Detecting Corrosion in Post-Tensioned Structures: Non-Destructive Evaluation Methods

In this article, we will explore how non-destructive testing (NDT) and evaluation methods help structural engineers and construction managers in detecting corrosion in post-tensioned structures. By using a multi-technology approach that includes Ground Penetrating Radar (GPR), Ultrasonic Pulse Echo (UPE), and Impact-Echo (IE), we can effectively identify soft grout, voids, or corrosion in post-tensioning structures.

Understanding Corrosion in Post-Tensioned Structures

Post-tensioning ducts are crucial components in reinforced concrete structures, housing high-strength steel tendons that enhance strength and flexibility. Despite their reliability, post-tensioning systems can develop certain anomalies, such as:

  • Duct Obstructions: Debris, grout, or rust particles within the duct can hinder tendon movement during tensioning or relaxation, affecting performance and durability.
  • Duct Misalignment: Improper installation or shifting can cause misalignment, leading to friction, bending, or stress concentrations that reduce effectiveness.
  • Duct Grout Voids: Inadequate grouting can create voids or air pockets, compromising the bond between tendons and concrete and increasing corrosion risk.
  • Duct Grout Debonding: Poor bonding due to inadequate preparation can lead to voids and reduced load transfer, heightening corrosion susceptibility.
  • Duct Leakage: Damaged or improperly sealed ducts can allow water ingress, causing tendon corrosion and grout loss, which diminishes system effectiveness.

Detecting Corrosion in Post-Tensioned Structures Using NDT Methods

Combining three NDT methods provides a comprehensive evaluation method for detecting corrosion in post-tensioned structures. Here’s how each method contributes:

1. Ground Penetrating Radar (GPR)

Ground Penetrating Radar (GPR) offers a reliable and cost-effective means of rapidly screening the precise location and orientation of post-tensioning ducts with minimal preparation. The GPR scan can help engineers map the profile of the duct along the girder length.

Detecting Corrosion in Post-Tensioned Structures

2. Ultrasonic Pulse Echo Tomography (UPE)

Ultrasonic Pulse Echo (UPE) uses acoustic stress waves to examine subsurface targets in concrete. It identifies defects by detecting anomalies in acoustical impedance, valuable for assessing grout conditions and detecting voids, corrosion, and soft grout in ducts. UPE helps determine damage extent, identify weak points, and guide maintenance or rehabilitation decisions.

Once the precise position of the post-tensioning ducts are mapped using GPR, UPE can be used to detect corrosion in post-tensioned structure. The UPE can be performed at regular intervals, providing key information about the sub-surface condition. The UPE scans can be analyzed for abnormal reflections that might be associated with voids, or corrosion in post-tensioned structure.

3. Impact-Echo Method (IE)

The Impact-Echo (IE) method is a rapid, cost-effective NDT technique for detecting corrosion in post-tensioned structures. It uses stress waves to evaluate grout quality, providing information on potential damage and aiding in grout installation quality assurance. IE results can be cross-referenced with GPR and UPE findings to pinpoint voids and corrosion accurately.

Validating NDT Findings

To confirm the presence of voids and corrosion in post-tensioning ducts, conduct additional investigations on at least 5% of test locations, such as coring or borescope video inspections. These methods help verify NDT program results, ensuring accurate detection and evaluation.