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 Current condition of a structure
 
current condition of a structure

The diagnosis of the condition state of a reinforced concrete work, makes it possible to characterise the origin and the extent of possible damages. As a rule, this diagnosis is carried out after some damages have appeared, but it is also asked within various frameworks, such as rehabilitation works or regular inspections.

In addition, a diagnosis has several objectives:

    to identify the origin of reinforcement corrosions,
    to evaluate their extent, in the structure,
    to predict their probable change, with time and space,
    to estimate their consequences on the safety concerning the structure and people
    to define the further actions to make.

A preliminary visit of the structure makes it possible to put forth assumptions on the possible causes of the damages, and thus to specify the operations to be carried out, by taking account of the functioning of this structure. The mechanical behaviour of the various structure parts, is a point very significant to examine. However, it is not developed here.

A diagnosis concerning the reinforced concrete relates to the concrete of coating, on the one hand, and them reinforcements, on the other hand. It is done within the framework of an inspection.


aaDiagnosis of condition state

The condition (state) of a reinforced concrete structure, which has a risk of corrosion, is determined with characterizing covering concrete and reinforcing steel. A diagnosis on the condition of a structure mainly makes it possible to determine the origin of steel corrosion (particularly, after concrete carbonation or chloride ingress).

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Concrete condition

Protective qualities of a concrete cover and its ageing conditions are assessed by various tests, which are carried out on site and which are not (or are not too much) destructive. Here are the tests and measurements:
    on site measurements of the concrete permeability to air. This makes it possible to characterize
      the ingress of air (and of cabon dioxide in atmosphere)
    on site measurements of the electrical resistivity of concrete. This makes it possible to locate
      wet or salted areas,
    determination of carbonation depth on cores taken from concrete,
    determination of content chloride profile, on cores taken from concrete, according to
     the European standard EN 13396.
    possibly, measurement of permeability to water, and on site measurements of the concrete
      surface cohesion,


Reinforcement condition

The reinforcement state of the is characterised by its geometry and its corrosion condition.

The reinforcement geometry concerns its cross section (diameter), its arrangement in concrete (number of mats) and especially the cover thickness above reinforcement close to the facing. This thickness is determined by various non-destructive methods.

The condition state of reinforcement is characterised by measuring half-cell potential with using various equipments This method makes it possible to locate the corroding areas.

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 Predicting the future condition of reinforcement
   


Predicting corrosion initiation

A reinforcement starts to corrode, when it is in contact with aggressive agents which are with contents higher than some critical values. The progressive deteriorations of the concrete cover, which lead to reinforcement corrosion are especially carbonation and chlorides penetration.

The penetration of the aggressive agents is characterised by tests and calculations, which are:

    on site measurements of concrete permeability to air or water,
    determination of the time when carbonation will reach reinforcement,
    determination of the time when, chloride concentration at the reinforcement level, will reach a critical       value (critical chloride content).

Corrosion of steel in concrete in initiated, when carbonation depth is not lower than cover thickness. In other words, corrosion starts when carbonation depth is equal to cover thickness.
Measuring permeability to air makes it possible a prediction, which is only qualitative, but without coring concrete.
When steel corrosion is due to chloride ingress, drawing chloride content profile (at a given age) makes it possible to calculate a diffusion coefficient. This parameter makes it possible to calculate the time, when a given chloride content (called « critical ») reaches the reinforcement.


Predicting the extension of corrosion

The measurement of half-cell potential, on a reinforced concrete structure, makes it possible to locate the areas considered as sound. Their dimensions can decrease with time. It is then convenient to forecast the corrosion initiation, at some given points of these areas (see the here above paragraph).

Another method for predicting the expansion of corrosion, is to measure the corrosion rate, which makes it possible to predict the time of first concrete cracking.


Possible further actions

When a diagnostic shows that in a reinforced concrete structure, reinforcement is corroding, several actions can be undertaken.

For example, probes can be embedded in concrete, for continuously (or semi-continuously) the condition (state) of reinforcement.

When the structure deterioration is too important, repair or rehabilitation are undertaken.

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 References
   

European standards :

    EN 13396 Products and systems for the protection and repair of concrete structures
      Test methods – Measurement of chloride ingress, 2004
    EN 14630 Products and systems for the protection and repair of concrete structures
      Test methods – Determination of carbonation depth in hardened concrete by the phenolphthalein
      method, 2006

Recommendations by RILEM TC 154-EMC “Electrochemical Techniques for Measuring Metallic Corrosion”,

    “Test methods for on site measurement of resistivity of concrete”, Materials and Structures,
       Vol. 33, Dec 2000, pp. 603-611
    “Half-cell potential measurements. Potential mapping on reinforced concrete structures”,
       Materials and Structures, Vol. 36, Aug-Sept 2003, pp. 461 – 471
    “Test methods for on-site corrosion rate measurement of steel reinforcement in concrete,
       by means of the polarization method”, Materials and Structures, Vol. 37, Nov 2004, pp. 623-643

ASTM document ASTM C876-09, Standard Test Method for Half-Cell Potentials of Uncoated
   Reinforcing Steel in Concrete, 2009

 
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