Augusto Marchetti

Crosslink density and fracture toughness of epoxy resins

A series of networks of diverse crosslink density were prepared using bifunctional epoxide prepolymers of different molecular weight, crosslinked with diamine diphenyl sulphone, and their fracture behaviour investigated. The same set of resins was also modified with a reactive rubber. The fracture toughness regularly decreased on increasing the crosslink density for all formulations. The addition of the rubber gave rise to a marked increase in toughness, though it magnified the influence of the molecular weight of the prepolymer. Tests performed with blunt notches showed that the fracture toughness was maximum at medium crosslink densities. A dispersion of rubber particles caused a toughness increase through the formation of microcavities ahead of the crack tip. Failure was preceded by a rapid volume increase caused by void coalescence.

Electrical properties of fluidified portland cement mixes in the early stage of hydration

Abstract The electrical conductivity and the dielectric constant of cement mixes have been studied in the initial stages of setting to correlate the time evolution of the electrical parameters with the chemical and microstructural modifications. The formulations, based on a mix of Portland cement and microsilica, were fluidified with various amounts of an acrylic polyelectrolite and had a w/c ratio of 0.24. Measurements were carried out at ambient temperature for times of up to 40 h, in the frequency interval 30 Hz–200 kHz. The conductivity showed a marked decrease correlated with the loss of connectivity of the macroscopic porosity (depercolation). The time to the drop of conductivity depended linearly on the concentration of superplasticizer whose retarding effect was clearly evidenced. A dielectric amplification phenomenon was observed which vanished after the porosity depercolated. The temperature profiles of mixtures with different concentrations of the superplasticizer were drawn and a simple model was developed to compute the specific thermal power as a function of time. A substantial coincidence was found, on the time axis, between depercolation and maximum rate of setting. Vicat tests were also carried out.

Broad band dielectric analysis of bituminous concrete

Abstract Bituminous concrete were irradiated with intense electromagnetic fields at 2.45GHz in a laboratory microwave oven The observed heating rate was similar to that of an equal volume of water put in the same conditions. Information useful for the comprehension of this behaviour were obtained by exposing individual constituents (bitumen and aggregates) to microwaves and by means of broad band dielectric spectroscopy performed on both concrete and single constituents. Collected data show that bitumen is the less dissipative component of the concrete, while the nature of aggregates and the effects of interfacial polarisation seem to be at the basis of both the finite dissipation observed in the radiofrequencies region of the spectra and the efficient heating obtained with microwaves.

Mechanical Properties and Fracture of Hybrid Particulate Composites

A rigid epoxy resin was modified by acrylic core-shell particles and glass beads (with and without surface treatment). Glass transition temperature, elastic modulus and fracture toughness were determined for all formulations. The fracture toughness increased linearly with the rubber content (from 0.82 MPa√m for the neat resin to 2.6 MPa√m for 20% rubber). Toughness also increased on adding glass particles (for 50% glass up to 1.9 and 2.3 MPa√m depending on the surface treatment). The stress-strain behaviour in compression depended on glass content and surface sizing. The fracture behaviour markedly depended on the radius of curvature of the crack tip. The toughness of sharply cracked samples increased on increasing the glass content. On the contrary samples containing blunt cracks were embrittled by the hard particles. This was attributed to crack tip sharpening caused by detached beads.