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It is often not possible to complete a job at one go, for example because of the size or complexity of the structure or because of limited materials or manpower. When work resumes it will be necessary to place fresh concrete on or against the previous pour, that will have already hardened. The resultant contact surface is known as a construction joint or daywork joint. Such joints must be formed carefully so that they transmit the required forces and, where appropriate, do not leak. Firstly the surface of the hardened concrete must be clean and free from laitance (the relatively soft surface layer) and any dust or debris. If it is too hard, hand-held tools or abrasive blasting will be necessary. (Note that the use of hand-held tools should be limited for Health and Safety reasons.) Care must be taken to avoid dislodging aggregate. Then the fresh concrete must be place and compacted so that it bonds properly to the surface of the previously place concrete.
Vertical joints, for example in walls, are usually formed at a stop end in the formwork. The stop end must be easily removable, without damage to the young concrete. An alternative approach uses expanded metal mesh to form the stop end instead of the traditional timber. This is particularly useful when the reinforcement is congested. The expanded metal is left in place (unlike timber that is removed). The resulting rough surface provides a good mechanical key and requires no further preparation.
For both horizontal and vertical joints, care must be taken to ensure that the layer of fresh concrete adjacent to the joint is adequately compacted.
Acknowledgement: The Concrete Society
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