Concrete as a new strain stress sensor

A new strain/stress sensor technology was developed, based on the concept of short electrically conducting fiber pull-out that accompanies slight and reversible crack opening. The fiber pull-out reversibly increases the composite's electrical resistance, which is the signal provided by the sensor under static or cyclic loading. The new technology was manifested in concrete and mortar containing electrically conducting short fibers (e.g. carbon fibers and steel fibers), but not in those containing no fibers or those containing non-conducting (polyethylene) fibers. Carbon fibers worked best. They served to greatly decrease the crack height, so that reversible pull-out of the crack bridging fibers occurred. Even in the elastic regime, a part of the resistance change was irreversible, such that it provided memory of the first deformation; this is due to permanent damage, probably associated with the increase in fiber/matrix contact electrical resistivity due to the interface bond weakening. The stress at which this damage began was much lower under tension than compression. The ratio of the contribution to the fractional resistance increase by the reversible part to that by the irreversible part was much higher under tension than compression. The irreversible part increased with increasing irreversible strain, which increased with increasing stress amplitude. The fractional increase in resistance at fracture was much larger under compression than under tension, was much larger for mortars than concretes at similar volume fractions, and was quite independent of the loading rate.