Adaptive prestressing of concrete beams

The design of concrete structures has gradually evolved from unreinforced concrete, to reinforced concrete, to more slender prestressed concrete structures. The prestressing system has many advantages over the earlier passive reinforced system but has three key disadvantages. Firstly, the amount of stress in the steel tendon reduces with time due to various losses such as relaxation of the steel and creep of the concrete. Secondly, the criteria which governed the maximum amount of prestressing force that can be used in an element are often limited by the stresses that develop as a result of the prestressing operation during fabrication before the component can be fully integrated into a structural system and before any super-imposed loads are applied. Thirdly, prestressed concrete beams often have much less ductility at failure than the reinforced concrete system which is undesirable in some applications. Therefore, the prestressed system while highly effective is not fully optimized.;The concept of adaptive prestressing was explored experimentally and theoretically in this study. The mechanics of an actively controlled structure were investigated using a total of 9 beams. Traditional post-tensioning was applied to some of the beams so that a base line could be developed for the results. Adaptively controlled prestressing was applied to the beams using hydraulic actuators. This allowed the prestressing force to be adjusted during the testing to balance externally applied loads. It was found that by using active control it was possible to increase the capacity of the sections, increase the cracking moments and reduce live load deflections. It was also possible to achieve similar strength and serviceability performance on smaller cross-sections. Several control methods were investigated to determine the most effective methods for implementing adaptive control and to examine if the ductility of a beam could improved. Current design equations were compared against the results that were obtained from the experimental testing.;The next step in this evolution of concrete design will be to actively control the amount of prestressing force that is applied to a structure in response to the amount of load at any given time creating an adaptive prestressing system. By actively controlling the prestressing force it is possible to maximize the benefits of a prestressed system while reducing or eliminating the disadvantages stated above.