Cracked concrete weakens faster in freeze-thaw cycles than previously thought

<p>This study investigated the interactive effects of pre-damage, water boundary conditions, and internal frost damage on concrete at dual-scale. The pre-damage included pre-cracking, which has not been studied experimentally before, and pre-compressive damage. Concrete specimens underwent pre-damage and had varied water boundary conditions during Freeze-Thaw Cycles (FTC). At the macro-scale, wedge-splitting tests combined with Digital Image Correlation (DIC) were conducted to assess post-FTC strength and fracture behaviour. At the meso-scale, X-ray CT scanning was employed to identify internal crack patterns. Results reveal that at the macro-scale, significant interaction between pre-damage and frost damage reduced splitting tensile strength compared to the internal frost damage alone. Besides, increased water exposure during FTCs reduced both splitting tensile strength and compressive strength, with a less pronounced reduction in splitting tensile strength. It also led to a diffuse crack pattern and increased tensile ductility. At the meso-scale, specimens subjected to the interactive effects of pre-damage and internal frost damage exhibited cracks along aggregate-cement interfaces and within the cement paste. Reference specimens displayed no internal cracks, while specimens exposed to only FTCs showed only cracks along aggregate-cement interfaces. Full submersion of specimens during FTCs induced more internal cracks than solely water on top. These findings on the interactions between pre-damage, water boundary conditions, and internal frost damage offer insight into the causes of frost damage, vital for the design and assessment of concrete structures in frost-prone environments. Furthermore, the results of these dual-scale tests can be used as a test case for the development of upscaling numerical models describing heat transfer and frost degradation in concrete.</p>