Prestressed
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Pre-stressed concrete was a major innovation that replaced conventional reinforced concrete, allowing for longer spans, higher impact resistance, and greater load capacity without tensile stresses. It involves casting concrete around high-strength steel that is placed under compression before use to counteract tensile stresses when in service. There are two main types: pre-tensioning applies tension before casting, while post-tensioning does so after casting, using ducts to hold the steel. Pre-stressed concrete enables more efficient structures through factory casting and reduced material needs.
Prestressed
- 1. Abstract—pre-stressed concrete can be considered as one of the greatest innovations in the field of concrete construction. This innovation lead to replace the conventional method of concrete construction (reinforced concrete) with lots of merits mainly in terms of possibilities of making long span structures, ability to resist impact, high live load carrying capacity and its tensile stress free character. Pre-stressedconcrete members are pre-casted and are helpful in decreasing the time for completion of a work. This method is mainly used for large scale projects such as metro, flyovers where completion time is very crucial. This in turn leads to more effective structural members andeffective use of resources in a way which is less harmful to the environment. Index Terms—pre stress concrete, off site construction I. INTRODUCTION Used high tensile steelwires, with ultimate strength as high as 1725 MPa and yield stress over 1240 MPa. In 1939, he developed conical wedges for end anchorages for post-tensioning and developed double-acting jacks. He is often referred to as the Father of Pre-stressedconcrete. Eugene Freyssinet (France) II. WHAT IS PRE-STRESSED CONCRETE. Pre-stressed concrete is a form of reinforced concrete that builds in compressive stresses during construction to oppose those found when in use. So it works on an application of an initial load on the structure so as to enable the structure to counteract the stresses arising during its service period. This combination of steel and concrete that takes advantages of the strengths of each materials. A. Concept of pre-stressing. The concept of pre stressing was invented years ago when metal brands were wound around wooden pieces to form barrels. The metal bands were tighten under tensile stress which creates compression between the staves allowing them to resist internal liquid pressure. B. Principles of pre-stressing. Pre-stressing is a method in which compression force is applied to the reinforced concrete section. The effect of pre stressing is to reduce the tensile stress in the section to the point till the tensile stress is below the cracking stress. Thus the concrete does not crack. Possibilities of Pre-stressed concrete in construction (April 2016) Vivek Paul, SEM VI student, KMEA College of Architecture Pre-stressed concrete is basically concrete in which internal stresses ofa suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree. 1938 Hoyer, E., (Germany) Developed ‘long line’ pre-tensioning method. 1940 Magnel, G., (Belgium) Developed an anchoring systemfor post-tensioning, using flat wedges.
- 2. It is then possible to treat concrete as an elastic material. The concrete can be visualized to have two compressive force i. Internal pre-stressing force. ii. External forces These two forces must counteract Each other. III. MATERIALS USED FOR PRE-STRESSING. Pre-stressed concrete was started to be used in building frames, parking structures,stadiums,railway sleepers,transmission line poles and other types of structures and elements. Materials for pre-stress concrete member 1. Cement, 2. Concrete, 3. Steel. 1. Cement: Ordinary Portland cement, Portland slag cement, Rapid hardening Portland cement, High strength ordinary Portland cement. 2. Concrete: Pre-stress concrete requires high strength concrete, which has high compressive strength comparatively higher tensile strength than ordinary concrete. The concrete is a material should be compose of gravels or crushed stones,sand, cement. In pre-stress concrete minimum grade of concrete M20. 3. Steel: High tensile steel,tendons,strands.In pre-stress concrete high tensile steel with tensile strength around 2000MPa. According to IS: 1343-1980 pre-stress concrete is design Pre-stressing wire is a single Unit made of steel. IV. TYPES OF PRE-STRESSING. 1. Pre-tensioning In Pre-tension, the tendons are tensioned against some abutments before the concrete is place. After the concrete hardened, the tension force is released. The tendon tries to shrink back to the initial length but the concrete resists it through the bond between them, thus, compression force is induced in concrete. Pretension is usually done with precast members 2. Post tensioning. In post tension,the tendons are tensioned after the concrete has hardened. Commonly, metal or plastic ducts are placed inside the concrete before casting. After the concrete hardened and had enough strength, the tendons was placed inside the duct, stressed and anchored against concrete. Grout may be injected into the duct later. This can either done in factories or in site itself. Two, three or seven wires are wound to form a pre- stressing strand. A group of strands or wires are wound to form a pre-stressing tendon. A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire.
- 3. V. COMPARISON BETWEEN PRE-TENSIONED AND POST TENSIONED. VI. ADVANTAGES OF PRE-STRESSED CONCRETE. Factory products are possible. Long span structureare possibleso that saving of wt is significant & thus it become economical. Pre-stressed member are tested before use. Dead load are get counter balanced by eccentric pre- stressing It has high ability to resist the impact. It has high fatigue resistance. It has high live load carrying capacity. It free from cracks from service loads and enable entire section to take part in resisting moments. Member are free from the tensile stresses. VII. DISADVANTAGES OF PRE-STRESSED CONCRETE. Required skilled builders & experienced engineers. Initial equipment cost is very high. Availability of experienced engineers is less. Required complicated formwork. It requires high strength concrete & steel. Pre-stressed concrete is less fire resistant. VIII. ADVANTAGESOF PRE-STRESSED CONCRETE. Bridges Slabs in buildings Water Tank Concrete Pile Thin Shell Structures Offshore Platform Nuclear Power Plant Repair and Rehabilitations