This paper presents an analytical model for simulating the bond between steel and concrete, in precast prestressed concrete elements, during the prestressing force release. The model establishes a relationship between bond stress, steel and concrete stress and slip in such concrete structures. This relationship allows us to evaluate the bond stress in the transmission zone, where bond stress is not constant, along the whole prestressing force release process. The model is validated with the results of a series of tests and is extended to evaluate the transmission length. This capability has been checked by comparing the transmission length predicted by the model and one measured experimentally in a series of tests.
Keywords:Bond, prestressed concrete, transmission length, modelling, bond strength
IntroductionPrecast prestressed concrete elements are widely used for construction in Europe. A frequent problem of the precast industry is to evaluate the real transmission length in precast prestressed concrete structural elements. The semi-empirical formulae proposed by codes are usually thought for conventional concrete and usual cast conditions, but high performance concrete (high-strength, self-compacting, etc.) and non-usual cast conditions (v.gr. accelerated curing processes) are becoming more and more frequent. In these cases experimental measurement is needed, though the standardised methods are expensive and so difficult to apply by industry. Analytical and numerical models, based on parameters measured experimentally with tests being simpler than complete transmission length tests, would be welcomed. This paper presents an analytical model for steel-concrete bond when the prestressing force is transmitted by releasing the steel (wire or strand). The model is applied to evaluate the transmission length.
Theoretical backgroundThe prestressing process of the precast prestressed concrete includes: a) steel wire prestressing, and b) prestressing force transmission after an accelerated curing process of concrete. Let be a prism of concrete with a single prestressed wire placed in the prism longitudinal axis (Fig. 1). Be P 0 the initial prestressing force in the wire and, for a given instant, the force at the end of the wire P 0 -∆P,