In most reinforced concrete (RC) structural applications, externally bonded FRP reinforcements are bond-critical. To mitigate debonding, fiber anchors (also known as fiber splay anchors or spike anchors) are typically employed. These anchors consist of an anchor dowel embedded in pre-drilled holes in the concrete substrate and the anchor splay/fan spreads out over the width of the anchored FRP laminate. Previous durability studies have indicated that after environmental exposure, fiber anchors commonly fail at the bend region (i.e., at the dowel-splay interface) due to combined normal and shear stresses. This study has been designed to investigate the underlying mechanisms behind this failure mode under four accelerated environmental conditions (i.e., hygrothermal, alkali, saltwater, and freeze-thaw) using bar samples fabricated to represent the dowel portion of the fiber anchor. The test variables are the type of sizing (coating) applied to carbon fibers, sample exposure conditions and duration. The specimens were tested in a three-point bending setup for interlaminar shear strength and flexural strength to investigate the effect of sizing and exposure. The findings will help in understanding the underlying mechanism behind the strength reduction of the fiber anchors under harsh environments and sizing which will be helpful for determining the environmental reduction factors in fiber anchors design.