Page:4SIGHT manual- a computer program for modelling degradation of underground low level waste concrete vaults (IA 4sightmanualcomp5612snyd).pdf/51

 satisfies eqns. 23 and 24 using an iterative process. Each iteration is composed of two steps: These two steps are repeated, as necessary, until both equations are satisfied.
 * 1) Determine how many moles of salt should be precipitated or dissolved to satisfy eqn. 23.
 * 2) Given $$K_{sp}$$ for water what concentration of hydroxyl ions ($$OH^-$$) is needed to satisfy eqn. 24

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.