I. Introduction of examples
A national highway K24 16800 is a hollow slab bridge with an oblique angle of 45 degrees and a span of 12M. The standard drawing adopted is JT/GQB.
00 1-93, precast prestressed concrete inclined hollow slab bridge superstructure, in the construction process, due to various reasons, two slabs were eventually scrapped. The constructors dissected and analyzed the scrapped slabs, and at the same time, according to the literature search, the quality problems of hollow precast slabs were deeply analyzed, from which it was found that the quality problems of hollow precast slabs were mainly manifested in the following eight aspects:
1. The bottom plate of precast hollow slab is too thick and the top plate is not thick enough. During the maintenance period, because the construction personnel did not operate according to the specifications and walked on it too early, the top of the slab collapsed, and the measurement found that it was only 2/3 of the design thickness;
2. The concrete at the bottom of the hollow slab is not dense, resulting in water seepage and leakage. Water leakage and honeycomb were found in different degrees after lifting the scrap plate;
3. The height of precast hollow slab is not strictly controlled, exceeding the design height;
4. The sealing end of prestressed hollow slab is not strictly controlled on the total length of beam and slab, which leads to different lengths, some sealing ends are not vertical, and the skew angle is inconsistent, which increases the difficulty of expansion joint installation. When the slab is installed, the slab cannot be put in place, so the hollow slab can only be put in place by digging the back of the abutment.
5. The embedding position of embedded parts is incorrect or even omitted;
6. Cracks appear in the transverse direction of the top plate or the longitudinal direction of the bottom plate of the hollow precast slab;
7. The thickness of the reinforced concrete protective layer of the bottom plate is insufficient, and the steel bar is polluted by the release agent. Even the phenomenon of exposed tendons appears;
8. The base surface is uneven, and the positions and heights of the supports at both ends of the plate are inconsistent, resulting in torsional force of the plate. This phenomenon is obvious when assembling prefabricated panels after installation.
Second, the reason analysis
The main reasons are as follows:
1. Polygonal hollow precast slab is formed once and concrete is poured once. Because of the width of the slab (1m), the bottom slab concrete under the bottom surface of the core mould cannot be vibrated directly, but the concrete on both sides (mostly cement mortar) is squeezed and flowed to fill the bottom slab of the hollow slab. If the specifications of concrete stone are too large and the water-cement ratio is improper, the concrete of the bottom plate will be not dense, and there will be water leakage or longitudinal shrinkage cracks. If it is not treated, the bottom slab reinforcement will rust easily, which will affect the service life of the bridge. Therefore, it is easy to ensure the construction quality by adopting the technological process of pouring the bottom plate first, then installing the core mould, and then pouring the concrete above the bottom plate.
2. The mandrel of the hollow precast slab is not firmly fixed. When the concrete is vibrated, the mandrel floats due to the extrusion pressure, which leads to the super-thick bottom surface of the hollow slab and insufficient thickness of the top plate. In order to ensure the thickness of the roof, some construction units artificially increase the height of the slab, which affects the thickness of the bridge deck pavement. Although it is convenient to install and demould the hollow plate by using the inflatable capsule as the core mold, it is difficult to fasten the capsule, and it is more prone to the material problems of the capsule itself, floating and local bulging. Therefore, unless a special structure is needed, it is better to use the inflatable capsule as the core mold, and generally it is better to use the steel template.
3. The precast hollow slab concrete has transverse cracks on the top plate and longitudinal cracks on the bottom plate. The main reasons for transverse cracks are as follows: firstly, excessive cement dosage, excessive temperature difference or untimely maintenance are prone to dry shrinkage cracks; Secondly, the foundation is not firm, and the settlement is uneven, resulting in transverse cracks; Third, improper hoisting or stacking causes the stress fulcrum to break; The main reason for the longitudinal cracks in the bottom plate is that the vibration is not dense, cement mortar or cement gathers together, and shrinkage cracks appear, resulting in water seepage and leakage of the bottom plate.
4. The geometric dimension of precast hollow slab is inconsistent with the design geometric dimension (mainly the length), and the uneven base surface is mainly caused by sloppy construction and no process inspection before, during and after construction. The geometric dimension of hollow precast slab is inconsistent with the design geometric dimension (mainly length), and the uneven base surface is mainly caused by sloppy construction and no process inspection before, during and after construction. Three, the hollow precast slab quality problem handling method
1. The concrete strength of hollow slab is unqualified or the thickness of the whole beam roof is less than 8cm, or the width of transverse crack exceeds the specification, so it should be scrapped and re-manufactured. This is the case with one of the two discarded boards.
2. For the hollow slab with the roof degree (locally) less than 7cm, local opening should be carried out, the part with insufficient thickness should be chiseled, the core mould should be reinstalled, the reinforcing ribs should be added, and the concrete with a higher grade than the original concrete label should be poured to make the roof thickness meet the design standards. The steel wire mesh with the diameter of 10× 10cm should be set for the roof deck pavement, which is difficult to operate and is not recommended.
3. If the hollow slab is not dense, there is water leakage or longitudinal local cracks or the reinforced concrete protective layer is insufficient, and if the concrete strength is qualified and there is no problem in the static load test, waterproof measures can be taken, and XYPEX waterproof material can be sprayed on the top surface of the non-dense concrete floor to improve the density and strength of concrete, thus playing the role of waterproof and air erosion prevention.
4. The height of precast hollow slab building exceeds the design standard, which directly affects the thickness of bridge deck pavement. If the thickness of bridge deck pavement can't meet the design requirements, the height of pier cap or cushion stone can be adjusted or the super-thick top plate can be chiseled away. If the superstructure has been installed and the pier cap and cushion stone cannot be adjusted, the longitudinal slope can be adjusted.
5. In order to ensure the driving safety and service life of the hollow slab bridge, it is suggested that the design department increase the pavement thickness of the hollow slab bridge from 8cm to 10cm, and double the steel mesh from 10 to 10, with a diameter of 8-10 (the span of 20 meters is10). Prefabricated hollow slabs vary in length. When installing, some expansion joints at the beam end have no expansion joints, and some are serrated, which increases the difficulty of installing expansion joints. For this kind of problem, the extra-long part should be sawed (or chiseled) neatly before being installed in place.
6. The bottom plate of the hollow precast slab is not on the same plane, and the main routes of the supporting points are inconsistent. If the bearing capacity of the shaft is uneven, the bearing elevation should be adjusted with a support pad (stainless steel) to make it bear evenly.
Four. Concluding remarks
The quality of bridge hollow precast slab is related to road traffic safety and people's life safety. Therefore, in the process of construction, we should carefully construct, operate strictly according to the specifications, correct the problems found in time, and actively take remedial measures. We should dare to take responsibility for uncertain quality problems, and we should not leave hidden dangers for road traffic because of temporary interests. Never be soft on scrapped hollow slabs. It is believed that through continuous practice and exploration, the quality problem of hollow slab will be effectively solved.
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