Columns or beams with rust stains running down, render bulging in sheets, or exposed reinforcement are signs that the steel inside is already corroding. Patching over the surface without fixing the cause usually brings the problem back within a few years. This article explains the mechanism, the test set used to assess the member before repair, and the repair sequence accepted in structural inspection (NDE) practice.
Why does concrete spall?
Concrete spalling is when the concrete cover over the reinforcement cracks and breaks away from the section. The most common cause is corroding reinforcement. Corrosion products occupy several times the volume of the original steel, and the expansive pressure first cracks the concrete along the bar, then breaks it off in slabs.
A common misconception is that the concrete only cracks once the rust has fully expanded 2–6 times. Research shows surface cracks appear when the steel has corroded only a few tens of micrometers, and before spalling becomes visible the concrete usually passes through a stage where it has already cracked in a plane parallel to the surface along the bars (delamination). This stage can be detected by hammer tapping or Impact Echo, and it is the most cost-effective time to repair.
Reinforcement in sound concrete is protected by high alkalinity (pH about 12.5–13.5), which forms a thin film on the steel surface. This film can be destroyed by 2 main mechanisms:
- Carbonation — carbon dioxide from the air penetrates the concrete and lowers its alkalinity. Once it reaches the steel, the protective film breaks down. Checked with the Carbonation Test
- Chloride ingress — sea-salt spray, saline groundwater or chemicals break the protective film locally and cause pitting corrosion. Checked by measuring chloride content in concrete
Common accelerating factors on site are concrete cover thinner than designed, existing cracks that let moisture in, and spots with regular ponding or leaks. Other causes unrelated to corrosion, such as fire heat or impact, can also cause spalling, but the damage looks different.
Cracks along the reinforcement with reddish-brown rust stains seeping out are a specific sign of corrosion, unlike shrinkage or settlement cracks, which usually show no rust staining.
What are the warning signs before concrete spalls?
Corrosion damage gives warning signs before the concrete breaks off in slabs. Watch for these 5 signs:
- Reddish-brown rust stains seeping from cracks or the concrete surface
- Long cracks running parallel to the reinforcement in columns, beams and slab soffits
- Render bulging in sheets that sounds hollow when tapped (delamination), even if it has not fallen off
- White deposits or salt efflorescence on the concrete surface, especially in seaside buildings and under parking deck slabs
- Earlier patch repairs cracking again in the same location
Light hammer tapping (hammer sounding) is the fastest and cheapest screening method for mapping areas that have delaminated but not yet fallen off, and should always be done before choosing detailed test locations.
What should be tested before repair?
A pre-repair investigation must answer 3 questions: what is the cause, how extensive is the damage, and has load capacity been reduced. The usual test set has 5 items:
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| Test item | What it answers | Reference standards |
|---|---|---|
| Concrete cover measurement | Is the cover too thin? Where are the bars? | BS 1881-204 · DPT 1505-51 |
| Carbonation depth measurement | Has the loss of alkalinity reached the steel? | EN 14630 / RILEM CPC-18 |
| Chloride content measurement | Does chloride exceed the corrosion-initiation level? | ASTM C1152 / C1218 |
| Half-Cell Potential | Probability that the steel is actively corroding | ASTM C876 · DPT 1506-51 |
| Concrete core drilling | Actual remaining compressive strength | ASTM C42 |
Results from Half-Cell Potential per ASTM C876 are read as probabilities, not definitive answers. The commonly cited criteria are values measured against a copper/copper sulfate reference electrode (Cu/CuSO₄ or CSE), as follows:
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| Potential (vs. CSE) | Meaning |
|---|---|
| More positive than −200 mV | Corrosion probability below 10% |
| −200 to −350 mV | Uncertain; check other results |
| More negative than −350 mV | Corrosion probability above 90% |
If another reference electrode such as Ag/AgCl is used, convert the values before comparing them with the criteria, and always use this result together with carbonation, chloride and concrete cover measurements.
Never conclude the cause from a single test result. For example, a strongly negative Half-Cell value can also be caused by water-saturated concrete. Interpretation must be done by an engineer, considering the full data set together with actual site conditions.
How is spalled concrete repaired?
An effective repair has 6 steps, starting with defining the extent and ending with protection against recurrence. In the Thai context, DPT 1901-51, the standard practice for concrete repair, and DPT 1902-62, the standard for inspection and strength assessment of old and damaged building structures, both issued by the Department of Public Works and Town & Country Planning, form the main framework. International guidance used for supporting detail is ACI 562 for assessment and repair design, ACI 546R for repair methods, ICRI 310.1R for surface preparation, and the EN 1504 series for repair materials.
- Define the extent — combine hammer sounding with test results to define the area that actually has to be broken out, which is usually wider than the visibly spalled zone
- Break out unsound concrete — break out about 20–25 mm behind the reinforcement so the bar can be repaired all around, and about 50–75 mm beyond the corroded zone, following ICRI 310.1R
- Clean the steel — remove rust down to clean steel, then assess the remaining section loss
- Decide on the steel — if section loss is large enough to affect strength, add new reinforcement as specified by the engineer
- Repair with a compatible material — choose a repair mortar whose mechanical and shrinkage properties are close to the existing concrete, to reduce the risk of cracking at the interface
- Prevent recurrence — apply an anti-carbonation or anti-chloride coating, and fix the leaks or ponding that caused the problem
There are 2 common mistakes. The first is not breaking out behind the bar, so rust at the back keeps spreading under the new repair mortar. The second is patching locally in a structure with widespread chloride. When patched with new high-alkalinity mortar, the steel inside the patch returns to a passive state, but the steel next to the patch edge is still in the original contaminated concrete, so it becomes a new anode that corrodes instead. This is called the incipient anode or ring anode effect, and it leads to renewed spalling at the edge of the repair within a few years.
It can be prevented by extending the breakout beyond the actual contaminated zone, or by adding a supplementary corrosion protection system, such as zinc anodes for cathodic protection (galvanic anodes), as specified by the engineer.
When does structural capacity need to be reassessed?
Strength must be reassessed when the damage is not just superficial but affects the reinforcement or primary members. There are 4 cases where an engineer should check by calculation:
- Significant loss of reinforcement cross-section, generally considered from about 20% upward
- Damage to primary members such as columns, main beams or column heads carrying high shear
- Broken stirrups or heavy stirrup section loss, which affects both shear strength and confinement
- Spalling spread across many locations on the same floor or throughout the building
The figures above are guideline thresholds for deciding whether a detailed investigation is needed. Conclusions on remaining strength, safe level of use and the strengthening scheme must come from calculations by the responsible engineer based on the building's actual condition, which may extend into a whole-building assessment of an old building.
Start with hammer sounding to map the extent, then check the cause with the basic test set, and only then choose the repair method. Investigating before repair usually costs less than repairing the wrong way and having to tear it all out and redo it.
How much does an assessment cost?
Fees depend on the number of test points, the tests selected, and the height and access of the site. Contact us for a free quotation within 24 hours. This price excludes travel and equipment mobilization costs, which are charged extra depending on the distance to site. Reports are certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and license number, and can be submitted to the relevant authorities.