Article number:1672-0407 (2012) 07-055-02 Date of receipt: 20 12-06-28.
1. Introduction
For the capping beam of cylindrical pier bridge, the commonly used support methods in construction are full support method and embedded screw method, but when the full support method meets the river beach or soft soil section, the ground hardening treatment and installation cost is high, the time is long and the safety is poor; The embedded lead screw method is less affected by the ground conditions, but after the capping beam is completed, the reserved hole above the column needs to be blocked with concrete, especially when the column is high, which is more difficult. When encountering a mass capping beam (for example, the concrete of a single capping beam of the Second Yellow River Bridge in Zhengzhou reaches more than 100), the diameter of the lead screw needs to be very large, and some even far exceed the spacing between the main bars of the column. At this point, this method cannot or should not be used alone. But in recent years, the hoop method can overcome the shortcomings of the above two methods. This paper introduces the construction technology of capping beam by hoop method in detail.
2. Introduction of hoop method
The hoop method refers to the construction of capping beam. Two semicircular steel belts welded with brackets are clamped on the column, and then Bailey pieces or I-beams are placed on the sand cylinder placed on the brackets. Its working principle is to support the self-weight of beam and capping beam formwork and the construction load of pouring concrete, construction personnel and construction tools by using the friction between steel strip and column, which is a kind of unsupported construction method. This method can overcome the shortcomings of high requirements for foundation bearing capacity of full-house support, and can also overcome the shortcomings of inclined reserved holes of screws or affecting the beauty of pier columns. At the same time, the hoop and capping beam bottom formwork can be installed on the ground, and chain block or pulley block can be used to hoist them in place along the pier. When the bottom die is removed, the hoop can be used as a scaffold to repair the defects at the bottom of the capping beam after falling to a certain height, and it can save a lot of expenses for mechanically removing the bottom die of the capping beam. The application scope of this method is the construction of all piers with cylindrical capping beams.
3. Artistic process
The technological process of capping beam construction by hoop method mainly includes: measuring and setting out → installing hoop → placing sand pipe → installing Bailey piece or I-beam → installing square timber → installing capping beam bottom die (the first piece needs stacking and preloading) → manufacturing and installing capping beam reinforcement → installing capping beam side impact → template installation inspection → measuring elevation → concrete pouring → inspection and acceptance.
4. Key points of operation
4. 1 Chisel away the laitance at the top of the column
Chisel away all laitance of the concrete at the top of the column until fresh and solid concrete is exposed, and clean it to ensure that the pier column is firmly connected with the capping beam concrete.
4.2 Install the bearing hoop
4.2. 1 hoop structure
When the hoop is installed on the pier, it must be close to the pier. Because the cross section of a pier cannot be absolutely circular, the roundness of each pier is different, even the cross section of the same pier is different. Therefore, in order to adapt to various out-of-roundness piers, the hoop body of the hoop should be a flexible hoop body without hoop stiffening, that is, the steel plate without stiffening plate is used as the hoop body. In this way, when the pretension is applied, the hoop body is flexible and easy to stick to the pier. In construction, geotextile is usually placed between the hoop and the column in order to ensure the more obvious effect of dense paste.
4.2.2 Arrangement of bolts on connecting plate
The pre-tightening force of the connecting bolt on the hoop must be able to ensure that the friction between the hoop and the pier can transfer the load reliably. Therefore, there should be enough bolts to ensure the pre-tightening force. If only the stress of the connecting plate and the hoop body is considered, it is best to arrange the bolts on the connecting plate vertically in a row. But in this way, the hoop height is bound to be larger. Especially when the load of capping beam is large, more bolts are needed, and the height of hoop will be larger, which will increase the investment of hoop, and too high hoop will also bring inconvenience to construction. Therefore, as long as the connecting plate with sufficient thickness is used and the necessary stiffening plate is provided, the bolts on the connecting plate are generally arranged in two rows vertically.
4.3 Fabrication of capping beam formwork
The capping beam template is made of steel plate with a thickness greater than 5mm and processed into steel mold. When the finished product is delivered, it shall be inspected and accepted according to the processing drawings, and samples shall be taken for trial assembly. Formwork should meet the following requirements: it has sufficient strength and stability to meet various loads generated in construction; With sufficient rigidity, it will not deform during construction, thus ensuring the design shape and size of the structure and the accuracy of the mutual position between the components of the template; The formwork joints are tight, the gap between adjacent formwork interfaces is not more than 2mm, and there is no slurry leakage, so the construction operation is convenient to ensure safety; The inner surface of the template is flat, smooth and straight. When installing the template, clean the template first and apply the release agent. Before the template is used, the internal surface must be carefully treated to meet the requirements of beautiful appearance of the capping beam; Easy installation and disassembly.
4.4 survey lofting
Before capping beam construction, the center point of the column is set out with total station at the top of the column according to the triangle intersection method to determine the deviation of the center of the column. At the same time, the inclination should be measured with a vertical ball. If the error meets the specification requirements, the center line of two columns can be used as the center line of the bottom plate along the transverse direction when the formwork is erected, otherwise it should be handled.
4.5 model
Before the model is erected, the support should be set up and the support and bottom plate should be copied. The order of vertical formwork is: bottom formwork → side formwork → end formwork. Brush template oil (isolation agent) before erecting the template. After steel bar installation, close the formwork at both sides and the formwork at both ends, and carefully check whether the connection of each part is firm and reliable. The installed formwork shall be inspected in strict accordance with the relevant requirements of the Technical Specification for Construction of Highway Bridges and Culverts. The verticality, surface elevation difference, length, width and height of adjacent templates must be checked to ensure that they meet the requirements of construction specifications and design drawings. Ensure the accuracy of structural design shape and size and the mutual position of components between templates.
4.6 Fabrication and installation of steel bars
The steel skeleton of capping beam shall be processed in workshop and bound (welded) on site. Before use, all kinds of strength indexes of steel bars must be checked, and rusted steel bars should be thoroughly derusted and dusted. The capping beam reinforcement is integrally bound and formed in the reinforcement yard. Before fabrication, steel bars of different specifications and models should be classified, marked and bound according to the requirements of drawings to ensure the accuracy of the size and quantity of steel bars. The welding of reinforcement shall be carried out in strict accordance with the specifications, and the welding position of main reinforcement shall avoid the unfavorable position of maximum span and bending distance. The processed steel bars are slowly lifted by crane and placed on the paved capping beam bottom plate. The spacing between pads should not be greater than 1.2m in both vertical and horizontal directions. The installed reinforcement shall be inspected and approved by the supervision engineer before installing the side formwork of capping beam. At the same time, pay attention to the embedding of embedded parts, the accuracy of the positions of components such as cushion stones and shock-proof stops.
4.7 Concrete pouring
Concrete mixing
Concrete shall be supplied centrally by the mixing station. When mixing, all kinds of weighing instruments should be kept accurate. The moisture content of aggregate should be tested frequently, and the number of tests should be increased in rainy days to adjust the amount of aggregate mixing water. The first plate of concrete material put into the mixer should contain a proper amount of cement, sand and water to cover the inner wall of the mixing drum without reducing the mud content in the mixture. Before each shift is officially weighed, it is important to check the measuring equipment. Measuring instruments should be tested regularly, and should also be tested after overhaul, medium repair or relocation to a new location. After concrete mixing, the slump of concrete mixture shall be sampled and tested at the mixing station and pouring site respectively, and each shift shall be no less than twice. The evaluation should be based on the measured values at the pouring site. When detecting slump, we should also observe the cohesiveness and water retention of concrete mixture.
Concrete transportation
Choose concrete mixer for transportation. The transportation time from unloading to pouring should not exceed 30 minutes.
concrete pouring
When pouring, the method of layered pouring is adopted, that is, all sections of the capping beam are obliquely segmented, horizontally layered and continuously poured. The pouring method should be placed at the top of the column first, and then slowly spread out to the middle and sides.
4.7.4 Concrete vibrating and form removal
When vibrating concrete, use plug-in vibrator (flat vibrator at the top). When vibrating in layers with an internal vibrator, the thickness of each layer should be controlled between 30cm and 40cm. Vibrators should be inserted into the concrete and slowly set aside to avoid cavities. Vibrators should be vertically inserted into the concrete and the previous layer of concrete to ensure that the new concrete is well combined with the first poured concrete. The insertion depth is generally 50 ~100 mm. Vibrating shall be carried out step by step, without leakage or excessive vibration, until the concrete stops sinking, no bubbles appear, the surface is smooth, and finally the slurry is collected.
The formwork and support at the bottom of the capping beam can only be removed when the concrete reaches 75% of the design mark, and the consent of the supervision engineer should be obtained before formwork removal, so as to prevent the concrete surface or edges from being damaged due to improper formwork removal.
4.8 concrete curing
After concrete pouring, straw bags or sacks should be covered and watered as soon as possible after slurry collection. Covering shall not damage or pollute the concrete surface. When the concrete surface is covered with formwork, the formwork shall be kept moist during curing. Sprinkling curing time is usually 7 days, which can be extended or shortened according to the humidity and temperature of the air. The number of times of sprinkling water every day can keep the concrete surface wet.
4.9 Problems in capping beam construction
4.9. 1 slurry leakage, the key lies in the template design, followed by installation problems;
4.9.2 Coarse aggregate is found on the surface after form removal, mainly including slump, workability and mixing time of concrete;
4.9.3 There are honeycomb pits at the chamfer, mainly because the details are ignored when vibrating;
4.9.4 There are slight cracks on the surface, so it is suggested that the construction measures of mass concrete should be fully considered, and the slurry should be collected again before the work is over to strengthen health preservation.
5. Concluding remarks
Through the practical application of Zhengzhou Yellow River Second Bridge, Ashen expressway Xixian Huaihe River Bridge, Hunan Yue Chang expressway New Bridge and other projects, the hoop method does have the characteristics of operability, safety, light appearance and easy inspection and acceptance. Compared with the support method, it can save a lot of man-hours and avoid the construction difficulty of reserving holes after the completion of capping beam by embedded method. For the case of poor foundation conditions or large capping beams, the economic benefit of hoop method is more prominent. Therefore, it has extensive popularization value.
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