Coupled model of initiation and propagation of corrosion in reinforced concrete

Resumen

Corrosion of reinforcing steel in concrete due to chloride ingress is one of the
main causes of the deterioration of reinforced concrete structures. Structures
most affected by such a corrosion are marine zone buildings and structures
exposed to de-icing salts like highways and bridges. Such process is
accompanied by an increase in volume of the corrosión products on the rebarsconcrete
interface. Depending on the level of oxidation, iron can expand as
much as six times its original volume. This increase in volume exerts tensile
stresses in the surrounding concrete which result in cracking and spalling of the
concrete cover if the concrete tensile strength is exceeded.
The mechanism by which steel embedded in concrete corrodes in presence of
chloride is the local breakdown of the passive layer formed in the highly alkaline
condition of the concrete. It is assumed that corrosion initiates when a critical
chloride content reaches the rebar surface. The mathematical formulation
idealized the corrosion sequence as a two-stage process: an initiation stage,
during which chloride ions penetrate to the reinforcing steel surface and
depassivate it, and a propagation stage, in which active corrosion takes place
until cracking of the concrete cover has occurred.
The aim of this research is to develop computer tools to evaluate the duration of
the service life of reinforced concrete structures, considering both the initiation
and propagation periods. Such tools must offer a friendly interface to facilitate
its use by the researchers even though their background is not in numerical
simulation.
For the evaluation of the initiation period different tools have been
developed:
Program TavProbabilidade: provides means to carry out a probability analysis
of a chloride ingress model. Such a tool is necessary due to the lack of data and
general uncertainties associated with the phenomenon of the chloride diffusion.
It differs from the deterministic approach because it computes not just a chloride
profile at a certain age, but a range of chloride profiles for each probability or occurrence.
Program TavProbabilidade_Fiabilidade: carries out reliability analyses of the
initiation period. It takes into account the critical value of the chloride
concentration on the steel that causes breakdown of the passive layer and the
beginning of the propagation stage. It differs from the deterministic analysis in
that it does not predict if the corrosion is going to begin or not, but to quantifies
the probability of corrosion initiation.
Program TavDif_1D: was created to do a one dimension deterministic analysis
of the chloride diffusion process by the finite element method (FEM) which
numerically solves Fick’second Law.
Despite of the different FEM solver already developed in one dimension, the
decision to create a new code (TavDif_1D) was taken because of the need to
have a solver with friendly interface for pre- and post-process according to the
need of IETCC.
An innovative tool was also developed with a systematic method devised to
compare the ability of the different 1D models to predict the actual evolution of
chloride ingress based on experimental measurements, and also to quantify the
degree of agreement of the models with each others.
For the evaluation of the entire service life of the structure: a computer
program has been developed using finite elements method to do the coupling of
both service life periods: initiation and propagation.
The program for 2D (TavDif_2D) allows the complementary use of two external
programs in a unique friendly interface:
• GMSH - an finite element mesh generator and post-processing viewer
• OOFEM – a finite element solver.
This program (TavDif_2D) is responsible to decide in each time step when and
where to start applying the boundary conditions of fracture mechanics module in
function of the amount of chloride concentration and corrosion parameters
(Icorr, etc). This program is also responsible to verify the presence and the
degree of fracture in each element to send the Information of diffusion
coefficient variation with the crack width.
• GMSH - an finite element mesh generator and post-processing viewer • OOFEM – a finite element solver.
The advantages of the FEM with the interface provided by the tool are:
• the flexibility to input the data such as material property and boundary
conditions as time dependent function.
• the flexibility to predict the chloride concentration profile for different
geometries.
• the possibility to couple chloride diffusion (initiation stage) with chemical
and mechanical behavior (propagation stage).
The OOFEM code had to be modified to accept temperature, humidity and the
time dependent values for the material properties, which is necessary to
adequately describe the environmental variations.
A 3-D simulation has been performed to simulate the behavior of the beam on
both, action of the external load and the internal load caused by the corrosion
products, using elements of imbedded fracture in order to plot the curve of the
deflection of the central region of the beam versus the external load to compare
with the experimental data.