Joints in concrete

Concrete moves; if this movement is restricted beyond its area of control or tied to another structure, we get what is called ‘restraint’. This causes tensile forces internally and invariably causes cracking. Restraint simply means that the concrete element/structure is not being allowed to freely shrink / move/settle as it needs to. Joints allow one concrete element to move independently of other parts of the structure and other structures as may be the case. Joints also let concrete shrink locally as it dries, preventing what is called ‘internal restraint’. Internal restraint is created when one part of a slab shrinks more than another or shrinks in a different direction.

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Isolation joints – They?completely isolate a structure from something else i.e. a wall, a column or cladding, etc., that has a different coefficient of movement compared to the concrete. If not isolated and the various parts/structures stay connected and they move separately, will cause cracking. Isolation joints are formed by placing a soft separation prior to pouring the slab. Isolation joints are formed using impregnated fiberboard, polyethylene, cork, neoprene, etc. Isolation joint material should go all the way through the slab and be sealed at the top using a sealant.

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Construction joints - When concrete into a slab cannot be poured all at once, there will be starting and stopping points. Construction joints are thus created using a temporary stop. These are usually screed rails used during the placement and finishing of the slab. Construction joints should be tried to be worked into the overall joint plan, where they can also possibly function as contraction/control joints. This can also be done in cases of equipment breakdown, an unexpected shortage of materials, or bad weather. If the slab will have no significant traffic crossing the joint, a plain butt joint, with no reinforcement crossing the joint, is acceptable. If there is to be traffic other than foot traffic, you will need to use some sort of load transfer device; generally, rebar installed perpendicular to the joint both vertically and horizontally.

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Expansion Joints - Concrete, once placed and cured, will expand and/or contract relative to the change in ambient temperature and other physical conditions. It is therefore advisable to incorporate a movement/expansion joint within larger slabs or structures. Joints can be positioned within the structure incorporating aesthetic reasons. These kinds of joints are wider than control/construction joints and allow expansion and contraction/movement of the concrete structure without generating potentially damaging forces within the structure itself or the surrounding structures. These joints are also created to assist them to structurally behave independently without creating potentially damaging forces within the structures. These kinds of joints are generally formed using a fibrous, compressible, flexible board, such as 'Flexcell' or “Fibre Expansion Joints”, foam-based fillers that are flexible, lightweight, non-staining based on polyethylene, closed-cell expansion technology. Some materials also offer chemical resistance, stability against ultraviolet rays, non-absorbent features with low-density properties that are a requirement under certain conditions.

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Control joints - are placed in concrete slabs to restrict and direct potential shrinkage and avoid random cracking. Fresh concrete is very mouldable but as the material undergoes curing, it hardens and there is a change in a volume called ‘shrinkage’. While concrete is very strong in compression, the tensile strength is only 8 - 12 % of its compressive strength. Therefore when shrinkage is restrained and tensile stresses develop within the concrete it tends to crack. This is called shrinkage cracking. The most widely used method to control random cracking in concrete slabs is to place control joints in the concrete surface at predetermined locations to create weakened planes where the concrete can crack in a straight line. This produces an aesthetically pleasing appearance since the crack takes place below the finished concrete surface, which means that the concrete has cracked but the absence of random cracks at the concrete surface gives the appearance of an un-cracked section.