Figure 2. Notice that under conditions of no-stress, fibers reduce the permeability of concrete, and the reduction appears to be proportional to the fiber volume fraction. This is often explained on the basis of internal curing that occurs due to hydrophilic cellulose fiber. Data further indicate that stress has a significant influence on the permeability of concrete. When stress was first increased to 0.3 fu, both plain and FRC showed a decrease in the permeability. However, when the stress was increased to 0.5fu, plain and FRC showed very different trends. At 0.5fu, the permeability of plain concrete increased substantially over that of the unstressed specimen, but for FRC, while there was an increase in the permeability over 0.3fu, the permeability still stayed below that of the unstressed specimen.