Microcapsule self-healing technology is one of the effective methods to solve the durability problem of cement-based composites. The evaluation method of the self-healing efficiency of microcapsule self-healing cement-based composites is one of the difficulties that limits the self-healing technology. This paper attempts to characterize the self-healing efficiency of microcapsule self-healing cement-based composites by acoustic emission (AE) parameters, which provides a reference for the evaluation of microcapsule self-healing technology. Firstly, a kind of self-healing microcapsules were prepared, and the microcapsules were added into the cement-based composites to prepare the compression samples. Then, the specimen with certain pre damage was obtained by compression test. Secondly, the damaged samples were divided into two groups. One group was directly used for compression tests to obtain the damage failure process. The other group was put into water for healing for 30 days, and then compression tests were carried out to study the influence of self-healing on the compression failure process. During the experiments, the AE signals were collected and the AE characteristics were extracted for the evaluation of self-healing efficiency. The results show that the compression pre damage test can trigger the microcapsule, and the compression strength of the self-healing sample is improved. The failure mechanism of microcapsule self-healing cement-based composites can be revealed by the AE parameters during compression, and the self-healing efficiency can be quantitatively characterized by AE hits. The research results of this paper provide experimental reference and technical support for the mechanical property test and healing efficiency evaluation of microcapsule self-healing cement-based composites.
Cement-based composites are one of the most widely used materials in civil engineering. However, due to the long service time and changeable service environment, the durability of cement-based composites has attracted much attention. With the development of self-healing microcapsule technology, microcapsule self-healing cement-based composites have developed rapidly, making it a hot and difficult point in the field of new materials in civil engineering [
Microcapsule self-healing cement-based composites have been preliminarily studied in engineering applications. A joint research team from Cardiff University, Cambridge University and Bath University conducted a series of field experiments using self-healing technology [
At present, the main nondestructive testing methods widely used to evaluate the self-healing efficiency include ultrasonic wave velocity method, acoustic emission (AE) test, coda interferometry, resonance frequency test, resistivity test, digital image correlation method (DIC), X-ray computed tomography (XCT), etc. In the ultrasonic wave velocity method, defects and discontinuities in the matrix can be detected by the change of ultrasonic wave velocity. Since the propagation speed of ultrasonic waves in liquid or gas phase is slower than that in cementitious matrix, the increase in propagation time means defects or cracks in the matrix. When the cracks are healed, the propagation time of ultrasonic wave is shortened, showing the healing behavior [
In this paper, the damage state of microcapsule self-healing cement-based composites and the whole process of compression failure of healed microcapsule self-healing cement-based composites are monitored by AE technology. By analyzing the characteristics of AE signals in different stages, the failure mechanism of self-healing microcapsule cement-based composites and the influence of microcapsule contents on its failure mechanism are revealed. In addition, the AE characteristic parameters were used as the evaluation index of to quantitatively characterize the self-healing effect of microcapsule cement-based composites with different contents.
The core materials, such as sodium silicate hexahydrate and expanded Portland cement were mixed evenly, and then the core particles were produced by extrusion and rounding. The ethyl cellulose solution was sprayed on the capsule core material with a syringe and dried by blast to obtain the finished microcapsules. Parameters such as the mix ratio of microcapsule core materials and microcapsule wall materials can be found in [
The manufacturing method of microcapsule self-healing cement-based composites mainly consults ISO 679-2009 (Test method for strength of cement mortar). The size of compression samples is 40 mm × 40 mm × 40 mm. The contents of microcapsules were 0%, 1%, 3%, 5% and 7%, respectively. Microcapsule content is defined as the percentage of microcapsule dosage and cement dosage. The compressive samples prepared in this paper are shown in
There are nine compressive samples of each content, of which three are used to test the strength of this batch of samples. The remaining six are divided into two groups. One group is unloaded after preloading to obtain the damaged specimens, and the other group is immersed in water for healing for 30 days after preloading and unloading to obtain the healed specimens. The pre damage method is to use 70% of the maximum compressive load as the load and maintain the load for 5 min. The purpose of this pre damage is to control the compression failure process and make the specimen reach the stage of stable propagation of microcracks. At this stage, microcapsules can be triggered with less damage, and microcapsules can give full play to their role.
In this experiment,
The test device is shown in
According to the AE characteristics of the specimens in the pre damage process and the failure process after pre damage in
After preloading and unloading, the damaged specimen was obtained. Comparing the failure process of the original state specimen and the damaged state specimen, it can be found that the AE hits curve of the damaged state specimen increases slowly in the early stage, because when the load reaches 70% of the maximum load, the specimen has gone through the first stage and the second stage, and the specimen has been compacted at this time. Although the characteristic curve has changed, the law of AE hits is still similar to that of the original state specimen. The AE hits first increase and then decrease, with the largest amount of 3% and the least amount of 7%. The reason is that the damage under 70% of the maximum load is small, slight damage has occurred in the specimen, the specimen still has well service performance, and the basic properties have not changed greatly in the damage process.
Another group of samples were pre damaged and put into water for healing for 30 days to obtain the healed samples. It can be seen from
Compared with the damage state, a large number of AE events occurred in the early stage of failure of specimens with 1%, 3% and 5% content. The reason may be that the internal self-healing produces restoration materials, it may also be that the load is removed and the pores develop again, or it may be caused by the expansion and healing of microcapsules. The expanded microcapsules can not only block the bonding micro cracks, but also flow into the interior to block the micro pores, so that the internal structure becomes compact. Some of the AE events in the early stage may be caused by the extrusion and collision of the expanded microcapsules during the compression of the specimen. The AE hits of 1%–7% samples first increase and then decrease, the AE hits of 3% samples are the most, and the AE hits of 7% samples are the least. The difference from 0% content specimen is that the whole process AE hits of other content specimens becomes more. The increase is mainly caused by the enhancement of compressive strength and the extrusion of expanded microcapsules and matrix (see
In this paper, the self-healing effect of microcapsule-based self-healing cementitious composites with different contents of microcapsules was evaluated using AE technique. The results show that the compression pre damage test can trigger the microcapsule, and the compressive strength of the self-healing samples is improved. The strength enhancement effect of cement-based composites is obvious after adding microcapsules, especially the specimens with 3% microcapsule content have the highest healing efficiency, and the strength increases by nearly 45%. The specimens with 1% content increase slightly, and the specimens with 5% and 7% content have an obvious enhancement effect. Comparing the damage state and healed state, it can be found that the AE hits curve becomes active in the early stage of healing state, which is due to the healing effect of microcapsules. The healing effect of microcapsules depends not only on the number of microcapsules, but also on the triggering efficiency of microcapsules. The failure mechanism of microcapsule self-healing cement-based composites can be revealed by the AE parameters during compression, and the self-healing effect can be quantitatively characterized by AE hits. The research results of this paper provide experimental reference and technical support for the mechanical property test and healing effect evaluation of microcapsule self-healing cement-based composites.