טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLeibovich Orit
SubjectLocal Interaction Between a Reinforcing Bar and Concrete
DepartmentDepartment of Civil and Environmental Engineering
Supervisors Professor Emeritus David Yankelevsky
Professor Avraham Dancygier
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

RC members are made of concrete and reinforcing steel rebars. A major factor that enables combined action of the concrete and the rebars is the ability to transfer loads between them. In deformed rebars it is based on mechanical interaction between the ribs of the rebar and the surrounding concrete (known as 'bond'). The rib applies an inclined force on the concrete, the longitudinal component in the rebar axis direction contributes to the pulling force resistance. The radial component produces a pressure strip on the concrete around the rebar and develops a global radial ring action.
Theoretical and experimental studies on the global longitudinal behavior provide qualitative and quantitative information on the global bond interaction. Most studies assume a continuous uniformly distributed "bond stress" along the steel-concrete interface. These arbitrary assumptions are essential to define "bond-slip relationships" that depend on the concrete properties, steel type and the "bond conditions”. 

The present research refutes the common assumption that the bond stress-slip relationship is a constitutive property of the interface and claims that it is an outcome of interaction between the rebar and the concrete, resulting from basic mechanisms that should be identified. The present investigation on the longitudinal global behavior identifies different bond-slip curves in their peak bond stress and descending branch slope depending on the concrete tensile strength (more moderate slope for lower tensile strength). Investigation of the global behavior in the radial direction examines the tangential strains distribution. This investigation demonstrates the ring action and supports derivation of a computational procedure to simulate the tangential strain-slip distribution. It identifies different strain-slip relationships depending on the concrete tensile strength that are in accordance with the above findings. The lower tensile strength curve remains constant at large slips compared to a descending branch in the higher tensile strength. Special effort was made to investigate the local behavior, using a new experimental method using a standard experimental setup with a cylinder specimen in which a "window" was opened. The "window" enables direct and continuous photography of the concrete near the rebar during a pullout experiment. Image processing provides digital images of displacement and strain fields and allows tracking local cracking development. Unique information is obtained shedding light on the interaction between ribs and concrete at all load levels.

The findings in the various experimental studies clearly identify two groups of specimens that differ in the distribution of the bond stress and tangential strain-slip curves. These groups are also different in the local interior damage and in the principal and shear strain fields and depend on the tensile strength. These findings confirm the assumption that the bond stress-slip relationship is not an interface property but a result of the interaction. Additionally, the local investigation indicated gradual participation of the ribs in the mechanism, when the resistance is concentrated around the ribs.

Much of the present research results and insights are new, and a major part of it is due to the innovative research technique developed in this study.