Abstract:

The fabrication of strengthening and toughening units using fiber-reinforced engineered cementitious composites is recognized as an effective approach to enhancing the mechanical performance of structural components. Polyvinyl alcohol engineered cementitious composite exhibits high ductility， pronounced strain-hardening behavior， and excellent crack-control capability， making it particularly suitable for embedding as localized functional reinforcement within normal concrete structures. Despite these advantages， existing techniques face substantial limitations. Achieving stable interface bonding while enabling flexible and directional placement of reinforcement units remains challenging， and further investigation is required to develop embedded directional strengthening methods capable of reinforcing structurally weak regions within concrete elements. Liquid nitrogen quick-freezing offers a potential solution by imparting controllable shape stability to the cementitious matrix. This approach allows strengthening and toughening units to be geometrically parameterized and optimally oriented according to localized stress distributions， enabling precise control over spatial placement， orientation， and scale. By rapidly arresting early hydration， quick-freezing preserves microstructural integrity and prevents the formation of coarse ice crystals that could compromise fiber-matrix interaction. Subsequent controlled thawing resumes hydration and restores mechanical performance. This technique introduces a novel strategy for directional toughening in engineered cementitious composites （ECC）-normal concrete （NC） composite systems and provides a framework for systematic material-level evaluation. To validate the feasibility of this concept， the present study focuses on the material performance of polyvinyl alcohol fiber-reinforced engineered cementitious composite units. Strengthening and toughening units were fabricated using two freezing protocols： slow-freezing and liquid nitrogen quick-freezing， followed by controlled thawing at predetermined temperatures. Mechanical performance was evaluated through 28 d compressive and uniaxial tensile tests， while multi-scale characterization was conducted using digital image correlation （DIC）， X-ray diffraction （XRD）， and scanning electron microscopy （SEM） to investigate strain localization， hydration progression， microstructure， and fiber-matrix interface integrity. Experimental results indicate that quick-freezing combined with subsequent thawing effectively preserves mechanical performance. Compressive strength of quick-frozen units recovered to 96.6% and 98.8% of the standard-cured reference values under two thawing conditions （40 and 60 ℃）， while tensile properties， including first-cracking stress， peak stress， and ultimate strain， were essentially comparable to the reference group. In contrast， slow-freezing induced substantial mechanical degradation： compressive strength decreased by 38.5%， and peak tensile stress and ultimate strain decreased by 42.3% and 34.3%， respectively. Multi-scale analyses revealed that quick-freezing and thawing maintained fine， uniformly distributed multiple-cracking patterns and a dense interfacial structure， whereas slow-freezing resulted in hindered hydration， a more porous matrix， and impaired fiber-bridging performance. These findings demonstrate that liquid nitrogen quick-freezing provides distinct advantages for fabricating strengthening and toughening units with controllable geometry and preserved performance. The results offer a robust experimental foundation for embedding pre-fabricated units in critical regions of concrete structures to enhance mechanical performance and support subsequent optimization of interface synergy with surrounding concrete. This study establishes the material basis and practical evidence necessary to inform further research on ECC-NC composite structural applications and interface engineering.

Key words: liquid nitrogen quick-freezing, PVA-ECC, strengthening and toughening unit, mechanical performance, digital image correlation technology, tensile energy