Influence of Beam Figure on Porosity of Electron Beam Welded Thin-Walled Aluminum Plates

Welded aluminum components in the aerospace industry are subject to more stringent safety regulations than in other industries. Electron beam welding as a highly precise process fulfills this requirement. The welding of aluminum poses a challenge due to its high tendency to pore formation. To gain a better understanding of pore formation during the process, 1.5 mm thick aluminum AW6082 plates were welded using specially devised beam figures in different configurations. The obtained welds were examined with radiographic testing to evaluate the size, distribution, and the number of pores. Cross-sections of the welds were investigated with light microscopy and an electron probe microanalyzer to decipher the potential mechanisms that led to porosity. The examined welds showed that the porosity is influenced in various ways by the used figures, but it cannot be completely avoided. Chemical and microstructural analyzes have revealed that the main mechanism for pore formation was the evaporation of the alloying elements Mg and Zn. This study demonstrates that the number of pores can be reduced and their size can be minimized using a proper beam figure and energy distribution

The influence of sodium and magnesium sulphate on the penetration of chlorides in mortar

Marine environments are very aggressive to concrete, mainly due to the presence of chlorides and sulphates. The influence of sulphates on chloride penetration in mortars was investigated by immersion in combined test solutions containing 165 g/l NaCl and 33.8 g/l SO42- (as MgSO4 or Na2SO4) at temperatures of 5, 20 and 35 degrees C. After immersion periods ranging from 7 weeks up to 37 weeks, chloride profiles were measured by means of potentiometric titrations, XRD analysis and electron probe micro analysis. In general, chloride ions penetrate much deeper into the mortar than sulphate ions. Nevertheless, chloride penetration is clearly influenced by the presence of sulphates in the environment. Sulphate ions compete with chloride ions to bind to aluminate phases. Therefore, the presence of sulphates initially increases chloride diffusion. When magnesium sulphate is present the formation of Mg-related reaction products such as brucite additionally influences the chloride penetration. Later, up to 37 weeks of immersion, a decreasing chloride diffusion is noticed compared to samples exposed to a single chloride solution, due to pore blocking products of the sulphate reaction. Contrarily, immersion periods longer than 37 weeks in combined solutions result in increasing chloride diffusion due to sulphate induced cracking at the outermost layers. Notwithstanding the reciprocal influence of chlorides and sulphates, the magnitude of the effect of sulphate on the chloride diffusion coefficient was limited. Chloride diffusion generally increases with increasing temperature. The presence of sulphates decreases chloride binding even more significantly at 5 and 35 degrees C than at 20 degrees C

Investigations regarding the pumping process of wet-mix shotcrete improvement and upgrading of underground traffic structures

Workability is a physical property of fresh concrete, which can’t be described with only one single parameter or measured with one single test method. Pumpability and sprayability plays an important role for wet-mix sprayed concrete. The material must be conveyed through the pipe without changing its properties and mix proportion. The mix should leave the nozzle in a uniform stream intermixed homogeneously with the accelerator. When concrete is pumped through a pipe, a thin layer of paste is lubricating the wall of the pipe. The interaction of the lubricating layer and the pipe wall is of great importance for pumpability. The shear of the layer allows the slipping of the concrete and leads to a reduction of the required pumping pressure. In this presentation a sliding pipe rheometer “Sliper” is used to determine the pumping capacity of concrete. The Sliper consists of a pipe and a guided piston which is standing on the ground. A pressure sensor is integrated onto the piston. When the pipe is sliding downwards, the pressure in the pipe as well as the speed of the pipe are recorded. Rheological parameters as well as the supposed pumping pressure may be estimated. The cohesiveness of concrete is important to avoid blockages in the conveying pipe. Blockages can occur, when the paste separates from the aggregate skeleton because of a high pressure in the pipe. The stability of the wet-mix was determined by a filter pressing test. Mixes tested by the Sliper and the filter press were then sprayed with a wet-mix shotcrete machine Sika PM 500. This machine was equipped with seven pressure sensors, which could monitor the pressure over time. The sensors measured the pressure of the hydraulic system, the mix at the beginning and end of the pumping line, the pressure of the accelerator shortly after the pump and at the end of the hose, the pressure of the air and finally the pressure in the aerosol converter. Simultaneously, the spraying was filmed by a high-speed camera. This slow-motion recording shows the homogeneity and pulsation of the spray jet. At its best, shotcrete should emerge from the nozzle in a steady, uninterrupted flow. Different durable and sustainable mixes, developed under the Austrian research project ASSpC, were sprayed at different output of the shotcrete machine. The results and lessons learned from the results will be final part of the presentation

Sulfate attack - Reaction mechanisms revealed by a multi proxy approach

The destructive effects of sulfate attack on concrete structures are well known, but the reaction paths and mechanisms that cause the deterioration are still under debate. The aim of this study is to contribute to a deeper understanding on investigating concrete damage by introducing a novel and promising multi proxy approach method. The methodology comprises advanced mineralogical and hydro-geochemical methods as well as stable isotope signals. Investigations were performed on various field case studies in Austria, where the locally occurring ground water was classified as slightly aggressive to concrete, in accordance to DIN EN 206-1. Nevertheless intense concrete damage related to sulfate attack was found. Severely damaged mushy concrete consisted mainly of thaumasite, secondary calcite, gypsum and relicts of aggregate. The expressed interstitial solutions from such material were extremely enriched in SO4 (up to >30000 mg L-1). Stable hydrogen and oxygen isotope were applied successfully and demonstrated that the degree of evaporation provoked enrichments in SO4 and other dissolved, potentially harmful ions such as Cl. Furthermore, the enormous accumulation of incompatible trace elements (e.g. Rb and Li) clearly indicated that numerous wetting and drying cycles had occurred. Such a highly dynamic system is known to induce severe destructive effects on concrete. In this study we demonstrate that the application of a multi proxy approach can provide a better understanding of the complexity of reaction mechanisms involving sulfate attack on concrete structures. More detailed knowledge on the individual reactions that promote concrete damage in field structures will help to find specific counter measures for already affected buildings and to develop tailored concrete recipes, applications and constructive measures for future projects

Fracture dolomite as an archive of continental palaeo-environmental conditions

The origin of Quaternary dolomites in continental environments (e.g. karst and lakes) is barely constrained compared to marine dolomites in sedimentary records. Here we present a study of dolomite and aragonite formations infilling young fractures of the ‘Erzberg’ iron ore deposit, Austria, under continental-meteoric and low temperature conditions. Two dolomite generations formed shortly after the Last Glacial Maximum (~20 kyr BP): dolomite spheroids and matrix dolomite. Clumped isotope measurements and U/Th disequilibrium ages reveal formation temperatures of 0–3 °C (±6 °C) and 3–20 °C (±5 °C) for the both dolomite types, and depositional ages around 19.21 ± 0.10 kyr BP and 13.97 ± 0.08 kyr BP or younger, respectively. Meteoric solution and carbonate isotope compositions (δ18O, δ13C and 87Sr/86Sr) indicate the dolomites formed via aragonite and high-Mg calcite precursors from CO2-degassed, Mg-rich solutions. Our study introduces low temperature dolomite formations and their application as a sedimentary-chemical archive.ISSN:2662-443

Estudio de las reacciones de hidratación temprana en el hormigón proyectado

[ES] El hormigón proyectado es un tipo de hormigón especial utilizado como soporte de rocas durante la construcción de túneles y excavación de minas, entre otras aplicaciones. Sus principales características son el fraguado rápido y el desarrollo de una resistencia mecánica muy alta a edades tempranas, lo cual permite que el hormigón se fije a la base sin necesidad de un soporte extra, endureciéndose en pocos minutos. Las reacciones de hidratación que ocurren en las primeras horas en el hormigón proyectado determinan el desarrollo de las propiedades mecánicas y su estudio es de crucial importancia para entender y optimizar el comportamiento de este tipo de hormigones. El estudio de estas reacciones, sin embargo, resulta relativamente complicado por varias razones: (i) el hormigón proyectado endurece muy rápido, lo cual dificulta su manejo durante los ensayos experimentales, y (ii) composiciones similares mezcladas en el laboratorio no se comportan de la misma forma que las proyectadas. En este trabajo se presenta una metodología experimental desarrollada para el estudio de las reacciones de hidratación que ocurren en el hormigón proyectado durante las primeras horas y su correlación con la resistencia mecánica.Los resultados que aquí se presentan forman parte del proyecto ASSpC (Advanced and Sustainable Sprayed Concrete) financiado por la ‘Sociedad austriaca para la tecnología de la construcción’ ÖBV y la ‘Agencia para la promoción de la investigación en Austria’ FFG (proyecto numero 856080). Los autores agradecen a SIKA Technology AG su colaboración para la realización de ensayos en sus instalaciones.Galan Garcia, I.; Stauffacher, A.; Mittermayr, F.; Thumann, M.; Kusterle, W.; Juilland, P.; Stenger, C.... (2018). Estudio de las reacciones de hidratación temprana en el hormigón proyectado. En HAC 2018. V Congreso Iberoamericano de hormigón autocompactable y hormigones especiales. Editorial Universitat Politècnica de València. 383-392. https://doi.org/10.4995/HAC2018.2018.6907OCS38339

Mechanisms and Processes of Concrete Corrosion in Sewers

Concrete corrosion in sewers is caused by the combination of chemical and biological processes including sulfide and carbon dioxide generation and partition in wastewater, sulfide oxidation, neutralizing reactions of carbon dioxide, hydrogen sulfide, and its oxidation products (mainly sulfuric acid) with concrete. Wastewater is a sulfate-rich environment with sufficient carbon sources. The metabolism of sulfate-reducing bacteria leads to the formation of hydrogen sulfide in wastewater under anaerobic conditions. During wastewater transport through the sewers, depending on the dissolved oxygen concentrations and pH in wastewater, hydrogen sulfide can be chemically or biologically oxidized in wastewater, or partition into sewer gas in gravity sewers. Due to the alkaline and porous nature of concrete sewer pipes, the hydrogen sulfide reacts with intact concrete and reduces the concrete pH, lowering the concrete surface pH through chemically induced corrosion. The additional outgassing of CO2 from the wastewater further accelerates these processes. The reduction of surface pH facilitates the colonization of sulfide-oxidizing microorganisms on concrete surfaces. Sulfide-oxidizing microorganisms can further biologically oxidize hydrogen sulfide into sulfuric acid, leading to microbiologically influenced concrete corrosion. This chapter describes and discusses the mechanisms of these processes in sewers

A new test for combined Ca-leaching and sulphate resistance of cementitious materials

Limitations in the understanding of chemical key controls on concrete damaging mechanisms exacerbate predictions on the long-term performance and durability of cementitious materials. Therefore, the scope of the project “ASSpC Advanced and Sustainable Sprayed Concrete” is to obtain a better mechanistic understanding of the processes underlying deleterious chemical attacks. The herein presented alternative test, loosely following the regulations of the German Building Authority (DIBt) testing procedure (the so-called SVA test) for sulphate resistance, investigates the resistance of concrete mixes with high levels of limestone substitution (35%, 50% and 65%) against sulphate attack in a 10 g L-1 Na2SO4 solution at ambient temperature. Powdered samples were used in favour of prisms or drill cores to accelerate alteration reactions and to eliminate variations in microstructure or porosity. Based on throughout chemical and mineralogical characterisation of the experimental solutions and solid materials, we identified and traced several mineral reactions taking place in a chronological order: (1) dissolution of portlandite and Ca-leaching from C-S-H started immediately at the beginning of the experiments and provided the physicochemical conditions favourable for (2) the precipitation of massive calcite and ettringite during the advanced stage of chemical attack. Ongoing changes in the aqueous composition indicate that C-S-H dissolves incongruently and may be transformed into Si-bearing hydrogarnet. The amount of precipitated ettringite is apparently controlled by the availability of calcium, sulphate and aluminium and the precipitation rate correlates with the superplasticiser demand of the concrete mixes and with the pH of the solution during the nucleation and crystal growth stages, respectively. Our test allows distinguishing between competing reaction paths and kinetics and is capable to provide new insights into concrete damaging mechanisms in sulphate-loaded aqueous environments

A new test for combined Ca-leaching and sulphate resistance of cementitious materials

Limitations in the understanding of chemical key controls on concrete damaging mechanisms exacerbate predictions on the long-term performance and durability of cementitious materials. Therefore, the scope of the project “ASSpC Advanced and Sustainable Sprayed Concrete” is to obtain a better mechanistic understanding of the processes underlying deleterious chemical attacks. The herein presented alternative test, loosely following the regulations of the German Building Authority (DIBt) testing procedure (the so-called SVA test) for sulphate resistance, investigates the resistance of concrete mixes with high levels of limestone substitution (35%, 50% and 65%) against sulphate attack in a 10 g L-1 Na2SO4 solution at ambient temperature. Powdered samples were used in favour of prisms or drill cores to accelerate alteration reactions and to eliminate variations in microstructure or porosity. Based on throughout chemical and mineralogical characterisation of the experimental solutions and solid materials, we identified and traced several mineral reactions taking place in a chronological order: (1) dissolution of portlandite and Ca-leaching from C-S-H started immediately at the beginning of the experiments and provided the physicochemical conditions favourable for (2) the precipitation of massive calcite and ettringite during the advanced stage of chemical attack. Ongoing changes in the aqueous composition indicate that C-S-H dissolves incongruently and may be transformed into Si-bearing hydrogarnet. The amount of precipitated ettringite is apparently controlled by the availability of calcium, sulphate and aluminium and the precipitation rate correlates with the superplasticiser demand of the concrete mixes and with the pH of the solution during the nucleation and crystal growth stages, respectively. Our test allows distinguishing between competing reaction paths and kinetics and is capable to provide new insights into concrete damaging mechanisms in sulphate-loaded aqueous environments