Concrete Repair for London Properties: Spalling, Carbonation, Rebar Corrosion, and Specification Guide

CONCRETE IS INHERENTLY ALKALINE — FRESH CONCRETE HAS A pH OF APPROXIMATELY 12 TO 13. THIS HIGH ALKALINITY CREATES A PASSIVE PROTECTIVE FILM ON THE SURFACE OF THE EMBEDDED STEEL REINFORCEMENT (REBAR), PREVENTING CORROSION. CONCRETE DETERIORATION OCCURS WHEN THIS PROTECTIVE ALKALINITY IS REDUCED OR WHEN AGGRESSIVE IONS (PRIMARILY CHLORIDE) PENETRATE TO THE REBAR AND BREAK DOWN THE PASSIVE FILM.

CARBONATION: ATMOSPHERIC CARBON DIOXIDE (CO2) REACTS WITH CALCIUM HYDROXIDE IN THE CEMENT PASTE MATRIX TO FORM CALCIUM CARBONATE — REDUCING THE pH OF THE CONCRETE FROM 12+ DOWN TO APPROXIMATELY 8 TO 9. AT pH BELOW 9, THE PASSIVE FILM ON THE REBAR BREAKS DOWN AND CORROSION CAN INITIATE. THE "CARBONATION FRONT" MOVES PROGRESSIVELY INWARD FROM THE CONCRETE SURFACE OVER TIME — THE RATE DEPENDING ON CONCRETE POROSITY (DETERMINED BY WATER-TO-CEMENT RATIO) AND THE CO2 CONCENTRATION IN THE ENVIRONMENT. IN LONDON, WHERE TRAFFIC AND URBAN CO2 LEVELS ARE ELEVATED AND WHERE MUCH CONCRETE CONSTRUCTION FROM THE 1950S TO 1980S USED HIGHER w/c RATIOS THAN MODERN STANDARDS, CARBONATION DEPTHS OF 20 TO 40MM ARE COMMON IN 40-YEAR-OLD CONCRETE. ONCE CARBONATION REACHES THE REBAR (TYPICALLY AT 20 TO 25MM COVER IN POORLY SPECIFIED HISTORICAL CONCRETE), CORROSION INITIATES AND IRON OXIDE (RUST) EXPANDS, CAUSING THE CONCRETE COVER TO CRACK AND SPALL.

CHLORIDE ATTACK: CHLORIDES (FROM DE-ICING SALTS ON ROADS AND CAR PARKS, FROM MARINE ENVIRONMENTS, OR FROM CALCIUM CHLORIDE ADMIXTURES USED IN SOME OLDER UK CONCRETE) PENETRATE THE CONCRETE AND DESTROY THE PASSIVE FILM ON REBAR AT MUCH LOWER CONCENTRATIONS THAN CARBONATION. CHLORIDE-INDUCED CORROSION IS TYPICALLY MORE AGGRESSIVE AND FASTER-PROGRESSING THAN CARBONATION-INDUCED CORROSION. LONDON CAR PARKS (PARTICULARLY THOSE WHERE DE-ICING SALT FROM WINTER ROAD GRITTING IS TRACKED IN BY VEHICLES) ARE PARTICULARLY VULNERABLE TO CHLORIDE ATTACK.

SPALLING: THE VISIBLE RESULT OF REBAR CORROSION — THE EXPANSIVE IRON OXIDE (RUST, WHICH HAS 2.5 TO 6 TIMES THE VOLUME OF THE ORIGINAL IRON) FRACTURES THE CONCRETE COVER HORIZONTALLY ALONG THE LINE OF THE REBAR. PATCHES OF CONCRETE FALL AWAY, EXPOSING RUSTY REBAR. ONCE SPALLING STARTS, WATER FURTHER ACCELERATES CORROSION AND REPAIR BECOMES INCREASINGLY EXPENSIVE THE LONGER IT IS DEFERRED.

A SYSTEMATIC DIAGNOSIS IS ESSENTIAL BEFORE SPECIFYING CONCRETE REPAIR — THE CAUSE, EXTENT, AND SEVERITY OF DETERIORATION MUST BE UNDERSTOOD TO SELECT THE CORRECT REPAIR SYSTEM AND ENSURE THE REPAIR WILL LAST.

CARBONATION DEPTH TEST: SPRAY FRESHLY BROKEN CONCRETE WITH A PHENOLPHTHALEIN INDICATOR SOLUTION (A 1% SOLUTION IN ETHANOL). UNCARBONATD CONCRETE TURNS PINK/PURPLE; CARBONATED CONCRETE REMAINS COLOURLESS. THE DEPTH OF THE COLOURLESS ZONE MEASURED FROM THE SURFACE IS THE CARBONATION DEPTH. FOR STRUCTURAL ASSESSMENT, COMPARE CARBONATION DEPTH AGAINST MEASURED CONCRETE COVER TO REBAR (USING A COVERMETER — AN ELECTROMAGNETIC DEVICE THAT LOCATES REBAR AND MEASURES COVER DEPTH).

HALF-CELL POTENTIAL MAPPING: AN ELECTROCHEMICAL TECHNIQUE USING A COPPER-COPPER SULPHATE REFERENCE ELECTRODE ON THE CONCRETE SURFACE TO MAP THE ELECTRICAL POTENTIAL OF THE EMBEDDED REBAR. HIGH NEGATIVE POTENTIALS (MORE NEGATIVE THAN -350 MV CSE PER ASTM C876) INDICATE A HIGH PROBABILITY OF ACTIVE CORROSION. HALF-CELL MAPPING IS USED FOR LARGE STRUCTURES (BRIDGES, PARKING DECKS) WHERE VISUAL INSPECTION ALONE CANNOT IDENTIFY DELAMINATED (ABOUT-TO-SPALL) AREAS.

CHLORIDE CONTENT TESTING: CORE SAMPLES TAKEN FROM THE STRUCTURE ARE ANALYSED (BY VOLHARD TITRATION OR ION CHROMATOGRAPHY) AT DIFFERENT DEPTHS TO PRODUCE A CHLORIDE PROFILE. THE CHLORIDE THRESHOLD FOR CORROSION INITIATION IN NORMAL CONCRETE IS APPROXIMATELY 0.4% CHLORIDE BY WEIGHT OF CEMENT. WHERE CHLORIDE CONTENT APPROACHES OR EXCEEDS THIS THRESHOLD AT REBAR DEPTH, CORROSION IS LIKELY OR IMMINENT.

SCHMIDT HAMMER REBOUND TEST: A SIMPLE SITE TEST USING A SPRING-LOADED HAMMER THAT MEASURES THE REBOUND OF A PLUNGER FROM THE CONCRETE SURFACE — CORRELATED WITH COMPRESSIVE STRENGTH. USEFUL FOR LARGE-AREA ASSESSMENT OF CONCRETE QUALITY AND IDENTIFICATION OF WEAK OR DELAMINATED ZONES (LOW REBOUND). NOT A SUBSTITUTE FOR CORE TESTING FOR DEFINITIVE STRENGTH ASSESSMENT.

CONCRETE REPAIR IS A SPECIALIST ACTIVITY — THE MATERIALS AND APPLICATION METHODS MUST BE SELECTED AND IMPLEMENTED CORRECTLY FOR THE REPAIR TO PERFORM AND LAST. PROPRIETARY REPAIR MORTARS ARE STRONGLY PREFERRED OVER SITE-BATCHED CEMENT-SAND MORTARS FOR ALL STRUCTURAL CONCRETE REPAIR.

SIKA MONOTOP SYSTEM (SIKA AG): THE SIKA MONOTOP RANGE IS ONE OF THE MOST WIDELY SPECIFIED CONCRETE REPAIR SYSTEMS IN THE UK. THE SYSTEM INCLUDES: SIKA MONOTOP-610 (CORROSION PROTECTION COATING — APPLIED DIRECTLY TO CLEANED REBAR BEFORE REPAIR MORTAR APPLICATION — CONTAINS ZINC-RICH PRIMER AND ACTIVE CORROSION INHIBITORS). SIKA MONOTOP-612 (CEMENTITIOUS REPAIR MORTAR — ONE-COMPONENT, POLYMER-MODIFIED, FIBRE-REINFORCED MORTAR FOR HAND APPLICATION IN LAYERS UP TO 40MM). SIKA MONOTOP-620 (LARGER AGGREGATE MORTAR FOR DEEPER REPAIRS UP TO 100MM IN A SINGLE LAYER). THE SIKA MONOTOP SYSTEM IS DESIGNED TO THE EN 1504 SERIES (PRODUCTS AND SYSTEMS FOR PROTECTION AND REPAIR OF CONCRETE STRUCTURES — THE EUROPEAN STANDARD FOR CONCRETE REPAIR).

FOSROC NITOCOTE EP402 (EPOXY ANTI-CARBONATION COATING): FOSROC NITOCOTE EP402 IS AN EPOXY SURFACE COATING APPLIED OVER REPAIRED OR SOUND CONCRETE TO ARREST FURTHER CARBONATION PENETRATION AND PROVIDE A DECORATIVE FINISH. EPOXY COATINGS FOR CONCRETE CARBONATION PROTECTION ARE WIDELY USED IN CAR PARKS AND INDUSTRIAL ENVIRONMENTS. FOR EXPOSED EXTERNAL WALLS, BREATHABLE COATINGS (FOSROC DEKGUARD ELASTIC) ARE PREFERRED OVER EPOXY — EPOXY COATINGS TRAP MOISTURE IF APPLIED TO DAMP CONCRETE.

MAPEI MAPE-ANTIQUE (LIME-BASED REPAIR MORTARS FOR HISTORIC CONCRETE): FOR REPAIR OF HISTORIC OR DECORATIVE CONCRETE STRUCTURES WHERE MATCHING THE ORIGINAL SURFACE APPEARANCE IS IMPORTANT (E.G., PRECAST CONCRETE ARCHITECTURAL DETAILS ON 1930S ART DECO BUILDINGS IN LONDON), MAPEI'S MAPE-ANTIQUE RANGE PROVIDES LIME-HYDRAULIC NATURAL CEMENT (NHL) REPAIR MORTARS WITH COLOUR MATCHING.

BEFORE ANY REPAIR MORTAR IS APPLIED OVER CORRODING REBAR, THE REBAR MUST BE PROPERLY TREATED.

CONGRESS STRUCTURAL REPAIR SPECIFICATION (EN 1504, PRINCIPLE 7C — PRESERVING OR RESTORING PASSIVITY): THE INTERNATIONAL STANDARD FOR CONCRETE REPAIR (EN 1504) PROVIDES A FRAMEWORK OF PROTECTION AND REPAIR PRINCIPLES. FOR REBAR CORROSION DUE TO CARBONATION OR CHLORIDE, THE KEY STEPS ARE:

1. BREAK OUT ALL DELAMINATED AND CONTAMINATED CONCRETE BEYOND THE CARBONATION FRONT / CHLORIDE CONTAMINATION ZONE — TYPICALLY TO A DEPTH OF 20 TO 25MM BEHIND THE REBAR (CHASING OUT "BEHIND" THE BAR). CONCRETE MUST BE REMOVED TO A MINIMUM EDGE DEPTH OF 5MM BEHIND THE REBAR (TO ALLOW THE REPAIR MORTAR TO FULLY ENCAPSULATE THE BAR). THIS REMOVAL IS TYPICALLY DONE BY HAND CHISEL, PNEUMATIC CHISEL, OR HYDRODEMOLITION (HIGH-PRESSURE WATER JET — PREFERRED FOR LARGE AREAS AS IT REMOVES CONTAMINATED CONCRETE WITHOUT DAMAGING SOUND CONCRETE OR THE REBAR).

2. REBAR CLEANING: EXPOSED REBAR MUST BE CLEANED TO SA 2.5 (NEAR WHITE BLAST CLEAN) PER BS EN ISO 8501-1 — REMOVING ALL VISIBLE RUST, MILL SCALE, AND CONTAMINATION. IN PRACTICE, ON SITE THIS IS DONE BY ANGLE GRINDER WITH A WIRE WHEEL OR CUP BRUSH (FOR ACCESSIBLE REBAR) OR SAND BLASTING (FOR LARGE AREAS).

3. REBAR PRIMER APPLICATION: SIKA MONOTOP-610 CORROSION PROTECTION COATING OR EQUIVALENT (FOSROC NITOPRIME ZINCRICH+ OR MAPEI MAPEFER) IS APPLIED TO CLEANED REBAR — TYPICALLY TWO COATS, PROVIDING A PHYSICAL BARRIER AND ELECTROCHEMICAL PROTECTION (ZINC-RICH PRIMER) AGAINST RE-INITIATION OF CORROSION.

4. BONDING BRIDGE APPLICATION: A SLURRY COAT (SBR-MODIFIED CEMENT SLURRY — SIKA MONOTOP-610 OR FOSROC RENDEROC S) IS APPLIED TO THE CONCRETE SUBSTRATE BEFORE THE REPAIR MORTAR IS PLACED — ENSURES GOOD ADHESION BETWEEN EXISTING CONCRETE AND REPAIR MORTAR.

CATHODIC PROTECTION FOR LARGE-SCALE CHLORIDE-CONTAMINATED CONCRETE: WHERE CHLORIDE LEVELS ARE HIGH ACROSS A LARGE AREA (E.G., A MULTI-STOREY CAR PARK DECK), IMPRESSED CURRENT CATHODIC PROTECTION (ICCP) OR GALVANIC CATHODIC PROTECTION SYSTEMS MAY BE THE MOST COST-EFFECTIVE LONG-TERM SOLUTION — THESE SYSTEMS APPLY A SMALL ELECTRICAL CURRENT TO THE REBAR TO SUPPRESS CORROSION WITHOUT PHYSICAL REMOVAL OF CONTAMINATED CONCRETE.

THE FOLLOWING SCENARIOS ARE COMMON IN LONDON RENOVATION AND EACH REQUIRES A SPECIFIC CONCRETE REPAIR APPROACH.

SPALLING CONCRETE LINTELS IN VICTORIAN TERRACE: MANY LONDON VICTORIAN TERRACES HAVE REINFORCED CONCRETE LINTELS (POURED IN SITU CIRCA 1900-1950) ABOVE WINDOW AND DOOR OPENINGS THAT ARE NOW SPALLING DUE TO CARBONATION. REPAIR APPROACH: BREAK OUT SPALLED CONCRETE AND SOUND DELAMINATED AREAS, CLEAN REBAR TO SA 2.5, APPLY SIKA MONOTOP-610 PRIMER, APPLY SIKA MONOTOP-612 REPAIR MORTAR IN LAYERS (MAX 40MM PER LAYER) UNTIL FLUSH WITH ORIGINAL PROFILE, SEAL WITH FOSROC DEKGUARD ELASTIC OR APPROVED SILICONE WALL COATING. IF THE LINTEL IS STRUCTURALLY INADEQUATE, TEMPORARY PROPPING MAY BE REQUIRED AND A STRUCTURAL ENGINEER'S ADVICE SHOULD BE SOUGHT BEFORE AND DURING THE WORK.

CONCRETE BASEMENT RETAINING WALL REPAIR: FOR WATER-PENETRATING AND SPALLING CONCRETE BASEMENT RETAINING WALLS IN LONDON, THE REPAIR MUST ADDRESS BOTH THE STRUCTURAL CONCRETE DEFICIENCY AND THE WATERPROOFING FUNCTION. APPROACH: APPLY NEWTON SYSTEM 500 OR SIKA WATER BAR INJECTION RESIN GROUTING TO ACTIVE CRACK/WATER INGRESS POINTS, REPAIR CONCRETE FACE WITH SIKA MONOTOP MORTAR, THEN APPLY A CEMENTITIOUS TANKING SYSTEM (NEWTON WATERPROOF RENDER 3-COAT) OVER THE REPAIRED SURFACE.

CONCRETE FLAT ROOF REPAIR: FOR LONDON PROPERTIES WITH REINFORCED CONCRETE FLAT ROOFS (OFTEN 1950S-1970S RESIDENTIAL BLOCKS), CONCRETE REPAIR ABOVE OCCUPIED SPACES REQUIRES CAREFUL STRUCTURAL ASSESSMENT AND TEMPORARY WATERPROOFING BEFORE REPAIR MORTAR IS APPLIED.

CONCRETE REPAIR COSTS IN LONDON VARY SIGNIFICANTLY BASED ON THE EXTENT OF DETERIORATION, ACCESS REQUIREMENTS, AND SPECIFICATION LEVEL.

SPALLED CONCRETE LINTEL REPAIR (SMALL SCALE, 1 TO 3 LOCATIONS, ACCESSIBLE FROM GROUND LEVEL OR SCAFFOLD ALREADY ERECTED): DIAGNOSIS (PHENOLPHTHALEIN TEST + VISUAL SURVEY): £200 TO £500. REPAIR MATERIALS (SIKA MONOTOP SYSTEM PER LINTEL): £50 TO £150 MATERIALS. LABOUR (2 TO 4 HOURS PER LINTEL AT LONDON SPECIALIST RATES): £300 TO £600. TOTAL PER LINTEL: £400 TO £900 INCLUDING MAKING GOOD ADJACENT DECORATION.

CONCRETE COLUMN OR BEAM REPAIR (EXPOSED STRUCTURAL ELEMENT, REQUIRING SCAFFOLD OR MEWP ACCESS): ENGINEER ASSESSMENT + SPECIFICATION: £500 TO £2,000. REPAIR WORKS (PER LOCATION, INCLUDING BREAKOUT, REBAR TREATMENT, MORTAR APPLICATION): £800 TO £3,000 DEPENDING ON AREA AND ACCESS.

CAR PARK DECK OR LARGE AREA REPAIR (MULTI-STOREY CAR PARK, SPECIALIST CONTRACTOR, HYDRODEMOLITION): SPECIALIST CONCRETE REPAIR CONTRACTOR RATES FOR LARGE AREAS: £150 TO £350 PER M² FOR PATCH REPAIR (BREAKOUT + REBAR TREATMENT + PROPRIETARY MORTAR). EPOXY OVERLAY FOR CAR PARK DECK (SIKA COMFORTFLOOR OR FOSROC NITOCOTE EP402 SYSTEM): £25 TO £60 PER M².