Scholarly article on topic 'Application Research on the Temperature Control and Crack Prevention of the Large-scale Aqueduct in China's South-to-North Water Diversion Project'

Application Research on the Temperature Control and Crack Prevention of the Large-scale Aqueduct in China's South-to-North Water Diversion Project Academic research paper on "Materials engineering"

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Procedia Engineering
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{"Large-scale Aqueduct" / "Temperature Control" / "Crack Prevention" / "Structural Optimization" / "Development Tendency"}

Abstract of research paper on Materials engineering, author of scientific article — Zheng Chongyang, Peng Hui, Liu Shaolin, Zhang Chao

Abstract A lot of investigations and practices indicate that temperature control has a positive effect on preventing and limiting concrete cracking. This paper summarizes some experience from the construction of China's South-to-North Water Diversion Project in the aspects of material optimization, temperature control measures in the construction period, surface maintenance in post-construction period and so on. At the same time, it attempts to predict the development tendency and provides some relative suggestions for the temperature control of aqueduct, which mainly comes from some new perspectives of the optimization of the material mix proportion, structural design for limiting cracking, concrete temperature monitoring and prediction, information sharing platform of temperature control and so on.

Academic research paper on topic "Application Research on the Temperature Control and Crack Prevention of the Large-scale Aqueduct in China's South-to-North Water Diversion Project"

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Procedía Engineering 28 (2012) 635 - 639

Procedía Engineering

www.elsevier.com/Iocate/procedia

2012 International Conference on Modern Hydraulic Engineering

Application Research on the Temperature Control and Crack Prevention of the Large-scale Aqueduct in China's South-to-North Water Diversion Project

ZHENG Chongyang, PENG Huia, LIU Shaolin, ZHANG Chao, a*

College of Hydraulic & Environmental Engineering of China Three Gorges University,Yichang 443002,China

Abstract

A lot of investigations and practices indicate that temperature control has a positive effect on preventing and limiting concrete cracking. This paper summarizes some experience from the construction of China's South-to-North Water Diversion Project in the aspects of material optimization, temperature control measures in the construction period, surface maintenance in post-construction period and so on. At the same time, it attempts to predict the development tendency and provides some relative suggestions for the temperature control of aqueduct, which mainly comes from some new perspectives of the optimization of the material mix proportion, structural design for limiting cracking, concrete temperature monitoring and prediction, information sharing platform of temperature control and so on. © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Society for Resources, 'Environment and Engineering

Keywords: Large-scale Aqueduct; Temperature Control; Crack Prevention; Structural Optimization; Development Tendency

1. Introduction

China's South-to-North Water Diversion Project is a world famous inter-basin water transfer project, which plays a significant role in the optimal allocation of water resources in China. A large number of large-scale prestressed concrete aqueducts are needed, since there are a lot of rivers and valleys needed to conquer for the water transferring in the path from the South to the North. Generally, the operation condition of aqueduct is complex. The forces it bears are not only from the effects of repeated water load, but also from the impact of temperature variation, freezing-melting circulation, alternation of wetting and drying, water erosion, wind and hurricane and etc.. The results of thermal stress and shrinkage distortion

* Corresponding author: PENG Hui. Tel: 13207203616. E-mail address: hpeng1976@163.com.

1877-7058 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Society for Resources, Environment and Engineering doi:10.1016/j.proeng.2012.01.782

lead to cracking in the aqueduct. However, as a water-carrying structure, aqueduct is constant under the effects of the dynamic load. Once a penetrating crack grows, the durability and safety of the structure will be reduced, more seriously, the transported water may be leaked [1].

The construction practices show that the temperature stress can foster the formation and development of the cracks and it is very necessary to control the temperature properly to prevent and limit the cracks. Based on the practices of temperature control and crack prevention of the aqueduct in the middle-line of China's South-to-North Water Diversion Project [2], this paper predicts the technological development trend of temperature control and crack prevention of the aqueduct with an aim of offering a reference for the subsequent project in this aspect.

2. Basic Experience of Temperature Control and Crack Prevention

China's South-to-North Water Diversion Project, initiated in 1950s, is the biggest water conveyance project all over the world. It has to cross a large amount of rivers due to the long distance; therefore, there are technological breakthroughs in the aspects of both amount and scale. Many countries accumulate a wealth of experience in the aspects of material optimization, design, construction measures, management and etc. Here is a brief introduction of the achievements and experience in temperature control and crack prevention in large-scale aqueducts of China's South-to-North Water Diversion Project.

2.1. Optimization of the Concrete Material

The large-scale aqueduct in the South-to-North Water Transfer Project operating in very harsh environment has the characteristics of large flow and large span. These lead to the higher demands for the concrete material which is expected to have the good quality of cracking prevention, frost resistance, impermeability and durability for guarding against alkali-aggregate reaction. Recently, C50 high performance and high strength pumping concrete has been mainly used. Because of the large shrinkage of this kind material, it is usually mixed with concrete water reducer, air-entraining agent and coal ash to reduce the usage amount of water and cement. Lower water cement ratio will cause lower heat of hydration, which is a good way for decreasing the concrete shrinkage and improving the cracking resistance.

The experimental result from State Key Laboratory of Silicate Materials for Architectures of Wuhan University of Technology demonstrates, by adding proper polypropylene fiber, the compressive strength, flexural strength and splitting tensile strength of the 28d concrete will increase 6.5%, 20.8%, 24.3% respectively. The elastic modulus and dry shrinkage decline. The impermeability and freezing resistance (durability) can be enhanced [1]. An investigation made by Li Xinggui from Hohai University is about an important problem of shrinkage deformation of the aqueduct. It manifests that appropriate water binder ratio and right amount of fly ash are greatly helpful to decline the shrinkage deformation and the possible shrinkage cracking, especially in the early stage.

2.2. Measures of Temperature Control ic Construction Period

2.2.1. The Temperature Control of Concrete Pouring

The temperature of concrete pouring varies with seasons changing. Also it is closely related to the temperature of concrete outlet, which is 3~4°C lower than it. Experiments show that the main factors that influence the temperature of outlet are the temperature of cement and aggregates. Here some main methods for the temperature control in the summer are listed as follows: storing the cement in advance for the purpose of heat dissipation and cooling; As for the fine aggregate (sand aggregate) , it should be

cooled in closed condition for the fear of overexposure to the sun, applying a spraying cooling fan for decreasing the temperature if necessary; about the coarse aggregate, apart from sprayed 4~6°C water in closed state, at the same time spray blower fans are needed to decrease the temperature in order to form an artificial micro-climate; finally, the pre-cooling concrete can be produced by adding cold water[3]. Moreover, the transportation distance should be shortened as much as possible. The heat insulation measures can be taken to reduce solar radiation during transportation period. In order to avoid the temperature recovery, the material should be stored as soon as possible after arriving at the destination. It is suggestive that the lowest temperature for the concrete pouring in winter shouldn't be less than 5 C.Generally, the concrete pouring should avoid the period of the daily top temperature. During the period of stronger solar radiation of the day, it is reasonable to spray water droplets on the surface of the warehouse to decrease the environmental temperature. Spraying is the top priority for temperature control, especially for the aqueduct body convolved with dense steel [3].

2.2.2. Pipe Cooling Technique

Pipe cooling is one of the most common and effective measure for temperature control and cracking prevention for the mass concrete, which is applied nowadays in the construction of aqueduct, pier and other thin-walled structures. The experimental result shows that this method has an apparent effect on thermal conducting and cooling through burying the pipes inside concrete to reduce the temperature difference between the inside and the surface for the purpose of lessening the stress and preventing cracking[4]. But the water temperature, cooling rate and time for water delivery should be controlled strictly when applying the pipe cooling. It may emerge penetrating cracks if the water temperature is too low, the cooling is too fast or the time for water delivery is too long [2]. Movable cold water station has the advantages of increasing the recycling rate of cooling water, economizing the construction cost, saving the construction time and realizing personalized water delivery. Therefore, it is worthy of popularization and application for its greater flexibility in field operation and lower cost [5].

2.3. Surface Maintenance in Post-construction Period

Surface maintenance includes measures of both heat insulation and moisture preservation. The measure taken in the Caohe River aqueduct project is pasting heat insulation board of 1.0~2.0cm thick on the outside of the concrete template for reducing the temperature difference and tensile stress of the surface of the concrete. In this way, it can not only prevent the earlier crack effectively but also can get rid of the surface shrinkage caused by temperature difference between day and night and cold wave. However, form removal can give rise to dramatic change of the surface temperature and tensile stress, which is as strong as the attack of cold wave. Thus, in the season with large temperature difference between day and night, prolonging the removal period (7~9d suggested) will reduce the cracks caused by the temperature difference of the concrete surface [6]. Moisture preservation on the concrete surface can prevent the desiccation fissure effectively. But the choosing of water temperature should be taken into consideration seriously. If it is too low, it will give a cold shock to the concrete which will lead to the surface cracks. One of the national scientific and technological achievements applied by Hebei Provincial Bureau of Water Conservancy Project adopts the construction techniques innovatively including burying the cooling pipe to reduce internal temperature, steaming in the shelter to lift the external temperature and testing analysis of the temperature and so on. These methods can decrease the temperature difference between the inside and outside, which has an obvious effect of crack prevention.

3. Development Tendency of Temperature Control and Crack Prevention

Combining the basic experience summarized and analyzed above with the practice of concrete dam and mass concrete structures in industrial and civil construction, this part attempts to predict the development trend and offer some suggestions for the follow-up project in the aspect of temperature control.

3.1. Optimization of the Mixture of Material

Due to the complicated operating environment and the required design life span of 150 years of the large-scale aqueduct in South-to-North Water Diversion Project, higher demands are required for the construction materials. Therefore, it is crucial to further enhance the optimization of material mix in order to improve concrete strength and control the temperature flexibly, such as adopting low-heat cement to retard the concrete temperature and exploiting the concrete with the quality of high cracking resistance, impermeability, frost resistance and durability.

3.2. Reasonable Design for Limiting Cracking

C50 high performance concrete is widely used in the construction of the aqueduct. Its temperature will increase greatly in the construction process, therefore it is quite hard to realize cracking prevention completely. Also the technical feasibility on the worksite may be limited. The newest Code for Construction of Mass Concrete, which complies with the principle of economy, reasonableness, safety and applicability, stipulates that the target of the concrete temperature control is to prevent the emergence of dangerous cracks. At the same time, according to the construction methods of concrete, the designers can collocate constructional reinforcing steel which can withstand the temperature stress and shrinkage stress caused by hydration heat of cement and shrinkage. By reducing the crack spacing, constructional reinforcing steel can keep the crack width under the range of safety and applicability. A report put forward by Japan Society of Civil Engineers indicates that the higher the collocated rate of reinforcing steel is, the better effect of the crack limiting is. Therefore, the aim of crack limiting can be reached by temperature stress simulating in the construction stage, designing for limiting cracking, fully considering the temperature effect and allocating enough temperature steel.

3.3. Reinforcement of Temperature Monitoring and Prediction for the Concrete

The experience from the South-to-North Water Transfer Project shows that temperature monitoring in the construction period plays a positive role in temperature control and crack prevention. The distributed optical fiber temperature measurement system has its own unique technical advantages, such as large quantity of data information, high sensitivity, real-time monitoring, auto warning and remote automation control and so on. In recent years it has been widely used in water delivery and hydropower engineering for dam concrete monitoring, seepage position monitoring, crack monitoring and so on[7,8]. Optical fiber temperature measurement system can monitor the process of early temperature change and distribution of the concrete. However, because the spatial resolution of this system normally is 1m, the fiber can only lay along the aqueduct vertically, while electronic thermometers are still used in cross section. In other words, this is a kind of integrated measurement by applying both the optical fiber and the thermometers [2].

Using the results of temperature measurement to inversely calculate the parameters of concrete's thermal properties can realize the full process simulation of the temperature field and stress field more easily. In the phase of aqueduct's construction, the stress field and security coefficient of the later phase can be predicted according to the actual stress field and pre-established construction plan to formulate the corresponding integrated control measures and construction scheme, which can be applied to conduct the

concrete pouring. While in the operation stage, the engineer should make safety evaluation which is expected to be in accordance with the reality by making the best use of observation data under various operating conditions.

3.4. Establishment of an Information Sharing Platform for Temperature Control

It is reasonable to establishing an information sharing platform for temperature control of aqueduct in case that the specific temperature control measurement is inconsistent with the practical operation. As a good way to realize information sharing, it provides a better reference and basis of decision-making for the design institutes, construction companies and operation organizations. The execution sequence in this platform is as follows: optical fiber temperature measurement, simulation prediction, assessment and early warning of cracking risk, information sharing platform. Currently, this information sharing platform for temperature control has been successfully put into use in the construction period of Xiluodu double curvature arch dam project successfully.

4. Conclusion

The practical results show that the integrated use of all the measures mentioned above can reduce the temperature difference between interior and exterior of concrete effectively. It has an obvious effect on preventing crack, which is worthy of popularizing. However, the measures for crack prevention change as different projects and operation condition. Therefore, it is still necessary to research and improve the techniques in this aspect. The development trend and suggestions for temperature control and crack prevention this paper has predicted will possess reference significance for the construction of future aqueducts and similar projects.

References

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[3] Jia Zhiying. Application of Temperature Control Measures on High Performance Pumped-concrete to IE bid Section of Caohe Aqueduct [J].Water Resources and Hydropower Engineering, 2009, 40(1):82-83, 93.

[4] Zhu Yueming, Xu Zhiqing, Cao Weimin, et al. Analysis of Water-pipe Cooling in Thin-walled Concrete Structures[J].Engineering Mechanics, 2004, 21(5):182-187.

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