Concrete Repair and Restoration

Chapter Introduction

Concrete repair and restoration make up an important part of the concrete finisher's work, particularly in the context of aging infrastructure in Canada. This chapter covers the fundamental principles, techniques, materials, and procedures that every Red Seal exam candidate must master. You will find the applicable Canadian standards, essential calculations, common pitfalls, and practical exam tips.

Fundamental Principles of Concrete Repair

Why Does Concrete Deteriorate?

Concrete is durable, but it is not permanent. Deterioration results from physical, chemical, and mechanical causes, often combined. To repair properly, you must first understand the cause of the degradation; otherwise, the repair will fail prematurely.

Physical Causes:

Freeze-thaw cycles: water freezes in the pores of the concrete, creating internal pressure that causes spalling and cracking.

Abrasion: traffic, water carrying particles, equipment.

Thermal shrinkage: temperature variations creating tensile stresses.

Chemical Causes:

Carbonation: atmospheric CO₂ reacts with calcium hydroxide (Ca(OH)₂), reducing the surface pH (from ~13 to ~8.5) and exposing the reinforcement to corrosion.

Chloride attack (de-icing salts, seawater): Cl⁻ ions penetrate to the reinforcement, breaking the protective passive layer even if the pH remains high.

Alkali-aggregate reaction (AAR): reaction between cement alkalis and certain siliceous aggregates, producing an expansive gel that cracks the concrete.

Sulfate attack: formation of expansive ettringite in soils or waters rich in sulfates.

Mechanical Causes:

Overloading, impacts, differential settlement.

Plastic shrinkage cracking (in the first hours after placement).

Condition Assessment of Concrete

Before any repair, a complete assessment is required. This step determines the extent of the work, the materials to use, and the application method.

Assessment Method	Purpose	Remarks
Visual inspection	Identify cracks, spalling, rust stains	Mandatory first step
Sounding hammer	Detect delaminated areas (hollow sound)	Tap gently, listen for the sound
Carbonation depth measurement	Determine depth of carbonation	Use a phenolphthalein solution
Corrosion potential measurement	Locate active corrosion areas	Half-cell method (Cu/CuSO₄)
Core sampling	Evaluate strength, depth of damage	Destructive test, proceed with caution
Pull-off adhesion test	Measure adhesion of existing layers	ASTM C1583 standard

Carbonation depth: Phenolphthalein turns pink if pH > 8.5 (sound concrete) and remains colorless if pH < 8.5 (carbonated concrete). If the carbonation depth reaches the reinforcement, corrosion is likely.

Surface Preparation

Surface preparation is the most critical step in any repair. Poor preparation is the leading cause of repair failure. The surface must be sound, clean, rough, and moist (or saturated surface dry, depending on the product).

Preparation Methods:

Method	Advantages	Disadvantages
Manual chipping (hammer, chisel)	Simple, inexpensive	Slow, can damage sound concrete
Pneumatic scaling	Fast for large areas	Risk of microcracking
Shot blasting	Clean and rough surface	Specialized equipment
Hydrodemolition (high-pressure water jet)	Removes deteriorated concrete without microcracks	Water management, cost
Sandblasting (abrasive blasting)	Cleans and roughens	Dust, containment required
Chemical stripping	Removes coatings	Hazardous products, rinsing required

Golden rule: Remove all delaminated, contaminated, or carbonated concrete down to sound concrete. The edges of the cavities must be cut at right angles (or dovetailed) to avoid thin edges that will spall off.

Reinforcement Protection

When reinforcement is exposed, it must be cleaned and protected before applying the repair mortar.

30.Cleaning: Remove rust by sandblasting or wire brushing. Rust must be removed down to bright metal (SSPC-SP6 or SP10 standard depending on severity).

31.Passivation: Apply a passivation coating (cement-based modified or epoxy) to protect the steel against future corrosion.

32.Cover: Ensure the reinforcement has a minimum clear cover of 20 mm (or more depending on exposure) from the surface of the repair mortar.

Repair Materials

Repair Mortars

The choice of mortar depends on the application, the required thickness, the exposure conditions, and the available time.

Mortar Type	Typical Thickness	Advantages	Disadvantages
Portland cement-based mortar (sand-cement)	10–40 mm	Economical, familiar	High shrinkage, poor adhesion if improperly applied
Polymer-modified mortar (PCM)	5–50 mm	Excellent adhesion, low permeability	Higher cost
Epoxy resin-based mortar	5–25 mm	High strength, exceptional adhesion	High cost, different coefficient of expansion
Shotcrete (gunite / shotcrete)	50 mm and more	Ideal for large areas, good compaction	Specialized equipment, skill required
Self-leveling concrete	5–50 mm	Easy application, smooth surface	Requires carefully prepared surface

Key properties to verify:

Compressive strength (MPa) — must be equal to or greater than the original concrete.

Modulus of elasticity — must be compatible with the original concrete to avoid differential stresses.

Coefficient of thermal expansion — must be similar to that of the original concrete.

Shrinkage — must be minimal to avoid cracking.

Chloride permeability — must be low to protect the reinforcement.

Admixtures and Additives

Water retention agents: reduce water loss from the mortar, improve hydration.

Water reducers (plasticizers): improve workability without adding water.

Air-entraining agents: create microscopic air bubbles to resist freeze-thaw cycles (important for exterior structures in Canada).

Set accelerators: for rapid repairs (e.g., road repairs).

Corrosion inhibitors: added to the mortar or applied to the surface to protect the reinforcement.

Curing Compounds

Curing is essential for any cement-based repair. It maintains the moisture needed for cement hydration and reduces plastic shrinkage.

Curing Method	Application	Minimum Duration
Water (immersion, spraying)	Horizontal and vertical structures	7 days
Wet coverings (burlap, geotextile)	Horizontal surfaces	7 days
Curing membranes (film-forming compounds)	All surfaces	7 days
Steam (accelerated curing)	Precast production	3–24 hours

Canadian standard: Curing must comply with CSA A23.1/A23.2 (Concrete: Materials and Methods of Concrete Construction / Test Methods and Standard Practices for Concrete).

Repair Techniques

Crack Repair

Cracks are classified according to their width and their activity (active or inactive).

Crack Width	Classification	Recommended Treatment
< 0.1 mm	Hairline	No treatment required (aesthetic only)
0.1–0.3 mm	Fine	Epoxy injection or surface sealing
0.3–1.0 mm	Medium	Epoxy or polyurethane injection
> 1.0 mm	Wide	Injection, then monitoring; may require structural repair

Active cracks: If the crack continues to move (thermal, settlement), you must use a flexible product (polyurethane, sealant) and create a movement joint. Injecting rigid epoxy into an active crack will cause a new crack to form adjacent to it.

Inactive cracks: Epoxy injection is the method of choice to restore structural integrity.

Epoxy injection procedure:

60.Clean the crack (compressed air, pressurized water).

61.Install injection ports at intervals of 150–300 mm along the crack.

62.Seal the surface of the crack with a sealant (epoxy paste or cement).

63.Inject the epoxy at low pressure (0.1–0.3 MPa) starting from the lowest point.

64.Allow to cure according to the manufacturer's recommendations.

65.Remove the ports and grind the surface.

Repair of Cavities (Spalls, Delaminations)

General procedure:

68.Outline the area to be repaired (cut with a diamond blade or concrete saw).

69.Remove the deteriorated concrete (chipping, hydrodemolition).

70.Clean the reinforcement and apply a passivation product.

71.Moisten the surface (saturated surface dry — SSD).

72.Apply the repair mortar in layers (if the thickness exceeds 25 mm, apply in multiple passes).

73.Finish the surface (floating, troweling) to achieve the desired texture.

74.Cure immediately.

Maximum thickness per pass: For polymer-modified mortars, the maximum thickness per pass is generally 25–30 mm. Beyond that, you must apply in multiple passes or use shotcrete.

Shotcrete Repair

Shotcrete is used for large-scale repairs: bridges, tunnels, retaining walls, reservoirs.

Two methods:

Dry mix (gunite): the dry mix is transported by compressed air, water is added at the nozzle. Advantage: water control, longer reach.

Wet mix: the concrete is pumped in a wet state, air is added at the nozzle. Advantage: less rebound, better mix control.

Key requirements:

The surface must be prepared and moistened before application.

Application is done in successive passes of 25–50 mm.

Rebound (material that falls off) must not be reused.

Curing must begin as soon as the surface is firm enough.

Joint Repair

Expansion, contraction, and construction joints must be inspected and repaired regularly.

Types of joints:

Expansion joint: allows movement between two elements.

Contraction joint: controls shrinkage cracking.

Construction joint: interface between two successive pours.

Joint repair procedure:

93.Clean the joint (remove old sealant, debris).

94.Install a backer rod made of polyethylene foam to control the sealant depth.

95.Apply a primer if required.

96.Install the sealant (polyurethane, silicone, polysulfide) according to the width and expected movement.

97.Smooth the sealant surface.

Joint sizing: The joint width must be at least 4 times the expected movement. The sealant depth must be ½ to ⅔ of the width.

Essential Calculations

Calculating Repair Mortar Volume

Formula: Volume = Length × Width × Depth (in meters)

Example: A cavity of 0.5 m × 0.3 m × 0.04 m (average depth).

Volume = 0.5 × 0.3 × 0.04 = 0.006 m³ = 6 litres.

Conversion: 1 m³ = 1000 L. For a 30 kg bag of mortar producing approximately 13 L of fresh mortar, you will need: 6 L ÷ 13 L/bag ≈ 0.46 bags, therefore 1 bag.

Calculating Mortar Mix Proportions

For a Portland cement-based repair mortar, the typical mix ratio is 1 part cement to 3 parts sand (by volume), with a water-cement ratio (w/c) of 0.40 to 0.45.

Example: For 0.1 m³ of mortar (fresh volume):

Cement volume = 0.1 ÷ 4 = 0.025 m³ (≈ 37.5 kg of cement, density ≈ 1500 kg/m³)

Sand volume = 0.025 × 3 = 0.075 m³

Water = 0.40 × 37.5 kg = 15 L

Caution: The fresh mortar volume is less than the sum of the constituent volumes (mixing shrinkage). Use the manufacturer's data.

Calculating Corrosion Inhibitor Quantity

The typical dosage is 5 to 15 L per m³ of concrete depending on the product. For a repair of 0.5 m³:

Dosage at 10 L/m³: 0.5 × 10 = 5 L of inhibitor.

Calculating Carbonation Depth

Carbonation depth approximately follows a √t law (square root of time):

d = k × √t

Where:

d = carbonation depth (mm)

k = carbonation coefficient (mm/√year), typically 3–8 for average quality concrete

t = age of the concrete (years)

Example: For a 20-year-old concrete with k = 5 mm/√year:

d = 5 × √20 = 5 × 4.47 = 22.4 mm.

If the reinforcement cover is 25 mm, carbonation has almost reached the reinforcement — repair is imminent.

Applicable Canadian Standards

The candidate must be familiar with the following standards, which govern concrete repair and restoration in Canada:

Standard	Title	Relevant Content
CSA A23.1/A23.2	Concrete: Materials and Methods of Concrete Construction / Test Methods and Standard Practices for Concrete	Requirements for concrete, curing, testing
CSA A23.3	Design of Concrete Structures	Structural requirements for repairs
CSA S413	Parking Structures	Repair of parking structures
CSA S6	Canadian Highway Bridge Design Code	Bridge repair
CSA A3000	Masonry Bedding and Mortars	Masonry mortars (if applicable)
ASTM C1583	Standard Test Method for Tensile Strength of Concrete Surfaces and Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-off Method)	Evaluation of repair adhesion
ACI 546R	Guide for Concrete Repair	Reference guide (non-mandatory in Canada)

Important note: The Canadian Electrical Code, Part I, and CSA B149.1 (natural gas and propane) do not apply directly to concrete repair, but the finisher may be called upon to work near electrical conduits or gas lines. In these cases, the safety rules of these codes apply (e.g., Rule 8-200 of the Canadian Electrical Code for clearances).

Repair Procedures — Detailed Steps

Repair of a Spalled Concrete Slab (Complete Example)

Situation: Exterior parking slab with surface spalling (delamination) over an area of 2 m × 1.5 m, average depth of 15 mm. Reinforcement exposed in places.

Step 1 — Assessment:

Visual inspection: spalling, rust stains.

Sounding hammer: hollow areas identified.

Carbonation depth measurement: depth of 10 mm (reinforcement is at 25 mm — not yet reached, but close).

Step 2 — Preparation:

Outline with a diamond blade (cut at 90° to a depth of 15 mm).

Chip out the delaminated concrete down to sound concrete (final depth: 20–25 mm).

Clean the reinforcement by abrasive blasting (SSPC-SP6).

Apply a passivation product to the reinforcement.

Final cleaning: compressed air + pressurized water.

Moistening: saturate the surface, allow to drain (SSD).

Step 3 — Application:

Mix the polymer-modified mortar (follow the manufacturer's instructions: water, mixing time).

Apply in a single pass of 20 mm (maximum thickness for this product: 25 mm).

Float to compact and finish.

Final smoothing with a trowel.

Step 4 — Curing:

Apply a curing membrane (film-forming compound) immediately after finishing.

Duration: 7 days minimum.

Protect against rain and frost during curing.

Step 5 — Quality Control:

Visual inspection: no cracks, no voids.

Pull-off adhesion test after 28 days: minimum value of 1.0 MPa (or per the specification).

Pitfalls to Avoid

Here are the most frequent errors on the Red Seal exam on this topic:

157.Neglecting the cause of deterioration: Repairing without addressing the cause (e.g., repairing spalling without correcting drainage) leads to premature failure. The exam tests your ability to diagnose.

158.Insufficient surface preparation: Not removing all delaminated concrete, not cleaning the reinforcement, or leaving thin edges. Remember: preparation is 80% of a repair's success.

159.Confusing active and inactive cracks: Injecting rigid epoxy into an active crack will cause a new crack. Always identify the crack type before choosing the product.

160.Forgetting curing: Curing is often overlooked in answers. Without adequate curing (7 days), the repair mortar loses strength and cracks.

161.Wrong water-cement ratio choice: Too much water increases porosity and reduces strength. The w/c ratio must be as low as possible while maintaining workability.

162.Applying too thick a layer: Each product has a maximum thickness per pass. Beyond that, the mortar sags or cracks. Apply in multiple passes if necessary.

163.Ignoring weather conditions: Do not apply repair mortar in freezing temperatures (< 5 °C) or extreme heat (> 30 °C) without precautions. Fresh concrete freezes and loses its strength.

164.Confusing units: Always check the units (mm, m, MPa, L/m³). An error by a factor of 1000 is common in volume calculations.

165.Not knowing the standards: The Red Seal requires knowledge of CSA A23.1/A23.2 standards and ACI good practices. Memorize the titles and scopes of application.

166.Forgetting safety: Chipping, sandblasting, and epoxy injection present risks (silica dust, chemicals, noise). The exam may include questions on personal protective equipment (PPE).

Exam Tips

Read each question twice: Red Seal exam questions are often worded with traps (e.g., "What is the FIRST step?").

Logical reasoning: If a question is about a repair, think in order: diagnosis → preparation → material → application → curing → quality control.

Memorize key values: Maximum thickness per pass (25–30 mm), curing duration (7 days), w/c ratio (0.40–0.45), carbonation depth (√t law).

Know your standards: CSA A23.1/A23.2 is the reference standard for concrete in Canada. Know what it covers.

Use the right vocabulary: Technical terms (spalling, delamination, carbonation, passivation) are used on the exam. Master them.

Summary

Concrete repair begins with a complete diagnosis: identify the cause of deterioration before choosing the repair method.

The main causes of deterioration are freeze-thaw cycles, carbonation, chloride attack, AAR, and sulfates.

Surface preparation is the most critical step: remove all deteriorated concrete, clean the reinforcement, moisten the surface (SSD).

Repair mortars are selected based on thickness, exposure, and compatibility with the original concrete: Portland cement, polymer-modified, epoxy, shotcrete.

Curing (7 days minimum) is mandatory for any cement-based repair.

Active cracks require flexible products; inactive cracks can be injected with epoxy.

Calculations for volume, mix proportions, and carbonation are simple but must be done carefully (units!).

CSA A23.1/A23.2 standards govern concrete in Canada; the candidate must know their scope.

Common pitfalls: neglecting the cause, poor preparation, forgetting curing, wrong product choice, unit errors.

End of Chapter 8. This chapter covers the essentials for the Red Seal exam in concrete repair and restoration. Review the sections on standards and pitfalls before the exam. Good luck with your preparation!

Concrete Repair and Restoration