Sheaths for Prestressed Concrete
A few decades have passed since the early ideas for prestressing concrete. Today the use of prestressed concrete is a tried and tested construction technique, indispensable for modern projects. The requirements for the prestressing processes are laid down in national and international regulations. This also includes the execution of prestressing work, including the applicable requirements for the companies that carry out the work and for specialized suppliers.
Accordingly, the requirements for sheathing pipes to hold prestressing steel are clearly defined and regulated in EN 523. Analogously to this European standard, the sheaths are subject to certain regulations relating to material and technical requirements. The high standards of quality of the HYDRA sheathing pipes are therefore maintained by monitoring through continuous production checks.
In order to document this quality standard independently, an extensive series of tests was carried out on the longitudinally welded pipes at the material testing institute at the University of Karlsruhe in 2008.
This affords us the chance and the security of applying the “CE mark” to HYDRA sheathing pipes in the future, and supplying our customers with the corresponding declaration of conformity. This in turn has that advantage of allowing our customers to document the required quality accordingly to construction supervisors and approval authorities.
Shape of the Profile
By means of an appropriate design, Hydra sheaths allow for an easy and total filling of the cavities between the tendons and the sheath walls with compressed mortar. This guarantees corrosion protection for the tendons and the connection required for the transmission of force from the tendon to the sheath as well as from the sheath to the surrounding structural concrete.
- Corrosion protection
- High expenditures in time and money
- Collecting of air bubbles to avoid their contact with the prestressing
- Sufficent shearing connection by sufficient profile heigth
- Short installation periods
Sheaths have to guarantee that
- no laitance intrudes into the prestressing duct during
- no mixing water is squeezed out during grouting since this
would cause segregation of the intrusion mortar components
- no leakage occurs even under various types of stress in harsh
con struction-site conditions,
- the sheath is free of buckling or indentations.
On the one hand, sheaths have to be sufficiently rigid toprevent their sagging between the support points, on theother hand, they have to be sufficiently flexible to adaptthemselves to the course of the tendon in the structure.Wherever sheaths are laid without internal stiffening andthe tendons are inserted after the concrete has hardened,longitudinal stiffness comes to the fore as a decisivecriterion. Flexibility is required whenever tendons aremanufactured in a factory and are supplied to theconstruction site on drums, as rings or in loops.
Resistance to Transversal Stress, Tensile Strength
Before being laid in structural concrete, sheaths aresubject to various types of mechanical stress. The wallthickness and the shape of the profile are decisive factorsin avoiding damage which will affect the proper functionof the sheaths, for example when the tendons areinserted into the sheath, when the sheath is installed inthe structure. In addition to these types of stress sheathsare also subject to tensile stress. The tensile strengththerefore is another important quality and testing criterion.
Corrosion Protection and Bonding
Profile shape, pitch and leakage resistance influence theflowing process of the intrusion mortar and reduce thedanger of irregular protrusions. The profile shape has toprovide sufficient space for the collection of air bubblesand water pockets in the corrugation crests in order tokeep them away from the prestressing wires. The pitchguarantees an optimum enclosing of the prestressingwires in the protecting mortar. The leakage resistanceprevents the segregation of intrusion mortar componentsdue to the squeezing out of mixing water.The interior surface of the sheaths have to be free of oiland grease films which are insoluble in water in order toensure a good bonding quality.