A, concrete cracks

The formation reason of 1

The general mechanism of cracks is generally believed that the internal stress caused by the constrained deformation of concrete materials (including cement stone and coarse and fine aggregate) is greater than the tensile strength of the materials. Generally, it can be explained by the mechanism of "bonding-sliding". The occurrence and development of visible cracks in concrete is the result of relative sliding between steel bars and concrete because they can no longer maintain deformation coordination. The crack width is essentially the difference between the tensile deformation of concrete and the tensile deformation of steel bars between cracks.

2 concrete crack repair method

(1) concrete mixing and transportation time is too long, pouring speed is too fast, false vibration, improper construction joint practices, formwork and other reasons cause cracks. General concrete crack reinforcement measures can be adopted, or concrete materials, steel anchors and even steel plates and prestressed steel bars can be filled for remedy.

(2) For cracks caused by dry climate, poor initial maintenance, premature freezing of concrete, changes in atmospheric temperature and humidity, measures such as surface treatment of cracks, filling concrete materials or injecting epoxy resin should be taken. Some severely frozen parts need to be removed, while others need to be reinforced before they can be used.

(3) For cracks caused by excessive uneven settlement of foundation, joints should be adopted, and reinforcement methods should be adopted due to load, such as grouting method, surface layer method, reinforced concrete sheath method, external steel method, steel plate sticking method, prestress method, changing force transmission path method, and component adding method.

Second, concrete pits.

Causes and characteristics of 1

It is due to the defects such as lack of slurry, sanding and peeling on the concrete surface, which is characterized by concave appearance, crisp texture, small area (less than or equal to 0.5m2), shallow depth (less than or equal to 5mm) and no exposed reinforcement. This defect is generally caused by insufficient wetting of the formwork, lax support, insufficient slurry leakage or vibration during tamping, and poor maintenance after air bubbles are discharged from tamping.

2. Preventive measures for pits

Template surface should be clean, can not be stuck with dry cement mortar and other sundry, before pouring concrete, template should be watered fully wet, template gap, application of linoleum paper, putty, etc. When plugging, the template isolation agent should be long-lasting, evenly coated, and no brush leakage is allowed; Concrete shall be vibrated and compacted evenly in layers until bubbles disappear; If there is paint on the surface, it can be left untreated. If the surface has not been painted, the pitted surface should be watered and fully wetted, and the stone mortar should be removed with the original concrete mixture ratio, and the pitted surface should be smoothed and calendered. It is possible to deal with the defect thickness below 5mm, and the surface layer can be repaired with engineer A2 durable thin-layer repair material. 5mm or more can be repaired with durable and high-strength repair materials from Engineer A3. Among them, the interface between new and old concrete needs to be treated to prevent hollowing and surface peeling in the later stage.

Third, concrete lacks edges and corners.

The formation reason of 1

Refers to the local damage of concrete at right angles of beams, columns, walls, slabs and holes. The reason for this is the following：

(1) Before concrete pouring, the formwork is not fully wetted, which leads to insufficient water loss or hydration of the concrete at the edges and corners, which reduces the strength and damages the edges and corners when the formwork is removed;

(2) Early form removal or core pulling, the concrete has not yet established enough strength, causing damage to the edges and corners;

(3) During hoisting and transportation, the components are not well protected, resulting in local shedding and splitting damage of corners.

2. Precautions against falling corners:

1) The formwork shall be fully wetted or evenly coated with release agent before concrete pouring, and shall be carefully watered and maintained after concrete pouring;

2) When dismantling the side non-bearing formwork, the concrete should have sufficient strength;

3) Don't use too much force when ripping. Pay attention to protecting edges and corners; When lifting, it is forbidden for the formwork to collide with edges and corners;

4) Strengthen the protection of molded concrete. Click here to download the construction technical data for free.

3-angle drop processing method

When the lack of edges and corners is small, it can be cleaned with clean water with a wire brush. After it is fully wetted, it can be plastered with cement mortar of 1: 2 or 1: 2.5. Unreal concrete and prominent aggregate particles can be chiseled away, washed and wetted with water, then repaired with fine stone concrete higher than the original concrete and carefully maintained.

Fourthly, concrete honeycomb.

Causes and characteristics of 1

There are three kinds of honeycomb: surface honeycomb, deep well honeycomb and through honeycomb. Horizontal, oblique and oblique single honeycomb and conjoined honeycomb are often encountered, which are characterized by local crisp surface and no cement slurry. The exposed depth of coarse aggregate is greater than 5mm (less than the thickness of concrete protective layer), and there are gaps between stones that are smaller than the maximum particle size of stones, which are honeycomb-shaped. The strength of honeycomb concrete is very low.

The main reasons for the formation of concrete honeycomb are as follows:

1, concrete vibrating is lax, especially not vibrating step by step;

2. When the concrete enters the mold, it will be layered due to the high pouring height;

3, the use of hard concrete, or the construction of the concrete mixture ratio control is lax, especially the water cement ratio is too low;

4, the template is not tight, after pouring concrete slurry leakage phenomenon, cement slurry loss;

5. There is no concrete during transportation.

2 measures to control the honeycomb phenomenon:

(1) The mix proportion of concrete should be adjusted according to the construction situation of concrete technology, such as vibrating mode, transportation mode, reinforcement size and reinforcement distribution. When it is found that the workability of concrete is not ideal, water should not be added just because it will damage the strength and durability of concrete, but the mixture ratio of concrete should be adjusted or the pouring method of concrete should be improved.

(2) The concrete should be carefully discharged. In any case, we should pay attention to the key point of concrete unloading, which is to avoid segregation. Concrete should be discharged vertically, and the closer the outlet is to the final position, the better. Concrete can't flow to the designated position, and if it needs to be moved, it should be carried out with a big shovel; After unloading a truck of concrete, the next truck should be next to the rear of the previous truck, instead of starting another truck and finally connecting them, because this will often produce a honeycomb phenomenon at the joint.

(3) The formwork shall have sufficient rigidity, stability and strength to avoid formwork displacement when vibrating concrete; Flat-fell seam should be sealed to ensure no slurry leakage.

(4) Carefully vibrate the concrete to avoid false vibration and excessive vibration. A new layer of concrete should be deeply vibrated in place, and the spacing between vibrating points should ensure that each layer of concrete is not vibrated.

3 Method of treating concrete honeycomb

Generally speaking, the best way to repair concrete honeycomb is to remove 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 air pressure spraying. When using this method, the honeycomb mouth should be cleaned or slightly chiseled outward; When filling by hand, the honeycomb mouth should be cleaned or chiseled straight with a depth of not less than 25mm. In addition, the concrete surface of the honeycomb mouth is all roughened, and all dust and sundries are cleaned up to ensure the reliable connection strength between the old and new surfaces. The new concrete should be in harmony with the old concrete in color and should be properly maintained.

Five, concrete exposure

1 Causes and characteristics of reinforcement exposure

In the process of pouring reinforced concrete, the vibration is not in place, the protective layer pad is not set or fixed firmly, the concrete slump is small, or the formwork is removed too early, and the external force causes the concrete to peel off before hardening, so that the reinforced bar is exposed after the component is formed.

The main reasons of concrete structure exposure are as follows:

(1) When pouring concrete, the protective pad of steel bar is shifted or the pad is too little or leaked, so that the steel bar is exposed close to the template;

(2) The cross section of the structural member is small, the steel bar is too dense, and there are stones stuck on the steel bar, so that the cement mortar can't fill around the steel bar, resulting in exposure;

(3) Improper mix proportion of concrete leads to segregation, slurry leakage of formwork or slurry leakage.

(4) The concrete protective layer is too small or the concrete at the protective layer is vibrated badly; Or the vibrating rod hits the steel bar or tramples on the steel bar, so that the steel bar is displaced and exposed;

(5) The wooden formwork is not watered and wet. Premature water absorption, bonding or demoulding, lack of edges and corners when demoulding, leading to steel bar leakage.

2. Preventive measures for exposed tendons:

(1) When pouring concrete, ensure the correct position of reinforcement and the thickness of protective layer, and strengthen inspection. When the steel bars are dense, stones with appropriate particle size should be selected to ensure the accurate mix proportion and good workability of concrete; When the sprinkler height is more than 2m, a string tube or chute should be used for discharging to prevent segregation; Template should be fully wet and carefully plug the gap; When vibrating concrete, it is forbidden to hit the steel bar. Avoid stepping on the steel bar during operation, and adjust the straightness in time if there is any bending or tripping. The protective layer concrete shall be vibrated and compacted; Grasp the demoulding time correctly to prevent premature demoulding and damage to edges and corners. Click here to download the construction technical data for free.

(2) After the surface leaks reinforcement, brush it clean, apply 1: 2 or 1: 2.5 cement mortar on the surface, and smooth the leaked reinforcement; Chisel away weak concrete and protruding particles with deep steel bar leakage, clean them, and fill them with fine stone concrete higher than the original one.

Six, concrete cavity

Causes and characteristics of 1 cavity formation

Because some parts of concrete are blocked when pouring, cavities will appear in the components, which are different from honeycomb, and are characterized by being in uncompacted concrete or water-deficient mud; And the cavity is partially or completely free of concrete. The size of the cavity is usually so large that all the steel bars are exposed, which leads to the fracture of the components and the overall failure of the structure.

The main reasons for the formation of concrete voids are as follows:

1. The vibration is not compact or leaks;

2. The spacing between steel bars is too small, so the concrete can't get down;

3. The aggregate gradation of concrete is not good;

4. The free dumping height of concrete exceeds the specification, resulting in the separation of concrete stones from cement slurry;

5. The outer protective layer of steel bar is not enough.

2. Measures to prevent depression

(1) The concrete mixture ratio should be accurate, the water-cement ratio should be strictly controlled, the feeding should be accurate, the mixing should be uniform, the workability should be good, and it should be vibrated and compacted after entering the mold.

(2) The surface of the formwork shall be smooth and clean, and shall not be stuck with hard cement and other sundries; Template flat-fell seam should be tight, wooden template should be fully wet before pouring concrete; Water-based release agent shall be used for steel formwork, which shall be painted evenly.

(3) Fine stone concrete with the same strength grade should be used for layered pouring of the over-dense parts of steel bars, and it should be carefully operated and vibrated to ensure the smooth and dense surface of the formed concrete;

(4) Formwork removal shall strictly control the strength of concrete, and it is forbidden to remove the form prematurely. Side formwork can only be removed when the strength of concrete can ensure that its surface and edges and corners are not damaged by formwork removal; After the concrete strength meets the design requirements or specification requirements, the bottom die can be removed.

Three ways to deal with tooth decay

The first step is to chisel the loose concrete to hard concrete, and remove the laitance on the concrete surface by grinding.

In the second step, the interfacial agent is used to enhance the interfacial adhesion. Tensile bond strength ≥3.3MPa (concrete failure).

The third step is the durability repair of concrete. Concrete surface treatment: repair with durable high-strength repair materials. The thickness is determined according to the depth of the hole on site, and the plastering thickness is about 20mm each time, which shall be subject to leveling at the grass-roots level. The compressive strength R28 is greater than or equal to 50mpa, and the minimum thickness required for durability is 10mm. Step 4: surface treatment and surface repair: after 24 hours, scrape a durable thin layer of repair material on the surface to ensure the beauty of the surface, with a thickness of about1mm.

Seven, concrete template flat-fell seam caused by quality defects.

1 Phenomenon and reasons of concrete quality defects caused by flat-fell seam of formwork;

Because the joints between templates are not tight and there are gaps, slurry leakage occurs during concrete pouring, and honeycomb appears on the concrete surface, which seriously leads to holes and exposed tendons. The main reasons are as follows:

(1) The template is not serious and correct, the template is not carefully made, and the joint is too large when assembling.

(2) The installation period of the wooden formwork is too long, and the wooden formwork shrinks to produce cracks.

(3) the wooden template is rough and flat-fell seam is lax.

(4) When pouring concrete, the wooden formwork is not watered and wetted in advance to make it expand.

(5) The deformation of steel formwork is not repaired in time.

(6) Improper joint measures of steel formwork.

(7) At the joint of beam and column, the joint size is not allowed or misplaced.

2 template flat-fell seam problem prevention measures:

(1) Translate the sample carefully, translate the details of each division into detailed drawings in strict accordance with the ratio of110-1/50, and attach detailed notes. After verification, make a detailed disclosure to the operators, strengthen the quality awareness of workers, and carefully make stereotypes and assembly.

(2) Strictly control the moisture content of wooden formwork and make it flat-fell seam tightly.

(3) The installation period of wooden formwork should not be too long. When pouring concrete, the wooden formwork should be watered and wetted in advance to make it expand and crack.

(4) The deformation of steel formwork, especially the deformation outside the side bar, should be trimmed straight in time.

(5) Sealing measures should be controlled between steel formworks, and linoleum, plastic sheets and cement bags should not be used for sealing and plugging. Click here to download the construction technical data for free.

(6) Support at the joint of beam and column should be firm, flat-fell seam should be tight (double-sided tape should be added between joints when necessary), and dislocation should be corrected.

Eight, quality defects caused by concrete expansion.

1 Phenomenon, location and causes of concrete quality defects caused by formwork expansion;

It is the template deformation produced when beams, columns, walls and floors are cast in place, which leads to the increase of the thickness of cast-in-place parts. The expansion of concrete formwork will cause the size of components to increase and the shape to be irregular. In severe cases, it needs to be removed, which will affect the appearance quality of concrete.

See the following table for the parts and reasons that are prone to die expansion:

2. Precautionary measures for concrete expansion formwork

1, template design measures.

2. Load combination: In addition to calculating the lateral pressure of concrete according to the specifications, the pouring method, stacking method and possible unexpected load of concrete should also be considered.

3. Formwork design: The layout of all-steel formwork panels, supports and tie bolts can be calculated according to the usual method. However, when the panel is made of wood and bamboo, because the technical indicators of the panel are generally measured under standard dry conditions, it will lose a little after soaking. Therefore, when using wood-bamboo board, it is suggested that the thickness should be increased to a certain extent, the purlin spacing and the column hoop spacing should be appropriately reduced, and the arrangement spacing of tie bolts should also be appropriately reduced.

4. Layout of pull bolts: The layout should be calculated in combination with the structure, and the parts where the pull bolts are not suitable for layout can be reinforced with supports. In addition, 18-25 steel supporting feet are embedded on the floor 200 ~ 250 ㎜ away from the wall and column formwork to fix the formwork.

5, floor support, should be on the basis of structural calculation, respectively, calculate the beam and plate support frame, and according to the actual design layout support frame stud. Using secondary support system or quick formwork removal system, it is necessary to draw detailed formwork drawings to assist construction.

6. Deal with the following problems: the form and size of corner formwork and the connection mode with wall and floor formwork; Splicing methods of templates, splicing methods between large templates and between large templates and corner templates; Internal and external templates and supporting methods for beam-column joints, floor and wall, and column joints.

7, doors and Windows hole template, reserved hole template and its support form design, installation and positioning.

8, according to the actual situation of building structure design, pre-arranged template, put the non-standard board in the middle of the easy to deal with.

9, determine the template installation order, to ensure the overall assembly quality.

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