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Over a year, a structure is exposed to different seasonal environments. For instance, structures are not exposed to chemically aggressive salts during the summer, whereas they endure substantial applications of deicing salts during winter. It is thus possible to define different time sequences to take into account these different exposure conditions. The user must first select the number of sequences during a year by clicking on Add or Delete (see Figure 9) and then enter the end of each sequence as a number of days. The default value for the last sequence is 365 days.
The next step consists in specifying for each sequence whether boundary conditions are applied at x=0 and/or x=L, i.e. the two slab extremities. If the structure is not exposed to an aggressive solution in a given sequence, neither x=0 nor x=L should be selected.
The user must then specify, for each ion and for each sequence, the concentration values to be imposed at the previously selected locations: x=0 and/or x=L, or none. The user may impose concentration values according to the following sine function relationship:
Figure 22. Main window for the ionic species boundary conditions
The following examples illustrate the use of boundary conditions. It is assumed that a parking structure is exposed to deicing salts (NaCl) for only a short period throughout the year. The exposure lasts from day 245 to 305 in a one-year period and is assumed to increase gradually to reach 1300 mmol/L before decreasing to zero. The user must define three sequences. During the first sequence, from day 0 to day 245, no salts are applied. This corresponds to Figure 22 where no boundary conditions are specified. For the second sequence (Figure 22), NaCl is applied as a half-sine function at the top surface of the slab (x=0). In this case, the following parameters should be used: offset: 245, period: 120.0, amplitude: 1300.0, and mean value: 0.0 for Na+ and Cl–. All the other species are set at zero. The corresponding screenshot is shown in Figure 22.
If the user wants to set boundary conditions as shown in Figure 23, i.e. a half-sine function for NaCl instead of a uniform application, the following parameters should be used: offset 240, period: 120.0, amplitude: 1300.0, and mean value: 0.0. The corresponding screenshot is shown in Figure 21.
Finally, an important case to consider is constant boundary conditions over the entire exposure period. Typical cases are structure continuously immersed in seawater and concrete samples immersed in chloride solution for laboratory testing. For this type of boundary conditions, simply select one sequence and assign a zero value to the amplitude, offset, and period parameters. The composition of the solution is entered through the “mean value” parameter.
Figure 23. Boundary Conditions window for chloride exposure: Half-sine chloride application-input parameters for the half-sine function
Note that the boundary conditions must not violate the electro-neutrality requirement for proper simulation results. This means that, at any time on a given boundary (x=0 or x=L), the sum of positive and negative charges must be zero.
Finally, you must consider the boundary conditions associated with chlorides. Unlike some chloride ingress models for cementitious materials, you must enter the composition of the solution at the surface of the structure. This is very different from the usual Co value, which represents the total chloride content measured at the surface of the material. The value required here is the sum of the chloride in the pore solution and the chloride that is chemically bound to the paste
Web site: http://www.simcotechnologies.com/
