Post-tensioning and prestressing are techniques used to apply stress to concrete or steel structures, aimed at improving their strength, durability, and ability to support heavy loads. For the Ironworker (Structural/Ornamental), these processes are essential in the construction of bridges, high-rise buildings, stadiums, and other major structures. This chapter covers the fundamental principles, installation procedures, basic calculations, Canadian Standards Association (CSA) requirements, and common pitfalls to avoid on the Red Seal exam.
2. Fundamental Principles of Prestressing
2.1 Definition and Purpose
Prestressing involves applying an initial compressive force to a structural element (concrete, steel) before it is subjected to service loads. This force is generated by high-strength steel cables (strands, bars, or wires) that are tensioned and anchored in the concrete or onto the structure.
The main objective is to counteract the tensile stresses that appear under load. Concrete is strong in compression but weak in tension; prestressing creates a permanent compression that prevents cracking and increases the achievable span.
2.2 Types of Prestressing
Type
Description
Typical Application
**Pre-tensioning**
Cables are tensioned **before** the concrete is poured, then released after curing. The bond between the steel and concrete transfers the force.
Precast beams, hollow-core slabs
**Post-tensioning**
Cables are placed in ducts, concrete is poured, then cables are tensioned **after** curing. Anchorage at the ends transfers the force.
Cast-in-place bridges, floor slabs, tanks
2.3 Essential Terminology
Strand: an assembly of twisted steel wires, typically 7 wires (1 center + 6 outer). Common diameters: 12.7 mm (½ in) and 15.2 mm (0.6 in).
Duct: a plastic or metal tube that protects the cable and allows its movement during tensioning.
Anchorage: a device fixed to the end of the cable to transfer the force to the concrete. Types: wedge, nut-head, plate.
Tensioning jack: hydraulic equipment that applies the tensile force to the cable.
Elongation: the measured stretch of the cable during tensioning, expressed in mm or inches.
Prestress loss: the decrease in initial force due to concrete shrinkage, creep, steel relaxation, and friction in the duct.
3. Materials and Components
3.1 Prestressing Steel
The steel used must have high tensile strength (typically 1860 MPa for strands). It is classified according to CSA G30.18 (reinforcing bars) or ASTM A416 (strands). Key characteristics:
Yield strength: approximately 85-90% of ultimate strength.
Modulus of elasticity: 195,000 MPa (strands) to 200,000 MPa (bars).
Relaxation: loss of stress at constant length, low for low-relaxation steels (≤ 2.5% after 1000 h).
3.2 Ducts and Grouting
Duct: made of high-density polyethylene (HDPE) or corrugated metal. Must be watertight to prevent corrosion.
Grouting: after tensioning, the duct is filled with cement grout to protect the steel and ensure bonding. The grout must have a water-cement ratio ≤ 0.45 and a minimum strength of 20 MPa at 7 days.
3.3 Anchorages
Anchorages are classified into two categories:
Active anchorages: allow tensioning (jack applied at the end).
Passive anchorages: fixed, located at the opposite end.
Wedge anchorages are the most common for strands. They consist of a bearing plate, conical wedges, and a stop. Installation must be precise: misalignment can cause premature failure.
4. Installation and Tensioning Procedures
4.1 General Post-Tensioning Steps
37.Duct placement: position the ducts according to the drawings, using supports (chairs) to maintain the exact profile. Curves must respect minimum radii (often 3 m for 15.2 mm strands).
38.Strand insertion: strands are pushed or pulled into the ducts. Lubricate if necessary (approved product, non-corrosive).
39.Concrete pouring: concrete must be carefully vibrated around the ducts to avoid voids. Ducts must be sealed at the ends.
40.Curing: wait until the concrete reaches the specified minimum strength (often 70-80% of the 28-day strength) before tensioning.
41.Strand tensioning: use a calibrated hydraulic jack. The force is applied gradually, in stages, with elongation measurement.
42.Anchoring: wedges are driven in to hold the strand in tension after the jack is released.
43.Grouting: fill the duct under pressure (0.5 to 1.0 MPa) to eliminate air and protect the steel.
44.Strand cutting: cut the excess strand 25-50 mm from the anchorage, then protect with a cap or coating.
4.2 Calculating Tension Force and Elongation
The tension force (P) is specified by the designer. The theoretical elongation (ΔL) is calculated as follows:
ΔL = (P × L) / (A × E)
Where:
P = tension force (N)
L = free length of the strand (mm)
A = cross-sectional area of the strand (mm²)
E = modulus of elasticity (MPa)
Example: 15.2 mm strand (A = 140 mm²), E = 195,000 MPa, L = 20,000 mm, P = 200,000 N.
The measured elongation must correspond to ± 5% of the theoretical value. A larger deviation indicates a problem (excessive friction, blocked strand, force error).
4.3 Tensioning Sequence
For structures with multiple cables, the tensioning sequence must be specified to avoid asymmetric stresses. In general:
Tension the cables symmetrically with respect to the centroid of the section.
Alternate sides to balance moments.
Never tension a cable adjacent to an already tensioned cable without checking the drawings.
5. Applicable Canadian Standards
5.1 CSA A23.1/A23.2 — Concrete and Materials
CSA A23.1 (Concrete: Constituents and Execution of Work) and CSA A23.2 (Test Methods) govern concrete quality, strength testing, and tolerances. Key points:
Minimum strength before tensioning: specified by the designer, often 30 MPa.
Compression tests: cylinders taken and tested at 7 and 28 days (Clause 5.5 of CSA A23.2).
Duct position tolerances: ± 10 mm vertical, ± 20 mm horizontal (Section 7.3 of CSA A23.1).
5.2 CSA S6 — Canadian Highway Bridge Design Code
CSA S6 is the reference for bridges and major structures. It specifies:
Durability requirements (minimum duct cover: 50 mm for bridges).
Prestress losses to consider (Clause 8.4.3): shrinkage, creep, relaxation, friction.
Anchorage acceptance tests (Clause 8.8.2): each anchorage must be visually inspected, and 10% must undergo a tension test at 110% of the service force.
5.3 CSA A23.3 — Design of Concrete Structures
CSA A23.3 provides the design rules for prestressed elements. Clause 18 deals specifically with prestressing:
Section 18.3: requirements for materials (steel, ducts, anchorages).
Section 18.4: calculation of prestress losses.
Section 18.5: verification of flexural and shear strength.
5.4 CSA W59 — Welding
For steel connections (welded anchorages, plates), CSA W59 (Welded Steel Construction) applies. Welds must be performed by qualified welders according to CSA W47.1 (certification of welding companies).
6. Safety and Precautions
6.1 Specific Hazards
Stored energy: a tensioned strand can release enormous energy if it fails. The tensioning area must be barricaded and off-limits to unauthorized personnel.
Hydraulic jacks: high pressure (up to 70 MPa). Check hoses, fittings, and gauges before each use.
Falling materials: anchorage plates and wedges are heavy; use appropriate lifting equipment.
6.2 Personal Protective Equipment (PPE)
Safety helmet with chin strap.
Safety glasses (risk of grout or fragment splashes).
Cut-resistant gloves (when handling strands).
Steel-toed safety boots.
Safety harness if working at heights (lifeline mandatory).
6.3 Emergency Procedures
In case of strand failure: stop tensioning immediately, evacuate the area, report to the supervisor.
In case of hydraulic leak: shut off the pump, do not touch the pressurized jet (risk of skin injection).
Grout is alkaline (pH > 12): in case of skin contact, rinse thoroughly with water for 15 minutes.
7. Practical Calculations for the Ironworker
7.1 Converting Forces and Pressures
1 MPa = 1 N/mm² = 145 psi.
Force (kN) = Pressure (MPa) × Piston area (mm²) / 1000.
Example: Jack with a 100 mm diameter piston (area = π × r² = 3.1416 × 50² = 7854 mm²). Pressure reading: 25 MPa.
Force = 25 × 7854 / 1000 = 196.35 kN.
7.2 Calculating the Number of Strands
For a total force of 1200 kN with 15.2 mm strands (ultimate strength = 260 kN, service force = 70% = 182 kN):
Number = 1200 / 182 ≈ 6.6 → round up to 7 strands.
Important: always round up to the next whole number to maintain safety.
7.3 Elongation Tolerances
The measured elongation must be within the range: theoretical ΔL ± 5%.
If theoretical ΔL = 150 mm, the acceptable range is: 142.5 mm to 157.5 mm.
Lower elongation may indicate excessive friction; higher elongation may indicate a damaged strand or excessive force.
8. Pitfalls to Avoid
113.Confusing pre-tensioning and post-tensioning: pre-tensioning is done before pouring, post-tensioning after. The exam often tests this distinction.
114.Ignoring prestress losses: the effective force is always lower than the initial force. Never use the initial force in service calculations.
115.Neglecting the tensioning sequence: tensioning cables in the wrong order can create cracks or permanent deformations.
116.Forgetting the minimum concrete strength before tensioning: tensioning too early can crush the concrete at the anchorages.
117.Using incorrect units: check whether the drawings are in metric (mm, MPa) or imperial (in, psi). A conversion error is fatal.
118.Not checking jack calibration: an uncalibrated jack gives false readings. Calibration must be done every 6 months or after 500 cycles.
119.Cutting strands before grouting: cutting must be done after grouting, otherwise the anchorage can loosen.
120.Forgetting duct position tolerances: a deviation of more than 10 mm can alter the structural behaviour.
121.Confusing the standards: CSA S6 is for bridges, CSA A23.3 for buildings. The exam may present specific scenarios.
122.Underestimating safety risks: the tensioning area is dangerous. Any safety-related question must be treated with the highest priority.
9. Summary
Prestressing improves the tensile strength of concrete by applying initial compression.
Post-tensioning is done after pouring, with cables in ducts and anchorages at the ends.
Strands are high-strength steel (1860 MPa), with common diameters of 12.7 mm and 15.2 mm.
Elongation (ΔL) is calculated using the formula ΔL = (P × L) / (A × E) and must be verified to ± 5%.
Prestress losses (shrinkage, creep, relaxation, friction) reduce the effective force.
Safety is paramount: barricaded tensioning area, calibrated jacks, mandatory PPE.
Duct position tolerances are ± 10 mm vertical and ± 20 mm horizontal.
Grouting must be done after tensioning to protect the steel against corrosion.
10. Exam Tips
Memorize the formulas: elongation, pressure-force conversion, number of strands.
Know the standards by heart: numbers and areas of application.
Read questions carefully: pitfalls are often in the units (mm vs in) or conditions (pre- vs post-tensioning).
Practice the calculations: redo the examples in this chapter without looking at the solutions.
Review technical terms in both French and English (post-tension / post-tensioning, toron / strand, gaine / duct) as the exam may be bilingual depending on the province.
11. Normative References
CSA A23.1/A23.2 — Concrete: Constituents and Execution of Work / Test Methods.
CSA A23.3 — Design of Concrete Structures (Clause 18 for prestressing).
CSA S6 — Canadian Highway Bridge Design Code (Clause 8 for prestressing).
CSA W59 — Welded Steel Construction.
CSA W47.1 — Certification of Welding Companies.
ASTM A416 — Specification for High-Strength Steel Strands.
This chapter prepares you for the theoretical and practical questions on post-tensioning and prestressing. A solid grasp of the concepts, calculations, and standards will give you a decisive advantage on the Red Seal exam.
