Concrete @ your Fingertips

Construction Joints - horizontal

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. Then the fresh concrete must be place and compacted so that it bonds properly to the surface of the previously place concrete.

For horizontal surfaces, the laitance should be brushed off while the concrete is still fresh, but has stiffened slightly, to produce a moderately exposed aggregate finish. The surface should be gently sprayed with water while the surface is brushed. Care must be taken to avoid dislodging particles of the coarse aggregate. If the laitance has hardened, but is still “green” (i.e. fairly young), it can be removed by wire brushing. 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, to avoid Vibration Induced White Finger.) Again care must be taken to avoid dislodging aggregate.

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.

Washing and brushing to remove laitance
Acknowledgement: The Concrete Society

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Concrete on site 7: Construction joints