Investigations on the Environmental Characteristics and Cracking Control of Plateau Concrete

Abstract

In the present study, first, the environmental challenges and cracking characteristics during the construction of plateau concrete on the Sichuan–Tibet route were revealed. Then, using a multi-field coupled shrinkage model with hydration temperature humidity constraints, the early and long-term cracking risks in the core of plateau pier bodies were investigated. Later, the effects of tensile strength, pouring interval age and adiabatic temperature rise on the cracking risk were analyzed. Finally, various control measures for high-altitude concrete cracking were proposed. The results indicated that the complex environment of the plateau led to different forms of cracks in the pier body, especially vertical cracks in the straight sections. The long-term risk of core cracking in the plateau pier body is significantly greater than the risk of early cracking. This risk was strongly influenced by factors such as the concrete tensile strength, pouring interval age and adiabatic temperature rise, which should be given more attention. Deformation compensation can significantly enhance the peak and residual deformation capacities of plateau concrete, with peak values greater than 900 με and residual deformation greater than 200 με at day 60, as well as its resistance to cracking. Strategies such as adopting radiant cooling techniques, improving construction techniques and implementing effective management measures can all play a vital role in improving the cracking resistance of highland concrete.

1. Introduction

The average altitude of the Qinghai–Tibet plateau is 3000 m above sea level, which is significantly higher than that in neighboring regions at the same latitude. Due to its distinctive geographical structure and location, the Qinghai–Tibet plateau presents extreme environmental conditions such as large temperature variations, low air pressure, dryness, intense ultraviolet radiation and high wind speeds. The serviceability of concrete engineering in the plateau region faces significant challenges [1,2], among which concrete cracking is the most prominent problem. The causes of concrete cracking are multifaceted, including factors such as the climate, environment, construction technology and raw material quality. Among these factors, the environment plays a crucial role in the occurrence of concrete cracking. In response to the unique environmental conditions of the plateau, several studies have investigated the behavior of concrete in high-altitude environments. Hu and Cao [3] examined the dynamic modulus and porosity of concrete surface layers in plateau regions. The results indicated that the surface layer of concrete in plateau regions was susceptible to environmental erosion and deterioration, resulting in a 10% decrease in the dynamic modulus and an increase in the proportion of harmful porosity. Li et al. [4] explored the effect of air pressure on the air content of concrete and found that when the ambient air pressure was decreased to 50 kPa, the air content of concrete decreased by about 20% to 49%, and the air content performed a linear decreasing pattern as the air pressure was further reduced. He et al. [1] investigated how the environment affects the strength and permeability of concrete in Lhasa, Tibet, and observed that concrete performed poorly under natural outdoor curing conditions at the plateau. The 28-day compressive strength decreased by about 21.9%, and the relative coefficient of permeability increased significantly. Ge et al. [5,6] also demonstrated the effect of low pressure on the strength and chemical resistance of concrete, showing that the effect of air pressure on these two properties was decreasing. It can be seen that with the development of railroad and highway projects on the Qinghai–Tibet plateau, the effect of a high-altitude environment on the deterioration of concrete mechanical properties has attracted more and more attention [7,8,9,10,11,12]. Improving the service performance of concrete in high-altitude environments has become a hot and difficult research topic in recent years.

In high-altitude regions such as the Qinghai–Tibet railway and the Sichuan–Tibet railway in China, great efforts have been made to improve the performance and cracking resistance of concrete [13,14], and some positive results have been achieved. However, some degree of cracking still exists in concrete structures in the plateau region, indicating that the risk of cracking and mitigation measures in the environment are not fully understood. In particular, little research has been conducted on the environmental conditions and risks of concrete use in the middle section between Sichuan and Tibet. Therefore, an in-depth study of the challenges facing concrete in plateau regions and the development of effective crack control strategies are imperative.

The challenges and in situ cracking characteristics in plateau concrete projects on the Sichuan–Tibet route were firstly investigated in the present study. Then, using a multi-field coupling shrinkage model with hydration–temperature–humidity constraints, the early and long-term cracking risks of plateau piers located at an altitude of more than 3500 m above sea level were quantitatively evaluated. Finally, crack control measures such as deformation compensation and radiation cooling technology for plateau concrete were proposed to provide insights and guidance for improving crack control and serviceability in plateau concrete projects.

2. Environmental Characteristics

2.1. Temperature and Wind Speed

The distribution of the average annual temperature in China in 2022 is depicted in Figure 1, showing that the temperature on the Sichuan–Tibet route is obviously low, and the temperature is below zero degrees in some areas. Meanwhile, due to the large temperature difference in the plateau region, it was necessary to shorten the construction period in order to ensure the quality of the concrete. For instance, in the high-altitude Xindu Bridge area in Kangding, Sichuan, which is 3500 m above sea level, the temperature can soar to 30 °C in summer and plummet to −15 °C in winter, and the difference in the temperature between day and night can be as much as 35 °C within a year. Thunderstorms and hailstorms occur frequently, and even in the summer months, there can be sudden icing, snow and sudden drops in the temperature. The harsh temperature conditions pose substantial risks to the use of concrete. According to the data from the micro meteorological station, the average wind speed in the Xindu Bridge area in 2020 ranged from 8 to 10 m/s, with a maximum speed of 20 m/s. Wind speeds are usually low from July to October. Strong winds can cause a significant drop in concrete slump, worsen drying shrinkage and trigger cracking. In addition, high wind speeds require more stringent, early concrete insulation measures due to increased heat exchange between the formwork and concrete surfaces. These environmental challenges are less pronounced in the plains. Therefore, controlling concrete cracking becomes particularly difficult in high wind-speed environments.

2.2. Severe Freeze–Thaw Cycles

The annual freeze–thaw cycle refers to the number of times in a year that the temperature drops from +3 °C to below −3 °C and then rises back to +3 °C again. The freeze–thaw characteristics of the Kangding–Bomi section in the plateau region were statistically analyzed based on the monitoring data from meteorological stations. The frequency of freeze–thaw cycles varies greatly from year to year, with the most severe freeze–thaw cycles generally occurring from November of one year to March of the next, accounting for more than 80% of the total number of freeze–thaw cycles in the year. Figure 2 shows the average annual number of freeze–thaw cycles in some highland areas along the Sichuan–Tibet route from 2020 to 2022. Litang, Mangkang, Zuogong and Bangda experienced the most significant freeze–thaw cycles, with more than 110 cycles per year, and even more than 150 cycles in Mangkang. The average annual number of freeze–thaw cycles in Bomi and Yajiang was relatively low, at 48 and 71 cycles, respectively. Although the annual freeze–thaw occurrences are minimized, severe freeze–thaw conditions are still encountered in the plateau regions, highlighting the need for greater attention to be paid to the consideration of freeze–thaw effects in the design of concrete durability.

The freeze–thaw failure of concrete is a gradual process in which the internal micro-particles fail due to expansion pressure and permeation pressure [15,16]. This process is accompanied by mortar detachment on the concrete surface, and the internal loosening, cracking and filling of cracks with frozen bodies, all of which can significantly reduce the durability of concrete engineering. Figure 3 illustrates the failure mode of C20 test concrete after one year of storage at a high altitude of 3500 m, where water is abundant, and driving loads and freeze–thaw conditions exist. Water diffuses and freezes in the gaps and cracks of the concrete particles, leading to the severe deterioration of the concrete structure and strength. This highlights the increased risk of freeze–thaw damage to highland concrete.

2.3. Low Air Pressure

The air pressure distribution in high-altitude areas on the Sichuan–Tibet route is shown in Figure 4. The air pressure level in these areas is about 30–40% lower than that in the plains, with Zuogong, Bangda and Basu having particularly low air pressure values, below 60 kPa. Other areas have slightly higher pressures, mostly around 60 kPa, with Bomi close to 70 kPa. A lower air pressure led to reduced air solubility and increased the surface tension of bubbles and decreased the foam volume upon bubble burst [17,18,19], affecting the effectiveness of air-entraining agents. In addition, under low air pressure conditions, the concrete hydration products decreased, and number of concrete micro-cracks increased, which seriously affected the mechanical and durability properties of concrete. This usually leads to a loss of slump in concrete mixtures and poses challenges to field workers in placing concrete, thus affecting concrete quality control in engineering practices.

2.4. Light Intensity and Temperature Differences

Figure 5 shows the average annual light intensity in various high-altitude regions on the Sichuan–Tibet route. The data revealed that Mangkang and Basu had the highest light intensity of more than 2500 h/year, while Kangding and Bomi had a light intensity of about 1250 h/year, which was significantly higher than that in many plains. Adequate sunlight can lead to moisture evaporation during concrete curing, accelerating drying shrinkage and potentially causing cracking. Moreover, light intensity is related to radiation intensity, and intense ultraviolet radiation at high altitudes may cause internal cracks in the concrete, reducing its performance and ultimately leading to concrete failure.

In fact, the temperature difference between the sunny side and the shaded side of a bridge abutment on the plateau has the potential to cause concrete damage. On-site monitoring has shown that the temperatures on the sunny side can reach 30–40 °C, while temperatures on the shaded side are still close to ambient temperature. The temperature difference between the two sides can affect the hydration strength of the poured concrete, resulting in a 10% reduction in the rebound strength [20]. Additionally, a significant temperature difference can lead to high tensile stresses from temperature gradients, which can be detrimental to preventing concrete cracking. Figure 6 presents the simulation results of the surface temperature stresses on the pier body, considering the sunny surface temperatures of 30 °C and 40 °C and the cloudy surface temperature of 10 °C. The results indicated that the tensile stress on the concrete surface was 3.08 MPa (positive for tension and negative for compression) at the sunny temperature of 30 °C, while at the sunny temperature of 40 °C, the tensile stress was about 4.63 MPa. Conventional concrete typically has a tensile strength of 2–4 MPa, suggesting that the temperature-induced tensile stress resulting from the temperature difference between the sunny and shady sides of a pier could potentially lead to micro-cracks or even outright cracking. Over time, this will bring a great risk of damage to the concrete structure. Based on observations of the post-construction concrete of a large number of bridge piers on the Qinghai–Tibet plateau, many bridge piers and abutments in a specific plateau project have shown extensive cracking in the years of construction, with more severe damage on the sunny side. Therefore, it is critical to investigate methods to regulate the temperature distribution on both sides of the pier body in a high-altitude environment.

2.5. Dynamic Disturbance

The faults and other discontinuous tectonic surfaces along the Sichuan–Tibet route are developed, with strong tectonic movements and frequent strong earthquakes. When rupture occurs, the stress drops instantaneously, and a large amount of energy is released in the form of dynamic stress waves. This causes significant additional dynamic loads on concrete structures. In addition, the opening of certain railway projects with speeds of up to 200 km/h will also cause additional dynamic stresses on concrete structures (1–300 kPa, acceleration of 0.5–20 m/s²). It has been shown that dynamic loading can accelerate crack initiation, coalescence or nucleation and causes intricate changes in the stress field near the crack tip, resulting in fatigue and degradation effects [21,22]. It has been observed that 80% to 90% of static stresses and micro-dynamic disturbances can initiate material failure: (1) under dynamic disturbance loading and material properties, crack propagation is time dependent, meaning that crack propagation accelerates as the dynamic disturbance time increases, and the material properties degrade over time; (2) dynamic stress waves can be reflected and refracted inside the concrete structure, which leads to a localized concentration of dynamic stresses, initiating adjacent micro-cracks, thus exacerbating concrete damage.

2.6. Other Factors

Precipitation is generally low in high-altitude areas such as the Qinghai–Tibet plateau. Areas such as Sichuan and Tibet receive less than 600 mm of rainfall, which is significantly lower than in central and eastern China. In addition, the relative humidity in high-altitude regions is also lower than that in the low-altitude eastern areas. Failure to timely maintain concrete in high altitudes can lead to a significant decrease in the rate of the hydration reaction and strength development. With the proliferation of construction projects in these regions in recent years, the challenges of transportation difficulties and the urgent need for raw material have led to problems of pile breakage and segregation in concrete structures. The key to solving these problems is to improve the frequency and standard of raw material quality monitoring to prevent such problems from occurring.

3. Investigation of Concrete Cracking

3.1. On-Site Cracking Investigation

The cracking characteristics of bridge piers in high altitudes are summarized from the on-site tests, as listed in

Table 1. Cracking of on-site piers.

Distribution
Cases
Descriptions	Irregular cracks	Vertical cracks	Horizontal cracks	Arc segment cracking	Honeycomb cracks

. Cracks were mainly found on the rectangular surfaces and circular arc segments of the piers, with most of the cracks occurring on the rectangular surfaces. Cracks were found in a variety of forms including vertical, horizontal, diagonal and irregular, with vertical cracks being the most common and horizontal cracks being less common. Cracks usually appeared about 1.2 m from the bottom of the pier. Cracks were formed by a variety of factors, including temperature shrinkage, dry shrinkage, chemical shrinkage and high-temperature stresses.

3.2. Risk Analysis of Cracking

Cracking in plateau concrete is a process that develops from continuous development to stabilization. Factors such as the temperature, humidity, wind speed and other environmental variables change from year to year at high altitudes. When assessing cracking, these factors must be taken into account. Engineers often assess the crack resistance of concrete by observing the early cracking conditions, especially after demolding. However, the long-term cracking risk of concrete structures is often overlooked, especially in high-altitude environments. To remedy this deficiency, the early cracking risk and long-term cracking risk of concrete in high-altitude environments have been systematically evaluated. Early cracking time is defined as within 60 days after pouring completion, while long-term cracking time is defined as at least one minimum winter temperature cycle. The risk of cracking was investigated using a plateau bridge pier as a case study. A numerical model (21,532 solid 65 units) was developed for the pier with C45 concrete for the lower 1.0 m of the pier and C35 concrete for the upper pier (Figure 7). The bottom of the numerical model was fixed in all directions, and the displacement of the node at x = 0 in the x-direction was fixed. A multi-field coupled shrinkage model with hydration, temperature and humidity constraints [23,24] was used in the numerical simulation process, taking into account the time interval between the pier body and pier cap casting (denoted by the interval age △t), the varying tensile strengths of the concrete σ_t, adiabatic temperature rise (ATR), construction and environment. The risk coefficient for concrete cracking is defined as

$$\eta ={\sigma }_t^T/f_t^T$$

(1)

where ${\sigma }_t$ and $f_t$ denote the tensile stress and tensile strength of concrete at time T, both of which depend on the degree of hydration. When η ≥ 1.0, the risk of concrete cracking is extremely high. When 0.7 < η < 1.0, the risk is high. When η ≤ 0.7, the concrete is unlikely to crack, and the crack-free guarantee is not less than 95% [24].

The early elastic modulus E and tensile strength $f_t$ of concrete are important parameters for determining the shrinkage stress and cracking risk and dependent on the hydration degree α [23,25,26]. The calculation formula is

$$E(\alpha )=E^{\infty }{(\frac{\alpha -{\alpha }_0}{{\alpha }^{\infty }-{\alpha }_0})}^p$$

(2)

$$f_t(\alpha )=f_t^{\infty }{(\frac{\alpha -{\alpha }_0}{{\alpha }^{\infty }-{\alpha }_0})}^q$$

(3)

where E^∞ and f_t^∞ denote the final elastic modulus and final tensile strength of concrete; p and q are exponential constants with values of 0.5 and 1, respectively; and α₀ and α^∞ denote the initial and final values of the degree of hydration α, respectively.

To simulate the maintenance measures after the pouring of a pier, the surface heat dissipation coefficient of the concrete was set to 20 kJ/(m²·h·K). The thermal conductivity coefficient of concrete was set to 1.5 W/(m·K). In assessing the risk of early cracking, the ambient temperature was set at 8 °C, and the concrete pouring temperature was set at 24 °C. In assessing the risk of long-term cracking, the annual temperature variation was taken into account and defined as

T = T₀ + (A/2)cos(π/6(t − t₀))

(4)

where T₀ is the average annual temperature (8.6 °C); A is the annual variation in the temperature (26 °C); t₀ is the highest temperature of the year in a month, usually in mid-July, with a value of 6.5. The annual variation in the temperature of the plateau project site can be seen in Figure 8. More calculation parameters are detailed in

Table 2. Model parameters.

Parameters	C35	C45
Final elastic modulus E∞	32 GPa	36 GPa
Final tensile strength ft∞	2.3 MPa	2.6 MPa
Poisson’s ratio	0.23	0.23
Pouring interval age	7/15/30 d	7/15/30 d
7-day adiabatic temperature rise	43 °C	47 °C
28-day shrink	150 με	200 με

.

Figure 9 displays the stress distribution of the pier body and pier cap at day 63 (positive values represent tensile stress; negative values represent compressive stress). It can be seen that the core tensile stresses of the pier body and pier cap are relatively high at day 63, posing a risk of cracking. However, the surface tensile stress is relatively low, reducing the risk of cracking, since the surface temperature matches the ambient temperature. Therefore, the risk of core cracking in the pier body and pier cap may exist for a long time. Frequently, the core cracking of the pier is often overlooked by engineers. Therefore, a comprehensive analysis of the early and long-term cracking risks of the core concrete of a pier is provided in this study.

Table 3. Effect of tensile strength on the maximum risk of core cracking (ATR = 43 °C, △t = 7 d).

Concrete	σt/MPa	Early Cracking	Long Term
C35	2.0	0.92	1.1
	2.3	0.69 (↓33%)	0.91 (↓21%)

,

Table 4. Effect of adiabatic temperature rise on the maximum risk of core cracking (σ_t = 2.3 MPa, △t = 7 d).

Concrete	Adiabatic Temperature Rise/°C	Early Cracking	Long Term
C35	43	0.69	0.91
	50	0.87 (↑26%)	1 (↑10%)

and

Table 5. Effect of pouring interval age on the maximum risk of core cracking (σt = 2.3 MPa, ATR = 43 °C).

Concrete	Pouring Interval Age/d	Early Cracking	Long Term
C35	7	0.69	0.91
	14	0.93 (↑35%)	1.15 (↑26%)
	30	1.24 (↑79%)	1.35 (↑48%)

lists the cracking risk in the core of the pier body and pier cap, with the higher of the two values indicated. The data indicated that at a tensile strength of 2.3 MPa and an age of 7 days, with an adiabatic temperature rise of 43 °C, the early cracking risk coefficient was less than 0.7. Conversely, with a tensile strength of 2.0 MPa, an adiabatic temperature rise of 50 °C or a pouring interval age of the pier body and pier cap of 14 days or more, the early cracking risk coefficient was greater than 0.7, indicating that the risk of cracking is higher. For an age of 30 days after pouring, the early cracking risk coefficient was 1.24, indicating a very high risk of cracking (Table 5). Additionally, the long-term risk of cracking in the core was higher than in the short term under different working conditions, which was especially noticeable when the pouring interval for the pier body and pier cap was 14 days or 30 days, where the cracking risk coefficients exceeded 1.0 (Table 5).

Increasing the tensile strength from 2 to 2.3 MPa can reduce the risk of early cracking by about 33% and long-term cracking by about 21% (Table 3). There is a positive correlation between adiabatic temperature rise and the risk of cracking, where the lower the temperature rise, the lower the cracking risk (Table 4). Extending the interval age between pier body and pier cap pouring from 7 to 30 days increased the risk of early cracking by 79% and long-term cracking by 48% (Table 5), suggesting that a decrease in the age of this interval was recommended to reduce the risk of concrete cracking. To maintain the early cracking risk coefficient of the concrete core below 0.7 and the long-term cracking risk coefficient below 1.0, it is crucial to improve the concrete mix ratio, control the adiabatic temperature rise and manage the pouring interval age.

4. Cracking Control Measures

More than 80% of cracks in concrete are caused by shrinkage [24,27]. Early shrinkage cracking poses a significant challenge in modern cement concrete engineering. There are two approaches to mitigate shrinkage-induced cracking in cement-based materials: one is to enhance the crack resistance of the material itself, e.g., by adding fibers to increase the tensile strength, and the other is to enhance the volume stability of the materials, e.g., by optimizing the raw materials, using shrinkage-reducing agents or curing agents and incorporating expansion components to reduce various shrinkage factors. In practical application, the crack resistance of concrete can be greatly improved by incorporating admixtures with deformation compensation, together with advanced curing construction techniques; thus, the cracking resistance of concrete can be significantly improved.

4.1. Increase Deformation Compensation

Concrete cracking is closely related to deformation shrinkage. Deformation compensation is an important approach to enhance crack resistance and minimize the risk of cracking [24,27]. Therefore, an anti-cracking agent (HΜΕ^®-V) with deformation compensation properties has also been used to develop an advanced high-altitude deformation compensation and crack control strategy, aiming at minimizing deformation shrinkage and potentially achieving zero shrinkage during the temperature drop stage after concrete pouring. A comparison was made between two site piers: pier #1 without an anti-cracking agent at a height of 7.5 m and pier #2 at a height of 7 m with an added anti-cracking agent. The cement used was Jiahua Low Heat P LH 42.5 with Class II fly ash, with three types of single aggregates with particle sizes of 5–10 mm, 10–20 mm and 16–31.5 mm and a continuous graded crushed stone size of 5–31.5 mm. The Shanxi Jinkeqi high-performance water-reducing agent was also used. The concrete had a slump of about 195 mm in the laboratory and showed good workability during on-site pouring. Details of the mix ratio are shown in

Table 6. Concrete mix ratio (kg/m³).

	Cement	Fly Ash	HΜΕ®-V	Sand	Aggregate	Water
Pier 1#	320	80	0	774	1070	150
Pier 2#	300	68	32	774	1070	150

. Geotextiles and membranes were used to cover the pier body to provide insulation and moisture retention. Additionally, intermittent water sprinkling was conducted for maintenance purposes. A strain monitoring point was placed at each pier body and pier cap core. Two mutually perpendicular rigid chord DY-YT-500B sensors were installed along the length direction (x-direction) and thickness direction (y-direction) to monitor the strain in the pier body and pier cap core. The sensor layout and on-site testing are shown in Figure 10.

Figure 11 shows the deformation monitoring results for the benchmark comparison pier (#1) and the test pier (#1). It is worth noting that the positive strains indicate expansion and compression, while negative strains indicate contraction and tension. The peak deformation of the test pier ranged from 600 to 900 με, indicating that the concrete was in compression (Figure 11b). The peak strain of the reference pier was about 400 με, indicating a significant deformation compensation effect of the anti-cracking agent before the peak strain was reached (Figure 11a). The deformation of the test pier stabilized around 50 days and remained in expansion and compression. Even after 60 days, the deformation still exceeded 200 με. The deformation of the benchmark pier gradually stabilized after about 65 days and transitioned to contraction and tension, with a deformation of less than −100 με. This highlighted the compensatory effect of the anti-cracking agent in the post-peak strain stage. The deformation curves of the test piers performed a relatively smooth pattern without abrupt fluctuations in the data (Figure 11b). The deformation curve of strain sensor #2 in benchmark pier showed a significant local variation around 40 days (Figure 11a), indicating that local cracks may exist in the pier. Concrete has limited tensile stress but can withstand significant compressive stresses. Therefore, anti-cracking agents help to compensate for the deformation and improve the crack resistance by keeping the concrete in a slightly expanded state internally.

The early and long-term cracking risks of core concrete with the addition of anti-cracking agents (HΜΕ^®-V) were analyzed. The results are listed in

Table 7. Effect of tensile strength on the maximum risk of core cracking after adding HΜΕ^®-V (ATR = 43°C, △t = 7 d).

Concrete	σt/MPa	Early Cracking	Long Term
C35-HME	2.0	0.73	0.92
	2.3	0.56 (↓30%)	0.77 (19%)

,

Table 8. Effect of adiabatic temperature rise on the maximum risk of core cracking after adding HΜΕ^®-V (σ_t = 2.3 MPa, △t = 7 d).

Concrete	Adiabatic Temperature Rise/°C	Early Cracking	Long Term
C35-HME	43	0.56	0.77
	50	0.72 (↑28%)	0.88 (↑14%)

and

Table 9. Effect of pouring interval age on the maximum risk of core cracking after adding HΜΕ^®-V (σ_t = 2.3 MPa, ATR = 43 °C).

Concrete	Pouring Interval Age/d	Early Cracking	Long Term
C35-HME	7	0.56	0.77
	14	0.77 (↑37%)	1.0 (↑30%)
	30	1.11(↑98%)	1.25(↑62%)

. The mix proportion was consistent with that of pier #2. Compared with the concrete without adding anti-cracking agents, the risk coefficient for both early and long-term cracking was significantly lower for the concrete with anti-cracking agents. For instance, at an adiabatic temperature rise of 43 °C, pouring interval of 7 days and tensile strength of 2.0 MPa, the risk coefficient of early cracking decreased from 0.92 to 0.73. In terms of long-term cracking, keeping the pouring interval between the pier body and pier cap below 14 days led to a risk of cracking lower than 1.0. Practical experience has demonstrated that maintaining an early cracking risk coefficient below 0.7 and a long-term cracking risk coefficient below 1.0 can effectively control concrete cracking. Therefore, the use of deformation compensation technology in plateau concrete can greatly reduce the risk of cracking.

4.2. Radiation Refrigeration Technology

Using radiation refrigeration technology, the temperature difference between the sunny and shaded sides of the bridge pier body can be controlled, thus mitigating the effect of thermal radiation on the temperature field of the concrete and ultimately improving the durability of the concrete. To verify the technology, concrete specimens with dimensions of 100 mm × 100 mm × 100 mm were prepared and sprayed with radiatively cooled polymer batch on their surrounding surfaces. The specimens without any spraying measures were also prepared. Both specimens were placed in the outdoor environment at the same time. The temperature field variations of the specimens under the two different working conditions were monitored, as shown in Figure 12.

Figure 13 shows the temperature field variations and temperature difference between the sunny and shaded sides of concrete specimens under different working conditions. The maximum temperature of the specimen was decreased by 12 °C after spraying with the radiation-cooled polymer materials compared with the specimen without the radiation-cooled polymer material. The temperature difference between the sunny side and shady side was significantly decreased, with a temperature difference of 5.4 °C for the unsprayed specimens and only 1.7 °C for the specimen the radiation-cooled polymer material. Therefore, the spraying of radiation-cooled polymer materials can effectively reduce the overall temperature of the concrete structure and minimize the temperature difference in different directions, thus reducing the risk of concrete cracking.

4.3. Improve Construction Technology

Figure 14 shows the shrinkage deformation of concrete under different plateau curing methods. During joint curing with insulation and film, the shrinkage strain of the concrete was 150 με and remained relatively constant over time. The shrinkage strain of concrete after 40 days in natural conditions exceeded 400 με, an increase of 167%. As mentioned earlier, shrinkage was a significant factor contributing to concrete cracking. Effective maintenance practices can greatly reduce this risk. Delayed curing of freshly poured concrete can lead to the rapid evaporation of water, preventing the full hydration of cement particles and formation of stable crystals. This inadequate hydration led to insufficient cohesion, resulting in spalling of the concrete surface. By controlling the proportion of cement clinker components, continuously optimizing the concrete mix proportion and implementing strategies such as increasing cooling water pipes, lowering the molding temperature and mixing mineral powder and silicon powder, the release of the hydration heat was controlled, and the serviceability of concrete was improved. These measures ultimately improved the quality and crack resistance of the concrete. The specific construction process control measures are outlined in

Table 10. Construction process control measures for concrete.

No.	Strategy	Stands
1	Water	Low-temperature water prepared using groundwater or cold-water machine.
2	Cement	With 3 d hydration heat ≤ 230 kJ/kg, 7 d hydration heat ≤ 260 kJ/kg, C2S ≥ 40% and C3A ≤ 6% [28].
3	Mixing time	The mixing time of each concrete mixer should not be less than 120 s.
4	Transportation	Concrete transport vehicles should have insulation or thermal insulation measures; the transportation time should not exceed 60 min.
5	Pumping	① The concrete molding temperature falls within the range of 5–25 °C. ② The pumping temperature should be lowered, or construction should be conducted during low temperature seasons. This can also mitigate the risk of long-term concrete cracking.
6	Pouring and vibration	① In cases where there is a substantial loss of slump that fails to meet the requirements, it can be rectified by adding a suitable amount of water-reducing agent twice, with direct water addition being strictly prohibited. ② Concrete should be poured continuously in layers, with each layer having a thickness of 30–50 cm. The time between layers should not exceed the initial setting time of the concrete. ③ The thickness of vibration pouring ranged from 30–50 cm, and the vibration time was determined based on the factors of concrete surface bleeding, absence of significant sinking and no appearance of bubbles.
7	Demolding and maintenance	① Concrete formwork should be removed during periods of high temperatures. After the removal of concrete formwork, a water energy film should be applied and covered with a minimum of two layers of geotextiles with plastic lining. ② In conditions of strong winds, rainy days and sudden temperature drops, it was recommended to extend the demolding time to prevent cold cracking. ③ The cooling rate of concrete should be ≤2.0 °C/d, and the temperature difference between the inner and outer surfaces should be maintained at ≤ 20 °C. ④ When mold curing was adopted, it was recommended to embed insulation materials such as a rubber sponge and XPS board in the steel formwork, and to wrap windproof tarpaulin around the exterior of the formwork. Additionally, post-dismantling of the formwork, it was recommended to continue long-term insulation and moisturizing curing. ⑤ The temperature difference between the curing water and the concrete surface should be less than 15 °C.

.

4.4. Improve Management Measures

Careful management is necessary to improve the crack resistance of concrete. Essential elements such as well-equipped laboratories, efficient mixing plants and effective site management are essential to maintain quality control of concrete. Moreover, proper secondary finishing techniques can help to minimize the formation of concrete micro-cracks. Usually, the use of an intelligent control system (MES) has proved beneficial in the production of concrete for highland projects. The system allows for precise adherence to proportions during automatic discharging, controls the water–cement ratio and ensures the overall quality of the concrete, such as the strength, slump and workability. Therefore, the workability of the concrete placed on site has been significantly improved, improving the crack resistance and overall quality of the piers after placement.

5. Conclusions

The challenges faced in concrete construction in plateau areas on the Sichuan–Tibet route were investigated. The cracking characteristics and early and long-term cracking risks of the plateau pier using a multi-field coupled shrinkage model were analyzed. Furthermore, crack control technologies such as deformation compensation and radiation cooling for plateau concrete were discussed. The main conclusions are outlined as follows.

(1)

Various factors such as the temperature, wind speed, freeze–thaw cycles, air pressure, lighting conditions, temperature differentials between sunny and shady sides and dynamic disturbances significantly affected the durability and service performance of the plateau concrete. These factors usually led to different types of cracking in the vertical, longitudinal and transverse directions of the bridge piers. Vertical cracks in the straight section of the pier body are the most common type of crack.

(2)

The risk of concrete cracking in the core of a plateau pier body was influenced by factors such as the tensile strength of the concrete, pouring interval age and insulation temperature rise. The long-term cracking risk of the core of the plateau pier was significantly higher compared with the early cracking risk. To mitigate this risk, it is necessary to optimize the concrete mix ratio, regulate the insulation temperature rise and carefully manage the pouring interval age. Through these measures, it is possible to ensure that the early cracking risk coefficient for pier concrete remains below 0.7 and that the long-term cracking risk coefficient does not exceed 1.0.

(3)

Deformation compensation can significantly enhance the peak and residual deformation capacities of plateau concrete, with peak values greater than 900 με and residual deformation greater than 200 με at day 60, which greatly reduced the possibility of early and long-term cracking of plateau concrete.

(4)

The implementation of radiation cooling technologies, advances in construction techniques and effective management practices can help to mitigate cracking in high-altitude concrete. However, further research is needed to fully understand the effects of air pressure, high-frequency freeze–thaw cycles and other factors on the degradation of plateau concrete.