טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentErez Berkover
SubjectFlexural Static Behavior of Normal Strength Reinforced
Concrete Beams with Steel Fibers
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Dancygier Avraham
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

This research deals with the “cracking localization” phenomenon in reinforced concrete with steel fibers (SFRC) beams. When the force in the longitudinal rebars was larger than their yield force, only a single crack, out of all the bending cracks which appeared at the service state, continued to widen significantly, much more than the other cracks did. This widening caused high local strains in the rebars that crossed that crack, up to their rupture, which led to failure of the beam. This phenomenon had been previously observed in high strength SFRC beams and the main objective of the current experimental study was to examine whether this phenomenon is resulted by the concrete strength. Therefore, the current research included a series of bending tests, in which the effect of adding steel fibers to normal strength concrete (NSC) beams was studied.

An additional aim of the research was to evaluate the prediction of the moment capacity of SFRC beams by common design procedures (EC2), which do not take into account the effect of fibers, and by calculation models that consider the effect of fibers in tension (RILEM).

The experimental program included a series of fourteen, four-point bending tests of NSC beams with and without steel fibers with different reinforcement ratios. The fibers were hooked-ended in a constant amount of 60 kg/m3 (0.76% in volume). Measurements included the load and mid-span deflection, and the curvature along the beam. The experimental results showed that there was a reduction of the flexural ductility in the NSC beams with the lower reinforcement ratio (0.28%) and steel fibers, which was accompanied by cracking localization. Furthermore, as the amount of conventional reinforcement was increased, cracking localization decreased. This result was manifested by increasing numbers of cracks that widened more-or-less uniformly, which led to larger beam deflections and larger flexural ductility. A possible explanation to the “crack localization” phenomenon can be related to the non-uniform distribution of the fibers along beam, which is likely to have a stronger effect in the presence of lower amounts of conventional steel.


The influence of adding steel fibers on the load carrying capacity of the beams in which fibers were added, was more significant in specimens with lower reinforcement ratios and was less pronounced in beams with the moderate and high reinforcement ratios.

Evaluating the moment capacity using the EC2 model had a very good agreement with the measured capacities of the control specimens and with the specimens that included relatively large reinforcement ratios. However, the results overpredicted the capacities of the beams that included smaller amounts of reinforcement, where the effect of fibers was more pronounced.

The RILEM model overpredicted the measured capacity by 35% but for the moderate and high reinforcement ratios, the calculated load carrying capacity of the beams was very close to the measured capacity. This result shows that the RILEM model is not accurate for the assessment of the effect of adding steel fibers to the load carrying capacity for the lower reinforcement ratios.