Limiting constraints
Eurocode 2: Reinforced concrete - Checking serviceability limits

To avoid the appearance of longitudinal cracks, microcracks or high creep, in the absence of other measures (increased encasement in the compressed zone or confinement of the concrete by transverse reinforcement), it may be appropriate to limit the compressive stress of the concrete under the characteristic action combination [C 60, § 5.8.4] to 0.6 f _ck in areas likely to be exposed to exposure class XD, XF or XS environments. .