On cold materials of pavement and high-temperature performance of asphalt concrete

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On cold materials of pavement and high-temperature performance of asphalt concrete
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  See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/269385184 On Cold Materials of Pavement and High-Temperature Performance of Asphalt Concrete  Article   in  Materials Science Forum · April 2009 DOI: 10.4028/www.scientific.net/MSF.620-622.379 CITATIONS 10 READS 70 3 authors , including:Meizhu ChenWuhan University of Technology 39   PUBLICATIONS   193   CITATIONS   SEE PROFILE Shaopeng WuWuhan University of Technology 358   PUBLICATIONS   2,338   CITATIONS   SEE PROFILE All content following this page was uploaded by Meizhu Chen on 22 January 2015. The user has requested enhancement of the downloaded file.    On Cold Materials of Pavement and High-temperature Performance of Asphalt Concrete Chen Mei-zhu 1, a ,  Wei Wei 1,b and Wu Shao-peng 1,c 1 Key Laboratory for Silicate Materials Science and Engineering of Ministry of Education (Wuhan University of Technology), Wuhan 430070, China  a chenmzh@whut.edu.cn, b xunyerenweiwei@163.com, c wusp@whut.edu.cn Keywords:  Cool pavement; asphalt concrete; Urban heat island; High-temperature deformation   Abstract.  With global climate becoming warmer more and more attention is being paid to cold materials. Lower surface temperature contributes to decrease the temperature of the ambient air as heat convection intensity from a cooler surface is lower. Such temperature reductions can have significant impacts on cooling energy consumption in urban areas, a fact of particular importance in hot climate cities. The black surface of asphalt pavement absorbs more heat from the sun, and higher temperature of pavement surface contributes to increase the effect of the urban heat island, but affects the performance and life span of a pavement. Asphalt pavements form an integral part of any transportation system and are typically engineered to last 15 years or more, but many have been failing early due to potholes, cracks, raveling and other problems. Cool pavement are mainly aimed to decrease the effect of asphalt pavement on the urban heat island, but the influence of cold materials on the high-temperature performance of asphalt concrete pavement is paid little attention relatively. In this paper, it’s discussed that the effect of asphalt-pavement high temperature and its improving measures. And the mechanism of cool pavements is introduced, and possible technologies applied to asphalt pavements are reviewed. The idea of asphalt concrete pavement with automatic temperature-control is put forward. Introduction Asphalt concretes are widely used for highways, airport runways, bridge decks and city roads. Temperature is a significant factor that affects the performance and life span of asphalt pavements. . Asphalt surfaces exhibit higher surface temperatures than other surfaces in summer days, and the asphalt is softer, and during the strong sunshine, the risk is high that heavy vehicles cause rutting, wave traction, sticking wheel and so on. In the other hand, this stored heat in asphalt pavements is released into the atmosphere during the night and thus acts as a heat reservoir, namely as an urban heat island (UHI). Asphalt pavements have become an important contributor to this effect by altering landcover over significant portions of an urban area. In the United States, the Environmental Protection Agency (EPA) has developed a three-prong approach of (1) cool pavements, (2) urban forestry and (3) cool roofs to mitigate the UHI. Reducing the UHI effect can benefit air quality, lower air conditioning needs, and enhance human health and comfort. As part of a heat island reduction strategy, cool pavements contribute to the general benefits of heat island mitigation, including increased comfort, decreased air conditioning needs, and likely improved air quality. Cool pavements also can be one component of a larger sustainable pavements program, or a “green” transportation infrastructure [1]. Many research works have been carried out to evaluate the  possible energy and environmental benefits of cool pavements; however, there have been few studies to understand the effect of cold materials on the performance of asphalt concrete pavement during the summer period. In contrast, in order to improve resistance to permanent deformation, more and more scholars and pavement engineers have been placing emphasis on the development of modified binder mixtures including crumb rubber modifier (CRM)   [2]. So, it’s an interesting and significant work to relate the development of cool pavement with the improvement of high-temperature performance of asphalt concrete. In this paper, it’s discussed that the effect of asphalt-pavement high temperature and  Materials Science Forum Vols. 620-622 (2009) pp 379-382online at http://www.scientific.net © (2009) Trans Tech Publications, Switzerland   All rights reserved. No part of contents of this paper may be repr oduced or transmitted in any form or by any means without the written permission of thepublisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 218.104.96.134-27/04/09,09:37:48)    its improving measures. And the mechanism of cool pavements is introduced, and possible technologies applied to asphalt pavements are reviewed. The idea of asphalt concrete pavement with automatic temperature-control is put forward. High Temperature of Asphalt Concrete Pavement Effects of pavement high-temperature . During the summer, pavements, sidewalks, dirt paths and metal structures are hotter than the ambient temperature because they absorb solar radiant energy. The asphalt pavement surface absorptivity for solar radiation may reach 0.90. According to investigation, the surface temperature of pavement is 24°C higher than that of air, and it can reach above 60°C or 71~72 when the air temperature is about 40°C. So, during solar irradiation in summer times, asphalt  pavements can often be heated up to 70 °C due to the excellent heat-absorbing property. The high temperature of asphalt pavement will cause some undesirable cases including the following:       Rutting (permanent deformation)  of asphalt pavement: according to investigation and statistics, pavement rutting occurs in high temperature seasons during the summer, and only during the several days of highest temperature. Usually, rutting will not occur when the air temperature is lower than 30°C or 35°C, namely when the surface temperature of pavement is lower than 55°C, rutting may be limited to several millimeters. However, the rutting will increase rapidly when the air temperature is higher than 38°C, and the serious rutting of  pavement will happen during several days if the air temperature is continually higher than 40°C.     Asphalt aging  : high temperatures experienced by the exposed pavements can significantly increase the asphalt aging due to the oxidation and volatilization of these materials. Somayaii [3] reported that the rates of oxidation and volatilization in asphalt pavements nearly double for every 10°C rise in temperature. These processes lead to losses in plasticity, resulting in hard and brittle pavements, susceptible to cracking under stress, affecting the durability and functional lifetime of asphalt pavements.     Heat Island Effect  : presently, asphalt pavement account for a significant percentage of the land surface in an urban area. Paved surfaces absorb large amounts of radiation during the day and the surface heat input is conducted rapidly to deeper layer, causing rise in temperature of the  pavement. This stored heat is later released in to the atmosphere during the night and thus acts as a heat reservoir. So, asphalt pavements have important localized environmental effects in urban areas.     Fire accidents involved  : William et al[4] reported that asphalt pavement was hot enough to cause burns from 9AM to 7PM during the summer months. It was hot enough to cause a second-degree burn within 35 seconds from 10AM to 5PM. And they obtained records of 23  pavement burn cases during a retrospective review of the logbook of all burn center admission for the years 1986 to 1992. Improving Measures of High-temperature Performances of Asphalt-pavement. Rutting is a leading cause of pavement deterioration in regions with warm climates and at intersections in urban areas, where the asphalt  pavement deforms inelastically under heavy and usually slow moving traffic[5]. Minimizing asphalt pavement deterioration and maximizing its durability requires high-quality asphalts, which are stiff enough to  be resistant to permanent deformation (rutting) at elevated temperatures and soft enough to respond elastically to large loads at low 25303540455055606570    1   2  :   0   0   1   3  :   2   0   1   5  :   2   0   1   7  :   0   0   1   9  :   0   0   2   1  :   0   0   2   3  :   0   0   1  :   0   0   3  :   0   0   5  :   0   0   7  :   2   0   8  :  4   0 Time of Day     T   e   m   p   e   r   a    t   u   r   e    (    °    C  ) )) ) Dense graded hot-mix asphaltGap graded asphalt rubber HMAPortland cement concrete Fig.1 Surface temperature for different pavement materials using standard thermocouple sensors (redrawn based on the datas from Ref.[8]) 380Eco-Materials Processing and Design X    temperatures[6]. The behavior of the asphalt binder in different conditions and its ability to preserve its initial properties strongly depends on its chemical composition[7]. So, it’s usually used that modified asphalts to improve the performance of asphalt concrete. In the other hand, the gradation of asphalt mixtures is also improved for increasing rutting –resistance, including Open-Graded Friction Course (OGFC). A porous or permeable surface that allows water to percolate through it can exert a cooling effect through evaporation of water in the pavement voids or from beneath. As shown in Fig. 1[8], the highest temperature is lower for gap graded asphalt rubber hot matrix asphalt (HMA) than for dense graded HMA. However, the construction variability of open-graded asphalt concrete is increased and is susceptible to water damage. Above all, using modified asphalt or OGFC is the  passive measure, and it’s urgent that how to actively control the temperature of pavement surface. Selecting Cool Pavement of Asphalt Concrete Mechanism of cool pavements . The materials’ thermal balance is determined mainly by their thermal performance including their optical and thermal characteristics. The albedo to solar radiation and the emissivity to long wave radiation are the most significant factors. For a surface under the sun and insulated underneath, the equilibrium surface temperature, T s  is obtained from the following equation[9]. (1 – α )  I  = εδ ( T  4s  - T  4sky ) + h c ( T  s  - T  a ) ( 1 )  Where α is solar-reflectivity or albedo of the surface;  I   is total solar radiation incident on the surface, W/m 2 ; is emissivity of the surface; δ  is Stefan-Boltzmann constant, 5.6685 × 10 -8 Wm -2 K  -4 ; T   s  is equilibrium surface temperature, K; T   sky  the effective radiant sky temperature; h c  is convection coefficient, W m -2  K  -1 ; T  a  is air temperature, K (ASHRAE, 1989). As shown in equation (1), some strategies seek to control the temperature of the pavement (hence its ability to transfer heat to the air above) by controlling one or more of the material properties that influence the way pavements absorb, store, and radiate heat, including albedo (the ability to reflect short-wave radiation), permeability (evaporation heat), conductivity (the rate of heat transferred), emissivity (the rate of heat radiated), thickness of pavement (influencing how much heat stored), and convective airflow. So, possible mechanisms for creating a cool pavement that have been studied to date are the following:    Increased surface reflectance, which reduces the solar radiation absorbed by the pavement. Albedo is measured as the ratio of incident to reflected radiation. The higher the albedo of  pavement surface is, the less sunlight will be absorbed, lowering the daytime temperature of the pavement.    Increased permeability, which cools the pavement through evaporation of water. In addition,  permeable or porous surfaces are sometimes more conductive to cooling from convective airflow.    A composite structure for noise reduction, which also has been found to emit lower levels of heat at night. This effect is accomplished by using a rubberized asphalt layer over a PCC base. Technologies Adaptable for asphalt pavement . Generally, the heat balance at the pavement surface is expressed by Q  g   = α (  E  0 +  E   sky ) -  E   sur -  E  h  -  L  E   - G (2) Where: Q  g   = total gained heat flux at the pavement surface; α =  the absorptivity of pavement surface on solar radiation;  E   sur    = the   long-wave radiative flux at the pavement surface;  E  h  = the convection heat of the pavement surface;  L  E = the latent heat flux; G  =the conduction heat flux into the pavement. Materials Science Forum Vols. 620-622381    In equation (2), when Q  g   = 0, the heat balance at the pavement is reached, namely the temperature of the pavement surface doesn’t rise or lower, if Q  g >0, the pavement temperature will rise otherwise lower. High-albedo pavements are mainly based on the principle of solar heat-reflection through increasing the albedo of the pavement surface namely decreasing the absorptivity of pavement α in equation, less sunlight will be absorbed, lowering the daytime temperature of the pavement. So, some technologies can applied to asphalt pavement, including using light-colored aggregate, chip seals with light aggregate, color pigments and seals, rubberized asphalt. However, the atmosphere temperature will be higher because of the heat reflected into air. Moreover, they may not be appropriate in places where people will be uncomfortablely exposed to the reflected radiation for long periods; and the surface characteristics that affect reflectance also affect the appearance of pavements, which is important for transportation safety, the effect of these technologies on the durability of asphalt  pavement is not studied in detail. As shown in equation (2), if some methods can be adopted to increase the latent heat stored in the pavement, namely  L  E  , the pavement temperature will not rise too, for latent heat storage doesn’t depend on the temperature difference but the change of microcrystalline structure. Summary High temperature of asphalt pavement will initiate two harms (1) performance deterioration of  pavement, including rutting and asphalt aging; (2) increasing UHI effect. In order to control the surface temperature of asphalt pavement, high-albedo pavements can be achieved based on the  principle of the solar heat-reflection, and will absorb less heat than conventional darker asphalt  pavements and thus stay cooler. However, they may not be appropriate in places where people will be uncomfortablely exposed to the reflected radiation for long periods, and the atmosphere temperature will be higher. If one functional material is used in asphalt pavement, the heat absorbed by the  pavements surface could be stored as the latent heat, which doesn’t cause the surface temperature to rise. And the use of phase change materials (PCM), which possess lot s of advantages such as larger energy storage density, higher efficiency and approximately constant temperature during phase changing is meant to control pavement temperature by increasing pavement latent heat. So, the idea of asphalt concrete pavement with automatic temperature-control is put forward. References [1]   Information on http://www.camsys.com,Cool pavement reports(2005) [2]   C. C. Wong, W.-G.. Wong: Construction and Building Materials Vol.21(2007),p.1741 [3]   S. Somayaii: in Civil engineering materials ( 2nd ed. Prentice Hall, New Jersey, 2001) [4]   W. Z. Harrington, B. L. Strohschein, David Reedy, et al.: Annals of Emergency Medicine Vol.26 (1995), p.563 [5]   W. Uddin: Appl. Rheol. Vol.13(2003), p. 191 [6]   P. Michalica, I. B. Kazatchkov, J. Stastna, et al.: Fuel Vol.87(2008), p.3247 [7]   I. Ishai, Y. A. Tuffour and J. Craus: Transport. Res. Board Transport Res. Rec. Vol.1391(1993),p.39 [8]   J. S. Golden, J. Carlso, K. E Kaloush, et al.: Solar Energy Vol.81 (2007), p.872 [9]   S. Bretz, H. Akabri and A. Rosenfeld.: Atmospheric Environment Vol.32(1998), p.95 382Eco-Materials Processing and Design X
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