How to find the special construction scheme of cylindrical high pier without support?

Construction technology of turnover formwork for high pier without support

Wang Bi, the Fourth Company of China Railway 11th Bureau Group

Combined with Huichang Shuidong Viaduct, this paper introduces the construction technology, construction survey method and construction safety measures of thin-walled pier of this bridge, which can provide reference for similar projects in the future.

Keywords:: thin-walled pier construction technology monitoring method turnover mode

1 project overview

Huichang Shuidong Viaduct is one of the key control projects in AS-3 section of Ruigan expressway. The total length of this bridge is 328m, and the superstructure adopts two (4×40+4×40m) simply supported and then continuous "T" beam bridges, of which 0 # and 8 # piers adopt ribbed slab piers, and 1 ~ 7 # piers adopt rectangular thin-walled piers, with the highest pier height of 47.9m. See table 1.

Altimeter table of each pier 1

Pier number left (m) right (m)

1 20.4 22. 1

2 4 1.5 4 1.5

3 39.6 39.6

4 42. 1 39.0

5 47.9 47.9

6 46.5 47.5

7 24.2 24.4

2 construction scheme selection

The cost of the bridge is 3 1.2% lower than the normal investment, and the unit price is lower, which is one of the key control projects. Therefore, the construction scheme should not only consider the economy, but also consider the appearance quality of the thin-walled pier. After comparison and selection, the thin-walled pier formwork adopts integral steel formwork. In order to improve the turnover rate of formwork, the formwork is used by each pier in turn. Because the thin-walled pier is relatively high, the crane below 50T can't meet the lifting height requirements of construction, and the cost of using extra-large crane is high, so 25T crane and QTZ3 15 jack-up tower crane are used as lifting equipment. In order to improve the use efficiency of tower cranes, according to the arm length and maximum lifting weight limit of elevators, the whole bridge is equipped with two tower cranes, each of which is responsible for the construction of a pier and used in turn.

3 construction technology

3. 1 Template production requirements

Because the formwork is the basis to ensure the size and appearance quality of each part of the pier, the following technical requirements must be met at the beginning of formwork production:

(1) has the necessary strength, rigidity and stability, can reliably bear all kinds of loads generated in the construction process, and ensure the accuracy of structural geometric dimensions;

(2) It is simple to manufacture and convenient to dismantle the formwork, which can minimize the damage to the formwork and members during formwork dismantling, thus improving the turnover rate of formwork use;

(3) The formwork panel is flat, with tight joints and no slurry leakage;

(4) During construction, it is convenient to operate and ensure safety.

3. 1. 1 formwork construction

The turnover formwork consists of two sections of formwork. The height of the first section of turnover formwork is 2.5m, which is mainly composed of external formwork, coaming, pull rod and operation platform.

Template adopts large steel template. In order to prevent deformation and facilitate transportation, two templates with the size of 2.2× 2.5m (width× height) are assembled in the direction of the pier 2.2m, and connected into a whole by M2 1 bolts. 7. Formwork in the direction of1m is assembled by two formworks of 7. 1×2.5 m (width× height) and connected into a whole by M 16×30 bolts. The front formwork and the side formwork are connected by a 45-degree tie rod.

[16 channel steel is used for lateral back reinforcement of side formwork, [20a channel steel is used for lateral back reinforcement of end formwork, the spacing between lateral back reinforcement is 90cm, and [10 channel steel is used for vertical back reinforcement of side formwork. The thickness of formwork panel is 6mm.

Tie bars are φ 27 tie bars, which are arranged at1.5m.

3. 1.2 setting of belt and tie rod

The formwork shall be used within a height of 2.5m ][ 10 channel steel shall be provided with three hoops, which shall be directly welded on the back of the formwork with a spacing of 80.0cm Each hoop shall be equipped with φ27 steel bars, and a tie rod shall be arranged according to1.5m. The tie rod shall be fixed between the two channel steels with nuts and gaskets.

The pull rod is wrapped with PVC pipe, and it can be pulled out for reuse when the formwork is removed.

3. 1.3 Operating Platform Settings

The operation platform is set up on the template fixing frame, and a template fixing frame is set for each layer of external templates. The fixed frame is welded into a triangle with ∠50 equilateral angle steel and fixed on the back edge and enclosure of the internal and external formwork. Construction platform erection of internal and external template fixed frame. The width of the external construction platform is 0.8 ~ 1.0m, the top surface of the platform is provided with protective railings along the periphery, and a closed safety net is provided from the outside of the railings to the bottom of the formwork fixing frame. The construction platform is paved with 5cm thick boards for operators to work, walk and store small machines and tools.

3.2 turnover formwork construction principle

The pier turnover formwork consists of three formwork sections, each of which is 2.5 meters high, and the pier column is first poured with concrete at a height of 5.0 meters. After that, the formwork will be lifted circularly, and the concrete pouring height will be 5m each time.

During construction, the first formwork is supported on the top of the pile cap, and the second formwork is supported on the first formwork. After measurement and positioning, concrete shall be poured at one time. When the concrete reaches the strength of form removal, dismantle the first formwork and the bottom tie rod of the second formwork at the same time. At this time, the load is transferred from the hardened pier concrete to the bottom of the pier. After the first section template is adjusted and polished, it will be hoisted to the third floor by tower crane and chain block. According to this cycle, continuous operations such as formwork removal, formwork lifting, formwork assembly, steel bar welding and binding, pumping pipeline extension, concrete pouring, maintenance, measurement and positioning, elevation measurement, etc. will be formed upward until the design height is reached.

3.3 the production of steel bar

Because the pier is relatively high, it is impossible to weld steel bars at one time. Therefore, the production and molding of steel bars are generally completed in the steel yard. After molding, welding and binding are carried out in sections, and positioning steel bars are embedded up and down to ensure the accurate position of steel bars. In the welding process, to ensure the welding quality and binding quality of steel bars, the production and installation requirements of steel bars should meet the technical specifications of steel bars.

3.3. 1 steel batching

(1) Be familiar with the construction drawing, understand the general situation of the project, check whether the drawing number is complete, remember the relationship between steel bars in each component, and be familiar with the connection between steel bars, formwork, scaffold configuration, etc.

(2) Check whether the dimensions of each component are consistent, whether the serial numbers of all steel bars in each component are repeated, and check whether the diameter, length and number of steel bars of each serial number are inconsistent with the bill of materials. If not, report to the supervisor for correction. According to the construction drawing, strictly calculate and determine the batching length of various steel bars.

3.3.2 Reinforcement processing

In order to prevent the steel bars from rusting, try to wipe the floating rust with a rag when necessary. Old rust must be removed by manual or mechanical derusting to ensure the meshing force between steel bar and concrete. Large-diameter steel bars shall be straightened manually, and φ 10 and φ8 shall be straightened and cut by numerical control straightening machine. Steel bars should be cut according to the quantity and length of ingredients before bending. And on the workbench, according to the shape and size indicated on the ingredient list and drawings, calibrate the position of each bending point. After the first steel bar is formed, it should be rechecked according to the shape and size indicated on the drawings, and then mass production can be carried out after verification.

3.3.3 Binding and installation of steel bars

(1) Preparation before binding. According to the needs of construction, the welding and binding of steel bars should be carried out in layers, and the size of welding and binding should be controlled according to quality standards. According to the height of each welding and binding, the corresponding operating platform is set up by means of bowl bracket and tower crane hoisting.

(2) Steel bar welding. According to the welding equipment on site, when lap welding is adopted, the steel bars should be pre-bent, so that the axes of the two steel bars are in the same straight line and fixed with two points. The lap length shall meet the specification requirements, and the single-sided welding shall be greater than 10d, and the rebar joint of the same section shall be set according to 50% of its cross-sectional area.

(3) steel binding operation. Draw the reinforcement position line on the main reinforcement of thin-walled hollow pier, and tie it with cross buckle and anti-cross buckle. When binding, pay attention to adjust the position of main reinforcement, and bind each intersection firmly with iron wire. If necessary, it can also be spot welded. In order to ensure the verticality of the steel skeleton, the vertical bars at the four corners of the skeleton must be vertically aligned to avoid binding the skeleton to tilt.

3.3.4 Protective layer control

In order to ensure the thickness of the protective layer, special-shaped mortar pads should be set between the steel bars and the formwork, and the steel bars should be firmly bound and staggered, or the top surface of the steel bars should be padded with wooden wedges. When pouring concrete, the wooden wedge should be removed, and the traces of padding after formwork removal should be ensured on the concrete surface.

3.4 template installation

After the steel binding is completed, the formwork can be erected only after passing the inspection. Before shuttering, if the surface of the pile cap is uneven, the sundries at the summer-planting place should be removed. Spread a layer of 1: 2 leveling mortar of about 2cm at the outer membrane position on the top surface of the cap, and put a sponge strip with a width of 10cm and a thickness of 5cm between the template and the leveling mortar to prevent slurry leakage. Steel formwork is supported by tower crane. First, support two side end templates, and then support two side front and back templates in turn. Template with 4mm steel wire temporarily fixed on the stud, template with connecting bolts (φ2 1mm) connection. Finally, adjust the verticality of the external formwork and tighten the connecting bolts.

After the first formwork is supported, assemble the second 2.5m high end formwork and side formwork. Insert a sponge strip with a width of 2cm and a thickness of 0.5cm between the two templates to prevent slurry leakage. If the two templates are uneven, use hemp fiber to block the seam and correct it. After the position correction, triangular brackets and safety railings are installed on the first and second templates, and scaffolding is laid on the top surface of the brackets as the later work platform. Check and adjust the verticality, spatial position and horizontal elevation of all templates.

3.5 concrete construction

Thin-walled pier concrete is labeled C30 concrete, and the concrete mixture ratio is adjusted by the laboratory of the project department according to the characteristics of raw materials and weather conditions to control slump. Thin-walled pier concrete is poured by pumping, which requires high workability and water-cement ratio of concrete. Use two 750 forced mixers for mixing. The slump of concrete shall be controlled at 130 ~ 150 mm in summer, 120 ~ 135 mm in winter, and the concrete mixing time shall be strictly controlled at 1.5.

3.5. 1 construction preparation

Before construction, the cement label, variety and net weight shall be reinspected to see whether they meet the design requirements and standards, and the water content of sand and stone shall be checked for ratio conversion, and the appropriate ratio shall be adjusted through experiments. Overhaul the operation of mixing equipment and debug the weighing system of dosing machine to ensure the accuracy of materials. Check the position, elevation and verticality of steel bars and formwork to ensure accuracy.

Concrete mixing

The mixing of concrete is to change cement sand and water into uniform concrete by mixing according to the mixture ratio of concrete. The mixing time of concrete should be 1 min-3 min to achieve uniform mixing and consistent color. At the same time, it is necessary to ensure that the workability and slump of the mixed concrete meet the specification requirements.

concrete pouring

Before concrete pouring, check the formwork and reinforcement. Template should be smooth and bright, steel bar should be clean and straight. When pouring, the concrete shall be poured evenly and symmetrically in layers, and the thickness shall be determined according to the pouring speed, which is generally about 3 0 cm. The concrete shall be poured until the formwork is flush, so that the construction joints are sewn at the same position with the formwork, and it is not allowed to exceed the formwork, which is generally lower than the formwork1mm..

Concrete should be vibrated fully and compactly, and vibrated according to the requirements of operation specifications, and no vibration leakage or mold smashing is allowed. The pier adopts 70-type plug-in vibrating rods, and the order of vibrating insertion points is staggered, with a depth of 5 ~ 10 cm and a time of 20 ~ 30 s at each point. The thickness of each layer of concrete is 1.25 times the length of vibrating rods.

3.6 demoulding and curing

The demoulding time should be well controlled. If it is too early, the concrete will crack or lose its strength. If it is too late, it will cause difficulty in demoulding and even local damage to concrete. At the same time, it is not conducive to the turnover of templates. Generally, when the concrete strength reaches 2. 5MPa, dismantle. Concrete curing: The primary curing of concrete is an important measure to ensure the early strength of concrete. Because the pier is high, we adopt the method of film covering for early maintenance and natural maintenance for later period.

4 construction monitoring of thin-walled pier

Due to the limitation of pier height, it is impossible to pour the pier column at one time, and it needs to be poured several times and in sections. In this way, the construction survey is particularly important. The cross line method is usually used to control the pier top platform, but the stability of the platform is poor when the pier height exceeds 20m, and it is difficult to measure and set out. Vertical ball method is often used to locate piers. Because the solid section at the bottom of the pier or the center point made on the foundation is often blocked by rain or construction waste, the light in the pier is dim and the measurement is very inconvenient. After many tests and experiments in the field, the four-point vertical ball method is summarized to control and detect the deviation between formwork and concrete construction, and good results are obtained. The so-called four-point positioning vertical ball method is to set four standard points on the concrete surface of thin-walled pier by coordinate method with total station. Generally, the corner points of the pier wall in four directions are measured, and the installation deviation of the template can be checked at any time during the template installation.

△L =L-δ /2

Where △L is the template deviation error, when △L > 0, it means that the template deviates from this point △L, when △ L

Take corresponding corrective and preventive measures according to the magnitude of the offset. This method is usually used to check the measuring frequency every time concrete is poured. The measurement method has achieved good control effect, and the dimensions, midline and elevation of each part of the pier meet the design and specification requirements.

5 safety measures

It is particularly important to protect the safety of the wharf working at high altitude. Therefore, a variety of emergency plans and safety management measures have been formulated to ensure the personal safety of construction personnel. Cantilever frame is welded on the outside of formwork with steel pipe as the working platform for pier top construction personnel; Dockers must wear safety helmets and safety ropes to ensure construction safety.

6 construction effect

(1) The above monitoring measures were taken in the bridge construction, and no safety quality accident occurred. The qualified rate of sub-projects is 100%, and the excellent rate is over 90%, which is well received by the construction unit, design unit and supervision unit, and has achieved good economic and social benefits.

(2) Since September 2007, the bridge has completed 14 pier in half a year. The average construction period is three days. The pier elevation is accurate, the concrete surface is smooth and smooth, and the edges and corners are smooth, which has been widely praised. The construction unit also saved a lot of labor and machinery costs because of this construction method, and achieved good economic benefits.