1. 研究目的与意义（文献综述包含参考文献）
I) Briefly talk about the cognition and experience of concrete structure.Concrete is perhaps the most widely used building material in the world today, thanks to its availability and versatility. Its also the material we have to thank for some of the worlds most impressive buildings, including the Sydney Opera house, the Lotus Temple in Delhi, and the Burj Khalifa in Dubai.Concrete is one of the most frequently used building materials worldwide. The distinctive characteristics like strength, durability, low-maintenance, energy-efficient, sustainability are the reasons for wide range usage of concrete in the field of civil engineering. You can find concrete almost anywhere you look, including buildings, bridges, walls, swimming pools, roadways, airport runways, floors, patios or even a cement house. All of these structures depend on a man-made material with a simple formula. How is all that concrete made?Concrete is composed of cement, water and coarse aggregates. When mixed together, they create a construction material that hardens over time. How much water and cement you use determine the properties of concrete, such as: Strength Durability Resistance to heat or radiation WorkabilityFresh concrete has many applications and can be cast into circles, rectangles, squares and more. It can also be used for staircases, columns, doors, beams, lentils and other familiar structures. Concrete is made in different grades, including normal, standard and high-strength grades. These grades indicate how strong the concrete is and how it will be used in construction. What kind do you need? Our guide can help you decide based on your project requirements.How Do You Make Concrete?When you make concrete, regardless of what you plan to use it for, you have to mix the right proportions to achieve the quality you desire. You can use different mixes to make concrete:1. Nominal mix2. Design mix3. Machine mixing4. Hand mixingUses of ConcreteThe major uses of concrete are1. Concrete Dams2. Residential Buildings3. Commercial Buildings4. Roads or Driveways5. Marine Construction6. Culverts and Sewers7. Foundations8. Fences9. Concrete BridgesOther Uses As the concrete is an important part of a building, it is prioritized over wood as a construction material. Concrete once cast and cured does not require any maintenance and can hold up against any weather condition. Concrete is a non-combustible or decaying material that makes it inert material that doesnt burn, mildew or feed rot. Its superior structural integrity provides an added degree of protection from the severe weather as well as an earthquake. Concrete is produced from locally available materials and leaves a small environmental footprint while still providing high-level durability. Concrete can be shaped in various forms when freshly mixed. A concrete floor can be stamped to create an attractive surface. It can admit natural light during the day and transmit artificial light after work. It is a durable and cost-effective material which is a necessity for underground use.Main Properties of Concrete for ConstructionConcrete is a mixture of several materials. At the hardened state, this heterogeneous material becomes stone-like mass. The extensive use of concrete in the construction field has made it a material of huge concern for engineers. To participate in the vast uses of concrete an engineer must know its properties.Strength, Workability, Elastic Properties, Durability, Impermeability, Segregation, Bleeding, FatigueHow Concrete Buildings Are Made?Concrete buildings are made with metal forms, which hold the concrete in place while it cures. Typically, the forms are set into place and then braced. Rebar, grooved steel rods of various thicknesses, are placed inside the forms to reinforce the concrete.The first step in the making of a concrete building is its design. Designers take into consideration how the characteristics of concrete, including its weight, strength and stability, will affect their design. This is important because the concrete walls and floors become the structure of the building.DesignThe first step in the making of a concrete building is its design. Designers take into consideration how the characteristics of concrete, including its weight, strength and stability, will affect their design. This is important because the concrete walls and floors become the structure of the building. In a skyscraper, the concrete must be able to withstand the weight of many floors.FormsConcrete buildings are made with metal forms, which hold the concrete in place while it cures. Typically, the forms are set into place and then braced. Rebar, grooved steel rods of various thicknesses, are placed inside the forms to reinforce the concrete. Some buildings are constructed of poured concrete pillars and floors, with other materials used to construct the walls.PouringConcrete is poured into the forms by large trucks. Some have booms that pump the concrete up from the truck container and deposit it into the forms. The foundation and the floor of the concrete building are poured first. Utility pipes are pre-installed before the concrete is poured. Once the floor and foundation have cured, the walls and pillars can be poured. Rebar from the floor is tied to the rebar set in the wall forms. Concrete buildings can be constructed level by level in this fashion.The Advantages of ConcreteYou dont become the second most used material on Earth for no reason. What are the key advantages of using concrete for building structures?Concrete is Easily AvailableAs stated above, concrete is made up of four incredibly simple components; cement, aggregates, sand and water. Since each of these key ingredients is readily available from a variety of sources, not only are they easy to acquire, theyre also affordable.Both the ease of getting the ingredients and how simple it is to make are key reasons why concrete is a popular choice when it comes to large structures like overpasses.Concrete is Easy to ShapeWhen concrete is in its green (wet) state, its incredibly easy to shape and mould, which means its a great choice for a variety of functions. While its in its wet state, concrete is also easy to transport from the place it was mixed to the place where it will be used and left to set.Concrete is Extremely VersatileAnother great advantage of concrete is that it can be manufactured specifically to a desired strength, depending on the structures requirements. For example, concrete is often reinforced with steel in order to create a material with a high compressive strength compared to others. Reinforced concrete also requires little to no maintenance and lasts a long time.From single blocks to entire flyovers, concrete structures are incredibly robust and durable, which is one reason its so widely used.Concrete is Non-CombustibleUsing concrete for structures adds extra fire safety since concrete is non-combustible by nature. Its ranked as a class A1 building material, which means it has the highest classification of fire resistance.Concrete can also withstand extremely high temperatures and does not exude any toxic fumes when affected by fire.Concrete is DurableNot only does concrete stand up to the test of time and fire, but it is also resistant to wind and water. This is why concrete is a popular material for building storm shelters or weatherproofing other structures such as barns.Disadvantages of ConcreteNow that weve looked at why concrete is the worlds favourite building material, were going to explore some of the disadvantages of concrete structures.Low Tensile StrengthWhen compared to other building materials, concrete is shown to have rather low tensile strength, which can lead to the concrete cracking under too much tension. Therefore larger concrete structures require the concrete to be reinforced with steel.Cracks in concrete can also be caused by drying, shrinkage and moisture expansion. This is another reason why most concrete needs to be reinforced.Concrete is Not SustainableUnfortunately, concrete is not the most environmentally sustainable choice when it comes to building materials. It has been claimed that because of its worldwide popularity, concrete is responsible for 5% of the worlds total carbon emissions.If youd like to learn about more sustainable alternatives to concrete, why not read our blog below?Concrete is Susceptible to EfflorescenceConcrete can sometimes contain soluble salts, which, when the water in the concrete evaporates, causes efflorescence.Efflorescence in concrete shows as a whitish coloured salt deposition that sits on the surface of the concrete when the moisture evaporates. However, efflorescence can be removed from the concretes surface simply by using a dilute solution of household white vinegar and water.While concrete isnt without its flaws, it still remains the most used building material across the globe and will probably stay that way for a little while more.2) General steps of structural concrete design.The primary objective of structural analysis and design is to produce a structure capable of resisting all applied loads without failure during its intended life. If improperly designed, elements of a structure would fail causing serious consequences such as large expenses or ultimately losses in lives which cannot be compared with any cost.Once the architectural engineer sets the function and layout of the structure, the role of the structural engineer begins which can be summarized in the following steps to develop a safe, functional and economic structures.STEP 1: CONCEPTUAL DESIGN In this stage, initial design of the building elements (e.g. slabs, beams, columns etc.) is performed based on code recommendations.It starts with selecting the appropriate columns locations and orientation in such a way that they do not interfere with the architectural drawings. For example, you do not want a column to be in the middle of a room or something like that. Also, one should consider that at least 30% of the total number of columns to be in either x- or y- directions to give adequate earthquake resistance of the building.After that, the type of structural system is selected. For instance, the slab might be chosen to be solid slab, hollow block or flat slab etc. Accordingly, the location of beams are determined.STEP 2: DETERMINING THE INTERNAL FORCES OF EACH ELEMENTAfter finishing the initial design, the exact dimensions of each building element shall be determined. First, an analysis model is created for the building with its initial dimensions as determined from STEP 1 on an analysis software such as SAP 2000.All the loads that act on the structure have to defined in the model such as dead loads, live loads, wind and earthquake loads etc.From the model, the internal forces [Normal Force, Shear Force, Bending Moments] on each elements is calculated.STEP 3: ITERATIVE DESIGNNow you have the straining actions on the elements based on the initial sizing and simply, you can use them to design according to the relevant code. The design process became so easy by using some spreadsheets or any other software that facilitates the design.Next, the analysis model should be modified to the new dimensions obtained from the previous design and the analysis is re-run. Internal forces are obtained and design is made again based on the new forces.This iterative process is repeated until the element design be the same in two following iterations.STEP 4: FOUNDATION DESIGNAfter the final dimensions of members are found, the foundation system type can be selected taking in consideration, the bearing capacity of the soil and the loading coming from the structure.STEP 5: DRAFTINGIn this step, the structural plans are created. These plans should be fully detailed such that the construction process in the site can proceed smoothly and not delayed due to missing data in the drawings.What are the basics of structural design?Structural design is the process of creating a safe and functional structure under any load that it may experience. Generally, there are four steps: (1) modelling, (2) load analysis, (3) structural analysis, and (4) design.3) Discuss the contents, problems and solutions of structural concrete design.Concrete is one of the most used building materials in the world. Concrete is a mixture of powdered cement, sand, aggregate such as stone, and water. It allows for treatment and has high compressive strength and low tensile strength. In contrast, reinforced concrete (RC) is a mixture of concrete with reinforcements (steel bar).RC is a building material discovered in late 19th century and was credited for inventing it into Joseph Louis Lampot in 1848 and has obtained a patent through the design of 1867 reinforced garden basins, and then beams-patented concrete beams and elements for railways and road fences. After that, the major developments of the armed concrete began since the year 1900 [3]. RC columns (RCC) are usually structural linear elements, which are generally casted vertically. RC is containing embedded plates, steel bars, or fibers that support the material. Also, RC is responsible for carrying loads from floors to foundations. The stability, inflexibility, and strength of any structure are closely associated with the strength and robustness of the columns. The capacity to carry loads is maximized by these materials, and because of this, RCC is widely used in all construction. In fact, building materials have become the most used. Therefore, the concrete columns must be reinforced identically to the global analysis of the structural system, in order to ensure quality, efficiency and security.RC, as a building material, is used extensively around the world because of its importance in the development of urbanization. It has a lot of pros and cons. For example, some advantages of armed concrete are: 1. Compared to other construction materials, RC has high compressive strength. 2. RC can withstand a great deal of tensile stress due to the reinforcement provided. 3. Fire resistance and weather. 4. Considered to be more robust building system than any other building system for RC. 5. Initially, RC, as liquid materials, can be formed in an almost unlimited range of shapes. 6. Less skilled labour to establish the structure, compared to using steel in the structure.On the other hand, some disadvantages: 1. Tensile strength up to ten compressive strength. 2. RC steps are mixing, casting, and processing, which undoubtedly have a significant impact on the final power. 3. The cost of models used for casting RC is quite higher. 4. Deflation causes crack expansion and loss of strength. There are many topics to be presented such as: concrete materials and different environments that are exposed to concrete and durability, concrete durability in the global building code, distressed RC tradition structures, concrete cracks, corrosion, and causes of deterioration, and solutions for the treatment of these problems, for example concrete injectors, repair by concrete, repair and prevention of rust, especially cathodic protection, reinforcing bars made of fiber reinforced plastic (FRP). Developed countries spend a huge proportion of their annual infrastructure budgets on repairs, maintenance and replacement of present and new structures, which is a significant indication of deficiencies in past construction practices, designs and techniques.
2. 研究的基本内容、问题解决措施及方案
There are many problems for using RC, and many researchers have identified problems and solutions to these problems.Problems with Concrete MaterialsVisual inspection of concrete will allow for the detection of distressed or deteriorated areas. Problems with concrete include construction errors, disintegration, scaling, cracking, efflorescence, erosion, spalling, and popouts.Construction ErrorsErrors made during construction can include adding improper amounts of water to the concrete mix, inadequate consolidation, and improper curing can cause distress and deterioration of the concrete. Proper mix design, placement, and curing of the concrete, as well as an experienced contractor are essential to prevent construction errors from occurring. Construction errors can lead to some of the problems discussed later in this fact sheet such as scaling and cracking. Honeycombing and bugholes can be observed after construction.Honeycombing can be recognized by exposed coarse aggregate on the surface without any mortar covering or surrounding the aggregate particles. The honeycombing may extend deep into the concrete. Honeycombing can be caused by a poorly graded concrete mix, by too large of a coarse aggregate, or by insufficient vibration at the time of placement. Honeycombing will result in further deterioration of the concrete due to freeze-thaw cycles because moisture can easily work its way into the honeycombed areas. Severe honeycombing should be repaired to prevent further deterioration of the concrete surface.Bugholes is a term used to describe small holes (less than about 0.25 inch in diameter) that are noticeable on the surface of the concrete. Bugholes are generally caused by too much sand in the mix, a mix that is too lean or excessive amplitude of vibration during placement. Bugholes may cause durability problems with the concrete and should be monitored.Disintegration and ScalingDisintegration can be described as the deterioration of the concrete into small fragments and individual aggregates. Scaling is a milder form of disintegration where the surface mortar flakes off. Large areas of crumbling (rotten) concrete, areas of deterioration which are more than about 3 to 4 inches deep (depending on the wall/slab thickness), and exposed rebar indicate serious concrete deterioration. If not repaired, this type of concrete deterioration may lead to structural instability of the concrete structure. A registered professional engineer must prepare plans and specifications for repair of serious concrete deterioration. For additional information, see the Concrete Repair Techniques fact sheet.Disintegration can be a result of many causes such as freezing and thawing, chemical attack, and poor construction practices. All exposed concrete is subject to freeze-thaw cycles, but the concretes resistance to weathering is generally determined by the concrete mix and the age of the concrete. Concrete with the proper amounts of air, water, and cement, and a properly sized aggregate, will be much more durable. In addition, proper drainage is essential in preventing freeze-thaw damage. When critically saturated concrete (when 90% of the pore space in the concrete is filled with water) is exposed to freezing temperatures, the water in the pore spaces within the concrete freezes and expands, damaging the concrete. Repeated cycles of freezing and thawing will result in surface scaling and can lead to disintegration of the concrete. Hydraulic structures are especially susceptible to freeze-thaw damage since they are more likely to be critically saturated. Older structures are also more susceptible to freeze-thaw damage since the concrete was not air entrained. In addition, acidic substances in the surrounding soil and water can cause disintegration of the concrete surface due to a reaction between the acid and the hydrated cement.CracksCracks in the concrete may be structural or surface cracks. Surface cracks are generally less than a few millimeters wide and deep. These are often called hairline cracks and may consist of single, thin cracks, or cracks in a craze/map-like pattern. A small number of surface or shrinkage cracks is common and does not usually cause any problems. Surface cracks can be caused by freeze-thaw cycles, poor construction practices, and alkali-aggregate reactivity. Alkali-aggregate reactivity occurs when the aggregate reacts with the cement causing crazing or map cracks. The placement of new concrete over old may also cause surface cracks to develop. This occurs because the new concrete will shrink as it cures. Surface cracks in the spillway should be monitored and will need to be repaired if they deteriorate further. Structural cracks in the concrete are usually larger than 0.25 inch in width. They extend deeper into the concrete and may extend all the way through a wall, slab, or other structural member. Structural cracks are often caused by settlement of the fill material supporting the concrete structure, or by loss of the fill support due to erosion. The structural cracks may worsen in severity due to the forces of weathering. A registered professional engineer knowledgeable about dam safety should investigate the cause of structural cracks and prepare plans and specifications for repair of any structural cracks.EfflorescenceA white, crystallized substance, known as efflorescence, may sometimes be noted on concrete surfaces, especially spillway sidewalls. It is usually noted near hairline or thin cracks. Efflorescence is formed by water seeping through the pores or thin cracks in the concrete. When the water evaporates, it leaves behind some minerals that have been leached from the soil, fill, or concrete. Efflorescence is typically not a structural problem. Efflorescence should be monitored because it can indicate the amount of seepage finding its way through thin cracks in the concrete and can signal areas where problems (i.e. inadequate drainage behind the wall or deterioration of concrete) could develop. Also, water seeping through thin cracks in the wall will make the concrete more susceptible to deterioration due to freezing and thawing of the water.ErosionErosion due to abrasion results in a worn concrete surface. It is caused by the rubbing and grinding of aggregate or other debris on the concrete surface of a spillway channel or stilling basin. Minor erosion is not a problem but severe erosion can jeopardize the structural integrity of the concrete. A registered professional engineer should prepare plans and specifications for repair of this type of erosion if it is severe.Erosion due to cavitation results in a rough, pitted concrete surface. Cavitation is a process in which subatmospheric pressures, turbulent flow and impact energy are created and will damage the concrete. If the shape of the upper curve on the ogee spillway is not designed close to its ideal shape, cavitation may occur just below the upper curve, causing erosion. A professional engineer should prepare plans and specifications for repair of this type of erosion if the concrete becomes severely pitted which could lead to structural damage or failure.Spalling and PopoutsSpalling is the loss of larger pieces or flakes of concrete. It is typically caused by sudden impact of something dropped on the concrete or stress in the concrete that exceeded the design. Spalling may occur on a smaller scale, creating popouts. Popouts are formed as the water in saturated coarse aggregate particles near the surface freezes, expands, and pushes off the top of the aggregate and surrounding mortar to create a shallow conical depression. Popouts are typically not a structural problem. However, if a spall is large and causes structural damage, a registered professional engineer should prepare plans and specifications to repair the spalling.Inspection and MonitoringRegular inspection and monitoring is essential to detect problems with concrete materials. Concrete structures should be inspected a minimum of once per year and after any significant weather event. The inspector should also look at the interior condition of concrete spillway conduit. Proper ventilation and confined space precautions must be considered when entering a conduit. It is important to keep written records of the dimensions and extent of scaling, disintegration, efflorescence, honeycombing, erosion, spalling, popouts, and the length and width of cracks. Structural cracks should be monitored more frequently and repaired if they are a threat to the stability of the structure or dam. Photographs provide invaluable records of changing conditions.A rapidly changing condition may indicate a very serious problem, and the State Dam Safety Agency should be contacted immediately. All records should be kept in the operation, maintenance, and inspection manual for the dam.Solutions of structural concrete designDesign of members and structures of reinforced concrete is a problem distinct from but closely related to analysis. Strictly speaking, it is almost impossible to exactly analyze a concrete structure, and to design exactly is no less difficult. Fortunately, we can make a few fundamental assumptions which make the design of reinforced concrete quite simple, if not easy.A problem unique to the design of reinforced concrete structures is the need to detail each member throughout. Steel structures, in general, require only the detailed design of connections. For concrete structures, we must determine not only the area of longitudinal and lateral reinforcement required in each member, but also the way to best arrange and connect the reinforcement to insure acceptable structural performance. This procedure can be made reasonably simple, if not easyReinforced concrete is one of the principal building materials used in engineered structures because: Low cost Weathering and fire resistance Good compressive strength Formability all these criteria make concrete an attractive material for wide range of structural applications such as buildings, dams, reservoirs, tanks, etc.
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