What are the applications of prestressed concrete in engineering?

Using prestressed concrete structures does not save steel, but it can improve the structural performance and address technical issues that are difficult to solve with other structural materials. Prestressed concrete is widely used in various engineering fields such as building construction, railways, highways, energy, water resources, electricity, communications, and marine structures. Practice has shown that there are hardly any structural problems that cannot be solved through prestressing, and prestressing often results in structures that are thoroughly innovative.

I. The Role of Prestress in Concrete Structures 　　 (1) What is Prestress? 　　 Prestress is short for pre-applied stress. Although this term is relatively new, having only been around for a few decades, the application of the principles of prestress dates back to ancient times; it is easy to find familiar examples of this in everyday life if one pays close attention. 　　 Wooden basins and barrels are typical examples. Wooden basins and barrels reinforced with bamboo rings, such as washbasins, laundry basins, bathtubs, and water buckets, have been used in daily life in our country for thousands of years. When the bamboo hoops are tightened, they experience tensile stress due to elongation, while the barrel walls made of wooden planks experience circumferential compressive stress. If the compressive stress applied in advance between the gaps of the wooden planks exceeds the tensile stress caused by water pressure, wooden basins and barrels will not crack or leak. The principle behind the manufacturing of such wooden basins and barrels is exactly the same as that of modern prestressed concrete circular pools. This is a typical example of using pre-compressive stress to resist the tensile stress that is expected to occur. 　　 A wooden saw is another familiar example. As the saw blade moves back and forth to cut wood, one part of the blade is under tension while another part is under compression. This thin and narrow saw blade does not possess much compressive strength on its own, but due to the pre-tension applied by the tightly tied rope, when this pre-tension exceeds the compressive stress generated during sawing, the blade remains in a tensile state, thereby preventing buckling and unstable failure. This is a typical example of using pre-tensile stress to counteract the compressive stress that occurs during use. 　　 Similar examples can be cited as well, such as the brick grippers used at construction sites to lift red bricks, which can hold 5 bricks at a time, and the spokes of bicycle wheels. All these examples make use of the principles and techniques of prestressing; prestressed forces can be employed to enhance a structure’s tensile and bending resistance, as well as its compressive strength. Therefore, with proper application, pre-stressing can be used to improve the service performance of structures and enhance their strength. 　　 (II) Why is prestressing required for concrete? Concrete is a structural material with high compressive strength but low tensile strength. Its tensile strength is not only very low, at only 1/15 to 1/10 of its compressive strength, but it is also highly unreliable. Its tensile deformation capacity is also very low; it is brittle like glass, with no obvious signs before failure. Therefore, plain concrete can only be used in applications that are primarily subject to compression, such as column footings, gravity retaining walls, floors, and road surfaces, and cannot be used in members and structures that are subject to bending, such as beams and slabs. 　　 To overcome the drawback of low tensile strength in concrete, the first idea that comes to mind is to reinforce the areas of concrete where tensile stress is expected to occur by using rebar, that is, to use rebar to take on the tensile forces instead of concrete. Concrete, which uses compression in the concrete and tension in the steel bars, has a wide range of applications and many advantages, but it also has an inherent and difficult-to-overcome flaw: cracking. All reinforced concrete flexural and tensile members, whether with little or much reinforcement, almost always crack under service conditions, which limits their range of application and future development. The problems caused by cracking are mainly reflected in the following three aspects: (1) It is not suitable for structures that require leak resistance, nor can it be used in structures exposed to aggressive media. 　　 (2) It deviates from the general trend of structural materials moving toward higher strength and lower weight. Since the width of cracks in concrete is generally proportional to the stress (strain) exerted by the rebar, the average crack width for grade I and II rebar under normal conditions is already 0.2 mm (with a stress of around 180 MPa). Using rebar with higher strength will result in wider cracks, and increasing the strength grade of the concrete also does not help; as a result, it is difficult to take advantage of the strengths of high-strength steel and high-strength concrete. 　　 (3) The structural self-weight is too large; when used in floor or roof structures, this results in large cross-sectional dimensions, and the self-weight load of beams and slabs becomes very substantial, often accounting for more than half of the total load. Therefore, it is not suitable for modern engineering projects such as long-span structures and high-rise building structures. 　　 Conventional structural techniques are ineffective due to cracking and the defects it causes. An obviously effective method is prestressing, that is, artificially applying compressive stress in those parts of the structure where tensile stress is expected to occur, in order to counteract or reduce the tensile stress that the concrete is likely to experience. Depending on the requirements of the structural service conditions, the magnitude of the pre-stress can be adjusted to ensure that the concrete is not subjected to tension, does not crack, or that the width of any cracks is controlled. This overcomes the defects of low tensile strength in concrete and the tendency of reinforced concrete to crack. Although there are various methods for applying prestress, it is generally achieved by tensioning high-strength prestressed bars. The purpose of this tensioning is to put the concrete under compression; thus, the strength of high-strength steel and concrete can be fully utilized, which enables reduction of the concrete cross-section, lowering of the structure’s weight, and expansion of its applicable range. It can be seen that this type of concrete with prestressed high-strength steel is a new variant within the category of reinforced concrete; it essentially overcomes the main disadvantages of both plain concrete and reinforced concrete, representing an optimization, advancement, and development of reinforced concrete.

Summary: Due to their small cross-section, high stiffness, good crack resistance, and durability, prestressed concrete structures are widely used in the field of civil engineering around the world. In recent years, the emergence of high-strength steel and high-strength concrete has promoted the development of prestressed concrete structures, and further contributed to the refinement and improvement of prestressed concrete construction techniques.