PSI - Issue 13

Myroslava Hredil / Procedia Structural Integrity 13 (2018) 1657–1662 Author name / Structural Integrity Procedia 00 (2018) 000–000

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3.2. Pull-out test after impressed cathodic current In the case of the pull-out test of RC specimens which were not undergone hydrogen charging, the shorter rod always withdrew from the specimen during loading. When hydrogenated specimens were tested – just the hydrogenated rod withdrew despite it was longer. Loading curves are shown in Fig. 5. Basing on the assumption that the bond stress is evenly distributed along the steel–concrete interface, the nominal bond strength can be calculated as the relation of fracture load to the contact surface of the corresponding embedded rod (Table 2). Bond strength in the range of several MPa attributed to the specimens not been cathodically charged is consistent with other researchers. In particular, the obtained values of bond strength are in agreement with those reported recently by Ma et al. (2017) for smooth reinforcement, while the values for hydrogenated ones are enormously low. In spite of data scattering which is inevitable for RC testing, a clear effect of cathodic polarization is observed on bond strength between rebar and concrete: the load needed for withdrawing of a rebar after cathodic polarization of RC specimen is lower at least in 4 times compared to the control one.

4

3

P , kN

3

2

4

2

1

1

0

0

2

4

6

8

L , mm

Fig.5. Load–displacement curves in pull-out tests for hydrogenated ( 1,2 ) and control ( 3,4 ) specimens of the type R2 with embedded rebar lengths 6 and 4 cm ( 1,3 ) or 8 and 2 cm ( 2,4 ).

Table 2. The results of the pull-out test of the type R2 specimens.

Specimen type

Hydrogenated

Control

Embedded rebars length, cm Pulled rebar length l , cm

8/2*

6/4*

8/2*

6/4*

8

6

2

4

Load P max, kN

1.87 0.62

0.72 0.32

2.74 3.63

3.37 2.23

Nominal bond strength, MPa

* Duble-rod specimen with steel rebars 8 and 2 cm (8/2), 6 and 4 cm (6/4)

It should be noted that application of impressed cathodic current to RC structures causes the outflow of Cl - ions from the cathode (reinforcement), and the well known and widely used methods, such as electrochemical chloride extraction and re-alkalization, are based on this effect. At the same time Na + та K + ions move towards the reinforcement. Chang (2002) noticed that the rising concentration of these cations at the steel–concrete interface leads to “softening” of concrete paste, bond deterioration in RC structures. Consequently, losing of bearing capacity of RC installation should be achieved under such conditions. Furthermore, bond strength degradation is shown by Rasheeduzzafar et al. (1993) to be proportional to the content of potassium and sodium ions in contaminated concrete. However it should be kept in mind that hydrogenation of steel rebar is possible during cathodic polarization. Chang (2002) emphasized that even under acceptable “safe” cathodic currents appearance of localized overprotected areas is eventual due to nonuniform distribution of protecting currents resulted from physico-chemical inhomogeneities and instabilities inherent to the concrete material. It is known that only a tiny part of hydrogen permeates a metal promoting hydrogen embrittlement, while a majority of it recombines and evolves as a gas at the steel–concrete interface. Evidently, it may destroy the