Answer:
This setting is used to reduce the effective concrete tensile strength fct,eff. If you expect crack formation in the first 28 days, you should reduce the effective tensile strength of concrete fct,eff according to EN 1992‑1‑1, 7.3.2 (2). You can enter the corresponding reduction factor. The German National Annex recommends using fct,eff = 0.50 ⋅ fctm (28d). If you cannot determine a definite point of time for crack formation within the first 28 days, a tensile strength of at least 3 N/mm² for normal concrete should be assumed in accordance with the German National Annex.
