Reinforcement of bridge’s prestressed Plate

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Reinforcement of bridge’s prestressed Plate of Highway from Panjgur to Hoshab in Pakistan

1. Introduction
No. 2 bridge Pakistan highway from Panjgur to Hosha is located in the place 8 kilometers away from Panjgur, leading to Hosha in the west, and the whole length of bridge is about 324.8 m.
However, for various reasons, the bridge had still not taken reinforcement renovation by March, 2016. From October 9, 2015 to November 2, 2015, the evaluation and bridge technique condition and load test were carried out. By comparison between the old and new test reports, bridge diseases are further serious, the quantity of cracks increases, and individual crack’s width is enlarged. The corrosive cracks of prestressed concrete box increase. Reinforced concrete cracks increase, the individual crack width. Structure concrete carbonation depth is increased, and the intensity is reduced. Bearing damage is increased, and bearing displacement is too large. The expansion cracks on the surface are seriously damaged. Main bridge’s bearing capacity is insufficient, and cannot meet the requirement of the designed load and carrying capacity.
2. Materials for Reinforcement
  carbon fiber plate, sticky trap glue, anchor plate and tensioning machine
3. Design of maintenance and reinforcement engineering
Based on the check of overall bridge structure, it can be seen that the bridge has the problems of different degrees in structural safety, durability and driving comfort. In order to ensure the bridge structure’s security, durability and driving comfort, and a and properly recover the original functions of various bridge components, the bridge reinforcement shall be immediately conducted, further to prevent diseases from being more serious; otherwise the service life for the bridge’s structure safety and driving safety cannot be guaranteed. The scheme of using prestressed Plate for repair and reinforcement engineering is as follows:
In order to improve the whole strength of the bridge, enhance the bearing capacity of components and slow crack propagation, the measure of using bonded prestressed carbon fiber plate is taken to reinforce and maintain. Within the scope of beam bottom of the first stage of the main bridge (1st ~ 6th holes of the main bridge), the second stage (7th -11th holes of the main bridge), and the third stage (13rd-14th holes of the main bridge), the bottom of the beam, the prestressed carbon fiber plate is bonded forward the bridge. Within the scope of prestressed concrete continuous box beam bottom of the first stage and the second stage, six prestressed carbon fiber Plates are made for every piece of box beam bridge, and the thickness of each carbon fiber board is 1.4 mm, its width is 100 mm, and its length is 17000 mm. Within the scope of the beam bottom of simple supported-continuous small box of the third stage, two prestressed carbon fiber Plates are made for each small box beam bottom. The thickness of each carbon fiber board is 2.0 mm, its width is 100 mm, and its length is 22000 mm. The tension control stress of prestressed carbon fiber is 1000 mpa, and the control strain is 6‰. The measured value of tensile elastic modulus of carbon fiber plate needs to be higher than 160 gpa, and the fiber volume content should be higher than 70%. The prestressed carbon fiber board system uses the matching prestressed anchor plate and tensioning machine, but the single-ended tensioning or tow-ended tensioning are made at the same time. Anchor plate adapts alloy anchor plate with high strength and corrosion resistance; the fixed end and tensioning end use high-strength chemical anchor bolt and the concrete beam structure for connection.
Construction technique of bonded prestressed carbon fiber plate
(1)Construction preparation: It shall set up the construction stent, purchase the main materials and process and manufacture anchorage steel components. According to the condition on site, the construction stent that is suitable for construction shall be set up, with construction layout. In the reinforced beam surface, according to the design drawings, the construction layout is made. It shall determine the position of carbon fiber plate and anchorage on both ends.
(2)Processing of concrete surface: The concrete structure of beam steel is directly exposed to air with certain humidity for a long time. During the construction of bonded carbon fiber plate reinforcement, it needs to remove embrittled and loose layers, so as to enhance the cohesive force. For the embrittled and loose layers of concrete surface, it should use hard diamond grinding wheel to grind them out and make the new surface shown, as well as completely remove loose floating layer with a wire brush. In order to ensure the design gap between the carbon fiber plate and the concrete (rubber thickness is 3~5 mm), and realize the carbon fiber plate is straight and unimpeded when tensioning, the concrete’s high convex shall be polished by diamond grinding piece, and epoxy repair glue should be adopted to repair concrete low concave. If concrete humidity is bigger when it is bonded, manual dry processing should be made for the bonded surface of the concrete.
(3)In position for the tensioning end of carbon fiber plate and fixed-end components, drilling and planting are made for high-strength screw in accordance with the design drawing: the method of planting reinforced bars is used for installation, and the drill with the drilling diameter that should be matched with the bolt diameter is used for drilling; positioning and drilling of planting reinforced bars: before drilling, it shall determine the position of the bean steel and mark it; when drilling hole has contradiction with reinforced location, appropriate adjustment is made for the hole position, and it should be vertical to the beam body when drilling. The blower nozzle stretches into the bottom of the hole and the clean air is blown into it, pulling out of the nozzle, which is repeated three times. The scrub brush is inserted into hole, rotating flush back and forth for three times. The hair dryer blowing nozzle gets into the bottom of the hole, which is repeated 3 times;
The oil dust on the planting bolt should be cleaned up. Before planting the bolt, acetone is used to clean the hole wall, hole bottom and bolt. The special injector for planting glue is adopted for perfusion, and the amount is as much as two-thirds of the hole depth, ensuring a little glue overflows after implantation of bolt. After injecting glue, it shall immediately make the one-way rotating to insert bolt, until it reaches the design depth, to ensure the top of the screw is at the same plane, and correct the bolt verticality. Before the glue is cured, it cannot touch or vibrate the embedded bolt, avoiding the viscidity.
(4)After screw curing meets the designed strength, tensioning-end and fixed-end steel parts shall be installed: the grease is brushed onto the contact part of anchorage parts and the carbon fiber board; acetone is used to clean the surface of concrete parts contacted by the carbon fiber plate; carbon fiber board is installed on the fixed end and tensioning end, and carbon plate glue is made between the carbon fiber plate and beam bottom. After the glue for carbon fiber plate is primarily fixed, compression bar can be installed; when installing compression bar, shim is installed at the bottom of the anchor bolt to keep 0.8 cm gap between the bar and concrete crack, so that the carbon fiber plate can slide freely when tensioning. At the same time, before the compression bar and prestressed carbon plate, rubber piece is stacked to prevent carbon plate from being damaged. After compression bar is installed in place, jack is installed at tensioning end, to make sure that jack midline mergers with carbon fiber board line in a horizontal position, with pressure tensioning till 15﹪of the designed tension stress value, and the anchorage tensioning moves to the starting line. Pressure tensioning makes it reach 30% of the designed tension stress value; it shall check whether there is glue liquid extrusion overflow between the carbon fiber plate edge and the beam surface; if there is no liquid extrusion overflow for some parts, it shall relieve pressure and then make tension; it shall check whether anchorage displacement corresponds to prestressed tension.
Tension is increased with certain phrase until 50﹪, 70%, 90﹪ and 100﹪ of the designed tension stress value. After each level of the tensioning is finished, screw is fixed with a wrench, and sustained load between each level is five minutes, to record the location of the tension-end clamp, with the comparison of the deviation between measured and calculated value. It shall check whether the concrete displacement of tensioning anchorage meets the requirements of theoretical elongation; according to the requirements of specification, the error of concrete displacement of tensioning anchorage and theoretical elongation should be not more than ±10﹪.
When prestressed force is applied to 100%, the final carbon fiber plate’s tensile value is calculated, and the sustained load is 5 minutes. After the tension, double nut is used to fix tensioning screw. Jack is removed. Too long tensioning screw is cut,  and the back-end shall be 3 cm.
(5)Curing maintenance: the curing time shall be reasonably controlled based on construction environment temperature and material properties of carbon fiber composite plastic; when construction environment temperature is below 25 ℃, curing time shall be relatively longer. In carbon fiber plate’ prestressed tensioning system, anchorage are steel part, prone to rust. Because carbon fiber plate is resin composite, and should avoid the erosion of ultraviolet ray; after completion of tensioning inspection, epoxy resin mortar with good cohesiveness, anti-corrosion and good durability is used, and the not less than 3mm epoxy resin mortar is coated for anchorage and carbon fiber plate.
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