PSI - Issue 10

Alk. Apostolopoulos et al. / Procedia Structural Integrity 10 (2018) 49–58 Alk. Apostolopoulos and T. Matikas / Structural Integrity Procedia 00 (2018) 000 – 000

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2. Experimental part

2.1. Procedure of artificial corrosion, mechanical testing procedure and mass loss.

In order to study the mechanical behavior of steel bars under conditions of corrosion damage and inelastic buckling on low cycle fatigue, a representative number of high strength steel specimens with B500Α and B500B steel grades and 12 mm diameter (Table 1) were selected for experimental tensile tests and LCF tests. Procedure of artificial corrosion for the above mentioned specimens took place in a salt spray corrosion chamber, according to ASTM B117 specification, for 45 and 90 days, with a 5% sodium chloride and 95% distilled water solution, pH range of 6.5-7.2, and temperature of 35 (+1.1 – 1.7 o C). During the experimental procedure a digital measurement of pH was monitoring chamber’s environment . A number of tensile tests were carried out on reference and corroded specimens to give a general idea of the effects of corrosion on the tensile mechanical properties of the material. Tensile tests were carried out according to ISO/FDIS 15630-1 (2002). Furthermore, a number of low cycle fatigue tests for each corrosion level were conducted for two different strain levels (imposed deformation) ± 2.5% and ± 4% (simulation of seismic events). All mechanical tests were conducted at room temperature using Instron dynamic servo-hydraulic system. All readings were recorded using a fully automated computer system. Tensile tests are presented in Table 2. Low-cycle-fatigue tests on non-corroded (control specimens) and corroded specimens, after exposure to the salt spray chamber for 45 and 90 days were carried out according to the experimental procedure of Apostolopoulos (2007) study in Tables 3 to 8. For each type of bar B500A and B500B two different sizes of free length were selected, 6 Φ (to count the rebars in buildings when designing for high ductility class) and 8 Φ (when designing for low ductility class, according to the limits set in Eurocode 8 for seismic design of distance between stirrups in reinforced concrete structures). The low cycle fatigue tests were carried out on the same dynamic servo-hydraulic Instron machine which was used for the tensile tests. The production of an oxide layer which covers the specimens and increases the thickness of the specimen is the result of exposure of the specimens to the salt spray environment. Mass loss was evaluated as M i -M f /M i , in which M i and M f represent respectively the mass of the specimen before and after corrosion tests. 3. Results and discussion The results of tensile tests confirmed the progressive decrease of the mechanical properties of corroded specimens in existing structures. The results of tensile tests are presented in Table 2, in terms of mechanical properties ( R p , R m , A gt and U ) and mass loss. Β 500 Α specimens present lower values of mass loss relative to Β 500 Β specimens. Mass loss values in B500A steel grade varied between 7.95 % and 10.40 % (mean value 8.83 %), on the other hand in B500B steel grade mass loss varied between 11.10 % and 11.55 % (mean value 11.34%). Table 2 presents that after 90 days of exposure to corrosion, both steel types, B500A and B500B, showed a reduction of strength properties ( R p , R m ) proportional to mass loss, in contrast to the ductility properties ( A gt , U ) which present a dramatic decline. More specifically, after 90 days of corrosion, yield strength of B500 Β maintained at values in excess of 500MPa and only 2.7% decline rate. In contrast, for the same time of corrosion, yield stress of B500 Α falls below the threshold, set by the regulations and presents decline rate 7.85% (equivalent to mass loss). The drop of the ultimate strength for B500B is 13.6 % and for B500A is 11.65 %, respectively. The mechanical property of ductility, A gt (elongation to maximum load), presents a dramatic decline rate of 83.12 %, (from 10.07 % to 1.70 %). A gt of Β 500 Α also presents a large drop of 77.5 %. After exposure to corrosion, A gt values are below the thresholds regulations pose. Energy density ( U ) presents corresponding class scale drops (83.82 % for Β 500 Α and 85.34 % for B500). In general, 3.1. Tensile tests

Table 1. Mechanical properties of B500A and B500B according to EC2. Class Yield stress, R p ( MPa )

Characteristic strain at maximum force, A gt

Minimun value of k=(R m /R p ) k

B500A B500B

≥500 ≥500

≥2.5% ≥5.0%

≥1.05 ≥1.08