I. Introduction
With the rapid development of urban construction, bridge construction has developed rapidly. With the increase of bridge service life, the number of dangerous bridges is also increasing sharply. Taking the reinforcement project of Dawangzhou Bridge in Dongguan as an example, it is of universal guiding significance to summarize the reinforcement technology of the bridge project to ensure the reinforcement quality and the safety of the construction process.
Second, the current situation analysis
1. The strengthening project of Dawangzhou Bridge is limited in construction facilities, working environment and working space due to the influence of the original structure. For example, all the construction materials and equipment in the beam body need to be moved in and out manually, and the mechanical equipment is basically unusable, making the safety state even more uncontrollable.
2. The reinforced part works together with the original structure, not only to ensure the construction quality of the reinforced part, but also to ensure that the original structure is not destroyed.
3. Web reinforcement is the key construction process of reinforcement project. Web reinforcement adopts adding 20cm thick concrete on the inner side of the original box girder, and applying vertical prestress on the reinforced web. The vertical prestress adopts cold-drawn grade IV steel bar with a diameter of 25mm, standard strength Rby=750Mpa, tension control force of 33 1KN, and adopts unbonded method and steel wire rolling anchor system.
Third, the preparation before construction
1. power supply line: 380V power supply line and 220V lighting line are laid in the box, one lighting lamp is set every 4m, and a simple mobile switchboard (several sockets) is set every 16m. Lighting and power lines are fixed at the top and lower edge of the beam (the center of the bridge) with expansion bolts to prevent the ground from being wet and leaking electricity.
2. Dredging and cleaning: Dredge the ventilation holes and drainage holes of the original box, make holes in impassable places or manually, and keep normal ventilation inside and outside the box to avoid water accumulation in the box. Clear the concrete residue and other sundries in the box.
3. Ventilation: 4 sets of ventilation facilities are installed in the box to keep the air circulation and normal ventilation in the box, so that the staff in the box have a good working environment.
4. Facilities: 5 construction orchids need to be installed outside the beam.
Fourth, the construction scheme
1, crack repair
First, mark the cracks larger than 0.02mm, cut a "V" groove with a width of 2cm and a depth of 1cm along the direction of the cracks, bury it in the grouting nozzle, and inject epoxy resin after the strength meets the requirements. After the epoxy resin is cured, remove the epoxy mortar and check the fullness of the grouting.
2, construction lofting
(1) Because this reinforcement project is to reinforce the existing structure, taking into account the errors in construction, in order to ensure that the prestressed tendons of the original main beam are not destroyed and achieve the reinforcement effect, relative plane control should be adopted for the main construction layout.
(2) According to the characteristics of the original main beam suspension casting construction: the overall dimensions of the beam, the positioning of prestressed tendons, and the positions of reinforcing bars and embedded parts are all controlled by the relative positions with the bottom surface of the bottom plate, so this reinforcement project takes the midpoint connecting line of the front end bottom surface of the original construction section as the central control line of the bridge.
(3) The mileage control point of this bridge is based on the center of pier No.5 and transmitted to the center of pier No.0, which will be used as the relative mileage control point of the whole bridge after closing adjustment (it can be used as the lofting basis within the allowable range of closing difference).
(4) According to the design requirements, control the elevation of the web nail hole and the relative size of the chamfer on the bottom surface of the main girder roof.
⑤ When placing the transverse hole of the diaphragm beam, the size of the transverse bridge will be released according to the measured size of the bridge centerline, and the elevation will be controlled based on the bottom surface of the beam top bridge, supplemented by the distance check of the bottom surface of the beam bottom plate (confirm to avoid the prestressed tendons in the beam).
3, drilling
(1) The φ27 mm nail holes of web reinforcement are drilled by impact drill electric hammer with a diameter of φ27mm, and 43,262 φ27 holes are drilled in the whole bridge. The designed drilling depth is 15cm. Considering the bit taper, the depth of the drill tip should not be less than 15.5cm during drilling. Measures should be taken to control the depth so as not to damage the prestressed tendons. After drying in the hole, φ16mm "L" spike was implanted and filled with epoxy mortar.
② Original prestress measures for drilling vertical prestressed holes to avoid collision: vertical prestressed reinforcement holes (φ40mm), concrete pouring holes (φ82mm) and vibrating holes (φ56mm) are all drilled by 130 diamond coring machine, and the drilling diameters are φ40mm, φ82mm and φ56mm respectively. Φ φ40m holes need to be drilled for the web reinforcement of the whole bridge. There are 392 φ82mm holes. The main girder of the original structure is a three-dimensional prestressed system, and the prestressed tendons (bundles) are arranged densely, and all the holes are drilled in the gaps between prestressed tendons. Even if the lofting construction position is accurate, there are deviations in the prestressed installation of the original bridge itself. Especially, the distance between the centers of 0 longitudinal prestressed tendons in the center of pier is 200mm, the original prestressed tendons are φ external 85mm corrugated pipes, and the theoretical gap between φ82mm concrete pouring hole and the original prestressed tendons is only 16mm, which is particularly strict for lofting and construction. Therefore, it is extremely important to adjust according to the actual situation during drilling.
(3) Because the original prestressing tendons of the main beam are too dense, the drilling accuracy is very high. In order to ensure that the prestressed beam does not break when drilling, drilling is operated by skilled workers. Before formal drilling, organize workers to try drilling, and explain the position of the original prestressed beam to the workers, so as to know fairly well. At the same time, strengthen workers' training on drilling rig performance knowledge, drilling rig positioning method and drilling footage.
4, the new and old concrete combination measures
(1) Adding new concrete to the original structure, how to ensure the force transfer of the joint surface is the key. Therefore, the method of chiseling away the joint surface of new and old concrete and planting nails at regular intervals is adopted.
(2) In order to ensure the good combination of the post-poured concrete and the original web, and achieve the common stress, the upper and lower chamfered mixed surfaces inside the original web and within the range of thickened web should be chiseled. When chiseling, the sundries that hinder the combination of new and old concrete should be removed first, and the epoxy cement on the closed surface should be chiseled, the grouting nozzle should be removed, and the sealing tape should be peeled off. Finally, the original concrete surface is chiseled, and the chiseling rate should reach 95%. Before grouting, the dust and impurities on the joint surface should be cleaned up.
③ Another measure to strengthen the combination of new and old concrete is planting nails.
Plum blossom spikes are arranged on the net with an interval of about 30cm, and the spikes are L-shaped; Welding with external stirrups, the spacing between upper and lower chamfers is about 20cm, and the skewers are straight ribs, which need to be welded with external stirrups; The nail spacing at the end of the steel mesh is 20cm, and the nail is L-shaped, which requires welding with the transverse horizontal bar of the steel mesh. The nail string is bent with φ 16mm steel bar, implanted into the drilled φ27mm nail string with a depth of 15cm, and anchored with epoxy cement.
5, concrete pouring and maintenance
The formwork is made of new steel with a width of 60cm and a maximum height of less than 200cm, which is convenient for manual movement. After the installation inspection is qualified, concrete pouring can be carried out. Web reinforcement adopts 50# micro-expansive concrete, and the concrete mixture ratio is prepared by the laboratory according to the design requirements and construction needs. The sinking degree of concrete into the mould shall be controlled at 12cm~ 14cm, and the initial setting time shall be 10 h. Micro-expansive cement shall be added to compensate the shrinkage of concrete and prevent shrinkage cracks.
Because the thickened part of the web is completely closed except the mortar hole and vibrating hole under the roof, all the concrete is manually rammed into the mold from the mortar hole below 56 mm, and the concrete is continuously cured for 14 days. When the concrete starts pouring, the bridge is closed and all vehicles are prohibited.
Step 6 be nervous
Tension operation can only be carried out after all cracks inside and outside the beam are repaired and grouted. The vertical prestress of web reinforcement is φ25mm cold-drawn IV unbonded reinforcement, with its end anchored at the bottom of beam bottom plate and its upper end anchored at the top of beam top plate.
Verb (abbreviation for verb) safety and quality
1. The working lighting in the box is sufficient and the ventilation facilities are good.
2. There are many wires in the cabinet, and the cabinet is wet. Therefore, it is necessary to strengthen the safety management of electricity consumption, ensure the safety of electricity consumption, and set warning signs for high-voltage electrical appliances.
3. Concrete slag and sundries shall not be dumped at will from the bottom drain hole, and sundries in the box shall be cleared in time.
4. When drilling, the top surface of the main girder bottom plate is prone to water accumulation, so measures such as drilling small-diameter drainage holes in the depressions between sawtooth blocks should be taken in advance to prevent water accumulation.
5. During the period of web reinforcement, designated persons will be assigned at both ends of the main bridge to direct traffic, and strictly control the vehicles not to exceed 20t, the vehicle speed not to exceed 10km/h, and the distance between vehicles not less than 20m. Overloaded vehicles are strictly prohibited from getting on the bridge. Traffic control of the whole bridge during web crack grouting: only one vehicle is allowed to pass on the deck of the main bridge, with the vehicle not exceeding 20t and the vehicle speed less than10 km/h. 。
6. Warning signs such as signal flag and red light should be set on the bridge deck bracket of each set of construction hanging basket, and all vehicles are prohibited from colliding to ensure the safety of the hanging basket. The construction hanging basket can only be used after it is installed and accepted, and all stressed parts should be checked regularly during use.
7. As the Haihe waterway is below, a large number of fishing boats pass by every day, and it is strictly forbidden to throw sundries under the river.
Ending of intransitive verbs
In the bridge reinforcement operation, the problems of difficult lofting and poor accuracy are overcome. In the process of drilling, the original prestressed system is ensured to be intact, the concrete is vibrated tightly, and the top pouring is full, and the reinforcement task is completed as planned.
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