This work presents the results of a research project aimed at assessing the correlation among fresh state behavior mechanical properties in the hardened state and fiber dispersion in steel fiber reinforced concretes. Three fiber reinforced concretes were hence designed and targeted to different levels of fresh state performance: a vibrated concrete, a self consolidating one and a third exhibiting segregation. Fiber reinforcement consisted in all cases of 50 kg/m3 hooked end steel fibers, 35 mm long and with an aspect ratio equal to 65. Square plates 600 mm wide and 60 mm thick were cast for each mix. The dispersion of fibers within the specimens was investigated through Alternate Current Impedance Spectroscopy (AC-IS). Finally, beams were cut from the plates and tested in 4-point bending. From the load-crack-opening and load-deflection response toughness and stiffness parameters were computed to assess the behavior at serviceability and ultimate limit states. The influence of fiber dispersion and orientation in thin plates on the measured mechanical properties is discussed and a correlation is attempted with parameters, such as fiber spacing, suitably defined to represent the dispersion, detected as above. The results clearly highlight the connections existing between fresh state behavior, fiber dispersion and mechanical properties of SFRC, pointing out their importance for a design of the material composition as well as of the casting process "tailored" to the specific structural application.

This paper presents an inverse analysis approach to obtain material properties of fiber reinforced concrete in terms of Young’s modulus, Poisson’s ratio and tensile stress crack width parameters from the load deflection response of a round panel test. The properties were then used in a nonlinear finite element model to simulate the test of a full scale elevated slab subjected to a point load at mid span of the central slab. The simulation reasonably agreed with the experimental test data measured in the field; the predicted load capacity was higher than the test result by 15.5% and the ascending response was also stiffer than the measurement in the field. An alternative simpler yield line analysis was also used to calculate the material strength from the round panel test and then used to predict the load capacity of the full scale test. The load capacity predicted by the yield line theory was in between the finite element simulation and the experimental result.

This paper is part of an on ongoing research project involving testing of small and large-scale beams to investigate shear behavior of reinforced concrete beams with synthetic macro fibers. Six full-scale tests were completed on longitudinally reinforced concrete beams without stirrups. The size of the beam was 280 mm x 460 mm x 3200 mm and tested with a shear span to depth (a/d) ratio of 3.5. Synthetic macro-fibers were added at two volume fractions of 0.5 % and 0.75 %, which is equivalent to 4.6 and 6.9 kg /m3. Strains and deflection were measured under monotonic loading of the beams and cracking was also monitored. The test results show that the synthetic macro-fibers improved the first diagonal shear cracking strength and ultimate shear capacity of the beams. Ultimate shear capacity of the reinforced concrete beams was increased by 12 to 25 % depending on the dosage of synthetic macro-fibers used. Embedded strain gauges in the concrete beams indicated the fibers effectively distributed the load, improved tensile strain capacity and thus increased the shear capacity of the concrete beams. Load-deflection measurements show that synthetic macro-fibers improve the post-diagonal cracking stiffness and toughness of the concrete beams and reduce the brittleness of the shear failure.

The load-deflection response of fiber reinforced cement composites generally starts by an initial portion that is linear elastic up to a certain load at which it deviates from linearity; this is often identified as the onset of first cracking in the matrix. If the cement matrix is not reinforced, first cracking is followed by a sudden drop in the load-deflection curve, and failure occurs. The addition of fibers mostly influences the response of the composite after cracking. For all practical purposes, the load-deflection response of fiber reinforced cement composites after first cracking can be simply classified as either "deflection-softening" or "deflection-hardening." This paper describes first the different types of load-deflection curves observed in various experimental tests and illustrates the influence of some fiber reinforcing parameters with steel and polymeric fibers. Then, an analytical formulation is suggested to predict the value of the critical volume fraction of a given fiber to achieve deflection-hardening behavior. Several parameters influence the “deflection-hardening” portion of the curve and include the fiber content, fiber aspect ratio, and fiber to matrix bond.

Fiber-reinforced concrete (FRC) has a post-cracking (residual) tensile strength which can provide extra stiffness to a reinforced concrete structure. This helps to reduce deflections and control cracking. Basic concepts of tension stiffening and the tensile capacity of the FRC at a crack are used to develop a rational model for both axial and flexural member stiffness. Axial member stiffness is defined by an effective concrete area and validated with experimental results. An effective moment of inertia is used to define the flexural stiffness, and the computed response of a plain reinforced concrete beam is compared with an FRC reinforced concrete beam. FRC is shown to increase member stiffness by between 10 to 50% depending on the amount and type of conventional reinforcement and post-cracking strength of the FRC used. The expressions developed for member stiffness are compatible with the ACI 318 approach of using an effective moment of inertia and can be easily incorporated into existing design procedures to ensure that deflection requirements are satisfied.