A Two-Axis Climatic Zonation for the Frost Durability Design of Concrete Structures in Romania Based on Extreme Value Analysis of Freezing Indices and Freeze–Thaw Cycles

Frost damage to concrete proceeds by two distinct mechanisms: deep sustained freezing, which governs buried elements, and repeated freeze–thaw cycling, which governs exposed ones. Romanian practice still assigns durability requirements from maps built for other purposes: a 1977 frost-depth zonation and a thermal zoning derived from building heat-loss temperatures. Daily records from 156 meteorological stations were processed by Gumbel extreme value analysis at 2–100-year mean recurrence intervals to map the freezing index (FI), freeze–thaw cycle (FTC) counts, day-counts below −5 °C and −10 °C, the 50-year minimum temperature, and frost penetration depth. The two hazards are spatially decoupled: across the 135 stations below 1000 m, the freezing index explains only 31% of the variance in cycle count, falling to 14% once altitude is partialled out. Low-altitude cold pockets in northern Oltenia reach top-class cycling at moderate minima (Apa Neagră, 250 m, FTC50 = 134.6) yet are currently placed in the second-mildest zone. A two-axis zonation with Jenks-calibrated thresholds is therefore proposed, aggregated into five zones that preserve the existing numbering while correcting four misclassifications, with the axes being assigned to buried and above-ground concrete. Requirements matched to local severity extend service life and avoid premature repair and over-specification.