Last reviewed 30 Sept 2026 · 8 min read
Deterioration — the main causes
Bridges age because of environment, use and defects built in at the start. Understanding the mechanism tells you whether the damage will grow, how fast, and how to stop it.
Corrosion of reinforcement — the dominant problem
Concrete gives steel a high-alkaline (pH 12–13) environment in which it forms a thin passive film and does not rust. Corrosion begins when this is broken:
- Carbonation — carbon dioxide from the air reacts with the calcium hydroxide in the concrete and lowers the pH to about 9. When the carbonation depth reaches the steel, the passive film is lost. It advances roughly as .
- Chlorides — from de-icing salts, sea water, coastal air or contaminated materials. When the chloride content at the bar exceeds a threshold (of the order of 0.4 % by mass of cement) the film is destroyed locally, giving pitting corrosion.
The two-stage model (Tuutti): an initiation period, during which carbonation or chloride advances to the steel, followed by a propagation period, in which the steel corrodes. Rust occupies 2–6 times the volume of the steel it replaces, producing expansive pressure that cracks the cover, causes rust stains, then delamination and spalling. The section of the bar is lost and the bond to the concrete is reduced; the structural capacity falls.
Factors: concrete quality (permeability), cover, cracks, moisture, oxygen, temperature, and stray currents (from DC traction).
Other concrete deterioration
| Mechanism | Description |
|---|---|
| Alkali–silica reaction (ASR) | Reactive silica in some aggregates reacts with alkalis in the cement, forming an expanding gel; produces map cracking and distress |
| Sulphate attack | Sulphates in soil or water react with the cement paste to form expansive products (ettringite, gypsum) — softening, expansion, spalling |
| Delayed ettringite formation | Formed in heat-cured or mass concrete later in life |
| Freeze–thaw | Water freezes in pores and causes scaling and cracking (cold regions) |
| Abrasion and erosion | Flowing water carrying sediment wears piers; wheels wear decks |
| Leaching | Water dissolves calcium hydroxide, leaving white deposits and a porous surface |
| Fire and high temperature | Loss of strength and spalling |
| Early-age and plastic cracking | Shrinkage and thermal cracks from poor curing or construction |
Steel deterioration
- Corrosion — uniform, pitting, crevice (in lap joints and under debris) and galvanic (at the contact of dissimilar metals); accelerated by leaking joints and poor drainage.
- Fatigue cracking at welds, bolt holes and details of low fatigue category.
- Overstress and distortion from impact or overload; buckling of members.
- Loose rivets and bolts in older bridges.
Structural and geotechnical causes
- Overloading by heavier traffic than the design.
- Scour of foundations by floods — the most frequent cause of collapse of river bridges.
- Differential settlement of foundations and approach embankments.
- Bearing failure or restraint of movement, causing distress in girders and piers.
- Earthquake and collision damage.
- Design and construction defects: inadequate detailing, insufficient cover, poor compaction and curing, poor grouting of tendon ducts.
Diagnosing the cause
Diagnosis — finding the cause — is essential: repairing the symptom without removing the cause means the damage returns. The approach is that of a forensic investigation:
- Collect the history — design, construction, loads, repairs, environment, and the sequence of events.
- Observe and map the distress — type, location, extent, and time-development.
- Formulate hypotheses consistent with the pattern (see below).
- Test — cores, chemical analysis (chloride, sulphate, pH), petrography, NDT, load test, analysis with actual properties.
- Confirm the cause and rule out the alternatives.
- Assess the effect on capacity and durability; predict the future.
- Decide the remedy: eliminate the cause, repair, strengthen, restrict.
Crack patterns and their meanings
| Pattern | Likely cause |
|---|---|
| Vertical cracks in the tension zone at mid-span | Flexure — normal service cracking, or excessive load |
| Diagonal cracks near the supports (about 45°) | Shear — insufficient stirrups or overload |
| Cracks along the reinforcement, rust stains | Corrosion of the bars |
| Map (random) cracking, gel exudation | Alkali–silica reaction |
| Longitudinal cracks over a tendon duct | Corrosion of tendons, or poor cover |
| Cracks with horizontal offsets in the pier | Settlement, bearing restraint |
| Fine, shallow cracks soon after casting | Plastic shrinkage, thermal effects |
| Cracks at the bearing seat, spalling of the pedestal | Bearing restraint or overload |
| Cracks in a steel member at a weld or hole | Fatigue |
Residual strength
Residual strength is the capacity that remains after deterioration. It is found by recalculating the capacity with the reduced properties:
- Reduced steel area — a bar of original diameter that has lost the fraction of its diameter (or a pit depth) has area . A 20 % loss in diameter means a 36 % loss in area .
- Reduced concrete section due to spalling and delamination.
- Bond loss, which affects the anchorage of the bars.
- Reduced steel section from corrosion in steel members: the thickness is measured by an ultrasonic thickness gauge, and the properties (area, moment of inertia, section modulus) are recomputed with the reduced thickness.
- Material strengths from cores and coupons instead of the design values.
- Effect on ductility — pitting corrosion reduces the ductility of the bar more than uniform loss; brittle failure is a concern.