Concrete Repair & Diagnostics Suite

A single tabbed StructCalc-style app combining diagnostics guidance, repair strategy, carbonation prediction and corrosion testing tools.

PAGE 0: OVERVIEW

Concrete Deterioration Diagnostics

Reinforced concrete fails primarily due to carbonation and chloride-induced corrosion of embedded steel. This guide explains testing methods, interpretation thresholds, and decision logic for assessing condition and planning repairs.

Why Diagnostic Testing?

🔍 Early Detection

Find problems before visible spalling or cracking — when repair costs are lowest.

📊 Quantified Risk

Measure corrosion likelihood and rate to prioritise intervention and set timelines.

💡 Repair Selection

Diagnosis informs repair strategy — protective coating vs. structural repair vs. replacement.

⏳ Service Life

Support remaining life calculations and monitor post-repair durability.

Failure Mechanisms at a Glance

Mechanism	Primary Cause	Visible Signs	Key Test
Carbonation	CO₂ penetration, pH drop <9	Map cracking, rust staining	Phenolphthalein
Chloride Attack	Salt ingress (marine, de-icing)	Rust staining, spalling patches	Chloride profiling
Combined	Both carbonation + chlorides	Widespread cracking & spalling	Multiple tests needed

PAGE 1: TESTING METHODS

Testing Methods

Click each card to expand details and interpretation guidance

👁️ Visual Inspection & Crack Mapping

What it is

Systematic observation of surface condition: cracks, staining, delamination, spalling geometry.

Standard: ACI 201, BS EN 1504-1

How to do it

Map all visible cracks (width, pattern, location)
Tap with hammer or chain drag for delaminated areas
Note rust staining, efflorescence, spalling
Photograph with scale reference

Interpretation

Crack pattern clues:

Map cracking (fine diagonal cracks) → likely carbonation + corrosion
Rust staining rings → active corrosion near surface
Spalling in patches → chloride-induced, progressing
Uniform discoloration → pollution, not necessarily corrosion

⚠ Limitation: Visual alone cannot quantify risk. Follow up with instrumental testing.

📏 Cover Meter (Rebar Locator)

What it is

Electromagnetic device measuring concrete cover above reinforcement.

Standard: BS 1881-204

Interpretation

Adequate cover for:

Aggressive environment (marine)	≥50 mm
Moderate (urban)	≥35 mm
Mild (sheltered)	≥25 mm

Note: Low cover accelerates carbonation/chloride ingress → higher corrosion risk.

🧪 Carbonation Depth Test

What it is

Fresh concrete fracture sprayed with phenolphthalein indicator.

Standard: BS EN 13295, ASTM C25

How to interpret

Pink/magenta colour: pH > 8.3, alkaline (uncarbonated, safe for rebar)
Colourless: pH < 8.3, carbonated (risk to rebar)

Critical threshold: Carbonation front reaches rebar = loss of passivity.

Assessment guidance

Measure carbonation depth from surface:

<10 mm: Low risk. Monitor every 5 years.

10–30 mm: Moderate risk. Combine with half-cell or chloride test.

>30 mm: High risk if cover < measured depth. Plan repair intervention.

⚡ Half-Cell Potential Mapping

What it is

Measures corrosion potential of rebar relative to Cu/CuSO₄ reference electrode. Rapid mapping of corrosion probability across large areas.

Standard: ASTM C876-15

Interpretation zones

> −200 mV	Low corrosion probability (<10%)
−200 to −350 mV	Intermediate (50% uncertain)
< −350 mV	High corrosion probability (>90%)

Limitations & pitfalls

Probability, not certainty: Many borderline (−250 to −300 mV) results are ambiguous.
Seasonal variation: Winter/wet periods can shift readings >50 mV.
Concrete age: Young concrete readings unreliable; wait >1 year if possible.
Cannot distinguish cause: Cannot tell if carbonation or chloride-induced.

Best used with: Combine with carbonation test or chloride profiling for diagnosis.

📊 Electrical Resistivity

What it is

Measures concrete resistivity (Ω·cm) — proxy for moisture, chloride, permeability.

Standard: ASTM C1876

Interpretation

>50 kΩ·cm	Low corrosion risk
10–50 kΩ·cm	Moderate risk
<10 kΩ·cm	High risk

Why it matters: Low resistivity enables rapid ion transport → accelerates corrosion if rebar is exposed.

📉 Linear Polarization Resistance (LPR)

What it is

Electrochemical method. Applies small potential to rebar, measures resulting current → corrosion rate in μA/cm².

Standard: ASTM G59, RILEM TC-154-EMC

Converting to rate

Corrosion current (i_corr, μA/cm²) converts to steel loss rate:

<0.1	Passive (negligible loss)
0.1–0.5	Low (0.001 mm/year)
0.5–1.0	Moderate (0.005 mm/year)
>1.0	High (0.01+ mm/year)

For perspective: 10 mm rebar losing 0.1 mm/year reaches critical loss of section (~30%) in ~30 years.

Advantages & limitations

✅ Quantitative: Gives actual corrosion rate, not just probability.
✅ Non-destructive.
❌ Cannot distinguish cause: Cannot tell if carbonation or chloride.
❌ Assumes accessible rebar (requires specialist electrode).

🧂 Chloride Profiling

What it is

Lab analysis of concrete dust or cores. Measures acid-soluble chloride content vs. depth.

Standard: BS EN 14629, ASTM C1152

Interpretation

Critical threshold: ~0.4% by cement weight triggers initiation of corrosion.

<0.1% Cl⁻	Safe
0.1–0.3%	Approaching threshold
0.3–0.5%	At/near threshold
>0.5%	High risk of active corrosion

Also assess: Depth of chloride front. Is it approaching rebar depth?

Pros & cons

✅ Direct measurement of the attacking ion.
✅ Can predict future ingress (diffusion model).
❌ Destructive (requires core or dust sample).
❌ Lab turnaround 1–2 weeks.
❌ Single point result; need multiple samples for mapping.

🌊 Non-Destructive Testing (NDT)

Ultrasonic Pulse Velocity (UPV)

Measures speed of sound through concrete. Lower velocity = poor quality, voids, delaminations.

>4.5 km/s	Good quality
3.5–4.5 km/s	Medium quality
<3.5 km/s	Poor quality / voids

Ground Penetrating Radar (GPR)

Electromagnetic imaging. Detects voids, delaminations, rebar position, concrete layer thickness.

Best for: Mapping internal defects without drilling.

Rebound Hammer (Schmidt Test)

Measures surface hardness → rough estimate of concrete strength (non-destructive, fast, cheap).

Caution: Only surface-sensitive. Cannot assess internal condition.

Thermography

Infrared imaging. Detects delaminations (trapped moisture changes thermal signature).

Best in: Clear weather, night-time (after solar heating subsides).

PAGE 2: INTERPRETATION & DIAGNOSIS

Diagnosis: Combining Test Results

No single test is definitive. Combine methods to confirm diagnosis and estimate severity.

Scenario 1: High Half-Cell Potential & Low Carbonation Depth

Finding: Half-cell <−350 mV, but phenolphthalein shows only 5 mm carbonation.

Diagnosis: Chloride-induced corrosion, NOT carbonation.

Implication: Rebar is still in alkaline environment but exposed to chloride ions (marine exposure, road salt).

Next steps: Profile for chloride content. Estimate time to loss of section using LPR.

Scenario 2: High Carbonation Depth Matching Rebar Depth

Finding: 35 mm carbonation, 40 mm cover measured, half-cell −280 mV (uncertain zone).

Diagnosis: Carbonation front at / near rebar.

Implication: Rebar passivity compromised. Corrosion may have initiated or be imminent.

Next steps: Run LPR to quantify active corrosion rate. Check resistivity (if high, corrosion will be slow).

Scenario 3: Low Resistivity, No Visible Cracking

Finding: Resistivity <10 kΩ·cm, half-cell −200 to −250 mV.

Diagnosis: High moisture, low permeability (poor quality concrete). Early-stage corrosion or passive.

Implication: Concrete provides poor protection. Even low driving force can initiate corrosion when chloride or carbonation front arrives.

Next steps: Assess chloride depth. Consider protective coating. Plan monitoring schedule.

Scenario 4: Visible Spalling, Rust Staining, High Chloride

Finding: Rust rings, surface spalling, chloride >0.5% at rebar depth, LPR = 1.0 μA/cm².

Diagnosis: Active chloride-induced corrosion with significant section loss.

Implication: Ongoing steel loss at ~0.01 mm/year. Without intervention, critical loss in 20–30 years.

Next steps: Plan REPAIR immediately — see Concrete Repair guide for strategies (cathodic protection, spall repair, overlay, etc.).

Test Selection Guide

Test	Cost	Speed	Destructive?	Use When...
Visual + Cover	Very Low	1 day	No	Initial condition assessment (always start here)
Carbonation	Low	1 day	Slight*	Suspect carbonation-induced corrosion
Half-Cell	Medium	2–3 days	No	Map corrosion probability across structure
Resistivity	Medium	1 day	No	Assess concrete quality & permeability
LPR	High	2–3 days	No	Quantify active corrosion rate (informs repair urgency)
Chloride Profile	High	1–2 weeks	Yes	Marine/de-icing exposure; confirm chloride ingress
GPR / UPV	High	2–3 days	No	Detect voids, delaminations, internal defects

*Carbonation requires small core or chip sample; cover meter helps minimize sampling points.

PAGE 3: TOOLS

Diagnostic Tools & Calculators

LPR to Corrosion Rate Converter

Convert electrochemical current to steel loss rate

Input: Corrosion current (i_corr) from LPR measurement in μA/cm²

i_corr (μA/cm²):

Conversion: corrosion rate (mm/year) = i_corr × 0.0116

Carbonation Diffusion Model

Estimate time for carbonation to reach rebar

Assumes square-root-of-time relationship: Depth² ∝ Time × Diffusion Coeff.

Measured carbonation depth (mm):

Years since exposure:

Concrete cover (mm):

Chloride Diffusion Estimator

Predict years to chloride threshold at rebar depth

Assumes Fickian diffusion with surface concentration boundary condition.

Current chloride at surface (%):

Current chloride at rebar depth (%):

Rebar depth (mm):

Chloride critical threshold (% by cement wt):

Steel Loss & Section Remaining

Estimate critical time to loss of section

Corrosion rate (mm/year):

Original rebar diameter (mm):

Acceptable loss of section (%):

PAGE 4: EXPOSURE CLASSES (BS 8500 & EN 206)

Exposure Classes: BS 8500 & EN 206-1

Exposure class determines concrete durability requirements (w/c ratio, cover, cement type). Understanding your structure's exposure class informs both testing strategy and repair urgency.

BS 8500 & EN 206-1 Exposure Classification System

Both standards use similar designations. Main categories are:

1. CARBONATION (XC Classes)

Class	EN 206 / BS 8500 Definition	Example Environments	Design w/c Ratio	Min. Cover (mm)*	Typical Diagnostic Focus
XC0	No exposure to carbonation	Interior, always dry	—	15	None needed (passive condition)
XC1	Dry or permanently wet	Interior heated buildings, underwater (fully immersed)	0.65	20 (int.) / 30 (ext.)	Visual inspection only; low risk
XC2	Wet, rarely dry	Exposed surfaces in humid climate, continuously splashed	0.60	30	Carbonation testing if age >20 yrs
XC3	Moderate moisture (most common outdoor)	Protected surfaces exposed to rain, sheltered under eaves	0.55	35	Standard testing: Half-cell + carbonation every 10–15 yrs
XC4	Cyclic wet/dry	Exposed surfaces (splash zones, weather-facing walls)	0.50	40	Enhanced testing: Annual half-cell mapping if industrial region

2. CHLORIDE INDUCED CORROSION — DE-ICING (XD Classes)

Class	EN 206 / BS 8500 Definition	Example Environments	Design w/c Ratio	Min. Cover (mm)*	Typical Diagnostic Focus
XD1	Moderate chloride exposure (spray zone)	Road surfaces exposed to salt spray, parking areas near roads	0.55	35	Chloride profiling recommended every 5–10 yrs
XD2	High chloride exposure (continuously wet or regularly wetted)	Submerged splash zone, car park decks, bridge decks in salt climate	0.50	40	Priority testing: Chloride profile + half-cell every 3–5 yrs
XD3	Very high chloride exposure (tidal/splash)	Tidal zone, heavily salted bridge decks, near de-icing plants	0.45	45	Aggressive monitoring: Annual chloride + half-cell + LPR. Plan repair/CP early.

3. CHLORIDE INDUCED CORROSION — SEAWATER (XS Classes)

Class	EN 206 / BS 8500 Definition	Example Environments	Design w/c Ratio	Min. Cover (mm)*	Typical Diagnostic Focus
XS1	Saltwater splash (not tidal)	Coastal area spray zone, above high water mark	0.50	40	Chloride profile + half-cell every 5 yrs
XS2	Submerged (fully immersed in seawater)	Below low water mark, permanently submerged	0.45	45	Half-cell mapping every 3–5 yrs. Salinity constant but oxygen limited (slower corrosion).
XS3	Tidal zone (cyclic wetting & drying)	Between high and low water mark, most aggressive marine exposure	0.40	50	Most aggressive: Annual testing. Early consideration of CP or protective overlay.

4. FREEZE-THAW (XF Classes)

Class	EN 206 / BS 8500 Definition	Example Environments	Design w/c Ratio	Air Content Required
XF1	Moderate saturation, no de-icer	Inland regions with freeze-thaw cycles (rainfall driven)	0.55	4–6%
XF2	High saturation, no de-icer	Coastal splash zone, continuous spray, freeze-thaw cycles	0.50	4–6%
XF3	Moderate saturation + de-icing salt	Road surfaces in freeze-thaw regions	0.45	4–6%
XF4	High saturation + de-icing salt (tidal or splash)	Bridge decks in cold coastal zones, heavily de-iced pavements	0.40	4–6%

5. CHEMICAL ATTACK (XA Classes)

Class	EN 206 / BS 8500 Definition	Example Environments	Design Measures
XA1	Weakly aggressive chemical environment	Soil with low sulphate, slightly acidic groundwater	Sulphate-resistant cement (SR); limit w/c
XA2	Moderately aggressive	Soil with medium sulphate, acidic soil, certain industrial waste	SR cement mandatory; lower w/c; increased cover
XA3	Highly aggressive	Soil with high sulphate, acidic mine drainage, chemical plant effluent	SR cement + protective lining or epoxy coating; specialist design

*Covers given are nominal for reinforced concrete; values vary by member type and design code version. See BS EN 1992-1-1 (Eurocode 2) for exact specifications.

BS 8500 vs. EN 206-1: Practical Comparison

Aspect	BS 8500	EN 206-1 (Eurocode)	Practical Notes
Status	UK standard (replaces BS 8110); adopted from EN 206 with UK amendments	European standard; mandatory in EU	BS 8500 generally aligns with EN 206, with UK-specific exposure class definitions
Exposure Classes	XC, XD, XS, XF, XA (same as EN 206)	XC, XD, XS, XF, XA (same)	Classes are identical; only implementation details differ
Design w/c Ratio	Specified in BS 8500-2 (Tables 4 & 5)	Specified in EN 206-1 (Table 4.3)	BS 8500 values typically slightly more conservative (lower w/c allowed)
Cover Specification	BS 8500-1:2015 (Table A.2); integrated with BS EN 1992-1-1	EN 1992-1-1 Eurocode 2 (Table 4.4)	BS 8500 cover values tend to be higher for XD/XS classes
Cement Type Requirements	SR (sulphate-resistant) for XA; CEM III or CEM IV often recommended for marine (XS)	Similar; can specify CEM I with additions, or CEM III (blast furnace slag)	BS 8500 more explicit on cement selection for durability
Chloride Ion Content	Max. 0.1% by cement wt. (XC); 0.05% (XD/XS)	Similar limits in EN 206-1 Annex F	Identical in practice; affects concrete mix design and material sourcing
Relevant for Repair/Diagnostics	Exposure class determines: Target corrosion potential (when CP is needed), minimum concrete quality for repair, expected service life of interventions		Always establish original design exposure class; it guides durability of repair strategy

Using Exposure Classes in Diagnostics & Repair

🧪 Diagnostics Planning

XC classes: Focus on carbonation testing every 10–15 years once age >20 years.

XD/XS classes: Prioritise chloride profiling and half-cell mapping every 3–5 years (higher exposure = higher frequency).

XF classes: Include visual inspection for freeze-thaw damage (scaling, pop-outs). Monitor air-void system integrity if available.

XA classes: If suspected chemical attack, test pH locally, examine concrete core for dissolution patterns.

🔧 Repair Strategy

XC1/XC2: Protective coating sufficient if no active corrosion. Plan re-coat every 10 years.

XC3/XC4: Spall repair + overlay for moderate damage. Consider cathodic protection if active.

XD2/XD3 (high chloride): Prioritise cathodic protection or heavy overlay. Annual monitoring mandatory.

XS2/XS3 (seawater): CP or replacement recommended for critical members. Protective coating + monitoring for secondary elements.

Quick Reference: Test Frequency by Exposure Class

Exposure Class	Annual Visual	Carbonation Test	Half-Cell Mapping	Chloride Profile	LPR (if Active)
XC1/XC2	Optional	Every 15 yrs (age >20)	No	No	If staining visible
XC3/XC4	Every 2–3 yrs	Every 10–15 yrs	Every 10 yrs or if cracking	No	If cracking/staining
XD1	Annual	No	Every 5–10 yrs	Every 10 yrs	If potential <−280 mV
XD2	Annual	No	Every 3–5 yrs	Every 5 yrs	Annual if potential <−250 mV
XD3 / XS2 / XS3	Annual	No	Every 2–3 yrs	Every 3 yrs	Annual or continuous monitor

PAGE 5: DECISION GUIDE

Decision Flowchart: From Observations to Repair

Use this logic tree to guide your diagnostic testing and determine if repair is needed.

START: Visible distress or asset age >30 years? │ ├─→ YES: Conduct Visual Survey │ │ ├─ Look for: cracks, rust staining, spalling, soft cover │ │ └─ Measure: cover with cover meter at multiple points │ │ │ ├─→ Significant visible damage or low cover (<25 mm)? │ │ │ │ │ ├─→ YES: Run HALF-CELL MAPPING │ │ │ ├─ >−200 mV: Low risk → Monitor. Go to STEP-2. │ │ │ ├─ −200 to −350 mV: Uncertain → Run CHLORIDE PROFILE │ │ │ └─ <−350 mV: High risk → RUN LPR + CHLORIDE │ │ │ │ │ └─→ NO: Run CARBONATION + RESISTIVITY │ │ ├─ Carbonation > 80% of cover? → Assess urgency │ │ └─ Resistivity <10 kΩ·cm? → High permeability risk │ │ │ └─→ STEP-2: Interpret Combined Results │ ├─ If LPR >0.5 μA/cm² + visual damage: REPAIR REQUIRED │ ├─ If carbonation at rebar + half-cell <−280 mV: REPAIR PLAN │ └─ If stable condition + low risk: MONITOR every 3–5 years │ └─→ NO: Routine assessment for preventive maintenance └─ Use RESISTIVITY + COVER SURVEY → Inform future monitoring

Repair Decision Matrix

Corrosion State	LPR (μA/cm²)	Visible Damage	Recommendation
Passive (Safe)	<0.1	None	✅ Monitor every 5 years. Apply protective coating if desired.
Initiation Phase	0.1–0.5	Incipient (staining)	⚠ Begin monitoring (2–3 year cycle). Plan coating/cathodic protection.
Active (Early)	0.5–1.5	Rust staining, light cracking	⚠ Repair within 3–5 years. Spall repair + coating or cathodic protection.
Active (Severe)	>1.5	Spalling, section loss visible	🔴 URGENT REPAIR. Spall removal + structural retrofit or replacement.

Next: Repair Strategy

Once diagnosis is complete and repair is indicated, consult the Concrete Repair guide for:

Repair method selection (spall repair, cathodic protection, overlay, etc.)
Material specifications and durability design
Post-repair monitoring and maintenance

PAGE 0: OVERVIEW

Concrete Repair Strategies & Methods

Once corrosion diagnosis is complete (see Concrete Diagnostics), repair strategy depends on damage severity, structural demands, and durability requirements. This guide covers repair modalities, material selection, and post-repair monitoring.

Repair Strategy by Damage Stage

🟢 Preventive

No visible damage, but diagnostics show initiation risk (e.g., carbonation approaching rebar).

Strategy: Protective coatings, cathodic protection.

No urgency

🟡 Early Active

Light staining, incipient cracks, LPR 0.5–1.0 μA/cm².

Strategy: Targeted spall repair + protective treatment.

3–5 years

🔴 Severe Active

Visible spalling, section loss, LPR >1.0 μA/cm².

Strategy: Urgent spall repair, structural retrofit, or partial replacement.

Immediate

Repair Hierarchy (Most to Least Effective)

Rank	Method	When to Use	Cost Estimate	Durability
1	Remove & Replace	Severe section loss; unsalvageable member	€€€€	50+ years
2	Cathodic Protection	Active corrosion, intact structure desired	€€€	20+ years
3	Spall Repair + Coating	Localized damage, early-stage corrosion	€€	10–20 years
4	Protective Coating	Preventive; no active corrosion yet	€	5–15 years
5	Epoxy Injection	Sealed cracks, low moisture	€	3–10 years

PAGE 1: REPAIR METHODS

Concrete Repair Methods

1. PROTECTIVE COATINGS (Preventive / Early Stage)

Purpose & When to Use

Seal concrete surface to reduce penetration of CO₂ and chloride ions. Used when corrosion risk is identified but rebar is still passive.

Best for: Initiation phase (carbonation approaching), low-moderate exposure, preventive maintenance.

Types of Coatings

Acrylic Emulsion	Breathable, water-based, cheap, short-lived (5–7 yrs)
Polyurethane	Durable, UV-resistant, moderate cost (10–15 yrs)
Epoxy	Hard, long-lived (12–20 yrs), difficult to remove if needed
Silane / Siloxane	Penetrating, breathable (5–10 yrs), allows concrete to dry

Key Steps & Precautions

Surface prep critical: Remove all dirt, loose concrete, algae (pressure wash or grit blast).
Moisture check: Concrete must be dry (carbonation tests should show where moisture sits).
Bond strength: Apply to sound concrete surface. Do not apply over spalling areas.
Recoat cycle: Plan for re-application every 8–10 years depending on exposure and coating type.

✅ Pros

Non-invasive — no structural disturbance
Cheap compared to other methods
Reversible — can remove and re-apply
Slow carbonation penetration

❌ Cons

Does NOT stop active corrosion (if rebar is exposed, coating won't help)
Limited life (5–20 years, then re-coat needed)
May trap moisture if chlorides already present (epoxy can worsen)
Surface damage voids coating benefit

2. SPALL REPAIR & PATCHING (Early–Moderate Active Corrosion)

Purpose & When to Use

Remove deteriorated concrete and rusted rebar. Clean rebar, apply corrosion inhibitor, repatch with repair mortar. Arrests visible damage and removes rust.

Best for: Localized spalling, early-stage corrosion with manageable section loss.

Spall Repair Procedure

Expose damage: Break away all spalled/deteriorated concrete and loose material (sound concrete edge ~20 mm beyond visible damage).
Clean rebar: Wire-brush to remove rust; achieve Sa 2.5 or better (mill scale visible but no rust).
Apply corrosion inhibitor: Zinc-rich epoxy primer or organic inhibitor over cleaned steel.
Apply repair mortar: Use shrinkage-compensated mortar in layers (max 50 mm per layer for thick repairs).
Finish: Level with surrounding surface; dome slightly if traffic expected.
Cure: Keep moist for 7 days minimum (critical for bond).
Apply surface protection: Coating over repair to match surrounding surface.

Repair Mortar Types

Cementitious Mortar	Cheap, alkaline environment protects rebar, shrinkage risk (~0.5%)
Shrinkage-Compensated Mortar	~0% shrinkage, better bond, standard for spalls
Epoxy Mortar	Low shrinkage, excellent bond, chemical-resistant, expensive, exothermic
Polymer-Modified Mortar	Intermediate performance, good durability, moderate cost

✅ Pros

Stops visible corrosion and rust bleeding
Restores section locally
Relatively cheap for small areas
Works immediately — no long cure needed

❌ Cons

Does NOT address root cause — corrosion may resume if moisture/salt continues ingress
Bond failure risk if surface prep poor
Shrinkage cracking possible (use shrinkage-compensated mortar)
Short life without coating (5–10 years typical)

3. CATHODIC PROTECTION (Active Corrosion, Structure to be Preserved)

Purpose & When to Use

Apply small electric potential to shift rebar potential to passive zone (>−100 mV Cu/CuSO₄), stopping corrosion even if chloride/carbonation are present.

Best for: Severe active corrosion, structures worth saving, marine/aggressive environments.

Two Main Types

1. Impressed Current Cathodic Protection (ICCP)

Apply external DC power (anode impressed into concrete or on surface).
Rebar becomes cathode; continuous power required.
Advantages: Can target specific areas, rapid (current density 1–5 mA/m²).
Disadvantages: Electricity cost (~€1–5k/year), complex installation, hydrogen embrittlement risk if overapplied.

2. Galvanic Anode (Passive)

Install zinc or aluminium anodes on surface connected to rebar.
Galvanic couple naturally shifts potential without external power.
Advantages: No electricity required, simple, lower cost.
Disadvantages: Weaker current (0.1–0.5 mA/m²), works best on high-resistivity concrete.

Design & Installation

Half-cell survey required to map current requirements across structure.
Anode layout: Ground electrodes or impressed current anodes spaced to achieve coverage.
Concrete resistivity: If low (<10 kΩ·cm), higher current needed → higher power costs.
Insulation & monitoring: Install reference electrodes to monitor potential, ensure −100 to −800 mV range.
Power supply: Constant current rectifier, hardwired or remote monitored.

✅ Pros

Stops active corrosion immediately
Works regardless of chloride/carbonation
No concrete removal needed
Can be applied retrofectively
Long-term durability (20+ years if maintained)

❌ Cons

High capital cost (€30–100k+ for large structures)
Ongoing electricity / maintenance for ICCP
Complex design — requires electrochemical expertise
Hydrogen embrittlement risk if overprotected (<−900 mV)
Anode consumption (galvanic anodes need periodic replacement)

4. CONCRETE OVERLAY / JACKETING (Moderate–Severe, Structural Reinforcement)

Purpose & When to Use

Apply new layer of concrete (50–150 mm) over corroded member. Provides physical barrier, increases depth to rebar, and can add structural capacity.

Best for: Columns, beams, slabs with section loss; structures needing structural reinforcement plus corrosion control.

Design Steps

Assess structural capacity: Can existing member support removal of deteriorated cover + overlay weight?
Prepare surface: Remove loose/damaged concrete; expose sound substrate and rebar.
Design overlay: Specify depth, reinforcement (mesh or bars), concrete strength (min. C35).
Bond preparation: Roughen surface (bushhammer or grit blast) and apply bonding agent.
Install formwork: Maintain accurate depth and alignment.
Cast overlay concrete: Ensure good consolidation, especially around existing rebar.
Cure properly: 7–14 days damp curing to develop bond strength.

✅ Pros

Large reduction in corrosion penetration (increased depth to rebar)
Stops chloride/carbonation ingress if sealed properly
Can add structural capacity (especially if reinforced)
Addresses multiple members in one phase

❌ Cons

Bond failure risk if surface prep inadequate
Adds weight (may require structural checks)
Formwork intensive → labour cost
Still requires surface protection (coating or sealant)
Long cure time before structure reopens

5. MEMBER REPLACEMENT (Severe Structural Loss)

Purpose & When to Use

Remove and replace heavily corroded member. Used when section loss is severe and structural capacity is compromised beyond repair.

Best for: Critical members (load-bearing columns, foundations), extensive corrosion, unsalvageable conditions.

When Warranted?

>30% section loss on primary reinforcement.
Corrosion rate >1.0 μA/cm² and member is critical (high live load).
Multiple corrosion sites affecting overall capacity.
Spalling extent >30% of member perimeter.

✅ Pros

Complete structural renewal
No residual corrosion risk in new concrete
Restored full capacity
Long service life ahead (50+ years if designed well)

❌ Cons

Very high cost
Extended downtime (supports, temporary structures)
Complex logistics (dismantling, loads, waste)
Risk of collateral damage during removal

PAGE 2: MATERIALS

Repair Materials: Selection & Specifications

Repair Mortars & Concretes

Standard Cementitious Repair Mortar (Non-Structural Spalls)

Composition: Portland cement, sand (0–2 mm), water, possibly pozzolanic additive (fly ash, silica fume).
Strength: C25–C35 (25–35 MPa at 28 days).
Shrinkage: ~0.5% — use shrinkage-compensating admixtures (calcium sulfoaluminate) to reduce to ~0.1%.
Typical spec: BS EN 1504-3 Class R1 (spray-applied) or R2 (placed).
Bond strength: Min. 1.5 MPa (BS EN 1504-10).
Cost: €50–150/m³.

Epoxy Mortar (High Performance, Structural Spalls)

Composition: Two-part epoxy resin + hardener + aggregates (sand).
Strength: 50–80 MPa (much higher than cement).
Shrinkage: ~0.05% — negligible.
Bond: 3–5 MPa (excellent adhesion if substrate is dry).
Curing: Fast (24 hrs to hard state), but temperature-dependent (warmer is faster).
Limitations: Cannot cure at <10°C; exothermic heat can crack thick sections; not breathable.
Cost: €300–600/m³.

Polyester or Vinyl Ester Mortar (Corrosive Environments)

Use when: Aggressive acid/sulphate environment, chemical plant, sewers.
Strength: 60–100 MPa.
Chemical resistance: Superior to cement-based (resists dilute acids, oils).
Cost: €400–700/m³ (expensive but justified in harsh conditions).

Corrosion Inhibitors & Primers

Zinc-Rich Epoxy Primer (for Rebar Cleaning)

Purpose: Passivate cleaned rebar, provide galvanic protection during repair.
Zinc content: 70–85% by volume of resin solids.
Application: Brush or spray onto wire-brushed rebar (Sa 2.5 or better).
Dry thickness: 75–150 μm (single coat).
Cure time: 7–14 days before patching mortar applied.
Cost: €20–40/litre (covers ~10–20 m²/litre).

Organic Corrosion Inhibitors (Migrating Type)

Purpose: Penetrate concrete, reach rebar, inhibit corrosion without galvanic element.
Mechanism: Form protective film on steel; reduce chloride or oxygen diffusion.
Applied to: Sound concrete (not spalls); works internally over time (weeks to months).
Typical products: Aliphatic amine compounds, amino alcohols.
Cost: €300–500/m³ of concrete (typically dosed at 1–2% by cement weight).
Benefit: Can treat large areas without aggressive surface prep.

Surface Coatings (Post-Repair Protection)

Coating Type	Thickness (μm)	Durability	Cost (€/m²)	Best for
Acrylic Emulsion	80–200	5–7 years	3–8	Sheltered, non-aggressive
Polyurethane	150–250	10–15 years	15–35	Moderate exposure, traffic
Epoxy (Two-Pack)	150–300	12–20 years	25–50	Wet/chemical environment, high durability need
Silane / Siloxane	50–150	5–10 years	8–20	Breathable coating, chloride reduction
Elastomeric Coating	2–3 mm	10–20 years	50–100	Crack bridging, movement tolerance

Concrete Specifications for Overlays & Jacketing

Overlay Concrete (50–150 mm)

Strength grade: Min. C35 (35 MPa) for durability.
Water/cement ratio: Max. 0.50 (reduces permeability).
Slump: 80–120 mm (workable but not overly fluid).
Air content: 4–6% (improves frost resistance).
Bonding agent: BS EN 1504-2 Type 2 (epoxy or cementitious slurry) — critical for adhesion.
Cover to new reinforcement: Same as original design (typically 35–50 mm for moderate exposure).
Cure: Keep wet for 7–14 days after casting to develop full bond and strength.

PAGE 3: SELECTION GUIDE

Repair Strategy Selection Flowchart

Use diagnostic findings (LPR, carbonation, chloride) to select the most appropriate repair method.

Decision Tree

START: Diagnostic findings from Concrete Diagnostics app
│
├─ STEP 1: Assess Damage Severity
│  │
│  ├─ Visible spalling or section loss >5%?
│  │  ├─ YES → Go to STEP 2
│  │  └─ NO → Check LPR or half-cell
│  │
│  └─ LPR available?
│     ├─ >1.0 μA/cm² (high corrosion rate) → URGENT REPAIR
│     ├─ 0.5–1.0 μA/cm² (moderate) → PLAN REPAIR IN 3–5 YEARS
│     └─ <0.5 μA/cm² (low) → PREVENTIVE / MONITOR
│
├─ STEP 2: Evaluate Environmental Exposure
│  │
│  ├─ Marine environment or de-icing salt zone?
│  │  ├─ YES → High chloride ingress expected → Plan for CP or overlay
│  │  └─ NO → Likely carbonation-driven → Coating may suffice
│  │
│  └─ Concrete resistivity <10 kΩ·cm (poor quality)?
│     ├─ YES → High future risk → Consider CP or overlay
│     └─ NO → May stabilize with coating
│
├─ STEP 3: Assess Structural Importance
│  │
│  ├─ Is member load-bearing (primary beam, column, foundation)?
│  │  ├─ YES, section loss >10% → REPLACEMENT or JACKETING
│  │  ├─ YES, section loss <5% → SPALL REPAIR + overlay or CP
│  │  └─ NO (secondary, cladding, etc.) → SPALL REPAIR + coating
│  │
│  └─ Is structure accessible for ongoing maintenance?
│     ├─ YES → CP or periodic coating re-application feasible
│     └─ NO → Prefer durable method (overlay, jacketing, replacement)
│
├─ STEP 4: Select Method
│  │
│  ├─ Active corrosion + deep member + access to rebar?
│  │  └─ → CATHODIC PROTECTION (ICCP or galvanic)
│  │
│  ├─ Localized spalling, early stage, secondary member?
│  │  └─ → SPALL REPAIR + protective coating
│  │
│  ├─ Moderate damage, structural reinf. needed?
│  │  └─ → OVERLAY / JACKETING
│  │
│  ├─ Section loss >20% or multiple spall sites?
│  │  └─ → REPLACEMENT (if critical) or CP (if preservation desired)
│  │
│  └─ No visible damage but risk detected (carbonation front, high chloride)?
│     └─ → PROTECTIVE COATING or corrosion inhibitor

        

Quick Reference: Repair Selection by Scenario

Scenario	Best Method(s)	Urgency	Cost Estimate
Visible rust staining, light cracking, LPR <0.5	Protective coating or organic inhibitor	Preventive / 5 years	€200–500/100m²
Small spall (<100 mm), rust visible, LPR 0.5–1.0	Spall repair + coating (epoxy repair mortar)	3–5 years	€50–200/m² (area)
Multiple spalls, section loss >5%, LPR >1.0	Overlay or cathodic protection	1–3 years	€300–1000/m² (area) or €30–100k (CP system)
Severe corrosion, section loss >20%, load-bearing member	Replacement or heavy-duty jacketing	URGENT (months)	€5k–50k+ depending on member size/access
Chloride-induced, high ingress, no visible damage yet	Cathodic protection (best) or organic inhibitor + monitoring	1–3 years (plan), preventive if early stage	€30k–150k (CP) or €500–1500 (inhibitor)
Marine structure, no visible damage, high chloride at depth	Cathodic protection (long-term) + protective coating	Preventive	€30–100k initial + €1–5k/year operating

PAGE 4: EXPOSURE CLASSES (BS 8500 & EN 206)

Exposure Classes: BS 8500 & EN 206-1

Exposure class determines durability requirements, material specifications for repair, and expected service life. A key reference for designing repairs that outlast the original damage.

Quick Reference: Exposure Class Definitions

Class	Type of Exposure	Example Environments	Min. Design w/c for New Concrete	Cover (mm)
XC3	Moderate carbonation (outdoor)	Protected surfaces exposed to rain, sheltered under eaves	0.55	35
XC4	High carbonation (cyclic wet/dry)	Exposed surfaces, splash zones, weather-facing walls	0.50	40
XD1	Moderate chloride (de-icing spray)	Road surfaces near salt spray, parking areas	0.55	35
XD2	High chloride (continuously wetted)	Car park decks, bridge decks in salt climate, submerged splash	0.50	40
XD3	Very high chloride (tidal/splash)	Heavily salted bridge decks, near de-icing plants	0.45	45
XS1	Marine saltwater spray	Coastal splash zone (above high tide)	0.50	40
XS3	Marine tidal zone (most aggressive)	Between high & low water mark	0.40	50

Exposure Class & Repair Material Selection

Rule 1: Repair Concrete Must Meet or Exceed Original Design Class Requirements

Example: Original member designed to XC4. Spall repair mortar must achieve w/c ≤ 0.50 and use cement suitable for XC4 (e.g., CEM II with pozzolanic addition).
Why: Repair is only as durable as its weakest link. Poor-quality patch will fail again quickly.

Rule 2: For Aggressive Exposures (XD/XS), Use High-Performance Repair Materials

XD1/XD2: Cementitious shrinkage-compensated mortar acceptable, but consider epoxy for critical areas. Zinc-rich epoxy primer mandatory on cleaned rebar.
XD3 / XS (tidal/marine): Epoxy mortar or polyester mortar preferred. Zinc-rich epoxy primer + organic inhibitor + polyurethane or epoxy topcoat.
Service life expectation: XD/XS repairs with standard coatings ~10–15 years. With CP: 20+ years.

Rule 3: Overlay / Jacketing Concrete Specification Matched to Class

XC3/XC4: Min. C35 concrete, w/c ≤ 0.55, 30–40 mm cover to new reinforcement.
XD2 / XS: Min. C40, w/c ≤ 0.45, 40–50 mm cover. Consider CEM III (blast furnace slag cement) for better durability.
XD3 / XS3: C45, w/c ≤ 0.40, 50+ mm cover. Slag or silica fume cement. Air-entrain for freeze-thaw (XF classes).

Exposure Class & Repair Strategy Trade-offs

Exposure	Most Economical Repair	Most Durable Repair	Service Life of Economic Option	When to Upgrade Strategy
XC3	Spall repair + acrylic coating	Overlay (80 mm) + polyurethane	8–12 years	If member is critical load-bearing or age >40 years
XC4	Spall repair (epoxy mortar) + polyurethane	Overlay (100 mm) + epoxy + periodic re-coat	12–15 years	If corrosion active (LPR >0.5) or next corrosion cycle expected <10 yrs
XD1 / XD2	Spall repair + coating (if early stage)	Cathodic protection (ICCP) or heavy overlay	10 years	Always upgrade to CP or overlay if LPR >0.8 or section loss >5%
XD3	Not recommended—use durable method	Cathodic protection + overlay	—	CP mandatory for long-term survival; patch alone will fail within 5 yrs
XS (Marine)	Not recommended—use durable method	Cathodic protection (critical); heavy overlay + CP (long-term)	—	Standard repairs fail rapidly in marine. Budget for CP system or replacement.

Exposure Class & Post-Repair Durability Management

Key principle: Higher exposure classes require more frequent monitoring and earlier re-coating cycles.

XC classes: Re-coat every 10 years. Monitor visually every 2–3 years.
XD1/XD2: Re-coat every 8 years. Annual visual + half-cell every 5 years.
XD3 / XS classes: If using coating alone, plan for coating failure within 5–7 years. Strongly recommend cathodic protection instead (self-maintaining, 20+ year life).

Material Specifications by Exposure Class

Exposure	Repair Mortar / Concrete Type	Rebar Treatment	Topcoat / Sealant	Re-coat Cycle
XC3/XC4	Shrinkage-comp. cementitious or epoxy mortar (C35)	Wire brush + epoxy primer (if exposed)	Polyurethane or acrylic	10 years (check condition at 8)
XD1/XD2	Epoxy mortar (better durability) or high-strength cementitious (C40, w/c ≤0.50)	Zinc-rich epoxy primer mandatory	Polyurethane (durable) or epoxy (very durable)	8 years; re-assess at year 6
XD3 / XS1	Epoxy or polyester mortar (high durability) + overlay if possible	Zinc-rich epoxy + organic inhibitor primer	Epoxy or elastomeric (crack-bridging)	7 years; or consider CP alternative
XS2/XS3 (Tidal)	Polyester mortar + heavy overlay preferred; or plan for CP instead	Zinc-rich epoxy + protective topcoat	Elastomeric or epoxy (high durability needed)	5–7 years; CP more cost-effective long-term

PAGE 5: DURABILITY & MONITORING

Post-Repair Durability & Monitoring

Key principle: Repair itself has finite life. Plan for inspection, maintenance, and potential re-repair cycles.

Expected Service Life by Method

Repair Method	Service Life (Years)	Maintenance Required	Failure Mode
Protective Coating Alone	5–15	Re-coat every 8–10 years; inspect for damage	Coating breakdown; chloride/carbonation breakthrough
Spall Repair + Coating	10–20	Inspect annually; re-coat substrate every 8–10 years	Bond failure; repeat corrosion in repaired area or adjacent
Overlay (50–100 mm)	15–30	Seal surface; inspect for edge spalling; monitor for new cracks	Overlay delamination or new corrosion at overlay edge
Cathodic Protection (ICCP)	20–30+ (renewable with power supply)	Annual electrical inspection; monitor potential; maintain anode connections	Power failure; anode deterioration; electrical issues
Cathodic Protection (Galvanic)	15–25	Inspect anode consumption; replace anodes as needed (every 5–10 yrs)	Anode consumed; potential rises above −100 mV
Member Replacement	50+ (new design life)	Same as new construction; apply protective coating if desired	Age-related decay (carbonation, chloride ingress in new concrete)

Post-Repair Monitoring Plan

Year 1 (Post-Repair Acceptance)

Visual inspection: Check for repair mortar cracks, spalling, bond issues (tap test with hammer).
If coated: Ensure coating is continuous and sound.
Photo documentation: Record baseline condition for future comparison.

Years 2–5 (Early Service)

Annual visual survey: Look for repeat corrosion (rust staining around repair), coating breakdown, new cracks.
Half-cell mapping (optional): At year 3–5, verify rebar still passive. If <−250 mV in repaired area, corrosion may be recurring.
Coating inspection: Minor touch-ups if damage found; plan re-coat around year 8–10 if needed.

Years 5–10+ (Ongoing Maintenance)

2–3 year cycle: Visual surveys + photographic record.
Coating re-application: At 8–10 years, plan full re-coat if original coating life approaching end.
If CP installed: Annual electrical inspection, potential monitoring, anode inspection (galvanic).
Chloride profiling (optional): At year 10, if marine environment, sample to see if ingress rate has slowed (indicates coating/repair working).

Red Flags: When to Re-Repair

Rust staining reappearing within 2–3 years of repair → indicates moisture ingress or incomplete spall removal.
Spall repair bond failure (hollow sound on tap test) → re-patch required.
New spalling adjacent to repair → suggests corrosion front has progressed; re-assess and expand repair scope.
Half-cell potential drops below −300 mV in repaired area → corrosion may have re-initiated; increase monitoring or switch to cathodic protection.
Coating loss >10% of area → plan re-coat immediately; don't wait for full failure.
CP potential outside safe window (<−100 mV or >−800 mV) for sustained period → electrical malfunction; investigate and correct.

Long-Term Strategy Recommendations

Structure Type / Exposure	Recommended Approach
Secondary/non-critical, sheltered (office building interior)	Spall repair + acrylic coating. Monitor annually. Re-coat every 10 years or as needed. Cost-effective, long life.
Moderate exposure (urban carpark, bridge deck)	Overlay (80–150 mm) with durable coating. Plan re-coat at 10–15 years. Good balance of protection and cost.
Aggressive exposure (marine, de-icing, chemical)	Cathodic protection (ICCP if ongoing corrosion, galvanic if stable). Annual monitoring. Provides long-term reliability.
Critical load-bearing members with severe damage	Replacement or heavy jacketing + CP. Ensures structural integrity and durability. Accept high cost.

Documentation & Maintenance Records

Keep a Repair & Maintenance Log

Date of repair: Method, materials used, area extent, cost.
Annual/biennial inspections: Visual findings, photos, half-cell or coating assessments if done.
Maintenance work: Touch-ups, re-coating, anode replacement (if CP).
Performance notes: Any unexpected corrosion, bond issues, coating failure.

This log informs future repair decisions and helps predict when next major intervention is needed.

Inputs

Assessment

Exposure age

years

Concrete cover to reinforcement

Chart / prediction horizon

years

Concrete quality

Characteristic strength f_ck

MPa

Water / binder ratio

CEM I / Portland cement

kg/m³

GGBS content

kg/m³

PFA / fly ash content

kg/m³

Silica fume content

kg/m³

Binder type multiplier

Curing quality

Environment

Relative humidity

CO₂ concentration

ppm

Mean temperature

°C

Wetting / rain exposure

Surface condition

Crack width

Crack severity λ

Coating / render protection

Advanced model constants

Reference carbonation coefficient k_ref

mm/√yr

Strength exponent α

w/b exponent β

cementitious exponent γ

Minimum RH factor

Maximum RH factor

Measured calibration

Use measured carbonation depth instead of model coefficient

Measured carbonation depth

Age at test

years

If switched on, the app uses k = measured depth / √measured age. This is usually better than a purely theoretical prediction if the test is reliable and representative.

Results

Model k

mm/√year

Used k

model

Carbonation depth

mm at assessment age

Current rate

mm/year at assessment age

Time to reinforcement

years from first exposure

Status

against cover input

Carbonation depth versus time

Calculated factors

Factor	Value	Comment

This is a predictive estimate, not a durability certificate. For real assessment of existing concrete, calibrate against measured carbonation depth and verify cover, concrete quality and exposure history.

Calculation method

Calculation text will appear after running the model.

OVERVIEW

Understanding Concrete Corrosion Testing

Reinforced concrete structures deteriorate primarily due to carbonation and chloride-induced corrosion of the embedded steel. This interactive guide explains the most important testing modalities used by engineers to assess risk and remaining service life.

Why Test for Corrosion?

🔍 Early Detection

Identify problems before visible damage (spalling, cracking) occurs.

📊 Risk Assessment

Quantify probability and rate of corrosion to plan maintenance.

⏳ Service Life Prediction

Support remaining life calculations and repair strategies.

TESTING METHODS

Testing Modalities

Click on each method to learn more

👁️ Visual Inspection & Basic Tools

Common techniques

Crack mapping and measurement
Delamination survey (hammer / chain drag)
Phenolphthalein test for carbonation depth
Cover meter (rebar locator)

Standards: ACI 201, BS EN 1504

🧪 Carbonation Depth

Phenolphthalein indicator

Spray fresh concrete fracture with phenolphthalein solution. Pink = alkaline (uncarbonated), colourless = carbonated.

Typical threshold: pH < 9 indicates risk to reinforcement.

⚡ Half-Cell Potential Mapping

ASTM C876

Measures corrosion potential of rebar relative to a reference electrode.

> -200 mV	Low corrosion risk
-200 to -350 mV	Intermediate
< -350 mV	High corrosion probability

📏 Electrical Resistivity

Concrete quality indicator

Lower resistivity = higher moisture / chloride / corrosion risk.

Typical ranges: <10 kΩ·cm = very high risk

📉 Linear Polarization Resistance (LPR)

Corrosion rate measurement

Gives quantitative corrosion current (μA/cm²) → corrosion rate in μm/year.

🧂 Chloride Profiling

Lab analysis on dust or cores

Acid-soluble or water-soluble chloride content vs. depth.

Critical threshold ~0.4% by cement weight for initiation.

🌊 Non-Destructive Testing (NDT)

UPV, GPR, Rebound Hammer, Thermography

Assess uniformity, voids, delaminations, and cover without damaging the structure.

COMPARISON

Method Comparison

Method	Destructive?	Quantitative?	Best For	Cost / Speed
Visual + Hammer	No	Qualitative	General condition	Very Low / Fast
Carbonation (Phenolphthalein)	Slightly	Yes	Carbonation front	Low / Fast
Half-Cell Potential	No	Probability	Corrosion likelihood	Medium / Medium
Resistivity	No	Yes	Permeability & risk	Medium / Fast
LPR / Corrosion Rate	No	Yes	Active corrosion rate	High / Medium
Chloride Profiling	Yes	Yes	Chloride ingress	Medium-High / Slow
GPR / UPV	No	Indirect	Internal defects	High / Medium

QUIZ

Knowledge Check

Question 1: Which test directly measures carbonation depth?