96
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
cover [4] [5]. Due to the mechanical deleterious effect of the corrosion phenomenon, it is
important to develop non-destructive techniques as well as predictive numerical modelling to
assess the corrosion evolution of RC structures. This could help the structure's end users to
provide an efficient maintenance policy. The main issue of this paper is to design a specific
protocol to generate a "controlled" corrosion evolution versus time in order to bring some
experimental evidences on the concrete cover cracking process due to corrosion and to
determine relevant input parameters for the numerical modelling.
2.
Experimental program
2.1
Materials and specimens
Twelve single-rebar specimens (500x125x100mm
3
) were casted with a 600mm long and
20mm diameter steel deformed rebar. The reinforcement was positioned to obtain a 30mm
concrete cover at two sides of the beam [Figure 1]. The specimens designed with a not
symmetric location of the rebar aim to get closer to reality, to represent the heterogeneity of
the mechanical environment of the reinforcement in a structure
.
A Portland cement and
siliceous aggregates were used for the concrete composition with
a water to cement ratio of
0.73. This ratio is representative of old reinforced concrete structures. Moreover, it allows the
penetration of the chloride ions during the accelerated corrosion test.
Figure 1. Schematic representation of RC specimens (dimensions in millimeters)
The cement type used for the concrete composition was CEM I 52.5 CP2 NF according to
European standards. Concrete was prepared with aggregates having different particle size
classes ((0/0.315 mm; 0.315/1 mm; 0.5/1 mm; 1/4 mm; 2/4 mm; 4/8 mm; 8/12 mm; 12.5/20
mm)). Compressive and tensile strengths were measured on concrete cylinders (160mm in
diameter, 320mm in height) after 28 days according to NF EN 12390-3 [6] and
NF EN 12390-
6 [7] standards. The mean compressive strength is 32 MPa ± 2.46, the tensile strength is 2.6
MPa ± 0.08 and the Young's modulus is 35 GPa ± 1.58. The Poisson's ratio is equal to 0.15.
2.2
Accelerated corrosion tests and monitoring system
The set-up used for accelerated corrosion test and the monitoring system are illustrated in
[Figure 2]. RC samples were corroded using a power supply (Agilent 6614C, 100V, 0.5A)
which delivered an imposed anodic current to the steel rebar. The counter electrode (cathode)
consisted in an inert platinum titanium mesh (275mm long, 75mm wide) placed into a PVC
tank containing the
alkaline
electrolyte (1 g/L of NaOH, 4.65 g/L of KOH, 30 g/L of NaCl)
which was glued on the top side of the concrete (in the middle of the specimen). All the
specimens were connected in series and a current density of 100 μA/cm² of steel (0.0172A for
a steel surface area of 172.8 cm²: diameter 20mm and length 275mm) was applied during the
97
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
chosen exposure time (7d, 14d, 21d, 28d and 35d). At the end of each considered exposure
time, two specimens were disconnected from the electrochemical test set-up. One of the
specimens was used for the non-destructive electrochemical characterization and the other one
was dedicated to destructive measurements. The accelerated corrosion test was monitored
using a data acquisition unit (Keysight 34970A): temperature, delivered constant current, each
sample voltage and total voltage were recorded every two hours. Moreover, two cameras were
used for the digital image acquisition of the front side of the two specimens subjected to a 35
days accelerated corrosion test.
Figure 2. Accelerated corrosion and monitoring system
2.3
Electrochemical characterizations
In order to determine the corrosion state of the rebar before the accelerated test, half-cell
potential measurements (E
corr
) linear polarization resistance measurements (LPR) and
impedance spectroscopy (Re) were carried out, using a potentiostat (Bio-Logic, PARSTAT
2263) and the usual electrochemical cell with three electrodes. The working electrode was the
steel rebar, the reference electrode was a KCl saturated calomel electrode (SCE, 242 mV /
SHE) and the counter electrode was a titanium platinum mesh [Figure 3]. The same
electrolyte as for the accelerated corrosion test was used. Then the corrosion current density
J
corr
(μA/cm²) was calculated based on the following equation:
J
corr
=
(1)
with B a constant (26mV), Rp (ohm) the linear polarization resistance and S (cm²) the steel
surface (172.78 cm² in this study).
Data acquisition unit
Power supply
Cameras