Effects of deicers on the performance of concrete pavements containing air-cooled blast furnace slag and supplementary cementitious materials

This study investigates the effects of continuous deicer exposure on the performance of pavement concretes. For this purpose, the differences in the compressive strength, the changes in the dynamic modulus of elasticity (DME) and the depth of chloride ingress were evaluated during and after the exposure period. Eight different concrete mixtures containing two types of coarse aggregates (i.e. air-cooled blast furnace slag (ACBFS) and natural dolomite) and four types of binder systems (i.e. plain Type I ordinary portland cement (OPC) and three combinations of OPC with fly ash (FA) and/or slag cement (SC)) were examined. These mixtures were exposed to three types of deicers (i.e. MgCl₂, CaCl₂, and NaCl) combined with two different exposure conditions (i.e. freezing-thawing (FT) and wetting-drying (WD)). In cold climates, these exposure conditions are the primary durability challenges that promote the physical deterioration of concrete pavements. The results indicated that among the studied deicers, CaCl₂ had the most destructive effect on the tested concretes while NaCl was found to promote the deepest level of chloride ingress yet was shown to have the least damaging impact on concretes. The microstructure evaluation revealed that the mechanism of concrete deterioration due to the deicer exposure involved chemical reactions between the deicers and concrete hydration products. The use of FA or SC as partial replacements for OPC can offset the detrimental effects of both deicers and FT/WD cycles.     

Backscattered electron imaging of cementitious microstructures: understanding and quantification

During the last 20 years, backscattered electron imaging of polished surfaces has become well established as a method for the study of cement and concrete microstructures. The technique has many advantages, including the visualisation of representative cross-sections over a wide range of magnifications and reproducible contrast dependent on atomic number. Nevertheless the limitations of observing a two-dimensional section of a three-dimensional structure must be borne in mind.

In this paper, the general microstructural features of hydrated cement pastes are described. Although the amount of aluminate phase (“C₃A”) in cement is comparatively minor, it plays an important role in determining many of the microstructural features of cement paste microstructure, for example in the formation of “Hadley” grains.

Despite the very heterogeneous nature of cement paste, it is important to be able to derive quantitative measures if the relationships between microstructure and properties are to be understood. The possibilities to quantify BSE images are described. The interface between paste and aggregates in concrete is particularly variable, but average features can be measured, which help to understand the processes of packing cement grains, which gives rise to this region. Finally an example of the potential for BSE images to study concrete durability is given.     

Fabrication and characterization of aluminum nitride polymer matrix composites with high thermal conductivity and low dielectric constant for electronic packaging

Polymethyl methacrylate (PMMA) composites filled with Aluminum Nitride (AlN) were prepared by powder processing technique. The microstructures of the composites were investigated by scanning electron microscopy techniques. The effect of AlN filler content (0.1–0.7 volume fraction (vf)) on the thermal conductivity, relative permittivity, and dielectric loss were investigated. As the vf of AlN filler increased, the thermal conductivity of the specimens increased. The thermal conductivity and relative permittivity of AlN/PMMA composites with 0.7 vf AlN filler were improved to 1.87 W/(m K) and 4.4 (at 1 MHz), respectively. The experimental thermal conductivity and relative permittivity were compared with that from simulation model.     

Fracture mechanisms of epoxy-based ternary composites filled with rigid-soft particles

Epoxy composites filled with different amounts of aggregate-free silica nanoparticles and phase-separated submicron rubber particles were fabricated to study the synergistic effect of multi-phase particles on mechanical properties of the composites. Compared with binary composites with single-phase particles, the ternary composites with both rigid and soft particles offer a good balance in stiffness, strength and fracture toughness, showing capacities in tailoring the mechanical properties of modified epoxy resins. It was observed that debonding of silica nanoparticles from matrix in the ternary composites was less pronounced than that in the binary composites. Moreover, the rubber particles became smaller and their shape tends to be irregular, affected by the presence of rigid silica nanoparticles. The toughening mechanisms in the epoxy composites were evaluated, and the enlarged plastic deformation around the crack tip, induced by the combination of rigid and soft particles, seems to be a dominant factor in enhancing fracture toughness of the ternary composites.