A MICROSTRUCTURAL STUDY OF FIBRE MORTAR INTERFACES DURING FIBER DEBONDING AND PULL-OUT/

Attempts were made to connect the change in interfacial properties dur ing fibre pull-out in cementitious material to the microstructural fea tures of the interface. The microstructural features of fibre (steel, nylon and polypropylene)/mortar interfaces were examined during the fi bre debonding and pull-out process. Because fibre pull-out was found t o be sensitive to lateral compression, microscopic studies were carrie d out on fibres pulled out with and without lateral compression. SEM a nd energy-dispersive X-ray (EDX) analyses were performed at four diffe rent stages: (a) before debonding; (b) immediately after debonding; (c ) at small sliding distance; and (d) at large sliding distance. For th e steel fibre/mortar interface, it was found that the mortar surface ( interfacial transition zone) was subjected to abrasion, while the stee l surface was subjected to plastic deformation. EDX analysis on the mo rtar interface showed that the ratio of calcium/silicon count first in creases within a short sliding distance and decreases thereafter, indi cating a process of CH layer abrasion and C-S-H layer exposure. The ra pid post-peak drop of the pull-out force at the beginning of sliding i s due to the ''grinding'' effect, which leads to crushing and abrasion of the CH crystals and a reduction of asperity on the mortar surface. The grinding and abrasion effect becomes more significant with the ap plication of lateral compression, which results in more rapid drop of the pullout force. For the nylon and polypropylene fibre/mortar interf aces, the fibre surface peels and the mortar surface experiences very little damage. Nylon fibre surface swells and is peeled with short whi skers on the surface, leading to significant increase in interfacial f riction causing the post-debonding pull-out force to increase. The pol ypropylene fibre surface is peeled and plowed with long whiskers and l ong scratch lines which also leads to an increase in interfacial frict ion. On applying lateral compression to the mortar during fibre pull-o ut, the abrasion and peeling effects are more severe. With lateral com pression, holes may form on the polypropylene surface over a longer sl iding distance. The ratio of calcium/silicon count on the mortar surfa ce by EDX does not show obvious trends with sliding distance indicatin g that the mortar surface experiences very little damage.