In order to ensure the appearance quality of pier concrete and speed up the construction progress, the external formwork is designed as turnover formwork, the panel is made of 5mm thick steel plate, and the stiffening framework is welded by [12 channel steel and ∠70 x50 x6 angle steel. The construction platform adopts [10 channel steel and ∠50 x50 x6 angle steel, which are integrated with the formwork. A single section of the formwork is 5m high, and consists of six integral large formwork. The side formwork along the bridge is 2934 (width) x5000 (height) mm, and the side formwork across the bridge is composed of two pieces, both of which are 3255 (width) x5000 (height) mm. The same set of formwork is all connected by tongue-and-groove joints and high-strength bolts. There are tie rods between the templates, and the density of the tie rods is calculated and determined according to the height of each concrete pouring.
2. Internal model design and manufacturing
In order to ensure the design size of the structure, because the appearance quality of the internal formwork is not high, the 2.0m x 1.5m composite steel formwork commonly used in the construction industry is used for site binding. Two sets of internal formwork are made for each pier, with a section height of 5m and no interface formwork.
3. Installation quality standards
1) Before pier construction, technical disclosure shall be made to the construction personnel to make them familiar with and master the construction and operation technology of steel formwork.
2) The layout and construction operation procedures of steel formwork shall be implemented in accordance with the formwork construction design and technical measures.
3) When pouring hollow section, the composite steel formwork should avoid opening as much as possible. If it is necessary to drill holes, please use machines and tools to drill holes, and do not use electric welding to cook holes.
4) After the formwork is dismantled, the formwork shall be overhauled in time.
5) Template should be coated with release agent before installation, with uniform coating and moderate consistency.
6) After the formwork is installed, check its axis position, horizontal elevation, size and verticality of each part until it meets the design and specification requirements. 1: Fabrication and installation of steel bars
Pier rebar is connected by rebar straight thread sleeve, which is pulled by rebar straight thread machine at both ends. The length of thread mouth is 4cm and the length of joint is 8cm. According to the specification requirements, when the main reinforcement is extended, the number of joints in the same section (any joint within a continuous height range of 0.98m shall be regarded as being in the same section) shall not exceed 50% of the number of main reinforcement in this section. In order to facilitate the construction, when making the first section of reinforcement, the number of joints of each reinforcement and the length of each reinforcement should be calculated and controlled in advance according to the above requirements. The main reinforcement of the pier is cluster reinforcement, and each beam consists of three reinforcement bars. Considering the uniform arrangement of the stressed steel bars, in order to meet the requirements of design specifications and appearance, when the pier reinforcement is embedded in the construction platform, according to the calculation results, the length of each beam exposed from the top surface of the platform by three reinforcement bars should be controlled at 3m, 4.5m and 6m respectively. Starting from the second paragraph, every time a section of steel bar is extended, the 9-meter-long fixed-length steel bar will be sleeved at one end of the threaded sleeve on the ground, and the other end will be tightened with a steel wrench during installation. As the outer side of the main reinforcement of the bridge pier in this project is designed as cold-rolled ribbed welded steel mesh, the steel mesh needs to be installed after the main reinforcement and stirrup are installed. Two-level control should be carried out when steel bars are installed. In the first stage, the main reinforcement joint, main reinforcement spacing and stirrup spacing are controlled within the scope specified in the specification, and the number of steel bars is controlled at the same time to avoid the phenomenon of less reinforcement; In the second stage, after the template installation and positioning is completed, check and adjust the spacing between internal and external steel bars and the thickness of protective layer again. Steel bars are hoisted in place by a small number of tower cranes, and then connected and installed. Because of the high pier height, the "rooting" of the inner frame is considered. The steel pipe is embedded in the poured pier, and the external steel pipe and steel bar are locked into a whole.
2. Improve the template
Template should be strictly checked before use, and check the geometric size, flatness, etc. Each part of the template should meet the design and specification requirements. First of all, the template should be pre-assembled, and it can be installed only after it is correct. Before installing the formwork every time, remove the cement mortar and other attached dirt on the surface and joints of the formwork. After cleaning, apply the release agent evenly on the surface of the template. The formwork is hoisted by tower crane. PVC pipes are used as sleeves for pull rods. On the one hand, it is convenient to reuse the pull rod, on the other hand, it can avoid the damage to the concrete surface when the pull rod is pulled out. The fixing and adjustment of the formwork are realized by connecting bolts between the pull rod and the two layers of formwork. When pouring the concrete of the bottom two piers, the formwork shall be corrected by pulling rope and wind rope, and the embedded retaining ring embedded in the top concrete of the bottom section shall be used from the third section. After the pier in the first section is accurately positioned, use 10 kg vertical ball to control the verticality of each template in the last section, and then use total station and level meter to check the coordinates and elevation of the four corners of the template. The hollow cross-section internal mold adopts composite steel mold, and the steel pipe bracket is tightly supported.
3. Concrete pouring
After the formwork installation is completed, concrete pouring can only be carried out after the supervision engineer checks that it meets the design and specification requirements. The concrete is C50, the cement is prepared by Duyun P.O425, and the mixing ratio is: 1 (cement): 1.448 (sand): 2.333 (crushed stone): 0.33 (water): 0.0095 (admixture UFN-3C), and the concrete is directly pumped by the delivery pump. Control the layered thickness of concrete. Each time the layered thickness of concrete is controlled between 30 and 40cm, it is distributed clockwise from the inside of the pier body, vibrated with an internal vibrator, and the vibrating points are arranged in parallel. The spacing between vibrating points is not more than 40 cm, and it is 5 ~ 10 cm away from the template edge. Vibration operation is divided into two groups. The vibration time of the first group is 45s ~ 60s/ point, and the vibration time of the second group is 20s~30s after 30 minutes. The vibration direction and distribution are the same as those in the first group. Vibrate until the concrete surface stops sinking, no bubbles appear, and the surface is smooth and pulpy. Every time a section of concrete is poured, the previous section of concrete shall be chiseled away until the new concrete is chiseled away. Fully wet the concrete on the top surface of the next segment with clear water in advance. When pouring the first layer of concrete, make the top surface of concrete flush with the top surface of formwork. When the concrete in this section reaches 75% of the design strength, the formwork can be removed, the vertical main reinforcement can be connected, the stirrups can be bound, and then the formwork in the previous section can be installed. Special attention should be paid to the correct positioning and embedding of embedded parts during pier construction. Immediately after concrete pouring, cover for maintenance; Immediately after form removal, it should be wrapped with plastic film for wet curing, and at the same time, it can avoid polluting the poured next pier when pouring the concrete of the previous pier.