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Joints in concrete structures such as contraction joints and expansion joints are required to be provided to prevent cracks in concrete at hardened state. To provide these joints, there certain specification which must be followed. These specifications are explained below:
Specifications for Construction joints:
A construction joint is defined as a joint in the concrete introduced for convenience in construction at which special measures are taken to achieve subsequent continuity without provision for relative movement (Hacking of laitance, air jetting and wetting).
Fig: Construction joint in concrete
Construction joints shall be located so as not to impair the strength of the concrete. The position of construction joints and the size of the formwork panels shall be so co-ordinated that where possible the line of any construction joint coincides with the line of a formwork joint and that in any case all construction joint lines and formwork joint lines appear as a regular and uniform series.
For all exposed horizontal joints and purposely inclined joints, a uniform joint shall be formed with a batten of approved dimensions to give a straight and neat joint line.
Concrete placed to form the face of a construction joint shall have all laitance removed and the large aggregate exposed prior to the placing of fresh concrete. The laitance shall wherever practicable be removed when the concrete has set but not hardened by spraying the concrete surface with water under pressure or brushing with a wire brush sufficient to remove the outer mortar skin and expose the large aggregate without disturbing it.
Where the laitance cannot be removed due to hardening of the concrete, the whole of the concrete surface forming the joint shall be treated by high pressure water j et, sand blasting, use of a needle gun or a scaling hammer to remove the surface laitance.
The other alternative but effective method is:
- Apply retarder by brush on the concrete surface and wash it by water jet. The laitance at top does not get set and is easily removed by washing. Before concreting is resumed, all loose matter on the existing concrete surface shall be removed by compressed air and the surface slightly wetted, but excess water removed by soaking or air jet (Never spray cement slurry).
Concrete shall not be allowed to run to a feather-edge and any vertical joints shall be formed against a stop-end. If a kicker or stub-end is used it shall be at least 70 mm high. Removal of laitance from vertical face, wetting etc. shall be same as for horizontal joint as explained above. No concreting shall be started until the positions and form of the construction joints (both horizontal &vertical) are inspected and found in order.
Specifications for Movement /expansion joints:
Movement joints are defined as all joints intended to accommodate relative movement between adjoining parts of a structure, special provision being made where necessary for maintaining the water tightness of the joint. The contractor shall pay due regard to the instructions of manufacturers of proprietary jointing materials and shall, if required by the Engineer, demonstrate that the jointing materials can be applied satisfactorily.
Fig: Expansion joint in concrete
As far as possible jointing of waterstops on Site shall be confined to the making of butt joints in straight runs. Where it is necessary for an intersection, or change of direction, or any joint other than a butt joint in a straight run, to be made on Site, a preliminary joint, intersection or change of direction piece shall be made and submitted to such tests as the Engineer may require.
Flexible waterstops shall be fully supported in the formwork, free of nails and clear of reinforcement and other fixtures. Any waterstops which after installation is found to be damaged shall be removed and replaced.
Concreting in the vicinity of waterstops shall be carried out with care to ensure that the waterstops do not bend or distort while the concrete is being placed and compacted in position.
The surface of set concrete in a movement joint shall, where specified on the drawing, be painted two coats of bituminous paint and new concrete shall be placed against it only when the paint is dry. Expansion joints shall be formed by a separating strip of approved preformed joint filler.
Caulking grooves shall be provided as shown on the Drawings. At all joints where a caulking groove is formed, the groove shall be wire brushed and loose material removed and blown out by compressed air immediately prior to caulking. After the groove has dried, it shall be primed and caulked with approved jointing compound. At all caulked joints, the face of the caulking strip and 50 mm width of concrete on either side shall be painted with two coats of paint having the same base as the caulking compound.