Analysis of the causes of the hottest floor concre

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Cause analysis and Countermeasures of floor concrete cracks and infilled wall cracks

at present, cracks in concrete floors and infilled walls in residential projects are relatively common. Now, according to relevant data and in combination with the situation of our company, the causes and Countermeasures of cracks in cast-in-place concrete floors and block infilled walls are analyzed as follows for your reference in work

I. types of cracks in residential cast-in-place concrete floor slab

1. Longitudinal cracks: that is, cracks along the longitudinal direction of the building, most of them appear in the lower skin of the slab, and some of them run up and down

2. Transverse cracks: that is, within 1/3 of the span, cracks along the transverse direction of the building mostly appear in the lower skin of the slab, and some of them run up and down

3. corner cracks: oblique cracks appear in the four corners of the room, mostly in the plate epithelium

4. Irregular cracks: cracks with irregular distribution and strike

5. transverse cracks at the root of the floor: cracks generated within 30cm from the support, located on the upper surface of the slab

6. cracks along the direction of the embedded conduit

II. Causes of floor cracks

1 In terms of design

1.1 when designing the structure, the safety reserve is too small, the reinforcement is insufficient or the section is small, which makes the rigidity of the beam and slab after forming poor, and the overall deflection is too large, causing cracks at the four corners of the slab

1.2 the design plate thickness is not enough, and the deflection checking calculation is not done. The overall deflection is too large, causing cracks at the four corners of the plate

1.3 when the house is long, expansion joints are not set, and shrinkage cracks occur in weak links. (according to the data of the American Concrete Institute, there are two kinds of concrete: dry shrinkage and temperature deformation. The dry shrinkage deformation shrinks by about 19mm every 30.48m. The deformation caused by temperature change is that the temperature change of 37 ℃ shrinks or lengthens by about 19mm every 30.48m. Some people in China believe that the longitudinal shrinkage of 40m long floor due to hardening and solidification is 8-20m. The global high-speed railway has entered the high-speed development period, and plastic materials usher in business opportunities of M.)

1.4 improper treatment of foundation design leads to uneven settlement, additional stress in the superstructure and floor cracks

1.5 the floor is stressed in two directions and reinforced according to one-way slab, causing cracks

2. Reasons for commercial concrete

2.1 large water cement ratio and large amount of cement

2.2 the amount of high-efficiency retarder is too large, and the stone sinks before solidification, resulting in shrinkage cracks, which often occur at the junction of beams and slabs

2.3 the quality of sand and stone is poor, the grading is poor, and the silt content is large

3 construction reasons

3.1 the curing is not in place. The mandatory specifications require that the concrete curing should be covered and watered. Now most of them are not covered, and watering cannot ensure regular wetting

3.2 the construction speed is too fast and the loading is early, especially for the floor slab of brick concrete residential buildings. The floor slab was poured one day before, and the bricks and cars were loaded the next day, causing damage to the early concrete

3.3 freezing during winter

3.4 early formwork removal or insufficient rigidity of formwork support system

3.5 the laitance on the concrete surface is too thick and the surface strength is not enough

3.6 during construction, the concrete cover bar of the floor slab is bent and stepped down, the protective layer is too thick, and the bearing capacity decreases

III. measures to prevent cracks in floor concrete

1. Design

1.1 in the case of using small-diameter reinforcement, appropriately increasing the reinforcement ratio can improve the ultimate tensile strain of concrete

1.2 the negative reinforcement at the corner is configured in both directions, and the negative reinforcement is also configured on all sides of the one-way slab

1.3 under the same reinforcement ratio, the crack resistance of cast-in-place slab can be improved by using reinforcement with smaller diameter and reducing the spacing of reinforcement

2. In terms of construction

2.1 try to set expansion joints in the cast-in-place floor slab. The spacing of expansion joints can be about 14m or the longitudinal length of a unit in the residential building. It is set at the floor support with a joint width of 10mm, and soft materials are added in the middle. The concrete is broken and the reinforcement is continuous

2.2 when binding reinforcement, ensure that the spacing is uniform and the position of negative reinforcement remains unchanged. When pouring concrete, set a berm and do not step on negative reinforcement

2.3 use a plate vibrator, and plaster and press twice, and the second plaster and press is carried out before final setting

2.4 add steel wire under the embedded wire pipe, and the embedded pipe shall be arranged along the direction of the stressed reinforcement as far as possible

2.5 the method of covering and watering is adopted for curing. Covering and watering is the requirement of mandatory specifications. At present, most of us only water without covering. After the pouring water is dry, it cannot be supplemented in time. During the curing period, it cannot ensure that the concrete is in a continuous wet state, which cannot achieve the due curing effect

2.6 no one is allowed to walk on the concrete below 1.2MPa, but the formwork shall be removed early, or the early removal system shall be adopted, and the vertical support shall be maintained after the formwork is removed

3 mixing plant

3.1 ensure production according to the designed slump, and conduct secondary mixing if segregation is found on site

3.2 ensure the quality of cement and sand and ensure the scientific and reasonable mix proportion

3.3 shrinkage reducing agent will be available soon, and shrinkage cracks can be reduced by adding shrinkage reducing agent into concrete

IV. treatment of floor concrete cracks

4.1 those with crack width less than 0.3mm can be sealed with 6202 mastic, etc

4.2 when the crack width is greater than 0.3mm, chemical grouting treatment is carried out as follows:

(1) chiseling: chisel along the crack, and chisel a V-shaped groove with a width of 15~20mm and a depth of 20mm according to the crack condition

(2) embedding grouting pipes: drill holes every 50cm along the crack direction, embed grouting nozzles, and fix them with glue.

(3) sealing cracks: use structural glue to ride the cracks and scrape them repeatedly, and seal the surrounding cracks and branch cracks at the same time

(4) air blowing and pressure test: seal the air leakage part

(5) grouting: prepare the grouting fluid and inject it into the grouting device. The relevant district and municipal governments) pressurize the air compressor by 0.2MPa, and carry out the grouting from the grouting nozzle at one end. Generally, stop the grouting after the grouting fluid overflows from the adjacent grouting nozzle, close the grouting nozzle, and carry out the next grouting in turn

(6) remove the nozzle and close the grouting nozzle

v. crack analysis of block filled wall

1. Crack type

1.1 cracks at the junction of masonry and columns and beams, horizontal joints or vertical joints

1.2 cracks in masonry itself, vertical joints or cracks along mortar joints

2. Causes of cracks

2.1 influence of block dry shrinkage: the dry shrinkage value of blocks is generally less than 0.4mm/m. Tests have proved that 30-40% of the total shrinkage rate can be achieved after curing at room temperature for one month, and about 95% of the shrinkage rate can be achieved after curing for about two months. If the construction speed is too fast, due to the poverty of the family and the use of blocks with a stagnation period of less than 28 days, the shrinkage cracks of block walls will be generated

2.2 the masonry mortar is not full: the rib of the block wall is narrow, and it is difficult to ensure that the mortar is full and uniform without careful construction. Once the wall is under the effect of stress, cracks along the mortar joint will occur at the insufficient mortar

2.3 temperature cracks: cracks caused by inconsistent deformation caused by the temperature difference inside and outside the outer wall, such as the horizontal joints under the beam and the cracks at the heating kiln under the windowsill

2.4 cracks caused by structural settlement: the overall stiffness of the structure is poor. The block wall and the frame column beam can only be connected by the tie bar on the column. Even if steel wires are added during plastering, it is difficult to resist the stress deformation caused by settlement

2.5 influence of plastering mortar: the shrinkage value of cement mortar or cement lime mixed mortar commonly used for plastering is generally 0.6~0.8mm/m, and its water retention and workability are poor. Bleeding often occurs at the interface during plastering, which leads to hollowing and cracking

2.6 the shrinkage deformation of the block is about 2~3 times that of the machine brick, and it is easy to crack when the block is mixed with the machine brick and the block is mixed with the beam column

3. prevention measures

3.1 the overall stiffness of the structural frame should be ensured in the design, and the deformation joints should be reasonably set for the complex buildings to prevent uneven settlement

3.2 starting from the design, strengthening measures should be taken at the parts that are easy to crack, such as adding core columns on both sides of door and window openings, setting horizontal tie bars in the mortar joints of the wall under the windowsill, and adding steel wires in the wall plastering, so as to increase the crack resistance

3.3 the block shall be dried before masonry to reduce internal shrinkage. The moisture content of the block is better equal to or lower than the average annual relative humidity of the outside air on the site, and the moisture content shall not be greater than 35%~40% just after masonry. The linear shrinkage of selected blocks is less than 0.03~0.065%

3.4 it is strictly forbidden to use blocks less than 28 days old. If possible, they can be used after curing for 2 months. When storing on site, the bottom should be padded, and attention should be paid to dampproof. Cover them in rainy days

3.5 in addition to using diagonal machine bricks to top the wall, steel wire sheets should also be added. In addition to tie bars, steel wire sheets should also be added at the column wall joints. The sheets should be firmly connected with the column wall with shooting nails, and the sheets should be clamped in the middle of the bottom ash

3.6 control joints can be set for large-area infilled walls, and continuous vertical weakened sections can be made in the walls. It can be made into tongue and groove joints or prefabricated caulking strips, so that the cracks appear at the control joints and are not noticeable

3.7 masonry mortar should have good water retention, which can restrict the block from absorbing water from the mortar. Generally, hydrated lime paste is added to the mortar or appropriate additives are determined through tests

3.8 adopt the method of reverse masonry, with the bottom of the block facing upward, which can make the block more in contact with the mortar

from the above analysis, it can be seen that the cracks in concrete floors and block filled walls are caused by many factors. We must start from the joint review of drawings, formulate a comprehensive treatment plan, actively combine with the owner and the design, strengthen the control of subcontractors and sub suppliers under pressure at both ends, and strictly control the construction process, so as to minimize the occurrence of cracks and make the quality of residential projects reach the satisfaction of users

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