Experiments on Tensile and Shear Characteristics of Amorphous Micro Steel (AMS) Fibre-Reinforced Cementitious Composites

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.

The slump flow of mortar containing AMS fibre was measured as given in Fig. 5, together with a photo of measurement. It was evident that an increase in the volume of AMS fibre resulted in a decrease in the fluidity, irrespective of the length of the fibre. Fibre-free mortar had the slump flow, accounting for about 680 mm, whilst the fluidity was dramatically reduced by the AMS fibre. It may suggest that overwhelming amount of fibre is limited due to clash between the fluidity and strength development.

The compressive strength of mortar containing AMS fibre is given in Fig. 6. It was evident that the fibre might not benefit in enhancing the strength of concrete; there was only, in fact, a reduction of the compressive strength in the mix containing 30 mm long AMS fibre. In particular, the compressive strength at 1.5% of AMS fibre was equated to 27.7 MPa, whilst fibre-free mortar indicated 33.9 MPa. The variation in the compressive strength was again observed for 15 mm long AMS fibre: the range of the compressive strength was 32.3–34.8 MPa. Substantially, it seemed that no benefit in raising or at least sustaining the compressive strength was gained by AMS fibre in concrete. Above the fibre volume of 0.75%, the compressive strength tend to reduce with increase of the fibre volume fraction which is mainly caused by insufficient fibre dispersion as observed in a literature (Yoo and Banthia 2017).

After curing of 28 days, tensile behaviors of hardened AMS fibre composite were examined and test results were presented by tensile stress–strain curves as given in Fig. 7. Non-fibre mortar and AMS fibre-reinforced composite with a fibre volume of 0.5% were exempt from the direct tensile test due to a very high level of the brittleness in tension. In preliminary test, the tensile stress had been sharply dropped after the initial crack was generated when the tensile strain ranged from 0.22 to 0.23%. All specimens exhibit apparent multiple cracking patterns accompanying tensile strain softening and ductile behaviors with strain capacities ranging from 0.57 to nearly 0.75%. These ductile characteristics remain much higher than that in normal concrete and conventional FRC composites.

The tensile strength and strain simultaneously measured for each mixture are given in Figs. 8 and 9, respectively. It is clearly shown that an increase in the volume of the AMS fibre in mixture resulted in a mostly linear increase in the tensile strength, irrespective of the length of AMS fibre. The tensile strength of non-fibre mixture was equated to about 1.32 MPa, while the tensile strength of AMS fibre mixtures with the fibre volume of 1.0, 1.25 and 1.50% exceeded 4.0 MPa. Even with a lower volume of AMS fibre (i.e. 0.50 and 0.75%) the tensile strength was in the range of 2.46–3.57 MPa. When it comes to the tensile strain, it is also observed that an increase in the volume of AMS fibre in the mixture resulted in an increase in the tensile strain. In particular, with up to 0.75% of the AMS fibre in volume, the tensile strain was significantly increased by the AMS fibre, which was subsequently marginal in the variation in the strain, ranging from 0.57 to 0.75%. AMS fibre-reinforced cementitious composites may represent a post-cracked ductile strain in direct tensile test, which could not be observed in fibre-free concrete (Batson et al. 1972; Shah 1980). However the ductile capacity of AMS fibre-reinforced cementitious composites was reduced for ECC and SHCC mixes, imposing high ductile and strain-hardening characteristics in direct tensile post-cracked region as the maximum tensile strain was above 2.0% (Fischer and Li 2003; Lee et al. 2012; Choi et al. 2014; Cho et al. 2012). AMS fibres in cementitious mixture were expected to be fracture before complete pullout as shown in Fig. 10. This would cause a lower post-peak ductility compared to PVA fibres with a pullout failure.

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.

After curing of 28 days, shear transfer capacity was examined to measure the shear strength. The shear crack was observed for all cases, as given in Fig. 11, together with the shear strength. It was clearly observed that an increase in the AMS fibre in the mix resulted in an increase in the shear strength, up to 1.25% of the AMS fibre in volume, irrespective of the length of fibre. In fact, the shear strength of the mixture with 1.25% of fibre in volume accounted for 11.77–11.20 MPa, while with 1.50% the shear strength ranged 9.30–9.77 MPa. The variation of shear strength for varying fibre contents was manifested as observed in a series of current test that AMS fibres mixed into cementitious composites were so efficient to promote the shear capacity of cementitious composites by suitable fibre contents. Mixtures of AMS fibre-reinforced cementitious composites exhibited up to 2.22 times and 2.35 times higher the average maximum shear strength than the mixture of non-fibre mortar, for the fibre length of 15 mm and 30 mm, respectively.

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.