Main mechanisms of deterioration

Main mechanisms of deterioration

References: [3] [4] [5] [6] [7]

Diffusion of CO2 (carbonation) into concrete

Carbonation lowers the pH of concrete, through a reaction with Ca(OH)2 to form CaCO3. Diffusion of CO2 into concrete takes place through capillary pores. It is therefore strongly dependent upon concrete porosity and relative humidity, besides temperature. When the pH has reached a value of about 9; the carbon steel rebar is no longer in a passive state and corrosion can take place.

Carbonation, alone, is rarely a source of concern for lifetimes below 50 years unless concrete quality is inadequate and/or workmanship is poor (including insufficient concrete cover). For longer lasting structures carbonation must be taken into account.

Diffusion of Chlorides into concrete

The main cause of concrete deterioration. As chlorides diffuse into the concrete some are trapped in a "bound" form while the rest, "free" chlorides, diffuse further and cause corrosion as they reach the carbon steel rebar, according to the following steps (and Figure) –from ref12.

  1. Diffusion of the corrosive ion (usually chloride) into the concrete
  2. Once it reaches the carbon steel rebar (t0), corrosion begins
  3. Corrosion products, which occupy a greater volume than steel, exert an outwards pressure
  4. Concrete cracking occurs (t1), opening easy access to chlorides
  5. Concrete cover cracks (spalling) (t3), exposing the rebar
  6. If unattended corrosion continues until the rebar cannot bear the applied tensile stresses and the structure collapses (t5)

Critical Chloride Threshold Level (CCTL) [8]:  This is the chloride content above which rebar corrosion starts.  There is some disagreement regarding how to measure it and about its value, but  the value of 0,4% per cement weight is generally accepted for C-steel in the absence of carbonation.

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