Chloride is the most severe cause of reinforcement corrosion in concrete and a recurring problem in coastal buildings, ports, bridges over salt water, chemical plants and seafood processing plants. Chloride testing is therefore a structural inspection (NDE) task that should be done before deciding on repairs, because if chloride has built up throughout the section, patch repairs alone will not work.

Collecting concrete powder in depth increments to find the chloride distribution, and reading the value at the reinforcement depth against the corrosion threshold The top-left image is a section through a coastal concrete wall. Salt spray and seawater carry chloride in from the surface; the concentration is high at the surface and decreases with depth. The reinforcement sits at a cover of about 45 millimeters and its surface has started to pit. Samples are taken with a drill, collecting powder separately at depth increments of 0 to 25, 25 to 50, 50 to 75 and 75 to 100 millimeters, after scanning for the bars first to avoid drilling into steel. The bottom-left image is a bar chart of chloride for each depth increment on the same depth scale; the values are hypothetical. The red dashed line is the corrosion threshold of about 0.20 percent acid-soluble chloride by weight of cement, per ACI 222R guidance. The value that governs is the one at the reinforcement depth, not at the surface. The right side summarizes the two test methods: ASTM C1152 measures total acid-soluble chloride, while ASTM C1218 measures only the water-soluble portion, which is always lower. The ACI 318-19 limits for new concrete refer to water-soluble chloride: prestressed concrete 0.06 percent, exposed to chloride 0.15, moist but with no chloride source 0.30, and dry 1.00 percent by weight of cementitious material. Limits must be compared with the matching test method, and results interpreted together with carbonation depth, concrete cover and Half-Cell Potential. Chloride Content: sample powder by depth, read the value at rebar level Not to scale Coastal wall section Cl⁻ 0–25 25–50 50–75 75–100 Rebar Pitting begins Drill powder per increment · scan rebar first 0.52 0.34 0.18 0.08 0 25 50 75 100 Depth from surface (mm) Chloride (%) → 0.20 Corrosion threshold (ACI 222R) Value at rebar = governing value % by wt. of cement (acid-soluble) · illustrative Match limit to test method ASTM C1152 · Acid-soluble Total chloride (free + bound) Existing: corrosion ~0.20% ASTM C1218 · Water-soluble Free (mobile) part only Always below C1152 ACI 318-19 · new concrete water-soluble, % of cementitious Prestressed 0.06 Chloride-exposed (C2) 0.15 Moist, no Cl⁻ source (C1) 0.30 Dry/protected (C0) 1.00 Interpret with: Carbonation · Cover · Half-Cell New-concrete limit ≠ demolition criterion
Example chloride profile with depth: chloride is highest at the surface and decreases with depth. What matters is the value at the depth of the reinforcement — whether it has already exceeded the level at which corrosion risk begins (figures in the image are illustrative only)

How Does Chloride Make Reinforcement Rust?

Chloride breaks down the protective film on the steel surface locally, causing pitting corrosion. Reinforcement in concrete that is still alkaline is coated with a thin passive film. When enough chloride ions reach the steel surface, this film breaks down at isolated spots, and those spots corrode deeper and faster than general surface corrosion.

Chloride is more dangerous than carbonation because it can corrode steel even while the concrete is still alkaline, and pitting can remove a large part of the bar cross-section while the concrete surface still looks normal. By the time concrete spalling is visible, the damage has usually already spread.

Chloride comes from two sources: chloride present in the mix from the start, such as poorly washed sand or contaminated water, and chloride that penetrates later from salt spray, seawater, saline groundwater or process chemicals.

📚 Key points

Chloride measured at the concrete surface is often very high, but the value that governs the risk is the one at the depth of the reinforcement. Samples must therefore be taken in depth increments, not from a single surface point.

What Types of Chloride Test Are There?

Building assessments use two main standards, ASTM C1152 and ASTM C1218, which measure different fractions of chloride.

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MethodMeasuresWhen to use
ASTM C1152
Acid-Soluble		All chloride in the sample, both the mobile fraction and the fraction bound in the cement pasteAssessing total chloride content and tracking penetration with depth
ASTM C1218
Water-Soluble		Only water-soluble chloride, which approximates the mobile fraction that drives corrosionComparing with ACI 318-19 limits, which refer to water-soluble chloride

Values from ASTM C1218 are always lower than those from ASTM C1152 on the same sample because they measure different fractions. Before comparing a result with any limit, check which method that limit is based on.

In the laboratory, the powder sample is ground finely, the chloride is extracted with dilute nitric acid (C1152) or water (C1218), filtered, and then titrated with silver nitrate solution to determine the chloride content. The result is a percentage by weight of the concrete sample. To compare it with a limit expressed as a percentage by weight of cement or cementitious material, the units must first be converted using the cement content of the mix.

2D diagram of the laboratory chloride analysis steps, from grinding the concrete powder, extracting with acid or water, filtering and titrating, to converting units before comparing with limits A three-part diagram. The top part is a five-step laboratory sequence. Step one, grind and sieve the concrete powder finely. Step two, weigh the sample on a balance. Step three, extract the chloride from the powder on a hot plate, with one beaker using dilute nitric acid per ASTM C1152 and another using water per ASTM C1218. Step four, filter to obtain a clear solution. Step five, titrate with silver nitrate solution using an electrode to find the end point, then calculate the chloride content. The bottom-left part is a bar showing that total chloride in the sample consists of a water-soluble, mobile portion and a portion bound in the cement paste; ASTM C1152 measures the whole bar, while ASTM C1218 measures only the water-soluble portion, so C1218 values are always lower than C1152 on the same sample. The proportions shown are hypothetical. The bottom-right part shows the unit conversion from percent by weight of concrete to percent by weight of cement, by multiplying by the ratio of concrete weight to cement weight. Hypothetical example: 0.03 percent times 2350 divided by 350 gives about 0.20 percent, so the cement content of the mix must be known. The bottom strip stresses three points: state the test method, state the units, and compare with a limit based on the same method. 2D diagram: in the lab, from concrete powder to % chloride Not to scale Laboratory sequence ASTM C1152 (acid-sol.) · ASTM C1218 (water-sol.) 1 Grind & sieve Fine and uniform 2 Weigh Record mass Acid Water 3 Extract Cl⁻ Acid (C1152) or water (C1218) 4 Filter Clear solution mV 5 AgNO₃ titration Find chloride Two methods, two fractions Water-soluble · mobile Bound in cement ASTM C1152 (acid-soluble) = total chloride ASTM C1218 (water-soluble) Same sample: C1218 always below C1152 Proportions illustrative Reported units: convert before comparing % chloride by wt. of concrete % chloride by wt. of cement multiply by concrete wt. ÷ cement wt. Example (illustrative): concrete 2,350 · cement 350 kg/m³ 0.03% × (2,350 ÷ 350) ≈ 0.20% by wt. of cement Cement content of mix must be known If unknown, engineer states assumption 1 State test method acid- or water-soluble 2 State units clearly per concrete or per cement 3 Compare like with like ACI 318-19 uses water-soluble
2D diagram of the laboratory work: grind the powder, weigh, extract with acid (C1152) or water (C1218), filter and titrate. Below, it shows that C1152 measures total chloride while C1218 measures only the water-soluble portion, plus an example of converting from percent by weight of concrete to percent by weight of cement (figures are illustrative)

Another standard used on bridge and highway work is AASHTO T 260, which covers both sampling and analysis, while ASTM C1556 is used to determine the chloride diffusion coefficient for long-term service-life assessment.

What Chloride Level Is Dangerous?

There is no single number for every case. New concrete is judged against ACI 318-19, while in-service buildings refer to the corrosion threshold given in ACI 222R. For new concrete, ACI 318-19 Table 19.3.2.1 sets the maximum water-soluble chloride content as a percentage by weight of cementitious material as follows:

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Concrete type (Exposure Class)Max. chloride (water-soluble)
Prestressed0.06%
Reinforced concrete exposed to chloride in service (C2)0.15%
Reinforced concrete exposed to moisture but with no external chloride source (C1)0.30%
Reinforced concrete that is dry or protected from moisture (C0)1.00%

For buildings already in service, reference is usually made to the level at which corrosion begins, which ACI 222R guidance gives as about 0.20% acid-soluble chloride (acid-soluble per ASTM C1152) by weight of cement (0.08% for prestressed concrete). Always state the test method alongside the value, because acid-soluble and water-soluble limits cannot be compared directly.

This value is not a fixed cutoff, because it depends on moisture, concrete pH, cement type and oxygen availability. Results must therefore be interpreted together with carbonation depth, concrete cover and Half-Cell Potential results.

⚠️ Cautions

The ACI 318-19 limits are requirements for new concrete at the time of construction, not criteria for deciding whether an existing building must be demolished or repaired. Applying new-concrete limits directly to in-service buildings is a common misunderstanding.

In the Thai context, assessment of in-service buildings refers to DPT 1902-62, the Department of Public Works and Town & Country Planning standard for inspecting and evaluating the strength of old and damaged building structures, which has dedicated clauses on chloride-induced reinforcement corrosion (clauses 4.2.3.3 and 5.3.3) and is used as the inspection framework. Repair work refers to DPT 1901-51, the standard practice for concrete repair.

How Should Samples Be Taken for Reliable Results?

Results are reliable when concrete powder is collected separately by depth increment, from both damaged and sound areas. Common practice follows five points:

  • Drill out concrete powder in depth increments, e.g. 0–25, 25–50 and 50–75 mm, to build a chloride profile with depth
  • Choose sampling points covering both damaged areas and areas that still look sound, so they can be compared
  • Avoid drilling into reinforcement by first locating the bars with a cover meter
  • Store powder in sealed containers, clearly labeled with location and depth increment, so samples are not mixed up
  • Record the environment at each point, such as distance from the sea, wind direction, and ponding or leaking water
3D model of on-site concrete powder sampling for chloride testing on a coastal column, drilling in depth increments at both a damaged point and a point that still looks sound, then bagging each increment separately A conceptual 3D view of a reinforced concrete wall-column on a concrete deck by the sea. The face of the column facing the sea receives wind-driven salt spray. Dashed lines on the surface show the bar positions from the scanner. Point A is in an area with cracks and rust stains, where a hammer drill with a depth stop is drilling powder in the gap between bars. Point B, higher up in an area that still looks sound, is marked for comparison sampling. Three sealed sample bags are lined up on the deck. The lower-left panel shows that one drill hole yields three bags by depth increment, 0 to 25, 25 to 50 and 50 to 75 millimeters, which are sent for analysis to ASTM C1152 or C1218. The lower-right panel shows the drill bit with a depth stop, stopping at each increment to collect powder separately. The right panel summarizes five practices: drill in depth increments, sample both damaged and sound points, scan for rebar before drilling, store in sealed labeled containers, and record the environment, and stresses that the governing value is the one at the reinforcement depth, not at the surface. 3D model: sampling concrete powder for chloride on a coastal column Not to scale Point B: sound area For comparison with damage Bar lines from scanner Drill between bars Sea wind carries salt Builds up, then soaks in Point A: damaged area Cracks and rust stains Powder drill With depth stop Sealed sample bags Point + depth range Sea Point A · 1 hole = 3 samples 0–25 mm 25–50 mm 50–75 mm Send for lab analysis ASTM C1152 or C1218 Depth stop Stop touches face = stop Collect, then drill next Reliable sampling 1 Drill by depth e.g. 0–25 · 25–50 · 50–75 mm 2 Damaged and sound points So results compare 3 Scan rebar first Avoid hitting rebar 4 Sealed, labeled Location and depth 5 Record environment Sea distance, wind, leaks The value governing risk is the one at rebar depth not at the concrete face Option: drill a core Slice in the lab Better depth control, more intrusive Conceptual, not actual design
3D model of concrete powder sampling on a coastal column: scan the bar layout first, drill with a depth-stop drill at both a damaged point (A) and a point that still looks sound (B), then put the powder from each depth increment in sealed, labeled bags (conceptual image)

Another option is to drill a concrete core to ASTM C42 and slice it into layers in the laboratory. This controls depth more precisely than drilling powder on site, but is more intrusive to the structure and takes longer.

How Does a Chloride Content Test Differ from RCPT?

The two methods answer different questions. A Chloride Content Test tells you “how much chloride is there now”, while the Rapid Chloride Permeability Test (RCPT) to ASTM C1202 tells you “how easily this concrete lets chloride through”, using the electrical charge passed through the specimen as an indicator.

Assessments of existing buildings that need to know whether the reinforcement is already at risk of corrosion require a Chloride Content Test, while RCPT suits acceptance testing of new concrete or comparing mix designs.

Which Buildings Should Be Tested for Chloride?

Testing is advisable when a structure is exposed to salt spray, salt water or chloride-bearing chemicals, or when rust is found and its cause is not yet known. Five groups are commonly encountered:

  • Buildings, condominiums and resorts within reach of sea salt spray
  • Ports, breakwaters, bridges and structures in direct contact with seawater or brackish water
  • Chemical plants, seafood processing plants and cold-storage facilities that use brine
  • Structures showing rust stains or spalling concrete of unknown cause
  • Buildings due for major repair or change of use that need a remaining-life assessment

In structures where chloride is distributed throughout the section, patch repairs can trigger corrosion at the edges of the repair. Test results therefore directly affect the choice of repair method — from patch repair and protective coatings to electrochemical corrosion protection systems — which must be specified by the engineer of record.

✅ SPN offers end-to-end chloride testing

From planning sampling points and on-site sampling to analysis to ASTM C1152 / C1218, through to a report certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and license number, and engineering conclusions that can be used directly for repair planning.

How Much Does Chloride Testing Cost?

The fee depends on the number of sampling points, the number of depth increments per point, the test method chosen and site access. Contact us for a free quotation within 24 hours. This price does not include travel and equipment mobilization, which are charged extra according to the distance to site.