Asphalt mixture is one of the more temperature sensitive mixtures, as high surface temperatures have a greater impact on the performance of asphalt pavement in regards to rutting, aging and fatigue [1
]. Numerous investigations have found that the performance of asphalt can be highly affected by environmental factors, specifically temperature [2
Phase change material (PCM) has the capability of storing and releasing thermal energy, so PCM can be used to store or release heat storage based on either the sensible heat principle or latent heat principle. Sensible heat is a specific thermal term that refers to a temperature rise due to heat absorption without the presence of a phase change. Latent heat refers to the absorbed or released heat during the process of a material’s phase transition under the condition of constant temperature [4
]. Therefore, PCM can be used in an asphalt mixture to proactively regulate the temperature of asphalt pavement and improve the temperature performance of pavement. There have been numerous studies on incorporating PCM in asphalt mixture. Using the heat transfer theory and simulation test, Ma and Wang studied the effects of PCM’s thermo regulation on asphalt [7
]. Numerous studies, carried out by Ma and Li, concluded that mixing organic solid–liquid PCM into an asphalt mixture was beneficial in adjusting the temperature of the asphalt pavement but it has negatively impacted the service performance of the asphalt mixture [8
]. The addition of PCM to an asphalt mixture can change the working temperature and improve the temperature resistance capacity of asphalt mixture as well as its ability to adapt to changes in the environment [11
]. Ma [12
] prepared the composited shape-stabilized phase change material (CPCM), which was developed using Tetradecane, silica, Ethyl Cellulose (EC) and dispersant. Such CPCM adjusted the temperature of the pavement, increasing its adaptability to environmental changes. It can be applied directly to the asphalt mixture [14
]. Therefore, CPCM was used as an admixture of asphalt mixture to improve the performance of asphalt pavement.
In previous studies [7
], temperature changing behaviors of PCM were used to analyze the influence of CPCM on asphalt mixture. However, limited by the test conditions, a computer is needed to simulate the real working conditions of roads. Furthermore, the analysis on a simple temperature regulation of CPCM is not enough; the specific heat capacity is the critical parameter for thermal analysis. To indicate the influence of CPCM on asphalt mixture intuitively, studies of specific heat capacity on CPCM and asphalt mixture mixed with CPCM are in urgent need.
Endothermic and exothermic reactions are the essence of temperature change, so temperature can be expressed as a quantity of heat by calculating the specific heat capacity. There is no universal method for measuring specific heat capacity. Differential Scanning Calorimetric (DSC) [15
] was used to test the phase change temperature, latent heat and thermal stability by measuring the physical appearance of the material with temperature change. Melting point, endothermic enthalpy, and exothermic enthalpy of docosane PCM were tested by DSC and Thermal Gravimetric Analysis (TGA). Alkan reviewed that the Microencapsulated phase change material using docosane as the core material has remarkable chemical stability and thermal stability [16
]. Bo [17
] studied liquid–solid phase equilibrium and phase transformation of Tetradecane and hexadecane binary mixtures by DSC. T-history was used to measure the solid point, specific heat, latent heat, thermal conductivity and thermal diffusion coefficient of phase change materials. The theory behind T-history testing of the latent heat of PCM is a comparison of the temperature history between the PCM and a reference material subjected to conditions capable of the lumped capacity method [18
]. Zhang YP [19
] studied the T-history method and stated that for the engineering and preparation of new PCMs, which is an especially useful method in choosing the best fit PCM.
In previous research, a Marshall specimen was used to determine the specific heat capacity of asphalt mixture by the solid standard test method [20
]. After the temperature of water and heated containers stabilize, a low temperature object was put into hot water and the heat exchanged until the temperature equilibrated. The water temperature and the initial temperature of the low temperature object were known, and the specific heat capacity of solid was calculated using the heat balance law. In summary, DSC has advantages of high maturity and high precision. The advantages of the T-history method are that only a simple apparatus is required (sealed tubes) and no sampling requirements, which make it useful when measuring the thermo-physical properties of materials that exhibit inhomogeneous phase changes [22
]. However, the integrity of the materials used in the T-history method was destroyed, likely to affect the measured data. As for solids with a quantity less than 20 mg and a particle size smaller than 6 mm, the most common method is DSC. As the quantity of solid ranges from 5 g to 10 g, T-history is more reliable.
The above test methods for specific heat capacity were just for a small quantity, which cannot be applied to test the specific heat capacity of solid materials with a greater quantity. As with the increasing of the sample quality, the peak shape of DSC curves moves to the right. The existence of a temperature gradient between the sample surface and the sample center, an increase in sample quality, an increase in the temperature gradient in the center of the sample center during the temperature-rise period, and an extended time of peak completed or transferred led to the overall peak shape shifting to the right [23
]. In addition, asphalt mixture is mainly composed of aggregates of different sizes, which ranged from 0.075 mm to 13.2 mm in this study. When the aggregate size is large, the Marshall specimen must to be large enough to ensure uniformity in the specimen. DSC is unable to test large sized material, though T-history test can obtain the specific heat capacity of each composition of asphalt mixture, as there are many factors that affect the specific heat capacity of asphalt mixture so the specific heat capacity of asphalt mixture cannot be obtained by averaging the specific heat capacity of each material. Obviously, the two test methods cannot measure the specific heat capacity of asphalt mixture directly. Meanwhile, the solid standard test method was applied to the materials with relatively small temperature changes, and an accurate specific heat capacity was not obtained. The data were imprecise, the results were unstable, and the calculated data were scattered and unreliable. Above all, a new method is urgently needed to test the specific heat capacities of CPCM and asphalt mixture.
Therefore, this paper proposed a heat exchange system to test real-time temperature change behaviors and calculate the specific heat capacities of CPCM and asphalt mixture mixed with CPCM. The specific heat capacity was used to study the influence of CPCM on asphalt mixture.
The shape-stabilized PCM is made up of, in certain proportions, PCM and silica as a carrier material [12
]. Ethyl Cellulose (EC) works as the membrane material and Ethyl alcohol (CH3
OH) as an organic solvent for EC to cover the PCM material. Plasticizer was used to improve the flexibility of the membrane material. The initial phase change temperature of CPCM used in this study was 5 °C, which is 5 °C CPCM for short. The DSC curves of 5 °C CPCM [14
] are shown in Figure 1
. During the exothermic process, the initial phase change temperature and final phase change temperature were 2 °C and −31 °C, respectively, and the compensation power peak appeared at −5 °C or so. The exothermic enthalpy was 80.31 J/g.
SBS (I-C) modified asphalt was selected as the original asphalt, which is asphalt modified by adding thermoplastic styrene-butadiene rubber (SBS) into the matrix asphalt while a percentage of the exclusive stabilizer is added to form SBS blend material. I-C indicates that SBS modified asphalt meet the I-C technical requirements of SBS modified asphalt; flash rock, mechanism sand and grounded limestone were selected as coarse aggregate, fine aggregate and slag, respectively. The technical indicators of the above materials were in accordance with the specification requirements [24
In this paper, the heat exchange system and the data acquisition system were used to test the temperature change behaviors and the specific heat capacities of 5 °C CPCM and asphalt mixture mixed with 5 °C CPCM.
It is important to note that the 5 °C CPCM temperature-rise curve has an obvious temperature plateau while that of the asphalt mixture mixed with 5 °C CPCM does not have a temperature plateau. The temperatures of boundary points on the temperature-rise curve are due to the phase change temperature and the thermal compensation on the DSC scanning curve. As for the 5 °C CPCM, with an increase in temperature, the specific heat capacity first increased and reached the maximum at the initial phase change temperature, then decreased and reached the minimum at the final phase change temperature. Finally, the specific heat capacity increased slowly. With increasing temperature, the specific heat capacity of the asphalt mixture first linearly increased and then decreased linearly. It can be concluded that the low dosage of 5 °C CPCM has little influence on the specific heat capacity of the asphalt mixture, and the delay in the rise in temperature is insignificant. This may be due to different masses. When compared with the mass of the Marshall specimen, the 5 °C CPCM mass is too small to significantly affect asphalt mixture, which leads to the temperature plateau being less obvious. This is of some applicable significance in studying the thermoregulation mechanism and efficiency of CPCM on asphalt mixture. Meanwhile, this work provides parameters and a theoretical foundation for further studies. The limitation of this study is that the specific heat capacity was calculated by multiple fittings of the original data so that there exists cumulative error. Therefore, the processing of original data needs further improvement.