Technical measures for crack prevention of the hot

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Technical measures for crack prevention and control of reinforced concrete cast-in-place floor

scene: after formwork removal, the layout concrete is roughly cracked due to the influence of temperature difference, load, settlement, maintenance and other notices. According to its target and shape, it can be divided into: degree cracks, vertical cracks, longitudinal cracks, transverse cracks and radial cracks. According to its crack depth, it can be divided into: rough cracks, deep cracks and through cracks, Especially in the concrete layout, deep cracks and through cracks should be prevented to ensure that the layout is safe

1 design

1.1 the design of cast-in-place concrete floor slab should be suitable for the provisions of the standard for design of concrete layout GB; The construction plane of the room should be designed to prevent sudden changes in plane shape. When there is a notch in the plane, the reinforcement of the floor slab around the notch should be properly strengthened. When the shape of the floor plane is irregular, the beam should be set to form a regular plane

1.2 in order to make the calculation sketch of the floor different from the actual stress environment, the crack resistance should be constructed according to the standard, and the deformed reinforcement should be extended

1.3 in order to prevent 45 inclined cracks at corners, radial reinforcement should be built at corners. The length is larger than 1/3 span. The negative bending moment reinforcement of the upper support should be set with a leading reinforcement every other one to resist the cracks in the plate and end cracks. At the same time, the diffuse reinforcement should be densified with a spacing of 150 ~ 200mm. Make the floor stress uniform, and strengthen the ability of concrete to withstand temperature and dry shrinkage deformation

1.4 at the places where the slab width of the cast-in-place slab is sharply changed, the hole opening is weakened, etc., which are easy to cause the extension stress concentration, the spacing of reinforcement should not be greater than 150mm, and the diameter should not be less than 6mm, and the temperature extension reinforcement of the vertical and horizontal targets should be roughly installed on the slab. Additional reinforcement shall be built at each side of the hole weakening. The reinforcement ratio of the upper and lower parts of the slab along the vertical and horizontal targets shall not be less than 0.15% of the cross-sectional area

1.5 the design thickness of reinforced concrete cast-in-place floor (hereinafter referred to as cast-in-place slab) should generally not be less than 100mm (kitchen, bathroom, balcony slab should not be less than 90mm), and the thickness of indoor corner plate at the outer corner and corner plate of well floor should not be less than 120mm. The floor slab at the sudden change of plane stiffness of the building should be appropriately thickened

1.6 when the pipeline needs to be embedded in the floor slab, the design thickness of the cast-in-place slab should not be less than 100mm, and the pipeline must be inserted between the upper base reinforcement sheets. The pipeline should not cross at grade, and an additional one should be roughly set up at the top and bottom of the plate along the target of the pipeline φ Reinforcement of [email protected]

3.6 the position and treatment of construction joints, the position of post cast strip and the pouring of concrete shall be strictly carried out in accordance with the design and construction technology scheme. The post cast strip shall be set at the part that has little impact on the layout, with a width of 700 ~ 1000mm. The concrete pouring of the post cast strip should be left behind after 60 days of pouring in the main layout. Micro shrinkage concrete should be used during pouring, and the strength grade of the concrete is one grade higher than that of the original concrete

3.7 effective shelter measures for strengthening the reinforcement under the floor. The double-layer two-way reinforcement of the floor (including the negative bending moment short reinforcement of split type construction) must be equipped with a steel stirrup, and its vertical and horizontal spacing should not be greater than 700mm (that is, not less than 2 per m2); yes φ 8 for class I small reinforcement, the spacing of small brace lead screws should be controlled within 600mm (i.e. not less than 3 per m2)

3.8 because the bleeding of concrete and the sinking of aggregate are easy to produce plastic extension cracks, at this time, the compaction and troweling of the concrete cast-in-place slab should be roughly stopped; The secondary vibration shall be stopped before the initial setting of the concrete, and more than twice pressing and plastering shall be carried out before the final setting of the concrete. When the pouring of cast-in-place slab concrete is completed and does not reach 1.2MPa, measures must be taken to prevent direct stepping on the concrete, and no one is allowed to stop the next process. When the concrete strength is less than 10MPa during continuous construction, it is strictly prohibited to stack construction equipment and materials exceeding the allowable value of construction load on the cast-in-place slab, and it is strictly prohibited to have the impact of strike load

3.9 at the end of the concrete pouring of the cast-in-place slab, the covering and watering curing shall be stopped within 12 hours, and the curing time shall not be less than 7 days; The concrete mixed with retarding admixture shall not be less than 14d. Measures that can ensure continuous watering and maintenance must be set up on the construction site. When it is impossible to ensure whether the watering is in line with the alignment accuracy for curing, the concrete should be roughly covered with plastic film, and it can ensure that there is consolidation water in the plastic cloth

3.10 control the formwork removal strength, and the concrete strength shall not be less than 1.2MPa when the formwork of wallboard and column side is removed. When the bottom formwork of cast-in-situ slab is removed, the pressure test of the same premise curing specimen on site shall be stopped, and the bottom formwork of cast-in-situ slab can be removed only when the pressure test results meet the minimum strength grade of formwork removal. The layout of the formwork and its support has been removed, and the full utilization load can be accepted before the concrete strength meets the design requirements; When the effect of construction load is more unlucky than that of using load, it must be accounted for, weighted and supported

3.11 concrete cracks and layering (scene: there is a loose or vertical mixed layer in the concrete. The notices: (1) the construction joint or deformation joint is not processed by the joint, the roots are broken, the cement film and loose stones are rough, and the concrete is poured without removing the weak and weak concrete layer and without sufficient moisture. (2) Sawdust, soil, bricks and other sundries at the construction joint are not broken or clean; 3) The bottom layer is not filled with joint mortar layer, and the concrete at the joint is not well vibrated.)

1) the outline of construction joints and deformation joints should be handled according to the requirements of construction acceptance standards. Debris such as sawdust, construction, bricks and so on at the joints should be cleaned and cleaned. Before pouring at the joints, mm thick stone mortar with strong innovation, pertinence and practicality should be poured first, so as to facilitate excellent contact and strengthen the vibration and compaction of concrete at the concrete joints

2) when the crack interlayer is not deep, the loose concrete can be chiseled away, washed and cleaned, and then filled tightly with 1:2 or 1:2.5 cement mortar. When the crack interlayer is deep, the loose part and external debris should be cut off, the hardware should be washed with pressure water, and then the formwork should be erected after cleaning, poured with fine aggregate concrete or roughly closed and left behind for grouting treatment

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