Design And Preparation Of High-Strength Self-Healing Capsules And Study On The Effects Of Capsule Incorporation On Concrete Properties

Concrete,as one of the building materials with the highest usage,plays an important role in the construction of national infrastructure.However,due to various environmental loads and its own properties,concrete is prone to produce microcracks.The existence of such cracks poses a huge potential threat to the safe and effective operation of concrete structures.In order to quickly repair microcracks and contain their potential threats at the initial damage stage,capsule-based self-healing technology for concrete has been extensively studied and has achieved a lot of beneficial results.The use of capsules can significantly enhance the selfhealing performance of concrete,but sometimes it can also reduce the original mechanical properties or durability of concrete.Therefore,in order to solve the problem of "incompatible enhancement of self-healing performance and inherent performance of the matrix" of concrete,this study designed a new type of high-strength self-healing capsule,proposed an efficient preparation process,systematically evaluated the influence of capsules on the mechanical properties,durability,and self-healing performance of concrete,and revealed the influence mechanism of capsules on various properties of concrete from the aspects of phase composition,micro-morphology,pore structure,and so on.The specific research content and conclusions are as follows:(1)A new type of high-strength self-healing capsule was designed,and a batch preparation method for the capsule was proposed.The method proposed the key technology of pre-pressing dry mixtures to improve the flowability of mineral powder repairing agents,achieved stable preparation of high-quality capsules,and utilized 100%of the raw materials.The prepared capsules are short cylindrical in shape and can withstand a compressive strength of more than 3.5 MPa,with good waterproof performance.(2)The influence of the new type of capsule on the basic mechanical properties of concrete,such as compressive strength,flexural strength,and elastic modulus,was studied.The study found that with the increase of the capsule dosage,the compressive strength of concrete only decreased by up to 5%,and the flexural strength of concrete increased slightly,and the stressstrain curve showed that the ductility was enhanced;with the increase of the capsule particle size,the compressive strength of the specimen showed a slight decrease,but the flexural strength increased,and the 3mm particle size capsule increased the flexural strength of the concrete specimen by about 20%;the elastic modulus of specimens with different capsule particle sizes was in the order of 4mm>3mm>2.5mm.Considering the degree of the influence of the new self-healing capsules on the mechanical properties of the concrete,the capsule particle size of 3mm is preferred.(3)The influence of the new capsules on the durability of concrete,including freeze-thaw resistance,carbonization resistance,permeability resistance,and chloride salt erosion resistance,was studied.The research found that the new capsules can significantly improve the freezethaw resistance and permeability resistance of concrete,and also enhance its carbonization resistance.During the freeze-thaw process,the relative mass loss of concrete containing the capsules was less,the elastic modulus was more stable,the remaining compressive and flexural strengths after freeze-thaw were larger,and the water penetration depth and carbonization depth were significantly reduced.As for chloride salt erosion resistance,the results of the natural immersion method showed that the capsules had a small influence on the distribution of chloride ions in concrete,and especially under the conditions of dry-wet alternation,the capsules would slightly reduce the content of chloride ions in concrete.(4)The influence of the new capsules on the self-healing performance of concrete(crack width,permeability,compressive strength,etc.)was studied.The research found that the addition of capsules can enhance the self-healing performance of concrete.When the initial crack width is less than 0.4mm,the specimens with capsules added can achieve complete repair at the repair age of 14 days,and the best effect of crack repair is achieved when the capsule dosage is 5%and the particle size is 3mm.The appropriate increase in capsule particle size can improve the repair rate of permeability in specimens.The capsule particle size of 2.5mm,the capsule dosage of 5%,and the water-cement ratio of 0.63 can achieve the highest recovery rate of the compressive strength of concrete,which can reach about 70%.Due to the difficulty in controlling the initial crack width of the specimens,there is no obvious difference in the ultrasonic wave velocity of the specimens with different capsule dosages.The influence of different capsule particle sizes on the change of ultrasonic wave velocity through the concrete specimens with the repair age is very small.In the first 3 days of the repair age,the order of ultrasonic wave velocity through the specimens was the one with the water-cement ratio of 0.63>0.53>0.43;in the later period of the repair age,the difference in ultrasonic wave velocity among the specimens with different water-cement ratios was small.(5)The MIP results show that the most recognizable pore size of the internal net slurry specimens with or without the addition of capsules is 47.15 nm.Therefore,the flexural and compressive strength of concrete will not change significantly after adding the capsules.The CT results show that some of the capsules were damaged after freeze-thaw,with fewer large pores and more small pores.The freeze-thaw damage process and the capsule repair process occurred simultaneously,so the freeze-thaw resistance was significantly enhanced.In addition,the pores inside the specimens were filled with repair materials,so the loss of mechanical properties of concrete was relatively small.The analysis of the repair materials at the crack repair location shows that the main repair products are C-S-H gel,CaCO3,calcium feldspar,and calcium aluminate.It is precisely these repair products that fill and bond the crack defects,resulting in different degrees of recovery in crack width,permeability,compressive strength,and ultrasonic wave velocity.