Restoration, Repair, and Cleaning of Masonry

Chapter Introduction

Restoration, repair, and cleaning of masonry constitute an essential part of the bricklayer's trade, particularly in the context of Canada's aging building stock. For the Red Seal exam, you must master not only new construction techniques but also intervention methods for existing structures. This chapter covers fundamental principles, assessment procedures, repair techniques, cleaning methods, and applicable Canadian standards. Particular attention is given to mix calculations, material compatibility, and common pitfalls that cause candidates to fail.

Assessment and Diagnosis of Existing Structures

Preliminary Inspection and Condition Survey

Before any intervention, you must conduct a systematic inspection of the structure. This step determines the nature and extent of the work. Examine the following elements:

Cracks: distinguish between shrinkage cracks (fine, superficial) and structural cracks (wide, through-wall, with displacement).

Efflorescence: whitish deposit on the surface, indicating the migration of soluble salts.

Spalling and crumbling: loss of surface material, often due to freeze-thaw cycles or poorly fired bricks.

Joint defects: hollowed, deteriorated, or missing joints.

Deformation or bulging: a sign of structural movement or internal pressure.

Moisture: presence of stains, mould, or saltpetre, indicating water infiltration.

For each defect, note its location, extent, depth, and progression. Use a field notebook and, if possible, a dimensioned sketch. Measure crack openings using a crack comparator (or fissurometer); record the width in millimetres (mm) and the approximate depth.

Non-Destructive Testing and Sampling

Non-destructive testing (NDT) allows you to assess the condition of the wall without damaging it:

Geologist's hammer: light tapping to detect hollow or delaminated areas (dull sound = void).

Moisture meter: measures the relative moisture content of the material.

Endoscopy: visual inspection of cavities and internal voids.

When necessary, take samples (cores) for laboratory analysis. These analyses determine the compressive strength of the bricks, the salt content, and the composition of the original mortar. This last piece of information is crucial for selecting a compatible repair mortar.

Analysis of Existing Mortar

The original mortar must be precisely identified. Historic mortars are often lime-based (air lime or hydraulic lime) with sand, sometimes with a small addition of cement. Chemical analysis (acid titration) determines the binder-to-sand ratio and the type of binder. As a general rule, the repair mortar must be weaker (less strong) than the brick, to allow moisture migration and avoid internal stresses. Mortar that is too hard traps water and causes the bricks to spall.

Material Compatibility Principles

Mechanical and Physical Compatibility Rule

The golden rule of restoration: the repair material must be compatible with the original material. This means:

Compressive strength: the mortar must not exceed the strength of the brick. A stronger mortar creates stress concentration points.

Modulus of elasticity: deformations under load must be similar to avoid cracking.

Water vapour permeability: the mortar must allow moisture to evaporate. An impermeable mortar (high cement content) traps water in the wall.

Coefficient of thermal expansion: materials must expand and contract at comparable rates to avoid detachment.

Types of Repair Mortars

The following table presents the standardized mortar types according to CSA A179 (Mortar and Grout for Unit Masonry):

Type	Composition (by volume)	Compressive Strength (MPa)	Recommended Use
S	1 cement : 2 lime : 9 sand	2.5 – 5.0	Repair of historic masonry, heritage joints
N	1 cement : 1 lime : 6 sand	5.0 – 10.0	Exterior exposed masonry, load-bearing walls
O	1 cement : 2 lime : 9 sand	2.5 – 5.0	Interior masonry, non-load-bearing
M	1 cement : 0.25 lime : 3 sand	17.0 – 25.0	Masonry in contact with soil, heavy loads

For restoration, types O and S are preferred, sometimes a pure lime mortar (Type K, not standardized in Canada but recognized in practice). Type N is acceptable for good-quality bricks. Avoid Type M except for specific structures (retaining walls).

Lime Mortars: Preparation and Application

The lime used is either air lime (CL 80 or CL 90) or hydraulic lime (NHL 2, NHL 3.5, or NHL 5). Natural hydraulic lime (NHL) hardens in the presence of water and is suitable for exposed joints. Air lime hardens slowly through carbonation (absorption of CO₂ from the air) and requires constant humidity.

Preparation of a typical lime mortar (by volume):

1 part NHL 3.5 hydraulic lime

3 parts clean, well-graded sand

Clean water, in sufficient quantity to achieve a plastic consistency

Setting time is long (several days to several weeks). Protect fresh joints from frost and rapid drying. The ideal temperature is between 5 °C and 25 °C.

Repair Techniques

Repointing

Repointing consists of replacing deteriorated mortar in the joints. It is the most common restoration operation. Procedure:

43.Raking out: remove the old mortar to a depth of at least 20 mm to 25 mm (or 2 to 2.5 times the joint thickness). Use a diamond-blade grinder or a chisel, being careful not to damage the brick edges.

44.Cleaning: brush the joints to remove dust and particles. Lightly dampen the joints before applying the mortar (the brick must not be saturated).

45.Application: fill the joint using a jointing gun or a trowel, compacting the mortar well. The mortar must be applied in successive layers if the depth exceeds 10 mm.

46.Finishing: after initial set (when the mortar leaves a fingerprint), smooth the joint with an appropriate tool (jointing iron, concave jointer, brushed joint). The profile must match the original joint.

47.Protection: keep the joint moist for 3 to 7 days (mist spraying) to ensure proper hydration.

Common mistake: repointing in hot, dry weather without dampening, which causes rapid shrinkage and hairline cracks.

Brick Replacement

When a brick is too deteriorated (spalling, frost damage, through-crack), it must be replaced. Procedure:

51.Extraction: cut the joints around the brick using a diamond-blade saw, then remove the brick with a chisel. Avoid hammering the brick itself to prevent damage to neighbouring bricks.

52.Bed preparation: clean the cavity, remove all residual mortar, and lightly dampen.

53.Installing the new brick: apply a mortar bed (compatible type), set the brick, and adjust its alignment with the existing wall. Fill the side and top joints.

54.Finishing: point and smooth as for repointing.

Choosing the replacement brick: it must have dimensions, colour, texture, and porosity similar to the original brick. If the original brick is no longer manufactured, look for a salvaged brick or a new brick with equivalent characteristics. Water absorption (%) must be comparable (immersion absorption test).

Crack Repair

The treatment depends on the nature of the crack:

Fine cracks (less than 0.5 mm): they are often superficial and non-structural. Careful repointing is sufficient.

Medium cracks (0.5 to 3 mm): clean the crack, dampen, and inject a cement grout or a fluid repair mortar using a syringe or pump. The grout must be compatible (low strength).

Large cracks (more than 3 mm): they indicate structural movement. Epoxy resin injection may be necessary, but this technique is reserved for structures where strength must be restored. In most cases, consult a structural engineer.

Note: never repair a crack without identifying its cause. If the cause (settlement, expansion, moisture) persists, the crack will reappear.

Corner and Edge Repair

Chipped corners can be repaired with a lime-based repair mortar or quick-setting cement for small areas. For edges, use a wooden or metal form to hold the mortar in place during setting. The mortar must be applied in thin layers (less than 10 mm) and dampened regularly.

Wall Consolidation

Consolidation is necessary when masonry loses its internal cohesion. Techniques:

Grout injection: to fill internal voids. The grout consists of hydraulic lime, fine sand, and water, sometimes with a small addition of white cement. Injection pressure must be low (less than 0.1 MPa) to avoid damaging the wall.

Tie rods and anchors: to stabilize walls that are leaning. Metal tie rods are installed through the wall and anchored at the ends. They must be stainless steel or protected against corrosion.

Underpinning: for failing foundations. This complex operation requires prior shoring and the involvement of an engineer.

Masonry Cleaning

General Principles

Cleaning must be as minimally invasive as possible. Overly aggressive cleaning damages the brick surface, increases porosity, and accelerates deterioration. Before choosing a method, identify:

The type of soiling: dust, soot, efflorescence, paint, graffiti, moss, lichen.

The type of brick: fired clay, concrete, calcium silicate, natural stone.

The surface condition: sound, spalled, frost-susceptible.

Cleaning Methods

The following table compares the main methods:

Method	Principle	Advantages	Disadvantages	Use
Manual brushing	Bristle or nylon brush	Gentle, controllable	Slow, ineffective on stubborn stains	Light soiling, fragile surfaces
Water cleaning (low pressure)	Water spray (less than 3.5 MPa)	Removes dust and salts	Does not remove adhered stains	Pre-washing, rinsing
Water cleaning (high pressure)	Water spray (3.5 to 10 MPa)	Fast, effective on moderate soiling	Risk of eroding joints and bricks	On hard, sound bricks
Chemical cleaning	Diluted hydrochloric acid (5 to 10%) or alkaline detergent	Removes efflorescence, cement stains, paint	Corrosive, must be neutralized	On resistant bricks, after testing
Abrasive blasting	Sand, microbeads, walnut shells	Removes stubborn stains and paints	Abrasive, risk of damage	On highly resistant surfaces, with caution
Steam cleaning	High-temperature water vapour	Gentle, eco-friendly	Slow, does not remove chemical stains	On fragile historic bricks

Chemical Cleaning: Precautions

Hydrochloric acid (muriatic acid) is commonly used to remove mortar residue and efflorescence. Mandatory rules:

Always dilute: concentrated acid must be poured into water (never the reverse) at a maximum concentration of 10% (1 part acid to 9 parts water).

Dampen the surface before application to prevent the acid from penetrating the pores.

Apply with a brush or low-pressure sprayer, allow to act for 5 to 10 minutes maximum.

Rinse thoroughly with clean water to neutralize the acid. Insufficient rinsing leaves salts that cause subsequent efflorescence.

Protect metal elements (windows, anchors) and fragile facing bricks.

Wear full PPE: rubber gloves, safety goggles, waterproof clothing, respiratory protection.

Prohibition: never use acid on calcium silicate bricks or limestone, as it dissolves them.

Abrasive Blasting (Sandblasting)

Sandblasting is reserved for extreme cases (thick paint, embedded soot). Use a soft abrasive (very fine silica sand, glass microbeads, crushed walnut shells) and low pressure (less than 0.3 MPa). Always test on an inconspicuous area. Sandblasting removes the fired skin of bricks, which increases their porosity and makes them vulnerable to frost.

Efflorescence Treatment

Efflorescence is white salts that appear on the surface. It results from the evaporation of water containing dissolved salts. Treatment:

91.Dry brushing with a bristle brush (do not wet, as this dissolves the salts and drives them deeper).

92.Rinsing with clean water after brushing, if necessary.

93.Application of a water repellent only after the wall has completely dried (several weeks). The product must be water-vapour permeable (siloxane or silane).

Prevention: efflorescence is a symptom. Treat the cause (water infiltration, rising damp) before applying a water repellent.

Calculations and Mix Proportions

Repair Mortar Mix Design

Mix proportions are done by volume (parts) or by mass (kg). For a Type N mortar (1:1:6), the proportions are:

1 volume of Portland cement

1 volume of hydraulic lime

6 volumes of sand

For a given mortar volume, calculate the quantities:

Example: You need to produce 0.5 m³ of Type N mortar. Approximate densities are: cement = 1,440 kg/m³, lime = 600 kg/m³, sand = 1,600 kg/m³.

Total number of parts = 1 + 1 + 6 = 8 parts.

Volume of one part = 0.5 m³ / 8 = 0.0625 m³.

Cement: 0.0625 m³ × 1,440 kg/m³ = 90 kg

Lime: 0.0625 m³ × 600 kg/m³ = 37.5 kg

Sand: 0.0625 m³ × 6 × 1,600 kg/m³ = 600 kg

Water: add approximately 20 to 25% of the total mass of binders (cement + lime). Here, binders = 90 + 37.5 = 127.5 kg. Water = 0.22 × 127.5 ≈ 28 litres. Adjust to achieve a plastic consistency.

Calculating the Joint Surface Area for Repointing

To estimate the quantity of mortar needed, calculate the volume of the joints. For a wall with surface area S (m²) with bricks of dimensions L × H × E (m) and joint thickness j (m):

Number of bricks per m² ≈ 1 / [(L + j) × (H + j)]

Mortar volume per m² ≈ (number of bricks) × (perimeter of one brick) × (joint depth) × (joint width)

Simplified example: bricks of 200 mm × 100 mm (0.2 m × 0.1 m), joints of 10 mm (0.01 m), repointing depth of 20 mm (0.02 m).

Number of bricks per m² = 1 / [(0.2 + 0.01) × (0.1 + 0.01)] = 1 / (0.21 × 0.11) = 1 / 0.0231 ≈ 43.3 bricks.

Perimeter of one brick = 2 × (0.2 + 0.1) = 0.6 m.

Joint volume per brick = 0.6 m × 0.02 m × 0.01 m = 0.00012 m³.

Total volume per m² = 43.3 × 0.00012 ≈ 0.0052 m³ (5.2 litres).

Add 10% for waste: 5.7 litres per m².

Calculating the Quantity of Replacement Bricks

Measure the surface area of bricks to be replaced (in m²) and multiply by the number of bricks per m² (calculated above). Add 5% for bricks broken during extraction.

Applicable Canadian Standards

CSA A179 – Mortar and Grout for Unit Masonry

This standard defines mortar types (S, N, O, M), their proportions, and their minimum strengths. It applies to new masonry and repair work. For restoration, refer to the tables in Annex A, which provide recommendations for repair mortars.

CSA A371 – Masonry Construction for Buildings

Standard CSA A371 covers the design and execution of masonry. It includes requirements on:

Substrate preparation

Brick installation

Expansion and control joints

Dimensional tolerances

For repair work, respect alignment and plumbness tolerances (maximum deviation of 10 mm over 3 m).

CSA S304 – Design of Masonry Structures

This standard addresses structural design. It is relevant when the repair affects the load-bearing capacity of the wall. Design strength values for bricks and mortar are defined here.

National Building Code of Canada (NBC)

The NBC, in Part 9 (Housing and Small Buildings) and Part 4 (Structural Design), imposes requirements on durability and moisture protection. For restoration, ensure that the work meets frost resistance requirements (bricks classified as F1, F2, or F3 based on their absorption) and water infiltration protection requirements.

Other Relevant Standards

CSA A82 – Fired clay bricks (classification, absorption, strength).

CSA A3001 – Hydraulic cements (types GU, MS, HE, etc.).

CSA A3002 – Hydraulic and air limes.

Safety and Protection

Personal Protective Equipment (PPE)

Safety glasses or face shield during cutting, chiselling, and chemical cleaning.

Gloves: leather for handling bricks, nitrile rubber for chemicals.

Respiratory protection: dust mask (N95) for grinding, filter cartridge for acid vapours.

Safety helmet on sites with falling object hazards.

Safety footwear with steel toe.

Risk Management

Working at height: use scaffolding compliant with CSA S269.2 (Access scaffolding for construction purposes). Check stability and guardrails.

Chemicals: read the Safety Data Sheets (SDS) before use. Store acids in labelled containers, away from bases.

Crystalline silica dust: grinding and sandblasting generate silica, a recognized carcinogen. Use tools with source extraction and wear an appropriate mask.

Asbestos: in older buildings (pre-1990), mortars and plasters may contain asbestos. When in doubt, have a sample analysed before any work.

Pitfalls to Avoid

152.Using mortar that is too strong: Type M mortar on historic bricks causes cracking and spalling. Always choose a mortar weaker than the brick.

153.Cleaning with acid without dampening: the acid penetrates the pores and causes permanent efflorescence.

154.Sandblasting fragile bricks: sandblasting removes the fired skin and accelerates frost deterioration.

155.Repointing without raking out sufficiently: a joint less than 20 mm deep will not hold.

156.Ignoring the cause of cracks: repairing a crack without addressing structural movement is useless.

157.Confusing air lime and hydraulic lime: air lime does not harden under water and requires CO₂.

158.Forgetting expansion joints: during repair, do not block existing expansion joints with mortar.

159.Neglecting winter protection: fresh mortar freezes and loses its strength. Protect the work with tarps and heaters if the temperature drops below 5 °C.

160.Not testing the cleaning method on a test area: every cleaning method must be tested on a small surface before general application.

161.Using dirty or hard water: mixing water must be clean and potable. Hard water (high in calcium) can cause efflorescence.

Summary

Diagnosis: inspect, measure, identify causes before repairing.

Compatibility: the repair mortar must be weaker than the brick (Types O or S for restoration).

Repointing: rake out to 20 to 25 mm, dampen, apply in layers, protect against frost.

Brick replacement: choose bricks with equivalent characteristics (absorption, strength, dimensions).

Cleaning: favour gentle methods (brushing, low-pressure water). Diluted hydrochloric acid (max 10%) is reserved for resistant bricks.

Efflorescence: dry brushing, rinsing, treat the cause, water repellent after drying.

Calculations: master volume mix proportions and joint surface area estimates.

Standards: CSA A179 (mortars), CSA A371 (execution), CSA S304 (structure), NBC (durability).

Safety: full PPE, silica management, asbestos vigilance.

Self-Assessment Questions

173.Which mortar type (S, N, O, M) is most appropriate for repointing a wall of old, low-strength bricks?

174.What is the minimum raking depth for repointing?

175.Why should you never pour water into concentrated acid?

176.What is the difference between air lime and hydraulic lime?

177.A wall shows cracks 5 mm wide. What is your approach?

178.Calculate the volume of mortar needed to repoint 25 m² of wall with 200 × 100 mm bricks and 10 mm joints (20 mm depth).

179.What are the three requirements of CSA A179 regarding repair mortars?

180.Why is sandblasting discouraged on historic bricks?

181.What is the role of a water repellent and when should it be applied?

182.What PPE is mandatory when cleaning with hydrochloric acid?

Answers: 1) Type O or S. 2) 20 to 25 mm. 3) The reaction is exothermic and violent, with risk of splashing. 4) Air lime hardens by carbonation (CO₂); hydraulic lime hardens by hydration (water). 5) Identify the cause (structural?), consult an engineer if necessary, inject grout or resin after stabilization. 6) 25 m² × 0.0052 m³/m² ≈ 0.13 m³ (plus waste). 7) Proportions, minimum strength, compatibility with masonry units. 8) It removes the fired skin and increases porosity. 9) It reduces water absorption while allowing vapour passage; apply after complete drying (several weeks). 10) Rubber gloves, sealed goggles, waterproof clothing, respiratory protection.

Restoration, Repair, and Cleaning of Masonry