Water and Wastewater Protective Coating Systems
Article 07 of 24
Evaluating Existing Concrete Structures
Before concrete can be repaired, prepared, or coated, the project team must determine what is sound, what is contaminated, what is deteriorated, and what requires evaluation beyond the coating contractor’s responsibility.
The Coating Can Only Be as Sound as the Concrete Beneath It
A high-performance lining applied over weak, contaminated, cracked, chemically deteriorated, or poorly repaired concrete can fail even when the coating material is correctly mixed and applied.
The purpose of the condition assessment is to identify the existing substrate, determine the extent and likely causes of deterioration, define repairs, establish preparation requirements, and confirm that the remaining concrete can support the proposed protective system.
Condition assessment should occur before the final scope and price are established. Once surface preparation begins, concealed damage may be discovered, but the project should not depend on blasting equipment to perform the initial investigation.
Know the Limit of the Contractor’s Evaluation
The coating contractor should recognize and document concrete deterioration, but should not make unsupported structural conclusions.
Conditions that may require evaluation by a licensed design professional or other qualified specialist include:
- Significant section loss
- Exposed or heavily corroded reinforcing steel
- Wide, displaced, active, or leaking cracks
- Movement at joints or structural connections
- Large areas of delamination or spalling
- Evidence of settlement, overloading, impact, or structural movement
- Unknown or severe chemical deterioration
- Damage affecting wall, roof, floor, beam, column, or tank integrity
- Any condition outside the contractor’s training, authority, or contractual responsibility
Begin with Records and Process Information
Before entering or cleaning the structure, collect available information about its construction, service, maintenance, and previous repairs.
- Original drawings and specifications
- Concrete mix and construction records
- Age of the structure
- Normal and maximum operating levels
- Water, wastewater, sludge, and chemical exposure
- Temperature and pH history
- Hydrogen-sulfide or odor-control records
- Previous coating and lining records
- Previous concrete repairs
- Leak, crack, or structural inspection reports
- Known operational upsets and chemical spills
Records may be incomplete or inaccurate. Use them to guide the investigation, not as a substitute for field verification.
Clean Enough to See the Concrete
Wastewater residue, biological growth, sludge, grease, scale, deposits, salts, and failed coatings can conceal the actual surface. A meaningful assessment may require preliminary cleaning before close visual examination or testing.
Preliminary cleaning is not necessarily final surface preparation. Its purpose is to expose the substrate sufficiently to identify deterioration, contamination, cracks, repairs, and existing coatings.
Cleaning wastewater structures can release hazardous contaminants and gases. The work must be planned with appropriate containment, ventilation, personal protection, waste handling, atmospheric monitoring, and confined-space controls.
Perform a Systematic Visual Survey
Inspect the structure in a planned sequence. Use drawings, grids, elevations, photographs, or marked diagrams so that every observation can be tied to a location.
Look for:
- Cracks and active leakage
- Spalls and delaminated concrete
- Soft, friable, or powdering surfaces
- Exposed fine or coarse aggregate
- Exposed or corroding reinforcing steel
- Rust staining
- Efflorescence and mineral deposits
- Acid attack or chemical erosion
- Abrasion, erosion, and cavitation damage
- Honeycombing, bugholes, fins, voids, and form offsets
- Previous patches and repairs
- Failed coatings or linings
- Wet areas, seepage, and hydrostatic pressure indicators
- Deteriorated joints and sealants
- Damage at penetrations, edges, corners, and transitions
Sound the Concrete
Sounding can help identify near-surface delamination, voids, and areas that may not be visibly detached. A hammer, chain drag, or other appropriate method may be used depending on the surface orientation and project procedure.
Sound concrete typically produces a different response from hollow or delaminated areas. Results should be marked directly on a drawing or surface map.
Sounding is a screening method, not a complete structural evaluation. Suspect areas may require additional testing, removal, coring, engineering review, or other investigation.
Determine Surface Strength
The concrete surface must have sufficient tensile strength to support the repair material, primer, coating, or lining. A coating can remain attached to a weak surface layer while the weak layer separates from the concrete below.
Project specifications may require pull-off testing or another method to evaluate surface tensile strength and adhesion. ASTM C1583 is commonly referenced for pull-off strength of concrete surfaces and repairs. Coating-related adhesion testing may involve other specified methods.
Record the measured value and the location and type of failure. Failure within weak concrete provides different information from failure at an adhesive interface or within a repair material.
Classify the Cracks
A crack is evidence of movement or stress, but its appearance alone does not establish the cause. Cracks may result from shrinkage, thermal movement, settlement, reinforcement corrosion, structural loading, chemical reaction, restraint, impact, or construction practices.
Document:
- Location and orientation
- Visible length
- Measured width where practical
- Dry, damp, leaking, or previously repaired condition
- Displacement across the crack
- Evidence of continuing movement
- Relationship to joints, penetrations, corners, reinforcement, or structural members
- Condition of previous crack repairs
Do not assume that every crack can be filled rigidly and coated. Active cracks and moving joints require details capable of accommodating expected movement. Structural cracks require appropriate evaluation and repair design.
Evaluate Concrete Deterioration by Exposure Zone
The pattern of deterioration can help identify contributing exposure conditions.
| Location |
Possible Conditions |
Assessment Focus |
| Immersion zone |
Chemical exposure, abrasion, erosion, biological deposits |
Surface loss, contamination, cracks, previous lining condition |
| Wet-and-dry zone |
Cycling, deposits, moisture movement, changing chemistry |
Scaling, salts, coating edges, cracks, debonding |
| Wastewater headspace |
Condensation, hydrogen sulfide, biogenic sulfuric acid |
Soft concrete, exposed aggregate, low-pH surface, crown deterioration |
| Exterior surface |
Weathering, freeze-thaw, moisture entry, carbonation |
Cracking, spalling, rust staining, previous repair condition |
Check for Chemical Deterioration
Chemical attack can alter the concrete surface and reduce its strength. Wastewater structures may experience biogenic sulfuric-acid corrosion, sulfate exposure, aggressive cleaning chemicals, treatment chemicals, industrial discharges, or other contaminants.
Surface pH testing can provide useful information, but a single reading does not define the complete depth or cause of deterioration. Testing may be affected by cleaning, moisture, deposits, carbonation, and localized conditions.
Where chemical attack is suspected, the repair limits should extend to sound concrete—not merely to a visually acceptable surface. Obtain qualified assistance when the depth, cause, or structural effect is uncertain.
Evaluate Reinforcing-Steel Corrosion
Rust staining, longitudinal cracks, delamination, and spalling may indicate corrosion of reinforcing steel. As steel corrodes, expanding corrosion products can create internal pressure and fracture the surrounding concrete.
When reinforcement is exposed, the project team should determine:
- The extent of concrete removal required
- The degree of steel section loss
- Required cleaning of the reinforcement
- Whether supplemental reinforcement is necessary
- Required corrosion-control treatment
- Repair-material placement and cover requirements
- Whether the repair requires engineering design
Do not conceal severely corroded reinforcement beneath repair mortar and coating without the required evaluation and repair.
Moisture Is a Condition—not a Single Number
Concrete in water and wastewater facilities may contain moisture from immersion, process leakage, groundwater, condensation, cleaning, rainfall, incomplete drying, or vapor movement.
The evaluation should identify:
- The likely moisture source
- Whether the source is temporary or continuing
- Visible dampness or active leakage
- Moisture moving through cracks, joints, penetrations, or walls
- Whether hydrostatic pressure may be present
- Whether the proposed system tolerates damp concrete
- What testing and acceptance criteria are required
ASTM D4263 provides a qualitative practice for indicating moisture in concrete using a plastic sheet. It does not identify every moisture source or guarantee coating compatibility.
Use the test methods, locations, duration, and acceptance limits required by the specification and coating manufacturer. Correct active water intrusion before applying a system that cannot tolerate it.
Evaluate Existing Coatings and Repairs
An existing coating may appear intact while concealing weak concrete, corrosion, moisture, contamination, or loss of adhesion. Previous repairs may also differ significantly in strength, porosity, profile, and compatibility.
Document:
- Known coating or repair-material identity
- Approximate age and service history
- Blistering, cracking, peeling, erosion, chalking, or discoloration
- Adhesion and cohesive condition
- Moisture or corrosion beneath the system
- Compatibility with the proposed repair or coating system
- Whether complete removal is required
A small successful test patch does not prove that every portion of a large existing coating is sound. The specification should define the investigation, acceptance, and removal requirements.
Create a Condition Map
A condition map turns observations into measurable project information. Use photographs, marked drawings, grids, elevations, or digital records to identify:
- Sound concrete
- Unsound or delaminated areas
- Depth and area of anticipated removal
- Cracks by type and condition
- Active leaks and damp areas
- Exposed reinforcement
- Existing coatings and repairs
- Joints, penetrations, edges, and transitions
- Contaminated areas
- Locations requiring additional testing or engineering review
Quantify the findings whenever possible. “Repair concrete as needed” is not an adequate basis for estimating a severely deteriorated wastewater structure.
Establish Acceptance Criteria Before Preparation
The parties should agree on how the prepared and repaired concrete will be accepted before large-scale work begins.
Acceptance criteria may address:
- Removal of unsound concrete
- Required surface tensile strength
- Surface cleanliness
- Required concrete surface profile
- Maximum permitted voids and surface irregularities
- Moisture condition
- Repair-material cure
- Treatment of cracks, joints, and penetrations
- Removal of dust and debris
- Required inspection hold point before coating application
Article 07 Knowledge Check
- Why should concrete condition assessment occur before final pricing?
- Which conditions should be referred for structural or engineering evaluation?
- What is the difference between preliminary cleaning and final surface preparation?
- What can sounding indicate, and what can it not establish?
- Why is the location and type of failure important during pull-off testing?
- Why should cracks be classified before selecting a repair method?
- What information should be included on a concrete condition map?
- Why should acceptance criteria be established before surface preparation begins?
Technical References and Further Study
- AMPP: SSPC-SP 13/NACE No. 6-2024, Surface Preparation of Concrete.
- American Concrete Institute: ACI CODE-562, Assessment, Repair, and Rehabilitation of Existing Concrete Structures.
- American Concrete Institute: ACI 546R, Guide to Concrete Repair.
- International Concrete Repair Institute: ICRI 310.2R, Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair.
- ASTM International: ASTM C1583, pull-off tensile strength of concrete surfaces and repairs.
- ASTM International: ASTM D4263, Standard Practice for Indicating Moisture in Concrete by the Plastic Sheet Method.
- Project documents and manufacturer requirements: Use the specified assessment procedures, acceptance criteria, repair details, moisture limits, surface-profile requirements, and coating-system instructions.
Standards and codes are revised. Always verify the edition required by the project and obtain current documents from the issuing organization.
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