HALF A CENTURY OF PROGRESS LEADING TO ULTRA-HIGH PERFORMANCE FIBER REINFORCED CONCRETE: PART 1- OVERALL REVIEW

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1 HALF A CENTURY OF PROGRESS LEADING TO ULTRA-HIGH PERFORMANCE FIBER REINFORCED CONCRETE: PART 1- OVERALL REVIEW Antoine E. Naaman Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan, USA Abstract Ever since the first patent on fiber reinforced concrete, the dream of civil engineers has been to mix fibers into concrete like sand or aggregate to achieve a moldable, strong, ductile and durable composite for construction applications with strength and ductility competitive with those of reinforced concrete or steel. Tensile strain-hardening ultra-high performance fiber reinforced concretes (UHP-FRC) provide a step closer towards that dream. Following the onset of modern developments of fiber reinforced concrete in the early 1960 s, there has been a continuous search for its improved performance. One can thus follow such progress in milestones along four inter-related paths: one path for the cementitious matrix, another for the fiber, the third for the interface bond between fiber and matrix, and the forth for the composite itself. After identifying some key milestones for each path leading to the record setting mechanical properties of today s UHP-FRCs, the tensile stress-strain response of typical fiber reinforced cement composites is taken as an example of property for review to illustrate progress since the 1960 s. Typical stress-strain curves are presented showing improvement over time, from strain-softening to strain-hardening, then to record breaking performance as we know it today. Over the last five decades, the post-cracking tensile strength and the corresponding strain capacity have increased about twenty five times and five times, respectively. Undoubtedly, these records will be exceeded in the future. It is noted that the full paper consists of two parts that are both published in these proceedings. The current paper provides an overall review while the second paper describes how the stress-strain curve in tension has evolved and improved over time leading to today s record breaking results. 1. INTRODUCTION The demand has never been greater for construction materials with combined properties of strength, toughness and ductility. Indeed high rise buildings reaching a mile height are in the advanced planning stage and bridges in conception spanning more than 3000 meters will utilize the boundaries of today s materials technology. Such progress demands higher limits 17

2 in the performance of construction materials in order for the next generation infrastructure to resist both static and dynamic loadings, including seismic, wind, and impact loading. This is Demand for high bending, shear and torsion; high rotational capacity; Demand for high strength. Photo: courtesy Bouygues Construction Figure 1: Typical structures where high performance fiber reinforced cement and concrete composites are needed, and particularly critical locations in these structures particularly challenging to concrete, the most used construction material worldwide, but it is expected that concrete will rise to the occasion. The past five decades mark the modern development and broad expansion of fiber reinforced cement and concrete composites, which led to today extensive applications and market penetration worldwide. Their success is due in part to significant advances in the fiber reinforcement, the cementitious matrix, the interface bond between fiber and matrix, fundamental understanding of the mechanics of the composite, and improved costeffectiveness. 18

3 It is strongly believed that high performance and ultra-high performance fiber reinforced cement composites are emerging materials well suited for use in the next generation of infrastructure. There is real need to tailor-design these composites to satisfy certain demands on strength, toughness, durability, ductility, and fracture energy. Figure 1 illustrates the types of applications where these materials can offer an effective technical solution. These include demand for combined axial and bending resistance at the base of columns in high rise buildings, demand for high rotational capacity, demand for combined plastic shear and plastic bending deformations at the base of shear walls, high shear and bending resistance at the continuous supports in long-span bridges, and, not shown in the figure, blast and impact resistant structures. Clearly high performance mechanical properties are needed. 2. DEFINITIONS AND CHARACTERISTIC QUALIFICATIONS Generally the attribute "advanced" or "high performance" when applied to engineering materials is meant to differentiate them from the conventional materials used, given available technologies at the time and geographic location considered for the structure. It also implies an optimized combination of properties for a given application and should be generally viewed in its wider scope. Combined properties of interest to civil engineering applications include strength, toughness, energy absorption, stiffness, durability, freeze-thaw and corrosion resistance, fire resistance, tightness, appearance, stability, construct-ability, quality control, and last but not least, cost and user friendliness. 2.1 High Strength Concrete and High Performance Concrete (HPC) For cement and concrete composites the term high strength refers to high compressive strength. There is no general consensus as to when the term high strength can be used since limits are regularly exceeded and are not line limits but rather smeared limits. Concrete with a compressive strength less than about 6 ksi (42 MPa) is considered normal strength in the US [1]. Concrete in the range of 6 to 12 ksi (42 to 84 MPa) is considered high strength. From about 12 to 16 ksi (84 to 112 MPa) concrete is also considered high strength but with qualifications about particular quality of aggregates (such as quartzite versus limestone) and curing conditions. Sometimes the term exotic production methods or requirements is used to differentiate that range. The term ultra high strength has been used for compressive strength exceeding the above listed limits. The American Concrete Institute provides a broad definition for high performance concrete as follows [2]: Concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practices. An additional explanation of about half a page qualifies the definition such as early strength, high toughness, low permeability, etc At this time, it seems that the term high performance used in parallel with high strength for non fiber reinforced cement matrices, often implies a concrete with better durability. 2.2 Fiber Reinforced Cement or Concrete (FRC) Composites, or FRCC For practical purposes and mechanical modeling, fiber reinforced cement or concrete (FRC) composites are generally defined as composites with two main components, the fiber and the matrix, while the bond between them is a paramount causal variable [3, 4]. While the 19

4 cementitious matrix may itself be considered a composite with several components, it is assumed to represent, in the context of the FRC composite, its first main component. The fiber represents the second main component. The fiber is assumed to be discontinuous and, unless otherwise stated, randomly oriented and distributed within the volume of the composite. Both the fiber and the matrix are assumed to work together, through bond, and provide synergy to make an effective composite. The matrix, whether it is a paste, slurry paste, mortar, or concrete is assumed to contain all the aggregates and/or additives specified. Air voids entrapped in the matrix during mixing are assumed to be part of the matrix. 2.3 High Performance Fiber Reinforced Cement Composites (HPFRCC) This acronym has been used since the late 1980 s and is defined as follows [5 to 8]: High performance fiber reinforced cement composites are a class of FRC composites characterized by a strain-hardening behavior in tension after first cracking, accompanied by multiple cracking up to relatively high strain levels. Strain-hardening behavior is a very desirable property, generally accompanied by multiple cracking and related large energy absorption capacity. Six international symposia have taken place using such a definition [6, 7, 9, 10, 11, 12]. One simple way to express the condition to achieve strain-hardening behavior is to write that the post-cracking strength of the composite σ pc σ cc is larger than its cracking strength, that is, [5, 8]. Note that the definition of high performance in FRC composites does not imply high strength. Indeed a low strength concrete containing fibers may show strain-hardening response in tension and thus would qualify as high performance according to the above discussion. This is one of the reasons Naaman and Reinhardt [3, 13, 14] suggested in 2003 to simply use the term tensile strain-hardening when it is the case and let the user qualify the term high performance as related to the property of interest such as high strength, or high durability, or low permeability, etc. 2.4 Ultra High Strength and Ultra High Performance Cement or Concrete (UHPC) and UHP-FRC Composites The foundation of UHPC is based on the principle that a material with smaller amounts of defects such as microcracks and internal pores leads to higher levels of performance. Performance implies first high strength and, because of improved permeability and low diffusion, high durability as well. Initially UHPC was obtained using fine particles ranging from less than 1 micron in size (silica fume) to about 1000 microns (fine sand) [15, 16, 17]. In between, particles of different sizes (glass powder, cement, fly ash, fine sand) are used to minimize the pore space, maximize packing density, and obtain a denser matrix. While room temperature curing is acceptable and becoming more common, curing under heat (such as o 90P PC for three days) generally leads to improved performance. With increased experience in the field, the range of particle sizes has been extended to include quartz sand particles up to 5 mm in diameter. Several examples of mixtures can be found in two recent symposia proceedings on the subject [18, 19]. Except for the above principle related to packing density, to date there is no general consensus among researchers as to the strength or durability limit that makes a cement matrix ultra high strength or ultra high performance. One attempt for a definition can be found 20

5 P column) P column) in a couple of papers by Rossi [20, 21]: ultra high strength or ultra high performance cement composites use a relatively high binder ratio, a water to cementitious ratio (or water to binder ratio) less than 0.2, and show a compressive strength in excess of 150 MPa (about 22 ksi). It should be pointed out that compressive strength values referred to by many researchers to describe their material, are often obtained using different standards and different size specimens. Therefore, it is reasonable to allow a broader range for the strength limit qualifying ultra-high strength or ultra-high performance Ultra high performance cement or concrete (UHPC) composites are very brittle and, as such, often compared to ceramics. Adding fibers to an UHPC matrix, assuming they lead to little or no deterioration in the above described properties, has led to the terminology ultra high performance fiber reinforced cement composite or UHP-FRC composites. However, UHP-FRC composites should follow the same classification as FRC composites to describe their tensile response, that is, either strain-hardening or strain-softening. It is clear today that the challenge for UHP-FRC composites is to achieve strain-hardening behavior in tension at least cost or equivalently with the least fiber content. Standard tests defining testing procedure and specimen dimensions are needed to help clearly identify a particular composite. 3. CHRONOLOGICAL DEVELOPMENTS: FIVE DECADES OF PROGRESS It is difficult to put specific limits at technical advances and progress on a particular subject, not only in terms of time but also geographic location. However, one can point out certain milestones that helped improve the performance of cement and concrete composites in general and somehow started a trend. These milestones can be followed along four paths and their combination, namely, the cement matrix, the fiber, the bond at the interface between fiber and matrix, and the resulting composite. 3.1 Concrete matrix and fiber In Table 1, the author lists in chronological order key advances related to the concrete nd rd matrix (2P and the fiber (3P since the 1960 s, mostly as encountered in the US. It is likely that a similar evolution took place elsewhere around the world, but some delay in adoption or implementation. Table 1 is self-explanatory and will not be discussed. Bond is reviewed in the next section. 3.2 Progress in Understanding and Improving Bond A review of research studies on the bond at the fiber-matrix interface since the 1960 s lead to the following observations : In the early research on bond during the 1960 s, it was mostly thought that bond at the fiber matrix interface is primarily adhesive and frictional. Early research using latex and other polymer additives to the cement matrix showed that while adhesive bond can be significantly increased for a single fiber under pull-out, it had little influence on the composite behavior in tension; this phenomenon was better understood later and can be explained by the fact that adhesive bond is brittle and requires very little slip before being destroyed; thus adhesive bond fails at very small crack widths. Moreover, for all fibers pulling out from a cement matrix, group effect can be significant: that is the more fibers are pulling out from the same area of cracked matrix, the less the bond performance per fiber. 21

6 Early research also showed that it is better to tailor the fiber geometry so that the fiber pulls out of the matrix (instead of failing) upon cracking of the matrix. This led to improved energy of the composite, and encouraged researchers to optimize the fiber geometry. Various mechanical deformations were attempted with steel fibers starting in the early 1970 s including Duoform fibers (stamped intermittently yielding a cross section changing from round to square not available any more); end hooked fibers; crimped fibers; paddled end fibers; buttoned end fibers; etc Examples are shown in Fig. 2. From numerous tests on the pull-out behavior of steel fibers, and the tensile or bending response of composites using these fibers, it became clear that the mechanical component of bond was paramount to the performance of steel fibers. Table 1: Chronological Advances in the matrix and fibers since the 1960 s Decade Cementitious Matrix and Concrete Fiber 1970 s Better understanding of hydration reactions; gel structure; Better understanding shrinkage, creep, porosity, High strength concrete to 50 MPa in practice Smooth steel fibers; normal strength Glass fibers 1980 s 1990 s 2000 s Increased development of chemical additives: HWRA, etc Increased utilization of fly ash and silica fume, and other mineral additives, etc Increased flowability (flowable concrete) Reduction in W/C ratio; High-Strength-Concrete terminology: up to 60 MPa; special high strength: up to 80 MPa; exotic high strength (special aggregate and curing): up to 120 MPa High-Performance-Concrete terminology: highstrength-concrete with improved durability properties. Increased development in chemical additives: superplasticizers; viscosity agents; etc. Increased use of supplementary cementistious materials as cement replacement UHPC: application of concept of high packing density; addition of fine particles; low porosity; lower water to cementitious ratio; Self consolidating concrete; self compacting concrete; Increased development of UHPC: Ductal and other proprietary mixtures UHPC: improved understanding of high packing density; application of nanotechnology concepts 2010 s Increased understanding of the cementitious matrix at the nano-scale???... Deformed steel fibers: normal and high strength Low-modulus synthetic fibers (PP, nylon, etc..) Increased use of glass fibers High performance polymer fibers (carbon, Spectra, Kevlar, etc..) New steel fibers with a twist (untwist during pull-out) PVA fibers with chemical bond to concrete Improved availability of synthetic fibers Ultra high strength steel fibers: smooth or deformed with diameters as low as 0.12 mm and strengths up to 3400 MPa Carbon nano-tubes; carbon nano-fibers Carbon nano-fibers???... 22

7 Figure 2: Typical shapes of steel fibers currently used in concrete composites Another component of bond was observed with steel fibers during the late 1970 s and early 1980 s in the production and utilization of SIFCON (Slurry Infiltrated Fiber Concrete). That was defined as the fiber-to-fiber interlock. The fibers formed a tight network or framework in space, and were interlocking on their own without a matrix in between. Fiber-to-fiber interlock occurs only if a continuous fiber network can form with fibers touching each other to form a continuous path. With SIFCON it became also clear that significant strain-hardening behavior can be achieved for FRC composites but its causal variables needed to be better understood. With the increasing use of synthetic fibers, as early as the 1980 s, both frictional and adhesive bond were improved. Frictional bond was improved by increasing the surface area per unit fiber cross-section, leading to flat fibrillated fibers. Strong adhesive bond was achieved with PVA fibers. The success of the utilization of high performance synthetic fibers such as carbon, Spectra, and Kevlar also greatly depends on bond, but no significant breakthrough can be mentioned so far, except perhaps for Spectra fibers. For close to three decades after the modern development of fiber reinforced concrete steel fibers, whether straight or deformed, were primarily made out of round wires. However, in the mid 1990 s a new fiber was introduced with polygonal cross section (triangular, square, rectangular) that allowed a surface area per unit cross section higher than round fibers [22, 23]. Because of its polygonal cross section, the new fiber called Torex, could be twisted inducing very effective mechanical bond, almost like a screw. Moreover, the twist ratio could be tailored so as to allow the fiber to un-twist during pull out generating optimal pull-out energy. Torex fibers showed that very effective sliphardening bond can be achieved resulting more easily in fiber reinforced cement composites with strain-hardening behavior in tension. There are other ideas that may affect bond and the performance of fiber reinforced cement composites. In particular, it is worth mentioning self-stressing composites in which the fibers made out of special shape-memory materials have the property of deformation recovery (shrinking) when subject to special heat or radiation, thus inducing prestressing through bond 23

8 [24] Progress Leading to Ultra High Performance Fiber Reinforced Concrete It has been a common aspiration for researchers dealing with cement and concrete composites to race for increasing compressive strength. In the early 1970 s very high compressive strengths of up to 510 MPa were reported from testing small specimens prepared under special conditions with vacuum, heat and pressure curing [25, 26]. Tabel 2 : Developments in high strength high performance cementitious composites from the 1970 s to the early 1990 s (in the US and Europe) In the early 1980 s the addition of special polymer and the use of very low water to cement ratios led to what was described as micro-defect-free cement with a compressive strength 24

9 exceeding 200 MPa [27]; no pressure or heat curing was needed. Such discoveries, however, while illustrating the potential of the material, did not translate into easily implemented applications. In Tables 2 and 3, the author summarizes various milestones related to numerous such composites developed since the 1970 s. Table 2 covers the period from 1970 to the early 1990s and Table 3 covers the period from 1994 (the year ultra-high performance concrete was introduced) to Table 3 Developments in high strength high performance cementitious composites from the 1990 s to date (in the US and Europe) These tables give the approximate date of introduction, the range of compressive strength reported, the reference, the name and/or acronym used for the material developed, if any, and the special conditions applied to achieve the reported properties. Related references can be found in the reference list [25 to 40]. Note that these tables are by no means exhaustive; they 25

10 covers what the author consider key developments in the US and Europe. The emphasis is on materials that have led to ultra-high performance concrete and ultra-high performance fiber reinforced concrete as understood at time of this writing. 4. CONCLUDING REMARK The reader is referred to Part 2 of this paper which describes how the stress-strain curve of fiber reinforced concrete in tension has evolved since about five decades leading to record breaking results with ultra-high performance fiber reinforced concrete. The entire list of references is also given at the end of Part 2. 26