Normal and High Volume Steel Fibre Concrete Composites for Special ApplicationsDr. V.S. Parameswaran, President and Chief Executive, Design Technology Consultants, Chennai Chief Executive, International Centre for FRC Composites (ICFRC), Chennai, Former Director, SERC & Past President, ICI.
In recent times, the sustained efforts of researchers all over the world to innovate and incorporate unmatched excellence in construction have led to development of several advanced concrete construction materials. Of these, composites containing steel fibres have come to stay and deserve special mention. This paper, besides outlining the properties and applications of normal fibre reinforced concrete (SFRC), also describes the emergence and potentials of high-volume fibre composites such as slurry infiltrated fibrous concrete (SIFCON), slurry infiltrated mat concrete (SIMCON), compact reinforced concrete (CRC) and reactive powder concrete (RPC).
Steel Fibre Reinforced Concrete (SFRC)Concrete is the most widely used structural material in the world with an annual production of over seven billion tons. For a variety of reasons, much of this concrete is cracked. The reason for concrete to suffer cracking may be attributed to structural, environmental or economic factors, but most of the cracks are formed due to the inherent weakness of the material to resist tensile forces. Again, concrete shrinks and will again crack, when it is restrained. It is now well established that steel fibre reinforcement offers a solution to the problem of cracking by making concrete tougher and more ductile. It has also been proved by extensive research and field trials carried out over the past three decades, that addition of steel fibres to conventional plain or reinforced and prestressed concrete members at the time of mixing/production imparts improvements to several properties of concrete, particularly those related to strength, performance and durability.
The weak matrix in concrete, when reinforced with steel fibres, uniformly distributed across its entire mass, gets strengthened enormously, thereby rendering the matrix to behave as a composite material with properties significantly different from conventional concrete.
The randomly-oriented steel fibres assist in controlling the propagation of micro-cracks present in the matrix, first by improving the overall cracking resistance of matrix itself, and later by bridging across even smaller cracks formed after the application of load on the member, thereby preventing their widening into major cracks (Fig. 1).
The idea that concrete can be strengthened by fibre inclusion was first put forward by Porter in 1910, but little progress was made in its development till 1963, when Roumaldi and Batson carried out extensive laboratory investigations and published their classical paper on the subject. Since then, there has been a great wave of interest in and applications of SFRC in many parts of the world. While steel fibres improve the compressive strength of concrete only marginally by about 10 to 30%, significant improvement is achieved in several other properties of concrete as listed in Table 1. Some popular shapes of fibres are given in Fig.2.
In general, SFRC is very ductile and particularly well suited for structures which are required to exhibit:
- Resistance to impact, blast and shock loads and high fatigue
- Shrinkage control of concrete (fissuration)
- Very high flexural, shear and tensile strength
- Resistance to splitting/spalling, erosion and abrasion
- High thermal/ temperature resistance
- Resistance to seismic hazards.
The degree of improvement gained in any specific property exhibited by SFRC is dependent on a number of factors that include:
- Concrete mix and its age
- Steel fibre content
- Fibre shape, its aspect ratio (length to diameter ratio) and bond characteristics.
Mix Design for SFRCJust as different types of fibres have different characteristics, concrete made with steel fibres will also have different properties.
When developing an SFRC mix design, the fibre type and the application of the concrete must be considered. There must be sufficient quantity of mortar fraction in the concrete to adhere to the fibres and allow them to flow without tangling together, a phenomenon called ‘balling of fibres’ (Fig. 6). Cement content is, therefore, usually higher for SFRC than conventional mixes Aggregate shape and content is critical. Coarse aggregates of sizes ranging from 10 mm to 20 mm are commonly used with SFRC. Larger aggregate sizes usually require less volume of fibres per cubic meter.
SFRC with 10 mm maximum size aggregates typically uses 50 to 75 kg of fibres per cubic meter, while the one with 20 mm size uses 40 to 60 kg.
Smaller sections less than about 100 mm in thickness should be considered as requiring 10 mm aggregate size only.
It has been demonstrated that the coarse aggregate shape has a significant effect on workability and material properties. Crushed coarse aggregates result in higher strength and tensile strain capacity.
Fine aggregates in SFRC mixes typically constitute about 45 to 55 percent of the total aggregate content.
Typical mix proportions for SFRC will be: cement 325 to 560 kg; water-cement ratio 0.4-0.6; ratio of fine aggregate to total aggregate 0.5-1.0; maximum aggregate size 10mm; air content 6-9%; fibre content 0.5-2.5% by volume of concrete. An appropriate pozzolan may be used as a replacement for a portion of the Portland cement to improve workability further, and reduce heat of hydration and production cost. The suggested mix proportions for making SFRC mortars and concretes is given in Table 2.
The use of steel fibres in concrete generally reduces the slump by about 50 mm. To overcome this and to improve workability, it is highly recommended that a super plasticizer be included in the mix. This is especially true for SFRC used for high-performance applications.
Generally, the ACI Committee Report No. ACI 554 ‘Guide for Specifying, Mixing, Placing and Finishing Steel Fibre Reinforced Concrete’ is followed for the design of SFRC mixes appropriate to specific applications.
In Scandinavian countries, shotcreting is done by the wet process and as much as 60% of ground support structures (tanks and domes) in Norway are constructed using steel fibres. In many countries including India, steel fibre shotcrete has been successfully used in the construction of several railway and penstock tunnels (Fig. 7).
Typical mix proportions for making fibre shotcrete with sand only, and with a combination of sand and coarse aggregate, is given in Table 3.
Applications of SFRCThe applications of SFRC depend on the ingenuity of the designer and builder in taking advantage of its much enhanced and superior static and dynamic tensile strength, ductility, energy-absorbing characteristics, abrasion resistance and fatigue strength.
Growing experience and confidence by engineers, designers and contractors has led to many new areas of use particularly in precast, cast in-situ, and shotcrete applications. Traditional application where SFRC was initially used as pavements, has now gained wide acceptance in the construction of a number of airport runways, heavy-duty and container yard floors in several parts of the world due to savings in cost and superior performance during service.
The advantages of SFRC have now been recognised and utilised in precast application where designers are looking for thinner sections and more complex shapes. Applications include building panels, sea-defence walls and blocks, piles, blast-resistant storage cabins, coffins, pipes, highway kerbs, prefabricated storage tanks, composite panels and ducts. Precast fibre reinforced concrete manhole covers and frames are being widely used in India, Europe and USA.
Cast in-situ application includes bank vaults, bridges, nosing joints and water slides. “Sprayed-in” ground swimming pools is a new and growing area of shotcrete application in Australia. SFRC has become a standard building material in Scandinavia.
Applications of SFRC to bio-logical shielding in atomic reactors and also to waterfront marine structures which have to resist deterioration at the air-water interface and impact loadings have also been successfully made. The latter category includes jetty armor, floating pontoons, and caissons. Easiness with which fibre concrete can be moulded to compound curves makes it attractive for ship hull construction either alone or in conjunction with ferrocement.
Use of SFRC for repair work is also a growing market. Several tunnels and bridges have been repaired with spraying of layers of shotcrete after proper surface preparation. A few most common applications of SFRC are illustrated in Fig. 8.
SFRC shotcrete has recently been used for sealing the recesses at the anchorages of post stressing cables in oil platform concrete structures. Recent developments in fibre types and their geometry and also in concrete technology and equipment for mixing, placing and compaction of SFRC and mechanized methods for shotcreting have placed Scandinavian and German consultants and contractors in a front position in fibre-shotcreting operations world wide.
Slurry Infiltrated Fibrous Concrete (SIFCON)SIFCON is a high-strength, high-performance material containing a relatively high volume percentage of steel fibres as compared to SFRC. It is also sometimes termed as ‘high-volume fibrous concrete’. The origin of SIFCON dates to 1979, when Prof. Lankard carried out extensive experiments in his laboratory in Columbus, Ohio, USA and proved that, if the percentage of steel fibres in a cement matrix could be increased substantially, then a material of very high strength could be obtained, which he christened as SIFCON.
While in conventional SFRC, the steel fibre content usually varies from 1 to 3 percent by volume, it varies from 4 to 20 percent in SIFCON depending on the geometry of the fibres and the type of application. The process of making SIFCON is also different, because of its high steel fibre content. While in SFRC, the steel fibres are mixed intimately with the wet or dry mix of concrete, prior to the mix being poured into the forms, SIFCON is made by infiltrating a low-viscosity cement slurry into a bed of steel fibres ‘pre-packed’ in forms/moulds (Fig. 9).
The matrix in SIFCON has no coarse aggregates, but a high cementitious content. However, it may contain fine or coarse sand and additives such as fly ash, micro silica and latex emulsions. The matrix fineness must be designed so as to properly penetrate (infiltrate) the fibre network placed in the moulds, since otherwise, large pores may form leading to a substantial reduction in properties.
A controlled quantity of high-range water-reducing admixture (super plasticizer)may be used for improving the flowing characteristics of SIFCON. All types of steel fibres, namely, straight, hooked, or crimped can be used.
Proportions of cement and sand generally used for making SIFCON are 1: 1, 1:1.5, or 1:2. Cement slurry alone can also be used for some applications. Generally, fly ash or silica fume equal to 10 to 15% by weight of cement is used in the mix. The water-cement ratio varies between 0.3 and 0.4, while the percentage of the super plasticizer varies from 2 to 5% by weight of cement. The percentage of fibres by volume can be any where from 4 to 20%, even though the current practical range ranges only from 4 to 12%.
Uniaxial Tensile StrengthUnlike the cracks which form in continuous reinforced cementitious composites such as ferrocement, the cracks in SIFCON generally do not extend through the whole width of the specimen. Instead, they can be short and randomly distributed within the loaded volume, i.e. on the surface and through the depth of the specimen. The ultimate tensile strength of SIFCON typically varies from 20 to 50 MPa, depending on the percentage of steel fibres and the mix proportions used.
Compressive StrengthThe cement slurry (without fibres) used in the making of SIFCON generally develops a one-day strength of 25 to 35 MPa, and a 28-day strength of 50 to 70 MPa. The corresponding values for SIFCON composites are 40 to 80 MPa and 90 to 160 MPa, respectively, depending on the percentage of steel fibres incorporated in the matrix. Generally, SIFCON exhibits an extremely ductile behavior under compression.
Flexural StrengthThe ultimate flexural strength of SIFCON is found to be very high and is in the order of magnitude higher than that of normal SFRC. The values observed by several researchers range from 25 to 75 MPa with an average of about 40 MPa. SIFCON is found to possess excellent ductility both under monotonic and high-amplitude cyclic loading.
Shear StrengthInvestigations carried out in USA, Denmark and India have shown that the ultimate shear strength of SIFCON specimens were 30.5, 28.1, 33.3 and 31.8 MPa, respectively, for fibre lengths of 30, 40, 50 and 60 mm, indicating thereby that the fibre length does not seem to affect the shear strength. The average shear strength of SIFCON can be taken as about 30 MPa as compared to just about 5 MPa for plain concrete.
Resistance to Abrasion, Impact, Fatigue, and Repeated LoadingSIFCON possesses extremely high abrasion and impact resistance, when compared with plain concrete and SFRC specimens. The resistance improves further drastically with the increase in the percentage of fibres. It is several times that of ordinary plain or reinforced concrete.
Design PrinciplesThe design methods for SIFCON members must take into account their application or end-use, the property that needs to be enhanced, mix proportion, strength, as well as its constructability and service life. In general, a high-strength SIFCON mix can easily be designed and obtained with virtually any type of steel fibres available today, if the slurry is also of high strength.
Like conventional concrete, the strength of the slurry is a function of the water-cement ratio; because the slurry mixes used in SIFCON usually contain significant percentages of fly ash or silica fume or both, the term “water-to-cement plus admixtures” is used when designing the slurry mix. In addition, the ratio of the “admixtures to cement” is also an important parameter in the design of SIFCON. It is also to be noted that higher volume percentages of fibres need lower viscosity slurry to infiltrate the fibres thoroughly. In general, the higher the strength of the slurry, the greater is the SIFCON strength.
Applications of SIFCONSIFCON possesses several desirable properties such as high strength and ductility. It also exhibits a very high degree of ductility as a result of which it has excellent stability under dynamic, fatigue and repeated loading regimes (Figs. 10 a & 10 b). It is also quite expensive. Because of this, SIFCON should be considered as an efficient alternative construction material only for those applications where concrete or conventional SFRC can not perform as may be expected/required by the user or in situations where such unique properties as high strength and ductility are required.
Since properties like ductility, crack resistance and penetration and impact resistance are found to be very high for SIFCON when compared to other materials, it is best suited for application in the following areas:
- Pavement rehabilitation and precast concrete products
- Overlays, bridge decks and protective revetments
- Seismic and explosive-resistant structures
- Security concrete applications (safety vaults, strong rooms etc)
- Refractory applications (soak-pit covers, furnace lintels, saddle piers)
- Military applications such as anti-missile hangers, under-ground shelters
- Sea-protective works
- Primary nuclear containment shielding
- Aerospace launching platforms
- Repair, rehabilitation and strengthening of structures
- Rapid air-field repair work
- Concrete mega-structures like offshore and long-span structures, solar towers etc.
Compact Reinforced Concrete (CRC)CRC is a new type of composite material. In its cement-based version, CRC is built up of a very strong and brittle cementitious matrix, toughened with a high concentration of fine steel fibres and an equally large concentration of conventional steel reinforcing bars continuously and uniformly placed across the entire cross section (Fig. 11).
CRC was initially developed and tested by Prof.Bache at the laboratories of Aalborg Portland cement factory in Denmark. The pioneering experiments carried out at this laboratory established the vast potential of CRC for applications that warrant high strength, ductility and durability.
CRC has structural similarities with reinforced concrete in the sense that it also incorporates main steel bars, but the main bars in CRC are large in number and are uniformly reinforced. Owing to this and also because of the large percentage of fibres used in its making, it exhibits mechanical behavior more like that of structural steel, having almost the same strength and extremely high ductility.
CRC specimens are produced using 10-20% volume of main reinforcement (in the form of steel bars of diameter from about 5 mm to perhaps 40 or 50 mm) evenly distributed across the cross section) and 5-10% by volume of fine steel fibres. The water-cement ratio is generally very low, about 0.18% and the particle size of sand in the cement slurry is between 2 and 4mm.The flow characteristics while mixing and pouring is aided by the use of micro silica and a dispersant. High-frequency vibration is often resorted to for getting a the mix compacted and to obtain homogeneity. Prolonged processing time for mixing, about 15-20 minutes, ensures effective particle wetting and high degree of micro-homogeneity.
Such highly fibre-reinforced concrete typically has compressive strengths ranging from 150 to 270 MPa, and fracture energy from 5,000 to as much as 30,000 N/m.
CRC beams exhibit load capacities almost equivalent to those of structural steel and remain substantially uncracked right up to the yield limit of the main reinforcement (about 3 mm/m), where as conventional reinforced concrete typically cracks at about 0.1-0.2 mm/m.
Some of the properties of CRC as obtained from extensive experiments carried out on CRC specimens are given in Table 4.
Design of CRCThe development and design of CRC is based on fracture mechanics principles/theories, that takes into account the coherent and ductile phase of the composite, cracked pattern and ultimate failure mode. The theories assume that, as in the case of metals, any single, micro crack developed owing to the presence of a local flaw can not propagate and cause sudden tensile failure because of the interlinked pattern of main steel and fibres, thereby rendering the composite highly elastic, ductile and strong.
Applications of CRC
CRC can probably be used especially in the form of large plates or shells designed, for example, to resist very large local loads with unknown attack position (from explosives, say, or mechanical impact) or to resist uniformly distributed pressure, either as pure compression or pure tension (e.g. large pressure tanks).
Because CRC has very high “strength-density ratios” (often greater than those of commonly used structural steel), it offers particularly interesting possibilities for members, where weight and inertia loads are decisive. It could, for instance, be used for different forms of transport (ships, vehicles, etc.), where low weight is essential, or for rapidly rotating large machine parts, where the performance is limited by the capacity of the materials to resist their own inertia loads.
The high degree of ductility of CRC, even at very low temperatures, will make CRC very interesting for large objects that have to resist large loads at low temperatures, where steel will fail due to brittleness or suffer functional deficiency due to progressive corrosion damage.
Because of the far better possibilities of forming CRC and combining it with several other components than those afforded by steel, CRC finds its principal use in hybrid constructions – for example, load-carrying parts in large machines, or special high-performance joints in conventional steel and concrete structures, where large forces have to be concentrated in small volumes.
Slurry Infiltrated Mat Concrete (SIMCON)SIMCON can also be considered a pre-placed fibre concrete, similar to SIFCON. However, in the making of SIMCON, the fibres are placed in a “mat form” rather than as discrete fibres. The advantage of using steel fibre mats over a large volume of discrete fibres is that the mat configuration provides inherent strength and utilizes the fibres contained in it with very much higher aspect ratios. The fibre volume can, hence, be substantially less than that required for making of SIFCON, still achieving identical flexural strength and energy absorbing toughness.
SIMCON is made using a non-woven “steel fibre mats” that are infiltrated with a concrete slurry. Steel fibres produced directly from molten metal using a chilled wheel concept are interwoven into a 0.5 to 2 inches thick mat. This mat is then rolled and coiled into weights and sizes convenient to a customer’s application (normally up to 120 cm wide and weighing around 200 kg).
As in conventional SFRC, factors such as aspect ratio and fibre volume have a direct influence on the performance of SIMCON. Higher aspect ratios are desirable to obtain increased flexural strength. Generally, because of the use of mats, SIMCON the aspect ratios of fibres contained in it could well exceed 500. Since the mat is already in a preformed shape, handling problems are significantly minimised resulting in savings in labour cost. Besides this, “balling” of fibres does not become a factor at all in the production of SIMCON.
Investigations using manganese carbon steel mats (having fibres approximately 9.5 in long with an equivalent diameter of about 0.01 to 0.02 in) and stainless steel mats (produced using 9.5 in long fibres with an equivalent diameter of about 0.01 to 0.02 in) have revealed that SIMCON has performed very well compared with SIFCON specimens that had a steel fibre content of 14% by volume as illustrated in Table 5.
It is clear from the table that the energy-absorption capacity of SIMCON is far superior to SIFCON. A reinforcement level in SIMCON of only 25% of that of conventional SIFCON is found to provide as much as 75% of the latter’s ultimate flexural strength.
Applications of SIMCONSIMCON offers the designer a premium building material to meet the specialised niche applications, such as military structures or industrial applications requiring high strength and ductility.
While the use of SIFCON is presently limited only to specialised applications owing to high material and labour costs involved in the incorporation of a very high volume of discrete fibres that are required for achieving vastly improved performance, SIMCON broadens these market applications by cutting the fibre quantity to less than half and there by substantially reducing the product cost.
Reactive Powder Concrete (RPC)Another recent development in concrete technology is the production of reactive powder concrete (RPC) containing steel fibres as macro-reinforcement. First developed by Bouygues-SA, Paris, its processing has been patented. A high degree of strength, compactness, refined microstructure and homogeneity is achieved by using dense and powder-like particles smaller than 600 microns, and in some cases 300 microns, and by the addition of 2 to 5% of steel fibres. RPC, therefore, do not contain any aggregates, and traditional sand is replaced totally by finely ground quartz of particle size less than 300 microns.
The compactness of an RPC mix is enhanced further by pressing the mix before and during setting, while still in the moulds/forms and by using a very low water-cement ratio (about 0.2%). By subjecting the material to low or high pressure steam curing and by applying pressures up to 50 MPa, the pozzalolanic reaction of the silica fume is accelerated resulting in further modifying of the structure of the hydrates and in concrete strengths as high as 500MPa.
Even though RPC is very strong, it exhibits a brittle failure when fibres are not present. By confining RPC (with steel fibres) in mild steel /stainless steel tubes and applying pressure-cum-heating techniques during its casting, the compressive strength and ductility can be improved tremendously. It is reported that very high strengths of 200 to 800 MPa can be obtained for RPC with cement contents of 955 to 1000 kg/m3. Typical composition of an RPC mix used in the construction of the very first RPC pedestrian bridge built in 1997 in Sherbrooke, Quebec, Canada is given in Table 6. A view of another bridge built in Japan using RPC filled stainless tube supporting columns is shown in Fig. 12.
In due course of time, RPC is expected to outperform normal high performance concretes (HPC) as illustrated in Table 7.