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Concrete

Joints in Concrete Structures – Part II

Contnd. from Part I …..

Construction joint was discussed in Part – I. Discussion on the other three types ie, expansion joint, isolation joint and contraction joint is continued in Part – II & III.

Expansion joint : Concrete structural members such as slabs, pavement, walls etc expand in hot environment. For members of smaller dimensions this expansion may be negligible. For members exceeding certain lengths, widths the expansion may be more than sufficient to induce flexural stresses and thus induce cracks in the structure unless they are given the opportunity to expand freely. These cracks, besides weakening the structure, can lead to corrosion of steel reinforcement due to ingress of moisture through them which can pose even bigger problems.

That is where the expansion joints come into the picture. An expansion joint is provided in order to allow a concrete member to expand freely in hot environment and thus relieve the unwanted stresses so that no damage is caused to the it or the structure as a whole. These joints are common features in masonry and metallic structures.

Expansion joints are essential elements of long bridges, railway tracks, long walls, piping networks etc. These joints are provided in such a way that the overall structure is not weakened due to their presence. In liquid retaining concrete structures expansion joints are equipped with water stops.

Diverse materials are used for preparing these joints. Typical expansion joints in concrete structures can be about half inch to 20mm wide. The joints are prepared by placing forms of required thickness through full depth of the concrete members.

A quite common as well as economic formwork material is fibre impregnated bitumenous boards. Other compressible joint fill materials used in expansion joints are cork, polystyrene, rubber etc.

Sometimes, wooden forms are used which are removed after the concrete sets. Thereafter, the concrete members on both sides of the joint are given sufficient time to go through expansion or contraction for some days. Then the compressible joint fill materials, as mentioned above, of adequate thickness are inserted in the gaps through full depth of the concrete members such that the top of the filler boards remain about 20mm below the top of the concrete member. This 20mm or so deep voids are later sealed with elastomeric compounds like polyurethane (PU) or polysulphide etc. These are applied to seal the joints completely from ingress of water, dust, dirt etc and also to render better appearance. These elastomeric sealants are quite capable of accommodating the expansion or contraction of the concrete structures.

On the other hand, some prefer to install the compressible fill materials as sacrificial forms, ie they are not withdrawn upon completion of casting of the members. Elastomeric sealants are applied after some days as mentioned above to seal the joints.

Reinforcing steel bars are always discontinued at expansion joints to enable free expansion of the concrete members. However, sometimes dowel bars have to be provided in order to prevent differential settlement of the members on both side of the joint. This is especially true for concrete members subjected to heavy load or high traffic.

When dowels are provided they are usually kept fixed inside concrete at one end while the other ends (on the other side of the joint) have to be kept free by some means, eg by installing hollow tube inserts inside the concrete. The other ends remain inside the hollow tubes so that they can move freely when the concrete members expand or contract. The forms or the compressible joint fills should have perforations accordingly to let the dowel bars pass through them. Construction drawings usually clearly show the details of dowel bars wherever they are required.

A little suggestion from this author in this regard. There are coatings available that can completely prevent bond between a bar and the sorrounding concrete. Such coatings can be applied to dowel bars and two small compressible caps can be fitted at both ends of these bars after installing them. Even one cap of adequate thickness at one end should serve the purpose. These compressible caps should be able to facilitate movements of the bars inside the concrete even as the coating prevents bonds. This would eliminate the cumbersome process of installing hollow tubes etc besides proving to be more economical. In fact, this looks so simple that such a practice could already be existing unless there are some shortcomings in the suggestion.

Concluded in Part III …..

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About clazcons

B. Tech. (Civil Engineering)

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