measurement of optical properties and thermal performance

Measurement of Optical Properties and ThermalPerformance of Coloured Thin Layer Asphalt Samples

and Evaluation of Their Impact on The UrbanEnvironment

A. Synnefa, T. Karlessi, N. Gaitani, M. Santamouris, C.Papakatsikas

National and Kapodistrian University of AthensSecond International Conference on Countermeasures to Urban Heat Islands

–Berkeley, USA, 21-23d September 2009

DEVELOPEMENT OF COOL ASPHALT MATERIALSDEVELOPEMENT OF COOL ASPHALT MATERIALS

Athens ischaracterised by a

continuoustemperature

increase mainly dueto the intensive

urbanisation,industrialisation,heat island andglobal climatic

change

Spatial Temperature Distribution in Athens


The number ofhours above 37 and

40 C increasescontinously


The number ofsuccessive hours

above 35 Cincreases

continously


The frequency ofheat waves

increases as well.

Heat Island is mainly due to :a) Use of high absorbing materials and

mainly asphaltb) High Anthropogenic Heatc) Lack of Green areasd) Lack of cool sinks


Several color thin layer asphaltsamples have been developed

by an industrial partner of theUniversity of Athens

The color thin layer asphaltsamples were developed by

mixing an elastomeric asphaltbinder (colorless) and adding

special pigments and aggregatesof special sizes and colors

A sample of conventional blackasphalt was also tested and

used as reference


Measurement of the spectral reflectance by UV-VIS-NIR Spectrophotometer Carry 5000, Varian fitted with

a 150mm diameter integrating sphere

Measurement of the surface temperature of thesamples on a 24 h basis by surface temperature

sensors (thermocouples type K) connected to a datalogging system and an infrared camera (AGEMA

Thermovision 570, 7.5– 13 μm wavelength)

The samples were placed outdoors on a speciallymodulated platform, horizontal, unshaded during the

whole day and insulated from below

Experiment during July 2008, typical Mediterraneansummer conditions

SR = 0.45 SR = 0.55 SR = 0.27

SR = 0.27 SR = 0.40 SR = 0.040


Sola

r Ref

lect

ivity

Material

The mean max daily surface temperature of the colored thinlayer samples ranges from 48οC for the off-white asphaltsample to 55.8οC for the red sample.

The corresponding temperature for the black conventionalasphalt sample is 60 οC.

During the day all thesamples have Ts higherthan the T ambient air

During the night Tair ishigher than the Ts of thetested samples. Becausethe samples have highvalues of emissivity and sothey release faster the heatthey have absorbed due tonight sky radiative cooling.



A CFD model (PHOENICS) hasbeen used in order to investigatethe impact on air temperatures atlocal scale between a base casewhere conventional asphalt wasused and another case using the off–white (SR=0.55) sample, and thegreen sample, (SR=.27).

The simulation was performed for amain, commercial road (Ag.Ioannou st.) situated in the suburbsof Athens, surrounded mainly byresidential area. There are 4-5 storybuildings adjacent to the road.

The grid of the calculation domainhas dimensions of1380(x)*1704(y)*150(z) m andconsist of 80x140x50 cells at eachaxis respectively.

Simulations were performed for thesummer period and for a north windof 2m/sec and the measuredsurface temperatures of thesamples have been taken intoaccount.



For conventional asphalt, and during theday time and for an undisturbed ambienttemperature close to 35 C, localtemperatures range between 37 to 47 C.Undisturbed wind was close to 2 m/sec at10 m height.


For the off white asphalt, and for the sameboundary conditions, local temperaturesrange between 36 to 42 C. Thus, theachieved temperature reduction comparedto the conventional asphalt was between 1to 5 C.


For the green asphalt, and for the sameboundary conditions, local temperaturesrange between 36 to 45 C. Thus, theachieved temperature reduction comparedto the conventional asphalt was between 1to 2 C.


5 coloured thin layer asphalt samplesthat can be applied on existing

and new asphalt pavements havebeen developed and tested in

order to evaluate their optical andthermal performance.

All the samples demonstrated highersolar reflectance values and lower

surface temperatures comparedto conventional black asphalt.

CFD simulations showed that replacingconventional asphalt in a road could leadto an average air temperature decreasebetween 1-5 °C for calm conditions.

The results of this study indicate that theuse of color thin layer asphalt in roadsand pavements can have significantimpact in lowering surface and airtemperatures, mitigating thus the heatisland effect and its consequences.


measurement of optical properties and thermal performance

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