Study on the Effect of Insulation Materials on the Temperature Field of Piles in Ice-Rich Areas

Abstract

Concrete piles in ice-rich areas exist in negative-temperature environments, which seriously affect the concrete’s strength. In order to maintain the quality of concrete piles in these areas, the temperature during the concrete strength formation period needs to be controlled. In this paper, the temperature field of the pile body of a test pile with double sheaths filled with polyurethane insulation and one without polyurethane insulation were measured. The temperature disturbance law of the pile base with/without insulation was obtained and comprehensively analyzed. The temperature of the pile body was shown to increase with the thickness of the insulation layer. Analysis of the thermal and physical properties of the insulation materials showed a linear relationship between pile temperature and thermal conductivity, in which a lower thermal conductivity resulted in a higher pile temperature. The effect of applying insulation around the pile perimeter in the ice-rich permafrost region on the concrete strength of the pile foundation was verified. The test pile with insulated double sheaths showed better strength at all ages than the test pile without insulation. The use of insulation maintained the temperature of pile foundations in ice-rich areas and ensured that the pile foundations were in better condition, thus improving the concrete strength at all ages. Adopting a double-sheathing configuration with polyurethane as an insulating layer can improve the concrete strength of the pile. This method is applicable to the ice-rich permafrost area in the Daxinganling Mountains and also has reference value for middle and low-latitude wetland permafrost areas.

1. Introduction

Permafrost is widely distributed throughout China [1]. In these areas, bored piles are used for the construction of highways, railroads, and power lines because of their strong horizontal and vertical bearing capacity, ease of construction, and wide range of applications [2]. However, the construction of conventional bored piles melts the permafrost around them. At the same time, the lower environmental temperature in the permafrost region has an extremely negative impact on the strength development of concrete in the early age of the pile body [3]. This, in turn, reduces the strength of the pile body, resulting in an insufficient bearing capacity of the pile and ultimately endangering its structural integrity. Therefore, in ice-rich environments, it is important to reduce thermal disturbances to the construction on the perimeter of the pile permafrost and increase the pile concrete maintenance temperature to ensure the formation of early concrete strength for construction projects in permafrost regions.

Many scholars have researched the early thermal stability of concrete bored piles in permafrost areas and their strength growth under negative-temperature conditions. The exothermic heat of the hydration of concrete has a more significant effect on the refreezing process, with a reduction in the total heat of hydration, the maximum temperature rise, and refreezing time. The higher the entry temperature in the concrete filling, the higher the maximum temperature rise and the longer the refreezing time [4]; therefore, it is recommended that the temperature of concrete in the mold should be as low as possible when the conditions permit, preferably controlled at 2–9 °C. The freezing time of a 1.2 m diameter test pile is 1.14 times longer than that of a 1.0 m diameter test pile at the same entry temperature. The refreezing time of the frozen soil around the pile is also related to the initial conditions of the frozen soil, where a lower frozen soil temperature results in a shorter refreezing time [5]. To avoid temperature cracks caused by the large temperature difference between the inside and outside of newly-cast mass concrete, physical temperature control methods are often used. For example, cold pipes can be laid inside before casting, or insulation materials can be wrapped outside to reduce the temperature difference between the inside and outside of mass concrete [6]. Nurse et al. experimentally found that the compressive strength of concrete increased as the product of temperature and time increased [7]. Yan Ning [8] conducted a more systematic study on the temperature field of a bored pile bank and pile-side soil in ice-rich areas based on field tests and heat conduction theory [8]. The subsurface temperature measured in the ice-rich soil layer at the time of boring was within the range of −2.0 to −3.0 °C. The paper noted that the 28-d compressive strength of concrete at 0 °C maintenance only reached 75.2% of that at 20 °C. As the temperature decreased, the strength loss rate of concrete rose [9,10,11].

Many researchers have mainly focused on adding antifreeze agents and other measures to ensure the concrete strength of piles in negative-temperature environments. They have also studied changes in the temperature field of concrete piles. However, measures to change the pile concrete temperature by adding insulation to reduce heat exchange between the concrete and surrounding ice-rich frozen soil have not been studied. In this paper, relying on the construction project of the bored pile of the Jingmo Highway Bridge, the temperature field of a test pile with double sheaths filled with polyurethane insulation and a test pile without polyurethane insulation was measured. Based on the pile-soil heat conduction theory, the temperature disturbance law of the pile body was calculated by finite element software with an insulation layer applied around the pile. The temperature field influence law of the insulation material on the pile body was systematically studied under ice-rich geological conditions. Then, the strength of the concrete pile with an insulation layer was studied. The conclusions obtained provide guiding suggestions to ensure the safety and performance of bored piles constructed in ice-rich permafrost areas. They also have important theoretical and practical significance for accurately understanding the stability of constructed pile foundations in permafrost areas.

2. Pile Testing Program

2.1. Experimental Design Plan

According to Ref. [12], the geology of the northeastern Daxinganling region is mostly island-shaped multi-year permafrost and ice-rich permafrost. The release of the heat of hydration melts the permafrost, which reduces the bearing capacity of the pile foundation [12]. The lower environmental temperature will also affect the early-age strength development of concrete. Based on the complex construction projects in the ice-rich permafrost region, we relied on the permafrost bridge project from Zhanling (Ta-Mo boundary) to the Silingi section of the Jing-Mo Highway for the experimental study.

The two test piles were located near pile 11 of the K424 + 380 permafrost bridge, with a pile spacing of 6 m. The diameter of test pile 1# was 1.4 m. The pile was formed with a double bail without insulation, and the mud specifications were determined according to the bored pile construction regulations. Test pile #2 had an inner diameter of 1.4 m and an outer diameter of 1.6 m. The borehole was formed with a double-bailer plus insulation (polyurethane foam) formation scheme. The mud specifications were identical to those of the test pile. Temperature measurement holes were set at the pile core and pile side, and also 0.5 m, 0.75 m, 1.0 m, 1.5 m, 1.75 m, and 2.0 m from the pile side. In order to provide a reference comparison to the frozen soil around the pile to determine the influence of pile construction and concrete heat of hydration, temperature measurement holes were also set at 3.0 m from the pile side as foundation temperature measurement holes. The depth of each measurement hole was 11.5 m, and 13–14 temperature measurement points were laid in each measurement hole according to the different geological soil layers. The detailed layout of the temperature measurement points during the construction of the pile foundation is shown in Figure 1 and Figure 2. Figure 3 shows the construction site situation.

2.2. Insulation Material Selection

Insulation materials generally have a coefficient of thermal conductivity of ≤0.12. Commonly used insulation materials include expanded polystyrene board, extruded polystyrene board, rubber powder polystyrene particle insulation slurry, polyurethane foam material, and rock wool board. Polyurethane has excellent thermal insulation, strong bonding ability, and excellent processability [13,14,15], so it was selected as the insulation material for this test. Polyurethane foam has one of the lowest thermal conductivities. According to the different blowing agents in the combined white material formula, the thermal conductivity of rigid polyurethane foam can reach 0.017–0.023 W/(m·°C), and its thermal insulation effect is 13.5 times greater than that of ordinary concrete. A comparison of the thickness of different insulation materials with the same thermal conductivity is shown in Figure 4.

3. Modeling of Pile-Soil Heat Transfer

3.1. Numerical Calculation Method

The infiltration and flow of soil moisture during the thawing and freezing of permafrost were not considered in any of the finite element calculations. The mathematical control equations of the entire computational model were described by the heat transfer equation in transient three-dimensional Cartesian coordinates with a phase transition [16,17] heat balance control differential equations for the problem of phase change temperature field are as follows:

The differential equation for the thermal conductivity of the transient temperature field in the solid phase region ${\Omega }_f$ was:

$$\frac{\partial T_f}{\partial t}=\frac{{\lambda }_f}{\rho C_f}\left(\frac{{\partial }^2{T}_f}{\partial x^2}+\frac{{\partial }^2{T}_f}{\partial y^2}+\frac{{\partial }^2{T}_f}{\partial z^2}\right)+\frac{q_v}{\rho C_f}$$

(1)

The differential equation for the thermal conductivity of the transient temperature field in the liquid phase region ${\Omega }_u$ was:

$$\frac{\partial T_u}{\partial t}=\frac{{\lambda }_u}{\rho C_u}\left(\frac{{\partial }^2{T}_u}{\partial x^2}+\frac{{\partial }^2{T}_u}{\partial x{y}^2}+\frac{{\partial }^2{T}_u}{\partial z^2}\right)+\frac{q_v}{\rho C_u}$$

(2)

where T is the transient temperature of the object (°C); t is the time for a process to occur (s); ${\lambda }_f, {\lambda }_u$ are the thermal conductivity of frozen soil and thawed soil in W/(m∙°C), respectively; ρ is the density of the material; $C_f, C_u$ are the heat capacities of frozen soil and thawed soil, respectively kJ/(kg∙°C), and $q_v$ is the internal heat source intensity (W/m³). The two temperature fields at the phase transition interface $\xi \left(t\right)$ need to satisfy both the temperature continuity condition and the energy conservation condition:

$$T_f\left(\xi \left(t\right),t\right)=T_u\left(\xi \left(t\right),t\right)=T_m$$

(3)

$${\lambda }_f\frac{\partial T_f}{\partial n}-{\lambda }_u\frac{\partial T_u}{\partial n}=L\rho \frac{d\xi \left(t\right)}{dt}$$

(4)

T_m is the freezing surface temperature (°C); n is the direction normal to the freezing surface; L is the latent heat of phase change of the substance, and the latent heat of phase change of water is 334.56 kJ/kg.

The calculation process uses the sensible heat capacity method to simulate the phase change calculation of the material model. That is, the phase change of water in permafrost does not occur at a fixed temperature but in a small range. We assume that the phase change occurs within a temperature range (T_m ± ΔT) around T_m so that we obtain the Expressions (5) and (6) for the heat capacity and thermal conductivity:

$$C=\left\{\begin{array}{c}{C}_f T\le T_m-\Delta \mathrm{T}\\ \frac{L}{2\mathsf{\Delta }\mathrm{T}}+\frac{C_f+C_u}{2} T_m-\Delta \mathrm{T}\le \mathrm{T}\le T_m+\Delta \mathrm{T}\\ C_u T\ge T_m-\Delta \mathrm{T}\end{array}\right.$$

(5)

$$\lambda =\left\{\begin{array}{c}{\lambda }_f T\le T_m-\Delta \mathrm{T}\\ {\lambda }_f+\frac{{\lambda }_u-{\lambda }_f}{2\mathsf{\Delta }\mathrm{T}}\left[T-\left(T_m-\mathsf{\Delta }\mathrm{T}\right)\right] T_m-\Delta \mathrm{T}\le \mathrm{T}\le T_m+\Delta \mathrm{T}\\ {\lambda }_u T\ge T_m-\Delta \mathrm{T}\end{array}\right.$$

(6)

3.2. Model Calculation Assumptions

The process of rounding off or simplifying the complex unknown factors that hinder the solution of a problem without affecting the essence of the actual problem is the assumption of the model. In this paper, the following five assumptions are made in the modeling process:

(1)

Both concrete and soil are homogeneous media with uniform temperature distribution in each layer of frozen soil.

(2)

No energy loss during heat transfer.

(3)

The pile relies entirely on the side surface for heat transfer (i.e., no heat sink on the surface).

(4)

The frozen soil is in complete contact with the entire lower buried pile body.

(5)

The upper boundary of the model considers only the effects of external air temperature, solar radiation and scattering, and wind speed on the pile and the frozen soil around the pile.

3.3. Parameter Measurement

We took samples of each soil layer from the field and brought them back to the laboratory to determine the thermal and physical parameters of permafrost, including its dry density, specific heat capacity, and thermal conductivity in the frozen and thawed state, as well as the permafrost porosity. The results are shown in

Table 1. Permafrost thermal property parameters.

Name	Dry Density (kg/m3)	Specific Heat Capacity (J/kg∙K)		Thermal Conductivity (W/m∙k)		Porosity (%)
		Frozen	Melt	Frozen	Melt
Peaty soils	1170	1338.0	1259.8	3.41	1.45	80
Ice layer	470	1799.5	1799.5	1.88	1.88	91
Powdery clay	640	710.1	715.0	2.02	1.48	63
Round grains	1830	466.8	485.6	2.07	1.55	41
Strongly weathered tuffs	2050	643.4	639.3	3.66	1.57	12

.

Except for the thermal and physical parameters of permafrost, the thermal and physical parameters of other building materials (concrete, polyurethane foam for insulation, and q235B steel for steel sleeves) are shown in

Table 2. Material thermal property parameters.

Name	Density (kg/m3)	Thermal Conductivity (W/m∙K)	Specific Heat Capacity (J/kg∙K)
Concrete	2500	1.24	970
Polyurethane foam	38	0.02	2000
Steel sleeve	7500	43	500

.

3.4. Hydration Heat Generation Rate of Concrete

The heat from the concrete’s hydration zone is the main heat source affecting the distribution of the permafrost temperature field. By performing a literature analysis, we adopted the exponential heat of hydration formula and implemented the concrete hydration heat release process using finite element software by defining the heat of hydration heat generation rate (HGEN) [18]. The formula is as follows:

$$Q\left(t\right)=Q_0\left(1-e^{-mt}\right)$$

(7)

$$HGEN=m{Q}_0{e}^{-mt}\left(W+kF\right)$$

(8)

Q(t)—1 kg cumulative heat of hydration of cement in kJ/kg;

t—Age;

Q₀—Heat dissipation per l kg of cement, in kJ/kg;

m—Heat of hydration coefficient, i.e., the coefficient of cement species related to the rate of the heat of hydration. See

Table 3. Reference value of m depending on the inlet temperature.

Inlet Temperature (°C)	5	10	15	20	25	30
m	0.295	0.318	0.340	0.362	0.384	0.406

for details;

HGEN—Hydration heat generation rate of concrete in kJ/(m³∙h);

W—Amount of cementing material per m³ concrete, kg/m³;

F—Amount of concrete mix per unit volume, kg/m³;

k—The discount factor for fly ash is taken as 0.25.

According to the test, the heat dissipation Q₀ of the cement was 315.8 kJ/kg. According to the concrete’s inlet temperature, m = 0.318. From

Table 4. Low heat of hydration C30 concrete ratio.

Test Fit Strength	Fitting Ratio (Mass Ratio)		Water-to-Glue Ratio		Sand Rate
38.2 MPa	Cement: fly ash: sand: gravel: water 1:0.177:2.478:3.419:0.534		0.453		45%
Material Name	Cement	Sand	Gravel	Water	Fly ash
Species specifications	P.O 42.5	Nakasand	1–3 cm accounted for 28%. 1–2 cm accounted for 72%	Drinking water	/
Place of origin	Monxi	Mohe	Huzhong	/	Qiqihar
Dosage (kg/m3)	322	798	1101	172	57
Additive species; doping amount		KMSP dosing 1.3%; KMSP-14 antifreeze dosing 5.3%

, we can see that the amount of cement used in each m³ of concrete in the project is 322 kg and 57 kg of fly ash. According to Equation (8), we obtain:

$$\begin{gathered} HGEN=m{Q}_0{e}^{-mt}\left(W+kF\right) \\ =33,768 · e^{-0.318t}\mathrm{kJ}/({\mathrm{m}}^3·\mathrm{h}) \end{gathered}$$

3.5. Boundary Conditions

3.5.1. Initial Temperature

The initial temperature of the set material was used as the basis for the temperature field change. The temperature of the concrete in the field was 10 °C. For numerical calculations, the initial temperature of the concrete was taken to be 10 °C. Similarly, the soil around the pile was taken to have an average ground temperature of −2.8 °C according to the original ground temperature tested in the field.

3.5.2. Upper Boundary Conditions

The upper boundary of the model was influenced by the local atmospheric temperature, solar radiation, wind speed, and other factors, which belong to the third type of boundary conditions. Here, we introduce the theory of surface layer, which involves discarding the soil surface layer with many disturbing factors and replacing it with a stable surface layer as the upper boundary [19]. According to the theory of the surface layer, the peaty ground soil at the test site was moist clay. In comdination with the local temperature at the time of the test, the temperature of −5.3 °C was applied to the upper boundary according to the first type of boundary conditions during the simulation.

3.5.3. Lower Boundary Conditions

The lower boundary conditions were treated as the first type of boundary conditions, i.e., $\frac{\partial T}{\partial z}$ = 0.

3.5.4. Perimeter Boundary Conditions

The perimeter was considered as the permafrost boundary at infinity, according to ref. [14]. In the Tibetan area, large-diameter concrete piles disturb the frozen soil within 1.43 times the pile diameter, and the Mohe area soil moisture is large. In the above analysis of the thermal disturbance of the frozen soil around the pile perimeter, it can be seen that the maximum disturbance radius reached 1.64 times the pile diameter, i.e., when the distance from the pile-soil interface was greater than this range, it can be regarded as the same as the external conditions. Therefore, it can be treated as adiabatic. In this paper, 3.6 times the pile diameter from the pile-soil interface was taken as the surrounding boundary of the model, so adiabatic treatment was performed, i.e., $\frac{\partial T}{\partial r}$ = 0.

3.6. Computational Model

In this paper, test pile 1# of the project was used as the analysis model. The perimeter soil of the pile was divided into six layers according to the site survey (see Figure 1 and Figure 2. for details). The pile length was 11.5 m, the pile diameter was 1.4 m, and the outer diameter was 1.6 m. The study showed that when the average ground temperature was −2.0 °C, the heat of the hydration action range of the concrete was 1 to 2 times greater than the pile diameter. The pile refreezing period was longer than 2 months. The depth of the soil at the bottom of the pile was 1/3 of the pile length, and the total depth was set to 14 m. Finally, we used Ansys Fluent to build a 3D cylindrical calculation model with a diameter of 5.7 m and a height of 14 m. Pile side according to the depth range of different soil layers so that the pile side unit nodes and soil unit nodes correspond to each other, the number of units of the whole calculation model is 29,214, and the number of nodes is 21,072, as shown in Figure 5. The calculation time step per step is set to 3600 s, i.e., 1 h, and each calculation number of 24 steps is one day.

3.7. Numerical Validation

In the field data, we found that at 7.4 m, the pile core temperature is no longer affected by the environmental temperature. So we took the field temperature experimental data and calculated data from 0–50 d of the pile core (7.4 m from the top of the pile) at the middle section of the infill pile and analyzed whether the calculated results were correct by comparing the calculated data with the field monitoring data. Figure 6 shows the comparison curves of the field test results and the finite element calculation results.

The finite element calculation temperature curve form of the pile foundation was basically consistent with the measured temperature curve. From 0–3 d, the temperature rose rapidly. From 3–21 d, the temperature decreased rapidly, and after 21 d, the temperature was stable, which indicates that changes in the calculated and experimental data were consistent. The pile body temperature peaked at 3–4 d, monitoring reached the maximum temperature of 21.1 °C on the third day, and the calculation reached the maximum temperature of 21.0 °C on the third day, with an error value of 0.1 °C. The monitored temperature at 21 d was 0.5 °C, and the calculated temperature was 0.1 °C. The calculated results were similar to the monitored data. After back-freezing was complete, both the calculated and monitored values were below 0 °C. Part of the error between the two was due to the complex structure of the permafrost at the site, and the actual situation inside the permafrost was not fully represented by the field borehole extraction test. Within a reasonable error range, we can assume that the numerical calculation results basically reflect the real situation during concrete infilling.

4. The Effect of Insulation on the Temperature Field of Pile Foundation

Based on the results of pile-soil thermodynamic numerical calculations, we set up different scenarios to study the thickness of the insulation layer and the thermal conductivity of the insulation material and analyzed their effects on the temperature field of the pile.

4.1. Effect of Insulation Thickness on the Temperature Field of Pile Foundation

Five scenarios were used to study the effect of different insulation layer thicknesses on the temperature of the pile foundation. Keeping the thermal conductivity coefficient of the insulation layer and other thermal physical parameters unchanged, the thickness of the insulation layer was set as 0 cm, 5 cm, 10 cm, 15 cm, and 20 cm, and the calculated results of the temperature field on the pile side were taken for analysis.

Figure 7 shows the effect of the insulation thickness on the pile body temperature at different periods. In the upper part of the pile, the temperature was low due to the influence of atmospheric temperature. The heat passes through the top of the pile to the atmosphere by heat conduction. Upon increasing the depth, the insulating effect of the insulation layer gradually became stronger. At a depth of 5 m, the temperature of the pile was basically unaffected by the atmospheric temperature. At 7 d, the temperature of the insulation in the thickness range of 0–20 cm was 3.0 °C, 10.0 °C, 15.6 °C, 16.9 °C, and 19.9 °C at a depth of 7.4 m. At 28 d, the temperature of the insulation within the thickness range of 0–20 cm was −1.1 °C, −0.5 °C, 0.7 °C, 1.4 °C, and 1.6 °C. In the same period, the pile temperature increased with the thickness of the insulation layer. The use of insulation also reduces the lateral heat flow during the release and refreeze phases of concrete, weakening the thermal disturbance of the concrete exotherm to the perimeter of the pile in the frozen soil.

Figure 8 shows that upon increasing the insulation thickness, the maximum temperature of the pile body was elevated, and the rate of refreezing became slower. The temperature of the pile body fell below 0 °C at 16 d for the pile foundation without filling in the insulation layer. The temperature of the pile body eventually reached 0 °C after adding the insulation layer, and the thicker the thickness of the insulation layer, the longer it took for the pile body to reach 0 °C. Adding insulation during the early stages of concrete placement can slow the rate of heat dissipation and significantly increase the temperature of the pile. Therefore, the pile will be at a more appropriate maintenance temperature for a long time, which is conducive to ensuring the early strength formation of concrete.

When the concrete temperature dropped to 0 °C, the water inside the concrete began to freeze, which reduced the strength of the concrete at a later stage. If the concrete can be pre-cured to a certain strength before freezing, its late compressive strength loss will be reduced. Generally, the freezing of concrete late compressive strength loss within 5% of the pre-cured strength value is taken as the “concrete frozen critical strength” [20]. The “Construction Engineering Winter Construction Regulations” (JGJT104-2011) were developed to make the winter construction of construction projects safe, economical, and environmentally friendly, which explains the concrete frozen critical strength. This experiment was conducted in winter. According to calculations performed according to “Construction Engineering Winter Construction Regulations” (JGJT104-2011), an insulation thickness of 5 cm is sufficient to ensure that the critical strength of concrete used in the project is reached. Figure 9 shows that the ratio of pile temperature to thickness is greater than 1 when the insulation layer is 0–10 cm thick. Increasing the thickness of the insulation layer was significantly less efficient than increasing the temperature of the pile foundation when the thickness was greater than 10 cm. This is because the heat of hydration of the pile concrete is fixed, and upon increasing the thickness of the insulation layer, the concrete’s heat of hydration loss is reduced. Thus, when the insulation layer reaches a certain thickness, it can retain most of the heat of hydration, so continuing to increase the thickness of the insulation layer reduces the amount of heat retained, so the pile body temperature increase is reduced. In real projects, we have to consider saving insulation material and reducing the construction period. Combined with the concrete ratio used in the project, we chose an insulation layer thickness of 10 cm to achieve the best effect.

4.2. Effect of the Thermal Conductivity of Insulation Material on the Temperature Field of Pile Foundation

The basic parameters of insulation materials are density, specific heat capacity, and thermal conductivity. The density and specific heat capacity of an insulation material account for less than 1‰ of the effect on the temperature of the pile. Thermal conductivity has a great influence on temperature. For this reason, we chose to analyze the thermal conductivity of the insulation. To study the effect of the thermal conductivity of the insulation material on the temperature of the pile, we took the most suitable insulation thickness of 10 cm from the finite element simulation and set its material thermal conductivity to 0.02 W/M·K, 0.045 W/m·K, 0.07 W/m·K, 0.095 W/m·K, 0.12 W/m·K, respectively. We took the temperature data of 7 d and 28 d for analysis.

Figure 10 shows that the pile body temperature gradually decreased upon increasing the thermal conductivity of the insulation material around the pile. At 7 d, the material insulation coefficients of 0.02 W/m·k, 0.045 W/m·k, 0.07 W/m·k, 0.095 W/m·k, and 0.12 W/m·k for pile temperatures of 15.5 °C, 13.7 °C, 12.2 °C, 11.1 °C, and 9.5 °C, respectively. At 28 d, the pile temperatures were 0.73 °C, 0.16 °C, −0.14 °C, −0.35 °C, and −0.57 °C. The use of materials with lower insulation coefficients has a significant effect on raising the temperature of the pile.

Figure 11 shows that as the thermal conductivity of the insulation increased, the maximum pile temperature decreased. At a lower thermal conductivity, the pile more quickly reached 0 °C refroze. This occurred because the greater the thermal conductivity of the insulation material, the faster the heat flow through the insulation and the faster the heat loss from concrete hydration under the concrete and permafrost temperature gradient. This decreased the pile-side temperature with increasing thermal conductivity at the same insulation thickness.

Figure 12 shows that changes in the thermal conductivity of the insulation material were roughly linear with changes in the pile body temperature. The maximum temperature of the pile base was still located away from the temperature that would damage the later strength of the concrete. This means that when the insulation layer was 10 cm, insulation material with a lower thermal conductivity could be used to ensure that the pile body concrete meets the adiabatic temperature rise rate and maximum maintenance temperature. The thermal conductivity of the insulation layer was reduced, allowing it to better retain the heat of hydration and also prevent ambient temperature loss. Thus, the temperature gradient was lower, and the rate of loss of hydration heat to the outside was reduced. This allowed the concrete’s temperature to be maintained and also reduced the radius of disturbance during the construction of bored piles.

4.3. Effect of Insulation Materials on the Early Strength of Concrete

To verify the effect of the protective layer on the strength of bored piles, we performed ultrasonic testing on two test piles. According to the “Ultrasonic rebound synthesis method for testing the strength of concrete technical regulations”, the strength of concrete was determined, and the experimental results are shown in

Table 5. 1# test pile frozen 7–28 d ultrasonic sound velocity and temperature record sheet.

Test Pile	Test Point Distance from the Top of the Pile [m]	Ultrasonic Speed of Sound [km/s] and Temperature [°C]								7–28 d Increase in Sound Velocity [%]
		7 d		14 d		21 d		28 d
Number		Ultrasound [km/s]	Temperature [°C]	Ultrasound [km/s]	Temperature [°C]	Ultrasound [km/s]	Temperature [°C]	Ultrasound [km/s]	Temperature [°C]
1#	4.15	4.165	14.5	4.184	3.5	4.386	1.2	4.45	0.6	6.8
	6.15	4.303	15.7	4.386	3.6	4.573	1.2	4.596	0.5	6.8
	7.55	4.461	14.1	4.631	3.6	4.631	1.1	4.643	0	4.1
	9.55	4.494	12.5	4.655	2.9	4.691	0.9	4.703	0.5	4.7
	10.55	4.417	10.5	4.584	1.2	4.655	0.2	4.643	0.5	5.1

and

Table 6. 2# test pile frozen 7–28 d ultrasonic sound velocity and temperature record sheet.

Test Pile	Test Point Distance from the Top of the Pile [m]	Ultrasonic Speed of Sound [km/s] and Temperature [°C]								7–28 d Increase in Sound Velocity [%]
		7 d		14 d		21 d		28 d
Number		Ultrasound [km/s]	Temperature [°C]	Ultrasound [km/s]	Temperature [°C]	Ultrasound [km/s]	Temperature [°C]	Ultrasound [km/s]	Temperature [°C]
2#	4.15	4.008	10.5	4.512	4.2	4.575	1.3	4.621	0.4	15.3
	6.15	4.838	14.9	5	4.5	4.936	1.5	5.11	0.4	5.6
	7.55	4.887	14.5	5.039	4.6	5.039	1.4	5.231	0.3	7
	9.55	5.052	11.8	5.201	3.8	5.215	1.2	5.322	0.2	5.3
	10.55	5.013	8.8	5.146	2.7	5.187	0.9	5.222	0.2	4.1

.

The data show that test pile #2, with a double-bailer filled with insulation, had greater strength at all ages than test pile #1 without insulation. At 28 days, the compressive strength of pile #2 was 19.5% greater than that of pile #1 at the same age. These results show that test piles using double sheaths with added protective layers kept the piles at relatively high maintenance temperatures. The results recorded in the field showed a significantly higher compressive strength of concrete in the same period for the pile foundations in which insulation was applied.

5. Conclusions

Based on the pile-soil heat transfer theory, a finite element numerical model was used to calculate the temperature disturbance by the pile’s foundation by applying insulation around the pile perimeter. We conducted a comprehensive analysis and obtained the following conclusions.

(1)

The finite element numerical calculation showed that the temperature of the pile foundation increased with the insulation thickness. In a real project, we must consider saving insulation material and reducing the construction period. Combined with the concrete ratio used in the project, we can reasonably choose the thickness of the insulation layer to achieve the best effect. In this project, the best thickness was 10 cm.

(2)

Analysis of the calculated thermal and physical parameters of the insulation material revealed that the use of insulation materials with a low thermal conductivity was conducive to the formation of a more suitable concrete curing temperature. Polyurethane foam had insulation performance and was suitable for the construction of bored piles in permafrost areas.

(3)

The effect of applying insulation around the pile perimeter on the concrete strength of pile foundations in ice-rich permafrost areas was verified. Test pile #2 with double sheathing and an insulation layer was used, and the strength at all ages was greater than that of test pile #1 without an insulation layer. The application of the insulation layer improved the maintenance temperature of the pile foundation in the permafrost region and ensured that the pile foundation was better maintained, thus improving the concrete strength. The technical measures of adopting double sheathing with polyurethane as an insulation layer effectively improved the concrete strength. This technical measure was applicable to the ice-rich permafrost area in the Daxinganling Mountains and also has reference value for the wetland permafrost area at middle and low latitudes.