Critical Chloride concentrations are of two types: One is the total chloride and the other is the free chloride. Threshold Free and Total chloride concentrations may be safely taken as 0.15% and 0.4% respectively by mass of cement. Thirdly a threshold value for [Cl- /OH- ] ratio can also be set as 0.6. Average European environment applies.
In the Life-365 model, neither fly ash nor slag are assumed to effect the chloride threshold. However, the threshold is actually influenced by various factors, including w/b ratio, pH of the pore solution, mineral admixtures, steel type, defects at the steel-concrete interface and so on. More discussions about this issue can be found in the following two reviews.
U. Angst, B. Elsener, C.K. Larsen, O. Vennesland, Critical chloride content in reinforced concrete - A review, Cement and Concrete Research 39 (2009) 1122-1138.
Y. Cao, C. Gehlen, U. Angst, L. Wang, Z. Wang, Y. Yao, Critical chloride content in reinforced concrete - An updated review considering Chinese experience, Cement and Concrete Research 117 (2019) 58-68.
Given the complexity of this issue, it is not so reasonalbe to choose a constant threshold when conducting the service life analysis. It is better to consider it as a random variable, and the statistical distribution can be obtained with the literatures mentioned in the aforementioned reviews.