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Fibre reinforced cement-based (FRC) composites after over40 years of development in building andcivil engineeringAndrzej M. BrandtInstitute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw, PolandARTICLEINFOAvailable online 12 March 2008Keywords:FibresCement-based matrixFibre reinforced concretes (FRC)High performance concretes (HPC)ABSTRACTFibres have been used since Biblical times to strengthen brittle matrices; for example straw and horsehair was mixed with clay to form bricks and floors. In modern technology, steel fibres were for the first time proposed as dispersed reinforcement for concrete by Romualdi in his two papers in 1963 and 1964.Since that time, the concept of dispersed fibres in cement-based materials has developed considerably:hundreds of books and papers, many dissertations, and also applications in building and civil engineering structures all over the world.After over forty years, it is interesting to review the present state of knowledge and technology of FRC.The balance of achievements and shortcomings is certainly positive. Our knowledge, based on theoretical solutions and experimental findings, is rich and quite large. Test methods that are transferred from the so called high-strength composites are very effective.However, practical applications are not so numerous as it was initially expected with developments not exactly in the foreseen directions.In this paper the main fields of application of FRC composites are examined and future perspectives discussed.After a brief review of various kinds of fibres and applied techniques, some attention is paid to computation methods and composite materials’ design approaches. Large practical application of FRC in construction is mostly hampered by insufficient development of relevant standards, based on performance concepts. It should also be admitted that the cost of fibre reinforcement and related technological operations is certainly an obstacle for use of FRC in ordinary structures. On the other hand, in successful applications in demanding structures very special requirements are satisfied; probably future development will go in this direction.2008 Elsevier Ltd. All rights reserved1. IntroductionConcrete is by far the most important building material and its consumption is increasing in all countries and regions in our globe.The reasons are multiple: its components are available everywhere and relatively inexpensive, its production may be relatively simple,its application covers large variety of building and civil infrastructure works. Moreover, since around 30 years, its development has gone in new directions: high performance concretes (HPC). This new kind of building materials is defined as ‘a concrete in which certain characteristics are developed for a particular application and environment’; these characteristics are not only strength, but improved durability, increased resistance to various externalagents, high rate of hardening, better aspect, etc.The only disadvantage of concrete is its brittleness, i.e. Relatively low tensile strength and poor resistance to crack opening and propagation. In the development of concrete-like materials the reinforcement with dispersed fibres plays an important role.Since Biblical times, approximately 3500 years ago, brittle building materials, e.g. clay sun baked bricks, were reinforced with horse-hair, straw and other vegetable fibres.The concept of fibre reinforcement was developed in modern times and brittle cement-based paste was reinforced with asbestos bfibres when in about 1900 the so called Hatschek technology was invented for production of plates for roofing, pipes, etc. Later, glass fibres were proposed for reinforcement of cement paste and mortar by Biryukovichs [1]. The ordinary E-glass fibres are not resistant and durable in highly alkaline Portland cement paste and the alkali- resistant (AR) glass fibres with addition of zircon oxide ZrO2 were invented by Majumdar and Ryder [2]. Important influences of the development of steel fibre reinforced cements (SFRC) are papers published by Romualdi and his co-authors [3,4] for the first times on this subject.It is not surprising that in such an excellent material as concrete, after many recent improvements of additions and admixtures, with considerable development of technology in precast factories and in situ, and with exploitation of highly sophisticated test methods, the application of dispersed fibre reinforcement results after three decades in a large variety of excellent building materials for different purposes –fibre reinforced cements and concretes (FRC).The aim of the paper is to describe the present state of knowledge and technology of FRC and to discuss main directions of their application. The attention is concentrated on structural concretes for high-rise buildings, long-span bridges, highway and airfield pavements, and many other kinds of outstanding structures. For obvious reasons, ordinary concretes without fibres but of improved quality are used for low performance structures and non-structural elements.2. Matrices and fibresCement-based matrices have developed considerably during last 40 years. The main components are still Portland cement and coarse and fine aggregate of different origin, and there are several other components: super plasticizers, admixtures and micro fillers.Also proportions between these components have changed.There are many kinds of Portland cements that may be selected for particular purposes. The national and international companies may furnish cements that are characterized by high or low strength,high-early strength or low heat of hydration, high sulfate resistance,low content of C3A, and large variety of blended cements, i.e. With addition up to 70% by weight of fly ash and ground blast furnace slag.The next groups of concrete components are additions and admixtures that create special properties of fresh mix and hardened concrete; these are super plasticizers, air-entraining agents,micro fillers and secondary cementing materials: fly ash, natural pozzolans, rice husk ash, metakaolin, etc. In fact, often binary, ternary or quarter nary concretes are distinguished, i.e. based on compositions of different binders.As aggregate, not only crushed stone and natural gravel with sand are used, but also various artificial materials, carefully selected and inserted into fresh mix in well determined proportions. In concrete, many kinds of waste materials are used, including recycled aggregate, in order to decrease cost and to satisfy increasing demands of sustainability and ecology.As a result, concretes and particularly concretes that have to satisfy special requirements, became rather complicated materials and are ‘tailor-made’ to provide the precise properties necessary for a particular project. The design of such a concrete is based on deep knowledge and substantial experience; with the same concerns regarding the selected applications of technology. At all stages high competence of the personnel is needed.In general, modern concretes are more brittle than those in the first half of 20th century, with higher rates of strength and higher heat of hydration, and often less durable, i.e. less resistant against intensive corrosive attacks from environment if not specially designed. As remedies, there are special kinds of concretes called high performance concretes, described hereafter, frequently with application of dispersed reinforcement in different forms. The main role of short dispersed fibres is to control the crack opening and propagation. Basic groups offibres applied for structural concretes andclassified according to their material are Brandt [5]:–steel fibres of different shapes and dimensions, also micro fibres;–glass fibres, in cement matrices used only as alkali-resistant (AR)fibres;–synthetic fibres made with different materials: polypropylene,polyethylene and polyolefin, polyvinyl alcohol (PV A), etc.;–carbon, pitch and polyacrylonitrile (PAN) fibres.Natural vegetable fibres are not suitable for high performance structural concrete, but are applied in ordinary concretes. Asbestos fibres are completely abandoned in construction because of their detrimental influence on human health and are replaced by other kinds of fibres, e.g. polymeric.Certainly the most important for structural concrete are steel fibres; a few examples are shown in Fig. 1; hooks at the ends and various modifications of shape improve fibre-matrix bond and increase efficiency of the fibres.The influence of the fibres on cracking of cement-based matrix is explained in Fig. 2: thanks to the fibres, large single cracks are replaced with dense systems of microcracks, which may be acceptable from both safety and durability viewpoints. The role of fibres is clear from the data presented in Table 1. Numbers of fibres distributed in one cube centimetre are shown for a few examples of fibres and their volume fractions. Only fibre volume up to 3% is considered, because higher volumes require special techniques that are described below. Fine fibres control opening and propagation of microcracks as they are densely dispersed in cement matrix. Longer fibres up to 50 or 80 mm control larger cracks and contribute to increase the final strength of FRC, as it is shown in Figs. 3 and 4.With the increase of fibre volume and efficiency, their influence on behaviour of a SFRC element modifies completely its behaviour under load, as it is described in Fig. 5 with strain–stress diagrams.The conventional SFRC element is characterized by initial linear increase of stress and after the 1st crackopening there is a slow decrease,the so called softening branch. In contrast, where the reinforcement is sufficient, after the 1st crack there is a strain hardening stage, which accompanies multiple cracking and considerable amount of energy is absorbed that is proportional to the area under the curve. The softening branch follows that stage. In Fig. 5 the main difference between conventional FRC and high performance fibre reinforced cement composites (HPFRCC) is defined. The effects of fibre reinforcement on the behaviour of an element under bending are discussed in Fig. 6, and it is shown how such beam may be designed for a particular purpose.Beside steel fibres, also high tenacity PV A fibres, either monofilament or fibrillated polypropylene, with lengths varying from 10 to80 mm and diameters for 0.5–1.5 mm are used in high volumes (0.5–2.0%) to enhance the strength and toughness, and also impact and fatigue strength of structural concrete elements.Fig. 1. Examples of deformed steel fibres, after Sujivorakul and Naaman [6].Fig. 2. Crack pattern in reinforced concrete (RC) and fibre reinforced concrete (FRC)elements subjected to tension.Table 1Micro-reinforcement of concretes and mortars, Brandt [7]Fig. 3. Structures of long and short fibres controlling the crack propagation; after Betterman et al. [8].Low-modulus polypropylene fibres have two different applications. They are used in small amount (up to 1.0 kg/m3) to control the so called shrinkagecracking of young concrete in few first hours of setting. In that initial period, the Young’s modulus of young concrete is similar to that of these fibres, Ramakrishnan et al. [11]. Polypropylene fibres are also dispersed in concrete walls in apartment buildings, because in case of fire and high temperature they melt and create channels that may relieve internal pressure and help to delay the destruction of concrete.3. Special kinds of high performance fibre concretes (HPFC)The engineered cementitious composites (ECC) or reactive powder concrete (RPC) are the general names of special concretes, in which very high performance is achieved even with relatively low amount (about 2–5 vol.%) of short random synthetic fibres and specially composed matrices with high amount of Portland cement and other complementary binders and super plasticizers. As examples, the patented products Ductal and BSI may be mentioned.Ultra-high-strength (UHS) concrete was developed in France in early 1990s with the possibility to obtain compressive strength up to 800 MPa and tensile strength of up to 100 MPa. There is no coarse aggregate, and the maximum aggregate particle size is 0.3 mm. To improve ductility the steel fibres are added of about 5 vol.%, including micro fibres <3 mm long; other components are silica fume, ground quartz, super plasticizer, and high amount of Portland cement (up to 1000 kg/m3). This material is commercialized under name of Ductal by Lafarge (France) and with compressive strength of 100–200 MPa is used for concrete bridge structures as well as for producing thin precast elements. The stress–displacement curves for ordinary concrete and Ductal element under bending are compared in Fig. 7.A similar kind of concrete is BSI , a self-setting, fibre reinforced concrete that uses premixed dry components which can be adjusted for specific applications, and is proposed by the company Eiffage Construction (France). BSI does not require heat treatment or vibration, and it can be poured into a mould tocreate modular structures. After 28 days it has a direct tensile strength of 8–10 MPa, achieved by using fibres in the mix, and the characteristic compressive strength is 150 MPa. Because the concrete has no longer to cover passive reinforcements, it is possible to construct BSI structural elements that are less than an inch thick.Self compacting concrete (SCC) or self levelling concrete (SLC) can flow on its own without additional vibration and no detrimental effects of segregation or bleeding occur. High content of fine grain aggregate (below 0.125 mm), high cement content and thanks to the appropriate combination of water and super plasticizer high slump values up to 270 mm are obtained. The SCC passes without difficulties through densely distributed reinforcing bars. The ductility is improved with an addition of microfibres. The SLC are particularly useful in execution of industrial floors where no additional work for levelling is needed.In these high performance concretes the hybrid reinforcement is composed of different sizes of steel fibres, which contribute to different mechanical properties: control of microcracks, increase of tensile strength, improvement of ductility, etc. The addition of fine microfibres has a significant influence on the compressive strength, but the tensile strength is only slightly affected. Longer steel fibres bring other mechanical effects –higher strength and improved toughness. There are synergistic effects in the hybrid fibres system.Silica fume (SF) is an active pozzolana that leads to an increased strength, and it is frequently used to replace 5–10% of Portland cement to obtain a high performance composite. Improved density and reduction of permeability to water and related detrimental agents is important when longterm durability of concrete structures is required that are exposed to climatic influences and various chemical attacks. SF is used as a prevention against alkali aggregate reaction (AAR), reduces carbonation and chloride ions intrusion, and controls delayed ettringite formation (DEF). Instead of SF other microfillers are used, e.g. metakaolin.Carbon fibre reinforced mortar (CFRM) and carbon fibre reinforced concrete (CFRC) are attractive composites due to their high flexural strength andtoughness and low drying shrinkage, in addition to its electrical properties such as voltage-sensitive effect.Application of relatively low-cost pitch carbon fibres is adequate for bridges and other civil engineering structures and also for cladding for buildings, Kucharska and Brandt [13].In the regions with corrosive influence of marine atmosphere and strong winds (e.g. In Japan) CFRC is used in bridge structural elements for better durability than is would be possible using steel fibres.SIFCON (slurry infiltrated fibre concrete) is a strong composite in which a very high volume of steel fibres is used by special technology. Fibres are pre-placed in a mould and the fibre system obtained is infiltrated by cement slurry. Fibre volume may reach 8–12%, sometimes even higher, and fibres 100–200 mm long may be used. The cement slurry is filled with fine sand,micro-aggregate and special additives like fly ash and silica fume. The high fluidity (low viscosity) of the slurry is necessary for adequate penetration of the dense fibre systems in a mould. Very high-strength and resistance against local impacts and penetration of projectiles characterize the elements made with SIFCON. When instead of single fibres the woven or plaited mats are used, then the name SIMCON (slurry infiltrated mat concrete) is used. The main applications of both materials are heavy-duty pavements, anti-terrorist shields, walls in bank treasuries, etc., where additional cost of materials and special technology are acceptable.Fig. 4. Structure of short and long fibres controlling microcracks and its influence on the stress –crack opening curve, after Rossi [9].with conventional FRCC, after Naaman [10].composites, after Naaman [10].Fig. 7. Comparison of the stress–displacement curves of beams made with ordinary concrete and Ductal , after Behloul [12].4.Design and composition of FRCFor various applications of FRC where strength is not necessarily verified, the composite material is designed using test results and experience. The effect of reinforcement is proportional to the volume and efficiency of the fibres. Because fibres are an important part of the cost, the fibre volume in ordinary applications (industrial floors, pavements, etc.) is usually limited to 0.5% or even lower.The difficulties in a correct distribution of fibres also increase with their volume and this aspect should be always considered.Flexural toughness of FRC is traditionally estimated according to ASTMC 1018 [14]. For that aim, standard test on a beam under bending is necessary, and after the load-deflection curve (Fig. 8), the so called flexural toughness indices I5, I10, I30, . . . may be calculated. The results allow estimating quantitatively the load-deflection curve –how do the fibres influence the descending branch of the curve. There are several objections as to the precision of the indices, but this approach is universally applied; it serves also to compare different fibres or mixture compositions as to their efficiency. In structural elements, where it is essential to verify the tensile stress, a few procedures are applied to determine the so called equivalent strength feq, in which input from the fibres is included. Forexample, according to Japanese Standard [15], the equivalent strength is calculated from the bending test of a beam and it is equal:where Tb –work of bending calculated after the area under the load-deflection curve up to the deflection dL/150 = L/150, b and hare width and depth of the beam, L is its span.Similar formulae are proposed in RILEM Recommendation [16] as a result of a few year work of the Technical Committee TC 162-TDF led by L. Vandewalle.The lack of a universally accepted approach to the calculation and strength verification of SFRC and respective standards is one of the major obstacles in large development of this material in structural design. For steel fibres, as well as for other fibres (glass ibres, carbon fibres, etc.), there are several experimental methods proposed and used by different contractors and fibre producers.5. Application of FRC in building and civil engineering structuresSteel fibres are largely used as dispersed reinforcement of industrial floors and pavements in many countries and this is probably the most important field of application. There are also several kinds of structural elements where steel fibres are used together with steel bars, e.g. structures exposed to impact and fatigue,columns in building in seismic zones, refractory structure, etc.Glass fibres are used mostly for external claddings, facade plate sand other elements where their strengthening effects are required particular during construction.Fig. 8. Computation of the flexural toughness indices, after ASTM [14].Fig. 9. Cross-section of the bridge deck in Bourg-lès-Valence (France) made with FRC, Vf = 3%, after Behloul [12].Natural vegetal fibres (cellulose pulp, sizal, bamboo, hemp, flax,jute, ramie fibres, etc.) are used in the countries where these fibres are easily available, Coutts [17]. Structural elements with vegetal fibres are important for construction of inexpensive buildings in developing regions of the world.In last few years, the dispersed steel fibres are successfully used in prestressed concrete bridge beams where they replace mild steel reinforcement. In Fig. 9 the cross-section of the bridge deck Bourg-lès-Valence (France) is shown. Ten prestressed beams form the main structure of the bridge. Additional plates are used as the lost shuttering for ordinary concrete cast in situ with ordinary reinforcement and covered with road pavement.The prestressed beams and plates are made with Ductal.Thanks to 3 vol.% of dispersed microfibres in Ductal , there are neither stirrups no other mild steel reinforcement needed against shearing and local stresses in the beams and considerable economy was obtained in time and cost of labour. Another example of prestressed concrete beams with 3 vol.% of fibres is shown in Fig. 10. This is certainly a new and important direction of future application of fibres in structural elements without any other reinforcement.6. Main directions of development of FRCExtension of application of FRC with development of technology in construction is ensured. Further investigations in a few selected directions are needed, aimed particularly at–development of reliable and relatively simple methods of calculation of FRC elements for strength and stiffness and their introduction to the recommendations and standards available for professional engineers;–use of modern test methods of FRC in general practice, particularly in civil engineering structures, in view of ensuring the quality control and improving their durability;Fig. 10. Cross-section of prestressed FRC bridge beams without mild reinforcement,Vf = 3%.–solution of a few problems for special structures where high performance is required, e.g. concerning hybrid reinforcement,compatibility between various components, and optimization in material design to determine the best mixture proportions.7. ConclusionsThe development of various kinds of high performance and ultra-high performance concretes, reinforced with dispersed fibres, results in creation of a group of very important building materials. At present, for many outstandingstructures or for construction in special conditions, application of FRC is considered as necessary,and this situation will be extended in the future.Successful use of various high performance materials based on cement matrix has a considerable positive influence on production of ordinary concretes. New components and technologies developed for special purposes are now, at least partly, applied in everyday production in ready-mix-concrete plants. Large variety and better quality of admixtures, improved precision of execution and adequate curing are the bases for ordinary concretes that are becoming inexpensive, strong and ensured improved durability of buildings and civil infrastructure.In general, concrete and particularly concrete with dispersed fibre reinforcement is becoming a high-tech material that provides excellent performance but requires competent design and execution. Various experimental and theoretical methods that are successfully applied will certainly be used in further research and development.AcknowledgementsThis work was supported by Project No. R04 013 01 coordinated by IFTR PAS and sponsored by The Ministry of Science and Higher Education (MniSW), Warsaw, Poland, to which the author is grateful.References[1] Biryukovich KL, Biryukovich YuL, Biryukovich DL. 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