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# Thermal behavior of pervious concrete in wet conditions

Khaled Seifeddine, Sofiane Amziane, Evelyne Toussaint

► To cite this version:

Khaled Seifeddine, Sofiane Amziane, Evelyne Toussaint. Thermal behavior of pervious concrete in wet conditions. Materials and structures, 2024, 57 (10), pp.225. 10.1617/s11527-024-02509-5. hal-04777044

HAL Id: hal-04777044

https://hal.science/hal-04777044v1

Submitted on 12 Nov 2024

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# Thermal behavior of pervious concrete in wet conditions

Khaled Seifeddine · Sofiane Amziane ·
Evelyne Toussaint

Received: 2 April 2024 / Accepted: 1 November 2024
© The Author(s), under exclusive licence to RILEM 2024

**Abstract** Conventional impervious pavements occupy a large proportion of most cities, due to urbanization and the extensive development of transportation infrastructure. These pavements cause environmental problems such as flooding and urban heat islands. Pervious concrete (PC) is a special type of concrete, characterized by a porous structure that allows water infiltration and has the potential to reduce the effects of urban heat islands through cooling by evaporation. However, due to the low water absorption and retention properties of PC, it cannot effectively meet evaporative cooling needs. This study investigates the thermal behavior of PC in dry and wet laboratory conditions under controlled climatic conditions. Three types of PCs were fabricated: gravel-based PC (PCG) and Pozzolan-based PC (PCP1), having almost the same particle size distribution, and less coarse Pozzolan-based PC with a monodisperse particle size distribution (PCP2). The results show that the surface temperature of pozzolan-based PC is up to 4 °C higher than that of PCG in dry conditions. The partial immersion test shows that the use of pozzolan aggregates in PC mixtures improves the water-absorption properties compared to PCG. In

addition, pozzolan-based PC can have a surface temperature up to 11.7 °C lower than PCG during the daytime and up to 3 °C lower during the nighttime. The use of lightweight aggregates with high water-absorption coefficients in the PC mix is only recommended in wet conditions.

**Keywords** Urban heat island · Pervious concrete · Lightweight aggregates · Water-absorption properties · Cooling effect

## 1 Introduction

The continuous development of urbanization stemming from socio-economic development, and the burgeoning world population, is a significant contributor to climate and environmental change [1]. This urban development has led to the emergence of urban heat islands (UHIs)—a phenomenon characterized by the fact that urban areas are often warmer than the surrounding rural areas, especially during heat waves [2]. Many factors are behind this phenomenon. The most significant ones are the complex geometry of the urban environment, the use of building materials, anthropogenic heat emissions and the reduction of vegetation [3]. In particular, conventional impervious pavements (such as asphalt) have high thermal inertia and relatively low reflectivity [4]. These pavements can be warmer than natural vegetation during both the daytime and

<footnote>K. Seifeddine (✉) · S. Amziane · E. Toussaint
Université Clermont Auvergne, CNRS, SIGMA Clermont,
Institut Pascal, 63000 Clermont-Ferrand, France
e-mail: charles.seifeddine@cideco.tech</footnote>

<footnote>K. Seifeddine
CIDECO, 2 Avenue Blaise Pascal, 63170 Aubière, France</footnote>
nighttime, due to the high absorption of incident solar radiation and their impermeable nature, which suppresses evapotranspiration [5].

There are several solutions to limit the formation of UHIs [6], such as optimizing urban morphology to promote heat dissipation and urban ventilation [7], decreasing anthropogenic heat [8], increasing urban vegetation [9, 10] and installing cool and green roofs, though this technique has little benefit for air temperature at pedestrian height [11]. Since pavements constitute 30–45% of the total surface area of cities [12], the use of cool pavements to mitigate UHIs is an interesting strategy in today’s world. There are several types of cool pavements, such as reflective pavements [13, 14], heat storage modifying pavements [15–17] and evaporative permeable pavements [18, 19].

Pervious Concrete (PC) is among the most widely used types of permeable pavements. It was first discovered for use in construction 100 years ago in Europe [20]. It has become popular in recent years because of its contribution to stormwater management and sustainable development [21]. It is a special type of pavement with sufficient macro/micro pores that allows water to infiltrate into the soil with purification, and helps replenish natural water sources [22]. Due to its porous structure that allows water retention, this type of pavement has been advocated as a solution to reduce the effects of UHIs through cooling by evaporation [23, 24].

In dry conditions, PC has a higher surface temperature than asphalt during hot summer days [25]. Other studies have found that PC has a higher surface temperature during the day but a lower surface temperature with a higher cooling ratio than ordinary concrete during the night [26–28]. On the other hand, in wet conditions, Qin and Hiller [28] show that PC can be cooler than ordinary concrete for 12–24 h. Due to the importance of the presence of water in permeable pavements, water-retentive filler has been used to improve the cooling effect of such pavements. Various materials have been used to improve the water-absorption properties of permeable pavements such as porous geopolymers [29], blast furnace slag [30, 31], steel by-products based on a silica compound [30], and allophane- and vermiculite-based materials [32]. In a recent study, Tan et al. [33] show that the use of 5% pulverized biochar particles as an admixture in the PC mixture increases the capillary water content from 100±2 to 117±8 kg/m³, and makes its

surface temperature up to 6 °C lower than that of PC without biochar.

In a previous study [26], the thermal behavior of PC and the influence of thermophysical factors on its surface temperature were investigated in detail, with the objective of decreasing its temperature in dry conditions. In this study, the thermal behavior of pozzolan-based PC was compared to that of conventional dense aggregate-based PC in dry and wet conditions under controlled laboratory weather conditions using an innovative experimental bench. This is the first study to investigate the improvement of the evaporative cooling effect of PC using lightweight pozzolan aggregates. These aggregates are characterized by a high water-absorption coefficient, and will be able to improve the absorption properties and cooling effect of PC. The factors affecting the evaporation rate and evaporative cooling effect were also investigated.

## 2 Materials and methods

### 2.1 Materials

Two types of natural aggregates were used in this study to fabricate the PC mixtures: gravel and pozzolan (Fig. 1). In this study, "pozzolanic aggregates" refers to volcanic materials from the Auvergne Rhone Alpes France region and especially from the Puy de Dôme department. These aggregates are characterized by their high porosity, low density, and significant mineral content including silica and alumina. They exhibit a specific surface area that contributes to their high water absorption capacity. Their physical properties include a bulk density of approximately 1.0–1.2 g/cm³ and a specific gravity of around 2.0–2.2. These properties are fundamental to their classification as pozzolanic aggregates.

The water-absorption coefficient WA (%) of the aggregates was determined according to the procedure described in ASTM C127-15 [34]. The bulk density $ρ_d$ (kg/m³) was determined according to ASTM C29 [35]. The results of these tests are presented in Fig. 1.

The absorption coefficient of pozzolan particles (P1 and P2) is much higher than that of natural gravel (G) due to their high intergranular porosity (Fig. 1b), which allows them to retain a greater amount of water.
<figure><img src="image_3.png" /><figcaption>**Fig. 1** Aggregates used in PC mixes: a gravel ‘G’, b pozzolan ‘P1’ and (c) pozzolan ‘P2’</figcaption></figure>

<table><caption>**Table 1** Dosage of PC Mixtures</caption><thead><tr><th>Sample</th><th>Sand (kg/m³)</th><th>Coarse aggregates (kg/m³)</th><th>Cement (C) (kg/m³)</th><th>Water (W) (kg/m³)</th><th>W/C</th></tr></thead><tbody><tr><td>PCG</td><td>0</td><td>1600</td><td>350</td><td>112</td><td>0.32</td></tr><tr><td>PCP1</td><td>0</td><td>1000</td><td>350</td><td>140</td><td>0.4</td></tr><tr><td>PCP2</td><td>0</td><td>1065</td><td>350</td><td>140</td><td>0.4</td></tr></tbody></table>

## 2.2 Experimental methods

### 2.2.1 Dosage and sample preparation

Table 1 shows the dosages of the gravel-based PC (PCG) and pozzolan (PCP1, PCP2) samples. An ordinary Portland cement CEM II/B-LL 32.5 R CE NF was used in all three mixes. The coarse aggregates used in the PCG and PCP1 mixtures have a particle size distribution between 5 and 12.5 mm. Those used in the PCP2 mix have a particle size distribution between 3.15 and 6 mm.

The particle size distributions of the aggregates used in the three mixtures are presented in Fig. 2. This test was performed by sieving (NF EN ISO17892-4 [36]). The selection of this granulometry facilitates the study of the influence of the type of aggregates (case of PCG and PCP1) and the size and specific surface of the aggregates (case of PCP1 and PCP2) on the thermal behavior of PC.

The PCG, PCP1 and PCP2 mixes were produced using a standard concrete mixer with a capacity of

<figure><img src="image_4.png" /><figcaption>**Fig. 2** Particle size distribution of the aggregates used: G, P1 and P2</figcaption></figure>

300 L. The coarse aggregates and the required amount of cement as well as a small amount of water were mixed first. After a quick mixing of these materials, the remaining water was added progressively. Then, these mixtures were mixed for 3 min. Once the mixing was complete, they were poured in layers into prepared molds. Each layer was gently compacted with a rubber mallet. The samples were removed from the mold after 24 h. They were then stored in water until the date of the tests.

3 types of samples were fabricated for each mix: 3 cylindrical samples (diameter=16 cm and height=32 cm) to measure compressive strength, density and open porosity; 2 parallelepiped samples (15×15×6 cm³) to measure thermal conductivity;
and 1 parallelepiped sample (40×30×5 cm³) to measure permeability and cooling performance. The fabricated samples and their dimensions are shown in Fig. 3.

## 2.2.2 Porosity measurement

A methodology matching that of previous studies [37, 38] was used to measure the open porosity $P_0$ (%) of PC samples. It consists of filling all the interstices of a sample with water to correlate the open porosity with the total volume of water introduced. The cylindrical samples were covered with a PVC film and inserted into non-deformable cylindrical molds to ensure the PVC film did not suffer any deformation. Then, the mold and the sample inside were placed on a balance and water was added to fill the voids of the PC. Knowing the volume of the sample $V_S$ (m³) and the volume of water $V_W$ (m³), the open porosity was calculated using Eq. (1).

$$P_0 = \frac{V_W}{V_S} \times 100 \qquad (1)$$

When using this method, it is recommended to perform this test on at least 3 samples to measure open porosity [39, 40]. In this study, three samples of each PC mixture were tested at 28 days of age, which meets the requirements of this method.

## 2.2.3 Strength and permeability tests

The compressive strength at 28 days (R28d) of the samples was measured with reference to the standard NF EN 12390-3 [41].

The permeability of PC samples was measured using the standard method described in ASTM C 1701 [42]. This method determines the water infiltration rate (mm/h) of PC in the field by recording the time required for a certain volume of water to pass through a defined area of a pavement (Eq. (2)). In this study, PC pavers measuring 40×30×5 cm³ were used to measure permeability.

$$I = \frac{\gamma M}{D^2 t} \qquad (2)$$

where $I$ (mm/h)=infiltration rate or permeability; $M$ (kg)=infiltrated water mass; $D$ (mm)=inner diameter of the infiltration ring; $t$ (s) time required for the measured amount of water to infiltrate the pavement; $\gamma$ (dimensionless)=4,583,666,000 in SI units.

## 2.2.4 Tests to determine thermophysical properties

### 2.2.4.1 Albedo
The albedo is the reflectivity of a surface, which is the ratio of reflected light energy flux to incident light energy flux. It is a dimensionless. This characteristic was measured using a pyranometer in accordance with ASTM E1918 [43].

<figure><img src="image_5.png" /><figcaption>Fig. 3 Prepared samples: a PCG, b PCP1, c PCP2</figcaption></figure>

<figure><img src="image_6.png" /></figure>
The tested pavers (PCG, PCP1 and PCP2) were dried in a ventilated oven at 105°C to obtain a constant mass. They were then cooled to laboratory room temperature. The pyranometer was then placed over the dry pavements to determine the radiative flux reflected from the surface. The incident shortwave radiative flux can be controlled, since the experimental bench (see “Appendix 1”) is equipped with infrared lamps with a power control graduated from 0 to 100% (see [26]). As an example, 100% corresponds to the maximum power of the infrared lamps, which is 2000 W/m² (with respect to the position of the pavement under the infrared lamps) and 0% means that the lamps are completely switched off. The values of the radiative flux corresponding to each graduation were provided by the company that designed the experimental bench. These values were verified in the laboratory using a pyranometer. To measure the albedo accurately and avoid any influence by the other surrounding conditions, the bin in which the pavements were placed was covered with a black fabric having high solar absorption, in accordance with the experimental protocol employed by Chen et al. [44].

### 2.2.4.2 Emissivity
Emissivity is the ratio of the energy flux emitted by any body to the energy flux emitted by a black body of identical shape and temperature. A pyrgeometer was used to measure the emissivity of the PC pavements. It was placed above the pavements to measure the long-wave radiative flux emitted from the surface $L_e$ (W/m²) during a 1-h period. By measuring the surface temperature of the pavements, the change in emissivity during this experiment was calculated by means of Eq. (3).

$$ \epsilon_s = \frac{L_e}{T_s^4 \times \sigma} \qquad (3) $$

where $T_s$ (K)=surface temperature; $\sigma$ (W m⁻² K⁻¹)=Stefan–Boltzmann constant equal to 5.67×10⁻⁸.

### 2.2.4.3 Thermal conductivity
The parallelepiped samples measuring 15×15×6 cm³ were used to measure the thermal conductivity $\lambda$ (W m⁻¹ K⁻¹) of the PC pavements using a hot-wire conductivity meter, in line with ASTM D5930-97 [45].

## 2.2.5 Water absorption test

The water-absorption coefficient is an important property for permeable pavements as it expresses the rate of water absorption due to capillary forces. This coefficient has a significant effect on the heat- and mass-transfer of evaporative pavements. The water-absorption properties of PC pavements were determined by the partial immersion test, in keeping with ISO15148 [46] (Fig. 4).

For each mixture, a parallelepipedal paver measuring 15×15×6 cm³ was used. The pavers were dried in an oven at 110±5 °C until a constant mass was obtained. They were then stored in a room with controlled climatic conditions: 20 °C ≥ Ambient temperature ≤ 23 °C; 35% ≥ Relative humidity ≤ 40%. Subsequently, they were sealed with waterproof adhesive tape on all surfaces except the bottom surface. Small holes with a diameter of less than 1 mm and spaced less than 1 cm apart were made in the tape covering the top. The sample was placed in the water bin on supports to ensure a contact height of 3–5 mm between the base of the sample and the water, according to ASTM C1585-13 [46].

<figure><img src="image_7.png" /><figcaption>Fig. 4 Partial immersion test: a photograph and b diagram</figcaption></figure>
The specimen was removed from the water at regular time intervals; then its surface was wiped with a wet towel and its weight was determined with an electric balance (accuracy ± 0.01 g). Initially, the sampling interval was short (e.g. 5 s) and then a longer interval was chosen (e.g. 30 s, 1 min, 5 min, 30 min, 1 h, etc.). The cumulative capillary water absorption height *i* (mm) was calculated by means of Eq. (4). At the end of each test, the sample was dried again in the oven and subjected to two further tests.

$$i = \frac{M_{(t)}}{A \times \rho_{\text{water}}} \qquad (4)$$

where $M_{(t)}(g)$ = gain in mass due to water absorption as a function of time; $A$ (mm²) = surface of the sample exposed to water; $\rho_{\text{water}}$ (g/mm³) = density of water.

### 2.2.6 Experimental setup to study the thermal behavior of pervious concrete

To investigate the thermal behavior of the PC samples, an innovative experimental bench (“Appendix 1”) has been designed to study their thermal behavior in dry and wet conditions. This bench is equipped with several devices: a pyranometer to measure the albedo and the shortwave radiative fluxes; a pyrometer to measure the emissivity and the longwave radiative fluxes; temperature sensors to measure the surface temperature and the temperature at depth; infrared lamps to simulate the solar flux; and a balance to measure the mass of evaporated water over

time. All these devices are connected to data-acquisition units that automatically record the data. All these devices establish a complete energy balance of a pavement in order to analyze the variation of different energy fluxes (see [26] for more information on the function of the experimental bench).

In order to simulate the real structure of a permeable pavement, the bin (“Appendix 1”) was filled with a layer of sand (0.08–5 mm), representing the reservoir layer on which a PC pavement is laid. The dimensions of each layer are shown in Fig. 5. At the beginning of each experiment, the side walls of the bin were insulated with rockwool thermal insulation ($\lambda$=0.03 W m⁻¹ K⁻¹) to ensure unidirectional heat transfer. The surface temperature at the center of the pavement was measured using an infrared temperature sensor placed on top of the bin. The temperatures at depth: $T_{5 cm}$ (bottom side of the pavement) and $T_{15 cm}$ (bottom of the bin) were measured by two temperature sensors inside the bin (Fig. 5c). To accurately measure the evaporated water mass through the PC pavements, the empty spaces between the side walls of the pavement and the side walls of the bin were sealed with a transparent silicone seal to eliminate lateral evaporation (Fig. 5a, b).

The experimental bench was placed in a room with controlled climatic conditions: 20 °C ≥ ambient temperature ≤ 23 °C; 35% ≥ relative humidity ≤ 40%. Table 2 shows the duration of the day phase (infrared lamps turned on) and the night phase (infrared lamps turned off) chosen to study the thermal behavior of the PC pavements in dry and wet conditions. This

<figure><img src="image_8.png" /><figcaption>**Fig. 5** The placement of PCP1 in the bin: a dry condition, b wet condition, c the dimensions of the complete PC pavement structure</figcaption></figure>
<table><caption>**Table 2** Characteristics of the day and night phases during the experiment</caption><thead><tr><th></th><th>Daytime phase</th><th>Nighttime phase</th></tr></thead><tbody><tr><td>Duration</td><td>8 h</td><td>16 h</td></tr><tr><td>Incident radiation I (W/m²)</td><td>520</td><td>0</td></tr><tr><td>Incident radiation L<sub>down</sub> (W/m²)</td><td>531</td><td>430</td></tr></tbody></table>

table also shows the values of the shortwave radiative fluxes (emitted by the infrared lamps) and the incident longwave radiative fluxes (emitted by all the materials surrounding the bin) during these two periods.

The following experimental protocol was adopted to study the thermal behavior of the pavements in dry and wet conditions: the power of the infrared lamps was adjusted at the beginning of each experiment to obtain the desired incident shortwave radiation (Table 2). The recording frequency of all sensors was set to one reading every two minutes. The first data recording was when the lamps were turned off. Immediately afterwards, the lamps were turned on using the switch, and left on for 8 h, to study the thermal behavior of the pavements during the daytime period. At the end of this period, the lamps were turned off for 16 h to study the thermal behavior of the pavements during the nighttime period. In wet conditions, the total mass of the sample decreases over time. This decrease corresponds to the mass of water evaporated. After 24 h, all recordings were stopped and the data were harvested.

# 3 Results and discussions

## 3.1 Physical properties

Table 3 shows that the average open porosity of PCP1 is 3.2% higher than that of PCG. The main reason behind this small deviation is that these two mixtures

are fabricated using aggregates with almost the same particle size distribution (see Fig. 2). However, the open porosity of PCP2 is 20.3% higher than that of PCP1, the latter being made with coarser pozzolan aggregates. Indeed, the aggregates of the P1 pozzolan (5–12.5 mm) have a more widely dispersed grain size than those of the P2 pozzolan (3.15–6 mm); P2 has a monodisperse formulation, which increases the percentage of pores in the PC structure.

The average compressive strength (3 samples/mix) of the three PC pavements is presented in Table 3. The average compressive strength of PCG is 13.9% greater than that of PCP1 and 44.5% greater than that of PCP2. Indeed, the mechanical strength of PCP1 and PCP2 is affected by the low mechanical strength of pozzolan grains and their low densities. This table shows that the compressive strength of the permeable concrete samples decreases with increasing open porosity, which is consistent with the results found by Sahdeo et al. [47] and Kevern et al. [48].

The permeability results (3 measurements/sample) are presented in Table 3. The permeability of the PC must be higher than 0.054 mm s⁻¹ which is a critical value, beyond which a concrete can be qualified as pervious [49]. In addition, to avoid the risk of clogging, the minimum permeability must be higher than or equal to 1 mm s⁻¹ [50]. The results show that PCG, PCP1 and PCP2 all have a permeability well above the critical values. They are therefore able to perform their main function: stormwater management. Table 3 shows that the permeability of pervious concrete increases with increasing open porosity, which is consistent with the results of several prior studies [51–53].

The deviation between the permeability of PCG and PCP1 is 0.07 cm/s as they have almost the same open porosity and the aggregates used in these two mixtures have almost the same particle size distribution. Meanwhile, the permeability of PCP2 is 21.9% higher than that of PCP1 and 28.5% higher than that of PCG. The main reasons are the high open porosity

<table><caption>**Table 3** Open porosity, density, compressive strength and permeability of PC samples</caption><thead><tr><th>Sample</th><th>Open porosity (%)</th><th>Density (kg/m³)</th><th>Compressive strength (MPa)</th><th>Permeability (cm/s)</th></tr></thead><tbody><tr><td>PCG</td><td>21.97 ± 1.67</td><td>1951 ± 42.33</td><td>11.6 ± 0.460</td><td>1.30 ± 0.051</td></tr><tr><td>PCP1</td><td>22.67 ± 0.21</td><td>1592 ± 66.45</td><td>10.18 ± 0.580</td><td>1.37 ± 0.050</td></tr><tr><td>PCP2</td><td>27.27 ± 0.25</td><td>1584 ± 94.03</td><td>8.03 ± 0.369</td><td>1.67 ± 0.059</td></tr></tbody></table>
of PCP2 and the uniform granulometry of the aggregates used in this mix, leading to a less tortuous pore network which allows faster infiltration of water through the pavement.

Table 4 shows the average measured albedo values (3 measurements/pavement). Figure 3 shows that the three pavements have almost the same gray color, while the results show that the albedo of PCP2 is 5% higher than that of PCG and 8.1% higher than that of PCP1. This increase in albedo is mainly due to the low roughness of PCP2, which is manufactured using less coarse aggregates than those used in the other two pavements. These results contradict those found by Zhang et al. [54] who show that the albedo of PC decreases linearly from 0.39 to 0.26 when the porosity varies from 7 to 32.4%. Indeed, larger porosity do not always lead to larger pore sizes and higher roughness (as in the cases of PCP1 and PCP2). It was deduced that porosity is an insufficient factor to predict the albedo of PC, which is more closely related to pore characteristics such as size, shape and distribution.

Figure 6 shows the variation in emissivity of PC pavements as a function of time. 31 emissivity values were calculated. The results show that the

average emissivity values of the three pavements are: $ε_s = 0.877 ± 0.005$ for PCG, $ε_s = 0.892 ± 0.004$ for PCP1 and $ε_s = 0.902 ± 0.004$ for PCP2.

As regards the thermal conductivity of the PC pavements, 5 measurements were performed for each type of PC in 5 different locations. The results obtained (Table 4) are in agreement with those found by Chen et al. [55], who show that the thermal conductivity of PC decreases with increasing porosity. Moreover, the thermal conductivity of PCP1 is 11.5% lower than that of PCG while its open porosity is slightly lower than that of PCP1. Indeed, this decrease is also attributable to the low thermal conductivity of pozzolanic aggregate ($λ = 0.141 W m⁻¹ K⁻¹$, ASTM C518 [56]), which is a lightweight aggregate with high intragranular porosity compared to gravel which is a denser aggregate with higher thermal conductivity ($λ = 0.179 W m⁻¹ K⁻¹$, ASTM C518). This means that with the same value of open porosity, the thermal conductivity of PC can vary depending on the type of aggregate used in the mix.

The monodisperse aggregate distribution (3.15–6 mm) in the PCP2 formulation allows for a more uniform packing of aggregates, leading to higher porosity and creating a larger volume of interconnected air voids, thus enhancing thermal insulation. This results in a 20.3% increase in open porosity compared to PCP1 and a lower thermal conductivity. Higher porosity reduces thermal conductivity because the pores filled with air act as insulators. Air, having low thermal conductivity, slows down heat transfer. With a greater volume of pores, the thermal path becomes more complex and less efficient, leading to lower thermal conductivity of the material. In contrast, the wider aggregate size distribution

<table><caption>**Table 4** Albedo, emissivity and thermal conductivity of PC pavements</caption><thead><tr><th>Sample</th><th>Albedo (dimensionless)</th><th>Emissivity (dimensionless)</th><th>Thermal conductivity (W m⁻¹ K⁻¹)</th></tr></thead><tbody><tr><td>PCG</td><td>0.203 ± 0.007</td><td>0.877 ± 0.005</td><td>0.613 ± 0.018</td></tr><tr><td>PCP1</td><td>0.197 ± 0.005</td><td>0.892 ± 0.004</td><td>0.549 ± 0.010</td></tr><tr><td>PCP2</td><td>0.213 ± 0.007</td><td>0.902 ± 0.004</td><td>0.452 ± 0.0147</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>**Fig. 6** Variation in emissivity of the pavements (PCG, PCP1 and PCP2) as a function of time</figcaption></figure>
(5–12.5 mm) in the PCP1 formulation leads to less uniform packing. Larger particles are interspersed with smaller ones, which fill the gaps between the larger aggregates, reducing the overall porosity of the permeable concrete. Consequently, the thermal conductivity of PCP1 is higher due to the reduced volume of air voids and lower porosity compared to PCP2.

## 3.2 Water-absorption properties

Figure 7 shows the variation of the cumulative capillary absorption height of PCG, PCP1 and PCP2 samples. This figure shows that the capillary absorption process of each type of sample can be divided into three stages. The first is the capillary absorption stage. During this stage, the absorption is governed by capillary and viscous forces, and the water front gradually approaches the upper surface of the sample. The absorption coefficient $A_{cap}$ (mm/s$^{0.5}$) can be calculated by linear regression of the slope of this stage [57]. The second stage corresponds to the transition process, during which water continuously fills the small pores of the sample until

the point of capillary saturation. The capillary water content $W_{cap}$ (kg/m$^3$) corresponds to the mass water content of the samples after the transition stage [58]. The final stage corresponds to the air diffusion process (Fig. 7), when the water reaches the upper surface of the sample. Any slight increase in water content during this period may be caused by the dissolution and removal of air trapped in the water.

Figure 7 shows that the change in the cumulative capillary absorption height of all PC samples is fast during the first stage and slow during the other two.

The water-absorption properties of PCG, PCP1, PCP2 and those obtained in other studies [33, 57] that performed similar tests on PC based on dense aggregates are presented in Table 5. The open porosities of the PC samples tested in these two studies are similar to those obtained in this experiment regarding PCG. This table shows that PCG has a low absorption coefficient and low capillary water content. The deviation between the results could be related to several factors: the type of aggregates used, the density of each sample, the cement dosage, the degree of compaction, and the pore characteristics (size, shape and distribution), which have a

<figure><img src="image_10.png" /><figcaption>**Fig. 7** Variation in cumulative capillary water-absorption height of PC samples as a function of time: a for PCG, b for PCP1 and (c) for PCP2</figcaption></figure>
<table><caption>**Table 5** Water absorption properties of PC samples</caption><thead><tr><th>Study</th><th>Aggregate type</th><th>Aggregate size (mm)</th><th>Open porosity (%)</th><th>A<sub>cap</sub> (mm/s<sup>0.5</sup>)</th><th>W<sub>cap</sub> (kg/m<sup>3</sup>)</th></tr></thead><tbody><tr><td rowspan="3">Experiment</td><td>G</td><td>5–12.5</td><td>21.97 ± 1.67</td><td>0.0186 ± 0.0024</td><td>85.17 ± 2.49</td></tr><tr><td>P1</td><td>3.15–6</td><td>22.67 ± 0.21</td><td>0.0283 ± 0.0036</td><td>133.61 ± 4.25</td></tr><tr><td>P2</td><td>5–12.5</td><td>27.27 ± 0.25</td><td>0.0311 ± 0.0019</td><td>151.50 ± 4.02</td></tr><tr><td>Tan et al. [33]</td><td>Gravel and limestone</td><td>4.75–9.0</td><td>22.05 ± 0.11</td><td>0.0199 ± 0.0025</td><td>100.58 ± 6.62</td></tr><tr><td>Wang et al. [57]</td><td>—</td><td>5–10</td><td>22.86 ± 0.004</td><td>0.010 ± 0.007</td><td>97.33 ± 2.74</td></tr></tbody></table>

direct influence on the water-absorption properties of PC.

Table 5 also shows the absorption coefficient $A_{cap}$ (mm/s0.5) and capillary water content $W_{cap}$ (kg/m3) of pozzolan-based PC samples. The PCP2 sample has the highest water-absorption coefficient which is 10% higher than PCP1 and 67% higher than PCG. Similarly, PCP2 has the highest capillary water content which is 13.38% higher than PCP1 and 78% higher than PCG. It is evident from these results that PCP2 can hold more water than the other two pavements in a shorter time. It was found that this sample is capable of a higher evaporation rate compared to the others and a more potent cooling effect as a consequence.

The main factors that cause the differences in the water-absorption properties of these pavements studied are the type of aggregates used and the pore size. PCP2 is manufactured with less coarse aggregates which increases the contact surface between aggregates and water. Moreover, when the aggregate size decreases, the pore size decreases which increases the capillary suction capacity. The open porosity of PCP2 is also higher compared to the other two samples which contributed to a higher capillary water content. It should be noted that the micropores that are present in the pozzolan aggregate structure improved the absorption coefficient and capillary water content of PCP1 and PCP2 compared to PCG which is made from dense aggregates with a low water-absorption coefficient, as shown in Fig. 1.

### 3.3 Thermal behavior of PC pavements in dry conditions

Figure 8 shows the variation in surface temperature of PC pavements as a function of time. The results show that in dry conditions, the surface temperature of PC pavements increases from: 23.5 °C to 59 °C for PCG, to 61.1 °C for PCP1 and to 63 °C for PCP2.

<figure><img src="image_11.png" /><figcaption>**Fig. 8** Variation in surface temperature of PC pavements as a function of time</figcaption></figure>

The temperature of pozzolan-based PC pavements is higher than that of gravel-based PC. Since the difference between the albedo of these pavements is small, the main reason for this temperature increase is the low thermal conductivity of PCP1 and PCP2 compared to that of PCG (Table 4). Indeed, the lower the thermal conductivity, the less heat will be transferred to the interior of the pavement which leads to an increase in surface temperature during the diurnal period. On the other hand, during the nocturnal period, the difference between the surface temperature of these three pavements is not significant.

It was deduced that in dry conditions the use of lightweight aggregates should be avoided. These results are consistent with those obtained by Chen et al. [59] who showed that increasing the thermal conductivity of PC from 0.59 to 0.85 W m⁻¹ K⁻¹ by adding steel fibers to the mix reduces its surface temperature by 1–3 °C.

It is important to note that thermal conductivity alone is not an indicator of surface temperatures. Other factors influencing surface temperature include

<figure><img src="image_12.png" /></figure>
the heating of the air around the surface due to the lamp, as well as differences in air transmittance through the samples due to porosity and the associated heat transfer coefficients. However, this study focuses on the key factors that differ from one sample to another.

## 3.4 Thermal behavior of PC pavements in wet conditions

### 3.4.1 Variation in albedo and evaporation rate

The thermal behavior of PC pavements in wet conditions was studied according to the same experimental protocol adopted in dry conditions. The only difference was that the experiment was carried out for 5 days while recording the variation of the total mass of the bin and the albedo of the pavement over time (pyranometer placed above the pavement). Figure 9 shows the variation in cumulative evaporated water mass (Fig. 9a), evaporation rate (Fig. 9b), and albedo (Fig. 9c) over 5 days. By monitoring the mass of evaporated water during the 5 days of the experiment,

the researchers were able to calculate the variation of the evaporation rate by means of Eq. (5) [23].

$$ER = \frac{m_{t1} - m_{t2}}{A(t_2 - t_1)} \qquad (5)$$

where ER (kg m⁻² h⁻¹)=evaporation rate; mᵢ₁-mᵢ₂ (kg)=difference between the total mass of the sample at time t₁ (s) and the total mass of the sample at time t₂ (s); A (m²)=top surface of the pavement exposed to the exterior.

Figure 9a shows that the cumulative evaporated water mass in sample PCP2 increases from 0 to 1.01 kg. The total evaporated water mass in this sample is 40% higher than that in PCP1 and 77% higher than that in PCG. In addition, the slope of the curve is steepest for PCP2, followed by PCP1, while the curve for PCG has the least slope (Fig. 9a). The change in the slope of the evaporated water mass curve over time is an important indicator of the evaporation process, because it can be used to calculate the change in the evaporation rate for each sample. Figure 9b shows that the evaporation rate of the PCP2 sample is

<figure><img src="image_13.png" /><figcaption>**Fig. 9** Variation in: a cumulative evaporated water mass, b evaporation rate, and c albedo, as a function of time</figcaption></figure>
greater than that of the PCP1 during both the daytime and nighttime phases. The lowest evaporation rate is that of the PCG sample, which becomes insignificant from the third day (ER < 0.0173 kg m⁻² h⁻¹).

The maximum evaporation rate of each sample was observed on the first day of each experiment when the samples were saturated with water. At saturation, free water is retained by capillary forces and ready to evaporate. Then, as the water content decreases, the gap between the saturated zone and the evaporation plane increases, preventing the water in the pores from reaching the evaporation plane. The plane recedes to a lower level and a new evaporation plane is formed. Evaporation continues within the pavements at the new evaporation plane, by diffusion of vapor through the pores present at the upper surface. As the water content decreases, the global resistance to evaporation becomes high [60], which explains the decrease in evaporation rate after the first day.

The albedo variation of wet PC was ignored in a previous study [33], looking at the thermal behavior of different types of PC in wet conditions. In that study [33], the authors relied on the albedo of pavements in dry conditions. This assumption may affect the accuracy of the energy balance analyses of the pavements, to study the variation of the shortwave energy flux absorbed by the pavements over time. In this experiment, the albedo of the pavements in wet conditions was measured during the 5 days of the experiment during the daytime phases of each day. The results are shown in Fig. 9c: at the beginning of each experiment, the wet-to-dry albedo ratio of the pavements varies from 22.8 to 30.7%. The main reason for the decrease in albedo is the color of the cement, which becomes darker when it is wet; this

finding is in agreement with the results of Lekner and Dorf [61]. Figure 9c shows that the albedo of PCG increases rapidly during the third day to 19.5%, which represents 96% of its albedo in dry conditions. This shows that the water on the surface of the PCG has evaporated and its surface has become dry, which explains the sharp decrease in the evaporation rate of PCG from the third day. Furthermore, Fig. 9c shows that the albedo of the pavements at the beginning of each diurnal phase is slightly lower than that corresponding to the end of the diurnal period of the previous day. This decrease is caused by the capillary absorption of water during the night period, which brings moisture to the pavement surface.

### 3.4.2 Variation in surface temperature

Figure 10 shows the variation in surface temperature (Fig. 10a) and the variation in water content (Fig. 10b) of PC pavements as a function of time. It has been shown previously that the surface temperature of pozzolan-based PC pavements is higher than that of PCG during the day. However, in wet conditions, the results show that the maximum surface temperature of PCG is 2.8–5.8 °C higher than that of PCP1 and 3.2–11.7 °C higher than that of PCP2. Furthermore, during the night, the surface temperature of PCG is 0.9–2.3 °C higher than that of PCP1 and 1.3–3.3 °C higher than that of PCP2 at the end of the night phases. These results show that the use of the pozzolanic aggregate P2 in the PC mix improves the cooling effect and results in a surface temperature up to 11.7 °C lower during the day and up to 3.3 °C lower at night compared to dense aggregate PC pavements.

<figure><img src="image_14.png" /><figcaption>**Fig. 10** Variation in: a surface temperature and b moisture content of PC pavements in wet conditions over the course of 5 days</figcaption></figure>
The deviations between the maximum surface temperature $\Delta T_{max}$ of PC pavements in wet conditions and that in dry conditions at the end of the diurnal periods of each day are presented in Table 6. This table shows that PCP2 has the highest evaporative cooling performance compared to PCP1 and PCG. At saturation, the maximum surface temperature of PCP2 is up to 26.4 °C lower than that in dry conditions. This gap decreased to 11.1 °C after 5 days, which represents 42% of initial cooling performance, while the cooling performance at the end of the experiment represents 22.4% for PCP1 and 12% for PCG compared to their cooling performance during the first day. PCP2 exhibits the best cooling performance due to its high open porosity, the porous nature of the pozzolan aggregate and the high specific surface area of the P2 aggregate, which allows a larger amount of water to be retained near the surface compared to PCP1 and PCG. In addition, the high capillarity of the PCP2 pavement, as shown in Table 5, can be attributed to its greater porosity and relatively smaller pore sizes compared to other pavements. This configuration enhances the transfer of free water to the surface of the pavement. Specifically, the smaller pores increase capillary forces, facilitating the movement of water through the material. This enhanced water transport capability helps maintain a larger reservoir of water close to the surface. Consequently, this water reservoir allows for a more effective prolongation of the initial evaporation phase, which significantly improves the overall cooling effect of the PCP2 pavement. In other words, the combination of high porosity and smaller pore sizes enhances water retention and evaporation management, leading to a more pronounced cooling effect.

It was found that the cooling performance is strongly related to the evaporation rate in these pavements. As the evaporation rate increases (Fig. 9b), the pavement temperature decreases due to the latent heat absorbed during the phase change of water (from liquid to gas).

Figure 10b shows that at the end of the experiment, the water contents of all pavements were above 6%. The cooling performance of PCG pavement was negligible on the last day (2.3 °C decrease in temperature), though this pavement was not completely dry. It was found that it is not sufficient for the permeable pavement to be wet to have a significant cooling effect; the effect is strongly dependent on the presence of water near the surface. In order to ensure a greater cooling effect over a longer period of time, PC pavements should be watered periodically. For example, the PCG pavement should be saturated after 5 days to increase its water content from 6.4 to 11.3% (saturation state); this would achieve a powerful cooling effect, decreasing its surface temperature by 19.2 °C. Periodic watering of PC pavements is essential to maximize the cooling effect and to increase the effectiveness of these pavements in mitigating the formation of UHIs.

### 3.4.3 Energy distribution

Heat exchange occurs between pavements and their environment. When permeable pavements are exposed to solar radiation, their surface temperatures gradually increase until they become higher than the air temperature. This increase causes a significant release of convective heat flux (sensible heat flux), which increases the air temperature and affects the thermal comfort of residents. However, when water is present within the permeable pavement, the latent heat flux (evaporation flux) releases the absorbed energy and lowers the surface temperature.

The shortwave radiative flux absorbed by the permeable pavements $I_{abs}(W/m^2)$ was calculated according to Eq. (6). The convective heat flux (sensible heat flux) $H(W/m^2)$ was calculated according to Eq. (7). The theoretical maximum latent heat flux lost by evaporation $E(W/m^2)$ was calculated according to Eq. (8) [23]. This equation shows that the latent heat loss by evaporation is proportional to the evaporation rate $ER(kg m^{-2} h^{-1})$.

<table><caption>**Table 6** Difference between the maximum surface temperature in wet and dry conditions</caption><thead><tr><th>Sample</th><th>ΔT<sub>max</sub> (day 1)</th><th>ΔT<sub>max</sub> (day 2)</th><th>ΔT<sub>max</sub> (day 3)</th><th>ΔT<sub>max</sub> (day 4)</th><th>ΔT<sub>max</sub> (day 5)</th></tr></thead><tbody><tr><td>PCG</td><td>19.2</td><td>13.9</td><td>8.2</td><td>3.9</td><td>2.3</td></tr><tr><td>PCP1</td><td>24.1</td><td>21.8</td><td>13.5</td><td>9.2</td><td>5.4</td></tr><tr><td>PCP2</td><td>26.4</td><td>24.8</td><td>23.5</td><td>19.3</td><td>11.1</td></tr></tbody></table>
$$I_{\text{abs}} = (1 - \alpha) I \qquad (6)$$

$$H = h_c(T_s - T_a) \qquad (7)$$

$$E = \frac{ER \cdot L}{3.6} \qquad (8)$$

where $\alpha$=albedo (dimensionless); $h_c$ (W m⁻¹ °C⁻¹)=convection heat transfer coefficient [62]; $T_s$ (°C)=surface temperature (°C); $T_a$ (°C)=air temperature near the surface; $L$ (kJ/kg)=heat of vaporization of water.

Figure 11a–c show that the sensible heat flux (H) which is responsible for UHIs development and the latent heat flux (E) vary inversely. The results show that PCG has the highest sensible heat flux during the 5 days of the experiment compared to that of the other permeable pavements. The sensible heat flux for the PCG sample exceeded 60 W/m² on the second day, while the sensible heat flux exceeded this value on the third day for PCP1 and on the fifth day for PCP2, which has the highest evaporative flux. Indeed, the

convective heat flux is converted into latent heat in wet conditions by evaporation, leading to a decrease in surface temperature. Moreover, during the night, the significant latent heat flux released by the PCP2 sample allowed it to have a surface temperature lower than that of the air, which led to the negative sensible heat flux observed during the first 4 days of the experiment. On the other hand, the sensible heat flux of the PCG at night was higher than 20 W/m² from the fourth day onwards, which intensifies the nocturnal UHI effect.

Figure 11d shows that the shortwave radiative flux absorbed by the PCP2 sample was up to 43 W/m² higher than that of the PCG during the third and fourth days. The main reason is the high capillary absorption coefficient and large open porosity of PCP2 compared to PCG, which made its surface wetter (lower albedo) over a longer time. In contrast, the PCP2 sample has the lowest surface temperature and sensible heat flux over the 5 days of the experiment. Indeed, the additional absorption of shortwave radiation by PCP2 compared to PCG that was observed during the third and fourth day was compensated

<figure><img src="image_15.png" /><figcaption>**Fig. 11** Variation in latent and sensible heat fluxes for **a** PCG, **b** PCP1 and **c** PCP2 pavements as well as the **d** variation in absorbed short-wave heat flux as a function of time</figcaption></figure>
by higher water evaporation. It was deduced that in order to achieve a greater cooling effect, the permeable pavement must provide a greater evaporative flux over a longer period of time.

## 4 Conclusion

In this study, the thermal behavior of three types of pervious concrete was investigated under controlled climatic conditions in the laboratory in dry and wet conditions using an innovative experimental bench. The factors that influence the cooling effect of these pavements were identified. The main conclusions of this study can be summarized as follows:

1. In dry conditions, the maximum surface temperature of PCP2 is 1.9 °C higher than that of PCP1 and 4 °C higher than that of PCG, due to the low thermal conductivity of pozzolan aggregates. The use of lightweight aggregates in the pervious concrete mix should be avoided in dry conditions.

2. The partial immersion test shows that the use of lightweight and less coarse aggregates can increase the specific surface and improve the absorption properties of pervious concrete. The capillary absorption coefficient of PCP2 is 67% higher than that of PCG, while its capillary water content is 78% higher than that of PCG.

3. Capillary water absorption significantly affects evaporation in permeable pavements. The use of less coarse pozzolan aggregates improves the structure of the internal capillary interstices and ensures hydraulic continuity over a long distance. In wet conditions, after 5 days, the total evaporated water mass of PCP2 is 40% higher than PCP1 and 77% higher than PCG due to its high open porosity and high water-absorption and retention capacity. Therefore, the evaporation rate of the PCP2 sample is higher during both daytime and nighttime periods throughout the experiment, compared to other permeable pavements.

4. The maximum surface temperature of PCP2 over the course of 5 days in wet conditions is 3.2–11.7 °C lower than that of PCG. During the night, its surface temperature is up to 3 °C lower than that of PCG. This type of pervious concrete is more effective in reducing urban heat islands

during the daytime and nighttime than ordinary pervious concrete based on dense aggregates.

5. The difference between the surface temperature of PCG in dry and wet conditions varied from 19.2 °C (first day) to 2.3 °C (last day). That of PCP2 varied from 26.4 to 11.1 °C due to its higher evaporation flux compared to PCG. After 5 days of the experiment, PCP2 was able to reduce its temperature by 18% in dry conditions; its water content was over 6% and the pavement was not completely dry in volume. This means that the presence of water near the surface is a key factor that controls the evaporation flux and the cooling effect. In addition, periodic watering is an essential factor to ensure a greater cooling effect over a longer period.

Wind has a major impact on the thermal convection coefficient and on the sensible heat exchange between pavements and air. In this study, due to limitations in experimental equipment and the lack of an accurate wind-speed control device, the thermal behavior study was performed under controlled laboratory weather conditions in the absence of wind (natural convection). However, as permeable pavements are being used increasingly worldwide to mitigate urban heat islands, the cooling effect of these pavements under various climatic conditions needs to be studied in more detail. In addition, it is important to focus on designing pervious concrete mixtures to optimize their water-absorption properties and improve the capillary effect to increase the evaporation rate and maximize the cooling effect.

**Acknowledgements** The authors would like to thank the Auvergne Regional Council (France) and the European Regional Development Fund (ERDF) for their financial support of this work through the CPER ECOMAT program.

## Declarations

**Conflict of interest** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Appendix 1: Experimental bench

Figure 12 shows the experimental bench used to study the thermal behavior of pervious concrete pavement in dry and wet conditions.
<figure><img src="image_16.png" /><figcaption>Fig. 12 Presentation of the experimental bench</figcaption></figure>

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