Adelina Apostu

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.

In this paper, we study the influence of densified microsilica and colloidal nanosilica admixtures on the mechanical strength and the microstructural characteristics of special mortars used for immobilizing radioactive concrete waste. The experimental program focused on the replacement of cement with micro- and/or nanosilica, in different proportions, in the basic composition of a mortar made with recycled aggregates. The technical criteria imposed for such cementitious systems, used for the encapsulation of low-level radioactive waste, imply high fluidity, increased mechanical strength and lack of segregation and of bleeding. We aimed to increase the structural compactness of the mortars by adding micro- and nanosilica, all the while maintaining the technical criteria imposed, to obtain a cement matrix with high durability and increased capacity for immobilizing radionuclides. The samples from all the compositions obtained were analyzed from the point of view of mechanical strength. Also, micro- and nanosilica as well as samples of the optimal mortar compositions were analyzed physically and microstructurally. Experimental data showed that the mortar samples present maximum compressive strength for a content between 6 and 7.5% wt. of microsilica, respectively, for a content of 2.25% wt. nanosilica. The obtained results suggest a synergistic effect of micro- and nanosilica when they are used simultaneously in cementitious compositions. Thus, among the analyzed compositional variants, the mortar composition with 3% wt. microsilica and 2.25% wt. nanosilica showed the best performance, with an increase in compressive strength of 23.5% compared to the control sample (without micro- and nanosilica). Brunauer–Emmett–Teller (BET) analysis and scanning electron microscopy (SEM) images highlighted the decrease in pore diameter and the increase in structural compactness, especially for mortar samples with nanosilica content or a mixture of micro- and nanosilica. This study is useful in the field of recycling radioactive concrete resulting from the decommissioning of nuclear research or nuclear power reactors.