Investigating cracking in a concrete structure involves more than measuring the width of a visible line. The investigation should establish where the cracks occur, how they are arranged, whether they are changing and what structural, material, environmental or construction factors may have caused them.
Concrete cracks for many different reasons. Some cracks are associated with early-age shrinkage, thermal movement or drying. Others may relate to restraint, corrosion, settlement, overloading, reinforcement detailing, impact, water ingress or movement within the supporting structure.
The significance of a crack therefore depends on its location, direction, depth, width, history and relationship with the structural system. A narrow crack in one location may have a different meaning from a similar-width crack near a support, tendon anchorage, construction joint or heavily loaded connection.
The key point is this: crack width alone does not identify the cause or structural significance of concrete cracking. A defensible investigation combines visual mapping, measured evidence, construction history, reinforcement information, monitoring and targeted testing before engineering conclusions are reached.

Why Does Concrete Crack?

Concrete is strong in compression but comparatively weak in tension. Cracking can occur when tensile stress, strain or movement exceeds the concrete’s capacity to accommodate it.
Possible causes include:
  • Plastic shrinkage.
  • Plastic settlement.
  • Drying shrinkage.
  • Thermal contraction or expansion.
  • Restraint from adjoining structural elements.
  • Flexural loading.
  • Shear loading.
  • Punching or local concentrated loads.
  • Foundation settlement or structural movement.
  • Reinforcement corrosion.
  • Freeze-thaw damage.
  • Chemical attack.
  • Alkali-related deterioration.
  • Impact or accidental damage.
  • Fire or heat exposure.
  • Poor construction joints.
  • Inadequate curing.
  • Excessive water content or poor mix control.
  • Inadequate reinforcement detailing.
  • Changes in use or loading.
Several mechanisms may act together. For example, low concrete cover and water ingress may contribute to reinforcement corrosion, while restrained movement may determine where the resulting cracking appears.

Structural Versus Non-Structural Cracking

The terms structural and non-structural cracking are useful, but they should not be assigned solely from appearance.
Crack Category Possible Characteristics Important Qualification
Potentially structural May follow bending, shear, punching, settlement or connection-related stress patterns. Significance depends on geometry, loading, reinforcement and movement history.
Potentially non-structural May relate to shrinkage, thermal movement, surface drying or finishing. It may still affect durability, water tightness, fire resistance or appearance.
Durability-related May run parallel with reinforcement or occur with rust staining and spalling. The extent of corrosion and section loss may require intrusive investigation.
Construction-related May follow joints, changes in section, poorly compacted zones or restraint points. The visible crack may reflect a concealed construction interface.
A crack described as non-structural should not automatically be treated as harmless. It may allow water, chlorides or carbon dioxide to reach the reinforcement and may require repair for durability or serviceability reasons.

Why Crack Width Alone Is Not Enough

Crack width is an important measurement, but it does not explain:
  • How deep the crack extends.
  • Whether it passes through the element.
  • Whether it is widening.
  • Whether it opens and closes with temperature or loading.
  • Whether reinforcement crosses it.
  • Whether corrosion is present.
  • Whether the crack follows a construction joint.
  • Whether the supporting structure is moving.
  • Whether similar cracks occur elsewhere.
The investigation should therefore record the complete crack pattern and surrounding structural context.

What Should Happen When Concrete Cracking Is First Identified?

The first response should be proportionate to the apparent risk. The area should be inspected before cracks are filled, covered or altered.
Initial actions may include:
  • Photographing the cracking.
  • Marking crack ends.
  • Recording crack widths.
  • Identifying the structural element.
  • Checking for loose or falling concrete.
  • Checking for water ingress.
  • Reviewing recent loading or construction activity.
  • Restricting access where immediate safety concerns exist.
  • Requesting structural-engineer review.
The crack should not be repaired before the cause, activity and required investigation have been considered. Early filling can conceal evidence and make later monitoring more difficult.

When Is Immediate Action Required?

Urgent review may be required where cracking is associated with:
  • Sudden or continuing movement.
  • Falling concrete or exposed reinforcement.
  • Significant deflection or distortion.
  • Cracks close to heavily loaded supports.
  • Shear-type diagonal cracking.
  • Punching patterns around columns.
  • Damage after impact, fire or overload.
  • Cracking in post-tensioned elements.
  • Loss of bearing or support.
  • Unusual noises or visible instability.
The responsible engineer should determine whether temporary propping, unloading, exclusion or emergency making-safe is required.

Information to Gather Before Site Investigation

Useful background information includes:
  • Structural drawings.
  • Reinforcement and post-tensioning drawings.
  • Concrete specifications.
  • Construction dates and pour sequences.
  • Previous repair records.
  • Historic photographs.
  • Loading changes.
  • Recent drilling, demolition or fit-out works.
  • Water-ingress history.
  • Fire, impact or flooding events.
  • Previous crack-monitoring data.
  • Foundation or movement records.
This information helps distinguish long-standing construction cracks from recent changes or load-related deterioration.

What Is Concrete Crack Mapping?

Crack mapping records the position, extent, direction and characteristics of visible cracking across a structural element or area.
A crack map should identify:
  • Each crack reference.
  • Start and end positions.
  • Orientation.
  • Length.
  • Measured width or width range.
  • Branching and intersections.
  • Relationship with supports, joints and openings.
  • Associated staining, spalling or leakage.
  • Whether the crack is visible on the opposite face.
The map may be prepared on floor plans, elevations, reflected ceiling plans, photographs or survey drawings.

Why the Wider Crack Pattern Matters

One isolated close photograph may not show whether the crack forms part of a larger pattern.
The wider pattern may indicate:
  • Repeated flexural cracking.
  • Movement at regular restraint points.
  • Cracking parallel with reinforcement.
  • Settlement across several elements.
  • Shrinkage distributed across a slab.
  • Concentration around openings.
  • Movement at construction joints.
  • A local impact or isolated defect.
The investigation should include both overall views and detailed measurements.

How Should Cracks Be Referenced?

Each crack or crack group should have a unique reference used consistently on:
  • Plans and elevations.
  • Photographs.
  • Measurement schedules.
  • Monitoring records.
  • Sample locations.
  • Repair drawings.
  • The final report.
The reference should remain unchanged throughout the investigation so later measurements can be compared reliably.

Crack Orientation and Structural Context

The direction of a crack should be recorded relative to:
  • The structural span.
  • Columns and walls.
  • Beam direction.
  • Reinforcement direction.
  • Construction joints.
  • Openings and penetrations.
  • Element edges.
  • Movement joints.
The same diagonal crack angle can have different meanings depending on whether it occurs near a beam support, wall opening, slab corner or settlement interface.

How Is Concrete Crack Width Measured?

Crack width may be measured using:
  • A crack-width comparison card.
  • A graduated crack microscope.
  • A measuring loupe.
  • Digital imaging with a calibrated scale.
  • A displacement gauge for monitored cracks.
The method and approximate measurement resolution should be recorded.

Where Should the Crack Be Measured?

A crack may vary in width along its length. Measurements should therefore be taken at identifiable positions, including where relevant:
  • The widest visible point.
  • Near each end.
  • At regular intervals.
  • Near supports or joints.
  • At monitoring-gauge positions.
  • Where water ingress or spalling occurs.
The report should not state one width for a long variable crack without explaining where it was measured.

Surface Preparation Before Measurement

Paint, dirt, laitance and previous repair material may obscure the true crack edges.
Where preparation is permitted, the surface may require:
  • Light cleaning.
  • Removal of loose paint.
  • Drying of wet surfaces.
  • Improved lighting.
  • Marking the measurement point.
Aggressive grinding should be avoided unless specified because it may widen the apparent crack, remove evidence or damage finishes.

Can Crack Depth Be Measured?

Crack depth is more difficult to establish than surface width.
Possible approaches include:
  • Inspection of an exposed edge.
  • Local drilling or coring.
  • Ultrasonic methods.
  • Impact-echo or specialist non-destructive testing.
  • Borescope inspection.
  • Controlled opening-up.
Each method has limitations. A visible surface crack may taper, branch or change direction internally.

Is the Crack Visible on Both Faces?

Where both faces are accessible, the survey should check whether the crack:
  • Appears in approximately the same position.
  • Has a different direction on the opposite face.
  • Is associated with leakage.
  • Passes through a joint or opening.
  • Is visible only within a surface finish.
Cracking on both faces may indicate through-thickness movement, but the internal crack path still may not be straight or continuous.

What Is the Difference Between an Active and Dormant Crack?

An active crack is changing in width, length or displacement. A dormant crack is not showing meaningful ongoing movement over the period and conditions assessed.
Cracks may also be:
  • Seasonally active.
  • Load-dependent.
  • Temperature-dependent.
  • Moisture-dependent.
  • Intermittently active.
  • Progressively widening.
A crack should not be labelled dormant based on one visit. Monitoring over a suitable period may be required.

Why Activity Matters for Repair Selection

A rigid repair may reopen if the crack continues to move.
The repair design may depend on whether the objective is to:
  • Restore structural continuity.
  • Seal against water.
  • Protect reinforcement.
  • Accommodate movement.
  • Restore appearance.
  • Prevent debris or contamination.
The repair method should therefore follow investigation and engineering review rather than being selected from width alone.

What Crack Patterns May Indicate Structural Behaviour?

Crack patterns can provide clues, but they should not be diagnosed without considering the structural system and loading.
Possible Pattern Possible Association Further Evidence Required
Cracks perpendicular to the span in a flexural zone Bending-related tensile cracking. Loading, reinforcement, depth, deflection and movement history.
Diagonal cracks near supports Possible shear or load-transfer effects. Structural geometry, reinforcement, loading and crack progression.
Radial or circumferential cracks near a column Possible punching, restraint or local support behaviour. Slab thickness, punching reinforcement, loading and deflection.
Cracks across several connected elements Possible foundation movement, frame movement or restraint. Level surveys, monitoring, foundation information and structural review.
Cracking near bearings or connections Possible concentrated load, restraint or connection movement. Opening-up, bearing details, load path and support condition.

Flexural Cracking

Flexural cracks generally form where bending creates tensile stress in the concrete.
The investigation may consider:
  • Crack direction relative to the span.
  • Position within the tensile zone.
  • Spacing and repetition.
  • Element deflection.
  • Reinforcement size, spacing and cover.
  • Current and historic loading.
  • Changes in use.
The presence of flexural cracking does not by itself establish failure. Reinforced concrete is often designed to crack under service loading, but the width, distribution and progression still require assessment.

Shear-Related Cracking

Diagonal cracking near supports, concentrated loads or openings may require urgent structural review.
The assessment may need:
  • Crack angle and position.
  • Beam or slab dimensions.
  • Link or shear-reinforcement arrangement.
  • Support and bearing details.
  • Loading history.
  • Evidence of widening or displacement.
Visual appearance alone should not be used to confirm a shear mechanism.

Cracking Around Columns and Supports

Cracking around columns may relate to several mechanisms, including:
  • Punching-related stress.
  • Negative bending over supports.
  • Restraint and shrinkage.
  • Column or foundation movement.
  • Construction joints.
  • Local reinforcement congestion.
  • Previous drilling or openings.
The investigation should establish the crack pattern over a sufficiently wide area and review the reinforcement and slab thickness.

Settlement and Movement Cracking

Cracking caused by movement may cross several finishes and structural elements rather than remaining within one isolated panel.
Useful evidence may include:
  • Level and verticality surveys.
  • Foundation information.
  • Movement-joint condition.
  • Cracking in adjacent masonry and finishes.
  • Drainage or ground-condition history.
  • Monitoring over time.
  • Changes in nearby excavation or loading.
A crack cannot normally be attributed to settlement solely because it is diagonal.

What Crack Patterns May Relate to Shrinkage or Thermal Movement?

Shrinkage and thermal cracks can occur where concrete movement is restrained or uneven.
Possible characteristics include:
  • Distributed cracks across a large slab.
  • Cracks extending from re-entrant corners.
  • Cracking around openings and penetrations.
  • Cracks near changes in section.
  • Cracks at restraint points.
  • Regular cracking between movement joints.
  • Early-age surface cracking.
The investigation should review pour size, joint arrangement, curing, construction sequence and environmental conditions.

Plastic Shrinkage Cracking

Plastic shrinkage cracking develops before the concrete has fully hardened where moisture is lost rapidly from the surface.
It may appear as:
  • Relatively shallow surface cracks.
  • Several approximately parallel cracks.
  • Irregular cracks across exposed slabs.
  • Cracking associated with hot, dry or windy placement conditions.
The investigation may review construction records, weather, curing methods and whether the cracking remains superficial.

Plastic Settlement Cracking

Plastic settlement cracks may occur where fresh concrete settles around restraint provided by reinforcement or changes in section.
They may follow:
  • Top reinforcement.
  • Deep beam bars.
  • Changes in slab depth.
  • Embedded items.
Scanning can help establish whether the crack aligns with shallow reinforcement or embedded features.

Drying Shrinkage and Restraint Cracking

Drying shrinkage occurs as hardened concrete loses moisture. Cracking develops where contraction is restrained.
The investigation may review:
  • Panel dimensions.
  • Joint spacing.
  • Wall and column restraint.
  • Re-entrant corners.
  • Reinforcement distribution.
  • Concrete age.
  • Environmental exposure.

Thermal Cracking

Thermal cracking can occur where heat generated or absorbed by the concrete produces temperature differences and restrained movement.
Relevant information may include:
  • Concrete pour thickness.
  • Cement content and mix characteristics.
  • Placement temperature.
  • Ambient temperature history.
  • Formwork removal time.
  • Restraint from adjoining pours.
  • Construction-joint sequence.
Historic records may be limited, so the conclusion may remain based on several converging indicators rather than one definitive test.

Cracking Along Construction Joints

Cracks at or near construction joints may relate to:
  • Differential shrinkage.
  • Insufficient preparation.
  • Movement between pours.
  • Water ingress.
  • Poor load transfer.
  • Inadequate reinforcement continuity.
Opening-up, cores or drawings may be needed to establish the joint detail and reinforcement arrangement.

Corrosion-Related Cracking

Corrosion products occupy a greater volume than the original steel and can create pressure within the concrete cover.
Possible signs include:
  • Cracks running parallel with reinforcement.
  • Rust staining.
  • Delamination.
  • Spalling.
  • Exposed corroded bars.
  • Hollow-sounding concrete.
  • Repeated defects at low-cover locations.
The investigation may require cover surveys, carbonation testing, chloride sampling, half-cell surveys and reinforcement exposure.

How Is Reinforcement Investigated Around Concrete Cracks?

Reinforcement information helps establish whether cracks align with bars, occur between bars or appear within a structurally significant zone.
Concrete scanning may identify:
  • Bar direction.
  • Approximate bar spacing.
  • Estimated concrete cover.
  • Additional support reinforcement.
  • Links or stirrups.
  • Possible post-tensioning tendons.
  • Areas of reinforcement congestion.
For wider guidance on scanner outputs, read What Can a Concrete Scanner Detect?.

Why Scan Across and Along the Crack?

Scanning in different directions may show:
  • Whether the crack runs parallel with reinforcement.
  • Whether bars cross the crack.
  • Whether spacing changes near the crack.
  • Whether an embedded item or service lies beneath it.
  • Whether the crack follows a tendon route.
The survey area should extend beyond the visible crack so the surrounding reinforcement pattern can be understood.

Concrete Cover and Cracking

Low or variable concrete cover can increase the vulnerability of reinforcement to carbonation, chlorides and moisture.
A cover survey may compare:
  • Cracked and apparently sound areas.
  • Interior and exposed elevations.
  • Wet and dry zones.
  • Original and repaired construction.
Low cover alone does not prove that corrosion caused the crack, but it may form part of the durability evidence.

Can Scanning Confirm a Broken Reinforcing Bar?

Concrete scanning does not normally confirm whether an individual bar is fractured.
A discontinuous response could result from:
  • A bar changing depth.
  • Congestion or masking.
  • A lap or coupler.
  • Limited survey coverage.
  • A genuine discontinuity.
Physical exposure or another specialist method may be required where bar fracture is suspected.

Post-Tensioning and Concrete Cracking

Cracking in post-tensioned concrete should be reviewed with particular care because tendon arrangement, prestress and anchorage zones influence structural behaviour.
The investigation may require:
  • Post-tensioning drawings.
  • Tendon-route mapping.
  • Anchorage and stressing-zone identification.
  • Crack mapping relative to tendon bands.
  • Review by a post-tensioning specialist.
  • Specialist tendon-condition investigation.
Drilling or opening-up near a tendon-like response should not proceed without appropriate specialist approval.

What Material Tests May Be Used?

Material testing should be selected according to the suspected deterioration mechanism or engineering question.
Test or Investigation Possible Evidence Main Limitation
Concrete core testing Strength, internal cracking, material build-up and physical samples. Local sampling may not represent the whole element.
Carbonation testing Depth of carbonation at the tested location. Does not directly prove corrosion or section loss.
Chloride testing Chloride concentration at selected depths. Results vary by location, depth and exposure.
Half-cell potential survey Probability-related evidence of active corrosion conditions. Does not directly measure remaining steel section.
Concrete resistivity Electrical-resistivity information relevant to corrosion environment. Affected by moisture, temperature and concrete composition.
Petrographic examination Detailed evidence of concrete composition and deterioration. Requires representative physical samples and specialist interpretation.

Concrete Core Sampling Through or Near a Crack

A core may be taken through a crack to investigate its internal path, surrounding concrete and material condition.
The core plan should consider:
  • Whether the crack itself must be intersected.
  • Whether a comparison core is needed away from the crack.
  • Reinforcement and tendon positions.
  • Core diameter and orientation.
  • Whether the core is intended for strength testing or visual examination.
  • How the hole will be repaired.
A core containing a crack may be unsuitable for some strength interpretations, depending on the test objective and sample condition.

Rebound Hammer Testing

A rebound hammer may be used to compare surface hardness across selected areas.
It may help identify relative variation between:
  • Cracked and uncracked areas.
  • Different pours.
  • Sound and visibly deteriorated zones.
It does not directly establish compressive strength without appropriate correlation and should not be used alone to diagnose the cause of cracking.

Delamination and Sounding Surveys

Hammer sounding or chain-drag surveys may help identify concrete that has debonded or delaminated beneath the surface.
The survey can support mapping of:
  • Hollow-sounding areas.
  • Loose cover concrete.
  • Zones around corrosion-related cracking.
  • Repair boundaries.
Results are operator-dependent and should be supported by local verification where important.

How Are Concrete Cracks Monitored?

Crack monitoring records whether a crack changes over time or under changing conditions.
Possible methods include:
  • Repeated manual crack-width measurements.
  • Fixed tell-tales.
  • Graduated crack monitors.
  • Demountable mechanical gauges.
  • Electronic displacement transducers.
  • Survey targets and precise level monitoring.
  • Remote data logging.

What Should a Monitoring Plan Record?

  • The crack reference and exact gauge position.
  • Initial crack width.
  • Date and time.
  • Temperature and weather where relevant.
  • Building occupancy or loading condition.
  • Measurement method and equipment.
  • Reading frequency.
  • Trigger levels.
  • Action required if triggers are exceeded.
Monitoring should be designed around the suspected movement mechanism. A weekly reading programme may miss rapid load-related movement, while a very short programme may not capture seasonal change.

Tell-Tales and Crack Monitors

A tell-tale or crack monitor bridges the crack and records relative movement between its sides.
The device may indicate:
  • Opening or closing.
  • In-plane shear movement.
  • Seasonal movement.
  • Progressive displacement.
Installation should not bridge loose paint or unsound concrete because movement of the surface finish may be mistaken for structural crack movement.

How Long Should Cracks Be Monitored?

The monitoring duration depends on:
  • The suspected cause.
  • The rate of observed movement.
  • Seasonal temperature and moisture variation.
  • Construction activities.
  • Loading cycles.
  • The urgency of the engineering decision.
The structural engineer should define the period, reading frequency and trigger criteria.

Monitoring Deflection and Movement

Crack monitoring may need to be combined with:
  • Floor or beam deflection measurements.
  • Level surveys.
  • Column verticality checks.
  • Foundation settlement monitoring.
  • Joint-movement monitoring.
  • Load records.
A stable crack width does not automatically prove that the wider structure is not moving.

When Is Intrusive Investigation Required?

Intrusive investigation may be required where the cause, depth or associated condition cannot be established using visual and non-destructive methods.
Possible methods include:
  • Concrete reinforcement breakouts.
  • Cores through or beside cracks.
  • Opening-up construction joints.
  • Exposing bearings or connections.
  • Removing previous repair materials.
  • Borescope inspection.
  • Foundation trial pits.
The work should be targeted to answer a defined question and should not begin with uncontrolled breaking.

Concrete Breakouts Beside Cracks

A breakout may expose reinforcement and surrounding concrete to confirm:
  • Bar diameter and cover.
  • Whether the crack follows the bar.
  • Corrosion or pitting.
  • Section loss.
  • Bond condition.
  • Delaminated or weak concrete.
  • Previous repairs.
The breakout should be preceded by scanning so the required reinforcement can be targeted while avoiding tendons and services.

Opening-Up Bearings and Connections

Cracking near supports may require exposure of:
  • Bearings.
  • Connection plates.
  • Bolts and welds.
  • Support reinforcement.
  • Construction joints.
  • Adjacent masonry or grout.
The opening dimensions and temporary works should be approved by the structural engineer.

Structural Investigations and Existing Buildings

Concrete cracking is often one part of a wider existing-building investigation involving incomplete drawings, later alterations and changes in use.
STRUCTinspect has discussed the importance of verified site evidence in Structural Investigation for Office Retrofit: Beyond the Energy Model.
A crack investigation may need to be coordinated with:
  • Reinforcement surveys.
  • Slab-thickness checks.
  • Connection opening-up.
  • Concrete strength testing.
  • Load assessments.
  • Façade and water-ingress investigations.
  • Foundation or movement surveys.

RAMS for Crack Investigation Works

Visual crack mapping may be low risk, but intrusive work, monitoring installation and access arrangements still require project-specific controls.
The RAMS may need to address:
  • Work-at-height access.
  • Falling or loose concrete.
  • Drilling and breakout depths.
  • Reinforcement, tendons and services.
  • Dust and silica.
  • Water and slurry containment.
  • Temporary works.
  • Exclusion zones.
  • Monitoring-gauge protection.
  • Stop conditions.
  • Reinstatement.
STRUCTinspect has explained the importance of connecting technical methodology with actual site controls in What a Structural Testing RAMS Must Contain Before Loading Starts. The same principle applies to crack investigations: the written method should reflect the real structural element, access, equipment and decision points.

What Should Cause Investigation Work to Stop?

Work should normally stop if:
  • Cracking widens or propagates during the work.
  • Unexpected movement is observed.
  • Loose concrete becomes unstable.
  • A tendon-like feature is identified.
  • An unidentified service is encountered.
  • Reinforcement is accidentally damaged.
  • The approved opening dimensions are reached without exposing the required detail.
  • Hazardous material is suspected.
  • Temporary support appears inadequate.
The condition should be recorded and referred to the responsible engineer before work continues.

How Is the Cause of Cracking Determined?

The cause is determined by comparing several types of evidence rather than relying on one measurement.
The assessment may consider:
  • Crack pattern and orientation.
  • Crack activity.
  • Structural geometry.
  • Reinforcement arrangement.
  • Current and historic loading.
  • Construction joints and pour sequence.
  • Concrete properties.
  • Environmental exposure.
  • Water ingress.
  • Corrosion evidence.
  • Foundation or support movement.
  • Previous alterations or repairs.
In some cases, the evidence supports one likely cause. In others, several contributing mechanisms may remain possible.

Can the Cause Always Be Confirmed?

Not always. Existing buildings may have incomplete records, inaccessible details and long histories of loading and repair.
The report may therefore need to state:
  • The most likely cause.
  • Alternative possible causes.
  • The evidence supporting each interpretation.
  • Remaining uncertainty.
  • Further investigation needed to improve confidence.
A transparent qualified conclusion is more reliable than an unsupported definitive diagnosis.

What Should a Concrete Crack Investigation Report Include?

  • Project name, address and report reference.
  • Client and instructing party.
  • Purpose and scope of the investigation.
  • Documents and drawings reviewed.
  • Inspection dates.
  • Structural elements inspected.
  • Crack references and locations.
  • Crack maps, plans and elevations.
  • Widths, lengths and orientations.
  • Associated staining, leakage, spalling or deformation.
  • Reinforcement and scanning findings.
  • Monitoring equipment and readings.
  • Material-test and sample results.
  • Opening-up findings.
  • Differences from drawings.
  • Observed changes over time.
  • Likely causes and alternative interpretations.
  • Immediate risks or restrictions.
  • Recommendations for repair, monitoring or further work.
  • Limitations.

How Should Crack Findings Be Described?

Useful report wording separates observation from interpretation.
Evidence Type Example Wording
Observed A diagonal crack was visible on the beam side near the support.
Measured The crack width ranged from approximately 0.15 mm to 0.35 mm at the recorded positions.
Interpreted The orientation is consistent with a possible shear-related stress pattern, subject to structural review.
Monitored No measurable change greater than the stated monitoring resolution was recorded during the observation period.
Assessed The responsible structural engineer concluded that strengthening was required.

Photographs for Crack Investigation

A useful photographic record includes:
  • A general view of the structure or room.
  • The complete crack or crack group.
  • Close views with a measurement scale.
  • Crack references and marked ends.
  • Associated staining or spalling.
  • Reinforcement-scanning markings.
  • Monitoring gauges.
  • Opening-up and core locations.
  • Exposed reinforcement or defects.
  • Repair condition where previous repairs exist.
Close photographs should be supported by contextual images so the crack can be relocated.

Crack-Monitoring Data Presentation

Monitoring results may be presented using:
  • A dated reading table.
  • A graph of width or displacement over time.
  • Temperature or loading records.
  • Trigger-level lines.
  • Notes of construction activity.
  • Photographs showing gauge condition.
The measurement resolution and any missing or unreliable readings should be stated.

Recommendations After Investigation

Recommendations should respond to the evidence and may include:
  • No immediate repair, with continued monitoring.
  • Structural calculations.
  • Temporary unloading or propping.
  • Additional crack monitoring.
  • Concrete breakouts.
  • Core or durability testing.
  • Foundation or level surveys.
  • Water-ingress investigation.
  • Crack injection or sealing.
  • Concrete repair.
  • Strengthening.
  • Removal and replacement of defective concrete.
The repair specification should be prepared after the cause, movement and required performance have been considered.

Should Cracks Be Filled Immediately?

Not unless immediate sealing or making-safe is required and the effect on the investigation has been considered.
Premature filling can:
  • Conceal ongoing movement.
  • Obscure the crack pattern.
  • Trap moisture.
  • Make monitoring difficult.
  • Fail if the underlying cause remains active.
Temporary protection may be more appropriate while the investigation continues.

Limitations of Concrete Crack Investigations

Crack investigations are affected by access, time, historic information and the local nature of testing.
Important limitations may include:
  • Finishes concealing cracks.
  • Only one face being accessible.
  • Previous repairs obscuring the original pattern.
  • Short monitoring periods.
  • Unknown loading history.
  • Incomplete construction records.
  • Limited material samples.
  • Reinforcement congestion masking deeper features.
  • Unverified foundation conditions.
  • Cracks changing with seasonal conditions outside the observation period.
The report should identify project-specific limitations and avoid implying certainty beyond the available evidence.

Can One Crack Represent the Whole Structure?

No. One crack provides evidence about one local condition.
The investigation should consider whether:
  • Similar cracks occur elsewhere.
  • The structural arrangement repeats.
  • Exposure conditions vary.
  • Different concrete pours are present.
  • Loading differs between areas.
  • Previous alterations affect only part of the structure.
Additional survey areas may be required before conclusions are applied more widely.

What Crack Investigation Cannot Confirm Alone

A crack survey does not automatically confirm:
  • Structural capacity.
  • Remaining reinforcement area.
  • Concrete compressive strength.
  • The full internal crack path.
  • Foundation movement.
  • The remaining service life.
  • The suitability of a repair.
  • The condition of uninspected areas.
These conclusions may require further testing and structural assessment.

Common Concrete Crack Investigation Mistakes

Mistake Why It Is a Problem Better Approach
Judging significance from width alone. Location, depth, activity and structural pattern are ignored. Assess the full crack context and supporting evidence.
Photographing only a close-up. The structural location and wider pattern cannot be understood. Use general, intermediate and detailed views.
Filling cracks before investigation. Evidence and movement may be concealed. Record, assess and monitor before permanent repair.
Calling a crack dormant after one visit. Seasonal or load-related movement may be missed. Use monitoring over a suitable period.
Ignoring reinforcement and tendons. The relationship between cracking and concealed structure remains unknown. Review drawings and complete targeted scanning.
Assuming rust staining proves severe section loss. The reinforcement condition has not been measured. Use durability testing and local exposure where required.
Monitoring without trigger levels. There is no defined response to movement. Agree thresholds, review responsibility and actions.
Selecting repair before identifying cause. The repair may reopen or fail to address deterioration. Link repair design to cause, movement and required performance.
Applying local findings to the whole building. Construction, exposure and loading may vary. Assess representative areas and state limitations.

Concrete Crack Investigation Checklist

  • Engineering objective: state what the investigation must determine.
  • Responsible engineer: identify who assesses structural significance.
  • Background information: collect drawings, loading, repair and incident history.
  • Immediate risk: check for instability, falling concrete, overload or sudden movement.
  • Crack references: assign a unique reference to each crack or crack group.
  • Mapping: record location, length, direction, branches and relationships with supports.
  • Width measurements: record method, positions, dates and ranges.
  • Photographs: include general, intermediate and close views.
  • Opposite face: inspect where access is available.
  • Associated defects: record leakage, staining, spalling, delamination and deformation.
  • Reinforcement: review drawings and complete scanning where relevant.
  • Post-tensioning: identify tendons, ducts and anchorage zones.
  • Monitoring: define equipment, frequency, duration and trigger levels.
  • Deflection and movement: include level or displacement surveys where required.
  • Material tests: select cores, carbonation, chloride or other tests against the suspected cause.
  • Intrusive work: define opening locations, dimensions, depths and stop conditions.
  • Services: complete detection and isolation before drilling or breakout work.
  • Temporary works: confirm whether unloading, propping or exclusion is required.
  • Samples: provide unique references and laboratory instructions.
  • Report: separate observations, measurements, interpretations and engineering conclusions.
  • Repair: select the method only after crack cause and activity have been considered.
  • Limitations: state access, monitoring, sampling and record constraints.

Evidence-Based Summary

Concrete cracking should be investigated as a structural, material and movement pattern rather than judged from crack width alone.
The investigation should record crack location, direction, length, width, associated defects and relationship with supports, reinforcement, joints and openings.
Monitoring may be required to determine whether cracks are active, dormant, seasonal or load-dependent.
Scanning can map reinforcement and tendons around cracks, while targeted breakouts, cores and durability tests can provide physical evidence.
Crack patterns may suggest flexural, shear, shrinkage, thermal, corrosion or settlement mechanisms, but engineering conclusions require supporting evidence.
Local findings should not automatically be applied to the whole structure, particularly where construction, exposure and loading vary.
The strongest crack investigations combine accurate mapping, monitoring, structural records, non-destructive surveys, targeted intrusive work and competent engineering review before repair is specified.

FAQ: Investigating Cracking in Concrete Structures

What causes cracks in concrete structures?
Possible causes include shrinkage, thermal movement, restraint, structural loading, settlement, corrosion, construction joints, impact, fire and material deterioration.
Does crack width show whether a crack is structural?
No. Width is one important measurement, but location, direction, depth, activity, reinforcement and structural context must also be considered.
How is crack width measured?
It may be measured using a comparison card, crack microscope, measuring loupe or calibrated digital image at identified positions.
Should the widest point be recorded?
Yes, but additional measurements should normally be taken along the crack because width can vary significantly.
What is crack mapping?
Crack mapping records the position, direction, length, width and associated features of cracking on plans, elevations or photographs.
How can you tell if a crack is active?
Repeated measurements or fixed monitoring gauges are used to determine whether the crack changes over time or under different loading and environmental conditions.
How long should concrete cracks be monitored?
The period depends on the suspected cause, rate of movement, seasonal conditions, loading cycles and urgency of the engineering decision.
Can concrete scanning help investigate cracks?
Yes. Scanning can map reinforcement, tendons and embedded features around the crack and help plan targeted intrusive investigation.
Can scanning show how deep a crack is?
Ordinary reinforcement scanning does not normally provide a definitive crack-depth measurement. Specialist ultrasonic methods, cores or opening-up may be required.
Do cracks parallel with reinforcement indicate corrosion?
They may be consistent with corrosion-related expansion, particularly where rust staining or spalling is present, but testing and local exposure may be needed to confirm the cause.
Are diagonal cracks always caused by settlement?
No. Diagonal cracking may relate to shear, restraint, openings, local loading, movement or several other mechanisms.
Are all narrow cracks harmless?
No. Narrow cracks may still be active, structurally relevant or important for durability and water tightness.
Should cracks be filled immediately?
Not normally before the crack pattern, cause and activity have been assessed. Early filling may conceal evidence or fail if movement continues.
When are concrete cores required?
Cores may be required where the investigation needs evidence about concrete strength, internal cracking, material condition, deterioration or construction build-up.
Can one crack be treated as representative of the whole building?
No. Additional areas may need to be surveyed because construction, loading, exposure and deterioration can vary.
Who should assess the structural significance of concrete cracking?
The responsible structural engineer should review the measured evidence, structural arrangement, loading, monitoring and test results.
What should a concrete crack report include?
It should include crack maps, locations, widths, directions, photographs, associated defects, monitoring, reinforcement findings, testing, likely causes, recommendations and limitations.

Source Context and Editorial Note

This article is a STRUCTinspect technical explainer covering the investigation of cracking in reinforced, prestressed and existing concrete structures.
It provides general construction information rather than a project-specific structural diagnosis, monitoring plan or repair specification. Inspection methods, monitoring periods, sampling, opening-up, temporary works and repairs should be selected for the particular structure and suspected cracking mechanism.
This article does not provide structural engineering, post-tensioning, materials, concrete-repair, temporary works, health and safety, contractual or construction advice. Concrete cracking should be assessed by the appropriate structural engineer, materials specialist, designer, contractor or competent professional responsible for the structure and proposed works.