International Concrete Abstracts Portal

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.

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.