Following points are helpful to provide construction joint in RC Slab, Beam and Raft Foundation:
- Construction joint shall be provided preferably at a location where stress are zero.
- Construction joint is provided at one-third or end of support.
- Also provide key and rebar to transfer shear stress if shear stress is not zero at point of construction joint.
- Provide construction joints as minimum as possible.
- Construction joint shall not be left on discretion of contractor. It must be approved by designer.
Definition:
Construction joints are placed at the end of a day’s work. In slabs, they may be designed to permit movement and/or to transfer load. Often in reinforced concrete a conscious effort is made to clean the joint and bond the next day’s work.[1].
Explanation:
The number of construction joints in concrete structures should be minimized. If construction joints are necessary to facilitate construction, it is normally aligned perpendicular to the direction of the member. For beams and slabs, construction joints are preferably located at about one-third of the span length. The choice of this location is based on the consideration of low bending moment anticipated with relatively low shear force. However, location of one-third span is not applicable to simply supported beams and slabs because this location is expected to have considerable shear forces and bending moment when subjected to design loads. Sometimes, engineers may tend to select the end supports as locations for construction joints just to simplify construction. [2].
Construction joint is not preferred at midspan. When formwork removed from construction pours (delay in next pouring) and the beam (or slab) has had to cantilever from the previous support to the new construction joint. Also, joints at midspan do not typically work for post-tensioned construction. you need to accommodate anchorages and at approx 1/3 span works best. [3].
You need to provide a joint with key and rebars crossing the joint. In raft footing, at construction joint, shear force may not be minimum/zero. Hence you will have to leave dowels from one pour to another pour to transfer shear. You may calculate area of reinforcement required based on permissible shear stress as 0.45 fy. [4].
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Reference:
- CADMAN
- A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
- eng-tips
- sefindia.org
Additional Reference:
- ACI 302.1, “Guide for Concrete Floor and Slab Construction,” ACI Manual of Concrete Practice.
- “Slabs on Grade,” ACI Concrete Craftsman Series, American Concrete Institute, Detroit, Mi.
- “Cracks in Concrete: Causes, Prevention, Repair,” a collection of articles from Concrete Construction Magazine, June, 1973.
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