CSLB General Building (B)Site Work, Foundations and ConcreteEasy
A concrete slab is being poured around a steel support column that will remain in place permanently. To prevent the slab from cracking as it shrinks and moves independently of the rigid column, the contractor installs a joint that fully separates the slab from the column with a compressible filler. What type of joint is this?
- AConstruction joint
- BIsolation joint
- CControl joint
- DCold joint
Show answer & explanationAnswer & explanation
Correct answer: B. Isolation joint
An isolation joint (also called an expansion joint) completely separates the slab from fixed elements like columns, walls, or footings, allowing independent movement and preventing cracking caused by differential settlement or thermal expansion. It is typically filled with a compressible material rather than being a planned crack line like a control joint.
Why the other options are wrong
- A. A construction joint marks a planned stopping point between separate concrete pours, not a permanent isolation gap.
- C. A control joint creates a weakened plane to control cracking within a continuous slab, not separate it from a fixed object.
- D. A cold joint is an unintended weak bond from delayed concrete placement, not a designed separation feature.
Isolation Joint
A joint that completely separates a concrete slab from fixed building elements (columns, walls, footings) using a compressible filler to allow independent movement.
- Also called an expansion joint
- Prevents cracking from differential movement
- Filled with compressible material, not just a saw cut
Memory trick: 'Isolate the fixed stuff' — give rigid columns their own space to move.