Question

In a pre-tensioned prestressed concrete cross-section,

Statement 1: The stress in wires is assumed to be zero at the end supports and increases to its final maximum value over its transmission length.

Statement 2: The bond stress between concrete and prestressed wires is maximum near the end supports and decreases to nearly zero over its transmission length.

1. Statement 1 and 2 are true
2. Statement 1 is true and statement 2 is false
3. Statement 1 is false and statement 2 is true
4. Statement 1 and 2 are false

Answer 1

Correct Answer - Option 1 : Statement 1 and 2 are true

Concept:

Pre-tensioned prestressed concrete:

• The stress in wires is assumed to be zero at the end supports and increases to its final maximum value over its transmission length.
• The different stages involved in pre-tensioning are depicted.
•  In the first stage, the tendons are stretched to the required tension and anchored to the rigid supports known as bulkheads (Stage 1).
• Then the concrete member is cast around the tendons (Stage 2).
• During the curing process, a bond develops between the tendon and concrete.
• When the concrete attains sufficient strength, the tendons are released from the bulkheads (Stage 3).
• Immediately after release, the tendons try to return to their original length.
• But the bond between the steel and concrete prevents this movement thereby forcing the concrete into compression.
• Since in the pre-tensioning method, the prestressing force from the tendon is transferred to concrete by bond.
• The bond stress between concrete and prestressed wires is maximum near the end supports and decreases to nearly zero over its transmission length.
• pre-tensioned members usually have large numbers of prestressing steel of smaller diameter so as increase the area of contact.