Structural Diagnostics & Acoustic Emission (AE) Monitoring for Prestressing Steel Corrosion
Guest: Gregor Schacht (Managing Director, Marx Krontal Partner – MKP)
Topics: Stepwise structural diagnostics, prestressing/concrete behavior, stress corrosion cracking (SCC), acoustic emission (AE) monitoring, data evaluation & actions, examples (incl. Carola Bridge Dresden)
Why Structural Diagnostics?
MKP’s mission is to preserve existing structures, extend service life, and protect resources and budgets. The key is systematic information acquisition: from condition capture through diagnostics to monitoring. By combining data (structural recalculation, lab values, non-destructive testing, monitoring) we obtain a realistic picture of load-bearing capacity—forming the basis for robust decisions instead of costly blanket replacements.
The Doctor Analogy: The Logical Path to the Right Diagnosis
The existing-structure specialist acts like a physician:
Anamnesis & visual check: “How is the patient?” – Files are rarely enough. You need to see the patient on site.
Diagnostics: If suspicion remains, run tests—from the “blood panel” (cores, material samples) to the “MRI” (NDT).
Monitoring: If the patient shows abnormalities, monitoring is added to detect changes early.
This analogy makes it clear: Not every structure is sick, but the sick ones must be identified and treated with priority.
The Stepwise Structural Diagnostics Plan
1) Visual Inspection (first contact)
Site walk-through, reconciliation with records
Capture geometry/dimensions and obvious damage (cracks, spalling, leakage)
Early hypotheses: “Where might a deficit be?”
2) In-Depth Diagnostics & NDT
Structural recalculation with realistic assumptions
Material properties & samples: cores (compressive strength), cover depth, as-built conditions
Corrosion-relevant parameters: carbonation depth, chloride content, moisture
Non-destructive testing (NDT):
Half-cell/potential mapping (indications of reinforcement corrosion)
Radar/GPR & ultrasonics (rebar layout, homogeneity, ducts, grout condition)
Outcome: reliable data for recalculation and clear indications where targeted openings/samples make sense (e.g., over supports/deflection zones).
3) Monitoring—especially Acoustic Emission (AE)
Used when risk exists or initial findings are present
Goal: detect active processes, e.g., wire breaks in prestressing strands
Continuous monitoring delivers timeliness, trend, and localization; it supports risk management and operations.
Prestressing Explained (why the prestressing cable?)
Concrete is strong in compression, weak in tension. Prestressing pre-compresses the concrete so that under traffic loads no tensile cracks occur if possible.
The “rubber band around a sponge” analogy: the taut “band” (prestressing steel) compresses the “sponge” (concrete).
In bridges this enables large spans with slender construction.
Critical aspect: The prestressing steel is typically encased and grouted in a duct—protected against corrosion, but not visible. Openings are delicate and should be targeted.
Stress Corrosion Cracking (SCC) in Prestressing Steel—what really happens?
Stress corrosion cracking differs fundamentally from “classic rust”:
Mechanism: Hydrogen embrittlement in high-strength prestressing steel under tensile stress. Hydrogen can enter the metal lattice (e.g., through moisture/condensation, unfavorable conditions during storage/installation).
Consequence: Brittle fracture without typical corrosion products. Failure is not ductile (no “warning elongation”) but sudden.
Hotspots: Support and deflection zones (strong deviations/stress states), tight guidance, moisture influence.
Pitfall: Opening at “convenient” spots often misses critical zones. Hence targeted openings and/or monitoring are essential.
After a break: Strand bond can re-anchor over short lengths; clustered local breaks are critical, distributed breaks much less so.
Acoustic Emission Monitoring (AE)—principle, setup, calibration
Principle: When a tensioned wire breaks, elastic energy is released → impulse (snap). This creates elastic waves in the concrete, captured by surface sensors.
Setup:
Sensors (cabled) at regular spacing (e.g., ~10 m) along relevant webs/boxes
Data logger on site: pre-filtering via thresholds (amplitude, energy, frequency, duration)
Cloud processing: events are stored, classified, and pre-evaluated with models/heuristics
Localization: Using time-of-arrival differences between sensors → position of the break
Calibration/verification: standardized pencil lead break (defined energy), optional artificial wire break on samples, rebound hammer impulses
Important: AE delivers event data (when? where? how strong?)—evaluation always occurs in the context of structural recalculation, material data, and the overall condition.
From Data to Actuable Recommendations
Before monitoring goes live, an escalation strategy is defined:
Set thresholds (from recalculation + condition):
allowable number of local breaks per section
sensitivity (info/warning/alarm thresholds)
response times and responsibilities
Agree on notification logic:
Info events (single distributed break, no immediate danger) → documentation/trend
Warning (multiple events in short time) → additional checks, traffic management, targeted inspection/opening
Alarm (clustered local breaks near analytical limit) → immediate actions (load restrictions, closure, temporary shoring, targeted opening, repair planning)
Regular reports:
event lists with time/location/intensity
cluster/trend analyses (rising activity = activated process)
Concrete action recommendations per section (e.g., “Section A: unremarkable—continue observation monthly” / “Section B: activity ↑—short-term inspection + temporary load reduction” / “Section C: critical cluster—immediate safeguarding + rehabilitation planning”).
Practical benefits:
- Early warning instead of surprises
- Plannable interventions (cost/traffic)
- Transparent risks for owners, politics, public
- Preservation instead of premature replacement (budget and CO₂ benefits)
Case Reference: Carola Bridge Dresden
After the collapse, MKP supported cause analysis (stress corrosion cracking) and securing the remaining superstructures. AE served as palliative monitoring to detect active damage processes and to safely reopen navigation beneath the bridge. Lesson: targeted monitoring of critical zones could have indicated breaks early—the decisive factor is the combination of diagnostics, recalculation, and monitoring with an escalation plan.
Who benefits most?
- Municipalities/state road agencies with limited budgets
- Owners of critical infrastructure (road/rail bridges, parking structures)
- Assets with known vulnerable prestressing systems or unclear documentation
Recommendation: Your path forward
- Conduct records & visual inspection, start structural recalculation
- Targeted diagnostics: samples & NDT, focus on supports/deflection zones
- Define monitoring concept (objectives, sensors, spacing, data flow, calibration)
- Set escalation strategy (thresholds, notification logic, action cascade)
- Regular reviews: trend, clusters, correlation with climate/traffic—derive actions
- Communication: present findings in plain language; convey safety & planning certainty