596 research outputs found
Influence of fiber orientation on the behavior of fiber reinforced concrete slabs
No full textAfter several decades of research studies, design rules for fiber reinforced concrete (FRC) are now available for several structural elements. Starting from standard tests, it is possible to determine design parameters and constitutive laws to perform simplified sectional verifications and/or numerical analyses. However, one of the main issues still open is represented by the fiber orientation in the real structure that could be different from the one present in standard tests. For this reason, building codes take into account orientation factors to modify the standard material properties. The present paper aims to shed some new lights on the effects of the orientation of steel and macro-synthetic fibers on the local variability of FRC residual strength properties in slabs made with two concretes: Vibrated (slump of 80 ± 20 mm, V-FRC) and self-compacting concrete (slump flow diameter of 700 ± 50 mm, SC-FRC). In V-FRC slabs, the orientation of polymer fibers was more influenced by pouring and compaction process compared to steel ones. In SC-FRC slabs, the flow and wall effects resulted more significant when long steel fibers were used. The post-cracking performances locally determined in different points of slabs were compared against the ones obtained on standard beams as well. Finally, the global response of these slabs was numerically studied by considering two different support configurations: simply-supported slabs and slabs on grade.Fil: Conforti, Antonio. University of Brescia; ItaliaFil: Cuenca, Estefania. Politecnico di Milano; ItaliaFil: Zerbino, Raul Luis. Universidad Nacional de La Plata. Facultad de Ingeniería. Departamento de Construcciones; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Plizzari, Giovanni A.. University of Brescia; Itali
Pull-out tests to evaluate the effects of crack healing on the steel-to-concrete bond on concretes exposed to chloride-rich environments
No full textThis paper presents an experimental methodology to evaluate the effects of the self-healing of cracks on the steel-to-concrete bond. Overall, 80 cylindrical concrete specimens with a coaxial steel bar were cast and subjected to pull-out tests. To this purpose, the experimental methodology envisaged the casting of two types of concrete specimens: one made with a reference concrete and the other type to investigate the effects of a crystalline admixture as a self-healing promoter. Through pull-out tests, the specimens were precracked and subjected, together with uncracked companion specimens, to wet/dry cycles in tap water or saltwater. To facilitate the evaluation of the self-healing of the specimens, indices relating to the physical-mechanical characteristics have been defined. The results show a better behavior of both, surface crack sealing and recovery of the bond performance, when specimens were subjected to wet/dry cycles in saltwater. Moreover, crystalline admixtures promoted a continuous recovery of the steel-to-concrete bond performance due the continuous generation of healing products inside the crack
Autogenous self-healing capacity of early-age ultra-high-performance fiber-reinforced concrete
No full textThis paper analyzes the autogenous self-healing capacity of early-age Ultra-High-Performance Fiber-Reinforced concretes (UHPFRCs) by measuring the crack closure and the possible mechanical recovery on healed specimens. The main parameters considered in this research were the healing exposure conditions (humidity chamber, immersion in tap water, immersion in seawater and heat curing) and the precracking levels (microcracks and macrocracks). For the microcrack level, four-point bending tests were performed on prismatic specimens (100 × 100 × 500 mm3 ) obtaining a multiple cracking pattern characterized by crack widths ranged from 10 to 20 μm. Whereas for the macrocrack level (behavior after crack localization), splitting tests were carried out on notched cubic specimens (100 × 100 × 100 mm3 ) obtaining crack widths of up to 0.4 mm. For both precracking levels, specimens were precracked at two days and were cured for one month in the mentioned exposure conditions. Healing products were analyzed on the specimen surface and also inside the cracks; to this purpose, their microstructure was analyzed by means of SEM and EDS analyses. The results have shown that the highest crack closure values were obtained for the heat-cured specimens and for the specimens immersed in water (tap water and seawater) whereas the less efficient condition was the humidity chamber
Relación entre CPOD y el estado de ánimo en escolares de 12 años de la parroquia Totoracocha Cuenca-Ecuador 2016.
No full textLas emociones se encargan de regular el comportamiento del ser humano, y se establece que
las emociones afectan la calidad de vida del escolar generando un vínculo entre la salud oral
y su estado anímico. El presente estudio tuvo como objetivo relacionar el índice CPOD y el
estado de ánimo en escolares de 12 años, de la parroquia Totoracocha, Cuenca-Ecuador 2016.
Se revisaron 175 fichas de escolares de la parroquia Totoracocha, las variables utilizadas en
el estudio fueron índice CPOD y estado de ánimo, para el análisis de los datos de ambas
variables se utilizó una estadística descriptiva y para determinar la correlación entre las
variables se aplicó la prueba estadística Kendall. Los resultados indicaron que del total de las
fichas analizadas el 36% presentó un nivel de CPOD muy bajo y el 82,9% no presentó
limitaciones en el estado de ánimo, en cuanto a la correlación entre la experiencia de caries y
el estado de ánimo, se determinó que no existe relación estadística relevante con un valor de
p= 0,310.Trabajo de investigació
Self-healing capacity of fiber reinforced cementitious composites. State of the art and perspectives
No full textDesign of building structures and infrastructures is mainly based on four concepts: safety, serviceability, durability and sustainability. The latter is becoming increasingly relevant in the field of civil engineering. Reinforced concrete structures are subjected to damages that produce cracks which, if not repaired, can lead to a rapid deterioration and would result into increasing maintenance costs to guarantee the anticipated level of performance. Therefore, self-healing concrete can be very useful in any type of structures, as it allows to control and repairing cracks as soon as they are likely to occur. The effectiveness of self-healing can be improved with the use of fibers due to their capacity to control crack width and enhance multiple crack formation. In that way, researchers should use advanced cement based materials (FRCC, HPFRCC, etc.) and techniques (autogenous and engineering healing) to satisfy all demands in which sustainability and durability are key factors. Compared to the large number of investigations on selfhealing of plain concrete, self-healing studies on Fiber Reinforced Cementitious Composites (FRCC) are still limited. Therefore, the main objective of this paper is to provide a deep literature review on this subject in order to clarify what is known (What now?) and finally to identify those gaps which still require further studies (What next?) such as: healing capacity under sustained stress, repeatability healing/cracking cycles as well as healing capacity for cracks and damages occurring at later concrete ages
Shear Behavior of Self-Compacting Concrete and Fiber-Reinforced Concrete Push-Off Specimens
No full textThe shear behavior of reinforced "Z"-shaped push-off specimens made with self-compacting concrete (SCC) and self-compacting fiber-reinforced concrete (SCFRC) was analyzed by means of experimental tests. Testing consisted of two phases. Firstly, specimens were precracked subjected to linear load along the shear plane. During this first phase three precracked levels were distinguished (without precrack, thin and thick precrack). Then, precracked specimens were tested under direct shear load. The shear behavior along the shear plane was analyzed by means of the crack opening and shear displacement versus shear load process. Variables were: the type of concrete (SCC, or SCFRC with different fibers contents: 40 kg/m3 or 60 kg/m3), the transversal reinforcement (TR) and the precrack width. The analysis was specially focused on the study of aggregate interlocking. The failure occurrence is better controlled thanks to the presence of fibers, the shear behavior is more ductile. © RILEM 2010
Shear database for reinforced and prestressed beams made with fiber reinforced concrete
No full textA total number of 215 structural elements were used to prepare a complete database to analyze the shear behavior and the influence of each parameter on shear out of 363 elements of the experimental database. 148 elements were eliminated for various reasons. Thus, the following items were removed: those with different failure modes to shear, those beams which are not known in some detail, also the beams containing a mixture of more than one fiber type, those for which values of strength are not available and all those elements with ratios a/d smaller than 2.5, where the arching action is dominant [1]. The database is made up of elements from databases of the University of Brescia and of RILEM, in addition to all the shear tests carried out within the Brite/Euram project [2], beams tested by Dupont & Vandewalle [3], other beams [4] and the tests presented in the Ph.D. thesis of Cuenca [5]. The input parameters used were: the shear span-to-depth ratio (a/d); the effective depth (d); the concrete cylinder compressive stress (fc); the residual flexural tensile strength (fR3) corresponding to a crack mouth opening displacement CMOD=2.5 mm, according to EN 14651 [6]; the longitudinal reinforcement ratio (ρl); the average stress acting on the concrete cross-section for an axial force due to prestressing actions (σc); the amount of steel fibers (kg/m3) and transverse reinforcement area per unit length (Asa/s). The output value was the safety margin (SM) obtained as Vtest/Vtheo (the shear test value divided by the shear theoretical value). The theoretical shear (Vtheo) was calculated for each of the beams according to three calculation codes: the Spanish Standard EHE-08 [7], the RILEM approach [8] and the first complete draft of Model Code 2010 [9]
Cellulose nanofibers to improve the mechanical and durability performance of self-healing Ultra-High Performance Concretes exposed to aggressive waters
No full textThe effects of including cellulose nanofibers (CNFs) in Ultra High Performance Concretes (UHPCs) have been studied in this paper in terms of recovery of both durability and mechanical properties because of the stimulated self-healing behavior. To this purpose, flexural tests on 4-point bending on 30 mm thin and 100 mm wide beam specimens have been carried out to evaluate the mechanical recovery due to self-healing, together with Double Edge Wedge Splitting tests that have been performed to identify the tensile stress crack-opening behavior. Moreover, water permeability and chloride diffusion tests have been performed to evaluate the recovery of durability properties on healed specimens subjected to extremely aggressive environments rich in chlorides and sulfates. Microstructural analysis has been also performed to confirm the enhancement on durability performance due to the presence of CNFs. The presence of CNFs has improved self-healing performance of UHPC since for the same crack width value and same healing period (1, 3 or 6 months) the specimens with CNFs reached higher crack sealing rates compared to those without nano-additions. As a matter of fact, the total sealing of the cracks over the 6-month investigated period was only observed on specimens with CNFs
Optimized data-driven method to study the self-healing and durability of ultra-high performance concrete
No full textMachine learning frameworks offer significant potential for predicting the self-healing properties of Ultra-High-Performance Concrete (UHPC) and can also serve in the design of advanced durability-based concrete. Existing machine learning models, while exhibiting good generalization, are limited in prediction accuracy due to training on small datasets with outliers. Moreover, many studies lack comprehensive interpretive analyses, resulting in inadequate extraction of physical insights related to self-healing. This paper introduces an advanced data-driven framework to predict crack closure and chloride diffusion in cracked concrete. The framework has three main components: (1) The Tree-structured Parzen Estimator and 10-fold cross-validation are employed to optimize the hyperparameters of the model. (2) Isolation Forest and forward stepwise selection are used for data cleaning, identifying and removing outliers and irrelevant variables in the dataset. (3) Feature importance analysis, SHapley Additive exPlanations (SHAP), and Partial Dependence Plots (PDP) are utilized to explain and quantify the impact of features on model composition and prediction. This research framework utilizing Light Gradient Boosting Machine (LightGBM) as the base model has been established, characterized by high accuracy and strong generalization in predicting concrete self-healing and durability. Using interpretive tools, the crack width thresholds for healing and chloride transport has been quantified at 60–125 μm and 35–60 μm, respectively
repetability of self healing in fiber reinforced concretes with and without crystalline admixtures: preliminary results
No full textThis paper analyzes the repeatability of autogenous and engineered self-healing in fiber reinforced concrete (FRC) with and without crystalline admixtures. To this purpose, the tensile behavior of two different mixes, differing by the presence of the crystalline admixture, has been investigated after a series of pre-cracking and conditioning cycles. An indirect testing methodology has been employed to the aforementioned purpose, i.e. Double Edge Wedge Splitting (DEWS) test. Three different exposure conditions were considered: open air exposure, water immersion and wet/dry cycles. Specimens were pre-cracked up to a crack width of 0.25mm (0.01 in.) (precrack cycle). Then, specimens were healed for one month and tested again up to a crack width of 0.25mm (0.01 in.) (cycle after 1st healing). After that, specimens were healed for two months further (2nd healing) and finally, they were cracked once again up to 0.25mm (0.01 in.). The highest healing rate was reached for specimens immersed in water; moreover, as expectable, the larger the initial crack width, the lower is the percentage of crack closure. Regarding the repeatability, a general better trend was found for the mix with crystalline admixtures, in which, in addition, the maximum load regain was measured after the 2nd healing cycle rather than after the 1st healing
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