PSI - Issue 11

Pablo Benítez et al. / Procedia Structural Integrity 11 (2018) 60–67 Benítez et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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This analysis has shown that almost half of the cases analysed in the structures present a risk of corrosion or imminent corrosion initiation caused by carbonation. The former condition means that the carbonated thickness in the concrete is greater than the cover thickness and that the durability of the structure would be compromised by the rebar corrosion, requiring an immediate and possibly expensive intervention. Thus, considering several studies about corrosion, it could be said that almost 50% of the buildings analysed in the urban area of Asunción, and that were built no more than 40 years ago, can be considered as structures in the second stage of its service life. Finally, the real carbonation rate was calculated for structures with characteristic strength values of 20, 25, 30 and 45 MPa. So, structures whose strength tests showed values close to these were considered, allowing to analyse the influence of concrete quality on the carbonation rate results. The results of this study are presented schematically in Table 2.

Table 2. Carbonation rates of characteristic strength for concrete structures in Asunción

Characteristic Strength Carbonation Rate (mm/year 0.5 ) 20 MPa 3.83 25 MPa 3.31 30 MPa 1.75 45 MPa 1.32

The table above clearly represents the influence of the concrete quality in the carbonation resistance properties of structures. The values obtained above enable the analysis of the structures service life from the perspective of corrosion initiation. Conversely, is important denoting that, in addition to a good quality concrete, a proper cover thickness is indispensable to resist carbonation and to ensure the structure's durability against harmful agents of the environment surrounding.

3. Durability strategies for maintenance

In order to face the problems mentioned previously, a two-stages maintenance planning was considered: a first stage where the number and interval of inspections are established, and a second stage where the repair probabilities according to the inspection results is analysed. Maintenance affects the reliability of components and the system: if too little is done, this can cause an excessive number of costly failures and poor system performance. Instead, if it is done too often, reliability may improve but the maintenance cost will increase drastically. Then, in a cost-effective scheme, both expenditures must be compensated (Endrenyi et al. 2001). In this context, this paper develops the first stage of maintenance planning, where an analysis regarding the number of inspections (inspection planning) that must be performed to detect the damage before failure is studied. Hence, perform interventions before corrosion reaches its critical value is established as the main objective. This inspection schedule is formulated through a quantification of cost components over the service life of structures. Likewise, the inspection tasks are formulated based on certain parameters such as carbonation rate, quality of the inspection technique, the cost of the inspection technique and the probability of failure of the structure. Generally, it can be expected that a high-quality inspection will have a higher probability of flaw detection. However, it is necessary to make a trade-off analysis between cost and effectiveness of inspection techniques that result in an optimal maintenance planning. This effectiveness can be treated as the detectability ( ) of an inspection technique, which describes the ability to detect damage in a structure given a certain intensity of damage. This intensity of damage can be expressed as a function of the damage degree factor ( ) , whose values range from zero (no corrosion damage) to one, which corresponds to a damage by total sectional-loss of reinforcement by corrosion (Frangopol, Lin, and Estes 1997):   ( ) 0 ( ) 0 / t t D D D    (1) 3.1. Detectability of the inspection technique