Post-Tensioning and Prestressing Systems

Post-Tensioning and Prestressing Systems

Module Introduction

This chapter covers post-tensioning and prestressing systems used to reinforce concrete structures. For the Red Seal exam, you must understand not only the theoretical principles, but also the installation procedures, tolerances, elongation calculations, and safety requirements specific to this specialized trade. This module represents approximately 5 to 8% of the exam questions for the reinforcing steel (rebar) trade.

1. Fundamental Principles of Prestressing

1.1 Definition and Purpose

Prestressing is a technique that involves applying an initial compression to concrete before it is subjected to service loads. This pre-compression cancels out or reduces the tensile stresses that would otherwise appear in the concrete, a material naturally weak in tension.

Prestressed concrete is divided into two main families:

For the reinforcing steel installer, post-tensioning is the most relevant method, as it is performed on the job site and involves handling ducts, strands, and hydraulic jacks.

1.2 Essential Terminology

1.3 Applicable Canadian Standards

The Canadian regulatory framework for post-tensioning is as follows:

Duct installation tolerances are generally specified in the engineering drawings, but CSA A23.1 requires precise alignment to ±10 mm in vertical and horizontal position, unless otherwise indicated.

2. Components of a Post-Tensioning System

2.1 Strands

Strands are manufactured from high-carbon drawn steel wires. Their ultimate tensile strength is 1860 MPa (grade 270 ksi). Common diameters are:

The initial tensioning force is typically 75 to 80% of the breaking force, approximately 1395 to 1488 MPa.

2.2 Ducts

Ducts have three functions: to guide the strand, to prevent bonding with the concrete, and to allow grout injection after tensioning.

2.3 Anchorages

Anchorages are classified according to their function:

2.4 Grout

After tensioning, ducts are grouted with a cementitious grout to:

The grout must have a water/cement ratio ≤ 0.45 and compensated shrinkage (expansive admixture). CSA A23.1 requires a minimum strength of 20 MPa at 7 days.

3. On-Site Installation Procedures

3.1 Duct Placement

Duct placement follows the passive reinforcement. Critical steps:

Installation Tolerances (CSA A23.1):

3.2 Strand Threading

Threading can be done before or after concrete placement:

Rule of thumb: Never thread more than 24 hours before placement, unless the strands are protected.

3.3 Stressing

Stressing is a dangerous operation that requires specialized training and a work permit. Typical procedure:

Theoretical Elongation Calculation Formula:

ΔL = (P × L) / (A × E)

Where:

Example: Strand Ø 15.24 mm (A = 140 mm²), length L = 20,000 mm, force P = 195 kN.

ΔL = (195,000 × 20,000) / (140 × 195,000) = 3,900,000,000 / 27,300,000 = 142.9 mm

The measured elongation must be within ±5% of the theoretical value. A greater deviation indicates a problem (excessive friction, stuck strand, length error).

3.4 Grouting

Grouting must be done within 48 hours of tensioning to prevent corrosion. Procedure:

Minimum temperature: Grout must not be injected if the ambient temperature is below 5 °C, unless protective measures are taken.

4. Calculations and Verifications

4.1 Tension Force and Stress

The initial tension force (P₀) is given by:

P₀ = 0.75 × f_pu × A

Where f_pu = ultimate strand strength (1860 MPa).

For a Ø 15.24 mm strand: P₀ = 0.75 × 1860 × 140 = 195,300 N ≈ 195 kN.

4.2 Tension Losses

Tension losses are inevitable and must be anticipated:

Friction Formula (curvature loss):

P(x) = P₀ × e^(−μθ − kx)

Where:

4.3 Elongation Verification

The measured elongation must be compared to the theoretical value. The tolerance is ±5%. If the deviation exceeds this value:

Common trap: Elongation must be measured on the free portion of the strand, i.e., between the jack face and the anchorage. Do not include the jack length itself.

5. Site Safety

5.1 Specific Hazards

Post-tensioning presents unique risks:

5.2 Mandatory Safety Rules

5.3 Corrosion Protection

Ungrouted strands are susceptible to corrosion. Storage rules:

6. Quality Control and Documentation

6.1 Stressing Records

Each tendon must have a stressing record containing:

6.2 Required Testing

6.3 Final Tolerances

7. Pitfalls to Avoid

Here are the most frequent errors on the Red Seal exam on this topic:

8. Summary

Exam Watch Points:

Post-tensioning is a specialized field that distinguishes the qualified reinforcing steel installer from the labourer. A solid understanding of these principles will allow you not only to pass the exam, but also to perform this type of work competently and safely on the job site.