CSLB General Building (B)Site Work, Foundations and ConcreteHard
A crew resumes pouring a concrete wall the next morning after the previous day's concrete has already begun to set. The interface between the two placements shows visible discontinuity and reduced bond strength. What defect has occurred?
- ASegregation
- BHoneycombing
- CCold joint
- DLaitance
Show answer & explanationAnswer & explanation
Correct answer: C. Cold joint
A cold joint forms when fresh concrete is placed against concrete that has already begun to set, preventing proper bonding between the two layers and creating a plane of weakness. This is prevented by maintaining continuous placement or properly preparing (roughening, wetting) the surface before resuming.
Why the other options are wrong
- A. Segregation refers to separation of aggregate from paste within a single placement, not between placements.
- B. Honeycombing is voids from inadequate consolidation around rebar/forms, not a placement interface issue.
- D. Laitance is a weak surface layer of cement fines and water, not an interface between pours.
Cold Joint
A weakened plane in concrete that forms when a new batch is placed against concrete that has already begun to set, preventing proper bonding between layers.
- Caused by delays in placement exceeding initial set time
- Prevention includes continuous pours, retarders, or proper joint preparation
- Weakens structural integrity and can allow water infiltration
Memory trick: A cold joint is concrete that showed up too late to the party.