Project managment

Construction and other joints

The resident engineer must agree with the contractor where construction joints should be placed; but he should not require them to be placed in impracticable positions and must allow for the manner in which formwork must necessarily be erected. There are positions for construction joints which are ‘traditional’ even though the position may not seem to be the most desirable from a structural point of view. For instance a construction joint usually has to occur at the base of a wall even though it cantilevers from a base slab, which is a point of maximum tensile stress in one face of the wall concrete.
This joint is best sited 150 mm above the base slab so as to give a firm fixing for the wall shutters and the best possibility of achieving a sound joint. In water-retaining work it is important to keep the number of construction joints to a minimum.
The bonding of one layer of concrete to a previous layer is usually accomplished by cleaning the surface of the old concrete with a high pressure water jet, and placing a layer at least 2 cm thick of mortar on the exposed surface immediately before the new concrete is placed. Sometimes a proprietary bonding mortar is used, especially when refilling cut-out portions of defective concrete. Wire brushing of the old surface is not so effective as water jetting, is laborious, and can seldom be properly done when reinforcement passes through a joint. Aproblem frequently encountered is that of finding debris on a construction joint at the bottom of erected formwork. Such debris must be  removed before the mortar layer and new concrete is placed. Usually it is the job of the resident engineer’s inspector to inspect formwork and the cleanliness of construction joints before permission is given to the contractor to start concreting. If the contractor runs ‘Quality Assurance’ one of his staff should act as inspector of formwork, but this does not relieve the resident engineer of his need to inspect on behalf of the engineer.

In liquid-retaining structures resilient plastic waterstops are usually provided at contraction joints. Fixing half their width in the stop-end shuttering to a narrow reinforced concrete wall often leaves a congested space for the concrete which must therefore be most carefully vibrated in place to ensure that the waterstop is bedded in sound concrete. If the concrete face of the joint is to be bitumen painted before the next wall section is built, bitumen must not get on the waterstop.
Floor joint grooves need cleaning out by water jetting, then surface drying as much as possible with an air blower before the priming compound supplied by the manufacturer of the joint filler is applied to the groove faces. It is essential that this primer is not omitted, and the filler must be pushed down to the bottom of the groove. Joint grooves are normally filled after the concrete has been allowed to dry out for 2 or 3 weeks when most shrinkage on drying should have taken place (see Section 19.11).
Leaks from liquid retaining concrete structures are most likely to occur from opening up of wall joints due to wall movement, especially at the corners of rectangular tanks; and puncturing of the floor joint filler under liquid pressure where the filler has not been solidly filled to the base of the groove.


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