Rebar corrosion and spalling concrete: why it happens in our buildings and how to prevent it from the start

A familiar sight in many homes: a brown stain on the bathroom ceiling, then a crack, then a piece falls and the steel appears corroded. This is not a "plaster" problem; it is a failure of the protection of the steel inside the concrete.

1) How does steel corrode inside concrete?

Steel embedded in concrete is in a non-corroding, "passive" state because of the high alkalinity of concrete (pH above 13). This protection is lost in two ways1:

• Chlorides: when water-soluble chlorides are present, the passive layer is disrupted and corrosion begins. They come from external sources (seawater, salts) or internal sources in the mix materials.
• Carbonation: carbon dioxide in the air reacts with lime in the concrete and lowers its alkalinity; when pH at the steel falls below 9, corrosion begins.

The association notes that chloride-induced corrosion is more common than carbonation, and that corrosion is aggravated by moisture, high temperatures and cracking1.

2) Why does it cause spalling?

Rust has two to four times the volume of the original steel and none of its mechanical properties; it creates internal stresses that lead to delamination and spalling of the cover, reduces the steel's cross-section and load capacity, and if left unchecked can affect the integrity of the structure1.

3) Why is it so common here?

• Coastal areas and sabkha: sabkha soils spread along the Red Sea and Arabian Gulf coasts and have a corrosive effect2, and seawater is an external source of chlorides1. See soil and foundations.
• Soils rich in sulphates and chlorides in some cities; a study of Makkah found sulphate and chloride contents higher in soils within the top 5 m, and recommended considering them when selecting the cement type for foundation concrete and reinforcement3.
• Heat and moisture, which accelerate the reactions1.
• Leaks from bathrooms, roofs and tanks that keep concrete wet for years.

4) Prevention from the start

From the association's recommendations1:

1. Evaluate the exposure of each element and set requirements (dry, moist, moist with chlorides).
2. Low-permeability concrete: a water-to-cementitious ratio of 0.40–0.50, and about 0.40 with supplementary cementitious materials where chlorides are present. See admixtures.
3. Adequate cover over the steel: increasing cover slows chloride penetration and carbonation considerably.
4. Good curing: preferably at least 7 days; longer curing reduces porosity and improves corrosion resistance.
5. Extra protection for critical elements: membranes and sealers, corrosion inhibitors, or corrosion-resistant steel (galvanised, epoxy-coated, stainless).
6. Drain water away from concrete members.

5) Early signs to watch

• Brown or rust stains on ceilings or columns.
• Longitudinal cracks along the line of the steel.
• A hollow sound when lightly tapping the concrete (delamination).
• Bulging plaster or paint under the ceiling.