Current Situation and Development of Prestressed Concrete Bridges
Prestressed concrete is an urgent need to resume the war after World War II wounds, rapidly developed from Western Europe. Than half a century, from theory, materials, application technology to civil engineering, have made great development. Especially with the PPC concept matures, broke the tension and cracking of concrete can not be constrained, greatly expanded its range of applications. Prestressed concrete has now become a major domestic and international civil engineering structural material, and prestressed technology has been applied to the expansion of steel, brick, stone, wood and other structural materials, and to deal with structural design and construction using conventional techniques difficult to solve various difficult problems. Prestressed concrete in China starting later than in Western Europe about 10 years, but the rapid development of the large number of applications. Investment in China in recent years in civil engineering, construction and scope from top in the world. In concrete engineering, prestressing technology has made great progress. Recent decades, China has developed rapidly in prestressed concrete bridges, regardless of the bridge type, span and construction methods and technology are a breakthrough, a number of prestressed concrete bridge construction technology has reached the international advanced level. This article focuses on its composition and properties of materials Status and Prospects of prestressed concrete.
China has built from prestressed concrete bridge of view, most have adopted 40 to 50 concrete, and then prepared using water-reducing agent such as plastic additives, concrete, and developed concrete pumping process. With the increase of bridge span, the bridge structure to reduce weight, high strength concrete gradually, light direction. Japan early 70s were built by several concrete span 80 to 45 simply supported prestressed concrete railway bridge in Germany in the main span of 136 rich Lingle bridge using lightweight concrete. China's current high-strength, lightweight concrete has been some success. Such as the construction of a large Buddhist temple in Chongqing Yangtze River Bridge, is a main span of 450 meters of the tower two-Prestressed Concrete Cable-Stayed Bridge. Chongqing Yangtze River Bridge by the large temples loyalty test successfully developed the first 60 micro silica fume high-strength concrete used in pouring the bridge girder. Modified materials as concrete, micro silica fume high-strength concrete are easy to cast, the overall dense, long-term stability and high strength characteristics, can improve the intrinsic quality of construction, the bridge construction market has great application value.
Currently used mainly high-strength prestressed steel wire strand and High Strength Reinforcement three categories. The use of prestressed steel bridge has been moving in high-strength, low relaxation, large diameter direction. Before the mid-80s, the performance of China's PC steel more than the international community behind the past 20 years, gradually narrowing the gap. Prestressed steel production process of the plant's continuous improvement and a better performance, more economical materials. To improve efficiency, in recent years, strength of materials has increased, but in some cases, the intensity of growth is lower ductility and toughness of the material cost. Prestressed high strength steel, and sometimes increase the risk of hydrogen stress corrosion. These features should be seriously detrimental. New materials such as fiber reinforced plastics, in the past mainly for aerospace and aviation industry, has now entered the construction industry. Using these materials are mainly due to the following benefits: in a variety of environments with the characteristics of durability and corrosion resistance, light weight, high strength and non-magnetic and other properties. Prestressed fiber reinforced plastic can be used with non-prestressed material. These materials have linear elastic stress - strain relationship, until the snap. Their performance and the performance of reinforced and prestressed steel different, but also a new design. Since 1939, first in France and Belgium's first system-type system, the PC technology from the pre-tensioned to the progress of post-tensioned, for a variety of large prestressed structure opened the way for the development. Anchorage and the corresponding anchoring tendons, into crude steel anchor, steel wire strand beam anchor and anchor in Class 3. In recent years, the group used to anchor strand anchorage system is widely used. As to continuously improve the quality of its anchor performance is also better. When used as needed by the form of multiple strand bundle, the whole bundle tension, the domestic has grown to 1,200. Large-tonnage prestressed steel beam greatly simplifies the use of post-tensioning process. Cantilever construction for the bridge with each cycle the number of prestressed beams can be significantly reduced, and prestressed beam through the flat curve to the anchor position in the section on the layout fixed, significant savings through beam, tensioning, grouting and other processes The length of time, thus speeding up the construction progress. Another use of large-tonnage prestressed beam, cloth beam is easy, after a reasonable choice can be done after the cloth beam and the increase due to difficult structural dimension, resulting in material waste, can reduce the complexity of the anchor tooth block, easy to simplify the template to speed up construction period. Unbonded tendons is coated with anti-rust lubricant post-tensioned tendons, the construction of these tendons can be installed directly in reinforced as the template. Unbonded tendons without Hole for, post through beam, the processes of grouting material can be reduced to speed up the construction progress. Therefore it is simple in construction, construction and high efficiency. But its strength and stiffness and corresponding lower compared with tendons. From the durability of view, should seriously deal with its rust and anchor terminated. Bonded prestressed grouting problem because there are durability issues, there are often prestressed grouting of compaction grouting dissatisfaction or problems, which may lead to corrosion of prestressing tendon can not be ignored. Unbonded Tendons in China in the application of long-span bridges is increasing. Unbonded tendons because of its advantages will be more and more attention, but on its strength and durability issues still need further research, and constantly improve. Tendons in prestressed concrete structures used in the construction industry is the recent development directions. Manner in vitro construction of prestressed concrete bridge in the international arena for nearly 90 years. But early due to inadequate anti-corrosion technology, because of higher costs to obtain the results are unsatisfactory. But since the 80's, due to technological progress, external prestressing technology improved several times, becoming more perfect, its use is also increasing. Increase from the pre-stress way of view, it is most of the prestressed steel beam cross section arranged in concrete outside, through the anchoring end and change to the device to deliver pre-applied stress. The method not only can be applied to new structures can also be used to reinforce the original structure. PC use in the early prestressing tendons have been used in bridge construction, however, due to technical conditions at that time constraints, this method in the 20th century, almost 50 years people gave up. Corrosion-resistant (fiber reinforced plastic) cable, high-performance cable and cable protection systems in vitro development of in vitro re-emergence of PC technology to provide favorable conditions. Use of external prestressing technology bridge project Melamine Based Plasticizer has the following advantages: 1) Since the board is not installed within the pipe, reduces the thickness of the plate, thus reducing the weight of the bridge; 2) prestressed cable easy to install; 3) easy to check the prestressed cable conducive to the conservation of cable; 4) prestressed cable Polycarboxylate superplasticizer
replacement or re-tension as possible; 5) significantly shorten the construction period, particularly in sub-assembly method using precast bridge construction. Prestressed Concrete Bridge technology is widely used in construction. And has been used for highway and bridge construction precast viaduct section. Another great potential prestressing technique is the use of the original concrete structure reinforcement and repair. In recent years, the technology has been used in many new structures, including: a large set of asymmetric structure with external prestressing cable in order to improve the mechanical properties of the structure, can be used by the concrete flange and the corrugated steel webs constitute the composite structure among the use of high-performance lightweight materials to reduce the structural weight.Sodium Naphthalene Formaldehyde