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The usage of Steel Fibre Reinforced Concrete (SFRC) in precast underground tunnel lining design is becoming more prevalent due to its vast benefits. The first adoption of SFRC bored tunnel precast segmental lining in Southeast Asia was for the Land Transport Authority Downtown Line Stage 3. By using SFRC, the productivity of construction was enhanced by about 25% comparing to conventional steel bar reinforced concrete precast segmental lining. This technique also lowered the demand of factory space, facilitated rapid automated segment carousel production, and delivered more robust and cost-saving tunnels. Since then, SFRC demonstrates huge prospects in other underground tunneling projects.

In the past few years, the material performance and design guide were not well set by both codes and standards. Singapore standard SS 674 was published in 2021 with great anticipation of the stakeholders in construction industry. The standard comprises not only design but also the handling and testing methods.

Steel fibres are short, discrete lengths with an aspect ratio from 20 to 100, with different cross-sections. They are randomly dispersed in an unhardened concrete using the conventional mixing procedures. A certain mixing portion of fibres significantly increases resistance to cracking, impact, fatigue, and bending, tenacity, durability, and other properties of concrete.

Steel fibres demonstrates superior resistance in crack mouth widening and propagation. The residual flexural tensile strength classes (R-classes), which relate to the characteristic residual flexural tensile strength can be determined from beam testing in accordance with BS EN 14651 at an age of 28 days (specified in SS 674). The R-classes are typically used for design of Ultimate Limit States (ULS).

The residual flexural tensile strength is determined from the load-crack mouth opening displacement (load-CMOD) curve obtained by applying a centre-point load on a simply supported notched prism. Two material characteristics can be found on the load-CMOD curve, the limit of proportionality (LOP) and the residual tensile strength. The LOP is dominated by the concrete mix design, while the dosage of the steel fibre determines whether a bending hardening or bending softening mechanism is applied on the other part of the curve.

Unlike other testing, the sample preparation process shall be precisely controlled. Unless specified otherwise, test sample shall be cured for a minimum of 3 days until no more than 3 h before testing. This allows the concrete around the notching location to further develop in strength and heal if any tiny defects. Positioning of the beam gently on the rollers to avoid additional impact induced. Only experienced and professional care with the samples will ensure the trusted test data. Contact our test specialist today to understand more about Steel Fibre Concrete Testing

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