Passive Cooling of Pavements; a Panacea to Mitigating Urban Heat Island in Malaysia

Passive Cooling of Pavements; a Panacea to Mitigating Urban Heat Island in Malaysia

Didel Mabilong Juliet2 Okoro Leonard Ofonedum1 Chukwuma Gerald Obiora1Buba Yenhor Alfred 1

1Nigerian Building and Road Research Institute, Abuja, 900001, Nigeria.

2 Department of Architecture, Universiti Teknologi Malaysia, Malaysia.

Abstract: – Urban cities can be several degrees warmer than surrounding regions due to the built environment and concentration of human activities; a phenomenon known as Urban Heat Island. Pavements made of heat absorption materials are one of the major contributors to this effect by altering natural land cover over significant portions of the urban area. This research presents an experimental study aimed at evaluating the effect of water on outdoor pavements as a strategy to mitigate the rise in surface temperatures of pavements. The experiment, attempts to quantify the thermal performance of three commonly used pavement materials within the study area before and after water was applied with respect to their surface temperature, air temperature and relative humidity above the pavement surface. These variables were observed manually using the Infrared thermometer and the Kestrel weather tracker. The data obtained were analyzed using statistical techniques. Results show that the surface temperature of the three pavements sampled were most elevated during the day, Asphalt was observed to be the hottest material with an average surface temperature of 58.28 0C, Interlocking concrete tiles was next with 44.140C and clay bricks was the coolest material at 39.870C. The evaporation rate from the wet surface was most rapid during the day; it took 20 minutes above asphalt, 30 minutes above interlocking concrete tiles and 40 minutes above clay tiles. A significant difference in surface temperature was observed on all samples. However, asphalt was seen to have the best performance after water was applied; with a temperature difference of 120C. Results from this experiment conclude that the surface temperature of pavement samples is significantly cooler in presence of water. In other words different pavements have different evaporation rates and classifying outdoor materials in terms of cooling effect is useful and will aid in mitigating heat gains of pavements.

Key words – Air, Surface, Temperature, Heat, Water, Pavement

1.0 INTRODUCTION

Environmental pollution assessment in recent times is a consequence of the degradation of water environment, thermal environment and contamination in urban areas (Taniguchi et al 2009). Findings have shown the urban heat island in particular has become a serious environmental problem; with the expansion of cities and industrial areas all around the world (Oke 1987, Golden 2003).This phenomenon is a consequence of the nature of the fabric of cities; which are cladded with materials that absorb large amounts of solar heat radiation and release to the atmosphere (Pomerantz et al 2000, Santamouris et al 2008). The excess heat has become a tough and critical

issue leading to thermal imbalance of the environment and high demand of passive cooling systems. There is a need therefore; to improve the urban fabric with new technologies for energy saving and improved thermal comfort (Naticchia et al 2010). Studies have shown Pavement materials heavily influence outdoor thermal environment (Lin et al 2007, Santamouris et al 2011, Asaeda and Ca 2000, Didel and Ossen 2011, Tan and Fwa 1992). They conceal about 25 – 30 percent of the urban land mark and conventionally gets to a surface temperature of 480C to 670C (Pomerantz et al 2000, Doulos et al 2001, EPA 2009b). The heat rises up the ambient air and transfer to the overlying atmosphere. However, since expansion of the paved surface are inevitable within cities, feasible means of improving the thermal properties of pavements by minimizing the solar heat gains and maximizing heat loss of cities can never be overemphasized (Santamouris et al 2008) This include: the use of high reflective materials to reduce albedo of cities ( Doulos et al 2001, Synnefa et al 2006, Synnefa et al 2007), and increasing the permeability of pavements which induce evaporation similar to evapotranspiration in plants (Santamouris et al 2011, Gartland 2008). The evaporation of water provides an important counter to the heating effect, and so open parks and water surfaces are vital in urban areas for creating urban cool-island (Spronken Smith and Oke 1999, Carrasco et al 1987) .

Permeable pavements which allow the passage of water to the soil below induces evaporation to the atmosphere; this occurs as part of the net downward radiation is converted into latent heat , similar to the phenomenon which occurs on natural surfaces covered by bare soil or vegetation (Asaeda and Ca 2000). This means that the temperature of permeable paved surfaces does not escalate as much and subsequently the underground heat storage and sensible heat exchange between the ground surface and atmospheres are lessened. It is evident however that most of urban fabrics are conventionally impermeable surfaces with high heat absorption capacities; the idea of retrofitting to the cooler permeable surfaces may not be feasible. Studies by Carrasco et al (1987) used a roof spray cooling system set up on an impermeable roof top; experiments were conducted to compare the sprayed and unsprayed conditions on the roof surface temperature. Results show that there was 60% reduction in the heat transfer through the roof and also a reduction in the indoor temperature. Further studies by Naticchia et al (2010) developed a water sprinkling system which was capable of reducing heat

gains on impermeable walls. A proper insulating layer setup acted as a porous surface to store water sprayed by the system and then gradually release it when needed for cooling. The experimental analysis showed a decrease in the summer energy load on buildings by cancelling conduction loads. For the purpose of this paper the water sprinkling medium was employed on impermeable paved surfaces as a passive cooling medium to determine the cooling effect of water on the surface temperature of pavements and to mitigate the solar heat gains on pavement.

2.0 MATERIALS AND METHODS

Field experiments were conducted from October 2010 – November 2010 at Universiti Teknologi Malaysia, located 20km north of Johor Bahru (1029N – 103044E) of the equator. The experiment involved three samples of pavement materials used within the premises; Asphalt, Interlocking Concrete tiles and Terracotta Clay bricks. It aimed at evaluating the thermal performance of the various materials and the cooling effect of water on the surface temperature. The samples were at different locations within the study area and the layouts of the samples are depicted in figure 1.

A. Interlocking tiles B. Terracotta clay tiles C. Asphalt

Figure 1: Layout of the pavements samples

The surface temperature and metrological conditions such as air temperature and relative humidity were measured physically using the infrared thermometer and the kestrel weather tracker. The experimental apparatus are listed in Table 1.

Table 1: Instrumentation and Description of the Experimental site

Figure 2: A sketch of the mapped boundary with water 10m by 10m and without water 10m by 10m at 20m by 10m apart.

The three pavement materials used were: Interlocking concrete sample (A) made of concrete; they were assembled in moulds and interconnected on the surface. They were used conventionally for pedestrian walkways and vehicular parking lots. The second was Terracotta

claybricks (B) made up of raw clay usually unglazed; used mostly indoor on floors and parking areas. Asphalt pavement sample (C) made up of a combination of mineral aggregates and bituminous surface treatment (BST). They are utilized on roads and car parks mostly in urban areas.

The weather forecast of Johor Bahru Malaysia was used to predict three sunny and rainless days for the experiments to be conducted. During the experiment 11 liters of water was used to spray 10m x 10m, the dry area was also 10m x 10m for each pavement sample separated at 20m apart (Figure 2). The experiment was carried out on different days during the: morning, afternoon and evening hours of the day. During the experiment a weather tracker was used to measure relative humidity at 1.2m above the ground and the ambient temperature at 1.2m above the ground. The surface temperature was observed at 1.2m above the ground using a hand held Infrared thermometer. For each session the observations were carried out 10 times at 5 minutes interval. Readings for the sprayed and unsprayed areas were carried out concurrently under the same weather condition for each of the pavement samples and the data documented manually. This was carried out on the three pavement samples; Asphalt, interlocking concrete tiles and terracotta clay bricks.

1. RESULTS AND DISCUSSION

2. Surface temperature variation of pavements

   During the experiment it was observed that the heat budget on pavements depends mainly on the intensity of solar radiation n. Incident solar radiation has significant effects on the thermal performance of paved surfaces (Byerley and Christian 1994). The exposed surface of the pavement was seen as the most sensitive variable to the solar radiation incidence on it; escalating the temperature of the surface. According to Lin et al (2007) the surface temperature of paved surface is nearly 100C higher than the bare soil and green areas. The surface temperature of pavements gets as high as 42.70C in summer. Figure 2 illustrates the surface temperature distribution of asphalt, interlocking concrete tiles and clay bricks. It shows the surface temperature of each of the samples for a whole day; the variation of surface temperature for each pavement was minimal during the morning and evening and significantly higher during the day especially on the asphalt pavements where a very steep slope was observed during the day.

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   9.05 9.15 9.25 9.35 9.45 1.15 1.25 1.35 1.45 1.55 6.05 6.15 6.25 6.35 6.45

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   Legend Asphalt Interlocking tiles Clay bricks Asphalt Interlocking tiles Clay bricks

   Figure 3.1: Surface temperature variations for three pavements

   Figure 4 shows the average surface temperature of each pavement during the three practical periods of the day. The average surface temperature evaluated for all samples shows that the temperatures were most elevated during the day. Asphalt was seen to be significantly higher with a temperature of 58.280C. This was mainly due to its low reflectivity and consequently high absorption rate of solar radiation as being documented by Brez et al (1997); a material with low solar reflectance when exposed to solar radiation will experience higher surface temperature than materials with high solar reflectance. The interlocking concrete tiles had a higher reflectance capacity; as it was

   lighter in color, it had an average surface temperature of 44.140C. Clay bricks computed the lowest average surface temperature of 39.870C as compared to other materials sampled. The values of the surface temperature displayed Asphalt as the hottest with 600C while other materials were 100C lower than asphalt. The above findings reveal that the average surface temperatures of the pavements were most pronounced during the day and had less heat output in the morning and evening. The albedo of the materials determined to a large extent the level of reflectivity of the various materials and its thermal performance.

   Figure 3.2: Average surface temperature of three pavement samples

3. Ambient Temperature Variation of Pavements

   Lin et al (2007) Studies on the seasonal effects of pavements revealed that surface temperature is one of the closest variable related to air temperature followed by the globe temperature and solar radiation. Wind velocity he stated, rarely affects the air temperature in all seasons and periods of the day. Figure 5 displays the variation of ambient temperature carried out at 1.2m above each of the pavements type sampled carried out at 5 minutes interval. The variation of air temperature was seen to be minimal in the morning and evening of a typical sunny day with low heat output to the outdoor space. According to reports from

   the metrological data in Senai Johor climatic profile, it indicates that air temperature typically rises from about 230C in the early morning to 300C in the afternoon. For the period of this experiment, the variation of air temperature was seen to be 290C on an average in the morning and evening of a typical sunny day. During the day it was slightly higher with an average temperature of 320C Figure

   6. Similar to the case of the surface temperature, however the surface temperature of each pavement was seen to be significantly higher than the air temperature in all cases.

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   Air temeperature(oC)

   Air temeperature(oC)

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   9.05 9.15 9.25 9.35 9.45 1.15 1.25 1.35 1.45 1.55 6.05 6.15 6.25 6.35 6.45

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   legend Asphalt Inter Clay Asphalt Inter Clay

   Figure 3.4: Ambient temperature variation of three pavement samples

4. Surface and Air temperature comparison pavement samples

   The elevation of air temperature was seen to be a consequence of the surface temperature of the surfaces with the closest proximity to it. Studies by Osanyintola and Simonson (2006) indicate that the air temperature distribution inside a street canyon is a function of the optical and thermal properties of materials used on buildings and streets. The surface temperature difference was measured across the street reaching about 300C and

   this favored the overheating of the lower air levels and subsequently the atmospheric temperature. The Figure 6 illustrates the response of the ambient temperature measured 1.2m above the surface to the temperature of the paved surface. The figure 6a shows that the air temperature was appreciably lower than the surface temperature for asphalt pavement; it however maintained an average temperature at 290C during the morning and afternoon

   observation where the average surface temperature was 33.90C. During the day however, the surface temperature raised to about 58.280C thereby increasing the ambient temperature to 32.730C. This occurred as a result of the intense solar radiation experienced during the day. Figure 6b illustrates the surface and air emperature of interlocking concrete tiles, similar to asphalt pavement, the surface and air temperature were seen to be most prominent during the day; it had a lower average surface temperature of 44.140C and 32.730C air temperature. The figure 6c further illustrates the response of clay bricks to the surface and air temperature; it shows that the morning and afternoon sessions were relatively the same.

   It computed an average surface temperature of 39.870C during the day and 36.680C in the morning, the air temperature was also 31.210C and 31.870C respectively. However during the evening observations, rainstorm were experienced which moderated the heat gains on the pavement, it had an average surface temperature of 27.950C and air temperature of 250C. This shows that on an average, the surface temperature of the pavement samples was appreciable higher than their ambient temperature; however any variation in the surface temperature affects the ambient temperature.

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   Surface temperrature Interlocking tiles

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      Surface temperature Clay bricks

      Air tempertaure Clay bricks

      Surface temperature Clay bricks

      Air tempertaure Clay bricks

      (c) Clay Terracotta Tiles

      Figure 3.5: Comparative analysis of surface and air temperature of three pavement samples

5. Paired sample comparison of sprayed and unsprayed surface temperature

   ASHRAE laboratory in an effort to minimize the inward heat flow of absorbed solar radiation, experiments were carried out; roof sprays were seen to improve substantially the comfort of the occupant indoor and outdoor. In this study the spray mechanism was employed to evaluate the degree of surface temperature reduction when water was sprayed on a pavement surface. Results show a significant difference in surface temperature after water was sprayed on each of the pavement samples especially during the day as shown in Figure 7.

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      (c) Clay Bricks

      Surface Temperature without water

      Surface Temperature with water

      Surface Temperature without water

      Surface Temperature with water

      Figure 3.6: Surface temperature variation of three pavement samples before and after water sprinkling

      To evaluate quantitatively the difference in temperature observed; the paired sample test was used to compare both scenarios and compute the variation at a significance level of 5% Table 2. It showed a change in surface temperature for all pavement materials sampled. The most significant difference was observed during the day. On asphalt its average temperature reduced by 11.480C after water was sprinkled at a significance P< 0.05 t=8.16 it evaluated an average temperature of 58.200C before water was sprinkled and 46.800C after wards. Terracotta clay tiles on the other

      hand having good insulation capacity maintained a cooler state than other pavement materials. During the day the average surface temperature was 39.870C before spraying and 38.770C after spraying which gave a difference of 1.090C at a significance P< 0.05 t=5.15; This showed no trivial variation in temperature after water was sprinkled, hence clay tiles are cool in its natural state making it a suitable material to be used for pavements.

      Table 3.1: Paired sample statistics

      Average surface Temperature
      		Condition		Average Temp Difference (B-A)	T values for T- test
      Pavement type	Period	Dry surface (B)	Wet Surface (A)
      Asphalt	Morning	33.990C	32.210C	1.780C	3.96**
      	Afternoon	58.200C	46.800C	11.480C	8.16**
      	Evening	34.580C	33.570C	1.380C	8.84**
      Interlocking Tiles	Morning	34.580C	32.560C	2.020C	3.75**
      	Afternoon	44.140C	40.190C	3.950C	3.68**
      	Evening	35.480C	31.80C	3.680C	15.4**
      Clay Tiles	Morning	36.990C	34.920C	2.070C	2.92**
      	Afternoon	39.870C	38.770C	1.090C	5.15**
      	Evening	27.950C	27.220C	0.730C	3.56**

      **p-Value 0.05

      Interlocking tiles which was the second hottest material tested after asphalt evaluated an average surface temperature of 44.140C before water sprinkling and 40.190C after water sprinkling giving a difference of 3.950C at a significance P< 0.05 t = 5.30. Finally, findings shows that water is a feasible cooling medium on paved surfaces with heat absorption properties. This will help in mitigating the solar heat gain on pavements transferred to the overlying atmosphere.

6. Relative Humidity and Water Evaporation rate Previous studies have shown the porous state of some building materials aid in moderating indoor air relative humidity amplitudes. It helps in reducing passive cooling loads and improving the thermal comfort sensation of building occupants (Osayintola and Simonson 2006) These

materials are able to exchange moisture within the environment; a condition known as Relative humidity. Relative humidity according to Padfield (1999) is the amount of water vapor concentration in the atmosphere. At saturation a dynamic equilibrium exist between the rate of condensation and evaporation. Studies shows the evaporation rate of moisture depends on the solar heat available, the water capacity of the material and the time scale in which the process takes place (Padfield 1999). Figure 7 shows the ambient relative humidity of three samples as it responds to the level of moisture in the atmosphere, during the experiment 25 liters of water was sprinkled on the surface of the three samples observed. Results show that the extra heat during the day caused dryness and increased the evaporation rate and in turn lowered the relative humidity.

Figure 3.7: a. Relative humidity of asphalt b. Relative humidity of Interlocking tiles c. Relative humidity of clay brick tiles

Observations illustrated in Figure 8a and b afternoon show a similar consequence for Asphalt and Interlocking concrete tiles; the relative humidity reduced after water was sprinkled and within a period of 20 minutes the water completed evaporated from the surface and went dry. However Figure 7 shows the surface still remained cooler than the dry surface till the 20minutes experiment elapsed. Terracotta clay cricks on the other hand had low relative humidity during the morning and afternoon. Evaporation rate was predominantly the same for whole day during the three practical periods Figure 8c. Evaporation from the surface was relatively slow and the surface temperature of the sprinkled area did not vary significantly like in the case of asphalt and Interlocking concrete tiles. In essence the higher the solar radiation and moisture in the atmosphere especially during the day the lower the relative humidity.

4.0 DISCUSSION

The study involved observing the thermal performance of impermeable pavement under a typical sunny day condition and in its wet state after water had been sprinkled on the surface. Findings from the experiment shows that water evaporation and permeability has an important counter to the heating effect on pavement as well as the materials used; the presence of moisture or water induces evaporation and in turn cools the surface of pavements.

Permeable pavements or periodic water sprinkling over impermeable pavements will aid in improving the thermal performance of pavements. Materials used were also seen to aid in the perceptible rise in temperature caused by pavements due to the solar reflectance. Solar reflectance which is the measure and ability of a surface to reflect back solar radiation incident on the surface. Therefore materials with higher reflectivity absorb less solar heat gains while materials with lower reflectivity attract and absorb heat on the surface. The color shade of pavements was the main determinant of solar reflectance; light colored materials such as concrete with higher reflectivity have lower surface temperatures than dark colored materials such as asphalt with lower reflectivity. Asphalt being the darkest among the other materials observed, experienced the highest surface temperature especially during the day with 58.200C with a solar reflectance of 5- 10%, interlocking concrete tiles being a lighter color experienced a lower surface temperature of 40.190C and a solar reflectance of 20-30%, on the other hand the thermal mass and moisture of clay tiles aids it to absorb and release heat slowly and thus keeps its surface temperature cooler than other pavement materials.

Water sprinkled on pavements was seen to significantly reduce the surface temperature of pavement materials sampled. During the experiment the most significant

difference was observed during the day and the evaporation rate was seen to occur rapidly due to the presence of heat available during this period. The surface temperature of all pavements was observed to have dropped after water was applied on the surface the t-test for independent samples evaluated a significant difference in temperature during the three practical periods of the day (morning, afternoon and evening) after water was applied. During the day, water evaporation was observed to be faster and was most rapid on asphalt pavement. However, after water went dry the pavement still remained cooler than the dry surface. The t- test for Independent samples evaluated an average temperature difference for all samples during the day after water application. During the evening hours the evaporation was observed to be slow in the absence of heat from solar radiation. However, a temperature difference was equally observed on all pavement samples. Clay brick tiles had a slower rate of evaporation as compared to other samples however it remained the coolest materials under all weather conditions. The application of water to induce evaporative cooling on impermeable pavements has been confirmed during the course of this research to be a feasible heat softening strategy a phenomenon experienced throughout the world. In reducing the surface temperature of pavements this will improve the immediate ambient temperature of the pavements and the comfort index of the inhabitants. However findings also show that the quantity of water applied determines the extent of cooling so the more the water the better the cooling effect.

5.0 CONCLUSION

The heat budget of pavements depends mainly on the intensity of incident solar radiation. During the day the solar heat was more severe; pavements absorb heat and releases to the atmosphere. The physical features of pavements examined shows that the albedo of paved surfaces determines to a great extent the solar heat absorption rate. Dark materials such as asphalt attract and absorb more heat than light colored materials such as concrete. Among the materials tested asphalt was observed to have experienced a surface temperature of 58.200C during the day while terracotta was the lowest with 390C. The heat settled on these materials was instantly released to the ambient environment making the air warmer than normal. As such the surface temperature was seen to be the closest variable related to ambient temperature. Any variation in surface temperature affects the ambient temperature.

During the experiment, Water evaporation was seen to be an important counter towards minimizing the inward heat flow of radiation. The surface temperature for all pavement samples became cooler after water sprinkling. The most significant difference was observed on asphalt which reduced by 11.480C after water was applied on the surface. The amount of moisture present determined the evaporation rate and was seen to be faster during the day due to high solar intensity. Results show that the extra heat during the day caused dryness and increased the

evaporation rate and in turn lowered the relative humidity. This research concludes water sprinkling on pavement surface can significantly optimize the solar heat gains on pavements and reduce the surface temperature. Thus it can aid in mitigating the excess heat released by pavements to the outdoor environment and causes thermal discomfort.

ACKNOWLEDGEMENT

I would like to sincerely thank Dr. Remaz Ossen for his guidance, during this studies as well as Universiti Teknologi Malaysia (UTM) who provided the support and grant used for this research. The product of this research paper would not have been possible without them. However any opinions, findings and conclusions expressed in this paper are those of the author and do not necessarily reflect those of UTM.

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