1. Concrete segregation
The segregation of concrete is that the cohesion between the components of the concrete mixture is insufficient to resist the sinking of the coarse aggregate, and the components of the concrete mixture separate from each other, resulting in internal composition and structural unevenness. It usually manifests as the separation of coarse aggregate and mortar from each other, for example, dense particles are deposited at the bottom of the mixture, or coarse aggregate is separated from the mixture as a whole.
2. Pockmarked surface
Cause analysis:
1) The surface of the template is rough or not clean;
2) Release agent The painting is uneven or partially missed;
3) The formwork joints are not tightly spliced, and slurry leaks from the gaps when pouring the concrete;
4) The vibration is not dense, and there are air bubbles in the concrete. Some of the air bubbles that have not been discharged remain on the surface of the template.
The concrete surface is partially rough due to lack of slurry and has small pits
Preventive measures:
1) Clean the surface of the formwork, and no dry matter must be allowed to stick to it. Hard cement, etc.
2) Before pouring concrete, wet the formwork with clean water, leaving no water, and tightly splice the gaps in the formwork
3) The release agent must be applied evenly without leakage Brush
4) The concrete must be vibrated and compacted in layers according to the operating procedures to strictly prevent vibration leakage. Each layer of concrete should be vibrated until bubbles are released
3. Honeycomb
p>Cause analysis:
1) The concrete mix ratio is inaccurate or the aggregate measurement is wrong;
2) The concrete mixing time is short, the mixture is not evenly mixed, and the concrete mixes easily. Poor performance and poor vibration density;
3) When pouring concrete, improper feeding or too much feeding at one time, without pouring in sections and layers, resulting in concrete leakage and segregation;
4) The pores of the formwork are not blocked, or the formwork support is not firm,
The formwork shifts, causing serious grout leakage or wall root rot.
Phenomena: The concrete is partially loose, with less mortar and more stones. There are gaps between the stones, forming honeycomb holes
4. Holes
Cause analysis:
1) Concrete pouring is not smooth in areas with dense steel bars or reserved holes and embedded parts
2) Failure to follow the application sequence and construction technology carefully, resulting in leakage vibration
3) Segregation of concrete, separation of mortar, piles of stones, or severe slurry
4) Mud, wood and other debris are mixed into the concrete
5 ) Failure to cut materials according to regulations, too much material is cut at one time, and the vibration is not in place
Prevention and control measures:
1) In places where it is difficult to cut materials, manual paving and pouring of concrete can be used
2) Correct vibration to prevent vibration leakage, and strengthen vibration at the corners
3) Prevent the inclusion of debris such as earth or wood blocks
4 ) Choose a reasonable cutting and pouring sequence
5) Strengthen construction technology management and quality inspection
5. Gap interlayer
Cause analysis:
1) Before pouring, the surface of the construction joint was not carefully treated
2) Insufficient tamping
3) The garbage was not cleaned before pouring
Prevention and Control Measures:
1) Before pouring concrete, carefully clean up the garbage and debris in the formwork, and treat the surface of the construction joints
2) During the pouring process, vibrate and compact; at the same time, prevent Wood blocks and other debris fell into the concrete
3) During winter construction, winter construction precautions should be formulated to prevent interlayers of ice and snow
6. Missing edges and corners
Cause analysis:
1) The wooden formwork was not moistened or not moistened enough before the concrete was poured, or the steel formwork was not coated with release agent or was applied unevenly
2) The concrete was not maintained properly Good;
3) Remove the side non-load-bearing formwork prematurely;
4) When removing the formwork, external force or heavy object impact, or poor protection, the edges and corners are knocked off
Phenomena: Concrete falls off partially at right angles to beams, columns, slabs, walls and openings, is irregular, and has defective edges and corners
Prevention measures:
1) The formwork is being poured The concrete should be fully moistened or evenly coated with release agent before pouring, and the concrete should be carefully watered and cured after pouring
2) When removing the side non-load-bearing formwork, the concrete should have sufficient strength;
3) Do not use too much force or haste when removing the mold. Pay attention to protecting the edges and corners; when lifting, it is strictly forbidden to hit the edges and corners of the formwork;
4) Strengthen the protection of the formed concrete.
7. Exposed reinforcement
Cause analysis:
1) When pouring concrete, the pads were displaced or the number was too small;
2) The cross-section of the structural members is small and the steel bars are too dense;
3) The concrete mix ratio is improper, resulting in segregation;
4) The protective layer is small or the concrete leaks vibration;
5) The wooden formwork was not watered and moistened before the concrete was poured, and it absorbed water and bonded
Phenomena: the main bars, auxiliary bars or stirrups inside the reinforced concrete structure are exposed on the concrete surface
Prevention measures:
1) Before pouring concrete, check carefully to ensure the position of the steel bars and the thickness of the protective layer;
2) When the steel bars are densely packed, use stones of appropriate particle size;
3) If wooden formwork is used, moisten the wooden formwork with water before pouring.
8. Loose top
Cause analysis:
1) Improper concrete mix ratio
2) The vibrating time is too long, causing Separation
3) The water in the concrete is not eliminated
Phenomena:
After the concrete columns, walls, and foundations are poured, rough, rough, and rough surfaces appear within a height of 50 to 100 mm from the fixed surface. It is loose, has obvious color changes, and is porous inside. It is basically mortar without stones distributed in it. It has low strength, which affects the mechanical performance and durability of the structure and cannot withstand external impact and wear.
Prevention and control measures:
1) Design the concrete mix ratio, and the water-cement ratio should not be too large to reduce the water-clearing property and good water retention
2) Add air-entraining agents or reduce water agent
3) Control the vibration time
4) Use vacuum water absorption process, etc.
9. The looseness falls off
Cause analysis:
1) The wooden formwork is not watered or moistened.��Enough
2) On a hot and windy day, after the concrete was demoulded, no watering was done for curing
3) When concrete was poured in winter, no insulation measures were taken
Prevention measures:
1) Pay attention to the reasons analyzed above
2) When constructing concrete in special weather, special construction measures should be formulated
3) Strengthen the maintenance and insulation of concrete
4) Develop treatment measures after common problems occur
10. Plastic shrinkage cracks:
Phenomena
Irregular shapes of varying lengths and incoherence appear on the surface of the structure, resembling a dry mud surface. Most of them occur in the early stages of concrete pouring (about 4 hours after pouring), when the concrete itself is very different from the outside temperature, or the temperature itself is too high for a long time (above 400 degrees) and the climate is very dry. Plastic cracks, also known as cracks, are strictly dry shrinkage cracks.
Cause analysis:
1) After the concrete was poured, the surface was not covered in time and exposed to wind and sun. The free water on the surface evaporated too quickly, resulting in a sharp volume shrinkage. At this time The early strength of concrete is low and it cannot resist this deformation stress, resulting in cracking
2) Use cement with a large shrinkage rate, use too much cement or use too much silt
3) The water-cement ratio of the concrete is too large and the template is too dry
Preventive measures:
1) When preparing concrete, strictly control the water-cement ratio and cement dosage, select well-graded stones, and reduce Small void ratios and sand ratios must be vibrated and compacted to reduce shrinkage and improve concrete crack resistance
2) Water the base and formwork thoroughly before pouring concrete
3) When constructing in weather with high temperature, low temperature or high wind speed, after the concrete is poured, water spraying and curing should be carried out in time to keep it moist. After pouring a large volume of concrete for a period, the surface troweling and curing work should be strengthened during the curing period
4) Concrete maintenance can be done by spraying chlorine partial emulsion curing agent on the surface, or covering it with straw bags, plastic films, etc. When fine cracks are found on the surface, they should be pressed once in time, and then covered and maintained
5) Set up wind-shielding facilities
Protection of finished products
(1) The watering and curing time of the watering and curing method shall not be less than 7 days, and the concrete tools must be sufficiently moist. ; During winter construction, the plane and facade structures will be covered with plastic films and straw curtains until the concrete reaches the design strength.
(2) Only after the poured concrete strength reaches 1.2MPa can the steel pipe supports and formwork be installed. If the concrete has not reached the critical strength but must be operated by someone, springboards or plywood must be laid on the surface of the concrete to increase the stress-bearing area to prevent the concrete from being stepped on.
(3) The side formwork of vertical components can only be removed when the concrete strength can ensure that its surface area and edges will not be damaged by removing the formwork. The form removal time is generally controlled to 8 hours in summer and 8 hours in winter. The specific time needs to be adjusted according to the conditions provided by the ready-mixed concrete mixing station.
(4) The stair steps that have been poured with concrete should be covered with 1.8cm thick plywood to prevent damage to their edges and corners.
Construction during rainy season
(1) The mixing station strictly controls the water consumption of concrete. The calculation takes into account the water content of sand and gravel, and strictly controls the slump to ensure the strength of the concrete.
(2) It is not advisable to pour concrete in the open when it rains. Be sure to check the weather changes before starting work to avoid sudden rains that may affect concrete pouring.
(3) The concrete that has been placed in the mold and vibrated must be covered in time to prevent sudden rain and rainwater
(4) If the concrete cannot be poured in time after the mold is closed, , Drainage holes should be reserved in appropriate parts of the formwork to prevent water from accumulating in the formwork due to sudden rain
(5) If it suddenly rains when pouring concrete, the work will not stop in light rain, and the rainproof construction will be installed in heavy rain. The construction joints of the shed or temporary construction can be closed before the construction continues after the rain. The construction joints must be treated before pouring.
(6) Windproof fastening measures should be strengthened during construction during rainy periods
(7) All construction machinery should take corresponding measures to prevent leakage.
Winter construction
(1) The sand, stone and cement used to mix concrete during winter construction should be kept at a positive temperature.
(2) The ready-mixed concrete is mixed by heating water, and the water temperature should not exceed 80 ℃.
(3) The aggregate must be clean and must not contain ice, snow and other frozen structures and minerals that are easily cracked by freezing. The mud content must not exceed the specified value and be mixed with early-strength antifreeze containing potassium and sodium ions. Reactive aggregates must not be mixed with concrete.
(4) Strictly control the water-cement ratio, which should not be greater than 0.6. The water consumption per cubic meter of concrete should be no less than 300kg. The moisture brought in by the aggregate and the moisture in the admixture solution should be removed from the mixture. deduction.
(5) Before mixing concrete, the mixer should be flushed with hot water.
(6) The temperature of the concrete mixer shall not be lower than 10℃, and the mold entry temperature shall not be lower than 5℃.
(7) Before pouring concrete, the ice, snow, frozen blocks and dirt on the steel bars and formwork should be removed in time
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