Joints are usually part and parcel of structures made up of concrete, steel, masonry etc. Concrete structures usually call for a varieties of joints. The most commonly experienced among them are a) Construction joint b) Expansion joint c) Contraction joint and d) Isolation joint.
Of these, expansion joint and isolation joint are sometimes found to be regarded as the same type. That could be because an expansion joint is also seen as an isolation joint as it too isolates one part of the structure from another. Similarly, an isolation joint is sometimes seen as an expansion joint as well because it too can accomodate heat-induced expansion of concrete members.
But, actually both are distinct joints. An expansion joint is meant to take care of the stresses developed purely due to the heat-induced expansion of concrete members while an isolation joint is provided to tackle the stresses likely to result from differential settlement of adjacent members or structures.
a) Construction joint : This type is the most commonly experienced joint in most concreting work in spite of the fact that this type of joint is actually not a must in concrete structures unlike the other three types. A construction joint is provided when concrete pouring needs to be stopped due to some reason and then is continued again later. This is done to retain the monolithic nature of the structure which otherwise would be broken due to the break in the continuity.
In other words, the joint could have been avoided if the entire concreting were completed without any stopping in between so that no part of the structure needs to be continued (by concreting) at a later date. Practically that is often not possible, especially for larger structures. Interruptions can happen due to varieties of reasons such as sudden breakdown of machinery, stopping of work at the end of the day, sudden heavy rain, installation of formwork for next lift (for wall, column etc.) and so on.
Sometimes provisions for future extention of a building or a structure are required to be kept. Construction joints are often required at the ends of beams, slabs, tie beams etc, in such cases, for the purpose of future extention.
Since, construction joints are practical requirements for most concrete structures they should preferably be planned beforehand, wherever possible, and shown in the construction drawings. That would help in planning the concrete pour-plan accordingly and also in preparing the joints properly. Construction joints resulting from unforeseen events like rain, machinery breakdown etc, however, can arise at any moment and one should just stay prepared for such cases.
A construction joint can be avoided if it can be coincided with an expansion joint as monolithic nature of structure is always broken at expansion joints. Where contraction joints are required to be provided, it is always better to plan construction joints to coincide with contraction joints so as to minimise the number of joints in a structure.
Concrete’s capacity to take bending stresses is negligible. In reinforced concrete the steel bars take the stresses due to bending moment (BM) while the concrete has to endure primarily the shear stresses. Hence, planes of minimum shear force (SF) in structures are the ideal locations for providing construction joints. One need not worry about BM stresses as there always are enough reinforcement to take care of it and concrete does not have to bear stresses due to BM. So, in case of a beam or a slab the middle third span is the zone for providing construction joints where shear stresses are the minimum. For a column, a construction joint may be provided 3 to 4 inches below the lowermost soffit of the adjoining beams.
The plane of a construction joint should be perpendicular to the steel reinforcement of the concrete member. The formwork for construction joints should have shear key blocks. Shear keys provide better interlocking and thus more efficient transfer of SF between the old and new concrete.
Forms should also have well located perforations to allow the reinforcement or dowel bars to pass through. Dowel bars are usually provided when a slab, pavement etc would be subjected to heavy loads or vehicular traffic. They may as well have to be provided when concreting has to be stopped at high SF zone due to sudden unforeseen reasons as mentioned above. Shear keys are not necessary where dowel bars are provided.
Before continuing concreting the formwork and shear key blocks are stripped. If concreting is continued within a day or two of stopping the work, the construction joint is thoroughly water cleaned and then a layer of rich mortar is applied onto it before proceeding with the concreting. For an older joint it’s surface is roughened first and then water cleaned well to get rid of all dirts, loose materials etc. Thereafter, a layer of rich mortar is applied and concreting is resumed.
Other three types of joints are discussed in Part – II …..