Application of Post Tensioned Concrete in Pakistan and its Comparison with Conventional Concrete

Abstract

The research project is industry based and is objectified at an application of Post tensioning of concrete in Pakistan and its comparison with the conventional concrete systems. Our work of research is limited to Unbonded Monostrand Post-Tensioned concrete floor in multi-storey Buildings (more specifically the two story Parking structure). Post-tensioning is a pre-stressing type in which steel tendons and cables/wires are positioned in a special made ducts or sleeves before the concrete is placed in a formwork. After that concrete is placed and gained enough strength, the tendons are stressed to a known magnitude and anchored at the ends. The tendons are coated with grease and bituminous material within the ducts to prevent the bonding of concrete with steel. Anyway the ducts or sleeves are enough to prevent the bonding of concrete with steel. Post tensioning systems has a vital importance in civil engineering world because of having many advantages over normal concrete systems. Importance includes long spans which causes the reduction of other structural elements like columns and foundations, also increasing its aesthetic and architectural nature. Minimization of thickness of floor system also provides the facility of ceiling for horizontal services and also reduces the self-weights and foundation loads. Deflections can also be controlled by constructing longer spans with minimum depths. Speed, efficient and rapid construction is achieved in multi-story buildings due to less congestion of slab construction. Layout flexibility in structure can be achieved as the design can cover with irregular grids and patterns. Majorly parking lots, bridges, multi storey buildings and tennis courts are covered by this technique of pre-stressing. The strength of concrete required to carry out the research was 5300 psi in 28 days and minimum strength of 2463 psi for 3 days. This was achieved in structural lab by the partial replacement of percentages of super plasticizer i.e. Glanium-51, in terms of ordinary Portland cement (OPC) while keeping the quantity of cement constant. The respective amounts of Glanium-51were 2%, 1% and 0.8% in terms of cement mass. Final selection was based on the slump and workability test as well as on the consideration of desired strength. Concrete was casted for the required number of samples and cured accordingly for required time period. Testing of each type of sample was done in three phases i.e. 3rd, 7th and 28th day. During each time of testing, we obtained more than desired results. The research was kept on to cover the comparison of Post tensioned concrete with conventional concrete regarding the generated moments in structural members, top and bottom stresses, required story heights, columns to columns spacing, total area of steel and volume of concrete, structural loads on foundations, safety and feasibility, and cost comparison and economy. Firstly, a model of conventional concrete system for a parking structure was studied for roof systems. A model was drawn on ETAB for beams calculations and another model on CSI-SAFE for slabs. Calculations were also evaluated manually by hand and cross checked to satisfy the results. In the end total area of steel and concrete as well as the structural stresses in members were calculated manually. As far as the Post tensioning is concerned, another model of given plan was generated on AutoCAD and exported to CSI-SAFE. A strip of structure was selected and its calculations of stresses and forces losses were studied as per ASTM criteria. Given the anchorage losses and friction losses according to the surroundings weather conditions and labor expertise, the force losses in tendons was manually evaluated and all the required data was compiled to compare it with the conventional concrete systems. Another software “Bentley RAM concept v8i” was also helpful for calculation of post tensioning concrete systems. In the end a clear comparison between conventional concrete systems and application of post tensioning of concrete was made and we got all those results for which our research was aimed at. There are large advantages of post tensioning of concrete systems in replacement of conventional concrete in terms of structural strength, stresses in members and as well as the cost and economy. Post tensioned concrete has a large number of advantages compared to conventional RCC construction. Post-tensioning construction is advancing more and more with increasing innovations and environment friendly materials. It is of course mandatory to use high quality materials which should be able to withstand the pre-stress. However, one thing is for certain, Post tensioned concrete is going to serve the building community for many more years to come given the numerous advantages it bears. As Engineers we have a responsibility our society to overview all methods of construction available. In some cases Post-tensioning will not be much suitable but it should be at least decently analyzed while considering other techniques.