טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentNavon Zeev
SubjectTensile Behavior of a Steel Fiber-Reinforced Concrete Bar
Containing Conventional Reinforcement
DepartmentDepartment of Civil and Environmental Engineering
Supervisors Professor Avraham Dancygier
Dr. Yuri Karinski
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

This research deals with the phenomenon of cracking localization in a tensile steel fiber-reinforced concrete bar containing conventional reinforcement. The purpose of adding steel fibers to concrete mixtures is to increase the material toughness, and it is therefore expected to improve also the structural ductility and lead to a better crack control in the element.

Yet, it was also found that reinforced concrete beams that included both conventional reinforcement and steel fibers (R/SFRC beams) with low amounts of conventional reinforcement have reduced ductility, compared with similar, but plain RC beams. This phenomenon was characterized by significant widening of a single or few cracks at increased loads beyond yielding of the rebars, and it is therefore denoted as “cracking localization”. While cracking localization was observed in R/SFRC members with relatively low amounts of conventional reinforcement, it diminished at higher reinforcement ratios, where similarly to plain RC members all cracks widened more-or-less uniformly.

This Thesis presents an experimental study that was aimed at verification and quantification of the cracking localization phenomenon in R/FRC bars and the verification and calibration of a previously developed analytical model of this phenomenon. The first, out of two experimental programs, included control RC specimens (without fibers), and specimens with a fiber content of 60 kg/m3. For each type of mixture, with and without fibers, the conventional reinforcement consisted of a single deformed rebar, where the reinforcement ratios ranged from 0.79 to 4.91%. These specimens were tested under axial tension up to post-yielding of the rebar.

            Results of the tests further support the existence of the cracking localization phenomenon and the trend of its decrease with the increasing of the reinforcement ratio.

The second experimental program included a series of tests described in this Thesis, which were aimed at a quantitative examination of a probabilistic model that had been proposed for the fibers distribution along a reinforced concrete bar element. The tests comprised twelve square cross-sectional bars, made of fiber reinforced concrete with centrally located deformed rebars of different diameters, as well as three specimens without conventional reinforcement. Each specimen was sawed into equal slices with a width equal to the length of a single fiber, and then the fibers that had been exposed in the cross-sections were counted. The statistical analysis included calculations of the mean values and the standard deviation of the fibers scatter along the specimens. One of the assumptions of the model refer to the fibers distribution in the presence of a rebar. This particular assumption that was made in the model was not trivial and not obvious prior to the tests. The empirical results prove its validity, and additionally, a good agreement was obtained between the cumulative distribution functions predicted by the model and those obtained from the fibers count.

            Further study and quantification of the cracking localization phenomenon may lead to setting a minimum reinforcement ratio, which is required to prevent cracking localization in R/FRC members, especially beams.