The impact of various environmental phenomena and the surrounding buildings on our daily lives and health is becoming an increasingly important topic. Attempts are being made to analyze the extent of the impact of artificially created materials and objects (buildings) on the development of mankind. Reflecting on data that more than 50% of the global population lives in cities and that this proportion will increase to ~66% by 2050 [1
], questions arise regarding the consequences of high urbanization and the environment on humanity, and particularly, how to reduce the negative consequences of living in cities.
Urbanization and urban lifestyles greatly influence and change the local climate, and the effects of urban heat islands (UHIs) have been pronounced recently. The term UHI refers to a significantly higher air temperature in an urban environment compared with a rural area, caused by a large number of buildings and roads that retain more heat than parks or water surfaces. The scale of the UHI effect is influenced by several factors, including the site location, i.e., geographic location, wind characteristics, sun radiation and vegetative cover, the degree of modernization, i.e., population, population density and number and type of vehicles, and the urbanization degree, i.e., pavement type and age and height and spacing of buildings. While the site location and the degree of modernization are factors invariably depending on the geographical location and economic development, the urbanization degree set of factors has been investigated to mitigate the influence of UHIs since the 1980s, with an increasing trend in the number of laboratory experiments seeking suitable materials and new techniques [1
The two most common urban pavement materials are asphalt mixtures and cement concrete, which can be inbuilt as monolithic slabs or as precast pavers. There is a significant difference in the thermal behavior of these materials. Therefore, to adequately address UHI mitigation, understanding the thermal properties and behavior of pavement materials is essential. Heat transfer in pavement structures is accomplished by three common modes, namely, conductivity, radiation and convection. As previously reported [2
], the dominant heat transfer mode within the structure is thermal conduction, while on a pavement surface, the dominant modes are radiation and convection. Thus, UHI mitigation associated with the characteristics of pavement materials can be achieved by increasing the pavement surface reflectivity (reducing solar absorption) or by increasing the thermal conductivity (efficiently transferring heat flux toward sublayers) of the surface materials.
The high reflectivity of pavement surfaces is considered to be the most effective method for UHI mitigation by reducing the sensible heat discharge from the pavement surface [3
]. Although reflective pavement surfaces will mitigate both the daytime and nighttime UHI effect, new findings presented in [5
] state that for reducing UHI during the day, it is better to use pavement materials with higher conductivity and thermal storage and vice versa for nighttime UHI effect reduction.
Concrete pavements have a brighter surface color, resulting in a higher solar reflectivity during the daytime, which is an advantage compared with a dark asphalt surface color, particularly just after laying. An increase in asphalt surface reflectivity as a method of UHI mitigation was investigated [6
], where reflective coating materials consisting of epoxy glue and different micro-TiO2
and nano-ZnO fillers were utilized. For concrete pavements, a potential surface reflectiveness increase was achieved by thermochromic coatings with reversible color change ability, which reflected solar energy during summer and absorbed solar energy in winter [7
]. However, the main limiting factor of such materials is their high cost [8
]. Brighter color can be achieved by replacing fine aggregates with lighter colored glass particles, also resulting in reduced pavement temperature [9
Pervious concrete pavements, as alternatives to conventional concrete, represent a sustainable paving solution in view of improved surface draining characteristics, recharging groundwater potential, reducing natural aggregate exploitation and potentially reducing the UHI effect in urban environments. The main feature of pervious concrete is its high porosity that results in an excellent drainage ability, but also reduces its strength capacity, thereby imposing the possibility of its application in low traffic pavements as opposed to structural concrete [10
]. However, its low strength caused by lack of fine aggregate fractions makes it a sustainable solution due to its natural resource depletion reduction. In addition, sustainability is enhanced in energy and natural resource depletion reduction caused by the use of untreated subbase layers within pervious concrete pavement structure, and lower energy and CO2
emission during its production and construction [13
]. Total embodied energy and greenhouse gas emission in pervious concrete pavement with an aggregate base layer can be reduced by up to 3%, compared with Portland cement concrete pavement with similar configuration. However, recent research related to the sustainability of pervious concrete has been related to the possibility of UHI mitigation. The authors of [14
] demonstrated a slower increase in surface temperature for pervious concrete compared with Portland cement concrete, with high wind influence on surface temperature reduction. Conversely, the rougher surfaces of pervious concrete pavements caused by their high porosity reflected less solar radiation and produced higher internal and surface temperatures compared with conventional concrete [6
] in dry conditions. Furthermore, this higher porosity resulted in a decrease in thermal conductivity [16
], which does not favor UHI mitigation. Thermal conductivity was improved by replacing crushed limestone aggregate with recycled concrete and coal bottom ash aggregates [17
]. Similar or lower surface temperatures were recorded in wet conditions due to evaporation of the water held in pervious concrete surface pores [15
]. Results presented in [18
] suggested that a higher thermal conductivity of pervious concrete increased the evaporative cooling effect. Evaporative cooling performance of pervious concrete was also achieved by replacing a small quantity of cement by pulverized biochar particles [19
]. A more effective method for UHI mitigation and thermal comfort improvement was achieved by sprinkling the pervious concrete surface with water [20
]. Rewetting the pervious concrete kept it cooler than convectional concrete [21
], suggesting an increase in water consumption to maintain user comfort. Increased need for wetting could be overcome by water-retaining paver blocks which were developed to hold water near the pavement surface in the concrete matrix by water-retentive fillers (blast furnace slags, pervious mortar, bottom ash, peat moss, hydrophilic fiber, and other) [22
]. These pavements help improve ambient and human thermal conditions and comfort, and demonstrated significant surface temperature reductions compared with conventional asphalt pavement [23
]. Additionally, the results presented in [24
] showed that the capillary columns and an internal water storage zone formed by a high-density polyethylene liner in innovative permeable pavement enhanced evaporation and lowered pavement surface temperature compared with conventional permeable pavements. Finally, the combination of highly reflective and pervious pavement surfaces is potentially optimal [25
While dense and pervious concrete pavements have been widely investigated for their thermophysical characteristics, there are limited data on typical and widely available and used concrete pavers. In [26
], the effect of pavement texture and color on thermal performance of concrete pavers was presented. Proper selection of color and texture of pavers provided reductions in surface temperature of up to 5 °C; when considering both paver color and texture, it was shown that the red colored jagged paver could minimize pavement contribution to the UHI effect.
Thermal behavior is investigated within this research, i.e., material behavior and response to different temperature conditions, and prediction of material ability for heat transfer when inbuilt in pavement structure. The objective of the presented research was to compare different concrete paving materials in view of the manner of temperature change influencing the UHI effect. The objective was also to complement results from tests in laboratory (controlled) conditions, with real, field condition research. The aim of this research was to supplement the existing knowledge on the thermal behavior of dense and pervious concrete pavers, with an emphasis on concrete pavers typically used within urban areas. The novelty of this research lies in the analysis of the characteristics of concrete pavers that are marginalized in previous research and in supplementing laboratory research results with real condition field research. Prominent novelty is the emphasis on the need to investigate pavement thermal characteristics and UHI effect through the overall analysis of pavement structure, and not simple analyses of wearing course characteristics.
In this study, three types of concrete paving materials were researched, namely, ordinary, dense concrete paving flags, concrete pavers and pervious concrete paving flags, with the aim of comparing their thermal properties and behavior. For UHI mitigation, proper paving material selection could be essential in contributing to user comfort improvement. According to all the presented research results, the following conclusions can be made:
There is a significant difference in the thermal properties, and behavior between different concrete paving materials and proper material selection could be essential for proper UHI mitigation;
The dense concrete paving material had the highest thermal conductivity coefficient, highest heat absorption capacity, and slowest heating and cooling speed, compared with the other paving materials;
The thermal characteristics and behavior of the pavers and pervious concrete were similar, therefore, the pervious concrete, due to its improved drainage properties, could present a better solution for urban areas;
There was a significant influence on the base layer and the surrounding characteristics on the pavement thermal behavior. Therefore, future laboratory and field tests should consider these parameters when addressing the UHI effect of different materials;
A good correlation was observed between the results of thermal conductivity measurement and the results of thermal properties measurements conducted in the field. Therefore, thermal conductivity measurement as a simple laboratory method can be used for prediction of thermal behavior of paving materials in real conditions.
Finally, the added value of this research lies in its contribution to construction practice, showing the competitiveness of pervious concrete pavers with commonly used concrete pavers. The behavior in the view of UHI influence of these two materials is very similar, but the comfort of the user is increased by providing a dry pavement surface for pedestrians and cyclists, as the main users. This can help decision makers (engineers and architects) when choosing proper pavement material, depending on its purpose and expected characteristics.