RESUMEN
Multi-recycling of concrete waste presents a promising avenue for carbon-negative development and a circular economy. This study comprehensively assesses the triaxial mechanical performance and environmental impact of multi-recycled concrete (Multi-RAC) through three recycling cycles. The results reveal a triaxial failure mode similar to natural aggregate concrete (NAC). The peak stress and peak strain monotonically increase with confinement stress, showing a significant impact (enlarged by 171.4 % to 280.6 % and 397.4 % to 412.0 %, respectively) from 0 to 20 MPa. All P-values for recycling cycles and confining pressure are less than 0.05, with the confining pressure having a more significant effect. Three best-fit multivariate mixed models predict mechanical properties, and a modified elastoplastic model introduces the recycling cycles factor. Numerical simulations confirm the model's accuracy in predicting the triaxial mechanical properties of Multi-RAC. Comparative analysis reveals that the elastoplastic model-derived non-integral high order failure criterion outperforms the Willam-Warnke failure criterion and other conventional criteria. Regarding environmental impact, all indicators (GWP, POCP, AP, EP, and CED) decrease favourably with the increasing number of recycling cycles, with CED and EP playing the most significant roles. Compared to NAC, the five environmentally favorable indicators for RACIII decrease by 3.24 % to 50.6 %, respectively. These findings provide valuable insights for future research on developing eco-friendlier Multi-RAC for sustainable and green infrastructure.
RESUMEN
Cracks in reinforced concrete structures can accelerate the local depassivation of reinforcement due to carbonation. Different approaches have been proposed to account for pre-existing cracks within engineering models to predict the carbonation depth. In this study, we provide a detailed comparative analysis of different extensions available for the fib carbonation model to account for cracks, viz., crack influence factor (CIF) approaches, a diffusion-based model and the crack depth adaption. The model extensions are first validated against a dataset of lab data collected from the literature and additional experiments performed as the part of this study. The CIF approaches achieved the highest accuracy for the carbonation depth prediction when compared against lab data. The diffusion-based model was inaccurate for low CO2 concentrations. The crack depth adaption provides overly conservative results. No model was found to be best performing, and large scatter was observed between predicted and experimental values. This emphasizes the need for more detailed multi-physics-based models to achieve accurate predictions. For further comparison, service life predictions were conducted for two structural scales, viz., the whole structure and the cracked area. It is concluded that the choice of model extension and the structural scale of analysis have a large influence on predicted probability of failure.
RESUMEN
The nanoindentation technique is already widely applied in the mechanical characterization of the microstructure of thin films with respect to different materials. Generally, by means of nanoindentation, the hardness and the elastic modulus of materials can be determined with high precision. The focus of these analyses is usually on the materials from the metal, ceramic, and plastics processing industry. The application of nanoindentation in construction science, especially in concrete technology, is a relatively new field of investigation. This study deals with the basic application of nanoindentation for the mechanical characterization of hardened cement paste. In particular, the effects of sample preparation and the selection of the nanoindentation measurement parameters on the obtained results are the main subjects of this investigation. The results re intended to show the opportunities and limitations of analyzing a heterogeneous material such as hardened cement paste. The findings are used to assess the suitability of the nanoindentation method for investigating durability-related damage (e.g., due to freeze-thaw or alkali-silica reaction) in concrete.
RESUMEN
Concrete structures are increasingly becoming exposed to organic acid attack conditions, such as those found in agriculture and food-related industries. This paper aims to experimentally verify the thermodynamic modeling of cement pastes under acetic acid attack. For this, a modeling approach implemented in IPHREEQC via Matlab is described, and results are compared with measured pH and compositions of equilibrated solutions (MP-AES) as well as unreacted/precipitated solids (XRF, XRD and STA) for a wide range of acid concentrations. The 11% replacement of cement by silica fume (SF) led to a 60 or 70% reduction (measured or modeled, respectively) of Portlandite content in the hardened cement paste due to the pozzolanic reaction resulting in higher content of CSH phases, which has effects on the progression of dissolution processes and a resulting pH with increased acid concentrations. Considering that no fitting parameter was used, the model predictions showed good agreement with measured values of pH, dissolved ion concentrations and composition of the remaining (degraded) solids overall. The discrepancies here were more pronounced at very high acid concentrations (equilibrium pH < ~4), i.e., after the full dissolution of hydrate phases due to limitations in the model used to describe Al-, Si- and Fe-gel phases and/or identified experimental challenges in precipitation of calcium and aluminum acetate hydrates.
RESUMEN
This paper investigates the development of the alkalinity and its impact on carbon steel reinforcement embedded in alkali-activated fly ashes (AAFA) and alkali-activated fly ashes with ten percentage mass (wt%) of blast furnace slag (AAFAS)-based materials (geopolymer-GP). The pH analysis of eluates indicates a remarkable decrease of alkalinity in AAFA and AAFAS in the first hours of the geopolymerization process. Phenolphthalein solution and pore solution tests on concretes also show a sharp decrease of alkalinity with increased Ca content in the binder due to carbonation. Micro X-ray computer tomography (µXCT) and electrochemical techniques indicate that the changed pH in the GP systems was accompanied by a decrease in the corrosion rates of steel reinforcement when compared to ordinary Portland cement (OPC) systems. In contrast to calcite and vaterite, which were detected in OPC and AAFAS after a carbonation process, only sodium carbonate natron was determined at lower levels in AAFA by X-ray diffraction (XRD).
