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CHAPTER 1: INTRODUCTION The resulting combination of two materials, 1.1 CONCRETE, REINFORCED known as reinforced concrete, combines CONCRETE, AND PRESTRESSED many of the advantages of each: the CONCRETE...

CHAPTER 1: INTRODUCTION The resulting combination of two materials, 1.1 CONCRETE, REINFORCED known as reinforced concrete, combines CONCRETE, AND PRESTRESSED many of the advantages of each: the CONCRETE relatively low cost, good weather and fire resistance, good compressive strength, and Concrete is a stonelike material obtained by excellent formability of concrete and the permitting a carefully proportioned mixture high tensile strength and much greater of cement, sand and gravel or other coarse ductility and toughness of steel. aggregate, and water to harden in forms of the shape and dimensions of the desired 60 ksi is most commonly used in steel structure. Construction known as prestressed The bulk of the material consists of fine and concrete, however, does use steels and coarse aggregate. concretes of very high strength in combination. The process of controlling conditions after placement is known as curing. Prestressing greatly reduces both the deflections and the tensile cracks at ordinary The factors that make concrete a universal loads in such structures and thereby enables building material are so pronounced that it these high strength materials to be used has been used, in more primitive kinds and effectively ways than at present, for thousands of years, starting with lime mortars from 12,000 to 1.2 STRUCTURAL FORMS 6000 BCE in Crete, Cyprus, Greece, and the Middle East. cylindrical shell - single curvature ​ ​ - similar to folded plate ​ ​ - once quite popular in the united states Doubly curved shell surfaces may be Dead loads are those that are constant in generated by simple mathematical curves magnitude and fixed in location throughout such as circular arcs, parabolas, and the lifetime of the structure. Usually the hyperbolas, or they may be composed of major part of the dead load is the weight of complex combinations of shapes. the structure itself. The hyperbolic paraboloid shape, defined Live loads consist chiefly of occupancy by a concave downward parabola moving loads in buildings and traffic loads on along a concave upward parabolic path, has bridges. They may be either fully or partially been widely used. in place or not present at all, and may also change in location. Bridge design has provided the opportunity for some of the most challenging and Tabulated live loads cannot always be used. creative applications of structural The type of occupancy should be considered engineering. The award-winning Napoleon and the probable loads computed as Bonaparte Broward Bridge, shown in Fig. accurately as possible. Warehouses for 1.8 , is a six-lane, cable-stayed structure that heavy storage may be designed for loads as spans St. John’s River at Dame Point, high as 500 psf or more Jacksonville, Florida. It has a 1300 ft center span. Live loads for highway bridges are specified by the American Association of State Figure 1.9 shows the Bennett Bay Highway and Transportation Officials Centennial Bridge, a four-span continuous, (AASHTO) in its LRFD Bridge Design segmentally cast-in-place box girder Specifications. structure Environmental loads consist mainly of 1.3 LOADS snow loads, wind pressure and suction, Loads that act on structures can be divided earthquake load effects (that is, inertia into three broad categories: dead loads, live forces caused by earthquake motions), soil loads, and environmental loads. pressures on subsurface portions of structures, loads from possible ponding of rainwater on flat surfaces, and forces caused 1.7 SAFETY PROVISIONS OF THE by temperature differentials ACI CODE 1.4 SERVICEABILITY, STRENGTH, A maximum load factor of 1.0 is used for AND STRUCTURAL SAFETY wind load W and earthquake load E because these loads are expressed at strength level. 1.5 DESIGN BASIS ASCE/SEI 7, Minimum Design Loads for This design concept is known as strength Buildings and Other Structures. design. D = dead load 1.6 DESIGN CODES AND E = earthquake SPECIFICATIONS L = live load R = rain The American Concrete Institute (ACI) S = snow has long been a leader in such efforts. As F = fluids one part of its activity, the American H = earth pressure Concrete Institute has published the widely T = cumulative effects (creep, shrinkage) recognized Building Code Requirements for Structural Concrete and The joint application of strength reduction Commentary. factors ( Table 1.3 ) and load factors ( Table 1.2 ) is aimed at producing approximate The design of railway bridges is done probabilities of understrength of the order of according to the specifications of the 1/100 and of overloads of 1/1000. This AREMA Manual of Railway Engineering. results in a probability of structural failure of the order of 1/100,000. 1.8 DEVELOPING FACTORED Portland cement is a finely powdered, GRAVITY LOADS grayish material that consists chiefly of calcium and aluminum silicates. The common raw materials from which it is made are limestones, which provide CaO, and clays or shales, which furnish SiO 2 and Al 2 O 3. The material is shipped in bulk or in bags containing 94 lb of cement. Over the years, five standard types of portland cement have been developed. Type CHAPTER 2: MATERIALS I, normal portland cement, is used for over 90 percent of construction in the United 2.2 CEMENT States. Concretes made with Type I portland cement generally need one to two weeks to A cementitious material is one that has the reach sufficient strength so that forms of adhesive and cohesive properties necessary beams and slabs can be removed and to bond inert aggregates into a solid mass of reasonable loads applied; they reach their adequate strength and durability. design strength after 28 days and continue to gain strength thereafter at a decreasing For making structural concrete, hydraulic rate. cements are used exclusively. To speed construction when needed, high Water is needed for the chemical process early strength cements such as Type III (hydration) in which the cement powder have been developed. They are costlier than sets and hardens into one solid mass. ordinary portland cement, but within 7 to 14 days they reach the strength achieved using Of the various hydraulic cements that have Type I at 28 days. Type III portland cement been developed, portland cement, which contains the same basic compounds as Type was first patented in England in 1824, is by I, but the relative proportions differ and it is far the most common. ground more finely. When cement is mixed with water to form a The chemical process involved in the setting soft paste, it gradually stiffens until it and hardening liberates heat, known as heat becomes a solid. This process is known as of hydration. setting and hardening. 2.3 AGGREGATES The water in the paste dissolves material at the surfaces of the cement grains and forms In ordinary structural concretes the a gel that gradually increases in volume and aggregates occupy 65 to 75 percent of the stiffness. This leads to a rapid stiffening of volume of the hardened mass. the paste 2 to 4 hours after water has been added to the cement Natural aggregates are generally classified as fine and coarse. For complete hydration of a given amount of cement, an amount of water equal to about Natural aggregates are generally classified 25 percent of that of cement, by as fine and coarse. Fine aggregate weight—that is, a water-cement ratio of (typically natural sand) is any material that 0.25—is needed chemically. will pass a No. 4 sieve, that is, a sieve with four openings per linear inch. Material For normal concretes, the water-cement ratio coarser than this is classified as coarse is generally in the range of about 0.40 to aggregate. 0.60, although for high-strength concretes, ratios as low as 0.21 have been used. In this ASTM C33, “Standard Specification for case, the needed workability is obtained Concrete Aggregates,” through the use of admixtures. The unit weight of normal weight concrete, The strength of the hardened paste decreases that is, concrete with natural aggregates, in inverse proportion to the fraction of the varies from about 140 to 152 pounds per total volume occupied by pores. cubic foot (pcf) and can generally be assumed to be 145 pcf. For special purposes, lightweight concretes, on one hand, and heavy concretes, on the other, are used.  X-radiation in nuclear reactors and similar A variety of lightweight aggregates are installations, for protective structures, and available. Some unprocessed aggregates, for special purposes, such as counterweights such as pumice or cinders, are suitable for of lift bridges. Unit weights of heavyweight insulating concretes, but for structural concretes with natural heavy rock lightweight concrete, processed aggregates aggregates range from about 200 to 230 pcf; are used because of better control. if iron punchings are added to high-density ores, weights as high as 270 pcf are ASTM C330, “Standard Specification for achieved. The weight may be as high as 330 Lightweight Aggregates for Structural pcf if ores are used for the fines only and Concrete.” steel for the coarse aggregate. Structural lightweight concretes have unit 2.4 PROPORTIONING AND MIXING weights between 70 and 120 pcf, with most CONCRETE in the range of 105 to 120 pcf. Lower density lightweight concretes typically have To reduce the free water while retaining the compressive strengths of 1000 to 2500 psi workability, cement must be added. and are chiefly used as fill, such as over light-gage steel floor panels. The weights of the fine and coarse aggregates are based on material in the Lightweight concretes with unit weights saturated surface dry condition. between 90 and 120 pcf have compressive strengths comparable to those of Trial-batch method - Selecting a normalweight concretes. water-cement ratio from information such as that in Fig. 2.1 , one produces several small trial batches with varying amounts of aggregate to obtain the required strength, consistency, and other properties with a minimum amount of paste Heavyweight concrete is sometimes Concrete consistency is most frequently required for shielding against gamma and measured by the slump test. Slumps for concretes in building Conveying of most building concrete from construction generally range from 2 to 5 in. the mixer or truck to the form is done in bottom-dump buckets or by pumping batching is carried out in special batching through steel pipelines. plants. The chief danger during conveying is that of The principal purpose of mixing is to segregation, the separation of the individual produce an intimate mixture of cement, components of concrete because of their water, fine and coarse aggregate, and dissimilarity. possible admixtures of uniform consistency throughout each batch. Placing is the process of transferring the fresh concrete from the conveying device to On the other hand, in construction under its final place in the forms. congested city conditions, on smaller jobs, and frequently in highway construction, Proper placement must avoid segregation, ready mixed concrete is used. displacement of forms or of reinforcement in the forms, and poor bond between A good guide for maximum mixing time is successive layers of concrete. Immediately to allow 1 hour at a temperature of 70 ˚ F, upon placing, the concrete should be plus (or minus) 15 min for each 5 ˚ F drop consolidated, usually by means of (or rise) in concrete temperature for vibrators. concrete temperatures between 40 and 90 ˚ F. Ten minutes may be used at 95 ˚ F, the practical upper limit for normal mixing and placing. Vibration is not needed for 2.5 CONVEYING, PLACING, self-consolidating concrete, a fluid COMPACTING, AND CURING concrete that consolidates under its own ASTM C31, “Standard Practice for weight Making and Curing Concrete Test Specimens in the Field.” The maintenance of proper conditions during this time is known as curing. The cylinders are moist-cured at about 70 ˚ F, generally for 28 days, and then tested in the laboratory at a specified rate of loading. Thirty percent of the strength or more can The compressive strength obtained from be lost by premature drying out of the such tests is known as the cylinder strength concrete; similar amounts may be lost by fc and is the main property specified for permitting the concrete temperature to drop design purposes. to 40 ˚ F or lower during the first few days unless the concrete is kept continuously 150yd^3 moist for a long time thereafter. 6 x 12 in. 4 x 8 in. Freezing of fresh concrete may reduce its strength by 50 percent or more. No strength test (the average of two or three cylinder tests depending on cylinder size) To prevent such damage, concrete should be falls below the required fc by more than 500 protected from loss of moisture for at least 7 psi if fc is 5000 psi or less or by more than days and, in more sensitive work, up to 14 0.10 fc if fc exceeds 5000 psi. days. ACI 301 “Specifications for Structural 2.6 QUALITY CONTROL Concrete” The main measure of the structural quality of concrete is its compressive strength. ASTM C172, “Standard Method of Sampling Freshly Mixed Concrete,” 2.7 ADMIXTURES Certain organic compounds are used to reduce the water requirement of a concrete Admixtures are often used to improve mix for a given slump. Such compounds are concrete performance. There are admixtures termed water-reducing admixtures or to accelerate or retard setting and hardening, plasticizers. improve workability, increase strength, improve durability, decrease permeability, High-range water-reducing admixtures, and impart other properties or superplasticizers, are used to produce high-strength concrete (see Section 2.12) ASTM C494, “Standard Specification for with a very low water-cement ratio while Chemical Admixtures for Concrete.” maintaining the higher slumps needed for proper placement and compaction of the Air-entraining agents are widely used. concrete. They cause the formation of small dispersed air bubbles in the concrete. When superplasticizers are combined with viscosity-modifying admixtures, they can be Accelerating admixtures are used to reduce used to produce self-consolidating setting time and accelerate early strength concrete. development. Fly ash and silica fume are pozzolans, Calcium chloride is the most widely used highly active silicas, that combine with accelerator because of its cost effectiveness. calcium hydroxide, the soluble product of cement hydration (Section 2.2), to form Set-retarding admixtures are used more calcium silicate hydrate, the insoluble primarily to offset the accelerating effect of product of cement hydration ( Refs. 2.17 and high ambient temperature and to keep the 2.18 ). concrete workable during the entire placing period. Pozzolans qualify as supplementary In present practice, the specified cementitious materials, also referred to as compressive strength fc is commonly in the mineral admixtures, which are used to range from 3000 to 6000 psi for replace a part of the portland cement in normalweight cast-in-place concrete, and up concrete mixes to about 10,000 psi for precast prestressed concrete members. ASTM C618, “Standard Specification for Coal Fly Ash and Raw or Calcified The high-strength concretes, with fc to Natural Pozzolan for Use in Concrete,” 15,000 psi or more, are used with increasing frequency, particularly for heavily loaded ASTM C1240, “Standard Specification columns in high-rise concrete buildings and for Silica Fume Used in Cementitious for long-span bridges (mostly prestressed) Mixtures,” where a significant reduction in dead load may be realized by minimizing member In contrast to fly ash, silica fume contributes cross section dimensions mainly to strength gain at early ages, from 3 to 28 days. Creep is the slow deformation of a material over considerable lengths of time at constant Slag cement is another supplementary stress or load. cementitious material. Because initial elastic strains are also ASTM C989, “Standard Specification for proportional to stress in this range, this Slag Cement for Use in Concrete and permits definition of the creep coefficient. Mortar,” 2.8 PROPERTIES IN COMPRESSION Performance of a structure under load depends to a large degree on the stress-strain relationship of the material from which it is made When concrete is subject to fluctuating importance of the fracture properties of the rather than sustained loading, its fatigue material as distinct from tensile strength. strength, as for all other materials, is considerably smaller than its static strength. 2.10 STRENGTH UNDER COMBINED STRESS When plain concrete in compression is stressed cyclically from zero to maximum If any one of them is zero, a state of biaxial stress, its fatigue limit is from 50 to 60 stress is said to exist; percent of the static compressive strength, for 2,000,000 cycles. if two of them are zero, the state of stress is uniaxial. 2.9 PROPERTIES IN TENSION 2.11 SHRINKAGE AND tensile strength has been measured in terms TEMPERATURE EFFECTS of the modulus of rupture fr. For structures in which a reduction in The splitting tensile strength test also cracking is of particular importance, such as provides a measure of the tensile strength of bridge decks, pavement slabs, and liquid concrete. storage tanks, expansive cement concrete may be appropriate. ASTM C845, “Standard Specification for Expansive Hydraulic Cement.” While reinforced concrete structures have 2.12 HIGH-STRENGTH CONCRETE been successfully designed and built for over 150 years without the use of fracture There are a number of applications in which mechanics, the brittle response of high-strength concrete will provide high-strength concretes (Section 2.12), in improved structural performance tension as well as compression, increases the Although the exact definition is arbitrary, 2.13 REINFORCING STEELS FOR the term generally refers to concrete having CONCRETE uniaxial compressive strength in the range of about 8000 to 20,000 psi or higher. The useful strength of ordinary reinforcing steels in tension as well as compression, that An essential requirement for high-strength is, the yield strength, is about 15 times the concrete is a low water–cementitious compressive strength of common structural material ratio. For normal concretes, this concrete and well over 100 times its tensile usually falls in the range from about 0.40 to strength. 0.60 by weight, but for high-strength mixes it may be 0.25 or even lower On the other hand, steel is a high cost material compared with concrete. To permit proper placement of what would otherwise be a zero slump mix, high-range For most effective reinforcing action, it is water-reducing admixtures, or essential that steel and concrete deform superplasticizers, are essential and may together, that is, that there be a sufficiently increase slumps to as much as 6 or 8 in. and strong bond between the two materials to even higher when viscosity-modifying ensure that no relative movements of the admixtures are used to produce steel bars and the surrounding concrete self-consolidating concrete. Other additives occur. include fly ash and, most notably, silica fume. This bond is provided primarily by the natural roughness of the mill scale on the surface of hot-rolled reinforcing bars and by the closely spaced rib-shaped surface deformations that provide a high degree of interlock between the bars and the surrounding concrete. The thermal expansion coefficients of the For many years, bar sizes have been two materials, about 6.5 × 10 - 6 per ˚ F designated by numbers, Nos. 3 to 11 being for steel vs. an average of 5.5 × 10 - 6 per commonly used and Nos. 14 and 18 ˚ F for concrete. representing the two special large-sized bars previously mentioned. While the corrosion resistance of bare steel is poor, the concrete that surrounds the A No. 5 bar, for example, has a nominal steel reinforcement provides excellent diameter of 5/8 in. Bar sizes are rolled into corrosion protection, minimizing corrosion the surface of the bars for easy problems and corresponding maintenance identification. costs. Reinforcing bars with 40 ksi yield strength, The fire resistance of unprotected steel is once standard, have largely been replaced by impaired by its high thermal conductivity bars with 60 ksi yield strength and by the fact that its strength decreases sizably at high temperatures. Bars with yield strengths of 75 and 80 ksi are often used in columns 2.14 REINFORCING BARS bars with a yield strength of 100 ksi are The most common type of reinforcing steel allowed to be used as confining (as distinct from prestressing steel) is in the reinforcement. form of round bars, often called rebars, available in a large range of diameters from Bars with a yield strength of 120 ksi are about 3/8 to 1 3/8 in. for ordinary also available but not yet recognized by the applications and in two heavy bar sizes of ACI Code. about 1 3/4 and 2 1/4 in. Grade 40 bars are no longer available in sizes larger than No. 6 (No. 19) and Grade 50 bars are available in sizes up to No. 8 (No. 25). Most reinforced concrete in the U.S. is have either two longitudinal lines or the constructed using ASTM A615 carbon number 75 (5); Grade 80 (550) bars have steel bars. either three longitudinal lines or the number 80 (6); Grade 100 (690) bars have either ASTM A706 low-alloy steel bars are three longitudinal lines or the number 100 usually specified, however, for structures (6) * ; and Grade 120 (830) bars have either designed for seismic loading because they four longitudinal lines or the number 120 are more ductile than A615 bars. (8). The identification marks are shown in Fig. 2.17 for Grade 60 (420) bars The ACI Code permits reinforcing steels up to fy = 80 ksi for most applications. The two chief numerical characteristics that determine the character of bar reinforcement are its yield point (generally identical in tension and compression) and its modulus of elasticity Es Es = 29,000,000 psi. With further strains, the stress begins to increase again, though at a slower rate, a process that is known as strain-hardening. \ The curve flattens out when the tensile strength is reached; it then turns down until fracture occurs. A615, A706, A996 for both rail and axle ASTM A775, “Standard Specification for steel, and A1035 and an additional marking Epoxy-Coated Reinforcing Steel Bars,” to identify higher-strength steels. Grade 60 (420) bars have either one longitudinal line or the number 60 (4); Grade 75 (520) bars ASTM A934, “Standard Specification for 2.16 PRESTRESSING STEELS Epoxy-Coated Prefabricated Steel Prestressing steel is used in three forms: Reinforcing Bars,” round wires, strands, and alloy steel bars. Prestressing wire ranges in diameter from ASTM A1055, “Standard Specification 0.192 to 0.276 in. It is made by cold drawing for Zinc-Epoxy DualCoated Steel high-carbon steel after which the wire is Reinforcing Bars,” stress-relieved by heat treatment to produce the prescribed mechanical properties. ASTM A767, “Standard Specification for Zinc-Coated (Galvanized) Steel Bars for ASTM A421, “Standard Specification for Concrete Reinforcement,” Uncoated Stress-Relieved Steel Wire for Prestressed Concrete” 2.15 WELDED WIRE REINFORCEMENT ASTM A416, “Standard Specification for Steel Strand, Uncoated Seven-Wire Apart from single reinforcing bars, welded Stress-Relieved for Prestressed Concrete” wire reinforcement (also described as welded wire fabric ) is often used for ASTM A722, “Standard Specification for reinforcing slabs and other surfaces, such as Uncoated HighStrength Steel Bar for shells, and for shear reinforcement in thin Prestressing Concrete.” beam webs, particularly in prestressed beams. The tensile strengths of prestressing steels range from about 2.5 to 6 times the yield ASTM Specification A1064 covers both strengths of commonly used reinforcing smooth and deformed welded wire bars. reinforcement Grade 250 ( fpu = 250 ksi), Grade 270, and Grade 300, although the last is not yet recognized in ASTM A416. Grade 270 strand is used most often. For alloy steel bars, two grades are used: the regular Grade 150 is most common, but special Grade 160 bars may be ordered. Round wires may be obtained in Grades 235, 240, and 250, depending on diameter. While the modulus of elasticity Es for deformed bars is taken as 29,000,000 psi, the effective modulus of prestressing steel varies, depending on the type of steel (for example, strand vs. wire or bars) and type of use, and is best determined by test or supplied by the manufacturer. When prestressing steel is stressed to the levels that are customary during initial tensioning and at service loads, it exhibits a property known as relaxation. Relaxation is defined as the loss of stress in stressed material held at constant length. To qualify as an alternative to minimum shear reinforcement, fiber- reinforced concrete must exhibit minimum values of residual strength when tested in flexure in accordance with ASTM C1609.