Concrete exposed to moisture and an external chloride source should be treated as a chloride exposure even when the salt concentration is far below seawater. In the ACI 318-19 framework, moist soil containing chlorides is classified as C2 because chlorides can move through concrete to reinforcing steel and start corrosion. The classification is not reduced by measuring a smaller salt concentration.
The numerical limits below are specifically the values reproduced for ACI 318-19(22). ACI has published ACI CODE-318-25, but the current C2 table and local adoption were not established by the cited material. Confirm the jurisdiction’s adopted edition, project specifications and structural requirements before using any value in a contract document.
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Start with the exposure, not the seawater comparison
Chloride design begins by identifying whether concrete will be exposed to both moisture and an external source of chloride. That combination is what makes the exposure significant: water provides a path for dissolved chlorides to enter pores and cracks, and reinforcement provides the steel that can corrode when chlorides reach it.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThe American Concrete Institute’s official answer to a soil-classification question is explicit: “ACI 318-19 does not take into account the amount of salt present. Exposure to any amount of salt is considered a concern for corrosion.” The same answer explains that chlorides in moist soil can penetrate concrete toward reinforcing steel and initiate corrosion at concentrations well below those in seawater. Read the complete FAQ at ACI’s chloride concentration in soil and exposure classes FAQ.
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Consequently, a specification should not assign a less severe class merely because a laboratory result shows less chloride than seawater. First establish the wetting condition and the chloride source; then apply the exposure provisions in the code edition adopted for the project.
Sources that can create chloride exposure
ACI identifies deicing chemicals, salt, brackish water, seawater and spray from those sources as examples of external chloride sources. The same logic can apply to chloride-bearing, wet soil in contact with a foundation, retaining wall or other element. A site may have several sources at once, such as tidal splash combined with airborne spray or seasonal deicing runoff.
- Deicing chemicals: pavements, parking structures, bridge decks, ramps and loading areas can receive repeated wetting with chloride solutions.
- Salt or contaminated water: industrial or storage areas may expose concrete to chloride-bearing wash water or process water.
- Brackish or seawater: immersion, tidal cycling, splash and spray all create different wetting patterns that the design team must describe.
- Moist chloride-bearing soil: below-grade concrete can be C2 even when the measured soil concentration is nowhere near seawater.
ACI’s historical Q&A material discusses brackish water terminology, but a project should rely on its current geotechnical, environmental and structural documents rather than an informal label. See the abstract at ACI’s Concrete Q&A resource.
When the chloride classification question may not apply
Concrete that remains genuinely dry and isolated from external chlorides does not have the same exposure as concrete that is periodically wetted by chloride-bearing water. “Dry” must be a defendable service condition, not an assumption based only on the absence of visible water on the day of inspection. Groundwater, capillary moisture, condensation, leaks, splash, spray, drainage changes and seasonal conditions can change the classification.
Record the expected wetting mechanism and the source in the design basis. If conditions are uncertain, resolve the uncertainty with the owner, geotechnical professional and engineer of record rather than silently selecting a milder class.
Confirm which code edition governs
Exposure limits are code requirements, so the edition matters as much as the number. ACI lists ACI CODE-318-25: Building Code for Structural Concrete—Code Requirements and Commentary as its 2025 edition. Publication does not by itself establish that a particular state, municipality or owner has adopted it.
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- Identify the authority having jurisdiction and the code edition named in the permit, contract or project criteria.
- Check amendments, referenced standards and any owner durability criteria that supplement the structural code.
- Open the exact exposure and chloride tables in that adopted edition; do not copy a value from an earlier edition without labeling it.
- Coordinate the structural use, reinforcement type, exposure class and specified strength with the drawings and specifications.
- Keep the edition and table reference in the concrete submittal so reviewers can audit the basis.
The figures in the next table are the values reproduced for ACI 318-19(22) in an ACI-published article. They are not presented as verified ACI 318-25 limits. The article is available at ACI’s “Evaluation” publication.
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ACI 318-19(22) C2 values to carry into a preliminary design
Use this table only when the project is governed by the cited ACI 318-19(22) provisions, or as a clearly labeled preliminary reference while the adopted edition is being confirmed.
| Requirement | C2 value reproduced for ACI 318-19(22) | How to apply it |
|---|---|---|
| Maximum water-cementitious materials ratio (w/cm) | 0.40 | Do not exceed the limit in the approved mixture for the applicable C2 exposure. |
| Minimum specified compressive strength | 5,000 psi (34.5 MPa) | Use as the specified strength requirement for the C2 condition, subject to the adopted code and project criteria. |
| Maximum water-soluble chloride-ion content by weight of cementitious materials for reinforced concrete | 0.15% | Account for chloride contributed by every mixture ingredient and verify the required test basis. |
| Maximum water-soluble chloride-ion content by weight of cementitious materials for prestressed concrete | 0.06% | Use the prestressed-concrete limit when that structural category applies; do not substitute the reinforced-concrete value. |
These numbers are edition-specific. The cited material does not reproduce the current C2 table in ACI 318-25, so it is not evidence that the limits are unchanged. A project team using the 2025 edition should obtain the code and commentary through the official ACI CODE-318-25 store listing or another authorized source and verify the adopted text.
Why w/cm and strength are separate checks
A maximum w/cm limits the amount of mixing water relative to cementitious materials; it is a durability control, not merely a strength target. Minimum specified compressive strength is a separate acceptance and design requirement. A mixture can meet a strength target while failing the exposure w/cm limit, or meet a w/cm limit while failing to achieve the specified strength. The approved mixture must satisfy both, along with workability, temperature, finishing and other project requirements.
Why the chloride-ion limit follows the concrete type
Reinforced and prestressed concrete are separate categories in the reproduced table. The prestressed limit is lower, so the mixture submittal must identify which category governs before calculating allowable chloride. Do not report a single “chloride limit” without naming the concrete type and code edition.
A step-by-step mixture-design workflow
1. Write the exposure statement
Describe the element, its location, moisture condition, chloride source and expected wetting pattern. For example: “Foundation wall in contact with moist chloride-bearing soil,” or “Parking-deck slab subject to wetting from deicing chemicals.” State whether the member is reinforced or prestressed and identify the structural design strength.
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2. Select the governing table
Using the adopted code, identify the exposure class and retrieve the corresponding w/cm, strength and chloride provisions. If the contract cites ACI 318-19(22), the reproduced C2 values are 0.40 w/cm and 5,000 psi minimum specified strength, with the ingredient-chloride limits shown above. If the contract cites another edition, replace those figures with the exact adopted table rather than assuming continuity.
3. Inventory chloride contributions
Request chloride information for cementitious materials, aggregates, mixing water, admixtures and any other ingredient that could contribute soluble chloride. Check the supplier’s test method, reporting units, sample date and lot or source. Calculate the total against the limit expressed as a percentage by weight of cementitious materials. Keep reinforced and prestressed calculations separate.
4. Proportion for the w/cm limit and the required performance
Set the proposed w/cm at or below the applicable maximum, then verify that the mixture can reach the specified strength under the project’s curing and testing regime. Adjust aggregate grading, paste volume, admixture dosage and placement properties through the qualified concrete producer’s normal proportioning process. Do not add water at the site to recover slump if doing so would raise the approved w/cm.
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5. Resolve competing requirements before approval
Low w/cm can make placement and finishing more demanding, especially where reinforcement is congested or sections are deep. Review slump or flow, pumpability, setting behavior, temperature control and finishing sequence with the contractor and producer. A mixture that is theoretically durable but cannot be consolidated around reinforcement is not a successful design.
6. Document the approved mixture
The submittal should identify the code edition and table, exposure class, concrete type, specified strength, maximum w/cm, calculated ingredient-chloride content, constituent sources, admixtures, aggregate moisture assumptions, batch tolerances and required verification tests. Include procedures for changes in source, moisture or dosage; an unreviewed source substitution can change chloride content or workability.
Durability depends on execution as well as proportioning
ACI’s durability topic resource states: “Durable concrete is a result of proper design, proportioning, placement, finishing, testing, inspection, and curing.” The statement is important because a low w/cm is not a substitute for construction quality. Read the topic resource at ACI’s durability of concrete page.
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Placement and consolidation
Plan delivery, discharge, pumping and vibration so concrete fully surrounds reinforcement and embedded items. Avoid segregation, cold joints and unplanned construction joints. The placement plan should identify access, lift sequence, consolidation equipment and the person authorized to reject water additions or unapproved retempering.
Finishing
Coordinate finishing with setting time and weather. Finishing bleed water into the surface or adding water to make finishing easier can create a weaker, more permeable surface. Establish the permitted finishing operations and timing in the method statement, particularly for slabs exposed to deicing chemicals or spray.
Curing
Curing supports development of the designed concrete properties and limits premature moisture loss. Specify the curing method, start time, duration and inspection records required by the adopted code and project specification. The material cited here does not establish one universal curing duration for every chloride-exposed member; use the project’s governing requirements and qualified engineering judgment.
Testing and inspection
Define sampling locations, frequency, acceptance criteria and responsibilities before the pour. Strength tests, w/cm control records, air or temperature checks where specified, and ingredient-chloride verification should be traceable to the batch and placement location. Inspection should document reinforcement condition, forms, joints, consolidation, finishing and curing—not only cylinder results.
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Before the pour
- Confirm the jurisdiction, adopted code edition and exposure classification.
- Verify reinforced versus prestressed designation and specified strength.
- Review the approved mixture’s maximum w/cm and calculated water-soluble chloride content.
- Confirm current certificates or test reports for cementitious materials, aggregates, water and admixtures.
- Inspect reinforcement, forms, joints, embeds, drainage details and access for consolidation.
- Approve the placement, finishing, testing, inspection and curing plans.
During placement
- Record delivery time, batch identification, measured slump or flow where specified, temperature and any authorized adjustments.
- Prevent unauthorized water addition or changes to admixture dosage.
- Observe discharge, lift thickness and consolidation around reinforcement and penetrations.
- Record construction joints, interruptions, rejected loads and corrective actions.
- Protect fresh concrete from rapid drying, contamination and chloride-bearing water.
After placement
- Start the approved curing procedure promptly and record its continuity.
- Complete required strength and other acceptance tests with traceable sample identification.
- Document defects, repairs, cracking, curing interruptions and their engineering disposition.
- Retain batch tickets, inspection reports, test results and approved substitutions with the project record.
Common specification errors and how to correct them
“The soil has less salt than seawater, so use a lower class.”
This is the central error addressed by ACI’s FAQ. If the soil is moist and supplies chlorides, classify the exposure under the adopted code’s chloride provisions; do not scale the class by seawater concentration.
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An online table may reproduce a different edition or an amended local requirement. Put the edition beside every numeric limit and verify the jurisdiction before issuing the specification.
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“The strength test proves durability.”
Strength acceptance does not replace the w/cm, ingredient-chloride, placement, curing and inspection requirements. Review each control independently.
“One chloride limit fits every concrete member.”
The reproduced ACI 318-19(22) table distinguishes reinforced concrete from prestressed concrete. Identify the structural category before calculating the allowable chloride content.
“A low w/cm guarantees a durable surface.”
Improper consolidation, finishing or curing can undermine the intended performance. Use an execution plan and inspection records, not proportioning alone, to demonstrate control.
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The cited material does not establish a universal current cover depth, chloride diffusion coefficient, migration-test threshold or curing duration. Obtain those requirements from the adopted code, project specification, exposure and design-life basis. Do not transplant a historical recommendation into a current project without checking its context.
Decision matrix for project teams
| Decision axis | Question to answer | Evidence to retain |
|---|---|---|
| Exposure source and wetting | What chloride source contacts the concrete, and how often can it become wet? | Site, geotechnical, environmental and drainage information; exposure statement. |
| Code edition | Which edition and amendments are legally or contractually adopted? | Permit basis, contract reference and cited table. |
| Structural category | Is the member reinforced or prestressed? | Structural drawings and mixture classification. |
| Mixture limits | Does the approved mixture meet the applicable w/cm, strength and ingredient-chloride limits? | Proportioning report, batch calculations and supplier test data. |
| Constructability | Can the crew place, consolidate, finish and cure the mixture without unauthorized changes? | Method statement, pre-pour inspection and placement records. |
| Verification | How will compliance be sampled, tested, inspected and recorded? | Inspection and testing plan, reports, tickets and corrective-action log. |
Suggested specification-basis wording
A project basis can be written in a form that prevents the most common ambiguity:
“Concrete elements exposed to moisture and an external chloride source shall be classified using the adopted edition of ACI 318 and applicable local amendments. The exposure determination shall not be reduced solely because measured chloride concentration is below seawater concentration. The concrete submittal shall identify the adopted code edition, exposure class, reinforced or prestressed classification, specified compressive strength, maximum w/cm, calculated water-soluble chloride-ion content from all ingredients, and required placement, finishing, testing, inspection and curing procedures.”
If the contract specifically uses ACI 318-19(22), add the applicable table values—C2 maximum w/cm 0.40, minimum specified compressive strength 5,000 psi (34.5 MPa), and the relevant reinforced or prestressed chloride limit—while stating that the values are from that edition. Replace them when the governing adopted edition requires different limits.
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- Can the team point to the moisture condition and external chloride source?
- Is the exposure class tied to the legally adopted code edition rather than to a generic web table?
- Are reinforced and prestressed members separated wherever their chloride limits differ?
- Does the mixture meet the applicable w/cm, specified strength and ingredient-chloride requirements?
- Are placement, finishing, testing, inspection and curing requirements written and assigned?
- Are source changes, water additions, rejected loads and curing interruptions controlled?
- Have unresolved items—such as cover, diffusion criteria, migration testing or curing duration—been obtained from the adopted code and project-specific design basis instead of guessed?
When these questions are answered in the design documents and verified in the field, chloride durability becomes a controlled chain from exposure classification through construction—not a single strength number chosen from a seawater comparison.
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