First, the time definition of construction joint setting
In the construction process of large-scale floor cast-in-place slab, it is often because of the construction technology and labor organization, or considering the sectional flow construction, which leads to the continuous casting of each floor cast-in-place slab as a whole. When the estimated intermittent time exceeds the specified time, the construction joints shall be set at appropriate positions in advance.
Second, the setting position of construction joints
The cast-in-place floor slabs of residential buildings, office buildings and other buildings with brick-concrete structure are mostly two-way slabs, and the lien position of construction joints comes from investigation. In the design and construction drawings, most drawings do not separately indicate the design requirements of construction joints of cast-in-place slabs. At this stage, there are mainly the following ways of lien:
1. The first lien method: the construction joint of the cast-in-place slab is set in the range of 1/3 in the middle of the cast-in-place slab. Although it is preserved where the shear force of the structure is small, it is difficult to ensure the mechanical assumption in the structural calculation (that is, the integrity of the floor cannot be completely guaranteed). Due to the shrinkage of concrete floor slab or under long-term load, cracks often appear at this construction joint, and even through cracks may appear, which makes the floor tile crack. In the worst case, it will affect the structural safety and service life of the building.
The main reason for the cracks in this kind of cast-in-place floor slab is that the brick wall of brick-concrete structure building is too rigid, which limits the free shrinkage of cast-in-place slab concrete (essentially, the linear expansion coefficient of concrete is too different from that of brick masonry). When the shrinkage reaches a certain degree, cracks first appear at the construction joints of the cast-in-place slab, which is because the bonding force between the old and new concrete in this section is poor and the combination is not firm. This kind of non-free shrinkage is not as free as the cast-in-place floor slab of frame structure building, because the frame column is more elastic and less rigid than the brick wall. Frame structure composed of columns, beams and plates made of the same material (reinforced concrete). The whole building is basically isotropic. Therefore, even if the construction joint of the cast-in-place slab of the frame structure foundation building is within the span of 1/3, it should be treated according to the specifications. There are generally no cracks.
This method of leaving construction joints in cast-in-place slab is difficult to construct. The treatment of construction joints should meet the specification requirements. When pouring concrete at the construction joint, the compressive strength of the poured concrete should be not less than 1.2MPa, and the loose stones and weak concrete layer should be chiseled away, then washed with pressure water, and it is required to be fully wet and moisture-proof. Not less than 24 hours before pouring concrete, a layer of cement mortar with the thickness of 10- 15mm shall be paved, and its mixture ratio shall be the same as that in concrete.
2. The second lien method: the cast-in-place slab construction joint is retained on the center line of the bearing transverse wall. This design method of construction joints meets the requirements of specifications, and can eliminate the cracks caused by poor treatment of construction joints in the range of mid-span 1/3. The specific approach is:
(1) usually in the center line of the bearing transverse wall set up stow-wood or steel pipe, can be used as a temporary support for negative reinforcement of cast-in-place slab, cast-in-place slab concrete pouring temporary support, cast-in-place slab concrete pouring temporary side form.
(2) After the completion of concrete pouring, when the concrete reaches 65438±0.2 MP, chisel away the excess concrete poured at the center line.
(3) The dense concrete chiseled away from the center line needs to be rinsed with pressurized water and kept moist for more than 24 hours, and the residual water on the concrete surface should be removed.
(4) Spread a layer of cement mortar with a thickness of 10- 15mm (the mixture ratio is the same as that of mortar in concrete).
⑤ Secondary pouring of cast-in-place slab concrete.
As we all know, if the design effect can be completely achieved by building construction and the mechanical assumptions in structural design can be met, then the structural safety and service life of the building can be guaranteed. On the other hand, if the original design effect can't be guaranteed even by the highest level construction team (or it is difficult to construct or guarantee), the assumption of the original design computational mechanics model should be changed, and the construction specification stipulates that construction joints should be set according to the design requirements.
Three, the floor cast-in-place slab design construction joint treatment method
This construction joint treatment method is comprehensively considered on the basis of summarizing the advantages of the above construction joint treatment methods for your reference:
Concrete method of construction joint design for cast-in-place floor slab;
1. Change the bearing of the reserved construction joint into a simply supported bearing, disconnect the negative reinforcement, and adjust the bottom reinforcement (the number of reinforcement increases) and negative reinforcement (the number of reinforcement decreases) here. In this way, the stress state of the floor is changed, and the calculation needs to be redesigned and approved by the design unit.
2. Lowering the elevation (lowering the elevation of the thickness of a cast-in-place slab) has little effect on the seismic performance of the structure, because the horizontal direction of the building is the ring beam and the vertical direction is the constructional column.
3. When pouring concrete at the construction joint, ring beam concrete can be poured first, and then cast-in-place slab concrete can be poured. The vertical side formwork of the cast-in-place slab can be directly set at the section, and the concrete can be vibrated and compacted.
4. Determine the spacing of design construction joints: When designing a large-area floor structure, a construction joint should be set every 20 meters along the longitudinal direction. This can not only ensure the mechanical assumption of the original design, but also eliminate the shrinkage cracks of the cast-in-place slab, and also solve the problem of splayed joints at the top window opening caused by the thermal expansion of the cast-in-place slab on the roof.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd