International Concrete Abstracts Portal

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 CD-ROM is a collection of papers organized for a session held at the ACI 2007 Fall Convention in Puerto Rico. Papers relate to material behavior and the structural implications of using fiber-reinforced concrete (FRC), and focus on the benefits of using fibers to enhance stiffness and reduce deflection of FRC members both with and without reinforcement. Used in combination with conventional reinforcement, FRC can increase stiffness and reduce deflection of cracked members as well as decrease the stress in the reinforcement. This is particularly important in thin sections and cement-based products where the geometry and profile play an important role in controlling deflection. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-248CD

Effect of open- or closed-loop deflection control on the measured flexural toughness of fiber reinforced concrete (FRC) was investigated. Third-point loading tests were performed as per ASTM C1609M on several high strength concrete mixtures containing low volume fractions of single, double and triple-fiber blends. A 3-stage loading sequence was adopted for the closed-loop deflection control tests to fully capture the load vs. deflection response immediately after the peak-load. The results indicate that the open-loop tests produce high instability in the load deflection curves after the peak-load. However, contrary to general belief, the open-loop tests also overestimated the flexural toughness compared to the closed-loop deflection control tests. Manually removing the instability from the open-loop curves helped bring the open-loop toughness values closer to the closed-loop toughness values.

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.