Experimental and numerical analysis of a waterproofing adhesive layer used on concrete-bridge decks

ARTICLE IN PRESS International Journal of Adhesion & Adhesives 29 (2009) 525–534

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Experimental and numerical analysis of a waterproofing adhesive layer used on concrete-bridge decks Qinwu Xu a,, Qinghua Zhou b, Cesar Medina a, George K. Chang a, Dan K. Rozycki a a b

The Transtec Group Inc., Austin, TX 78731, USA The Key Laboratory for Special Area Highway Engineering of China’s Ministry of Education, Chang’an University, Xi’an 710064, China

a r t i c l e in fo

abstract

Article history: Accepted 20 December 2008 Available online 29 January 2009

This paper studies the adhesive behavior of a waterproof layer used between a concrete-bridge deck and an asphalt–concrete overlay. The laboratory direct-shear and pull-off tests were designed to measure the interface adhesive strengths. A three-dimensional, finite-element model was developed to analyze the interfacial shear stress and tensile stress in response to vehicle loading. Results indicate that an interface friction coefficient of 0.5 could achieve high interfacial shear strength with relatively low shear stress. The safety factor (strength/stress) decreases significantly with increasing environmental temperatures. The effects of compaction temperature, modulus, and thickness of overlay on the adhesive strengths and stresses were also examined. & 2009 Elsevier Ltd. All rights reserved.

Keywords: B. Concrete C. Finite-element stress D. Mechanical properties of adhesives

1. Introduction Adhesive materials are used in new constructions or rehabilitation of bridge decks. These adhesive agents include epoxy materials used to bond steel plates with concrete substrates [1,2], and asphalt binders used to bond asphalt–concrete (AC) overlays with Portland cement concrete (PCC) decks [3]. Asphalt is a highly viscous liquid or semi-solid with acceptable bonding properties, and has been primarily used as a binder between aggregate particles to form AC mixture—a composite material used in road pavements and bridge overlays. Asphalt is also used as an impervious material for manufacturing roofing shingles. In the transportation industry, AC overlay is commonly used as a wearing course constructed on concrete-bridge decks. Cracking and debonding to overlays are particular distresses that engineers have to continuously repair. Overlay cracking is generally caused by vehicle loading and the effect of changing environmental conditions like temperature and moisture. Additionally, overlay debonding occurs when the shear stress and/or normal tensile stress exceed the interfacial shear strength and/or pull-off strength, as illustrated in Fig. 1. In this figure, the shear mode shows how water usually intrudes into the structure through the existing cracks, penetrating the AC overlay and the bridge deck to cause steel corrosion in the PCC deck. To prevent this problem, a waterproofing adhesive layer (WAL) can be placed as an interlayer between the bridge deck and the AC overlay to prevent water and

 Corresponding author. Tel.: +1 512 709 4155; fax: +1 512 451 6234.

E-mail addresses: [email protected] (Q. Xu), [email protected] (Q. Zhou). 0143-7496/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2008.12.001

enhance interface adhesion. The types of WAL used for concretebridge decks generally fall into two categories: roll sheet and plaster coat. These WALs have been successfully used in the steel and timber bridges [4–6]. The effectiveness of WALs can be evaluated in different ways. Laboratory and field tests for evaluating the engineering properties of WALs have been investigated, including the ones conducted by the National Cooperative Highway Research Program (NCHRP) in the United States [7,8]. The engineering properties tested for WALs include tensile strength, durability, toughness, elasticity, water impermeability, puncture resistance, temperature susceptibility, etc. [7,8]. Laboratory test methods for evaluating material properties and repair techniques in field construction were comprehensively investigated in England in the 1990s [9–11]. However, the primary performance criterion to assess the benefits of WALs is measuring the interface adhesive strength. Accordingly, laboratory and field test equipments and methods have been investigated [3]. Unfortunately, other than laboratory and field tests, not much has been done to evaluate the performance of WALs when applied in a given system. For instance, minimum attention has been paid to the research of structure modeling and stress analysis of WALs. Due to the complicated multi-state loading conditions applied on bridges and the membrane structure of WALs as an interlay, it is very difficult to effectively measure the interface shear and normal tensile stresses between WAL and bridge deck or AC overlay as shown in Fig. 1. Therefore, it would be very meaningful to capture these critical stresses using numerical modeling. Meanwhile, the adhesive capability of WALs is highly affected by a myriad of field conditions, which include construction

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used to improve asphalt’s tolerance to high temperatures and the aging effect. SBS polymers have been frequently used in the highway construction to improve engineering properties of the asphalt mixture such as tensile strength, rutting resistance at high temperature, and cracking resistance at low temperature [12]. SBS can also improve the viscosity and adhesive strength of asphalt binders [13,14]. The WAL was applied to the concretebridge deck using the ‘‘thermal cutoff construction’’ method, in which the WAL surface was heated until it was melted, resulting in adherence to the bridge deck.

Water Vehicle tire Asphalt concrete overlay Concrete bridge deck Bridge beam Shear stress

Bridge bearing

3. Experimental program

Vehicle tire

Tensile stress There are some standard test methods to measure the adhesion strengths of coatings and tapes, including ASTM D5321, ASTM D 4541, and ASTM D3359. However, these methods are not specially defined to measure a WAL used on the concretebridge deck, and they have some limitations [3], e.g. test results may significantly depend on the test apparatus [15]. Accordingly, in this research the laboratory tests, including the direct-shear and pull-off tests, were designed to measure the interface adhesive strengths of WALs used on the concrete-bridge deck.

Box-girder Bridge

Fig. 1. Interface debonding mechanisms: (a) shear mode—side view and (b) tensile mode—front view.

APP modified asphalt AC overlay Polyester felt

WAL PCC deck

SBS modified asphalt

Bridge beam Fig. 2. Surface structures on concrete bridge.

conditions, material property, vehicle loading, and bridge structure. Therefore, studies on the influences of these critical factors on the mechanical behaviors of WALs are considered essential in order to design more reliable materials and structures. Accordingly, this paper aims to describe the adhesive behavior of WALs. A roll-sheet-type WAL was investigated to study the influences of critical factors that affect the adhesive behavior of WAL system, using both laboratory testing methods and finiteelement (FE) modeling technique. Some of these critical factors include the compaction temperature of AC overlay, interface normal pressure, interface shear speed, surface roughness (SR) of bridge deck, and modulus and thickness of AC overlay. Designs of materials and structures are also discussed in terms of the testing and modeling results.

3.1. Direct-shear adhesion test The direct-shear adhesion test was designed to determine the interfacial shear strength using the laboratory-loading machine (LLM) invented by the research team at the Chang’an University, as shown in Fig. 3. More details about the test equipment and mechanism were presented elsewhere [16,17]. In this test, a concrete slab measuring 320 mm (width) by 320 mm (length) by 150 mm (height), and an AC slab measuring 300 mm (width) by 300 mm (length) by 50 mm (height) were prepared. A 3-mm WAL was heated and adhered to the top of the PCC slab. Likewise, the AC slab was heated and then compacted on the WAL. Consequently, the AC slab was fixed by a steel box, to which a compressive force was applied on the top and a shear force was applied on the side, as shown in Fig. 3. The LLM can accurately control the magnitudes of shear force, compressive force, and shear speed that are applied to the specimen. Four different values of compressive stresses were used, including 0.1, 0.3, 0.5, and 0.7 MPa. Meanwhile, different shear speeds varying from 3.5 to 50 mm/min were applied. In addition, concrete slabs with different surface roughnesses or textures were prepared to study

Compressive force Steel mold Shear force AC slab WAL

2. Description of structure and materials The structure under investigation consists of a PCC bridge, an 8 cm-thick PCC deck placed on the bridge beam, a 3 mm-thick WAL bonded to the PCC deck, and an 8 cm-thick AC overlay placed on the WAL, as displayed in Fig. 2. The WAL used in this system is composed of a substrate of polyester fiber immersed in and covered by an APP (heat-weldable atactic polypropylene)modified asphalt, a styrene–butadiene–styrene (SBS)-polymermodified asphalt underneath as a subcoat, and mineral particles scattered on the upper surface of the WAL (Fig. 2). APP is primarily

PCC slab

Steel frame

Fig. 3. Direct-shear test.

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its effects on adhesion strengths. In this research SR was expressed by the friction coefficient u of the PCC slab on the surface, and was measured using the pendulum friction tester. The value of interfacial shear strength, tmax (MPa), was calculated as follows:

tmax ¼ F=A

(1)

where F is the maximum shear loading (N) and A the interfacial contact area (mm2) 3.2. Pull-off adhesion test The pull-off adhesion test was designed to determine the interface pull-off strength, as depicted in Fig. 4. The test samples included an AC cylinder with a diameter of 100 mm and a height of 64 mm, a PCC slab with a width of 150 mm, a length of 150 mm, and a height of 50 mm. The AC cylinder and PCC slab were bonded with a WAL as an interlay, using the same method as explained for the shear adhesion test.

Tensile force

smax ¼ F=A

(2)

where F is the maximum applied load (N), and A the contact area (mm2).

4.1. Structure of the model

AC cylinder

WAL

Fig. 4. Pull-off test.

In the field construction, the compaction temperature of the hot AC mixture is determined in terms of the viscosity of asphalt binder [18,19]. The compaction temperatures of ordinary AC mixtures have commonly fallen into a certain range such as 130–165 1C, and this value is higher for the polymer-modified asphalt binder due to its higher viscosity, requiring more compaction efforts [20,21]. In order to account for the influence of AC compaction temperature on the adhesion strength of WALs, in this test four compaction temperatures were used, those being 100, 130, 160, and 190 1C. A Universal Testing Machine (UTM), with a closed-loop servo-hydraulic system, was used as the load frame to apply a tensile force to pull the AC cylinder off the PCC slab. Then, the value of pull-off strength, smax (MPa), was calculated as follows:

4. FE modeling program

Steel cup

PCC slab

527

The ANSYS program was used to build a full-scaled, threedimensional, finite-element model (3D-FE) to simulate the continuously supported, four-spanned (4  20 m), box-girder bridge, as shown in Fig. 5. An eight-node solid element (SOLID 45) was used to model the rubber bearing, steel seat, bridge beam, PCC deck, and AC overlay. A four-node membrane element (SHELL 41) was used to model the WAL. A sensitivity analysis of stress response was conducted to determine the FE mesh size by reducing the element size gradually, until the calculated stresses were stable (no significant changes of stress values occur after refining mesh sizes, e.g. within 5% difference). As a result, the tire contact areas and other critical stress positions (i.e. the positions aligned with bridge ribs and bearings) were meshed with

Loading

AC overlay

Waterproof interlay

Steel cushion Rubber bearing Fig. 5. FE model: (a) front view, (b) side view.

Bridge beam

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dimensions of 2  2  2 cm3 for each element, but other positions were meshed with coarse mesh sizes. In total, 80,444 elements and 101,420 nodes were used for the FE model. 4.2. Material model Concrete is a brittle material and performs in an elastic way at a small stress level that does not reach its ultimate strength. The elastic modulus E (MPa) of concrete was estimated from the measured compressive strength using the American Concrete Institute (ACI) method [22], as follows: qffiffiffiffiffi 0 E ¼ g1:5 fc (3) c  0:043  0

where gc is the material density (kg/m3) and f c the compressive strength (MPa). Both the density and strength were determined by laboratory tests. The Poisson’s ratio of the concrete was set as 0.18 according to the bridge design specification [23]. The direct tensile test was performed on a 220-mm long and 50-mm wide WAL, following the method described by ASTM D412. Results indicate that the WAL primarily performs elastic behavior (stress has a linear relationship with strain) before breakage, as shown in Fig. 6. Therefore, the WAL is assumed to be an elastic material in the FE model, and its elastic modulus is determined form the stress–strain relationship plotted in Fig. 6. The determined material parameters are summarized in Table 1. Asphalt–concrete is a viscoelastic–plastic material, and its mechanical behavior is highly dependent on time and temperature. Generally AC exhibits linear-viscoelastic behavior without

LogðaT Þ ¼

C 1 ðT  T r Þ ½C 2 þ ðT  T r Þ

0.9 0.8 0.7 0.6

where C1, C2 are two constants, T is the temperature, and Tr the reference temperature (e.g. 25 1C). Then, the reduced frequency is determined as follows: f r ¼ f aT

(5)

LogðE Þ ¼ 8:4964 þ

0.5

0.3 0.2 0.1 0.0 0.0

6.6

13.2 19.8 Deformation (mm)

26.4

33.0

Fig. 6. Laboratory test results of tensile behavior of WAL.

Table 1 Material parameters.

(6)

Parameter

Concrete Concrete Concrete Concrete Concrete WAL WAL Interface

f ca Eb 0 fc E uc E u

bridge bridge deck deck

Compressive strength. Elastic modulus. c Poisson’s ratio. d Interfacial shear strength.

where e and v are deviatoric strain and volumetric strain, respectively; G is shear relaxation modulus and K is bulk relaxation modulus. The Prony series are used to express G and K in the numerical implementation. The relationship among these three complex moduli (shear complex modulus, bulk complex modulus, and dynamic modulus) is expressed as follows: G ¼

Material/phase

b

12:7699 1 þ eð2:4907þ0:2849Logðf r ÞÞ

The governing equation for the stress–strain relationship of linearviscoelastic materials can be expressed as follows: Z t Z t de dv sðtÞ ¼ 2Gðt  t0 Þ 0 dt0 þ I Kðt  t0 Þ 0 dt0 (7) dt dt 0 0

0.4

a

(4)

where aT is a shift factor (Eq. (4)), f is the frequency at temperature T, and fr the reduced frequency at the reference temperature Tr. The dynamic modulus versus the reduced frequency at the reference temperature (i.e. 25 1C) can be expressed in a sigmoidal function, which is generally used in the pavement industry [27]. A master curve of complex modulus resulting from the test data is expressed as follows and depicted in Fig. 8:

1.0

Tensile force (kN)

noticeable deterioration when it is subjected to a small strain level [24]. However, if the material of AC overlay is regarded as linear elastic, it may result in overestimated stresses since this ignores the effects of stress relaxation during the loading time [25]. Therefore, in order to more accurately capture the adhesive behaviors of the WAL and AC overlay, the AC overlay was considered to be a viscoelastic material in the FE model. A uniaxial frequency test was conducted by applying a sinusoidal compressive loading on a cylindrical AC specimen with dimensions of 100 mm (diameter) and 150 mm (height), as shown in Fig. 7. The test was performed at four temperatures (5, 5, 25, and 40 1C), and five loading frequencies (0.1, 0.5, 1, 5, and 10 Hz) for each temperature, in order to capture the viscoelastic behaviors of the AC material at different temperature/time levels that will be needed in the FE modeling discussed later. The test results of dynamic modulus and phase angle are plotted in Fig. 8. A temperature–time superposition based on the WLF rule [26] was used to convert loading frequencies at different temperatures to reduced frequencies at the same reference temperature of 25 1C. A shift factor aT used for this conversion is calculated as follows:

0

tmaxd

Value 43.3 MPa 30,700 MPa 50.1 MPa 33,021 MPa 0.18 172 MPa 0.30 0.27 Mpa

3K  E 9K   E

(8)

The relationship between bulk modulus and elastic modulus is expressed as follows: K¼

s DV=V

¼

s E ¼ xx þ yy þ zz 3ð1  2nÞ

(9)

Eq. (9) was used to approximate the relationship between bulk complex modulus and dynamic modulus. Consequently, the shear complex modulus can be calculated in terms of Eq. (8). The ANSYS program performed Fourier transformations to convert the frequency-dependent complex modulus to the time-dependent relaxation modulus. Afterwards, the responses of stress and strain of AC materials under vehicle loading were calculated using Eq. (7).

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529

Compressive force

100mm

Aluminum cup

 = 0eit

Stress

Stress Strain

75mm

Strain

37.5m

Extensometer 1 37.5m Extensometer 2

Time lag  =  0e

i(t −)

Teflon sheet Steelplate

Time

Fig. 7. Dynamic modulus tests on asphalt concrete. Note: (1) Extensometer is used to measure deformation. (2) Extensometer 3 located on the back of the specimen surface is not shown in this graph.

4.3. Loading model 10000 Dynamic Modulus (MPa)

9000 8000 -5 Deg.C

7000

5 Deg.C

6000

25 Deg.C

5000

40 Deg.C

4000 3000 2000 1000 0 0

2

4

6

8

10

12

10

12

Frequency (Hz)

35

Phase angle (0)

30 25

-5 Deg. C

20

5 Deg. C

15

40 Deg.C

25 Deg. C

10 5 0 0

4

2

6

8

Frequency (Hz)

The HS-20, a vehicle-fleet distribution model used to simulate the traffic loading for a first-class-highway bridge, was adopted in the FE model according to the bridge design specification [23]. This vehicle fleet consists of four trucks as shown in Fig. 9. From the left to the right, Trucks 1, 2, and 4 have two axles, and Truck 3 has five axles. In the FE model, HS-20 was moved both longitudinally and transversely at different bridge positions, in order to determine the most unfavorable loading positions. Modeling results showed that the maximum flexural stress in the WAL occurred at a position aligned with the third bridge bearing (from the left to the right) as shown in Fig. 9. Meanwhile, the maximum interface shear stress appeared under the second 14-ton axle (heaviest axle) of the third truck (heaviest truck), as shown in Fig. 9. The loading pulse of vehicle can be expressed as a halfsinusoidal function with time [28]. The loading pulse was uniformly distributed on the entire tire contact area on the AC overlay, as shown in Fig. 10. One cyclic loading time of 0.018 s was applied in the FE model, which is a result of dividing the tire contact length (24 cm) by the vehicle speed (80 km/h). The contact areas of the dual tires were simulated by two rectangles with a space of 10 cm between two tires, and each rectangle has a dimension of 18 cm (width) by 24 cm (length), as shown in Fig. 10. The tire-pavement friction coefficient can reach 0.5 at the vehicle acceleration or braking [29]. In order to account for the tire friction force, a truck is assumed to brake suddenly, which results in an additional tangible force applied on the surface of the AC overlay.

12000

4.4. Interface contact

Dynamic Modulus (MPa)

10000 8000

Sigmoidal Fit -5C

6000

5C 25C

4000

40C

2000 25°C reference

0 0

0

0

1

10

100

1,000

10,000 100,000

Frequency (Hz) Fig. 8. Measured dynamic modulus and phase angle of asphalt concrete. (a) Dynamic modulus at different frequency and temperature, (b) phase angle at different frequency and temperature, and (c) master curve of dynamic modulus.

Generally the interface adhesion between the WAL and the AC overlay is stronger than that between the WAL and the PCC deck [3]. Therefore, in the FE model, it is assumed that debonding will occur only at the interface between the WAL and the PCC deck, and thus this interface is regarded as frictionally contacted (relative slide is allowed). However, the interface between the WAL and the AC overlay is considered fully bonded. The nonlinear, surface-to-surface contact model was used to simulate the frictionally contacted WAL–PCC interface. In the ANSYS program, Coulomb’s friction law was used to describe the interface frictional stress t [30]:

t ¼ C þ us ¼ C þ tan j s;

tpjtj

(10)

where C is the cohesive strength, s the interface normal pressure,

f the material inner friction angle, and m the friction coefficient.

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Traffic direction

13ton

4m

15m

4m

13ton

7tons

10m 7tons

7m

1.4m 14tons

14 tons

1.4m

10m

3m

12 tons 12 tons

3tons

4m

13ton

6.6m

7tons

Overlay Bridge Max interface shear stress

Max flexural stress

20m

20m

20m

20m

Fig. 9. Truck distribution on a bridge (four spans, 4  20 m).

Table 2 Interfacial shear strength at different normal pressures. Normal pressure (MPa)

0.1 0.3 0.5 0.7

Interfacial shear strength (MPa)

Mean

Sample 1

Sample 2

Sample 3

(MPa)

0.202 0.273 0.356 0.392

0.174 0.264 0.408 0.392

0.185 0.228 0.375 0.395

0.187 0.255 0.380 0.393

The C, f, and t of the materials used here were determined by the direct-shear test as discussed previously (as results, C ¼ 0.1531 MPa, m ¼ tan(f) ¼ 0.361, and t ¼ 0.27 MPa at 25 1C).

5. Laboratory test results and analysis

temperature or higher loading frequency asphalt possesses higher shear modulus, and thus induces higher shear stress for interface failure. Previous research reported that for the PCC overlay constructed on the concrete bridge, interface debonding could appear in the concrete substrate, the interface between PCC overlay and concrete substrate, and the PCC overlay [15]. However, for the AC overlay constructed on concrete bridge, AC material is much ‘‘softer’’ than the concrete substrate, and thus generally interfacial failure would not occur in the concrete substrate. Test results showed that the interface failures primarily occur in the asphalt films (SBS-modified asphalt) at the bottom surface of the WAL (see Fig. 2), and some failures occur between the PCC deck and the bottom surface of the WAL (Fig. 2). In this paper both failures are called interface failures. Fig. 12 also shows that tmax has a linear relationship with the natural-logarithmic-scaled shear speed. The slope of this linear function can be used as an index to evaluate the response of the interfacial shear strength to the shear speed: the greater the slope, the higher the dependence of tmax on shear speed.

5.1. Interfacial shear strength 5.1.1. Influence of normal pressure The measured interfacial shear strength tmax is about 0.393 MPa under the normal pressure of 0.7 MPa at 60 1C (a high temperature of the AC material in summer). Test results of tmax for three samples at different normal pressures are presented in Table 2. This table shows that tmax decreases with decreasing interface normal pressure, and they have a closely linear relationship as illustrated in Fig. 11. Therefore, Coulomb’s friction law can be used to describe this linear relationship as follows:

tmax ¼ C þ s tan j

(11)

where s is the interface normal stress (theoretically it is the same as the normal pressure applied on the top of AC slab in the laboratory test), C the cohesive strength (0.153 MPa), and f the friction angle (19.851). 5.1.2. Influence of shear speed Test results show that tmax increases with increasing shear speed, and the increase rate drops gradually, as illustrated in Fig. 12. This trend is also notified when using the 901 peel adhesion test [3] and direct pull-off test [15]. This result is primarily due to the viscoelastic property of asphalt material. The mechanical behavior of asphalt is highly dependent on temperature and time/frequency. At a lower

5.1.3. Effect of SR of bridge deck Experiments find that tmax initially increases and then decreases with increasing friction coefficient of the PCC slab surface (surface roughness), as shown in Fig. 13. It shows that tmax reaches the greatest value at a friction coefficient of 0.4–0.6. Other researchers also reported that the SR significantly affects the interface adhesion between the stone/aggregate and the asphalt material, and there exists an optimum surface roughens [31]. This phenomenon can be explained as follows: with increasing SR, the interfacial shear strength increases due to the improved interlock effect [32]; however, when the PCC deck surface is rough enough, the asphalt films cannot effectively penetrate into the deep grooves of the PCC deck surface; as a result the actual contact area between the WAL and the PCC deck surface reduces and therein the interfacial shear strength decreases. Therefore, it is essential to design an appropriate SR (texture) to achieve high adhesion strengths. Laboratory testing is recommended to determine an appropriate SR for field construction. 5.1.4. Effect of environmental temperature Test results show that the environmental temperature has significant effect on tmax. As illustrated in Fig. 14, tmax obviously decreases with increasing environmental temperature, e.g. tmax decreases 64.36% when temperature increases from 0 to 60 1C.

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531

Shear adhesive strength (MPa)

0.28

Pressure

kPa

y = -3.2957x 2 + 3.4444x -0.6545

0.26

R² = 0.8462

0.24 0.22 0.20 0.18

3mm WAL Poly. (3mm WAL)

0.16 0.14 0.40

0.45

0.50 0.55 0.60 Friction coefficient

0.65

0.70

Fig. 13. Laboratory test results of interfacial shear strength versus interface friction coefficient at 60 1C.

Fig. 10. Vehicle axle loading and tire contact area in the FE models. (a) One cycle of half-sinusoidal vehicle loading. (b) Tire contact area of axle loading.

0.40 C = 0.1531 Mpa tanϕ = 0.361

0.35

0.6 0.5 0.4 0.3 0.2 0.1 0

0.30

τ = 0.361σ + 0.1531 R2 = 0.9797

5

10

15 20 25 30 35 40 45 Enviornmental temperature (°C)

50

55

60

Fig. 14. Laboratory test results of interfacial shear strength versus environmental temperature.

0.25 0.20

0.70 0.65

0.15 0

0.1

0.2

0.3 0.4 0.5 0.6 Normal Pressure (MPa)

0.7

0.8

Fig. 11. Laboratory test results of interfacial shear strength versus normal pressure at 25 1C.

0.30 y = 0.0829Ln(x) - 0.0891

0.25 Shear strength (MPa)

0.7

0

2

R = 0.9291

Pull-off adhesive strength (MPa)

Shear adhesive strength (MPa)

0.45

Shear adhesive strength (MPa)

0.8

0.60 0.55

WAL-3mm WAL-4mm

0.50 0.45 0.40 0.35 0.30 0.25

0.20

0.20 100

0.15

130 160 AC compaction temperature (°C)

190

Fig. 15. Test results of pull-off adhesive strength versus AC compaction temperature.

0.10 0.05 0.00 0

10

20 30 40 Shear speed (mm/minute)

50

60

Fig. 12. Laboratory test results of interfacial shear strength versus shear speed at 60 1C.

This trend was also observed in other tests performed on the epoxy asphalt and the SBS-modified asphalt used as WAL materials [33,34]. Asphalt is a viscoelastic material and its mechanical behavior is dependent on temperature and frequency; thus the shear modulus of asphalt decreases with increasing temperature or reducing loading frequency. As a result, interfacial shear strength decreases.

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5.2. Interface pull-off strength 5.2.1. Effect of compaction temperature The compaction temperature of AC mixture in construction may affect the interfacial adhesion strength. Test results show that pull-off strength smax increases with increasing AC compaction temperature, and the increase rate drops gradually, as illustrated in Fig. 15 (at 25 1C). It is also noted that interface debonding occurs at the AC–WAL interface when the compaction temperature is relatively low (e.g. 100 and 130 1C). This is because the asphalts of AC mixture are not melted completely at these relatively low temperatures to form sufficient bonding effects at the AC–WAL interface. It is known for asphalt that the fluidity increases, while the viscosity decreases with increasing temperature. It is believed that microvoids at the surface can be filled by adhesives only after sufficient wetting, followed by solidification to form interface adhesion according to the mechanical interlock theory [35]. Other researches also reported that the asphalt binder with lower viscosity can penetrate deeper and fill more voids in the AC mixture [35]. However, interface debonding occurs at the PCC–WAL interface when the AC compaction temperature is equal to or higher than 160 1C, which is because a strong adhesion forms at the AC–WAL interface at a relatively high temperature. Meanwhile, this shows that the asphalt material of the WAL could be damaged and even burned if the compaction temperature is too high (e.g. higher than 190 1C), which is due to the aging effect of asphalts at high temperatures. As a result, the adhesion strength eventually drops with increasing compaction temperature, as shown in Fig. 15. Therefore, an optimum compaction temperature of the AC overlay is essential to achieve large interface adhesions. Based on the test results obtained in this study, a temperature larger than 165 and less than 190 1C would be recommended. 5.2.2. Effect of environmental temperature Experiments find that the environmental temperature has pronounced effect on smax. As illustrated in Fig. 16, smax decreases dramatically with increasing environmental temperature (i.e. smax decreases 96.17% when environmental temperature increases from 0 to 60 1C). This trend is in agreement with researches on other WAL materials [33,34]. This result is due to the viscoelastic nature of asphalt material as discussed previously. 6. FE modeling results and analysis

under the contact areas of dual tires is plotted in Fig. 17 (these contours were automatically generated by the ANSYS program). This figure shows that t drops rapidly towards the outside direction of the tires, which indicates that the local-area effect of the tire pressure on the interfacial shear stress is significant.

6.2. Effect of interface friction Modeling results indicate that t increases with increasing interface friction coefficient m (e.g. t increases 30% after m increases from 0.07 to 0.23), and it reaches the maximum value at the fully bonded condition. This trend is in agreement with previous simulations performed on PCC overlays on concrete bridges [36]. However, the increase rate of t drops continuously, and t value is almost a constant after m reaches a value of 0.5, as shown in Fig. 18. In order to achieve a more durable interface system, the design should aim at reaching higher interfacial shear strength with lower shear stress, Laboratory test results discussed previously have indicated that interfacial shear strength tmax increases first and then decreases with increasing m, and tmax reaches the maximum value when m is between 0.4 and 0.6 (see Fig. 13). Therefore, a m value of 0.5 is suggested for the WAL material used in this study.

6.3. Effect of AC modulus The value of dynamic modulus of the AC overlay is dependent on the environmental temperature and loading frequency as illustrated previously (Fig. 8). Therefore, in the FE simulation the AC modulus was changed in order to capture its influence on interface stresses. Simulation results indicate that AC modulus has minimal effects on both t (interface shear stress) and s (interface pull-up stress); s slightly increases by 1.96%, and t slightly increases by 3.63% when the AC modulus increases from 1000 to 20,000 MPa. After AC modulus reaches 10,000 MPa, values of s and t are almost constant without changing, as depicted in Fig. 19. With lower modulus, the AC material has higher shear deformation/strain; while with higher modulus, the AC material has lower shear deformation/strain. As a result, the induced shear stress (it is a function of modulus times strain based on Hooke’s law) does not vary too much with changing modulus value.

6.1. Stress distribution FE modeling results show that the maximum interfacial shear stress t occurs at the edges of the tire contact areas. The t contour

Pull-off adhesive strength (MPa)

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MX I

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10

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20 25 30 35 40 45 50 Enviornmental temperature (°C)

55

60

65

Fig. 16. Test results of pull-off adhesive strength versus environmental temperature.

B=38438 C=58774 D=15701 E=22366 F=29031 G=114633 H=135624 I=318905 Fig. 17. Contour of interfacial shear stress under the dual tire (unit: Pa).

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0.45

0.30

0.40 Interface stresses (MPa)

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Fig. 20. Interface stresses versus AC thickness (FE modeling results).

Fig. 18. Interfacial shear stress versus friction coefficient (FE modeling results).

Interface shear stress (MPa)

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6 5 4 3 2 1 0 0

5

AC modulus (MPa)

10

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20 25 30 35 40 45 Enviormental temperature (°C)

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Fig. 21. Safety factor.

Fig. 19. Influence of AC modulus on interface stresses (FE modeling results).

Results show that AC overlay thickness has a significant influence on both t and s. With increasing AC overlay thickness, t and s are pronouncedly reduced and the decrease rate drops gradually, as shown in Fig. 20. In addition, the effect of the overlay thickness on t is more evident than that on s; e.g. t decreases by 66.39% and s decreases by 24.89% when the overlay thickness increases from 4 to 26 cm. This result is explained as follows: the friction force of vehicle tire applied on the AC overlay is the primary cause for the interfacial shear stress, and its effect on the interfacial shear stress decreases with increasing AC thickness. However, the effect of increasing overlay thickness on reducing shear stress drops gradually until being minimal or ignorable after 16 cm. Therefore, as regards the engineering cost and the additional weight loading of overlay structure applied on bridge beam, it is not recommended to design an AC overlay with a thickness higher than 16 cm.

strength–stress relationship would also depend on the environmental temperature. In order to study this property, SF at four different AC moduli resulting from four different environmental temperatures (0, 25, 40, and 60 1C) are computed and compared. As shown in Fig. 21, SF dramatically decreases with increasing environmental temperature, which indicates that the probability of interface failure at higher temperature is much higher than that at lower temperature. However, it should be noted that the safety factor below 1.0 at 60 1C shown in Fig. 21 does not necessarily mean that interface failure will occur since the shear speed/ frequency used in these tests may not exactly simulate the real frequency of stress responses under the traffic loading, and most possibly the shear frequency used in the tests (e.g. 50 mm/min) has underestimated the real condition as regards the vehicle speed of 80 km/h. Meanwhile, it has always been a challenge to accurately simulate the frequency/speed of stress response in laboratory tests, and has not been investigated here.

6.5. Influence of environmental temperature on safety factor

7. Conclusions

Higher strength and lower stress result in a safer structure, and the safety factor SF (strength/stress) is used as an index to evaluate the safety of the WAL, expressed as follows:

This paper has studied the adhesive behavior of a WAL under influences of critical factors in order to design more reliable materials and structures. This behavior was evaluated through laboratory testing and FE modeling. The results from the testing and the modeling support the following conclusions:

6.4. Effect of overlay thickness

SF ¼ tmax =t and SF ¼ smax =s

(12)

AC modulus varies due to the change of environmental temperature, resulting in changed adhesion strengths and interface stresses as discussed previously. Therefore, SF as a

 Interfacial shear strength as a function of normal pressure of vehicle loading follows the Coulomb’s friction law;

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 Adhesive strength increases with increasing compaction   

  

temperature of AC overlay, but it may decrease at compaction temperatures higher than 160 1C; Both interfacial shear strength and pull-off strength significantly decrease with increasing environmental temperature; However, both interfacial shear stress and normal tensile stress have minor changes with increasing environmental temperature or reducing AC modulus; As a result, the safety factors (strength/stress) significantly decrease with increasing environmental temperatures, which indicate that at higher temperatures the interface failure has a much higher chance to occur. Therefore, improving the adhesive strengths of the WAL at high temperatures would be one of the key objectives in designing materials and structures; Increasing thickness of AC overlay significantly reduces both the interfacial shear and normal tensile stresses; Interfacial shear strength increases first and then decreases with increasing the SR of bridge deck; However, interfacial shear stress continuously increases with increasing SR, and the increased rate drops gradually until being minimal. Design procedures would aim at a higher adhesive strength at lower interface stress, and as a result a friction coefficient of 0.5 is recommended for the WAL material used in this research.

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