Table of Contents

Chapter1: Introduction2

1.1General Description:2

1.2Stages of Post tensioning4

1.3History of Post Tension:5

Chapter2: Basic information6

2.1 Benefits of the Post Tension6

2.2 Advantages of Post Tension6

2.3Disadvantages of Post Tension7

2.4Devices7

2.5. Review of Previous Work7

Chapter3: Methodology15

3.1 Experimental Program15

3.2 Test Setup and Instrumentation15

Chaprter 4: Expectations17

4.1. Test Results17

4.2. Shake Table Response of a Post-Tension Gravity Dam Model19

Chapter 5: Conclusion20

5.1. Load-Displacement Response of Small Joint Specimens and 3.4 m Dam Model20

5.2. Limitations22

References23

Appendix A: Figures27

Appendix B: Tables31

Post Tension

Chapter1: Introduction

General Description:

Post tensioning is a procedure for reinforcing concrete. Post tensioning steel cables are positioned in the forms before the concrete is placed. After pouring the concrete and it gets dry and gained strength without any extra service load are applied, the cables are pulled in a specific measurement, and anchored against the edge of the concrete. Pre-stressed concrete means that the steel is stressed, and is ready before pouring the concrete. While Post-tensioning means that steel is tensioned after the concrete is poured.

Post-tensioned anchors are widely used to strengthen existing concrete gravity dams with poor lift joints that are subjected to severe loading to conform to new design criteria. Several dams throughout the world were strengthened because of their material degradation, and 3 deficiency in design and construction, as well as for 4 more stringent safety standards Xu and Benmokrane 1996. Moreover, the need to maintain structural integrity for often increasing predictions of extreme loads, such as a probable maximum flood PMF and a maximum credible earthquake MCE, has required a number of rehabilitations.

Dams were thus post-tensioned to increase their stability and to control anticipated failure mechanisms such as concrete cracking, sliding, and overturning. Post-tensioned cables were also used to increase, temporarily or permanently, the postseismic stability of some dams damaged by strong earthquakes; the dams include the Koyna gravity dam 103 m, India, 1967, Menjil buttress dam 106 m, Iran, 1990, and Pacoima arch dam 113 m, U.S., 1971, and 1994. However, there is no report on the seismic response of post-tensioned dam components, that have been subjected to severe earthquakes. There is therefore a lack of historical evidence regarding the seismic behavior and potential failure mechanisms of post-tensioned gravity dams.

In the case of a severe earthquake, controlled concrete cracking is accepted for dams. Cracks generally develop along lift joints in the upper part of the dam body and at the dam-foundation interface. Thus after cracking, simple rigid body motion of blocks restrained by post-tensioning can be used to investigate the dynamic stability. Post-tensioned cables should then limit sliding and rocking displacements to acceptable levels. Fig.1 gives the seismic response of post-tensioned dams and their anticipated failure mechanisms. To design the cables, the effect of post-tension is most often represented by an external clamping force, thus increasing the normal load on the sliding/rocking failure surface. However, in a comprehensive safety reevaluation study of existing post-tensioned dams, knowledge of the complete load-displacement response up to the failure of post-tensioned concrete-concrete lift joints is required to determine the load carrying capabilities of the dam and its displacement limitations. The behavior is influenced ...