- Email: [email protected]
Assessing the feasibility of incorporating phase change material in hot mix asphalt
Sustainable Cities and Society 19 (2015) 11–16
Contents lists available at ScienceDirect
Sustainable Cities and Society journal homepage: www.elsevier.com/locate/scs
Assessing the feasibility of incorporating phase change material in hot mix asphalt Bryan J. Manning a,∗ , Paul R. Bender b , Sarah A. Cote b , Rachel A. Lewis a , Aaron R. Sakulich b , Rajib B. Mallick b a b
Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA, USA Department of Civil and Environmental Engineering, Worcester Polytechnic Institute, Worcester, MA, USA
a r t i c l e
i n f o
Article history: Received 23 January 2015 Received in revised form 10 June 2015 Accepted 11 June 2015 Available online 20 June 2015 MSC: 00−01 99−00 Keywords: Phase change materials Hot mix asphalt Thermal stress reduction Lightweight aggregate
a b s t r a c t This paper investigated the feasibility of using lightweight aggregate (LWA) as a medium for incorporating phase change material (PCM) in hot mix asphalt (HMA) in order to extend pavement life by reducing the magnitude of temperature fluctuations. First, evaporation and absorption tests were conducted to investigate the absorption properties of LWA and the loss of PCM during heating. Then, batches of samples with varying concentrations of PCM were created and the thermal properties were evaluated using a guarded longitudinal comparative calorimeter (GLCC). Data from thermal testing indicated that the incorporation of PCM slowed the rate of cooling and reduced low temperatures when compared to control samples. Further research should investigate PCM incorporation methods, improve the mix design, and experiment with different combinations of PCMs and thermal fluctuations. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The construction of the Eisenhower interstate system helped place roads and highways at the forefront of transportation in the United States (Federal Highway Administration, 2014). Although hot mix asphalt (HMA) pavements are designed to withstand various uses and climates, failure of the pavement is often inevitable due to normal wear and tear (Mallick & El-Korchi, 2009). While repairing damaged pavement incurs capital and environmental costs, the combination of economic losses due to frequent traffic diversion for repairs, and environmental damages caused by reduced fuel efficiency on poor roadways adds up to a significant life-cycle impact (Hakkinen & Makela, 1996; Inamura, 1999). Therefore, increasing the life of pavement can improve overall sustainability. Phase change materials (PCMs) have the potential to extend pavement life and, therefore, reduce long-term maintenance costs. PCM can be used to regulate temperature because of its ability to
∗ Corresponding author. Tel.: +1 508 831 5912. E-mail addresses: [email protected] (B.J. Manning), [email protected] (P.R. Bender), [email protected] (S.A. Cote), [email protected] (R.A. Lewis), [email protected] (A.R. Sakulich), [email protected] (R.B. Mallick). http://dx.doi.org/10.1016/j.scs.2015.06.005 2210-6707/© 2015 Elsevier Ltd. All rights reserved.
release and absorb heat depending on the environment (Sakulich & Bentz, 2012a). This release or absorption occurs during the phase change, which makes the proper selection of a PCM for a given application largely dependent on the phase change temperature. Although relatively large amounts of heat are either released during solidification or absorbed during melting, the temperature of the composite ideally remains at the phase change temperature of the PCM (Sakulich & Bentz, 2012b). The time during which the temperature remains at this value depends on the value of the latent heat of fusion of the PCM (Sakulich & Bentz, 2012b). Thus, a PCM with a higher latent heat of fusion can delay a temperature change for a longer period of time and mitigate temperature fluctuations (Sakulich & Bentz, 2012a). This paper explored the incorporation of PCM in HMA by absorption into a porous lightweight aggregate (LWA). The incorporation of PCM was expected to lessen damages from thermal stresses, reduce debris from deterioration, and decrease the amount of time, money, and resources required to maintain pavements. PCM-6, a blend of paraffin waxes that changes phase at 6 ◦ C (42.8 ◦ F), was selected due to its potential for delaying the freezing of infiltrated water because the phase change temperature was slightly above the freezing temperature of water. The failure mechanisms addressed were thermal fatigue cracking, low-temperature cracking, and freeze-thaw cycling. Thermal
12
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16 Table 1 Preliminary mix design for control samples. Mix component
Mass (g)
Normal-weight aggregate Crushed stone Natural sand
699 556 143
Binder
73
Table 2 Preliminary mix design for 1.25% PCM-6 samples.
Fig. 1. Theoretical temperature profile of asphalt with and without PCM.
fatigue cracking is caused by repeated high\low-stress cycles produced by temperature fluctuations and loading (Mallick & ElKorchi, 2009). Low-temperature cracking results from high tensile stress and weakened tensile strength at temperatures below the operational threshold of the binder (Mallick & El-Korchi, 2009). Freeze-thaw cycling occurs when repeated temperature cycles cross the freezing point of water and cause the pavement to crack and break. PCMs may be able to reduce susceptibility to thermal fatigue, low-temperature, and freeze-thaw cracking because the material can potentially dampen temperature cycling in the pavement by reducing the magnitude of extreme temperatures. Ideally, the PCM would prevent or reduce the frequency of temperatures below the operational threshold of the binder and\or temperatures below the freezing point of water (Fig. 1), thus reducing damage and maintenance costs.
2. Methodology 2.1. Sample production Absorption and evaporation tests were conducted to determine the amount of PCM-6 absorbed by LWA. Three trials with four combinations (12 samples) of LWA were tested: LWA retained by No. 4 sieve, No. 8 sieve, No. 16 sieve, and an equal mixture of the aforementioned sieves. The LWA was soaked in PCM-6 for 24 h and the final weight of PCM-6 absorbed was recorded. The samples were then placed in an oven at 150 ◦ C (302 ◦ F), which is a temperature reached in HMA mixing procedures, and the amount of weight loss was measured at 5-minute intervals. Preliminary cubic samples (5.08 cm [2 in]) were then compacted to investigate the feasibility of incorporating PCM-6 in HMA using LWA. The samples contained normal-weight aggregate (NWA) retained on sieve sizes below No. 4 and a binder content of 9% by mass. The aggregate included 20% by mass natural sand and 80% by mass crushed stone. A limit of 10% LWA by mass was used because samples tested with greater amounts crumbled upon removal from the mold. The sample concentrations were 0% (control), 1.25%, and 2.5% PCM-6 by mass. The required amounts of PCM-6 by mass were estimated by using the water absorption of 100 g of the LWA sample (17.5% by mass, particle distribution below No. 4 (Sor, Sor, Micklus, Arhan, & Rowbotham, 2006)) to backcalculate an estimated absorption of PCM-6 into the LWA. Since PCM-6 (0.763 g cm−3 [47.6 lb ft−3 ]) (ScienceLab, Inc., 2014) was less dense than water, less PCM-6 by mass would be absorbed by the same volume of LWA. After accounting for the differences in densities, the estimated absorption of PCM-6 into LWA was 13.3% by mass.
Mix component
Mass (g)
Normal-weight aggregate Crushed stone Natural sand
894 715 179
Lightweight aggregate No. 4 No. 8 No. 16
105 21 42 42
PCM-6
14
Binder
95
To prepare for mixing, the LWA was soaked in PCM-6 for 24 h (agitated every 6 h) and the binder and NWA aggregates were heated at 150 ◦ C (302 ◦ F) for three h. Before mixing, the LWA was heated at 150 ◦ C (302 ◦ F) for 2–5 min to remove PCM from the surface and hypothetically improve strength by preventing surface PCM from interfering with the binder. The binder, NWA, and LWA were then mixed and hand-compacted into cubes. For the control samples, the LWA was replaced with an equivalent volume of NWA. An initial binder content of 5.5% by mass was used for the controls, while 9% by mass was used for the PCM-6 samples to account for excess fines in the mix. The calculated concentrations of PCM were used to identify samples, but the evaporation and absorption tests indicated actual amounts were lower due to mixing and heating losses. The compositions of the samples are in Tables 1, 2, and 3. Because samples with 2.5% PCM-6 by mass compacted poorly during preliminary testing, a final batch of samples containing 1.25% PCM-6 by mass was produced. To improve the uniformity and strength of the samples, aggregates were graded to comply with ASTM C-136 (C136-06, 2006). A binder content of 5.5% by mass was used for controls and the PCM samples. By utilizing this gradation, each final sample contained the same composition of material which limited thermal differences to the addition of PCM-6. The final samples were created using the same mixing process as the preliminary samples. The final mix design for the control batch is found in Table 4 and the final mix design for the PCM-6 samples is found in Table 5. 2.2. Thermal testing To evaluate the thermal properties of samples, a guarded longitudinal comparative calorimeter (GLCC) was used to generate a Table 3 Preliminary mix design for 2.5% PCM-6 samples. Mix component
Mass (g)
Normal-weight aggregate Crushed stone Natural sand
633 506 127
Lightweight aggregate No. 4 No. 8
180 90 90
PCM-6
24
Binder
81
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16
13
Table 4 Final mix design for control samples. Mix component
Mass (g)
Normal-weight aggregate Coarse aggregate Crushed stone Natural sand
840 141 360 339 48
Binder
Table 5 Final mix design for 1.25% PCM-6 samples. Mix component
Mass (g)
Normal-weight aggregate Coarse aggregate Crushed stone Natural sand
720 141 360 219
Lightweight aggregate No. 4
81 81
PCM-6
11
Binder
45
Fig. 3. Heating and cooling profiles for preliminary samples and final samples.
pyroceram standards was 4.18 W m−1 K−1 (2.42 BTU ft−1 h−1 ◦ F−1 ) and the thickness (Z) was 2.54 cm (1 in) (Sharifi et al., 2014). The heat flows through the standards were calculated using Eq. 1, which allowed for the computation of average heat flow through the sample using:
fluctuating heat flow through the cubic samples (Sharifi, Sakulich, & Mallick, 2014). To measure the heat flow, a sample was placed inside the device and six thermocouples measured temperatures at various locations (Fig. 2). The sample was placed between two pyroceram standards (5.08 cm × 5.08 cm × 2.54 cm [2 in × 2 in × 1 in]) and the thermocouples were held in contact using 0.3175 cm (0.125 in) of TC3008 elastomer. In general, to compute steady-state conductive heat transfer through a material: Q = A·
T Z
Qsample =
(2)
where Qsample (W [BTU h−1 ]) is the heat flow through the sample, Qtop (W [BTU h−1 ]) is the heat flow through the top standard, and Qbottom (W [BTU h−1 ]) is the heat flow through the bottom standard (E1225-09, 2009). In addition, the thermal conductivity of each sample (W m−1 −1 K [BTU ft−1 h−1 ◦ F−1 ]) was computed by rearranging Equation 1 and Equation 2 to yield:
(1)
where Q (W [BTU h−1 ]) is the heat flow through the material, A (m2 [ft2 ]) is the area measured normal to the direction of the heat flow, (W m−1 K−1 [BTU ft−1 h−1 ◦ F−1 ]) is the thermal conductivity of the material, and T/Z (◦ C m−1 [◦ F ft−1 ]) is the temperature gradient (D5470-11, 2011). The thermal conductivity of the
Qtop + Qbottom 2
sample =
Qsample A
·
Z T
(3)
where Qsample (W [BTU h−1 ]) is the heat flow through the sample, A (m2 [ft2 ]) is the area measured normal to the direction of the heat Z (m ◦ C−1 [ft ◦ F−1 ]) is the inverse of the temperature flow, and T gradient. The GLCC was programmed to replicate thermal cycling (Fig. 3). The preliminary samples were subjected to a high and low of 23 ◦ C (73.4 ◦ F) and −25 ◦ C (−13 ◦ F), respectively. The temperature change was 0.3 ◦ C h−1 (0.54 ◦ F h−1 ). Each sample underwent three cooling and heating cycles which lasted for a total of 43 h. Two control samples, three samples of 1.25%, and three samples of 2.5% PCM-6 by mass were created. Only two control samples were produced due to logistical reasons. The final samples were subjected to a high and low of 25 ◦ C (77 ◦ F) and −25 ◦ C (−13 ◦ F). The temperature change was 2 ◦ C h−1 (3.6 ◦ F h−1 ) for cooling and 4 ◦ C h−1 (7.2 ◦ F h−1 ) for heating. Each sample underwent one freezing and thawing cycle which lasted 45.5 h. The gradual temperature change was based on previous research, and was expected to produce more consistent temperature measurements and allow for accurate computation of thermal conductivity due to better steady-state conditions (Farnam, Bentz, Sakulich, Flynn, & Weiss, 2014). One control and two samples containing 1.25% PCM-6 by mass were evaluated. 2.3. Volumetric testing
Fig. 2. Pictorial representation of guarded longitudinal comparative calorimeter (GLCC).
The “Superpave” (Cooley et al., 2009) specification was used to evaluate the volumetric properties of the final batches of PCM-6 and control samples. First, the theoretical maximum density (TMD) test was performed on a loose sample of HMA to determine the maximum density possible for the control mix. The sample was mixed, allowed to cool under constant stirring, and then vacuum
14
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16
Fig. 4. Average temperature versus time for preliminary samples.
Table 6 PCM-6 absorption for various sieve sizes. Sieve size
PCM-6 absorption (% Mass)
No. 4 No. 8 No. 16 Blend
10.1 (±0.51) 13.3 (±0.53) 18.1 (±0.63) 11.9 (±0.28)
sealed in a CoreLok machine. The density was calculated by the water displacement method described in the CoreLok Operator’s Guide (Instrotek, Inc., 2011). The expected TMD of the PCM-6 mix was back-calculated using the TMD of the control mix and physical properties of the aggregates and binder. The TMD values were used to batch quantities of material that would yield samples with the desired density (96% of TMD) and air void content (4%). The cubes were cooled overnight, and then the bulk specific gravity (BSG) test (g cm−3 [lb ft−3 ] was completed following ASTM D7063 (D7063/D7063M, 2011) and the CoreLok Guide (Instrotek, Inc., 2011). The BSG values for each sample were used to calculate the percent voids in the total mix (VTM), percent voids in the mineral aggregate (VMA), and percent voids filled with asphalt (VFA). Comparisons were made between control and PCM-6 samples to determine the effects of PCM-6 on volumetric properties of HMA. These sample properties were also evaluated against the volumetric requirements of the Superpave specification to determine compliance of the mix.
3. Results and discussion 3.1. Evaporation and absorption testing results Absorption tests determined the amount of PCM-6 absorbed by each LWA sieve size and combination of sieve sizes. Finer LWA retained more PCM-6 than the coarser LWA most likely due to PCM adhering to the larger surface area for a given volume of material (Table 6). In addition, all samples were visibly glossy after soaking. The test also indicated the estimated 13.3% by mass of absorption of PCM-6 by LWA was a sufficient approximation. Evaporation tests found that PCM-6 evaporated quickly (Fig. 5). After 5 min of heating, the LWA was no longer glossy indicating approximately 10% to 15% of the PCM adhered to the surface and had evaporated. Because of the evaporation rate, the PCM-soaked LWA would be added to the HMA mix after minimal heating (2–5 min) in order to reduce losses from evaporation.
Fig. 5. PCM-6 evaporation rates for various sieve sizes.
3.2. Preliminary thermal testing results To evaluate the thermal effects of the PCM, the average temperatures of the samples were plotted as a function of time. Each of the individual cooling and heating cycles (Fig. 3) were separated into individual trials and averaged (Fig. 4). The solid lines represent the averages of all the samples of a given concentration and the dotted lines represent one standard deviation (). The approximate time for a sample to freeze was estimated as the time for the average temperature to reach 0 ◦ C (32 ◦ F) during cooling. The average freezing time, for both the controls and the PCM-6 samples, was approximately 2.3 h. The freezing times were similar most likely due to the small amounts of PCM-6 absorbed by the samples and because the PCM-6 did not fully change phase before the average temperature indicated 0 ◦ C (32 ◦ F). Although no PCM-6 concentration completely prevented freezing, a noticeable change in the cooling rate was observed at approximately 30 min after the sample reached the phase change temperature. The reduction in the cooling rate caused the average temperatures of the PCM samples to be higher than the temperature of the control. The extreme low temperatures for 2.5% PCM-6, 1.25% PCM-6, and control were, respectively, −13.11 ◦ C (8.40 ◦ F), −13.24 ◦ C (8.17 ◦ F), and −15.28 ◦ C (4.49 ◦ F). Although PCM-6 was chosen specifically to delay or prevent freezing and reduce the extreme low temperature, the extreme high temperature of the samples was also analyzed. The extreme high temperatures for 2.5% PCM-6, 1.25% PCM-6, and control were 22.08 ◦ C (71.74 ◦ F), 21.62 ◦ C (70.92 ◦ F), and 21.59 ◦ C (70.86 ◦ F). Because PCM-6 changes phase at 6 ◦ C (42.8 ◦ F), it was expected that all samples would reach approximately the same temperature (due to external variations or differences in thermal conductivity values)
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16
15
Table 7 Summary of volumetric results. Property Target Control PCM-6
BSG
VTM (%)
Control: 2.25 g cm−3 (140.46 lb ft−3 ) PCM-6: 2.20 g cm−3 (137.34 lb ft−3 ) 1.97 (± 0.11) g cm−3 [122.98 (± 6.87) lb ft−3 ] 1.92 (± 0.01) g cm−3 [119.86 (±.624) lb ft−3 ]
4 16 (± 4.8) 16.2 (± 0.5)
VMA (%)
VFA (%)
15
65–78
31.1 (± 4.1) 32.7 (± 0.4)
49.3 (± 8.3) 50.5 (± 1.0)
Table 8 Summary of thermal data for samples with a mix design. Sample 1 Max. Temp. Min. Temp. Conduction coeff. ()
◦
Sample 2 ◦
24.8 C (76.6 F) −13.4 ◦ C (7.88 ◦ F) 0.65 mWK (± 0.07) (0.38
◦
BTU ft h ◦ F
[± 0.04])
Control ◦
24.8 C (76.6 F) −13.8 ◦ C (7.16 ◦ F) 0.83 mWK [± 0.11] (0.48 ftBTU [± 0.06]) h ◦F
24.9 ◦ C (76.8 ◦ F) −13.8 ◦ C (7.16 ◦ F) 0.83 mWK [± 0.11] (0.48 ftBTU [± 0.06]) h ◦F
because most of the energy was dissipated by the time the samples reached the extreme high temperature. Data from the preliminary samples proved that, within one standard deviation, changes to the thermal properties were obtained by using LWA to incorporate PCM-6 in HMA. The results showed PCM-6 reduced the extreme low temperature and the rate of cooling. There was no conclusive evidence that utilizing PCM-6 reduced the extreme high temperature, but PCM-6 was not expected to reduce it. However, in order to have more certainty of the effects of PCM-6, final samples with a uniform mix design were tested.
Because Control and Sample 2 exhibited roughly the same results (Table 8), it was likely none or very little PCM was incorporated into Sample 2. When Sample 2 was produced, it was noted that the LWA appeared glossy, which indicated PCM was coated on the surface. Because it was hypothesized PCM-binder interactions reduced the strength of the sample, the LWA was placed in the oven for 10 min. This additional heating most likely evaporated most of the PCM from the LWA. Additional research is required in order to evaluate if the changes in thermal properties were primarily caused by the amount of PCM-6 absorbed or by other variations during testing.
3.3. Final thermal testing results
3.4. Volumetric testing results
The final batch of 1.25% by mass of PCM-6 samples reached approximately the same high and low temperatures, and thawed at approximately the same time as the control (Fig. 6). Because of the length of the cycle, it was expected that all samples would reach approximately the same high and low extreme temperatures, with potential variations caused by differing thermal conductivity values. For Sample 1, the rate of cooling was reduced which prevented freezing for approximately 35 min longer when compared to Control and Sample 2. The average thermal conductivity of Sample 1 was 0.65 (± 0.07) W m−1 K−1 (0.38 [± 0.04] BTU ft−1 h−1 ◦ F−1 ), which was lower than 0.83 (± 0.11) W m−1 K−1 (0.48 [± 0.06] BTU ft−1 h−1 ◦ F−1 ) for Control and Sample 2 (Table 8). The lower conductivity, theoretically, should correspond to a reduction in the rate of cooling and heating. Experimentally, the rate of cooling was reduced more noticeably than the rate of heating, but this discrepancy could be attributed to Sample 1 not reaching the same starting temperature as the rest of the samples. Although a lower thermal conductivity is effective for insulation, it increases the time the sample is frozen. Additionally, the thermal conductivity is difficult to measure during the phase change because the system becomes transient as energy is absorbed or released by the sample. Thus, the measured thermal conductivity values better represent the sample during approximately steady-state conditions.
After performing the TMD test on the control mix from the final batch of samples, the TMD was reported as 2.34 (± 0.11) g cm−3 (146.08 [±6.87] lb ft−3 ). The back-calculated TMD for the PCM batch was 2.29 g cm−3 (142.96 lb ft−3 ). Once the cubes were formed and cooled, the BSG values were found and the volumetric properties were calculated. Table 7 contains the results of the BSG tests and the calculated volumetric properties. Target values for the calculated volumetric properties were taken from the Superpave specification. The bulk specific gravity (BSG) results revealed that the densities of the control and PCM samples were actually 84% of the theoretical maximum, which resulted in an air void content (VTM) of 16% instead of the target 4%. This result was expected due to material losses during sample production and resulted in a skewing of the other volumetric properties. Additionally, the averages of the control and the PCM batches were comparable, indicating that the incorporation of PCM may not have significantly effected the volumetric properties of the mix (Table 7). The standard deviations of the PCM samples were low, indicating high precision in these values, while the standard deviations of the control batch were higher due to one sample which had to be retested during the BSG test due to a leak in the vacuum-sealed bag.
Fig. 6. Average temperature versus time for samples with a mix design.
4. Conclusions Results for the preliminary samples indicated PCM-6 reduced the extreme low temperature of a sample, reduced the rate of cooling/heating, and decreased the time the sample was frozen. For the final samples, with a longer cooling\heating cycle, the results also indicated potential reductions in the rate of cooling and the rate of heating, but no conclusive reduction in the extreme low temperature or time the sample was frozen. Comparing Fig. 4 with Fig. 6, preliminary sample testing produced more noticeable reductions in temperature and the rate of cooling. This difference could be caused by the time taken for the PCM-6 to completely change phase, which was estimated by when both sides of the sample reached at least 6 ◦ C (42.8 ◦ F) and by sample composition. For
16
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16
comparison, the preliminary samples required approximately 5 h to change phase, while the mix design samples required approximately 7 h. The collective ability of the PCM-6 to warm or cool the sample was diminished by the length of time required to change phase. Overall, the incorporation of PCM-6 in HMA using LWA was found to be feasible. As a whole, the data indicated that PCM-6 could reduce the extreme low temperature of a sample and the rate of cooling\heating. Additionally, it was found that PCM-6 behaved differently when subjected to different rates and patterns of thermal cycling, indicating that the proper selection and integration of PCM depends on the specific conditions to which the asphalt is subjected, as well as the phase change temperature of the PCM. For example, for an environment that changes quickly, one type of PCM may be sufficient, but, for gradual changes, multiple types of PCM may be required due to the different temperatures within the sample. Potentially, if a method is developed that maintains the strength of HMA and ensures consistent absorption of PCM, then HMA incorporated with PCM may improve the longevity and sustainability of asphalts that are subjected to frequent thermal cycling. 5. Future work Before HMA incorporated with PCM is practical, reliable incorporation methods that maintain the material properties of the asphalt and reduce PCM losses during production should be developed. Although no strength tests were conducted, some PCM samples compacted poorly and failed during normal handling. Potentially the strength of the samples could be improved if hand-compaction was replaced with mechanical compaction and if the PCM-binder interactions were better understood. Further research is required to determine the practicality of using LWA as an incorporation medium. For example, alternative incorporation mediums should be investigated, such as encapsulated PCM, which may increase the strength of the mix. In addition, composite samples comprised of multiple types of PCM could be layered within the material to activate at different temperatures within the asphalt. The composite sample would have to utilize multiple phase change materials with different phase change temperatures in order to contend with the application and conditions in which the
material would be used. Finally, a testing method should be developed to accurately measure the amount of PCM actually incorporated into the HMA in order to further analyze the differences in thermal properties. References Cooley, L. A., Jr., Ahlrich, R. C., James, R. S., Prowell, B. D., Brown, E. R., & Kvasnak, A. (2009). . Implementation of superpave mix design for airfield pavements (Vols. 1–3) Burns Cooley Dennis, Inc. C136-06. (2006). Standard test method for sieve analysis of fine and coarse aggregates. West Conshohocken, PA: ASTM Standard. D5470-11. (2011). Standard test method for thermal transmission properties of thermally conductive electrical insulation materials. West Conshohocken, PA: ASTM Standard. D7063/D7063M. (2011). Standard test method for effective porosity and effective air voids of compacted bituminous paving mixture samples. West Conshohocken, PA: ASTM Standard. E1225-09. (2009). Standard test method for thermal conductivity of solids using the Guarded-Comparative-Longitudinal Heat Flow Technique. West Conshohocken, PA: ASTM Standard. Farnam, Y., Bentz, D., Sakulich, A., Flynn, D., & Weiss, J. (2014). Measuring freeze and thaw damage in mortars containing deicing salt using a low temperature Longitudinal Guarded Comparative Calorimeter and Acoustic Emission (AE-LGCC). Journal of Advances in Civil Engineering Materials (ASTM), 3(1), 316–337. Federal Highway Administration. (2014). Eisenhower interstate highway system. https://www.fhwa.dot.gov/interstate/homepage.cfm Hakkinen, T., & Makela, K. (1996). Environmental adaption of concrete – Environmental impact of concrete and asphalt pavements. Helsinki: Technical Research Centre of Finland (Report). Inamura, H. (1999). Life cycle inventory analysis of carbon dioxide for a highway construction project using input–output scheme. In A case study of the tohoku expressway construction works, Case study. Sendai: Graduate School of Information Sciences, Tohoku University. Instrotek, Inc. (2011). CoreLok Operator’s Guide, 2011 (23rd ed.). Raleigh, NC: Instrotek, Inc (User manual). Mallick, R. B., & El-Korchi, T. (2009). Pavement engineering: Principles and practice. Boca Raton, Florida: CRC Press. Sakulich, A. R., & Bentz, D. P. (2012a]). Incorporation of phase change materials in cementitious systems via fine lightweight aggregate. Construction and Building Materials, 35, 483–490. Sakulich, A. R., & Bentz, D. P. (2012b]). Increasing the service life of bridge decks by incorporating phase-change materials to reduce freeze–thaw cycles. Journal of Materials in Civil Engineering, 24, 1034–1042. ScienceLab, Inc. (2014). Tetradecane (PCM-6), Material Safety Data Sheet. ScienceLab. Sharifi, N. P., Sakulich, A. R., & Mallick, R. (2014). Experimental apparatuses for the determination of pavement material thermal properties. In Green Streets, Highways, and Development (pp. 117–124). Sor, K., Sor, O., Micklus, P. G., Arhan, Y., & Rowbotham, K. (2006). Laboratory testing of Solite aggregates and preparation of concrete mix design. Sor Testing Laboratories, Inc. Tested in accordance with ASTM C-330. Report 05-5187A.
Contents lists available at ScienceDirect
Sustainable Cities and Society journal homepage: www.elsevier.com/locate/scs
Assessing the feasibility of incorporating phase change material in hot mix asphalt Bryan J. Manning a,∗ , Paul R. Bender b , Sarah A. Cote b , Rachel A. Lewis a , Aaron R. Sakulich b , Rajib B. Mallick b a b
Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA, USA Department of Civil and Environmental Engineering, Worcester Polytechnic Institute, Worcester, MA, USA
a r t i c l e
i n f o
Article history: Received 23 January 2015 Received in revised form 10 June 2015 Accepted 11 June 2015 Available online 20 June 2015 MSC: 00−01 99−00 Keywords: Phase change materials Hot mix asphalt Thermal stress reduction Lightweight aggregate
a b s t r a c t This paper investigated the feasibility of using lightweight aggregate (LWA) as a medium for incorporating phase change material (PCM) in hot mix asphalt (HMA) in order to extend pavement life by reducing the magnitude of temperature fluctuations. First, evaporation and absorption tests were conducted to investigate the absorption properties of LWA and the loss of PCM during heating. Then, batches of samples with varying concentrations of PCM were created and the thermal properties were evaluated using a guarded longitudinal comparative calorimeter (GLCC). Data from thermal testing indicated that the incorporation of PCM slowed the rate of cooling and reduced low temperatures when compared to control samples. Further research should investigate PCM incorporation methods, improve the mix design, and experiment with different combinations of PCMs and thermal fluctuations. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The construction of the Eisenhower interstate system helped place roads and highways at the forefront of transportation in the United States (Federal Highway Administration, 2014). Although hot mix asphalt (HMA) pavements are designed to withstand various uses and climates, failure of the pavement is often inevitable due to normal wear and tear (Mallick & El-Korchi, 2009). While repairing damaged pavement incurs capital and environmental costs, the combination of economic losses due to frequent traffic diversion for repairs, and environmental damages caused by reduced fuel efficiency on poor roadways adds up to a significant life-cycle impact (Hakkinen & Makela, 1996; Inamura, 1999). Therefore, increasing the life of pavement can improve overall sustainability. Phase change materials (PCMs) have the potential to extend pavement life and, therefore, reduce long-term maintenance costs. PCM can be used to regulate temperature because of its ability to
∗ Corresponding author. Tel.: +1 508 831 5912. E-mail addresses: [email protected] (B.J. Manning), [email protected] (P.R. Bender), [email protected] (S.A. Cote), [email protected] (R.A. Lewis), [email protected] (A.R. Sakulich), [email protected] (R.B. Mallick). http://dx.doi.org/10.1016/j.scs.2015.06.005 2210-6707/© 2015 Elsevier Ltd. All rights reserved.
release and absorb heat depending on the environment (Sakulich & Bentz, 2012a). This release or absorption occurs during the phase change, which makes the proper selection of a PCM for a given application largely dependent on the phase change temperature. Although relatively large amounts of heat are either released during solidification or absorbed during melting, the temperature of the composite ideally remains at the phase change temperature of the PCM (Sakulich & Bentz, 2012b). The time during which the temperature remains at this value depends on the value of the latent heat of fusion of the PCM (Sakulich & Bentz, 2012b). Thus, a PCM with a higher latent heat of fusion can delay a temperature change for a longer period of time and mitigate temperature fluctuations (Sakulich & Bentz, 2012a). This paper explored the incorporation of PCM in HMA by absorption into a porous lightweight aggregate (LWA). The incorporation of PCM was expected to lessen damages from thermal stresses, reduce debris from deterioration, and decrease the amount of time, money, and resources required to maintain pavements. PCM-6, a blend of paraffin waxes that changes phase at 6 ◦ C (42.8 ◦ F), was selected due to its potential for delaying the freezing of infiltrated water because the phase change temperature was slightly above the freezing temperature of water. The failure mechanisms addressed were thermal fatigue cracking, low-temperature cracking, and freeze-thaw cycling. Thermal
12
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16 Table 1 Preliminary mix design for control samples. Mix component
Mass (g)
Normal-weight aggregate Crushed stone Natural sand
699 556 143
Binder
73
Table 2 Preliminary mix design for 1.25% PCM-6 samples.
Fig. 1. Theoretical temperature profile of asphalt with and without PCM.
fatigue cracking is caused by repeated high\low-stress cycles produced by temperature fluctuations and loading (Mallick & ElKorchi, 2009). Low-temperature cracking results from high tensile stress and weakened tensile strength at temperatures below the operational threshold of the binder (Mallick & El-Korchi, 2009). Freeze-thaw cycling occurs when repeated temperature cycles cross the freezing point of water and cause the pavement to crack and break. PCMs may be able to reduce susceptibility to thermal fatigue, low-temperature, and freeze-thaw cracking because the material can potentially dampen temperature cycling in the pavement by reducing the magnitude of extreme temperatures. Ideally, the PCM would prevent or reduce the frequency of temperatures below the operational threshold of the binder and\or temperatures below the freezing point of water (Fig. 1), thus reducing damage and maintenance costs.
2. Methodology 2.1. Sample production Absorption and evaporation tests were conducted to determine the amount of PCM-6 absorbed by LWA. Three trials with four combinations (12 samples) of LWA were tested: LWA retained by No. 4 sieve, No. 8 sieve, No. 16 sieve, and an equal mixture of the aforementioned sieves. The LWA was soaked in PCM-6 for 24 h and the final weight of PCM-6 absorbed was recorded. The samples were then placed in an oven at 150 ◦ C (302 ◦ F), which is a temperature reached in HMA mixing procedures, and the amount of weight loss was measured at 5-minute intervals. Preliminary cubic samples (5.08 cm [2 in]) were then compacted to investigate the feasibility of incorporating PCM-6 in HMA using LWA. The samples contained normal-weight aggregate (NWA) retained on sieve sizes below No. 4 and a binder content of 9% by mass. The aggregate included 20% by mass natural sand and 80% by mass crushed stone. A limit of 10% LWA by mass was used because samples tested with greater amounts crumbled upon removal from the mold. The sample concentrations were 0% (control), 1.25%, and 2.5% PCM-6 by mass. The required amounts of PCM-6 by mass were estimated by using the water absorption of 100 g of the LWA sample (17.5% by mass, particle distribution below No. 4 (Sor, Sor, Micklus, Arhan, & Rowbotham, 2006)) to backcalculate an estimated absorption of PCM-6 into the LWA. Since PCM-6 (0.763 g cm−3 [47.6 lb ft−3 ]) (ScienceLab, Inc., 2014) was less dense than water, less PCM-6 by mass would be absorbed by the same volume of LWA. After accounting for the differences in densities, the estimated absorption of PCM-6 into LWA was 13.3% by mass.
Mix component
Mass (g)
Normal-weight aggregate Crushed stone Natural sand
894 715 179
Lightweight aggregate No. 4 No. 8 No. 16
105 21 42 42
PCM-6
14
Binder
95
To prepare for mixing, the LWA was soaked in PCM-6 for 24 h (agitated every 6 h) and the binder and NWA aggregates were heated at 150 ◦ C (302 ◦ F) for three h. Before mixing, the LWA was heated at 150 ◦ C (302 ◦ F) for 2–5 min to remove PCM from the surface and hypothetically improve strength by preventing surface PCM from interfering with the binder. The binder, NWA, and LWA were then mixed and hand-compacted into cubes. For the control samples, the LWA was replaced with an equivalent volume of NWA. An initial binder content of 5.5% by mass was used for the controls, while 9% by mass was used for the PCM-6 samples to account for excess fines in the mix. The calculated concentrations of PCM were used to identify samples, but the evaporation and absorption tests indicated actual amounts were lower due to mixing and heating losses. The compositions of the samples are in Tables 1, 2, and 3. Because samples with 2.5% PCM-6 by mass compacted poorly during preliminary testing, a final batch of samples containing 1.25% PCM-6 by mass was produced. To improve the uniformity and strength of the samples, aggregates were graded to comply with ASTM C-136 (C136-06, 2006). A binder content of 5.5% by mass was used for controls and the PCM samples. By utilizing this gradation, each final sample contained the same composition of material which limited thermal differences to the addition of PCM-6. The final samples were created using the same mixing process as the preliminary samples. The final mix design for the control batch is found in Table 4 and the final mix design for the PCM-6 samples is found in Table 5. 2.2. Thermal testing To evaluate the thermal properties of samples, a guarded longitudinal comparative calorimeter (GLCC) was used to generate a Table 3 Preliminary mix design for 2.5% PCM-6 samples. Mix component
Mass (g)
Normal-weight aggregate Crushed stone Natural sand
633 506 127
Lightweight aggregate No. 4 No. 8
180 90 90
PCM-6
24
Binder
81
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16
13
Table 4 Final mix design for control samples. Mix component
Mass (g)
Normal-weight aggregate Coarse aggregate Crushed stone Natural sand
840 141 360 339 48
Binder
Table 5 Final mix design for 1.25% PCM-6 samples. Mix component
Mass (g)
Normal-weight aggregate Coarse aggregate Crushed stone Natural sand
720 141 360 219
Lightweight aggregate No. 4
81 81
PCM-6
11
Binder
45
Fig. 3. Heating and cooling profiles for preliminary samples and final samples.
pyroceram standards was 4.18 W m−1 K−1 (2.42 BTU ft−1 h−1 ◦ F−1 ) and the thickness (Z) was 2.54 cm (1 in) (Sharifi et al., 2014). The heat flows through the standards were calculated using Eq. 1, which allowed for the computation of average heat flow through the sample using:
fluctuating heat flow through the cubic samples (Sharifi, Sakulich, & Mallick, 2014). To measure the heat flow, a sample was placed inside the device and six thermocouples measured temperatures at various locations (Fig. 2). The sample was placed between two pyroceram standards (5.08 cm × 5.08 cm × 2.54 cm [2 in × 2 in × 1 in]) and the thermocouples were held in contact using 0.3175 cm (0.125 in) of TC3008 elastomer. In general, to compute steady-state conductive heat transfer through a material: Q = A·
T Z
Qsample =
(2)
where Qsample (W [BTU h−1 ]) is the heat flow through the sample, Qtop (W [BTU h−1 ]) is the heat flow through the top standard, and Qbottom (W [BTU h−1 ]) is the heat flow through the bottom standard (E1225-09, 2009). In addition, the thermal conductivity of each sample (W m−1 −1 K [BTU ft−1 h−1 ◦ F−1 ]) was computed by rearranging Equation 1 and Equation 2 to yield:
(1)
where Q (W [BTU h−1 ]) is the heat flow through the material, A (m2 [ft2 ]) is the area measured normal to the direction of the heat flow, (W m−1 K−1 [BTU ft−1 h−1 ◦ F−1 ]) is the thermal conductivity of the material, and T/Z (◦ C m−1 [◦ F ft−1 ]) is the temperature gradient (D5470-11, 2011). The thermal conductivity of the
Qtop + Qbottom 2
sample =
Qsample A
·
Z T
(3)
where Qsample (W [BTU h−1 ]) is the heat flow through the sample, A (m2 [ft2 ]) is the area measured normal to the direction of the heat Z (m ◦ C−1 [ft ◦ F−1 ]) is the inverse of the temperature flow, and T gradient. The GLCC was programmed to replicate thermal cycling (Fig. 3). The preliminary samples were subjected to a high and low of 23 ◦ C (73.4 ◦ F) and −25 ◦ C (−13 ◦ F), respectively. The temperature change was 0.3 ◦ C h−1 (0.54 ◦ F h−1 ). Each sample underwent three cooling and heating cycles which lasted for a total of 43 h. Two control samples, three samples of 1.25%, and three samples of 2.5% PCM-6 by mass were created. Only two control samples were produced due to logistical reasons. The final samples were subjected to a high and low of 25 ◦ C (77 ◦ F) and −25 ◦ C (−13 ◦ F). The temperature change was 2 ◦ C h−1 (3.6 ◦ F h−1 ) for cooling and 4 ◦ C h−1 (7.2 ◦ F h−1 ) for heating. Each sample underwent one freezing and thawing cycle which lasted 45.5 h. The gradual temperature change was based on previous research, and was expected to produce more consistent temperature measurements and allow for accurate computation of thermal conductivity due to better steady-state conditions (Farnam, Bentz, Sakulich, Flynn, & Weiss, 2014). One control and two samples containing 1.25% PCM-6 by mass were evaluated. 2.3. Volumetric testing
Fig. 2. Pictorial representation of guarded longitudinal comparative calorimeter (GLCC).
The “Superpave” (Cooley et al., 2009) specification was used to evaluate the volumetric properties of the final batches of PCM-6 and control samples. First, the theoretical maximum density (TMD) test was performed on a loose sample of HMA to determine the maximum density possible for the control mix. The sample was mixed, allowed to cool under constant stirring, and then vacuum
14
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16
Fig. 4. Average temperature versus time for preliminary samples.
Table 6 PCM-6 absorption for various sieve sizes. Sieve size
PCM-6 absorption (% Mass)
No. 4 No. 8 No. 16 Blend
10.1 (±0.51) 13.3 (±0.53) 18.1 (±0.63) 11.9 (±0.28)
sealed in a CoreLok machine. The density was calculated by the water displacement method described in the CoreLok Operator’s Guide (Instrotek, Inc., 2011). The expected TMD of the PCM-6 mix was back-calculated using the TMD of the control mix and physical properties of the aggregates and binder. The TMD values were used to batch quantities of material that would yield samples with the desired density (96% of TMD) and air void content (4%). The cubes were cooled overnight, and then the bulk specific gravity (BSG) test (g cm−3 [lb ft−3 ] was completed following ASTM D7063 (D7063/D7063M, 2011) and the CoreLok Guide (Instrotek, Inc., 2011). The BSG values for each sample were used to calculate the percent voids in the total mix (VTM), percent voids in the mineral aggregate (VMA), and percent voids filled with asphalt (VFA). Comparisons were made between control and PCM-6 samples to determine the effects of PCM-6 on volumetric properties of HMA. These sample properties were also evaluated against the volumetric requirements of the Superpave specification to determine compliance of the mix.
3. Results and discussion 3.1. Evaporation and absorption testing results Absorption tests determined the amount of PCM-6 absorbed by each LWA sieve size and combination of sieve sizes. Finer LWA retained more PCM-6 than the coarser LWA most likely due to PCM adhering to the larger surface area for a given volume of material (Table 6). In addition, all samples were visibly glossy after soaking. The test also indicated the estimated 13.3% by mass of absorption of PCM-6 by LWA was a sufficient approximation. Evaporation tests found that PCM-6 evaporated quickly (Fig. 5). After 5 min of heating, the LWA was no longer glossy indicating approximately 10% to 15% of the PCM adhered to the surface and had evaporated. Because of the evaporation rate, the PCM-soaked LWA would be added to the HMA mix after minimal heating (2–5 min) in order to reduce losses from evaporation.
Fig. 5. PCM-6 evaporation rates for various sieve sizes.
3.2. Preliminary thermal testing results To evaluate the thermal effects of the PCM, the average temperatures of the samples were plotted as a function of time. Each of the individual cooling and heating cycles (Fig. 3) were separated into individual trials and averaged (Fig. 4). The solid lines represent the averages of all the samples of a given concentration and the dotted lines represent one standard deviation (). The approximate time for a sample to freeze was estimated as the time for the average temperature to reach 0 ◦ C (32 ◦ F) during cooling. The average freezing time, for both the controls and the PCM-6 samples, was approximately 2.3 h. The freezing times were similar most likely due to the small amounts of PCM-6 absorbed by the samples and because the PCM-6 did not fully change phase before the average temperature indicated 0 ◦ C (32 ◦ F). Although no PCM-6 concentration completely prevented freezing, a noticeable change in the cooling rate was observed at approximately 30 min after the sample reached the phase change temperature. The reduction in the cooling rate caused the average temperatures of the PCM samples to be higher than the temperature of the control. The extreme low temperatures for 2.5% PCM-6, 1.25% PCM-6, and control were, respectively, −13.11 ◦ C (8.40 ◦ F), −13.24 ◦ C (8.17 ◦ F), and −15.28 ◦ C (4.49 ◦ F). Although PCM-6 was chosen specifically to delay or prevent freezing and reduce the extreme low temperature, the extreme high temperature of the samples was also analyzed. The extreme high temperatures for 2.5% PCM-6, 1.25% PCM-6, and control were 22.08 ◦ C (71.74 ◦ F), 21.62 ◦ C (70.92 ◦ F), and 21.59 ◦ C (70.86 ◦ F). Because PCM-6 changes phase at 6 ◦ C (42.8 ◦ F), it was expected that all samples would reach approximately the same temperature (due to external variations or differences in thermal conductivity values)
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16
15
Table 7 Summary of volumetric results. Property Target Control PCM-6
BSG
VTM (%)
Control: 2.25 g cm−3 (140.46 lb ft−3 ) PCM-6: 2.20 g cm−3 (137.34 lb ft−3 ) 1.97 (± 0.11) g cm−3 [122.98 (± 6.87) lb ft−3 ] 1.92 (± 0.01) g cm−3 [119.86 (±.624) lb ft−3 ]
4 16 (± 4.8) 16.2 (± 0.5)
VMA (%)
VFA (%)
15
65–78
31.1 (± 4.1) 32.7 (± 0.4)
49.3 (± 8.3) 50.5 (± 1.0)
Table 8 Summary of thermal data for samples with a mix design. Sample 1 Max. Temp. Min. Temp. Conduction coeff. ()
◦
Sample 2 ◦
24.8 C (76.6 F) −13.4 ◦ C (7.88 ◦ F) 0.65 mWK (± 0.07) (0.38
◦
BTU ft h ◦ F
[± 0.04])
Control ◦
24.8 C (76.6 F) −13.8 ◦ C (7.16 ◦ F) 0.83 mWK [± 0.11] (0.48 ftBTU [± 0.06]) h ◦F
24.9 ◦ C (76.8 ◦ F) −13.8 ◦ C (7.16 ◦ F) 0.83 mWK [± 0.11] (0.48 ftBTU [± 0.06]) h ◦F
because most of the energy was dissipated by the time the samples reached the extreme high temperature. Data from the preliminary samples proved that, within one standard deviation, changes to the thermal properties were obtained by using LWA to incorporate PCM-6 in HMA. The results showed PCM-6 reduced the extreme low temperature and the rate of cooling. There was no conclusive evidence that utilizing PCM-6 reduced the extreme high temperature, but PCM-6 was not expected to reduce it. However, in order to have more certainty of the effects of PCM-6, final samples with a uniform mix design were tested.
Because Control and Sample 2 exhibited roughly the same results (Table 8), it was likely none or very little PCM was incorporated into Sample 2. When Sample 2 was produced, it was noted that the LWA appeared glossy, which indicated PCM was coated on the surface. Because it was hypothesized PCM-binder interactions reduced the strength of the sample, the LWA was placed in the oven for 10 min. This additional heating most likely evaporated most of the PCM from the LWA. Additional research is required in order to evaluate if the changes in thermal properties were primarily caused by the amount of PCM-6 absorbed or by other variations during testing.
3.3. Final thermal testing results
3.4. Volumetric testing results
The final batch of 1.25% by mass of PCM-6 samples reached approximately the same high and low temperatures, and thawed at approximately the same time as the control (Fig. 6). Because of the length of the cycle, it was expected that all samples would reach approximately the same high and low extreme temperatures, with potential variations caused by differing thermal conductivity values. For Sample 1, the rate of cooling was reduced which prevented freezing for approximately 35 min longer when compared to Control and Sample 2. The average thermal conductivity of Sample 1 was 0.65 (± 0.07) W m−1 K−1 (0.38 [± 0.04] BTU ft−1 h−1 ◦ F−1 ), which was lower than 0.83 (± 0.11) W m−1 K−1 (0.48 [± 0.06] BTU ft−1 h−1 ◦ F−1 ) for Control and Sample 2 (Table 8). The lower conductivity, theoretically, should correspond to a reduction in the rate of cooling and heating. Experimentally, the rate of cooling was reduced more noticeably than the rate of heating, but this discrepancy could be attributed to Sample 1 not reaching the same starting temperature as the rest of the samples. Although a lower thermal conductivity is effective for insulation, it increases the time the sample is frozen. Additionally, the thermal conductivity is difficult to measure during the phase change because the system becomes transient as energy is absorbed or released by the sample. Thus, the measured thermal conductivity values better represent the sample during approximately steady-state conditions.
After performing the TMD test on the control mix from the final batch of samples, the TMD was reported as 2.34 (± 0.11) g cm−3 (146.08 [±6.87] lb ft−3 ). The back-calculated TMD for the PCM batch was 2.29 g cm−3 (142.96 lb ft−3 ). Once the cubes were formed and cooled, the BSG values were found and the volumetric properties were calculated. Table 7 contains the results of the BSG tests and the calculated volumetric properties. Target values for the calculated volumetric properties were taken from the Superpave specification. The bulk specific gravity (BSG) results revealed that the densities of the control and PCM samples were actually 84% of the theoretical maximum, which resulted in an air void content (VTM) of 16% instead of the target 4%. This result was expected due to material losses during sample production and resulted in a skewing of the other volumetric properties. Additionally, the averages of the control and the PCM batches were comparable, indicating that the incorporation of PCM may not have significantly effected the volumetric properties of the mix (Table 7). The standard deviations of the PCM samples were low, indicating high precision in these values, while the standard deviations of the control batch were higher due to one sample which had to be retested during the BSG test due to a leak in the vacuum-sealed bag.
Fig. 6. Average temperature versus time for samples with a mix design.
4. Conclusions Results for the preliminary samples indicated PCM-6 reduced the extreme low temperature of a sample, reduced the rate of cooling/heating, and decreased the time the sample was frozen. For the final samples, with a longer cooling\heating cycle, the results also indicated potential reductions in the rate of cooling and the rate of heating, but no conclusive reduction in the extreme low temperature or time the sample was frozen. Comparing Fig. 4 with Fig. 6, preliminary sample testing produced more noticeable reductions in temperature and the rate of cooling. This difference could be caused by the time taken for the PCM-6 to completely change phase, which was estimated by when both sides of the sample reached at least 6 ◦ C (42.8 ◦ F) and by sample composition. For
16
B.J. Manning et al. / Sustainable Cities and Society 19 (2015) 11–16
comparison, the preliminary samples required approximately 5 h to change phase, while the mix design samples required approximately 7 h. The collective ability of the PCM-6 to warm or cool the sample was diminished by the length of time required to change phase. Overall, the incorporation of PCM-6 in HMA using LWA was found to be feasible. As a whole, the data indicated that PCM-6 could reduce the extreme low temperature of a sample and the rate of cooling\heating. Additionally, it was found that PCM-6 behaved differently when subjected to different rates and patterns of thermal cycling, indicating that the proper selection and integration of PCM depends on the specific conditions to which the asphalt is subjected, as well as the phase change temperature of the PCM. For example, for an environment that changes quickly, one type of PCM may be sufficient, but, for gradual changes, multiple types of PCM may be required due to the different temperatures within the sample. Potentially, if a method is developed that maintains the strength of HMA and ensures consistent absorption of PCM, then HMA incorporated with PCM may improve the longevity and sustainability of asphalts that are subjected to frequent thermal cycling. 5. Future work Before HMA incorporated with PCM is practical, reliable incorporation methods that maintain the material properties of the asphalt and reduce PCM losses during production should be developed. Although no strength tests were conducted, some PCM samples compacted poorly and failed during normal handling. Potentially the strength of the samples could be improved if hand-compaction was replaced with mechanical compaction and if the PCM-binder interactions were better understood. Further research is required to determine the practicality of using LWA as an incorporation medium. For example, alternative incorporation mediums should be investigated, such as encapsulated PCM, which may increase the strength of the mix. In addition, composite samples comprised of multiple types of PCM could be layered within the material to activate at different temperatures within the asphalt. The composite sample would have to utilize multiple phase change materials with different phase change temperatures in order to contend with the application and conditions in which the
material would be used. Finally, a testing method should be developed to accurately measure the amount of PCM actually incorporated into the HMA in order to further analyze the differences in thermal properties. References Cooley, L. A., Jr., Ahlrich, R. C., James, R. S., Prowell, B. D., Brown, E. R., & Kvasnak, A. (2009). . Implementation of superpave mix design for airfield pavements (Vols. 1–3) Burns Cooley Dennis, Inc. C136-06. (2006). Standard test method for sieve analysis of fine and coarse aggregates. West Conshohocken, PA: ASTM Standard. D5470-11. (2011). Standard test method for thermal transmission properties of thermally conductive electrical insulation materials. West Conshohocken, PA: ASTM Standard. D7063/D7063M. (2011). Standard test method for effective porosity and effective air voids of compacted bituminous paving mixture samples. West Conshohocken, PA: ASTM Standard. E1225-09. (2009). Standard test method for thermal conductivity of solids using the Guarded-Comparative-Longitudinal Heat Flow Technique. West Conshohocken, PA: ASTM Standard. Farnam, Y., Bentz, D., Sakulich, A., Flynn, D., & Weiss, J. (2014). Measuring freeze and thaw damage in mortars containing deicing salt using a low temperature Longitudinal Guarded Comparative Calorimeter and Acoustic Emission (AE-LGCC). Journal of Advances in Civil Engineering Materials (ASTM), 3(1), 316–337. Federal Highway Administration. (2014). Eisenhower interstate highway system. https://www.fhwa.dot.gov/interstate/homepage.cfm Hakkinen, T., & Makela, K. (1996). Environmental adaption of concrete – Environmental impact of concrete and asphalt pavements. Helsinki: Technical Research Centre of Finland (Report). Inamura, H. (1999). Life cycle inventory analysis of carbon dioxide for a highway construction project using input–output scheme. In A case study of the tohoku expressway construction works, Case study. Sendai: Graduate School of Information Sciences, Tohoku University. Instrotek, Inc. (2011). CoreLok Operator’s Guide, 2011 (23rd ed.). Raleigh, NC: Instrotek, Inc (User manual). Mallick, R. B., & El-Korchi, T. (2009). Pavement engineering: Principles and practice. Boca Raton, Florida: CRC Press. Sakulich, A. R., & Bentz, D. P. (2012a]). Incorporation of phase change materials in cementitious systems via fine lightweight aggregate. Construction and Building Materials, 35, 483–490. Sakulich, A. R., & Bentz, D. P. (2012b]). Increasing the service life of bridge decks by incorporating phase-change materials to reduce freeze–thaw cycles. Journal of Materials in Civil Engineering, 24, 1034–1042. ScienceLab, Inc. (2014). Tetradecane (PCM-6), Material Safety Data Sheet. ScienceLab. Sharifi, N. P., Sakulich, A. R., & Mallick, R. (2014). Experimental apparatuses for the determination of pavement material thermal properties. In Green Streets, Highways, and Development (pp. 117–124). Sor, K., Sor, O., Micklus, P. G., Arhan, Y., & Rowbotham, K. (2006). Laboratory testing of Solite aggregates and preparation of concrete mix design. Sor Testing Laboratories, Inc. Tested in accordance with ASTM C-330. Report 05-5187A.