Effects of microfiber reinforcement on mechanical properties and durability of cementitious composite

Fiber reinforced cementitious materials exhibit quasi-brittle behavior, enhanced durability, flaw tolerance and toughness. However, most studies focus on the effects of macrofibers. The improvements of mechanical and fracture behavior of mortars due to steel and polymer microfibers were investigated in this study by cylinder, flexural, and compact tension test. During the compact tension test, in-situ crack propagation was observed under an optical microscope and the toughening mechanisms of the microfibers were identified.; In addition, the effect of steel microfiber reinforcement on the alkali silicate reaction (ASR) was investigated. The reactivity of ASR in mortars with 0.07 volume fraction (V_f) and without steel microfiber reinforcement was studied. The expansion and the reaction products were characterized by expansion test, optical microscopy, scanning electron microscopy (SEM), microprobe analysis, and induced coupled plasma (ICP) spectrometry. The flexural strength and fracture behavior of the two mortars subject to ASR was also investigated.; The test results revealed that incorporation of both steel and polymer microfibers enhanced the strength of the mortar. The crack growth resistance increased with increasing V_f of microfibers. Interground polymer fibers despite their very low V_f were very effective in resisting crack growth resistance. Multiple cracking due to successive debonding of the fiber/matrix interface was the dominant toughening mechanism in mortars reinforced with steel microfibers while fiber bridging and pullout dominated in the Fibar cement reinforced with interground polymer fibers. Furthermore, aggregate bridging and pullout and secondary crack formations associated with aggregate bridging sites were observed as well in both types of mortar specimens. The steel microfiber reinforced (SFR) mortar subject to ASR showed considerably less expansion and less reaction products were formed. The ICP analysis of the liquid obtained from the ASR site revealed a higher concentration of alkali-silicate complex in SFR mortar compared to the plain mortar, which explained the lower dissolution of reactive silica in SFR. The SEM image of layered structure that consists of small crystals observed in dry sodium rich gel supports the double layer theory as the ASR expansion mechanism.