5 research outputs found
Variations in dermatoglyphic patterns in oral submucous fibrosis and leukoplakia patients with and without adverse oral habits
No full textIntroduction: The present study was conducted to determine the comparative variations in dermatoglyphic patterns in patients without oral submucous fibrosis (OSMF) and leukoplakia and those having lesions, as well as to predict the occurrence of these diseases and initiate preventive measures in these high-risk patients. Materials and Methods: Dermatoglyphic patterns were collected from randomly selected 120 patients using 3M™ CSD200i. Single-digit Optical Scanner (3M™, Canada, 2015) with automatic capture mechanism was applied to capture finger prints of all the 10 fingers of patients, who were divided in control and test group with respective subgroups of leukoplakia and OSMF. Qualitative analysis of dermatoglyphic patterns in the different groups showed loops, arches, and whorls. Results: The collected data was subjected to analysis using Chi-square test for comparison between the groups; significant difference in P value was observed on comparison between dermatoglyphic patterns in patients with leukoplakia and those with adverse oral habits but without oral lesions (P = 0.00005), patients with OSMF and individuals with adverse oral habits but without oral lesions (P = 0.03), patients with OSMF and individuals without adverse oral habits and without oral lesions (P = 0.004), leukoplakia and OSMF (P = 0.007). Quantitative analysis including total finger ridge count was done by counting the number of ridges in all 10 fingers for all the patients in all the groups. Conclusion: The present study showed weak association in the loop pattern of patients with OSMF than leukoplakia, whorl pattern with adverse oral habits, without oral lesions, and arch pattern with OSMF. More controlled prospective trials are needed to affirm the association, if any, at larger homogeneous Indian sample in future to validate the finding
Long-Term Mechanical and Durability Behaviour of Two Alkali-Activated Types of Concrete
No full textA promising solution for reducing the carbon footprint of concrete is the use of alkali-activated concretes (AAC). Before this material can be widely applied, its long-term behaviour needs to be understood, especially since some studies reported a decrease of mechanical properties over time. Similarly, Prinsse et al. reported decreasing mechanical properties, especially elastic modulus and flexural and splitting tensile strength for the studied slag-based AAC (S100) and the blended slag- and fly-ash-based AAC (S50) up to the tested age of 2 years. They hypothesized that these decreases could be only temporarily. To test that hypothesis, this study continued to monitor the mechanical properties of both AACs up to the age of 5 years. As a reference, two OPC-based concretes (OPCC), with different strength classes, are monitored up to the age of 3.5 years. In addition, the internal structures of the concretes are assessed for carbonation and internal micro cracking. S100 shows stabilization of the elastic modulus and the compressive strength, whereas the tensile splitting strength continued to decrease up to 5 years. This is attributed to a combination of carbonation and drying, since the microscopic analysis showed increased porosity around the ITZ and in the carbonated region. In addition, S50 shows an ongoing decrease of all tested mechanical properties, which is attributed to carbonation. No decreases in mechanical properties are found for OPCC.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Concrete Structure
Development of Mechanical Properties of Concrete with Time - Experimental and Numerical Study
No full textThe performance of concrete is usually governed by its "strength" and "stiffness" properties. As most of the concrete structures are usually designed to last in service for a period of about half a century, it is important to understand the development of these "strength" and "stiffness" properties over the course of time. Based on the standard codes and practices, these properties are assumed to continuously increase with age assuming standard conditions (moist curing environment and temperature of 20° C), or at least remain constant in case of other curing conditions. However, few studies [27] [13] have reported a reduction in the trend of the strength and stiffness properties with time for the ordinary concrete of different strength mixes, when exposed to drying at different relative humidities after an initial moist curing period. Moreover, such decrease in trend has also been reported for a Geopolymer concrete upon drying at controlled lab conditions (20° C and 50%RH) after an initial moist curing period of 28 days [35]. It is not clear if the reduction in the trend observed is only temporary as a result of the imposed shrinkage deformations and eigen stresses, or if it is a permanent reduction as a result of microcracking. In this regard, it is aimed to understand the uncertainty in the development of the strength and stiffness properties of particularly the ordinary concrete mixes, which have been used across the world for a long time. Accordingly, it is hypothesized that a similar reduction upon drying in the trend of mechanical properties as reported by Prinsse [35] for a Geopolymer might also occur for the ordinary concrete mixes of various grades, which are cured and tested in the same way as the aforementioned Geopolymer concrete. Based on that, parameters such as the curing conditions, specimen size and the grade of concrete mix are assumed to influence the trend and a comprehensive literature study is performed in order to understand the influence of these assumed parameters on the mechanical properties - namely the compressive strength, splitting strength and elastic modulus over time. From the literature study, it is understood that there is some uncertainity in understanding the trend which is aggravated by the complex inter-dependency between the assumed parameters. Accordingly, an experimental program is set up to firstly verify whether there is a similar reduction in trend like the Geopolymer concrete and secondly to give more clarity on the complex inter-dependency between the assumed parameters. Further, the development of eigen stresses is understood to play a huge role in the trend of the measured mechanical properties over time and their influence might vary for the different tests. However, due to its temporary occurrence and larger testing frequency days, they are difficult to capture in the experiments. Accordingly, a FEM tool known as "FEMMASSE" is used, which can simulate the concrete behaviour by incorporating the heat and moisture models. Apart from considering the material effects like hydration, FEMMASSE can also capture the eigen stresses developed as a result of the thermal and moisture gradients. The software mostly finds application in the design of bridges and tunnels. Although, in this research study, the main purpose of using FEMMASSE is to firstly capture the shrinkage induced eigen stresses by simulating the tests at smaller intervals and secondly single out the influence of the studied parameters in order to give more clarity on the obtained experimental results. The obtained experimental results indicate no major reduction in the trend of the studied mechanical properties in time as compared to the Geopolymer concrete. With regard to the studied parameters, it is found that for the chosen curing regime (28DM regime), the trend of both compressive and splitting strength is seen to be affected by the development of the eigen stresses. Interestingly, for the trend of splitting strength, a general pattern of increasing and stabilizing behaviour is observed which seems to be dependent also on the specimen size. However, due to the lack of knowledge of the actual magnitude of the eigen stresses coupled with other phenomena like the size effect and current state of hydration, it becomes challenging to explain the trend merely on the basis of experimental results. In this regard, the use of numerical tool FEMMASSE is shown to help understand the experimental results at greater depth. Owing to the limitations of the software, only the tensile tests are simulated. Initially, it is understood that the attainment of the hygral equilibrium is dependent on the specimen size and the type of concrete,with the smaller specimen sizes and lower strength mixes reaching the equlibrium relatively quickly. The model is then extended to simulate the tensile tests - splitting and flexural and direct tension tests. It is concluded that drying after a moist curing period of 28 days affects the these tensile tests differently. In case of the splitting tests, drying results in the temporary increase of the strength owing to the apparent prestressing at the core of the specimen, with the gradual stabilization of the trend as the specimen attains moisture equilibrium. This trend of temporary increase and gradual stabilization is dependent on the specimen size with the smaller specimens stabilizing at a relatively quicker time due to the faster attainment of the moisture equilibrium. However, in case of the flexural and direct tensile test, drying results in the temporary reduction in the flexural strength due to the reduction in the tensile capacity at the surface, followed by an increasing trend as the specimen starts attaining hygral equilibrium. This indicates that the surface failure tests like the direct tension and flexural are negatively affected (temporarily) by drying as compared to the interior failure tests like splitting test which is positively affected (temporarily) by drying. From the research study, it is evident that there are no major reduction in the trend of strength and stiffness properties over time obtained experimentally until the period of 155 days. However, the presence of the eigen stresses, which not captured in the experiments, does influence the trend of the measured material properties over time. As long as these eigen stresses are present, the material strength determined experimentally might be deceptive, as evident from the simulations of the tensile tests performed in the study. It is understood that unless there is hygral equilibrium (no eigen stresses) across the specimen, the measured material properties might underestimate the actual material strength in case of the flexural and direct tension tests and overestimate in case of the splitting tensile tests. In the engineering practice, especially in mass concrete structures, the eigen stresses could be present throughout the service life of the structure. Thus, the experimentally obtained material strength used for designing the structure might be deceptive due to the influence of the eigen stresses.Civil Engineering | Structural Engineerin
Multiscale analysis of long-term mechanical and durability behaviour of two alkali-activated slag-based types of concrete
No full textAlthough alkali activated concretes (AACs) are promising for reducing the carbon emissions of concrete, in order to enable their wide application it is vital to understand their long-term behaviour. Herein, we report the development of mechanical properties of a ground granulated blast furnace slag (GGBFS)-based AAC and a binary fly ash (FA) /GGBFS-based AAC exposed to 55% relative humidity and 20 °C up to the age of 5 years. For comparison, two ordinary Portland cement (OPC) concretes were monitored for 3.5 years. For the GGBFS-based AAC, after an initial decrease within the first 6 months the elastic compressive modulus stabilized, while its tensile splitting strength continued to decrease for the tested period of 5 years. The binary AAC showed a continuous decrease in its tensile splitting strength for 5 years and a reduction in its compressive strength after 2 years. No decreases in mechanical properties were observed in OPC-based concretes. To reveal underlying mechanisms, additional analyses were performed. Permanent degradation was observed in both AACs; the binary AAC mainly suffered from carbonation, and the GGBFS-based AAC showed microcracking. These cracks were probably caused by drying shrinkage and drying-induced chemical changes. Based on the measured mechanical properties of AAC, crack widths and stiffness of reinforced AAC beams under bending were analytically evaluated and compared to experiments. Decreases in bending stiffness and increases in crack width were observed for reinforced AAC beams tested at later ages. A bimodular approach is proposed to predict the reduction of bending stiffness in the studied AACs over time. These findings are relevant to understand serviceability limit states of reinforced AACs.Concrete Structure
The role of eigen-stresses on apparent strength and stiffness of normal, high strength, and ultra-high performance fibre reinforced concrete
No full textConcrete is characterized in terms of its engineering properties, mainly strength and stiffness, which are subsequently used in structural design. However, the apparent (i.e., measured) concrete properties are not intrinsic but dependent on the conditions under which the measurement is performed. Herein a combined experimental and numerical study is performed to clarify the effects of hygral gradients and resulting eigen-stresses in self-restrained concrete on its apparent strength. Compressive strength, splitting tensile strength and Young's modulus are tested. Three mixes of varying strength grades are studied: normal strength, high strength, and ultra-high performance fibre reinforced concrete. Samples are subjected to two different curing conditions. To investigate the effect of size on the apparent properties, 50 mm, 100 mm and 150 mm cubes are tested. Depending on the size of the specimen there can be an underestimation or overestimation of up to 25% of the real concrete (e.g., splitting, direct tensile) strength
