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Influence of the thermal expansion of bitumen on asphalt self-healing
Accepted Manuscript Influence of the thermal expansion of bitumen on asphalt self-healing D. Grossegger, A. Garcia PII: DOI: Reference:
S1359-4311(18)36491-3 https://doi.org/10.1016/j.applthermaleng.2019.04.034 ATE 13624
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
23 October 2018 26 March 2019 8 April 2019
Please cite this article as: D. Grossegger, A. Garcia, Influence of the thermal expansion of bitumen on asphalt selfhealing, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.04.034
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Influence of the thermal expansion of bitumen on asphalt self-healing D. Grossegger a, A. Garciaa*
a
Nottingham Transportation Engineering Centre, Department of Civil Engineering, University of Nottingham, Nottingham NG7 2RD, United Kingdom
* Corresponding author. Tel: +44 (0) 0115 95 13914. E-mail addresses: [email protected]
Abstract: The self-healing of cracks in asphalt mixture is mainly due to the drain of the bitumen contained in the space between the aggregates, into the cracks. Until now, it was believed that the physical principles affecting the flow of bitumen are influenced by gravity and surface energy of bitumen and the aggregates. In this study, we show that the thermal expansion of bitumen plays an important role in the self-healing of asphalt mixture. To demonstrate this, asphalt mortar beams were manufactured and broken in two pieces by means of three-point bending tests. Self-healing was induced in asphalt mixture by increasing its temperature using a convection oven, at temperatures that ranged from 40 °C to 120 °C. The self-healing ratio was calculated by comparing the force required to break the test specimens before and after heating. Furthermore, a test was designed that consisted of bitumen raising through a capillary tube from a bitumen container. To account for the effect of thermal expansion, the bitumen container was fully enclosed except for the capillary tube. To account for the effect of surface energy on the bitumen’s capillary flow, the capillary tube was placed in a container that was open to the atmosphere. The rise of bitumen was monitored at temperatures that ranged from 40 °C to 120 °C. Finally, activation energies were derived from the rise of bitumen in the capillaries, viscosity changes and the selfhealing progression. It was found that the activation energy of asphalt self-healing is similar to that of bitumen rising due to thermal expansion, which confirms the contribution of thermal expansion on asphalt self-healing by the effect of increasing temperature. Keywords: asphalt mortar, self-healing, activation energy, induced heating, thermal expansion
Highlights: Surface energy and contact angle decrease with increasing temperature. Pressure due to thermal expansion reduced the activation energy for bitumen to flow through a capillary. The activation energy of self-healing for asphalt mortar is similar to the activation energy for confined capillary flow of bitumen.
1
1.
Introduction
Bituminous mixtures are well-established composite materials [1], which are mainly used for road constructions and other applications, such as roof constructions [2]. The mixtures used for road constructions consist of mineral aggregates, bitumen and a range of bitumen additives. The size of the mineral aggregates ranges from a few microns up to approximately 30 mm. Bitumen is the adhesive component in the asphalt mixture and is a viscoelastic liquid, which viscosity is temperature-dependant [3]. To produce hot or warm mixed asphalt, the aggregates and bitumen are mixed above 130 °C and compacted at a sufficiently high temperature. Moreover, thanks to the addition of additives, asphalt can be also produced and processed at lower temperatures [1]. Temperature changes, moisture, radicals, radiation and cyclic traffic loads may induce stresses [4, 5], ageing [6, 7] and stripping [8, 9] in asphalt road structures, mainly affecting the surface layer. Furthermore, due to ageing and the stresses induced by excessive traffic and decrease to low temperature cracks may develop in road structures [1]; these cracks may start at the bottom or at the top of the road structures, or at a material level, at the interface between mineral aggregates and bitumen. On the other hand, asphalt is a self-healing material. When a crack develops in the pavement, it may heal if enough time without traffic loading is allowed [10]. The reason for this is that bitumen can drain from the mixture into the cracks. The ratio of crack self-healing will depend of the length and width of the cracks, aggregate type and gradation, and on the viscosity of bitumen [11-14] and will vary depending on the environmental temperature, because the viscosity of bitumen is temperature-dependant. In addition, a limitation for rapid asphalt self-healing is due to the high viscosity of bitumen during service life, which delays the drain of bitumen into the cracks. Hence, the drain of bitumen into cracks may require several days to be completed and cracks may develop faster than they are able to self-heal. Finally, only cracks that are not caused by a structural failure of the road, such as those starting at the top and at the interface between the mineral aggregates and bitumen, are of importance for self-healing. The draining of bitumen into cracks occur above a certain threshold temperature. And identified driving mechanism for the bitumen flow into cracks are surface energy [15] and gravity [16]. The threshold temperature for flow has been correlated to either the glass transition temperature of bitumen [17] or the temperature when the flow behaviour of bitumen changes from non-Newtonian to Newtonian [18]. In addition, factors affecting the flow of bitumen are its fractional composition, temperature, ageing and additives content [1, 3]. Finally, the self-healing properties of asphalt at a certain temperature improve with the reductions in the viscosity of bitumen [19]. Therefore, to trigger and enhance the self-healing recovery of asphalt, the viscosity of bitumen must be reduced and to do that, two technologies have been developed. The first one, heating by magnetic or electromagnetic energy, increases the asphalt mixture’s temperature by indirectly heating metallic particles embedded in the asphalt [20-23]. In this technology, the temperature of bitumen may increase locally above 100 °C in less than 30 s and macroscopic cracks are
2
repaired in periods of time that range from seconds to minutes [22]. Furthermore, in [24], it has been hypothesized that the local heat generation in the metallic particles is responsible for bitumen to locally expanse and to flow into the crack, which accelerates the self-healing greatly. The second technology makes use of encapsulated bitumen solvents embedded in the asphalt mixture [25, 26]. Therefore, bitumen can easily flow and drain into the cracks. Thermal expansion in asphalt road structures is considered for thermal cracking, as bitumen becomes stiffer at lower temperatures crack can occur due to the thermal induced stress and the reduced relaxation characteristic [1]. The linear thermal expansion coefficient of asphalt is affected by the bitumen content, aggregate type and aggregate quality [27]. Furthermore, the expansion of bitumen is indirectly considered during the asphalt mixture design, as the amount of bitumen determines the air void content, density, deformation and stability of the compacted mixture [1]. The aim of this study is to demonstrate the effect of thermal expansion of bitumen on the self-healing of an asphalt mixture. The main hypothesis of the study is that the activation energy required by asphalt to self-heal through the effect of temperature increase will be: (i)
comparable to the activation energy for bitumen to flow through a capillary tube due to the effect of thermal expansion and,
(ii)
lower than the activation energy for bitumen to rise in a vertical capillary tube due to the effect of surface energy.
To demonstrate these points, the activation of energy for asphalt mixture self-healing has been calculated and compared to the activation energy of bitumen for confined and unconfined capillary flow.
2. 2.1.
Materials and methods Materials
The asphalt mortar mixture consisted of bitumen 70/100 and crushed limestone aggregates. The bitumen was obtained from Total Petroleum and had a needle penetration of 73 dmm, a softening point of 46.4 °C and a density of 1.02 gcm-3 at 25 °C. The gradation of the limestone aggregates is provided in Table 1. The maximum particle size was 4 mm and the aggregates had a density of 2.36 gcm-3. To manufacture the mortar mixture, bitumen and aggregates were heated to 180 ˚C and manually mixed together in the proportion of 15 wt.% bitumen and 85 wt.% aggregates. Prismatic beams with dimensions 0.10 x 0.03 x 0.03 m3 were manufactured from the mixture by pouring it into polytetrafluorethylene (PTFE) moulds. The moulds had a rectangular notch in the middle that created a predetermined breaking point in the beams. The mixture was manually compacted and let to cool down to ambient temperature (20 °C ±2 °C) for at least 1 hour. Afterwards, the beams were demoulded and stored at -17 °C ± 1°C in a freezer, to ensure a brittle and clean fracture during the three-point bending test.
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Table 1: Aggregate gradation
sieve size [μm] 4000 2800 2000 1000 500 250 125 63
2.2.
percent passing [wt.%] sand 1 100.0 99.8 90.9 65.4 43.5 19.6 5.6 1.8
Asphalt mortar’s self-healing characterisation
Asphalt self-healing was characterised according to the procedure described in [28]. To create a brittle fracture that can self-heal, the beams were tested under three-point bending while they were still at -17 °C ±1 °C. The three-point bending force was centred vertically above the breaking notch and applied at the constant rate of 10.2 Ns-1 ± 0.6 Ns-1. After breaking the beams in two halves, they were let to rest at ambient temperature (20 °C ±2 °C) for a minimum time of 16 h. The initial strength recovery, which represented the first seconds of contact without heating, was measured by assembling the broken beam halves by hand to its initial geometry at ambient temperature, with care of not applying any additional pressure, and subjecting them afterwards to the three-point breaking test. The duration from assembling to testing was less than 10 s and took on average 5 s. For the heat induced self-healing, beams were placed inside the manufacturing moulds and introduced in a convection oven (Hot Box oven, Gallenkamp), to accelerate the self-healing process, for times that ranged from 60 s to 101 h. The temperatures at which the oven was set were 40 °C, 60 °C, 80 °C, 100 °C and 120 °C. Furthermore, the average surface temperature of the test samples was measured with an infrared camera (micro-epsilon, TIM 160, UFPA, 160x120 pixels) after the heating period. Immediately after heating, the beams within the moulds were stored at -17 °C, to prevent any further self-healing. Afterwards, the beams were broken again by three-point breaking testing. The healing level (S) was defined as the ratio between the maximum breaking load after (FH) and before the heating period (FI). (1)
2.3.
Flow of bitumen through a capillary tube
To account for the influence of thermal expansion and surface energy of bitumen on asphalt self-healing, the movement of bitumen inside a capillary tube of 1 mm diameter was monitored. With this purpose, two different set-ups were used, see Figure 1(a) and 1(b).
4
2r ϑ
Capillary pressure and pressure due to thermal expansion
2r
ϑ
Capillary pressure
Petri dish (a) First set-up: open container
Culture test tube, 15 ml (b) Second set-up: sealed container
Figure 1: Scheme of the capillary flow test (a) without and (b) with thermal expansion pressure.
The first set-up was designed to test the influence of surface energy on the flow of bitumen through a capillary tube (Figure 1(a)). It consisted of a capillary tube glued perpendicular to the bottom edge of a clear glass tile. Afterwards, the glass tile with the capillary was introduced vertically on a petri dish filled with bitumen at the testing temperatures, 60 °C, 80 °C, 100 °C, 120 °C and 140 °C, respectively. The whole setup was stationed inside a convection oven with glass door at the testing temperature. The bitumen inside the capillary tube was recorded with a full HD camera (AW920, Ausdom). The second set-up was designed to test the influence of bitumen’s thermal expansion on its flow through a capillary tube (Figure 1(b)). To do that, a capillary tube was inserted in a 15 ml culture test tube that was fully filled with bitumen, through its lid. Any gap between the capillary and the lid was sealed with silicone, so that bitumen was hermetically enclosed in the capillary and no air remained inside the culture test tube. To quantify the effect of thermal expansion, the culture test tube with the bitumen at the ambient temperature was placed inside a convection oven. The rise of bitumen inside the capillary tube was recorded with a full HD camera (AW920, Ausdom). Different oven temperatures were tested: 40 °C, 60 °C, 80 °C and 100 °C, respectively.
2.4.
Calculation of the activation energy
The Arrhenius equation can be used to calculate activation energy of temperature dependent processes [29], including the self-healing of the mortar beams fractured, and the viscosity and movement of bitumen in the capillary tubes [16]. The Arrhenius-type relation [30], originating form molecular kinetics, is shown below:
(2)
5
where k is a rate coefficient, which is substituted by viscosity respectively time for the calculations; A is the preexponential factor; EA (Jmol-1) is the activation energy; R (8.314 JK-1mol-1) is the universal gas constant; and T (K) is the absolute temperature. To obtain the activation energy, the parameters (time or viscosity), were plotted in a logarithmic graph against the reciprocal of absolute temperature. The slope of the linear regression is equal to EA/R [31].
2.5.
Surface energy of bitumen
The surface energy of bitumen was calculated from the equilibrium heights of the capillaries (Figure 1(a)). According to Jurin’s law, the equilibrium height of the meniscus h can be calculated from the equilibrium between capillary pressure and hydrostatic pressure [32]:
(3) where γ (N·m-1) is the surface energy; θ (rad) is the contact angel between glass capillary and bitumen; ρ (kg·m-3) is the density; g (9.81 m·s-2) is the gravity acceleration; and r (m) is the capillary radius. Equation 3 was solved for the surface energy under the assumption of a linear decreasing density with increasing temperature [33]. The contact angle was determined with an accuracy of 5° from the external contact of the glass capillary with the surface bitumen in the petri dish, which is the same as the contact angle between bitumen and glass inside the capillary tube. The pictures obtained were analysed with a graphic program (Gimp 2.8.14, GNU Image Manipulation Program).
2.6.
Viscosity
In order to calculate the activation energy for viscous flow, the dynamic viscosity of bitumen was measured on a viscometer (DV-II+ Pro, Brookfield) and a dynamic shear rheometer (C-Vor 200, Bohlin Instruments). For temperatures form 70 °C to 120 °C in 10 °C increments, viscosity was measured by a viscometer and for 40 °C to 80 °C in 10°C increments, viscosity was determined by a dynamic shear rheometer in parallel plate mode with plate diameter of 20 mm at a shear stress of 0.2 Pa. The height of bitumen between the parallel plates was 1 mm. The average of two measurements per instrument were used to obtain a viscosity against temperature plot. According to [34, 35] viscosity results are independent of the particular type or make. Therefore, the two methodologies used are comparable.
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2.7.
Optical microscopy
To approximately characterise the width of cracks, a beam was cut into two halves parallel to the long side of the beam, broken and assembled in the same procedure as described before. The crack was magnified by an optical microscope (Nikon eclipse LV100ND, control Nikon LV-NCNT2) with different objective lenses (5x/0.15, 10x/0.3 and 50x0.80). The distance between the crack surfaces was measured at several positions and pictures were taken by an implemented camera (Nikon digital sight DS-Ri1).
2.8.
Theoretical background on asphalt self-healing
In [28], an analytical model was developed to predict the self-healing levels of asphalt mixture. It was based on the assumption that the drain of bitumen into a crack during the asphalt self-healing can be modelled as the flow of bitumen through a parallel plate capillary tube by using the Navier-Stokes equation. Assuming bitumen drains into the crack mainly due to capillary pressure γ
θ/a, hydrostatic and additional pressure p and is opposed by viscous losses
3ηlc/a2(dh/dt) and the hydrostatic pressure of the bitumen inside the crack ρgl until an equilibrium is reached. In this study, we will identify the thermal expansion of bitumen as the source for the additional pressure. Neglecting inertia in the Navier-Stokes Equation [36] and assuming a linear dependency of the dynamic contact angle on the velocity of bitumen [37], leads to the following simplified equation for bitumen flowing through a vertical parallel plate capillary: 3
(4)
where γ (N·m-1) is the surface energy of material in the crack; ß (kg·m-1·s-1) is a friction coefficient derived from the dynamic contact angle θ (rad); l (m) is the length/height flown through by the material in the crack; t (s) is time; a (m) is half the width of the crack; η (Pa·s) is the viscosity of the flowing material within the crack; lc (m) is a constant length, accounting for the material permeable walls of the fracture (draining of material into the crack); ρ (kg·m-3) is the density; g (m·s-2) is the gravity acceleration; and p (Pa) is an additional pressure, which could be caused by extra loading, or thermal expansion. Proceeding from Equation 4 by following the derivations described in [28], the self-healing level for a prismatic beam is calculated as:
(5) with
7
(6)
(7)
3
(8)
where S0 (kN) is the initial healing level due to adhesion force Fadhesive; σu (N·m-2) is the maximum stress resisted by the beam; cp (-) are the number of contact points of the crack surfaces, which were assumed to be constant; n (m) is the vertical distance from any contact point to the beam’s neutral axis; l0 (m) is the initial radius of the contact points; L (m) is the distance between the supports for three-point bending; and H (m) is the vertical distance between the healing fronts furthest apart. Equation 5 was used to model the self-healing of the mortar beams tested.
3.
Results
3.1.
Influence of temperature on the surface energy of bitumen
It is well known that the surface energy of bitumen is affected by temperature, decreasing as temperature increases [38]. Further, the contact angle and the equilibrium height are as well temperature dependent, which was identified for long chain n-alkenes as contact angle decreases with increasing temperature [39]. This could have deep implications on the capacity of bitumen to fill cracks in asphalt mixtures during self-healing, as the wetting affinity is a function of contact angle and surface energy [40]. Though, wettability is mostly determined by the surface texture [41], which refers to the microscopic features from the material that may affect the contact angle between bitumen and the walls of the crack or the glass capillary tube. Figure 3 shows the influence of temperature on equilibrium height, contact angle measured and the surface energy calculated, which was calculated by the use of Equation 3. The equilibrium height and the contact angle for bitumen in a capillary tube decreases approximately linearly with temperature (Figure 2(a)). The surface energy, which was calculated from the equilibrium heights of 4 capillaries per temperature, showed a significant linear dependency with temperature (Figure 2(b)). According to these results and from Equation 5 to 8, increasing temperature will have an adverse effect regarding surface energy and contact angle on asphalt self-healing, leading to a lower selfhealing level. Therefore, a reduced surface energy and contact angle allows better wetting of crack surfaces through mineral aggregates.
8
(a)
(b)
Figure 2: (a) Temperature dependency of the equilibrium height measured, shown for 0.5 mm capillary diameter, and contact angle measured. (b) Linear dependency of surface energy calculated with temperature.
3.2.
Influence of temperature on the viscosity of bitumen
Figure 3(a) shows the change of the viscosity of bitumen with temperature for the methods used. The results of both methods are comparable due to the high variation obtained by the dynamic shear rheometer (DSR). Furthermore, Figure 3(b) presents the Arrhenius plot for the influence of temperature on the viscosity of the bitumen used. This graphic suggests a linear relationship, which confirms that the effect of temperature on the viscosity can be represented by the Arrhenius equation in the measured temperature range, similar to other liquids [31]. For a wider temperature range the Williams-Landel-Ferry model [3] or the Masuko-Magill model [42] provide a more accurate viscosity temperature relationship.
9
l
(a)
33
3
(b)
Figure 3: (a) Viscosities of the bitumen obtained through viscometer (SP) and dynamic shear rheometer (DSR) plotted against temperature and (b) Arrhenius plot for the viscosities of the bitumen studied.
The activation energy found for bitumen pen 70/100 was 107,8 kJ·mol-1, which is in the range of values obtained in the literature, such as 116.52 kJ·mol-1 for bitumen pen 70/100 [16], 69,1 kJ·mol-1 for bitumen pen 85/100 and 88,5 kJ·mol-1 for bitumen pen 0 [43]. The activation energy increases with increasing viscosity, due to increased asphaltene content [44], polymer modification or ageing [43], which is explains why the self-healing of asphalt is increasingly more difficlult with increasing viscosity of bitumen [19].
3.3.
Influence of temperature on the unconfined capillary flow of bitumen
Washburn [45] was among the first to formulate an equation for the liquid rise h(t) in a cylindrical capillary. The complex analytical solution for a vertical capillary can be approximated by a square root function for the short-time limit and an asymptotic function for the long-time limit [46]:
(9)
where r (m) is the radius of the capillary tube. Short-time refers to the asymptotic function if time approaches 0. Longtime is the asymptotic approaches of the equilibrium capillary at infinite times. In this study we assumed that the drain of bitumen into a crack is equivalent to the flow of bitumen through a capillary tube. In Figure 4(a) the heights reached by bitumen in an unconfined capillaries at different temperatures against time are fitted by Equation 9. The approximation by Equation 9 is accurate for the short and long-time limit solution in Figure 4(b), although differences between theory and measurements may derive from the assumption of a static contact
10
angle and not accounting for pining effects, which were observed only for the 1 mm capillary diameter at 100 °C (Figure 4(b)).
(a)
(b)
Figure 4: (a) Measured height increase with time for a capillary with diameter of 0.5 mm at a range of temperatures and (b) for capillaries at 100 °C for different diameters. The data points were fitted with Equation 9, which is accurate for the short-time limit for 0.5 mm capillary at 60 °C and the 0.1 mm capillary at 100 °C and long-time limit.
Furthermore, in Figure 4(a) it can be seen that the capillary flow increases with higher temperatures, as the viscosity decreases. This is in coherence with Equation 9. In addition, regarding Equation 5, it can be seen that reducing the viscosity results in an increase of the self-healing rate, as viscosity influences the exponent in the equation. It may be considered as the main factor affecting the asphalt self-healing rate. The dependence of capillary flow on the capillary radius is shown in Figure 4(b) for 100 °C. According to Washburn’s equation [47] and Equation 9, the flow velocity in a capillary tube is proportional to the square of the radius. The reduced rise in height in a capillary tube resulting from a reduced flow can be clearly seen for the 0.1 mm capillary compared to the other capillary diameters used.
11
l
l
3
(a)
(b)
Figure 5: (a) Arrhenius plot of times for bitumen to reach 3 mm respectively 10 mm in a 0.3 mm capillary for different temperatures. (b) Activation energy of bitumen to reach 3mm and 10mm through capillary tubes with a range of diameters.
In addition, the time required by bitumen to reach 3 mm and 10 mm height in a 0.3 mm capillary tube at a range of temperature between 60 °C and 140 °C was used to generate an Arrhenius plot, see Figure 5(a). The activation energy changed with the height of the bitumen in a 0.3 mm capillary tube, with 41.0 kJ·mol-1 and 51.7 kJ·mol-1 for 3 mm and 10 mm height, respectively. This is shown in Figure 5(b) and is due to the influence of the flow rate on the capillary diameter. Activation energy decreases with increasing capillary diameter, which is an effect that was previously reported in [16] and further, is in agreement with the decrease of activation energy for flow with increasing gap size between spindle and container wall of a viscometer [43]. Hence, longer or wider cracks in asphalt require more energy to self-heal than shorter or narrower cracks. As a conclusion, two main aspects have been identified to characterise the self-healing capacity of an asphalt mixture. The first aspect is the critical radius. Capillary flow and self-healing occur below the critical radius, because an increasing radius implied a decrease in the equilibrium capillary height. The second aspect is the total crack length. If the crack is very long, it will not self-heal, as it requires a high amount of energy to heal. Both criteria, which are interdependent, will depend of the viscosity, contact angle and surface tension of bitumen and the type of aggregates, which will affect the movement of bitumen. This implies that simplified criteria for self-healing, such as the Newtonian temperature threshold [48] can only be used quantitatively to compare mixtures with the same type of aggregates and gradation.
3.4.
Influence of temperature on the confined capillary flow of bitumen
In the confined capillary flow experiment, shown in Figure 2(b), a temperature increase resulted in the development of an internal pressure inside the culture test tube, due to thermal expansion [33]. This internal pressure contributed to the
12
flow of bitumen in the capillary. Hence, pressure causes an accelerated movement of the meniscus as shown in Figure 6. Figure 6(a) shows the increase of flow rate with increasing temperature. The flow rate observed for the confined capillaries in Figure 6(a) is several times higher than the flow rate for the unconfined capillary flow in Figure 4(a). Furthermore, the pressure developed caused that the equilibrium height exceeded the capillary length of 10 cm. In addition, Figure 7(b) shows the influence of the capillary diameter on the flow rate of bitumen. Findings were similar to the results of the unconfined flow experiment and in according to Equation 9. The flow through a capillary tube is reduced as the diameter decreases.
(a)
(b)
Figure 6: (a) Movement of the bitumen meniscus for the confined capillary experiment in a 1 mm capillary tube at different temperature. (b) The influence of different diameters on the movement at 80 °C.
In addition, the activation energy for confined capillary flow was calculated similarly to the unconfined flow experiment. Unlike to the open container of the unconfined capillary flow experiment, the confined capillary experiment with a sealed container was not in thermal equilibrium, because the temperature at the beginning and at the end of the test had to change in order to quantify thermal expansion, which only will provide an approximated value for the activation energy. The reason for this is that the experiment had to start at ambient temperature and reach the oven temperature in a few minutes. The approximate activation energies calculated for the 1 mm capillary diameter for different heights is shown in Figure 7. The activation energies were 18.4 kJ·mol-1 for 30 mm, 24.3 kJ·mol-1 for 50 mm and 30.1 kJ·mol-1 for 70 mm height to reach in the capillaries. The activation energy increases with increasing height, which shows that longer cracks will require more energy to be filled. A comparison of activation energy for a 1.0 mm capillary tube at 10 mm height for unconfined conditions (46.2 kJ·mol-1) and 20 mm height for confined conditions (11.5 kJ·mol-1) showed that the activation energy for the unconfined flow is approximately 4 times higher.
13
l
3333
3
l
l
Figure 7: Arrhenius plot of time for bitumen to reach 30 mm, 50 mm and 70 mm in a 1.0 mm capillary for 30 mm, 50 mm and 70 mm height.
3.5.
Microscope images of the crack
The microscope images of the crack showed that the average crack width was 101 µm. Figure 8(a) shows the crack through an aggregate after self-healing. 40 °C resulted that cracks through aggregates that were not filled with bitumen. At temperatures above 60 °C, the cracks were partly filled with bitumen. In addition, Figure 8(b) shows a cohesive crack through mortar after initially assembling the beam halves. The not perfectly fitting crack sides may be due to losses of material due to fracturing of the beams at low temperature, which led to contact points and vacancies during the assembling of the beam halves.
14
mastic/mortar
crack
crack
aggregate
mastic/mortar
aggregate
500 μm (a)
500 μm (b)
Figure 8: Microscope images of the crack (a) through an aggregate of about 44 µm width after self-healing at 40 °C for 1 day and (b) through mortar with a crack width of about 126 µm at the initial start of self-healing. The images shown were located in the lower third of the beams.
3.6.
Influence of temperature variations on the self-healing of asphalt mortar beams
The self-healing level evolution with time for asphalt mortar showed a progression similar to that of bitumen moving through a capillary tube, which was fitted by Equation 5, as shown in Figure 9(a). The self-healing level at time 0 s ranged from 0.01 to 0.02, due to the adhesion effect of bitumen at the initial contact points where the beam halves were assembled. A rapid increase of the self-healing level up to a maximum of 0.11 was observed for the first 60 seconds of healing. This may be due to stress relaxation and the coalescence of bitumen at the contact points, which increases the contact area. Finally, the self-healing level increased until a steady stage is reached, where the self-healing level of the beams did not show further improvement. The self-healing level did not reach 1 because the cracks were partly through mineral aggregates. Furthermore, the steady state self-healing level was temperature dependant and, as the temperature increased, it increased as well, as it is shown in Figure 9(b). This is similar to the confined capillary experiment, where higher meniscus heights were reached for higher temperature increases.
15
3
(b)
(a)
Figure 9: (a) Self-healing levels obtained for temperature from 40 °C to 120 °C and fitted with Equation 5 and (b) the average selfhealing level of the steady stage. Error bars represent the standard deviation.
The time to reach the steady self-healing stage is shorter for higher temperatures and was in the range of 1.6 hours for 120 °C and approximately 5 hours for 40 °C. Comparing these times to those required by the capillary experiments in Table 2, it can be concluded that the self-healing is in the range of unconfined capillary flow for 80 °C till 120 °C. However, for 60 °C asphalt self-healing is faster than the unconfined capillary flow, but still slower than the confined capillary flow. This is a first evidence that the self-healing of mortar beams ranges between confined and unconfined capillary flow, considering that smaller capillary diameters required more time to reach the equilibrium height. The capillary pressure built during heating, as it was proposed in [16], is one of the main factors affecting asphalt selfhealing. Table 2: Comparison of times to reach the steady self-healing stage respectively the equilibrium capillary height for 1 mm capillary
temperature 120 °C 100 °C 80 °C 60 °C 40 °C
time [hours] steady selfunconfined healing stage capillary 1.6 1.25 3 4.2 3 3.6 4 24 5
16
outlier
l
3
l
3
(b)
(a)
Figure 10: (a) Arrhenius plot of time for aspahlt mortar beams to reach a self-healing level of 0.15 and 0.40 and (b) the dependency of the activation energy to reach a defiend self-healing level. The outlier (marked) at self-healing level of 0.15 and 0.6 are due to the convergen of healing function at the beginning and the assumptotic approch to the steady stage.
The activation energy for self-healing was obtained by generating Arrhenius plots with times required to reach a defined self-healing level. Figure 10(a) shows the Arrhenius plot for two self-healing levels, namely 0.15 and 0.40. The activation energies obtained were 25.4 kJ·mol-1 for 0.15 and 22.9 kJ·mol-1 for 0.40. A slight dependency of the activation energy with self-healing level was noticed, as shown in Figure 10(b). The outlier are due to the scatter during the beginning of self-healing and the convergence to the steady stage self-healing level at for higher defined self-healing levels with the reduction in points for the regression. The activation energy calculated for the self-healing was comparable to the activation energy calculated for the confined capillary flow, which is summarised in Table 3. Table 3: Comparison of activation energies
activation energy [kJ·mol-1] highest value lowest value average value
unconfined capillary flow 60.0 30.1 46.6
confined capillary flow 30.2 11.5 23.9
asphalt mortar self-healing 34.7 16.4 21.7
Furthermore, the crack size was on average 101 µm and hence, is in the range of the smallest capillary diameter tested for the unconfined conditions. The additional pressure in the confined capillaries reduced the activation energy to a similar range as self-healing of asphalt mortar. Moreover, the distance, which bitumen must flow through is related to the self-healing level, which influences the activation energy. Hence, shorter crack lengths may require less energy for healing. Based on the findings in this study and from the literature, a scheme of the self-healing process is shown in Figure 11. After the crack initiation, the surface of bitumen would smoothen due to minimising the surface energy [39], which results in a texture and colour change of the crack surface [49]. Bringing both crack surfaces together as the beam
17
halves are assembled, creates the first initial contact points [16] (Figure 11(a) and 11(b)). Starting from these contact points, bitumen drains into the crack due to surface energy and gravity [16]. In addition, while temperature increases, bitumen and aggregates expand [50]. As the volumetric thermal expansion coefficient for bitumen (for example, 6104
K-1) is 1 to 2 magnitudes higher than that for mineral aggregates, depending on the mineral composition (for example,
limestone 610-6 K-1, quartz 3010-6 K-1) [1, 51], the expanding bitumen may promote the formation of additional contact points (Figure 11(b)) and promotes the flow into cracks (Figure 11(c)). Reduced viscosities at higher temperatures increase the self-healing rate of asphalt. After thermal equilibrium is reached, pressure due to thermal expansion may be reduced due to relaxation and creep [52]. A prolonged time at the steady state temperature may reduce the self-healing level, as bitumen is drained to the bottom of the mixture due to gravitational flow, which reduces the load bearing properties of asphalt [16, 24]. Furthermore, during cooling, contraction of bitumen and aggregates occurs. The volumetric changes of asphalt due to temperature are significantly affected by the aggregate type and depend of the temperature, as the expansion coefficient can be slightly larger than the contraction coefficient [27, 53, 54]. As a result of the flow and drainage of bitumen into the cracks during thermal expansion, air voids can be generated during cooling, due to the contraction of the materials and the preference of bitumen to remain in the smaller pores, including cracks (see Figure 11(d)). bitumen
a
b
thermal expansion and capillary pressure
crack
d
Cooling
Heating aggregate
c
contact point
partially filled crack
new air void due to bitumen’s contraction and migration due to gravity
Assembling of the beam
During
heating,
halves. Initial adhesion is
points
are
contact
Further heating increases
During cooling,
as
the pressure due to thermal
contracts. Thus, new air
caused by initial contact
bitumen and to a minor
expansion and enhances the
voids can be created.
points.
degree aggregates expand.
flow of bitumen into the
The start of bitumen flow,
crack.
created
bitumen
draining into the crack. Figure 11: Schematic diagram of the self-healing process induced by uniform heating.
4.
Summary and conclusions
This study showed that thermal expansion of bitumen is one of the main contributors to the self-healing of asphalt mixtures by comparing activation energies for the flow of bitumen in capillaries to the activation energy of self-healing. Therefore, three different main tests were performed to determine the activation energies for bitumen flow and asphalt self-healing. The first two tests determined the flow of bitumen (i) at atmospheric pressure through a capillary tube and
18
(ii) at an increased pressure, due to its thermal expansion, through a capillary tube. The third test investigated the healing of asphalt mortar beams, induced by heat in a convection oven. The following conclusions were obtained:
The surface energy and the contact angle of bitumen decreased linearly with increasing temperature, resulting in a linear decrease of capillary height with temperature. The decreasing contact angle entails that at higher temperature the wetting of surfaces by bitumen is promoted.
The activation energy decreases with increasing capillary diameter and decreasing capillary height, meaning that bitumen flows faster in wider cracks and requires less energy for self-healing of shorter cracks. In addition, the activation energy for bitumen to flow through a capillary was significantly decreased if bitumen was confined, due to the additional pressure caused by thermal expansion. The self-healing performance of mortar beams was improved at higher temperatures, because of (i) increased pressure due to a higher temperature difference and (ii) lower viscosity.
Reaching higher self-healing levels requires more energy, similar to the higher energy required by bitumen to flow through a longer distance in the capillary tube. Furthermore, the activation energy for self-healing is in the same range as the activation energy for the confined capillary flow of bitumen. The authors hypothesised that the thermal expansion of bitumen and the pressure developed is a main factor affecting asphalt self-healing.
This study provides evidence on the contribution of the thermal expansion of bitumen to the self-healing of asphalt and a theory of how the thermal expansion affects the self-healing. The authors believe that in order to advance in this field and optimise asphalt for self-healing, research must continue by modelling the phenomenon using Multiphysics software, to determine the exact interaction between all the mentioned factors above.
5.
Acknowledgements
The authors like to acknowledge that this research received funding from Infravation under the grant agreement no. 31109806.0003 – Healroad. Further, the authors like to thank the Faculty of Engineering at the University of Nottingham for the research fellowship granted to one of the authors to undertake a PhD.
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22
S1359-4311(18)36491-3 https://doi.org/10.1016/j.applthermaleng.2019.04.034 ATE 13624
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
23 October 2018 26 March 2019 8 April 2019
Please cite this article as: D. Grossegger, A. Garcia, Influence of the thermal expansion of bitumen on asphalt selfhealing, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.04.034
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of the thermal expansion of bitumen on asphalt self-healing D. Grossegger a, A. Garciaa*
a
Nottingham Transportation Engineering Centre, Department of Civil Engineering, University of Nottingham, Nottingham NG7 2RD, United Kingdom
* Corresponding author. Tel: +44 (0) 0115 95 13914. E-mail addresses: [email protected]
Abstract: The self-healing of cracks in asphalt mixture is mainly due to the drain of the bitumen contained in the space between the aggregates, into the cracks. Until now, it was believed that the physical principles affecting the flow of bitumen are influenced by gravity and surface energy of bitumen and the aggregates. In this study, we show that the thermal expansion of bitumen plays an important role in the self-healing of asphalt mixture. To demonstrate this, asphalt mortar beams were manufactured and broken in two pieces by means of three-point bending tests. Self-healing was induced in asphalt mixture by increasing its temperature using a convection oven, at temperatures that ranged from 40 °C to 120 °C. The self-healing ratio was calculated by comparing the force required to break the test specimens before and after heating. Furthermore, a test was designed that consisted of bitumen raising through a capillary tube from a bitumen container. To account for the effect of thermal expansion, the bitumen container was fully enclosed except for the capillary tube. To account for the effect of surface energy on the bitumen’s capillary flow, the capillary tube was placed in a container that was open to the atmosphere. The rise of bitumen was monitored at temperatures that ranged from 40 °C to 120 °C. Finally, activation energies were derived from the rise of bitumen in the capillaries, viscosity changes and the selfhealing progression. It was found that the activation energy of asphalt self-healing is similar to that of bitumen rising due to thermal expansion, which confirms the contribution of thermal expansion on asphalt self-healing by the effect of increasing temperature. Keywords: asphalt mortar, self-healing, activation energy, induced heating, thermal expansion
Highlights: Surface energy and contact angle decrease with increasing temperature. Pressure due to thermal expansion reduced the activation energy for bitumen to flow through a capillary. The activation energy of self-healing for asphalt mortar is similar to the activation energy for confined capillary flow of bitumen.
1
1.
Introduction
Bituminous mixtures are well-established composite materials [1], which are mainly used for road constructions and other applications, such as roof constructions [2]. The mixtures used for road constructions consist of mineral aggregates, bitumen and a range of bitumen additives. The size of the mineral aggregates ranges from a few microns up to approximately 30 mm. Bitumen is the adhesive component in the asphalt mixture and is a viscoelastic liquid, which viscosity is temperature-dependant [3]. To produce hot or warm mixed asphalt, the aggregates and bitumen are mixed above 130 °C and compacted at a sufficiently high temperature. Moreover, thanks to the addition of additives, asphalt can be also produced and processed at lower temperatures [1]. Temperature changes, moisture, radicals, radiation and cyclic traffic loads may induce stresses [4, 5], ageing [6, 7] and stripping [8, 9] in asphalt road structures, mainly affecting the surface layer. Furthermore, due to ageing and the stresses induced by excessive traffic and decrease to low temperature cracks may develop in road structures [1]; these cracks may start at the bottom or at the top of the road structures, or at a material level, at the interface between mineral aggregates and bitumen. On the other hand, asphalt is a self-healing material. When a crack develops in the pavement, it may heal if enough time without traffic loading is allowed [10]. The reason for this is that bitumen can drain from the mixture into the cracks. The ratio of crack self-healing will depend of the length and width of the cracks, aggregate type and gradation, and on the viscosity of bitumen [11-14] and will vary depending on the environmental temperature, because the viscosity of bitumen is temperature-dependant. In addition, a limitation for rapid asphalt self-healing is due to the high viscosity of bitumen during service life, which delays the drain of bitumen into the cracks. Hence, the drain of bitumen into cracks may require several days to be completed and cracks may develop faster than they are able to self-heal. Finally, only cracks that are not caused by a structural failure of the road, such as those starting at the top and at the interface between the mineral aggregates and bitumen, are of importance for self-healing. The draining of bitumen into cracks occur above a certain threshold temperature. And identified driving mechanism for the bitumen flow into cracks are surface energy [15] and gravity [16]. The threshold temperature for flow has been correlated to either the glass transition temperature of bitumen [17] or the temperature when the flow behaviour of bitumen changes from non-Newtonian to Newtonian [18]. In addition, factors affecting the flow of bitumen are its fractional composition, temperature, ageing and additives content [1, 3]. Finally, the self-healing properties of asphalt at a certain temperature improve with the reductions in the viscosity of bitumen [19]. Therefore, to trigger and enhance the self-healing recovery of asphalt, the viscosity of bitumen must be reduced and to do that, two technologies have been developed. The first one, heating by magnetic or electromagnetic energy, increases the asphalt mixture’s temperature by indirectly heating metallic particles embedded in the asphalt [20-23]. In this technology, the temperature of bitumen may increase locally above 100 °C in less than 30 s and macroscopic cracks are
2
repaired in periods of time that range from seconds to minutes [22]. Furthermore, in [24], it has been hypothesized that the local heat generation in the metallic particles is responsible for bitumen to locally expanse and to flow into the crack, which accelerates the self-healing greatly. The second technology makes use of encapsulated bitumen solvents embedded in the asphalt mixture [25, 26]. Therefore, bitumen can easily flow and drain into the cracks. Thermal expansion in asphalt road structures is considered for thermal cracking, as bitumen becomes stiffer at lower temperatures crack can occur due to the thermal induced stress and the reduced relaxation characteristic [1]. The linear thermal expansion coefficient of asphalt is affected by the bitumen content, aggregate type and aggregate quality [27]. Furthermore, the expansion of bitumen is indirectly considered during the asphalt mixture design, as the amount of bitumen determines the air void content, density, deformation and stability of the compacted mixture [1]. The aim of this study is to demonstrate the effect of thermal expansion of bitumen on the self-healing of an asphalt mixture. The main hypothesis of the study is that the activation energy required by asphalt to self-heal through the effect of temperature increase will be: (i)
comparable to the activation energy for bitumen to flow through a capillary tube due to the effect of thermal expansion and,
(ii)
lower than the activation energy for bitumen to rise in a vertical capillary tube due to the effect of surface energy.
To demonstrate these points, the activation of energy for asphalt mixture self-healing has been calculated and compared to the activation energy of bitumen for confined and unconfined capillary flow.
2. 2.1.
Materials and methods Materials
The asphalt mortar mixture consisted of bitumen 70/100 and crushed limestone aggregates. The bitumen was obtained from Total Petroleum and had a needle penetration of 73 dmm, a softening point of 46.4 °C and a density of 1.02 gcm-3 at 25 °C. The gradation of the limestone aggregates is provided in Table 1. The maximum particle size was 4 mm and the aggregates had a density of 2.36 gcm-3. To manufacture the mortar mixture, bitumen and aggregates were heated to 180 ˚C and manually mixed together in the proportion of 15 wt.% bitumen and 85 wt.% aggregates. Prismatic beams with dimensions 0.10 x 0.03 x 0.03 m3 were manufactured from the mixture by pouring it into polytetrafluorethylene (PTFE) moulds. The moulds had a rectangular notch in the middle that created a predetermined breaking point in the beams. The mixture was manually compacted and let to cool down to ambient temperature (20 °C ±2 °C) for at least 1 hour. Afterwards, the beams were demoulded and stored at -17 °C ± 1°C in a freezer, to ensure a brittle and clean fracture during the three-point bending test.
3
Table 1: Aggregate gradation
sieve size [μm] 4000 2800 2000 1000 500 250 125 63
2.2.
percent passing [wt.%] sand 1 100.0 99.8 90.9 65.4 43.5 19.6 5.6 1.8
Asphalt mortar’s self-healing characterisation
Asphalt self-healing was characterised according to the procedure described in [28]. To create a brittle fracture that can self-heal, the beams were tested under three-point bending while they were still at -17 °C ±1 °C. The three-point bending force was centred vertically above the breaking notch and applied at the constant rate of 10.2 Ns-1 ± 0.6 Ns-1. After breaking the beams in two halves, they were let to rest at ambient temperature (20 °C ±2 °C) for a minimum time of 16 h. The initial strength recovery, which represented the first seconds of contact without heating, was measured by assembling the broken beam halves by hand to its initial geometry at ambient temperature, with care of not applying any additional pressure, and subjecting them afterwards to the three-point breaking test. The duration from assembling to testing was less than 10 s and took on average 5 s. For the heat induced self-healing, beams were placed inside the manufacturing moulds and introduced in a convection oven (Hot Box oven, Gallenkamp), to accelerate the self-healing process, for times that ranged from 60 s to 101 h. The temperatures at which the oven was set were 40 °C, 60 °C, 80 °C, 100 °C and 120 °C. Furthermore, the average surface temperature of the test samples was measured with an infrared camera (micro-epsilon, TIM 160, UFPA, 160x120 pixels) after the heating period. Immediately after heating, the beams within the moulds were stored at -17 °C, to prevent any further self-healing. Afterwards, the beams were broken again by three-point breaking testing. The healing level (S) was defined as the ratio between the maximum breaking load after (FH) and before the heating period (FI). (1)
2.3.
Flow of bitumen through a capillary tube
To account for the influence of thermal expansion and surface energy of bitumen on asphalt self-healing, the movement of bitumen inside a capillary tube of 1 mm diameter was monitored. With this purpose, two different set-ups were used, see Figure 1(a) and 1(b).
4
2r ϑ
Capillary pressure and pressure due to thermal expansion
2r
ϑ
Capillary pressure
Petri dish (a) First set-up: open container
Culture test tube, 15 ml (b) Second set-up: sealed container
Figure 1: Scheme of the capillary flow test (a) without and (b) with thermal expansion pressure.
The first set-up was designed to test the influence of surface energy on the flow of bitumen through a capillary tube (Figure 1(a)). It consisted of a capillary tube glued perpendicular to the bottom edge of a clear glass tile. Afterwards, the glass tile with the capillary was introduced vertically on a petri dish filled with bitumen at the testing temperatures, 60 °C, 80 °C, 100 °C, 120 °C and 140 °C, respectively. The whole setup was stationed inside a convection oven with glass door at the testing temperature. The bitumen inside the capillary tube was recorded with a full HD camera (AW920, Ausdom). The second set-up was designed to test the influence of bitumen’s thermal expansion on its flow through a capillary tube (Figure 1(b)). To do that, a capillary tube was inserted in a 15 ml culture test tube that was fully filled with bitumen, through its lid. Any gap between the capillary and the lid was sealed with silicone, so that bitumen was hermetically enclosed in the capillary and no air remained inside the culture test tube. To quantify the effect of thermal expansion, the culture test tube with the bitumen at the ambient temperature was placed inside a convection oven. The rise of bitumen inside the capillary tube was recorded with a full HD camera (AW920, Ausdom). Different oven temperatures were tested: 40 °C, 60 °C, 80 °C and 100 °C, respectively.
2.4.
Calculation of the activation energy
The Arrhenius equation can be used to calculate activation energy of temperature dependent processes [29], including the self-healing of the mortar beams fractured, and the viscosity and movement of bitumen in the capillary tubes [16]. The Arrhenius-type relation [30], originating form molecular kinetics, is shown below:
(2)
5
where k is a rate coefficient, which is substituted by viscosity respectively time for the calculations; A is the preexponential factor; EA (Jmol-1) is the activation energy; R (8.314 JK-1mol-1) is the universal gas constant; and T (K) is the absolute temperature. To obtain the activation energy, the parameters (time or viscosity), were plotted in a logarithmic graph against the reciprocal of absolute temperature. The slope of the linear regression is equal to EA/R [31].
2.5.
Surface energy of bitumen
The surface energy of bitumen was calculated from the equilibrium heights of the capillaries (Figure 1(a)). According to Jurin’s law, the equilibrium height of the meniscus h can be calculated from the equilibrium between capillary pressure and hydrostatic pressure [32]:
(3) where γ (N·m-1) is the surface energy; θ (rad) is the contact angel between glass capillary and bitumen; ρ (kg·m-3) is the density; g (9.81 m·s-2) is the gravity acceleration; and r (m) is the capillary radius. Equation 3 was solved for the surface energy under the assumption of a linear decreasing density with increasing temperature [33]. The contact angle was determined with an accuracy of 5° from the external contact of the glass capillary with the surface bitumen in the petri dish, which is the same as the contact angle between bitumen and glass inside the capillary tube. The pictures obtained were analysed with a graphic program (Gimp 2.8.14, GNU Image Manipulation Program).
2.6.
Viscosity
In order to calculate the activation energy for viscous flow, the dynamic viscosity of bitumen was measured on a viscometer (DV-II+ Pro, Brookfield) and a dynamic shear rheometer (C-Vor 200, Bohlin Instruments). For temperatures form 70 °C to 120 °C in 10 °C increments, viscosity was measured by a viscometer and for 40 °C to 80 °C in 10°C increments, viscosity was determined by a dynamic shear rheometer in parallel plate mode with plate diameter of 20 mm at a shear stress of 0.2 Pa. The height of bitumen between the parallel plates was 1 mm. The average of two measurements per instrument were used to obtain a viscosity against temperature plot. According to [34, 35] viscosity results are independent of the particular type or make. Therefore, the two methodologies used are comparable.
6
2.7.
Optical microscopy
To approximately characterise the width of cracks, a beam was cut into two halves parallel to the long side of the beam, broken and assembled in the same procedure as described before. The crack was magnified by an optical microscope (Nikon eclipse LV100ND, control Nikon LV-NCNT2) with different objective lenses (5x/0.15, 10x/0.3 and 50x0.80). The distance between the crack surfaces was measured at several positions and pictures were taken by an implemented camera (Nikon digital sight DS-Ri1).
2.8.
Theoretical background on asphalt self-healing
In [28], an analytical model was developed to predict the self-healing levels of asphalt mixture. It was based on the assumption that the drain of bitumen into a crack during the asphalt self-healing can be modelled as the flow of bitumen through a parallel plate capillary tube by using the Navier-Stokes equation. Assuming bitumen drains into the crack mainly due to capillary pressure γ
θ/a, hydrostatic and additional pressure p and is opposed by viscous losses
3ηlc/a2(dh/dt) and the hydrostatic pressure of the bitumen inside the crack ρgl until an equilibrium is reached. In this study, we will identify the thermal expansion of bitumen as the source for the additional pressure. Neglecting inertia in the Navier-Stokes Equation [36] and assuming a linear dependency of the dynamic contact angle on the velocity of bitumen [37], leads to the following simplified equation for bitumen flowing through a vertical parallel plate capillary: 3
(4)
where γ (N·m-1) is the surface energy of material in the crack; ß (kg·m-1·s-1) is a friction coefficient derived from the dynamic contact angle θ (rad); l (m) is the length/height flown through by the material in the crack; t (s) is time; a (m) is half the width of the crack; η (Pa·s) is the viscosity of the flowing material within the crack; lc (m) is a constant length, accounting for the material permeable walls of the fracture (draining of material into the crack); ρ (kg·m-3) is the density; g (m·s-2) is the gravity acceleration; and p (Pa) is an additional pressure, which could be caused by extra loading, or thermal expansion. Proceeding from Equation 4 by following the derivations described in [28], the self-healing level for a prismatic beam is calculated as:
(5) with
7
(6)
(7)
3
(8)
where S0 (kN) is the initial healing level due to adhesion force Fadhesive; σu (N·m-2) is the maximum stress resisted by the beam; cp (-) are the number of contact points of the crack surfaces, which were assumed to be constant; n (m) is the vertical distance from any contact point to the beam’s neutral axis; l0 (m) is the initial radius of the contact points; L (m) is the distance between the supports for three-point bending; and H (m) is the vertical distance between the healing fronts furthest apart. Equation 5 was used to model the self-healing of the mortar beams tested.
3.
Results
3.1.
Influence of temperature on the surface energy of bitumen
It is well known that the surface energy of bitumen is affected by temperature, decreasing as temperature increases [38]. Further, the contact angle and the equilibrium height are as well temperature dependent, which was identified for long chain n-alkenes as contact angle decreases with increasing temperature [39]. This could have deep implications on the capacity of bitumen to fill cracks in asphalt mixtures during self-healing, as the wetting affinity is a function of contact angle and surface energy [40]. Though, wettability is mostly determined by the surface texture [41], which refers to the microscopic features from the material that may affect the contact angle between bitumen and the walls of the crack or the glass capillary tube. Figure 3 shows the influence of temperature on equilibrium height, contact angle measured and the surface energy calculated, which was calculated by the use of Equation 3. The equilibrium height and the contact angle for bitumen in a capillary tube decreases approximately linearly with temperature (Figure 2(a)). The surface energy, which was calculated from the equilibrium heights of 4 capillaries per temperature, showed a significant linear dependency with temperature (Figure 2(b)). According to these results and from Equation 5 to 8, increasing temperature will have an adverse effect regarding surface energy and contact angle on asphalt self-healing, leading to a lower selfhealing level. Therefore, a reduced surface energy and contact angle allows better wetting of crack surfaces through mineral aggregates.
8
(a)
(b)
Figure 2: (a) Temperature dependency of the equilibrium height measured, shown for 0.5 mm capillary diameter, and contact angle measured. (b) Linear dependency of surface energy calculated with temperature.
3.2.
Influence of temperature on the viscosity of bitumen
Figure 3(a) shows the change of the viscosity of bitumen with temperature for the methods used. The results of both methods are comparable due to the high variation obtained by the dynamic shear rheometer (DSR). Furthermore, Figure 3(b) presents the Arrhenius plot for the influence of temperature on the viscosity of the bitumen used. This graphic suggests a linear relationship, which confirms that the effect of temperature on the viscosity can be represented by the Arrhenius equation in the measured temperature range, similar to other liquids [31]. For a wider temperature range the Williams-Landel-Ferry model [3] or the Masuko-Magill model [42] provide a more accurate viscosity temperature relationship.
9
l
(a)
33
3
(b)
Figure 3: (a) Viscosities of the bitumen obtained through viscometer (SP) and dynamic shear rheometer (DSR) plotted against temperature and (b) Arrhenius plot for the viscosities of the bitumen studied.
The activation energy found for bitumen pen 70/100 was 107,8 kJ·mol-1, which is in the range of values obtained in the literature, such as 116.52 kJ·mol-1 for bitumen pen 70/100 [16], 69,1 kJ·mol-1 for bitumen pen 85/100 and 88,5 kJ·mol-1 for bitumen pen 0 [43]. The activation energy increases with increasing viscosity, due to increased asphaltene content [44], polymer modification or ageing [43], which is explains why the self-healing of asphalt is increasingly more difficlult with increasing viscosity of bitumen [19].
3.3.
Influence of temperature on the unconfined capillary flow of bitumen
Washburn [45] was among the first to formulate an equation for the liquid rise h(t) in a cylindrical capillary. The complex analytical solution for a vertical capillary can be approximated by a square root function for the short-time limit and an asymptotic function for the long-time limit [46]:
(9)
where r (m) is the radius of the capillary tube. Short-time refers to the asymptotic function if time approaches 0. Longtime is the asymptotic approaches of the equilibrium capillary at infinite times. In this study we assumed that the drain of bitumen into a crack is equivalent to the flow of bitumen through a capillary tube. In Figure 4(a) the heights reached by bitumen in an unconfined capillaries at different temperatures against time are fitted by Equation 9. The approximation by Equation 9 is accurate for the short and long-time limit solution in Figure 4(b), although differences between theory and measurements may derive from the assumption of a static contact
10
angle and not accounting for pining effects, which were observed only for the 1 mm capillary diameter at 100 °C (Figure 4(b)).
(a)
(b)
Figure 4: (a) Measured height increase with time for a capillary with diameter of 0.5 mm at a range of temperatures and (b) for capillaries at 100 °C for different diameters. The data points were fitted with Equation 9, which is accurate for the short-time limit for 0.5 mm capillary at 60 °C and the 0.1 mm capillary at 100 °C and long-time limit.
Furthermore, in Figure 4(a) it can be seen that the capillary flow increases with higher temperatures, as the viscosity decreases. This is in coherence with Equation 9. In addition, regarding Equation 5, it can be seen that reducing the viscosity results in an increase of the self-healing rate, as viscosity influences the exponent in the equation. It may be considered as the main factor affecting the asphalt self-healing rate. The dependence of capillary flow on the capillary radius is shown in Figure 4(b) for 100 °C. According to Washburn’s equation [47] and Equation 9, the flow velocity in a capillary tube is proportional to the square of the radius. The reduced rise in height in a capillary tube resulting from a reduced flow can be clearly seen for the 0.1 mm capillary compared to the other capillary diameters used.
11
l
l
3
(a)
(b)
Figure 5: (a) Arrhenius plot of times for bitumen to reach 3 mm respectively 10 mm in a 0.3 mm capillary for different temperatures. (b) Activation energy of bitumen to reach 3mm and 10mm through capillary tubes with a range of diameters.
In addition, the time required by bitumen to reach 3 mm and 10 mm height in a 0.3 mm capillary tube at a range of temperature between 60 °C and 140 °C was used to generate an Arrhenius plot, see Figure 5(a). The activation energy changed with the height of the bitumen in a 0.3 mm capillary tube, with 41.0 kJ·mol-1 and 51.7 kJ·mol-1 for 3 mm and 10 mm height, respectively. This is shown in Figure 5(b) and is due to the influence of the flow rate on the capillary diameter. Activation energy decreases with increasing capillary diameter, which is an effect that was previously reported in [16] and further, is in agreement with the decrease of activation energy for flow with increasing gap size between spindle and container wall of a viscometer [43]. Hence, longer or wider cracks in asphalt require more energy to self-heal than shorter or narrower cracks. As a conclusion, two main aspects have been identified to characterise the self-healing capacity of an asphalt mixture. The first aspect is the critical radius. Capillary flow and self-healing occur below the critical radius, because an increasing radius implied a decrease in the equilibrium capillary height. The second aspect is the total crack length. If the crack is very long, it will not self-heal, as it requires a high amount of energy to heal. Both criteria, which are interdependent, will depend of the viscosity, contact angle and surface tension of bitumen and the type of aggregates, which will affect the movement of bitumen. This implies that simplified criteria for self-healing, such as the Newtonian temperature threshold [48] can only be used quantitatively to compare mixtures with the same type of aggregates and gradation.
3.4.
Influence of temperature on the confined capillary flow of bitumen
In the confined capillary flow experiment, shown in Figure 2(b), a temperature increase resulted in the development of an internal pressure inside the culture test tube, due to thermal expansion [33]. This internal pressure contributed to the
12
flow of bitumen in the capillary. Hence, pressure causes an accelerated movement of the meniscus as shown in Figure 6. Figure 6(a) shows the increase of flow rate with increasing temperature. The flow rate observed for the confined capillaries in Figure 6(a) is several times higher than the flow rate for the unconfined capillary flow in Figure 4(a). Furthermore, the pressure developed caused that the equilibrium height exceeded the capillary length of 10 cm. In addition, Figure 7(b) shows the influence of the capillary diameter on the flow rate of bitumen. Findings were similar to the results of the unconfined flow experiment and in according to Equation 9. The flow through a capillary tube is reduced as the diameter decreases.
(a)
(b)
Figure 6: (a) Movement of the bitumen meniscus for the confined capillary experiment in a 1 mm capillary tube at different temperature. (b) The influence of different diameters on the movement at 80 °C.
In addition, the activation energy for confined capillary flow was calculated similarly to the unconfined flow experiment. Unlike to the open container of the unconfined capillary flow experiment, the confined capillary experiment with a sealed container was not in thermal equilibrium, because the temperature at the beginning and at the end of the test had to change in order to quantify thermal expansion, which only will provide an approximated value for the activation energy. The reason for this is that the experiment had to start at ambient temperature and reach the oven temperature in a few minutes. The approximate activation energies calculated for the 1 mm capillary diameter for different heights is shown in Figure 7. The activation energies were 18.4 kJ·mol-1 for 30 mm, 24.3 kJ·mol-1 for 50 mm and 30.1 kJ·mol-1 for 70 mm height to reach in the capillaries. The activation energy increases with increasing height, which shows that longer cracks will require more energy to be filled. A comparison of activation energy for a 1.0 mm capillary tube at 10 mm height for unconfined conditions (46.2 kJ·mol-1) and 20 mm height for confined conditions (11.5 kJ·mol-1) showed that the activation energy for the unconfined flow is approximately 4 times higher.
13
l
3333
3
l
l
Figure 7: Arrhenius plot of time for bitumen to reach 30 mm, 50 mm and 70 mm in a 1.0 mm capillary for 30 mm, 50 mm and 70 mm height.
3.5.
Microscope images of the crack
The microscope images of the crack showed that the average crack width was 101 µm. Figure 8(a) shows the crack through an aggregate after self-healing. 40 °C resulted that cracks through aggregates that were not filled with bitumen. At temperatures above 60 °C, the cracks were partly filled with bitumen. In addition, Figure 8(b) shows a cohesive crack through mortar after initially assembling the beam halves. The not perfectly fitting crack sides may be due to losses of material due to fracturing of the beams at low temperature, which led to contact points and vacancies during the assembling of the beam halves.
14
mastic/mortar
crack
crack
aggregate
mastic/mortar
aggregate
500 μm (a)
500 μm (b)
Figure 8: Microscope images of the crack (a) through an aggregate of about 44 µm width after self-healing at 40 °C for 1 day and (b) through mortar with a crack width of about 126 µm at the initial start of self-healing. The images shown were located in the lower third of the beams.
3.6.
Influence of temperature variations on the self-healing of asphalt mortar beams
The self-healing level evolution with time for asphalt mortar showed a progression similar to that of bitumen moving through a capillary tube, which was fitted by Equation 5, as shown in Figure 9(a). The self-healing level at time 0 s ranged from 0.01 to 0.02, due to the adhesion effect of bitumen at the initial contact points where the beam halves were assembled. A rapid increase of the self-healing level up to a maximum of 0.11 was observed for the first 60 seconds of healing. This may be due to stress relaxation and the coalescence of bitumen at the contact points, which increases the contact area. Finally, the self-healing level increased until a steady stage is reached, where the self-healing level of the beams did not show further improvement. The self-healing level did not reach 1 because the cracks were partly through mineral aggregates. Furthermore, the steady state self-healing level was temperature dependant and, as the temperature increased, it increased as well, as it is shown in Figure 9(b). This is similar to the confined capillary experiment, where higher meniscus heights were reached for higher temperature increases.
15
3
(b)
(a)
Figure 9: (a) Self-healing levels obtained for temperature from 40 °C to 120 °C and fitted with Equation 5 and (b) the average selfhealing level of the steady stage. Error bars represent the standard deviation.
The time to reach the steady self-healing stage is shorter for higher temperatures and was in the range of 1.6 hours for 120 °C and approximately 5 hours for 40 °C. Comparing these times to those required by the capillary experiments in Table 2, it can be concluded that the self-healing is in the range of unconfined capillary flow for 80 °C till 120 °C. However, for 60 °C asphalt self-healing is faster than the unconfined capillary flow, but still slower than the confined capillary flow. This is a first evidence that the self-healing of mortar beams ranges between confined and unconfined capillary flow, considering that smaller capillary diameters required more time to reach the equilibrium height. The capillary pressure built during heating, as it was proposed in [16], is one of the main factors affecting asphalt selfhealing. Table 2: Comparison of times to reach the steady self-healing stage respectively the equilibrium capillary height for 1 mm capillary
temperature 120 °C 100 °C 80 °C 60 °C 40 °C
time [hours] steady selfunconfined healing stage capillary 1.6 1.25 3 4.2 3 3.6 4 24 5
16
outlier
l
3
l
3
(b)
(a)
Figure 10: (a) Arrhenius plot of time for aspahlt mortar beams to reach a self-healing level of 0.15 and 0.40 and (b) the dependency of the activation energy to reach a defiend self-healing level. The outlier (marked) at self-healing level of 0.15 and 0.6 are due to the convergen of healing function at the beginning and the assumptotic approch to the steady stage.
The activation energy for self-healing was obtained by generating Arrhenius plots with times required to reach a defined self-healing level. Figure 10(a) shows the Arrhenius plot for two self-healing levels, namely 0.15 and 0.40. The activation energies obtained were 25.4 kJ·mol-1 for 0.15 and 22.9 kJ·mol-1 for 0.40. A slight dependency of the activation energy with self-healing level was noticed, as shown in Figure 10(b). The outlier are due to the scatter during the beginning of self-healing and the convergence to the steady stage self-healing level at for higher defined self-healing levels with the reduction in points for the regression. The activation energy calculated for the self-healing was comparable to the activation energy calculated for the confined capillary flow, which is summarised in Table 3. Table 3: Comparison of activation energies
activation energy [kJ·mol-1] highest value lowest value average value
unconfined capillary flow 60.0 30.1 46.6
confined capillary flow 30.2 11.5 23.9
asphalt mortar self-healing 34.7 16.4 21.7
Furthermore, the crack size was on average 101 µm and hence, is in the range of the smallest capillary diameter tested for the unconfined conditions. The additional pressure in the confined capillaries reduced the activation energy to a similar range as self-healing of asphalt mortar. Moreover, the distance, which bitumen must flow through is related to the self-healing level, which influences the activation energy. Hence, shorter crack lengths may require less energy for healing. Based on the findings in this study and from the literature, a scheme of the self-healing process is shown in Figure 11. After the crack initiation, the surface of bitumen would smoothen due to minimising the surface energy [39], which results in a texture and colour change of the crack surface [49]. Bringing both crack surfaces together as the beam
17
halves are assembled, creates the first initial contact points [16] (Figure 11(a) and 11(b)). Starting from these contact points, bitumen drains into the crack due to surface energy and gravity [16]. In addition, while temperature increases, bitumen and aggregates expand [50]. As the volumetric thermal expansion coefficient for bitumen (for example, 6104
K-1) is 1 to 2 magnitudes higher than that for mineral aggregates, depending on the mineral composition (for example,
limestone 610-6 K-1, quartz 3010-6 K-1) [1, 51], the expanding bitumen may promote the formation of additional contact points (Figure 11(b)) and promotes the flow into cracks (Figure 11(c)). Reduced viscosities at higher temperatures increase the self-healing rate of asphalt. After thermal equilibrium is reached, pressure due to thermal expansion may be reduced due to relaxation and creep [52]. A prolonged time at the steady state temperature may reduce the self-healing level, as bitumen is drained to the bottom of the mixture due to gravitational flow, which reduces the load bearing properties of asphalt [16, 24]. Furthermore, during cooling, contraction of bitumen and aggregates occurs. The volumetric changes of asphalt due to temperature are significantly affected by the aggregate type and depend of the temperature, as the expansion coefficient can be slightly larger than the contraction coefficient [27, 53, 54]. As a result of the flow and drainage of bitumen into the cracks during thermal expansion, air voids can be generated during cooling, due to the contraction of the materials and the preference of bitumen to remain in the smaller pores, including cracks (see Figure 11(d)). bitumen
a
b
thermal expansion and capillary pressure
crack
d
Cooling
Heating aggregate
c
contact point
partially filled crack
new air void due to bitumen’s contraction and migration due to gravity
Assembling of the beam
During
heating,
halves. Initial adhesion is
points
are
contact
Further heating increases
During cooling,
as
the pressure due to thermal
contracts. Thus, new air
caused by initial contact
bitumen and to a minor
expansion and enhances the
voids can be created.
points.
degree aggregates expand.
flow of bitumen into the
The start of bitumen flow,
crack.
created
bitumen
draining into the crack. Figure 11: Schematic diagram of the self-healing process induced by uniform heating.
4.
Summary and conclusions
This study showed that thermal expansion of bitumen is one of the main contributors to the self-healing of asphalt mixtures by comparing activation energies for the flow of bitumen in capillaries to the activation energy of self-healing. Therefore, three different main tests were performed to determine the activation energies for bitumen flow and asphalt self-healing. The first two tests determined the flow of bitumen (i) at atmospheric pressure through a capillary tube and
18
(ii) at an increased pressure, due to its thermal expansion, through a capillary tube. The third test investigated the healing of asphalt mortar beams, induced by heat in a convection oven. The following conclusions were obtained:
The surface energy and the contact angle of bitumen decreased linearly with increasing temperature, resulting in a linear decrease of capillary height with temperature. The decreasing contact angle entails that at higher temperature the wetting of surfaces by bitumen is promoted.
The activation energy decreases with increasing capillary diameter and decreasing capillary height, meaning that bitumen flows faster in wider cracks and requires less energy for self-healing of shorter cracks. In addition, the activation energy for bitumen to flow through a capillary was significantly decreased if bitumen was confined, due to the additional pressure caused by thermal expansion. The self-healing performance of mortar beams was improved at higher temperatures, because of (i) increased pressure due to a higher temperature difference and (ii) lower viscosity.
Reaching higher self-healing levels requires more energy, similar to the higher energy required by bitumen to flow through a longer distance in the capillary tube. Furthermore, the activation energy for self-healing is in the same range as the activation energy for the confined capillary flow of bitumen. The authors hypothesised that the thermal expansion of bitumen and the pressure developed is a main factor affecting asphalt self-healing.
This study provides evidence on the contribution of the thermal expansion of bitumen to the self-healing of asphalt and a theory of how the thermal expansion affects the self-healing. The authors believe that in order to advance in this field and optimise asphalt for self-healing, research must continue by modelling the phenomenon using Multiphysics software, to determine the exact interaction between all the mentioned factors above.
5.
Acknowledgements
The authors like to acknowledge that this research received funding from Infravation under the grant agreement no. 31109806.0003 – Healroad. Further, the authors like to thank the Faculty of Engineering at the University of Nottingham for the research fellowship granted to one of the authors to undertake a PhD.
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22