Why Choosing the Right Post-Tensioning Subcontractor Matters

Post-tensioning has transformed modern concrete construction, enabling sustainability benefits through larger spans and thinner slabs in high-rise buildings, bridges, and complex structures.

However, unlocking these benefits relies heavily on selecting the system and the subcontractor behind it.

Interspan is proud to lead the market in the design, supply, and installation of post-tensioning (PT) systems across Australia. We are the only PT contractor nationally accredited under CARES APT04—a testament to our commitment to Quality, Traceability, and Safety at every stage.

When executed correctly, post-tensioning enhances structural performance and contributes to sustainability through material efficiency and reduced embodied carbon.

What to Look for in a PT Subcontractor

Post-tensioning is a specialist activity. Best practice requires experience, rigorous processes, and high-quality materials. Selecting the right subcontractor means safeguarding your project’s structural integrity, service life, and sustainability.

Here are the hallmarks of a quality PT subcontractor:

Accreditations and Memberships

1. ISO 9001:2015 (Quality Management System) certification – with regular internal and external audits.
2. ISO 14001:2015 (Environmental Management System) certification – with regular internal and external audits.
3. ISO 45001: 2018 (Occupational Health and Safety (OH&S) management system) certification – with regular internal and external audits.
4. Membership in the Post-Tensioning Institute of Australia (PTIA).
5. Pathway to CARES accreditation, with at least a Stage 1 audit completed.
6. Ongoing training and competency assessment of site personnel, with CARES-issued cards confirming qualifications.
7. Task-specific supervision by competent, accredited operatives. QA system details that each post-tensioning activity is supervised by an operative at or above a specified competency level for that task.

Best Practice Post-Tensioning Requirements

1. Ducts shall be blown through with oil-free compressed air. Blowing through of all tendons shall be documented.
2. Grouting procedures include strength, fluidity, wick, and sieve tests.
3. Full traceability of all materials—from purchase order to pour—is mandatory.
4. Stressing jacks must be NATA-calibrated within six months and comply with AS1349 standards to a tolerance of ±3%.
5. Grout bleed and volume change tests must be routinely conducted.
6. Only CARES APT03-certified prebagged grout is used to ensure consistent grout properties.
7. PT kits are routinely audited to confirm compliance with material and dimensional specs.
8. Anchor kits must be tested as complete systems to AS1314 or ETA standards, covering static, load transfer and fatigue testing.

A Specialist Process-Driven Discipline

Unlike traditional reinforced concrete slab installation, post-tensioning of slabs is a highly specialized, process-driven method that requires skilled execution and strict quality assurance. Due to the precision involved, there are multiple hold points throughout the construction process to ensure compliance, including:

1. Pour Watch – Monitoring concrete placement to ensure tendons aren’t damaged.
2. Stressing Operations – Ensuring tendons are tensioned correctly during both initial and final stress operations.
3. Grouting of Tendons – A properly grouted duct provides an alkaline environment which protects the strand from corrosion.

Each step relies on the use of approved products, standardized processes, and expert workmanship.

Read More about Interspan’s Quality Assurance