Concrete is a complex building material. Under normal curing conditions, concrete strength shows a nonlinear development process at an early age (1∼28 d). In the first few days after the completion of pouring, the strength of concrete increases slowly. Subsequently, the strength of concrete increases rapidly, reaching about 90% of its age strength. Finally, its strength gradually stabilizes. This paper introduces the experiment of 28-day concrete age strength monitoring based on embedded piezoelectric smart aggregate (SA). Two piezoelectric SAs were embedded in a concrete-filled GFRP (glass fiber reinforced polymer) tube column, one of which emitted a sinusoidal sweep signal and the other SA received the signal. With the hydration reaction of concrete, the stress wave would be significantly different when passing through concrete, and the received signal is changing constantly. Through power spectral density and wavelet packet energy analysis, the monitoring signal of concrete age within 28 days was analyzed. The experimental results show that the wavelet packet energy and power spectral density of the sensor monitoring signal show a nonlinear growth trend with time during the concrete target age. It can be divided into three stages, and the fifth day and the fourteenth day are the demarcation point of energy growth. And the trend of energy change corresponds well with the change of actual concrete strength and age. Comparing and analyzing the received signal energy of the sensor and the power spectral density function of the stress wave signal of the concrete specimen, the trend of the amplitude in the natural frequency domain is found to be the same in the three stages.

中文翻译：

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基于功率谱密度和小波包分析的早期混凝土强度监测

混凝土是一种复杂的建筑材料。在正常养护条件下，混凝土强度在早期（1〜28 d）呈现非线性发展过程。在浇筑完成后的头几天，混凝土的强度缓慢增加。随后，混凝土的强度迅速提高，达到其老化强度的90％。最终，其强度逐渐稳定。本文介绍了基于嵌入式压电智能骨料（SA）的28天混凝土龄期强度监测的实验。将两个压电SA嵌入到混凝土填充的GFRP（玻璃纤维增​​强聚合物）管柱中，其中一个发射正弦波扫描信号，另一个SA接收该信号。随着混凝土的水化反应，穿过混凝土时的应力波会明显不同，并且接收信号不断变化。通过功率谱密度和小波包能量分析，分析了28天内混凝土龄期的监测信号。实验结果表明，在具体目标年龄内，传感器监测信号的小波包能量和功率谱密度呈非线性增长趋势。它可以分为三个阶段，第五天和第十四天是能量增长的分界点。能量变化的趋势与实际混凝土强度和龄期的变化非常吻合。比较并分析了传感器的接收信号能量和混凝土试件应力波信号的功率谱密度函数，发现自然频率域中的振幅趋势在三个阶段中是相同的。