C. Paglia

Strength development of mixtures accelerated with alkali-free and alkaline admixtures is clarified. Alkali-free admixtures do not interfere with calcium silicatehydrate gel development. A good dissolution of the admixture (e.g., a calciumsulfoaluminate-rich one) causes the ettringites to be homogeneously distributed within the cementitious mass, thus acting as a crack arrester and enhancing the strength. Conversely, an inhomogeneous distribution of the admixture results in the formation of high voluminous ettringite enrichments that weaken the microstructure and result in the strength decrease of the alkali-free accelerated mixtures. Addition of the alkaline accelerator delays the hydration of the calciumsilicate components to calciumsilicatehydrate gel and its densifying process. As a result, a general high porosity persists up to 6 months, resulting in the lowest strength among the accelerated mixtures.

An evaluation of two innovative concrete reinforcement methodologies has been carried out. This study showed that a combination of polypropylene CoRe™ fibres and (resistance butt welded stainless steel) twisted closed loops, (with a threedimensional effect) provided the best energy absorption performance of all the systems examined. Furthermore, this feasibility study showed that concrete slabs reinforced using a combination of conventional rebar and CoRe™ polypropylene fibres lessened the risk of fragmentation and significantly improved energy absorption. In this paper two novel anchorage methods leading to improved structural integrity were investigated.

The dimensional stability and durability of Acrylamide- and Methacrylate-based acrylic resins have been studied. The dimensional stability was characterized by measuring the volume expansion of samples immersed in water for a period up to 240 days or by exposing the samples to 23oC and 50 per cent, 90 per cent relative humidity and by monitoring the shrinkage. The durability was investigated by exposing the resins to cyclic variations of temperature in air and in water. The resins generally exhibit a significant volume change up to 160 per cent of the initial volume when immersed in water or exposed to a relatively dry atmosphere (23oC and 50 per cent). A general increase in the material stiffness and/or crack formation on the surface of the resin is observed. On a long-term basis, the durability of the resins may significantly vary with occasionally a partial or complete deterioration of the some resins. A general better dimensional stability and durability is observed for the Meth...

The corrosion behavior of a friction stir welded Al-2.05Li-2.70Cu-0.6Mg-0.3Zn alloy AF/C458 has been characterized using several corrosion techniques. Vicker's hardness profiles collected transverse to the weld direction and optical microscopy helped distinguish different zones within the weld. Zones identified were the parent material, advancing heat affected zone (HAS), trailing heat affected zone (HTS), the thermo-mechanically affected sides on both sides of the weld (TMAZ) and nugget. The nugget and parent material were found to have Vicker's hardness values of 118 ± 4 HV and 170 ± 4 HV, respectively. The environmentally assisted cracking (EAC) and the corrosion behavior were investigated using constant extension rate testing (CERT), intergranular corrosion immersion test, and potentiodynamic scans. The CERT experiments were completed in air and 10mM NaCl solution on round tensile bar samples cut from within the weld micro-zones parallel to the weld and also transverse (...

ABSTRACT The temperature gradient occurring for a short time during friction stir welding greatly affects the localized corrosion properties of welded 7050-T7451 plates. An immersion experiment in a salty solution was carried out in order to verify the influence of short-term post-weld heat treatments at temperatures similar to those taking place during friction stir welding on the corrosion behaviour of friction stir welded 7050-T7451. The experiment consisted of inserting thermocouples at different weld regions for the registration of the temperature development with time, and partially immersing the welded plate in a salty solution at 480°C. In this manner, the weld experienced different temperature expositions at different locations. It was found that a temperature exposition above 180°C for 20min significantly increases the general corrosion resistance of friction stir welded 7050-T7451. The re-exposition of 7050-T7451 friction stir weld to a time–temperature combination similar to that occurring during the welding process on the thermomechanically heat affected zones of the weld, significantly improves the corrosion properties and the environmental cracking resistance. After the short-term temperature exposition, the fracture location of samples tested in a 3.5wt.% NaCl solution moved from the corrosion susceptible thermomechanically affected zone to the heat affected zone.

ABSTRACT The influence of alkali-free and alkaline accelerators on the setting behavior of cementitious mixtures is investigated. The aluminium sulfate present in the alkali-free accelerators promotes the crystallization of ettringite prisms (2–7 μm) between the clinkers, thus connecting them in a compact mass and accelerating the setting. The very rapid setting promoted by the addition of the alkali KAl(OH)4-rich admixture is mainly the result of the precipitation of CH plates and amorphous KCASSH hydrates. In this case, the crystallization of elongated ettringite rods contributes to a smaller extent to the setting.

The microstructure, the microchemistry and the environmental assisted cracking susceptibility was investigated for friction stir welded 2219-T87 plates. It was found that the coarsening of the Al2Cu intragranular precipitates occurring within the thermo-mechanically and the heat affected zones after friction stir welding correlates with a decrease in the hardness values. The regions with the lowest hardness are relatively narrow in size and are limited to the boundary between the thermo-mechanically affected zone and the nugget. The weld microzones exhibited a different microstructure but a similar environmental assisted cracking susceptibility. The environmental cracking susceptibility of the weld was generally low. The constant extension rate tensile tests were carried out in air and in a 3.5wt.% NaCl solution. For samples tested transverse to the welding direction failure occurred within the thermo-mechanically affected zone–nugget boundary. The thermo-mechanically affected zone–...