual COD is due to the
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deformation of the asphalt concrete beam. Moreover, the autonomous fully closure of the
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crack offers the opportunity to investigate the self healing behaviour of the asphalt mixture at
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any crack status.
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crack crack 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 1000 2000 3000 4000 5000 6000 Time[s] CO D [m m ] COD level of 0.2mm COD level of 0.6mm COD level of 0.9mmMonotonic response with loading-healing-reloading procedure
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A monotonic test with a loading-healing-reloading procedure was carried out. After self
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healing at various conditions, the reloading was carried out under the same loading condition.
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The reloading tests were done at a temperature of 5oC and a COD speed of 0.001mm/s. The
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reloading curve was then compared with the initial curve to investigate the strength gaining
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process through the monotonic response.
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Prior to the comparison, the loading and reloading LC curves were smoothened with
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using a power function as shown in Equation 2.
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nL
m C
(2)11
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Where, L is the load, N; C is the value of the COD, mm; m and n are the regression
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parameters.
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Table 2 shows the regression parameters of the loading and reloading curves. The
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regression parameters of the monotonic LC curve shown in Figure 3 are calculated as
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reference values.
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In addition, the reloading curves are normalized using the function given in Equation
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3. As shown in Equation 3, the reference LC curve is used as a normalized loading curve. All
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the normalized reloading curves are compared to avoid sample to sample variation.
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(3)22
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In figures 8a to 8c the normalized LC curves for all the healing conditions are given.
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An increase of the slope of the LC curve can be observed for increasing healing time and
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increasing healing temperature. It can also be seen that the reloading after high temperature
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healing shows a slight tendency to decrease compared with the initial loading curve. This can
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be explained by the following reasons: a. the conditioning time at 5oC before the reloading
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was not enough; b. some permanent deformation occurred during the high healing
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temperature.
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In figure 8d the area change of the LC curves for each healing conditions is given. In
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order to minimize the unexpected tendency at high temperature, the area change of the
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reloading curve up to a COD level of 0.2mm was calculated. The following observations can
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be made:
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o The increase of the area is dependent on healing time and healing temperature.
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o Because of the limited damage, the area change at a target COD level of 0.2mm is
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limited and all values are approaching the one for the initial loading LC curve.
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o The increase of the area of the target COD level of 0.6mm is slightly faster than
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that of the target COD level of 0.9mm.
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2 1 2 1( )
( )
( )
( )
ref n n reloadingnorm ref n ref
loading
L
C
m C
L
C
L
C
m C
L
C
m C
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TABLE 2 Regression results for different LC curves
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FIGURE 8 Comparison of normalized loading and reloading curves: (a) COD level of
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0.2mm; (b) COD level of 0.6mm; (c) COD level of 0.9mm; (d) Comparison of the area
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ratio under the reloading LC curve
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Loading Reloading Code m1 n1 m2 n2 0.2-5C-3h 3.37 0.40 3.64 0.48 0.2-5C-24h 3.72 0.51 4.20 0.55 0.2-40C-3h 3.93 0.44 3.89 0.47 0.2-40C-24h 4.16 0.50 3.88 0.49 0.6-5C-3h 3.91 0.44 4.13 0.62 0.6-5C-24h 3.82 0.48 4.18 0.60 0.6-40C-3h 3.93 0.44 3.82 0.48 0.6-40C-24h 3.87 0.47 3.51 0.48 0.9-5C-3h 3.54 0.50 3.23 0.67 0.9-5C-24h 3.92 0.45 3.73 0.60 0.9-40C-3h 3.95 0.48 3.46 0.55 0.9-40C-24h 3.76 0.41 3.66 0.49 Reference 4.21 0.45 0 0.5 1 1.5 2 2.5 3 0 0.05 0.1 0.15 0.2 COD [mm] Lo ad [ k N ] Loading Reloading-0.2-5C-3h Reloading-0.2-5C-24h Reloading-0.2-40C-3h Reloading-0.2-40C-24h 0 0.5 1 1.5 2 2.5 3 0 0.05 0.1 0.15 0.2 COD [mm] Lo ad [ k N ] Loading Reloading-0.6-5C-3h Reloading-0.6-5C-24h Reloading-0.6-40C-3h Reloading-0.6-40C-24h 0 0.5 1 1.5 2 2.5 3 0 0.05 0.1 0.15 0.2 COD [mm] Loa d [ k N ] Loading Reloading-0.9-5C-3h Reloading-0.9-5C-24h Reloading-0.9-40C-3h Reloading-0.9-40C-24h 0 0.2 0.4 0.6 0.8 1 1.2 5c-3h 5c-24h 40c-3h 40c-24h A re a r a ti o [-] COD level of 0.2mm COD level of 0.6mm COD level of 0.9mm (a) (b) (c) (d)Dynamic response
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Figure 9 shows the results of the apparent modulus of the MCOD obtained from the dynamic
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tests for different loading/healing conditions. The MCOD of the original beams (Dyna0) was
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used as a reference of 100%. It can be seen that the MCOD decreases with the loading and
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reloading process and increases with the healing process. When comparing different COD
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levels, a higher COD level shows a lower MCOD at Dyna1 (MCOD after unloading). Since
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different levels of COD indicate different levels of damage, one can conclude that the MCOD
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can be used to characterize different levels of damage.
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Figure 10 illustrates the development of the MCOD during the healing process at a
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healing temperature of 5oC for a rest period of 24 hours. Firstly, all the MCOD developments
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follow the same trend, fast recovery in the beginning and slowing down afterwards. A
semi-12
log relationship was used to model this trend. Secondly, for a COD level of 0.2mm, the MCOD
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is approaching 100% due to no visible cracking. The MCOD increase is slightly faster for the
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target COD level of 0.9mm. However, the difference between COD levels of 0.6mm and
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0.9mm is limited when considering the variation of the measurements.
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FIGURE 9 Dynamic response of the COD during the BOEF test under dynamic loading
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of 0.3kN and a frequency of 5Hz36
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FIGURE 10 Development of the MCOD at a healing temperature of 5oC for a rest period
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of 24 hours55
60 70 80 90 100 0 5 10 15 20Healing time [hours]
M c od [ % ] 0.2-5c-24h 0.6-5c-24h 0.9-5c-24h 0 20 40 60 80 100 120
Dyna0 Dyna1 Dyna2 Dyna3
Mc o d [ % ]
Figure 11 compares the MCOD increase as healing percentage for different conditions.
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It should be noted that each test condition was carried out on an individual asphalt concrete
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beam. Because of the uniqueness of the crack distributions in each beam the data was
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analyzed with care considering the sample-to-sample variation. It can be seen that in general,
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a longer time and higher temperature results in more MCOD improvement. However, the
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improvement is limited. The influence of the crack phases on the MCOD is not so clear because
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of the variation of the measurements. For a healing period of 24 hours, an increase in
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temperature shows limited improvement for the stiffness of the BOEF asphalt concrete beam.
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When comparing the recovery of the reloading curves obtained from the monotonic
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loading-healing-reloading tests and the recovery of the dynamic response from the dynamic
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modulus test, the recovery of the dynamic response to be related to the crack closure due to
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the autonomous process. The recovery of the reloading curves benefits more from healing
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time and healing temperature, which is believed to be related to the strength regained of the
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healing process [5, 14].14
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FIGURE 11 Healing percentages in terms of MCOD increase for different healing times
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and temperatures34
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CONCLUSIONS36
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The self healing capability of asphalt mixtures was investigated with an autonomous healing
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setup called Beam on Elastic Foundation (BOEF) test setup. Based on the test data and the
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performed analysis, the following can be concluded:
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o The BOEF setup is capable of investigating the self healing capability of asphalt
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mixtures under different crack conditions. The following advantages can be
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shown: the setup determines different damage phases by means of a load-COD
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relationship and the corresponding crack development. The BOEF setup allows
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full closure of the crack due to the confinement of the rubber foundation. The
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setup allows easy application and variation of healing time and healing
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temperature without disturbing the crack situation.
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o The self healing capability from recovery of the reloading curves is continuously
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increasing with increasing time and temperature. It is observed that the healing
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temperature is more important than the healing time.
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o The self healing capability from the dynamic response is at the beginning of the
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healing process, and limited increase can be observed with increasing time and
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temperature.
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o When comparing the self healing capability of the dynamic tests and the
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monotonic loading-healing-reloading tests, the recovery of the dynamic load
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0 2 4 6 8 10 12 14 5C-3h 5C-24h 40C-3h 40C-24h H e ali n g pe rc e n ta g e [ % ] COD level of 0.2mm COD level of 0.6mm COD level of 0.9mmresponse is believed to be related to crack closure and the recovery of the
1
monotonic reloading curves is believed to be related to strength gain, which is
2
dependent on the viscosity of the material [14].
3
In the future, the research will be focused on further implementing the BOEF setup
4
for the development of new self healing components and to compare the self healing
5
capacities of different types of materials.
6
In addition, since healing of the dynamic load response does not mean completion of
7
the healing process, but only the crack closure, further implementation of the dynamic load
8
response for healing characterization should be done with care. This can be important for
9
fatigue related healing tests and pavement response (such as FWD) related healing tests.
10
11
12
REFERENCES13
14
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