Studying Initiation and Growth of Shear Cracks in Reinforced Concrete Beams Using Full-Field Digital Imaging

The mechanisms which lead to shear failure of longitudinally reinforced concrete beams are complex and often misunderstood. In this paper, a combination of a special closed loop test technique and machine vision are applied to tests on two reinforced concrete beams failing in shear: one made of normal strength concrete and the second of high strength concrete. To control the progression of the shear crack, a closed-loop test technique was devised and employed such that the crack propagation was stable even during the post peak. To locate the crack and to measure the crack displacements, machine vision, a nondestructive measuring technique, was employed which gives full-field displacements of a viewed area. The displacement measurements are obtained by first grabbing digital images of the observed area at different stages of loading during the test. By comparing discrete subsets of the reference and target images using normalized cross correlation matching, the displacement fields of the images are calculated. Shear crack initiation and progression are monitored for the two beams tested through complete failure, and differences in the shear failures between reinforced beams made of normal and high strength concretes are noted

Flexural Cracks in Fiber-Reinforced Concrete Beams with Fiber-Reinforced Polymer Reinforcing Bars

Fiber-reinforced polymer (FRP) reinforcing bars have attracted considerable attention for applications where corrosion of steel reinforcement is problematic. Due to the generally low elastic modulus and poor bond characteristics of FRP as compared to steel reinforcing bars, the use of FRP results in larger crack widths under service loads. Fiber-reinforced concrete (FRC) is proposed for use with FRP to reduce crack widths. The work presented herein includes the results from 16 beams tested under four-point bending with either Grade 420 (Grade 60) steel or FRP reinforcing bars, and either plain concrete or FRC. A modified Gergely-Lutz model was applied to the measured crack widths to calculate bond coefficients that were used to quantify the effectiveness of FRC in reducing crack widths. In the beams with steel reinforcing bars, the FRC was found to have little influence on crack widths. In the beams with FRP reinforcing bars, the FRC was found to significantly reduce maximum crack widths

Opportunities for CO2 capture through oxygen conducting membranes at medium-scale oxyfuel coal boilers

AbstractEcofys and ECN conducted a study commissioned by the IEA Greenhouse Gas R&D Programme to identify the most suitable combinations of medium-scale CO2 sources (1–100 MWth) and capture technologies, with respect to potential and costs (see parallel paper by Hendriks et al.). An industrial coal-fired Circulating Fluidized Bed (CFB) boiler with oxyfuel combustion and Oxygen Conducting Membranes (OCM) appeared to be an economically attractive combination to capture CO2. This paper describes the principle and economic evaluation of this combination in comparison with a reference coal boiler without CO2 capture, as well as a coal boiler with CO2 capture based on amine scrubbing

Influence of initial vaccination with 13-valent pneumococcal conjugate vaccine or 23-valent pneumococcal polysaccharide vaccine on anti-pneumococcal responses following subsequent pneumococcal vaccination in adults 50 years and older

AbstractBackgroundUnlike free polysaccharide vaccines, pneumococcal polysaccharide conjugate vaccines (PCVs) induce a T cell-dependent immune response and have the potential to provide an extended duration of protection with repeated vaccinations.MethodsThis was an extension of a previous study in pneumococcal vaccine-naïve adults aged 50–64 years in which adults 60–64 years of age were given 13-valent PCV (PCV13) or 23-valent pneumococcal polysaccharide vaccine (PPSV23) and adults aged 50–59 were given PCV13. In this follow up study conducted about 4 years later, the 60–64 year olds initially given PCV13 received PCV13 or PPSV23, and those initially given PPSV23 received another PPSV23. All adults aged 50–59 years were re-vaccinated with PCV13. Anti-pneumococcal opsonophagocytic activity (OPA) titers were measured before and 1 month after vaccination.ResultsA second PCV13 given about 4 years after a first vaccination induced OPA titers that were significantly higher than those following the initial vaccination for 7 of 13 serotypes in the older group, and 6 of 13 serotypes in the younger group, and responses to the remaining serotypes were largely non-inferior. In contrast, OPA titers following revaccination with PPSV23 were statistically significantly lower for 9 of the 13 serotypes, and non-inferior for the remaining serotypes, when compared to the responses to the first PPSV23. OPA titers in the older adults who received PPSV23 after initial PCV13 were significantly higher than those following a first PPSV23 for 10 of the 13 serotypes.ConclusionIn adults 50 to 64 years of age, initial vaccination with PCV13 establishes an immune state that results in recall anti-pneumococcal responses upon subsequent vaccination with either conjugated or free polysaccharide vaccine. In contrast, initial vaccination with PPSV23 results in an immune state in which subsequent PPSV23 administration yields generally lower responses compared with the initial responses

Seawater softening of suture zones inhibits fracture propagation in Antarctic ice shelves

Suture zones are abundant on Antarctic ice shelves and widely observed to impede fracture propagation, greatly enhancing ice-shelf stability. Using seismic and radar observations on the Larsen C Ice Shelf of the Antarctic Peninsula, we confirm that such zones are highly heterogeneous, consisting of multiple meteoric and marine ice bodies of diverse provenance fused together. Here we demonstrate that fracture detainment is predominantly controlled by enhanced seawater content in suture zones, rather than by enhanced temperature as previously thought. We show that interstitial seawater can reduce fracture-driving stress by orders of magnitude, promoting both viscous relaxation and the development of micro cracks, the incidence of which scales inversely with stress intensity. We show how simple analysis of viscous buckles in ice-penetrating radar data can quantify the seawater content of suture zones and their modification of the ice-shelf’s stress regime. By limiting fracture, enhancing stability and restraining continental ice discharge into the ocean, suture zones act as vital regulators of Antarctic mass balance

The Incidence of Highly-Obscured Star-Forming Regions in SINGS Galaxies

Using the new capabilities of the Spitzer Space Telescope and extensive multiwavelength data from the Spitzer Infrared Nearby Galaxies Survey (SINGS), it is now possible to study the infrared properties of star formation in nearby galaxies down to scales equivalent to large HII regions. We are therefore able to determine what fraction of large, infrared-selected star-forming regions in normal galaxies are highly obscured and address how much of the star formation we miss by relying solely on the optical portion of the spectrum. Employing a new empirical method for deriving attenuations of infrared-selected star-forming regions we investigate the statistics of obscured star formation on 500pc scales in a sample of 38 nearby galaxies. We find that the median attenuation is 1.4 magnitudes in H-alpha and that there is no evidence for a substantial sub-population of uniformly highly-obscured star-forming regions. The regions in the highly-obscured tail of the attenuation distribution (A_H-alpha > 3) make up only ~4% of the sample of nearly 1800 regions, though very embedded infrared sources on the much smaller scales and lower luminosities of compact and ultracompact HII regions are almost certainly present in greater numbers. The highly-obscured cases in our sample are generally the bright, central regions of galaxies with high overall attenuation but are not otherwise remarkable. We also find that a majority of the galaxies show decreasing radial trends in H-alpha attenuation. The small fraction of highly-obscured regions seen in this sample of normal, star-forming galaxies suggests that on 500pc scales the timescale for significant dispersal or break up of nearby, optically-thick dust clouds is short relative to the lifetime of a typical star-forming region.Comment: Accepted for publication in ApJ; emulateapj style, 30 pages, 18 figures (compressed versions), 3 table