Expansion of mortar bars in the presence of potassium acetate deicer

Carboxylate-based deicing and anti-icing chemicals became widely used in the mid 1990s, replacing more environmentally burdensome chemicals. Within a few years of their adoption, distress of portland cement concrete runways was reported by a few airports using the new chemicals. Distress manifested characteristics identical to that of alkali silica reactivity (ASR), but onset occurred early in the pavement’s operating life and with pavements thought to contain innocuous aggregate. The carboxylate-based deicing chemicals were suspected of exacerbating ASR-like expansion. Innocuous, moderately, and highly reactive aggregates were tested using modified ASTM C1260 and ASTM C1567 procedures with soak solutions containing deicer solutions and sodium hydroxide or potassium hydroxide. ASR-like expansion is exacerbated in the presence of potassium acetate. The expansion rate produced by a given aggregate is also a function of the alkali hydroxide used. Petrographic analyses were performed on thin sections prepared from mortar bars used in the experiments. Expansion occurred via two mechanisms; rupture of aggregate grains and expansion of paste.

In vitro precision of fit of computer-aided design and computer-aided manufacturing titanium and zirconium dioxide bars

OBJECTIVES Optical scanners combined with computer-aided design and computer-aided manufacturing (CAD/CAM) technology provide high accuracy in the fabrication of titanium (TIT) and zirconium dioxide (ZrO) bars. The aim of this study was to compare the precision of fit of CAD/CAM TIT bars produced with a photogrammetric and a laser scanner. METHODS Twenty rigid CAD/CAM bars were fabricated on one single edentulous master cast with 6 implants in the positions of the second premolars, canines and central incisors. A photogrammetric scanner (P) provided digitized data for TIT-P (n=5) while a laser scanner (L) was used for TIT-L (n=5). The control groups consisted of soldered gold bars (gold, n=5) and ZrO-P with similar bar design. Median vertical distance between implant and bar platforms from non-tightened implants (one-screw test) was calculated from mesial, buccal and distal scanning electron microscope measurements. RESULTS Vertical microgaps were not significantly different between TIT-P (median 16μm; 95% CI 10-27μm) and TIT-L (25μm; 13-32μm). Gold (49μm; 12-69μm) had higher values than TIT-P (p=0.001) and TIT-L (p=0.008), while ZrO-P (35μm; 17-55μm) exhibited higher values than TIT-P (p=0.023). Misfit values increased in all groups from implant position 23 (3 units) to 15 (10 units), while in gold and TIT-P values decreased from implant 11 toward the most distal implant 15. SIGNIFICANCE CAD/CAM titanium bars showed high precision of fit using photogrammetric and laser scanners. In comparison, the misfit of ZrO bars (CAM/CAM, photogrammetric scanner) and soldered gold bars was statistically higher but values were clinically acceptable.

Complications with computer-aided designed/computer-assisted manufactured titanium and soldered gold bars for mandibular implant-overdentures: short-term observations

BACKGROUND Implant-overdentures supported by rigid bars provide stability in the edentulous atrophic mandible. However, fractures of solder joints and matrices, and loosening of screws and matrices were observed with soldered gold bars (G-bars). Computer-aided designed/computer-assisted manufactured (CAD/CAM) titanium bars (Ti-bars) may reduce technical complications due to enhanced material quality. PURPOSE To compare prosthetic-technical maintenance service of mandibular implant-overdentures supported by CAD/CAM Ti-bar and soldered G-bar. MATERIALS AND METHODS Edentulous patients were consecutively admitted for implant-prosthodontic treatment with a maxillary complete denture and a mandibular implant-overdenture connected to a rigid G-bar or Ti-bar. Maintenance service and problems with the implant-retention device complex and the prosthesis were recorded during minimally 3-4 years. Annual peri-implant crestal bone level changes (ΔBIC) were radiographically assessed. RESULTS Data of 213 edentulous patients (mean age 68 ± 10 years), who had received a total of 477 tapered implants, were available. Ti-bar and G-bar comprised 101 and 112 patients with 231 and 246 implants, respectively. Ti-bar mostly exhibited distal bar extensions (96%) compared to 34% of G-bar (p < .001). Fracture rate of bars extensions (4.7% vs 14.8%, p < .001) and matrices (1% vs 13%, p < .001) was lower for Ti-bar. Matrices activation was required 2.4× less often in Ti-bar. ΔBIC remained stable for both groups. CONCLUSIONS Implant overdentures supported by soldered gold bars or milled CAD/CAM Ti-bars are a successful treatment modality but require regular maintenance service. These short-term observations support the hypothesis that CAD/CAM Ti-bars reduce technical complications. Fracture location indicated that the titanium thickness around the screw-access hole should be increased.

(Table 1) Grain size parameters of intertidal channel sand bars near Sylt, North-Frisian Wadden Sea

The aim of the present study is to investigate directional asymmetric properties and internal structures of the bedforms on the intertidal sand bars in comparison with the migration problems of the sand bodies developed in the channel systems of the tidal basin off the west coast of Schleswig-Holstein. The tidal channel sand bodies studied have 'V'-shaped outlines and are asymmetric in cross-section. Based on such knowledge it was hoped to understand and find possible factors for application to recent and ancient tidal depositional environments. The V-shaped intertidal channel sand bodies developed in the tidal environments between Sylt and Föhr Island are constantly migrating sand bars. The migration directions are in good agreement with the resultant vector mean directions of internal cross-stratification structures of asymmetric sedimentary bedforms. Finally, it is shown that the orientation of the apex of V-shaped sand bar as an equilibrium form alone can not indicate the migration direction, but that the orientation of the resultant vector mean of internal structures of sedimentary bedforms does indicate the migration direction. Based on the analyses of textural parameters of the migrating intertidal bar sands, it seems that sands of typical intertidal sand bars are negatively skewed and well sorted. The high rounding of quarz sand grains of these tidal channel sand bars seems to be an additional characteristical criterion for tidal depositional environments, as also indicated by Balazs and Klein (1972).

Development of corrosion of steel bars embedded in mortar made with slag from secondary metallurgy

The aim of this work is to study the evolution of the corrosion rate of reinforcements embedded in mortar specimens that have been partly or fully replaced by the sand ladle furnace white slag. Prisms are manufactured mortar 6cm x 8cm x 2cm in which are embedded reinforcing steel bars of 6mm diameter B500SD. At the time of mixing were added varying amounts of chloride ion content by weight of cement (0%, 0.4%, 0.8%, 1.2%, 2%). The specimens were made totally or partially replacing the white slag, getting four different mixes depending on the degree of substitution. After curing the specimens for 28 days in moist chambers proceeded to dry up naturally. Here are gradually dampened by its conservation in a moist chamber, periodically measuring the corrosion rate of the bars using the technique of polarization curve. The results, in terms of corrosion current and corrosion potential, were compared with those obtained on standard samples, without replacement by slag aggregate. The analysis of results allows us to know, depending on the type of mortar used, the chloride threshold with the depassivation produced steel and the corrosion rates achieved in steels in the active state in terms of mortar moisture, obtained from qualitatively using gravimetric techniques. The results achieved to date support the conclusion that no significant differences in the behavior against corrosion induced by chloride ions, between the steel bars embedded in standard samples and the steel bars embedded in samples including with aggregates from slag. Both the chloride threshold resulting in the depassivation steel as the corrosion rate reached through the bars in an active state are very similar in both types of mortars when they have the same moisture content.

Failure analysis of prestressed anchor bars

Three broken steel bars from a sewed crack in a dam are reported. The inspection of the fracture surfaces of the prestressed bars suggests that fractures were triggered by small cracks and by the inherent brittleness of the bars, as fracture toughness was about 40 MPa m1/2. The analysis of the failures shows that the usual design requirements for prestressing bars fail to warn against brittle failures if some damage exists. Some recommendations, based on the concept of damage tolerance, are suggested to avoid similar unfortunate incidents.

A fracture model for pearlitic steel bars using a cohesive model

The fracture of ductile materials, such as metals, is usually explained with the theory of nucleation, growth and coalescence of microvoids. Based on this theory, many numerical models have been developed, with a special mention to Gurson-type models. These models simulate mathematically the physical growth of microvoids, leading to a progressive development of the internal damage that takes place during a tensile test. In these models, the damage starts to develop in very early stages of the test. Tests carried out by the authors suggest that, in the case of some eutectoid steels such as those used for manufacturing prestressing steel wires, the internal damage that takes place as a result of the growth of microvoids is only noticeable in very late stages of the tensile test. In the authors’ opinion, using a cohesive model as a failure criterion may be interesting in this case; a cohesive model only requires two parameters to be defined, with the fracture energy being one of them, which can be obtained experimentally. In addition to this, given that it is known that the stress triaxiality has a strong influence on the fracture of ductile materials, a cohesive model whose parameters are affected by the value of the stress triaxiality can be considered. This work presents a fracture model for steel specimens in a tensile test, based on a cohesive behaviour and taking into account the effect of stress triaxiality, which is different at each point of the fracture plane.

Fracture behaviour of notched round bars made of PMMA subjected to torsion at -60 °C

This paper presents seventy new experimental results from PMMA notched specimens tested under torsion at 60 C. The notch root radius ranges from 0.025 to 7.0 mm. At this temperature the non-linear effects previously observed on specimens of the same material tested at room temperature strongly reduce. The averaged value of the strain energy density over a control volume is used to assess the critical loads to failure. The radius of the control volume and the critical strain energy density are evaluated a priori by using in combination the mode III critical stress intensity factor from cracked-like specimens and the critical stress to failure detected from semicircular notches with a large notch root radius

Experimental validation of a fracture model for pearlitic steel bars based on the cohesive zone model

Congreso internacional celebrado en Praga sobre modelos numéricos de fractura en el campo de la ciencia de materiales y estructuras.

Experimental validation of a fracture model for pearlitic steel bars based on the cohesive zone model

Steel is, together with concrete, the most widely used material in civil engineering works. Not only its high strength, but also its ductility is of special interest, since it allows for more energy to be stored before failure. A better understanding of the material behaviour before failure may lead to better structural safety strategies.

Experimental and numerical study of bond response in structural concrete with corroded steel bars

Corrosion can affect the bond between reinforcing bars and concrete and hence the transfer of longitudinal stresses. Although a number of experimental studies on bond failure have been conducted in recent years, the findings have diverged rather widely, due primarily to differing test conditions. The present paper reports on an experimental programme consisting of eccentric pull-out tests run on corroded steel bars in specimens subjected to accelerated or natural corrosion. An axisymmetric bi-dimensional FE model with finite deformations initially developed to study bond mechanics with sound steel bars, has been enhanced to consider bond effects in corroded steel bars. The model simulation is compared to some of the experimental results for corroded and sound bars and the findings are analysed.

Use of Headed Reinforcement Bars in Contruction. A Theoretical Approach to Determine the Dimensions of Anchorage Plates and Experimental Tests on Knee Joints Subjected to a Closing Moment

In order to reduce costs and time while improving quality, durability and sustainability in structural concrete constructions, a widely used material nowadays, special care must be taken in some crucial phases of the project and execution, including the structure design and calculation, the dosage, dumping and curing of concrete: another important aspect is the proper design and execution of assembly plans and construction details. The framework, a name designating the whole reinforcement bars cage already assembled as shown in the drawings, can be made up of several components and implies higher or lower industrialization degree. The framework costs constitute about one third of the price per cubic meter placed in concrete works. The best solutions from all points of view are clearly those involving an easier processing to achieve the same goal, and consequently carrying a high degree of industrialization, meaning quality and safety in the work. This thesis aims to provide an indepth analysis of a relatively new type of anchoring by plate known as headed reinforcement bars, which can potentially replace standard or L-shaped hooks, improving the cleaning of construction details and enabling a faster, more flexible, and therefore a more economical assembly. A literature review on the topic and an overview of typical applications is provided, followed by some examples of specific applications in real projects. Since a strict theoretical formulation used to provide the design plate dimensions has not yet been put forward, an equation is proposed for the side-face blowout strength of the anchorage, based on the capacity of concrete to carry concentrated loads in cases in which no transverse reinforcement is provided. The correlation of the calculated ultimate load with experimental results available in the literature is given. Besides, the proposed formulation can be expanded to cases in which a certain development length is available: using a software for nonlinear finite element analysis oriented to the study of reinforced concrete, numerical tests on the bond-bearing interaction are performed. The thesis ends with a testing of eight corner joints subjected to a closing moment, held in the Structures Laboratory of the Polytechnic University of Madrid, aiming to check whether the design of such plates as stated is adequate for these elements and whether an element with plate-anchored reinforcement is equivalent to one with a traditional construction detail.

Noncollinear magnetic phases and edge states in graphene quantum Hall bars

Application of a perpendicular magnetic field to charge neutral graphene is expected to result in a variety of broken symmetry phases, including antiferromagnetic, canted, and ferromagnetic. All these phases open a gap in bulk but have very different edge states and noncollinear spin order, recently confirmed experimentally. Here we provide an integrated description of both edge and bulk for the various magnetic phases of graphene Hall bars making use of a noncollinear mean field Hubbard model. Our calculations show that, at the edges, the three types of magnetic order are either enhanced (zigzag) or suppressed (armchair). Interestingly, we find that preformed local moments in zigzag edges interact with the quantum spin Hall like edge states of the ferromagnetic phase and can induce backscattering.