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The concrete filled pultrusion-GFRP (Glass Fiber Reinforced Polymer) tubular column (CFGC) is popular in hydraulic structures or regions with poor environmental conditions due to its excellent corrosion resistance. Considering the influence of concrete hydration heat, shrinkage, and creep, debonding may occur in the interface between the GFRP tube and the concrete, which will greatly reduce the cooperation of the GFRP tube and concrete, and will weaken the mechanical property of CFGC. This paper introduces an active monitoring method based on the piezoelectric transducer. In the active sensing approach, the smart aggregate (SA) embedded in the concrete acted as a driver to transmit a modulated stress wave, and the PZT (Lead Zirconate Titanate) patches attached on the outer surface of CFGC serve as sensors to receive signals and transfer them to the computer for saving. Two groups of experiments were designed with the different debonding areas and thicknesses. The artificial damage of CFGC was identified and located by comparing the value of the delay under pulse excitation and the difference of wavelet-based energy under sweep excitation, and the damage indexes were defined based on the wavelet packet energy to quantify the level of the interface damage. The results showed that the debonding damage area of CFGC can be identified effectively through the active monitoring method, and the damage index can accurately reflect the damage level of the interface of GFRP tube and concrete. Therefore, this method can be used to identify and evaluate the interface debonding of CFGC in real time. In addition, if the method can be combined with remote sensing technology, it can be used as a real-time remote sensing monitoring technology to provide a solution for interface health monitoring of CFGC.

No abstract

The lifetime of hollow section tubular joints frequently can be shortened owing to the occurrence of the welded cracks and the plastic deformation of chords under the cyclic loading, because of the deficient radial bearing capacity of the steel tube. To avoid such failures, this paper proposes a novel method to strengthen the chord with double plates at the intersection of the chord and braces. To further investigate the efficiency of this strengthening method on hysteretic performance and energy depletion ability of the overlapped K-joints with hollow sections, two unreinforced K-joints and two reinforced K-joints were fabricated. By loading on the braces with collaborative cyclic loading, the joints failure modes, hysteresis curve, and skeleton curve were obtained. The bearing capacity, ductility, and energy depletion of the joints were assessed and the restoring force model of joints was proposed. The results show that the failure mode of the unreinforced joint is the plastic failure of the surface of the chord. For the K-RC1 (double-plate reinforced square hollow section tubular K-joints), cracks appeared at the junction weld between the through brace and the overlapped brace. However, cracks extended along the weld at the intersection of the chord and the through brace for K-CC1 (double-plate reinforced circular hollow section tubular K-joints). There is no obvious deformation on the chord surface of reinforced joints. Experimental results reveal that the mechanical properties of the joints can be improved effectively by such reinforcement measures and that the plastic deformation of the chord can also be restrained. Meanwhile, the reinforcement measures demonstrate the ability to avoid the risk of large stress concentration of the chord in the area where the braces and chords are intersected. The bearing capacity of the joint was increased; however, the ductility of the joint was weakened.

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.