The Deformation and Processing of Structural Materials

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However, the Northridge and the Hyogoken-Nanbu Kobe earthquakes resulted in considerable damage and nonductile performance of steel buildings despite the material ductility. These events showed that ductile materials do not automatically ensure ductile structures because the material must be used in ways that permit the ductile material properties to control system performance. As a result, the search for even greater ductility is always in progress. One contributing factor noted in these studies was that the continuing evolution toward higher yield strength steels as illustrated in Fig 2.

This figure shows the average measured yield stress and tensile stress measured in steel test specimens reported in the literature of recent decades. The ultimate tensile strength, Fu, is commonly associated with fracture or tearing of steel, and therefore an increased yield to tensile ratio reduces the amount of strain hardening reserve during seismic deformation of steel.

This limits the extent of yielding during inelastic deformation and causes larger local inelastic strain demands on the locations where yield occurs FEMA D As a result, greater ductility demands on welds and other critical elements may occur, and steels with higher yield to tensile strength ratios have a greater tendency to fracture during earthquake loading. Steel producers have responded to this need. In comparison to conventional mild steel, the new steel, designated as SN steel, was intended to satisfy the following requirements. To To To To To set an upper limit for the ratio of yield strength to tensile strength; reduce scatter of yield strength; lower carbon equivalent Ceq and weld crack sensitivity Pcm ; increase through-thickness ductility; limit the tolerance of geometrical dimensions.

A small scatter of yield strengths is beneficial in securing the yielding, failure mechanism, and accordingly the ductility of the steel frame. SN steel achieves significantly narrower bands relative to conventional steel. Requirement 3. Requirement 5. Table 2. Class C has higher performance in the through-thickness properties by controlling Sr to lower values 0. Class B steel is used primarily for beams that sustain flexure.

The Hyogoken-Nanbu Kobe earthquake occurred in , in which many instances of welded beam-to-column connection failures were reported Nakashima and Bruneau , Nakashima et al. The use of SN steel accelerated significantly after the earthquake, and most major steel constructions in Japan have used SN steel in recent years. Box sections are very commonly used in Japanese steel moment frames primarily because they have equal bending resistance in two orthogonal directions, and Japanese engineers make extensive use of three-dimensional moment framing.

Unless they are very thick, box sections are cold-formed. Two strength grades of cold-formed box sections, MPa and MPa in tensile strength, have been available for many years.

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Two types of cold forming are adopted, i. In roll-forming, the plate is initially formed and welded into a circular tube, and then the circular tube is press-formed into a box section. In press-forming, a flat plate is bent to form a C-section, and two C-sections are welded longitudinally to form a box section.

Both processes involve significant cold working and local strain hardening of the steel. In the process of cold forming, corners of the cross-section sustain large plastic strains, and the ability to sustain elongation is decreased. When combined with welding, the corners become the weak spots that are susceptible to brittle fracture. Japan developed new grades of cold-form tubes designated as BCR for roll-forming and BCP for pressforming whose material conforms to the SN grade series.

The use of energy dissipating dampers is becoming very popular in Japanese building construction. Viscous dampers, visco-elastic dampers, and hysteretic dampers are the common choices, but hysteretic dampers in which steel is used as the energy dissipating material is by far the most popular in steel building construction. According to statistics, all steel high-rise buildings constructed in Japan for the past few years have been equipped with some types of hysteretic dampers BCJ Among various types of hysteretic dampers, shear panel dampers and buckling restrained braces are most frequently adopted.

Buckling restrained braces are commonly used as members that add damping and supplement stiffness in Japanese seismic design, and they are discussed in greater detail later in this chapter. The dampers are designed to take hysteretic energy even in medium earthquake events; hence they have to yield with a relatively small force. To ensure the relatively early yielding, low-yield LYP steel was developed in Japan. Low-yield steel is characterized by a small initial yield strength MPa in Fig. Two grades of yield strength MPa and MPa are commercially available, and they are used primarily as materials for hysteretic dampers, buckling restrained braces, and other specialty items.

Corrosion resistant steels Steel corrodes when exposed to the environment, and the painting and protection of steels has long been a significant economic cost and occasionally a deterrent to the use of steel in exposed structural systems. It is intended primarily for use in welded bridges and buildings where added durability is important. It has been used for many years in bridge construction. The atmospheric corrosion resistance of this steel in most environments is substantially better than that of carbon structural steels.

When properly exposed to the atmosphere, this steel forms a patina which retards and delays further corrosion and increases the service life of most exposed structural elements. A steel has been shown to be particularly effective in environments where the steel remains dry for much of its service life, the patina layer is protected against wear or abrasion, and the local atmosphere does not include salt water or harsh chemical content AISI In some cases, A steel is painted during fabrication to provide supplemental protection during early service life, and the patina layer forms after the initial paint coat wears away.

This practice has been shown to extend the service life in most applications. Other new steels for increased corrosion resistance have been developed by Japanese steel companies. The use of weathering steel started in the s in Japan primarily for use in steel bridges. However, research has shown that basic weathering steel does not provide the intended performance when exposed to airborne salt; hence the applications of conventional weather steel are limited to environments with low deposition of airborne salt. In recent years, Japan has instituted a complete ban of studded tires in automobile production, and this forces the increased use of road de-icing salt in cold regions during the wintertime, which inevitably increases airborne salt.

In consideration of these circumstances, a new type of weathering steel was developed to enhance salt corrosion resistance JISF c. Three strength grades i. The new weathering steel has an enhanced capacity to inhibit the penetration of chloride ions into the steel substrate. This is achieved by means of ion exchange such that the chloride ions concentrate in the outer rust layers and the sodium ions are embedded in the inner rust layers.

Welding material that contains the same level of Ni was also developed. To improve the crack sensitivity of weld metal, the carbon content of new weathering steel is reduced to 0. Field exposure tests have been conducted for the past ten years at several coastal locations. In one test location, the corrosion thickness reached 1. Applications of the steel for new bridge construction has increased during the past few years. This subsection will describe some ways in which steel is effectively used to provide the greater benefits of the material.

Often these are simply new ways of viewing and using existing materials. Buckling restrained braces Earthquakes induce inertial loading, and large earthquakes can cause internal forces that are much larger than those required for resistance of gravity loads. As a result, economical seismic design requires a balance of strength, stiffness and ductility, and steel structures are commonly used for seismic design. Concentrically braced frames CBFs are attractive for seismic design, because they provide more lateral stiffness and resistance than other structural systems with similar building cost and weight.

However, the inelastic performance of most CBFs is dominated by inelastic post-buckling deformation of the brace. Inelastic post-buckling behavior can lead to rapid deterioration of stiffness and resistance and premature tearing or fracture of the brace or its connections under large inelastic deformations. This rapid deterioration limits the benefits and complicates the design of the CBF system.

Buckling restrained braces BRBs are a new component that has been developed to overcome this deficiency and significantly improve the inelastic performance of CBFs. BRBs employ patented braces where the axial member yields in tension and compression without brace buckling as depicted in Fig. This is accomplished by encasing the slender brace bar to prevent lateral deformation and buckling without bonding the bar to the encasing element.

This ensures that the yield strengths of the bar develop in both tension and compression and the deterioration in stiffness and resistance due to buckling is avoided. It increases the inelastic energy dissipation, improves axial yield performance and permits development of large inelastic axial deformations. The BRB curves show a large energy dissipation capacity and no deterioration in stiffness and resistance as compared to the deterioration noted with the buckling brace element.

Many engineers believe that BRBs provide superior seismic performance to that of traditional bracing systems, and they are enjoying increasing use in practice. The seismic performance of BRBs also depends on the connection design. CBFs typically use gusset plate connections as depicted in Fig. These gusset plate connections may have significant in-plane rotational restraint, while being relatively flexible to out-of-plane deformations.

Excessive inplane rotational stiffness and inadequate out-of-plane stiffness may be detrimental to the performance of the BRB system, and therefore the connection must be designed to control these competing requirements. BRB connections Encasing material Steel core yields in axial load and is unbonded to encasing material Steel tube Connecting end segment Connecting end segment 2. The connection must have adequate stability and lateral restraint to prevent out-of-plane deformation, and it cannot buckle or fracture prior to the development of the full resistance and ductility of the brace.

Rotation or out-of-plane deformation of the buckling restrained concentrically braced frame BRCBF connection cannot be tolerated, because these actions may inhibit development of system resistance and ductility. Continuing work on these BRB connection issues is in progress. BRBs are one of the most promising alternatives for both seismic design of new structures and seismic retrofitting. Additional alternative BRB elements are being developed with time Merritt et al. BRBs can significantly improve the seismic performance of many structures.

US have employed these elements in recent years. The inelastic performance of the structural component is excellent, but they are a new system and the engineer must verify the performance that they provide. A recent guideline SEAONC proposes some testing and acceptance criteria that can be used to verify that the BRB is appropriate for the proposed seismic design application.

This section will briefly describe several new and innovative structural systems and note the specific aspects of their construction that relate to advanced steel construction. The new systems discussed here are primarily variations of composite and hybrid elements. Concrete filled steel tubes CFT The greatest benefits of structural steel are often achieved in composite or hybrid applications, because concrete can be combined with the steel to produce significant economic and performance advantages. Concrete has relatively low compressive strength compared to structural steel, and as a result the larger volume of concrete needed to support compressive loads provides increased structural stiffness and resistance to buckling.

Concrete has limited shear strength, virtually no tensile strength and limited ductility. Steel has excellent capabilities in these areas, and the resulting composite or hybrid elements are usually significantly stronger, more ductile, and more economical than steel or reinforced concrete elements acting alone. High strength steel tubes permit the use of smaller, lighter members with increased stiffness, stability and resistance to buckling. CFT may result in reduced construction costs, because the tube serves as both formwork and reinforcing to the concrete, and this results in a reduction of labor costs as required to reinforce the concrete element.

The CFT member is much lighter than the reinforced concrete required for the same load and deformation, and this further reduces seismic design forces and design dead loads. The concrete fill adds compressive stiffness to that achieved with steel columns, and as a result the concrete fill adds axial stiffness to the column and inhibits buckling of the steel. CFT columns are particularly desirable in braced frames or as supercolumns or collector columns in taller buildings, because of their increased axial column compressive strength and stiffness.

The concrete fill permits increased ductility from lighter and more slender tubes than could be achieved with steel acting alone.

Structural Materials: Properties, Microstructure and Processing

The confining action of the tube increases the shear strength and inelastic strain capacity of the concrete within the tube, and this further increases system ductility. CFT column Light steel erection column Steel beam Reinforced concrete column Steel framing a b Reinforced concrete shear wall Normal steel framing c 2. A major requirement of all composite and hybrid systems is that applied loads must be appropriately shared and distributed between the two different materials, and this distribution is commonly facilitated by the connection in CFT construction.

Japanese engineers commonly use internal diaphragm connections such as illustrated in Fig. Japanese engineers primarily use CFT columns in moment resisting frames, but variations of the internal diaphragm connection have been used in braced frames. US engineers primarily use CFT columns in braced frames of taller buildings, because of the large axial stiffness and compressive load capacity of CFT.

Penetrating gusset plates, such as illustrated in Fig. US engineers occasionally use CFT columns in moment resisting frames of shorter buildings, and penetrating beam connections such as depicted in Fig. These penetrating connections are very desirable for CFT construction because they facilitate load sharing and composite action between the steel and the concrete. Research has shown that bond stress between the steel and concrete is very limited in CFT construction Roeder et al. CFT columns are a very promising alternative for cost effective steel construction, but steel tubes are still one of the more difficult elements to connect to other structural elements and erect in the field.

As a result, additional work on new and improved connections are required to further improve this system. This discussion of CFT has focused on CFT with circular steel tubes, but rectangular box columns such as discussed in the earlier sections on new materials are also filled with concrete to form CFT columns. These applications are quite common in Japan, because Japanese engineers make extensive use of bi-directional bending of columns and rectangular tubular sections. The benefits of CFT with rectangular box sections are significant, but somewhat different than those achieved with circular steel tubes.

Rectangular tubes provide reduced confinement to the concrete and bond stress between the steel tube and concrete fill to that achieved with circular steel tubes, but the benefits of shear reinforcement and composite action are significant. Other composite and hybrid systems Other forms of composite or hybrid construction have recently been developed. Development of a hybrid reinforced concrete-steel reinforced concrete RCS construction as depicted in Fig. Deierlein et al. RCS construction normally starts with erection of a steel frame with either light steel columns or widely spaced steel columns.

Moment resisting frames develop resistance to lateral wind and earthquake load through transfer of bending moment and shear from beams to columns and through the large bending resistance and stiffness of the column. In seismic design, inelastic deformation is expected in the beams adjacent to the column.

Reinforced concrete RC columns can economically provide benefits to the steel frame in RCS construction. The RC columns are cast between widely spaced steel columns, or the RC column may encase the very light steel columns which were used to temporarily support the steel framing during erection.

The RC column incorporates the steel beam to form a moment resisting frame as illustrated in the figure. The RC column can economically be built to provide large flexural stiffness and resistance, but the connection design details are an important issue in the design of the RCS system.

Connection design is important because large compressive bearing stresses are expected at the steel-concrete interfaces within the connection, and the shear design of the connection must consider the shear capacity of both the steel and concrete. Special stiffeners are frequently required in the steel beam to provide bearing surfaces between the steel and concrete as illustrated in Fig. Column reinforcement is needed to provide shear, flexural, and axial resistance, to confine the concrete, and to ensure continued contact and interaction at the steel-concrete interfaces.

Finally hybrid systems such as RC shear walls combined with steel framing as depicted in Fig. Older variations of this alternative have been used in prior years. The steel framing may be purchased and fabricated while the wall is being constructed. Steel erection may then proceed very quickly after the wall is complete. The wall provides great lateral resistance and stiffness to the structure, and therefore the steel frame can be very light.

The steel frame provides additional ductility to the RC wall and it supports the gravity load even as the wall is damaged during or deformed during earthquake deformation. The connection design is again an issue in obtaining good performance from the structural system. Considerable research has been performed on these connections, and guidelines and recommendations have been developed. This hybrid RC wall-steel frame system is less frequently used than the CFT system, but it offers clear benefits for a range of structural applications.

The reader is referred to references noted in this section for additional information and guidance regarding connection design for these alternative composite systems. A brief discussion of a few of these structures is provided. Taipei building As noted in prior comments, steel dominates the civil engineering construction market for taller, bigger, or unusual structures. The Taipei building was recently completed as a steel frame building in Taipei, Taiwan Fig.

This building is m tall with , m2 of floor area, and it is particularly noteworthy since it is one of the tallest buildings in the world. It was designed by C. The steel was a high strength material as discussed earlier in this chapter. Taiwan is susceptible to large earthquakes, and design for lateral loads is a major element of the structural design. The high rise portion of the building is built in a series of eight-storey modules. Each eight-storey module has internal bracing that provides shear resistance within that module and effectively transmits gravity and lateral loads within the module to a series of composite 2.

The composite supercolumns were made of concrete filled box steel sections. The supercolumns act together with the outriggers to resist wind and earthquake loads. The composite supercolumns of this building effectively gather a large portion of loads on the building, and they permit a relatively open structure with good visibility and exposure. These supercolumns are very stiff and strong, and they provide a relatively stiff, strong structure for lateral load and drift control.

The steel was a high strength alloy SMM as discussed in the new materials section, and the concrete was also a high strength material of 70 MPa. The building also uses a tuned mass damper to further control wind vibration. The tuned mass damper is located above the 87th and suspended by cable from the 92nd floor. The mass is a 5. It has a mass of , kg. Structural Engineer Seattle central library Another new and very unusual steel building is the Seattle Central Library shown in Fig.

The general contractor was Hoffman Construction Company. This building makes extensive use of A steel described earlier in this chapter, and it employs some innovative concepts of fire protection. The architectural geometry of the building is very unusual. The architect started with the concept that libraries will have many uses beyond books, and he proposed independent but connected spaces for these diverse uses.

He accomplished this by assembling the 12 stories into five overlapping platforms. These platforms can be seen through the skin of the building in Fig. Each platform consisted of several floors, and the platforms had truss framing to permit large column spacings and open spaces within the building. The trusses within each platform provided the lateral load integrity and gravity load resistance within the platform. The platforms and the columns supporting the platforms were not aligned within the building as seen in the photo.

Seattle is susceptible to large earthquakes, and the architects wanted to use a diamond shaped steel seismic grid around the perimeter of the building to provide full lateral resistance to the individual platforms of the structure. The seismic grid is fully exposed on the interior of the building. The platforms and building envelope were aligned to provide a large eightstorey central atrium. The sloped seismic grid was integrated with the exterior window-walls or envelope of the structure, and the combined interaction of the seismic grid and window mullion support system reduces the cost and complexity of the window support system over these open spaces.

The diamond pattern seismic grid carries the full seismic load, but it supports no gravity load. Therefore, fire protection was avoided for the lateral load grid, since the Seattle Building Code requires fire protection only for members supporting gravity load. Sprinklers were installed in the building as a primary form of fire protection.

A sophisticated fluid dynamics computer model for smoke flow was completed to limit the flow requirements and cost of this sprinkler system. However, past trends suggest broad future trends for steel bridge and building construction. Economic competition is increasing throughout the world, and so it is increasingly important to build more economical structures, which provide good system performance. Structural steel has long been the strongest and most ductile of the common construction materials. Steel has high strength in shear and tension, and if properly designed, it has good resistance in compression.

Steel has a relatively heavy unit weight, but its higher strength makes steel structures relatively light in weight per unit area compared to most other construction materials. Further, the unit cost of steel per unit load supported, is significantly lower for steel than other new composite materials. As a result, steel is likely to continue its common usage for many years into the future.

Steel is ideally suited for bigger, taller, and longer span structures, and steel is likely to continue to dominate this portion of the civil engineering structures market in the near future. Composite and hybrid systems such as CFT offer considerable benefits in design and constraint of larger, longer and taller structures, because the benefits of composite behavior often exceed the sum of the benefits of the materials acting alone. As a result, engineers are likely to use these systems with increasing frequency in the future. The goal of increased economy in civil engineering construction is partly achieved through higher strength steel, composite action, and improved materials, but it is partly achieved through more accurate prediction of element and structural performance.

Properties and Grain Structure

Engineers today use more sophisticated analytical tools in predicting system performance, and they assume greater accuracy from these predictions. Higher strength materials increase the likelihood of many alternative failure modes such as buckling, fatigue, and fracture. As a result, structural designers must consider a wider range of behaviors, and building codes and specifications are likely to become increasingly complex.

The figure plots the page count of the American Institute of Steel Construction Specification and Commentary over the past 80 years. There has been a dramatic increase in length and complexity in the design requirements for steel structures, and this has been accompanied by increased design stress levels and an increased number of failure modes that must be considered in design.

It is likely that this trend will continue. Codes and specifications will likely become even more complex to reflect this practice. As a result, it is expected that engineers will increasingly resort to methods of ameliorating this trend. Increased use of composite and hybrid structures is one clear way this may be achieved. Increased use of hybrid and composite systems may reduce the probability of many adverse performances, without necessarily increasing the cost or complexity of design.

Increased use of specialty items such as buckling restrained braces may also relieve some aspects of this trend. However, the cost of new developments can be high in our increasingly competitive environment. The reader is referred to these references for more detailed information on those issues. Doctor Mamoru Kono of Building Research Institute, Japan, provided the authors with information about fire-resistant steel.

The authors wish to thank these individuals and organizations and others who provided information for this chapter. Azizinamini, A. Deierlein, G. M, Yura, J. JISF website. Kihira, H. Kohno, M. Lie, T. MacRae, G. McKinlay, B. Merritt, S. Milke, J. Nakashima, M. Nishiyama, I. Roeder, C. Ruddy, J. Sakumoto, Y.

Schneider, C. Sheikh, T. M, Deierlein, G. Takanashi, K. For example, of the more than , bridges in America, , are structurally deficient, another , plus are functionally obsolete, and , are more than 50 years old and unsuitable for current or projected traffic demands Zureick et al. A study in the United States estimated that transportation agencies could repair or replace only 5, of these nonfunctioning bridges annually; however, in the meantime, 6, more bridges were added to the deficiency list.

Such rapid infrastructure degradation is not unique in the US — it is becoming a global issue. One of the main causes of bridge deterioration is the corrosion of reinforcement steel in concrete decks, exacerbated by road salt used to combat winter ice and snow, especially in the northern US states with their severe weather conditions and heavy use of de-icing salt.

Cracks in reinforced concrete often develop in the early years of structures from various shrinkage mechanisms such as moisture movement or thermal changes under restrained boundary conditions. Subsequently, crack openings cause durability problems in the reinforced concrete structure from corrosion of the rebars, spalling of the surface and increased permeability through the cracks. Although modern Portland cement was discovered in the nineteenth century, its ancient history can be traced back several thousand years Mindess and Young Due to their brittleness, concrete materials are designed solely to carry compressive loads; reinforcing bars are frequently incorporated in the design to carry tensile loads.

However, concrete cracking and spalling often lead to exposure of steel bars. Aggressive agents such as water and chloride ions easily migrate and attack steel reinforcement, the corrosion of which causes further matrix cracking and spalling. Eventually, the integrity of the structure is lost.

Material development in response to the call for more durable infrastructures has been an active research area for decades. High strength concrete typically with silica fume or fly ash added with compressive strengths exceeding MPa or higher has been steadily developed. The brittleness number of high strength concrete can be several times higher than normal strength concrete Elfgren et al. High strength concrete is therefore potentially more vulnerable to cracking.

Although high strength concrete may have very low permeability when it is intact, a cracked concrete will have similar crack related problems as normal strength concrete. Parallel to the developments of high strength concrete, efforts have been made to explore fiber reinforcement Marshall et al.

Significant improvements on toughness have been achieved through incorporation of various types of short fibers such as glass, steel, or polymers Shah , Li et al. In addition, advances in micromechanics modeling facilitate the creation of high ductility concrete Aveston et al. In recent years, advanced FRC composites have been developed and utilized in a wide spectrum of construction applications. These composites are typically composed of two major constituents, namely fiber and matrix. High toughness of composite is generally attributed to fibers that usually accounts for less than two volume percent of the composite, whereas a matrix phase holds fibers together and transfers loads to the high-stiffness fiber and protects them from harsh environments.

Matrix is cement-based with several supplementary substitutes. In this section, the function of individual constituents of the composites will be critically reviewed and discussed from a mechanics viewpoint to shed light on optimal selection of composite constituents for various kinds of infrastructure construction. Tensile behavior When a fiber reinforced cementitious composite is loaded beyond its tensile strength, a macroscopic crack is formed in the matrix.

The composite load will then be shared by the bridging fibers. Subsequently, there are two possible scenarios. In the first scenario, the composite load will be large enough to either rupture or pull out the bridging fibers, leading to a rapidly or gradually declining post-peak behavior. This composite is still considered quasi-brittle. In the second scenario, the composite load can be sustained by the bridging fibers.

These fibers then transfer the load via their interface back into the matrix. If enough load is transferred, the matrix may crack again and the process will repeat until the matrix is broken by a series of subparallel cracks Aveston et al. During the process of multiple cracking, the composite load can even rise and exceed the first cracking strength of the composite Wu and Li This composite is considered pseudo strain-hardened. For multiple cracking to occur, there are some conditions that must be met.

For instance, a critical fiber volume fraction, V fcrit , has been defined as the minimum fiber quantity required for achieving multiple cracking Li and Leung , Li and Wu , Li Similar results have also been obtained by others see e. Naaman and Reinhardt The exact magnitude of this quantity depends on all relevant material parameters, including fracture toughness of matrix, fiber properties and interfacial bonds.

For a short fiber randomly distributed concrete showing complete fiber pull-out when the concrete is tensioned beyond its ultimate strength, V fcrit is given in eqn 3. Low fracture toughness of the matrix is in favor of low V fcrit. Consequently, a weak cement paste is expected to achieve multiple cracking more easily than mortar Wu and Li , Wu It would be desirable to incorporate more sand and aggregate to the mix to reduce hydration heat, shrinkage and cost.

Critical fiber volume fraction V fcrit is closely related to fiber and matrix material properties. Strain-hardening is only possible with matrices with suitable toughness for a given fiber. In other words, with the same type and same amount of fiber in different matrices, pseudo-strain hardening may or may not occur depending on the toughness of the matrix and the interfacial bond strength. Therefore V fcrit can be used to guide the selection of desired material constituents. This is particularly true when both the social and environmental costs of premature repairs to our infrastructure are taken into account.

It will take some time before they become a routine part of concrete engineering practice. This technology was first developed in Japan in the late s, and has gained widespread acceptance, as it can be used to produce both ordinary concretes and high-strength concretes, and even fiber reinforced concretes Banthia et al. From a material point of view, SCC is considerably more expensive than ordinary concrete, but this is compensated for by the much greater ease of placement, and consequent reduction in labor requirements. It also permits proper concrete placement in areas of highly congested reinforcement, as occurs when seismic reinforcement must be used, or in other complex structures.

Li and H. Stang The concept behind the production of SCC is basically a simple one. The volume of fine material in the concrete is increased without particularly changing the amount of mixing water, combined with the use of relatively large amounts of superplasticizer.

Severe Plastic Deformation - 1st Edition

Typically, the ratio of fine to coarse aggregate in SCC is in the range of 1. In particular, there must be a relatively large amount of material with particle sizes below 0. Filling ability, or the ability of the SCC to flow into the forms and around the reinforcing bars with no externally applied vibration, and to fill the formwork completely.

Passing ability, or the ability of the SCC to pass through narrow spaces in the formwork, or through and around reinforcing bars, without either blocking or segregating. Segregation resistance, both while it is flowing and after it has come to rest. These objectives can be met by the appropriate selection and proportioning of the concrete mixture.

The high fluidity, combined with the segregation resistance, thus makes possible the production of SCC. Of course, these values may have to be modified to meet the strength and durability requirements of the hardened concrete. It should be noted that currently there are no well-defined or agreed upon procedures to determine the rheological properties of SCC. Thus, for mix design purposes or for quality control, a number of completely ad hoc or empirical tests are used, such as those suggested by EFNARC This assumption, unfortunately, is simply not true.

There is far more distress to concrete and, alas, far more litigation due to poor durability than there is to low strength. In designing high-performance concrete or any other concrete , it would be more appropriate to focus first on the environmental conditions to which the concrete will be exposed, and only then to worry about strength. Fortunately, at least some modern design codes are beginning to require such an approach. For instance, the American Concrete Institute Building Code ACI Committee , is now written to indicate that durability requirements shall take precedence over strength requirements.

This reduces the porosity and permeability of the concrete, making the ingress of aggressive chemicals much more difficult Gjorv, Of course, permeability is not the only factor that controls durability. We must always keep in mind the other well-known durability requirements: sulfate resistant cements, air entrainment for freeze-thaw durability, low alkali cements when the aggregates are susceptible to alkali-aggregate reactions, and so on. If the concrete is not cured properly, this will increase the severity of surface cracking due to early drying shrinkage.

Abrasion resistance The abrasion resistance of concrete is an important parameter in highway pavements, dam spillways and stilling basins, and so on. In some areas, abrasion due to ice is also of importance. Abrasion resistance is one of the few durability parameters that is in fact almost entirely proportional to the strength of the concrete, though the coarse aggregate properties and volume concentration are also of importance. Thus, high-strength concretes, particularly those made with the incorporation of silica fume, are particularly abrasion resistant.

It has been found that at compressive strengths of about — MPa, concrete has about the same abrasion resistance as granite. For instance, Laplante et al. However, there is still no standard test which can be used to predict the abrasion resistance of any particular aggregate, though tests such as the Los Angeles abrasion test may be helpful at least for screening purposes.

However, though the mechanisms of freezing and thawing in concrete are now pretty well understood Pigeon and Pleau, , this remains an open question, and the experimental studies to date have shown mixed performance. The experimental evidence on the necessity of using air entrainment in high-strength mixes has been summarized by Aitcin In the intermediate range, the necessity of air entrainment will depend on the particular cement, the presence of supplementary cementing materials, and so on, and this can be determined only by carrying out appropriate freeze-thaw tests.

The zinc coating acts both as a barrier and as a sacrificial coating, as the zinc itself slowly oxidizes corrodes. The effectiveness of the coating depends on its thickness, with its effective life expectancy being linearly proportional to its thickness. While galvanizing is effective against corrosion induced by carbonation of the concrete cover, it is apparently less effective when the corrosion is induced by chlorides Bentur et al. As well, if the coating is too thin, then it may break when the rebars are being bent or handled at the jobsite, which can lead to very rapid localized corrosion.

Finally, galvanized steel cannot be welded. Nonetheless, despite their relatively high cost, galvanized rebars seem to find a ready market. In epoxy coated reinforcing bars, the epoxy acts as a barrier to isolate the steel from an aggressive environment. The epoxy must be flexible enough to permit the bar to be bent without rupturing.

If the coating is ruptured, this can lead to severe localized corrosion and failure of the rebar. There have also been some instances of the epoxy coating debonding from the steel when used in warm marine environments. In addition to the high cost, the major problem with epoxy coating is that the bond between the concrete and the steel is substantially reduced, increasing the possibility of cracking, and requiring the use of larger anchorage and lap lengths. Stainless steel reinforcing bars are occasionally used in extreme exposure conditions.

While they are very effective in preventing corrosion, their high cost severely limits their use. Corrosion inhibitors of various types are now becoming increasingly common. These act not as barriers to aggressive agents, but as chemicals that reduce the corrosion of the steel. There are two types of corrosion inhibitors: Anodic inhibitors stabilize and reinforce the passivating film which forms on the steel surface in the high pH environment of concrete. Cathodic inhibitors are adsorbed onto the steel surface, where they act as a barrier to the reduction of oxygen which is the principal cathodic reaction for steel in concrete.

However, it must be remembered that corrosion inhibitors are effective only if used in otherwise good concrete; they are not a panacea for corrosion in poorly designed or placed concrete. The most common anodic corrosion inhibitor is currently calcium nitrite, which acts essentially by increasing the level of chloride necessary to initiate corrosion Berke Cathodic inhibitors are less effective than anodic ones.

They are primarily amines, phosphates, zincates and phosphonates, which have the unfortunate side effect of severe set retardation at the high dosages required for them to provide effective corrosion control. The low permeability of such concretes should provide adequate corrosion protection. In particular, for more extreme exposure to chlorides, a high-performance silica fume concrete should provide good protection. There are two reasons for this. As well, such concretes have high electrical resistivity about an order of magnitude higher than that of ordinary concrete , which reduces the chloride diffusion rate.

If necessary, the silica fume concrete can be combined with one of the other chemical or mechanical means of reducing corrosion described earlier. Thus, if it could be modified by the introduction of polymers, which are ductile and relatively strong, a composite material which is both strong and tough would result. As well, the incorporation of polymers should reduce the permeability, and hence improve the durability of concrete. However, this turns out to be both difficult and expensive about three to six times as expensive as plain concrete in practice, and so polymer-cement composites are not now widely used.

However, as both durability and repair considerations become more important, these materials will find an increasing market share. Latex-modified concrete The most common, and easiest, way of combining a concrete with a polymer is to add the polymer latex to the concrete during the mixing process. The result is latex-modified concrete, or LMC. The most common polymers for this purpose are vinyl acetate, styrene-butadiene, vinylidine chloride and acrylic esters.

The resulting materials are used primarily as bonding agents, overlays and for patching. A schematic of how the polymer film forms in LMC is given in Fig.

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As the mixing water evaporates, flocculation of the latex particles occurs; the particles form a continuous film as further water is removed by the cement hydration reactions. Note that, rather than extended moist curing, the concrete should be allowed to dry out after a day or two to permit film formation. The films thus formed are relatively strong and tough, and will thus increase the energy required to propagate cracks through the matrix.

The strength of LMC is higher than that of unmodified concrete under the same curing conditions in compression, and particularly in tension and flexure. LMC has a lower modulus of elasticity, but a higher strain at failure than plain concrete. Given that it is used primarily as an overlay for repair purposes, it is significant that LMC displays better bond both to old concrete and to reinforcing bars. On the other hand, it will tend to lose strength upon prolonged immersion in water. Perhaps of greater importance, LMC is more durable than plain concrete for several reasons. The polymer film reduces the permeability of the concrete.

The improved resistance to tensile cracking reduces the number of cracks that can transport water into the material, and helps to keep them narrower by bridging across them analogous to the way in which fibers work. Work by Xu et al. Polymer-impregnated concrete Polymer-impregnated concrete PIC is prepared by impregnating the concrete with a liquid monomer, and then polymerizing the monomer in situ to form a solid polymer within the concrete pores. The polymerization can be induced either by using gamma radiation, or by the use of a catalyst and heat. The most common monomers used are methyl methacrylate MMA , styrene, and the monomer of plexiglas, as they all have very low viscosities, and thus can be made fairly readily to penetrate the concrete pore system.

PIC can be much stronger two to four times than plain concrete, but is also more brittle. As with LMC, polymer impregnation strengthens both the hydrated cement paste and the cement-aggregate interface. More importantly, it too greatly improves the durability of the concrete, since the polymer effectively fills all of the available porosity, and prevents the ingress of aggressive chemicals. PIC is a very expensive material, and thus has seen little use in the field. Again, it will be used mostly in applications in which a long maintenance-free service life is desired, or where repairs would be difficult to undertake.

Each tonne of Portland cement requires about 1. Portland cement production is also a major contributor of greenhouse gases, as each tonne of cement produced involves the release into the atmosphere of about one tonne of CO2. One approach to this problem is to use much greater proportions of fly ash or granulated blast furnace slag in concrete; this would be desirable for both economic and environmental reasons. Because of their high pozzolanic content, these concretes reach their full strength potential rather more slowly than conventional concretes, which must be taken into consideration in the construction scheduling.

It appears to make no particular difference whether the fly ash is added at the batching plant or is preblended with the cement. It is not clear what maximum strength can be attained with these concretes, but day compressive strengths in excess of 40 MPa have certainly been obtained. Properly produced high-volume fly ash concretes have a low permeability, and thus are highly durable. They are also more resistant to cracking than ordinary concretes of the same strength.

The reasons for this may be explained by comparing the two typical mixes shown in Table 1. For both of these reasons, the drying shrinkage and the heat of hydration are substantially reduced, leading to less likelihood of cracking for the fly ash concrete. However, it must be emphasized that even more than for ordinary concretes, proper curing of such high-volume fly ash mixes is essential. These are primarily considered as replacements for conventional natural aggregates. There are many sources of such potential aggregates: mineral wastes, slags, incinerator residues, recycled concrete, and so on.

However, there are three main considerations that must be kept in mind when evaluating such waste materials: 1. Economy: how much material is available, how far will it have to be transported, how much beneficiation will it require, and how will it affect the basic concrete mix design?

Compatibility with other materials: will it react adversely with other components of the mix cement, admixtures? Concrete properties: will the material decrease the strength, the elastic modulus, or the volume stability shrinkage, creep of the concrete? With our current understanding of basic principles, and with the materials and production technologies now available, we can produce an enormous range of Portland cement-based materials. The strength and durability levels that we can now achieve will almost certainly be surpassed in the future; we are limited only by our imaginations.

The Concrete Construction Engineering Handbook edited by Nawy provides a broad overview of all aspects of concrete construction. By far the best single book devoted to high-performance concretes is that by Aitcin The collection of papers edited by Shah and Ahmad also contains much useful information. For fiber reinforced concretes, the most complete treatment is to be found in Bentur and Mindess For durability issues, the book by Richardson provides a comprehensive overview. Of course, any practitioner must make reference to the national codes and standards relevant to any particular country.

Aitcin, P-C. Bache, H. Banthia, N. Proceedings of the first international conference on innovative materials for construction and restoration, Vol. Bentur, A. Berke, N. Burg, R. Charron, J. High performance concretes and applications, Edward Arnold, — Haneharo, S. Holland, T.

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Laplante, P. Li, V. Advanced Concr. Technology 1 3 , — Malhotra, V. Malinowski, R. Mehta, P. Mindess, S. Naaman, A. Nawy, E. Parant, E. Pigeon, M. Richard, P. Romualdi, J. ASCE 89 — Russell, H. Sakai, E. Shah, S. Skarendahl, A. Taylor, H. Thibaux, T. Xu, H. Prior to about , cast and wrought iron components were used as columns and other elements in some civil engineering structures, but these structural elements generally had variable resistance, somewhat unpredictable failures, and limited ductility when compared to modern steel structures.

During the s, cast and wrought iron columns disappeared from common usage, and structural steel became the construction material of choice. This occurred because of improvements in the Bessemer process that permitted economical production of large quantities of steel required for building and bridge construction. Structural steel provided high strength with great ductility and consistent material behavior. These properties resulted in increased usage of steel in construction, but initial development of these steel structures followed construction practices employed with cast iron and wrought iron elements.

Experienced structural engineers designed buildings and bridges based upon simple calculations and nominal stress levels that had evolved from cast and wrought iron elements Schneider The cost of structural steel was relatively high, while the cost of labor was relatively low, and so steel was produced as plate, bars and angles, and most structural elements were built-up members assembled from these limited shapes as depicted in Fig.

Built-up members and connections between members were riveted, and the construction was labor intensive. The built-up elements were light and required small amounts of material, but they were cast in concrete or encased in masonry for fire protection. This fire protection provided increased strength and stiffness to the completed structure. Nominal design stresses in the order of MPa were employed, and the nominal yield stress of the most common structural steels was MPa during this period.

During the s, steel design practice began a significant change in much of the world, because of increased labor costs. Local customs and economic conditions delayed this evolution in some countries, and a few countries retain remnants of the earlier practices today. Standard connections such as those depicted in Fig.

These early specifications employed the allowable stress method, and they generally employed permissible stress levels at approximately MPa for most structural applications. Standard structural shapes such as I beams, H beams and channels reduced the labor required to assemble the structure.

Further, improvements in the manufacturing process of steel produced further savings in cost and increased usage of the material. Riveted connections and concrete encasement for fire protection were labor intensive construction practices. Increased labor costs combined with the large mass and dead load caused by the heavy concrete encasement led to further changes in engineering practice during the s and s. High strength bolts replaced rivets, because a much smaller number of bolts were required, and this reduced the labor costs and construction noise.

Lightweight fire protection and nonstructural elements were developed to reduce the weight of the structure and further reduce labor costs. Increasing emphasis on seismic design and ultimate strength design methods led to more accurate estimates of structural resistance and ductile structural performance. Electric arc welding had been developed many years earlier, but it became commercially viable during this period, since welds could be economically completed with more uniform quality and fewer flaws.

Welding permitted greater structural continuity and redundancy, and this further reduced material costs and increased the available resistance within the system. As a result, high strength bolts combined with electric arc welding became the normal construction techniques for connecting and joining steel elements. Factored load and plastic design methods evolved during this period to take advantage of the increased strength obtainable in indeterminate structures AISC Lightweight fire protections and nonstructural elements reduced the dead loads, and increased yield stress levels MPa in the s and allowable stress levels commonly MPa to MPa in the s produced lighter and bigger steel structures.

These trends have continued into the current practice. Today, civil engineering construction is a continual economic competition between steel, concrete and other construction materials. As a result, steady increases in the strengths of these materials have occurred. Today the standard mild steels used in civil engineering construction have nominal yield stress values of approximately MPa. The equivalent allowable stress levels for these steels are approximately to MPa. Operating stress levels are approximately twice as large as those used when steel was first employed.

Increased operating stress levels increase the potential for stability failures, fatigue and brittle fracture. Therefore structural engineers must respond to these design issues, and building codes and specifications have generally become more complex. Labor costs have continued to increase, and engineers are today asked to create light, economical and quickly erected structures.

Nevertheless, greater material strength, higher strength to weight ratio, and greater available ductility result in steel dominating the market for bigger, taller, and longer span civil engineering structures. Steel is particularly dominant in the design and construction of bigger structures and structures with unusual geometry because of its large strength to weight ratio and the ability to economically fabricate and erect complex structures.

The steel industry is continually pressed to develop improved material performance, higher strength, and more ductile materials and structural systems, because these attributes enhance this evolution. Great advances have been made in lighter and more economical fire protection methods, but fire protection is still viewed as an issue of greater concern for steel structures than for other construction materials, because of the smaller members and thermal mass associated with steel structures. Fire protection adds to the structural cost, and engineers must work toward reducing these costs, while assuring adequate resistance to elevated temperatures expected during a fire.

Steel structures have smaller members and larger operating stress levels than other construction materials. Buckling and stability become more critical as higher strength materials evolve. Fatigue and fracture. Crack initiation, crack growth and propagation were not considered issues of importance in structural engineering until the last 40 or 50 years. However, the increasing yield stress, operating stress levels, emphasis on plastic and ultimate capacity, and use of welded construction have resulted in increased frequency of fatigue and fracture in bridges and industrial systems.

Fatigue is presently not a major concern for building design, but fracture of steel in buildings has become a major issue for seismic design after recent earthquakes. Corrosion resistance. Steel may corrode when exposed to the environment, and this may lead to deterioration, increased maintenance, and increased construction costs. Galvanization, paint, and coatings inhibit corrosion, but they may increase the fabrication costs of the steel by several percent, Therefore, engineers are continually seeking methods of reducing these costs.

Increased steel yield strength places new and continuing demands upon welding methods, because higher strength steels are usually more difficult to weld without adversely affecting the ductility and performance of the system. Ductility for seismic and other loading. Seismic design is today a requirement for many civil engineering structural systems, and steel is an ideal material for seismic design because of its material strength, stiffness, and ductility. Buckling, weldability and other issues may affect the seismic performance and ductility, and engineers are continually developing new methods to improve inelastic seismic performance of steel structures.

These factors influence the choice and consequences of using steel as a construction material, and they are often the focus of developments that are made to improve the performance of steel. As a result, many recent advances and developments in steel respond in part to the recurring issues noted above. A number of recent advances are discussed here in terms of developments in new materials, new components and new structural systems. The combination of these developments provides a more versatile and more economical construction material.

In general, these alloys address the issues noted above. Some of these material developments are discussed here. Several hundred bridges have been designed or built with this material since its inception, and increased usage in the future is expected. Other HPS alloys with yield stress up to MPa have been developed, but they have not been produced in commercial quantities to date.

Higher strength welds are more costly and frequently have reduced CVN toughness and greater weld flaws than achievable with lower strength steel. Further, welded splices in bridge girders typically occur at regions where fatigue rather than ultimate strength is the critical design parameter. As a result, undermatched electrodes are usually used in HPS70 bridge design practice to provide economy combined with good fatigue resistance. HPS steels are often used as flanges in hybrid plate girders with lower strength A grade 50 or MPa yield strength steel used in the webs.

HPS steels permit the design of lighter bridges with longer spans and more economical bridge construction. However, the higher operating stress levels require changes in bridge design practice. Increased stress levels result in more slender flanges and webs, and therefore lower slenderness limits and stockier elements are required to develop their full plastic capacity. The strength and ductility of the HPS70 alloy has been investigated, and revised design limits have been proposed to effectively use this higher strength steel in bridge design practice while assuring economical design and construction Barth et al.

Some design specifications also require deflection limits for bridge serviceability and vibration control, and research Roeder et al. This research has also shown that deflection limits are not a desirable method for controlling bridge vibrations. Serviceability and durability are important, but the impact of deflections on bridge durability is not apparent. As a result, engineers are presently moving away from these deflection based design limits for HPS steels.

They are directly addressing bridge vibration and pedestrian comfort issues, but there is still ambiguity regarding the role deflections and deformations play in determining the service life of the bridge. Increased operating stress levels also increase the propensity for fatigue cracking.

HPS steels reduce the risk of fatigue at these higher stress levels by using undermatched weld metals at fatigue sensitive weld locations, improved fatigue resistant details, and the increased CVN toughness of the HPS materials. In some cases, HPS steel tubes are being used, because tubes are inherently more stable against local buckling than other structural shapes. Therefore, tubular elements may be used as chords in arches or trusses, or in other systems where they can better utilize the benefits of the higher strength material. HPS steels may also be combined with concrete in hybrid and composite systems, because this reduces deflections, vibrations and local buckling potential, while providing an economical lightweight structure.

Fire-resistant steels As illustrated in Fig. This means that buildings are susceptible to collapse during fires Lie ; hence the fire-resistance of steels and the fire protection of steel are crucial considerations of structural design. The IBC requires adequate fire protection to assure a duration of time where the members will support the required gravity loads while subject to a standard fire ASTM E , and this duration varies depending upon the building type and function.

The bold line is for conventional mild steel. Fire-resistant steel was developed in Japan, with the aim of reducing or eliminating the amounts of costly fire-protection in some situations Sakumoto et al. Addition of a few alloying elements of Mo, Nb, and Cr and strict control of heat-treatment processing was adopted to increase the yield stress in elevated temperature. The grey line of Fig. Figure 2. The yield strength of fire-resistant steel exceeds MPA, the target value equal to two-thirds of yield stress at room temperature i.

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Welding materials for submerged arc welding and CO2 arc welding and high-strength bolts that conform to elevated temperature performance of fire-resistant steel were also developed. The Charpy V-notch values are also higher than those obtained for conventional weld metals. High-strength bolts conforming to fire-resistant steel were developed for a common repertoire of high-strength bolts, namely torque-shear bolts, hexagon head bolts, and hot-dip galvanized bolts JIFS b, Sakumoto et al. Fire-resistant steel has been commercially available for more than ten years in Japan.

The steel is primarily used in parking structures, because of their open space and direct exposure to the atmosphere JISF website. In many of these structures, fire-protection is no longer required once fireresistant steel is adopted. While fire-resistant steels offer promise for reducing the cost of fire protection, their benefits are presently not available in many parts of the world, and their benefits may not be adequate for some applications.

Therefore, economical fire protection remains a critical concern. Fire protection today is lighter and less costly than that used in past practice.

Nevertheless, it is still a costly element of some steel construction, and engineers are continually searching for new methods to reduce the costs and to reduce the amount of fire protection required in the structural design Ruddy et al. Reduced costs of fire protection have sometimes been achieved by using gypsum board and other materials that may serve a dual role as fire protection and an architectural element. However, another promising advance for the future appear to be in the direction of advanced analytical techniques.

Material fire ratings are commonly based upon a standard fire test such as ASTM E , and the measured temperatures of actual fires may vary dramatically from these idealized temperatures as illustrated in Fig. Actual fire temperature depends upon the type and available quantity of combustible materials, the available oxygen and ventilation, as well as the building application and geometry. The actual fire temperatures may exceed or be significantly less than the standard fire temperatures, because the elevated temperatures suggested by standard design fires cannot develop if there is inadequate fuel and oxygen supply to support the fire growth.

Advanced computational methods permit more accurate estimates of the expected member temperatures and general structural performance during fires, and these analytical techniques show that significant reductions in the cost of fire protection may be possible in many future building designs Milke Steels developed for increased ductility Steel is one of the strongest, most ductile construction materials, and light weight and ductility are particularly important for seismic design.

However, the Northridge and the Hyogoken-Nanbu Kobe earthquakes resulted in considerable damage and nonductile performance of steel buildings despite the material ductility. These events showed that ductile materials do not automatically ensure ductile structures because the material must be used in ways that permit the ductile material properties to control system performance.

As a result, the search for even greater ductility is always in progress. One contributing factor noted in these studies was that the continuing evolution toward higher yield strength steels as illustrated in Fig 2. This figure shows the average measured yield stress and tensile stress measured in steel test specimens reported in the literature of recent decades. The ultimate tensile strength, Fu, is commonly associated with fracture or tearing of steel, and therefore an increased yield to tensile ratio reduces the amount of strain hardening reserve during seismic deformation of steel.

This limits the extent of yielding during inelastic deformation and causes larger local inelastic strain demands on the locations where yield occurs FEMA D As a result, greater ductility demands on welds and other critical elements may occur, and steels with higher yield to tensile strength ratios have a greater tendency to fracture during earthquake loading. Steel producers have responded to this need. In comparison to conventional mild steel, the new steel, designated as SN steel, was intended to satisfy the following requirements. To To To To To set an upper limit for the ratio of yield strength to tensile strength; reduce scatter of yield strength; lower carbon equivalent Ceq and weld crack sensitivity Pcm ; increase through-thickness ductility; limit the tolerance of geometrical dimensions.

A small scatter of yield strengths is beneficial in securing the yielding, failure mechanism, and accordingly the ductility of the steel frame. SN steel achieves significantly narrower bands relative to conventional steel. Requirement 3. Effective parameters for the success of severe plastic deformation methods 7. Ghader Faraji is an Assistant Prof. He serves as the head of the center for processing and characterization of Nanostructured metals and has spent the last ten years performing high impact research on severe plastic deformation processing, publishing over 80 Journal papers in the field.

Kim, Ph. He is a distinguished professor who has published more than journal paper in the field and is considered one of the top researchers in the area of severe plastic deformation. Hessam Torabzadeh is a Ph. He achieved the Iran National Elite Foundation award as a superior technician graduated in During his research, he worked in several fields such as metal forming, characterization of nanostructured materials and Severe Plastic Deformation SPD methods, which resulted in published papers and conference presentations. We are always looking for ways to improve customer experience on Elsevier.

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