Joints in Concrete Structures

Transcription

1 Joints in Concrete Structures 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.

2 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.

3 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. 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

4 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. Construction joints and expansion joints (aka movement joints) were discussed in Part-I & II respectively. Isolation joints and contraction joints are discussed in this post. Isolation joint : In concrete structures isolation joints are provided in order to relieve the stresses developed due to differential movement of the adjacent members or structures, thus preventing cracks (or damage) in the members or the structure as a whole. Isolation joints around machine foundations also perform the function of cutting off the vibrations generated by the machines, thus safeguarding the adjoing slabs or other structural elements from possible damage.

5 Isolation joints may be required at the junctions of horizontal members (slabs etc) and vertical members (columns, walls etc) when there are possibilities of significant differential movements. These joints may very well be required when a slab or a roof of a newly built structure has to meet an existing structure, or, a new block has to be extended from an old building in relatively softer soil or in a filled area. In both the cases there is every likelihood that the newly built slab or the newly extended block would settle more as compared to their old counterparts. Unless isolated, these differential movements can develope sufficient stresses causing cracks and damage to the structural members. The same issue might develope at the junction of large columns and slabs. While the slab would tend to move horizontally the column would always move vertically due to expansion or load related settlement causing differential movement. So, if the same is expected to be significant, an isolation joint along the periphery of the column can prove quite worthy. Isolation joints are seen in bridges as well. Isolation joints are usually about half an inch to 20mm wide. Like expansion joints these too are filled with compressible joint fillers such as fibre impregnated bituminous boards, cork, polystyrene, rubber etc. Either the joint fill boards are installed about 20mm short of top of the concrete surface or after installing the compressible boards through full depth of the joint the top 20mm (or so) portion is cut off. This void is later filled with elastomeric sealing materials like Polyurethane (PU) or Polysulphide (PS) sealants for better appearance as well to render the joints water and dirt proof. For best results, joint fillers and elastomeric sealants are to be installed only after the concrete members are cast and full loads are transferred through them. Contraction Joint : When concrete sets and dries it tends to shrink. This shrinkage or contraction can cause hair cracks on the surface of the concrete. Besides affecting the appearance of the finished surface these cracks could lead to other problems in the course of time. This is not an issue for concrete members with small surface area such as beams, columns etc. That s because the quantum of shrinkage is negligible. But cracks due to drying shrinkage can be a major issue for concrete structures with large surface area such as concrete roads, other large concrete pavement or floors etc. By forming contraction joints on the surface this issue is sorted out. Contraction Joints are also known as control joints. These joints are created in the concrete surface by inserting thin strips in fresh concrete, by saw cutting using either dry-cut or wet-cut saws, by grooving using grooving tools etc.

6 An important aspect is the depth of these joints as unless the depth of joint is adequate they won t serve the purpose well. According to a popular thumb rule the depth of a contraction joint should be at least one fourth of the depth of a concrete slab. Similarly, thumb rules are also there regarding spacing of control joints. Unlike an isolation joint or an expansion joint, a contraction joint is quite thin as the purpose of it is just to guide the cracks to a predetermined weak plane which is the joint itself. As already mentioned these joints should be preplanned to coincide construction joints so as to minimise the number of joints in a structure. Source: