Faculty of Engineering
Civil Engineering
Graduation Project 1
Mentor: Prepared By:
Assist. Prof. Dr. Jelena Ristic
Skopje, 2018
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Pre-Stressed Concrete: A Comprehensive Guide to its History, Properties, and Applications, Thesis of Interface between Computer Science and Economics
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Faculty of Engineering Civil Engineering Graduation Project 1 Mentor: Prepared By: Assist. Prof. Dr. Jelena Ristic Skopje, 2018
Table of Contents Abstract………………………………………………………………………………………..
- Introduction…………………………………………………………………………………
- Materials for Pre-Stressed Concrete Elements....................................................................5- 2.1 Concrete 2.2 Steel For Pre-stressed Concrete
- Pre-Stressed Concrete Elements and Application in Structure …………………………...
- Methodology of Pre-Stressing ………….............……………………………………..11- 4.1 Pre-Tensioning Systems 4.2 Post-tensioning systems
- Shapes of Pre-Stressed Concrete Cross-sections………………………………………16-
- Design of Members ……………………………………………………………..……..18-
- Conclusion ……………………………………………………………………………..…. Bibliography.............................................................................................................................
Introductıon Considering the history and development of reinforced concrete and pre stressed concrete; It is seen that the concrete is not the application, but firstly it comes from practice. The problem of the tensile strength of the concrete is very low compared to the compressive strength is eliminated by steel reinforcement. In other words, the cracks formed in the tensile regions are stitched by steel seam placed perpendicular to these cracks. While exploring these practices, it has developed steadily through the advancement of intensive laboratory. So today we have so much information. Construction technology is developing rapidly in various areas. Pre stressed concrete technology is one of the applications in these areas. In the areas where reinforced concrete carrier systems are inadequate, it meets with pre stressed. Pre stressing is a construction technique that is used in most building areas and is very beneficial in large span systems. The demands of the people throughout the world have increased more each time. As these needs increased, they tried to meet their needs in many areas. However, continuous improvement of these needs brings about the development of technology. Building sector is one of the sectors which continuously develops in line with these needs. This type of wide-span structures, which were difficult to accomplish before, started to become widespread by increasing the carrying capacity of the sections with the pre tensioning technique. This advancement technique has provided a great contribution and convenience. Although the pre-stress technique is more widely used due to the special properties of the concrete and pre-stress technique is known as a technique that can only be applied to concrete material, pre stressing can also be applied in steel structures. However, the pre-stressing technique in the steel structure under the shadow of the benefits provided by the concrete in the concrete has not been sufficiently investigated and there is not enough study on the benefits it will provide. Therefore, there is not much information about this issue. In this study, we examine the structures and methods which are applied by using the pre-stressing technique.
2. Materials for Pre-Stressed Concrete Elements The pre-stress is usually the force obtained by pulling high-strength steel with a certain force and clamping it together and transferring it to the concrete. If the stress process of Steel is done before the concrete is poured, the pre-stress takes the name pre-tensioning if done after the concrete is poured and durability is achieved. In the pre-grinding process, if the steel is drawn, it is stretched and clamped. After the concrete is poured and gained the necessary durability, the pre-tensile steel is broken. When we draw within Elastic Limits, The Steel wants to return to its original state so that the stress that exists in the steel is passed to the concrete. In the pre-hardening technique, if the pre-hardening steel is pulled and clamped before the concrete starts to break, pre-tensioning and post tensioning stretching is applied to the element. 2.1 Concrete Concrete; it is a composite building material produced by using aggregate, water, cement and chemicals when necessary. When we talk about the formation of concrete, aggregate, crushed mineral materials such as gravel, cement and water are mıxed together ın concrete mıxture. A mixture of cement and water is called cement paste. When the cement paste is first mıxed with aggregate, we can easily shape it. This concrete is called fresh concrete. The most important feature of fresh concrete is the workability of concrete. The ability of the concrete to be processed is easy to mix, homogenous, so that it is easily spread in the mould, with little energy and less space. After a few hours, the concrete turns ınto solid and begins to harden. After days, the hardness increases and concrete gains strength. The concrete which has been hardened is called strengthened concrete. Concrete has an important place in the development of the history of mankind and in the works of ancient civilizations which have survived until today. There are 3 main properties in concrete: 1 - Must have good workabılıty Fresh concrete should be easily mixed, placed easily, should not be decomposed when transported and placed.
Figure 2. Production Stages of concrete Mıxıng concrete In order to prepare the concrete to be used in the construction of the building, a measure of cement, two measures of sand and three parts of gravel or crushed stone are taken. They are shovelled in small buildings and mixed with water by special machines in large buildings. Concrete transmıssıon The most important issue is that the homogeneity of the concrete is not disturbed. The concrete can be conveyed by hand, machine and conveyed with the pump. There is no coarse aggregate in the concrete conveyed by the pump. Transmission may be horizontal or vertical. Figure 3. Casting concrete
Concrete Class and Properties C20 concrete properties C20 concrete is now the weakest, tough concrete used in the construction industry. The 1 cubic centimetre of the C20 concrete is able to withstand a load of 200 kilos. In today's construction technology, this rate is very low. C35 concrete properties C35 concrete is one of the most durable concrete used today. 1 square centimetre of C35 concrete is able to withstand 325 kilos. C35 concrete buildings, earthquakes and other natural or artificial negativities, disasters are one of the most powerful structures to withstand. C40 concrete properties C40 concrete is durable concrete used today. 28 days, The lowest 15x15 cm cube result should be 470 kg / cm2. C40 concrete propertıes ıs 15x15 cm Equivalent cube strength 500 kg / cm2. CONCRETE CLASS CYLINDER PRESSURE RESISTANCE (N/MM ²) CUBE PRESSURE RESISTANCE. (N/MM ²) C14 14 16 C16 16 20 C18 18 22 C20 20 25 C25 25 30 C30 30 37 C35 35 45 C40 40 50 Why Prestressed Concrete To reduce the height of the beam, to pass the large spans economically and safely, by greatly reducing the tensile stresses. Where to use Bridges, high-rise buildings, factory halls, towers etc. 2.2 Steel for Pre-Stressing General use of pre-stress steel: It increases the resistance quality and is susceptible to tensile and compression stresses.
case of releasing the cables in the case of releasing are generally provided with Pre-Stressed concrete by stretching the batch.
3. Pre-Stressed Concrete elements and applıcatıon ın structure Today, works towards safer structures and high strength materials have reduced the cross- sectional dimensions and structure, reduced. One of the most important applications of these developments are structures. In these structures, a large part of the total design load is the own weight of the elements. Thoughts about the application of pre-pressure in the section to reduce or prevent unwanted tensile stresses. Pre-stressed concrete between the structural System materials used in civil engineering. Increasing research has evolved with advancing technologies and has become one of the most efficient load bearing member system materials. Concrete is an important building material. As a defect, the characteristic is that the strength under tensile stresses is too small for its strength under pressure stresses. Material used in pre-stressed concrete should be considered as a special material group. Although the use of the materials used in the classical reinforced concrete construction can be seen as natural, both the high quality of the concrete and the use of high strength steel bring the material to the first plan. Cement: Normal Portland cement can be used in pre-stressed concrete. Aggregate: Durable aggregate should be used in pre-stressed concrete. If the appropriate aggregate is difficult to obtain, crushing stone can also be used, provided that it meets the specific requirements of washing, screening and appropriate classification. Water: Any drinkable water is suitable. Additives: It is necessary not to use additives in pre-stressed concrete. If additive necessary, the compatibility with experimental mixtures, especially in water tanks, should be determined. Pre-stressing equipment
Pre-tensioning Wire: High strength, straight and curved steel wire manufactured in various diameters. Pre-tensioning Bar: High strength, special alloy steel bar manufactured in various diameters. Pre-tensioning Cable: The pre-stressing wire is the use of a Bar or rod as a group. Cables It is used in the art method. Pre-tensioning Steel: The pre-stressed wire is the use of bars as a group. These cables are used in post drawing methods. Cover: It is a pipe made of metal or plastic which is used in the art-pulling elements to pass holes in the pre-tensioning device in the concrete. Transfer: Transferring the force generated by pulling the pre-stressing steel to concrete. First pre-stress: Transferring the force generated by pulling the pre-stressing steel to concrete.
4. Methodology of Pre-Stressing Methods of pre-stressing concrete fall into two main categories pre-tensioning and post- tensioning 4.1 Pre-Tensioning Systems Stages of Pre-tensioning In pre-tensioning system, the high-strength steel tendons are pulled between two end abutments (also called bulkheads) prior to the casting of concrete. The abutments are fixed at the ends of a pre-stressing bed. Firs the concrete attains the desired strength for pre-stressing, the tendons are cut loose from the abutments. The pre-stress is transferred to the concrete from the tendons, due to the bond between them. During the transfer of pre-stress, the member undergoes elastic shortening. If the tendons are located eccentrically, the member is likely to bend and deflect (camber). The various stages of the pre-tensioning operation are summarized as follows. 1) Anchoring of tendons against the end abutments 2) Placing of jacks 3) Applying Cutting of the tendons. 4) tension to the tendons 5) Casting of concrete During the cutting of the tendons, the pre-stress is transferred to the concrete with elastic shortening and camber of the member.
Figure 5. Pre-stressing bed, end abutment and mould 4.2 Post-tensioning systems Stages of Post-tensioning In post-tensioning systems, the ducts for the tendons (or strands) are placed along with the reinforcement before the casting of concrete. The tendons are placed in the ducts after the casting of concrete. The duct prevents contact between concrete and the tendons during the tensioning operation. Unlike pre-tensioning, the tendons are pulled with the reaction acting against the hardened concrete. If the ducts are filled with grout, then it is known as bonded post-tensioning. The grout is a neat cement paste or a sand-cement mortar containing suitable admixture. In unbounded post-tensioning, as the name suggests, the ducts are never grouted and the tendon is held in tension solely by the end anchorages. The following sketch shows a schematic representation of a grouted post-tensioned member. The profile of the duct depends on the support conditions. For a simply supported member, the duct has a sagging profile between the ends. For a continuous member, the duct sags in the span and hogs over the support. Figure 6. Post-tensioning
Figure 7. Post-tensioning ducts in a box girder The various stages of the post-tensioning operation are summarized as follows.
- Casting of concrete.
- Placement of the tendons.
- Placement of the anchorage block and jack.
- Applying tension to the tendons.
- Seating of the wedges.
- Cutting of the tendons. The stages are shown schematically in the following figures. After anchoring a tendon at one end, the tension is applied at the other end by a jack. The tensioning of tendons and pre- compression of concrete occur simultaneously. A system of self-equilibrating forces develops after the stretching of the tendons. Figure 8. Stages of post-tensioning
Anchoring Devices In post-tensioned members the anchoring devices transfer the pre-stress to the concrete. The devices are based on the following principles of anchoring the tendons.
- Wedge action
- Direct bearing
- Looping the wires Figure 10. Anchoring devices (Reference: Collins, M. P. and Mitchell, D., Pre-stressed Concrete Structures) Figure 11. Jacking and anchoring with wedges (Reference: Collins, M. P. and Mitchell, D., Pre-stressed Concrete Structures)
5. Shapes of Pre-Stressed Concrete Beam Beams are supported on columns; they are built horizontally. Reinforced concrete frame structures, foundation, flooring, beams and columns are composed of a bearing structural members. This system is the same on all stores. The beams of any story are constructed according to the mold plan of that story and the beam details. The location and dimensions of the beams in the mold plan are clear. But it is not clear how the beams will be fitted. How to assemble each beam in the mold plan is indicated by drawing the details of these beams.
I-beams I-beam is a structural bearing member used in construction. I-beams take their names from the distinctive shape. It consists of a long and vertical piece of metal called WEB and shorter horizontal parts called flanges. These pieces fit together to create a beam cross section looks like the ‘ I ‘. But mention pre-stressed concrete I-beams are the most popular I-beam type. I- beams made of materials such as aluminum and wood are also available and used. I-beams are classified according to their size. Their names change from country to country. I-beam Uses I-beams are generally used as support in buildings and provide structural integrity. An I-beam strength reduces the need for numerous supports, creating a pleasant construction area. Fıgure 12. Pre-stressed I-beam T-beams Pre-stressed concrete floor systems consist of slabs and beams placed on the ground. The beams move together to resist the forces. In effect, the beams have extra widths at their tops, called flanges, and the resulting T-shaped beams are called T beams. In effect, the beams have extra widths at their tops, called flanges, and the resulting T-shaped beams are called T beams. The part of a T beam below the slab is referred to as the web or stem. The beams may be inverted L shaped if it is edge or spandrel beam.( Russell H. Brown) Analysis of T-beams is very similar to the analysis of rectangular beams.
Design of Pre-stressing Force First, a preliminary dimension of the member is selected based on the architectural requirement. The pre-stressing force at transfer (P0) should be such that the compressive stress in concrete is limited to the allowable value. At service, the designed pre-stressing force (Pe) should be such that the tensile stress in concrete should be within the allowable value. The amount of pre-stressing steel (Ap) is determined from the designed pre-stressing force based on the allowable stress in steel. At transfer In absence of non-pre-stressed reinforcement, the stress in concrete (fc) is
Fc =-
P 0
Ac Ac = net area of concrete P0 = pre-stress at transfer after short-term losses. In presence of non-pre-stressed reinforcement, the stress in the concrete (fc) can be calculated as follows.
Fc=
− P 0
Ac (
Es
Ec )^
As As = area of non-pre-stressed reinforcement Es = modulus of elasticity of steel Ec = modulus of elasticity of concrete. At service, the stress in concrete (fc) can be calculated as follows.
Fc = -
Pe Ac
P
At At = transformed area of section P = external axial force Pe = external axial force
CONCLUSION Pre-stressed concrete technology, which is the advanced stage of the transformation of the concrete with low tensile strength against the compressive strength with reinforced concrete reinforcement, has made the transportation systems more economical, more aesthetic and more useful. In my project, I first mentioned the properties of concrete and how it was formed. I gave information about the durability levels of concrete. I explained why pre- stressed concrete is used. I explained the theories I found about this. Giving information about pre-stressed steel, I gave information about the good and bad sides and the areas in which it is used. I investigated concrete applications of pre-stressed concrete structures. I explained the pre-tensioning systems and gave information about the factors that affect the factors and the advantages and disadvantages. This study taught me how concrete is being transferred and how important the stages are. I learned more about pre-stressed concrete and pre-stressed steel. I had the opportunity to learn important points about methods and research.