Fiber addition has become one of the most prevalent methods for enhancing the tensile behavior of concrete. Fibers reduce cracking phenomena and improve the energy absorption capacity of the structure. On the other hand, the introduction of fibers can introduce a negative impact on concrete workability, whose loss is influenced by different parameters (among which are fiber content and fiber type). In this context, an exploratory study on the influence of steel (high stiffness) and macro-synthetic (low stiffness) fibers on the fresh properties of concrete was carried out, considering workability and air content, as well as resultant mechanical performance. Four fiber types at two volume fractions (0.5% and 1.0%) were studied in two base concretes with different water-to-cement ratios (0.45 and 0.50) by using the slump test, DIN flow table test and air content meter. An additional parameter for the DIN flow table test is proposed herein to quantify the potential preferential flow direction caused by fiber orientation and entanglement. Air meter results showed that the fibers caused only a slight increase in concrete air content; this agreed well with the results of mechanical testing, which showed no apparent effect on measured compressive strength. In addition, it was captured that, for a given fiber volume fraction, steel fibers more adversely affected Fiber Reinforced Concrete (FRC) workability as compared to polypropylene ones, while the opposite result was obtained considering FRC toughness.Fibers 2018, 6, 47 2 of 14 mixing, placing and compacting. In addition, there can be challenges in dispersion, where fibers entangle and result in non-uniform distribution in the concrete matrix. Consequently, an FRC that cannot be placed and compacted easily will affect constructability and will not lead to the required strength or durability characteristics of the material and, consequently, of the overall structure [28]. This aspect underscores the importance of carefully evaluating the fresh properties of FRC related to workability. Workability is influenced by different factors, among which are water content, cement content, aggregate characteristics, admixtures, fiber type and content.In comparison to the numerous studies present in the literature about the hardened properties of FRC, only a relatively small number of studies are focused on its workability [25,[29][30][31][32][33][34][35][36][37][38]. In addition, most of these studies are focused on steel fibers [25,[29][30][31][32][33][34][35][36][37][38], while macro-synthetic fibers were studied only in [34]. In these investigations, different test methods were adopted to measure the fresh properties related to workability, including slump [25,[29][30][31][32][33][34]37,38], Vebe test [25,29,30,33,[36][37][38], inverted cone test [29,33,34,36], compacting factor test [36], DIN flow table test [34,35] and rheometers [34,36]. All these tests were mainly developed for Plain Concrete (hereafter PC). The slump test is the most common method for both PC and FRC, even i...

The use of fibers can improve the behavior at serviceability limit states (crack and deflection control) and ultimate limit states (bearing capacity) of reinforced concrete (RC) elements under flexure. Fibers reinforcement provides a postcracking resistance, leading RC element to have a more diffused crack patterns characterized by narrower and more closely spaced cracks. Some doubts are instead related to RC element ductility, which can be affected by crack localization after rebar yielding. However, most of experiments present in literature relate only to elements in steel fiber reinforced concrete with significant residual strengths (f R,1 and f R,3 greater than 3.5-4.0 MPa). This paper aims to evaluate the influence of different fiber type (steel, glass, or polymer macrofibers) on the cracking and strength capacity of RC beams under flexure by using a broad range of Fiber Reinforced Concrete (FRC) toughness (1.6 ≤ f R,1 ≤ 5.1 MPa and 0.8 ≤ f R,3 ≤ 4.5 MPa). Twenty-one small scale RC beams with a typical value of longitudinal reinforcement ratio (0.87%) were tested under flexure. Crack and deflection control, as well as bearing capacity and crack localization were evaluated as a function of FRC toughness. Finally, results were compared against mean crack spacing and strength capacity predictions of fib Model Code 2010.

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