1 Introduction
Prestressed concrete is to apply compressive stress to the tensile area in advance in the use stage. When the component bears the tensile stress caused by the service load, the preloading stress of concrete should be offset first, and then with the increase of the load, the concrete in the tensile area will produce tensile stress, which can delay the appearance and development of concrete cracks.
2 factors affecting the quality of prestressed concrete bridges
2. 1 influence of raw materials
The quality and performance of raw materials are the main factors that directly affect the quality of prestressed concrete, and harmful impurities in water and sand have corrosive effects on cement concrete. Harmful impurities react with concrete to produce water-soluble substances, which corrode concrete. If the cement clinker is not fully calcined, more free calcium oxide will be produced. Because the hydration process of free calcium oxide is very slow, it will lead to the volume expansion of cement after solidification and hardening, resulting in uneven volume change and cracks.
2.2 The influence of material creep on quality control of prestressed bridges
Material creep has great influence on deformation and structural internal force, which is mainly caused by the small loading age of concrete in bridge construction and the great difference in different stages. It should be carefully studied and reasonable and practical creep parameters and calculation models should be adopted.
2.3 Influence of temperature change on quality control of prestressed bridge
The temperature change has a great influence on the stress and deformation of bridge structure, and this influence changes with the temperature change. If the influence of temperature change on the bridge is ignored in construction control, it will be difficult to obtain the real state data of the structure, thus ensuring the effectiveness of quality control. Therefore, the influence of temperature change must be considered in prestressed concrete construction.
2.4 The influence of construction process on the quality of prestressed bridge
In the process of construction, construction quality management has great influence on prestressed bridges. Every construction process is very important, such as formwork support, concrete pouring, prestress stretching and concrete curing.
3 prestressed bridge quality control measures
3. 1 Ensure the quality of concrete
3. 1. 1 Quality control before concrete construction.
The quality and performance of materials are the main factors that directly affect the engineering quality. The main materials are gravel, cement and additives. Aggregate used in concrete mixture can be divided into fine aggregate and coarse aggregate according to its particle size. The maximum particle size of coarse aggregate used in concrete shall not exceed 1/4 of the minimum size of the member section, and shall not exceed 3/4 of the minimum spacing of steel bars. At the same time, considering the pumping characteristics of ready-mixed concrete, the maximum particle size of stones should be controlled below 3 1.5 mm, and the variety and quality of cement is one of the keys to control the strength of concrete. Choose the cement that meets the requirements according to the design requirements. When cement enters the site, check its variety, grade, packaging or bulk warehouse number, factory date, etc. And re-check its strength, stability and other necessary performance indicators. Its quality must conform to the current national standards and other regulations.
3. 1.2 concrete construction quality control.
(1) mixture ratio
An important factor affecting coagulation quality is the mixing quality of mixture, that is, the determination of mixing ratio. The construction mixture ratio should be determined according to the design strength grade and quality inspection of concrete and the workability of concrete construction. Fully consider the engineering characteristics, climatic conditions, concrete delivery methods and other factors to prepare.
(2) concrete mixing
The sand and gravel entering the mixer must be accurately weighed, and the weighing should be checked and corrected before use. Bulk cement must be weighed in heavy bags, and the weight should be checked when measuring in bags. Strictly control the water addition. Before each shift starts, the water content of sand and gravel shall be determined according to the weather change, and the construction mixture ratio shall be determined by the site laboratory.
(3) Concrete pouring and curing
When pouring concrete, attention should be paid to prevent concrete from delamination and segregation. After the concrete mixture is transported to the pouring place, it should be poured into the mold immediately. The concrete discharged from the hopper and funnel for pouring concrete should not exceed 2 m in height. The vibrating of concrete should be carried out with the cooperation of flat vibrator, plug-in vibrator and vibrating beam with power not less than 22 kW. Concrete pouring shall be carried out continuously. If it is necessary to shorten the intermittent time, the second layer of concrete should be poured before the first layer of concrete sets. The total time of concrete transportation, pouring and intermittence shall not exceed the specification, and construction joints must be set when the specified time is exceeded. Measures should be taken to prevent cracks in the process of concrete pouring and standing. Non-structural surface cracks caused by concrete settlement and drying shrinkage should be repaired before the final setting of concrete.
After the concrete surface is repaired, it should be cured to make the concrete slab have enough strength and quality before it can be opened to traffic. If the curing is not good or timely, the concrete will dehydrate. The curing mode and time of concrete are related to natural temperature and concrete composition. When the average temperature is higher than 5℃, cover the concrete surface with suitable materials and water it. The watering should be carried out within 65+/-02h after concrete pouring, at least for 7 days.
3.2 Prestressed pipeline installation
Whether the installation of prestressed pipeline is accurate directly affects whether the stress of prestressed bridge beam meets the design requirements, which is related to the operational safety after the bridge is completed and is the focus of prestressed concrete construction. Before installation and concrete pouring of prestressed pipeline, the position, flatness, slurry leakage and sealing of pipeline should be strictly controlled.
3.3 Stretching and Grouting
(1) Before tensioning, the concrete strength must reach more than 75% of the design strength, and the tensioning adopts double control, namely stress control and elongation control.
(2) In the process of construction, if the jack tool clamp is worn, the clamping is not tight and the thread slips, the oil should be returned immediately, the tool clamp should be replaced, and whether there are sundries between the anchor hole and the clamp should be checked, and then it should be re-tensioned after being removed.
(3) Pay special attention to safety during tensioning, especially when tensioning or retracting the wedge, no one can stand behind the jack to prevent the prestressed tendon from breaking or the anchor wedge from popping up and hurting people.
(4) Due to the damage of bellows, the beam leaks, which leads to the steel strand being wrapped by concrete, resulting in excessive friction. At this time, the anchor rod should be tensioned repeatedly and kept for a period of time to overcome the influence of excessive friction, and pay attention to hole cleaning in time.
(5) During the construction, the grouting of prestressed duct must be carried out in strict accordance with the working procedure. Prestressed tunnel grouting has two functions: one is to protect prestressed tendons from corrosion; The second is to ensure that prestressed tendons and structures work together; However, there are some phenomena in the construction site, such as loose grouting, insufficient grouting and slurry leakage, which have become common problems of prestressed concrete bridges. To prevent loose grouting, the following measures can be taken: 1) Before grouting, flush the tunnel with high-pressure water to remove impurities and make the whole pipeline clear and wet.
2) To prepare high-quality cement slurry, ordinary portland cement with strength grade of not less than 32.5MPa should be used, and the cement slurry should have good fluidity, and the water cement ratio of the cement slurry should be controlled at 0. 35 ~ 0.45. It is best to add appropriate amount of water reducing agent and micro-expansive agent to improve its fluidity, reduce water cement ratio and increase compactness.
3) If the first grouting is not ideal, the second grouting can be carried out, and the second grouting should be carried out after the initial setting of the first grouting.
3.4 Prevent steel wire from slipping and breaking.
The main reasons for steel wire slippage are serious corrosion of steel strand, sundries and oil stains on the surface, or large size deviation of work clips. The main reason of broken wire is uneven stress caused by winding or mechanical damage of steel strand. In order to prevent the steel wire from slipping excessively and breaking, the following measures should be taken.
(1) During construction, the diameter deviation, ellipticity and hardness of steel strand should be included in the inspection contents. If the deviation exceeds the standard and the quality is unstable, we should consider changing the product supplier. Steel strand should be protected during transportation, blanking and installation to reduce damage.
(2) In addition to checking the factory certificate, the hardness and appearance of the clip should also be tested on site.
(3) If the number of slip lines and broken wires does not exceed the specification, it can be left untreated. If there is a whole bundle or a large number of slippery or broken wires, the anchor head should be removed, and the replacement steel bundle should be checked and re-tensioned.
3.5 Anchoring seal
After the grouting process is completed, the anchor should be sealed in time. Before sealing the anchor, the concrete at the anchor should be cut open. The standard of chiseling is that a large amount of gravel is exposed on the concrete surface, but the integrity of the concrete under the anchor pad should be protected. Before pouring concrete, the chiseled concrete surface should be wetted to facilitate the combination of new and old concrete and ensure the quality of sealed concrete.
4 abstract
The construction technology of prestressed bridge is complex, so the quality of the whole project must be strictly controlled from the aspects of raw material detection, steel bar production, concrete pouring, tension grouting, concrete maintenance, anchor sealing and so on, and the construction should be carried out according to the specifications and design drawings, so as to effectively ensure the overall engineering quality of the construction.
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