Preface i About the Author v
1 Introduction 1
1.1 The Significance of Thermal Stress in Mass Concrete 1
1.2 The Features of Thermal Stresses in Concrete Structures 3
1.3 The Variation of Temperature and Thermal Stress of Mass Concrete with Time 4
1.3.1 The Variation of Temperature of Mass Concrete with Time 4
1.3.2 The Variation of the Thermal Stress in Mass Concrete 5
1.4 Kinds of Thermal Stress 6
1.5 Analysis of Thermal Stress of a Massive Concrete Structure 6
1.6 Thermal Stress—The Cause of Crack 7
1.7 Technical Measures for Control of Thermal Stress and Prevention of Cracking 8
1.8 The Experience of the Temperature Control and Crack Prevention of Mass Concrete in the Last 30 Years 10
2 Conduction of Heat in Mass Concrete, Boundary Conditions, and Methods of Solution 11
2.1 Differential Equation of Heat Conduction, Initial and Boundary Conditions 11
2.1.1 Differential Equation of Heat Conduction 11
2.1.2 Initial Condition 12
2.1.3 Boundary Conditions 13
2.1.4 The Approximate Treatment of the Third Kind of Boundary Condition 14
2.2 Surface Conductance and Computation of Superficial Thermal Insulation 16
The cracking of massive concrete structures due to thermal stresses is a problem which had puzzled engineers for a long time. “No dam without crack” is the actual state of concrete dams in the world. The theory of thermal stress and temperature control of mass concrete is established by the writer in this book under the direc-tion of which the problem of cracking of massive concrete structures had been solved, and several concrete dams without crack have been successfully constructed in China in recent years which indicates that the history of “No dam without crack” has ended.
Mass concrete is important for the economical construction of a country. For example, more than 10 million cubic meters of mass concrete are placed in the hydraulic engineering projects in China every year. In addition, a large amount of mass concrete is placed every year in the engineering of harbors, foundation of high buildings heavy machines, nuclear reactors, etc.
The thickness of a massive concrete structure is immense, e.g., the thickness of a concrete dam may be 100-200 m, the depth of the region under tension may be 10-30 m; if all the tensile stresses are undertaken by steel reinforcement, the amount of steel will be considerable, and the cost will be very high. In the process of construction, if there are many vertical steel reinforcements on the top of a con-crete block, the spreading and placing of the new concrete lift will be very difficult. Thus in the design of massive concrete structures, such as concrete dams, generally it is required that the tensile stresses do not exceed the allowable tensile stress of concrete so that no steel reinforcement is used. If there are only concrete weight and water pressure acting on the dam, the above-mentioned requirement is easy to achieve, but the period of construction of a high concrete dam may be several years. Due to the heat of hydration of cement and the variation of the ambient tem-perature, large tensile stresses may appear in the massive concrete structure. As a result, cracks developed in almost all the concrete dams.
The concrete dams are divided into blocks and each block is constructed in horizontal lifts with thickness 1-3 m. The intermissions between two lifts are 5-10 days. As the mechanical and thermal properties of concrete vary with age and have different values in different layers, so the computing of thermal stresses in concrete dams is rather complicated. In the past, there were no methods to com-pute the thermal stresses in the period of construction of concrete dams, although some temperature control measures had been adopted, but the thermal stresses in the dam are unknown. Actually the tensile stresses are so large that many cracks developed in almost all the dams.
Now a perfect system of the theory of thermal stress and temperature control is established by the writer in this book which includes the following parts:
A series of methods for computing the temperature field and the thermal stress field, especially the simulation method for computing the temperature field and stress field of the structure taking account of the influences of all the factors including (a) the process of construction, (b) the mechanical and thermal properties varying with the age of con-crete, (c) the variation of ambient air and water temperature, (d) the various measures of temperature control.
The law of variation and peculiarity of thermal stresses of different types of massive con-crete structures, such as gravity dams, arch dams, buttress dams, concrete blocks, locks, sluices, concrete beams on elastic foundations, concrete pipes, and concrete linings of tunnels. Understanding these issues by engineers is favorable for the construction of mas-sive concrete structures without crack.
Various technical measures to prevent cracking of mass concrete, such as choice of raw materials, precooling, pipe cooling, and superficial thermal insulation.
The experiences of many practical concrete dams, particularly the success of the con-struction of several concrete dams without crack in China in recent years.
Many new ideas and new methods for prevention of cracking and temperature control of mass concrete.
Comprehensive analysis of different schemes of construction of concrete dams with dif-ferent combinations of the measures of temperature control.
In the design stage of a massive concrete structure, several schemes of temperature control may be given and computed the temperature field and stress field in detail by the methods given in this book, after comprehensive analysis, a rational scheme may be obtained. Otherwise, a new scheme with improved combination of temperature control may be given and analyzed, until a good scheme of temperature control is obtained which will lead to the possibility that there will be no crack in the dam in the construction and operation period. By this method, several concrete dams without crack have been constructed in China in recent years. This is an important and valu-able experience in the construction of massive concrete structures.