Cracks in brick-concrete structures generally exist in two places: first, cracks in brick masonry; The second is the crack of reinforced concrete cast-in-place slab. Cracks in brick-concrete structure not only affect the beauty of the building, but also cause leakage of the building, even affect the structural strength, stiffness, stability and durability of the building, and also cause greater psychological pressure and burden to the users of the building. Therefore, we must carefully analyze this and properly handle it. ...
2. Analysis and prevention of wall cracks
2. 1 The cracks in the bottom wall caused by uneven settlement of foundation mainly include oblique cracks.
A crack in the wall between two diagonally paired windows. Vertical cracks in the lower wall of the bottom window. The main causes of cracks are improper foundation treatment or uneven settlement of buildings caused by the difference of foundation bearing capacity and structural stiffness. Concrete strip foundation is widely used. However, in the design, when calculating the foundation force, it is still transmitted at an angle of more than 45 degrees. Therefore, when the compressive strength of the wall is insufficient, the foundation soil will react to the upper wall. Cracks often appear. Prevention and control measures: do a good job in exploration. It is strictly forbidden to design and construct without exploration, and reasonable layout and modeling should be carried out according to exploration data during design. Reasonable settlement joints should be set in the parts with large soil changes and load changes. Pay attention to soft foundation in construction. Treatment of special foundation such as collapsible loess foundation. Increase the tensile strength of the masonry part. Long reinforced concrete is used in the wall at the elevation of the window sill.
2.2 Cracks caused by insufficient strength of brick masonry and irregular construction.
Vertical or oblique cracks often appear in the bottom wall of beam end. This is due to the insufficient bearing capacity of the masonry at the bottom of the beam end after compression. Due to the thickness of mortar joint in masonry construction, the density of mortar is uneven. Uneven compression of masonry, shrinkage deformation of masonry will cause local cracking of the wall. Prevention and control measures in masonry structure design. The local bearing capacity of masonry under beam and brick column should be calculated. When the wall can't reach the bearing capacity, beam pad can be set and transverse reinforcement can be set in the wall. Before construction, the brick block should be watered and wetted, and the operation should be carried out in strict accordance with the specification requirements.
2.3 temperature cracks
Wall cracks caused by temperature changes are common in the top longitudinal wall, splayed oblique cracks on both sides of doors and windows, and horizontal cracks at the root of parapet. And staggered floors, vertical cracks in stairwells, etc. The main reason for this kind of crack is that the linear expansion coefficient of concrete is nearly twice that of clay brick wall. Under the irradiation of the scorching sun in summer, the concrete on the roof and the top forms a huge expansion force to squeeze the masonry, and when the temperature drops at night, the concrete produces a huge contraction stress, so the brick wall is in a complicated state of shear and tension. When these stresses exceed the tensile and shear strength of brick masonry, cracks will appear in masonry.
Prevention and control measures: ① Select insulation materials with excellent insulation performance, increase the thickness of roof insulation layer, and effectively control the change of roof temperature. Sun-shading ventilation boards can be installed, and roof plate stiffeners can be appropriately added to control deformation. In addition, the temperature expansion joint should be set in strict accordance with the requirements of the specification. (2) Strengthen the tensile strength of brick masonry, increase the mortar label of the top masonry, and control the deformation difference between the roof panel and the brick wall. Reinforced brick masonry can be used. (3) For the horizontal cracks at the parapet root, the constructional column can be raised to the top of the parapet (which has been popularized and implemented now), so that it can be poured together with the coping to ensure that the parapet does not fall.
3. Prevention and treatment of cracks in cast-in-place concrete floor slab
Cracks in the cast-in-place slab not only affect the appearance, but also easily make residents feel psychologically uneasy, which has been bothering residents, developers and construction units. In view of the causes of the above cracks, we put forward a set of targeted prevention measures under the guidance of relevant experts and on the basis of conscientiously implementing unified detailed practices, focusing on strengthening cooperation with design departments and making necessary strengthening treatment for key parts prone to cracks.
3. 1 raw materials
(1) Strengthen the management of suppliers of commercial concrete, strictly implement the inspection system for sand, stone, admixture and admixture, strictly control the mixture ratio, cement dosage and slump, minimize the internal defects of concrete, and thus reduce the occurrence of cracks.
(2) Strengthen the management in the process of concrete construction, check the slump and workability of commercial concrete in each vehicle, and collect and record concrete receipts as required to ensure continuous pouring of concrete before initial setting.
3.2 Measures taken in the design
(1) From the point of view of design, ask for design approval in the blue prints.the design.
Take various measures to enhance the foundation stiffness and overall stiffness of multi-storey brick-concrete structure. In foundation treatment, the thickness of backfill graded sand should be reduced, the compaction height of each layer should be strictly controlled, and the compactness of graded sand should be improved to reduce the final settlement and deformation of the bearing layer after bearing the upper load. In raft foundation, 490-mm-high constructional columns are added to the foundation ring beam at the inward position of each big corner, so as to increase the length and number of radial ribs at the big corner, thus increasing the overall stiffness of the foundation at the big corner.
(2) Although the large corner cast-in-place floor slab of the unit external wall at both ends of each floor has been equipped with radial negative reinforcement, in view of this phenomenon, 45. Most of the cracks occurred in this area. At the same time of increasing radial reinforcement, we added double-layer bidirectional reinforcement with the same specifications as the stressed reinforcement, with a length of 2.5m as reinforcement. Make the stress direction of the crack consistent with the reflection rib, and effectively control the crack. Due to the addition of double-layer bidirectional steel bars, the generation and propagation of cracks have been obviously improved. After adopting this method, the four large corners at both ends of each floor of three residential buildings delivered for use basically put an end to 45 cracks.
(3) Reinforce the structural steel bars of the living room panel with large width and depth. In addition to the structural negative reinforcement at the bearing, the temperature-resistant reinforcement is added. The negative reinforcement of the floor slab with design thickness of 130mm or more (including 130mm) shall be set according to the length, and the length below 100- 130mm shall be set as 50%, and the length below 100mm (excluding100).
(4) When laying the embedded PVC conduit in the cast-in-place floor slab, tie it with cushion blocks and steel bars to ensure the position of the conduit in the floor slab, and try to stagger the conduit in the same direction without overlapping. The staggered spacing shall not be less than l00mm, and it shall be reinforced from top to bottom with a steel mesh with a width of 250ram.
3.3 Strengthen the process quality management of the construction site
(1) Strengthen the process management of formwork construction: the formwork support must have sufficient rigidity, and there should be no gap between the square wood and the formwork. Each contact surface should have reliable support points, so that the formwork is flat and the allowable deviation is not more than 3 mm During concrete pouring, assign special personnel to guard the formwork to prevent the fasteners from loosening and sinking. The formwork removal must reach the design allowable value, and the weak parts such as platform entrance, formwork and support can only be removed after reaching the design strength of 100%, effectively preventing cracks caused by structural damage caused by premature formwork removal.
(2) Strengthen the quality management of negative reinforcement in steel binding construction. The spacing between encrypted negative reinforcement supporting horse stools is not more than 1m, so as to ensure the thickness of negative reinforcement protective layer on the board surface and avoid cracks caused by the increase of negative reinforcement protective layer thickness and sinking at the bearing.
(3) Strengthen the management during concrete construction and the maintenance after concrete is cast. In the process of concrete pouring, the thickness of concrete slab is continuously controlled by moving signs to ensure that the design requirements are met; After the concrete is cast, it is leveled with 4m aluminum alloy scraping bars according to the thickness control points, and the construction technology of plastering for three times before the final set of concrete is adopted to reduce the plastic shrinkage cracks on the concrete surface. Concrete curing is an indispensable link in the whole project. Especially at high temperature, the watering frequency should be increased to keep the surface moist for a long time, and the condensed water in the plastic sheet should be covered, so as to reduce the cracks caused by temperature difference, reduce the constraint stress caused by concrete shrinkage and effectively control the cracks.
(4) Protection of finished concrete products: The cast-in-place slab can only be carried out after the concrete strength reaches 1.2MPa. If the construction period is really tight, measures should be taken to prevent the cast-in-place slab from cracking prematurely due to concentrated load. If bricks are used for hanging, the brick stack should not exceed 3 layers. Mortar should not be placed directly on the cast-in-place floor by trolley, but put into the ash tank by effective measures such as shovel.
4. Concluding remarks
Combined with the practice of construction process. This paper analyzes the causes of common cracks in brick-concrete structure walls and floors, and based on this, advances some preventive measures for avoiding these cracks. Although most of these cracks affect the use function and aesthetics and will not cause structural problems, if these cracks are allowed to develop without taking preventive measures, they may cause structural hidden dangers. Resulting in structural safety accidents. Therefore, the construction unit should carry out the construction in strict accordance with the construction norms and standards, and only in this way. In order to reduce or avoid cracks and ensure the safe use of residential buildings.
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