Lecture Notes: Pre-tensioned vs. Post-tensioned Concrete

Introduction

Presented by Tyler Lay
Focuses on the differences between pre-tensioned and post-tensioned concrete
Concrete is stronger in compression than tension
- Romans used concrete arches to leverage this property

Modern solution to enhancing concrete's strength
Two types: Pre-tensioned and Post-tensioned
Discusses benefits of prestressed concrete in reducing cracks and enhancing durability

Characteristics:
- Involves stretching steel and then releasing to compress concrete
- Common in repetitive structures like girders, beams
- Utilizes high-strength seven-wire strands instead of rebar
Process:
- Anchor one end of the strand, apply force (~30,000 lbs per strand)
- Cast concrete around it, wait for it to gain strength, then release load
- Losses occur (difference between applied and actual load)
Benefits:
- Economical for repetitive use
- Less concern for reinforcement protection
- Improved durability and reduced cracking
Challenges:
- Requires large equipment and specialized factories
- Cracking at points of release can occur

Characteristics:
- Used in slabs and complex structures like segmental box girders
- Involves concrete with ducts filled with high-strength strands
Process:
- Anchor one end (dead end), apply load at the other (live end)
- Use wedges to hold strands in place
- Losses are fewer compared to pre-tensioning
Types:
- Unbonded: Contains oil or grease, anchors covered with grout
  - Can be adjusted/retightened if necessary
- Bonded: Ducts filled with grout post-stressing
  - More prone to issues with empty spaces causing corrosion
Benefits:
- Reduced cracking
- Versatile in structural designs
- Field stressing allows adjustment
Challenges:
- Requires special equipment and careful design
- Protecting strands is crucial to structural integrity
- Risk of cracking at anchorage points