Performance Characterization of Half Warm Mix Asphalt Using Foaming Technology

Transcription

1 Performance Characterization of Half Warm Mix Asphalt Using Foaming Technology V. S. Punith, A.M.ASCE 1 ; Feipeng Xiao ; and David Wingard 3 Downloaded from ascelibrary.org by Tongji University on 1/19/1. Copyright ASCE. For personal use only; all rights reserved. Abstract: The objective of this study was to evaluate the moisture susceptibility, rutting, and fatigue resistance of laboratory-made half warm mix asphalt (HWMA) mixtures containing moist aggregates and recycled materials such as manufactured roofing shingles and reclaimed asphalt pavement (). The test properties evaluated in this study included weight loss (%), indirect tensile strength (ITS), tensile strength ratio (TSR), wet flow, wet toughness, percent toughness loss, rut depth of dry and wet conditioned specimens, rut index, and beam fatigue life. The experimental design in this study included the use of foaming technology and three hydrated lime contents (, 1, and % by weight of aggregate), three aggregate sources (designated as A, B, and C), three fractioned sources from the same aggregate sources, and one PG 6- binder. A total of 7 mixtures was designed, and a total of 1 ITS samples, 16 asphalt pavement analyzer (APA) samples, and 36 fatigue beams were tested in this study. Results indicated that the addition of recycled materials such as or roofing shingles can be effectively employed in HWMA mixtures, which did not reduce wet ITS and TSR values, although the compaction temperature was only 85 C. The influence of the moist aggregate used in HWMA on rut depth can be neglected and even results in a better rut resistance when incorporating recycled materials. Stiffness values of mixtures with recycled materials were generally dependent on the aggregate. Based on the statistical analysis for the selected aggregates, no significant difference in fatigue life was observed for mixtures with recycled materials and control mixtures. DOI: 1.161/(ASCE)MT American Society of Civil Engineers. CE Database subject headings: Aggregates; Foam; Tensile strength; Asphalts; Mixtures. Author keywords: Moist aggregate; Foaming technology; Toughness; Flow; Indirect tensile strength; Half warm mix asphalt. Introduction Almost 55 million tons of asphalt are produced each year in the United States. The Federal Highway Administration (FHwA), state departments of transportation (DOTs), the Environmental Protection Agency (EPA), and the asphalt pavement industry have continually sought to lower the fuel usage and pollutant emissions from mixing, transporting, and placing asphalt pavement, in addition to lowering its cost (Copeland et al. 1). Using recycled materials can often improve the performance properties of a material and lessen the consumption of raw materials. Because flexible pavements constitute more than 9% of all paved roadways in the United States, the increased utilization of recycled materials such as reclaimed asphalt pavement () or roofing shingles would substantially increase the opportunity for overall usage of recycled materials in asphalt pavements [National Asphalt Pavement Association (NAPA) ]. The production and compaction at substantially lower temperatures can allow for longer mixture hauling 1 Research Assistant Professor, Asphalt Rubber Technology Service (ARTS), Glenn Dept. of Civil Engineering, Clemson Univ., Clemson, SC 963 (corresponding author). pshivap@clemson.edu Research Assistant Professor, Asphalt Rubber Technology Service (ARTS), Glenn Dept. of Civil Engineering, Clemson Univ., Clemson, SC Research Assistant Professor and Program Administrator, Asphalt Rubber Technology Service (ARTS), Glenn Dept. of Civil Engineering, Clemson Univ., Clemson, SC 963. Note. This manuscript was submitted on December 1, 11; approved on May, 1; published online on May, 1. Discussion period open until August 1, 13; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, Vol. 5, No. 3, March 1, 13. ASCE, ISSN /13/ /$5.. distances/times and may prolong the paving season, particularly in colder regions of the United States and Canada. Ideally, an asphalt pavement that is easier to compact should also experience an extension in its service life in terms of all major asphalt distresses: rutting, fatigue, low temperature damage, thermal cracking, and moisture damage (USDOT 5). In practice, using warm mix techniques, asphalt mixing temperatures can be reduced to approximately 1 13 C, whereas with half warm mix techniques, the mixing temperatures are below 1 C. For half warm mix asphalt (HWMA), the aggregates still contain considerable amounts of moisture (Jenkins et al. ; Xiao et al. 11; Romier et al. 6; Van De Ven 7). Currently, these new techniques are becoming increasingly accepted alternatives to hot mix asphalt (HMA), appealing to those who seek more sustainable solutions, less dependence on fossil fuels, lowered cost, and greater flexibility (Romier et al. 6; Van De Ven 7; Gaudefroy et al. 7; Soenen et al. 1). Many researchers believe that lowering temperatures may not allow for proper drying of aggregates, especially at the mixing temperature of 9 11 C. If the aggregate is not dried completely during the mixing process, the presence of moisture can prevent the binder and aggregate from adequately bonding, which can lead to moisture damage of mixtures and pavement failure (Soenen et al. 1; Prowell et al. 7; Xiao et al. 9; Punith et al. 11a; Bennert et al. 11; Cooper et al. 11). In addition, there is concern that the compounding effect of lowered mix production temperatures (leading to less oxidation of the asphalt binder) and the possibility of residual moisture in the aggregate may result in moisture damage and rutting failure of the asphalt pavement (Xiao et al. 1; Prowell et al. 7; Punith et al. 11b), which needs to be evaluated in detail. At present, many DOTs increasingly use recycled materials in asphalt concrete, and state agencies are interested in adopting 38 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13

2 HWMA technologies in asphalt pavements. As such, it has become extremely important to better understand the performance properties such as rutting, fatigue cracking, and moisture susceptibility of recycled mixtures using HWMA technologies. The primary objective of this study was to investigate the laboratory performance characteristics of HWMA mixtures containing moist aggregate and recycled materials, including roofing shingles and. The influence of moist aggregate and various hydrated lime contents on moisture susceptibility and rutting resistance of HWMA was also evaluated. Experimental Program and Procedures Materials The experimental design in this study included three hydrated lime contents (, 1, and % by weight of aggregate), three aggregate sources (designated as A, B, and C), three fractioned sources from the same aggregate sources, and one PG 6- binder. Roofing shingles were collected from a locally available source. These recycled materials were selected for this study because they are commonly accepted by DOTs for paving purposes. The three crushed aggregate s used for the present study were classified for Sources A, B, and C as granitic gneiss, schist, and granite, respectively. Aggregate Source A is a gneissic rock with a general granitoid composition that was created through the metamorphic process. Aggregate Source B was also created through the metamorphic process and is primarily composed of schist with secondary s of phyllite, quartzite, marble, amphibolite, and other calcium-silicate rocks. Aggregate Source C is an igneous plutonic rock that falls into the peraluminous granite category (USGS 1). The engineering properties of the coarse and fine aggregates used in this study are shown in Table 1. Macroscopic and microscopic images of the crushed aggregate samples from each source (i.e., A, B, and C) have been reported in Figs. 1 3, respectively. Mix Design, Sample Fabrication, and Testing The mix design used was the specifications set forth by the South Carolina Department of Transportation (SCDOT) for a B surface mixture (Nominal Maximum Aggregate Size = 1.5 mm; N design ¼ 75). The design gradations for each aggregate source (A, B, and C) are shown in Table. The designated aggregate gradations were within the limits of SCDOT specifications and the amount of recycled materials ( or 5% roofing shingles) was the same for each aggregate source. The or roofing shingles were mixed with the virgin aggregate and heated in an oven at 17 C for 75 min until the aggregate temperature reached 93 C, as per the preliminary procedure developed in the laboratory to simulate aggregate temperatures to the actual field conditions for HWMA. In addition, this procedure was adopted for these recycled materials to avoid further binder aging when preheating at mixing temperatures for longer durations. Xiao et al. (9) reported that approximately.5% retained aggregate moisture content can be produced in the laboratory to better simulate the field conditions for WMA mixtures. In similar fashion, to simulate the HMWA technology in the field, a preliminary study was conducted in the lab. In this study, the oven dried aggregates were initially hydrated with water (5% by weight of aggregate), and the aggregates were heated in the oven at 17 C for 75 min to achieve the target aggregate temperature of 93 C. Aggregate was mixed with foamed asphalt binder using the laboratory foaming machine. Test results showed that at 93 C, the moisture content in the aggregates (before mixing) for sources A, B, and Table 1. Physical Properties of Coarse and Fine Aggregates Used for this Study Aggregate Coarse aggregate Physical property of aggregate Aggregate A B C LA abrasion loss (%) Absorption (%) Specific gravity Dry (BLK) SSD (BLK) Apparent Soundness loss at 5 cycles (%) 3= 3= = Hardness Fine aggregate Fineness modulus Chemical composition of aggregate Absorption (%)...6 Specific gravity SSD (BLK) Soundness loss (%) Sand equivalent Al O CaO Fe O K O MgO Na O SiO TiO LOI Note: Aggregate A is crushed granitic gneiss, B is crushed schist, and C is crushed granite; LA = Los Angeles Abrasion, BLK = Bulk, SSD = Saturated Surface Dry. C were 1.8, 1.3, and 1.%, respectively. This procedure was developed for HWMA mixtures in the lab to simulate the actual production conditions, because asphalt plants will use aggregate temperatures below 1 C for mixing purposes while adopting this technology. The mixing and compaction temperatures for the various mixtures are presented in Table 3. To achieve the target mixing temperatures (9 95 C), the asphalt binder initially was heated up to C because the water used for foaming was at room temperature. During the mixing process, it was observed that the aggregate was very well coated by the bubbled asphalt binder in a short duration of 1 min. Fig. shows a sequence of pictures taken during the laboratory mixing process for a typical HWMA mixture. The optimum binder content (OBC) was defined as the amount of binder required to achieve % air voids, in accordance with SCDOT volumetric specifications. The test results of the mix designs are presented in Table. After the mix designs were completed, six gyratory compacted specimens for each mixture were prepared for rut testing with 7 1% air voids, in accordance with AASHTO T3-1 (1). During specimen preparation, the weight loss (%) of each mixture was recorded to monitor the evaporation of moisture during the -h short-term aging (STA) process at 85 C. These moisture evaporation values from various mixtures were helpful in determining their effects on the performance properties of HWMA mixtures. In addition, during specimen preparation, the number of gyrations to achieve a specimen height of 75 mm was recorded for each specimen, to assess the compactability of each mixture. Three rut samples were tested after dry conditioning and the other three were tested after wet conditioning. The conditioning of wet APA samples was similar to the moisture susceptibility test as per AASHTO T83 (7b), without a JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13 / 383

3 mm Downloaded from ascelibrary.org by Tongji University on 1/19/1. Copyright ASCE. For personal use only; all rights reserved. freeze-thaw cycle. The APA samples were then tested in accordance with AASHTO T3-1 (1). In addition, four Superpave gyratory compacted ITS specimens were also prepared with 7 1% air voids for moisture susceptibility testing, in accordance with AASHTO T83 (7b). Four fatigue beams were compacted for each mixture using a vibratory compactor and tested as per AASHTO T31 (7a). The test used a controlled strain mode, a test temperature of.5 C, and frequency of 5 to 1 Hz. Analysis of Test Results Statistical Considerations The ITS and rut depth values of dry and wet conditioned specimens were statistically analyzed at the.5 level of significance (5% probability of a Type I error, t-test) with respect to the effects of the aggregate, lime content, and mixture. For these comparisons, all specimens were produced at OBC. The error bars on the graphs indicate standard deviation. Weight Loss Analysis As reported earlier, the HWMA mixtures contain moist aggregate and some moisture might be partially evaporated during the mixing 16 mm (c) Fig. 1. Representative macroscopic (a and c) and microscopic (b and d) images for crushed granitic gneiss aggregate samples from Source A (d) process, whereas some might be dried during the -h STA process. Table 5 shows the weight loss (%) values for mixtures after the STA process, conditioned at 85 C. This weight loss (%) value might include the combined evaporation of moisture and volatilization of light binder fractions. In general, the weight loss (%) of the HWMA mixtures was less than.5% regardless of aggregate, lime content, and mixture. The highest weight loss (%) was observed for mixtures with 5% shingles using Aggregate C. Considering the aggregate effect, an increased weight loss (%) was observed for mixtures made with Aggregate C. Also, Table 5 shows that the weight loss (%) of mixtures containing 5% roofing shingles was found to be higher than control mixtures and mixtures with. Compaction Effort Analysis NCHRP Report 78 indicated that an appropriate compaction parameter, gyration number, is related to the stiffness and rutting resistance of an asphalt mixture (Anderson et al. ). For gyratory samples, an increase in gyration number typically reduces its air void content. Table 5 presents the required gyration number for APA samples to reach the target air void content of 7 1% for each mixture. The results indicate that irrespective of the mixture, higher gyration numbers were required for mixtures using Aggregate B than for mixtures from Aggregate A. One possible reason might be that the samples from Aggregate A have a relatively 38 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13

4 9 mm Downloaded from ascelibrary.org by Tongji University on 1/19/1. Copyright ASCE. For personal use only; all rights reserved. higher asphalt binder content than mixtures from Aggregate B, and thus, are more easily compacted. In addition, regarding the moist aggregate effect, especially with Aggregate A, having a higher water absorption property (Table 1) might make the sample easier to compact because the moisture in the mixture softens the binder. Test results indicated that aggregate plays a key role in determining the amount of compaction effort required for HWMA mixtures containing moist aggregates. In most cases, control samples with % lime required more compactive effort, regardless of aggregate and mixture, indicating that the addition of hydrated lime to the mixtures improves compactability. For the selected aggregates, mixtures with 5% shingles required fewer gyrations than control mixtures. In most cases, a similar compactive effort was required for samples using either 5% roofing shingles or. Dry ITS Analysis The dry ITS values of the HWMA mixtures are shown in Fig. 5. In general, all mixtures showed dry ITS values greater than 5 kpa. For the selected aggregates, results indicated no significant differences in the dry ITS values for control mixtures. Fig. 5 shows that with an increase in lime content, the ITS values of all control mixtures increased. Considering, mixtures using Aggregate B showed higher dry ITS values than other mixtures in this study. Based on the statistical analysis (Table 6), for the selected aggregates, significant differences in dry ITS values were (c) mm Fig.. Representative macroscopic (a and c) and microscopic (b and d) images for crushed marble schist aggregate samples from Source B (d) generally observed for mixtures with recycled materials, thereby indicating that the use of recycled materials helps to improve the dry ITS values. For the selected aggregates, irrespective of the lime content and mixture, no significant differences in dry ITS values were observed for mixtures using recycled materials. Results indicated that the utilization of HWMA technology containing moist aggregates did not weaken the dry ITS values for the mixtures tested for this study. Wet ITS Analysis Fig. 5 summarizes the wet ITS strength values of HWMA mixtures. The figure shows that all control mixtures without lime failed to meet the minimum wet ITS requirement of 8 kpa (65 psi), as per SCDOT specifications for surface Type B mixtures. All mixtures met the minimum wet ITS value when hydrated lime was added. Marginal increases in wet ITS values were observed for mixtures with an addition of % lime, thereby indicating that % lime in HWMA may not be necessary, because mixtures with 1% lime showed wet ITS values greater than 8 kpa (65 psi). In addition, higher wet ITS values were observed for mixtures from Aggregate B with % hydrated lime and 5% shingles. Furthermore, mixtures from granite Aggregates A and C showed no difference in wet ITS values between control and mixtures using recycled materials. Based on the statistical analysis (Table 6), regardless of mixture, significant differences in wet ITS values were JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13 / 385

5 18 mm Downloaded from ascelibrary.org by Tongji University on 1/19/1. Copyright ASCE. For personal use only; all rights reserved. 16 mm (c) Fig. 3. Representative macroscopic (a and c) and microscopic (b and d) images for crushed granitic aggregate samples from Source C Table. Gradations Used for Different Aggregate Sources Specification for Passing of aggregate from after ignition test (%) B mixtures Aggregate source Roofing source: A source: B source: C Sieve size Limits A B C shingle þ þ þ 19 mm mm mm mm mm mm mm (d) Table 3. Mixing and Compaction Temperatures of HWMA Mixtures Aggregate (A, B, and C) For lime contents (%) Mixture Control % L, 1% L, % L or 5% Recycled Asphalt s Initial binder temperature ( C) Binder after foaming ( C) Compaction temperature ( C) observed for mixtures without lime and with lime. In addition, for the selected aggregate, no significant differences in the wet ITS values were observed for mixtures using either 5% shingles or. The chemical and physical properties of aggregates play an important role in determining the moisture resistance of the mixtures. It can be concluded that the addition of antistripping additives (e.g., hydrated lime) is extremely important in affecting the moisture susceptibility of HWMA mixtures when using recycled materials, regardless of aggregate moisture content. 386 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13

6 TSR Analysis The TSR results are presented in Table 7. In general, the specimens containing 1 or % lime have TSR values higher than 85%, the minimum specification value set forth by SCDOT. The TSR values of all mixtures without lime are lower than 85%, indicating that Fig.. Laboratory mixing process for a typical HWMA mixture Table. Superpave Mixture Design for HWMA Mixtures Type B: high volume road primary (75 gyrations) SCDOT specification requirements Aggregate Properties Control 5% shingles A OBC at % Air voids 5.7% 5.% 5.% Air void: 3 %; minimum VMA: 1.5%; VFA: 7 8%; binder content:.5 6.% VMA 17.% 15.9% 15.6% VFA 77.% 75.% 77.5% B OBC at % Air voids.6%.8%.5% VMA 15.3% 1.5% 1.5% VFA 73.% 8.% 75.% C OBC at % Air voids.6%.7%.6% VMA 1.5% 1.% 1.5% VFA 75.% 77.% 7.% Note: Voids in mineral aggregate (VMA); voids filled in asphalt (VFA). Table 5. Results of Weight Loss (%) and Gyration Numbers for HWMA Mixtures Lime (%) Mixture Gyration numbers Weight loss (%) for APA pills A B C A B C % L Control % L % L % L 5% % L shingle % L % L % L % L Dry ITS (kpa) Wet ITS (kpa) Fig. 5. ITS values for HWMA mixtures: dry ITS; wet ITS JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13 / 387

7 Table 6. Statistical Analysis for ITS and Rut Depth of HWMA Mixtures Statistical analysis Mixture Dry ITS Wet ITS Dry rut depth Wet rut depth Aggregate effect α ¼.5 A B B C C A A B B C C A A B B C C A A B B C C A Control NS NS NS NS NS NS S S NS S S S 5% shingle S NS S S NS NS S S S NS NS NS S S NS NS NS NS S S NS NS S S Lime effect α ¼ Control NS S S S S NS S S NS S S NS 5% shingle NS NS NS S S NS NS NS NS S S NS NS NS NS S S NS S S NS S NS S Mixture effect α ¼.5 CO SH CO R SH R CO SH CO R SH R CO SH CO R SH R CO SH CO R SH R A NS NS NS NS NS NS NS S NS NS S NS B NS S NS S S NS NS NS NS S NS NS C S NS NS NS NS NS S S S S S NS Note: Control (CO); 5% shingles (SH); (R); significant difference (S); no significant difference (NS) (α ¼.5). Table 7. Results for TSR, Toughness Loss, and Rut Index of HWMA Mixtures Aggregate Mixture such mixtures were susceptible to moisture-induced damage. In addition, although the HWMA mixtures contain moist aggregate, the addition of hydrated lime promoted improvement in the TSR values for mixtures, thereby indicating improved resistance to moisture susceptibility. In general, for the selected aggregates, the addition of recycled materials such as or 5% roofing shingles in HWMA mixtures does not affect the TSR values. Flow Analysis The flow (deformation) resistance of wet ITS samples, a measure of the material s resistance to permanent deformation in service (Punith et al. 11a), was used in this investigation to analyze the moisture susceptibility of the mixture. Fig. 6 shows that control mixtures showed higher dry flow values than mixtures made with recycled materials. With the incorporation of stiffer materials, like either 5% shingles or, the dry flow values for HWMA mixtures showed better resistance to dry deformation values. As shown in Fig. 6, test results indicate that, in general, the addition of hydrated lime resulted in an increase in wet flow values of the mixtures. For the selected aggregate s, the wet flow values of HWMA mixtures ranged between 3.5 and 5 mm. Test results showed that control specimens made with Aggregate C were found to be higher than specimens made with either Aggregates A or B. In other words, the distorting and shoving may occur more easily for control mixtures made with Aggregate C, as shown in Fig. 7, resulting in higher dry or wet rut depth values of control mixtures using Aggregate C. The flow values observed for mixtures with 5% shingles were found to be higher than mixtures with. As shown in Fig. 6, in most cases, wet flow values were found to be TSR (%) Toughness loss (%) Rut index (%) Lime content Lime content Lime content % L 1% L % L % L 1% L % L % L 1% L % L A Control Failed % shingle B Control % shingle C Control % shingle Dry Flow (mm) Wet Flow (mm) Fig. 6. Flow values for HWMA mixtures: dry flow; wet flow higher than dry flow values for the selected mixtures, irrespective of lime (%) and recycled material, due to the combined effects of wet conditioning of specimens and the presence of moisture in the aggregates. 388 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13

8 Dry Rut Depth (mm) Wet Rut Depth (mm) Fig. 7. Rut depth values for HWMA mixtures: dry rut depth; wet rut depth Toughness Analysis Toughness is defined as the area under the tensile stress deformation curve up to a deformation double that incurred at maximum tensile stress (Xiao et al. 9). The toughness values of the dry ITS specimens are shown in Fig. 8. In most cases, test results indicate that the addition of hydrated lime improved the dry toughness values of the mixtures. Mixtures with 5% roofing shingles made with Aggregate B showed the highest dry toughness values compared to the other investigated mixtures. Dry toughness values of control mixtures were found to be different for the selected aggregate s. In most cases, mixtures made with 5% Dry Toughness (N/mm) Wet Toughness (N/mm) Fig. 8. Toughness values for HWMA mixtures: dry toughness; wet toughness shingles showed higher dry toughness values than control mixtures and mixtures with. The toughness results of wet ITS specimens are presented in Fig. 8. Wet toughness values for the tested mixtures ranged between 1 and.5 N=mm. In most cases, the wet toughness values of specimens made with Aggregate B were found to be higher than specimens made with either Aggregates A or C, for control mixtures and mixtures with recycled materials containing 1% hydrated lime. Wet toughness values for mixtures were largely dependent of the aggregate. Significant improvements in the wet toughness values were observed for mixtures with the addition of hydrated lime. For the selected aggregates, the use of recycled materials improved the wet toughness values for mixtures containing moist aggregates, indicating that specimens undergoing warm water bath treatments (6 C for h) did not present any negative effects on the wet toughness values for HWMA mixtures made with recycled materials. Percent Toughness Loss Analysis As shown in Table 7, the percent toughness loss (PTL) values are positive, because the dry toughness values are greater than wet ones. Table 7 shows that, in most cases, the PTL values are positive. However, the PTL values from the specimens containing hydrated lime are negative, indicating that the wet toughness values from these specimens are higher than the dry values. In general, PTL values were found to be different for selected aggregates and s of recycled materials. In most cases, the control mixtures containing moist aggregates showed positive PTL values, indicating that these mixtures were more susceptible to moisture-induced damage, thereby indicating the necessity of using an antistripping agent like hydrated lime in control mixtures. All mixtures made with Aggregate B using 5% shingles showed negative PTL values, indicating that these mixtures were less susceptible to moisture-induced damage. Test results showed that with the addition of % hydrated lime, improvements in negative PTL values were observed, indicating that the resistance to moisture susceptibility of HWMA mixtures can be improved with the addition of hydrated lime while using recycled materials. Dry Rut Depth Analysis Fig. 7 shows the dry rut depths of HWMA mixtures. All mixtures from Aggregate B showed lower dry rut depth values, less than 8 mm. Mixtures from Aggregates A or C showed higher dry rut depths. Furthermore, mixtures with Aggregate A showed higher dry rut depths than mixtures with Aggregate B, because these mixtures have higher asphalt binder content (Table ), thus increasing the possibility of higher rut depths. Control mixtures without lime showed lower dry rut depths than mixtures with hydrated lime. This correlates with the data from the compaction effort analysis (Table 5), because mixtures with % lime required higher compaction efforts than mixtures with lime. For the selected aggregate sources, mixtures with 5% roofing shingles showed different dry rut depths. Furthermore, irrespective of the lime content, similar dry rut depths were observed for mixtures containing 5% shingles. Control mixtures from Aggregate C showed higher dry rut depths than other mixtures. Control mixtures generally showed higher dry rut depths; this may possibly be because these mixtures were conditioned at a lower temperature (85 C), experiencing relatively lower binder aging conditions during a -h STA process. Based on the statistical analysis (Table 6), in general, significant differences in the dry rut depth values were observed for mixtures between any two aggregates, indicating that aggregate source plays a key JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13 / 389

9 role in determining the dry rut depths of mixtures containing moist aggregates. With respect to lime effect, with an increase in lime content from 1 to %, no significant differences were observed in the dry rut depths for control and recycled mixtures. For mixtures from Aggregate B, no significant differences in the dry rut depths were observed for mixtures with or shingles, compared to control mixtures. In general, the utilization of recycled materials such as or 5% shingles can improve the dry rutting resistance of HWMA mixtures. control mixtures showed significantly different wet rut depth values. The wet rut depth values observed for mixtures with lime were different from mixtures without lime. No major differences in the wet rut depth values were observed for mixtures using either 1 or % lime. With respect to mixture effect, the majority of the mixtures showed no difference in the wet rut depths compared to mixtures with recycled materials. In addition, Fig. 7 illustrates that the influence of moist aggregate on wet rut depths is generally not significant, although these mixtures were treated in a warm water bath at 6 C for h. Downloaded from ascelibrary.org by Tongji University on 1/19/1. Copyright ASCE. For personal use only; all rights reserved. Wet Rut Depth Analysis The rutting resistance for a HWMA mixture containing moist aggregates using foaming technology under wet conditioning is not clearly understood. In this study, Fig. 7 shows all of the wet rut depths of various mixtures after a warm water bath treatment (6 C for h). The majority of the mixtures showed rut depths lower than 8 mm, although APA samples were compacted at 7 1% air voids. Mixtures from Aggregate B showed slightly lower rut depths than mixtures from Aggregates A or C when using moist aggregates, regardless of lime content and recycled material. The mixture from various aggregate sources exhibited significantly different rutting resistances. In Fig. 7, test results showed that the wet specimens generally had lower rut depths than dry specimens, thus exhibiting a better rutting resistance and confirming previously published research (Xiao et al. 1). This may be due to the possibility that high saturation levels (7 8%), adopted as per AASHTO T83 (7b), resulted in pore water pressure in the samples during the cyclic loading, which could help support the load, resulting in reduced rut depths for the wet conditioned samples. In addition, test results indicated that the mixtures using recycled materials generally showed lower susceptibility to rutting after a warm water bath treatment (6 C for h). As per Table 6, regardless of aggregate, no significant differences in the wet rut depths were observed for mixtures using 5% shingles, whereas Rut Index Analysis In this study, rut indices were evaluated to explore the effects of warm water bath conditioning of APA samples at 6 C for h. The rut index is the ratio of wet to dry rut depth values. These index values are shown in Table 7. Mixtures with % lime showed higher wet rut depths than mixtures with and 1% lime. In addition, 66% of the mixtures with 5% shingles showed rut index values higher than 1%, thereby indicating that the effect of warm water bath treatment was significant for these mixtures. Correlations between Gyration Number and Rut Depth In this study, although the compaction temperatures were the same for all s of HWMA mixtures, the gyration numbers were significantly different for each mixture. Based on the gyration number data collected in this study for each mixture, the correlation between gyration numbers and rut depth for each mixture was analyzed and equations were developed for various mixtures with respect to effect of aggregate and lime content; these are presented in Tables 8 and 9. The general trends indicated that regardless of the aggregate, lime content, or mixture, an increase of gyration number in the mixture reduces its potential to rutting, although the coefficient of determination (R ) values of regression models are generally low. Table 8. Relationship between Gyration Numbers and Rut Depth of HWMA Mixtures for the Selected Aggregate Types Mixture Dry rut depth Wet rut depth Equation R Equation R Control y ¼.67x þ y ¼.519x þ y ¼.67x þ y ¼.8x þ y ¼.17x þ y ¼.15x þ % shingles y ¼.6x þ y ¼.1918x þ y ¼.7x þ y ¼.163x þ y ¼.91x þ y ¼.15x þ y ¼.1x þ y ¼.38x þ y ¼.1x þ y ¼.x þ y ¼.379x þ y ¼.95x þ 9..9 Table 9. Relationship between Gyration Numbers and Rut Depth of HWMA Mixtures for Varying Lime Contents Mixture Lime (%) Dry rut depth Wet rut depth Equation R Equation R Control % L y ¼.9x þ y ¼.x þ % L y ¼.787x þ y ¼.87x þ % L y ¼.867x þ y ¼.153x þ % shingles % L y ¼.761x þ y ¼.1x þ.1.5 1% L y ¼.1669x þ y ¼.77x þ % L y ¼.99x þ y ¼.5x þ % L y ¼.58x þ.85.9 y ¼.58x þ.31. 1% L y ¼.187x þ y ¼.55x þ % L y ¼.63x þ y ¼.7x þ / JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13

10 Table 1. Flexural Beam Fatigue Test Results of HMWA Mixtures Aggregate Mixture Measured value Beam Fatigue Analysis The fatigue characteristics of asphalt mixtures are usually expressed as relationships between the initial stress or strain and the number of load repetitions to failure, determined by using repeated flexure tests performed at different stress or strain levels. Based on the moisture susceptibility and rutting test results, no statistically significant differences in the test results were observed for mixtures using either 1 or % lime. Hence, for the next level of testing, only mixtures with 1% lime were used for fatigue analysis in this study. Fatigue life was defined as the number of load cycles, N f, at the 5th cycle, at which the stiffness of the specimen was reduced to half of its initial value. For the fatigue test, a strain level of 35 microstrains was adopted in this study. The initial flexural stiffness of each mixture was calculated and test results are presented in Table 1. Results indicated that higher initial stiffness values were observed for control and mixtures with recycled materials when using Aggregate C. In general, for the selected aggregate, mixtures using showed different stiffness values than control mixtures, whereas no difference in the stiffness values were observed between control and mixtures using 5% roofing shingles. Stiffness values for the mixtures with recycled materials were largely dependent on the aggregate. As shown in Table 1, with an increase in the frequency rate from 5 to 1 Hz, a decrease in the fatigue life for the selected mixture was observed. The fatigue life of control mixtures from Aggregate A was found to be higher than control mixtures with other aggregate sources, because the required binder contents of these mixtures were higher than the other two aggregate s (B and C). Higher asphalt binder contents used in control mixtures for Aggregate A (Table ) would be helpful in improving the resilience to fatigue properties of these mixtures. Test results indicated that the aggregate source plays an important role in the fatigue characteristics of HWMA mixtures. For the selected aggregate, the fatigue life of the mixture containing 5% shingles and was found to be similar. Knowing the variability in the fatigue test results (Table 1), statistical analysis was performed to study the mixture effect. For the selected aggregates at C, analysis indicated no significant differences in the fatigue lives for the control and mixtures using recycled materials. Based on the statistical analysis, it was concluded that the addition Air voids (%) Frequency (Hz) of recycled materials did not negatively weaken the fatigue resistance of the mixtures. Further studies are needed to better understand the low-temperature cracking behavior of the mixtures using these recycled materials. Findings and Conclusions Initial stiffness (kpa) Fatigue life (cycle) A Control Average ,5,75 1,88 STDEV a.1 978,96 3,765 5% shingle Average ,173,95 13,18 STDEV.6 75,16,6 Average ,8, 6,53 STDEV. 71,565,66 B Control Average ,398, 17,68 STDEV.79,55, 9,631 5% shingle Average ,193,667 13,983 STDEV.3,1,35 3,793 Average ,56, 1,31 STDEV. 1,37,99,75 C Control Average ,5, 1,31 STDEV. 1,56,865,915 5% shingle Average ,8,5 9,66 STDEV.37 77,31 5,3 Average ,59,933 9, STDEV.3 1,55,738 5,7 a STDEV: standard deviation. The following conclusions were drawn based upon the experimental results obtained from this study on HWMA mixtures containing moist aggregate, produced at a mixing temperature of 95 C: In general, the weight loss (%) of mixtures was found to be less than.5%, regardless of aggregate, lime content, and mixture. HWMA mixtures using stiffer materials such as or 5% shingles can be compacted using similar or lower compaction efforts than control mixtures, to achieve target air voids at 85 C. Mixtures with 1% lime required fewer gyrations to reach target air voids than mixtures with % and % lime. For the selected aggregates, the addition of lime improved the ITS values of control mixtures. The use of recycled materials such as or shingles helps to improve the ITS values of HWMA mixtures. Marginal increases in wet ITS values were observed for mixtures with the addition of % lime. In general, the addition of or shingles in HWMA mixtures did not affect the TSR values. The inclusion of lime in the HWMA significantly reduced the susceptibility of all mixtures to moistureinduced damage, compared to mixtures without lime. The flow (deformation) values observed for mixtures with 5% roofing shingles were found to be higher than mixtures with. Wet flow values were found to be higher than dry flow values for HWMA mixtures, irrespective of hydrated lime (%) and recycled material, due to the combined effects of moist aggregate and wet conditioning of the specimens at 6 C for h. The use of recycled materials in HWMA resulted in an improvement in wet toughness values for mixtures containing moist aggregates. Based on the PTL analysis, control mixtures were more susceptible to moisture damage, thereby indicating the necessity of using an antistripping agent like hydrated lime in HWMA mixtures. JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / MARCH 13 / 391

11 The influence of moist aggregate on rutting resistance can be neglected and even results in a better rut resistance when incorporating recycled materials in HWMA mixtures, which generally showed lower susceptibility to rutting after a warm water bath treatment (6 C for h). For the selected aggregates, no significant differences in the wet rut depths were observed for mixtures using 5% shingles. The mixtures from different aggregate sources exhibited significantly different rutting resistance. Mixtures with Aggregate B exhibited better resistance to rutting than mixtures with Aggregates A or C. The rut depths of these mixtures were generally lower than 8 mm. The increase of lime content does not significantly affect the rut resistance of the mixture. For the selected aggregates, significant differences in initial stiffness values were observed between mixtures with and control mixtures. Stiffness values of mixtures with recycled materials were generally dependent on the aggregate. Based on the statistical analysis for the selected aggregates, no significant difference in fatigue life was observed for mixtures with recycled materials and control mixtures. Further studies are required to evaluate the low temperature cracking performance of HWMA mixtures using recycled materials like or roofing shingles. From the limited laboratory study, it can be concluded that with the incorporation of recycled materials, HWMA mixtures can satisfy the demand of field performance and help to reduce emissions, increase energy savings due to lowering production temperatures, and promote recycling materials such as or roofing shingles in the asphalt pavements. Acknowledgments The authors would like to acknowledge the support of the Civil Engineering Department Chair, Dr. Nadim M. Aziz for his challenge to the authors to address more sustainable asphalt paving technologies that are based on scientific evidence. 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