119 / 2022-04-26 19:15:08

Multifunctional stainless steel wires reinforced ultra-high performance concrete for upgrading structural longevity

Ultra-high Performance Concrete (UHPC),Stainless Steel Wires,Mechanical Properties,Multifunctional Performance,Structural Longevity

As the main structural material for structures, concrete is in an important stage of transition to sustainable development due to its significant impact on the resilience of resources and energy as well as environment. Upgrading the performance of concrete (i.e., improving the mechanical properties and endowing multifunctional properties) to enhance structural longevity is an effective approach to indirectly reduce carbon emissions caused by the concrete production and usage. For example, if the service life of concrete structures is increased from 50 years to 100 years, the carbon emission will be reduced by half. Despite its remarkable fresh and hardened properties, the performance of ultra-high performance concrete (UHPC) needs to be continuously modified due to the complexity of service environments and the demand for structural performance improvement and expansion. Owing to the microscale diameter, high aspects, large specific surface area, similar thermal expansion coefficient to concrete matrix, high strength and modulus, resistance to corrosion, and good electrically/thermally conductive properties, stainless steel wires (SSWs) can not only modify the interface transition zone and microstructures of concrete but can also form a widely distributed enhancing, toughening, conductive and thermal network in UHPC at a low content level, thus endowing UHPC with multifunctional properties including enhanced mechanical performances and durability as well as smart property. Especially, the stainless profile of wires is beneficial to maintain the long-term stability of properties of UHPC, thus avoiding the adverse effects of steel fiber corrosion on the long-term properties of concrete structures.

In our study, the static/dynamic mechanical behaviors as well as electrically conductive, self-sensing, and thermally conductive characteristics of SSWs reinforced UHPC are investigated, and the modification mechanisms of SSWs on UHPC are revealed through microstructure, intrinsic electrical and thermal conductivity analysis. The experimental results demonstrate that the flexural strength, flexural toughness of UHPC unnotched beams and equivalent flexural strength of UHPC notched beams can be increased by 103.2%, 146.5% and 80.0%, respectively, due to the addition of 1.5 vol. % of SSWs with diameter of 20 mm (Fig.1). The average flexural fatigue life of UHPC is enhanced by 636.6%, 558.3% and 1010.7% at the maximum stress levels of 0.7, 0.8 and 0.9, and the calculated ratio of flexural fatigue endurance limit to static flexural strength for SSWs reinforced UHPC reaches up to 0.64. In addition, incorporating only 0.5 vol.% SSWs into UHPC enables the uniaxial compressive fatigue life and energy dissipation capacity to increase by 252.0% and 262.3%. The strain sensitivity of 0.5 vol.% SSWs (with diameter of 8 μm) reinforced UHPC is 22.5, 94.9 and 43.6 under cyclic compression, monotonic compression and flexure, respectively, much higher than the gauge factor of commercial strain gauge. Meanwhile, the cracking opening process of composites under flexural loading can be real-timely monitoring (Fig. 1). The results of finite element simulations indicate that the temperature gradients for UHPC pavement slab with a size of 4.5 m×5 m×0.4 m is dropped by 6.9 ℃, and the maximum thermal stress is reduced by 0.90 MPa as 1.5 vol% of SSWs with diameter of 20 μm are added (Fig. 2). Microstructure analyses reveal that the modification mechanisms of SSWs on UHPC mainly come from the microstructure refinement effect, the widely connecting network, the restraining effect on initiation and convergence of microcracks, and the pull-out and stripping of SSWs under loading.

The developed multifunctional SSWs reinforced UHPC has great potential to enhance the structure safety, durability, multifunctionality/intelligence, resilience and sustainability, as well as update structural longevity to reduce the life cycle cost of structures. Under the same bearing capacity, the employment of multifunctional SSWs reinforced UHPC can also decrease section size of structural elements and optimize structural design to reduce the relative demand for concrete materials. This is an effective approach to address the ecological issues of concrete materials and structures and reduce environmental footprint (especially carbon emission) caused by the extensive use of concrete.