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Post tensioned concrete forms one of the most important mixtures that form the basis of most structural engineering works. Extant literature has been dedicated to the study of post-tensioned concrete. In this paper we analyse the various forms and types post-tensioned concrete and the ideal mix ratio that result in the most durable post-tensioned concrete. Our major aim is to improve the performance of post-tensioned concrete structures through the use of optimal design in terms of material and shape. This paper starts with the basic definition and introduction to the concept of post-tensioned concrete structures. It then proceeds to the study of relevant literature that concerns post-tensioned concrete structures with a particular focus to smart modelling. This is then followed with a recommendation on the best design parameters and conditions for the production of the highest quality/best pre-tensioned concrete structure. A conclusion is then drawn from the prescribed recommendations.
The process of designing a structural building system involves a careful selection and design by both engineers and architects and a proper understanding of materials that are used. The appropriate utilization of structures such as post-tensioned concrete coupled with their possible the effects on the durability and stability of the whole structure to be built is one of the most important concepts for the designers. A proper analysis and assembly of the concrete structures coupled with high quality materials that can provide a unique, robust structure results to unprecedented superior combination of material for better durability, sound control and standard fire safety that is highly required in the contemporary building market. A consideration of the the current construction market and global economics highlights the need to pay great attention to factors such as the construction cost, material supply process and the averagely reduced floor-to-floor heights.The factors combined with the lack of proper developer financing mechanisms leads contractors to select concrete as the more cost effective material to use in construction over steel (Olson and Smith,1997).
Technological advancement is one of the main drivers for the continuing increase in the use of post-tensioned concrete systems (Crigler,2007). This is because the use of technology has continually expanded the limits of durability of structures that are designed and developed using post tensioned concrete.
How post-tensioned concrete work
It is paramount to understand what post-tensioned concrete is composed of and how it achieves its working condition. Concrete in itself is very strong under compression and weak in terms of its tensile properties; steel on the other hand is very strong in tension while relatively weak in compression. However, the framing members in all construction structures must resist the various loads through a combined action of both compressive and tensile forces. Various forms of concrete framing are reinforced with steel in order to improve their load bearing capabilities. The development of post-tensioned concrete was therefore to make up for concrete's structural weakness under tension through the imposition of a permanently compressive load on its main structural members. This type of concrete arrangement is made up if extremely durable steel tendons with increased strength. This arrangement in conjunction with several reinforcing steel bars are then embedded and then securely anchored in the solid concrete. After the concrete has attained adequate strength, a process which occurs in about 3 or 4 days from the moment of placement, the steel tendons are the appropriately tensioned through the imposition of a compressive force on the concrete structure. Later on, tendons remain would stressed all through the life of the structure thereby producing a counterbalancing tension for future loads. The tendons together with the usual rebar reinforcing allows the s the engineer to come up with a shallower design for the structural members which posses an increased load capacities with a characteristically small deflection under loading.
Benefits of Post-Tensioned Concrete
Post-tensioned structures provides several advantages. Among these advantages are reduced dead load coupled with member depth due to the decreased amount concrete that is needed. The other advantage is the better control of deflection with a far greater crack control. Capabilities. The better crack control leads to improved durability which is an added merit. Apart from these merits, post-tensioning gives room for increased span to depth ratios. A fact that has the effect of lowering the building heights and hence leading to a reduction of both the heating and cooling volume of the buildings. The overall achievement is a decrease in the façade area of whole the structure.
For developers and owners of commercial buildings, the advantages of post-tensioning can make it a preferred reinforcing system. "Since the slab thickness is reduced, a developer building a high rise structure can easily add more floors without increasing the overall building height," said Scott Greenhaus, president of VSL. "Over the course of the building's life, this can represent significantly increase leasing revenue for the owner."
A traditional reinforced concrete building with two-way slabs requires more concrete and thus, more weight. As a lighter alternative, post-tensioned slabs require less concrete to achieve the same performance, thereby creating a structure with fewer shear walls, smaller columns and lower foundation loads. This results in more durable, efficient structures with longer clear spans.
"For commercial building operators, residential developers and hotel operators, they can have more usable space within the building envelope," Greenhaus noted.
Further, in corrosive environments, encapsulated bonded systems offer significant design advantages that lead to life-cycle savings. Because the amount of mild steel is reduced, particularly at the top zone of slabs, there is less steel to corrode should the concrete crack or spall. This is particularly important in parking garages where significant maintenance costs are due to repairs associated with spalled concrete from corroded rebar.
Another advantage of bonded post-tensioning is the inherent capacity to provide resistance to progressive collapse. This may be especially important in the event of localized blast loading. Like mild steel reinforcement, a bonded post-tensioning tendon is capable of developing its force in a relatively short distance along its length. In the event that an anchorage fails or a tendon is severed, the loss of tendon force would be localized. The remainder of the tendon would retain its force at the development length away from the failure point and would remain functional. This functionality can be considered in the design of a structure.
Bonded post-tensioning systems also allow for flexibility when future modifications to the building are needed. Tenant build-outs, remodeling and changes in a building's use may require modifications to the floor slabs. The use of bonded post-tensioning systems has allowed owners the flexibility to make these changes quickly, easily and cost-efficiently.
Both the bonded together with the unbounded systems can be combined within a given structure. The overall effect is the achievement of .perfect and durable design. An example of how bonded and unbonded systems were utilized together for the achievement of better economics, design efficiency and construction requirements is the W Victory Hotel & Residences located in Dallas, Texas. The building's concrete frame structure included a mixture of both monostrand and unbonded post-tensioning systems coupled with bonded and multistrand post-tensioning concrete systems. In the construction,the unbonded post-tensioned systems were utilized at the typical levels. However the bonded post-tensioning systems were used and implemented in the girders's transfer on three levels so as to give optimal crack and deflection control;elements that are paramount for transfer of the girders necessary for carrying the loads from the architecturally multi-story structure. The bonded post-tensioning systems were also utilized in the exterior applications that were susceptible to corrosion attack.
Post-tensioning undergone a lot of development and improvements over the last fifty years, this has the effect of improving and giving rise to the current methods that now serve as the most significant feature in the major construction industry.
Durability of post-tensioned concrete
Several studies have been devoted to the study of the durability of tensioned concrete. As an illustration we can point out the various numerous and extensive studies that were carried out by the U.S Army Corps of Engineers who conducted various exposure test with the aim of testing the effects of tides on the post-tensioned concrete structures.The test was carries out in a tidal flat in main(Treat island).The results was desirable since all the test specimens exhibited good qualities since they had no indication of a deterioration in their service integrity.The fact tha there has been no indication of a deterioration in post-tensioned concrete structure under exposure to a decade long period of cyclic freezing conditions and chlorides is in deed a great performance(Freyermuth,1991)
Another report that was compiled by the NCHRP in 1989 gave the result of exposing post-tensioned concrete beams to a year long test of corrosion protection (Perenchio et al 1989). In the test, very severe and highly accelerated corrosion tests were carried out on numerous corrosion protection systems. Perenchio discovered that ducts that were coated with epoxy gave the best performance of corrosion resistance.It is worth noting that the North American post-tensioned concrete industry is made up of structures that have a different post-tensioned structure standards. 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 percent 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 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)