Laboratory evaluation on performance of porous polyurethane ...

Construction and Building Materials 169 (2018) 436–442

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Laboratory evaluation on performance of porous polyurethane mixtures and OGFC Cong Lin a,⇑, Wang Tongjing a, Tan Le b, Yuan Junjie c, Shi Jiachen a a

Key Laboratory of Road and Traffic Engineering of the Ministry of Education, Tongji University, Shanghai 201804, China Highway Administration of Huzhou, Huzhou 313000, China c School of Materials Science and Engineering and Key Laboratory of Advanced Civil Engineering Materials, Ministry of Education, Tongji University, Shanghai 201804, China b

h i g h l i g h t s The laboratory test results show that PPM2 has the potential to substitute traditional OGFC. The fatigue life of PU-bonded porous mixtures was over one order of magnitude greater than that of asphalt porous mixtures. Generally, the mechanism of PU is between that of asphalt and cement. By increasing Tg of PU binder, the severe moisture damage problems in PPM were solved.

a r t i c l e

i n f o

Article history: Received 7 September 2017 Received in revised form 13 February 2018 Accepted 21 February 2018

Keywords: Polyurethane (PU) OGFC Porous pavements Cantabro loss Dynamic mechanical analysis (DMA) Fatigue test

a b s t r a c t The application of open grade friction course (OGFC) is limited by problems such as mixture disintegration and relatively poor fatigue performance. In this study, polyurethane (PU), a polymer binder, and PUbonded mixtures were studied for overcoming the limitations of traditional asphalt-bond OGFC mixtures. The viscosity-time and mechanical properties of two PU binders were characterized using Rheometer, tensile test and dynamic mechanical analysis (DMA). Then a laboratory evaluation of the PU-bonded porous mixtures (PPM) and asphalt-bonded porous mixtures (APM) were undertaken using the Marshall stability test, the Cantabro Loss test, and the fatigue test. The test results show the PPM obtained three times higher stability and over one order of magnitude greater fatigue life than APM. When compared with the low glass-transition temperature ðT g Þ PU binder, the high T g PU binder significantly improved resistance to moisture damage and lowered Cantabro loss of PPM in the 60 °C immersion conditions. Results of this study will provide an important reference for utilizing PU porous mixtures as OGFC surface layer, and discover ways to improve OGFC durability. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Open grade friction courses (OGFC) have shown to be of great benefit in terms of traffic safety, environmental-friendliness, noise-reduction, and economy [1–4]. The porous structures of OGFC are able to drain water faster from pavement surface, reducing the risk of wet skidding and splash. However, problems such as clogging and mixture disintegrations have obstructed the widespread use of OGFC. Research has been done to improve the long-term performance of OGFC [5–7]. Qureshi et al. [8] considered indirect tensile (IDT) strength as criteria for performance evaluation of OGFC, through both field and lab investigations. Coleri et al. [9] identified layer ⇑ Corresponding author. E-mail address: [email protected] (L. Cong). https://doi.org/10.1016/j.conbuildmat.2018.02.145 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

thickness as a major factor affecting the anti-rutting performance using full-scale APT. Punith and Veeraragavan [10] found that reclaimed polyethylene (PE) as an additive is able to improve the performance of OGFC on tensile strength and anti-deformation performance. Reducing air voids and increasing layer thickness have been effective in improving durability, however, reducing air voids weakens drainage performance, and a thicker layer increases cost [4,11–14]. Other strategies have focused on adding polymers to asphalt, such as rubber and fiber, and have shown some level of durability enhancement [15–18]. We focused on searching for better performance binders to entirely substitute asphalt binders. One of these new binders showing great potential to overcome the failures of traditional asphalt binders is polyurethane (PU). PU is a type of polymer joined by carbamate (urethane) links, that has been used in a myriad of applications worldwide, from automobiles to airplanes.

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L. Cong et al. / Construction and Building Materials 169 (2018) 436–442

(1) to examine rheological property and characterize the mechanical properties of PU binders. (2) to compare and evaluate the mixture performance of PPM and APM. (3) to compare the fatigue life between PPM and APM in dry and after freeze–thaw cycle conditions.

2. Introduction of PU The cohesion of the PU binders came from covalent bonds between polymer chains, which increased with the extent of reaction until the reaction was completed. Polyurethane polymers are most commonly formed by reacting a di- or poly-isocyanate with a polyol:

R1 AN@C@O þ R2 AOH ! R1 ANHCOOAR2 Curing time varies from minutes to days, depending on multiple factors, such as temperature and functional groups [28]. During the curing process, by synthesizing short urethane prepolymers, PU prepolymers start providing adhesion until fully cured. A threedimensional hybrid structure is formed during the curing process wherein the inorganic and organic phases are bonded with covalent bonds [29]. This structure generally presents strong chemical, radiation, and heat resistance [30,31]. The process is irreversible

and irrecoverable. Commonly used tests for asphalt binders such as Dynamic Shear Rheometer (DSR), which require materials to be in a liquid state, are not appropriate for PU binders. 3. Experiments 3.1. Materials Two kinds of PU binders were used in this study, PU1, and PU2, which were developed by BASF Polyurethane Specialties (China) Company Ltd. (BASF). A pre-calculated amount of part-A was mixed with part-B at a ratio of 100:60 using a vacuum mixer at 2050 rpm for 2 min at room temperature. Three different mixtures were compared in this study, which were bonded by PU1 and PU2, and SBS (Styrene–Butadiene– Styrene) modified asphalt binder with additional anti-strip additives at 0.3%, respectively. These were named PPM1, PPM2, APM. A 13 mm nominal maximum aggregate size of OGFC gradation was used in this study for preparing mixture samples and the designed air void was set at 18%. The gradation is shown in Fig. 2. This gradation is commonly used in Shanghai, China. The coarse aggregate used was basalt from Guizhou, China and the fine aggregate used was limestone from Jiangsu, China. The asphalt content ratio and PU ratio are defined as the percentage of aggregate weight. 3.1.1. PU mixture preparation For preparing PU mixtures, the PU prepolymer was mixed with aggregates according to standard practice for OGFC (ASTMD7064) [32]. However, the freshly mixed PU prepolymer does not provide enough adhesion to aggregates after the first side is compacted to allow the mixtures to be turned upside down for the second side compaction, as is done in ASTMD7064. Turning the PU mixtures upside down would compromise the previous compaction. Thus only one side of the Marshall specimens had 50 blows applied, and the mixtures were left until fully cured. Once PU prepolymer starts curing, any disturbance may affect the final strength of the bonding. Thus to ensure the binders were fully cured, the compacted Marshall specimens were left upright in their mold collars on special base plates for over 24 h before extracting specimens from the mold collars, as shown in Fig. 3. The base plates were used to prevent mixture slumps in the mold collars during the curing process.

100 90

Cumulative Passing Rate (%)

The high versatility of PU is due to its ability to form various types of molecular architectures [19–23]. In the construction industry, PU has been widely used as an additive for bitumen [24] and in foam injections for crack repairs [22]. Tentative experiments on PU as a binder for porous sidewalk pavement have been conducted in many countries such as China, South Korea, and Sweden [25–27]. Those studies have indicated that PU sidewalk pavements have better anti-skid performance than traditional asphalt or cement sidewalk pavements. Additionally, a PU mixture paving project in Chongqing, China has shown that traditional in-suit asphalt mixture mixers and pavers are suitable for PU-bonded mixtures [17]. However, the mechanical properties of PU-bonded mixtures and the relationship between PU and mixtures have not yet been explored. Further analysis of the mechanism of PU porous pavement and improvements to the PU formula needs to be done to fully exploit the potential of this extremely versatile elastomer. The object of this study is to characterize PU rheological and mechanical properties and to compare mixture performance between PU-bond porous mixtures (PPM) and asphalt-bond porous mixtures (APM), based on laboratory testing. Results of this study will provide an important reference for utilizing PU porous mixtures as OGFC surface layer, and discover ways to improve OGFC durability. This study’s experiments contained three parts. The strategy of this research is illustrated in Fig. 1.

80

Upper Limit Obtained Lower Limit

70 60 50 40 30 20 10 0 10-2

Fig. 1. Methodology of evaluation on the PPM.

10-1

100 Sieve Size (mm)

101

Fig. 2. Gradation Curve of Mixtures.

102

438


3.3.1. Marshall stability test Samples were prepared in two conditions: the standard condition and the 60 °C immersion condition. In the standard condition, samples were immersed in 60 °C for half an hour before testing. In the 60 °C immersion condition, samples were immersed in a 60 °C thermostatic water tank for 48 h before testing. Then tests were conducted to compare the performance of PPM1, PPM2, and APM according to Technical Specifications China T 0709–2011 [35].

3.3.2. Cantabro test Samples were prepared in the standard condition and 60 °C immersion condition. In the standard condition, samples were immersed in 20 °C for 20 h before testing. The 60 °C immersion condition samples were immersed in a 60 °C thermostatic water tank for 48 h before testing, as in the Marshall Stability test. The tests were conducted according to Technical Specifications China T 0733–2011 [36].

Fig. 3. The compacted Marshall Samples.

3.2. Tests for PU binders 3.2.1. Rheological property measurement Viscosity-time is an important factor affecting the mixing ability of aggregates with binders [33]. For achieving good mixing ability and compaction, PU-bonded mixtures should be produced before the PU starts hardening and the mixtures should be kept undisrupted until the PU is cured. Thus it is necessary to understand the time-dependent viscosity of PU in order to estimate the amount of time available for PU mixture production. We defined this as the operation time for PU mixtures. A Haake Rheometer 90 was used for measuring the viscositytime characteristics. The rheometer was conducted at 20 C. The freshly mixed PU prepolymer was placed into the rheometer, and the machine was set to rotate at 20 r=min. It measured the resistance (torque) exerted by the curing PU prepolymer on the cylinder every 6s. The torque was representative of the viscosity of the PU binders during the curing process.

3.3.3. Fatigue test The fatigue test was set at strain-controlled mode to determine the fatigue life of PPM2 and APM. First, an Indirect Tensile (IDT) Test (ASTM D6931-17) was conducted to measure the IDT strength of PPM2 and APM Marshall samples, before and after one freeze–thaw cycle (18 C for 16 h then 60 C for 24 h) [37]. Then the fatigue test was conducted using the Material Test System 810 (MTS 810), and the loading cycles to failure were calculated. The fatigue performance at a stress-strength ratio of 0.2, 0.3, 0.4, 0.5 was evaluated based on PPM2 Marshall samples with 5% PU2 ratio and the loading frequency was set at 10 Hz, which is a commonly adopted frequency for fatigue tests.

3.2.2. Tensile test The mechanical properties of PU binders were commonly represented by tensile test results. Tensile tests were conducted according to the ASTMD638 [34] using type IV specimen at room temperature. The specimens were cut into rectangular shapes with dimensions:

4.1.1. Rheology of PU prepolymer The test results in Fig. 4 show that the torques (viscosity) of the PU1 and PU2 prepolymer started increasing rapidly after 25 min and 30 min, respectively. This indicates that the mixing and compaction time of PU-bonded aggregates is relatively short compared with traditional asphalt mixtures. For achieving even mixing of the PU-bonded mixtures, ensuring sufficient compaction, and avoiding damage to the curing binders, it is better to mix and pave in situ.

thickness ¼ 4 mm; width ¼ 10 mm; length ¼ 100 mm

4. Results and discussion 4.1. PU test results

Specimens were loaded quasi-statically until failure, using an MTS809 axial/torsional testing system at a crosshead speed of 2 mm/min. Properties such as tensile strength, strain at rupture, and E-modulus were calculated. Four replicates were prepared for obtaining reliable results.

900 800

PU1 PU2

3.2.3. Dynamic mechanical analysis (DMA) The DMA test is used for measuring the glass transition temperature (T g ) and charcterizing the viscosity-temperature behavior of PU1 and PU2. This test was conducted on a TA Q800 dynamic mechanical analyzer at the frequency of 1 Hz, temperature range 1

of 20—100 C, and the heating rate was set at 2 K min . The size of the specimens was 4 mm 10mm 50mm.

Torque(Nm)

700 600 500 400 300 200 100 3.3. Tests for PU-bonded mixtures Four replicates were prepared for each of these mixture tests. Test results of the Marshall Stability, Cantabro loss, and Fatigue life were averaged and evaluated.

0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Time(min) Fig. 4. Hakke Rheometer Test Results.

439


1200

PU1 PU2

800 600 (a) Samples Before Testing

400

Fig. 6. Tensile Test Process.

200 106

0

20

40

60

PU1 Storage Modulus PU1 Loss Modulus PU1 Tanδ PU2 Storage Modulus PU2 Loss Modulus PU2 Tanδ

80 Storage/Loss Modulus(MPa)

0 -20

(b) Samples After Testing

strain(%) Fig. 5. Tensile Test Samples.

4.1.2. Tensile response The tensile stress–strain curves of PU1 and PU2 binders are shown in Fig. 5. A sharp contrast of viscoplasticity was observed between PU1 and PU2. PU1 binders have relatively high tensile strains up to the point of rupture, which means PU binders have better ductility. On the other hand, PU2 obtained a low strain but much higher stress, which indicates that PU2 tends to be more brittle. High tensile strength also indicates high adhesion within PU binders. The modulus of elasticity was calculated from the slope of the initial linear part to 5% strain. Summary of the tensile results is shown in Table 1. The specimens during and after testing can be seen in Fig. 6. The mechanical properties of PU are affected by numerous factors. The difference of two PU binders in Table 1 is due to different lengths of the polymer chain segments, molecular weights and degrees of crosslinking etc. [31]. 4.1.3. Dynamic mechanical properties of PU The dynamic mechanical properties of PU1 and PU2 were measured by DMA, where the dynamic mechanical indexes, such as the loss factor (tan d) and the storage modulus (E0 ), indicate the elasticity of PU. The loss modulus(E00 ), indicates the plasticity of PU. The main objective is to investigate the glass-transition temperature (T g ). The T g indicates a range of temperature over which this glass transition occurs, which in general includes three temperatures, the extrapolated onset of the sigmoidal change in storage modulus (T g ), as the peak of the loss modulus signal (T l ), and as the peak of the tangent delta signal (T t ) [30]. The test result is shown in Fig. 7. There are four points to be noted. (1) Two peaks in PU2 Tan d were observed. This is because the tested PU binders are composed of two different polymer segments. Tan d of PU1 also contains two peaks, but the two peaks have some overlap.

105

1 0.9 0.8 0.7

104

0.6 0.5

103

Tan δ

Stress/MPa

1000

0.4 0.3

2

10

0.2 101 20

0.1 40

60

80

100

120

140

0 160

Temperature ( C) Fig. 7. DMA Curve.

(2) The relatively broad PU1 tan d peak ðT t Þ indicates that this type of binder has a relatively wide range of polymer chain lengths and chain branching within the epoxy. (3) The PU2 T t was over 100 C, which was much higher than PU1 T t , indicating that PU2 binders have better high temperature stability. (4) Fig. 7 also shows the viscosity-temperature behavior of E0 and E00 with the increase of temperature. In contrast to the sharp decrease of both of PU1 E0 and E00 , PU2 showed relatively stable viscosity-temperature behavior up to 70 C.

4.2. Mixture performance of PPM 4.2.1. Marshall stability test results The Marshall test results of PPM1, PPM2, and APM are shown in Fig. 8. As shown in Fig. 8, the stability of PPM1 and PPM2, with very low binder ratio, was already much higher than that of APM. With the increase of binder ratio, the stability of both of PPM1 and PPM2 first improved from about 10 kN to over 36 kN, and then kept on 36kN, which is about 4 times higher than that of APM, in general. This test shows that PPM obtained great performance to resist excessive deformation.

Table 1 Summary of the Tensile Test Results. Material

Test Feature

Mean Value

Std.-dev

Unit

PU1

Tensile Strength Strain at Rurpture E-modulus Test speed

25.3 67.2 390 20

0.4 1 30.1 0

MPa % N=mm2 mm/min

PU2

Tensile Strength Strain at Rurpture E-modulus Test speed

66.2 9.8 2416.7 20

0.3 2.1 20.8 0

MPa % N=mm2 mm/min

440


6 PPM2 Before Freeze-thaw PPM2 After Freze-thaw APM Before Freeze-thaw APM After Freze-thaw

f

log10 (Cycles,N )

5.5

Fig. 8. Marshall Stability Test Results.

5

4.5

4

3.5

3 0.1

0.2

0.3

0.4

0.5

0.6

Stress Ratio Fig. 11. Fatigue Test Results.

interface between the PU1 and the aggregates, compromising adhesion and reducing the stability of PPM1.

Fig. 9. Cantabro Test Results.

(a) 5% PPM before Test

(b) 5% PPM after Test

4.2.2. Cantabro test results As shown in Fig. 9, the Cantabro loss of both of PPM1 and PPM2 decreased with the increase of the binder ratio. This is because the adhesion between the binder and aggregates strengthened with the increase of the binder film thickness. Similar results were observed in the Marshall Stability test. Although the Cantabro loss was similar in standard Cantabro Tests, in Immersion Cantabro Tests the PPM2 samples showed better resistance to disintegration than PPM. The test results showed that the particle loss resistance of this new type of mixture is slightly lower than APM. This is because the strong cohesion provided by the PU binder also increased the strength of the mixtures, which led to brittleness. This weakness should be considered and improved upon in the future design of PU molecule structures. The Cantabro loss also relates to the adhesion between stone and binders. As shown in Fig. 10, both of the 2% and 5% PU1 films of PPM after Cantabro tests were observed, and the fracture surfaces were smooth, which demonstrates that fracture energy was low [38].

4.3. Fatigue test results

(c) 2% PPM before Test

(d) 2% PPM after Test

Fig. 10. Cantabro Test Specimens.

In the Immersion Marshall Stability test, a sharp contrast in stability was discovered between that of PPM1 and PPM2. The stability of the PPM1 samples dropped drastically after immersion, but the PPM2 samples maintained high stability. The Immersion Index shows that the PPM2 kept over 95% Marshall stability after immersion, but PPM1 kept only 30% and APM 88%. The PU1 T g was only about 50 °C, so in the 60 °C immersion condition, the high temperature water softened PU1, which caused water seepage into the

Previous test results indicated that PPM2 has better overall performance than PPM1. Thus PPM2 was selected for comparison with APM for the evaluation of the long-term mechanical performance. The fatigue life was calculated and plotted on a log scale in Fig. 11. As shown in Fig. 11, the fatigue life of PPM2 was over one order of magnitude greater than that of APM, and with the increase of stress-strength ratio, PPM2 maintained good fatigue performance, versus APM, which showed a linear loss of fatigue life. Nevertheless, after one freeze–thaw cycle, the fatigue life of PPM2 dropped at a relatively high stress-strength ratio. This could be explained as the thermoset PPM2 bond was damaged by water expansion on freezing, and could not self-recover. This type of damage was negligible in low stress-strength ratio levels but magnified with increased stress-strength ratio levels. Overall the fatigue performance of PPM2 still significantly outperformed APM.


5. Conclusion The characteristics of the two PU binders and the moisture damage performance of PU mixtures were investigated and compared with traditional OGFC in this paper. The rest results can be summarized in the following points: (1) The freshly mixed PU prepolymer does not provide adhesion and the viscosity is low. However, after approximately 30 min, the viscosity of PU started increasing rapidly. For achieving good mixing ability and compaction, it is better to produce the PU-bonded mixtures before PU starts hardening and keep mixtures undisrupted until PU is cured. Therefore, PU mixtures should be paved in situ. (2) The laboratory test results show that PPM2 has the potential to substitute traditional OGFC since PPM2 obtains three times higher stability and over one order of magnitude greater than OGFC. The weakness of PPM2 was the relatively high particle loss resistance due to its brittleness. (3) The test results on PU binders show that the mechanical properties of PU binders can be modified in a wide range, but improving the strength generally results in more brittleness. In general, the mechanism of PU is between that of asphalt and cement, which means this material has the potential to strike a balance between elasticity and strength, and create more tenacious mixtures. (4) The comparative studies show that by increasing T g of PU binder, the severe moisture damage problems in PPM were solved. This paper shows that PPM2 is able to satisfy OGFC mixture standards for highway use, however, more experiments should be designed to test the special characteristics of this new generation binder, studying UV aging and interface mechanics. Field tests are also required to study the long-term performance of PPM2 since the laboratory-based tests are not sufficient to effectively simulate field conditions and accurately evaluate the performance in complex field conditions. A technical issue limiting the application of PPM on pavement is that isocyanate also reacts with water and releases carbon dioxide:

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