Why Ultrasonic Tomography is Essential for Verifying Drossbach Duct Grouting

Ultrasonic tomography is the only proven way to verify Drossbach duct grout fill, ensuring safety and protecting engineers from liability. Read about why GPR falls short and why ultrasonic testing is the professional standard.

September 8th, 2025

Category: NewsServices

The Engineer’s Responsibility

Structural consulting engineers are frequently asked to sign PS4 Producer Statements confirming that critical structural elements meet compliance. In post-tensioned (PT) concrete, one of the most vital elements is the Drossbach duct system—the corrugated galvanized steel conduits that house tendons or reinforcing starter bars. The duct must be properly aligned and fully grout-filled to ensure tendon protection, load transfer, and long-term durability.

The above image is an example of incomplete grouting inside a Drossbach duct, leaving tendons vulnerable.

If ducts are partially filled, misaligned, or left voided, the risks include corrosion, tendon failure, stress concentration, and even progressive collapse in extreme cases. For engineers, sign-off carries both professional liability and public safety consequences.

The Technology Question: GPR vs Ultrasonic Tomography

A widespread misconception in the New Zealand market is that Ground Penetrating Radar (GPR) alone can confirm whether ducts are grout-filled. In reality, this is scientifically impossible:

  • What GPR Can Do:
    • Locate ducts and reinforcement.
    • Provide broad imaging of alignment and discontinuities.
    • Detect anomalies influenced by moisture and density contrasts.
  • What GPR Cannot Do:
    • Differentiate between fully filled vs partially filled ducts.
    • Detect small or irregular voids within grout.
    • Distinguish a steel bar inside a duct from the duct wall itself.

By contrast, Ultrasonic Tomography (e.g., MIRA) was specifically developed to solve this problem.

  • Capabilities of Ultrasonic Tomography:
    • Three-dimensional imaging up to 1 m depth in reinforced concrete.
    • Quantitative verification of grout fill levels.
    • Detection of voids >20 mm, delamination, and inclusions.
    • Reliable Probability of Detection (PoD) validated for PT ducts.

Only ultrasonic tomography can scientifically determine whether a Drossbach duct is:

  1. Fully grout-filled,
  2. Partially filled, or
  3. Obstructed by a tendon or reinforcing bar.

The below image shows exposed Drossbach ducts in concrete showing grouped tendons inside — verifying complete grout fill is essential to prevent long-term failures.

Why Misrepresentation Matters

Some NDT providers market GPR as a complete solution. This is misleading, and worse, dangerous. Engineers relying on such results risk:

  • Signing PS4s on inaccurate data – unknowingly endorsing defective ducts.
  • Exposure to liability if defects later cause structural issues.
  • Compromised safety in seismic or high-load environments, where voided ducts accelerate deterioration.

In Australia, systemic failures in duct grouting have already been reported (CROSS-AUS Report 961), highlighting the consequences of inadequate QA.

The Professional Standard Engineers Should Demand

To protect both structures and professional liability, consulting engineers should insist on:

  1. Ultrasonic Tomography as the primary verification method for duct grout fill.
  2. GPR as a complementary tool—useful for locating ducts but not for grout verification.
  3. Qualified interpretation by trained professionals—NDT results are only as reliable as the expertise behind them.
  4. QA Documentation—comprehensive reports with tomographic reconstructions and acceptance criteria.

Conclusion: Science, Not Shortcuts

The evidence is clear: Drossbach ducts present inherent risks, with up to 70% of large PT projects experiencing duct installation issues. Grout verification is not optional—it is essential.

For structural consulting engineers, the path forward is straightforward:

  • Reject misrepresentations that GPR alone can “prove” grout integrity.
  • Insist on ultrasonic tomography for duct assessment.
  • Safeguard both the structures you certify and your professional reputation.

At CSI, we use advanced ultrasonic tomography (MIRA) alongside GPR to deliver science-based, reliable verification of Drossbach ducts—giving engineers the confidence their PS4s rest on solid ground.

We would be keen to hear what NDT solutions you would like for your structural resilience investigations.
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