What are the requirements for the location of concrete construction joints?

In the project, the construction joint is left at the "place" in the middle of the span, that is, the position in the middle of the support position.

The rationale is:

1. Concrete construction joints have a great influence on the shear force and little on the bending moment. In the flexural components of reinforced concrete beams, the bending moment of the beam is formed by the compression of the reinforced concrete and the tensile force of the reinforcement, and the concrete construction joint has little influence on the compressive strength of the concrete, so the concrete construction joint has little influence on the bending moment bearing capacity of the reinforced concrete component. Because the concrete on both sides of the construction joint is poured twice, the combination of new and old concrete has a great influence on the shear strength of the concrete. When the construction joint is retained, there is no need to consider the bending moment, only the shear force can be considered.

2. The specification stipulates two principles for determining the location of construction joints: small shear force and convenient construction. Article 1 of the Code for Acceptance of Construction Quality of Concrete Structure Engineering GB50204-2002 stipulates:

Concrete construction joints should not be left at will, and their positions should be determined in advance in the construction technical scheme. The principle of determining the position of the construction joint is: leave it in the part with less shear force as much as possible; The retention site should be convenient for construction.

Where is the shear force low for a beam member that is subjected to load? Obviously the middle of the span, the shear force in the middle of the span is minimal. Therefore, it is the most suitable position for the construction joint to be left in the middle of the span.

The old code "Code for Construction and Acceptance of Concrete Structure Engineering GB50204-92" stipulates that the construction joints of beams are retained within the "third of the span". Note that it is "within the middle third", not "at the middle third". From this provision, it can be seen that the middle third of the span is the most suitable part, and the "place" of the middle third of the span is the boundary point of this suitable range, that is, the most unfavorable point of this range.

The specification stipulates the principle that "the retention site should be convenient for construction". Before concrete pouring, the construction joint of the beam needs to be blocked with a template, and the operation of the mid-span section is the most convenient, and at the mid-span one-third place, there is often a negative moment reinforcement, and the plugging template is inconvenient, and the concrete interface of the construction joint is also inconvenient to be cleaned, and the operation is slightly deviated, and the mid-span one-third range may be deviated. Therefore, from the perspective of convenient construction, the "place" in the middle of the span is not a suitable location.

3. To sum up, it is inappropriate for the construction joint of the beam to be left in the middle third of the span. The "place" in the middle of the span is neither the position with the smallest shear force nor the most convenient position for construction, which is inconsistent with the principles of the construction specification. Some people even think that the construction joint retention in the middle of the span position is not allowed, which is even more contradictory to the principle of the specification, on the contrary, the middle of the span is the most suitable position for the construction joint to be retained.

The position of the construction joint should be set in the part where the shear force of the structure is small and convenient for construction, and the following provisions should be met: Horizontal joints should be left in columns and walls, and the concrete of beams and slabs should be poured at one time without leaving construction joints.

The construction joint should be left on the top surface of the foundation, below the corbel of the beam or crane beam, above the crane beam, and below the column cap of the beamless floor slab.

For the large-section beam that is connected with the floor slab, the construction joint should be left 20mm 30 mm below the bottom surface of the slab. When there is a beam bracket under the slab, it is left in the lower part of the beam bracket.

For one-way slabs, the construction joints should be left anywhere parallel to the short side of the slab.

The floor slab with the main and secondary beams should be poured along the direction of the secondary beam, and the construction joint should be left in the range of the middle of the secondary beam span 1 3.

The construction joint on the wall should be left in the range of the lintel span 1 3 of the door opening, and can also be left at the junction of the longitudinal and horizontal walls.

The construction joints on the stairs should be left in 1 3 places of the tread board. The concrete of the staircase should be poured continuously. If it is a multi-storey staircase, and the upper floor is a cast-in-place floor slab and has not been poured, the construction joint can be retained; It should be left in the middle of the staircase 1 3 parts, but it should be noted that the joint surface should be diagonally perpendicular to the direction of the stair axis.

The reason for the dispute in the construction is that the old specification stipulates that the staircase construction joint must be left in the middle 1 3 section, and the traditional construction is left in the upper and lower 3 steps, and when it is left in the middle of the staircase, theoretically the shear force is smaller, but the quality of the construction joint is not well controlled during construction, and it is easy to produce the short-term "cantilever" of the poured part when the secondary formwork is supported, which is not conducive to the quality control of the component. ）

The construction joints of the pool wall should be left on the vertical wall 200 mm and 500 mm above the surface of the bottom plate.

For two-way stressed floor slabs, large-volume concrete, arches, shells, silos, equipment foundations, multi-layer rigid frames and other complex structures, the location of construction joints should be set up according to the design requirements.

Summary. Good morning, dear. Construction joints refer to the gaps set up in buildings or structures in order to avoid deformation and cracks caused by factors such as temperature, humidity, and **.

The location of the construction joint needs to be specified according to the structure, materials, use environment and other factors of the building, and the following aspects are considered: 1Structural Factors:

Factors such as the structural form, height, and span of the building will affect the location of the construction joint. 2.Material Factors:

Factors such as the thermal expansion and contraction coefficient and elastic modulus of the materials used in the building will also affect the location of the construction joints. 3.Environmental factors:

The environmental factors in which the building is located, such as temperature, humidity, etc., will also affect the location of the construction joints. The location of the construction joint should be at the weak point of the building, such as columns, wall corners, windows, etc. The width of the construction joint also needs to be specified according to the structure of the building and the use environment, generally speaking, the width should be between 5mm and 20mm.

What is a construction joint and how to specify the location of the retention? How to deal with it?

Good morning, dear. Construction joints refer to the gaps set up in buildings or structures in order to avoid deformation and cracks caused by factors such as temperature, humidity, and **. The location of the construction joint needs to be specified according to the structure, materials, use environment and other factors of the building, and the following aspects are considered:

Nuclear Bi 1Structural factors: The structural form, height, span and other factors of the building will affect the location of the construction joint.

2.Material factors: The thermal expansion and contraction coefficient of the materials used in the building, the elastic modulus and other factors will also affect the location of the construction joints.

3.Environmental factors: The environmental factors in which the building is located are changed, such as temperature, humidity, etc., will also affect the location of the construction joints.

The location of the construction joint should be at the weak point of the building, such as columns, wall corners, windows, etc. The width of the construction joint also needs to be specified according to the structure of the building and the use environment, generally speaking, the width should be between 5mm and 20mm.

The treatment of construction joints can be done in the following ways:1Caulking:

Use caulking materials to fill construction joints, such as silicone sealants, polyurethane sealants, etc. 2.Coverage:

Covering materials are used to cover construction joints, such as aluminum alloy plates, PVC plates, etc. 3.Hide:

Hide the idea of construction joints in the decorative materials of the building, such as wallpaper, tiles, etc.