Shotcrete, a pneumatically applied concrete mixture, is widely used in tunneling, mining, and structural rehabilitation due to its rapid application and adaptability. However, like conventional concrete, it is inherently susceptible to cracking under various stress conditions. The question "Is cracking normal?" requires a balanced exploration of material science, construction practices, and mitigation technologies.
1. Why Cracking Occurs in Shotcrete
• Material Limitations:
As a quasi-brittle material, shotcrete lacks ductility. Internal stresses from shrinkage during hydration or thermal contraction can exceed its tensile strength, leading to microcracks.
• Structural and Environmental Factors:
① Dynamic Loading: In tunneling, ground movement or seismic activity induces shear-tension stresses.
② Improper Application: Inadequate compaction or uneven thickness during spraying creates weak zones prone to cracking.
• Design Constraints:
Standard mix designs prioritize early strength over long-term durability, sometimes accelerating crack formation.
2. Is Cracking "Normal"? When Should You Worry?
• Acceptable Microcracking:
Hairline cracks (<1 mm wide) are common and often harmless, as they don’t compromise structural integrity.
• Problematic Macrocracks:
Cracks exceeding 1 mm width—caused by severe overloading or poor quality control—require intervention. These allow water ingress, accelerating corrosion of embedded steel.
3. Modern Solutions to Minimize Cracking
• Material Innovations:
① Microbial Self-Healing: Bacillus subtilis bacteria added to shotcrete precipitate calcium carbonate, autonomously sealing cracks.
② Micro-Nano Bubble Water (MNBW): Enhances cement hydration, producing denser matrices with 20–30% fewer voids.
Cracking in shotcrete is often normal and manageable, but not all cracks should be ignored. Understanding the type, cause, and severity of cracks is essential to determine whether they are a cosmetic issue or a sign of deeper structural problems. With proper installation, material selection, and monitoring, shotcrete can remain a reliable and long-lasting solution.