PSI - Issue 67

Jiří Němeček et al. / Procedia Structural Integrity 67 (2025) 17 – 22 J. Neˇmecˇek et al. / Structural Integrity Procedia 00 (2024) 000–000

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The main sources of this deterioration are neutron and gamma irradiation, heating and drying. Neutron irradiation primarily a ff ects the aggregates, causing crystallographic and volume changes in minerals (Maruyama et al. (2017)). The cement matrix in concrete is mainly a ff ected by gamma irradiation, which causes radiolysis of water and subse quent drying of Calcium-Silicate-Hydrates (C–S–H) (Kontani et al. (2013)). Since the macroscopic deterioration mechanisms originate at the micro-scale, it is necessary to perform experi ments on a smaller scale. Nanoindentation provides an excellent tool for characterizing the micro-mechanical prop erties of materials (Oliver and Pharr (1992)). So far, only a limited number of studies on irradiated concrete have been conducted. Hunnicutt et al. (2020) performed nanoindentation on synthetically made C–S–H pellets exposed to gamma doses up to 0.784 MGy and 11% relative humidity (RH), reporting minimal changes in micro-mechanical properties. Recently, the same authors in Baral et al. (2022) reported new findings with higher exposure doses of 24 MGy and 189 MGy. In this case, a significant increase in nanoindentation Young’s modulus by approximately 15%–25% was reported for the highest dose of 189 MGy. Mortar samples exposed to doses of 12–15 MGy were studied by Khmurovska et al. (2021). The samples were exposed to 40%–60% RH conditions during one year of gamma irradiation, with negligible changes in nanoindentation Young’s modulus and creep compliance observed. Micro-indentation experiments with a relatively high testing force of 2000 mN were performed by Hilloulin et al. (2018) with gamma ray doses up to 0.257 MGy. Similar to Khmurovska et al. (2021) and Hunnicutt et al. (2020), no changes in Young’s modulus between irradiated and control samples were observed, but an increase in the creep modulus of irradiated samples was reported. The information about environmental conditions was not provided. Based on the limited research conducted at the micro-scale, with the role of relative humidity (RH) conditions still unclear, this study aims to clarify the influence of RH during the gamma irradiation of cement paste on micro mechanical properties, as assessed by nanoindentation. Ordinary Portland cement CEM I 42.5R was used to prepare cement paste samples with a water-to-cement ratio of 0.4. Fresh paste was cast into cylindrical molds with a height of 70 mm and a diameter of 27 mm. The samples were demolded after one day and placed in water for the next 27 days. After this curing period, the samples were cut into 8 mm thick slices using a diamond blade. These slices were then placed in acetone for 7 days to stop hydration. After acetone treatment, the samples were dried at 50°C to evaporate any residual acetone. The dried slices were then placed in plastic containers under specific RH conditions. Saturated salt solutions were used to achieve the following RH levels: 11% (LiCl), 33% (MgCl 2 · 6H 2 O), 43%(K 2 CO 3 ), and 76% (NaCl). A 4% RH was achieved using dry LiCl, and 100% RH was achieved by placing samples in a water bath. Samples stored with salt solutions were placed in plastic holders to prevent contact with the salts, while samples in water were in full contact with the liquid. After placement in the containers, the samples were left for 28 days to reach equilibrium conditions. More details about experimental campaign can be found in Neˇmecˇek et al. (2023). The gamma irradiation experiment was conducted at the Research Center Rˇ ezˇ in the Czech Republic using a 60 Co radiation source. The experiment lasted 120 days with an average gamma dose rate of 1 kGy / h, resulting in a total absorbed dose of 2.88 MGy. Meanwhile, a set of control samples was exposed to the same conditions (relative humidity and temperature) outside the irradiation chamber. Due to the harsh environment inside the irradiation chamber, the relative humidity (RH) was not measured si multaneously during irradiation, but a post-irradiation check was performed. It was found that the salts MgCl 2 · 6H 2 O ( + 15% RH) and K 2 CO 3 ( + 14% RH) were slightly degraded, leading to an increase in RH conditions. This suggests that these salt solutions should be replaced during irradiation breaks. Despite these changes, the RH levels still rep resented medium RH conditions. Examination of the plastic containers and holders revealed changes in transparency and some fragility to touch, but there was no significant mechanical damage. 2.2. Gamma irradiation and post-irradiation examination 2. Experiments and methods 2.1. Material and sample preparation