There are many technical varieties of modern concrete, but historic buildings and bridges generally used three main types: plain or unreinforced concrete, reinforced concrete, and prestressed concrete. Concrete, like stone, is very strong in compression and works well when used as a vertical column or supporting post, for example. When used horizontally as a slab or beam, concrete can typically span only short distances before it begins to crack and fail unless it is made thicker. The depth and weight of a plain concrete beam soon become too large and impractical for longer horizontal spans required in buildings and bridges. Builders learned that the addition of metal reinforcing bars in a concrete beam or slab would allow it span greater distances before cracking. As a result, reinforced-concrete became an important structural material for bridge construction after 1900. Virtually all modern concrete is reinforced with metal.
Prestressed Concrete Advantages And Disadvantages Pdf Free
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The post-tensioning system required simple equipment and could be done almost anywhere, including at the bridge site. In fact, the first prestressed-concrete bridge in Minnesota was post-tensioned. Because post-tensioning was limited to smaller beams and slabs, the alternate method of pretensioning became the industry standard after the early years.
Figure 5- Walnut Lane Bridge, Philadelphia, Historic American Engineering Record (HAER) photo. This documentary photo, taken in 1968, shows the Walnut Lane Bridge span, comprised of a parallel series of prestressed, post-tensioned concrete beams, aligned closely together. Image retrieved from the Library of Congress.
In 1952, brothers Norbert and Leonard Soukup established the Northern States Prestressed Concrete Co. to build the first prestressed-concrete bridge of any kind in Minnesota, using the post-tensioning method. They assembled rows of specially designed concrete blocks, tensioned them together in a long row with cables, and created a series of prestressed-concrete block-beams. The beams created the span for a bridge carrying local traffic from U.S. Highway 61 to a Boy Scout Camp outside Lake City. The bridge has since been replaced.
Created by the Federal Aid Highway Act of 1956, the new Interstate Highway System demanded thousands of new bridges across the nation. Designed as freeways, the new highways would have no traffic intersections anywhere. All highways and railroads would go over or under the new Interstates. Because the design of the new Interstate was so uniform and consistent with an original four-lane system, most of the bridges were similar in shape and length. The requirements created a perfect match for the new prestressed-concrete system, which readily turned out quantities of uniform beams, cast in off-site plants, under controlled factory conditions, independent of the weather and site conditions. Also, the new prestressed-concrete beams proved competitive with steel beams in size and cost, especially when steel was scarce and expensive in the 1950s. The Interstate System helped create a major prestressed-concrete industry virtually overnight. The same plants often fabricated beams, slabs, and planks for roofs and floors to build the many new shopping centers, schools, stadiums, offices, and other structures for the suburbs accessed by the Interstate Highways.
Structural engineering depends on the knowledge of construction materials and their corresponding properties for us to better predict the behavior of different materials when applied to the structure. Generally, the three (3) most commonly used materials in structural engineering are steel, concrete and wood/timber. Knowing the advantages and disadvantages of every material is important in ensuring a safe and cost-effective approach to designing structures. So let's take a look at the pros and cons of steel vs timber vs concrete!
Concrete is extremely strong in compression and therefore has high compressive strength of about 17MPa to 28MPa. With higher strengths up to or exceeding 70 MPa. Concrete makes it possible to design very robust and durable buildings, and taking advantage of its thermal mass by keeping it inside the building envelope can help regulate interior temperatures. There is also an increasing use of precast concrete in the building industry, which offers advantages in terms of environmental impact, cost and speed of construction.
While there are many advantages to the Just-In-Time manufacturing methodology, there are also some drawbacks to it as well. Listed below are some of the disadvantages of Just-In-Time (JIT) manufacturing.
Prestressed concrete is a type of concrete in which an appropriate amount and distribution of internal stress are supplied to counteract the stresses caused by external loads to the desired degree. In the concrete section, high-strength steel wire or alloys (known as 'tendons') are provided so they generate the initial compression. Because an initial load is applied to the structure prior to its use, prestressed concrete differs from traditional RCC structures.
Prestressing is a technique for preventing cracks, increasing the part's strength, and reducing deflection. Prestressing is a technique for achieving a lower depth, longer span, and improved strength in a structure. Let us discuss more prestressed concrete in the upcoming sections.
Prestressed concrete requires high-strength concrete since the material is very resistant to tension, shear bond, and bearing. High-strength concrete is generally selected over cost-cutting in the zone of anchorage where bearing stresses are being imposed. High-strength concrete has a lower risk of shrinkage cracks, a lower modulus of elasticity, and a lower ultimate creep strain, resulting in less prestress loss in steel. With the use of high-strength concrete, the cross-sectional dimensions of prestressed concrete structural elements are lowered, and the material's dead weight is reduced, allowing for a greater span to be technically and economically feasible.
Tensile forces are produced in an RC beam by dead loads, imposed loads, deformations, and load-independent processes such as temperature changes and shrinkage. Steel reinforcing bars are inserted in the concrete to carry all internal tensile forces due to the low tensile strength of concrete. Prestressed concrete is a form of reinforced concrete that has undergone additional development. The advantages of prestressing are, it controlled or reduced cracking, controlled deflection, reduces dead load, etc.
The stem and base slab are two components of the cantilever retaining wall. Reinforced concrete, prestressed concrete, or precast concrete can construct a cantilever retaining wall. Cantilever retaining walls are often built of concrete that has been prepared on-site by forming formwork. As a cantilever retaining wall, a precast retaining wall is employed.
Gravity walls are similar to these types of retaining walls. Individual interconnecting boxes made of precast concrete or wood make up the structure. The boxes are filled with crushed stone or coarse granular materials to create a free-draining structure. Timber retaining walls and reinforced precast retaining walls are two common types of crib retaining walls. It is appropriate for sustaining planter areas but unsuitable for supporting slopes or structures.
Prestressed concrete units are used for many large concrete projects. To create prestressed concrete, you must use a special technique. Like reinforced concrete, it includes bars or tendons. But these bars or tendons are stressed before the actual application of the concrete.
This compression enhances the strength of the lower section of the unit and improves its resistance against tensile forces. However, this process requires skilled labor and heavy equipment. Normally, prestressed units are created and assembled on-site. Prestressed concrete is used to build bridges, heavy-loaded structures or roofs that have long spans.
Some types of concrete hold billions of microscopic air cells in every cubic foot. These tiny air pockets relieve the internal pressure on the concrete. They provide tiny chambers where water can expand when it freezes.
Because this concrete is mixed at the site of application, the mixing and entraining process requires careful engineering supervision. The entrained air adds up to about 3% to 6% of the volume of the concrete. Almost all concrete used in a freezing environment or where there are freeze-thaw cycles is air-entrained.
It is a reliable, efficient and economical way to apply concrete and is often the only way that concrete can be placed in certain locations. Very fine aggregates are used in pumped concrete. The finer the aggregate used in the mix, the freer the concrete flows from the pipe.
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First used in the late-nineteenth century,[1] prestressed concrete has developed beyond pre-tensioning to include post-tensioning, which occurs after the concrete is cast. Tensioning systems may be classed as either monostrand, where each tendon's strand or wire is stressed individually, or multi-strand, where all strands or wires in a tendon are stressed simultaneously.[5] Tendons may be located either within the concrete volume (internal prestressing) or wholly outside of it (external prestressing). While pre-tensioned concrete uses tendons directly bonded to the concrete, post-tensioned concrete can use either bonded or unbonded tendons. 2ff7e9595c
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