Keywords

1 Introductory

In recent years, with the continuous acceleration of urbanization and the high development of industrial construction, some phenomena have exacerbated the development of the urban heat island phenomenon, such as the reduction of green space; atmospheric pollution; the increase of artificial waste heat from automobiles and air conditioning equipment brought about by the high level of industrial development; the increase in the number of concrete buildings; the change of the wind direction in the city due to the influence of the buildings; and the use of low-reflective paving materials and building materials; The use of paving materials such as asphalt and concrete [1]. At the same time, the external thermalization and the heat generated by indoor machines and equipment lead to an increase in the energy consumption of air conditioning inside the building, which in turn contributes to the increase in the temperature of the city, creating a vicious circle. In addition, asphalt and concrete surface paving contribute to the increase of the urban heat island phenomenon. These paving materials have a high heat capacity, and even at night, the heat that is not fully emitted during the day continues to be emitted to the atmosphere, resulting in urban surfaces remaining at high temperatures almost all day long [1]. Therefore, new pavement structures and materials are needed to reduce pavement distress and mitigate the heat island effect, which can improve road performance while suppressing the increase in pavement temperature [2]. Cities are currently facing major and severe threat of environmental stability that many researches focus on finding ways to help save, protect cities, and bring back local biodiversity and human breathtaking places [3].

Water retention and cooling pavement generally refers to a functional pavement that retains water inside the pavement structure, inhibits the rise of surface temperature and reduces air temperature through evaporation of internal water [4]. Sun Gaofeng studied the slag powder, fly ash, slaked lime mixed with water as a water retention emulsion grout infused OGFC matrix formed of water retention pavement materials, indoor simulation of the pavement light and heat environment than dense graded asphalt mixture has a cooling effect of 10 ℃ [5]. Zhang Liangliang et al. studied the steel bridge deck asphalt concrete pavement layer infused with water retaining mortar as well as the combination of water sprinkling can effectively reduce the pavement creep deformation and rutting damage, and improve the economic benefits [6], Oliveira studied the effect of the use of a cellulose-based water immersion agent on the construction of concrete blocks [7].

Combined with the above research of several pavement cooling ideas, this paper adopts a large pore space asphalt mixture filled with water retention mortar, with reference to the “CJJ/T 206–2013 Urban Road Low Heat Absorption Pavement Technical Specification” [8] of the Appendix C Cooling Effect Measurement Methods, with 220V275W diffused electronic lamps to simulate the sun, to carry out water retention evaporation of the pavement temperature reduction test.

2 Environmentally Friendly Semi-flexible Pavement Material Specimen Production

2.1 Water Retaining Mortar

The water retention mortar used in this paper consists of cement, fly ash, blast furnace slag powder, diatomaceous earth, slow-setting high-efficiency water-reducing agent, microsilica powder and other materials. After the ratio optimization design finally determined to meet the target design requirements of the water retention mortar ratio of cement: fly ash: water: blast furnace slag powder: diatomite: slow-setting high-efficiency water reducing agent: micro-silica powder = 1626.8: 697.2: 1790: 414: 112.8: 27.9: 185.9. Technical requirements to meet the “semi-flexible mix with cementitious grouting materials” (JT/T1238- 2019) [9] specification requirements, water retention mortar performance is shown in Table 1.

Table 1. Comparison of required and measured values of water-retaining mortar performance indexes
Full size table

2.2 Environmentally Friendly Semi-Flexible Pavement Mixes

In this paper, the environmentally friendly semi-flexible pavement mixture using rubber powder compounded SBS, olefin polymers and petroleum resins to prepare high viscosity modified asphalt to meet the specification requirements; through the ratio design to determine the matrix asphalt mixture of aggregate grading range of SFAC-13, and its specific composition is shown in Table 2; by the Kentenburg fly-away test and Cherenburg precipitation leakage test to determine the grading limit of the optimal oil-rock ratio of 3.4% (the best asphalt dosage is 3.3%) The performance indexes of matrix asphalt mixture meet the technical requirements.

Table 2. SFAC-13 gradation scope of mother asphalt mixture
Full size table

2.3 Water Retaining Mortar Production and Forming of Base Asphalt Mixtures

Water retention mortar production according to “highway engineering cement and cement concrete test procedures” (JTG E30–2005) [10] in the cement mortar molding method, the specific steps are first of all will be cement, water retention agent and additives are added to the mixing pot, start the mixer mixing contract for 1~2 min, to be homogeneous, with the mixing to the mixing pot to join the amount of water designed to be added, and add the material used for the time should not be more than 2 min, the water is added to the whole, and then continue to mix the contract for 2 min, turn off the mixer.

Base asphalt mixture according to “highway engineering asphalt and asphalt mixture test procedures” (JTG E20–2011) [11] in the T 0702–2011 asphalt mixture specimen production methods (compacting method) to produce Marshall specimens and T 0703–1993 asphalt mixture specimen production methods (wheel milling method) [12] to produce rutted plate specimens. The base asphalt mixture in 130~160 ℃ mixing, 120~150 ℃ under the compacting or rolling molding, Marshall specimen double-sided compacting each 50 times. Mixed rutted specimen plate rolling molding density by compacting Marshall specimen density control, after starting the wheel mill, rutted specimen plate in one direction first 2 round trips (4 times), unloading, and then lift the milling wheel, the specimen will be reversed direction, and then add the same load crushed 12 round trips (24 times), to the specimen to achieve the Marshall standard compactness of 100 ± 1% until.

Wait until the specimen is cooled to below 60 ℃ before grouting, when the mortar penetration is complete, use a rubber scraper to scrape off the excess slurry to expose the unevenness of the surface of the asphalt mixture specimen is appropriate. Since the cement mortar will remain on the paving surface, which may affect the appearance of the road surface to a certain extent and reduce the anti-skid performance of the asphalt pavement, the surface can be rinsed with water before the initial setting of the slurry after the cement slurry is grouted. The specimen after grouting will be left at room temperature for more than 24 h, until the slurry hardening, put into the temperature of 20 ℃ ± 3 ℃, humidity greater than or equal to 90% of the sustenance room sustenance, to the age of the demolding, measured its road performance indicators.

3 Test Methods

3.1 Test Preparation for Water Retention and Cooling of Environmentally Friendly Semi-Flexible Pavements

In order to accurately simulate the sunlight irradiation, so that the water retention and cooling specimens get uniform light, simulating the sunlight of the electric lamp using 220V275W diffusion type electronic lamp, its irradiation height can be automatically adjusted to about 100 cm. The temperature data were recorded by XMTD temperature readout meter with 0.1℃ resolution. Adjust the irradiation height, so that the dense standard specimen AC-13 in the electronic lamp directly under the surface temperature of 60 ℃ after 3 ± 1 h of irradiation, and fix the height of the electronic lamp.

3.2 Correction Test for Temperature Readout Data

Considering that the thermocouple of the temperature reading instrument will produce test error when connecting the asphalt pavement surface, specially through a cup of high temperature heating of water, with the range of 100 ℃ thermometer for the comparison of the correction test, and its test data are shown in the following table 3, the data comparison graph is shown in Fig. 1.

Table 3. XMTD temperature correction of the data
Full size table
Fig. 1.
figure 1

Correct testing temperature contrast figure

Full size image

As can be seen in Fig. 1, the difference between the real-time temperature measured by the XMTD temperature reader and the thermometer is not significant, and the XMTD temperature reader is slightly lower than the thermometer, in the range of 0.6 to 1.0 ℃.

its average value \(\mu = \frac{1}{N}\sum\limits_{i = 1}^N {x_i = 0.822}\).

(statistics) standard deviation \(\sigma = \sqrt {{\frac{1}{N}\sum\limits_{i = 1}^N {\left( {x_i - \mu } \right)^2 = 0.015} }}\).

The standard deviation is less than 0.02, indicating that the difference between the XMTD temperature reader and the thermometer is relatively centralized, and the average value of + 0.8℃ can be taken as the correction value of the XMTD temperature reader.

4 Experimental Process and Result Analysis

After the preparatory work, will be immersed in water for more than 12 h to keep the surface of the cooling specimen with a wet towel dry, placed in the diffuse light type electronic lamp directly under the arrangement of the thermocouple position of the temperature readout meter, turn on the power supply of the temperature recorder, 2~3 min, turn on the diffuse light type electronic lamp switch, and at the same time began to record the temperature data.

Comparing the cooling effect under each test condition, the comparison graphs before and after the upper limit grouting are shown in Fig. 2, before and after the median grouting are shown in Fig. 3, before and after the lower limit grouting are shown in Fig. 4.

From Fig. 2, it can be seen that before and after the upper limit grouting has no obvious effect on the cooling effect, and the difference is only 3℃. From Fig. 3, it can be seen that the cooling effect before and after the middle value grouting is relatively obvious, which can reach 7℃. From Fig. 4, it can be seen that the lower limit grouting before and after the cooling effect is more prominent, 5 min after the cooling reached 7 ℃, and has been sustained at about 9~10 ℃, up to 11 ℃, it can be seen that, with the increase of the void ratio, the water retention mortar filling amount increases, its cooling effect is more obvious.

Fig. 2.
figure 2

Upper limit of before and after grouting cooling contrast figure

Full size image
Fig. 3.
figure 3

Median grouting and cooling contrast figure

Full size image
Fig. 4.
figure 4

The lower the temperature before and after grouting contrast figure

Full size image

In the natural phenomenon, the sun exposure time is generally not more than 10 h a day [10], in the morning there is dew, night cooling, rainy weather, water vapor in the atmosphere, rain or artificial sprinkling of water can be stored in the asphalt pavement in the water-preserving mortar, the formation of a pavement is always inside the state of the benign cycle of water, the water-preserving pavement will be a sustained and durable cooling function, to achieve energy-saving and environmental protection of the natural evaporation of the temperature reduction.

5 Concluding Remarks

This paper analyzes the mechanism of water retention and cooling, water storage is achieved by natural rain or artificial sprinkling, cooling is achieved by evaporation of water in the mortar to take away the latent heat of the pavement, forming a natural environmentally friendly water retention and cooling effect. Through the study of the cooling performance of water retention and cooling semi-flexible pavement, the matrix asphalt mixture before and after grouting is compared, and the following conclusions are obtained:

  1. (1)

    The correction value of XMTD temperature reading meter is + 0.8℃.

  2. (2)

    The upper limit (void ratio of 16.1%) is not a very obvious cooling due to the small void ratio, and the median (void ratio of 19.9%) and the lower limit (void ratio of 23.4%) have a continuous temperature difference between 10 and 16 ℃ after 80 min of irradiation, which can last for more than 24 h.

  3. (3)

    The temperature difference before and after grouting is determined by the void ratio of the base asphalt mixture, and the cooling effect is positively proportional to the size of the void ratio or the amount of mortar grouted under the condition of meeting the road performance.

  4. (4)

    The water-preserving mortar can circulate and repeatedly absorb and release water, which provides a virtuous cycle of water storage and evaporation for the water-preserving and cooling semi-flexible pavement, and achieves the natural evaporation cooling of energy-saving and environmental protection of the pavement.