Measurement and Modeling of the Ability of Crack Fillers to Prevent Chloride Ingress into Mortar.
A common repair procedures applied to damaged concrete is to fill cracks with an organic polymer. This operation is performed to increase the service life of the concrete by removing a preferential pathway for the ingress of water, chlorides, and other deleterious species. To effectively fulfill its mission of preventing chloride ingress, the polymer must not only fully fill the macro-crack, but must also intrude the damage zone surrounding the crack perimeter. Here, the performance of two commonly employed crack fillers, one epoxy, and one methacrylate, are investigated using a combined experimental and computer modeling approach. Neutron tomography and microbeam X-ray fluorescence spectrometry (μXRF) measurements are employed on pre-cracked and chloride-exposed specimens to quantify the crack filling and chloride ingress limiting abilities, respectively, of the two polymers. A two-dimensional model of chloride transport is derived from a mass balance and solved by the finite element method. Crack images provided by μXRF are used to generate the input microstructure for the simulations. When chloride binding and a time-dependent mortar diffusivity are both included in the computer model, good agreement with the experimental results is obtained. Both crack fillers significantly reduce chloride ingress during the 21 d period of the present experiments; however, the epoxy itself contains approximately 4 % by mass chlorine. Leaching studies were performed assess the epoxy as a source of deleterious ions for initiating corrosion of the steel reinforcement in concrete structures.
Examining Curing Efficiency using Neutron Radiography
Many state highway agencies use prescriptive specifications for the curing of concrete bridge decks, pavements, flatwork, or structural elements. For example, concrete pavements are frequently specified to have a curing compound applied shortly after placement and bridge decks typically require seven days of wet curing. These specifications are often based on historical practices that have developed over the last century as opposed to quantitative measurements of performance. New approaches to curing are being introduced which include advanced curing compound formulations or internal curing, for example. However, clear information is not always available as to how this may affect curing requirements. This paper demonstrates the potential to use neutron radiography to quantify the degree of hydration at various distances from the finished surface. It describes how different curing approaches affect cement hydration in terms of both time and distance from the surface. The results show that in a sample exposed to drying after one day the top 12.5 mm (1/2 in) of the mortar was dramatically affected by evaporation, and the degree of hydration in this region was 32% lower than in a 14-day moist-cured sample. Also, the use of superabsorbent polymers increased degree of hydration by about 3.7–7.8% for sealed samples and samples exposed to drying. While these results are preliminary, it is believed by the authors that neutron radiography provides a powerful approach that could be used to determine equivalent curing requirements for new materials.
Evolution of carbide precipitates in Hastelloy N joints during welding and post weld heat treatment
Abstract The effects of welding and post weld heat treatment (PWHT) on the morphology, composition and lattice parameter of the precipitates in the welding joints of Hastelloy N alloy have been investigated. Joint samples were prepared using a gas tungsten arc welding process with ERNiMo-2 fillers, and then solid-solution treated at 1100 °C for 20 min. After welding, eutectic M6C carbides can be observed in the heat affected zones (HAZ) and weld zones (WZ). These eutectic M6C carbides become spheroidized during the following PWHT. The concentration of Si decreases during the transformation from the primary ones to eutectic ones, and then increases during the spheroidizing process. The total concentration of Si + Cr in M6C carbides remains stable during welding and PWHT. At the same time, the heat leads to the outward diffusion of Mo from the M6C carbides, which results in a decrease of the lattice parameter.