This paper presents results of an extensive experimental parametric study of the mechanical response of various types of fiber-reinforced concrete tested under triaxial stress combinations. The objective of this study was to characterize the constitutive properties of these materials, with respect to fiber type and content, load path, condition at testing, and specimen size. A total of 250 tests were done on cylindrical specimens made of concretes with fiber contents (volumetric ratio): (1) steel microfiber, 1 and 2%; (2) mix of steel microfiber and long hooked steel fiber, 1 and 2%; (3) polypropylene fiber, 1.5 and 4%; and (4) mix of steel microfiber and polypropylene fiber, 2.5 and 5%. Triaxial stresses were applied either by hydraulic pressure or by means of passive confinement. In the latter procedure, lateral stress was provided by means of carbon fiber reinforced polymer jackets, wrapped to a fixed number of layers. Parameters of this component of the test program were (1) the number of wrap layers (one, two, or three); and (2) specimen size (two sizes were considered). Experimental results were used to quantify the relative influences induced by the various fiber additives and document the effectiveness of confinement provided by carbon fiber reinforced polymer wraps in comparison with actively applied lateral pressure. By mixing fibers of drastically different lengths, changes were effected both on the microstructure of the cement paste (by increasing its toughness) and on the macroscopic structure of concrete, thereby increasing the postpeak deformability and ductility of the material.
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