1. Purpose
This document sets out a technical methodology for the investigation and diagnosis of existing car park slabs before specification of repair or strengthening works. The objective is to establish the condition, causes and extent of deterioration, determine the structural significance of identified defects, and provide sufficient evidence to develop an appropriate repair specification.
The investigation shall be undertaken by suitably qualified and experienced structural and materials professionals. The extent of investigation should be proportionate to the age, construction, exposure conditions, observed deterioration, structural importance and proposed future use of the car park.
The investigation shall distinguish between:
- Observed defects — what is physically present.
- Measured condition — the quantified extent and severity of deterioration.
- Probable mechanisms — the processes responsible for deterioration.
- Structural consequences — the effect on capacity, durability and serviceability.
- Confirmed causes — mechanisms supported by inspection, testing and evidence.
- Repair requirements — interventions developed only after the preceding matters have been established.
No repair specification should be finalised solely from visual inspection where concealed deterioration or structural capacity may be affected.
2. Scope of Investigation
The investigation should, as applicable, address:
- Existing drawings, specifications and historical records.
- Structural form and load paths.
- Concrete slab construction and reinforcement.
- Waterproofing and drainage arrangements.
- Surface finishes and traffic loading.
- Concrete deterioration and cracking.
- Reinforcement corrosion.
- Carbonation and chloride contamination.
- Concrete strength and material condition.
- Delamination and debonding.
- Water ingress and leakage.
- Previous repairs and their performance.
- Movement joints and construction joints.
- Bearings, beams, columns, walls and supporting elements where relevant.
- Local and global structural capacity where deterioration may be significant.
- Environmental exposure and future operational requirements.
The investigation should consider both the slab itself and the systems that influence its durability, particularly drainage, waterproofing, joints and surface protection.
3. Stage 1 – Desk Study
Prior to intrusive investigation, all reasonably available documentation should be reviewed.
3.1 Existing Documentation
The following information should be sought:
- Original structural drawings.
- Reinforcement drawings and schedules.
- As-built drawings.
- Concrete specifications and construction records.
- Waterproofing specifications.
- Previous condition surveys.
- Previous repair specifications and completion records.
- Structural assessment reports.
- Records of water ingress or leakage.
- Maintenance records.
- Records of de-icing salt use.
- Records of changes to vehicle loading or use.
- Previous incidents, impacts or overloading.
- Building control or statutory records where available.
Where original documentation is unavailable or unreliable, the structure should be treated as an existing structure of uncertain construction until verified by investigation.
3.2 Historical Review
Particular attention should be given to:
- Date of construction.
- Changes in use.
- Alterations to drainage.
- Changes in waterproofing.
- Previous concrete repairs.
- Repeated leakage locations.
- Areas subject to winter salting.
- Changes in ventilation or environmental exposure.
- Any known structural movement.
Historical patterns can provide important evidence regarding the underlying deterioration mechanism.
4. Stage 2 – Preliminary Visual Inspection
A systematic visual inspection shall be undertaken before intrusive testing.
The inspection should cover all accessible slab areas, including:
- Top surfaces.
- Undersides of slabs where accessible.
- Beam/slab junctions.
- Columns and supports.
- Expansion and movement joints.
- Construction joints.
- Drainage outlets.
- Falls and ponding areas.
- Upstands and parapets.
- Ramps.
- Areas around service penetrations.
- Previous repair locations.
4.1 Defect Mapping
Defects should be recorded on scaled drawings or a suitable digital survey platform.
The survey should identify, as applicable:
- Cracking.
- Spalling.
- Delamination.
- Exposed reinforcement.
- Rust staining.
- Damp areas.
- Efflorescence.
- Surface scaling.
- Abrasion.
- Chemical attack.
- Localised loss of concrete.
- Joint failure.
- Failed sealants.
- Failed coatings.
- Debonded repairs.
- Leakage.
- Ponding.
- Surface wear.
Cracks should be mapped with their:
- Location.
- Orientation.
- Approximate width.
- Length.
- Pattern.
- Depth where established.
- Whether active or apparently dormant.
- Relationship to reinforcement, supports, joints and openings.
Photographs should be referenced to the defect drawings and include sufficient information to establish location and scale.
5. Structural Form and Load Path
The investigation shall establish the structural arrangement sufficiently to understand how deterioration could affect structural performance.
The survey should identify:
- Slab type.
- One-way or two-way spanning behaviour.
- Flat slab or beam-and-slab construction.
- Span dimensions.
- Slab thickness.
- Support conditions.
- Columns and walls.
- Drop panels, ribs or thickened regions.
- Cantilevers.
- Ramps and transitions.
- Openings and penetrations.
- Movement joints.
Particular attention should be given to deterioration near supports, concentrated loads, openings and changes in geometry, where structural demand may be high.
6. Concrete Cover and Reinforcement Investigation
Concrete cover should be established using suitable non-destructive techniques, such as electromagnetic cover measurement, supplemented by local opening-up where necessary.
The investigation should establish:
- Nominal and actual concrete cover.
- Reinforcement diameter.
- Bar spacing.
- Reinforcement orientation.
- Reinforcement continuity.
- Top and bottom reinforcement where relevant.
- Presence of additional reinforcement.
- Reinforcement condition.
Local breakout should be undertaken where necessary to verify reinforcement identified by scanning and to assess corrosion directly.
Care should be taken to avoid damaging critical reinforcement during opening-up.
7. Concrete Condition Assessment
Concrete testing should be selected according to the deterioration mechanisms identified during the visual inspection.
7.1 Concrete Strength
Where structural assessment requires confirmation of concrete strength, representative cores should be obtained and tested in accordance with the applicable standards.
Core locations should be selected to represent:
- Apparently sound concrete.
- Deteriorated concrete.
- Different construction zones.
- Areas subject to different exposure conditions.
Core results should not be interpreted in isolation. Existing concrete strength should be assessed in the context of the original design, construction history and structural assessment.
7.2 Surface and Internal Defects
Non-destructive techniques may include:
- Rebound hammer testing.
- Ultrasonic pulse velocity.
- Impact-echo or similar techniques.
- Ground penetrating radar.
- Infrared thermography where appropriate.
These methods can assist in identifying areas requiring intrusive confirmation but should not automatically be treated as direct measurements of structural capacity.
8. Delamination and Spalling
Areas of hollow-sounding or delaminated concrete should be systematically identified.
Investigation should establish:
- Extent of delamination.
- Depth of deterioration.
- Condition of reinforcement.
- Remaining sound concrete cover.
- Relationship between delamination and corrosion.
- Whether deterioration extends beyond visually damaged areas.
Where corrosion is suspected, apparently sound concrete adjacent to visible spalling should also be investigated because corrosion-induced deterioration frequently extends beyond the visible defect.
9. Carbonation Assessment
Carbonation testing should be undertaken where carbonation-induced reinforcement corrosion is considered possible.
Testing should establish:
- Carbonation depth.
- Concrete cover.
- Relationship between carbonation front and reinforcement depth.
- Variation across different exposure zones.
The results should be used to determine whether reinforcement has been exposed to a lower-alkalinity environment and whether carbonation is likely to have contributed to corrosion.
10. Chloride Contamination
For car parks exposed to de-icing salts or other chloride sources, chloride testing should be considered a key component of the investigation.
Testing should establish chloride concentration at appropriate depth intervals, particularly around reinforcement.
Sampling should consider:
- Surface condition.
- Concrete cover.
- Reinforcement depth.
- Different exposure zones.
- Areas with and without visible corrosion.
- Areas adjacent to joints, drainage points and vehicle routes.
Chloride results should be interpreted alongside reinforcement corrosion observations and concrete cover measurements.
11. Reinforcement Corrosion Assessment
Where corrosion is suspected, reinforcement condition should be assessed using a combination of:
- Visual inspection.
- Local breakout.
- Cover measurements.
- Carbonation testing.
- Chloride profiling.
- Half-cell potential measurements where appropriate.
- Concrete resistivity measurements where appropriate.
Electrochemical surveys can assist in identifying areas with a higher probability of active corrosion but should not be interpreted as a direct measurement of reinforcement section loss.
Where structural capacity is potentially affected, the actual reinforcement section loss should be established at representative critical locations.
12. Water Ingress and Drainage Investigation
Water ingress is frequently a major contributor to car park slab deterioration and should be investigated as a potential cause, rather than simply treated as a symptom.
The investigation should examine:
- Drainage falls.
- Drainage outlets.
- Blocked or damaged drains.
- Ponding.
- Failed waterproofing.
- Failed joints.
- Cracked surfaces.
- Upstands.
- Penetrations.
- Interfaces between different construction materials.
- Leakage through the slab.
- Water staining and efflorescence.
Where necessary, waterproofing systems should be opened up to establish:
- Type and thickness of waterproofing.
- Condition.
- Adhesion.
- Continuity.
- Substrate condition.
- Presence of trapped moisture.
- Compatibility with proposed repair systems.
13. Crack Investigation
Cracking should be classified according to likely mechanism.
Potential causes include:
- Drying shrinkage.
- Thermal movement.
- Restraint.
- Reinforcement corrosion.
- Structural flexure.
- Shear.
- Punching.
- Settlement or differential movement.
- Construction defects.
- Excessive loading.
- Movement at joints.
- Chemical or environmental deterioration.
Crack width alone should not be used to determine structural significance.
Cracks should be assessed in relation to their:
- Location.
- Orientation.
- Depth.
- Continuity.
- Pattern.
- Relationship with reinforcement.
- Relationship with supports.
- Water ingress.
- Historical evidence.
- Evidence of movement.
Where required, crack monitoring should be installed to establish whether movement is ongoing.
14. Previous Repairs
Existing repairs shall be separately mapped and assessed.
The investigation should determine:
- Repair material.
- Repair depth.
- Repair extent.
- Bond to existing concrete.
- Cracking.
- Debonding.
- Delamination.
- Compatibility with substrate.
- Evidence of renewed corrosion.
- Coating condition.
- Whether the original deterioration mechanism remains active.
Previous repair failure should be treated as evidence requiring diagnosis rather than simply as a requirement for replacement.
15. Drainage, Waterproofing and Joints
The investigation should establish the condition and functionality of:
- Expansion joints.
- Movement joints.
- Construction joints.
- Joint sealants.
- Waterproofing transitions.
- Drainage channels.
- Gullies.
- Downpipes.
- Upstands.
- Edge details.
Defective details may permit continued water and chloride ingress even where concrete repairs are otherwise correctly executed.
Consequently, repair specifications should address the source of deterioration as well as the resulting concrete damage.
16. Structural Assessment
A structural assessment should be undertaken where investigation identifies deterioration that may reduce structural capacity.
Potentially critical issues include:
- Loss of reinforcement section.
- Significant concrete section loss.
- Reduced concrete strength.
- Cracking in critical structural zones.
- Punching shear deterioration.
- Loss of anchorage or development length.
- Deterioration at supports.
- Excessive deflection.
- Structural movement.
- Changes in imposed loading.
- Unrecorded alterations.
The assessment should consider both the existing condition and the proposed future loading/use.
Where deterioration is localised, assessment should consider whether local repair is sufficient or whether redistribution of forces requires a wider structural intervention.
17. Investigation Sampling Strategy
Sampling should be based on engineering judgement and the observed variability of the structure.
The investigation should avoid relying solely on a small number of convenient test locations.
Test locations should be selected to represent:
- Severe deterioration.
- Moderate deterioration.
- Apparently sound areas.
- Different exposure conditions.
- Different structural zones.
- Different construction phases, where relevant.
- Areas of previous repair.
- Locations around drainage and joints.
- Critical structural regions.
A test plan should be prepared before intrusive works and updated if early findings indicate greater or different deterioration than anticipated.
18. Investigation Records
A complete investigation record should include:
- General arrangement drawings.
- Defect plans.
- Crack maps.
- Photographic records.
- Test location plans.
- Concrete cover results.
- Reinforcement survey results.
- Core locations and results.
- Carbonation measurements.
- Chloride profiles.
- Corrosion survey results.
- Waterproofing findings.
- Drainage findings.
- Joint condition.
- Previous repair assessment.
- Structural observations.
- Laboratory certificates.
- Calibration records where relevant.
All test results should be traceable to their physical location.
19. Diagnosis
The diagnosis should integrate the results rather than treating individual test results independently.
For each significant defect, the investigation report should identify:
Observation → Evidence → Mechanism → Consequence → Required intervention
For example:
Chloride contamination is present at reinforcement depth, with corrosion staining and local reinforcement section loss. The evidence indicates chloride-induced reinforcement corrosion, with associated cracking and delamination. Continued ingress is likely to result in progressive deterioration unless the chloride source and corrosion mechanism are addressed.
The diagnosis should distinguish between:
- Primary causes.
- Contributing factors.
- Secondary damage.
- Consequences.
- Uncertainties requiring further investigation.
20. Risk Classification
Defects should be categorised according to their significance.
A suitable classification may include:
Category A – Critical
Defects with potential immediate or significant structural safety implications, such as substantial reinforcement loss, severe structural cracking or major loss of section in critical locations.
Immediate engineering review and, where necessary, temporary restrictions or propping should be considered.
Category B – Significant
Defects that may materially affect structural durability, serviceability or future capacity but do not appear to present an immediate safety concern.
These should be incorporated into the repair strategy and structural assessment.
Category C – Moderate
Deterioration affecting durability or serviceability but with limited immediate structural consequence.
Category D – Minor
Cosmetic or low-consequence defects requiring routine maintenance or monitoring.
The classification should be based on engineering judgement and the particular structural arrangement.
21. Determination of Repair Extent
The repair boundary should not automatically be defined by the visible extent of damage.
Where reinforcement corrosion is present, the repair extent should account for:
- Delaminated concrete.
- Carbonated concrete.
- Chloride-contaminated concrete.
- Corroded reinforcement.
- Loss of reinforcement cross-section.
- Poor-quality surrounding concrete.
- Access requirements.
- Compatibility with the proposed repair system.
The objective should be to remove or otherwise address the concrete and reinforcement conditions responsible for ongoing deterioration while maintaining the required structural performance.
22. Pre-Specification Information Requirements
Before preparing the repair specification, the investigation should establish, as far as reasonably practicable:
- Existing construction.
- Existing structural capacity or relevant design information.
- Concrete condition.
- Concrete strength.
- Reinforcement arrangement.
- Concrete cover.
- Carbonation depth.
- Chloride contamination.
- Reinforcement corrosion condition.
- Extent of delamination and spalling.
- Crack characteristics.
- Waterproofing condition.
- Drainage performance.
- Joint condition.
- Previous repair performance.
- Environmental exposure.
- Traffic/loading requirements.
- Required design life.
- Constraints on access and working.
- Phasing requirements.
- Health and safety constraints.
- Requirements for maintaining car park operation.
Any unresolved uncertainty that could materially affect the repair design should be clearly identified.
23. Reporting
The investigation report should contain, as a minimum:
23.1 Executive Summary
A concise statement of:
- Overall condition.
- Principal deterioration mechanisms.
- Structural significance.
- Key risks.
- Recommended next steps.
23.2 Description of Structure
Including:
- Construction.
- Approximate age.
- Structural system.
- Exposure.
- Use and loading.
- Known alterations.
23.3 Investigation Methodology
Describe:
- Visual inspection.
- Non-destructive testing.
- Intrusive investigation.
- Laboratory testing.
- Structural assessment methodology.
23.4 Findings
Present test results and observations by location and defect type.
23.5 Diagnosis
Explain the likely deterioration mechanisms and supporting evidence.
23.6 Structural Implications
Identify areas where deterioration affects or may affect:
- Strength.
- Stability.
- Serviceability.
- Durability.
- Future design life.
23.7 Repair Strategy
At pre-specification stage, recommendations should generally be expressed as repair principles and performance requirements, rather than prematurely prescribing proprietary products or detailed construction methods.
23.8 Further Investigation
Clearly identify any additional investigation required before final design or specification.
24. Repair Specification Interface
The investigation report should provide a clear technical basis for the subsequent repair specification.
The specification should ultimately define:
- Areas requiring repair.
- Concrete removal requirements.
- Reinforcement treatment requirements.
- Concrete repair material performance.
- Crack repair requirements.
- Waterproofing requirements.
- Joint repairs.
- Drainage improvements.
- Protective coating requirements.
- Surface preparation.
- Compatibility requirements.
- Quality control and testing.
- Acceptance criteria.
- Inspection and hold points.
- Environmental limitations.
- Curing requirements.
- Required service life.
The investigation should therefore avoid specifying a repair system before the deterioration mechanism and substrate condition have been adequately established.
25. Quality Assurance
All investigation works should be undertaken using appropriate calibrated equipment and competent personnel.
Laboratory testing should be carried out by appropriately accredited or technically competent laboratories.
The investigation should include suitable quality-control measures covering:
- Test location identification.
- Sample identification.
- Chain of custody.
- Equipment calibration.
- Laboratory testing.
- Data validation.
- Photographic records.
- Drawing revisions.
- Review of anomalous results.
Any limitations in access, sampling or testing should be explicitly recorded in the final report.
26. Conclusion
The purpose of a pre-specification investigation is to establish a defensible understanding of what is deteriorating, where it is deteriorating, why it is deteriorating, and what structural and durability consequences may result.
For car park slabs, particular emphasis should normally be placed on the interaction between:
water ingress + chloride exposure + concrete cover + reinforcement corrosion + cracking/delamination + waterproofing/drainage performance.
A robust investigation should therefore combine visual inspection, targeted non-destructive testing, intrusive examination, material testing and structural assessment where necessary.
Only once the condition and deterioration mechanisms have been sufficiently established should the findings be translated into a detailed repair and protection specification.
Appendix A – Suggested Investigation Schedule
| Investigation item | Typical purpose |
|---|---|
| Visual condition survey | Establish defect distribution and severity |
| Crack mapping | Establish cracking pattern and likely mechanism |
| Concrete cover survey | Assess reinforcement protection |
| Reinforcement scanning | Establish reinforcement arrangement |
| Local breakout | Verify reinforcement and concrete condition |
| Concrete cores | Establish in-situ concrete properties |
| Carbonation testing | Assess carbonation-induced corrosion risk |
| Chloride profiling | Assess chloride contamination |
| Half-cell potential survey | Identify areas with probable active corrosion |
| Concrete resistivity | Support corrosion assessment |
| Delamination survey | Establish hidden concrete deterioration |
| Waterproofing investigation | Identify water-ingress mechanisms |
| Drainage inspection | Identify persistent water sources |
| Joint inspection | Assess leakage and movement-related defects |
| Crack monitoring | Establish ongoing movement where required |
| Structural assessment | Determine implications for capacity |
| Previous repair investigation | Determine causes of repair failure |
Note: The final investigation scope, test frequency and acceptance criteria should be determined by the appointed engineer with reference to the applicable project standards, structural design, exposure environment and intended repair design life.