Locating the site of diagonal tension crack initiation and path in reinforced concrete beams

The most favorable site of diagonal tension crack initiation has been attempted to be located. Due to the numerous interacted parameters affecting both site and angle of diagonal tension crack initiation, twelve possible sites were investigated, at midheight of the shear span and at the bottom surface near the support of the beam with vertical and diagonal orientations. The first diagonal tension crack initiated from the bottom tip of the diagonal pre-crack at midheight of the beam as a result of constraint release. To verify the previous finding, a single diagonal pre-crack has been created at midheight of only one side of the shear spans in both normal and fiber reinforced concrete (FRC) beams. FRC beam showed different behavior, where couple of diagonal tension cracks initiated at both sides from the tip of flexural cracks regardless of the existence of pre-crack at one side of the beam

Mixed mode fracture behavior of fiber reinforced concrete; Experimental and numerical analysis adopting cracked Brazilian disc specimen

Fiber reinforced concrete (FRC) is a material of prime importance to enhance fracture resistance of structural elements. It is essential to consider crack initiation and fracture resistance experimentally and select the most convenient and applicable numerical criterion to predict such behavior in FRC. In this study, a total of 40 cylinders and 540 cracked Brazilian discs (CBD) were divided into four groups to evaluate mechanical and fracture properties. The investigated groups include plain concrete (PC), glass fiber reinforced concrete (GRC), polypropylene fiber reinforced concrete (PRC), and hybrid fiber reinforced concrete (HRC) of glass and polypropylene fibers. The CBD was adopted due to its capability to achieve a varied range of mode mixity. Different crack length to disc radius ratios (a/R) and crack inclination angles (β) were investigated. Fracture loads were determined experimentally then applied to analytical and numerical models to calculate fracture toughness parameters. An elastic two-dimensional finite elements analysis embedded in ABAQUS program was utilized to evaluate fracture behavior regarding mode mixity for the tested range of CBDs. Generalized maximum tangential stress (GMTS) shows supreme fit to the tested data than those of maximum tangential stress (MTS) including all a/R, β and material types regarding fracture resistance and crack initiation angle

Structural Behavior of RC Beams Containing a Pre- Diagonal Tension Crack

<div><p>Abstract This study focuses the light on the shear behavior of pre-cracked beams, and examines the feasibility of applying fracture mechanics concepts to widen the understanding of shear behavior and mechanisms. The experimental program contains ten beam specimens of high strength concrete (HSC) and steel fiber reinforced concrete (SFRC). Pre-cracks were embedded with different sizes and locations along the favorable path and orientation to shear failure. Three main scenarios of shear failure were reported with minute effect of pre-cracks. The geometrical effect is dominant with marginal effect of the material’s nonlinearity in case of severe pre-diagonal crack in HSC while the nonlinearity of the material is supreme to that of the geometrical effect for SFRC and shorter cracks. For verification, numerical simulation was conducted to examine the geometrical effect of the pre-diagonal tension crack in shear span on the structural behavior of RC beams. It is found numerically that, when the crack tip of the tensile crack is away from the tensile reinforcement, the closuring moment of tensile reinforcement increases, and as a result reduces the strain energy release rate. Therefore, the tensile cracks stop and the shear cracks keep propagating leading the failure mechanism to the end failure point.</p></div

Determination of some metal ions in aquatic environs by atomic absorption spectrometry after concentration with modified silica

137-142N-propylsalicylaldimine based on porous silica, as ion exchanger, is used for the separation and preconcentration of Fe3+, Co2+, Ni2+ and Zn2+ from surface water samples collected from eleven locations at Nile river, Mediterranean and Red seas and Mansoura city tap water. The effects of pH, time of stirring, concentration of eluting acids and some common ionic species on the separation and preconcentration of the investigated metal ions in synthetic aqueous solutions are studied. The results indicate that the optimum conditions for separation are pH = 9.0-9.5 and time of stirring = 30 min. HCl is better eluent for the investigated metal ions than HNO3 except for Fe3+.Citrate and EDTA show severe interference on the recovery of the metal ions, therefore, the water samples are previously oxidized to digest organic matter prior to the application process

Ameliorative effects of l-carnitine on rats raised on a diet supplemented with lead acetate

Lead intoxication has been a major health hazard in humans. It affects people at all ages. Its toxicity is associated with various organs of the body and affects different metabolic pathways. Based on histological data, l-carnitine reduced the severity of tissue damage produced as a result of exposure of rats to lead acetate. The main objective of this study was to evaluate the underlying mechanism of protection offered by l-carnitine against lead acetate intoxication using male Sprague–Dawley rats.Forty male Sprague–Dawley rats were randomly divided into four groups with ten rats in each. The first group (G1) served as the control group and animals received standard diet only. The second group (G2) received lead acetate in their diet. The third group (G3) was the l-carnitine treated group and received the normal standard diet supplemented with l-carnitine. While the fourth group (G4) had a diet supplemented with both lead acetate and l-carnitine. At the end of each experiment, blood (serum and whole blood) were collected from each animal and analyzed for the following parameters: serum testosterone levels, serum nitric oxide and serum malondialdehyde. This is in addition to looking at the enzymatic activities of two important enzymes (superoxide dismutase and catalase) and on (glutathione reductase) which are indicative of the antioxidant activities in the whole blood. The results indicated that l-carnitine will counteract the undesirable effects of lead intoxication. It exerted its antioxidant potential by reducing the production of ROS and scavenging free radicals by maintaining and protecting the level of the of antioxidant enzymes SOD, CAT and glutathione peroxidase.Conclusion:l-Carnitine may play an important role in reversing the undesirable effects of lead intoxication. Future studies should be conducted to see whether such an effect is applicable in humans exposed to lead poising. Keywords: l-Carnitine, Sprague–Dawley rats, Lead toxicity, Antioxidants, Lead acetat

Protective Effect of Starch-stabilized Selenium Nanoparticles against Melamine-induced Hepato-renal Toxicity in Male Albino Rats

Melamine and its analogues are illegally added to raise the apparent protein content in foods. The elevated concentrations of these compounds cause adverse effects in humans and animals. In this contribution, the protective effects of the synthesized starch-stabilized selenium nanoparticles (Se-NPs@starch) on melamine-induced hepato-renal toxicity have been systematically investigated. The Se-NPs@starch were characterized by X-ray photoelectron spectroscopy (XPS) analysis, energy dispersive spectroscopy (EDS) mapping analysis, TEM, and FT-IR. Starch plays a crucial role in the stabilization and dispersion of Se NPs, as noticed from the TEM and EDS investigations. Furthermore, the atomic ratio of Se distribution over the starch surface is approximately 1.67%. The current study was conducted on four groups of adult male rats, and the oral daily treatments for 28 days were as follows: group I served as control, group II received Se-NPs@starch, group III was exposed to melamine, while group IV was treated with melamine and Se-NPs@starch. The results reveal a significant alteration in the histoarchitecture of both hepatic and renal tissues induced by melamine. Furthermore, elevated liver and kidney function markers, high malondialdehyde, and increased expression levels of apoptosis-related genes besides a reduction in GSH and expression levels of antioxidant genes were observed in the melamine-exposed group. Interestingly, the administration of the Se-NPs@starch resulted in remarkable protection of rats against melamine-induced toxicity through increasing the antioxidant capacity and inhibiting oxidative damage. Collectively, this study provides affordable starch-stabilized Se-NPs with potent biological activity, making them auspicious candidates for prospective biomedical applications