International Concrete Abstracts Portal

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.

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.

Steel fiber-reinforced self-compacting concrete (SFRSCC) was developed and applied on the manufacture of structural façade panels composed of a grid ribbed system covered by a layer of 30 mm thickness. Panel prototypes of this structural system were tested using loading configurations that promote the flexural and the punching failure modes in order to assess the benefits of fiber reinforcement to the flexural and shear resistance of thin SFRSCC structural systems. A smeared multi-fixed crack model, implemented into a FEM-based computer program, was used to simulate the deformational behavior of the tested panel prototypes up to their failure. The fracture parameters characterizing the SFRSCC post-cracking behavior were obtained from inverse analysis, using the data derived from three point notched beam tests. The punching failure mode was well captured by adopting a softening diagram for both out-of-plane shear components.

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.

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.