UNIT 4: RETROFITTING AND REHABILITATION OF STRUCTURES - SHORT NOTES

Based on analysis of CE-803(C) past papers (Nov 2023, May 2022). Focus on definitions, mechanisms, techniques, and NDT methods.

1.0 FUNDAMENTALS & CONCEPTS

1.1 Definitions and Scope

Philosophy: Rehabilitation is a systematic, cost-effective approach to extend life-cycle, balancing initial cost vs. long-term maintenance & failure cost.

1.2 Importance & Need for Rehab

• Life-Cycle Cost: Minimize total cost (construction + maintenance + failure) over structure's life.

• Key Prompting Factors:

  1. Age & natural deterioration.

  2. Design deficiencies (old codes, seismic/load inadequacy).

  3. Change in use (increased loads, new function).

  4. Natural hazards (earthquake, flood, fire damage).

  5. Accidents/overloads.

  6. Poor construction quality.

1.3 Classification of Distress

• By Severity: Minor (cosmetic), Moderate (functional), Severe (safety-critical).

• By Extent: Localised, Widespread, Systemic.

• By Cause: Load-induced, Environmental (chemical, physical), Material defects, Construction errors.

2.0 CAUSES & MECHANISMS OF DETERIORATION

2.1 Concrete Deterioration

A. Physical Causes:

B. Chemical Causes:

C. Surface Deterioration & Cracking:

• Scaling: Flaking of surface mortar (freeze-thaw, de-icing salts).

• Spalling: Break-off of larger concrete pieces (corrosion, fire).

• Crazing: Fine surface cracks (shrinkage, carbonation).

• Cracking Types:

  • Plastic: During setting (shrinkage, settlement).

  • Drying: Shrinkage after hardening.

  • Thermal: Temperature gradients.

  • Structural: Overstressing (flexure, shear).

  • Corrosion-induced: From rebar expansion.

Preventive Measures: Adequate concrete cover, proper mix design (low w/c ratio), control joints, curing, drainage.

2.2 Steel Deterioration (Corrosion)

Electrochemical Mechanism:

1. Anode: $$\displaystyle Fe \rightarrow Fe^{2+} + 2e^- $$ (Iron dissolution).

2. Cathode: $$\displaystyle O_2 + 2H_2O + 4e^- \rightarrow 4OH^- $$.

3. Result: Rust ($$\displaystyle Fe_2O_3 \cdot nH_2O $$) occupies >6x volume → cracking & spalling.

Factors Accelerating Corrosion:

• Environmental: Humidity, $$\displaystyle Cl^- $$ ingress, $$\displaystyle CO_2 $$ (carbonation), temperature.

• Metallurgical: Inconsistent steel quality.

• Stray Currents: From DC sources (trams, pipelines).

Corrosion Protection Methods:

3.0 ASSESSMENT, DIAGNOSIS & INVESTIGATION

3.2 Physical Inspection & Preliminary Survey

• Steps: Documentation review → Visual inspection (map cracks, defects, leakages) → Identify probable cause → Plan detailed investigation.

• Diagnostic Approach: Observe pattern of distress (e.g., diagonal cracks → shear; vertical at supports → flexure).

3.4 Material Testing & Strength Assessment

Classification of Tests:

Key NDT Methods (Frequently Asked):

Role of NDT after Retrofit: To verify repair quality, assess bond of new materials (FRP, concrete), and confirm reduction in corrosion activity.

4.0 REPAIR & SURFACE TREATMENT TECHNIQUES

4.2 Surface Preparation

1. Removal: Chipping, scarification, hydro-demolition (water jet – less vibration).

2. Cleaning: Wire brush, compressed air, water jet.

3. Substrate Prep: Ensure sound, clean, rough surface. Apply bonding agent (cement slurry, epoxy).

4.3 Common Repair Techniques

5.0 STRENGTHENING & RETROFITTING TECHNIQUES

5.1 Jacketing Techniques

Column Jacketing:

• Process: Remove cover → clean steel → install additional longitudinal & transverse reinforcement → cast new concrete/steel/FRP jacket.

• Materials:

  • Concrete: Increased section, improved confinement.

  • Steel: Steel plates/sections welded/bolted → high strength increase.

  • FRP: CFRP/GFRP wraps → excellent confinement, corrosion resistance, minimal section increase.

• Effect: Increases axial load capacity & ductility.

Beam Strengthening with Jacketing:

• Increase Cross-Section: Add new concrete & reinforcement to sides/bottom.

• Enhancement: Increases flexural strength (tension face) & shear capacity (sides).

5.2 Shear Transfer & Flexural Strengthening

• Shear Strengthening (External Bonded Reinforcement - EBR):

  • Steel Plates/FRP Strips: Bonded to beam sides (45° to horizontal for shear).

  • Mechanism: Provides additional tensile force to resist shear.

• Flexural Strengthening:

  • External Post-Tensioning: Tendons (strands) placed externally & tensioned → creates compressive stress.

  • FRP Laminating: CFRP sheets/plates bonded to tension face (bottom) → acts as tensile reinforcement.

5.3 Advanced Composite Materials (FRP)

Bonding Process:

1. Surface prep (grind, clean).

2. Apply primer (epoxy).

3. Apply adhesive (epoxy resin) & place FRP.

4. Roll to remove air bubbles.

5. Cure.

6.0 SPECIALIZED REHABILITATION SCENARIOS

6.1 Seismic Rehabilitation of Existing Buildings

Objectives: Increase strength, stiffness, ductility, energy dissipation. Common Techniques:

• Jacketing: Columns & beams (concrete/steel/FRP).

• Bracing: Add steel braces or FRP straps (X, V, K-configurations).

• Shear Walls: New reinforced concrete walls (core or perimeter).

• Base Isolation (Concept): Insert isolators (elastomeric) between foundation & superstructure → decouple from ground motion.

• Energy Dissipators: Viscous dampers, yielding devices.

6.3 Rehabilitation for Change in Use

• Assessment: Re-analyze structure for increased dead/live loads and potentially new seismic/wind requirements.

• Approaches:

  • Global: Strengthen multiple elements (e.g., add shear walls).

  • Local: Strengthen specific critical members (e.g., column jacketing, beam FRP).

7.0 PREVENTIVE MEASURES & MAINTENANCE

7.1 For New Structures (Design & Construction)

• Durable Design: Adequate cover (per exposure), low w/c ratio, proper joints (construction, expansion), effective drainage.

• Protective Systems: Epoxy-coated rebar, stainless steel rebar, corrosion inhibitors in concrete.

• Quality Control: Strict oversight of materials, mixing, placing, curing.

7.2 Maintenance Strategies

• Routine: Cleaning, drainage clearance.

• Preventive: Scheduled inspections, sealing cracks, recoating.

• Corrective: Repair after damage/deterioration.

• Plan: Develop maintenance manual with inspection frequency, checklists, and repair protocols.

8.0 DEMOLITION & ENGINEERED TECHNIQUES

Planning: Structural survey → determine sequence → safety (shoring, debris chutes, public protection) → environmental controls (dust, noise).

📝 EXAM TIPS & COMMON PITFALLS (Based on Past Papers)

[!TIP]

Distinguish Clearly:

• Repair = Fix damage. Rehabilitation = Comprehensive upgrade. Strengthening = Increase capacity. Retrofitting = Modify for new hazards.

• Corrosion Mechanism: Anodic dissolution ($$\displaystyle Fe \rightarrow Fe^{2+} + 2e^- $$). Carbonation lowers pH; Chlorides break passive layer.

• NDT Selection: Rebound Hammer → strength estimate (surface). UPV → integrity/homogeneity. Half-Cell → corrosion activity.

• Jacketing: For columns (axial) & beams (flexure/shear). FRP is lightweight, non-corrosive; concrete increases section; steel is strongest.

• Seismic Rehab: Aim for ductility (not just strength). Techniques: Jacketing, Bracing, Shear Walls. Base isolation is a concept (not common in typical retrofits).

[!CAUTION]

Avoid These Mistakes:

1. Confusing sulfate attack (expansive) with carbonation (lowers pH).

2. Stating epoxy injection for wide/active cracks (use stitching/routing instead).

3. Forgetting substrate preparation as a critical step in any repair/FRP bonding.

4. Not linking NDT results to action (e.g., high half-cell potential → need corrosion mitigation).

5. In seismic rehab, mentioning only "strengthening" without specifying ductility/energy dissipation enhancement.