Behavior of spirally prestressed concrete columns

Experimental work was performed to verify the initial idea of using a spiral post-tensioning system for creating an active confined pressure. Three spirally prestressed concrete cylinders were tested to study their strength and behavior under mid-span loading. Each specimen had four clockwise and four counterclockwise post-tensioning strands each making a complete revolution from one end of a specimen to the other. Results were compared to those of a nominally identical control specimen consisting of a similar concrete cylinder with the same number of longitudinally aligned post-tensioning strands. The specimens were designed to study the effect of: (1) the orientation of prestressing strands, and; (2) the percentage magnitude of applied strand prestressing force. A finite difference method was used for determining the capacity of specimens for setting up a test program. This method was simple and readily gave results based on approximating and simplifying assumptions. The finite element method which including a concrete plastic model, was performed to more closely study the behavior of spirally confined concrete. Using a system identification approach, a concrete model was calibrated by adjusting plastic parameters to correspond to known triaxial behavior. Subsequently the finite element model was compared to the test results to confirm that the concrete plastic model could be used for parametric studies. Finite element analysis was used to perform a parametric study for uniaxial compression tests. This parametric study focused on determining the uniaxial compressive model of concrete cylinders under spiral prestress confinement. A series of variables and parameters were set up to determine the relations of those parameters to the uniaxial compressive strength of concrete cylinders under spiral prestressed confinement.