First, cracks in concrete
1. The general mechanism of the formation of cracks is generally believed to be due to the fact that the internal stress caused by the deformation of concrete materials (including cement stone and coarse aggregate) is greater than the tensile strength of the material. This can generally be explained by the "stick-slip" mechanism. The occurrence and development of visible cracks in concrete is the result of the relative sliding between rebar and concrete that can no longer maintain deformation coordination. The width of the crack is essentially the difference between the tensile deformation of the tensioned concrete and the tensile deformation of the tensioned steel between the cracks.
2. Repair method of cracks in concrete 1 Cracks in concrete are transported for a long time, the speed of pouring is too fast, the vibrations are not true, the construction joints are improperly handled, the formwork moves, etc. Form cracks, and general concrete cracks can be used to reinforce or fill concrete materials. , steel bolt reinforcement, and even steel plate reinforcement, prestressed reinforcement and other methods to remedy.
2 For dry weather, poor initial maintenance, early freezing of concrete, cracks caused by changes in atmospheric temperature and humidity, application of crack surface treatment, filling of concrete materials, or injection of epoxy resin, etc., severely frozen components, To be removed, some need to be reinforced before use.
(3) For excessively large uneven foundation settlements, excessively wide cracks will produce excessive cracks, seams shall be adopted, and reinforcement methods such as grouting method, surface layer method, addition of reinforced concrete jackets, outsourcing type steel method Paste steel plate method, pre-stress method, changing force transmission method method, additional component method, etc.
Second, the concrete surface
1. The causes and characteristics of the formation are due to the defects of lack of slurry, sanding, and peeling on the surface of the concrete. The appearance of the component is a crisp, pit-shaped pit with a small area (less than or equal to 0.5m2) and a depth that is not deep (less than Or equal to 5mm), and no bar exposed. This kind of defect is generally caused by insufficient wetness of the template, loose support, insufficient slurry leakage or vibration during tamping, and no well-maintained bubbles after discharge and solidification.
2. The prevention and control measures for pockmarks The surface of the template should be cleaned, and no cement such as dry hard cement mortar should be stuck. Before pouring the concrete, the template should be fully moistened with water, the template should be cracked, and the linoleum paper, putty, etc. should be used to strictly block the mold. The agent should be long-acting, even brushing, and should not leak brush; concrete should be layered and evenly vibrated and compacted, until the elimination of bubbles; the surface for painting, can not be treated, the surface is not painted, should be fully watered in the pockmarks After being wetted, the gravel mortar was grinded with the original concrete mix ratio and polished. The defect thickness can be less than 5mm. Engineers can use A2 durability thin layer repair material to repair the surface layer. More than 5mm can be repaired with Engineer A3 durable high strength repair material. In the intersection of new and old concrete, interface treatment is also needed to prevent the occurrence of hazards such as empty drums and surface shedding.
Third, the lack of concrete corner off corner
1. The reason for the formation is that the concrete on the right-angled side of beams, columns, walls, slabs and holes is partially damaged and dropped. The causes are as follows: 1 The template is not fully wetted before the concrete is poured, resulting in water loss or insufficient hydration of the concrete at the edges and corners, and the strength is reduced. The angularity is damaged when the mold is removed; 2 The mold is removed or the core is drawn too early and the concrete is not yet established. Strength, resulting in damaged edges and corners; 3 lifting, transportation, protection of the structure is not good, resulting in the local part of the corner off, cracking and other damage.
2, out of corner prevention measures
1) The template should be fully wetted or brushed with a uniform release agent before pouring the crucible, and should be carefully sprinkled and cured after pouring.
2) When the side non-load-bearing form is removed, it should have sufficient strength;
3) Do not use too much force to remove the mold. Pay attention to the protection of edges and corners; when lifting, it is forbidden that the template will hit the edges and corners;
4) Strengthen the protection of the forming crucible. (3) When the angle of missing corners is small, clean water can be used to wipe the net thoroughly with a wire brush, and then use a 1:2 or 1:2.5 cement mortar to repair Qi Zheng. The unreal radon and prominent aggregate particles can be minced, rinsed clean and moist with water, and then filled with fine burrows one level higher than the original raft, and carefully conserving.
Fourth, concrete honeycomb
1. Causes and Characteristics of Formation There are three types of honeycombs: surface, deep wells, and penetrating, and they often encounter horizontal, inclined, skewed individual honeycombs and connected honeycombs, which are characterized by partial surface crispness and no cement. Pulp, coarse aggregate exposed depth greater than 5mm (less than the thickness of the concrete protective layer), there are less than the maximum stone particle size of the gap between the stone, honeycomb, honeycomb concrete strength is very low. The main reasons for the formation of concrete honeycombs are as follows: 1. The concrete vibrates poorly during vibration, especially without vibrating layer by layer; 2. When the concrete is thrown into the mold, it is stratified by the falling height; The use of dry hard concrete, or poor control of the mix ratio of concrete materials during construction, especially the water-cement ratio is too low; 4. The formwork is not tight, run slurry phenomenon occurs after the pouring of concrete, the cement slurry is lost; 5, the concrete in the transport process There have been departures.
2. Measures to control cellular phenomena (1) The mix ratio of concrete should be adjusted according to the construction conditions of the concrete process, such as vibrating mode, transport mode, rebar size, and distribution of reinforcing bars; if the workability of the concrete is not found Ideally, water should not only be added because it will impair the strength and durability of the concrete, but should adjust the mix ratio of the concrete or improve the method of pouring the concrete.
(2) Carefully unload concrete. In any case, the key point when paying attention to the unloading of concrete is to avoid segregation. The concrete should be unloaded vertically. The outlet should be as close to the final position as possible. The concrete should not flow to its designated location. If it is necessary to move them, a large spade should be used. After the completion of the unloading of a concrete vehicle, the next vehicle should be tight. Take advantage of the tail of the previous place, and do not have to start another one. Finally, connect them together because doing so tends to create a honeycomb at the junction.
(3) The formwork should have sufficient rigidity, stability and strength to avoid shifting of the formwork when vibrating the concrete; joints should be properly sealed to ensure that no leakage occurs.
(4) Carefully perform vibrating operation of concrete to avoid false vibration and excessive vibration. The new layer of concrete should be fully vibrated in place at the depth, and the distance from the vibrating point should ensure that no concrete will be left out.
3. Methods of Handling Concrete Honeycomb In general, the best way to repair a concrete honeycomb is to remove those defective concrete and fill it with new concrete. If the honeycomb surface is relatively large and the depth is relatively deep, mortar and fine stone concrete should be filled into the honeycomb by pneumatic spraying. In this method, the honeycomb mouth should be cleaned or knocked slightly outward; When filled by hand, the honeycomb mouth should be cleaned or knocked into a relatively straight, depth of not less than 25 mm. In addition, the concrete surface of the honeycomb mouth is all roughened, and all the dust debris is cleaned to ensure a reliable new and old surface connection strength. New concrete colors and old concrete should be coordinated and properly maintained.
Fifth, concrete exposed bar
1. The causes and characteristics of the formation of exposed reinforcements are not well-restored in the process of placing reinforced concrete, and the protective layer pads are not provided or are not firmly fixed. The concrete slump is small, or the demoulding is early, and the concrete is subject to spalling before being hardened by external forces. The appearance of reinforcing bars after molding the components. The main reasons for the formation of concrete structures are as follows: (1) When the concrete is poured, the displacement or pad of the protective layer of the steel bar is too little or leaked, which causes the steel bar to stick out to the template; (2) The cross section of the structural member is small, and the steel bar is dense The stone card is stuck on the steel bar, so that the cement mortar can not be filled with the surrounding of the steel bar, causing the bar to be exposed; (3) The concrete mix ratio is improper, and segregation occurs, and the template part is short of pulp or the template is leaked. (4) The concrete protection layer is too small or the concrete in the protective layer is not vibrated; or the vibrating rod hits the steel or treads on the steel bar, so that the steel bars are displaced, resulting in the dew; (5) The wooden formwork is not wet and wet. Absorbent or stripping is too early, and edges and corners are missing when the mold is removed, resulting in leakage of ribs.
2, prevention and treatment measures
(1) When pouring concrete, ensure that the position of reinforcing bars and the thickness of the protective layer are correct, and strengthen the inspection. When the reinforcement is dense, stones of proper size should be selected to ensure the concrete mix ratio is accurate and good workability; watering height is more than 2m, Application of string, or chute under the material, in order to prevent segregation; template should be fully wet and carefully plug the gap; concrete vibrator is strictly prohibited hit the steel, operation, to avoid trampling steel, such as stepping or tripping and timely adjustment of straight; The concrete of the protective layer should be vibrated and compacted; the demoulding time should be correctly mastered, and premature demoulding should be prevented and the corners should be damaged.
(2) The surface is drained. After scrubbing, apply 1:2 or 1:2.5 cement mortar on the surface to smooth the surface of the leaky rib; allow the deeper cut to remove weak coagulation and protruding particles. After being cleaned, compacted with finer concrete than the original one.
Six, concrete hollow
1. Causes and Characteristics of Void Formation Due to the imperfections in some parts of the concrete during pouring, voids are formed in the components. The voids are different from the honeycomb. The honeycomb is characterized by unconsolidated concrete or lack of grout. The void is Part or all without concrete. The dimensions of the voids are usually so large that the steel bars are all exposed, resulting in the lack of continuity in the components and the overall destruction of the structure.
The main reasons for the formation of concrete voids are as follows:
1. Vibration is not dense or leaky; 2. Spacing between steel bars is too small and concrete does not go down; 3. Sand and stone gradation for preparation of concrete is not good; 4. Concrete free-fall height exceeds specification, causing concrete stones and grout Separation; 5. Reinforced outer protective layer is not enough.
2, empty preventive measures
(1) The mix ratio of concrete should be accurate, the water-cement ratio should be strictly controlled, the feed must be accurate, the mix must be uniform, the workability must be good, and the vibrating compaction should be completed after entering the mold.
(2) The surface of the formwork should be smooth and clean, and must not be stuck with dry cement or other debris; the joints of the formwork should be tight and the wooden formwork should be fully wetted before pouring the concrete; the steel formwork should be applied with a water-based release agent and be evenly applied.
(3) Reinforced dense parts should be layered with fine-strength concrete with the same strength and should be carefully handled and carefully vibrated to ensure smooth and dense concrete surface after molding;
(4) The strength of the concrete should be strictly controlled during demolding and it is forbidden to remove the mold prematurely. The side mold can be removed only when the concrete strength can ensure that the surface and corners are not damaged due to the removal of the formwork; the bottom mold should be removed only after the concrete strength meets the design requirements or the specification requirements.
3, the method to deal with holes in the first step of the base treatment first remove the loose concrete to the hard place, grinding to remove the surface of the concrete laitance.
In the second step, interface treatment was performed using a rugged interface agent to enhance the interfacial adhesion. Tensile bond strength ≥ 3.3 MPa (concrete damage).
The third step Concrete durability repair concrete surface treatment: Wipe the durability of high-strength repair materials for repair. Thickness in accordance with the depth of the site to determine the depth of each plastering: 20mm or so, and ultimately to the level of the base level repair prevail. The compressive strength R28 ≥ 50MPa, the minimum thickness required for durability is 10mm.
The fourth step surface treatment surface layer repair: 24 hours after the surface scrape a layer of durable thin layer of repair material to ensure that the surface aesthetics, thickness: about 1mm.
VII. Quality defects caused by joints in concrete formworks Concrete defects caused by joints in the formwork. Phenomenon and cause: Due to loose joints in the joints of the formwork, leakage occurs during pouring of the concrete. Honeycombs appear on the surface of the concrete, and holes and dew occur in the concrete. The phenomenon of ribs. The main reasons for this are as follows:
(1) The sample is not meticulous or error-free. The template is made sloppy and the joint is too large when assembled.
(2) The installation time of the wooden formwork is too long, and cracks are caused by shrinkage of the wooden formwork.
(3) The wood formwork is rough and the joints are not strict.
(4) When the concrete is poured, the wooden formwork is not moistened in advance so that it expands.
(5) The deformation of the steel formwork was not repaired in time.
(6) Improper measures for joints of steel formwork.
(7) Beam, column junctions, inaccurate and misaligned joints.
The prevention and treatment measures of the joints of the template:
(1) The sample should be carefully conversed, and the detail of each sub-item should be turned into a detailed drawing in strict accordance with the ratio of 1/10 to 1/50. The details should be compiled and carefully reviewed. Styling template and assembly.
(2) Strictly control the moisture content of the wood formwork and make the joints tightly when making.
(3) The installation period of the wooden formwork should not be too long. When the concrete is poured, the wooden formwork must be moistened in advance to make it expand and open.
(4) The deformation of the steel formwork, especially the deformation of the side bar, must be straightened and trimmed in time.
(5) Control measures for caulking between steel formworks must be controlled. Do not use linoleum, plastic cloths, cement bags, etc. to caulking.
(6) The support of beams and columns at the intersection should be firm and the seams should be tight (if necessary, double-sided adhesive tape should be added between the seams).
VIII. Quality defects caused by concrete bulge
1, the phenomenon of concrete quality defects caused by the problem of template expansion, the location and causes: the deformation of the template occurred in the cast-in-place beam, column, wall, floor, resulting in increased thickness of cast-in-place parts. The concrete expansion mold will cause the size of the components to increase, the appearance is irregular, and the serious ones need to be removed to affect the appearance quality of the concrete.
2. Prevention measures for concrete bulge
1) Measures for template design.
2) Combination of loads: In addition to calculating the lateral pressure of concrete in accordance with the specifications, the concrete pouring method, stacking method, and accidental load that may occur should also be considered.
3) The design of the template: The layout of all steel template plates, supports, and pull-bolts can be calculated according to the normal method. However, when the panel is a wooden bamboo plate, the technical indicators of the plate are generally determined under standard drying conditions, after the blistering. There will be a slight loss, so when using wood, bamboo panels, it is recommended that the thickness of a certain increase over the calculated value, the cofferdam spacing and spacing of the column hoop to make appropriate reduction, the spacing of the pull bolt is also appropriately reduced.
4) The layout of the pull bolts: should be calculated in conjunction with the structural calculations. For those locations where it is not appropriate to lay the pull bolts, support can be used for reinforcement. In addition, on the floor from 200 to 250 meters away from the wall and column formwork, 18-25 steel reinforcement feet must be embedded to fix the lower profile of the template.
5) For the support of the floor slabs, the beam and slab support frames shall be calculated separately on the basis of the structural calculations, and the support frame poles shall be laid out according to the actual design conditions. The use of secondary support system or quick release system, to draw a detailed template diagram assisted construction.
6) Handle the following issues: angle form form, size and connection with wall and floor formwork; splicing method of formwork plate, splicing method between large formwork and large formwork and angle form; beam-column joint, floor and wall, At the junction of the column inside and outside the template and support methods.
7) Design the template of the door and window openings, the template of the reserved openings, their supporting forms, and the installation and positioning methods.
8) According to the actual situation of the building structure design template pre-discharge, the non-standard board placed in the cross-center easy to deal with the site.
9) Determine the installation order of the template to ensure the overall assembly quality.
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