Expansion, isolation and construction joints are created by formwork before the PCC is placed. Since these joints are designed to completely separate adjacent masses of PCC, they are usually made by inserting a small non-PCC piece of material such as a strip of wood.
Construction joints, sometimes called “headers” are usually made to separate successive construction activities; they do not serve any design purpose. However, with proper planning, construction joints can often be made to coincide with other planned joints, such as a transverse contraction joint. For instance, in slipform paving a construction joint is made at the end of the day as a transverse piece of formwork used to shape the last slab. If enough PCC is available at the end of the day, the construction joint can be placed at a planned transverse contraction joint. Construction joint considerations include:
- Ensure adequate PCC is available to finish the last slab of the day. The construction joint is set before all the PCC is placed. If truck delivery is stopped to soon, the temptation may be to use the lower-quality PCC that has been pushed in front of the screed to fill the remaining volume. This low-quality PCC may contain little portland cement, excess water, low air and/or segregated aggregate.
- Ensure proper consolidation. Typically, the paver does not traverse the construction joint. Therefore, the construction joint is not consolidated by the paver vibrators and it should be manually consolidated.
- Dowel placement should not segregate or cause air voids in the adjacent PCC. Sometimes, dowel bars are placed after the PCC has been placed up to the header. If dowel bars are pounded in by hammer, the resultant vibrations may cause air pockets or segregated aggregate.