Abstract
Cracking is the most common visible symptom of distress in concrete and masonry construction, and also the most frequently misinterpreted. Concrete and masonry are strong in compression but inherently weak in tension, so some degree of cracking is an expected characteristic of these materials rather than an automatic indication of failure. At the same time, certain crack types — horizontal cracks in foundation walls, diagonal shear cracks in beams, or displaced stair-step cracks in masonry — can be the first outward sign of a serious structural deficiency.
This white paper provides a systematic overview of cracking in three building element groups: reinforced and plain concrete elements; concrete and concrete masonry unit (CMU) foundation walls; and brick veneer and loadbearing brick masonry walls. For each crack type, the paper describes the characteristic pattern and orientation, typical crack widths, the underlying causation, whether the crack is generally structural or non-structural, and appropriate repair strategies.
Two principles frame the discussion. First, a crack is a symptom, not a diagnosis: durable repair is only possible once the mechanism that produced the crack has been identified and, where the mechanism is ongoing, arrested. Second, no single attribute — width, length, or location — is sufficient on its own to classify a crack; reliable assessment integrates crack pattern, width, taper, displacement, age, activity, and the load path and exposure conditions of the element.
1. Introduction
The practical challenge for building owners, property managers, insurers, and contractors is distinguishing between benign, non-structural cracking and cracking that signals an active structural or geotechnical problem. The discussion draws on established technical literature, including American Concrete Institute (ACI) committee reports, Building Research Establishment (BRE) guidance, classical journal and conference papers on foundation movement and building damage, and authoritative reference texts on concrete and masonry behaviour.
2. Crack Width: Measurement, Significance, and Classification
Crack width is the most readily measured attribute of a crack and the usual starting point of any assessment. Widths are measured with a crack comparator card, an optical crack microscope (accuracy of approximately 0.02 mm), or calibrated crack gauges, and should be recorded at several points along the crack because width commonly varies along its length. Width also varies seasonally and diurnally with temperature and moisture, so classifying a crack as dormant (stable) or active (progressing or cyclically moving) frequently requires monitoring over weeks or months.
2.1 Descriptive Width Terminology
Cracks narrower than approximately 0.1 mm are described as hairline and are generally of no structural consequence in themselves; cracks up to about 1 mm are described as fine; cracks of 1 mm to 5 mm are moderate; and cracks wider than 5 mm are wide and warrant engineering attention regardless of cause.
2.2 Damage Classification for Buildings (BRE Digest 251)
For low-rise buildings and masonry walls, the most widely used severity framework is the six-category damage classification originally developed by Burland, Broms and de Mello (1977) and codified in BRE Digest 251. The framework is summarized in Table 1. Its central lesson is that visible crack width is used only as a convenient index: the categories are actually defined by ease of repair and functional consequence, and the assessor must also account for crack number, location, and evidence of distortion.
Category Severity Approx. crack width Typical description / repair implication
0 Negligible < 0.1 mm Hairline cracks; no action required. 1 Very slight ≤ 1 mm Fine cracks treated during normal redecoration. 2 Slight ≤ 5 mm Cracks easily filled; may be visible externally; doors/windows may stick slightly; repointing may be required. 3 Moderate 5–15 mm (or several > 3 mm) Cracks require opening up and patching by a mason; service pipes may fracture; weather-tightness often impaired. 4 Severe 15–25 mm Extensive repair involving breaking-out and replacing wall sections; distorted openings; walls lean or bulge noticeably. 5 Very severe > 25 mm Major repair or partial/complete rebuilding; beams lose bearing; danger of instability.
2.3 Durability-Based Width Limits for Reinforced Concrete
For reinforced concrete, tolerable crack widths at the tension face under service loads are governed primarily by exposure conditions, because crack width influences the ingress of moisture, chlorides, and carbon dioxide toward the reinforcement. These are guides to reasonable serviceability rather than strict pass/fail criteria; the correlation between surface crack width and long-term corrosion is weaker than once assumed, with concrete cover quality and depth being at least as influential.
Exposure condition Tolerable crack width
Dry air or protective membrane 0.41 mm (0.016 in.) Humidity, moist air, soil 0.30 mm (0.012 in.) De-icing chemicals 0.18 mm (0.007 in.) Seawater and seawater spray; wetting and drying 0.15 mm (0.006 in.) Water-retaining structures 0.10 mm (0.004 in.)
3. Cracking in Concrete Elements
Cracks in concrete are conventionally divided into those that occur before hardening (plastic-state cracks) and those that occur after hardening (drying shrinkage, thermal, chemical, and load-induced cracks). The distinction matters because plastic-state cracks are almost always non-structural, whereas hardened-state cracks span the full range from cosmetic to critical.
3.1 Plastic Shrinkage Cracks
Pattern and width. Plastic shrinkage cracks appear on horizontal surfaces — slabs, toppings, slabs-on-grade — within roughly one to six hours of placement. They are typically straight or slightly ragged, parallel to one another, spaced 0.3 m to 1.0 m apart, often oriented diagonally across the slab, and rarely extend to the slab edges. Surface widths of 0.5 mm to 3 mm are common, but depth is usually shallow (25–50 mm) and the cracks are discontinuous.
Figure 1 — Plastic shrinkage cracking in a slab-on-grade (plan view) Causation. These cracks form when the rate of surface evaporation exceeds the rate at which bleed water rises, so the surface dries and contracts while the concrete is still plastic and has essentially no tensile strength. Hot, dry, windy conditions are the classic trigger; evaporation rates above approximately 1.0 kg/m² per hour are flagged as critical.
Structural significance and repair. Plastic shrinkage cracks are non-structural, but in exposed or trafficked slabs they can admit water and chlorides and may propagate with subsequent drying shrinkage. Dormant cracks in exterior slabs should be routed and sealed or filled with a low-viscosity polymer; in interior slabs of no durability concern they may be left or cosmetically filled. Prevention — windbreaks, fog sprays, evaporation retarders, and prompt curing — is far more economical than repair.
3.2 Plastic Settlement Cracks
Pattern and width. Plastic settlement cracks mirror the layout of restraint within fresh concrete: they appear directly above reinforcing bars, form ties, or abrupt changes in section depth, typically within the first few hours after placement, with widths from hairline up to about 1 mm and, importantly, a void or weakened zone beneath the restraining bar.
Figure 2 — Plastic settlement cracking over a top reinforcing bar (cross-section) Causation. As plastic concrete bleeds and consolidates, solids settle; where settlement is locally prevented by a rigid inclusion such as a top reinforcing bar, the concrete above the bar is held up while adjacent concrete subsides, tearing the surface. Deep sections, high slumps, low cover, and inadequate vibration or revibration aggravate the phenomenon.
Structural significance and repair. The cracks themselves are non-structural, but because they trace the reinforcement and may be accompanied by voiding under the bars, they create a preferential corrosion path. In exposed structures they should be repaired by epoxy injection or by routing and sealing, and cover-zone quality should be verified.
3.3 Drying Shrinkage Cracks
Pattern and width. Drying shrinkage cracking is the most widespread crack type in hardened concrete. In slabs it produces roughly straight cracks subdividing large panels, frequently initiating at re-entrant corners, penetrations, and saw-cut joints that were installed too late or too shallow. In walls it produces vertical cracks at fairly regular spacing — often at mid-length between restraints and radiating from window and door corners — commonly extending through the full thickness. Widths are typically 0.1 mm to 1 mm and relatively uniform along the crack.
Causation. Hardened concrete loses adsorbed water to the environment and contracts by roughly 400 to 800 microstrain over months to years; where that contraction is restrained — by the subgrade, by footings and intersecting walls, or by reinforcement — tensile stress develops and cracks form when it exceeds the tensile strength. High water content, high paste volume, poorly graded aggregate, and inadequate curing all increase shrinkage magnitude.
Structural significance and repair. Drying shrinkage cracks are non-structural in the sense that they are not caused by external load, and once moisture equilibrium is reached they become dormant. Their significance is chiefly serviceability: leakage through foundation walls and water-retaining elements, and reinforcement exposure in aggressive environments. Dormant shrinkage cracks are effectively repaired by epoxy injection where structural continuity or rigidity is desired, by polyurethane injection where the crack is leaking, or by routing and sealing where flexibility and water-tightness are the goal.
3.4 Early-Age Thermal Cracks
Pattern and width. Thermal cracks are characteristic of thick sections and long restrained walls: vertical cracks in walls restrained along their base (foundations, retaining walls), often widest at mid-height or at the base and appearing days to a few weeks after casting; in mass placements, wide surface cracks over a heated core.
Causation. Cement hydration is exothermic. The interior of a placement heats and expands relative to cooler surfaces, and as the whole element subsequently cools toward ambient it contracts against external restraint. When the restrained thermal contraction strain exceeds the tensile strain capacity of the young concrete, through-cracks form. Walls cast on mature footings are a textbook case: the footing restrains the cooling wall, producing vertical cracks at intervals roughly one to two times the wall height.
Figure 3 — Restrained drying-shrinkage and early-age thermal cracking in a wall cast on a mature footing Structural significance and repair. Early-age thermal cracks are usually not load-induced, but through-thickness thermal cracks in liquid-retaining or below-grade walls compromise water-tightness, and in heavily loaded elements they can reduce shear friction capacity. Repair follows the drying-shrinkage logic: epoxy injection for dormant cracks requiring monolithic behaviour, flexible injection or sealing where movement continues.
3.5 Crazing (Map Cracking)
Crazing is a network of very fine, shallow, hexagonal surface cracks — individual cells 10 mm to 75 mm across, widths well under 0.1 mm, and depths of a few millimetres — on troweled or formed surfaces. It results from minor surface shrinkage of a weak, laitance-rich skin, typically caused by over-trowelling, premature finishing of bleed water, or rapid early surface drying. Crazing is cosmetic; no structural repair is required, although penetrating sealers can reduce moisture ingress and improve appearance. It must, however, be distinguished from the superficially similar map cracking of alkali–silica reaction, which is progressive and structurally significant.
Figure 4 — Map-pattern cracking: (a) benign surface crazing; (b) progressive alkali–silica reaction (ASR) 3.6 Corrosion-Induced Cracking
Pattern and width. Corrosion-induced cracks run parallel to and directly over the reinforcement, most often at edges, soffits, and other low-cover locations. They are typically accompanied by rust staining bleeding from the crack, hollow-sounding delaminated zones detectable by hammer or chain drag, and eventually spalling that exposes corroded bars. Widths increase progressively with time.
Figure 5 — Corrosion-induced cracking, delamination, and spalling along a reinforcing bar (cross-section) Causation. Reinforcing steel in sound concrete is protected by a passive oxide film maintained by the high alkalinity of the pore solution. That protection is destroyed either by carbonation of the cover concrete or by chloride ingress from de-icing salts or marine exposure. The resulting corrosion products occupy two to six times the volume of the parent steel, generating internal bursting pressures that crack the cover along the bar.
Structural significance and repair. This is a structurally significant and progressive mechanism: it reduces bar cross-section, bond, and anchorage, and in advanced states has caused element failures. Repair must address the cause, not merely the crack: remove delaminated and chloride-contaminated concrete to beyond the bar, clean or supplement the reinforcement, reinstate with low-shrinkage repair mortar or concrete, and control future corrosion through membranes, sealers, galvanic anodes, or impressed-current cathodic protection where contamination is extensive. Simply injecting the cracks with epoxy without removing the corrosion cell is a recognized failure mode of repairs.
3.7 Alkali–Silica Reaction (ASR) Cracking
Pattern and width. ASR produces map (pattern) cracking on unrestrained surfaces; where the element is restrained or heavily reinforced in one direction, the cracks align with the restraint. Distinguishing features include dark reaction rims around aggregate particles, white silica-gel exudations at cracks, and surface discolouration. Cracks widen over years, commonly reaching several millimetres.
Causation. ASR is a chemical reaction between the alkaline pore solution of the cement paste and reactive amorphous or strained silica in certain aggregates. The reaction product is a hygroscopic gel that absorbs water and swells, generating internal expansion. Three ingredients are required simultaneously: sufficient alkalis, reactive aggregate, and moisture (relative humidity above roughly 80 percent).
Structural significance and repair. ASR is structurally significant because the expansion is bulk and ongoing: it can cause misalignment of equipment and bearings, closure of joints, and reduction in concrete tensile properties. Management options include restricting moisture access (drainage, coatings, cladding), sealing cracks against water, in some cases lithium treatment, and structural strengthening or confinement of critical members; monitoring of expansion is essential to confirm whether the reaction is continuing.
3.8 Freeze–Thaw and D-Cracking
Pattern and width. Freeze–thaw distress presents as surface scaling and mortar flaking, crumbling at edges, and — in pavements and slabs with susceptible coarse aggregate — "D-cracking": closely spaced crescent-shaped cracks paralleling joints and free edges where the concrete stays wettest. In vertical elements such as porch foundations, parapets, and retaining walls, freeze–thaw acts as a powerful accelerant of pre-existing cracks: water enters a crack, freezes, expands by approximately nine percent, and wedges the crack progressively wider each cycle.
Causation. Freezing of water in the capillary pore system generates hydraulic and osmotic pressures as ice forms and unfrozen water is expelled through the paste; when these pressures exceed the paste tensile strength, microcracking accumulates. Non-air-entrained concrete, high water–cement ratios, and critical saturation are the governing risk factors. In climates such as Ottawa's, structures may experience dozens of freeze–thaw cycles per year, and de-icing salts compound the damage by increasing surface saturation and adding chloride-driven corrosion.
Structural significance and repair. Freeze–thaw damage is progressive and, if unchecked, structurally consequential through loss of section. Repair requires removal of disintegrated concrete, reinstatement with air-entrained, low-permeability material, sealing of cracks and surfaces against water ingress, and — critically — correction of the moisture source: drainage, slope, flashing, and joint sealing.
3.9 Load-Induced (Structural) Cracks: Flexure, Shear, and Bearing
Flexural cracks form perpendicular to the tension face of a member in bending — vertical cracks in the bottom of a simply supported beam near midspan, or in the top over continuous supports — widest at the tension face and tapering toward the neutral axis. In properly designed reinforced members, flexural cracks at service load are an expected consequence of the design philosophy; widths noticeably exceeding guideline values, or cracks accompanied by visible deflection, indicate overload, under-reinforcement, or loss of reinforcement section and require structural evaluation.
Shear (diagonal tension) cracks form at roughly 45 degrees to the member axis in webs near supports, often initiating from a flexural crack and turning diagonally toward the load. Because shear failure of concrete members can be sudden and brittle, any pronounced diagonal crack in a beam, girder, or deep foundation element should be treated as structurally significant and referred for engineering assessment without delay.
Figure 6 — Load-induced crack patterns in a simply supported beam: flexural and diagonal shear cracks Bearing and bursting cracks occur under concentrated loads — posts, beam seats, anchorages — as vertical splitting cracks radiating from the load point. In residential construction, a classic example is vertical cracking in a foundation or porch wall directly beneath a roof-support post where the load has no adequate footing beneath it; the crack maps the compression strut splitting the unreinforced concrete. These cracks are structural and typically indicate a load-path deficiency that must be corrected, not merely sealed.
4. Cracking in Foundation Walls
Foundation walls — cast-in-place concrete or CMU — combine every mechanism described above with soil–structure interaction: differential settlement, lateral earth and hydrostatic pressure, frost action, and expansive or collapsible soils. Crack orientation is the single most informative attribute in a foundation wall.
Figure 7 — Characteristic crack orientations in foundation walls and their typical significance 4.1 Vertical Cracks
Near-vertical cracks of uniform width (typically 0.1–0.5 mm) occurring near the mid-length of a wall panel, at form-tie locations, or at the corners of window openings are overwhelmingly restrained shrinkage or early-age thermal cracks. They usually appear within the first year, stabilize, and are non-structural; their practical consequence is water leakage, for which polyurethane or epoxy injection from the interior is a reliable repair. Two variants change the diagnosis: a vertical crack that is distinctly wider at the top than the bottom (or vice versa) indicates differential vertical movement — settlement or frost heave — of one wall segment relative to the other; and a vertical splitting crack directly beneath a concentrated load indicates a bearing deficiency. Both variants are potentially structural and warrant investigation of the founding conditions.
4.2 Diagonal Cracks
Diagonal cracks in foundation walls are the signature of differential foundation movement. When one portion of a footing settles (or heaves) relative to another, the wall spans the differential like a deep beam and cracks diagonally, with the crack generally pointing toward the zone of greater movement and tapering — wider at one end than the other. Common causal scenarios include consolidating fill or compressible clay beneath part of the footing, moisture-driven shrink–swell of clay soils around large trees or leaking drains, frost heave of inadequately protected footings, and adjacent excavation. Diagonal cracks radiating from the corners of basement window openings are a partial exception: openings concentrate shrinkage stress, and narrow, uniform-width diagonal cracks at these locations are often non-structural. Active or tapered diagonal cracking is structural in origin and requires geotechnical-structural assessment before repair; sealing the crack without addressing the movement guarantees recurrence.
4.3 Horizontal Cracks
A horizontal crack in a below-grade foundation wall is the most serious common crack pattern in residential and light-commercial construction and should always be treated as structural until proven otherwise. The wall spans vertically between the basement slab and the floor diaphragm; lateral soil pressure produces maximum bending near mid-height, and a horizontal crack there — frequently accompanied by measurable inward bowing — indicates that the wall's flexural capacity has been reached.
Figure 8 — Mechanism of horizontal cracking in a below-grade foundation wall under lateral pressure (cross-section) Recognized causal mechanisms include excessive lateral earth pressure from saturated or surcharged backfill, hydrostatic pressure due to failed perimeter drainage, frost action and adfreeze on the backfill face, expansive clays, and construction-stage damage from compaction equipment or premature backfilling before the floor restraint was in place. In CMU walls the crack follows a bed joint at or below grade level, commonly between the second and fourth courses below grade in frost-affected climates.
Horizontal cracking must be distinguished from a horizontal cold joint — a plane between successive concrete lifts cast too far apart in time. Cold joints are planes of weakness and potential leakage rather than load-induced cracks; they are typically visible as a change in texture or colour across a straight, full-length horizontal line and are frequently the first location to open under subsequent shrinkage, frost wedging, or minor movement. A leaking or weathering cold joint merits sealing and moisture management; a horizontal crack with inward displacement merits structural intervention.
4.4 Stair-Step Cracks in CMU Foundation Walls
In concrete block walls, tensile cracks follow the path of least resistance — the mortar joints — producing the familiar stair-step (stepped) diagonal pattern. Stepped cracks that taper and align with other movement indicators reflect differential settlement or heave; stepped cracking combined with a horizontal bed-joint crack and inward bulging reflects lateral pressure failure; narrow, uniform stepped cracks confined to a small area may be shrinkage of the units themselves (CMU, unlike clay brick, shrinks after manufacture).
5. Cracking in Brick Veneer and Masonry Walls
Masonry crack diagnosis rests on one material fact with far-reaching consequences: fired clay brick undergoes permanent, irreversible moisture expansion over its service life (of the order of 0.02 to 0.09 percent), while concrete, CMU, and mortar shrink. Whenever brickwork is attached to, supported by, or confined by concrete elements, these opposing movements must be accommodated by movement joints and flexible connections; where they are not, the wall accommodates them by cracking.
5.1 Stair-Step Cracks from Differential Settlement
Stepped cracks following the mortar joints, tapering in width and typically emanating from the corners of openings or from the base of the wall, indicate differential foundation movement exactly as described for foundation walls. The direction of taper and the offset across the crack identify which portion of the structure has moved.
Figure 9 — Stair-step cracking in brick masonry caused by differential foundation settlement Stepped cracks up to about 5 mm are generally repairable by repointing once movement has stabilized, while wider or active cracks call for investigation and stabilization of the foundation first.
5.2 Vertical Cracks from Restrained Brick Expansion
Long brick walls and veneer elevations constructed without adequate vertical movement (expansion) joints develop essentially vertical cracks, characteristically near corners, offsets, and long uninterrupted runs, as cumulative moisture and thermal expansion of the brickwork is restrained. At external corners, expansion of the two intersecting walls can shear the corner, producing a vertical crack a short distance from the corner on one or both faces.
Figure 10 — Vertical cracking near a corner from restrained brick expansion, with retrofit expansion joint (EJ) placement Industry guidance recommends vertical expansion joints in clay brick veneer at spacings generally not exceeding about 7.6 m (25 ft), with closer spacing near corners and at changes in height. These cracks are non-structural in the load-carrying sense but progressively degrade weather-tightness and tie engagement; the durable repair is retrofit installation of soft joints, not rigid patching.
5.3 Horizontal Cracks at Shelf Angles and Lintels: Corrosion Jacking
In veneer construction supported on steel shelf angles or loose lintels, horizontal cracking, spalling of brick edges, and outward rotation of courses immediately above the steel are the classic signature of oxide (rust) jacking: corrosion products on the embedded steel expand and lift the masonry above.
Figure 11 — Oxide (rust) jacking at a corroding steel shelf angle supporting brick veneer (cross-section) A related pattern occurs where veneer bears rigidly on a shelf angle with no compressible joint beneath it — brick expansion against frame shortening crushes and spalls the bearing course. Both mechanisms are structurally significant for the veneer because they compromise its support; repair requires exposing, cleaning or replacing, and protecting the steel and reinstating a properly sized soft joint, not cosmetic repointing.
5.4 Cracks Above Openings
Vertical or stepped cracking rising from the ends or centre of a lintel span, sometimes with sagging of the courses above, indicates lintel deflection, corrosion, or inadequate bearing. Steel lintel corrosion is again a frequent culprit in older buildings. The masonry arching over the opening redistributes load to the jambs, and the crack pattern maps that redistribution. Repair ranges from lintel cleaning and painting through full lintel replacement with temporary needling of the masonry above.
5.5 Wall Tie Corrosion and Veneer Bulging
Corroded or missing ties between a veneer and its backup manifest as horizontal cracking at regular vertical intervals (often coinciding with tie courses), outward bulging visible by sighting along the wall, and in advanced cases veneer detachment. Because tie failure converts a laterally supported veneer into a free-standing slender wall, it is a life-safety issue in tall walls. Investigation (metal detection, borescope, tell-tale removal of units) and retrofit helical or mechanical ties are the standard remediation.
5.6 Freeze–Thaw and Moisture-Driven Deterioration
Saturated brickwork exposed to freezing — parapets, chimneys, sills, walls below failed flashings — develops face spalling, cracked units, and disintegrating mortar joints. As with concrete, the durable repair couples material reinstatement (matching brick; mortar batched to be weaker and more vapour-permeable than the units, typically a lime-rich mix in heritage work) with elimination of the moisture source. Hard cement repointing of soft historic brickwork is a well-documented cause of accelerated unit spalling and should be avoided.
6. Distinguishing Structural from Non-Structural Cracks
No single measurement classifies a crack. A defensible determination integrates at least six lines of evidence:
Pattern and orientation relative to the load path: horizontal cracks in below-grade walls, diagonal cracks near supports, and cracks parallel to reinforcement rank among the highest-concern patterns; uniform vertical mid-panel cracks rank among the lowest.Width and taper: uniform-width cracks suggest volumetric (shrinkage/thermal) origins; tapered cracks indicate rotation or differential movement; widths beyond guideline thresholds elevate the response regardless of cause.Displacement across the crack: any out-of-plane offset (one face proud of the other) or in-plane shear offset is prima facie evidence of structural movement.Activity: dormant cracks admit simple repair; active cracks demand cause correction first. Activity is established by monitoring — calibrated tell-tale gauges, pin sets measured with a vernier or DEMEC gauge, or glass/plaster tabs — read over at least one seasonal cycle where frost or expansive soil is suspected.Corroborating distress: sticking doors and windows, sloped floors, separated trim, racked frames, leaning chimneys, and exterior grade or drainage defects transform the interpretation of an otherwise minor crack.Environment and history: age of structure, timing of crack appearance relative to construction, renovations, excavation nearby, tree growth or removal, and climate exposure (freeze–thaw cycling is a first-order accelerant of every crack type in Canadian service).
Figure 12 — Calibrated tell-tale gauge for monitoring crack activity As a practical triage rule for owners: a professional structural assessment should be obtained without delay for any horizontal crack in a foundation wall, any diagonal crack in a beam or near a support, any crack wider than about 3 mm, any crack showing offset or measurable growth, and any cracking accompanied by bowing, leaning, or bearing distress. Conversely, hairline and fine vertical cracks of uniform width, stable over time and unaccompanied by other symptoms, are in the great majority of cases non-structural.
7. Repair Recommendations
Repair selection follows a strict logic: identify the mechanism; determine whether the crack is dormant or active; arrest the cause if active; then select a repair compatible with the residual movement, the exposure, and the performance objective (structural continuity, water-tightness, or appearance).
7.1 Epoxy Injection
Pressure injection of low-viscosity structural epoxy can restore monolithic behaviour across cracks as narrow as 0.05 mm and is the reference repair for dormant structural cracks in concrete. It is unsuitable for active cracks — the concrete simply re-cracks alongside the rigid epoxy — and for cracks that are actively leaking unless moisture-tolerant formulations and proper surface sealing are used.
7.2 Polyurethane (Chemical Grout) Injection
Hydrophilic and hydrophobic polyurethane grouts react with water to form a flexible, watertight seal and are the technique of choice for leaking, damp, or slightly moving cracks in foundation walls and water-retaining structures. They provide sealing, not strength; where both are required, structural repair precedes or accompanies injection.
7.3 Routing and Sealing; Flexible Joint Sealants
For dormant, non-structural cracks in exposed slabs and walls where appearance and water exclusion are the objectives, the crack is widened with a V-groove (typically 6–25 mm), cleaned, and filled with an appropriate sealant. Cracks expected to continue moving are treated as joints: routed, fitted with a bond-breaker, and sealed with an elastomeric sealant sized for the anticipated movement.
7.4 Stitching, Doweling, and External Reinforcement
Where tensile capacity must be re-established across a crack in concrete, options include stitching dogs or dowels grouted across the crack, added conventional reinforcement set in epoxy, post-tensioning across the cracked zone, and bonded FRP or steel plates. These are engineered repairs requiring analysis of the force to be transferred and of the consequence of stiffening one crack in a field of shrinkage strain.
7.5 Masonry Repairs: Repointing and Helical-Bar Stitching
Stabilized cracks in brickwork are repaired by raking joints to sound mortar (typically 20–25 mm minimum) and repointing with mortar matched to be slightly weaker and more permeable than the units. Cracked units are cut out and replaced. Stepped and vertical cracks across which modest tensile continuity is desired are stitched with stainless helical bars set in polymer-modified grout in raked bed joints at vertical intervals (commonly every fourth to sixth course), extending at least 500 mm beyond the crack each side. Corroded shelf angles, lintels, and ties are cleaned or replaced and isolated from moisture before any cosmetic work proceeds.
7.6 Movement Joints
Where cracking is caused by restrained volume change — long veneer runs, brick over concrete frames, abutting stiff elements — the durable repair is to legalize the movement: saw-cut and install vertical expansion joints in the brickwork, install horizontal soft joints beneath shelf angles, and provide slip planes or flexible anchorage at interfaces. Rigid patching of a restraint crack simply relocates it.
7.7 Foundation Stabilization: Underpinning, Bracing, and Drainage
Cracks caused by foundation movement are repaired from the ground up. Depending on cause and soil profile, stabilization measures include: correction of site grading, downspout discharge, and perimeter drainage (the least invasive and most frequently sufficient intervention); root or moisture management for shrink–swell clays; mass-concrete, helical-pile, or hydraulically driven resistance-pier underpinning to transfer load to competent strata; and, for laterally overloaded basement walls, interior steel channels, carbon-fibre straps, wall anchors, or partial rebuild combined with drainage and backfill correction. Only after monitoring confirms stability are the cracks themselves injected, stitched, or repointed.
7.8 Corrosion-Damage Repair
Corrosion-induced cracking is repaired by concrete removal to beyond the corroded bar, reinforcement cleaning or supplementation, reinstatement with compatible low-shrinkage material, and a corrosion-control strategy (coatings, sealers, galvanic anodes, or cathodic protection) proportional to the chloride or carbonation exposure.
Table 3 — Summary of Common Crack Types
Crack type Typical pattern / width Structural? Primary cause Typical repair
Plastic shrinkage Parallel/diagonal, shallow; 0.5–3 mm No Surface evaporation exceeding bleed Route and seal; prevention by curing Plastic settlement Over rebar/ties; ≤ 1 mm No (durability risk) Solids settling around restraint Epoxy injection or route and seal Drying shrinkage Vertical, uniform width; 0.1–1 mm No Restrained moisture loss Epoxy or PU injection; route and seal Early-age thermal Vertical in restrained walls; through-thickness Rarely Restrained cooling after hydration Injection once dormant Crazing Fine hexagonal map; < 0.1 mm No Weak surface skin; over-finishing None / sealer Corrosion-induced Parallel to bars; rust staining; spalls Yes (progressive) Chloride or carbonation attack on steel Concrete repair + corrosion control ASR Map cracking; gel exudate; grows over years Yes (progressive) Reactive aggregate + alkalis + moisture Moisture control; monitor; strengthen Freeze–thaw Scaling; D-cracking; widening of existing cracks Yes if advanced Ice pressure in saturated concrete Remove/replace; seal; fix moisture source Flexural Perpendicular to span, tension face; taper to NA Yes if wide/growing Bending stress Assess capacity; injection; strengthen Shear (diagonal) ≈ 45° near supports Yes — urgent Diagonal tension Immediate engineering assessment Bearing/splitting Vertical beneath concentrated load Yes Load-path/footing deficiency Correct load path; underpin; repair Foundation vertical Uniform; mid-panel or tie/opening Usually not Shrinkage/thermal restraint PU/epoxy injection (leak control) Foundation diagonal Tapered; from corners/openings Often Differential settlement or heave Stabilize foundation, then repair Foundation horizontal Bed-joint/mid-height; inward bowing Yes — urgent Lateral earth/frost/hydrostatic pressure Brace/rebuild; drainage correction Masonry stair-step Along joints; tapered Often Differential settlement Underpin/stabilize; stitch; repoint Masonry vertical (corners) Near corners; long runs without joints No (serviceability) Restrained brick moisture expansion Retrofit expansion joints Shelf angle/lintel horizontal Above embedded steel; spalled edges Yes (veneer support) Rust jacking; missing soft joint Replace/protect steel; soft joint
8. Conclusions
Cracking in concrete and masonry spans a spectrum from the purely cosmetic to the structurally critical, and the two ends of the spectrum can look deceptively similar to the untrained eye. The reliable discriminators are pattern and orientation read against the load path, width and taper, displacement across the crack, and — decisively in ambiguous cases — monitored activity over time. Volumetric mechanisms (plastic and drying shrinkage, early thermal contraction, crazing) produce the majority of cracks and are rarely structural, but durability mechanisms (corrosion, ASR, freeze–thaw) are progressive and will become structural if neglected, while movement- and load-induced patterns (horizontal foundation cracks, tapered diagonal cracks, shear cracks, bearing splits, rust jacking at veneer supports) demand engineering assessment before any repair is attempted. In every case, the governing rule of durable repair is the same: treat the cause, then the crack.
9. About Avid Structural Engineering Ltd.
Avid Structural Engineering Ltd. (ASE) is a structural engineering consultancy based in Ottawa, Ontario, providing professional assessment, forensic investigation, and repair design for cracking and structural distress in residential, commercial, heritage, and industrial buildings. Our services include visual condition assessments, crack mapping and monitoring programs, foundation movement investigations, masonry and veneer evaluations, repair specifications, and sealed engineering reports suitable for permits, insurance claims, and real-estate transactions.
If you have observed cracking in your building's concrete, foundation walls, or brickwork and are unsure of its significance, ASE can provide a clear, evidence-based diagnosis and a practical repair strategy. Contact us at info@avse.ca.
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Disclaimer: This white paper is provided for general information only and does not constitute engineering advice for any specific structure. Assessment of cracking in a particular building requires site-specific investigation by a licensed professional engineer.
