硅酸盐学报

高延性水泥基复合材料(ECC)具有高延性、高韧性等特点，大量细密裂缝的产生为其自愈合提供条件，海洋环境中硫酸盐含量较高、北方部分地区土壤呈碱性，为了揭示环境与荷载共同作用下ECC的自愈合性能，分别开展了3种环境(5%Na_2SO₄、饱和Ca(OH)₂、清水)下、不同预加载水平的拉伸试件(0%、1%、2%)及四点弯曲试件(0%、20%、40%)在不同龄期的性能试验研究。结果表明：ECC自愈合后抗拉性能恢复良好，预拉应变1%有利于恢复试件各龄期的抗拉强度，5%Na_2SO₄及饱和Ca(OH)₂干湿循环作用有利于提升ECC抗拉强度，且Ca(OH)₂环境的恢复效果更好；清水环境对于完好试件(预拉0%)长龄期下的延性恢复效果明显，5%Na_2SO₄及饱和Ca(OH)₂干湿循环作用削弱有裂缝试件的延性，预拉应变2%时极限拉应变降幅最大，可达52.78%～65.39%。预加载40%对于四点弯曲薄板的初裂抗弯强度、极限抗弯强度恢复作用最显著，且5%Na_2SO₄干湿循环作用的效果更佳，预加载20%明显提升极限挠度，且清水环境最具优势；抗弯延性与多缝开裂模式正相关；5%Na_2SO₄及饱和Ca(OH)₂干湿循环作用对于试件短龄期的抗弯强度恢复作用明显，对微裂缝愈合具有积极效果；自愈合90 d后，预加载20%薄板的初始弯曲韧度比、残余弯曲韧度比恢复效果最佳，Ca(OH)₂环境有利于提升峰前弯曲韧性，清水环境有利于提升峰后弯曲韧性。

Introduction ECC is characterized by high ductility and high toughness, and the generation of a large number of fine cracks gives the material a significant self-healing ability, thereby improving its safety and durability. In some northern regions of China, the soil pH value exceeds 9 in alkaline environments, sulfate content in marine environments can generally reach 1400–2700 mg/l, structures are typically subjected to dry–wet cycles in areas of water level fluctuations, and water is a necessary condition for the occurrence of self-healing. In order to comprehensively and systematically analyze the self-healing performance of ECC under different environments and preloading conditions, the tensile and flexural properties of ECC after self-healing under three environments(5% Na₂ SO₄, saturated Ca(OH)₂, and water) were investigated in this paper, providing a basis for the further application of ECC materials in engineering practice. Methods Three environments were designed: 1) 5% Na₂ SO₄ solution(by mass) under dry–wet cycles; 2) Saturated Ca(OH)₂ solution under dry–wet cycles; 3) Water environment which served as a control group. The preloaded specimens were placed in them individually. A total of 9 batches of dog-bone specimens under three different environments were prepared. The group preloading to 1% and 2% strain and the intact specimens without preloading were placed in the self-healing environment. After reaching the corresponding age, tensile tests were conducted to obtain stress-strain curves under different conditions, and the recovery of tensile strength and ultimate deflection was analyzed. Besides, 9 batches of four point bending thin plates under three different environments were prepared, the ultimate strength was obtained from the intact specimens, and 20% and 40% of the value were selected as the preloading levels for bending tests. The preloaded group and intact specimens without preloading were placed in the self-healing environments for the flexural tests to obtain the load-deflection curves, the recovery of properties such as flexural strength, ultimate deflection, and flexural toughness were investigated. Results and discussion Preloading to 1% strain gained a significant advantage in restoring the tensile strength of specimens at different ages in three different environments. The tensile strength of ECC was improved when subjected to dry–wet cycle in both Na₂ SO₄ and Ca(OH)₂ solutions, with a supreior recovery ability observed in the Ca(OH)₂ environment. It is also noted that under a 1% preload, the tensile strength increased by about 36.33% throughout the 90 d aging process. Reloading after pre-tensioning led to a significant improvement on the tensile ductility of specimens at 0 d, with an increase in ultimate tensile strain of up to 138.4% with the preloading level increasing from 0% to 2%. The water environment had a significant effect on the ductile healing of intact specimens without preloading at long ages, with an increase in ultimate tensile strain of 137.52% with age from 0 d to 90 d, while the Na₂ SO₄ and Ca(OH)₂ environments weakened the ductility of cracked ECC specimens, with the largest decreases of 52.78%–65.39% at 2% preloading, and the Na₂ SO₄ environment had a better effect on the maintenance of ductility in the specimens. At 90d of self-healing, increasing preloading levels weakened the tensile ductility of the specimens, with the greatest decrease in water. Under the unsoaked condition, 20% preloading increased the flexural strength of the bent plate, but weakened the flexural ductility. As the age of self-healing increased, the initial flexural strength and ultimate flexural strength of the specimens under 40% preloading were improved, and Na₂ SO₄ environment was more effective, with an increase of 28.91% and 40.35%, respectively, compared with that at 0 d. Preloading by 20% significantly increased the ultimate deflection, and the best effect was achieved in water, with an increase of up to 202.83% compared with 0 d. Flexural ductility was positively correlated with multi-seam cracking pattern. Na₂ SO₄ and Ca(OH)₂ environments were effective in restoring the flexural strength of the specimens at a short age, and had a positive effect on the healing of microcracks. After 90 d of self-healing, the initial flexural toughness ratio and residual flexural toughness ratio of the preloaded 20% thin plate recovered best. Ca(OH)₂ environment was favorable to enhance the pre-peak flexural toughness, and the water environment was favorable to enhance the post-peak flexural toughness. Conclusion The tensile and flexural properties of ECC after self-healing in three environments were investigated. The results showed good tensile and flexural self-healing properties. A 1% preloading level was clearly beneficial for tensile strength recovery at all ages in the three environments. Although Dry–wet cycling in Na₂ SO₄ and Ca(OH)₂ enhanced the tensile strength of ECC, it reduced the tensile ductility of preloaded ECC specimens. The greatest decrease in ductility occured at 2% preloading, and the Na₂ SO₄ environment was more effective in maintaining it. After 90 d of self-healing, tensile ductility of the specimens decreased with increasing preloading levels. A 40% preloading level had the most significant effect on flexural strength recovery after long-term aging, particularly in the Na₂ SO₄ environment. The ultimate deflection recovery of the specimen was most significant at a 20% preloading level, with better performance in water. Furthermore, 20% preloading significantly improved the flexural toughness after 90 d of self-healing.