4SIGHT Manual: A Computer Program for Modelling Degradation of Underground Low Level Waste Concrete Vaults/Failure Criteria

by Kenneth A. Snyder and James R. Clifton

9Failure Criteria

Once the rules for propagating ions through the slab have been defined criteria for terminating the calculations are needed for each degradation mechanism. Termination occurs when suitable failure criteria have been met.

9.1Reinforcement Corrosion

Corrosion of the rebar is due to the presence of chloride ions and proceeds in two stages: initiation and corrosion. The initiation stage is the time during which chloride ions are diffusing through the concrete. During this time there is an insufficient concentration of chloride ions at the rebar for corrosion to occur. As the concentration of chloride ions at the rebar increases, the pH decreases to insure charge neutrality. When a sufficient concentration of chloride ions reaches the rebar, corrosion begins. The corrosion stage begins at the onset of corrosion and lasts until failure of the of the steel reinforcement, which is typically only a few years hence. Since the initiation stage may last for hundreds of years, the duration of the corrosion stage is insignificant to the total lifetime of the reinforcement. Therefore, an accurate estimate of the service life of the concrete can be approximated from the duration of the initiation period.

9.2Sulfate Attack

As the sulfate front proceeds into the concrete, the effective thickness of the concrete decreases. At some point in time the roof has an insufficient thickness to support its load. At this time, structural failure of the roof will occur. This critical depth of sulfate penetration can only be determined through detailed structural analysis. Therefore, it is the user's responsibility to provide this information.

9.3Joint Failure

The failure criteria for joints is based upon the service life of the joint compound. Since joint widths will be centimeters wide, as previously stated, upon joint failure the permeability of the roof slab will be overwhelmed by the flow through the joints. Therefore, the calculations terminate upon when the internal time variable has reached the limit of the joint service life.