Estimation of Remaining Years

 

ESTIMATION OF REMAINING YEARS BEFORE CORROSION STARTS

Why structures corrode?
Concrete structures may suffer from different deterioration mechanisms. However, the main deterioration mechanism corrosion in the reinforcing steel (rebar corrosion). Corrosion may start by the ingress of the Chloride ion or Carbon Dioxide.

Chloride Ion induced corrosion
In salty environments, under the adequate conditions of oxygen and moisture, the Chloride ions penetrate through concrete until they reach the reinforcement steel. As the chlorides reach the steel, the passivation layer between steel and concrete starts degenerating, then the steel starts corrosion.

Carbon Dioxide COinduced corrosion
In urban environments, under the adequate moisture conditions, the Carbon Dioxide penetrates through the concrete until reaching the reinforcing steel. As the Carbon Dioxide is penetrating, the pH of concrete is decreasing. This phenomena is called Carbonation. When Carbonation reaches the reinforcing steel and the low pH level degenerates the passivation layer between steel and concrete, the reinforcing steel starts corroding.

How can we determine the Remaining Years of a Structure before Corrosion starts?

The Service Life of a structure is the time lapse between its construction and the start of corrosion in the reinforcing steel. This time is regulated mostly by the Diffusion Coefficient of Chloride Ions or the penetration rate of Carbonation in the concrete placed in the structure. Each concrete has a different Difussion Coefficient based on the quality and durability of the cementitious matrix. Once the corrosion has started, there is no way back. As the result of corrosion, the generated Iron Oxide in the reinforcing steel will generate stresses in concrete and create delaminations or surface opened cracks.

How do we Determine the Remaining Years of a Concrete structure?

  1. We drill out concrete powder in small increments of depth, until we reach the reinforcing steel depth (coverlayer)
  2. Chloride Ion percentage or pH number is determined for every increment of drilled out powder. Alternatively a simpler process can be done for Carbonation evaluation by means of just extracting a little concrete core.
  3. Based on the mathematical analysis of the results vs depth, the estimation of remaining years before corrosion starts can be determined. The “years” depend on the conditions of the structure when it was tested. If the conditions change, further analysis has to be done.
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