Benjamin Varela

Durability and Testing – Physical Processes

Concrete is well known to be strong in compression but weak in flexion and tension. However, by the use of steel reinforcing, often in combination with techniques such as pretensioning, and through appropriate structural engineering design methodologies, it is possible to compensate for this weakness by ensuring that the binder and aggregate of the concrete are subjected to minimal tensile load. This means that the relationship between compressive strength, flexural strength and other mechanical properties of concrete is used as an essential basis for civil and structural engineering design purposes. In practice, and with the current almost-universal use of Portland cement-based concretes in civil infrastructure applications, many of these relationships are codified in standards as empirical power-law relationships involving the 28-day compressive strength of the material, sometimes as the sole property used in the predictive equations or sometimes along with a small number of additional physical parameters. For example, the American Concrete Institute [1] specifies the prediction of elastic modulus as a function of compressive strength and concrete density, but an equation solely based on compressive strength is also provided, and is probably more widely used in practice. More sophisticated and more detailed theoretical models, or empirical correlations involving larger numbers of parameters, are often published in the academic literature, but are not in widespread application. An excellent discussion of phenomena and models for Portland cement concrete is presented by Neville [2], and the reader is referred to that text for further information.

Comparing the Environmental Impacts of Alkali Activated Mortar and Traditional Portland Cement Mortar using Life Cycle Assessment

Since the year 1908 there has been research into the use alkali activated materials (AAM) in order to develop cementitious materials with similar properties to Ordinary Portland Cement. AAMs are considered green materials since their production and synthesis is not energy intensive. Even though AAMs have a high compressive strength, the average cost of production among other issues limits its feasibility. Previous research by the authors yielded a low cost AAM that uses mine tailings, wollastonite and ground granulated blast furnace slag (GGBFS). This mortar has an average compressive strength of 50MPa after 28 days of curing. In this paper the software SimaPro was used to create a product base cradle to gate Life Cycle Assessment (LCA). This compared the environmental impact of the AAM mortar to an Ordinary Portland Cement mortar (PCHM) with similar compressive strength. The main motivation for this research is the environmental impact of producing Ordinary Portland Cement as compared to alkali activated slag materials. The results of this LCA show that the Alkali Activated Material has a lower environmental impact than traditional Portland cement hydraulic mortar, in 10 out of 12 categories including Global Warming Potential, Ecotoxicity, and Smog. Areas of improvement and possible future work were also discovered with this analysis.

Durability of Alkali Activated Blast Furnace Slag

The alkali activation of blast furnace slag has the potential to reduce the environmental impact of cementitious materials and to be applied in geographic zones where weather is a factor that negatively affects performance of materials based on Ordinary Portland Cement. The scientific literature provides many examples of alkali activated slag with high compressive strengths; however research into the durability and resistance to aggressive environments is still necessary for applications in harsh weather conditions. In this study two design mixes of blast furnace slag with mine tailings were activated with a potassium based solution. The design mixes were characterized by scanning electron microscopy, BET analysis and compressive strength testing. Freeze-thaw testing up to 100 freeze-thaw cycles was performed in 10% road salt solution. Our findings included compressive strength of up to 100 MPa after 28 days of curing and 120 MPa after freeze-thaw testing. The relationship between pore size, compressive strength, and compressive strength after freeze-thaw was explored.

Effective Damping of a Flexible-Matrix-Composite Laminate With a Negative Effective Poisson’s Ratio

Two very important factors which determine the effectiveness of a pump are its volumetric and energy efficiencies. Yin and Ghoneim constructed a prototype of a flexible body pump with a very high volumetric efficiency or pumping potential (the relative volume reduction due to a relative input stroke) [1]. The high volumetric efficiency is attributed to the geometry of the pump’s structure (hyperboloid) as well as the high negative effective Poisson’s ratio of the 3-layer ([θ/β/θ]) flexible-matrix-composite (carbon/polyurethane) laminate adopted for the body of the pump. The energy efficiency was not evaluated. An important factor in assessing the energy efficiency of flexible-body pumps is the effective damping (a measure of the energy dissipation per cycle) of the flexible body material. An objective of the current work is to determine the effective damping inherent in the 3-layer laminate, as a function of the two angle orientations θ and β, employed for the design of the flexible body pump. Thereby, the best fiber angle orientation, for the highest volumetric as well as energy efficiency, can be considered. The contribution of this work is twofold: 1) viscoelastic characterization (longitudinal, transverse, and shear complex moduli, as well as the in-plane complex Poisson’s ratio) of the polyurethane/carbon composite, used in the Geo-polymer lab at RIT; the results may be utilized as benchmarks for other researchers using similar carbon/polyurethane in dynamic applications, and 2) provide a comprehensive study of the effect of the two angles θ and β on the effective damping factors of the three-layer laminate. Together with the similar study on the negative Poisson’s ratio [1], a better design of the laminate for the most efficient flexible-body pump performance can be established.Copyright

Multiple Phase Identification in Alkali Activated Slag by SEM-EDS

Alkali-activated slag is studied using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray microanalysis. Attention is focused on delineating the phases induced by the alkali activation, as these phases are important in determining the mechanical properties of the material. The starting material, slag, is found to be a heterogeneous material with at least two phases. Upon alkali activation the material becomes more heterogeneous, now exhibiting at least four phases with significant different chemical composition. Furthermore, the alkali activation is found to modify the phase rich in Ca in the unactivated slag more than the other. Alkali activation of the slag produced mostly an amorphous material with some crystalline phases such as hydrotalcite and calcite, also some nanocrystalline structures were detected by TEM.

EDS-Based Phase Analysis of Alkali Activated Slag

Alkali-activated materials are considered third-generation binders after lime and ordinary Portland cement (OPC). There are two main categories of alkali activated binders: the first one is the activation of industrial by-products such as fly ash and blast furnace slag with a mild alkaline solution and calcium silicate hydrate (C-S-H) as the main reaction product. The second one is the alkali activation of natural aluminosilicates such as metakaolin with medium to high alkaline solutions. Alkali-activated materials have been known as an alternative to Portland cements due to the higher compressive strength (exceeding 100 MPa in 28 days), durability and environmental benefits [1,2]. However, to understand the development of mechanical strength and durability, an understanding of the phase formation and chemical composition of these phases it required. [6]