Interaction between pavement subsurface layers and the radioactive balance of urban canyons: Outdoors study of a physical model

Abstract

The interplay between the components of the urban landscape remains one of the most difficult analyses in thermal field research. Possibly due to the various possibilities of heat transfer between them and the unpredictability of anthropogenic heat production. The thermal field of a city can be impacted by cultural and economic factors and mitigated by urban policy control. 

In this context, the rise in air temperature in urban cores has significant effects on the health and wellbeing of city dwellers. Additionally, the rise in anthropogenic heat production from human activities promotes the establishment of urban heat islands (ICU). According to Mohajerani, Bakaric, and JeffreyBailey (2017), the effect of the ICU also leads to an increase in the energy requirements for cooling buildings, which further contributes to the heating of the urban landscape, as well as the environmental and public health effects of the issue.

Oke’s (1978) idealized equation of the radiative balance of the urban environment has served as the foundation for studies on the modification of the urban microclimate and provides a broad explanation of how energy is transported over the urban surface. Moreover, according to Duffie and Beckman (1984), thermophysical qualities are parameters for the description of heat transfer events based on Fourier’s law, which are essential for comprehending materials and their interaction with the built environment.

As a result, this relationship allows for an examination of prospective improvements in the city, whether by controlling the Net Radiation (Q*) that enters the canyon and the formation of anthropogenic heat (QF) within it, or by dispersing sensible heat (QH), latent heat (QE), heat stored in the intra-urban grid (QS), or by varying horizontal convective circulation (advection) (QA).

In this regard, the pavement has been one of the objects having the greatest impact on microclimate changes, particularly via the emission of sensible heat. According to Qin et al. (2019), traditional gray concrete has a high solar radiation absorption rate and releases the absorbed heat into the air as sensible heat, allowing for the growth of the ICU.

In addition, the paved surface already accounts for around 40 percent of the urban fabric, with 75 to 80 percent of roadways having a dark coating (CHEELA et al., 2021), which has an even greater heat absorption rate during the day.

In addition, the process of densification results in an expansion of waterproofed regions. And one of the most significant repercussions on the urban environment is the growth in problems associated with water infiltration into the soil and, as a result, the rise in drainage volumes (TATARANNI, SANGIORGI, 2019).

By evaluating the main climate variables interacting under the urban canopy layer with the other elements that make up the canyons, floors, and facades, which have naturally different types of geometric, thermal, and optical properties, the thermal quality of the urban environment and the living spaces can be affected.

This research is guided by the question, «What thermophysical features of pavements have the greatest impact on the radiative balance of the urban canyon in microclimate studies?» The proposed hypotheses on this question are:

• The thermal inertia of the pavement is a determining factor for heat storage; thus, the density of the materials, the surface texture, the color, and the thickness of the layers that make up the pavement interact with the air close to the surface in different ways;
• The albedo is an important optical property because its capacity to reflect short waves throughout the day reduces heat accumulation in the coating layer and nighttime emission of long waves;
• Due to a change in their optical properties, particularly in the near infrared region of spectral reflectance, aging of pavements produces a change in their thermal performance;
• The use of low thermal conductivity materials in the casing layer promotes the disposal of heat accumulated throughout the day via conduction in the contact of the base layer with the subgrade.

Thus, the goal of this study is to assess the level of microclimatic interaction between the pavement and the facades using a physical scale model in open space.

The study was carried out in an experimental model created specifically to assess the thermal behavior of urban canyons. Eight physical scale models were built with four different types of pavements and distinct geometric configurations that resemble streets and buildings in generic urban environments.

In terms of the effects of changing the coating material with the addition of rubber to the asphalt, it should be noted that:

• Because of its low thermal conductivity, the rubber pavement’s subsurface layer had lower surface temperatures during the day, and despite having a higher heat storage capacity, it did not cause a higher air temperature in the evening. By conducting heat to the ground, heat is dissipated.

In terms of the effects of urban form and its interaction with pavements:

• The lowest record in morphology with aspect ratio of 0,33 (M1) was the red pavement (26°C), and the lowest record in the morphology with aspect ratio of 0,66 (M2) was the gray pavement (22.05°C).

The following are the main findings when investigating the changes in heat transfer processes in various types of pavements:

• The highest total heat flow values have been set on all pavements of morphology M2. One theory is that the higher the height of the façades, the more difficult it is to dissipate heat inside the canyon in relation to M1.
• The canyons with graphite concrete pavement have the highest values of average total heat flow, followed by asphalt pavement. And M2 was the most critical morphology. And that the canyons with gray pavement had the lowest average total flow values.

Keywords: Cool Pavements; Outdoor Physical Model; Subsurface Layers; Urban Pavements.

References

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