Heat of the hydration-induced temperature evolution of a 3.30 m thick raft concrete foundation for wind turbines at the early ages was monitored in situ through a temperature sensor testing system. The temperature variation patterns and risk of cracking were studied. Finite element analysis (FEA) conducted on the temperature fields determined the lower thickness threshold requiring temperature control. A comprehensive temperature control approach suitable for thick raft foundations was proposed based on a practical engineering project. Temperature monitoring and analysis results showed that the early temperature field evolution featured two characteristic phases: heat accumulation and heat release. A remarkable temperature gradient was observed along the vertical direction of the foundation. The maximum temperature difference between the concrete core and the top surface was approximately 35°C, indicating a risk of cracking. The accuracy of the FEA was ensured by adopting the concrete heat generation rate obtained from the adiabatic temperature rise test. A further FEA performed on foundations with various thicknesses demonstrated that a thicker foundation corresponded to a higher vertical temperature gradient. Moreover, a raft thickness larger than 2.50 m corresponded to a maximum temperature difference between the concrete core and the surface higher than 25°C, above which cracking prevention measures should be taken. Field test results proved the applicability of a suite of temperature feedback regulation measures proposed herein, including layered pouring, thermal insulation, and in situ real-time temperature monitoring, to thick raft mass concrete structures with relatively small volumes. Good control of temperature difference was achieved using this approach.
The concept of mass concrete stems from water resource projects. Initially, it mostly referred to concrete used for dams, such as the Arrowrock Dam in the US that opened in 1915, the Hoover Dam in the US that opened in 1933, the Toktogul hydropower station in the former Soviet Union that opened in 1977, and the world-renowned Three Gorges Dam in China that opened in 1994 [
With the rapid social and economic development, large-sized, high-volume, and poured concrete structures have been increasingly used in sectors other than water resource projects. Thick raft concrete foundations for high-rise buildings, concrete diaphragm wall for subway tunnels, and thick raft concrete foundations for power generation facilities have been widely employed particularly in industrial and civil engineering. For instance, the foundation rafts of the Jin Mao Tower in Shanghai and the new office building of CCTV in Beijing are as thick as 5 m and 7 m, respectively. Although these raft foundation structures are smaller than dams in size, a relatively large temperature difference between the inside and the concrete surface can still be observed because of their remarkable thickness. Such temperature difference results in temperature cracks, which hinders the normal use of the structure. Engineering practices have shown a noticeable increase in the probability of temperature cracking occurrence when the temperature difference is roughly larger than 25°C [
Massive water resource constructions, such as dams, have been extensively probed in the field of concrete engineering. In contrast, only a few studies in the literature have reported on the thick raft mass concrete foundation structures used in general industrial and civil constructions, which are of course significantly smaller in size [
Based on a practical engineering project, this study obtained the real-time temperature monitoring data within 18 days after concrete pouring by arranging temperature sensors in a thick raft foundation for a wind turbine. The temperature variation patterns were observed by analyzing the test data. The concrete cracking risks were also discussed. A finite element analysis (FEA) was performed on the temperature fields of foundations with various levels of raft thickness, from which a thickness threshold requiring temperature control was determined. Finally, effective temperature control measures for thick raft foundations were proposed for practical engineering applications.
The engineering background of this study was a wind turbine foundation project in North China, which had 21 identical wind turbines supported by thick raft concrete foundation structures that were also identical. Among them, Foundation 1, without any temperature control, was used for the temperature field test, whereas foundations 2–21 had temperature control measures based on the temperature test results obtained from Foundation 1. The foundation was 18 m in both length and width and has a thickness of 3.3 m. Its upper part was a cylinder with a 7 m diameter. The concrete had a C35 grade, whose mixing ratio is shown in Table
Concrete mixing ratio (unit: kg).
Cement | Fly ash | Aggregate | Sand | Water | Admixture |
---|---|---|---|---|---|
385.0 | 68.0 | 106.5 | 725.0 | 179.0 | 10.4 |
Photos of Foundation 1. (a) Before pouring. (b) After demolding.
Before pouring, multiple sensors were set inside Foundation 1 to monitor the temperature variations. They were installed at different vertical positions of the foundation, as shown in Figure
Layout of the temperature sensors: (a) top view and (b) side view.
In situ installation of the temperature sensors.
The adiabatic temperature rise test aims at obtaining the temperature evolution inside the concrete specimens under adiabatic conditions, where the value of temperature rise measures the heat released from the hydration reaction of the cementing material of the concrete [
The temperatures at various sensor installation points of Foundation 1 within 18 days after pouring were recorded through in situ real-time monitoring. Figures Heat accumulation phase (i.e., temperature rise phase): this phase primarily featured heat build-up and temperature increase. A considerable amount of heat was generated because of the hydration reaction occurring in a large volume of concrete. The rate of heat generation inside the foundation was higher than the rate of heat dissipation from the foundation surface. Consequently, heat continuously built up within the foundation, leading to the rising and peaking of temperature at all monitoring points. Figures Heat release phase (i.e., temperature decay phase): this phase primarily featured heat conduction and dissipation. That is, heat generated from hydration transferred to the ambient environment and the neighboring earth, leading to the slow decline of the foundation’s temperature. After heat transfer from the concrete, the temperature curves of the neighboring earth showed different fluctuation levels. A more intense fluctuation was observed at locations closer to the foundation surface. For example, Figure
Measured temperature evolution of Points 001–005.
Measured temperature evolution of Points 006–010.
Measured temperature evolution of Points 011–013.
The temperature evolution of the foundation top and bottom was different from that of the internal locations. The sensor at the top had the lowest temperature peak (approximately 44°C), becoming sinusoidal after roughly 50 h, followed by a sharp decline. This was caused by the top being more susceptible to the environmental impact, thereby having a higher heat release rate compared to other locations. In contrast, the temperature at the foundation bottom declined in a slow fashion after peaking because the earth below the foundation had a relatively low heat conductivity, where heat dissipation was slower than heat accumulation.
Temperature gradient is one of the main reasons leading to the cracking of mass concrete structures [
Temperature variation along the vertical direction of the foundation.
Figures
Temperature comparison between Points 002 and 007.
Temperature comparison between Points 004 and 009.
Based on the actual temperature evolution of Foundation 1 discussed earlier, the cracking risk of the thick raft concrete foundation can be analyzed from the perspective of the two identified phases: Temperature rise phase: according to the spatial variation of temperature, the internal part of the concrete was the high-temperature zone, whereas the part close to the foundation surface was the low-temperature zone. The high-temperature zone was dominated by compressive stress because of the difference in temperature rise, whereas the low-temperature zone was dominated by tensile stress. However, the elastic modulus at the early ages was small; hence, the compressive and tensile stress values in this phase were quite small. Temperature decay phase: the elastic modulus of the concrete gradually increased to as large as 90% of the eventual value [
As a result, the compressive stress inside the concrete, which was generated during the temperature rise phase, was counteracted by the tensile stress generated during the temperature decay phase. The eventual combined effect was the creation of a tensile stress field with fairly large values inside the concrete while the surface was turned into a compressive stress field. A larger temperature gradient between the core and the surface led to a higher tensile stress inside the foundation, making it more likely to reach the ultimate tensile stress of the concrete and the crack. Previous studies have shown that concrete is highly susceptible to cracking when the temperature difference between the core and the surface is higher than 25°C [
The finite element method was used in our simulation to further investigate the temperature variation patterns under different raft thicknesses. The precision of the temperature FEA mostly depends on the input values of the thermal properties of concrete, among which the heat generation rate is of paramount significance [
Based on the principle of heat balance, the heat conductivity equation for the heat of hydration of concrete is [
The key to the current FEA is the determination of the heat generation rate of concrete,
Adiabatic temperature rise test curve.
The measured temperature values shown in Figure
Under adiabatic conditions, equation (
Based on equations (
An FEA simulation was performed on the temperature field of Foundation 1 after pouring. Construction and solving of the computational model were both conducted in FEA software, ANSYS. According to geometric symmetry, 1/4 of the model consisting of concrete and earth was computed. Figure
FEA meshing.
Main thermal parameters.
Properties | Value adopted in the analysis |
---|---|
Heat conductivity of concrete (kJ/(m·h·°C)) | 8.0 |
Specific heat of concrete (kJ/(kg·°C)) | 0.97 |
Density of concrete (kg/m3) | 2400 |
Heat conductivity of soil (kJ/(m·h·°C)) | 5.0 |
Specific heat of soil (kJ/(kg·°C)) | 1.273 |
Density of soil (kg/m3) | 1800 |
Concrete-air heat transfer coefficient (kJ/(m2·h·°C)) | 100 |
Steel mold-air heat transfer coefficient (kJ/(m2·h·°C)) | 95 |
The calculated temperature evolution of some representative monitoring points shown in Figure
Comparison between the calculated and measured values for Point 001.
Comparison between the calculated and measured values for Point 002.
Comparison between the calculated and measured values for Point 003.
In practical construction, different mixing ratios correspond to different hydration heat characteristics [
An FEA was performed on the temperature fields of five foundations with various thicknesses (i.e., 1.65 m, 3.30 m, 4.95 m, 6.60 m, and 9.90 m) but similar material properties and horizontal cross-section size with Foundation 1 to examine the role of foundation thickness in temperature variation. Figure
Core temperature evolution of foundations with various raft thicknesses.
Figures
Temperature difference between the core and the upper surface of the 1.65 m thick foundation.
Temperature difference between the core and the upper surface of the 2.5 m thick foundation.
Temperature difference between the core and the upper surface of the 9.9 m thick foundation.
As stated earlier, 25°C is a commonly recognized temperature difference threshold, above which temperature control measures need to be taken to avoid thermal cracking. Therefore, the raft thickness corresponding to the 25°C temperature difference should be considered as the thickness threshold requiring temperature control. According to the current analysis (Figure
Various methods for controlling thermal cracking have been proposed based on the previously reported case studies on cracking of concrete dams [
Through these measures, the temperature differences of the wind turbine foundations were controlled within 25°C. Table
Temperature measurement results of representative foundations.
Foundation | Maximum temperature at the foundation core (°C) | Maximum temperature difference between the foundation core and top (°C) |
---|---|---|
Foundation 5 | 60.5 | 20.5 |
Foundation 10 | 62.0 | 22.0 |
Foundation 15 | 58.6 | 18.2 |
Foundation 20 | 57.2 | 18.0 |
Wind turbines and foundation after the completion of construction: (a) two of the constructed wind turbines and (b) a constructed foundation.
The field test results prove the applicability of the proposed suite of temperature feedback regulation measures, including layered pouring, thermal insulation, and in situ real-time temperature monitoring, to thick raft mass concrete structures with relatively small volumes, thereby achieving good control of the temperature difference between the concrete core and surface, as well as preventing thermal cracking at early ages. Compared to other temperature-based cracking control methods, this approach is simpler and more cost-effective. This approach is expected to be applied more widely with the emergence of novel environmentally friendly insulation materials.
The following conclusions are obtained in this study: In situ monitoring showed that the temperature evolution of the concrete foundation at early ages could be categorized into two characteristic phases: heat accumulation and heat release phases. The temperature at the concrete core peaked at 73°C 4 days after pouring, then slowly declined to an approximately steady level. An evident temperature gradient in the vertical direction of the foundation was observed. The maximum temperature difference between the concrete core and the top surface at an age of 9 days was as high as 35°C. Such large temperature gradients led to cracking risks. The concrete heat generation rate used in the temperature-field FEA was based on the adiabatic temperature rise test, ensuring high accuracy of the numerical simulations. The simulated results were in good agreement with the measured data. The FEA on the temperature fields of foundations with various thicknesses showed that thicker foundations corresponded to higher temperature gradients. Thermal cracking was likely to occur when the raft thickness was larger than 2.50 m, which corresponds to a maximum temperature difference between the concrete core and surface higher than 25°C; hence, temperature control measures should be taken. The field test results validated the applicability of the proposed suite of temperature feedback regulation measures, including layered pouring, thermal insulation, and in situ real-time temperature monitoring, to thick raft mass concrete structures with relatively small volumes, achieving good control of the temperature difference between the concrete core and the surface, as well as prevention of thermal cracking at early ages. This approach is simpler and more cost-effective compared to other temperature-based cracking control methods.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest regarding the publication of this paper.
This study was supported by the National Natural Science Foundation of China (no. 51178287) and the Natural Science Foundation of Shanxi Province, China (no. 2011011024-1).