自密实混凝土（SCC）是由水泥、集料、水（可含外加剂）和粉煤灰等掺合料按照一定比例搅拌而成的多相复合材料，在浆体凝结硬化前，能够依靠自重作用流动并充满模板空间。但是SCC的凝结硬化过程受到粉煤灰掺量的显著影响，粉煤灰掺量不仅影响SCC的填充性能和早期水化特性，还影响其强度的发展。机电阻抗法（EMI）是一种灵敏度高、应用广泛的无损检测方法，能够通过压电陶瓷（PZT）阻抗特性间接分析结构固有属性的变化。本文通过智能骨料对不同粉煤灰掺量的SCC在凝结硬化过程的电导信号进行了监测，研究了粉煤灰掺量对SCC早期水化特性和强度特性的影响。主要研究内容如下：

（1）不同封装材料对PZT压电性能和监测灵敏度的影响。本文制作了水泥浆、环氧树脂和大理石封装层智能骨料，通过分析内部PZT在封装前后电阻、电容的变化量和混凝土水化期间电导信号的峰值频率偏移量（∆f），对其压电性能和监测灵敏度进行了研究。结果表明：环氧树脂封装层对PZT的电阻和电容影响最小，水化过程的电导信号峰值频率偏移量最大，相应的环氧树脂封装层智能骨料监测灵敏度最高。

（2）基于机电阻抗法研究了不同粉煤灰掺量的SCC在凝结过程的水化特性。本文制备了不同粉煤灰掺量的SCC试块，利用智能骨料对凝结过程（0~24 h）的电导信号进行了监测。通过电导信号共振频率、幅值和相关系数（CC）随龄期的变化规律，对SCC的早期水化特性进行了研究，并与贯入阻力试验结果进行了对比。结果表明：机电阻抗法可准确捕捉不同粉煤灰掺量的SCC初凝和终凝状态，且随着粉煤灰掺量的增加，SCC的初凝和终凝时间均有所延长。

（3）基于机电阻抗法研究了不同粉煤灰掺量的SCC在硬化过程的强度发展和加载过程的刚度退化。利用智能骨料对硬化过程（1~28 d）和加载过程的电导信号进行监测，通过谐振频移指数（RFS）、均方根偏差值（RMSD）和1-CC等量化指标随龄期的变化规律，对SCC试块在硬化过程的强度发展和加载过程的刚度退化进行了研究。结果表明：随着粉煤灰掺量的增加，SCC的强度先增加后减小。上述量化指标与SCC早期强度的发展趋势基本一致，与强度值有很好的相关性。在单轴压缩荷载作用下，电导信号变化与混凝土试块刚度退化过程基本相符，可以反映混凝土试块的刚度退化。

Self-compacting concrete (SCC) is a multi-phase composite material made of cement, aggregates, water (which can contain admixtures) and fly ash and other admixtures mixed in a certain proportion, which can flow and fill the formwork space by self-weight before the slurry sets and hardens. However, the setting and hardening process of SCC is significantly influenced by the amount of fly ash admixture, which affects not only the filling properties and early hydration characteristics of SCC, but also the development of its strength. Electromechanical impedance (EMI) is a sensitive and widely used non-destructive testing method that can indirectly analyse changes in the inherent properties of a structure through the impedance characteristics of piezoelectric ceramics (PZT). In this paper, the influence of fly ash dosing on the early hydration properties and strength properties of SCC was investigated by monitoring the conductivity signal of SCC with different fly ash dosing during the setting and hardening process by means of smart aggregates. The main studies are as follows.

(1) The effect of different encapsulation materials on the piezoelectric properties and monitoring sensitivity of PZT. In this paper, smart aggregates of cement paste, epoxy resin and marble encapsulation layers were produced, and the piezoelectric properties and monitoring sensitivity of the internal PZT were investigated by analysing the amount of change in resistance and capacitance before and after encapsulation and the peak frequency shift of the conductivity signal (∆f) during the hydration of the concrete. The results show that the epoxy encapsulation layer has the least effect on the resistance and capacitance of PZT, the peak frequency shift of the conductivity signal during hydration is the largest, and the corresponding smart aggregate monitoring sensitivity of the epoxy encapsulation layer is the highest.

(2) The hydration characteristics of SCC with different fly ash admixtures in the setting process were investigated based on the electromechanical impedance method. In this paper, SCC specimens with different fly ash admixtures were prepared and the conductivity signals of the setting process (0~24 h) were monitored using smart aggregates. The early hydration characteristics of SCC were investigated by the variation pattern of resonance frequency, amplitude and correlation coefficient (CC) of conductivity signal with age and compared with the results of penetration resistance test. The results show that the electromechanical impedance method can accurately capture the initial and final setting states of SCC with different fly ash admixtures, and the initial and final setting times of SCC are prolonged with the increase of fly ash admixture.

(3) The strength development and stiffness degradation during the hardening process and loading process of SCC with different fly ash admixtures were investigated based on the electromechanical impedance method. The conductivity signals of the hardening process (1~28 d) and the loading process were monitored using smart aggregates, and the strength development of SCC specimens during the hardening process and the stiffness degradation during the loading process were investigated by the variation patterns of quantitative indicators such as resonance frequency shift index (RFS), root mean square deviation value (RMSD) and 1-CC with age. The results show that the strength of SCC increases and then decreases with the increase of fly ash admixture. The above quantitative indicators are basically consistent with the development trend of the early strength of SCC and correlate well with the strength values. Under uniaxial compression load, the change of conductivity signal is basically consistent with the degradation process of concrete specimen stiffness, which can reflect the degradation of concrete specimen stiffness.

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