I. Time definition of construction joint setting
In the construction process of large-area floor cast-in-place slabs, it is often because of construction technology and labor organization reasons, or it is considered that the construction of flowing water is adopted in stages, and the entire cast-in-situ slabs on each floor cannot be continuously poured and completed. When it is expected to exceed the prescribed time, appropriate construction sites should be pre-selected for construction joints.
Second, the setting position of construction joints
Most of the floor cast-in-place slabs of brick-concrete residential buildings, office buildings and other buildings are two-way slabs, and the location of the construction joints is determined from the investigation. In the design drawings, most of the drawings are not indicated separately. Pavement construction joint design requirements. At this stage, there are mainly the following kinds of retention methods:
1. The first method of indwelling: The construction joints of cast-in-place slabs are arranged along the transverse direction in the middle 1/3 of the span of the cast-in-place slab. Although they are placed in the structures where the shear force is small, it is difficult to guarantee the mechanical assumptions in structural calculations. (ie, the integrity of the floor cannot be completely guaranteed.) Due to shrinkage of the concrete floor or under long-term loading, cracks may often occur at such construction joints, or even open joints may crack the floor tiles, which may affect the appearance. It will seep again and cause corrosion of steel bars. Seriously affect the structural safety, affect the life of the building.
The main reason for the cracking of cast-in-place floor slabs is that the brick walls of brick-concrete structure buildings are too rigid, which limits the free shrinkage of cast-in-place concrete (essentially because of the linear expansion coefficient of concrete and the line of brickwork The difference in the coefficient of expansion is caused by a large difference). When the shrinkage reaches a certain level, cracks are first generated at the construction joints of the cast-in-place slabs, because the old and new concretes on this section have poor adhesion and weak bonding. This non-free shrinkage is not as free as the cast-in-place slab of the building structure on the building floor because the frame columns are more flexible and less rigid than the brick walls. Frame structure consisting of columns, beams and plates of the same material (reinforced concrete). The whole building is basically isotropic. Therefore, even if the construction joints of the cast-in-place slabs of the basic structure of the frame structure are located within the mid-1/3 of the span, they shall be treated according to the regulations. In general, no cracks will appear.
This method of retaining cast-in-situ slab construction joints is difficult to construct. The treatment of construction joints shall be conducted in accordance with the requirements of the code. When concrete continues to be poured at the construction joints, the compressive strength of the poured concrete shall reach not less than 1.2 MPa and the loose stones and weak concrete layers shall be cut off and then washed with pressure water. Clean, and require full moisture conservation, not less than 24h before pouring concrete should be a layer of 10-15mm thick cement mortar, the mix ratio and mortar in the same composition.
2. The second method of retention: the construction joints of cast-in-place slabs are placed on the midline of the transverse wall of the load-bearing wall. This method of retaining the construction joints complies with the specifications in the place where the shear force is small and can eliminate the construction joints in the mid-1/3 of the span. Cracks caused by poor handling within range. The specific approach is:
1 First set up the usual wood or steel pipe on the midline of the load-bearing transverse wall. The first is to provide temporary support for the negative reinforcement of the cast-in-place slab, the second is to provide temporary support for the cast-in-place concrete, and the second is to use it as a cast-in-place concrete. Temporary side template.
2 After the concrete is poured, when the concrete reaches 1.2MP. The excess cast concrete at the centerline is cut off.
3 When the solid concrete at the centerline is removed, it needs to be washed with pressure water, and it is required to keep it wet for more than 24 hours. The residual water remaining on the concrete surface should be removed.
4 Lay a layer of 10-15mm thick cement mortar (its composition is the same as that of mortar in concrete).
5 Secondary pouring cast-in-place concrete.
As we all know, if we can fully achieve the design effect and meet the mechanical assumptions in structural design through construction, then we can ensure the safety of the structure and the service life of the building. On the contrary, through construction, even if the construction technology team with higher levels can not guarantee (or is difficult to construct, it is difficult to guarantee) the original design effect, the assumptions of the original design calculation mechanical model should be changed, and the construction specification regulations Construction joints shall be retained according to design requirements.
Third, floor cast-in-place plate design construction joint processing method
This method of dealing with construction joints is based on summing up the advantages of the above-mentioned construction joint treatment methods, and is comprehensively considered for your reference.
Floor concrete slab design construction joints specific practices:
1. Change the bearing support of the reserved construction joints to simply support the bearing, disconnect the negative reinforcement, adjust the bottom bar (increasing the amount of reinforcement) and the negative reinforcement (reducing the amount of reinforcement). In this way, the state of stress on the floor was changed and the calculations should be redesigned and the consent of the design unit should be obtained.
2. Lower the elevation (reduce the elevation of a cast-in-place slab). This adjustment affects the seismic performance of the structure to be very small, because the horizontal direction of the building is pulled by the ring beam, and the building is pulled up by the structural column.
3. When concrete is poured at the construction joints, the concrete can be cast first and then the cast-in-place concrete can be placed. The vertical side form of the cast-in-place slab can be set directly at the cross section to vibrate the concrete.
4. The spacing between the design and construction joints shall be determined: When designing a large-area floor structure, a construction joint shall be set every 20 meters in the longitudinal direction. This will not only guarantee the assumption of the original design mechanics, but also eliminate the shrinkage cracks of the cast-in-place slabs, and at the same time, it can also solve the problem of the eight-character seams at the top-level window openings caused by thermal expansion of the cast-in-place slabs on the roof.
Through the above several methods and measures, it is possible to basically overcome the quality accidents such as cracks caused by improper positions left by construction joints.
In the construction process of large-area floor cast-in-place slabs, it is often because of construction technology and labor organization reasons, or it is considered that the construction of flowing water is adopted in stages, and the entire cast-in-situ slabs on each floor cannot be continuously poured and completed. When it is expected to exceed the prescribed time, appropriate construction sites should be pre-selected for construction joints.
Second, the setting position of construction joints
Most of the floor cast-in-place slabs of brick-concrete residential buildings, office buildings and other buildings are two-way slabs, and the location of the construction joints is determined from the investigation. In the design drawings, most of the drawings are not indicated separately. Pavement construction joint design requirements. At this stage, there are mainly the following kinds of retention methods:
1. The first method of indwelling: The construction joints of cast-in-place slabs are arranged along the transverse direction in the middle 1/3 of the span of the cast-in-place slab. Although they are placed in the structures where the shear force is small, it is difficult to guarantee the mechanical assumptions in structural calculations. (ie, the integrity of the floor cannot be completely guaranteed.) Due to shrinkage of the concrete floor or under long-term loading, cracks may often occur at such construction joints, or even open joints may crack the floor tiles, which may affect the appearance. It will seep again and cause corrosion of steel bars. Seriously affect the structural safety, affect the life of the building.
The main reason for the cracking of cast-in-place floor slabs is that the brick walls of brick-concrete structure buildings are too rigid, which limits the free shrinkage of cast-in-place concrete (essentially because of the linear expansion coefficient of concrete and the line of brickwork The difference in the coefficient of expansion is caused by a large difference). When the shrinkage reaches a certain level, cracks are first generated at the construction joints of the cast-in-place slabs, because the old and new concretes on this section have poor adhesion and weak bonding. This non-free shrinkage is not as free as the cast-in-place slab of the building structure on the building floor because the frame columns are more flexible and less rigid than the brick walls. Frame structure consisting of columns, beams and plates of the same material (reinforced concrete). The whole building is basically isotropic. Therefore, even if the construction joints of the cast-in-place slabs of the basic structure of the frame structure are located within the mid-1/3 of the span, they shall be treated according to the regulations. In general, no cracks will appear.
This method of retaining cast-in-situ slab construction joints is difficult to construct. The treatment of construction joints shall be conducted in accordance with the requirements of the code. When concrete continues to be poured at the construction joints, the compressive strength of the poured concrete shall reach not less than 1.2 MPa and the loose stones and weak concrete layers shall be cut off and then washed with pressure water. Clean, and require full moisture conservation, not less than 24h before pouring concrete should be a layer of 10-15mm thick cement mortar, the mix ratio and mortar in the same composition.
2. The second method of retention: the construction joints of cast-in-place slabs are placed on the midline of the transverse wall of the load-bearing wall. This method of retaining the construction joints complies with the specifications in the place where the shear force is small and can eliminate the construction joints in the mid-1/3 of the span. Cracks caused by poor handling within range. The specific approach is:
1 First set up the usual wood or steel pipe on the midline of the load-bearing transverse wall. The first is to provide temporary support for the negative reinforcement of the cast-in-place slab, the second is to provide temporary support for the cast-in-place concrete, and the second is to use it as a cast-in-place concrete. Temporary side template.
2 After the concrete is poured, when the concrete reaches 1.2MP. The excess cast concrete at the centerline is cut off.
3 When the solid concrete at the centerline is removed, it needs to be washed with pressure water, and it is required to keep it wet for more than 24 hours. The residual water remaining on the concrete surface should be removed.
4 Lay a layer of 10-15mm thick cement mortar (its composition is the same as that of mortar in concrete).
5 Secondary pouring cast-in-place concrete.
As we all know, if we can fully achieve the design effect and meet the mechanical assumptions in structural design through construction, then we can ensure the safety of the structure and the service life of the building. On the contrary, through construction, even if the construction technology team with higher levels can not guarantee (or is difficult to construct, it is difficult to guarantee) the original design effect, the assumptions of the original design calculation mechanical model should be changed, and the construction specification regulations Construction joints shall be retained according to design requirements.
Third, floor cast-in-place plate design construction joint processing method
This method of dealing with construction joints is based on summing up the advantages of the above-mentioned construction joint treatment methods, and is comprehensively considered for your reference.
Floor concrete slab design construction joints specific practices:
1. Change the bearing support of the reserved construction joints to simply support the bearing, disconnect the negative reinforcement, adjust the bottom bar (increasing the amount of reinforcement) and the negative reinforcement (reducing the amount of reinforcement). In this way, the state of stress on the floor was changed and the calculations should be redesigned and the consent of the design unit should be obtained.
2. Lower the elevation (reduce the elevation of a cast-in-place slab). This adjustment affects the seismic performance of the structure to be very small, because the horizontal direction of the building is pulled by the ring beam, and the building is pulled up by the structural column.
3. When concrete is poured at the construction joints, the concrete can be cast first and then the cast-in-place concrete can be placed. The vertical side form of the cast-in-place slab can be set directly at the cross section to vibrate the concrete.
4. The spacing between the design and construction joints shall be determined: When designing a large-area floor structure, a construction joint shall be set every 20 meters in the longitudinal direction. This will not only guarantee the assumption of the original design mechanics, but also eliminate the shrinkage cracks of the cast-in-place slabs, and at the same time, it can also solve the problem of the eight-character seams at the top-level window openings caused by thermal expansion of the cast-in-place slabs on the roof.
Through the above several methods and measures, it is possible to basically overcome the quality accidents such as cracks caused by improper positions left by construction joints.
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