Identifying the root cause

Typical Concrete Problems

Rust staining
Cracking
Spalling




Root Cause

Steel Reinforcement Corrosion
Corrosion of Steel in Concrete.
Concrete is a highly alkaline material when first produced, (pH range 12-13), and embedded steel is well protected by the pasive oxide layer formed by the initial high alkalinity at the surface of the steel.

While this alkaline protection is usually effective for a long period of time, it is dependent on the absence of surface defects, permeability and the depth of concrete cover over the steel reinforcement. As an electrochemical process, corrosion of steel in concrete requires an electrolyte. Concrete is full of small pores which contain moisture and is an effective electrolyte. A small electrical current flows between anode and cathode with corrosion activity taking place at the anode.

The corrosion products of steel are known as "RUST" chemically, iron oxides and hydroxides), which have a much greater volume than the steel (up to 8-12 times the volume). This increase in volume is progressive, gradually exerting greater and greater expansive forces within the concrete. The ultimate result is stress relief through cracking, first visible as rust staining, followed by larger cracks, and then spalling over the corroded reinforcement.

Both the relative humidity and the temperature have a significant bearing on the speed of corrosion in concrete with, as with most processes, the corrosion rate increasing with temperature rise and reducing with a drop in relative humidity.

Root Cause

Chemical Exposure


Carbonation

Carbon dioxide (CO2) in the atmosphere, reducing passivating alkalinity.

Chlorides-Marine Environment/Deicing Salts

Accelerate reinforcement and freeze / thaw damage.

Chemical Contact and Pollution

Concrete must withstand chemical attack from industrial processes and atmospheric pollution, particularly sulfur dioxide (SO2).

Physical Exposure


Loading and Dynamic Loading

If these are not correctly quantified at the design stage or are changed in use, serious problems may occur.

Impact

Some structures, such as multi-story parking garages, must withstand impact and remain safe.

Abrasion

Heavy traffic on floors or the abrassive effects of water flow.

Thermal Movement

In different temperature and weather conditions, concrete is subjected to substantial and frequent volume changes due to thermal expansion and contraction.

Original Construction


Exposure

Known exposure conditions are not given adequate consideration.

Joints

Movement joint detailing is inadequate.

Compaction and Vibration

Concrete was not sufficiently compacted leaving honeycombing or voids.

Water Cement Ratio

The concrete contained excessive water when placed ans is porous with higher surface absorption.

Curing and Protection

The concrete has been inadequately cured leading to increase porosity.

Cover

Adequate cover to the reinforcement has not been maintained.

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