Lecture 8 - ARCH 106 Materials of Construc6on...

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Unformatted text preview: ARCH 106 Materials of Construc6on Concrete 13 CONCRETE CONSTRUCTION CONSTRUCTION CONCRETE Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. HISTORY Roman Concrete •  Earliest concrete, 300 BC •  Pozzolanic ash: Volcanic ash originally found at Pozzuoli, Italy; high in silica and alumina content •  When pozzolanic ash was mixed with lime, water, sand, and rock, the resul4ng mixture would harden to an unusually strong material, even when fully submerged under water. •  Hydraulic cement: Cement that when hardened, is not water soluble Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. HISTORY Modern Concrete •  Knowledge of hydraulic cements is lost with the fall of the Roman Empire. •  Hydraulic cements are rediscovered in late 18 c. England. •  1824: Joseph Aspdin patents Portland Cement, an ar4ficial hydraulic cement made from a finely ground mixture of limestone and clay. This name for hydraulic cement remains in use to today. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. HISTORY Modern Concrete •  The embedding of steel reinforcing in concrete, to increase concrete's tensile strength and resilience, is the other key component of modern concrete. •  Steel reinforced concrete first appears in the 1850's. •  Right: Francois Hennebique's 1892 patent for reinforced concrete construc4on Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. 13 CONCRETE CONSTRUCTION CEMENT AND CONCRETE Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Concrete ingredients: •  Fine aggregate (sand) •  Coarse aggregate (gravel) –  Coarse and fine aggregate provide the structural mass of the concrete and cons4tute the majority of the concrete volume. •  Portland cement –  Cement binds the aggregate. •  Water –  Water is necessary for the chemical hydra4on of the cement and the hardening of the concrete. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Portland Cement •  Primary ingredient: Calcium silicates •  Lesser amounts of compounds of aluminum, iron, and magnesium •  Raw materials can be extracted from a variety of sources depending on the locale, such as limestone, chalk, marl, marble, sea shells, clays, shales, industrial waste materials, iron ore, bauxite, and others. •  Materials are crushed, ground, propor4oned and blended into a very fine powder. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Portland Cement •  The blended materials are burn or sintered in a high ­temperature, rota4ng kiln, crea4ng clinker. •  The clinker is cooled and ground to a fine powder. •  Small quan44es of The cement kiln is gently sloped, so that as it rotates, the gypsum are added to cement ingredients travel down the slope at a controlled rate. adjust the cement's seeng rate. •  In the U.S., a standard bag of cement contains 1 cubic foot of material and weighs 94 lb. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Air ­Entraining Cement •  Ingredients added to the cement generate bubbles during concrete mixing that create small, distributed voids in the finished concrete. •  Air content in the range of 2 to 8 percent of the total concrete volume is typical. •  Air ­entrained concrete has greater resistance to freeze ­ thaw damage. It is use for concrete exposed to wet, freezing condi4ons. •  Air ­entrained concrete also has improved workability when wet. •  Air entraining reduces concrete strength, unless the propor4ons of other ingredients in the concrete mix are adjusted to compensate. Copyright © 2009 J. Iano. All rights reserved. Enlarged section of air-entrained concrete: Note the air voids within the cement paste matrix. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on CEMENT AND CONCRETE ASTM C150 Cement Types •  Type I: General purpose •  Types II and Type V: For concrete in contact with soils or water with high sulfate concentra4ons •  Type III: For cold weather construc4on, concrete precast in plants, accelerated construc4on schedules •  Type IV: For massive structures, such as dams, where the heat generated during hydra4on must be limited to avoid excessive temperatures ASTM C150 Type I Type IA Type II Type IIA Type III Type IIIA Type IV Type V Description Normal Normal, air entraining Moderate resistance to sulfate attack Moderate resistance to sulfate attack, air entraining High early strength High early strength, air entraining Low heat of hydration High resistance to sulfate attack Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE ASTM C150 Cement Types •  Ninety percent of U.S. manufactured cements are Type I, Type II, or Type I/II (cement that meets all the requirements of both types). •  Most air ­entrained concrete is made with admixtures added separately to the concrete mix rather than with air ­entraining cements. Separate admixtures allow greater control over amount and quality of air entraining in the final concrete. ASTM C150 Type I Type IA Type II Type IIA Type III Type IIIA Type IV Type V Description Normal Normal, air entraining Moderate resistance to sulfate attack Moderate resistance to sulfate attack, air entraining High early strength High early strength, air entraining Low heat of hydration High resistance to sulfate attack Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Portland Cement and Carbon Dioxide Emissions •  Cement produc4on is a significant source of emissions of carbon dioxide (CO2), a greenhouse gas. •  The burning of fuel to heat the cement kiln generates CO2. •  The chemical conversion of limestone (calcium carbonate) within the kiln releases addi4onal large quan44es of CO2. •  In total, manufacturing 1000 pounds of portland cement generates on average roughly 900 pounds of carbon dioxide. Simplified chemistry of the cement kiln: Limestone + silica (2700 deg F) = portland cement + carbon dioxide 5CaCO3 + 2SiO2 (3CaO, SiO2) (2CaO, SiO2) + 5CO2 Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Portland Cement and Carbon Dioxide Emissions •  The subs4tu4on of industrial waste products with cemen4ng proper4es (fly ash, silica fume, blast furnace slag) for some por4on of the cement in concrete reduces the total CO2 emissions associated with concrete produc4on. •  When concrete is crushed and recycled, it is capable of absorbing up to half of the CO2 originally generated in the cement manufacture. •  New, experimental cements aim to reduce CO2 emissions, even to the Cement clinker after emerging from the cement kiln extent that concrete becomes carbon ­nega4ve, absorbing more CO2 during its life4me than is emined during its manufacture. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Aggregates •  Coarse and fine aggregate make up 60 to 80 percent of the total concrete volume. •  Most aggregate comes from natural sand and gravel deposits or is made from crushed rock. •  Ar4ficial aggregates may come from: –  –  –  –  Blast furnace slag Fly ash Recycled concrete Thermally treated clay, shale, and other minerals •  Strength of concrete is heavily dependent on the quality of the aggregates: –  –  –  –  –  Porosity Size distribu4on Moisture absorp4on Shape and surface texture Strength, elas4city, density, soundness –  Contamina4on or detrimental substances Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Maximum Aggregate Size •  Generally, with larger aggregates, less cement is required in the concrete mix. Reducing the quan4ty of cement reduces cost, since cement is the most expensive ingredient in the mix. •  The largest aggregate must fit comfortably between reinforcing bars and within the overall thickness of the concrete. Largest aggregate size should not exceed: –  1/5 distance between form faces –  3/4 of the space between reinforcing bars –  1/3 the depth of a slab •  Common maximum aggregate size for concrete used in buildings ranges from 3/8 to 1 ­1/2 in. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Lightweight Aggregates: Used to make lighter ­weight concrete •  Structural lightweight concrete: –  Roughly 80 percent or less of the weight of ordinary concrete –  Reduced structure weight saves costs –  Lower thermal conduc4vity increases resistance to building fires –  Roughly 60 percent or less of the weight of ordinary concrete –  Insula4ng roof toppings –  Fill material •  Nonstructural lightweight concrete: •  Expanded shale, clay, slate, slag: Minerals or industrial waste products thermally treated such that they expand and take on a less dense, cellular structure •  Cinders and other volcanic rocks: Naturally occurring light ­weight volcanic materials •  Vermiculate: Thermally expanded mica •  Perlite: Thermally expanded volcanic glass Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Water •  Water is an essen4al ingredient in concrete, that combines chemically with the cement as the concrete hardens. •  Water must be free of contaminants. •  Water that is potable is acceptable for use in making concrete. •  ASTM C 1602 permits the use of other sources such as waste water from washing out concrete trucks or storm water runoff at concrete produc4on facili4es that may contain limited amounts of concrete waste material. •  The quan4ty of water in the concrete mix must be controlled as closely as any other ingredient: Adding unneeded water dilutes the cement paste, weakening the hardened concrete. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. CEMENT AND CONCRETE Admixtures •  Other ingredients in the concrete mix used to alter or improve concrete proper4es in various ways: –  –  –  –  –  –  –  –  Air ­entraining Water ­reducing Cure accelera4ng or retarding Workability modifying Shrinkage ­reducing Corrosion inhibi4ng Freeze protec4ng Coloring •  E.g., High ­strength concrete for very tall buildings: –  Supplementary Cemen44ous Materials (SCMs), for greater strength –  Water reducers, to increase concrete strength while maintaining workability –  Admixtures to improve pumpability –  Retarding admixtures, to allow adequate 4me for placing Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. Burj Dubai, world's tallest concrete framed building (800+ meters, 2600+ feet) 13 CONCRETE CONSTRUCTION MAKING AND PLACING CONCRETE Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. MAKING AND PLACING CONCRETE Concrete Quality •  Workability: Ease of placing, consolida4ng, and finishing wet concrete •  Structural proper4es when hardened: Strength, s4ffness, •  Other important proper4es: –  –  –  –  –  –  –  –  –  –  –  Rate of early strength gain Degree shrinkage during curing Flatness, for slabs and paving Surface hardness, for industrial slabs Porosity Density Surface appearance, for architectural concrete Resistance to freeze/thaw and weather, for exterior concrete Water4ghtness, for dams, tanks, exterior walls Cost and more… durability Concrete for an exterior planter can be of rela4vely low strength, but must have good resistance to weathering and, aser formwork is removed, a reasonably smooth and pleasing surface appearance. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. MAKING AND PLACING CONCRETE Concrete Strength •  Concrete strength varies with design of the concrete mix. •  Normal strength concrete –  Up to 6000 psi compressive strength –  Made with conven4onal ingredients •  High ­strength concrete •  Specially formulated, ultra ­high performance concretes have compressive strengths as high as 30,000 psi. –  Greater than 6000 psi to roughly 20,000 psi –  Supplementary Cemen44ous Materials are required to reach higher strengths. –  Lower water content, required for higher strength, results in a s4ff, unworkable mixture when wet. To compensate, water reducing admixtures or high ­range water reducing admixtures (superplasLcizers) are used to improve workability. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. MAKING AND PLACING CONCRETE Water ­Cement Ra4o •  The water ­cement raLo (w/c raLo) is the most important determinant of concrete strength. Lowering the propor4on of water to cement: –  Increases concrete strength and durability –  Decreases workability –  Increases cost •  W/C ra4o is measured by weight, not volume. •  When supplementary cemen44ous materials are added to the mix, the ra4o is measured as the water ­ cemenLLous materials raLo. •  Why use higher ­strength concrete: –  Reduce column dimensions in tall buildings. –  Achieve higher earlier strength, allowing construc4on to proceed more quickly. –  Sa4sfy more stringent structural requirements. Copyright © 2009 J. Iano. All rights reserved. (A ­E Concrete: Air ­entrained concrete) Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on MAKING AND PLACING CONCRETE Mixing Concrete •  Most concrete is prepared at batch plants and delivered to the construc4on site in transit ­mix (ready mix) trucks. •  The concrete ingredients are mixed in the rota4ng drum of the truck so that the concrete is ready to pour on arrival at the construc4on site. •  Smaller batches of concrete may be prepared on site by hand or with the aid of portable power mixers. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. MAKING AND PLACING CONCRETE Slump Test •  The slump test provides a rough measure of the workability of concrete while wet. •  Concrete is placed into a conical cylinder; the cylinder is removed, and the loss in height of the concrete mass is measured. –  Concrete with too low slump may be difficult to place. –  Concrete with too high slump may have had too much water added. •  Specified maximum slump is usually in the range of 3 to 5 inches. •  Slump tests are performed on batches of concrete as the arrive on the concrete site. •  Slump that varies excessively from one batch to the next may indicate quality control problems in the concrete mixes. Copyright © 2009 J. Iano. All rights reserved. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on MAKING AND PLACING CONCRETE Strength Test Cylinders •  Test cylinders may be cast from each batch of concrete delivered to the construc4on site. –  The most common cylinder size is 6 in. diameter x 12 in. high. –  Concrete is placed in a cylindrical mold and consolidated to eliminate voids. Test cylinders prepared and labeled on site. •  Cylinders are returned to the laboratory, cured under controlled condi4ons, and then strength ­tested at appropriate 4mes. Test results for laboratory ­ cured cylinders verify the quality of the concrete as delivered to the site. •  Cylinders can also be cured on site in condi4ons similar to that for the cast concrete. Results from these tests can be used to determine when it is safe to remove formwork or subject cast concrete to construc4on loads. Copyright © 2009 J. Iano. All rights reserved. Laboratory cylinder testing apparatus Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on MAKING AND PLACING CONCRETE Placing Concrete •  Avoid delays, during which concrete can s4ffen and become difficult to place. Depending on condi4ons, concrete can placed up to 90 minutes aser mixing commences. •  Concrete that does s4ffen can have water added prior to placing, provided that: –  Maximum w/c ra4o is not exceeded –  Maximum slump is not exceeded –  Agita4on limits are not exceeded •  Concrete may be placed on site directly from the discharge chute of a transit ­mix truck, or by combina4on of wheelbarrows, power buggies, crane ­lised buckets (right), conveyor belts, pumpers, or other devices. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. MAKING AND PLACING CONCRETE Placing Concrete •  SegregaLon, separa4on of large aggregate from the finer por4ons of the mix, must be avoided. •  Place concrete as close to final posi4on as possible. •  Do not push concrete over large horizontal distances. •  Avoid dropping concrete from high heights or discharging against obstacles (use drop chutes if needed). •  Right: Concrete delivered to its final loca4on by pumping. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. MAKING AND PLACING CONCRETE Consolida4ng Concrete •  ConsolidaLon (compacLon): The elimina4on of voids and air pockets within the concrete pour. –  –  –  –  Hand rodding or tamping Screeding (top) Internal vibra4on (bonom) External vibra4on •  Consolida4on is especially cri4cal with s4ff concrete mixes or when concrete is placed around densely packed reinforcing arrays. •  Over ­consolida4on must be avoided, as it can lead to segrega4on of aggregate as larger par4cles descend and finer components rise to toward the surface. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. MAKING AND PLACING CONCRETE Curing Concrete •  Concrete hardens by hydraLon, the chemical bonding of water and cement. •  If concrete dries out prematurely, the hydra4on process stops and maximum strength is not achieved. •  Hydra4on, along with increasing strength and durability, can con4nue for a very long 4me, even years. Concrete strength is normally specified at 28 days. •  Exposed surfaces of newly poured concrete must be protected from evapora4on and drying. Concrete may be regularly misted, covered with moisture ­retaining materials, or treated with a chemical surface sealer. •  In very hot or cold weather, steps may be taken to moderate the temperature of the concrete, such as pre ­hea4ng of ingredients, adding water as ice, or the use of retarders or accelerators to adjust cure rate. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. MAKING AND PLACING CONCRETE Formwork •  Formwork: Construc4on, usually temporary, to hold freshly poured concrete in the desired shape un4l the concrete achieves sufficient strength to support itself. •  Formwork also frequently helps to protect newly poured concrete from drying too quickly. •  Formwork must be strong and s4ff enough to support the weight and fluid pressure of the concrete. •  In conven4onal concrete construc4on, formwork can account for half or more of total concrete construc4on costs. •  Form release compounds are oils, waxes, or plas4c coa4ngs applied to formwork surfaces to prevent adhesion of the formwork to the concrete and ease formwork removal. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. 13 CONCRETE CONSTRUCTION REINFORCING Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. REINFORCING The Concept of Reinforcing •  Concrete has no useful tensile strength—ability to resist pulling forces. (See table below.) •  Steel bars or wires are laid into concrete along lines of tension, to provide resistance to these forces. •  Steel and concrete work well together. –  The two materials have similar rates of thermal expansion/contrac4on. –  The alkaline chemistry of the concrete protects the steel from corrosion. –  The two materials bond well, allowing them to work as a single structural composite. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. REINFORCING Steel Reinforcing Bars •  Steel reinforcing bars (rebar): The most common concrete reinforcing material. Rebar are hot ­rolled steel, deformed with surface ridges so as to bener bond with concrete. •  Bar sizes are numbered, with numbers generally corresponding the diameter of the bar in 8ths of an inch. E.g., –  #4 bar is 1/2 in. diameter –  #8 bar is 1 in. diameter •  Bars are made with steel grades of varying strength. •  Higher strength bars are used to reduce rebar congesLon, where reinforcing becomes crowded and concrete placement becomes more difficult. Use of higher strength steel allows for bars smaller in diameter or with greater spacing. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. REINFORCING Welded Wire Reinforcing •  Welded wire reinforcing (WWR) or welded wire fabric (WWF): Prefabricated, welded grids of reinforcing bars or wires •  Especially common for concrete slab reinforcing. •  Smooth wire designa4ons: From W1.4=0.134 in. diameter , to W20=0.505 in. diameter •  Deformed wire designa4ons: From D4= 0.226 in. diameter, to D20=0.505 in. diameter. •  Designa4on example: 6x12 ­W12xW5 –  W12 wires spaced at 6 in., crossed by –  W5 wires spaced at 12 in. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. REINFORCING Reinforcing Fabrica4on and Erec4on •  Concrete reinforcing requirements are shown on the structural drawings. •  A reinforcing fabricator prepares shop drawings that are reviewed by the structural engineer. •  Reinforcing is cut to length, bent as needed, and possibly par4ally assembled before being transported to the construc4on site. •  Fabrica4on con4nues on site. Eventually reinforcing is assembled in its final configura4on. •  Aser inspec4on by the engineer, concrete may be poured. Shop-fabricated spiral reinforcing, for drilled concrete piers. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. REINFORCING Reinforcing Fabrica4on and Erec4on Heavy reinforcing for a concrete shear wall is being pre ­assembled on site. Once complete, the assembly will be lised into place by a construc4on crane. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. REINFORCING Reinforcing a Simple Beam •  Top: In a simply supported beam, the greatest tension forces occur at the bonom middle of the beam. •  Where tension forces cross compression forces closer to the beam ends, shear forces also occur. •  BoQom: The placement of reinforcing in a concrete beam approximates the lines of tension, but is simplified to reduce fabrica4on costs. Theoretical lines of tension and compression in a simple beam. Simple concrete beam reinforcing, including heavy bottom bars, lighter top bars, and U-shaped shear stirrups Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. REINFORCING Reinforcing a Column •  VerLcal or column bars: Larger ­diameter bars placed ver4cally in the column •  Ties or spirals: Wrap around the ver4cal bars •  The ver4cal bars add to the strength of the column in compression, and resist tensile forces that are introduced from wind or seismic forces, or from connec4ons to beams. •  Ties prevent the ver4cal bars from buckling outward. •  Note how, where the ver4cal bars extend beyond the 4es, they are bent slightly inward toward the center of the column. As the column height is extended further, the next sec4on of reinforcing will nest and overlap with these ends. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. REINFORCING Fibrous Reinforcing •  Fibrous reinforcing: Short fibers of glass, steel, or polypropylene, added to the concrete mix •  Microfiber reinforcing: Rela4vely low amounts of fibers, to aid concrete in resis4ng plasLc shrinkage cracking that occurs during early curing •  Macrofiber reinforcing: Greater concentra4ons of fibers, that also resist longer ­term cracking due to drying and thermal stresses •  Steel fiber reinforcing also increases the durability of the concrete surface Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. 13 CONCRETE CONSTRUCTION PRESTRESSING Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. PRESTRESSING Prestressing •  Prestessing: Applying an ini4al compressive stress to a concrete member, so as to improve its structural efficiency. •  High ­strength steel strands are stretched 4ghtly and then restrained by the concrete, pueng the concrete into ini4al compression. •  When normal service loads are added, the concrete in prestressed members is subject to less tensile force. •  Prestressed members are typically more slender and lighter than comparable conven4onally reinforced members. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Copyright © 2009 J. Iano. All rights reserved. PRESTRESSING Prestressing •  Pretensioning: Steel strands are tensioned before concrete is cast. This requires heavy abutments to restrain the strands and is normally only done with concrete in precas4ng plants, but not on the construc4on site. •  PosQensioning: Steel strands are tensioned aser concrete has been cast and reached adequate strength. Concrete cast on the construc4on site can be posnensioned. Copyright © 2009 J. Iano. All rights reserved. Posttensioning strands, encased in green plastic sheaths, for reinforcing a slab. The strands are purposely draped to more closely follow the natural lines of tension in the slab. Fundamentals of Building Construc4on, Materials & Methods, 5th Edi4on Reading Assignment: Chapter 10 Page 516 ­554 Concrete •  History •  Making and Placing •  Formwork •  Reinforcing •  Prestressing ...
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