Draft · 2019-03-14 · Draft 31 order to decrease the occurrence of these cracks on the pavement...

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Evaluation of factors affecting tack coat bond strength

Journal: Canadian Journal of Civil Engineering

Manuscript ID cjce-2018-0290.R2

Manuscript Type: Article

Date Submitted by the Author: 18-Aug-2018

Complete List of Authors: Biglari, Moein ; Tarbiat Modares University, Tehran, Iran, Civil EngineeringAsgharzadeh, Seyed Mohammad ; Tarbiat Modares University, Tehran, Iran, Civil EngineeringSharif Tehrani, Saleh; Kharazmi University, Civil Engineering

Keyword: tack coat, sand asphalt, Roller Compacted Concrete, Emulsified binder, Crumb Rubber

Is the invited manuscript for consideration in a Special

Issue? :Not applicable (regular submission)

https://mc06.manuscriptcentral.com/cjce-pubs

Canadian Journal of Civil Engineering

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1 Evaluation of factors affecting tack coat bond strength

2 Moein Biglari, MSc student, Tarbiat Modares University, Tehran, Iran3 [email protected] Seyed Mohammad Asgharzadeh, Assistant professor, Tarbiat Modares University, Tehran, Iran 6 [email protected] Saleh Sharif Tehrani*, Assistant professor, Kharazmi University, Tehran, Iran9 [email protected]

10 *corresponding author11

12 Abstract

13 Different types of distress occur in asphalt concrete pavements due to lack of bonding between existing old layer

14 and overlay. Therefore, this paper evaluates the bonding strength between sand asphalt mixture as overlay and

15 Roller Compacted Concrete (RCC) as the existing layer. Four types of tack coat including crumb rubber modified

16 (CRM), grade 60/70 binder, cationic slow-setting (CSS) and cationic rapid-setting (CRS) emulsion were considered,

17 with 200, 400 and 600 gr/m2 dosages. Different RCC surface temperatures including 0, 25 and 60°C were chosen to

18 evaluate the effect of ambient temperature on the bonding strength. Results showed that CRM and 60/70 binders

19 have higher bonding strength in comparison to emulsions. The bonding strength at 0°C for all types of tack coat was

20 significantly lower than other temperatures. The optimum application rates of 200 gr/m2 and 400 gr/m2 were

21 selected for the CSS and CRS emulsified binders respectively. The optimum application rate for the 60/70 and CRM

22 binders was selected as 600 gr/m2.

23 Keywords: Tack Coat, Sand Asphalt, Roller Compacted Concrete, Emulsified binder, Crumb Rubber, Bond Strength

24 1-Introduction

25 Road pavements are required to be constructed in several layers with different physical properties and characteristics

26 usually because of some technical and economic considerations (Diakhaté et al. 2011). Municipalities and

27 Departments of Transportations (DOTs) also use different remedial actions and treatments, such as overlays on old

28 pavements, to maintain a reasonable level of service. Besides, common overlays cannot fully eliminate the cracks

29 that occur on the pavement surface and the reflecting cracks usually will appear after a while. An asphalt layer

30 named Stress Absorbing Membrane Interlayer (SAMI) is applied between the old pavement and the new overlay in

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31 order to decrease the occurrence of these cracks on the pavement surface (Nithin et al. 2015). A proper bonding

32 among pavement layer interfaces is essential for pavements in order to withstand different stresses while the weak

33 bonding between pavement layers can result in several damages. Commonly, slippage of pavement layers is known

34 to be the major deterioration that occurs due to weak bonding between pavement layers. This distress type usually

35 occurs in locations where significant traffic acceleration/declaration exists. This is also common in curves and U-

36 turns (West et al. 2005). A research performed by King and May (2004) on the simulation of the pavement

37 structure showed that decreasing the bonding state from a full bond (no slippage) to a 90% bonding level (partial

38 slippage) significantly increased the stress and strain values in the pavement and decreased the final service life by

39 around 50% (King and May 2003). Roffe and Chaignon (2002) performed a similar study and showed that low

40 bonding between pavement layers can decrease the pavement service life from 20 years to 7 years (Roffe and

41 Chaignon 2002). Another research performed by Hachiya et al (1997) on the modeling of an airport runway

42 pavement being loaded under Boing 747 wheels, indicated that the top course asphalt layer deteriorated rapidly

43 when the bonding among the underlying layers started to decrease. Increasing the bonding levels using a different

44 type of tack coat and increasing the layers thicknesses was proposed in order to increase the pavement service life

45 (Hachiya et al. 1997).

46 Factors such as the improper base layer aggregate type, improper compaction of sub-base, base or subgrade layers,

47 segregation in the base layer due to large aggregate size, weather condition when pavement is placed, contamination

48 among layers, water flow between the layers, type of bitumen used in the wearing course and finally insufficient or

49 excessive amount of tack coat are believed to affect the debonding at the layer interface of asphalt pavements

50 (Canestrari et al. 2013). Tack coat, in general, is a diluted emulsified binder or a neat asphalt binder, which is used

51 to bond the new overlay to the existing layer. Based on a study by Asphalt Institute in 2007, tack coat is the key

52 element in increasing the bonding between pavement layers in order to decrease slippage cracks (Asphalt Institute

53 2007). Leng et al (2008) studied the interface bonding of pavement layers using tack coat applications and indicated

54 that three major elements including proper type of tack coat, proper application rate and proper tack coat curing time

55 can increase the bonding strength among pavement interfaces (Leng et al. 2008). Hu et al (2017), designed a device

56 to measure the shear strength of interface using a Universal Test Machine and evaluated the effects of temperature,

57 dosage of tack coat and type of tack coat on the shear properties of the interface. Based on results, shear strength

58 decreases when the dosage increases.

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59

60 Emulsified binder is the most commonly used type of tack coat in pavements (Roffe and Chaignon 2002). Different

61 types of emulsions such as Rapid Setting (RS), Medium Setting (MS), Slow Setting (SS) and Quick Setting (QS)

62 emulsions as well as polymer modified emulsions and crumb rubber modified emulsions are used as tack coat

63 materials (West et al. 2005; Hachiya et al. 1997). In addition, common bitumen grades, which are used in asphalt

64 concrete construction, can also be applied as a tack coat. Cut back bitumen was also used as tack coat until the last

65 decade when it was discarded due to the negative environmental effect of the solution (West et al. 2005). Based on a

66 study by Paul and Scherocman in USA (1998), the most common binders used as tack coat include SS-1, SS-1h,

67 CSS and CSS1-h and only the State of Georgia uses AC-20 and AC-30 bitumen (Paul and Scherocman 1998).

68 Several studies showed that emulsified binder performed better than PG asphalt binder as tack coat, but West et al.

69 (2005) declared that fine-graded mixtures had better bonding strength using PG asphalt tack coat. Today, emulsified

70 binder is the most used tack coat binder in the world (Roffe and Chaignon 2002) and this could be attributed to its

71 lower viscosity which makes it possible to be easily spread at ambient temperature. Wang et al (2017), mentioned

72 that upgrading tack coat material using modified asphalt, epoxy resin, rubber asphalt mastics can be used to improve

73 bonding strength with the overlay.

74 Using the proper application rate of tack coat can also improve the bonding between the layers and result in the total

75 increase of pavement service life. The bonding strength between pavement layers is a function of both cohesion

76 between layers and the available friction between them. Application of tack coat provides the required cohesion

77 between layers while an excessive dosage of tack coat can decrease the interlocking forces and friction between

78 layers and increase the chance of slippage between layers (Figure 1). High amounts of tack coat can also increase

79 the chance of pavement bleeding and decrease skid resistance of pavement and result in traffic collisions

80 (Raposeiras et al. 2013). Therefore choosing a proper type of tack coat with optimum application rate plays the key

81 role in pavement layer bonding.

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83 Figure 1: Effect of tack coat application rates on bonding strength.

84

85 The optimum tack coat application rate should be selected by the agency to achieve proper bonding between layers.

86 Different amounts of tack coat applied by researchers and agencies include a range of as low as 0.1 and up to 0.5

87 liters per square meter (Hachiya et al. 1997; HASIBA 2012; Uzan et al. 1978; Chen and Huang 2010; Asphalt

88 Institute (1989); De Bondt 1999; NLT-328/08 2008; Mohammad 2012). Previous research showed that the

89 application rates for the PG asphalt binders are generally more than the emulsified binders (Uzan et al. 1978). It

90 should also be noted that the optimum application rate of tack coat mainly depends on the pavement surface type

91 and surface condition, so that a worn-out old pavement surface will increase the amount of tack coat required

92 (Raposeiras et al. 2013; Raab and Partl 2004; Mohammad 2012). Besides, concrete pavements were shown to

93 require higher amounts of tack coat binder compared to asphalt pavements (Mohammad 2012). Mohammad (2012)

94 declared the required tack coat application rate for a new asphalt concrete layer as 0.1 liters per square meter, for an

95 old asphalt concrete or a newly milled asphalt concrete layer as 240 gr/m2, and for a cement concrete layer as 190

96 gr/m2 (Mohammad 2012).

97 Tack coat curing time is the final parameter affecting the quality of interface bonding. It is clear that there is no

98 complete agreement among researchers and agencies on the proper time of laying the second asphalt concrete layer

99 after application of tack coat. Based on a research by Roffe and Chaignon in 2001, curing time for emulsified binder

Low: just the friction Proper: both cohesion and friction Excessive: just the cohesion

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100 has significant effects on its performance. This time can vary among 20 minutes for broken emulsion to a few hours

101 for dry emulsion (Roffe and Chaignon 2002). Deysarkar showed that placing the second layer five minutes after

102 spraying tack coat would not change the interface bonding strength when compared to immediate placing. He

103 indicated that the bonding strength would significantly increase when placing the second layer starts 30 and 60

104 minutes after application of tack coat (Deysarkar 2004). Chen and Huang also showed that the bonding strength

105 would considerably increase when the second layer is spread and compacted after the emulsion tack coat breaking

106 time (Chen and Huang 2010). Raposeiras et al (2012) reviewed several researches on the effects of curing time on

107 bonding strength of pavement layers and declared that the optimal curing time of 40 to 50 minutes should be

108 considered to achieve a proper bonding strength (Raposeiras et al. 2012). Based on Paul and Scherocman (1998),

109 many DOTs spend specific amount of time for curing and setting of emulsified binder before laying the second layer

110 (Paul and Scherocman 1998). Three DOTs stated that they consider the maximum possible time for tack coat curing.

111 Alaska DOT considers 2 hours for CSS-1 emulsion, Arkansas DOT considers 72 hours for curing SS-1 emulsion

112 and Texas DOT uses 45 minutes for curing SS-1 and MS-2 emulsions (Cho 2016).

113 By now, it was mentioned that using tack coat increases the bonding between the pavement interlayers. However,

114 there are still some researches showing different and contrary effects of tack coat application. Molenaar et al (1986)

115 examined the effects of using interlayers on bonding strength of a multilayer pavement system and indicated that the

116 specimens with and without tack coat showed the same shear strength (Molenaar et al. 1986). While there are

117 several studies on different aspects of bonding strength of pavement layers using tack coat application, there are still

118 many gaps in the literature including the unknown effect of pavement surface temperature (or ambient temperature)

119 during tack coat application. Furthermore, the bonding strength between different materials such as Roller

120 Compacted Concrete (RCC) and sand asphalt mixture used as a SAMI materials was not exclusively evaluated by

121 other researchers. Different types of tack coats were used in this research to find the most proper one along with the

122 RCC and sand asphalt layers. Using different dosages of each tack coat, the most proper application rate of tack

123 coats was also determined. Besides, all above factors were evaluated at different surface temperatures of the existing

124 layer during tack coat application.

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125 2- Scope and objectives

126 As mentioned earlier, bonding strength between pavement layers can be a major factor that can result in distress.

127 An asphalt concrete layer is commonly laid on RCC layer, due to its rough surface. As a result, providing a proper

128 bonding strength between RCC and the asphalt concrete layer is of importance. As a result, this paper aims at

129 evaluating the effects of tack coat dosage, surface temperature and tack coat types on bonding strength between sand

130 asphalt mixture and RCC as existing layer. Therefore three types of tack coat including grade 60/70 binder, CRM

131 binder and two emulsions, CRS-1 and CSS-1 were selected. Three surface temperatures and three different tack coat

132 dosages used to evaluating effect of different parameters on tack coat bonding strength.

133 3- Materials and methods

134 Failure in the vicinity of pavement layer bonding zones can occur due to tension, torsion or shear. This happens due

135 to the composition of vertical and horizontal forces applied on the pavement surface in vehicle turning movements

136 and curves. Therefore, researchers have conducted various types of tests to evaluate the quality of tack coat in

137 different loading types and levels. These tests can be divided into three sets including tension, shear and torsion tests

138 (Raposeiras et al. 2013). In the tension mode test (Schenck-Trebe Test (Raposeiras et al. 2013)), a steel plate is

139 placed on the asphalt layer after tack coat is applied. When emulsion is dried, steel plate is pulled up using a loading

140 device. In these sets of tests, the surface roughness of two layers has no impact on the bonding strength. On the other

141 hand, in the shear mode test (Direct Shear Test (Romanoschi and Metcalf 2001), LCB test (NLT-328/08 2008) and

142 Lautner Test (Leutner 1979)) and torsion mode test (ATacker test device ('InstroTek Inc. 2003, ATACKER™ A

143 Tack Coat Testing Device, USA.')), both cohesive strength and surface roughness are influential factors. Among

144 these tests, shear mode test is more common for evaluating the bonding strength between layers (Cho 2016). Most of

145 these tests are carried out by constant loading rate but some researchers have also used cyclic loading for more

146 accurate simulation of vehicle movements (Donovan et al. 2000; Romanoschi and Metcalf 2001; Wheat 2007).

147 Loading rates from 1.27 to 720 mm/min have been proposed in different studies (Leng et al. 2008; Canestrari et al.

148 2005) and cylindrical specimens (100 to 150 mm diameter) and cubic samples are both used (Leng et al. 2008).

149 As previously mentioned, the bonding strength between a sand asphalt SAMI and a RCC surface was examined in

150 this research via using different tack coat applications. RCC was used as the existing bottom layer in this study and

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151 the sand asphalt layer was used as the overlay. It should be noted that the sand asphalt mixture used in this research,

152 works primarily as a Stress Absorbing Membrane Interlayer, which independently requires having a good bonding

153 to the underlying RCC layer. To determine the shear bonding strength between the two layers, cylindrical specimens

154 (Figure 2-a) were prepared. LPDS1 test device (Kim et al. 2010) (Figure 3) was also used to apply the load.

a b 155 Figure 2: a) Final double layered specimen and b) Application of tack coat on RCC156157

Figure 3: LPDS loading device

158 RCC samples were prepared using a 5cm height mold based on ASTM C192, cured for 28 days, and dried for 72

159 hours. Aggregates used for production of RCC and sand asphalt mixture were prepared from a mine in west of

1Layer-Parallel Direct Shear

Shear forces

Core head

Specimen(Φ10)

Clamp

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160 Tehran, including about 53% Silica in its chemical composition. Type I cement was used to prepare RCC samples

161 and cement content of 350 km/m3 and water to cement ratio of 0.42 were selected for this purpose. The compressive

162 strength, indirect tensile strength and flexural strength of RCC were 32.13, 2.9 and 3.91 MPa respectively. RCC

163 aggregate gradation together with the sand asphalt aggregates gradation were presented in Table 1 based on

164 percentage of passing aggregates (Rooholamini et al. 2018). Sand asphalt Marshall Strength and indirect tensile

165 strength were 830 Kg and 690 kPa respectively. The optimum binder percentage, Gmb and air void percentage were

166 8.4%, 2.45gr/cm3 and 3% respectively.

167 Table 1: RCC and sand asphalt aggregates gradations by percentage of passing aggregatesSieve No. RCC Sand Asphalt

25 100 -19 82 -

12.5 90 -9.5 80 1004.75 65 902.36 38 82.51.18 36 600.6 31 450.3 24 23.50.15 16 11.50.075 5 5.5

168

169 The RCC sample surface was in new condition and was not milled. Before the application of tack coat, RCC surface

170 was cleaned using a soft brush to remove any dust and tack coat binder was applied over the RCC surface using a

171 simple brush (Figure 2-b). Four types of binder were used as tack coat in order to increase bonding between RCC

172 and sand asphalt layer. These binders include a Grade 60/70 binder, a CRM (Crumb Rubber Modified) binder and

173 two emulsions, CRS-1 and CSS-1. Tables 2 and 3 show the physical properties of all four binders used as tack coat

174 in this research. Three different tack coat dosages of 200, 400 and 600 gr/m2 were considered and applied to the

175 surface. For grade 60/70 binder and CRM binders, the total weight of binder and for the emulsified binders, the

176 weight of residual binder after breaking and drying was considered in calculating the tack coat application rate. The

177 effects of RCC surface temperature on interface bonding was also evaluated in this research. For this purpose, the

178 tack coat binders were applied over the RCC sample at three different surface temperatures of 0, 25 and 60°C. To do

179 that, RCC samples were kept in a constant temperature chamber for 60 minutes before applying the tack coat

180 binders. These temperatures resemble the construction of the paved road in different months of the year, from the

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181 coldest to warmest weather conditions. The application temperature of grade 60/70 binder, CRM binder and

182 emulsions were 150±1°C, 170±1°C and 60±1°C respectively to achieve proper viscosity.

183

184

185 Table 2: Grade 60/70 bitumen and CRM binder specificationsBitumen Test Standard Grade 60/70 CRM

Penetration @ 25 °C ASTM D5 63.5 57.4Softening point (°C) ASTM D36 49.3 55.3Ductility @ 25 °C ASTM D113 107 28PG grades ASTM D6373 PG 64-22 PG 76-28

186187 Table 3: CRS-1 and CSS-1 emulsified binder specifications

Bitumen Test Standard CRS-1 CSS-1Primary emulsified bitumen

Saybolt Furol Viscosity (sec) ASTM D244 40 22Particle Charge ASTM D244 Positive PositiveOversized Particles (%) ASTM D6933 0 0Storage Stability (%) ASTM D6930 0.4 0Residue by Evaporation (%) ASTM D6934 2.5 0.2Distillation of Emulsion (%) ASTM D6997 63 63

Distillation residuePenetration @ 25 °C ASTM D5 105 115

188

189 It should be mentioned that the crumb rubber used in this research was prepared from recycled heavy weight vehicle

190 tires by crushing and grinding at ambient temperature. Particles passing sieve #40 were selected in order to modify

191 the Grade 60/70 neat binder. The moisture and steel tire wires that usually exist in crumb rubber sources can

192 significantly influence the performance of CRM binder and therefore their percentage should be limited to 0.1 and

193 0.75 respectively (Kavussi et al. 2014). These limits were also met in this study. The bitumen was mixed with 15%

194 weight of crumb rubber particles in a high shear mixer with 4000 rpm rotation speed for 45 minutes. Relatively high

195 percentage of crumb rubber and high specific surface area of fine crumb rubber particles were the main reasons

196 behind selecting the long blending duration time of 45 minutes (Amirkhanian et al. 2015) (Nejad et al. 2012). A mix

197 temperature of 180°C was also selected to achieve proper viscosity and better blending.

198 In the next step after applying the tack coat binder over RCC, samples were placed again in a mold. A curing time of

199 45 minutes was considered for the emulsion tack coat binders before placing the sand asphalt mixture while no

200 curing time was considered for neat binder. The curing time was selected based on the literature at room temperature

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201 to simulate the filed condition. The sand asphalt mixture was then laid over RCC and compacted using Marshall

202 Hammer. Considering that only the upper side of sand asphalt mixture could receive hammer blows, the number of

203 blows was determined based on the target air void content computed for the 98% of the average bulk density (EN

204 12697).

205 The layered specimens were demolded after being cooled at room temperature for another 30 minutes and then

206 became ready for the shear loading by LPDS test device. The loading was performed in a constant rate of 51

207 mm/min at a constant temperature of 20°C for all samples.

208 4- Results and discussion

209 As mentioned above, bonding strength between RCC and sand asphalt concrete layer was evaluated and four types

210 of tack coat and three surface temperatures of 0, 25 and 60°C were considered. In addition, three different dosages

211 of tack coat including 200, 400 and 600 gr/m2 were applied to each specimen surface. Three replicates of each

212 specimen were made and tested in similar conditions. Table 4 shows the average bonding strength of three

213 specimens and their corresponding coefficient of variation for all test conditions. Furthermore, average bonding

214 strength of specimens with 200, 400 and 600 gr/m2 tack coat was illustrated in Figures 4, 5 and 6 respectively.

215 Table 4: Bonding strength of all specimensType of tack coat Specimen

No.Dosage (gr/m2)

Bottom layer temperature (°C)

Bonding strength (N)

CV (%)

1 200 0 5740 3.72 200 25 6790 2.33 200 60 6700 1.54 400 0 6170 2.15 400 25 7010 1.26 400 60 7140 3.07 600 0 6800 6.48 600 25 7270 4.8

Grade 60/70

9 600 60 7550 3.410 200 0 5520 2.911 200 25 6840 2.812 200 60 6810 6.413 400 0 5930 4.814 400 25 7320 3.315 400 60 7640 1.316 600 0 6790 2.7

CRM binder

17 600 25 7450 1.4

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18 600 60 7670 2.319 200 0 6560 4.020 200 25 6620 3.221 200 60 6420 3.322 400 0 6530 6.023 400 25 7150 4.124 400 60 7060 3.425 600 0 5680 6.526 600 25 6720 3.7

Emulsion CRS

27 600 60 6800 3.728 200 0 6140 3.929 200 25 6800 2.330 200 60 7030 2.731 400 0 5840 2.232 400 25 6730 2.033 400 60 6880 1.234 600 0 5270 4.235 600 25 6230 2.6

Emulsion CSS

36 600 60 6690 1.5

216217 As it can be seen in Figures 4 to 6, bonding strength at 25°C and 60°C does not show a significant difference but

218 when the RCC surface temperature is 0°C, bonding strength significantly decreases. This clearly indicates that

219 spreading overlay which requires a tack coat application should be avoided at very low temperatures or at least

220 performed with special considerations if necessary. Furthermore, a comparison between all three figures shows that

221 the best type of tack coat, which reveals higher bond strength, depends on the tack coat application rates.

222 Considering Figure 4 (200 gr/m2 application rate), it seems that emulsified binders, especially CSS, shows slightly

223 better performance in comparison to grade 60/70 and CRM binder. However with increasing the application rates to

224 400 gr/m2 and 600 gr/m2 (Figures 5 and 6), it becomes clear that two emulsified binders cannot hold their superiority

225 over the two others. The CRM binder especially shows to be the best option at higher rates of application. These

226 finding also reveal that when emulsified binders are applied, lower application rates should be selected while

227 approaching the highest levels of bond strength is only possible with grade 60/70 and CRM binders. Bonding

228 strength graphs for each type of tack coat were illustrated in Figure 7 in order to find the optimum binder content

229 and application temperature.

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CRM CRS 60/70 CSS4000

4500

5000

5500

6000

6500

7000

75000 C 25 C 60 C

Bond

ing

stre

ngth

(N)

200 gr/m2

231 Figure 4: Bonding strength for 200 gr/m2 tack coat232

233CRM CRS 60/70 CSS

400045005000550060006500700075008000 0 C 25 C 60 C

Bond

ing

stre

ngth

(N)

400 gr/m2234 Figure 5: Bonding strength for 400 gr/m2 tack coat235

236CRM CRS 60/70 CSS

4000

4500

5000

5500

6000

6500

7000

7500

80000 C 25 C 60 C

Bond

ing

stre

ngth

(N)

600 gr/m2 237 Figure 6: Bonding strength for 600 gr/m2 tack coat

238239 Figure 7-a shows the bonding strength of CRM binder. As it is clear in this figure, increasing the surface

240 temperature and tack coat dosage both increased the bonding strength. The reason for the higher bonding strength at

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241 high temperatures is the lower viscosity of CRM binder and its deeper penetration into the RCC surface. It should

242 however be noted that the high strength of 7600 N is only available at very high temperatures of about 60°C which

243 usually occurs only at hot climate zones. The dosage of 400 gr/m2 to 600 gr/m2 both can reach this target. So it

244 seems that at high temperature conditions there is no need for higher application rate of 600 gr/m2, and the

245 application rate of 400 gr/m2 seems to be the more economical option.

246 Figure 7-b shows the bonding strength for grade 60/70 binder. Like the CRM binder, increasing the RCC surface

247 temperature and dosage of tack coat both improved the bond strength, although the bond strength seems to be

248 slightly less than that of CRM binder. The more uniform distribution of tack coat at higher temperatures and also its

249 deeper penetration into RCC surface are the main reasons behind it. However, unlike CRM binder, increasing the

250 application rate from 400 gr/m2 to 600 gr/m2 at high temperatures of about 60°C still increased the bond strength.

251 This is because of the lower viscosity of grade 60/70 binder at about 60°C compared to CRM binder which makes

252 its penetration into RCC surface much easier even at high dosage of 600 gr/m2.

253 According to Figure 7-c, CRS emulsion tack coat shows different response to surface temperature and tack coat

254 dosage. It can be seen in this figure that with an increase in the tack coat dosage from 200 gr/m2 to about 400 gr/m2,

255 bonding strength initially increased but then after increasing the tack coat dosage to about 600 gr/m2, bonding

256 strength started to decrease. Therefore, it seems that an optimum tack coat dosage of about 400 gr/m2 would be

257 recognized. The initial increase in the tack coat bond strength could be attributed to the residual binder content

258 increase after setting the emulsion. But then further increasing the tack coat dosage increased the water content

259 between the layers and this resulted to the decrease of bond strength among the layers. On the other hand, higher

260 surface temperature increased bonding strength especially when higher tack coat dosage was used. As mentioned

261 above, in higher emulsion dosage, more water is available and emulsion setting is slower. Higher surface

262 temperature increases setting rate and water evaporation increases the bonding strength.

263 CSS emulsion tack coat bonding strength is illustrated in Figure 7-d. It shows that unlike the 60/70 grade and CRM

264 binders, higher tack coat dosages reduced the bonding strength so that 200 gr/m2 application rate became the

265 optimum dosage for CSS emulsion.

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266 When slow setting emulsified binder was applied as the tack coat, 45 minutes breaking time before laying the sand

267 asphalt over the tack coat was not enough because excessive amount of water remained captive between layers. So

268 increasing the tack coat dosage indeed increased the captive water which reduced the bond strength. Increasing the

269 surface temperature however increased the bonding strength. The reason is that higher surface temperature has

270 increased setting rate and water evaporation.

271

272 (a) CRM binder (b) Grade 60/70

273

274 (c) CRS (d) CSS275 Figure 7: Bonding strength graphs for each type of tack coat276277 Failure at the interface of pavement layers under loading can show the behavior of layers and the tack coat and few

278 studies have evaluated the pavement layer interface characteristics and bonding strength. As an example, Leng et al

279 (2009), evaluated the bonding strength between a PCC and HMA layer and used different tack coats and different

280 RCC surface textures. To quantify the potential for interface slippage, tensile strength at the bottom of HMA was

281 measured. Ktari el al (2016), used Digital Images Correlation (DIC) analysis to evaluate the interface behavior in the

Dos

age

(gr/m

2 )

Temperature (°C) Temperature (°C)

Temperature (°C) Temperature (°C)

Dos

age

(gr/m

2 )

Bon

d St

reng

th (N

)B

ond

Stre

ngth

(N)

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282 pavement layers under monotonic tension loading. Mohamad et al (2015), considered the friction and cohesion

283 coefficients of different surface textures on the bonding strength of two concrete layers.

284 The interface failure is also evaluated in this study. Figure 8 shows the failure interface of different specimens with

285 600 gr/m2 tack coat dosage under shear load induced by LPDS testing device. The fracture surface can occur in any

286 of three conditions, in RCC layer (colored light gray), in the interface (colored dark gray) or in the sand asphalt layer

287 (colored black). If the fracture occurs inside the concrete (light gray) or sand asphalt layer (black), it means that the

288 tack coat can provide a good bonding between the layers and failure does not occur at the interface of two layers.

289 However, when debonding occurs at the interface (dark gray), the tack coat bonding strength seems not to be

290 sufficiently high. The dark gray stain from the binder remained over the debonding interface is the sign of this type

291 of failure.

292 For the CRM binder, it can be seen in this figure that a small part of the fracture surface occurred inside the RCC at

293 0°C. Then a small part occurred inside the RCC and a relatively larger part occurred inside the sand asphalt at 25°C,

294 and finally a very large area occurred inside the RCC layer at 60°C. For the grade 60/70 binder, again a small

295 surface fracture occurred inside the RCC surface at 0°C, then two small parts of fracture occurred inside the RCC

296 and sand asphalt layers at 25°C. Finally, a large area of fracture occurred inside the sand asphalt layer at 60°C. It can

297 be concluded from these observations that when CRM or grade 60/70 binders were used as tack coat, part of the

298 failure surface occurs either inside the RCC or the inside the sand asphalt layer (randomly) and this area almost

299 increases with increasing the temperature. This further shows that these two binders possess higher tack coat bond

300 strength compared to the emulsified binders. CSS emulsion has revealed a more or less steady behavior at all

301 temperature levels and that was the failure at the interface, which implies lower bond strength of this binder. That is

302 the same for the other emulsified binder, CRS, except at high temperature level of 60°C. CRS emulsion fracture area

303 occurred at the bonding interface (indicating low bond strength) at 0°C and 25°C, and occurred partly inside the

304 concrete layer at 60°C (indicating relatively higher bond strength). These findings are more or less consistent with

305 the results shown in Figure 7 for the dosage of 600 gr/m2.

Bottom layer temperature CRM binder 60/70 CSS CRS

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0°C

25°C

60°C

306 Figure 8: Failure interface of specimens with different types of tack coat

307 5- Summary and conclusions

308 This study evaluated the effects of influential parameters on interface bonding strength between RCC and sand

309 asphalt SAMI using LPDS testing device. Four types of tack coat including grade 60/70 neat binder, CRM binder,

310 and CRS and CSS emulsions were used as tack coat. To determine the optimum dosage of tack coat, three dosages

311 of 200, 400 and 600 gr/m2 were applied. Three different RCC surface temperatures of 0, 25 and 60°C were also

312 considered to account for different weather temperatures of construction site. Based on the results, the following

313 conclusions were made:

314 Among the above-mentioned tack coats, in most cases CRM binder provided the highest bonding strength at the

315 interface of two layers.

316 CSS emulsion can also be selected as a good tack coat only at very low application rate of 200 gr/m2. The best

317 performance from the CRS emulsion can also be achieved at the application rate of 400 gr/m2. In selecting the

318 best dosage for the emulsified binders, the remaining water between the layers should be considered as a key

319 parameter.

320 Increasing tack coat dosage for grade 60/70 neat binder and CRM binder to 600 gr/m2 increased the bonding

321 strength.

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322 Bottom layer surface temperature (RCC layer), showed to have significant effects on the bonding strength

323 between layers. When the surface temperature was around 0°C, bonding strength decreased almost for all binders

324 and application rates. Applying the tack coat at higher surface temperatures significantly improved the bonding

325 strength for CRM and grade 60/70 binders when high dosages of tack coat were applied, and for CSS emulsion

326 at all dosages.

327 Analysis of fracture surfaces showed that when the surface fracture occurred partly inside the sand asphalt or

328 RCC layers, the bond strength were also higher and whenever the debonding from the interface occurred, the

329 bond strength started to decrease.

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