Insight into the Optimization of Implementation Time in Cob Construction: Field Test and Compressive Strength Versus Drying Kinetics

Mastering construction times is of paramount importance in making vernacular earth construction techniques attractive to modern clients. The work presented here is a contribution towards the optimization of the construction time of cob buildings. Therefore, this paper follows the evolution of a cob’s mechanical properties during its drying process in the case of a double-walling CobBauge system. Laboratory tests and in situ measurements were performed, and further results were described. Volumetric water content sensors were immersed in the walls of a CobBauge prototype building during its construction. The evolution of the cob layer’s compressive strength and Clegg Impact Value (CIV) as a function of its water content has been experimentally studied and discussed. These studies showed that compressive strength and CIV are correlated with water content, and both properties decrease exponentially with time. In this study, a new tool to evaluate cob’s mechanical performances in situ has been proposed, Clegg Impact Soil Tester. This was linked to compressive strength, and a linear relationship between these two properties was found. Finally, appropriate values of compressive strength and CIV to satisfy before formwork stripping and re-lifting were proposed. For this study’s conditions, these values are reached after approximately 27 days

Machine Learning-Based Indoor Relative Humidity and CO2 Identification Using a Piecewise Autoregressive Exogenous Model: A Cob Prototype Study

The population of developed nations spends a significant amount of time indoors, and the implications of poor indoor air quality (IAQ) on human health are substantial. Many premature deaths attributed to exposure to indoor air pollutants result from diseases exacerbated by poor indoor air. CO2, one of these pollutants, is the most prevalent and often serves as an indicator of IAQ. Indoor CO2 concentrations can be significantly higher than outdoor levels due to human respiration and activity. The primary objective of this research was to numerically investigate the indoor relative humidity and CO2 in cob buildings through the CobBauge prototype, particularly during the first months following the building delivery. Both in situ experimental studies and numerical predictions using an artificial neural network were conducted for this purpose. The study presented the use of a piecewise autoregressive exogenous model (PWARX) for indoor relative humidity (RH) and CO2 content in a building constructed with a double walling system consisting of cob and light earth. The model was validated using experimental data collected over a 27-day period, during which indoor RH and CO2 levels were measured alongside external conditions. The results indicate that the PWARX model accurately predicted RH levels and categorized them into distinct states based on moisture content within materials and external conditions. However, while the model accurately predicted indoor CO2 levels, it faced challenges in finely classifying them due to the complex interplay of factors influencing CO2 levels in indoor environments

Impact of a sulphidogenic environment on the corrosion behavior of carbon steel at 90 °C

The influence of sulphide ion concentration on the behavior of carbon steel in a synthetic solution at 90 °C has been investigated using the methods of weight loss, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), confocal micro-Raman spectroscopy and X-ray diffraction (XRD). Corrosion batch experiments were conducted at 90 °C for 1 month with steel coupons immersed in Na2S solutions. Weight loss measurements revealed that the corrosion layer resistance is strongly dependent on both the sulphide concentration and the physicochemical properties of the corrosion products. In the absence of sulphide ions, a magnetite (Fe3O4) corrosion product layer was formed on the steel coupon, while in presence of sulphide ions (1 mg l−1), we observed the formation of a less protective mackinawite corrosion layer. At higher sulphide concentrations (5-15 mg l−1), highly protective pyrrhotite and pyrite are formed, inhibiting the steel corrosion process. Thus, it has been suggested that the formation of pyrite and/or pyrrhotite could be a promising strategy to protect against carbon steel corrosion in sulphidogenic media

Insight into the mechanism of carbon steel corrosion under aerobic and anaerobic conditions

We particularly focused our study on identifying the corrosion products formed at 30 °C on carbon steel under aerobic and anaerobic conditions and on following their evolution with time due to enhanced microbial activity under environmental and geological conditions. The nature and structural properties of corrosion products were investigated by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD) and confocal micro-Raman spectroscopy. Structural characterisation clearly showed the formation of iron oxides (magnetite and maghemite) under aerobic conditions. Under anaerobic conditions, the first corrosion product formed on the steel surface was nanocrystalline mackinawite, which was then followed by a fast transformation process into the pyrrhotite phase, and the Raman spectrum of monoclinic pyrrhotite was proposed for the first time. Finally, this study also shows that in the context of geological disposal of radioactive waste, the corrosion of carbon steel containers in anoxic and sulphidogenic environments sustained by sulphate-reducing bacteria may not be a problem notably due to the formation of a passive layer on the steel surface

Assignment of Raman-active vibrational modes of tetragonal mackinawite: Raman investigations and ab initio calculations

International audienceThe mackinawite mineral was prepared as a carbon steel corrosion product in sulfidogenic waters at 90 degrees C after 2 months. The tetragonal crystal structure of the material was confirmed by Rietveld refinement of X-ray diffraction (XRD) data, and vibrational modes were analysed by micro-Raman spectroscopy. Despite a large number of studies on the formation and the stability of tetragonal mackinawite, the interpretation of the Raman spectra remains uncertain. In the present study, we report on the first calculation of the Raman-active vibrational modes of mackinawite using Density Functional Perturbation Theory and direct methods with BLYP + dispersion correction. Based on the comparison between calculated and experimental results, the four fundamental vibrational modes were assigned as 228 cm(-1) (B-1g), 246 cm(-1) (E-g), 373 cm(-1) (A(1g)) and 402 cm(-1) (E-g)

The effect of temperature on carbon steel corrosion under geological conditions

International audienc

Corrosion of carbon steel under sequential aerobic-anaerobic environmental conditions

We investigated sequential aerobic and anaerobic microbiologically induced corrosion of carbon steel to simulate deep geological disposal conditions. Under limited oxygen supply, lepidocrocite and magnetite corrosion products formed on the steel coupon, while under continuous oxygen supply, a mixture of lepidocrocite, maghemite and magnetite was identified. Upon oxygen consumption and establishment of sulphidogenic conditions, due to sulphate-reducing bacteria activity, all these oxides disappeared via transformation into pyrrhotite. Corrosion rate of steel in direct anaerobic cultures was higher than that of steel initially corroded in aerobic condition, suggesting a protective role of corrosion product layer formed under sequential aerobic-anaerobic conditions