Davide Zampini

Fracture processes of hybrid fiber-reinforced mortar

Subregion Scanning Computer Vision (SSCV), a digital image based method for measuring surface deformation is used to examine the role of the fibers in the fracture process of mortars reinforced with hybrid blends of microfiber (less than 22 μm in diameter) and macrofiber (500 μm in diameter). Closely-spaced microfibers interact with cracks at the microstructural level and hamper the widening of coalesced microcracks, thus encouraging the growth of multiple cracks. The microfibers improved pre-peak mechanical performance and strength by delaying the formation of a through-specimen macrocrack. Macrofibers were most effective at bridging macrocracks and imparting ductility to the composite due to their geometry and greater length. Compared to mortar reinforced with a single fiber type, an increase in strength and toughness was seen with a blend of steel macrofibers and either steel or PVA microfibers. Finally, based on the crack topography observed, the reduction in water permeability of cracked mortar achieved with hybrid fiber-reinforcement, measured directly in a parallel study, was governed by multiple crack development.RésuméLa visualisation assistée par ordinateur (en anglais Subregion Scanning Computer Vision (SSCV), méthode basée sur les images numériques pour mesurer les déformations de surface, est utilisée ici pour examiner les processus de fissuration de mortiers de ciment renforcés à l’aide de mélanges hybrides de fibres: microfibres (diamètre inférieur à 22μm) et macrofibres (diamètre supérieur à 500μm). Les microfibres faiblement espacées interagissent avec les microfissures et rendent difficile l’élargissement de celles-ci et leur éventuelle union en fissures plus importantes. Ces microfibres ont amélioré les performances mécaniques et la résistance du mortier en deçà de la contrainte de rupture en retardant l’apparition de macrofissures traversant l’ensemble de l’échantillon.Les macrofibres ont été les plus efficaces pour «coudre» les macrofissures tout en conférant une ductilité accrue au composite en raison de leur géométrie et de leur longueur plus importantes. Par comparaison avec des mortiers renforcés avec un seul type de fibre, ceux renforcés avec un mélange de macrofibres en acier et microfibres en alcool polyvinylique (PVA) ont révélé une résistance et une robustesse accrues.Le renforcement avec un mélange hybride de fibres a permis aussi d’observer une diminution de la perméabilité du mortier fissuré. Cette diminution de perméabilité est due au développement d’une fissuration multiple par opposition a des microfissures qui se transforment en macrofissures.

EARLY AGE MICROSTRUCTURE OF THE PASTE-AGGREGATE INTERFACE AND ITS EVOLUTION

The sequence of microstructural changes occurring at the wet paste-aggregate interface is documented at an age as early as 5 min using the environmental scanning electron microscope (ESEM). Unlike other microscopic techniques, the ESEM allows pastes of normal water: cement ratio to be observed at early ages without reducing the paste to a powder. Evolution of the paste-aggregate microstructure is followed up to an age of 24 h. The region adjacent to the aggregate surface contains a phase with a morphology referred to as a “sheaf of wheat” morphology. The same interfacial region in a 10-day-old specimen has a microstructure similar to the interfacial transition zone (ITZ) reported in the literature. Variations of the “sheaf of wheat” morphology due to original water-to-cement ratio, mixing energy, incorporation of silica fume, and drying are documented. As revealed by energy dispersive x-ray analysis (EDS), the microstructure contains significant amounts of calcium and silica. These results indicate that the observed morphology is likely to be a calcium silicate hydrate (C-S-H) product that is a precursor to type I C-S-H. A description of the evolution of the observed microstructural features is presented. The “sheaf of wheat” morphology appears to be a general precursor to morphologies commonly seen in mature pastes.

Characterization of the paste-aggregate interfacial transition zone surface roughness and its relationship to the fracture toughness of concrete

Notched concrete beams containing varying amounts of pea gravel aggregate were tested under three-point bend, and their fracture toughness determined. The roughness of the region near the interface between the cement paste and the aggregate was evaluated by digitizing images from a confocal tandem scanning microscope. The average roughness of the paste was found to be related to the fracture parametersKIC (critical stress intensity factor) and Δac (critical crack extension), as determined by the two-parameter fracture model. The roughness in the proximity of the paste-aggregate interface was generally higher than that of the paste far from the aggregate, and it decreased with the distance from the aggregate. This study indicates that aggregate particles increase the toughness of the cement paste portion of concrete, and that this is an important mechanism for toughening concrete.

The Interfacial Transition Zone and its Influence on the Fracture Behavior of Concrete

Concrete specimens of constant water-to-cement ratio and varying amounts of gravel aggregate were tested under 3-point bend. The fracture toughness of the composite and surface roughness of the paste are determined. Fracture parameters obtained from the Two ParameterFracture Model (TPFM) such as the critical stress intensity factor (K Ic ) and critical effective crack extension (Δa o ) are found to be related to the average surface roughness of the paste. The Interfacial Transition Zone (ITZ) and the bulk paste are distinguished by different values of roughness. The surface roughness of the area adjacent to the aggregate particle is evaluated as a function of the distance from the aggregate surface. In particular, the surface roughness of the paste near the aggregate is greater than that of the paste far from the aggregate, and it decreases with distance from the aggregate. The higher roughness of the paste near the aggregate indicates that aggregate particles, and more specifically the ITZ associated with them, act to toughen the paste in concrete.