Do you known what causes shotcrete to crack?  Shotcrete, a widely used construction material in tunneling and mining, is prone to cracking due to its inherent brittleness and complex interaction with environmental/structural factors. Based on research findings, the primary causes can be categorized as follows:

   1. Shrinkage and Drying: As shotcrete cures, it loses moisture and shrinks. If too much water is used in the mix or if it dries too rapidly due to environmental conditions (sun, wind, high temperature), tensile stresses develop that can lead to cracking. This is especially common in the early curing phase when shrinkage is most pronounced.

   2. Improper Mix Design: The water-to-cement ratio and aggregate proportions are critical. Excess water weakens the final product, increasing the likelihood of shrinkage cracks, while an overly dry mix can hinder proper compaction and adhesion, both of which may cause cracking.

   3. Application Technique: Shotcrete is applied in layers, and any issues with improper layering, inadequate compaction, or poor reinforcement placement can create weak zones. If the layers do not bond properly or if the initial layer restrains the subsequent layers unevenly, differential shrinkage can cause cracks.

   4. Environmental and Structural Factors: Factors such as temperature fluctuations, wind, and even soil movement (in cases like shotcrete pools or retaining walls) can impose additional stresses. Improper curing practices—such as not keeping the surface adequately moist—can exacerbate cracking by allowing rapid drying and uneven moisture loss.

   Mitigation Strategies

   ① Material Optimization: Use bacterial self-healing agents , steel fibers, and low-permeability admixtures.

   ② Structural Reinforcement: Integrate stud shear connectors to enhance steel-concrete synergy .

   ③ Environmental Control: Apply protective coatings to resist chemical erosion and freeze-thaw damage .