What Is a Construction Joint?

Construction joint refers to the joint formed between the first and last concrete during the concrete pouring process due to design requirements or construction needs to be poured in sections. Construction joints are not a kind of real "slots". It is just because the concrete is poured first over the initial setting time, and there is a joint surface with the concrete poured later. This joint surface is called construction joint.

Construction joint

The position of the construction joint shall be set at a location where the structure is subject to small shear forces and is easy to construct, and shall meet the following requirements: Horizontal joints shall be left in columns and walls, and concrete for beams and slabs shall be poured at one time, leaving no construction joints.

The construction joint shall be left on the top surface of the foundation, under the corbel of the beam or crane beam, on the crane beam, and under the beam cap of the beamless floor.

For large cross-section beams connected with the floor slab, the construction joints should be left at a distance of 20 mm to 30 mm below the bottom surface of the slab. When there is a beam support under the plate, it is left under the beam support.

(3) For unidirectional boards, the construction joints should be left at any position parallel to the short sides of the boards.

Floor slabs with primary and secondary beams should be poured in the direction of the secondary beams. Construction joints should be left in the middle 1/3 of the span of the secondary beams.

The construction joint connection method shall meet the design requirements. When the design has no specific requirements, for plain concrete structures, a connection steel bar with a diameter of not less than 16mm should be buried in the construction joint. The embedded depth and exposed length of the connecting bars should not be less than 30d of the diameter of the bars, and the spacing should not be greater than 20cm. When using round bars, semi-circular standard hooks should be set at both ends. When using ribbed bars, hooks can be omitted. The treatment of concrete construction joints should also meet the following requirements:

The concrete aggregates at the construction joints are concentrated, the concrete is loose, the old and new concrete are stubble, and water leaks along the joints.

1.Cutting method for standing seam surface

After the concrete is finally set, the baffle is removed, and the surface is chiseled with a chopping axe or steel rod to clean loose stones. At this time, the concrete strength is very low. It is easier to chisel to a depth of 20 ~ 30MM. When the concrete is secondly poured, use pressure water in advance Rinse the seam surface and brush the plain cement slurry while pouring to increase the bite force.

2.Add coarse aggregate method

The large volume of beams and plates causes the thickness of the indwelling joints to be thick, and the surface slurry layer and bleeding layer are also correspondingly thick. It is difficult to handle the construction joints. For example, if the surface slurry is scraped or the secondary vibration is not effective, The method of adding coarse aggregate can be used to scatter grading clean crushed stones into the slurry and re-vibrate to prevent the stones from concentrating. In this way, the thick and fine aggregate will be uniform when the concrete is poured at the joints, the cement slurry will not be lost and the strength will not be reduced, and the adhesion and bite force of the new and old interfaces will be improved.

Construction joint

3.Removing the slurry method

When the volume of the concrete is small, the simple method is to scrape the surface of the slurry with an iron trowel, the depth is less than 25MM, and excavate the stripe, which can improve the bonding quality of the horizontal construction joints, which is beneficial to the combination of old and new concrete .

4. Secondary development vibrating method

Master the time, and perform the second re-vibration after the initial setting and before the final setting. This will re-stir the sunken stones and the floating slurry water to make it more uniform and dense. The re-vibration of the joints proves to be effective One of the measures.

5. When the concrete is continuously poured at the construction joints, the following requirements shall be met:

The compressive strength of the poured concrete shall not be less than 1.2 MPa.

On the surface of the hardened concrete, the cement film and loose stones and the weak concrete layer should be removed, and fully wet and rinsed, and no water should be accumulated. That is to do: Remove the milky skin, reveal the rough sand, and the surface is rough.

(3) Before pouring, the horizontal construction joint should be covered with a layer of cement mortar with a thickness of 10 mm to 15 mm. Its mixing ratio is the same as that of the mortar in the concrete.

The concrete should be finely vibrated and compacted to ensure the close combination of the old and new concrete.

The design of waterproof concrete structure, the arrangement of its steel bars and the thickness of the wall should be considered for easy construction and easy to ensure the quality of construction.

Waterproof concrete should be continuously poured, and fewer construction joints should be left. When the construction joints need to be retained, the following requirements shall be observed: First, construction joints shall not be left on the floor and roof, and longitudinal construction joints shall not be left on the bottom and top arch. Second, the wall should not leave vertical construction joints. The horizontal construction joint shall not be left at the place where the shear force and bending moment are maximum or where the floor and the side wall meet. It shall be left on the wall not less than 300 mm above the floor surface. When there is a hole in the wall, the construction joint shall not be less than 300 mm from the edge of the hole. Horizontal construction joints with arch-wall joints should be left 150 mm to 300 mm below the seam line of the arch (slab) wall. Construction joints of the arch-back wall can be left at the arch-line, but attention must be paid to strengthening waterproof measures. Adopt the waterproof water-proofing tape outside, apply waterproof paint and mortar outside. Third, the foundation of the equipment under the action of power should not be left with construction joints.

Walls with a height greater than 2 m should be unloaded with a tube or vibration tube.

5. Check calculation of shear capacity of beam construction joints

V0.85fA

V--the value of the shear force acting on the construction joint

f--Design value of longitudinal joint strength of construction joint section

A--The total area of steel bars that intersects the construction joints perpendicularly.

When the shear capacity of the beam construction joint does not satisfy the above formula, insert reinforcement bars in the construction joint with a diameter 12mm and a spacing of 200mm. The anchorage length of the mortise on both sides of the joint shall be la.