Experiments on Tensile and Shear Characteristics of Amorphous Micro Steel (AMS) Fibre-Reinforced Cementitious Composites
© The Author(s) 2017
Received: 19 December 2016
Accepted: 21 August 2017
Published: 7 December 2017
Amorphous micro-steel (AMS) fibre made by cooling of liquid pig iron is flexible, light and durable to corrosion, then to be compatible with high flowable and disperable states of mixing as well as high ductile post-cracked performances to apply in fibre-reinforced cementitious composites. In the current research, AMS fibre-reinforced cementitious composites based on cement and alkali-activated ground granulated blast furnace slag mortars were newly manufactured and evaluated for the strength and ductile characteristics mainly by direct tensile and shear transfer tests in the variation in the volume of AMS fibres with two different lengths of 15.0 and 30.0 mm. As a result, it was found that 1.0–1.25% fibre volume fractions were recommendable for AMS fibre-reinforced cementitious composites to maximize direct tensile strength, ductile tensile strain, and shear strength of the composites. However, a further fraction of AMS fibre lowered these mechanical characteristics. Simultaneously, it could be said that AMS fibre-reinforced cementitious composites exhibited up to about 3.7 times higher in direct tensile strength and up to 2.3 times higher in shear strength, compared to AMS fibre-free specimens.
Attempts to utilize fibre cementitious or concrete composites mixed with metallic or non-metallic fibres had been greatly made in fields of high rise building and infra structures in order to enhance additional requirements of high ductility, performance and durability (Narayanan and Darwish 1987; Ashour et al. 1992; De Hanai and Holanda 2008; Fischer and Li 2003; Kim et al. 2009; Lee et al. 2012; Choi et al. 2014). For fibre-reinforced cementitious composites, non-metallic fibres such as synthetic fibres were used to mainly develop high ductile characteristics after cracking, with no improvement of strength, known as engineered cementitious composites (ECC) or strain-hardening cementitious composites (SHCC) (Fischer and Li 2003; Lee et al. 2012; Choi et al. 2014; Cho et al. 2012; Kim et al. 2014). On the other hand, steel fibres are often used to mix in concrete or cementitious composites to refine their brittle characteristics by enhancing tensile and shear strength (Narayanan and Darwish 1987; Ashour et al. 1992; De Hanai and Holanda 2008; Özgür and Khaled 2009; Özcan et al. 2009; Lim and Hong 2016; Lu et al. 2016).
Most of steel products used in construction fields such as deformed reinforcing bars, steel fibres, and steel plates, etc., in general, are crystalline metals, of which properties are mainly characterized by adjusting the cooling speed after crystalline metals are liquefied under high temperature. A crystal in metal is formed when liquid metal is cooled slowly at thousands of degrees per second (Won et al. 2012; Seo 2006). These crystalline steels are basically an anisotropic material, consisting of regular arrays of atoms, so that their mechanical characteristics are dependent on crystal directions, such as in exhibiting the modulus of elasticity, electrical and heat conductivities, and refractivity.
In the manufacturing process, steel fibres produced from wires are subjected to a repeated heating and cooling process of lengthening and thinning, then to achieve suitable shape, yield and rupture strength as well as elastic modulus. Considering in fibre-reinforced cementitious composites, steel fibre is, however, susceptible to corrosion in a humid atmospheric condition. Moreover, high level of the specific gravity may lower dispersion of steel fibres in fresh binders; a suitable quality of fibre-reinforced cementitious composites may not be achieved (Won et al. 2012; Morga et al. 1999; Yoo et al. 2016).
Amorphous micro-steel is fundamentally different in the manufacturing process that cools liquid pig iron at the bottom of a furnace under fast rotation and do not make crystalline grain boundaries in the metal, when the metal liquid is cooled rapidly above 105 K/s (Won et al. 2012; Seo 2006). Therefore, the micro-steel is a pure isotropic as independent to material directions and an amorphous metal as exhibiting liquid characteristics in solid, called as a liquid metal. Amorphous micro-steel has not only a superb strength and toughness with the relatively low specific gravity in mechanical characteristics, but also has distinguished durability to resist corrosion in humidity, acid and exposed atmospheres (Won et al. 2012; Seo 2006). Some researchers attempted to evaluate mechanical properties of mortar or concrete including AMS fibres (Redon and Chermant 1999; Won et al. 2013; Dinh et al. 2016; Yoo et al. 2016).
In this study, the feasibility of a new micro-steel fibre (i.e. amorphous micro-steel fibre made by cooling of liquid pig iron) reinforced cementitious composite based on cement and alkali-activated ground granulated blast furnace slag (GGBS) mortars was experimentally assessed for the strength and ductile characteristics mainly by direct tensile and shear transfer tests in the variation in the volume of AMS fibres with two different lengths of 15.0 and 30.0 mm. In addition to optimize the volume of the steel fibre in the mix, the slump flow, the compressive strength, the direct tensile behavior, and the shear transfer were examined.
2 Amorphous Micro-Steel Fibre-Reinforced Cementitious Composites
2.1 Manufacturing of Fibre-Reinforced Cementitious Composites
Oxide composition of OPC and GGBS.
2.2 Measurement of Slump and Mechanical Tests
The flow and mechanical characteristics of fresh and hardened AMS fibre composite were tested and measured varying with the fibre volume(0, 0.5, 0.75, 1.00, 1.25 and 1.50%) for two different fibre length of L = 15 mm and L = 30 mm. The fluidity of mortar containing AMS fibre was measured immediately after mixing, casting in a cone and removing the cone, of which the taper-shaped margin was Ø100 × Ø200 × 300 mm (KS F 2402 2007). Uniaxial compressive strength was measured by a 50 mm cube specimen for two specimens for each mix, according to ASTM C109-07 (ASTM 2007).
3 Characteristics of Slump Flow and Compressive Strength
4 Test and Discussions on Direct Tensile Characteristics
When the ECC or/and SHCC were manufactured to mix PVA fibres into cementitious composites about 1.5–3.0% in volume of the mix, the tensile strength was not much affected (Fischer and Li 2003; Lee et al. 2012; Choi et al. 2014; Cho et al. 2012). From observations of current direct tensile test, it was supposed that AMS fibre-reinforced cementitious composites had ambivalent post-cracked tensile characteristics between steel fibre reinforced concrete and ECC mixture. For ductile post-cracked tensile strain capacities, the AMS fibre cementitious composite was superior to steel fibre reinforced concrete, but was inferior to ECC or SHCC mixtures, whilst the AMS fibre-reinforced cementitious composites could greatly enhance tensile strength, which cannot be observed in ECC or SHCC mixtures. Considering the fibre dispersion, mixtures at more than 1.5% fibre volume fraction were not recommendable to increase direct tensile strength as well as improve deformation capacities.
5 Test and Discussions on Shear Transfer Strength
For mixtures with less than 1.25% fibre volume fraction, the shear strength was gradually increased according to the increase of fibre volume fraction because AMS fibres could be suitably dispersed into the cementitious mixture with below the fibre contents. A reduction in the shear strength with 1.50% fibre volume fraction was presumably due to a poor dispersion of fibres, which might get tangled and made fibre balls within cementitious mixtures. It could be commented from experiments that 1.0–1.25% fibre volume fractions were suitable to improve shear transfer capacity in AMS fibre-reinforced cementitious composites with the fibre length of 15 and 30 mm, and the mixtures with 1.25% fibre volume fraction could exhibit most excellent to develop shear transfer capacities in mixtures.
In the present study, a series of experimental programs was investigated to establish the feasibility of developing a new micro-steel fibre cementitious composite using OPC and GGBS mixing mortar and amorphous micro-steel fibres with the length of 15 mm or 30 mm at 0.5–1.5% in volume, and the followings were concluded.
The flexible, light weighted properties of amorphous micro-steel fibres could allow excellent in flowable and disperable states of mixing with cementitious composites. The actual slump flow measured immediately after mixing was about 640 mm, as meeting the flowable workability for fresh concrete. It was increased the tensile strain and shear transfer strength of AMS fibre-reinforced cementitious composites by fiber bridging effects. The fibre dispersion seemed poor at exceeding 1.5% of AMS fibre in volume rather than lower values: the shear strength was adversely reduced at 1.5% of AMS fibre than at 1.25%. Moreover, the compressive strength seemed not affected by the fibre, whereas an increase in the fibre in the mix resulted in an increase in the tensile strength.
AMS fibre-reinforced cementitious composites had ambivalent post-cracked tensile characteristics between steel fibre reinforced concrete and ECC mixtures. For high ductile post-cracked tensile strain capacities, AMS fibre-reinforced cementitious composites was superior to steel fibre reinforced concrete but was inferior to ECC or SHCC mixtures. For the tensile strength, the ultimate tensile strength of AMS fibre-reinforced cementitious composites could be greatly enhanced varying with the fibre contents, as also observed in steel fibre reinforced concrete, but these phenomena of tensile strength enhancement cannot be fundamentally observed in ECC or SHCC mix.
This research was supported by a Grant (17RDRP-B076268-04) from Regional Development Research Program funded by Ministry of Land, Infrastructures, and Transport of Korean government, and supported by research fund from Chosun University, 2015.
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