These standards require the post-tensioned concrete structure to have a bare anchorage and must be made up of galvanized steel ducts which bear duct taped joints. The prepressing strands must be bare and the grout must be cementitious and be adequately protected.
A more comprehensive analysis of the durability of the post-tensioned concrete was later carried at the University of Texas at Autin.This was concentrated on the methods of improving the durability of the standard bridge decks by means of a transversal prestressing system (Poston et al., 1997) .The finding were that the use of grouted and galvanized ducts resulted in better and adequately protected tendons that lie between the various anchorage points. It was also found that thin concrete covers provided enough protection against corrosion. In fact it was recommended by the authors that complete encapsulation of the post-tensioning concrete components be provided for effectiveness in increasing the structural durability of the component. The University of Texas experiment also highlighted the negative role that the cracking of the concrete has on the corrosion rate of the component/structure.The effect of cracking of concrete was found to be more severe on the mild reinforcement in the system of slabs. The penetration of chloride ions was also found out to be reduced or rather hampered wherever the cracks had a limitation in terms of their width.
Later on, it was found out that the following are bound to be achieved on the post-tensioned concrete systems.
The test have revealed that for various forms of external tendon systems, there exists an uncertain level of protection that can be observed on the grout whenever there is a compromise of the grout.The second observation is that the installation of plastic duct system in the internal tendons has the effect of eliminating the problems associated with secondary durability such as staining and spalling.
The tests have revealed that steel ducts are capable of providing a comprehensive and long-term protection against corrosion. Finally, the test revealed that the use of various forms of fluidizing admixtures together with pozzolans aid in the improvement of the grouting operations and the overall effect of increasing the protection of the grout's value.
Post-tensioned concrete has been utilized for over than forty years in the United States in a wide ranger of construction projects (Crigler,2007).. Initially employed in the construction of bridges its applications have by now increased far beyond the construction of bridges and now encapsulated the building of tanks, offices condos, hotels, parking garages, pavements, various forms of masonry, durable seismic walls, family homes among many other applications. The fact that post-tensioning can be effectively combined with other structural materials coupled with he fact that it has employed in the strengthening of steel, reinforcing of concrete, applied in masonry and the strengthening of other timber structures, together with the enhancement and extension of the basic capabilities of precast, pre-tensioned structures, the techniques of utilization will ever increase.
Salas et al. (2004) found out that in order to have better durability through the adoption of efficient and high performance grouts. In a study that they carried out at center for Transportation Research at the University of Texas at Austin, they realized that certain small alterations in the composition of the grout can result in an increase of the durability of the post-tension concrete. They found out that a thirty percent fly ash which contained approximately 0.35 water content resulted in a very excellent performance of post-tensioned concrete on a horizontal ground. They also realized that a 2% antibleed grout that contained approximately 0.33% water had very commendable performance in various vertical applications. This in turn rendered the standard and average grout to be below average in terms of its performance. Their study revealed that the voids in the grout that were caused by the entrapped air pockets, bleed water and the incompletely filled grout fluidity resulted in very detrimental scenario. This adversely affected the prestressing strand and the galvanized duct too.Grout that is never prepressed coupled with the dominant grout cracks in it became more evident on loading.In order to provide better and enhanced long-term strand structure it is necessary to provide adequate corrosion protection.
Further research by Salas et al. (2004) shown that the quality and treatment that is given to the internal post-tensioned ducts is very crucial in creasing the durability of the post-stressed concrete structures. Their analysis pointed out the weaknesses in galvanizing the steel ducts. They found out that there is superior performance recorded from the use of plastic ducts. In order to provide better corrosion protection, completely epoxy-filled joints coupled with unspliced plastics ducts are to be employed.
In order to avoid the cracking at the joints, it is necessary to take a number of steps to curtail the negative effects of these cracks. Studies have shown that transverse cracking has very negative effects on the various forms of corrosion of the post-tensioned concrete structure. This is because the existence of very large crack widths on the concrete structure results in very severe cases of localized and highly uniform corrosion incidents. The existence of longitudinal and splitted cracks on the concrete surface act as a sure indicator of the very severe corrosion that occurs within the member. The existence of dry joints and epoxy joints that are incompletely filled results in very poor performance of the member.
The next factor that must be considered in the improvement of the durability of the post-tensioned member is the level of post-tensioning. Studies have shown that as the level of post-tensioning is increased, there is a corresponding increase of the corrosion protection action. This is also has an effect of providing improved wicking resistance.
In order to provide better durability to the post-tensioned concrete meber, it has been found that a proper selection of the right kind of concrete can do the trick of increasing durability.This is because the high performance concrete has the effect of reducing the penetration of chloride into the member.This in turn increases corrosion resistance. The thickness of the concrete cover has also been identified to have a direct influence on the durability of the post-tensioned steel members. This is because it results in improve performance as a result of better encapsulation (Less corrosion)
Breen, J.(1999).State of the Art Durability of Post-Tensioned Bridge Substructures,
CSA (1990) CSA Standard A23.1-M90, A23.2-M90, Concrete Materials and Methods
of Concrete Construction, Methods of Test for Concrete, 1990
Crigler, J (2007).Post-Tensioning Revisited. Structure Magazine, July 2007
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Salas, R, Andrea Schokker, A, West, J Breen, J and. Kreger, M (2004).Durability Design
of Post-Tensioned Bridge Substructure Elements: Summary; Center for TransportationResearch, The University of Texas at Austin;September 2004
Olson, G and Smith L (1997).Post-tensioned concrete for today's market