COMSOL 6.3 - Oxide Jacking of Reinforced Concrete

Oxide jacking is the process by which reinforced concrete cracks, due to the corrosion of the reinforcement bars. The corrosion process causes growth of an oxide layer on the rebar, which in turn causes internal stresses in the concrete. If the corrosion process is allowed to continue, the concrete will eventually crack, compromising the structure.

This tutorial investigates the oxide jacking of reinforced concrete. The corrosion process is driven by oxygen reduction, forming an oxide layer. Charge and oxygen transport are modeled in the concrete domain, where the electrolyte conductivity and oxygen diffusivity depend on the moisture content.

The rebar and concrete are modeled as linear elastic materials, where the volumetric strain or the rebar in each time-step is based on the thickness of the oxide layer. Cracking in the concrete caused by the volumetric expansion due to oxide formation is accounted for using a scalar damage model.

Model Definition

Figure 1 shows the model geometry, defining concrete as an electrolyte domain and a circular steel rebar. The geometry is defined in 2D.

Figure 1: Model geometry.

Oxygen diffuses from one side of the concrete block, which is assumed to have a water pore saturation of 60%. The model is defined using the interface, solving for the electrolyte phase potential and the oxygen concentration, assuming a supporting electrolyte charge conservation model, defined in a similar way as in the Cathodic Protection of Steel in Reinforced Concrete tutorial.

The oxygen concentrations at the left most boundary is set to a reference concentration value of 8.6 mol/m3.

The following three different electrode reactions on the steel rebar boundary are considered: oxygen reduction, hydrogen evolution and the net iron oxidation.

Oxygen reduction reaction:

(1)

Hydrogen evolution reaction:

(2)

The net iron oxidation reaction is considered to follow the following reaction scheme.

Iron oxidizes and dissolves:

(3)

Dissolved iron ions then react with water and oxygen:

(4)

The final corrosion product iron oxide is the formed according to:

(5)

Thus, the net iron oxidation reaction considered in the model is:

(6)

The iron oxide layer thickness, sb,tot (m) along the rebar-concrete interface is computed using the functionality of the node.

The interface is used for structural analysis using two nodes to model the steel rebar and concrete domains. A scalar damage model is used to describe cracking of the concrete domain, using a subnode.

The average areal strain of the rebar is computed based on the oxide thickness layer obtained from the interface according to

(7)

The node of the Linear Elastic Material node defining the rebar domain is used to define the strain ε0 in the x and y directions according to

(8)

The model is solved in a time-dependent solver, simulating the oxide jacking during 1500 days.

Results and Discussion

Figure 2 shows the line plot of iron oxide corrosion product layer thickness along the rebar surface for different times. It can be seen that the iron oxide layer continuously grows with time.

Figure 2: Iron oxide layer thickness at different times.

Figure 3 shows the surface plot of first principal stress in the concrete domain after 1500 days.

Figure 3: First principal stress distribution in the concrete domain after 1500 days.

Figure 4 shows the surface plot of damage over the concrete domain after 1500 days, clearly showing the detrimental effect of corrosion on the surrounding concrete.

Figure 4: Damage distribution over concrete domain after 1500 days.

Figure 5 shows the surface plot of nonlocal equivalent strain over the concrete domain after 1500 days. It can be seen that a crack is formed at the rebar surface as the stiffness of concrete degrades. The crack is seen to be propagating toward the outer surface of concrete.