Introduction
Post-tensioned concrete silos are critical infrastructure elements widely used for the storage of granular and powdered materials. Over time, these structures may experience deterioration due to environmental exposure, loading conditions, or material aging. A thorough condition assessment is vital for ensuring structural safety, functionality, and longevity.
This case study presents the application of Non-destructive Evaluation of Post-Tensioned Concrete Silos in mining facilities in Quebec, Canada. It focuses on identifying defects such as cracks, delaminations, and corrosion while determining the health of the post-tensioning system. The findings serve as a basis for recommending remedial actions to maintain structural integrity.
Scope of the Assessment
The primary objectives of this study included:
- Assessing the extent of concrete defects, including cracks and delamination.
- Evaluating the uniformity and quality of concrete using NDT-E methods.
- Investigating the condition of the post-tensioning system to ensure structural safety.
- Providing recommendations for remedial measures and maintenance.
Key areas of focus included the foundation walls, pilasters, and connections between the silos’ concrete walls and metal roofs. The assessment was carried out over two stages, combining visual inspections and advanced NDT-E methods.
Methodology
A systematic inspection and evaluation approach was adopted, employing the following NDT-E techniques:
- Ultrasonic Pulse Velocity (UPV):
- Purpose: To assess concrete quality, detect delaminations, and measure crack depths.
- Process: Stress waves were transmitted through the concrete, with their velocity analyzed to evaluate material integrity.
- Rebound Hammer Testing:
- Purpose: To measure surface hardness and indirectly assess compressive strength.
- Process: A spring-loaded hammer applied controlled impacts to the concrete surface, and rebound values were recorded.
- Ground Penetrating Radar (GPR):
- Purpose: To investigate subsurface anomalies such as voids, delaminations, and rebar positioning.
- Process: Electromagnetic waves were transmitted into the concrete, with reflected signals analyzed to identify internal irregularities.
- Half-Cell Corrosion Potential (HCP) Survey:
- Purpose: To evaluate the likelihood of active corrosion in steel reinforcements.
- Process: Measurements of electrical potential were taken at various locations to identify areas of high corrosion risk.
- Visual Inspection:
- Purpose: To map surface-level defects, including cracks and delaminated zones.
- Process: Digital imaging, hammer sounding, and crack width measurements were performed.
Key Findings
1. Foundation Walls
- Cracks: Cracks were identified, and recorded – Crack severity (width) were measured.
- Crack Depths: Measurements showed significant crack depths, with some extending beyond the reinforcement level.
- Concrete Quality: Rebound hammer and Ultrasonic Pulse Velcoity results indicated uniform concrete quality, with no major anomalies in compressive strength.
- Water-Stop Conditions: The condition of waterstops were visually examined.
2. Pilasters
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- Pilasters exhibited vertical cracks and delaminated zones. Cracks exceeding the critical width were prevalent.
- Delaminated areas were observed, with defects running deeper than the rebar level in some locations. The NDT methods suggest that the extent of the defects were limited.
- Half-cell corrosion potential testing on a pilaster showed no significant active corrosion. Most readings indicated a low likelihood of corrosion, with no areas showing high-risk potential. It was understood that the deterioration of embedded steel tubes in the pilasters may have contributed to premature cracking of concrete.
4. Post-Tensioning System
- No exposed or damaged post-tensioning cables, headers, or ducts were observed during the inspections. Further investigations were recommended for specific pilasters to ensure ongoing safety.
Recommendations
Based on the findings, the following maintenance and remedial actions are recommended:
- Crack Repairs:
- Inject epoxy into significant cracks to restore structural integrity and prevent moisture ingress.
- Prioritize repairs on pilasters showing critical defects. Delaminated areas were identified and quantifed helping owner and contractor with cost estimate.
- Delamination Mitigation:
- Remove loose and delaminated concrete in affected areas and repair with high-strength repair mortar.
- Apply protective coatings to prevent further degradation.
- Water-Stop Replacement:
- Replace deteriorated water-stops in silo foundations to improve waterproofing.
- Post-Tensioning System Monitoring:
- Conduct detailed investigations into specific pilasters to ensure post-tensioning components remain unaffected by defects.
- Regular Inspections:
- Establish a routine NDT-E inspection schedule to monitor crack progression, corrosion, and overall structural health.
Conclusion
This case study demonstrates the effectiveness of non-destructive evaluation methods in assessing the condition of post-tensioned concrete silos. By employing techniques such as UPV, GPR, and corrosion potential surveys, critical defects were identified, and targeted recommendations were developed to address structural vulnerabilities. The insights gained from this assessment will help extend the lifespan of these silos and ensure their safe operation for years to come.