| 1. |
- Hedlund, Jakob, 1991-, et al.
(författare)
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Nurse anaesthetists' experiences of student nurse anaesthetist learning during clinical practice : a qualitative interview study
- 2024
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Ingår i: BMC Nursing. - : BioMed Central (BMC). - 1472-6955. ; 23:1
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: The professional role of a nurse anaesthetist involves taking a pedagogical approach towards students, including supervision during clinical practice. Although supervisors are facilitators of student learning, they are offered little training in adult learning principles. The aim of this study was to describe supervisors' experience of student nurse anaesthetist learning during clinical practice in the operating room.METHOD: In this qualitative interview study, 12 semi-structured individual interviews were carried out with clinical supervising nurse anaesthetists. The data were analysed inductively using thematic analysis.RESULTS: The results are illustrated with one theme and five sub-themes. The clinical learning situation of student nurse anaesthetists is described as a reflection of different cultures coming together. The operating room environment is a new context to students, and students enter with different clinical background and experiences. There is tension in facilitating student learning due to demands for productivity; supervisors suggest the use of separate operating rooms with a special focus on learning in the future.CONCLUSION: Clinical practice facilitates student learning and is a parallel process to routine care. Thus, it requires the cultures of higher education and healthcare organizations to co-exist. This is illustrated with the theme "Contributing to students' future professional roles by bridging the hospital and university cultures". In the operating room, student learning is challenged by a new context and time pressure as shown by subthemes. To overcome challenges and support student learning in the operating room from a supervisors' perspective, interprofessional student teams are suggested as a future approach and need to be further investigated.
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| 2. |
- Hedlund, Jakob, 1991-, et al.
(författare)
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Student Nurse Anesthetists' and Supervisors' Perspectives of Learning in the Operating Room : An Integrative Review
- 2024
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Ingår i: Journal of Perianesthesia Nursing. - : Elsevier. - 1089-9472 .- 1532-8473. ; 39:2, s. 303-310.e8
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Forskningsöversikt (refereegranskat)abstract
- PURPOSE: The purpose of this review was to identify supporting and hindering factors for student nurse anesthetists' (SNAs') learning in the operating room during clinical practice, from students' and supervisors' perspectives.DESIGN: An integrative review.METHODS: Systematic searches were conducted in Medline, Cinahl, PsycInfo, and ERIC. Search terms were related to nurse anesthetist, education, operating room context, and clinical setting. Searches were performed at three points in time and in total 1,530 unique articles were identified. After screening using Covidence and using Joanna Briggs Institute appraisal tools, 34 articles remained. These were analyzed inductively using a constant comparison method.FINDINGS: Supporting factors include preparation before clinical practice, clearly stated expectations, a respectful relationship with the supervisor, daily planning and communication, and constructive feedback. Hindering factors include lack of time, disruptive behavior from supervisors or other team members, and environmental factors such as a high room temperature and noisy environment.CONCLUSIONS: SNAs' learning situation in the operating room resembles undergraduate nurses' learning during clinical practice. Educators and supervisors can take several actions to promote SNAs' learning. Further research is warranted on the effect of teamwork on SNAs' learning.
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| 3. |
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| 4. |
- Kothari, Ankit, et al.
(författare)
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A Review of the Mechanical Properties and Durability of Ecological Concretes in a Cold Climate in Comparison to Standard Ordinary Portland Cement-Based Concrete
- 2020
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Ingår i: Materials. - Basel, Switzerland : MDPI. - 1996-1944. ; 13:16
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Forskningsöversikt (refereegranskat)abstract
- Most of the currently used concretes are based on ordinary Portland cement (OPC) which results in a high carbon dioxide footprint and thus has a negative environmental impact. Replacing OPCs, partially or fully by ecological binders, i.e., supplementary cementitious materials (SCMs) or alternative binders, aims to decrease the carbon dioxide footprint. Both solutions introduced a number of technological problems, including their performance, when exposed to low, subfreezing temperatures during casting operations and the hardening stage. This review indicates that the present knowledge enables the production of OPC-based concretes at temperatures as low as −10 °C, without the need of any additional measures such as, e.g., heating. Conversely, composite cements containing SCMs or alkali-activated binders (AACs) showed mixed performances, ranging from inferior to superior in comparison with OPC. Most concretes based on composite cements require pre/post heat curing or only a short exposure to sub-zero temperatures. At the same time, certain alkali-activated systems performed very well even at −20 °C without the need for additional curing. Chemical admixtures developed for OPC do not always perform well in other binder systems. This review showed that there is only a limited knowledge on how chemical admixtures work in ecological concretes at low temperatures and how to accelerate the hydration rate of composite cements containing high amounts of SCMs or AACs, when these are cured at subfreezing temperatures.
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| 5. |
- Kothari, Ankit, et al.
(författare)
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Effects of sodium nitrate and OPC-GGBS concrete mix composition on phase transition of pore water at subzero temperatures
- 2022
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Ingår i: Construction and Building Materials. - : Elsevier. - 0950-0618 .- 1879-0526. ; 327
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Tidskriftsartikel (refereegranskat)abstract
- Lowering the freezing temperature of the mixing water is crucial for concrete works at subzero temperatures. In this study, formation of ice was examined for various pastes and concretes of OPC-GGBS based, while exposed to a constant temperature of −15 °C. Sodium nitrate antifreeze admixture was added as 0, 6, 10, 15, 20, 25, 30 wt% by the total binder amount. The ice formation and its effects on the binder matrix microstructure was studied using differential scanning calorimetry (DSC), ultrasonic pulse velocity (UPV) and Scanning Electron Microscopy – Energy Dispersive Spectrometry (SEM-EDS). Several curing procedures were applied to samples before commencing tests. Results showed that, addition of 25 wt% of the sodium nitrate caused the most extensive delay of the ice growth. Mixes containing less admixture showed an increasing amount of the forming ice which in some cases lead to the development of the false strength. The hydration rate has been the highest for the mix with 25 wt% of the sodium nitrate and tended to be limited at lower additions. The porosity of the hydrated binder matrix tended to be lower for mixes characterized by a lower amount of the forming ice. In general, application of above freezing temperature resulted in resuming of the hydration process that led to densification of the microstructure and strength increase. This trend was more pronounced for mixes having lower amounts of the formed ice.
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| 6. |
- Kothari, Ankit
(författare)
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Low Portland cement content concretes at freezing and subfreezing temperatures
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Concrete is the most used building material. With the increasing growth of industries and urbanization globally; the demand for concrete is increasing significantly. Ordinary Portland Cement (PC) is the binder used to produce typical concrete. Unfortunately, for every ton of manufactured cement about 0.61-ton CO2 is emitted into the earth’s atmosphere. As a result, several solutions have been implemented to reduce the usage of this material in the production of concrete. This includes its partial or full replacement with supplementary cementitious materials (SCMs) or alternative binders. Some of these combinations could be problematic to be used in cold climates due to a lower developed hydration heat, slower setting, or worse frost durability.In winter the immediate exposure of fresh concrete to freezing temperatures results in pore ice formation and could delay or completely stop the hydration process. This is commonly prevented by using an additional heating system installed in concrete or the formwork. Unfortunately, usually, it adds complexity, increases the price, and depending on the used power source, could increase the CO2 footprint. Another potentially simpler and more sustainable solution is to modify the concrete itself by adjusting the mix design, by using certain chemical admixtures and special cementitious binders.This research aimed to better understand how partial replacement of Portland cement with GGBFS and/or CSA cement affects the properties of concretes exposed to freezing and subfreezing temperatures in a fresh state and at a young age. The secondary aim was to evaluate a possible application of UHPC to protect new and existing concrete structures from frost damage.The research included a literature review of methods used to produce concrete structures at zero and subzero temperatures. A special emphasis was on the application of chemical and mineral admixtures that could eliminate the need to use heat treatments. The output of this analysis enabled to narrow the scope of the research.The experimental program focused on the optimization, testing, and analysis of mixes containing various combinations of chemical admixtures, CSA cement, and Portland cement. Tests included exposure to freezing and subfreezing temperatures. The aim was to lower the freezing point of water and promote faster hydration and strength gain. Fresh and hardened properties were determined for all produced concretes. The phase transition of pore water into ice, the ice-forming temperature, and their effects on the binder matrix were studied using differential scanning calorimetry (DSC). Other tests included ultrasonic pulse velocity measurements (UPV), bond test (pull-off), scanning electron microscope (SEM) for analysis of the microstructure and phase composition, frost durability evaluation with Båras test, semi-adiabatic calorimetry to study hydration processes, compressive strength measurements.
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| 7. |
- Kothari, Ankit, et al.
(författare)
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Partial replacement of OPC with CSA cements – effects on hydration, fresh-, hardened-properties
- 2023
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Ingår i: Advances in Cement Research. - : ICE Publishing. - 0951-7197 .- 1751-7605. ; 35:5, s. 207-224
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Tidskriftsartikel (refereegranskat)abstract
- The effects of a partial replacement of Ordinary Portland cement (OPC) with three types of calcium sulfoaluminate (CSA) cements (40 wt% and 20 wt%) were investigated. The obtained results were generally in agreement with previously published data but with few interesting exceptions. Setting times were shortened due to the formation of ettringite. The maximum hydration temperature increased for concretes containing 40 wt% of CSA but decreased when 20 wt% replacement was used. The decrease was related to the deficiency of the available sulfates, which limited the formation of ettringite. The presence of extra anhydrite and calcium oxide was associated to the delayed establishment of the second temperature peak in contrast to OPC-based concretes. Their surplus delayed calcium aluminate and belite reactions, and triggered renewed formation of ettringite, C-S-H and portlandite. Effects of aluminum hydroxide were also indicated as possibly important, although not proved experimentally in this research. The slightly lower compressive strength measured for mixes containing 40 wt% of CSA were linked with more formed ettringite. The same factor was indicated as the key to the reduction of the total shrinkage in mixes containing 40 wt% of CSA and increased for the lower CSA replacement level. In that case, the insufficient amount of formed ettringite caused too small expansion, which could not efficiently mitigate or compensate the developed shrinkage.
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| 8. |
- Murray, Alison E., et al.
(författare)
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Roadmap for naming uncultivated Archaea and Bacteria
- 2020
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Ingår i: Nature Microbiology. - : NATURE PUBLISHING GROUP. - 2058-5276. ; 5:8, s. 987-994
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Tidskriftsartikel (refereegranskat)abstract
- The assembly of single-amplified genomes (SAGs) and metagenome-assembled genomes (MAGs) has led to a surge in genome-based discoveries of members affiliated with Archaea and Bacteria, bringing with it a need to develop guidelines for nomenclature of uncultivated microorganisms. The International Code of Nomenclature of Prokaryotes (ICNP) only recognizes cultures as 'type material', thereby preventing the naming of uncultivated organisms. In this Consensus Statement, we propose two potential paths to solve this nomenclatural conundrum. One option is the adoption of previously proposed modifications to the ICNP to recognize DNA sequences as acceptable type material; the other option creates a nomenclatural code for uncultivated Archaea and Bacteria that could eventually be merged with the ICNP in the future. Regardless of the path taken, we believe that action is needed now within the scientific community to develop consistent rules for nomenclature of uncultivated taxa in order to provide clarity and stability, and to effectively communicate microbial diversity. In this Consensus Statement, the authors discuss the issue of naming uncultivated prokaryotic microorganisms, which currently do not have a formal nomenclature system due to a lack of type material or cultured representatives, and propose two recommendations including the recognition of DNA sequences as type material.
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| 9. |
- Rajczakowska, Magdalena, et al.
(författare)
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Autogenous self-healing of low embodied energy cementitious materials: Effect of multi-component binder and crack geometry
- 2023
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Ingår i: Construction and Building Materials. - : Elsevier. - 0950-0618 .- 1879-0526. ; 376
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Tidskriftsartikel (refereegranskat)abstract
- Concrete's ability to auto-repair the cracks reduces the need for maintenance and repair. Autogenous self-healing is an intrinsic property of concrete highly dependent on the binder composition. The urgent necessity to decrease CO2 emissions of concrete by replacing cement with “greener” materials provides challenges and opportunities for self-healing cementitious materials. This research aims to verify the self-healing behavior of environmentally friendly multi-component binders. An experimental study is conducted to test the effect of binder composition-related parameters (e.g., phase composition, porosity) and crack geometry on the self-healing efficiency of the “green” mortars. Cementitious materials with 50 wt.%cement replacement with limestone powder blended with fly ash, blast furnace slag, and silica fume are investigated. Sorptivity change, compressive strength regains, and crack closure after self-healing are used to quantify the self-healing efficiency. Quantitative analysis and correlations between chemical composition/microstructural features, geometrical crack characteristics, and self-healing measures are investigated. The results indicate that “green” binder composition affects the self-healing mechanism leading to different levels of performance recovery. Some SCMs-limestone binder formulations enable a better self-healing efficiency than pure OPC or OPC/limestone cementitious materials, presumably due to a synergistic effect between the limestone and the mineral additions. Correlation analysis indicated that geometrical complexity characterized by fractal dimension and tortuosity of the crack does not affect the external crack closure, whereas the fractal dimension and maximum crack width are correlated with the internal crack healing.
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| 10. |
- Rajczakowska, Magdalena, et al.
(författare)
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Autogenous self-healing of thermally damaged cement paste with carbon nanomaterials subjected to different environmental stimulators
- 2023
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Ingår i: Journal of Building Engineering. - : Elsevier Ltd. - 2352-7102. ; 72
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Tidskriftsartikel (refereegranskat)abstract
- Autogenous self-healing of post-fire damaged concrete enables structure performance auto-recovery leading to reduced repair costs, less generated waste, and lower CO2 emissions. In this paper, to improve the efficiency of this process and understand the underlying mechanism, the self-healing of 0.1 wt% MWCNT-modified and pure cement paste subjected to novel environmental stimulators was tested. High-temperature damage was induced at 200 °C and 400 °C, followed by a self-healing cyclic treatment with water, a mixture of water with phosphate-based retarding admixture, and limewater. The self-healing efficiency of the proposed solutions were compared based on crack closure, strength regains, porosity, and chemical composition changes. The surface crack closure after 200 °C varied between 33% and 60%, whereas for 400 °C, only retarding admixture exposure obtained over 50% crack closure and the most considerable decrease in average crack width of 33% for MWCNT-modified paste. The highest values of compressive strength recovery, equal to 18% and 14%, exceeding the intact specimen's compressive strength, were observed for the MWCNT-modified paste healed in water and limewater. Water exposure with an extended wetting phase enhanced the compressive strength recovery of the MWCNT-modified materials. Strong (r = 0.87) and moderate (r = 0.52) positive correlations were observed between temperature loading and compressive and flexural strength recovery parameters, respectively. Higher porosity and interconnected crack network, caused by high temperature, facilitated the self-healing process. Porosity changes before and after healing were pronounced in contrast to the amount of unhydrated cement, which did not exhibit noticeable changes. The healing mechanism included three processes: calcite formation, further hydration inside the cracks, and rehydration of the bulk cement paste.Previous article in issue
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| 11. |
- Rajczakowska, Magdalena, et al.
(författare)
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Improved self-healing of mortars with partial cement replacement
- 2020
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Konferensbidrag (refereegranskat)abstract
- Making the European Union’s economy sustainable is the “European Green Deal” strategy announced by The European Commission. One of the major aims is becoming climate-neutral by 2050. Since global cement production accounts for approximately 8% of anthropogenic carbon dioxide emissions, the development of concrete with waste by-products as alternative binders and efficient self-healing properties would be a significant milestone towards the circular economy. The self-healing efficiency of cementitious composites with alternative binders requires further improvement as there is still insufficient information on this topic. The latest results for cement mortars showed the promotion of crack closure, both internally and externally, when the healing medium is a mixture of phosphate-based retarding admixture and water. The current study verifies whether satisfactory healing may also be achieved for cementitious composites with 20%cw slag and fly ash replacement subjected to phosphate-based exposure. The efficiency of the proposed solution is compared with other types of environmental conditions such as deionized or lime water immersion. The self-healing process is quantitatively assessed after 4 weeks of healing based on the crack closure and flexural strength regain. All exposure conditions applied resulted in efficient external crack closure; however, the phosphate-based retarding admixture showed the most impressive internal filling of the crack (Figure 1). Based on the Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopy (EDS) analysis, the majority of the self-healing products were identified as calcium carbonate crystals. Calcium phosphate compound and calcium silicate hydrate (C-S-H) were visible inside the crack in case of retarding admixture exposure contributing also to a limited flexural strength recovery.
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| 12. |
- Rajczakowska, Magdalena, et al.
(författare)
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Interpretable Machine Learning for Prediction of Post-Fire Self-Healing of Concrete
- 2023
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 16:3
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Tidskriftsartikel (refereegranskat)abstract
- Developing accurate and interpretable models to forecast concrete’s self-healing behavior is of interest to material engineers, scientists, and civil engineering contractors. Machine learning (ML) and artificial intelligence are powerful tools that allow constructing high-precision predictions, yet often considered “black box” methods due to their complexity. Those approaches are commonly used for the modeling of mechanical properties of concrete with exceptional accuracy; however, there are few studies dealing with the application of ML for the self-healing of cementitious materials. This paper proposes a pioneering study on the utilization of ML for predicting post-fire self-healing of concrete. A large database is constructed based on the literature studies. Twelve input variables are analyzed: w/c, age of concrete, amount of cement, fine aggregate, coarse aggregate, peak loading temperature, duration of peak loading temperature, cooling regime, duration of cooling, curing regime, duration of curing, and specimen volume. The output of the model is the compressive strength recovery, being one of the self-healing efficiency indicators. Four ML methods are optimized and compared based on their performance error: Support Vector Machines (SVM), Regression Trees (RT), Artificial Neural Networks (ANN), and Ensemble of Regression Trees (ET). Monte Carlo analysis is conducted to verify the stability of the selected model. All ML approaches demonstrate satisfying precision, twice as good as linear regression. The ET model is found to be the most optimal with the highest prediction accuracy and sufficient robustness. Model interpretation is performed using Partial Dependence Plots and Individual Conditional Expectation Plots. Temperature, curing regime, and amounts of aggregates are identified as the most significant predictors.
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| 13. |
- Rajczakowska, Magdalena, et al.
(författare)
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Is Cement Paste Modified with Carbon Nanomaterials Capable of Self-Repair after a Fire?
- 2022
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Ingår i: Nordic Concrete Research. - : Walter de Gruyter. - 0800-6377 .- 2545-2819. ; 67:2, s. 79-97
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Tidskriftsartikel (refereegranskat)abstract
- This manuscript presents preliminary results on the cement paste potential, with and without carbon nanomaterials, to heal high-temperature cracks. Cement paste beams were subjected to thermal loading of 200 & DEG;C and 400 & DEG;C after 28 days of water curing. High temperature caused the formation of microcrack networks on the specimen's surface. Self-healing was achieved by exposing the cracked samples to cyclic water immersion. The efficiency of the process was evaluated based on the crack closure and mechanical properties recovery after 24 days. The results indicated a distinct dependence of the healing on the loading temperature. Carbon nanotubes had a positive effect on self-repair efficiency.
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| 14. |
- Rajczakowska, Magdalena
(författare)
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Stimulated autogenous self-healing of mechanically and thermally cracked cementitious materials
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is estimated that each year, approximately 8 billion cubic meters of concrete are produced worldwide, a vast number comparable to 1 m3 per person, making the construction industry a major contributor to overall global CO2 emissions. Throughout the manufacturing process of the most common cement binder, ordinary Portland cement (OPC), CO2 emissions reach 842 kg per ton of clinker produced. Besides production-related emissions, concrete is a brittle material prone to cracking, wherein the mechanical performance and durability of the material degrade. In addition, maintenance and repairs of concrete structures require material resources, adversely affecting the concrete's overall environmental impact.At the same time, concrete is a very popular building material, primarily due to its low price, accessibility, and multifunctionality, enabling it to be used in most construction environments. Given its versatility and widespread use, decreasing its carbon footprint is essential. It can be achieved through different methods, such as partially replacing OPC with industrial by-products or activating waste materials, using low-carbon cement, or reusing and recycling. Another area of interest in achieving increased service life for concrete is developing and utilizing cementitious materials with self-healing properties.Cementitious materials have an inherent ability to self-repair cracks up to widths of 150 μm. However, wider cracks can be healed by employing various "stimulators" to boost the self-healing process, such as adding specific types of fibers, crystalline admixtures, or particular exposure conditions. Partial healing can also be achieved in extreme conditions. For example, structures that sustained high-temperature damage can be partially healed by executing post-fire curing. The recovery mechanism involves rehydration and self-healing of high-temperature cracks. Several variables define the process efficiency, such as the curing conditions, binder type, loading temperature, and post-fire cooling. The goal of this Ph.D. research project was to investigate the physicochemical processes and mechanisms behind the autogenous self-healing of cementitious materials. Two types of damage were evaluated: mechanical Cracking and high-temperature damaged binders. Furthermore, identifying potentially novel stimulators for enhanced self-healing properties was one of the project objectives. The application of low-carbon cementitious materials was of primary interest.A comprehensive exploratory and experimental program was devised and implemented to evaluate factors affecting autogenous self-healing, including the age of the material, exposure conditions, amount of unhydrated cement, and self-healing duration. Environmentally friendly binders were primarily used for the different mix compositions. Observations were made at the crack mouth and deep inside the crack by analyzing the crack closure and chemical composition of the newly formed self-healing products. In addition, the strength recovery and durability of the specimens were investigated. Quantitative analysis and correlations were examined between microstructural features, geometrical crack characteristics, and self-healing efficiency parameters. Physicochemical mechanisms for thermally and mechanically cracked cementitious materials were studied. Machine Learning techniques were used to predict the compressive strength recovery after high-temperature exposure numerically. Four algorithms were deployed and trained on a database of results collected from the literature review, and corresponding hyperparameters were tuned for optimized model results. Individual Conditional Expectation and Partial Dependency plots were used to visualize and interpret the results.It was observed that high cement content in the concrete mix does not guarantee an efficient autogenous self-healing of cracks. A dense, impermeable binder microstructure constrained the transport of silicon and calcium ions to the crack and reduced the precipitation of the healing products. With the addition of fly ash, the crack closure ratio close to the crack mouth increased, but recovery of flexural strength was not supported, presumably due to the small number of loadbearing phases inside the crack. All SCM-limestone cementitious materials have shown superior self-healing efficiency compared to pure OPC or OPC/limestone binders, presumably due to a synergistic effect between the limestone and the mineral additions. The binder composition affected the self-healing mechanism, leading to varying levels of performance recovery. Calcium carbonate was detected mainly at the crack mouth, whereas ettringite and calcium silicate hydrate (C-S-H) were found deeper inside the crack. Flexural and compressive strength was regained, presumably because of C-S-H and ettringite formation.On the other hand, after calcite crystals sealed the crack at the surface, the concentration of the ions inside the crack presumably increased, leading to better self-healing performance. Healing based on pure water exposure had limited efficiency despite applying various water volumes and temperature cycles. The highest crack closure was observed with the addition of a retarding admixture in the curing water. The admixture supposedly blocked the formation of a dense hydration shell on the surface of the unhydrated cement grains. Phosphorus and calcium were detected in the self-healing phases within the crack. Recovery of flexural strength by forming C-SH in the crack was recorded when using water mixed with micro silica particles.Using lime water with a small dosage of carbon nanomaterials displayed marginally improved high-temperature crack closure and mechanical performance compared with ordinary cement paste and tap water curing. Two distinct processes were identified for the recovery process of a thermally cracked cementitious material, i.e., rehydration and self-healing of the cracks. Phase assemblage and the cement paste porosity were exposed to changes with increasing loading temperature. These changes were presumably partially reversed upon application of a water re-curing process after cooling, i.e., the unhydrated cement grains further hydrate, forming new hydrates, pores are filled with new hydration products, and existing phases react to form new ones, e.g., CaO reacted with water to form Ca(OH)2. It can be hypothesized that the mechanism of the crack healing is the same as in the mechanically cracked concrete, i.e., based on diffusion-dissolution-precipitation processes.The developed machine learning model interpretation indicated that strength recovery depends on the temperature range that caused the damage, re-curing conditions, and the amount of fine and coarse aggregate.
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| 15. |
- Sayahi, Faez, et al.
(författare)
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Effect of Admixtures on Mechanism of Plastic Shrinkage Cracking in Self-Consolidating Concrete
- 2020
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Ingår i: ACI Materials Journal. - : American Concrete Institute. - 0889-325X .- 1944-737X. ; 117:5, s. 51-59
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Tidskriftsartikel (refereegranskat)abstract
- This research studies the effect of retarder, accelerator, stabilizer, air-entraining agent, and shrinkage-reducing admixture (SRA) on plastic shrinkage cracking in self-consolidating concrete (SCC). The main objective is to identify the dominant cracking cause—that is, plastic settlement or plastic shrinkage—in an SCC containing a particular admixture. During experimentation, crack-free concretes were achieved by adding air-entraining agent and SRA, while accelerator and retarder increased the crack area. The impact of admixtures on the cracking mechanism was identified by comparing the respective vertical and horizontal deformations. It was observed that the crack-free concretes had moderate settlement and horizontal shrinkage, while the cracked specimens exhibited significant deformation either vertically or horizontally.
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| 16. |
- Sayahi, Faez, et al.
(författare)
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Effect of Steel Fibres Extracted from Recycled Tyres on Plastic Shrinkage Cracking in Self-Compacting Concrete
- 2021
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Ingår i: Magazine of Concrete Research. - : Thomas Telford. - 0024-9831 .- 1751-763X. ; 73:24, s. 1270-1282
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates and compares the effect of steel fibres obtained through recycling waste tyres (known as RTSF), and a commercially available hooked steel fibre (HSF), on plastic shrinkage cracking in self-compacting concrete. The volumetric deformations of the specimens, bleeding, and the mass loss have been quantified. Mixtures containing 2.5, 5, 7.5 and 10 kg/m3 of RTSF, and 5 and 7.5 kg/m3 of HSF have been tested. The results show that an almost similar reduction of the crack area can be attained if HSF is replaced by a slightly higher amount of RTSF. However, the former seems to be more effective in restraining plastic shrinkage. Both fibres decreased the volumetric shrinkage and the bleeding capacity of the specimens.
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| 17. |
- Sayahi, Faez, et al.
(författare)
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Experimental validation of a novel method for estimating the severity of plastic shrinkage cracking in concrete
- 2022
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Ingår i: Construction and Building Materials. - Amsterdam : Elsevier. - 0950-0618 .- 1879-0526. ; 339
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Tidskriftsartikel (refereegranskat)abstract
- Plastic shrinkage cracking in cementitious materials is caused mainly by rapid and excessive moisture loss during mixture’s early ages, before sufficient tensile strength is gained. A novel model has been previously developed by the authors to estimate the severity of plastic shrinkage cracking in concrete. This paper presents findings of a series of full-scale experiments carried out to validate the accuracy of the proposed model. The experiments included investigating the impact of cement type, water-cement ratio (w/c), and admixtures (i.e., accelerator, retarder, and superplasticizer). The tests were performed in three rounds under similar ambient conditions using 3 slabs (3 m × 2 m) and 3 ring test moulds at each round. The results confirm the accuracy of the model in anticipating/comparing the cracking severity of the tested concretes.
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| 18. |
- Sayahi, Faez, et al.
(författare)
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Modelling the Severity of Plastic Shrinkage in Cementitious Materials
- 2020
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Ingår i: Proceedings of the international Conference on Civil Infrastructure of Construction (CUSC 2020). - : Qatar University Press. ; , s. 566-573
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Plastic shrinkage cracking in cement-based materials occurs between the mixing and the final setting of the mixture, where rapid evaporation of the mixed water is the main cause behind the phenomenon. The induced cracks may impair the durability and sustainability of the structure by facilitating ingress of harmful materials into the concrete bulk. In this paper a new model for estimating the cracking severity of plastic cementitious materials is presented based on the mixture’s initial setting time and the amount of the pore liquid evaporated from within the concrete mass. Results of experiments performed by the authors in another study, in addition to results of tests performed by other researchers are used to control the validity of the model. It is concluded that the model can anticipate the cracking severity of plastic concretes with good precision. The new method can provide practical tools for designers and contractors to predict and compare the cracking risk of the concretes prior to casting.
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| 19. |
- Sayahi, Faez, et al.
(författare)
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The Severity of Plastic Shrinkage Cracking in Concrete : A New Model
- 2021
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Ingår i: Magazine of Concrete Research. - : Institution of Civil Engineers (ICE). - 0024-9831 .- 1751-763X. ; 73:6, s. 315-324
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Tidskriftsartikel (refereegranskat)abstract
- Plastic shrinkage cracking in concrete is mainly a physical process, in which chemical reactions between cement and water do not play a decisive role. It is commonly believed that rapid and excessive moisture loss, due to evaporation is the primary cause of the phenomenon. This paper presents a new model to estimate the severity of plastic shrinkage cracking, based on the initial setting time and the amount of the evaporated water from within the concrete bulk. A number of experiments were performed under controlled ambient conditions, during which the water-cement ratio, cement type, and the dosage of superplasticizer were altered. The results, alongside those reported by other researchers, were utilized to check the validity of the proposed model. According to the outcomes, the model could predict the cracking severity of the tested concretes with a relative precision.
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| 20. |
- Stelmarczyk, Marcin
(författare)
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Applied Modeling of Moisture Phenomena in Concrete
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis contains new and improved calculation models, based on applied research merging numerical modeling and material research, in the areas of hydration and chemical binding of water, sorption of moisture and transport of moisture in concrete. The proposed model for hydration and chemical binding of water is based on general kinetics modeling and calculates degree of hydration directly, without using of equivalent time of maturity. It handles dependencies on temperature as well as availability of binder and water. It models the dormant phase in the beginning of hydration. It allows also for linear variability with temperature of how much water is bounded per amount of binder, which models cross-over effects in development of chemical binding of water, measured at different temperatures. In the area of moisture sorption a family of models is proposed, building on a domain-based approach combined with explicit modeling of sorption site concentration for various desorption and absorption conditions by a matrix of adaptable values. The proposed modeling idea offers a spline-like possibility of choosing the amount of adaptation parameters and the precision of the model. Two formulations of the sorption model are presented – for both isothermal and non-isothermal conditions. A selection of methods to adapt the parameters to measured data is proposed for various situations, also covering both isothermal and non-isothermal conditions. Their properties are to some degree mathematically shown/proved and their performance is discussed.In the area of moisture transport, a family of models is proposed, conceptually similar to the proposed sorption models. It also builds on a domain-based approach combined with explicit modeling of concentration of contributions to the overall transport, for various desorption and absorption conditions by a matrix of adaptable values. The proposed modeling idea offers a spline-like possibility of choosing the amount of adaptation parameters and the precision of the model. Three formulations of the transport model are presented – one for isothermal conditions, one generalized for non-isothermal conditions and one simplified for non-isothermal conditions where capillary suction is assumed to dominate transport phenomena. A selection of methods to adapt the parameters to measured data is proposed for various situations, covering all three proposed model versions. Their properties are to some degree mathematically shown/proved and their performance is discussed.
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| 21. |
- Telhaj, Klaudja, et al.
(författare)
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Hybrid Interfacial Transition Zone between Wet—On—Wet Casted Concrete—Microstructure and Mechanical Properties
- 2022
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 15:19
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Tidskriftsartikel (refereegranskat)abstract
- The manufacture of elements containing two types of concrete allows for the minimization of the amount of Portland cement by matching the properties of concrete with local structural and durability requirements. The most common production method of the hybrid element is wet–on hard and wet–on–wet. Casting wet–on–dry is the most common approach while casting wet–on–wet has been used mostly for concrete overlays and screeds. The study focuses on the wet–on–wet method but is applied in the production of vertical and horizontal elements. Bond-behavior and micro properties of the wet–on–wet casting interface of ultra–high–performance concrete (UHPC)–normal strength concretes are investigated. The obtained results indicate the formation of a hybrid interfacial transition zone between the two types of casted concrete. The binder matrix located in this zone appeared to combine properties of both used concrete. Porosity, phase composition, and presumably also strength, changed gradually. Furthermore, despite significant differences in shrinkage, no microcracking or delamination was observed in that zone. The ultimate flexural and compressive strength of the produced elements were either equal to the stronger concrete or were higher than the weaker of the used concrete.
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