EFFECT OF FIBER LENGTH VARIATION ON TENSILE PROPERTIES OF CARBON-FIBER CEMENT COMPOSITES

Unlike other fiber types, carbon fibers tend to possess a distribution of fiber lengths even before mixing into a cementitious matrix. Durin g mixing, some amount of fiber rupture may be expected. Along with oth er fiber parameters, fiber length distribution may be expected to play an important role in governing the composite properties. This paper d escribes the results of an experimental program which measures the len gth distribution of carbon fibers both before and after mixing. A data base of composite tensile stress-strain behavior is established for a number of commercially available carbon fibers in a cementitious matri x. Special emphasis is placed on pseudo strain-hardening behavior and the associated ultimate tensile strength beyond first cracking. A theo retical model of this ultimate strength which explicitly account for f iber length distribution is developed and shown to account for the red uced tensile strength of experimentally observed results reasonably we ll. Results of this research suggest that for the purpose of brittle c ementitious matrix reinforcement, carbon fibers with higher strain cap acity may be more important than high modulus typically demanded for p olymer matrix composite reinforcement.