slides construction engineering 2024.pptx
- 1. CONSTRUCTION TECHNOLOGY (CIVE 3405) PROJECT 2: A STUDY ON THE PRE-STRESSED CONCRETE AUTHORS: SIVAPRAKASH (202022090) KEERTHY (202022126) NOR ALI MOHAMED (202922095) YUVANESWARAN (202022091)
- 2. 1.0 INTRODUCTION • 1.0 BACKGROUND OF THE STUDY Prestressed concrete is a form of concrete used in construction. It is substantially "prestressed" (compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service. When compared to simple reinforced concrete, prestressed concrete allows for longer spans, reduced structural thicknesses, and material savings. High-rise structures, residential slabs, foundation systems, bridge and dam structures, silos and tanks, industrial pavements, and nuclear containment structures are all examples of typical applications. • 1.1 OBJECTIVES OF THE STUDY Too get to know what is Prestressed concrete To identify the importants of the prestressed concrete in modern day construction • 1.2 SCOPES AND SIGNIFICANCES OF THE STUDY To understand what is Prestressed concrete To understand concept of prestressing To get to know pretensioning equipment and procedures to use it To identify the differences between pretension and post tension concrete To determine the application of prestressed concrete
- 3. 2.0 PRE-STRESSED CONCRETE • Although prestressed concrete was patented by a San Francisco engineer in 1886, it did not emerge as an accepted building material until a half-century later. • The shortage of steel in Europe after World War II coupled with technological advancements in high-strength concrete and steel made prestressed concrete the building material of choice during European post-war reconstruction. • North America's first prestressed concrete structure, the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania, however, was not completed until 1951. • The high tensile strength of steel is combined with the considerable compressive strength of concrete in traditional reinforced concrete to create a structural material that is strong in both compression and tension. • Prestressed concrete works on the idea that compressive stresses produced in a concrete member by high- strength steel tendons before loads are applied balance tensile stresses imposed in the member during service. • Post-tension removes many design limitations that traditional concrete imposes on spans and loads, allowing the construction of long unsupported span roofs, floors, bridges and walls. • This allows architects and engineers to design and build lighter, flatter concrete structures without sacrificing strength.
- 4. 2.0.1 COMPRESSIVE STRENGTH ADDED • Compressive stresses are induced in prestressed concrete either by pretensioning or post-tensioning the steel reinforcement. • Before the concrete is poured, the steel is stretched. Steel tendons are stretched to 70 to 80 percent of their maximal strength and inserted between two abutments. • The tendons are encased in concrete and allowed to harden in moulds. Stretching forces are released once the concrete has reached the appropriate strength. • Tensile tensions are transferred into compressive stresses in the concrete as the steel reacts to restore its former length. Roof slabs, piles, poles, bridge girders, wall panels, and railroad ties are common examples of pretensioned concrete products. • The steel is stretched after the concrete solidifies in post-tensioning. Concrete is poured all around the steel, but not in contact with it. Ducts are frequently produced in concrete units using thin-walled steel forms. • The steel tendons are inserted and stretched against the ends of the unit and anchored off externally once the concrete has hardened to the required strength, putting the concrete into compression. • Bridges, huge girders, floor slabs, shells, roofs, and pavements are all constructed with post-tensioned concrete.
- 5. 2.0.2 CONCEPT OF PRESTRESSING • As previously stated, prestressing is the application of an initial load to a concrete structure so that it can withstand or counteract the strains caused by service loads. The use of a barrel as an example clarifies the notion. • Metal bands tightly bind a barrel used to transport liquids and grains in the past, as seen in figure-1. These metal bands are so closely fitted that they form a hoop around the barrel. • Hoop tension is created when this barrel is filled with liquid. The metal bands help to counterbalance the hoop tension caused by the fluid within by compressing the hoop. • Before the concrete structure is subjected to any service loads, effective internal stresses are created into the concrete using tensioned steel bars. External pressures are counteracted by this stress.
- 6. 2.0.3 NEED FOR PRESTRESSING CONCRETE The need for prestressing in concrete can be justified by the following issue Concrete is weak in tension and strong in compression. This is a weak point of concrete that results in early flexural cracks mainly in flexural members like beams and slabs. To prevent this, the concrete is induced with compressive stress deliberately (prestressing) and this stress counteracts with the tensile stress the structure is subjected to during service condition. Hence the chances of flexural cracks are reduced. 1. The pre-compression that is induced as a part of prestressing helps to enhance the bending capacity, the shear capacity and the torsional capacity of the flexural members. 2. A compressive prestressing force can be applied concentrically or eccentrically in the longitudinal direction of the member. This prevents cracks at critical midspan and supports at service load. 3. A prestressed concrete section behaves elastically. 4. The full capacity of the concrete in compression can be used over entire depth under full loading in the case of prestressed concrete. METHODS OF PRESTRESSING The prestressing can be performed by two methods: 1. Pretensionong 2. Post-Tensioning
- 7. 2.1 PRE-TENSIONING • Pretension in concrete is the method when the concrete is prestressed with tendons before the placing of the concrete. and it is a suitable method for small structural elements. • the pre-tensioning members are produced in the mold. It is achieved by either pre-tensioning or posttensioning processes. • In pre-tensioning, lengths of steel wire, cables, or ropes are laid in the empty mold and then stretched and anchored. • After the concrete has been poured and allowed to set, the anchors are released and, as the steel seeks to return technology with the introduction of pre-tensioning. • In this process, the reinforcing wires were stretched in tension, and the concrete was poured around them; when the concrete hardened, the wires were released, and the member acquired an upward deflection and was entirely in compression. • When the service load was applied, the floors and roofs are usually pretensioned, another prestressing technique, which is similar in principle to posttensioning. • The reinforcement is again steel wire, but the wires are put into tension (stretched) on a fixed frame, formwork is erected around the taut wires, and concrete is poured into it. • Pre-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned prior to the concrete being cast. • The concrete bonds to the tendons as it cures, following which the end-anchoring of the tendons is released, and the tendon tension forces are transferred to the concrete as compression by static friction. • Pre-tensioning is a common prefabrication technique, where the resulting concrete element is manufactured remotely from the final structure location and transported to site once cured. • It requires strong, stable end-anchorage points between which the tendons are stretched. • These anchorages form the ends of a "casting bed" which may be many times the length of the concrete element being fabricated. • This allows multiple elements to be constructed end-to-end in the one pre-tensioning operation, allowing significant productivity benefits and economies of scale to be realized.
- 8. • Pre-tensioning is accomplished by stressing wires or strands, called tendons, to predetermined amount by stretching them between two anchorages prior to placing concrete as shown in fig.1. • The concrete is then placed, and tendons become bonded to concrete throughout their length. After concrete has hardened, the tendons are released by cutting them at the anchorages. • The tendons tend to regain their original length by shortening and in this process transfer through bond a compressive stress to the concrete. • The tendons are usually stressed by the use of hydraulic jacks. • The stress in tendons is maintained during the placing and curing of concrete by anchoring the ends of the tendons to abutments that may be as much as 200m apart. • The abutments and other formwork used in this procedure are called prestressing bench or bed. • Most of the pre-tensioning construction techniques are patented although the basic principle used in all of them is common and is well known. • In pre–tensioning the reinforcement, in the form of tendons or cables, is stretched (put into tension) across the concrete formwork before the concrete is placed. • After the concrete has hardened and a suitable strength developed, the tendons are released. • A compressive force is therefore induced into the concrete.
- 9. 2.1.1 PRE-TENSIONING EQUIPMENT AND PROCEDURES • Pre-tensioning, is the tenn used for the process of making prestressed concrete in which the prestressed reinforcement is stressed before the concrete is placed. • Pretensioned concrete is most commonly made in Penn anent precasting plants, but, on single construction projects that include a large quantity of pre tensioned members, it has been found feasible to construct a pre- tensioning facility on the job site and amortize it on the one project. • Penn anent pre-tensioning facilities were first constructed in the United States in the early 1950s. • Like the early producers of pretensioned concrete, most present-day manufacturers do not limit their products to prestressed concrete but produce other precast concrete products as well. • The number of plants and the volume of pretensioned products made in them have steadily increased over the years. • The precast prestressed-concrete producers constitute an important part of the U.S. construction industry through their involvement in the production of structural members for buildings, bridges, and piers, as well as structural products such as piling and railroad ties. • Many manufacturers of prestressed concrete structural products make architectural concrete products as well. • The stress in pre-tensioning tendons must be maintained as nearly constant as possible during placing and curing of the concrete. This can be accomplished in two ways: • (1) The tendons are stressed and anchored to individual steel molds designed to withstand the prestressing force as well as the stresses resulting from the plastic concrete. • (2) The tendons, after being stressed, are restrained by a special device, called a pre-tensioning bench or bed. Pre-tensioning benches also provide a level surface on which the concrete forms are supported. In addition to these devices, other equipment peculiar to pretensioned construction, including the mechanisms used to stress and release the prestressing tendons, the forms, the vibrators, and the tendon deflectors.
- 10. 2.1.2 PRETENSIONING WITH INDIVIDUAL MOLDS • Apart from a few firms that have employed stress-resisting molds in the manufacture of double-tee roof slabs and pretensioned spun piles, this technique has not received wide use in the United States. • In plants where pretensioned concrete railroad ties and small joists for residential construction are produced, this method has the advantage of allowing individual units to be mass-produced, with products (and molds) moving through the plant in a production cycle, rather than requiring the materials and plant be brought to the molds or forms, as is done when pre-tensioning benches are used. • Another advantage of this method, when employed on small pretensioned products, is that the prestressing plant need not be as large and as elongated as that required in using a conventional pre-tensioning bench because small pretensioned products in their individual molds can be stacked and need not be arranged in long rows. • This advantage applies, but to a lesser degree, with large products that can only be handled with very large cranes and cannot be stacked very high, if at all.
- 11. 2.1.3 PRETENSIONING BENCHES • Pre-tensioning benches normally are designed to withstand a specific maximum force applied at a specific maximum eccentricity. • Therefore, it is customary, when establishing the capacity of a pre-tensioning bench, to give the maximum permissible force (shear) and maximum permissible moment that the bench is to safely withstand. • The maximum moment normally is expressed in terms of the bench proper (slab portion of the bench, which extends between the uprights at the abutments) and not necessarily in terms of the top surface at the abutments, which may be recessed to accommodate the stressing mechanism.
- 12. 2.2 POST TENSIONING • Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast. • The tendons are not placed in direct contact with the concrete, but are encapsulated within a protective sleeve or duct which is either cast into the concrete structure or placed adjacent to it. • At each end of a tendon is an anchorage assembly firmly fixed to the surrounding concrete. • Once the concrete has been cast and set, the tendons are tensioned "stressed" by pulling the tendon ends through the anchorages while pressing against the concrete. • The large forces required to tension the tendons result in a significant permanent compression being applied to the concrete once the tendon is "locked-off" at the anchorage
- 13. • The method of locking the tendon-ends to the anchorage is dependent upon the tendon composition, with the most common systems being "button-head" anchoring for wire tendons), split-wedge anchoring for strand tendons, and threaded anchoring for bar tendons. • A T-shaped section of bridge being constructed over a river Balanced-cantilever bridge under construction. • Each added segment is supported by post-tensioned tendons. • Tendon encapsulation systems are constructed from plastic or galvanised steel materials, and are classified into two main types: those where the tendon element is subsequently bonded to the surrounding concrete by internal grouting of the duct after stressing bonded post- tensioning; and those where the tendon element is permanently debonded from the surrounding concrete, usually by means of a greased sheath over the tendon strands unbonded post-tensioning. • Casting the tendon ducts/sleeves into the concrete before any tensioning occurs allows them to be readily "profiled" to any desired shape including incorporating vertical and/or horizontal curvature. • When the tendons are tensioned, this profiling results in reaction forces being imparted onto the hardened concrete, and these can be beneficially used to counter any loadings subsequently applied to the structure
- 14. 2.2.1 BONDED POST TENSIONING • In bonded post-tensioning, tendons are permanently bonded to the surrounding concrete by the in-situ grouting of their encapsulating ducting (after tendon tensioning). • This grouting is undertaken for three main purposes: to protect the tendons against corrosion; to permanently "lock-in" the tendon pre-tension, thereby removing the long-term reliance upon the end- anchorage systems; and to improve certain structural behaviors of the final concrete structure • Bonded post-tensioning characteristically uses tendons each comprising bundles of elements (e.g., strands or wires) placed inside a single tendon duct, with the exception of bars which are mostly used unbundled. • This bundling makes for more efficient tendon installation and grouting processes, since each complete tendon requires only one set of end-anchorages and one grouting operation. • Ducting is fabricated from a durable and corrosion-resistant material such as plastic (e.g., polyethylene) or galvanised steel, and can be either round or rectangular/oval in cross-section (Federation Internationale du Beton (February 2005). • The tendon sizes used are highly dependent upon the application, ranging from building works typically using between 2 and 6 strands per tendon, to specialized dam works using up to 91 strands per tendon. • Fabrication of bonded tendons is generally undertaken on-site, commencing with the fitting of end- anchorages to formwork, placing the tendon ducting to the required curvature profiles, and reeving (or threading) the strands or wires through the ducting. • Following concreting and tensioning, the ducts are pressure-grouted and the tendon stressing-ends sealed against corrosion
- 15. 2.2.2 UNBONDED POST TENSIONING • Unbonded post-tensioning differs from bonded post-tensioning by allowing the tendons permanent freedom of longitudinal movement relative to the concrete. • This is most commonly achieved by encasing each individual tendon element within a plastic sheathing filled with a corrosion-inhibiting grease, usually lithium based. • Anchorages at each end of the tendon transfer the tensioning force to the concrete, and are required to reliably perform this role for the life of the structure • Unbonded post-tensioning can take the form of individual strand tendons placed directly into the concreted structure such as buildings, ground slabs. • Bundled strands, individually greased-and-sheathed, forming a single tendon within an encapsulating duct that is placed either within or adjacent to the concrete such as restress able anchors, external post-tensioning for individual strand tendons, no additional tendon ducting is used and no post-stressing grouting operation is required, unlike for bonded post-tensioning. • Permanent corrosion protection of the strands is provided by the combined layers of grease, plastic sheathing, and surrounding concrete. • Where strands are bundled to form a single unbonded tendon, an enveloping duct of plastic or galvanised steel is used and its interior free-spaces grouted after stressing. • In this way, additional corrosion protection is provided via the grease, plastic sheathing, grout, external sheathing, and surrounding concrete layers
- 16. 2.2.3 COMPARISON BETWEEN BONDED AND UNBONDED POST TENSIONING • Both bonded and unbonded post-tensioning technologies are widely used around the world, and the choice of system is often dictated by regional preferences, contractor experience, or the availability of alternative systems. • Either one is capable of delivering code-compliant, durable structures meeting the structural strength and serviceability requirements of the designer • The benefits that bonded post-tensioning can offer over unbonded systems are reduced reliance on end-anchorage integrity, following tensioning and grouting, bonded tendons are connected to the surrounding concrete along their full length by high- strength grout. • Once cured, this grout can transfer the full tendon tension force to the concrete within a very short distance approximately 1 metre. • As a result, any inadvertent severing of the tendon or failure of an end anchorage has only a very localised impact on tendon performance, and almost never results in tendon ejection from the anchorage
- 17. • Improved crack-control in the presence of concrete cracking, bonded tendons respond similarly to conventional reinforcement (rebar). With the tendons fixed to the concrete at each side of the crack, greater resistance to crack expansion is offered than with unbonded tendons, allowing many design codes to specify reduced reinforcement requirements for bonded post-tensioning • Reinforcement in Post-Tensioned Building Design"). Improved fire performance the absence of strain redistribution in bonded tendons may limit the impact that any localised overheating has on the overall structure. • As a result, bonded structures may display a higher capacity to resist fire conditions than unbonded ones • "Comparison of unbonded and bonded post-tensioned concrete slabs under fire conditions"). • The benefits that unbonded post-tensioning can offer over bonded systems are ability to be prefabricated. • Unbonded tendons can be readily prefabricated off-site complete with end-anchorages, facilitating faster installation during construction. • Additional lead time may need to be allowed for this fabrication process. • improved site productivity the elimination of the post-stressing grouting process required in bonded structures improves the site-labour productivity of unbonded post- tensioning
- 18. 2.3 APPLICATIONS • Prestressed concrete is a highly versatile construction material because it is an almost perfect combination of its two main constituents: • high-strength steel that has been pre-stretched to allow for easy realisation of its full strength and modern concrete that has been pre-compressed to reduce cracking under tensile forces. • Its broad scope of use is shown in its inclusion in major design codes for buildings, bridges, dams, foundations, pavements, piles, stadiums, silos, and tanks, among other structural and civil engineering applications.
- 19. 2.3.1 APPLICATIONS ON BUILDING STRUCTURES • Building structures must often meet a variety of structural, aesthetic, and financial constraints. • A minimum number of (intrusive) supporting walls or columns; low structural thickness (depth), allowing space for services or additional floors in high-rise construction; fast construction cycles, especially for multi-story buildings; and a low cost-per-unit-area, to maximise the building owner's return on investment are all important considerations to counter in-service loadings, prestressing concrete permits "load- balancing" forces to be introduced into the structure. • Building constructions benefit from this in a range of methods such as longer spans for the same structural depth. • Load balancing results in lower in-service deflections, which allows spans to be increased and the number of supports reduced without adding to structural depth. Reduced structural thickness. • For a given span, lower in-service deflections allow thinner structural sections to be used, in turn resulting in lower floor-to-floor heights, or more room for building services. Stripping time will decrease. • Within five days, prestressed concrete building parts are typically fully stressed and self-supporting. They can now have their form work scraped and re-deployed to the next part of the building, reducing "cycle-times" in the construction process. • Material prices are lower. When compared to other structural materials, prestressed concrete often shows significant cost savings in building constructions due to the combination of reduced structural thickness, reduced traditional reinforcing quantities, and quick construction.
- 20. Foundation • Foundation is Part of a structural system that supports and anchors a building's superstructure and transmits its loads straight to the ground. • The bottom of the foundation must be below the frost level to avoid damage from repeated freeze-thaw cycles. • Spread footings, wide bases (typically of concrete) that support walls or piers and distribute the load over a larger area, are almost universally used to support the foundations of low-rise residential buildings. • To support the outer wall, a concrete grade beam supported by isolated footings, piers, or piles High-rise buildings also use spread footings, but in a much larger scale. • Piles, concrete caisson columns, and constructing directly on exposed rock are also other options for bearing enormous loads. • A floating foundation, which consists of stiff, box like structures positioned at a depth so that the weight of the soil removed to place it equals the weight of the superstructure supported, can be used to produce soil.may be constructed at ground level, especially in a building without a basement.
- 21. Bridges • In the construction of bridge superstructures, prestressed concrete makes a significant contribution. It's been utilized widely in rail and road bridges. • The prestressing process is well suited to the building of many types of bridges. • Simply Supported Bridge They are used for medium and short periods of time. These beams' cross-sections could be I, T, two T's, or Box. • Pre-tensioned or post-tensioned girders are available. • These beams can be precast or cast-in-place, and they are commonly supported at both ends by neoprene or other types of bearings. • This technology is typically used for bridges with larger spans. There will be cantilevers protruding from each of the piers in this technique. • To connect the cantilevers, a suspended span of the shorter length will be used. Cantilevers are typically extended by anchoring short-length precast parts. • Each piece is secured to the pier's balancing extension on the opposite side.
- 22. Marine Structures • Its durability, strength, and cost, prestressed concrete has gained popularity in the field of marine structures. • It has previously been described in relation to foundations in the previous section. Prestressed concrete is increasingly being used in superstructures for marine constructions. • The following are some examples of marine structures that have used prestressed concrete are Coastal jetties, Wharves, Bulkheads, Offshore platforms, Navigation structures, Protective fenders. • It is common knowledge that marine building has a slew of issues with its construction processes. The difficulties of quality control on the job site, particularly in terms of material and worker movement, makes it a difficult task.
- 23. Industrial Structures • The use of prestressed concrete in the construction of industrial structures is growing in popularity. • The trusses' tie members are frequently prestressed. • The following are some of the benefits of using prestressed concrete are Trusses with longer spans can be built and the structure's visual appearance is improved.
- 24. Water tank carrying structures • Aqueducts are Prestressed concrete is found to be the ideal choice for the construction of aqueducts due to its water tightness and crack-free surface. • Prestressed concrete, due to its high strength, enables the construction of long- span aqueducts with high water carrying capacity. • Water tanks are Prestressed concrete is also used to build circular water tanks. • They are more resistant to circumferential stress than R.C.C. Because of its great strength, prestressed concrete tanks have a much thinner wall than R.C.C. tanks. • Because of these benefits, prestressed concrete is becoming more common for the building of overhead water tanks and reservoirs.
- 25. Pre tensioned products • Much progress has been made in the field of pretense to considerable profit. • One of the many applications of pretension is the widespread production of prestressed electric transmission poles. • The train sleeper is a recent and significant addition to the list. A slew of plants producing these sleepers are popping up all across the globe. • Prefabricated houses also make considerable use of precast pretensioned members. • This application has a lot of potential and a lot of room for growth and diversity.
- 26. Nuclear structure • The notion of prestressed concrete lives up to its reputation as a technique capable of solving even the most difficult and complex challenges encountered in civil engineering. • The advantages of prestressed concrete have been recognized by reactor designers, who are now promoting the use of prestressed concrete in the design of their pressure vessels and container vessels. • This application should demonstrate the flexibility and superiority of the prestressed concrete concept over traditional approaches.
- 27. 3.0 CONCLUSION • As an alternative to in-situ cast construction, pre-stressed concrete precast members consisting of slabs, beams, and columns have been proposed. • Due to a labor scarcity, this is gradually becoming a must. • These precast methods also enable building to proceed at a faster pace. • The structural features of pre-stressed concrete columns have been thoroughly discussed. In this report we have shown about prestressed concrete, pre tension and post tension. • These precast systems also allow the construction to be carried out at a rapid rate. • In many circumstances, this can result in increased structural capacity and/or serviceability when compared to conventionally reinforced concrete. • Internal stresses are introduced in a prestressed concrete member in a controlled manner. Internal stresses are introduced in a prestressed concrete member in a predetermined manner to counterbalance the stresses caused by the imposed loads to the correct degree. • When compared to simple reinforced concrete, prestressed concrete allows for longer spans, reduced structural thicknesses, and material savings. • High-rise structures, residential slabs, foundation systems, bridge and dam structures, silos and tanks, industrial pavements, and nuclear containment structures are all examples of typical applications. • The paragraph above shows us that this type of cement is much stronger than other form of cement where it can make the building or any construction use it will be much more safer than usually uses material constriction. • All the modern construction have started using this type of cement when building a building because all have known its strength and pros of using this type of cement.