The surface where two successive placements of concrete meet is known as:
Correct Answer: C. Construction joint
📚 Detailed Explanation: The Interface Between Two Successive Concrete Placements is a Construction Joint
Why C (Construction joint) is correct: A construction joint is, by definition, the hardened surface of previously placed concrete against which new concrete is cast. It represents the boundary between two distinct, non-simultaneous pours. The key characteristic is that it is not a deliberate movement-accommodating gap (like an expansion joint) nor a controlled crack plane (like a contraction joint) — it is the natural consequence of interrupted or staged concreting.
Distinguishing Construction Joint from Other Joint Types
| Joint | How Formed | Gap Present? | Allows Movement? | Purpose |
|---|---|---|---|---|
| C. Construction Joint | Two successive concrete placements; second cast against hardened first | No gap — monolithic bond intended | No | Work-sequence joint; structural continuity maintained |
| A. Contraction Joint | Saw-cut or formed groove in slab; induces crack at planned location | Partial (groove depth 1/4 to 1/3 of slab thickness) | Partial (controlled crack opens) | Controls shrinkage cracking |
| B. Expansion Joint | Pre-formed gap with compressible filler; complete separation of structure | Yes — full gap (20–25 mm typical) | Yes (thermal expansion/contraction) | Prevents buckling from thermal expansion |
| D. Both contraction & expansion | Not applicable | — | — | Wrong answer choice |
Examples of Construction Joints in Practice
| Structure | Why Construction Joint Occurs |
|---|---|
| Continuous beams / road slabs | Poured in segments; joint between segments |
| Tall columns | Poured in lifts; joint between each lift (typically at floor level or beam junction) |
| Deep foundations / raft slabs | Large volume; poured in sections; joint at section boundary |
| Retaining walls | Horizontal joints between daily pour lifts |
- Surface where two successive concrete placements meet = Construction Joint.
- No gap; no deliberate movement provision; reinforcement continuous through joint.
- Surface must be roughened and cleaned before new pour to restore structural continuity.