Considered as an effective real-time monitoring tool, Acoustic Emission (AE) measurements is a promising technology for reinforced concrete (RC) structures. However, its application on real RC structures is still limited. Due to the lack of knowledge on the crack induced acoustic emission in large scale structures.

The aim of this study is to explore the relationship between the fracture energy and the energy of AE signals at structural level. This serves as a basis for the quantification and localization of cracking activities at structural level.

This study is based on the AE and crack propagation measurement of a series of large scale RC specimen tests. To avoid the influence of existing cracks, the first part of study focuses on the development of the first flexural crack. It was assumed that the amount of energy required for unit length of crack opening is linearly proportional to the energy of the AE signals that are generated upon the opening of this segment of the crack. These signals can only be monitored AE sensors at given locations. By then, they have travelled through the bulk concrete and possibly already existing cracks, thus their energy has attenuated due to the geometric spreading and the damping property of the material. When these effects are taken into account, the total energy of the AE signals that were obtained by the AE sensors at given location (defined as cumulative signal strength CSS) has a potential of reflecting the fracture energy of the corresponding crack. In the part of the study, this process is theoretically studied first. The theoretical result was further validated by the AE measurement obtained from experiments.

Theoretical investigation is carried out based on a simplified model considering only length of the crack and the horizontal distance between crack and AE sensor. Theoretical result shows that both crack length and horizontal distance would affect CSS. However, when the horizontal distance is big enough, crack length is no longer the dominant factor, and CSS drops significantly with the increase of horizontal distance. The CSS of different sensors in a row in the experiments are used to validate this attenuation phenomenon. Exponential curve fitting is carried out to describe the attenuation of experimental results in different tests. Finally, a comparison of attenuation in percentage terms between curve fitting results and theoretical results is carried out. In the uncracked specimens, the results fit each other well.

Furthermore, the effect of the existing cracks to the attenuation of the CSS is studied as well. In that case, a dramatic drop of CSS is observed compared to the uncracked structures.

The study shows CSS detected by AE sensors could partly indicate the cracking behavior of RC structures. The attenuation tendency gives a guidance for sensor installation in future tests.

It has been reported that due to the rapid urbanization and economic growth the municipal solid waste (MSW) would double in volume from 1.3 billion tons per year (in 2012) annually by the end of 2025, challenging environmental and public health management worldwide. Given that, most of the MSW incineration (MSWI) bottom ash (BA) are disposed in landfill currently, and technically and economically viable techniques for the reuse and recycling of MSWI BA is still at a premium. This issue would seriously challenge the environmental and public health management worldwide.
In some European countries and the US, MSWI BA has been utilized as aggregate in pavement construction or as aggregate in concrete. Previous studies also proved the feasibility of using MSWI BA in concrete, either as aggregates or binder substitute materials. However, it is worth noticing that there are several significant drawbacks of using MSWI BA in concrete, including the potential risk of leaching due to the existence of heavy metals and harmful salts, the low reactivity due to high content of quartz and unburned organic matters, and the metallic aluminum-induced expansion.
Therefore, in this study, a characterization of as-received MSWI BA was conducted at the beginning to find out the potential problems when used in concrete, namely the metallic aluminum content, low reactivity and unburned organics. A comprehensive pretreatment was performed subsequently to solve the problems. Specifically, both physical and chemical treatments were carried out to get rid of the metallic aluminum in BA. Afterwards, thermal treatment was conducted to enhance the reactivity of BA and remove the unburned organics. Pre-treated BA samples were characterized again to reveal the effectiveness of pretreatment. The results showed that both chemical and physical treatment were highly effective in removing metallic aluminum. Meanwhile, thermal treatment was proved to be a proper activation method which also removed the remaining organic matters through the high-temperature process.
Subsequently, the investigations of the effects of pre-treated BA addition on compressive strength, reaction products and hydration heat development were conducted on cement paste level by varying the replacement material (BA with different treatment methods) and ratio. A proper method of pretreatment was proposed as well as an optimization of a maximum replacement level of BA in cement paste without detrimentally influence the performance of concrete paste was studied. Compared with nonreactive micronized sand (only works as filler) and pure cement, the addition of physically treated BA has a certain amount of contribution to the hydration process from the viewpoint of heat release. Results show that BA do have pozzolanic activity but is much lower than cement, and physically treated BA is suitable to be used as filler in concrete. Additionally, physically treated BA was further activated through thermal treatment according to the result of compressive strength test, which delivered the highest strength among all the treated BA under the same replacement ratio.
Finally, to extend the application of MSWI BA in concrete, the mix design was made by blending treated BA with the highest compressive strength into concrete and make it suitable for structural application. The effects of treated BA addition on the workability and compressive strength of concrete were investigated. The addition of treated BA brought slight negative impact both in workability and strength due to the existence of nonreactive phases in BA (quartz and organics).
Accordingly, this study proved the potential of BA with proper treatment to be used as a cement substitute material in concrete as well as promoted the understanding of the influence of BA on the hydration process, which also brings the possibility that BA could be widely reused in concrete system in future industry.