Contnd. from Part II …..
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.
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.
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.