My Understanding of Prestressed Construction
Pay attention to the reinforcement and concrete behind the anchor plate
Many construction and supervision personnel only pay attention to the middle part of the component, but ignore the reinforced steel bar and concrete behind the anchor plate at the end of the component. Considering the pressure on the beam end during tension and the transmission of stress, the designed reinforcement is densified at the end of the component and equipped with spiral reinforcement behind the anchor plate. However, in the actual construction process, due to the dense reinforcement of the place, the spiral reinforcement is difficult to put in, so the construction staff casually put the spiral reinforcement on the corrugated duct, some even did not put it, and the reinforced steel bars are not placed according to the design drawings. In addition, the concrete is difficult to pour in because of the concentrated anchor plate, spiral reinforcement, dense steel bars, and honeycombs and holes that are often formed behind the anchor plate. These are very serious quality risks. Therefore, my requirement for all construction personnel is to ensure the position of the spiral reinforcement and the dense insertion reinforcement. If the spiral reinforcement is indeed not in place, it should be adopted by inserting multiple reinforcements.
"Dual control" during tension
When tensioning with prestressed equipment, the “dual control” is used, that is, the control of tension force and elongation. When the tension reaches the design force, see whether the error between the actual elongation and the theoretical is within the allowable range, if it exceeds the range, the reasons must be analyzed until the requirements are met. According to my many years of practical experience, if the actual elongation exceeds the theoretical elongation, it may be caused by an error in the initial elongation; if the actual elongation is smaller than the theoretical, it may be caused by the large frictional resistance at the edge of the hole. One thing that needs special attention is: not that the “dual control” indicators meet the requirements, the tension must be qualified. For example, once the strands are blocked, the tension and elongation may meet the requirements, but if they are mistaken as qualified, it will be a very dangerous quality problem. Therefore, before tensioning, the strands must be pulled to see if they can slide freely in the tunnel.
Strictly control the grouting process
Some people believe that the key to prestressed components is tensioning, as long as the tensioning is successful, everything will be fine, and the subsequent grouting is ignored. In fact, grouting is also a crucial part of prestressed construction. The main function of grouting is to protect the steel strands from corrosion. There are numerous examples of bridge collapse caused by corrosion of steel strands: In 1967, Bickton Meadows Foot-Bridge in Hampshire, a footbridge suddenly collapsed, but the steel beams were only slightly corroded; in 1985, the Welsh Bridge suddenly collapsed due to the corrosion of the steel beam at the indirect joint. In 1992, the River Schelde bridge in Belgium suddenly collapsed. In addition, the collapse of the Rainbow Bridge in Chongqing, Sichuan, which shocked China and foreign countries in 1998, was found to be caused by the severe corrosion of the steel strands. It can be seen that the grouting process of prestressed channels must be strictly controlled and paid great attention to.
A little suggestion
In view of my past experience and lessons in post-tension construction, my suggestions for post-tension technology are as follows:
- It is best to use the products of regular and well-known manufacturers for steel strands and anchorage
- In order to ensure the success rate of tension, it’s better to use wedge-type anchors.
- In order to avoid the overall collapse of the bridge, the prestressed bridge should use a prefabricated beam and plate structure instead of a tied-arch or a cable-stayed structure as far as possible under the conditions of the allowable span, so that even if one or two beams have an accident, the overall collapse is impossible.
- For bridges with tied arches, cable-stayed structures, etc., during construction, the tunnel should be embedded with electronic sensing elements that can observe the changes in the prestress of the bridge.
- In order to reduce the trouble of piercing the tunnel, ensure the construction quality, and avoid the corrosion of the steel strands, the more advanced prestressed construction technology- unbonded prestressed construction should be adopted as far as possible.