IntroductionCement-based materials are widely used because of their relatively low cost and strong mechanical properties. However, during hydration hardening and service, such materials are often subjected to multiple effects such as plastic shrinkage, self-shrinkage, drying shrinkage, freezing and thawing cycles, and external loading, which in turn induces the formation of internal microcracks. The emergence and development of internal microcracks in cement-based materials can easily become a penetration channel for erosion media such as sulfate and chloride salts, which adversely affects the stability, durability and safety of the structure. Therefore, the characterization and evaluation of internal cracks in cement-based materials have received extensive attention from scholars at home and abroad. Traditional methods still face certain challenges in measuring the width of internal microcracks in samples quickly and accurately. This paper proposes a micro-crack testing method based on the directional transport of the inert gas argon, which can rapidly and accurately determine the micro-crack width by analyzing the diffusion behavior of the gas in the cracks and the cement matrix through the comprehensive consideration of the sample size and the pressure gradient at both ends.MethodsThe cement paste cracking samples with specific crack widths were preset by the insert method, and the preset crack widths were 0.03, 0.05, 0.10, 0.15 mm and 0.20 mm. A cylinder mold with a diameter of Φ50 mm×100 mm was adopted. Through the insert method, steel sheets with different thicknesses were placed in the center of the mold in advance to control the formation of penetrating cracks with different standard widths.The gas transport system was mainly composed of sample chamber, gas transport control system and computer data acquisition system. During the test, the sample needed to be placed on a closed sample table and connected to the upper and lower outlets through a thermoplastic tube. The sample room maintained an oil bath environment to ensure sealing. The gas mass flowmeter was adjusted to the required flow rate, so that the gas passed through the device and recorded the pressure gradient changes at different gas flow. The crack width was calculated by Darcy’s law.Results and discussionWhen the crack width is in the range of 0.03-0.15 mm, the gas transport method can measure the width value more smoothly. When the crack width is 0.1 mm, the gas transport test results are closest to the actual crack width, and the error is less than 5%. When the crack width reaches 0.15 mm, the gas transport test can still maintain a high measurement accuracy, and the error is less than 10%. When the crack width reaches 0.2 mm, the error is more than 110% because the pressure gradient at both ends is not obvious. When the gas flow is 10-50 mL/min, with the increase of crack size, the increase of gas flow helps to reduce the error. Combined with the crack width and gas flow, it can be concluded that when the crack width is about 0.03 mm, the gas flow of 50 mL/min should be adopted; when the crack width is about 0.05 mm, the gas flow of 30 mL/min should be adopted. When the crack width is 0.10-0.15 mm, the gas flow of 20 mL/min should be adopted. In addition, there is a certain complementarity between the gas transport method and the optical microscope test in measuring the crack width. The optical microscope is suitable for the measurement of the width value of the larger crack surface, and the gas transport test can obtain the statistical average value of the penetrating crack inside the sample.In the self-healing test, with the increase of gas flow (≥20 mL/min), the equivalent crack width obtained by the test is basically in a stable state. This is because the high gas flow is more conducive to the passage of argon through the gap between SAP hydrogel particles and the formation of channels, thereby further increasing the gas fluidity in the sample and improving the gas transport effect. By comparing the crack width before and after self-healing, it can be found that the healing effect of SAP hydrogel significantly reduces the crack equivalent width, and the healing rate calculated according to the crack equivalent width.ConclusionsAt the gas flow of 20-50 mL/min, the error is less than 10%, and the test error gradually decreases with the increase of crack width. The gas transport test can accurately measure the crack volume equivalent width of the crack in a short time (400 s), and has good rapidity and reliability. When the crack width reaches 0.2 mm, the detection accuracy is significantly reduced because the pressure difference gradient at both ends is not obvious, and the error is more than 110%. When the crack width is 0.03-0.15 mm, the fluctuation range of the test results increases with the increase of the crack size, and the appropriate increase of the gas flow is helpful to reduce the test error. Therefore, the gas transport method can better quantify the microcracks within 0.15 mm. The suitable gas flow range for crack self-healing in gas transport test is recommended to be 20-50 mL/min, and the average self-healing rate of 24 h is 39.1%, indicating that SAP hydrogel can effectively seal the microcracks inside cement-based materials in a short time, reduce the equivalent crack width, and play a good self-healing sealing effect.