In recent years, rubber capsule, as a hole-making tool, has been widely used in the prefabrication of concrete members, especially bridge beams (slabs). As a core mold (or internal mold), it has great potential to replace the steel internal mold. It has the advantages of light weight, simple structure, convenient installation and disassembly, and is more suitable for the requirements of continuous concrete construction. Especially, the structural forms and types of rubber capsules have been greatly enriched in recent years. They are no longer limited to circular cross-section structures, but also can be made into polygonal and irregular cross-section structures. It is reasonable to gradually replace the heavy steel internal model with rich products and increased functions. However, the biggest disadvantage of rubber capsule is its lack of rigidity, which makes it easier to float than steel formwork. As a core mold (or internal mold), its surface will expand outward after inflation, making the original plane convex. If the manufacturing process is worse, such as uneven distribution of tire cord (or tire net) or uneven thickness of rubber in the capsule wall, the inflated capsule will be partially bulged, and the rubber core template will float up, resulting in uneven thickness of precast beam and slab. Prefabricated beam (slab) roof, as the bearing plate of vehicle load on the bridge deck carriageway, will affect the service life of the bridge and even the driving safety of vehicles if its uneven thickness makes it locally ultra-thin or the thickness of the roof is seriously insufficient. Although rubber core dies with circular cross-section have been widely used, polygonal and irregular rubber core dies (or internal dies) still have not completely replaced the role of steel formwork, which is the reason. Based on my years of construction experience and research on concrete hollow slab and rubber core formwork, the author found a method to solve the thickness defect of hollow slab in view of the defects of polygonal and irregular cross-section hollow slab in rubber core formwork construction, and summarized it to discuss with the majority of bridge engineering, civil engineering construction and researchers.
In the superstructure of several bridges on a national highway in southern Xinjiang, a large number of 20m post-tensioned concrete hollow slabs are designed, which are prefabricated, simply supported and then continuous. Prestressed concrete hollow slab box is symmetrical octagon. In order to improve the working efficiency, the internal mold adopts rubber capsule core mold, and its cross-sectional structure is as follows:
According to the established construction technology, the hollow slab concrete is poured and molded in two times, and the bottom slab concrete is poured for the first time, and it is vibrated and compacted by a flat vibrator. Then install the core module bag and anti-floating pressure bar. The anti-floating pressure bar is made of I80- 100 I-beam, with the spacing of 1- 1.5m, then inflate the bag, add anti-floating pressure blocks, and finally pour the web and roof concrete symmetrically. As a result of this construction, the thickness and uniformity of the bottom plate can be guaranteed, but the thickness of the web plate and the top plate is not uniform (Figure 3). After careful observation, it is found that the concave top plate under the core die is super thick, but the web is thin. The mandrel is convex between the coal balls, and the thickness is basically opposite. There are three main reasons for the analysis:
1. The rubber mandrel is flexible. After inflation, there is a great tension on the surface, and the side of the compressed concave part at the top will be convex, while the uncompressed part at the top is just the opposite, showing a convex side concave;
Secondly, the anti-floating pressure bar passes through the press block and the core mold capsule.
Thirdly, the pouring sequence of concrete has formed a secondary concave-convex effect. First compress and then press down the first dumping position, so that the gas in the capsule flows to the back dumping position and bulges here.
It is not difficult to see that the uneven thickness of the hollow plate is caused by the concave-convex effect of the core mold bag, and the concave-convex effect of the core mold bag is mainly that the contact area between the anti-floating block and the core mold bag is small, and no surface contact is formed. In order to solve the problem of uneven thickness of hollow slab, it is necessary to increase the contact area between pressing block and core mold bag as much as possible and improve the overall stiffness of core mold bag.
According to this idea, we put pressure on the top surface of the core mold capsule. In the plane range of 32cm wide at the top of the core mold, wood blocks or strips were used to increase the contact area between the briquette and the core mold capsule, but the effect was not ideal, because the wood board was too thick, which would affect the geometric size of the core mold capsule, too thin and easy to deform, and the wood board was easily embedded in the top concrete, which was not convenient to disassemble. Later, the effect of bamboo veneer has been greatly improved, but it is still not ideal; Finally, through the comparative experiment of weight and stiffness, it is determined that it is ideal to use 300mm× 1500mm×5mm steel plate. First of all, the thickness of 5mm steel plate is small, and adding a layer between the capsule and the roof concrete will not affect the structural dimensions of the core mold and the hollow slab box room; Secondly, the steel plate has high rigidity, which can effectively strengthen the rigidity of the core mold capsule; Third, each steel plate of this size is very light (only 17.66kg), which is convenient for manual installation and disassembly; Fourthly, the concrete of the top plate is not buried, and the top surface of the steel plate is brushed with a release agent, so that the steel plate can automatically fall by its own weight after the core mold capsule is deflated; Fifth, the construction is convenient: when installing the core mold capsule, the steel plate is placed on the top of the capsule, and the steel plate floats in place with the capsule after the core mold capsule is inflated, which can be manually adjusted, and an anti-floating pressure bar and a cushion block are added. When disassembled, the steel plate falls down due to its own weight along with the deflation of the mandrel capsule, remains on the capsule and is pulled out together with the capsule.
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