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Potential of recycled concrete aggregate pretreated with waste cooking oil residue for hot mix asphalt
Journal of Cleaner Production 221 (2019) 469e479
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Potential of recycled concrete aggregate pretreated with waste cooking oil residue for hot mix asphalt Jianmin Ma a, Daquan Sun a, *, Qi Pang b, Guoqiang Sun a, Mingjun Hu a, Tong Lu a a b
Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, Shanghai, 200092, PR China China Changjiang Construction Investment. Corp. Ltd, Chengdu, 610218, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 September 2018 Received in revised form 24 February 2019 Accepted 25 February 2019 Available online 2 March 2019
Application of recycled concrete aggregate (RCA) in hot mix asphalt (HMA) is a sustainable alternative to natural aggregate. However, the increased asphalt consumption and decreased mechanical properties induced by the incorporation of RCA limit the extensive substitution of natural aggregates with RCA in HMA. In this paper, an experimental program was carried out to investigate the potential of recycled concrete aggregate pretreated with waste cooking oil residue (WCOR) with the expectation of reducing asphalt consumption and enhancing performance of HMA. The optimum pretreatment process for coarse and fine RCA in terms of content of WCOR, curing time and curing temperature was determined by an orthogonal design method and the grey correlation analysis. Subsequently, a laboratory characterization of HMA incorporating 40% coarse RCA, 40% coarse RCW (RCA pretreated with WCOR), 20% fine RCA and 20% fine RCW was further conducted in terms of optimum asphalt content, resistance to permanent deformation, cracking resistance at low temperature, fatigue performance, moisture sensitivity and dynamic modulus. Finally, scanning electron microscopy (SEM) test and fluorescence microscope (FM) test were performed to provide insight into the mechanism of WCOR-induced performance alteration. The experimental results showed that the pretreatment of RCA with WCOR decreased the optimum asphalt content from 5.4% to 4.5% for HMA incorporating 40% coarse RCA, from 5% to 4.5% for HMA incorporating 20% fine RCA. Moreover, RCA pretreated with WCOR enhanced significantly fatigue life and low temperature performance of HMA, while had a slight adverse effect on moisture sensitivity, permanent deformation resistance and dynamic modulus. Based on the SEM and FM tests, WCOR was observed to penetrate into tiny voids of cement mortar attached on RCA, which contributed to the reduction of optimum asphalt content and enhancement of fatigue life and low temperature performance of HMA. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Asphalt Recycled concrete aggregate Waste cooking oil residue Pretreatment Performance evaluation
1. Introduction With the rapid industrialization and urbanization in China, more than 18 billion tons of construction and demolition (C&D) wastes were generated during urban construction and reconstruction activities every year, while the recycling rate is less than 5%, far less than that of more than 95% in Europe and America (Huang et al., 2018). On the one hand, the increasing amount of construction waste and debris occupy plenty of land resources, which may restrict urban development. On the other hand,
* Corresponding author. Department of Road and Airport Engineering, Tongji University, 4800 Caoan Road, Shanghai, 201804, PR China. E-mail address: [email protected] (D. Sun). https://doi.org/10.1016/j.jclepro.2019.02.256 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
uncontrolled disposal of C&D wastes incorporated with hazardous substance may contaminate soils and water supplies, deteriorating the environmental condition and endangering human health rez et al., 2010; Pe rez and Pasandín, (Marinkovi c et al., 2010; Pe 2017). Therefore, the handling of construction waste disposal is particularly important nowadays in promoting sustainable development (Poon et al., 2006). Highway construction is a dominant industry consuming substantial amounts of natural resources especially mineral aggregates derived from quarry extraction (Blankendaal et al., 2014). Recently, the development of highway construction in China has made virgin aggregate increasingly scarce, the depletion of natural sources has also brought to the universal attention for the concern of sustainable development (Ding and Xiao, 2014). Accordingly, the environment protection department in China has issued a series of
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policies to restrict stone mining for the sake of environmental protection, which further exacerbates the scarcity of natural aggregate for pavement engineering. It is worthwhile mentioning that the unfinished highway construction and maintenance of the enormous highway network still requires a huge quantity of aggregates. Therefore, it is urgent to develop a new alternative to substitute virgin aggregates or reuse recycled aggregates from building debris. Among the proposed approaches, recycling was proved an effective way to reduce C&D waste disposal in landfills and preserve natural resources (Zhang et al., 2016; Zhu et al., 2012). Recycled concrete aggregate (RCA) is a representative of recycled aggregates obtained from C&D waste (Mills-Beale and You, 2010; Afonso et al., 2016). In general, the physical and mechanical properties of RCA are worse than those of natural aggregates due to the porous and weak mortar attached to RCA (Lee et al., 2012; rez Aljassar et al., 2005), e.g. lower resistance to fragmentation (Pe rrez, et al., 2012a), higher water absorption (De Juan and Gutie 2009) and lower density (Rafi et al., 2011). Additionally, owing to the heterogeneous sources and variant crushing process of RCA, the composition of RCA varies extensively and asphalt mixtures incorrez et al., porating RCA exhibit controversial characteristics (Pe 2010, 2012b; Paranavithana and Mohajerani, 2006; Shen and Du, 2004). Consequently, controversial conclusions were obtained for the impact of RCA on performance characteristics of HMA, including moisture damage resistance (Cho et al., 2011; Wu et al., 2013; Zhu et al., 2012) and resistance to permanent deformation (Mills-Beale and You, 2010; Radevi c et al., 2017; Zhu et al., 2012). rez et al., 2007) and low While the fatigue life (Chen et al., 2011; Pe temperature resistance (Chen et al., 2011; Wu et al., 2013; Zhu et al., 2012) of RCA mixtures exhibit a similar trend of improvement compared with conventional mixture. It is noteworthy that HMA incorporating RCA induces higher asphalt consumption compared with conventional asphalt mixtures (Bhusal et al., 2011; Cho et al., 2011; Lee et al., 2012; Wong et al., 2007). This difference was more apparent for finer RCA mixture that has a larger specific surface area (Rafi et al., 2011). To mitigate the problem of high asphalt consumption and performance degradation, Bhusal et al. (2011) suggested that it is better to incorporate coarse RCA rather than fine RCA in HMA. Several other studies have also tried different methods to pretreat RCA in recent years, these methods can be divided into two categories: calcination pretreatment (Pasandín rez, 2013) and surface pretreatment (Pasandín and Pe rez, and Pe 2014). In calcination pretreatment, RCA is calcined at high temperature, part of the calcium carbonate on the surface of RCA is decomposed into calcium oxide, eventually making the surface denser and enhancing the adhesion to asphalt (Wong et al., 2007). However, it still remains challenging to overcome the large energy consumption for the consideration of sustainable development. In surface pretreating pretreatment, different materials including liquid silicone resin (Zhu et al., 2012; Hou et al., 2014), cement mortar (Lee et al., 2012) and emulsified asphalt (Pasandín and rez, 2014; Eisa, 2018; Giri et al., 2018) et al. were adopted to Pe pretreat RCA to enhance the adhesive properties of RCA and asphalt. To further decrease the potential degradation of asphalt mixture resulted from the poor nature of RCA, Kareem et al. (2018) developed a double coating technique in which the first layer is cement slag paste and the second layer is acrylic based bitumen waterproofing membrane. These measures have been proved to be effective for the improvements in mechanical performance and durability. Nevertheless, further efforts are required to be devoted to the balance of cost and operational feasibility in plant before practical use. In this paper, a kind of industrial waste named waste cooking oil residue (WCOR) was used as the pretreatment material of RCA. The optimum asphalt content for both fine and coarse RCA mixtures
before and after pretreatment with WCOR was investigated. Subsequently, performance characteristics including permanent deformation resistance, cracking resistance at low temperature, fatigue performance, moisture sensitivity and dynamic modulus were evaluated to analyze the effectiveness of the pretreatment. Finally, the microscale morphology differences in interfaces of RCAasphalt mastic before and after treatment were analyzed by SEM, the distribution of WCOR in the interfaces of RCA-asphalt mastic was captured by fluorescence microscope (FM) to provide insight understanding of morphological change in asphalt mastic-RCA interface.
2. Materials 2.1. Asphalt The neat asphalt with penetration grade 70 was provided by China Petroleum & Chemical Corporation Co. Ltd. The basic properties of the neat asphalt were presented in Table 1.
2.2. Waste cooking oil residue (WCOR) WCOR, a kind of black viscous oily liquid (Sun et al., 2017a), was provided by Shanghai Environmental Protection Technology Co. Ltd. It was the byproduct of biodiesel refined with waste cooking oil through alkaline catalysis process. The chemical element of WCOR mainly includes C, H, O and a relative small proportion of N and S (less than 10%). Based on Fourier Transform Infrared Spectroscopy (FTIR) analysis in our previous study (Sun et al., 2017b), the main compounds in WCOR were found to be saturated fatty acids and esters. Moreover, the estimated moisture content of WCOR is approximately 3% by mass. This kind of soft waste oil residue was mainly used to modify asphalt binder (Gong et al., 2016; Sun et al., 2016) or serve as a potential substitute for petroleum asphalt in pavement engineering (Sun et al., 2017b).
2.3. Aggregate Two types of aggregates including natural aggregate and recycled concrete aggregate (RCA) were used in the preparation of HMA. Natural aggregates are crushed limestone with five different particle sizes: 0~3 mm, 3~5 mm, 5e10 mm, 10e15 mm and 15e25 mm, respectively. RCA is produced following steps of crushing, screening and sieving. The particle size distribution of fine RCA and coarse RCA are 0e4.75 mm and 4.75e13.2 mm, respectively. The results of X-ray fluorescence (XRF) chemical composition analysis indicated that the main constitutions of RCA are: SiO2 (49.40%), CaO (20.65%) and Al2O3 (12.00%). The limestone natural aggregates are primarily composed of CaO (60.3%), SiO2 (7.88%) and MgO (6.16%) and other substances. The physical and mechanical properties of RCA and natural aggregate were listed in Table 2. It was found that both coarse and fine RCA present a lower density and a higher water absorption, which results from the poor nature of cement mortar attached to rrez, 2009; Pe rez the recycled aggregate in RCA (De Juan and Gutie et al., 2012a). Due to the larger specific area for fine RCA, the density of fine RCA is lower than coarse RCA and the water absorption of fine RCA is larger than coarse RCA, in accordance with results in references. Additionally, the attached porous and weak mortar layer on recycled aggregate causes a lower mechanical property compared with natural aggregate and the corresponding crushing value of RCA is larger than natural aggregate.
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Table 1 Basic properties of neat asphalt. Penetration (25 C, 100 g, 5s)/0.1 mm
Viscosity (135 C)/Pa$s
Ductility (5 cm/min,15 C)/cm
67.4
0.569
>100
Table 2 Material properties of RCA and nature aggregate. Properties
Fine RCA
Limestone (0~3 mm)
Coarse RCA
Limestone (5e15 mm)
Apparent density (g/cm3) Bulk density (g/cm3) Water absorption (%) Aggregate crushing value (%)
2.639 1.957 11.63 24.4
2.573 2.441 2.06 19.1
2.539 2.257 4.9 31.2
2.706 2.673 0.46 23.7
3. Methodology 3.1. RCA pretreated with WCOR The pretreatment process of RCA using WCOR mainly includes three steps: 1) put preheated RCA into mixing pot; 2) pour a certain amount of WCOR into mixing pot and perform mixing process; 3) transfer the mixed RCA to oven and cure for a period of time. The pretreatment procedure was indicated in Fig. 1. The mixing temperature was controlled at 160 C for the following two objectives: 1) Removing residual water from WCOR effectively to avoid negative impact on the moisture stability of asphalt mixture (Raouf and Williams, 2009); 2) Avoiding the potential composition loss and excessive oxidation of WCOR at a relative high temperature larger than 180 C (Hidalgo et al., 2018). In order to obtain the optimum pretreatment, the orthogonal design method was previously occupied in this investigation and a three-level orthogonal table L9 (34) was employed considering three impact factors: curing time, curing temperature and content of WCOR. The correspondent factors and levels were shown in Table 3. The indirect tensile strength (ITS) under 15 C was obtained to evaluate the optimum WCOR pretreatment considering the stripping performance of HMA is related to the incorporation of WCOR. The blending ratio of aggregates was initiated as 40% coarse RCA plus 60% limestone, the asphalt content was set as 4.5% (by mass of aggregate) to fabricate asphalt mixture for ITS test. On the basis of orthogonal design, the optimum pretreatment of coarse and fine RCA using WCOR was further examined by grey relational analysis (Shen and Du, 2005). Parameters including
Table 3 Level of factors. Factors
levels
Content of WCOR Curing temperature Curing time
3% 25 C 1h
4% 60 C 3h
5% 160 C 6h
indirect tensile strength, void ratio and content of WCOR were set as comparable sequence, in contrast, the reference sequence was composed of the optimum values of the three indexes, as shown in Table 4. The target void ratio was specified as 4% and the measured void ratios were processed according to Eq. (1). 0
VV ¼ jVV 4j
(1) 0
Where VV is measured void ratio and VV is processed void ratio. Content of WCOR and indirect tensile strength were preprocessed according to Eq. (2) to be normalized by maximum value. Void ratio was preprocessed according to Eq. (3) to be normalized by minimum value, the corresponding results were presented in Table 5. Grey correlation coefficient and grade of the relation of the three indexes were calculated using Eq. (4) and Eq. (5) and the weights of the three index were specified as b1 ¼ b2 ¼ b3 . The derived results were shown in Table 8. ð0Þ
ð1Þ
xi ðkÞ ¼
ð1Þ xi ðkÞ
¼
xi ðkÞ h i ð0Þ Max xi ðkÞ
(2)
h i ð0Þ Min xi ðkÞ
(3)
ð0Þ
xi ðkÞ
3 min min Di ðkÞ þ r max maxDi ðkÞ i¼1;mk¼1;n 5 xi ½x0 ðkÞ; xi ðkÞ ¼ 4 Di ðkÞ þ r max maxDi ðkÞ 2
i¼1;mk¼1;n
(4)
i¼1;mk¼1;n
rji ¼
n X
bk x0i ðkÞ
(5)
k¼1 n X
bk ¼ 1
(6)
k¼1
Where, x0 is comparable sequence and xi ði ¼ 1; 2; :::; mÞ is reference sequence, min min Di ðkÞ is bipolar minimum difference, i¼1;mk¼1;n
Fig. 1. Pretreatment process of RCA using WCOR.
max maxDi ðkÞ is bipolar maximum difference, r is discrimination
i¼1;mk¼1;n
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Table 4 Comparable sequence and reference sequence of fine and coarse RCA asphalt mixture.
Coarse RCA
Fine RCA
Index
Comparable sequence
WCOR content (%) Void ratio (%) Indirect tensile strength (MPa) WCOR content (%) Void ratio (%) Indirect tensile strength (MPa)
0 4.15 1.53 0 1.13 1.61
Reference sequence
1 2.90 1.34 2 0.45 1.29
2 0.21 1.19 4 0.03 1.19
3 0.53 1.08 6 0.84 0.84
4 1.71 0.86 8 1.79 0.54
5 1.97 0.65 10 1.75 0.43
5 0.21 1.53 10 0.45 1.61
Table 5 Comparable sequence and reference sequence of fine and coarse RCA asphalt mix after normalization.
Coarse RCA
Fine RCA
Index
Comparable sequence
WCOR content (%) Void ratio (%) Indirect tensile strength (MPa) WCOR content (%) Void ratio (%) Indirect tensile strength (MPa)
0 0.05 1.00 0 0.02 1.00
Reference sequence
0.2 0.07 0.88 0.2 0.06 0.80
0.4 1.00 0.78 0.4 1.00 0.74
0.6 0.40 0.71 0.6 0.03 0.52
0.8 0.12 0.56 0.8 0.01 0.33
1 0.11 0.43 1 0.01 0.27
1 1 1 1 1 1
Table 6 Results of intuitive analysis. Test No.\ Factor
1 2 3 4 5 6 7 8 9 T1 T2 T3 t1 t2 t3 R
Content of WCOR
Curing temperature
Curing time
(%)
( C)
(h)
3 3 3 4 4 4 5 5 5 1.87 0.88 1.14 0.62 0.29 0.38 0.33
25 60 160 25 60 160 25 60 160 0.77 1.07 2.05 0.26 0.36 0.68 0.43
1 3 6 3 6 1 6 1 3 0.97 1.52 1.39 0.32 0.51 0.46 0.18
y
0.353 ¼ y1 0.563 ¼ y2 0.956 ¼ y3 0.242 ¼ y4 0.266 ¼ y5 0.375 ¼ y6 0.172 ¼ y7 0.245 ¼ y8 0.720 ¼ y9
1 2 3 3 1 2 2 3 1 1.34 1.11 1.44 0.45 0.37 0.48 0.11
Table 7 Results of variance analysis. Resource
Sum of squares S
Degree of freedom f
Mean square MS
F ratio
Factor A Factor B Factor C Deviation e T
0.176 0.300 0.056 0.019 0.55
2 2 2 2 8
0.09 0.15 0.03 0.01 F0.90(2,
9.08 15.46 2.87
Table 8 Grey relational coefficient and grey correlation degree of coarse and fine RCA. RCA
Index
Content of WCOR (%) 0
1
2
3
4
5
Coarse
WCOR content (%) Void ratio (%) Splitting strength (MPa) rji WCOR content (%) Void ratio (%) Splitting strength (MPa) rji
0.33 0.33 1.00 0.56 0.33 0.34 1.00 0.56
0.38 0.34 0.70 0.48 0.38 0.34 0.65 0.46
0.45 1.00 0.57 0.67 0.45 1.00 0.58 0.68
0.56 0.44 0.50 0.50 0.56 0.34 0.43 0.44
0.71 0.35 0.39 0.49 0.71 0.33 0.35 0.47
1.00 0.35 0.33 0.56 1.00 0.33 0.33 0.56
Fine
2) ¼
9.0, F0.95(2,
2)
¼ 19.0
coefficient specified as 0.5, rji is grey correlation degree of comparable sequence and reference sequence, n is the number of factor in comparable sequence, bk is the weight of each sequence. 3.2. Mix design In this study, two sets of aggregates which are substituted with 20% fine RCA and 40% coarse RCA are selected for analyzation based on previous study (Sun et al., 2018). The combination gradation curves of the two sets of aggregates with the substitution of 40% coarse RCA and 20% fine RCA are shown in Fig. 2. The target gradation for the two sets of aggregate was AC-20. The asphalt concrete mixture incorporating 40% coarse RCA and 40% coarse
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Fig. 2. Gradation curve of asphalt mixtures incorporating 20% fine RCA and 40% coarse RCA.
RCW as well as 20% fine RCA and 20% fine RCW was prepared according to Marshall design method. 3.3. Optimum asphalt content The optimum asphalt content of the four groups of asphalt mixtures, namely, 40% coarse RCA, 40% coarse RCW, 20% fine RCA and 20% fine RCW, was obtained using the Marshall procedure. Three parallel specimens in each group were tested for their volumetric parameters. The optimum asphalt content was determined by evaluating the Marshall stability and flow number along with the volumetric parameters. 3.4. Experimental programs 3.4.1. Permanent deformation resistance The permanent deformation resistance of the four groups of asphalt mixtures was characterized with dynamic stability derived from wheel tracking test. Three parallel standard specimens (300 mm 300 mm 50 mm) were fabricated in each group. The wheel tracking test was conducted according to Chinese standard Test Procedures for Asphalt and Asphalt Mixtures of Highway Engineering (JTG E20-2011). 3.4.2. Cracking resistance at low temperature The cracking resistance of the four groups of asphalt mixtures at low temperature was evaluated by ultimate strain at bending failures. The test was performed using servo hydraulic test system (MTS-810) shown in Fig. 3 (a) according to Chinese specification Test Procedures for Asphalt and Asphalt Mixtures of Highway Engineering (JTG E20-2011) and 4 parallel specimens were included in each group. 3.4.3. Fatigue performance The fatigue life of the aforementioned four groups asphalt mixture was evaluated by repeated indirect tensile fatigue test (ITFT). The ITFT was performed using a servo hydraulic test system (MTS-810) as shown in Fig. 3 under stress-controlled mode. The test was conducted in accordance with AASHTO T245. The cylindrical specimens with a diameter of 100 mm and a control height of 50 mm were fabricated using a Superpave gyratory compactor (SGC). A repeated haversine waveform load with three stress ratios (0.3, 0.4 and 0.5), was applied on the prepared specimens for 0.1 s,
Fig. 3. (a) Servo-loading System (MTS-810), (b) Loading procedure.
followed by a rest period of 0.9 s. The loading frequency was set as 10 Hz. Fatigue life of the four groups of asphalt mixtures was obtained by Eq. (7)
ε0 ¼ k,N n f
(7)
Where, Nf represents the number of load cycles to failure, k and n are the constants of material obtained from ITFT. ε0 is the initial tensile horizontal strain at the centre of the specimen.
3.4.4. Moisture sensitivity To evaluate the moisture sensitivity of RCA and RCW mixture, indirect tensile strength (ITS) test and Hamburg Wheel Tracking test (HWT) (Chaturabong and Bahia, 2017) were conducted. In ITS test, a set of 8 parallel Marshall specimens were manufactured for each of the four groups of asphalt mixtures: 40% coarse RCA, 40% coarse RCW, 20% fine RCA and 20% fine RCW. In each group, 4 of the 8 Marshall specimens were kept at room temperature, while the other 4 Marshall specimens undergone 4 freeze-thaw cycles. Each freeze-thaw cycle comprises of retention in water for 0.5 h under vacuum environment, retention for 16 h under 18 C in environmental cabinet and retention for 24 h in water bath under 60 C. Finally, the 8 Marshal specimens in each group were saturated in the water bath for 2 h under 25 C prior to the conduction of ITS
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test. In ITS test, tensile stress ratio (TSR) was measured to characterize moisture resistance. TSR was defined as the loss of indirect tensile strength as shown in Eq. (8).
TSR ¼
ITSw ITSd
(8)
Where, TSR is tensile stress ratio (%), ITSw is the average tensile strength of the four conditioned specimens (MPa) and ITSd is the average tensile strength of the four unconditioned specimens (MPa). In HWT test, two parameters were calculated to evaluate moisture sensitivity of HMA, respectively: (1) Stripping inflection point (SIP) defined as the number of wheel passes where the rutting depth undergoes a sharp decrease; (2) Creep slope defined as slope of rutting curve. The corresponding results were shown in Fig. 10. 3.4.5. Dynamic modulus To evaluate the influence of WCOR treatment on the stiffness of asphalt mixture, the test was conducted using Universal Testing Machine (UTM) in accordance with AASHTO TP 62e03. Cylindrical specimens with the diameter of 150 mm and height of 180 mm were first compacted with a Superpave Gyratory Compactor. Then the cylindrical specimens with the diameter of 100 mm and the height of 150 mm were cored from the compacted cylindrical specimen. Three parallel specimens were prepared in each of the four groups as mentioned above. The load with 6 different levels of frequency: 0.1 Hz, 0.5 Hz, 1 Hz, 5 Hz, 10 Hz and 25 Hz was applied to the specimens under controlled temperature of 5 C, 20 C and 35 C. The corresponding results were presented in Fig. 11 . 3.4.6. Scanning electron microscope (SEM) test and fluorescence microscope (FM) test Microscopic morphology of the interface between RCA and asphalt mastic as well as RCW and asphalt mastic was captured using Field Emission Scanning Electron Microscope (VEGA3 TESCAN) and fluorescence microscopy. Three Marshall specimens were firstly manufactured with 40% coarse RCW, 20% fine RCW and 40% untreated coarse RCA. Then the three Marshall specimens were cut into small cubic blocks as shown in Fig. 4. The facets containing interface of RCA and asphalt mastic before and after treatment were
analyzed. The facets of cubic samples were polished by abrasive paper and then cleaned with acetone to remove dust on the surface of the specimens. Then the selected facets of the cubic samples were scanned with SEM after being sputtered with a 10 nm thick gold layer as shown in Fig. 12. Additionally, the dispersion of WCOR on the surface of coarse RCA and fine RCA was observed by FM as shown in Fig. 13. 4. Results and discussion 4.1. Optimum pretreatment combination and pretreating volume 4.1.1. Optimum pretreatment process As shown in Table 6, it's obvious that the combination of A1B3C3 exhibit the excellent potential of water resistant in the 9 combinations of pretreatment. In order to determine the optimum combination in the whole 27 combinations, factors (A, B and C) and corresponding error analysis column were classified into three groups based on different levels (e.g. factor C: 1.(y1, y6, y8), 2.(y2, y4, y9), 3.(y3, y5, y7)). Comprehensive comparison in Table 6 indicated that the optimum pretreatment of RCA was A1B3C2, which means that pretreating with 3% of WCOR and curing at 160 C for 3 h. In addition, the range of the three factors was: RB > RA > RC, that is, the curing temperature has the greatest influence on the indirect tensile strength of the mixture, followed by the WCOR content, and the least curing time. It can be observed from Table 7 that both FA and FB are greater than F0.90 (2, 2) ¼ 9.0, while FC was less than F0.9 (2, 2) ¼ 9.0. Consequently, Factor A and B were significant at a significance level of 0.1, while Factor C was not significant at a significance level of 0.1. Variance analysis showed that the optimum combination of factors was A1B3C. In summary, the optimum pretreatment of RCA was preliminarily determined as adding 3% WCOR into RCA and curing at 160 C for 1 h. 4.1.2. Optimum content of WCOR As shown in Table 8, the highest grey correlation of the WCOR content for coarse RCA asphalt mixture was 2%, while the highest grey correlation of WCOR content for fine RCA was 4%. Therefore, the optimum WCOR content of coarse RCA was 2% and that for fine RCA was 4%. To sum up, the pretreatment of RCA with WCOR was
Fig. 4. Inspection on interface transition zone (ITZ) using SEM and FM.
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Fig. 5. Optimum asphalt content of asphalt mixture with RCA and pretreated RCA.
Fig. 6. Dynamic stability of RCA and RCW mixtures.
Fig. 7. Maximum flexural tensile strain of RCW and RCA mixtures.
determined: adding an appropriate amount of WCOR (2% by mass of coarse RCA and 4% by mass of fine RCA) to the RCA and curing for 1 h at 160 before mixing. 4.2. Optimum asphalt content The pretreatment of RCA with WCOR was proved to change the optimum asphalt content of asphalt mixture. The optimum asphalt content for 40% coarse RCA mixture and 20% fine RCA were 5.4% and 5%, respectively. After pretreating coarse and fine RCA with the optimum volume of WCOR following the proposed procedure, the optimum asphalt content decreased to 4.5% for both 40% coarse RCW mixture and 20% fine RCW mixtures as shown in Fig. 5. The
result of optimum asphalt content was consistent with the expectation and it can be explained as follows: Compared with asphalt binder, WCOR has a better chemical affinity with cement mortar attached to RCA, when the porous structure is filled with WCOR in place of asphalt, the effective asphalt to pretreat aggregates will decrease and the optimum asphalt content will eventually be reduced. 4.3. Performance characteristics 4.3.1. Permanent deformation resistance The resistance of permanent deformation of the four groups of asphalt mixtures was evaluated by dynamic stability obtained in a
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wheel tracking test. The test results were shown in Fig. 6. It can be seen that the dynamic stability of RCW mixture was lower than that of RCA mixture, but still met the specification requirements (DS 800 cycles/mm). The slight reduction of high temperature may be resulted from the softening potential of WCOR on the surface of the RCW under the combined action of vehicle loading and high temperature.
4.3.2. Cracking resistance at low temperature The low-temperature cracking resistance of RCW asphalt mixture was evaluated using the ultimate strain at bending failure under low temperature. The test results are presented in Fig. 7, from which we know that the RCW mixture had a higher lowtemperature bending failure strain than RCA asphalt mixture. The main reason was that the viscosity of WCOR is insensitive to
temperature change, thus the interlocking effect of WCOR between asphalt mastic and RCW will not be affected notably by the low temperature. Consequently, the flexural tensile resistance was enhanced extensively.
4.3.3. Fatigue performance Fatigue life of asphalt mixture is defined as the number of load cycles when failure occurs, it implies the ability to withstand cyclic vehicle loading of asphalt mixture. It is noteworthy that the fatigue life increased for HMA mixture incorporating 40% coarse RCW and HMA mixture incorporating 20% fine RCW compared with the corresponding RCA mixture in Fig. 8. The decrease in indirect tensile strength may result from the lower bonding strength between WCOR and asphalt binder. The increased fatigue life of RCW mixture may be attributed to the lubricant effect of WCOR, which
Fig. 8. Fatigue life of RCA and RCW mixture.
Fig. 9. Indirect tensile strength ratio of RCW and RCA mixtures.
Fig. 10. Stripping point SIP and creep slope of RCW and RCA mixtures.
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makes the mixture softer and less susceptible to repeated loading cycles. 4.3.4. Moisture sensitivity Moisture sensitivity has an effect on the durability of asphalt pavement. The moisture sensitivity of RCA and RCW mixtures was first evaluated by freeze-thaw splitting ratio (TSR) in ITS test. Then stripping inflection point (SIP) and creep slope in HWT test was measured to further analyze the moisture sensitivity of RCA and RCW mixtures. The results in Fig. 9 indicated that TSR of RCW mixture was smaller than that of RCA mixture, which illustrated that the incorporation of WCOR treated RCA decreased the moisture resistance of asphalt mixture but still met the requirements of Chinese specification (TSR75%). Similarly, the decreased SIP and increased creep slope in HWT test indicated that the pretreatment of coarse and fine RCA with WCOR will adversely impact the moisture stability of asphalt mixture. This degradation of moisture resistance may be resulted from the weak adhesion of the RCW surface and asphalt binder. 4.3.5. Dynamic modulus The dynamic modulus of RCA and RCW mixtures under 5 C, 20 C, 35 C and 50 C was depicted in Fig. 11. It can be found that the overall dynamic modulus of the four asphalt mixture specimens decreased with the increase of temperature due to the temperature susceptibility of asphalt binder. Similarly, the dynamic modulus of the four groups of specimens undergone a sharp increase and then grew steadily with the increase of loading frequency under different temperatures. It was observed that the dynamic modulus of fine RCW and RCA mixtures was higher than that of coarse RCW and RCA mixtures under different levels of frequency, this was mainly resulted from the low substitution ratio of fine RCA, in which case, the overall dynamic modulus of the mixture was attributed from nature aggregate. In addition, the dynamic modulus of RCW mixture was lower than that of RCA mixture with the same substitution ratio. This was mainly resulted from the incorporation of liquid state WCOR, which is also consistent with the increase of the fatigue life. 4.4. SEM and FM analysis 4.4.1. SEM test The microstructure of asphalt mixture with the addition of RCW was captured by SEM. Fig. 12 (a) demonstrates the interface between RCW and asphalt mastic with WCOR treatment and Fig. 12 (b) demonstrates that without treatment. It can be observed that WCOR has penetrated into the fissure of cement mortar attached on RCA compared with untreated RCA. WCOR, unlike asphalt, is a liquid state organic matter with lower surface tension, making it possible to flow into micro cracks in place of asphalt binder, thus, the liquid like WCOR significantly decreases the optimum asphalt content. Additionally, due to the interlocking effect between RCA and WCOR, the strengthened interface of RCA and asphalt has a positive effect on the fatigue life and cracking resistance. Also, it was found that the pretreatment of RCA with WCOR leads to a decrease in the permanent deformation resistance, moisture resistance and dynamic modulus, which can be explained by the lubricant effect of the liquid state organics. Fig. 11. Dynamic modulus of RCA and RCW mixtures: (a) 5 C; (b) 20 C; (c) 35 C; (d) 50 C.
4.4.2. FM test In FM test, WCOR was labelled with fluorescent oil to show the dispersion of WCOR in asphalt mixture. Two groups of cylindrical specimens were made with 40% coarse RCW and 20% fine RCW. The cubic sample for microscope observation was manufactured following the similar procedure in SEM test. The cubic sample
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Fig. 12. SEM figure of interfacial transition zone: (a) ITZ of WCOR treated RCA and asphalt mastic, (b) ITZ of untreated RCA and asphalt mastic.
Fig. 13. Fluorescence microscopic photograph: (a) Interface of coarse RCW and asphalt mastic; (b) Distribution of fine RCW in asphalt mastic.
containing interface of WCOR treated RCA and asphalt mastic was detected as shown in Fig. 13 (a), in which the light green represented the WCOR. We found that WCOR has penetrated into the surface of coarse RCA to a considerable depth and reached a saturated condition. Fig. 13 (b) shows that the fine RCA pretreated with WCOR distributed in the asphalt mastic and WCOR has immerged into the full depth of fine RCA. 5. Conclusions RCA is a sustainable alternative to natural aggregates. However, RCA could cause the optimum asphalt content increased and some performance reduced. In this paper, a method of RCA pretreated with WCOR was proposed, and the orthogonal design and grey relational method were employed to determine the optimum pretreatment. The optimum asphalt content and the performance of asphalt mixture with RCA and pretreated RCA were evaluated. Moreover, SEM and FM were conducted to capture the morphology change in interface of RCA and asphalt mastic before and after treatment. The main conclusions were summarized as follows: (1) An optimum process of RCA pretreated with WCOR for HMA was developed, i.e. blending an appropriate amount of WCOR (2% by mass of coarse RCA or 4% by mass of fine RCA) with RCA and then curing for 1 h at 160 C. (2) Compared with RCA, the pretreated RCA decreased the optimum asphalt content from 5.4% to 4.5% for HMA incorporating 40% coarse RCA, from 5% to 4.5% for HMA incorporating 20% fine RCA. (3) The pretreated RCA enhanced significantly fatigue life and low temperature performance of HMA, while had a slight adverse effect on moisture sensitivity, permanent deformation resistance and dynamic modulus.
(4) Based on the SEM and FM, the WCOR was observed to penetrate into tiny voids of cement mortar attached on RCA, which contributed to reducing optimum asphalt content and enhancing fatigue and low temperature performance of HMA. In view of the results, the pretreatment of RCA with WCOR was proved an effective approach to reduce the asphalt consumption of RCA mixture, meanwhile, potentially making good use of the two types of industrial wastes. Nevertheless, further investigations should be devoted to the mechanism of variation of performance characteristics of HMA incorporating pretreated RCA. Acknowledgements The work described in this paper is supported by the National Natural Science Foundation of China (Nos.51878500 and No.51378393), the Fundamental Research Funds for the Central Universities, and Research Project of Transport Department of Zhejiang Province (2015J25). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.02.256. References Afonso, M.L., Dinis-Almeida, M., Pereira-de-Oliveira, L.A., Castro-Gomes, J., Zoorob, S.E., 2016. Development of a semi-flexible heavy duty pavement surfacing incorporating recycled and waste aggregatesePreliminary study. Constr. Build. Mater. 102, 155e161. Aljassar, A.H., Al-Fadala, K.B., Ali, M.A., 2005. Recycling building demolition waste in hot-mix asphalt concrete: a case study in Kuwait. J. Mater. Cycles Waste Manag.
J. Ma et al. / Journal of Cleaner Production 221 (2019) 469e479 7 (2), 112e115. Bhusal, S., Li, X., Wen, H., 2011. Evaluation of effects of recycled concrete aggregate on volumetrics of HMA. J. Transp. Res. Board 2205 (2205), 36e39. Blankendaal, T., Schuur, P., Voordijk, H., 2014. Reducing the environmental impact of concrete and asphalt: a scenario approach. J. Clean. Prod. 66, 27e36. Chaturabong, P., Bahia, H.U., 2017. The evaluation of relative effect of moisture in Hamburg wheel tracking test. Constr. Build. Mater. 153, 337e345. Chen, M., Lin, J., Wu, S., 2011. Potential of recycled fine aggregates powder as filler in asphalt mixture. Constr. Build. Mater. 25 (10), 3909e3914. Cho, Y.H., Yun, T., Kim, I.T., Choi, N.R., 2011. The application of recycled concrete aggregate (RCA) for hot mix asphalt (HMA) base layer aggregate. KSCE J. Civ. Eng. 15 (3), 473e478. rrez, P.A., 2009. Study on the influence of attached mortar De Juan, M.S., Gutie content on the properties of recycled concrete aggregate. Constr. Build. Mater. 23 (2), 872e877. Ding, T., Xiao, J., 2014. Estimation of building-related construction and demolition waste in Shanghai. Waste Manag. 34 (11), 2327e2334. Eisa, M.S., 2018. Evaluation of hot mix asphalt made with recycled aggregates from demolition waste (RADW) coated with bitumen emulsion. Innov. Infrastruct. Solution. 3 (1), 17. Giri, J.P., Panda, M., Sahoo, U.C., 2018. Performance of bituminous mixes containing emulsion-treated recycled concrete aggregates. J. Mater. Civ. Eng. 30 (4), 04018052. Gong, M., Yang, J., Zhang, J., Zhu, H., Tong, T., 2016. Physicalechemical properties of aged asphalt rejuvenated by bio-oil derived from biodiesel residue. Constr. Build. Mater. 105, 35e45. Hidalgo, J.M., Hora cek, J., Matousek, L., Vr ablík, A., Tisler, Z., Cerný, R., 2018. Catalytic hydrocracking of vacuum residue and waste cooking oil mixtures. Monatshefte für Chemie - Chemical Monthly 149 (6), 1167e1177. Hou, Y., Ji, X., Su, X., Zhang, W., Liu, L., 2014. Laboratory investigations of activated recycled concrete aggregate for asphalt treated base. Constr. Build. Mater. 65, 535e542. Huang, B., Wang, X., Kua, H., Geng, Y., Bleischwitz, R., Ren, J., 2018. Construction and demolition waste management in China through the 3R principle. Resour. Conserv. Recycl. 129, 36e44. Kareem, A.I., Nikraz, H., Asadi, H., 2018. Evaluation of the double coated recycled concrete aggregates for hot mix asphalt. Constr. Build. Mater. 172, 544e552. Lee, C.H., Du, J.C., Shen, D.H., 2012. Evaluation of pre-coated recycled concrete aggregate for hot mix asphalt. Constr. Build. Mater. 28 (1), 66e71. Marinkovi c, S., Radonjanin, V., Malesev, M., Ignjatovi c, I., 2010. Comparative environmental assessment of natural and recycled aggregate concrete. Waste Manag. 30 (11), 2255e2264. Mills-Beale, J., You, Z., 2010. The mechanical properties of asphalt mixtures with Recycled Concrete Aggregates. Constr. Build. Mater. 24 (3), 230e235. rez, I., Toledano, M., Gallego, J., Taibo, J., 2007. Mechanical properties of hot mix Pe asphalt made with recycled aggregates from reclaimed construction and den 57 (285), 17e29. molition debris. Mater. Construccio rez, I., Gallego, J., Toledano, M., Taibo, J., 2010. Asphalt mixtures with construction Pe and demolition debris. Proc. Inst. Civ. Eng.-Trans. 163 (4), 165e174. rez, I., Pasandín, A.R., Medina, L., 2012a. Hot mix asphalt using C&D waste as Pe coarse aggregates. Mater. Des. 36, 840e846.
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rez, I., Pasandín, A.R., Gallego, J., 2012b. Stripping in hot mix asphalt produced by Pe aggregates from construction and demolition waste. Waste Manag. Res. 30 (1), 3e11. rez, I., Pasandín, A.R., 2017. Moisture damage resistance of hot-mix asphalt made Pe with recycled concrete aggregates and crumb rubber. J. Clean. Prod. 165, 405e414. Paranavithana, S., Mohajerani, A., 2006. Effects of recycled concrete aggregates on properties of asphalt concrete. Resour. Conserv. Recycl. 48 (1), 1e12. rez, I., 2013. Laboratory evaluation of hot-mix asphalt containing Pasandín, A.R., Pe construction and demolition waste. Constr. Build. Mater. 43, 497e505. rez, I., 2014. Mechanical properties of hot-mix asphalt made with Pasandín, A.R., Pe recycled concrete aggregates pretreated with bitumen emulsion. Constr. Build. Mater. 55, 350e358. Poon, C.S., Qiao, X.C., Chan, D., 2006. The cause and influence of self-cementing properties of fine recycled concrete aggregates on the properties of unbound sub-base. Waste Manag. 26 (10), 1166e1172. Radevi c, A., Ðurekovi c, A., Zaki c, D., Mladenovi c, G., 2017. Effects of recycled concrete aggregate on stiffness and rutting resistance of asphalt concrete. Constr. Build. Mater. 136, 386e393. Rafi, M.M., Qadir, A., Siddiqui, S.H., 2011. Experimental testing of hot mix asphalt mixture made of recycled aggregates. Waste Manag. Res. 29 (12), 1316e1326. Raouf, M.A., Williams, R.C., 2009. Determination of pre-treatment procedure required for developing bio-binders from bio-oils. In: Proc., 2009 Mid-continent Transportation Research Symposium. Shen, D.H., Du, J.C., 2004. Evaluation of building materials recycling on HMA permanent deformation. Constr. Build. Mater. 18 (6), 391e397. Shen, D.H., Du, J.C., 2005. Application of gray relational analysis to evaluate HMA with reclaimed building materials. J. Mater. Civ. Eng. 17 (4), 400e406. Sun, D., Lu, T., Xiao, F., Zhu, X., Sun, G., 2017a. Formulation and aging resistance of modified bio-asphalt containing high percentage of waste cooking oil residues. J. Clean. Prod. 161, 1203e1214. Sun, D., Sun, G., Du, Y., Zhu, X., Lu, T., Pang, Q., Dai, Z., 2017b. Evaluation of optimized bio-asphalt containing high content waste cooking oil residues. Fuel 202, 529e540. Sun, D., Tian, Y., Sun, G., Pang, Q., Yu, F., Zhu, X., 2018. Performance evaluation of asphalt mixtures containing recycled concrete aggregates. Int. J. Pavement Eng. 19 (5), 422e428. Sun, Z., Yi, J., Huang, Y., Feng, D., Guo, C., 2016. Properties of asphalt binder modified by bio-oil derived from waste cooking oil. Constr. Build. Mater. 102, 496e504. Wong, Y.D., Sun, D.D., Lai, D., 2007. Value-added utilisation of recycled concrete in hot-mix asphalt. Waste Manag. 27 (2), 294e301. Wu, S., Zhong, J., Zhu, J., Wang, D., 2013. Influence of demolition waste used as recycled aggregate on performance of asphalt mixture. Road Mater. Pavement Des. 14 (3), 679e688. Zhang, Z., Wang, K., Liu, H., Deng, Z., 2016. Key performance properties of asphalt mixtures with recycled concrete aggregate from low strength concrete. Constr. Build. Mater. 126, 711e719. Zhu, J., Wu, S., Zhong, J., Wang, D., 2012. Investigation of asphalt mixture containing demolition waste obtained from earthquake-damaged buildings. Constr. Build. Mater. 29, 466e475.
Contents lists available at ScienceDirect
Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Potential of recycled concrete aggregate pretreated with waste cooking oil residue for hot mix asphalt Jianmin Ma a, Daquan Sun a, *, Qi Pang b, Guoqiang Sun a, Mingjun Hu a, Tong Lu a a b
Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, Shanghai, 200092, PR China China Changjiang Construction Investment. Corp. Ltd, Chengdu, 610218, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 September 2018 Received in revised form 24 February 2019 Accepted 25 February 2019 Available online 2 March 2019
Application of recycled concrete aggregate (RCA) in hot mix asphalt (HMA) is a sustainable alternative to natural aggregate. However, the increased asphalt consumption and decreased mechanical properties induced by the incorporation of RCA limit the extensive substitution of natural aggregates with RCA in HMA. In this paper, an experimental program was carried out to investigate the potential of recycled concrete aggregate pretreated with waste cooking oil residue (WCOR) with the expectation of reducing asphalt consumption and enhancing performance of HMA. The optimum pretreatment process for coarse and fine RCA in terms of content of WCOR, curing time and curing temperature was determined by an orthogonal design method and the grey correlation analysis. Subsequently, a laboratory characterization of HMA incorporating 40% coarse RCA, 40% coarse RCW (RCA pretreated with WCOR), 20% fine RCA and 20% fine RCW was further conducted in terms of optimum asphalt content, resistance to permanent deformation, cracking resistance at low temperature, fatigue performance, moisture sensitivity and dynamic modulus. Finally, scanning electron microscopy (SEM) test and fluorescence microscope (FM) test were performed to provide insight into the mechanism of WCOR-induced performance alteration. The experimental results showed that the pretreatment of RCA with WCOR decreased the optimum asphalt content from 5.4% to 4.5% for HMA incorporating 40% coarse RCA, from 5% to 4.5% for HMA incorporating 20% fine RCA. Moreover, RCA pretreated with WCOR enhanced significantly fatigue life and low temperature performance of HMA, while had a slight adverse effect on moisture sensitivity, permanent deformation resistance and dynamic modulus. Based on the SEM and FM tests, WCOR was observed to penetrate into tiny voids of cement mortar attached on RCA, which contributed to the reduction of optimum asphalt content and enhancement of fatigue life and low temperature performance of HMA. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Asphalt Recycled concrete aggregate Waste cooking oil residue Pretreatment Performance evaluation
1. Introduction With the rapid industrialization and urbanization in China, more than 18 billion tons of construction and demolition (C&D) wastes were generated during urban construction and reconstruction activities every year, while the recycling rate is less than 5%, far less than that of more than 95% in Europe and America (Huang et al., 2018). On the one hand, the increasing amount of construction waste and debris occupy plenty of land resources, which may restrict urban development. On the other hand,
* Corresponding author. Department of Road and Airport Engineering, Tongji University, 4800 Caoan Road, Shanghai, 201804, PR China. E-mail address: [email protected] (D. Sun). https://doi.org/10.1016/j.jclepro.2019.02.256 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
uncontrolled disposal of C&D wastes incorporated with hazardous substance may contaminate soils and water supplies, deteriorating the environmental condition and endangering human health rez et al., 2010; Pe rez and Pasandín, (Marinkovi c et al., 2010; Pe 2017). Therefore, the handling of construction waste disposal is particularly important nowadays in promoting sustainable development (Poon et al., 2006). Highway construction is a dominant industry consuming substantial amounts of natural resources especially mineral aggregates derived from quarry extraction (Blankendaal et al., 2014). Recently, the development of highway construction in China has made virgin aggregate increasingly scarce, the depletion of natural sources has also brought to the universal attention for the concern of sustainable development (Ding and Xiao, 2014). Accordingly, the environment protection department in China has issued a series of
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policies to restrict stone mining for the sake of environmental protection, which further exacerbates the scarcity of natural aggregate for pavement engineering. It is worthwhile mentioning that the unfinished highway construction and maintenance of the enormous highway network still requires a huge quantity of aggregates. Therefore, it is urgent to develop a new alternative to substitute virgin aggregates or reuse recycled aggregates from building debris. Among the proposed approaches, recycling was proved an effective way to reduce C&D waste disposal in landfills and preserve natural resources (Zhang et al., 2016; Zhu et al., 2012). Recycled concrete aggregate (RCA) is a representative of recycled aggregates obtained from C&D waste (Mills-Beale and You, 2010; Afonso et al., 2016). In general, the physical and mechanical properties of RCA are worse than those of natural aggregates due to the porous and weak mortar attached to RCA (Lee et al., 2012; rez Aljassar et al., 2005), e.g. lower resistance to fragmentation (Pe rrez, et al., 2012a), higher water absorption (De Juan and Gutie 2009) and lower density (Rafi et al., 2011). Additionally, owing to the heterogeneous sources and variant crushing process of RCA, the composition of RCA varies extensively and asphalt mixtures incorrez et al., porating RCA exhibit controversial characteristics (Pe 2010, 2012b; Paranavithana and Mohajerani, 2006; Shen and Du, 2004). Consequently, controversial conclusions were obtained for the impact of RCA on performance characteristics of HMA, including moisture damage resistance (Cho et al., 2011; Wu et al., 2013; Zhu et al., 2012) and resistance to permanent deformation (Mills-Beale and You, 2010; Radevi c et al., 2017; Zhu et al., 2012). rez et al., 2007) and low While the fatigue life (Chen et al., 2011; Pe temperature resistance (Chen et al., 2011; Wu et al., 2013; Zhu et al., 2012) of RCA mixtures exhibit a similar trend of improvement compared with conventional mixture. It is noteworthy that HMA incorporating RCA induces higher asphalt consumption compared with conventional asphalt mixtures (Bhusal et al., 2011; Cho et al., 2011; Lee et al., 2012; Wong et al., 2007). This difference was more apparent for finer RCA mixture that has a larger specific surface area (Rafi et al., 2011). To mitigate the problem of high asphalt consumption and performance degradation, Bhusal et al. (2011) suggested that it is better to incorporate coarse RCA rather than fine RCA in HMA. Several other studies have also tried different methods to pretreat RCA in recent years, these methods can be divided into two categories: calcination pretreatment (Pasandín rez, 2013) and surface pretreatment (Pasandín and Pe rez, and Pe 2014). In calcination pretreatment, RCA is calcined at high temperature, part of the calcium carbonate on the surface of RCA is decomposed into calcium oxide, eventually making the surface denser and enhancing the adhesion to asphalt (Wong et al., 2007). However, it still remains challenging to overcome the large energy consumption for the consideration of sustainable development. In surface pretreating pretreatment, different materials including liquid silicone resin (Zhu et al., 2012; Hou et al., 2014), cement mortar (Lee et al., 2012) and emulsified asphalt (Pasandín and rez, 2014; Eisa, 2018; Giri et al., 2018) et al. were adopted to Pe pretreat RCA to enhance the adhesive properties of RCA and asphalt. To further decrease the potential degradation of asphalt mixture resulted from the poor nature of RCA, Kareem et al. (2018) developed a double coating technique in which the first layer is cement slag paste and the second layer is acrylic based bitumen waterproofing membrane. These measures have been proved to be effective for the improvements in mechanical performance and durability. Nevertheless, further efforts are required to be devoted to the balance of cost and operational feasibility in plant before practical use. In this paper, a kind of industrial waste named waste cooking oil residue (WCOR) was used as the pretreatment material of RCA. The optimum asphalt content for both fine and coarse RCA mixtures
before and after pretreatment with WCOR was investigated. Subsequently, performance characteristics including permanent deformation resistance, cracking resistance at low temperature, fatigue performance, moisture sensitivity and dynamic modulus were evaluated to analyze the effectiveness of the pretreatment. Finally, the microscale morphology differences in interfaces of RCAasphalt mastic before and after treatment were analyzed by SEM, the distribution of WCOR in the interfaces of RCA-asphalt mastic was captured by fluorescence microscope (FM) to provide insight understanding of morphological change in asphalt mastic-RCA interface.
2. Materials 2.1. Asphalt The neat asphalt with penetration grade 70 was provided by China Petroleum & Chemical Corporation Co. Ltd. The basic properties of the neat asphalt were presented in Table 1.
2.2. Waste cooking oil residue (WCOR) WCOR, a kind of black viscous oily liquid (Sun et al., 2017a), was provided by Shanghai Environmental Protection Technology Co. Ltd. It was the byproduct of biodiesel refined with waste cooking oil through alkaline catalysis process. The chemical element of WCOR mainly includes C, H, O and a relative small proportion of N and S (less than 10%). Based on Fourier Transform Infrared Spectroscopy (FTIR) analysis in our previous study (Sun et al., 2017b), the main compounds in WCOR were found to be saturated fatty acids and esters. Moreover, the estimated moisture content of WCOR is approximately 3% by mass. This kind of soft waste oil residue was mainly used to modify asphalt binder (Gong et al., 2016; Sun et al., 2016) or serve as a potential substitute for petroleum asphalt in pavement engineering (Sun et al., 2017b).
2.3. Aggregate Two types of aggregates including natural aggregate and recycled concrete aggregate (RCA) were used in the preparation of HMA. Natural aggregates are crushed limestone with five different particle sizes: 0~3 mm, 3~5 mm, 5e10 mm, 10e15 mm and 15e25 mm, respectively. RCA is produced following steps of crushing, screening and sieving. The particle size distribution of fine RCA and coarse RCA are 0e4.75 mm and 4.75e13.2 mm, respectively. The results of X-ray fluorescence (XRF) chemical composition analysis indicated that the main constitutions of RCA are: SiO2 (49.40%), CaO (20.65%) and Al2O3 (12.00%). The limestone natural aggregates are primarily composed of CaO (60.3%), SiO2 (7.88%) and MgO (6.16%) and other substances. The physical and mechanical properties of RCA and natural aggregate were listed in Table 2. It was found that both coarse and fine RCA present a lower density and a higher water absorption, which results from the poor nature of cement mortar attached to rrez, 2009; Pe rez the recycled aggregate in RCA (De Juan and Gutie et al., 2012a). Due to the larger specific area for fine RCA, the density of fine RCA is lower than coarse RCA and the water absorption of fine RCA is larger than coarse RCA, in accordance with results in references. Additionally, the attached porous and weak mortar layer on recycled aggregate causes a lower mechanical property compared with natural aggregate and the corresponding crushing value of RCA is larger than natural aggregate.
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Table 1 Basic properties of neat asphalt. Penetration (25 C, 100 g, 5s)/0.1 mm
Viscosity (135 C)/Pa$s
Ductility (5 cm/min,15 C)/cm
67.4
0.569
>100
Table 2 Material properties of RCA and nature aggregate. Properties
Fine RCA
Limestone (0~3 mm)
Coarse RCA
Limestone (5e15 mm)
Apparent density (g/cm3) Bulk density (g/cm3) Water absorption (%) Aggregate crushing value (%)
2.639 1.957 11.63 24.4
2.573 2.441 2.06 19.1
2.539 2.257 4.9 31.2
2.706 2.673 0.46 23.7
3. Methodology 3.1. RCA pretreated with WCOR The pretreatment process of RCA using WCOR mainly includes three steps: 1) put preheated RCA into mixing pot; 2) pour a certain amount of WCOR into mixing pot and perform mixing process; 3) transfer the mixed RCA to oven and cure for a period of time. The pretreatment procedure was indicated in Fig. 1. The mixing temperature was controlled at 160 C for the following two objectives: 1) Removing residual water from WCOR effectively to avoid negative impact on the moisture stability of asphalt mixture (Raouf and Williams, 2009); 2) Avoiding the potential composition loss and excessive oxidation of WCOR at a relative high temperature larger than 180 C (Hidalgo et al., 2018). In order to obtain the optimum pretreatment, the orthogonal design method was previously occupied in this investigation and a three-level orthogonal table L9 (34) was employed considering three impact factors: curing time, curing temperature and content of WCOR. The correspondent factors and levels were shown in Table 3. The indirect tensile strength (ITS) under 15 C was obtained to evaluate the optimum WCOR pretreatment considering the stripping performance of HMA is related to the incorporation of WCOR. The blending ratio of aggregates was initiated as 40% coarse RCA plus 60% limestone, the asphalt content was set as 4.5% (by mass of aggregate) to fabricate asphalt mixture for ITS test. On the basis of orthogonal design, the optimum pretreatment of coarse and fine RCA using WCOR was further examined by grey relational analysis (Shen and Du, 2005). Parameters including
Table 3 Level of factors. Factors
levels
Content of WCOR Curing temperature Curing time
3% 25 C 1h
4% 60 C 3h
5% 160 C 6h
indirect tensile strength, void ratio and content of WCOR were set as comparable sequence, in contrast, the reference sequence was composed of the optimum values of the three indexes, as shown in Table 4. The target void ratio was specified as 4% and the measured void ratios were processed according to Eq. (1). 0
VV ¼ jVV 4j
(1) 0
Where VV is measured void ratio and VV is processed void ratio. Content of WCOR and indirect tensile strength were preprocessed according to Eq. (2) to be normalized by maximum value. Void ratio was preprocessed according to Eq. (3) to be normalized by minimum value, the corresponding results were presented in Table 5. Grey correlation coefficient and grade of the relation of the three indexes were calculated using Eq. (4) and Eq. (5) and the weights of the three index were specified as b1 ¼ b2 ¼ b3 . The derived results were shown in Table 8. ð0Þ
ð1Þ
xi ðkÞ ¼
ð1Þ xi ðkÞ
¼
xi ðkÞ h i ð0Þ Max xi ðkÞ
(2)
h i ð0Þ Min xi ðkÞ
(3)
ð0Þ
xi ðkÞ
3 min min Di ðkÞ þ r max maxDi ðkÞ i¼1;mk¼1;n 5 xi ½x0 ðkÞ; xi ðkÞ ¼ 4 Di ðkÞ þ r max maxDi ðkÞ 2
i¼1;mk¼1;n
(4)
i¼1;mk¼1;n
rji ¼
n X
bk x0i ðkÞ
(5)
k¼1 n X
bk ¼ 1
(6)
k¼1
Where, x0 is comparable sequence and xi ði ¼ 1; 2; :::; mÞ is reference sequence, min min Di ðkÞ is bipolar minimum difference, i¼1;mk¼1;n
Fig. 1. Pretreatment process of RCA using WCOR.
max maxDi ðkÞ is bipolar maximum difference, r is discrimination
i¼1;mk¼1;n
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Table 4 Comparable sequence and reference sequence of fine and coarse RCA asphalt mixture.
Coarse RCA
Fine RCA
Index
Comparable sequence
WCOR content (%) Void ratio (%) Indirect tensile strength (MPa) WCOR content (%) Void ratio (%) Indirect tensile strength (MPa)
0 4.15 1.53 0 1.13 1.61
Reference sequence
1 2.90 1.34 2 0.45 1.29
2 0.21 1.19 4 0.03 1.19
3 0.53 1.08 6 0.84 0.84
4 1.71 0.86 8 1.79 0.54
5 1.97 0.65 10 1.75 0.43
5 0.21 1.53 10 0.45 1.61
Table 5 Comparable sequence and reference sequence of fine and coarse RCA asphalt mix after normalization.
Coarse RCA
Fine RCA
Index
Comparable sequence
WCOR content (%) Void ratio (%) Indirect tensile strength (MPa) WCOR content (%) Void ratio (%) Indirect tensile strength (MPa)
0 0.05 1.00 0 0.02 1.00
Reference sequence
0.2 0.07 0.88 0.2 0.06 0.80
0.4 1.00 0.78 0.4 1.00 0.74
0.6 0.40 0.71 0.6 0.03 0.52
0.8 0.12 0.56 0.8 0.01 0.33
1 0.11 0.43 1 0.01 0.27
1 1 1 1 1 1
Table 6 Results of intuitive analysis. Test No.\ Factor
1 2 3 4 5 6 7 8 9 T1 T2 T3 t1 t2 t3 R
Content of WCOR
Curing temperature
Curing time
(%)
( C)
(h)
3 3 3 4 4 4 5 5 5 1.87 0.88 1.14 0.62 0.29 0.38 0.33
25 60 160 25 60 160 25 60 160 0.77 1.07 2.05 0.26 0.36 0.68 0.43
1 3 6 3 6 1 6 1 3 0.97 1.52 1.39 0.32 0.51 0.46 0.18
y
0.353 ¼ y1 0.563 ¼ y2 0.956 ¼ y3 0.242 ¼ y4 0.266 ¼ y5 0.375 ¼ y6 0.172 ¼ y7 0.245 ¼ y8 0.720 ¼ y9
1 2 3 3 1 2 2 3 1 1.34 1.11 1.44 0.45 0.37 0.48 0.11
Table 7 Results of variance analysis. Resource
Sum of squares S
Degree of freedom f
Mean square MS
F ratio
Factor A Factor B Factor C Deviation e T
0.176 0.300 0.056 0.019 0.55
2 2 2 2 8
0.09 0.15 0.03 0.01 F0.90(2,
9.08 15.46 2.87
Table 8 Grey relational coefficient and grey correlation degree of coarse and fine RCA. RCA
Index
Content of WCOR (%) 0
1
2
3
4
5
Coarse
WCOR content (%) Void ratio (%) Splitting strength (MPa) rji WCOR content (%) Void ratio (%) Splitting strength (MPa) rji
0.33 0.33 1.00 0.56 0.33 0.34 1.00 0.56
0.38 0.34 0.70 0.48 0.38 0.34 0.65 0.46
0.45 1.00 0.57 0.67 0.45 1.00 0.58 0.68
0.56 0.44 0.50 0.50 0.56 0.34 0.43 0.44
0.71 0.35 0.39 0.49 0.71 0.33 0.35 0.47
1.00 0.35 0.33 0.56 1.00 0.33 0.33 0.56
Fine
2) ¼
9.0, F0.95(2,
2)
¼ 19.0
coefficient specified as 0.5, rji is grey correlation degree of comparable sequence and reference sequence, n is the number of factor in comparable sequence, bk is the weight of each sequence. 3.2. Mix design In this study, two sets of aggregates which are substituted with 20% fine RCA and 40% coarse RCA are selected for analyzation based on previous study (Sun et al., 2018). The combination gradation curves of the two sets of aggregates with the substitution of 40% coarse RCA and 20% fine RCA are shown in Fig. 2. The target gradation for the two sets of aggregate was AC-20. The asphalt concrete mixture incorporating 40% coarse RCA and 40% coarse
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Fig. 2. Gradation curve of asphalt mixtures incorporating 20% fine RCA and 40% coarse RCA.
RCW as well as 20% fine RCA and 20% fine RCW was prepared according to Marshall design method. 3.3. Optimum asphalt content The optimum asphalt content of the four groups of asphalt mixtures, namely, 40% coarse RCA, 40% coarse RCW, 20% fine RCA and 20% fine RCW, was obtained using the Marshall procedure. Three parallel specimens in each group were tested for their volumetric parameters. The optimum asphalt content was determined by evaluating the Marshall stability and flow number along with the volumetric parameters. 3.4. Experimental programs 3.4.1. Permanent deformation resistance The permanent deformation resistance of the four groups of asphalt mixtures was characterized with dynamic stability derived from wheel tracking test. Three parallel standard specimens (300 mm 300 mm 50 mm) were fabricated in each group. The wheel tracking test was conducted according to Chinese standard Test Procedures for Asphalt and Asphalt Mixtures of Highway Engineering (JTG E20-2011). 3.4.2. Cracking resistance at low temperature The cracking resistance of the four groups of asphalt mixtures at low temperature was evaluated by ultimate strain at bending failures. The test was performed using servo hydraulic test system (MTS-810) shown in Fig. 3 (a) according to Chinese specification Test Procedures for Asphalt and Asphalt Mixtures of Highway Engineering (JTG E20-2011) and 4 parallel specimens were included in each group. 3.4.3. Fatigue performance The fatigue life of the aforementioned four groups asphalt mixture was evaluated by repeated indirect tensile fatigue test (ITFT). The ITFT was performed using a servo hydraulic test system (MTS-810) as shown in Fig. 3 under stress-controlled mode. The test was conducted in accordance with AASHTO T245. The cylindrical specimens with a diameter of 100 mm and a control height of 50 mm were fabricated using a Superpave gyratory compactor (SGC). A repeated haversine waveform load with three stress ratios (0.3, 0.4 and 0.5), was applied on the prepared specimens for 0.1 s,
Fig. 3. (a) Servo-loading System (MTS-810), (b) Loading procedure.
followed by a rest period of 0.9 s. The loading frequency was set as 10 Hz. Fatigue life of the four groups of asphalt mixtures was obtained by Eq. (7)
ε0 ¼ k,N n f
(7)
Where, Nf represents the number of load cycles to failure, k and n are the constants of material obtained from ITFT. ε0 is the initial tensile horizontal strain at the centre of the specimen.
3.4.4. Moisture sensitivity To evaluate the moisture sensitivity of RCA and RCW mixture, indirect tensile strength (ITS) test and Hamburg Wheel Tracking test (HWT) (Chaturabong and Bahia, 2017) were conducted. In ITS test, a set of 8 parallel Marshall specimens were manufactured for each of the four groups of asphalt mixtures: 40% coarse RCA, 40% coarse RCW, 20% fine RCA and 20% fine RCW. In each group, 4 of the 8 Marshall specimens were kept at room temperature, while the other 4 Marshall specimens undergone 4 freeze-thaw cycles. Each freeze-thaw cycle comprises of retention in water for 0.5 h under vacuum environment, retention for 16 h under 18 C in environmental cabinet and retention for 24 h in water bath under 60 C. Finally, the 8 Marshal specimens in each group were saturated in the water bath for 2 h under 25 C prior to the conduction of ITS
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test. In ITS test, tensile stress ratio (TSR) was measured to characterize moisture resistance. TSR was defined as the loss of indirect tensile strength as shown in Eq. (8).
TSR ¼
ITSw ITSd
(8)
Where, TSR is tensile stress ratio (%), ITSw is the average tensile strength of the four conditioned specimens (MPa) and ITSd is the average tensile strength of the four unconditioned specimens (MPa). In HWT test, two parameters were calculated to evaluate moisture sensitivity of HMA, respectively: (1) Stripping inflection point (SIP) defined as the number of wheel passes where the rutting depth undergoes a sharp decrease; (2) Creep slope defined as slope of rutting curve. The corresponding results were shown in Fig. 10. 3.4.5. Dynamic modulus To evaluate the influence of WCOR treatment on the stiffness of asphalt mixture, the test was conducted using Universal Testing Machine (UTM) in accordance with AASHTO TP 62e03. Cylindrical specimens with the diameter of 150 mm and height of 180 mm were first compacted with a Superpave Gyratory Compactor. Then the cylindrical specimens with the diameter of 100 mm and the height of 150 mm were cored from the compacted cylindrical specimen. Three parallel specimens were prepared in each of the four groups as mentioned above. The load with 6 different levels of frequency: 0.1 Hz, 0.5 Hz, 1 Hz, 5 Hz, 10 Hz and 25 Hz was applied to the specimens under controlled temperature of 5 C, 20 C and 35 C. The corresponding results were presented in Fig. 11 . 3.4.6. Scanning electron microscope (SEM) test and fluorescence microscope (FM) test Microscopic morphology of the interface between RCA and asphalt mastic as well as RCW and asphalt mastic was captured using Field Emission Scanning Electron Microscope (VEGA3 TESCAN) and fluorescence microscopy. Three Marshall specimens were firstly manufactured with 40% coarse RCW, 20% fine RCW and 40% untreated coarse RCA. Then the three Marshall specimens were cut into small cubic blocks as shown in Fig. 4. The facets containing interface of RCA and asphalt mastic before and after treatment were
analyzed. The facets of cubic samples were polished by abrasive paper and then cleaned with acetone to remove dust on the surface of the specimens. Then the selected facets of the cubic samples were scanned with SEM after being sputtered with a 10 nm thick gold layer as shown in Fig. 12. Additionally, the dispersion of WCOR on the surface of coarse RCA and fine RCA was observed by FM as shown in Fig. 13. 4. Results and discussion 4.1. Optimum pretreatment combination and pretreating volume 4.1.1. Optimum pretreatment process As shown in Table 6, it's obvious that the combination of A1B3C3 exhibit the excellent potential of water resistant in the 9 combinations of pretreatment. In order to determine the optimum combination in the whole 27 combinations, factors (A, B and C) and corresponding error analysis column were classified into three groups based on different levels (e.g. factor C: 1.(y1, y6, y8), 2.(y2, y4, y9), 3.(y3, y5, y7)). Comprehensive comparison in Table 6 indicated that the optimum pretreatment of RCA was A1B3C2, which means that pretreating with 3% of WCOR and curing at 160 C for 3 h. In addition, the range of the three factors was: RB > RA > RC, that is, the curing temperature has the greatest influence on the indirect tensile strength of the mixture, followed by the WCOR content, and the least curing time. It can be observed from Table 7 that both FA and FB are greater than F0.90 (2, 2) ¼ 9.0, while FC was less than F0.9 (2, 2) ¼ 9.0. Consequently, Factor A and B were significant at a significance level of 0.1, while Factor C was not significant at a significance level of 0.1. Variance analysis showed that the optimum combination of factors was A1B3C. In summary, the optimum pretreatment of RCA was preliminarily determined as adding 3% WCOR into RCA and curing at 160 C for 1 h. 4.1.2. Optimum content of WCOR As shown in Table 8, the highest grey correlation of the WCOR content for coarse RCA asphalt mixture was 2%, while the highest grey correlation of WCOR content for fine RCA was 4%. Therefore, the optimum WCOR content of coarse RCA was 2% and that for fine RCA was 4%. To sum up, the pretreatment of RCA with WCOR was
Fig. 4. Inspection on interface transition zone (ITZ) using SEM and FM.
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Fig. 5. Optimum asphalt content of asphalt mixture with RCA and pretreated RCA.
Fig. 6. Dynamic stability of RCA and RCW mixtures.
Fig. 7. Maximum flexural tensile strain of RCW and RCA mixtures.
determined: adding an appropriate amount of WCOR (2% by mass of coarse RCA and 4% by mass of fine RCA) to the RCA and curing for 1 h at 160 before mixing. 4.2. Optimum asphalt content The pretreatment of RCA with WCOR was proved to change the optimum asphalt content of asphalt mixture. The optimum asphalt content for 40% coarse RCA mixture and 20% fine RCA were 5.4% and 5%, respectively. After pretreating coarse and fine RCA with the optimum volume of WCOR following the proposed procedure, the optimum asphalt content decreased to 4.5% for both 40% coarse RCW mixture and 20% fine RCW mixtures as shown in Fig. 5. The
result of optimum asphalt content was consistent with the expectation and it can be explained as follows: Compared with asphalt binder, WCOR has a better chemical affinity with cement mortar attached to RCA, when the porous structure is filled with WCOR in place of asphalt, the effective asphalt to pretreat aggregates will decrease and the optimum asphalt content will eventually be reduced. 4.3. Performance characteristics 4.3.1. Permanent deformation resistance The resistance of permanent deformation of the four groups of asphalt mixtures was evaluated by dynamic stability obtained in a
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wheel tracking test. The test results were shown in Fig. 6. It can be seen that the dynamic stability of RCW mixture was lower than that of RCA mixture, but still met the specification requirements (DS 800 cycles/mm). The slight reduction of high temperature may be resulted from the softening potential of WCOR on the surface of the RCW under the combined action of vehicle loading and high temperature.
4.3.2. Cracking resistance at low temperature The low-temperature cracking resistance of RCW asphalt mixture was evaluated using the ultimate strain at bending failure under low temperature. The test results are presented in Fig. 7, from which we know that the RCW mixture had a higher lowtemperature bending failure strain than RCA asphalt mixture. The main reason was that the viscosity of WCOR is insensitive to
temperature change, thus the interlocking effect of WCOR between asphalt mastic and RCW will not be affected notably by the low temperature. Consequently, the flexural tensile resistance was enhanced extensively.
4.3.3. Fatigue performance Fatigue life of asphalt mixture is defined as the number of load cycles when failure occurs, it implies the ability to withstand cyclic vehicle loading of asphalt mixture. It is noteworthy that the fatigue life increased for HMA mixture incorporating 40% coarse RCW and HMA mixture incorporating 20% fine RCW compared with the corresponding RCA mixture in Fig. 8. The decrease in indirect tensile strength may result from the lower bonding strength between WCOR and asphalt binder. The increased fatigue life of RCW mixture may be attributed to the lubricant effect of WCOR, which
Fig. 8. Fatigue life of RCA and RCW mixture.
Fig. 9. Indirect tensile strength ratio of RCW and RCA mixtures.
Fig. 10. Stripping point SIP and creep slope of RCW and RCA mixtures.
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makes the mixture softer and less susceptible to repeated loading cycles. 4.3.4. Moisture sensitivity Moisture sensitivity has an effect on the durability of asphalt pavement. The moisture sensitivity of RCA and RCW mixtures was first evaluated by freeze-thaw splitting ratio (TSR) in ITS test. Then stripping inflection point (SIP) and creep slope in HWT test was measured to further analyze the moisture sensitivity of RCA and RCW mixtures. The results in Fig. 9 indicated that TSR of RCW mixture was smaller than that of RCA mixture, which illustrated that the incorporation of WCOR treated RCA decreased the moisture resistance of asphalt mixture but still met the requirements of Chinese specification (TSR75%). Similarly, the decreased SIP and increased creep slope in HWT test indicated that the pretreatment of coarse and fine RCA with WCOR will adversely impact the moisture stability of asphalt mixture. This degradation of moisture resistance may be resulted from the weak adhesion of the RCW surface and asphalt binder. 4.3.5. Dynamic modulus The dynamic modulus of RCA and RCW mixtures under 5 C, 20 C, 35 C and 50 C was depicted in Fig. 11. It can be found that the overall dynamic modulus of the four asphalt mixture specimens decreased with the increase of temperature due to the temperature susceptibility of asphalt binder. Similarly, the dynamic modulus of the four groups of specimens undergone a sharp increase and then grew steadily with the increase of loading frequency under different temperatures. It was observed that the dynamic modulus of fine RCW and RCA mixtures was higher than that of coarse RCW and RCA mixtures under different levels of frequency, this was mainly resulted from the low substitution ratio of fine RCA, in which case, the overall dynamic modulus of the mixture was attributed from nature aggregate. In addition, the dynamic modulus of RCW mixture was lower than that of RCA mixture with the same substitution ratio. This was mainly resulted from the incorporation of liquid state WCOR, which is also consistent with the increase of the fatigue life. 4.4. SEM and FM analysis 4.4.1. SEM test The microstructure of asphalt mixture with the addition of RCW was captured by SEM. Fig. 12 (a) demonstrates the interface between RCW and asphalt mastic with WCOR treatment and Fig. 12 (b) demonstrates that without treatment. It can be observed that WCOR has penetrated into the fissure of cement mortar attached on RCA compared with untreated RCA. WCOR, unlike asphalt, is a liquid state organic matter with lower surface tension, making it possible to flow into micro cracks in place of asphalt binder, thus, the liquid like WCOR significantly decreases the optimum asphalt content. Additionally, due to the interlocking effect between RCA and WCOR, the strengthened interface of RCA and asphalt has a positive effect on the fatigue life and cracking resistance. Also, it was found that the pretreatment of RCA with WCOR leads to a decrease in the permanent deformation resistance, moisture resistance and dynamic modulus, which can be explained by the lubricant effect of the liquid state organics. Fig. 11. Dynamic modulus of RCA and RCW mixtures: (a) 5 C; (b) 20 C; (c) 35 C; (d) 50 C.
4.4.2. FM test In FM test, WCOR was labelled with fluorescent oil to show the dispersion of WCOR in asphalt mixture. Two groups of cylindrical specimens were made with 40% coarse RCW and 20% fine RCW. The cubic sample for microscope observation was manufactured following the similar procedure in SEM test. The cubic sample
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Fig. 12. SEM figure of interfacial transition zone: (a) ITZ of WCOR treated RCA and asphalt mastic, (b) ITZ of untreated RCA and asphalt mastic.
Fig. 13. Fluorescence microscopic photograph: (a) Interface of coarse RCW and asphalt mastic; (b) Distribution of fine RCW in asphalt mastic.
containing interface of WCOR treated RCA and asphalt mastic was detected as shown in Fig. 13 (a), in which the light green represented the WCOR. We found that WCOR has penetrated into the surface of coarse RCA to a considerable depth and reached a saturated condition. Fig. 13 (b) shows that the fine RCA pretreated with WCOR distributed in the asphalt mastic and WCOR has immerged into the full depth of fine RCA. 5. Conclusions RCA is a sustainable alternative to natural aggregates. However, RCA could cause the optimum asphalt content increased and some performance reduced. In this paper, a method of RCA pretreated with WCOR was proposed, and the orthogonal design and grey relational method were employed to determine the optimum pretreatment. The optimum asphalt content and the performance of asphalt mixture with RCA and pretreated RCA were evaluated. Moreover, SEM and FM were conducted to capture the morphology change in interface of RCA and asphalt mastic before and after treatment. The main conclusions were summarized as follows: (1) An optimum process of RCA pretreated with WCOR for HMA was developed, i.e. blending an appropriate amount of WCOR (2% by mass of coarse RCA or 4% by mass of fine RCA) with RCA and then curing for 1 h at 160 C. (2) Compared with RCA, the pretreated RCA decreased the optimum asphalt content from 5.4% to 4.5% for HMA incorporating 40% coarse RCA, from 5% to 4.5% for HMA incorporating 20% fine RCA. (3) The pretreated RCA enhanced significantly fatigue life and low temperature performance of HMA, while had a slight adverse effect on moisture sensitivity, permanent deformation resistance and dynamic modulus.
(4) Based on the SEM and FM, the WCOR was observed to penetrate into tiny voids of cement mortar attached on RCA, which contributed to reducing optimum asphalt content and enhancing fatigue and low temperature performance of HMA. In view of the results, the pretreatment of RCA with WCOR was proved an effective approach to reduce the asphalt consumption of RCA mixture, meanwhile, potentially making good use of the two types of industrial wastes. Nevertheless, further investigations should be devoted to the mechanism of variation of performance characteristics of HMA incorporating pretreated RCA. Acknowledgements The work described in this paper is supported by the National Natural Science Foundation of China (Nos.51878500 and No.51378393), the Fundamental Research Funds for the Central Universities, and Research Project of Transport Department of Zhejiang Province (2015J25). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.02.256. References Afonso, M.L., Dinis-Almeida, M., Pereira-de-Oliveira, L.A., Castro-Gomes, J., Zoorob, S.E., 2016. Development of a semi-flexible heavy duty pavement surfacing incorporating recycled and waste aggregatesePreliminary study. Constr. Build. Mater. 102, 155e161. Aljassar, A.H., Al-Fadala, K.B., Ali, M.A., 2005. Recycling building demolition waste in hot-mix asphalt concrete: a case study in Kuwait. J. Mater. Cycles Waste Manag.
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