UNIT 2: ASSESSMENT, DIAGNOSIS & INVESTIGATION
Based on RGPV past papers (Nov 2023 & May 2022), this unit focuses on the systematic process of evaluating existing structures to identify distress, determine causes, and plan appropriate interventions.
2.1 Preliminary & Detailed Investigation
Situations Calling for Structural Investigation:
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Age of structure exceeding design life.
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Visible damage (cracks, spalling, deflection).
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Change in use or loading (e.g., residential to commercial).
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Natural disaster impact (earthquake, flood).
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Code updates requiring higher safety standards.
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Construction quality doubts.
Steps in Evaluation of Structures:
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Preliminary Survey: Visual inspection, documentation of obvious distress, review of available drawings & records.
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Detailed Investigation: Material testing (NDT/DT), crack mapping, structural analysis, load testing if needed.
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Analysis & Assessment: Compare findings with design codes, evaluate remaining strength & serviceability.
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Reporting: Prepare comprehensive report with findings, causes, and recommended repair/strengthening measures.
Physical Inspection of Distressed Structures:
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Procedure: Systematic walk-through, photographing all defects, measuring crack widths & lengths, mapping spalls & corrosion stains.
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Documentation: Create crack maps (location, pattern, width, activity) and spall maps. Note environmental conditions and accessibility issues.
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Key Observations: Crack patterns (tell-tale signs of cause), rust stains, efflorescence, water leakage paths, deformation.
[!TIP] Exam Focus: Questions often ask to differentiate between preliminary and detailed investigation steps. Emphasize that preliminary is visual/desk study, while detailed involves quantitative testing.
2.2 Classification of Structural Cracks & Damage
Different Types of Cracking in Structures:
| Crack Type | Primary Cause | Typical Pattern & Appearance |
|---|---|---|
| Plastic Shrinkage | Rapid surface drying before setting | Random, shallow, parallel to reinforcement, at early age |
| Thermal | Temperature gradients & restraint | Long, parallel cracks at regular intervals (e.g., in walls) |
| Settlement | Differential foundation movement | Diagonal cracks from corners, often over openings; varies in width |
| Flexural | Bending moments exceeding concrete tension strength | Vertical cracks in beams at tension face, near supports or mid-span |
| Shear | Shear stress exceeding concrete shear strength | Diagonal cracks (45°) in beams near supports, web criss-cross |
| Corrosion-Induced | Expansion of rust from rebar corrosion | Cracks following rebar pattern, often with rust staining; map cracks |
Damage under Accidental and Cyclic Loads:
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Impact/Explosion: Localized crushing, spalling, shear failure; requires forensic investigation to distinguish from other causes.
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Fatigue: Progressive crack growth under repeated cyclic loads (e.g., bridges). Assessment involves identifying crack initiation points, propagation paths, and final fracture.
[!TIP] Common Pitfall: Students often confuse shear and flexural cracks. Remember: Flexural = vertical (tension face), Shear = diagonal (near supports). Settlement cracks are typically diagonal from corners.
2.3 Material & Structural Testing
Classification of Tests:
| Category | Definition | Examples |
|---|---|---|
| Destructive (DT) | Test destroys part of the structure. | Core cutting for strength, load testing, pull-out test. |
| Non-Destructive (NDT) | No damage to structure. | Rebound Hammer, Ultrasonic Pulse Velocity (UPV), Radiography, Half-cell potential. |
| Partially Destructive | Minor, localized damage. | Pull-off test, breakout test. |
Key Non-Destructive Testing (NDT) Methods:
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Rebound Hammer (Schmidt Hammer):
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Principle: Measures surface hardness via rebound of a spring-driven mass.
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Use: Quick, relative assessment of concrete surface strength and uniformity. Not for absolute strength.
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Limitation: Affected by surface condition, carbonation, orientation, and aggregate type.
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Ultrasonic Pulse Velocity (UPV):
- Principle: Measures time taken by an ultrasonic pulse to travel through concrete. Velocity $V$ is given by:
$$V = \frac{L}{t}$$
where $L$ = path length, $t$ = transit time.
* **Use:** Assess **concrete quality, homogeneity, and presence of cracks/voids**. Higher velocity indicates better quality.
* **Interpretation:** Can estimate strength via empirical correlations. Combine with rebound hammer for better accuracy.
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Half-Cell Potential (HCP) Mapping:
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Principle: Measures voltage difference between a reference copper/copper sulfate electrode and the steel rebar (half-cell). More negative potential indicates higher corrosion probability.
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Use: Corrosion mapping of reinforcement in existing structures. Identifies areas of active corrosion for targeted repair.
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Radiography (X-ray/Gamma):
- Use: Detect internal flaws like voids, honeycombing, and rebar location/cover. Useful for post-tensioned ducts.
Role of NDT in Qualifying Structures after Retrofitting:
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Quality Control: Verify repair material placement and bonding (e.g., using UPV across epoxy injection).
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Verification of Design Intent: Confirm that strengthening materials (FRP, steel plates) are properly bonded and have no voids.
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Assessment of As-Built Condition: Compare post-retrofit NDT results with pre-retrofit baseline to quantify improvement.
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Long-Term Monitoring: Periodic NDT (like HCP) to check effectiveness of corrosion protection systems.
[!TIP] Exam Focus: Be prepared to explain any one NDT method in detail (principle, procedure, applications, limitations). UPV and Rebound Hammer are favorites. Always link HCP specifically to corrosion assessment.
2.4 Supporting Factors in Assessment
Importance of Concrete Cover in RCC Structures:
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Primary Function: Provides physical protection to reinforcement against corrosion (delays chloride/carbonation ingress) and fire resistance (insulation).
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Minimum Cover: Specified in codes (IS 456) based on environmental exposure class. Inadequate cover is a major cause of early corrosion.
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Assessment Check: During inspection, measure cover depth at several locations using a cover meter. Compare with design drawings and code requirements.
Factors to be Considered at Construction Site during Assessment:
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Safety: Secure the site, assess stability of distressed elements before inspection.
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Accessibility: Ensure safe access to all structural elements (slab undersides, beam sides, columns).
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Environmental Conditions: Temperature and humidity affect NDT results (e.g., UPV). Note conditions during testing.
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Material Variability: Expect in-situ concrete to have higher variability than lab-cast specimens; tests should reflect this.
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Documentation Availability: Presence/absence of original design drawings, construction records, and previous repair history.
[!TIP] Key Link: Concrete cover depth directly influences the rate of carbonation and chloride ingress, linking Unit 2 (Assessment) to Unit 1 (Corrosion Mechanisms). Always mention this connection.
Integrated Decision Framework (Link to Unit 3)
The diagnosis from Unit 2 (e.g., "corrosion-induced cracking due to inadequate cover in a marine environment") directly dictates the selection of repair technique in Unit 3 (e.g., remove concrete, treat steel with inhibitor, patch with polymer-modified mortar, and possibly apply a protective coating system). NDT results are used to verify the success of this intervention.