Prestressing technology is widely used in highway bridge construction because of its advantages of saving materials, light weight, simple structure, safety and reliability, and convenient installation. However, the prestressed tensioning construction is relatively complicated, which requires a strong professionalism of prestressed structure construction, and there are many quality problems in actual construction.
I. Construction of Prestressed Bridges
1. In order to improve the early strength of prestressed concrete, prestress is generally applied after 3 days of concrete pouring, and early strength agent is added. Because the strength growth of concrete takes a certain time, and the growth of strength and elastic modulus is not synchronous, the strength growth is fast, the elastic modulus is long and slow, and the early deformation of concrete is large. Premature prestress will increase prestress loss, lead to insufficient bearing capacity of bridges and produce many cracks.
2. Tension technology at one end of prestressed super-long beam. In China, the cast-in-place prestressed continuous box beam with large span (3 ~ 5 spans, 30 ~ 50m each span) is generally tensioned at one end. According to relevant domestic and foreign codes, the prestressed bridge with a span greater than or equal to 30m requires symmetrical tension at both ends to ensure the establishment of effective prestress of the middle span and the resisting bending moment required by the bridge under dead load and live load, otherwise, the bearing capacity of the middle span will be insufficient, resulting in normal section cracks. In the highway bridges that have been opened to traffic, there are also a lot of cracks caused by tensioning technology.
3. At present, the operation of prestressed construction is not standardized, especially the tension control is not strict, which has a great influence on the quality of prestressed bridges. Generally, the tension and elongation of prestressed tendons are controlled at the same time, with the tension as the main factor, and the tension is checked by the elongation value. Tension is usually measured by 15 oil pressure, which has a large error. Some jacks are tensioned even without measurement and calibration, and most tensioning personnel have not received professional training. If they don't concentrate on their work, they will make mistakes, read the wrong table, and the tension will rise and fall. Especially when multi-beam is tensioned, because of the different tension of each beam, the calculation of the elongation of prestressed tendons is often inaccurate and the elastic modulus is chaotic. It is difficult to achieve the specified elongation during actual tensioning.
4, prestressed duct grouting: first, it has the function of protecting prestressed tendons from corrosion; Second, it has the function of ensuring the prestressed reinforcement and structure to work together; However, in practical engineering, the phenomenon of insufficient grouting, non-compactness, slurry leakage and slurry leakage is widespread, which has become a common problem of prestressed structures. The main reason is that the construction unit has not paid enough attention to the hole grouting process, and there are also problems in grouting process, reserved hole quality and slurry configuration. Especially the water cement ratio of cement slurry is too high (0.4 ~ 0.45) in the specification. After adopting the water-cement ratio specified in the specification, the pore slurry oozes out, and the pores are not easy to be full and dense.
5. Because the concrete protective layer is generally small, and the protective layer cement pad used in construction has been damaged and displaced, resulting in the failure of the beam and slab protective layer. In addition, the grouting of most prestressed channels is not in place, which leads to many longitudinal and transverse cracks in the bottom plate of box girder and plate girder, and the concrete protective layer of post-tensioned prestressed structure is out of control.
Second, the prestressed construction technology
1. Construction of anchorage end beam, mid-span steering transverse rib and pier top guide groove. The spatial position of steel strand is determined, and the equivalent load is determined by cable shape and tensile stress. The steel strand in the mid-span steering transverse rib and the pier top guide groove is skewed. In order to bear the local compressive stress, it is required that the embedding position and direction of the anchor pad at the anchorage end beam are accurate. The fabrication of steering transverse rib and guide groove at pier top shall be carried out in strict accordance with the requirements of drawings. The radius of curvature of bending must be ensured, and the ends must be polished to make them smooth, so as to prevent the ends from squeezing and slipping the steel strand during tensioning.
2, steel strand blanking and wear beam. In bridge reinforcement, after tensioning, the anchor pad and steel pipe should be grouted to form a bonded section. Therefore, it is difficult to control the length and position of the bonding section when cleaning the PE layer and grease of the steel strand during blanking, because it is necessary to consider the influence of the sagging of the steel strand in the process of beam penetration in advance, ensure that the PE protective layer enters the sealing cover in advance, and also consider the influence of tensile elongation, ensure that the elongation parts at both ends are consistent, and ensure that the bonding force of the two bonding sections is roughly equal. In the process of beam threading, because the length of steel strand is more than 150m, it is impossible to thread 12 steel strand in box girder through multiple pier top guide grooves and mid-span steering devices, so the method of single cable threading is adopted.
The winding of steel strand will affect the establishment of effective prestress, so it is necessary to ensure that steel strand is not wound within the whole bridge length. In actual construction, the steel strand, working anchor plate hole and cover plate hole are numbered separately in advance, and each bundle of 65,438+02 steel strands adopts a single rope. Every once in a while, the position of the steel strand is limited by the rubber pad corresponding to the hole of the sealing cover. After tensioning, it is found by this method that each strand of steel strand is straight without winding.
3, steel strand tension. In order to keep the steel strand straight from the loose state to the completion of tensioning, pre-tensioning should be carried out before formal tensioning, and the quality of pre-tensioning determines the overall reinforcement effect. Even if the necessary measures are taken, the steel strand is still very long and the sag is still very large. In order to ensure that the bonding sections at both ends are approximately equal in length, preloading is carried out symmetrically at both ends. The magnitude of the pre-tightening force should not only ensure that the steel strand is tight and not entangled in the process of pre-tightening, but also ensure that the steel strand is not in place during high-stress tensioning, and the purpose of pre-tightening can not be achieved if the pre-tightening force is too large or too small.
Because the bridge is reinforced by long epoxy coated steel strand, it needs multi-stroke continuous tensioning and the working clip needs multiple anchorage. When the working clip is temporarily fixed, the chips formed by the protective film coated with epoxy resin will attach to the teeth of the clip. With the repeated clamping of the steel strand by the working clamp, the epoxy coating debris between teeth increases, which will cause the phenomenon of wire slipping, thus affecting the anchoring effect of the working clamp. In view of this situation, a temporary anchoring device was developed. In the middle stroke, the steel strand is clamped by the tool holder of the temporary anchorage, which avoids the temporary anchorage in the middle of the work holder and ensures the anchorage effect.
4. Grouting. The external cable anchorage beam adopts the form of local bonding. In order to meet the requirements of the design and the owner for the bonding force of the steel strand in the local bonding section, the grouting work of the local bonding section is a very important process after tensioning: the model test of 1: 1 is conducted before construction, and the bonding force of the local bonding section can reach 108% of the designed tensile force, which meets the anchoring requirements; the manual dynamic grouting machine is used for grouting to ensure the uniformity and stability of the grouting process and the requirements of grouting pressure.
Third, the construction quality control measures
1, material quality control. Strictly control the quality of materials and adopt the products of manufacturers with good reputation and quality. Before use, the on-site materials should be inspected, and the strength, stiffness, tightness and bite fastness of spiral pressure joint meet the quality standards. Strengthen the protection of bellows and reduce its damage. Reduce welding operations. When laying corrugated pipes after forming ordinary steel skeleton, avoid corrugated pipes and corrugated pipe joints when vibrating concrete with vibrating rods. Use the corrugated pipe of the first size as the casing, the casing is 20 ~ 30cm long, and the pipe joint should be aligned and centered in the casing. The circumferential gap between the two ends is sealed with adhesive tape.
2. Preparation before tensioning. Before prestressing tendons, members should be inspected, and the external dimensions should meet the requirements of quality standards. When tensioning, the strength of component concrete should meet the design requirements; When the design has no requirements, it should not be less than 75% of the design strength grade value. When mortar joint is used for vertical joints of block assembly members, the mortar strength shall not be lower than 15MPa. The reserved passage should be checked by using a hole opener, air pressure and water pressure. Welding slag, burr, concrete residue, etc. The embedded iron plate at the end in contact with the anchorage and pad shall be removed.
When the beam should pass first, it is best to use compressed air and water. Before the steel bar passes through the beam, the threaded part of the threaded end rod should be wrapped with cement bag paper for 2 ~ 3 layers and fastened with fine iron wire; Before steel strand bundle, steel strand bundle, steel bar bundle, etc. Go through the bundle, find a flush end and number it in sequence. For longer beams, the perforator should be put on and pulled out from the other end by lead wire and traction equipment. For clip anchorage, the good clip should be flush with the steel pipe and compacted before tensioning. The tensioning sequence of prestressed tendons should meet the design requirements.
3. Control of construction influencing factors
Structural parameters are the basic data of structural construction control, and their accuracy directly affects the accuracy of analysis results. Structural parameters mainly include: section size of structural members, elastic modulus of structural materials, bulk density of materials, thermal expansion coefficient of materials, construction load and prestress or cable force. Construction control serves the construction, in turn, the construction quality directly affects the realization of control objectives. The construction technology must meet the control requirements, and the construction state must be controlled in the construction control. Construction inspection is one of the important means of bridge construction monitoring. Detection includes stress detection and deformation monitoring. Due to the installation of measuring instruments and meters, data collection of measuring methods.
The above are collected and sorted by Zhong Da Consulting Company.
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