CORROSION OF STEEL IN R.C. STRUCTURES

CORROSION OF STEEL IN R.C. STRUCTURES

Repairs in Construction - A growing concern

  Bad construction practices

  Standards not adhered to

  Poor  maintenance

  Increasing pollution

SEM micrographs of microcracks propagating from a pore 

Deterioration of concrete due to corrosion results because of the product of corrosion – ferric oxide, brown in color – occupies a greater volume 9 more than 2 to 10 times) than steel and exerts substantial bursting stressed on the surrounding concrete.

SEM micrographs from the no-load specimen 

Internal voids

Pollution affects Durability

Contaminated Water       Chlorides

Effluents                           Chemicals

Soils                                   Sulphates

Vehicles                                     CO₂

CORROSION OF REINFORCEMENT IN CONCRETE

                Protection of the reinforcements from corrosion is provided by the alkalinity of the concrete, which leads to passivation of steel.  However, environmental influences and CO₂ in particular, will reduce the concrete’s pH value from 12.6 to 8.0, and thus remove the passivating effect.

•New steel is anodic to Old one

•Brightly cut one is anodic to un-cut steel

•Steel anodic to mill scales

•Highly stressed steel is anodic to unstressed steel

Stray direct current

•Stray direct current passes from them to electrolyte.

•Current arises from crane systems , embedded electrical conduits and improperly grounded equipments.

•Causes accelerated corrosion- Should be checked and corrected 

Differential environment

•Different levels of aeration ( oxygen content)

•Different salinities

•Corrosion occurs at locations of lower oxygen content- just below mean low tide level

Erosion corrosion

•Scouring action of sand

•Commonly found just above mud line of piles- riser chains of moorings

•High wind effect in sandy areas

Biological corrosion

•Marine organisms accelerate corrosion

•Creates differential oxygen levels

•Produces corrosive products for their metabolism

•Organisms remove protective layer on steel