Self-sensing of impact damage in carbon fiber cement mortar
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Damage monitoring of the civil infrastructure is of critical national importance. Cement mortar containing short carbon fiber and silica fume has been found to be effective for sensing its own impact damage through the effect of impact on the DC or AC electrical resistance, provided that the region of resistance measurement contains the point of impact. The carbon fiber mortar may be in bulk form or the form of a coating of thickness as small as 5 mm. The carbon fiber can be made from pitch or PAN, provided that the resistivity of the mortar is in the range 10 4 - 10 5 Ω.cm, as provided by pitch-based carbon fiber (15 μm diameter, 5 mm long, unsized) at 0.5% or 1.0% by mass of cement or by type A PAN-based carbon fiber (7 μm diameter, 8 mm long, desized) at 0.5% by mass of cement. Due to the low mortar resistivity of 10 3 Ω.cm, pitch-based carbon fiber at 1.5% by mass of cement and type B PAN-based carbon fiber (7 μm diameter, 8 mm long, unsized) at 0.5% by mass of cement are less effective. In the absence of fiber, there is no sensing ability. The surface resistance measured at the surface receiving the impact is a particularly effective indicator of the impact damage, as shown for impact energy 880-3990 J. Due to 2660 J impact, for example, the flexural strength decrease is 20%, the flexural toughness decrease is 81% and the indent depth is 64 μm, with no visible cracks, as shown for pitch-based carbon fiber. The four-probe method gives higher fractional increase in the surface resistance upon damage than the two-probe method. The oblique resistance and the longitudinal volume resistance are much less effective. The surface resistance increases abruptly upon impact, though it decreases abruptly upon impact after a sufficient number (5-40) of impacts have been inflicted. The higher is the impact energy, the smaller is the number of impacts needed to change from the abrupt increase behavior to the abrupt decrease behavior. The abrupt increase is due to defect generation and the consequent increase in resistivity (1800% increase at 2660 J) of the region being damage. On the other hand, the abrupt decrease is due to the subtle damage of the cement matrix between adjacent fibers and the consequent increase in the degree of fiber-fiber contact and decrease in the resistivity, as previously reported for fatigue damage. When the impact energy is higher than that of prior impacts, the impact damage sensitivity is better than the case of the impact energy being lower than that of prior impacts.