ASSESSEMENT AND REHABILITATION OF STRUCTURES Course Code: CIVL524 Dr. Aeid A. Abdulrazeg

EXTERNAL PRESTRESSING

Introduction “Pre-stressed concrete is reinforced concrete in which the steel reinforcement has been tensioned against the concrete. This tensioning operation results in a self-equilibrating system of internal stresses (tensile stress in the steel and compressive stresses in the concrete) which improves the response of the concrete to external loads. ” There are two different ways of prestressing, pre-tensioning and post-tensioning. Pre-tensioning: Pre-tensioning is when the cables are stressed prior to casting of the concrete. The cables remain stressed until the concrete has cured and then they are released or cut.

Post-tensioning: Post-tensioning is when the cables are stressed on an existing structural element, for example after the concrete has cured if it is a concrete structure. The cables can be located both inside and outside of the structure. Here a mechanical anchorage in the end must be used to hold the cables in place and to keep the prestress active. An externally stressed cable is defined as post-tensioned. External tendons can be used in new structures but also on existing structures.

EXTERNAL PRESTRESSED REINFORCEMENT External prestressing refers to a post-tensioning method in which tendons are placed on the outside of a structural member. It is an attractive method in rehabilitation and strengthening operations because: It adds little weight to the original structure Its application poses little disturbance to users It allows the monitoring, re-stressing and replacement of tendons.

Advantages and disadvantages with external prestressing External prestressing, both for new and existing structures, has proven to be an effective technique. Concreting of new structures is improved as there are no or few tendons and bars in the section Dimensions of the concrete section can be reduced due to less space needed for internal reinforcement. Profiles of external tendons are simpler and easier to check during and after installation.

Grouting is improved because of a better visual control of the operation. External tendons can be removed and replaced if the corrosion protection of the external tendons allows for the release of the prestressing force. Friction losses are significantly reduced because external tendons are linked to the structure only at the deviation and anchorage zones.

Advantages and disadvantages with external prestressing But it is also important to understand the weaknesses of the technique. The following disadvantages should be kept in mind: • External tendons are more easily accessible than internal ones and, consequently, are more vulnerable to sabotage and fire. • External tendons are subjected to vibrations and, therefore, their free length should be limited. • Deviation and anchorage zones are cumbrous additions to the cross section. These elements must be designed to support large longitudinal and transverse forces. • In the deviation zones, high transverse pressure acts on the prestressing steel. The saddles inside the deviation zones should be precisely installed to reduce friction as much as possible and to avoid damage to the prestressing steel.

In the case of internal grouted tendons, the long-term failure of anchor heads has limited consequences because prestressing may be transferred to the structure by bond. In the case of external tendons, the behaviour of anchor heads is much more critical. Therefore, anchor heads should be carefully protected against corrosion. At ultimate limit states, the contribution of external tendons to flexural strength is reduced compared to internal grouted tendons. The stress variation between the cracking load and ultimate load cannot be evaluated at the critical section only, as is done for internal bonded tendons. At ultimate limit states, failure with little warning due to insufficient ductility is a major concern for externally prestressed structures. The actual eccentricities of external tendons are generally smaller compared to internal tendons.

Moment capacity