Large Volume, High-Performance Applications of Fibers in Civil Engineering VICTOR C. LI ACE-MRL, Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109 Received 8 November acce Nouem ber 2000 ABSTRACT: This article presents an overview of fiber applications in cementitious composites. The socio-economic considerations surrounding materials development in civil engineering in general, and fiber reinforced cementitious materials in particular appliations are fibers are used in these applications, are documented. An attempt is made to extract common denominators among the widely varied applications. The R&D and industrial trends of applying fibers in enhancing structural performance are depicted. An actual case study involving a tunnel lining constructed in Japan is given to illustrate how a newly proposed structural design guideline takes into account the load carrying con- tribution of fibers. Composite properties related to structural performance are de cribed for a number of FRCs targeted for use in load carrying structural members Structural applications of FRCs are currently under rapid development. In coming years, it is envisioned that the ultra-high performance FRC, with ductility matching that of metals, will be commercially exploited in various applications. Highlights of such a material are presented in this article. Finally, conclusions on market trends are drawn, and favorable fiber characteristics for structural applications are provided o 2002 John Wiley Sons, Inc. J Appl Polym Sci 83: 660-686, 2002 Key words: FRC, ECC; fiber; composites; structure INTRODUCTION Fibers are generally used in one of two forms short staple randomly dispersed in the cement The use of fibers to reinforce a brittle material can tious matrix of a bulk structure, or continuous be traced back to egyptian times when straws or mesh used in thin sheets. In recent years, some horsehair were added to mud bricks. Straw mats attempts to weave synthetic fibers into three-di- serving as reinforcements were also found in mensional reinforcements have been made In ad- early Chinese and Japanese housing construc- dition, fiber-reinforced plastic rods are currentl tion. The modern development of steel fiber rein- entering the market as replacement of steel bar forced concrete may have begun around the early reinforcements. Beyond cementitious matrix, fi- 1960s, preceded by a number of patents. Poly- ber-reinforced plastics are finding increasing use meric fibers came into commercial use in the late in the civil engineering industry. However, this 970s, glass fibers experienced widespread use in article will focus only on the material with the the 1980s, and carbon fiber attracted much atten- currently largest consumption of fiber--randomly che early 1990s oriented fiber-reinforced cementitious matrix (ce- ment, mortar, and concrete) materials (hereafter Contract grant sponsor: National Science Foundation. breviated as FRCs). Based on industrial sources, the amount of fibers used worldwide at Joumal of Applied Polymer Science, Val. 83, 660-686(2002) o 2002 John wiley Sons, Inc. present is estimated at 300, 000 tons per year, and
Large Volume, High-Performance Applications of Fibers in Civil Engineering VICTOR C. LI ACE-MRL, Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109 Received 8 November 2000; accepted 18 November 2000 Published online 15 November 2001; DOI 10.1002/app. 2263 ABSTRACT: This article presents an overview of fiber applications in cementitious composites. The socio-economic considerations surrounding materials development in civil engineering in general, and fiber reinforced cementitious materials in particular, are described. Current FRC appliations are summarized, and the where, how, and why fibers are used in these applications, are documented. An attempt is made to extract common denominators among the widely varied applications. The R&D and industrial trends of applying fibers in enhancing structural performance are depicted. An actual case study involving a tunnel lining constructed in Japan is given to illustrate how a newly proposed structural design guideline takes into account the load carrying contribution of fibers. Composite properties related to structural performance are described for a number of FRCs targeted for use in load carrying structural members. Structural applications of FRCs are currently under rapid development. In coming years, it is envisioned that the ultra-high performance FRC, with ductility matching that of metals, will be commercially exploited in various applications. Highlights of such a material are presented in this article. Finally, conclusions on market trends are drawn, and favorable fiber characteristics for structural applications are provided. © 2002 John Wiley & Sons, Inc. J Appl Polym Sci 83: 660–686, 2002 Key words: FRC; ECC; fiber; composites; structure INTRODUCTION The use of fibers to reinforce a brittle material can be traced back to Egyptian times when straws or horsehair were added to mud bricks. Straw mats serving as reinforcements were also found in early Chinese and Japanese housing construction. The modern development of steel fiber reinforced concrete may have begun around the early 1960s, preceded by a number of patents.1 Polymeric fibers came into commercial use in the late 1970s, glass fibers experienced widespread use in the 1980s, and carbon fiber attracted much attention in the early 1990s. Fibers are generally used in one of two forms— short staple randomly dispersed in the cementitious matrix of a bulk structure, or continuous mesh used in thin sheets. In recent years, some attempts to weave synthetic fibers into three-dimensional reinforcements have been made. In addition, fiber-reinforced plastic rods are currently entering the market as replacement of steel bar reinforcements. Beyond cementitious matrix, fi- ber-reinforced plastics are finding increasing use in the civil engineering industry. However, this article will focus only on the material with the currently largest consumption of fiber—randomly oriented fiber-reinforced cementitious matrix (cement, mortar, and concrete) materials (hereafter abbreviated as FRCs). Based on industrial sources, the amount of fibers used worldwide at present is estimated at 300,000 tons per year, and Contract grant sponsor: National Science Foundation. Journal of Applied Polymer Science, Vol. 83, 660–686 (2002) © 2002 John Wiley & Sons, Inc. 660
HIGH PERFORMANCE APPLICATIONS OF FIBERS 661 is projected to increase. In North America, the plications of FRCs are currently under rapid de- growth rate has been placed at 20% per year. velopment In coming years, it is envisioned that However, it should be pointed out that FRC re- the ultrahigh-performance FRC, with ductility mains a small fraction of the amount of concrete matching that of metals, will be commercially used each year in the construction industry exploited in various applications. Highlights of ibers may have been originally introduced in such a material are described in the section enti n attempt to "strengthen"the matrix, without tled Strain- Hardening Cementitious Composites consciously distinguishing the difference between In the final section, conclusions on market trend became generally recognized that the most signif- placed on the need for fiber and surface charac cant effect of fiber addition to the brittle cemen- teristics most suitable for the ensuing applica titious matrix is the enhancement of toughness. tions and performance needs of future FRCs For most FRCs, this means the capability of the material to carry tensile load, albeit at a decreas- ing level with opening of a crack after its forma- SOCIOECONOMIC CONSIDERATIONS tion. For certain FRC with continuous and/or high-fiber volume fractions, the ability of fibers in Civil infrastructures are organic, in the sense substantially increasing the tensile ductility has that they grow with the years. The Akashi-Kaikyo been recognized since the work of Aveston et al. Bridge in Kyoto, Japan, recently completed in However, it is only in recent years that such duc- April 1998, has the longest suspended span(1990 tility accompanied by strain-hardening can be de- m) of all bridges in the world. At 450 m, the rived by a moderately low amount of randomly Petronas twin Tower in Malaysia(completed in oriented discontinuous fibers(e. g, less than two 1996)is the tallest building in the world. No volume percent) by carefully tailoring the matrix, doubt these records will be shattered in the near interface, and fiber via the help of micromechan- future. Behind this growth is the development of ics. As a result, a new class of economically viable, advanced construction materials field processable, high-performance damage-tol Unfortunately, when put in perspective, civil erant material is emerging. Emphasis on compos- and building engineering materials development ite tailoring also brings with it the need to control does not have a good track record, in comparison fiber characteristics to meet the performance with other industries. Part of the reason comes need and economic constraints in construction ap- from the lack of cooperation/coordination between plications of this new type of FRO the construction industry and the construction n the next section, broad socioeconomic con- material supplying industry. Especially in the siderations surrounding materials development United States, joint research and development in civil engineering in general, and fiber rein- between materials suppliers and the construction forced cementitious materials in particular, are dustry is relatively nonexistent. Such fragmen- described. The section entitled Current Applica- tation between materials development and infra- tions of FRCs summarizes current Frc applica structures is not conducive to the healthy growth tions worldwide and documents where, how, and and maintenance of our societies infrastructures why fibers are used in these applications. An at- The negative impact of this stance on construc- tempt is made to extract common denominators tion productivity, durability, and public safet among the widely varied applications. Most cur- cannot be underestimated rent use of frcs is in nonstructural or. at most The magnitude of our infrastructure need is semistructural applications. The following section enormous. Put in economic terms, about 10% describes the research and development and gross domestic product derives from infrastruc dustrial trends of applying fibers in enhancing ture construction worldwide. In the United States structural performance. An actual case study in lone, infrastructure construction is a $400 bi volving a tunnel lining constructed in Japan is lion industry involving 6 million jobs. We have given to illustrate how a newly proposed struc- approximately $17 trillion worth of infrastruc ural design guideline takes into account the load- tures in place. Advanced construction materials carrying contribution of fibers Composite proper- must contribute to the organic growth of our new ties related to structural performance are de- infrastructures, and at the same time, contribute scribed for a number of FRCs targeted for use in to maintaining the health of our infrastructure load-carrying structural members. Structural ap- inventory. The implications of advanced civil en-
is projected to increase. In North America, the growth rate has been placed at 20% per year. However, it should be pointed out that FRC remains a small fraction of the amount of concrete used each year in the construction industry. Fibers may have been originally introduced in an attempt to “strengthen” the matrix, without consciously distinguishing the difference between material strength and material toughness. As the study of FRC evolved into a scientific discipline, it became generally recognized that the most significant effect of fiber addition to the brittle cementitious matrix is the enhancement of toughness. For most FRCs, this means the capability of the material to carry tensile load, albeit at a decreasing level with opening of a crack after its formation. For certain FRC with continuous and/or high-fiber volume fractions, the ability of fibers in substantially increasing the tensile ductility has been recognized since the work of Aveston et al.2 However, it is only in recent years that such ductility accompanied by strain-hardening can be derived by a moderately low amount of randomly oriented discontinuous fibers (e.g., less than two volume percent) by carefully tailoring the matrix, interface, and fiber via the help of micromechanics. As a result, a new class of economically viable, field processable, high-performance damage-tolerant material is emerging. Emphasis on composite tailoring also brings with it the need to control fiber characteristics to meet the performance need and economic constraints in construction applications of this new type of FRC. In the next section, broad socioeconomic considerations surrounding materials development in civil engineering in general, and fiber reinforced cementitious materials in particular, are described. The section entitled Current Applications of FRCs summarizes current FRC applications worldwide, and documents where, how, and why fibers are used in these applications. An attempt is made to extract common denominators among the widely varied applications. Most current use of FRCs is in nonstructural or, at most, semistructural applications. The following section describes the research and development and industrial trends of applying fibers in enhancing structural performance. An actual case study involving a tunnel lining constructed in Japan is given to illustrate how a newly proposed structural design guideline takes into account the loadcarrying contribution of fibers. Composite properties related to structural performance are described for a number of FRCs targeted for use in load-carrying structural members. Structural applications of FRCs are currently under rapid development. In coming years, it is envisioned that the ultrahigh-performance FRC, with ductility matching that of metals, will be commercially exploited in various applications. Highlights of such a material are described in the section entitled Strain-Hardening Cementitious Composites. In the final section, conclusions on market trends are drawn, and favorable fiber characteristics for structural applications are provided. Emphasis is placed on the need for fiber and surface characteristics most suitable for the ensuing applications and performance needs of future FRCs. SOCIOECONOMIC CONSIDERATIONS Civil infrastructures are organic, in the sense that they grow with the years. The Akashi-Kaikyo Bridge in Kyoto, Japan, recently completed in April 1998, has the longest suspended span (1990 m) of all bridges in the world. At 450 m, the Petronas Twin Tower in Malaysia (completed in 1996) is the tallest building in the world. No doubt these records will be shattered in the near future. Behind this growth is the development of advanced construction materials. Unfortunately, when put in perspective, civil and building engineering materials development does not have a good track record, in comparison with other industries. Part of the reason comes from the lack of cooperation/coordination between the construction industry and the construction material supplying industry. Especially in the United States, joint research and development between materials suppliers and the construction industry is relatively nonexistent. Such fragmentation between materials development and infrastructures is not conducive to the healthy growth and maintenance of our societies’ infrastructures. The negative impact of this stance on construction productivity, durability, and public safety cannot be underestimated. The magnitude of our infrastructure need is enormous. Put in economic terms, about 10% of gross domestic product derives from infrastructure construction worldwide. In the United States alone, infrastructure construction is a $400 billion industry involving 6 million jobs. We have approximately $17 trillion worth of infrastructures in place. Advanced construction materials must contribute to the organic growth of our new infrastructures, and at the same time, contribute to maintaining the health of our infrastructure inventory. The implications of advanced civil enHIGH PERFORMANCE APPLICATIONS OF FIBERS 661
ineering materials in the world economy are sig. large amount of materials used in construction the negative impact(through energy consumption There are a number of unique characteristics of and pollution)on our environment can be signif- civil/building engineering materials which set icant. However, we can enable sustainable infra them apart from those used in other industries. structures to be developed by using more recycled These characteristics include materials (e. g, fly ash, silica fumes, and waste bers (or seconds) in infrastructure with en- Low cost-for example, concrete costs $o 1/kg hanced durability (in contrast to eye contact lens which cost n summary, construction materials can, and $100,000/kg) should, play an important role in our infrastruc- Large volume application-e. g, on a ture development and renewal. The obvious im- wide basis. 6 billion tons of concrete pact on society in economics, public safety, and half billion tons of steel are used in the environment must be recognized structure construction annually Durability requirement--our infrastructures generally are designed for much longer life CURRENT APPLICATIONS OF FRCS than consumer goods, e.g., most bridges are Most current applications of fibers are nonstruc designed with a 75-year service life,com tural Fibers are often used in controlling (plastic pared with an automobile with a typical de- and drying) shrinkage cracks, a role classically sign life of 10-20 year played by steel reinforcing bars or steel wire- Public Safety-it goes without saying that mesh. Examples include floors and slabs, large the general public will not tolerate failure of concrete containers, and concrete pavements. In infrastructures. The experiences from the re- cent Northridge earthquake in the United general, these structures and products have ex- States and the Kobe earthquake in Japan tensive exposed surface areas and movement con- straints, resulting in high cracking potential. For serve important lessons such applications, fibers have a number of advan- processed into infrastructures. Construction These include: (a) uniform reinforcement distri workers generally do not have the same kind bution with respect to location and orientation, of training ceramics engineers have. This im- (b)corrosion resistance especially for synthetic plies that the material, if processed at a con- carbon, or amorphous metal fibers, and(c)labor struction site, must be tolerant of low-preci- saving by avoiding the need of deforming the re- sion processing. inforcing bars and tying them in the form-work Thich often leads to reduction of construction The above unique characteristics need to be time. Elimination of reinforcing bars also relaxes bserved when developing advanced construction constraints on concrete element shape. This func- materials. They may be regarded as overall con- tional value of fibers has been exploited in the straints. Only materials meeting such constraints curtain walls of tall buildings. The Kajima Cor will be successfully adopted in the real world. For poration ( Japan) has taken advantage of fibers i FRC, the first two constraints on cost and appli- the manufacture of curvilinear-shaped wall pan cations in large-scale structures imply that fibers els valued for their aesthetics(see, e. g Fig. 1). In cannot be overly expensive and must be used in some applications, the use of fibers enables the relatively small volume content elimination or the reduction in the number of Viewed in a more positive light, some of the cut -joints in large continuous structures such as above constraints also make materials serve as containers(Fig. 2) and pavements. Especially enabling technology for infrastructures. Proper pavements, joints are locations of weaknesses at selection of fiber and matrix materials is critical which failure frequently occurs. Thus, fibers have in producing durable infrastructures. FRCs with been exploited to enhance the durability of con- high ductility lead to safer infrastructures. Mate- crete elements. Some additional representative rials can even lend themselves to improving con- industrial applications of FRCs are shown in Fig struction productivity. For example, the replace- ures 3-5. These examples are chosen to illustrate ment of re-bars in reinforced concrete(R/C)struc- the wide range of fiber used(steel, glass, polymer, tures with FRCs have led to reduction in labor amorphous metal, carbon) and the international cost in construction sites. Finally, because of the nature of FRC applications
gineering materials in the world economy are significant. There are a number of unique characteristics of civil/building engineering materials which set them apart from those used in other industries. These characteristics include: ● Low cost—for example, concrete costs $0.1/kg (in contrast to eye contact lens which cost $100,000/kg). ● Large volume application—e.g., on a worldwide basis, 6 billion tons of concrete and a half billion tons of steel are used in infrastructure construction annually. ● Durability requirement—our infrastructures generally are designed for much longer life than consumer goods, e.g., most bridges are designed with a 75-year service life, compared with an automobile with a typical design life of 10–20 years. ● Public Safety—it goes without saying that the general public will not tolerate failure of infrastructures. The experiences from the recent Northridge earthquake in the United States and the Kobe earthquake in Japan serve important lessons. ● Construction labor—materials have to be processed into infrastructures. Construction workers generally do not have the same kind of training ceramics engineers have. This implies that the material, if processed at a construction site, must be tolerant of low-precision processing. The above unique characteristics need to be observed when developing advanced construction materials. They may be regarded as overall constraints. Only materials meeting such constraints will be successfully adopted in the real world. For FRC, the first two constraints on cost and applications in large-scale structures imply that fibers cannot be overly expensive and must be used in relatively small volume content. Viewed in a more positive light, some of the above constraints also make materials serve as enabling technology for infrastructures. Proper selection of fiber and matrix materials is critical in producing durable infrastructures. FRCs with high ductility lead to safer infrastructures. Materials can even lend themselves to improving construction productivity. For example, the replacement of re-bars in reinforced concrete (R/C) structures with FRCs have led to reduction in labor cost in construction sites. Finally, because of the large amount of materials used in construction, the negative impact (through energy consumption and pollution) on our environment can be significant. However, we can enable sustainable infrastructures to be developed by using more recycled materials (e.g., fly ash, silica fumes, and waste fibers (or seconds)) in infrastructure with enhanced durability. In summary, construction materials can, and should, play an important role in our infrastructure development and renewal. The obvious impact on society in economics, public safety, and the environment must be recognized. CURRENT APPLICATIONS OF FRCS Most current applications of fibers are nonstructural. Fibers are often used in controlling (plastic and drying) shrinkage cracks, a role classically played by steel reinforcing bars or steel wiremesh. Examples include floors and slabs, large concrete containers, and concrete pavements. In general, these structures and products have extensive exposed surface areas and movement constraints, resulting in high cracking potential. For such applications, fibers have a number of advantages over conventional steel reinforcements. These include: (a) uniform reinforcement distribution with respect to location and orientation, (b) corrosion resistance especially for synthetic, carbon, or amorphous metal fibers, and (c) laborsaving by avoiding the need of deforming the reinforcing bars and tying them in the form-work, which often leads to reduction of construction time. Elimination of reinforcing bars also relaxes constraints on concrete element shape. This functional value of fibers has been exploited in the curtain walls of tall buildings. The Kajima Corporation (Japan) has taken advantage of fibers in the manufacture of curvilinear-shaped wall panels valued for their aesthetics (see, e.g., Fig. 1). In some applications, the use of fibers enables the elimination or the reduction in the number of cut-joints in large continuous structures such as containers (Fig. 2) and pavements. Especially in pavements, joints are locations of weaknesses at which failure frequently occurs. Thus, fibers have been exploited to enhance the durability of concrete elements. Some additional representative industrial applications of FRCs are shown in Figures 3–5. These examples are chosen to illustrate the wide range of fiber used (steel, glass, polymer, amorphous metal, carbon) and the international nature of FRC applications. 662 LI
HIGH PERFORMANCE APPLICATIONS OF FIBERS 663 Figure 1 Japanese curvilinear carbon-FRC curtain Figure 3 French Metglas FRC underground tunnel Durability is an important performance-en hancement characteristic in many industrial FRC applications. Naturally, durability has different connotations in different application contexts. For xample, for containers durability implies the lifetime prior to unacceptable leakage. For pave- Figure 2 Danish pp-FRC containers Figure 4 U.S. glass-FRC wall panels
Durability is an important performance-enhancement characteristic in many industrial FRC applications. Naturally, durability has different connotations in different application contexts. For example, for containers, durability implies the lifetime prior to unacceptable leakage. For paveFigure 1 Japanese curvilinear carbon–FRC curtain walls. Figure 2 Danish pp–FRC containers. Figure 3 French Metglas FRC underground tunnel linings. Figure 4 U.S. glass–FRC wall panels. HIGH PERFORMANCE APPLICATIONS OF FIBERS 663
at. eo in building foundation cost, hoisting ma- steel reinforcement, and transportation example, the Kajima Corporation claims a reduction In extern structural steel requirement of 4000 tons for the Tokyo Ark- Mori building which used 32, 000 m-of CFRC (carbon fiber FRC)wall panels. Reduction in construction time is highly valued (e. g, in fiber shotcreting of tunnel linings common in Sweden a and Austria) and represents major cost advan- tage in the construction industry There is no question that fibers lead to concrete element performance improvements in a wide range of applications, providing the benefit part of the cost/benefit ratio consideration. Apart from durability against shrinkage cracks, fibers are Figure 5 German steel-FRC airfield pavement. valued for their imparting the concrete element with energy absorption capability-often de- scribed in terms of their impact resistance(e. g ments, durability implies the repair time interval foors and slabs), and delamination and spall re- in order to maintain rideability. The cause of loss sistance (e. g, concrete structure repair). Other of durability is also very much dependent on the rformance improvements include corrosion and specific application and field conditions fatigue resistance. Repair of concrete structures appears to be a To achieve such performance enhancements sizable application of FRCs. This includes resto- two essential properties of FRCs are utilized. As ration of pavements, airfields, bridge decks, and replacements for steel reinforcements and joints floor slabs. With the decaying infrastructure cou- fibers contribute to the shrinkage crack resis- pled with increasing demand in their perfor nce property of the FRC Impact resistance per mance in most industrialized countries, it is ex- formance (and to a certain extent bending pected that the need for durable repairs will in- strength) is linked to the fracture toughness of crease over time. Fundamental understanding of the composite. Fibers are very effective in this durable repairs is lacking at present. However, it respect, much more so than in increasing compos is generally agreed that repair failures are often ite tensile strength or ductility( strain capacity) in related to mechanical property incompatibilit urrent frcs. IThe exception to this“rule”is between the repair material and substrate con- being realized in the laboratory; see Strain-Hard crete. Dimensional stability of the repair material ening Cementitious Composites below.] The and delamination resistance are often cited as shrinkage crack resistance and toughness prop- some of the controlling factors. Fibers can be, and erty of FRCs are well recognized and exploited in have been, used to advantage in this area current concrete element applications in the con- The adoption of new materials in the highly struction industry. Because of the utilization of cost-sensitive construction, building, and precast proved mechanical properties of FRC, some products industries(grouped together as the"con- the"con- dustrial applications can be considered semi struction industry hereafter) generally requires structural. These properties are needed to carry justification of cost advantage. The dollar value of dead loads, handling (or construction) loads, loads durability is difficult to quantify, but durability related to restrains from dimensional changes demand clearly represents one of the driving ete. Wall panels and some pavement applications forces in the use of fibers, especially when shrink- belong to this category. However, in most of these age crack resistance is considered. As mentioned applications, the fibers are not expected to con above, labor saving via elimination of joints or tribute to load-carrying function in the element re-bars provides extra financial incentives. Other Some examples of current industrial applica cost advantages in the use of fibers include ele- tions of FRCs are summarized in Table I. 7 This ment thickness and/or weight reduction, such as table provides a broad overview of wide-ranging in concrete pipes, pavements, and building cur- applications in different parts of the world. How- tain wall panels. In the case of building curtain ever, it is by no means exhaustive. Some of these walls, weight reduction can lead to significant applications are experimental, in the prototyping
ments, durability implies the repair time interval in order to maintain rideability. The cause of loss of durability is also very much dependent on the specific application and field conditions. Repair of concrete structures appears to be a sizable application of FRCs. This includes restoration of pavements, airfields, bridge decks, and floor slabs. With the decaying infrastructure coupled with increasing demand in their performance in most industrialized countries, it is expected that the need for durable repairs will increase over time. Fundamental understanding of durable repairs is lacking at present. However, it is generally agreed that repair failures are often related to mechanical property incompatibility between the repair material and substrate concrete. Dimensional stability of the repair material and delamination resistance are often cited as some of the controlling factors. Fibers can be, and have been, used to advantage in this area. The adoption of new materials in the highly cost-sensitive construction, building, and precast products industries (grouped together as the “construction industry” hereafter) generally requires justification of cost advantage. The dollar value of durability is difficult to quantify, but durability demand clearly represents one of the driving forces in the use of fibers, especially when shrinkage crack resistance is considered. As mentioned above, labor saving via elimination of joints or re-bars provides extra financial incentives. Other cost advantages in the use of fibers include element thickness and/or weight reduction, such as in concrete pipes, pavements, and building curtain wall panels. In the case of building curtain walls, weight reduction can lead to significant savings in building foundation cost, hoisting machinery, steel reinforcement, and transportation cost. For example, the Kajima Corporation claims a reduction in external wall load of 60% and structural steel requirement of 4000 tons for the Tokyo Ark-Mori building which used 32,000 m2 of CFRC (carbon fiber FRC) wall panels. Reduction in construction time is highly valued (e.g., in fiber shotcreting of tunnel linings common in Sweden and Austria) and represents major cost advantage in the construction industry. There is no question that fibers lead to concrete element performance improvements in a wide range of applications, providing the benefit part of the cost/benefit ratio consideration. Apart from durability against shrinkage cracks, fibers are valued for their imparting the concrete element with energy absorption capability—often described in terms of their impact resistance (e.g. floors and slabs), and delamination and spall resistance (e.g., concrete structure repair). Other performance improvements include corrosion and fatigue resistance. To achieve such performance enhancements, two essential properties of FRCs are utilized. As replacements for steel reinforcements and joints, fibers contribute to the shrinkage crack resistance property of the FRC. Impact resistance performance (and to a certain extent bending strength) is linked to the fracture toughness of the composite. Fibers are very effective in this respect, much more so than in increasing composite tensile strength or ductility (strain capacity) in current FRCs. [The exception to this “rule” is being realized in the laboratory; see Strain-Hardening Cementitious Composites below.] The shrinkage crack resistance and toughness property of FRCs are well recognized and exploited in current concrete element applications in the construction industry. Because of the utilization of improved mechanical properties of FRC, some industrial applications can be considered semistructural. These properties are needed to carry dead loads, handling (or construction) loads, loads related to restrains from dimensional changes, etc. Wall panels and some pavement applications belong to this category. However, in most of these applications, the fibers are not expected to contribute to load-carrying function in the element. Some examples of current industrial applications of FRCs are summarized in Table I.7 This table provides a broad overview of wide-ranging applications in different parts of the world. However, it is by no means exhaustive. Some of these applications are experimental, in the prototyping Figure 5 German steel–FRC airfield pavement. 664 LI
HIGH PERFORMANCE APPLICATIONS OF FIBERS 665 stage of development. They are indicated with an (-$0.004/b ) even for the lowest-cost fiber), and asterisk. The properties utilized, application per- processability(measured in terms of workability formance improvements, and cost advantage or for concrete mixing and casting). In special prod justifications are also included for each applica- uct lines, such as thin roofing tiles and other tion. In this table, the nature of how the FrC has thin-sheet products, as well as in FRC protective been used is identified by the symbols n= non- shields and other products that can tolerate structural, Ss= semistructural, and S= struc- higher cost for additional performance needs tural. Many applications lie in the gray zone be- larger amounts of fibers have been used. Exam tween nonstructural, semistructural, and struc- ples include SifCon(slurry infiltrated concrete tural applications. The classification is therefore invented in the United States and used in airfield somewhat subjective. Even so, it is clear that at pavements) and CRC(compact reinforced con the present time, straight structural applications crete, invented in Denmark and used in safet of FRC are in the minority, but growing vaults). These FRCs have fiber content ranging It is noted that most fibers currently in use are from 5% to 20%. Special processing techniques either steel or polypropylene fibers. These are are required. SIfCON requires bedding of fibers relatively low-cost fibers and generally satisfy the into a concrete form followed by infiltration of the composite property needs and the concrete ele- fiber bed by a high w/e ratio mortar slurry. CRC ment performance needs as described above requires high frequency vibration applied directly Glass fibers have been used extensively in wall to a dense array of steel reinforcements to reach panel type applications. However their real/per cceptable material compaction. For thin-sheet ceived problems in durability appear to have products, the Hatchek technique is common in slowed down their market expansion, at least for processing the composite with high-content fibers the near future. A number of litigation cases in which serve as the main reinforcement in such the United States involving time-delayed crack- products ing of wall panels with glass fibers have added to One of the major drawbacks in many current concerns by end-users. Other fibers used in large FRC applications is that the development of the quantities include cellulose fiber, often used as FRC is often decoupled from the design of the processing aids rather than for their reinforcing concrete element. Furthermore, the detailed ef- capability. As is well known, asbestos fibers (often fect of fiber addition to the composite property, used in thin-sheet elements)are increasingly dis- and hence to the performance improvement of the placed, at least in the United States and in many concrete element is often not quantified. Instead countries in Europe, because of carcinogenic decisions on the choice of fibers and the fiber health hazard potential. Newcomers on the mar- content chosen are often reflections of experience ket for concrete reinforcements are Metglas on the part of the user. Unfortunately, this often (amorphous metal), carbon, and certain high-per- leads to results that fail to meet expectations. A formance polymer fibers. Metglas is produced in good example is the use of steel fibers in pave- France and its applications appear mostly limited ments. Many successful uses of fibers in pave to france at the present time. Production of car- ments have been reported, in some cases even bon and polyvinyl alcohol (PvA) fibers is cur- with the pavement thickness reduced. However, rently led by Japa nese manufacturers, although just as many cases have shown disappointing re- ome production facilities of these fibers have sults. There are a variety of ns for this to the last few years been started up in China. Each happen. The loading condition(environmental or of these fibers has their limitations. For example, mechanical)can be different, e.g., for pavements most carbon fibers are brittle (low bending located in different states. Because of this, there strength or tensile strain capacity), and some is a certain amount of luck factor involved in studies have suggested durability problems in successful applications. A ramification of this re- composites reinforced with certain PVA fibers. sult is that users become disenchanted over the However, manufacturers of these fibers are con- use of fibers The lack of systematic design guide- tinuously advancing the properties of these fibers lines and mixed experience in FRC applic to so that some of these problems may be expected to concrete elements are responsible for slowing the be overcome in the future spread of FRCs to even broader industrial appli- In most applications, fibers are used in less cations, despite their many advantages as de- than 1% by volume. Fiber content in FRC is lim- scribed above ited by cost(cost of fibers are much higher than Although the current application of fibers in Portland cement 03/1b )and aggregates concrete elements is limited in scope, it appears to
stage of development. They are indicated with an asterisk. The properties utilized, application performance improvements, and cost advantage or justifications are also included for each application. In this table, the nature of how the FRC has been used is identified by the symbols N 5 nonstructural, SS 5 semistructural, and S 5 structural. Many applications lie in the gray zone between nonstructural, semistructural, and structural applications. The classification is therefore somewhat subjective. Even so, it is clear that at the present time, straight structural applications of FRC are in the minority, but growing. It is noted that most fibers currently in use are either steel or polypropylene fibers. These are relatively low-cost fibers and generally satisfy the composite property needs and the concrete element performance needs as described above. Glass fibers have been used extensively in wall panel type applications. However their real/perceived problems in durability appear to have slowed down their market expansion, at least for the near future. A number of litigation cases in the United States involving time-delayed cracking of wall panels with glass fibers have added to concerns by end-users. Other fibers used in large quantities include cellulose fiber, often used as processing aids rather than for their reinforcing capability. As is well known, asbestos fibers (often used in thin-sheet elements) are increasingly displaced, at least in the United States and in many countries in Europe, because of carcinogenic health hazard potential. Newcomers on the market for concrete reinforcements are Metglast (amorphous metal), carbon, and certain high-performance polymer fibers. Metglas is produced in France and its applications appear mostly limited to France at the present time. Production of carbon and polyvinyl alcohol (PVA) fibers is currently led by Japanese manufacturers, although some production facilities of these fibers have in the last few years been started up in China. Each of these fibers has their limitations. For example, most carbon fibers are brittle (low bending strength or tensile strain capacity), and some studies have suggested durability problems in composites reinforced with certain PVA fibers. However, manufacturers of these fibers are continuously advancing the properties of these fibers so that some of these problems may be expected to be overcome in the future. In most applications, fibers are used in less than 1% by volume. Fiber content in FRC is limited by cost (cost of fibers are much higher than Portland cement (; $0.03/lb.) and aggregates (; $0.004/lb.), even for the lowest-cost fiber), and processability (measured in terms of workability for concrete mixing and casting). In special product lines, such as thin roofing tiles and other thin-sheet products, as well as in FRC protective shields and other products that can tolerate higher cost for additional performance needs, larger amounts of fibers have been used. Examples include SIFCON (slurry infiltrated concrete, invented in the United States and used in airfield pavements) and CRC (compact reinforced concrete, invented in Denmark and used in safety vaults). These FRCs have fiber content ranging from 5% to 20%. Special processing techniques are required. SIFCON requires bedding of fibers into a concrete form followed by infiltration of the fiber bed by a high w/c ratio mortar slurry. CRC requires high frequency vibration applied directly to a dense array of steel reinforcements to reach acceptable material compaction. For thin-sheet products, the Hatchek technique is common in processing the composite with high-content fibers which serve as the main reinforcement in such products. One of the major drawbacks in many current FRC applications is that the development of the FRC is often decoupled from the design of the concrete element. Furthermore, the detailed effect of fiber addition to the composite property, and hence to the performance improvement of the concrete element is often not quantified. Instead, decisions on the choice of fibers and the fiber content chosen are often reflections of experience on the part of the user. Unfortunately, this often leads to results that fail to meet expectations. A good example is the use of steel fibers in pavements. Many successful uses of fibers in pavements have been reported,3 in some cases even with the pavement thickness reduced. However, just as many cases have shown disappointing results.4 There are a variety of reasons for this to happen. The loading condition (environmental or mechanical) can be different, e.g., for pavements located in different states. Because of this, there is a certain amount of luck factor involved in successful applications. A ramification of this result is that users become disenchanted over the use of fibers. The lack of systematic design guidelines and mixed experience in FRC applications to concrete elements are responsible for slowing the spread of FRCs to even broader industrial applications, despite their many advantages as described above. Although the current application of fibers in concrete elements is limited in scope, it appears to HIGH PERFORMANCE APPLICATIONS OF FIBERS 665
Table I FRC Industrial Applications Performance Applications Ve(%) S? Properties Utilized Cost reduction ement""full pp monofils SS MOR, toughness pete with steel n Shrinkage crack res. Durability Crackstop o shrinkage control steel n Wear resistance ability Less than complete Canada op rutting of Life-cost Denmark Stang. 94 SIFCON 0.75 nergy absorption Impact Fatigue Life-cost = n Elastic modulus Compatibility with substrate conerete N Toughness Reduced thickne Canada Johnston. 9 1.7-25 SS Toughness Denmark Ramboll ustrial Metglas Adhesion 3MPa. Self-leveling, Chem. Life-cost France Lanko MPa. MOR 12 Shock, abrasion Industrial floor steel Ductility isting base mage res from Rendering, flo PP N No need for sand Denmark screeding resist, crack stop ngth and Floors Slabs pp 0.1 Shrinkage crack re ife-cost 300K m2 Glavind enmark Stang, 94 replaces mesh, reduce nsitop Broc. liability metglas Chemical res Parking garage pp n Shrinkage crack re Durability No steel mesh Denmark mm thick slabs 16x8 Shrinkage crack res. Seis mic force red. Build, weight resion Ss Shrinkage crack res. Durability Replace steel rein Mufti et al. 93 hear resistance Curtain wall carbon CFRC Broe Transp. oost red. all panels Low density inless Clubhouse n shells 65Km2 uilding foundation cost
Table I FRC Industrial Applications Applications Fiber Vf (%) S? Properties Utilized Performance Improvements Cost Reduction Amount Appl. & Location Reference Pavement overlay* 35 m long, 10 cm thick pp 1steel 0.75 1 0.75 SS Shrinkage crack res. Tensile strain cap. Durability: no failure at joints Thick. red. to 1⁄2 No steel reinf. No cut joint Denmark Glavind, ’93; Ramboll Hannemann & Hojlund, ’92 Pavement 75– 175 mm thick steel 0.5–1 SS Shrinkage crack res. Flexural strength Durability? Greater joint spacing Canada Balaguru & Shah, ’92 Pavement* “full depth” Thin bridgedeck overlay pp monofils 1 SS MOR, toughness Impact resistance chemical resistance fatigue resistance Compete with steel fibers US Van Mier, ’95 Ramakrishnan, ’93 Pavement 200 mm thick, 80 m long pp 0.7 N Shrinkage crack res. Durability No steel No shrinkage control joint U.K. Crackstop News, No. 1, ’90 Pavement whitetopping 100 mm steel 1 N Wear resistance Durability Stop rutting of asphalt Noise reduction Less than complete replacement of flex. pavement Canada Johnston, ’95 Repair Pavement pp 1–1.5 N Toughness Interface bond Crack res. Delamination res. Life-cost Denmark Glavind & Stang, ’94 Repair Airfield pavement SIFCON steel 5 0.75 SS Toughness Interface bond Energy absorption MOR Tensile strain cap. Crack & spall res. Delamination res. Impact res. (dyn. load from planes) Fatigue res. Life-cost Simplicity in slipform paving Incr. in joint spacing Glavind & Stang, ’94 Balaguru and Shah, ’92 Germany Harex fiber broc. Repair Airfield runway patch steel ? N Elastic modulus COE Compatibility with substrate concrete Durability Life-cost US Balaguru & Shah, ’92 Repair Bridge substructure steel pp 0.3 0.2 N Toughness Fatigue res. Impact res. Reduced thickness Canada Johnston, ’95 Repair General pp 1 steel 1.7–2.5 SS Toughness Interface bond Crack res. Delamination res. Life-cost No shrink. reinf. Thick. red. Denmark Ramboll Hannemann & Hojlund, ’92 Industrial floor restoration Metglas 1 SS Adhesion 3MPa, Compress. 80 MPa, MOR 12 MPa Self-leveling, Chem. corr. res. Shock, abrasion, crack res. Life-cost France St. Gobain, Lanko Industrial floor rehabilitation Thin overlay steel 0.5 SS Ductility Toughness Long term bond to existing base Spall res. Damage res. from forklift Reduce production facility downtime Long term performance US Smith, R., ’95 Rendering, floor screeding pp ? N Shrinkage crack resist, crack stop Durability No need for sand 3strength and bond Denmark D. Davis, Danaklon lit. Floors & Slabs pp 0.1 n Shrinkage crack res. Toughness Durability Life-cost 300K m2 , Denmark Glavind & Stang, ’94 steel 0.3–0.5 “1 energy absopt., MOR, toughness Impact res. (dyn. wheel load form fork lift); Easy processing, High reliability Spall res. Fatigue res. replaces mesh, reduce labor, slab thickness, incr. joint spacing, faster construction, lower maint. cost Bache, ’92 Densitop Broc. Bekaert Broc. Metglas Chemical res. St. Gobain, Lanko Parking garage floor 150 mm thick slabs 16 3 8 m pp 0.9 N Shrinkage crack res. Durability No steel mesh Denmark Crackstop News, No. 1, ’90 Bridge deck slabs* pp 0.88 SS Shrinkage crack res. Shear resistance Durability Replace steel reinf., Caontrol corrosion Canada Mufti et al, ’93 Curtain wall panels carbon 2–4 SS Low density Strength Shrinkage crack res. Light-weight Seismic force red. Fire res. Dim. stability Durability Build. weight red. Found. cost red. Steel red. Transp. cost red. Erec. cost red. Constr. time red. Shape flexibility 300K m2 Japan CFRC Broc., Kajima Wall panels carbon 2 SS MOR Low density Avoid corner damage & cracking Durable against sunlight, heat and salt Increase design flexibility Light-weight Kitakyus hu Prince Hotel 12 tons Japan Mitsubishi Kasei Broc. Ando, Lightweight cladding panels skin 40,75 mm thick stainless steel 1.3 SS MOR Low density Durability, Res. wind load Sail Clubhouse Australia Fibresteel, Vol 4 No. 1, ’92 Thin shells & facades AR glass 4–5 SS Shrinkage crack res. Tensile strength Toughness MOR Durability Shape desg flex. No steel reinf. Low weight per unit area, building deadload, foundation cost 65K m2 , Denmark Glavind & Stang, ’94 666 LI
HIGH PERFORMANCE APPLICATIONS OF FIBERS 667 Table i continued Performance S? Properties Utilized Cost reduction Reference Tunnel linings ste Strength, MO Durability Energy absorption Better bonding to 3: Maage Broe Metglas >25 kg/'m Shoterete. St Gobain lit. Crack resistance ability fatigue re Van Mier et chem. resistance n Abrasion rs n Shrinkage crack res Poland s出mw 25-30K m. Glavind Stang, 94 Fiberbeton steel reinf. Bending load res Wall thick. rec Australia No. 1, 92 0.3-0.5 Bending load res. nical w/o wire- Belgium Dramix broe. steel 1.75 Bending load res ck. red to 2 Denmark oors Metglas S Ductility Lightweight, Replace steel, sam France St. Gobain lit. aname energ weight, lower cost Dsorption CRC steel High-cost Denmark nergy Absorp. steel? Ss Energy absorption Impact Plastic shrinkage Denmark SS Tensile Thermal shock re Belgium thermal shock resist Columns(RPC steel 2-4 s Strength, Toughness Seismic resistance Slender columns France Richard & Cheyney, Column/slab CRC steel S Toughness Short development Building system Van Mier et al S Truss members Netherlands van et al Roofing tiles 4-5 Life-cost enmark Glavind extruded or engi age crack res. Durability rength steel reinf. poss. Stang, 94 Light-weight Life-cost products for o steel reinf. po cladding Toughness Ferrocem N rst erack Life-cost Shirai and Ohama, 95 Pakeaging and Metglas n Microcrack resist dioactivity 300 yr containers St, Gobain lit. Stair treads PVA Rust resistance Light-weight (Kajima KaTRI Broe *Experimental
Table I Continued Applications Fiber Vf (%) S? Properties Utilized Performance Improvements Cost Reduction Amount Appl. & Location Reference Tunnel linings steel 0.5–1 S Strength, MOR Toughness Energy absorption Shrinkage crack res. Safety Durability Better bonding to underlay Maintain contour Water tightness Replace wire-reinf., No reinf. corrosion Constr. time & labor red. (with shotcreting) Thickness red. Constr. safety 80K m3/yr., Norway Skerendale, ’94 Horri & Nanakorn, ’93; Maage, M., ’94 Bekaert Broc. EE-fiber Broc. St. Gobain Broc. Metglas .25 kg/m3 Sewage network linings* Metglas SS MOR Crack resistance Corrosion resistance Durability Replace wire-mesh, labor, Reduce thickness Shotcrete, France St. Gobain lit. Drainage canal in tunnel CRC steel 6 S MOR fatigue resistance durability (100 yrs) non-conducting (elec. sys. of train) chem. resistance Thin cover 40K element 500x400x 40 mm3, Denmark Van Mier et al., ’95 Wear linings, hydraulic structures steel N Abrasion rs. Durability Life-cost Denmark Glavind & Stang, ’94 Underground railway system pp “sm. amt.” N Shrinkage crack res. Durability Poland Brandt et al, ’94 Containers agriculture process sludge purifying pp 2 SS Shrinkage crack res. Elastic modulus Durability Water-tightness Mat’l cost 2x thickness red. No cut joint’ No steel reinf. 25–30K m3 , Denmark Glavind & Stang, ’94 Fiberbeton R&S, Denmark Septic tanks steel ? SS MOR Bending load res. Wall thick. red., Labor & mat’l red. Mass & weight red. Cost less than mesh Australia Fibresteel, V.4 No. 1, ’92 Pipe* steel 0.3–0.5 SS MOR Crack res. Ductility Bending load res. Spall res. Corrosion res. Economical w/o wiremesh reinforcement Belgium Dramix broc. and design doc. Pipe* steel 1.75 SS MOR Bending load res. Wall thick. red. to 1⁄3 of normal Denmark Thygesen, ’93 Pedersen & Jorgen, ’92 Anti-blast doors military shelters Metglas ? S Ductility Lightweight, Dynamic energy absorption Replace steel, same weight, lower cost France St. Gobain lit., Dynabeton by Sogea Security products CRC steel 6 S Strength Ductility Energy Absorp. Impact Res. High-cost Denmark Bache, ’87 Vaults and safes steel 1–3 S Energy Absorp. Impact Res. Thickness reduction (;2⁄3) U.S. Balaguru & Shah, ’92 Tetrapods (dolosse) steel? ? SS Energy absorption Impact res. 30K m3 Australia Engineer update, ’84 Sea defense work concrete pp 0.9 N Plastic shrinkage crack res. Durability against wind/exposure 4 tonnes, Denmark Crackstop News, No. 1, ’90 Refractories Stainless steel ? SS Tensile MOR Thermal shock resis. Spall resist. Durability Reliability Belgium Bekaert Broc. Refractories e.g. lip rings for iron ladles, furnace hearths stainless steel ? SS thermal shock resist. U.S. Lankard, ’92 Columns (RPC in steel tube) steel 2–4.5 S Strength, Toughness Seismic resistance Slender columns France Richard & Cheyrezy, ’94 Column/Slab cast-in-place joint CRC steel ? S Toughness Short development length for reinf. Building system flexibility Denmark Van Mier et al, ’95 Truss-system* SIFCON ? S Tensile Compress. strength Truss members Netherlands Van Mier et al, ’95 Roofing tiles (extruded or Hatschek) cellul. pp wollas. 4–5 N Shrinkage crack res. Tensile strength Toughness MOR Durability Life-cost No steel reinf. poss. Light-weight Denmark Glavind & Stang, ’94 Thin sheet products for cladding asbestos cellul. pp glass PVA carbon varies N Shrinkage crack res. Tensile strength Toughness MOR Durability Life-cost No steel reinf. poss. Light-Weight Europe Bentur, A., ’95 Ferrocement column steel carbon 1 5 N First crack res. MOR Corrosion res. Durability Red. in crack width Life-cost Japan Shirai and Ohama, ’95 Pakcaging and storage nuclear waste Metglas N Microcrack resist. Contain radioactivity Durability 300 yr. containers France St. Gobain lit. Sogefibre Stair treads PVA 2 SS Low denstiy Strength Toughness Rust resistance Light-weight Boltable Life-cost Japan (Kajima) Yurugi et al, ’91 KaTRI Broc. *Experimental. HIGH PERFORMANCE APPLICATIONS OF FIBERS 667
be gaining ground with documented successes in ments in first crack and ultimate member various parts of the world. The sluggish growth in strength, impact resistance, and shear resistance FRC applications is influenced by many factors, If properly designed, fibers can add to member including: 1. the high cost of fiber compared with structural performance even when used together other constituents of concrete: 2. the cost-sensi- with conventional steel main reinforcements (re- tive nature of the construction industry; 3. the bars). Some highlights of these laboratory find mixed experience in the use of FRC in certain ings are summarized in the section below. applications; and 4. the unclear linkage between Currently, several construction projects are fiber and concrete element performance. Both contemplating the application of fibers in load end-users and fiber suppliers need to be realistic carrying concrete members. The concrete tunnel- in what each type of fiber can do to concrete ning project in Japan appears to be the most element performance. Research is needed to con- advanced one, both in time and in implementing tinuously improve the benefits brought about by the fiber load-carrying capacity into the design fibers, while reducing the cost of fiber applica- calculation. This project is described in a later tions. Users need to be educated that part of the section. This case, together with the laboratory fiber cost can be offset by reduction or elimination studies of FRC structural members, suggest that of other costs using conventional concrete without the a-8 relation is the most useful property char fibers. as described in this section. acterization of FRCs for structural design Means The cost pressure will always be present. One of FRC structural performance comparison are way of overcoming this pressure is to continu- indicated at the end of this section ously and systematically enhance the benefit/cost ratio. Structural load-bearing capacity of fibers appears to be a significant benefit reaching be. Laboratory Studies of Structural Applications of frc yond laboratory curiosity and emerging in indus trial settings at the present time Fibers designed There have been a large amount of laborator with this function in mind, with proper surface studies of applications of FRCs in R/C and pre- treatment for fresh(FRC rheological) properti stressed concrete structural members This sec- and hardened composite properties, can contrib- tion summarizes the highlights of these studies ute significantly to future advanced structural which demonstrate without a doubt that fibers members. The emerging trend of structural appli- can be effective in enhancing structural strength cations of frc is described in the next section and ductility in load-carrying members studies include members under fexural torsion, and combined loads. Additionally STRUCTURAL APPLICATIONS OF FRC tural component responses under cyclic load and bond property of reinforcing steel bars have also At present. despite much research in the labora- been studied. Most of these studies have been tory, the use of FRC in load-carrying structural limited to steel fibers. More detailed descriptions members is very limited. Using fibers to carry of the test methods and parameters as well as the load across cracks in a hardened concrete in original references can be found in Balaguru and structural design is still a novel practice. This is Shah.3 because of a lack of clear understanding in how In fexural r/c members the addition of fibers fibers contribute to load-carrying capacity, confu- improves the modulus of rupture (bending sion between material and structural strengths, strength). Especially in over-reinforced concrete lack of structural design guidelines for FRC mem- beams, the significant gain appears to be in the bers, uncertain cost/benefit ratio, and insufficient enhancement of post-peak structural ductility material property specification, characterization, (Fig. 6), a quantity valued by structural and test standards. These deficiencies not only for safety reasons. This ductility improvement limit the broader use of fibers in structural appli likely a result of the delay in compression crush cations, but also make it difficult for fiber suppli by pression strain capacit ers to optimize their fibers for concrete structural due to fiber reinforcement. The potential for over- reinforcement is greater when higher strength Research findings in the last decade clearly steel or FRP(fiber reinforced plastic rod)is used establish that ductility of certain structural mem- as reinforcement. For under-reinforced beams or bers can be greatly enhanced with the use of beams with no main reinforcement at all, flexural fibers. In addition, fibers generally favor improve- strength enhancement and post-peak ductility
be gaining ground with documented successes in various parts of the world. The sluggish growth in FRC applications is influenced by many factors, including: 1. the high cost of fiber compared with other constituents of concrete; 2. the cost-sensitive nature of the construction industry; 3. the mixed experience in the use of FRC in certain applications; and 4. the unclear linkage between fiber and concrete element performance. Both end-users and fiber suppliers need to be realistic in what each type of fiber can do to concrete element performance. Research is needed to continuously improve the benefits brought about by fibers, while reducing the cost of fiber applications. Users need to be educated that part of the fiber cost can be offset by reduction or elimination of other costs using conventional concrete without fibers, as described in this section. The cost pressure will always be present. One way of overcoming this pressure is to continuously and systematically enhance the benefit/cost ratio. Structural load-bearing capacity of fibers appears to be a significant benefit reaching beyond laboratory curiosity and emerging in industrial settings at the present time. Fibers designed with this function in mind, with proper surface treatment for fresh (FRC rheological) properties and hardened composite properties, can contribute significantly to future advanced structural members. The emerging trend of structural applications of FRC is described in the next section. STRUCTURAL APPLICATIONS OF FRC At present, despite much research in the laboratory, the use of FRC in load-carrying structural members is very limited. Using fibers to carry load across cracks in a hardened concrete in structural design is still a novel practice. This is because of a lack of clear understanding in how fibers contribute to load-carrying capacity, confusion between material and structural strengths, lack of structural design guidelines for FRC members, uncertain cost/benefit ratio, and insufficient material property specification, characterization, and test standards. These deficiencies not only limit the broader use of fibers in structural applications, but also make it difficult for fiber suppliers to optimize their fibers for concrete structural applications. Research findings in the last decade clearly establish that ductility of certain structural members can be greatly enhanced with the use of fibers. In addition, fibers generally favor improvements in first crack and ultimate member strength, impact resistance, and shear resistance. If properly designed, fibers can add to member structural performance even when used together with conventional steel main reinforcements (rebars). Some highlights of these laboratory findings are summarized in the section below. Currently, several construction projects are contemplating the application of fibers in loadcarrying concrete members. The concrete tunnellining project in Japan appears to be the most advanced one, both in time and in implementing the fiber load-carrying capacity into the design calculation. This project is described in a later section. This case, together with the laboratory studies of FRC structural members, suggest that the s–d relation is the most useful property characterization of FRCs for structural design. Means of FRC structural performance comparison are indicated at the end of this section. Laboratory Studies of Structural Applications of FRC There have been a large amount of laboratory studies of applications of FRCs in R/C and prestressed concrete structural members. This section summarizes the highlights of these studies, which demonstrate without a doubt that fibers can be effective in enhancing structural strength and ductility in load-carrying members. These studies include members under flexural, shear, torsion, and combined loads. Additionally, structural component responses under cyclic load and bond property of reinforcing steel bars have also been studied. Most of these studies have been limited to steel fibers. More detailed descriptions of the test methods and parameters as well as the original references can be found in Balaguru and Shah.3 In flexural R/C members, the addition of fibers improves the modulus of rupture (bending strength). Especially in over-reinforced concrete beams, the significant gain appears to be in the enhancement of post-peak structural ductility (Fig. 6), a quantity valued by structural engineers for safety reasons. This ductility improvement is likely a result of the delay in compression crushing by increasing the compression strain capacity due to fiber reinforcement. The potential for overreinforcement is greater when higher strength steel or FRP (fiber reinforced plastic rod) is used as reinforcement. For under-reinforced beams or beams with no main reinforcement at all, flexural strength enhancement and post-peak ductility 668 LI
HIGH PERFORMANCE APPLICATIONS OF FIBERS 669 登,5 v/bdF f A 1.5%fiber Figure 7 Enhancement of shear capacity of struc Figure 6 Enhancement of structural ductility in R/C tural elements by fiber reinforcement. FRC beam Fibers are perhaps most effective for structural members under combined bending, shear, and are associated with fiber bridging action on the torsion loads(e.g. a concrete utility pole under tensile cracks activated when the beam is flexed to beyond the elastic limit. Proper design of r/c wind loads and electric wire tension). This is be- cause the combin ned load often makes the exact and prestressed beams with deliberate exploita- location of concrete cracking difficult to predict tion of advantages afforded by fibers requires fur- Even if predictable the changing direction of the ther research There is evidence that fibers can be effective principal tensile stress makes conventional con- replacements for shear steel stirrups commonly tinuous steel reinforcement difficult to place in used in r/c beams and other structural element optimal orientation. Instead, fibers with its vir such as shear keys and corbels. Shear failure by tual advantage of random orientation bridge ten diagonal cracking is often structurally unstable sile cracks whichever directions they form and Shear fracture is prevented in current structural wherever they form on the structural member. design practice by the use of"shear reinforce- Figure 9 shows an example of the effect of fibers ments-often in the form of U-shaped stirrups or on the behavior of a member under combined helical windings in cylindrical-shaped elements torsion, bending, and shear However the use of shear reinforcements are la- In R/C structures or prestressed concrete struc bor intensive and reinforcement effects are direc tural components, the bond between concrete and tional and discrete (location-wise). As a result steel reinforcement is paramount. When bond is replacement of shear steel reinforcement by ibers lost, the concrete/steel composite action also van has been attempted. As soon as diagonal cracks are formed, fibers are activated to provide bridg emanation of radial cracks from deformed slugs ing across the concrete cracks. If this bridging action and the resulting shear resistance are high enough, the ductile bending failure mode can be restored and the brittle shear fracture (18 KN-m failure can be avoided. The effect of fibers on shear strength of R/C beams depends on the span/ depth ratio, but can be as much as 100% improve-s 100 ment (Fig. 7) The failure mode of torsion members is similar that under direct shear, in the sense that diag onal cracks form in response to the principal ten- sile stress. Hoop reinforcement or helical wind ings are conventionally provided to resist torsion 04080120160200240280320 failure. Again, fibers can be very effective in bridging against the opening of these diagonal Angle of twist(x 10 degin.) cracks, and delay the ultimate failure of the struc- Figure8 Enhancement of torsional capacity of struc tural member, as shown in Figure 8
are associated with fiber bridging action on the tensile cracks activated when the beam is flexed to beyond the elastic limit. Proper design of R/C and prestressed beams with deliberate exploitation of advantages afforded by fibers requires further research. There is evidence that fibers can be effective replacements for shear steel stirrups commonly used in R/C beams and other structural elements such as shear keys and corbels. Shear failure by diagonal cracking is often structurally unstable. Shear fracture is prevented in current structural design practice by the use of “shear reinforcements”—often in the form of U-shaped stirrups or helical windings in cylindrical-shaped elements. However, the use of shear reinforcements are labor intensive and reinforcement effects are directional and discrete (location-wise). As a result, replacement of shear steel reinforcement by fibers has been attempted. As soon as diagonal cracks are formed, fibers are activated to provide bridging across the concrete cracks. If this bridging action and the resulting shear resistance are high enough, the more ductile bending failure mode can be restored and the brittle shear fracture failure can be avoided.6 The effect of fibers on shear strength of R/C beams depends on the span/ depth ratio, but can be as much as 100% improvement (Fig. 7). The failure mode of torsion members is similar to that under direct shear, in the sense that diagonal cracks form in response to the principal tensile stress. Hoop reinforcement or helical windings are conventionally provided to resist torsion failure. Again, fibers can be very effective in bridging against the opening of these diagonal cracks, and delay the ultimate failure of the structural member, as shown in Figure 8. Fibers are perhaps most effective for structural members under combined bending, shear, and torsion loads (e.g., a concrete utility pole under wind loads and electric wire tension). This is because the combined load often makes the exact location of concrete cracking difficult to predict. Even if predictable, the changing direction of the principal tensile stress makes conventional continuous steel reinforcement difficult to place in optimal orientation. Instead, fibers with its virtual advantage of random orientation bridge tensile cracks whichever directions they form and wherever they form on the structural member. Figure 9 shows an example of the effect of fibers on the behavior of a member under combined torsion, bending, and shear. In R/C structures or prestressed concrete structural components, the bond between concrete and steel reinforcement is paramount. When bond is lost, the concrete/steel composite action also vanishes. The loss of bond is associated with the emanation of radial cracks from deformed slugs Figure 6 Enhancement of structural ductility in R/C FRC beam.5 Figure 7 Enhancement of shear capacity of structural elements by fiber reinforcement.7 Figure 8 Enhancement of torsional capacity of structural elements by fiber reinforcement.8 HIGH PERFORMANCE APPLICATIONS OF FIBERS 669
