Bridge diseases caused by 1. formwork
Formwork engineering is a series of technical work and completed entities such as the design, installation and removal of concrete formwork and support. The appearance quality of bridge depends on the quality of formwork to a great extent, and the quality of formwork production and installation is the key. The flatness, smoothness and color difference of concrete are directly related to the formwork, such as irregular formwork, loose joints, water seepage, slurry leakage, and even loose support, demoulding and deformation, which will cause poor quality of concrete or rough appearance.
At present, the diseases caused by templates cannot be completely eradicated, and only preventive and remedial measures can be taken. The following are the bridge diseases caused by formwork and the corresponding treatment methods for analysis.
1. 1 disease forms and causes
1. 1. 1 pits
Pockmarked surface refers to numerous irregular pits the size of mung beans on the concrete surface. Generally, the diameter is not more than 5 mm Due to the dense steel bars, the concrete is not vibrated sufficiently, and the bubbles are not completely discharged, and some bubbles remain between the concrete and the formwork. After the newly poured concrete enters the mold, the residence time is too long, and some parts have solidified when vibrating; Before pouring, the formwork is not watered and wet, or it is not wet enough, and the concrete moisture is sucked away by the formwork, and the steel mold isolation agent is not evenly coated and the exhaust is not smooth; Template flat-fell seam leakage, flat-fell seam near the component surface leakage less; The surface of the template is not clean, and there are impurities such as cement slurry residue.
1. 1.2 honeycomb
Refers to the concrete surface without cement slurry, there are gaps between aggregates, forming more or less holes, such as honeycomb, irregular shape, exposing stones to a depth of more than 5mm, and the depth does not leak the main reinforcement, which may leak the stirrup. Template slurry leakage, coupled with excessive vibration, slurry leakage is serious; The slump of concrete is small, coupled with insufficient excitation force or vibration leakage; Improper concrete pouring method, not using slurry blanking method and slurry vibrating method; Insufficient mixing and vibrating of concrete makes the concrete uneven and not dense, resulting in too little local mortar.
1. 1.3 hole and dog hole
Hole refers to the defect that the concrete surface exceeds the thickness of the protective layer, but does not exceed the cross-sectional size of 1/3. There are cracks in the structure, and there is no concrete locally or partially. Dog eye refers to the narrow spacing between internal and external templates, which is not easy to vibrate. The aggregate particle size is too large, and the web reinforcement is too dense, which leads to the concrete being stuck by reinforcement and conduit, and holes are formed in the lower part; Poor fluidity of concrete, or segregation of concrete components and coarse aggregate at the same time, resulting in poor concrete pouring; Not vibrating according to the pouring sequence, causing vibration leakage points; There is no layered pouring, or layered concrete is too thick, so that the vibrating radius of the lower concrete can not be reached, forming a loose state; Cement is caked, and the aggregate contains impurities such as ice and mud.
1. 1.4 "rotten edges" and "rotten roots"
Rotten edges and roots are mainly caused by loose flat-fell joints of the formwork, poor grout stopping at the joints, poor combination between the bottom edge of the formwork and the pedestal angle steel, and grout leakage on the concrete surface during vibrating. When there is less slurry leakage, "burrs" appear at the corners, and the concrete honeycomb pits are serious when slurry leakage occurs. The bottom of the template is not cleaned or not cleaned before pouring; In addition, the arrangement of attached vibrators is too dense, too low and too close to flat-fell joints, which is also a reason for the "rotten edges" and "rotten roots" of beams.
1. 1.5 The platform and the mold run alternately.
After repeated disassembly, the tongue-and-groove deformation of the formwork is serious or the verticality of the formwork is not well controlled during formwork support, and the adjacent two formwork itself is blocked; The tightness of two adjacent templates for pulling screws is different, and the expansion degree of templates after excitation is different; The lateral pressure of concrete is relatively large; The sliding thread of the pull rod and the nut thread are damaged, and the nut falls off during vibration.
1. 1.6 tangent angle
Corner cutting refers to the corner falling after the horseshoe-shaped concrete at the bottom of the beam is destroyed by the inclined section after the beam is tensioned and arched.
1. 1.7 Crack and corner drop
In some projects, due to the lack of formwork materials, in order to speed up the turnover of formwork, formwork removal is often carried out before the concrete reaches the strength required for formwork removal, which is unsafe and can not guarantee the engineering quality. In the process of formwork removal, the impact of formwork on members or concentrated stacking load will also lead to cracks in concrete members.
1.2 control measures
1.2. 1 formwork support
The appearance of concrete, that is, the formation of exposed surface, directly depends on formwork, and the selection of formwork and the quality of support directly affect the appearance of concrete. If the formwork itself is of poor quality, rusty and deformed, and the support is not standardized, it will lead to the appearance defects of concrete such as honeycomb, pitted surface, dull surface and formwork running. Construction focuses on the following aspects:
(1) The formwork shall be made of plates with sufficient rigidity and strength, not easy to deform and smooth surface, so as to prevent obvious deflection and deformation when pouring concrete. Plywood or steel formwork is suitable for formwork surface of exposed concrete surface.
(2) For a large area of concrete, it is appropriate to choose a template with small modulus and large surface area, which can reduce the number of flat-fell joints of the template, thus reducing the dislocation and unevenness of the concrete surface.
(3) Formwork flat-fell seam should ensure that the lap is flat and tight, and there should be no dislocation and big gap. For places where local gaps are difficult to fuse, rubber strips should be added to ensure no slurry leakage.
(4) The support must be stable and firm, and can resist occasional collision and vibration during construction. Support columns should be installed on the basis of sufficient bearing capacity to ensure that the allowable settlement specified in the design is not exceeded after concrete pouring. Drainage measures should be taken for the support standing on land to prevent the support from loosening due to rain soaking in soft soil.
(5) The support of the support on the back of the template should be distributed reasonably, and the flat-fell seam of two templates can be supported by battens to prevent the template from moving in the process of pouring and vibrating, resulting in dislocation. be transferred to a new job
1.2.2 Apply release agent
(1) It is best to choose clean engine oil, salad oil or special release agent sold in the market, and its release effect is far better than that of general release agent.
(2) The release agent shall be painted 30 minutes before the formwork erection and protected to prevent pollution. Should be daubed evenly, the thickness is consistent, there can be no leakage, stains and dripping.
(3) The release agent should not be changed at will, and the same one should always be used to reduce the color difference.
1.2.3 concrete pouring
(1) When pouring, the concrete surface should be carefully and thoughtfully operated to make the mortar close to the formwork and make the concrete surface smooth without bubbles, air bags or honeycombs. The thickness of layered concrete pouring shall not exceed 30cm. Pouring should be carried out continuously to prevent the formation of fault lines at long intervals.
(2) When pouring concrete, special personnel must be assigned to check the stability of supports, templates, steel bars and embedded parts. If looseness, deformation or displacement is found, it should be handled in time.
(3) After the initial setting of concrete, the formwork shall not vibrate, and the protruding steel bars shall not bear any external force.
1.2.4 concrete tamping
(1) It is not allowed to use vibrators to make concrete flow in the formwork for a long distance, resulting in segregation. The sign of concrete vibrating compaction is that the concrete stops sinking, does not bubble, does not boil, and the surface is flat.
(2) Check the flat-fell seam of the formwork before pouring, and try to block the seam that may leak slurry; Template surface is clean, isolation agent should be evenly coated.
(3) During the casting process, special personnel shall be assigned to check the quality of the formwork and strictly control the vibrating time limit every time; The performance of vibrating tools must be compatible with the working degree of concrete; One vibrating hand closely follows the ash hopper for coarse vibration, and the other closely follows the fine vibration, and carefully vibrates according to the specified vibration spacing, especially strengthening the vibration of the corners and joints of the template; Strictly carry out the methods of blanking with slurry and vibrating with slurry, and pay attention to the five properties of concrete vibrating; Concrete mixing time should be enough; The layered thickness shall not exceed the specification to prevent vibration from being out of place and not dense.
(4) Concrete with good fluidity shall be used, and fine stone concrete shall be used for the place with dense steel bars. When vibrating, designated persons shall knock outside the formwork to assist in vibrating and check the compactness of concrete. Focus on checking the corners and joints of components, and pay special attention to vibration. When mechanical vibration cannot be used, it can be replaced by manual tamping, and it should be tamped several times repeatedly to ensure that concrete does not appear pores; Adding superplasticizer into the concrete mixture ratio to ensure the fluidity of concrete to meet the work requirements; Take measures to ensure that concrete will not separate during transportation and pouring; Vibrating should be dense, and there is no vibration leakage point. The skip should move around to discharge the ash according to the thickness of the ash distribution, so as to prevent the skip from directly discharging the concrete into the formwork and discharging too much at one time. Prevent sundries from appearing in the mixed concrete.
(5) Rubber sponge strips or non-woven geotextile are entrained at the joints to stop the grout, and the flat-fell seam is designed to overlap tongue-and-groove joints, and the grout is stopped by steel-wood pressure plates and rubber layering, and the rubber sponge is stuck on the surface of the angle steel of the prefabricated pedestal to prevent the bottom edge from leaving the pedestal when the formwork is excited; Under the condition of ensuring horseshoe vibration, the first four attached vibrators between every two partitions will be reduced to three, and the position of vibrators will be improved to avoid the vibrators being too close to the bottom edge of the template and flat-fell seam.
(6) The pouring sequence of precast beams generally starts from one end of the formwork and is poured horizontally in layers. When the beam height exceeds 2.5m, horseshoe shape, web plate and wing plate are used for simultaneous casting. Starting from one end of the formwork, a horseshoe is poured first, then the web section and the wing plate are poured, then the horseshoe is poured forward, and then the web and the wing plate are poured backward, so that the pouring slope is repeatedly promoted. Due to the concentration of the horseshoe-shaped part of the normal section, the web at the beginning of the variable section is thin, and the reinforcement of the diaphragm is dense. The fluidity of large concrete is not as good as that of small concrete, and it is easy to get stuck during the blanking process, resulting in holes in the horseshoe-shaped part and the lower ear of the diaphragm. Therefore, for these parts, two proportions of small and large concrete should be adopted. The first two cans of ash should be mixed with small materials, and the horseshoe-shaped part should be poured horizontally in layers and vibrated by a vibrator. Concrete in other parts shall be poured with aniseed according to the above method, and vibrated with plug-in vibrator; Continue pouring concrete after stopping. Although it does not exceed the stop time of construction joints, it should also be inspected according to the requirements of construction joints. When the situation is serious, the construction joints should be treated in strict accordance with the provisions of the construction specifications; Carefully organize vibrating, pay attention to the five-point performance of concrete tamping, and insert the vibrating rod into the lower concrete 5- 10 cm to avoid concrete segregation; When applying engine oil, the template should be polished and smooth, and the engine oil with good quality should be applied evenly.
1.2.5 Trim the template regularly.
Ensure that the flatness of the bottom edge and flat-fell seam of the template meets the specification requirements; Fasten the formwork by designated personnel, and keep the tightness and strength of the pull screw consistent; When assembling the mold, tell the operator to check the working condition of the pull rod to prevent the use of the pull rod nut with broken wires and the deformed pull rod. When the lateral pressure of concrete is high, double nuts are installed on the pull rod. When strongly excited, add a damping spring gasket under the nut to prevent the pull rod from breaking wires and demoulding.
Step 2 crack
The causes of concrete bridge cracks are complex and varied, and even many factors influence each other, but each crack has one or several main reasons. According to the causes of cracks, the types of cracks can be roughly divided into the following categories.
2. 1 Cause analysis of cracks
2. 1. 1 cracks caused by load
Cracks caused by static and dynamic loads and secondary stresses in concrete bridges are called load cracks, which mainly include bending cracks, shear cracks, torsion cracks and cracks caused by local stresses. Load cracks show different characteristics according to different loads. This kind of crack often appears in tension area, shear area or serious vibration area. If peeling occurs in the compression zone or there are short cracks along the compression direction, it is often a sign that the structure has reached the limit of bearing capacity and a precursor to structural failure. According to the different stress modes of the structure, the characteristics of cracks are: (1) center compression, and short and dense parallel cracks parallel to the stress direction appear along the member.
(2) The center is in tension, and the cracks run through the cross section of the member, with approximately equal spacing and perpendicular to the stress direction.
(3) When bending, cracks perpendicular to the tensile direction begin to appear near the section with the largest bending moment in the tensile zone.
(4) Under shear, when the stirrup is too dense, it will be destroyed by oblique pressure, and oblique cracks will appear along the end and abdomen of the beam in the direction greater than 45; When the stirrup is suitable, shear and compression failure occurs, and oblique cracks parallel to each other in the direction of about 45 appear along the middle and lower part of the beam end.
(5) Under torsion, a number of oblique cracks in the direction of about 45 appear on one side of the member, and they spread to the adjacent surface in a spiral shape.
(6) By stamping, the inclined plane in the direction of about 45 degrees is pulled along the inner side of the column head plate to form a stamping surface.
(7) Local compression, in which several short cracks approximately parallel to the pressure direction appear.
2. 1.2 cracks caused by temperature change
Concrete has the characteristics of thermal expansion and cold contraction. When the temperature of concrete changes due to the release of hydration heat of cement, sunlight, atmosphere, environmental temperature and other factors, it will shrink and expand to produce temperature stress, and when the temperature stress exceeds the strength of concrete, cracks will occur. Temperature cracks are different from other cracks mainly because they expand and close with the change of temperature.
2. 1.3 cracks caused by shrinkage
When concrete solidifies, concrete shrinks sharply due to the rapid evaporation of water on the surface of concrete or the rapid absorption of water by foundation and formwork, as well as the high hydration heat of concrete itself. At this point, the strength of concrete is too small to resist this deformation stress, leading to cracking.
2. 1.4 cracks caused by raw material quality
Concrete is mainly composed of cement, sand, aggregate, water and additives. Unqualified materials used in concrete may lead to cracks in the structure, which are mainly manifested as excessive silt content in sand and gravel, poor grading of sand and gravel, muddy silicide in aggregate, chloride and other impurities in mixing water or admixture, which have great influence on the corrosion of steel bars.
2. 1.5 cracks caused by poor quality of construction technology
In the process of pouring, formwork lifting, transportation, assembly and hoisting of reinforced concrete structures, if the construction technology is unreasonable and the construction quality is not high, various forms of cracks may appear, especially for slender thin-walled structures. The location, direction and width of cracks vary due to reasons, such as the thickness of reinforced concrete protective layer is too thick, the concrete is not compacted and vibrated unevenly, the concrete is cast too fast, the water-cement ratio is too large, the concrete is frozen in the early stage, and the formwork is removed too early.
2.2 Repair of cracks in concrete bridges
In the design, construction and operation stages, preventive measures should be taken according to different reasons. Many cracks can be overcome and controlled, but some cracks are inevitable. Some cracks continue to expand under service load or external physical and chemical action, resulting in peeling of protective layer and corrosion of steel bars, reducing the performance and durability of concrete and endangering the normal use of bridges, which must be repaired as soon as possible.
2.2. 1 surface sealing method
(1) Surface sealing method is the simplest and most commonly used method to repair cracks. It seals cracks with small width to prevent the invasion of water vapor, air and chemicals.
(2) Before sealing, the crack surface should be treated, and the dust, scum, loose layer and other dirt on the crack surface should be removed with tools such as wire brush, and then the two sides of the crack should be scrubbed with organic solvents such as alcohol to keep it clean.
(3) After the crack treatment, coat a layer of epoxy resin on both sides of the crack first, and then coat a thin layer of epoxy resin cement, and scrape the surface to ensure tight.
Grouting method
(1) Wider cracks should be pressure grouted. Grouting treatment generally adopts epoxy resin for grouting.
(2) The surface of the crack is treated, and a V-shaped groove or a U-shaped groove is chiseled along the crack. The width and depth of the groove can be determined according to the depth of the crack and the convenience of sealing. When cutting hair, first open along the crack and then widen it to both sides. After chiseling, use a wire brush and an air pump to clean concrete chips and dust.
(3) The spacing of grouting nozzles can be determined according to the crack depth. When buried, the grouting nozzle shall be pasted in the predetermined position with epoxy mortar.
(4) After the cracks are closed, pressure leakage test shall be conducted after the sealant mud reaches a certain strength to check the sealing effect.
(5) Before grouting, check the grouting machines and instruments, and use them only after they are in normal operation. Immediately after grouting, remove the pipeline and clean the pipeline and equipment with acetone.
(6) After grouting, smooth the grouting place with epoxy mortar, and finally brush epoxy cement slurry on the surface of each crack to ensure tight sealing and try to match the color of bridge concrete.
Conclusion: In the process of concrete construction, the quality control of raw materials and templates should be strengthened, the construction site should be inspected in time, and effective measures should be taken to strengthen the prevention of common quality defects. When problems or matters affecting the construction quality of concrete structures are found, they should be solved immediately to keep the construction quality of concrete structures under control, improve the construction quality of concrete structures and prevent diseases.
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