Research On Piezoelectric Smart Aggregate Monitoring For Frost Resistance Of Concrete-Filled Steel Tubes

In order to study the frost resistance mechanism and deterioration process of concrete-filled steel tube,the whole process of concrete-filled steel tube freeze-thaw damage was monitored by piezoelectric smart aggregate,and the effectiveness of this method was verified by the experiment of mechanics.To explore the validity of the test for the freeze-thaw damage of concrete-filled steel tube,first from the simple prism concrete specimen,two piezoelectric smart aggregates are embedded inside the structure,and the damage signal under the freezing and thawing state is compared with the reference signal under the condition of water-saturated,and the law of the stress wave propagation is studied.(1)For the detection of freeze-thaw damage on concrete,the energy analysis and root mean square analysis are adopted to process signal by acuating low frequency signal,which is found that both of them shows the two stage of development.Compared with the traditional relative dynamic modulus test method,it is indicated that the number of freeze-thaw cycles corresponding to the turning point of two stages is almost identical to the number when the relative dynamic modulus decreases to 60%.The explanation is provided from the mechanism of freeze-thaw damage,which gives a basis for the detection of freeze-thaw of concrete-filled steel tube.(2)For the detection of freeze-thaw damage on concrete-filled steel tube,the five cycle sinusoidal modulation signal of 50 KHz is finally selected by manually scanning the frequency.Wavelet packet energy analysis is carried out for signals in different freeze-thaw state,and the damage index based on wavelet packet energy and wavelet packet energy spectrum are established.The analysis results are basically in the first development stage of freeze-thaw damage,which indicates the good frost resistance of concrete-filled steel tubes.In addition,the total propagation time of the first wave of the signal is extracted,and it is found that the total propagation time is less sensitive to the freezing thawing damage than the migration of water in the structure.(3)In the light of the pull-out tests on interface of the concrete-filled steel tube after freezing and thawing,the load slip curve of load side concrete and the longitudinal and transverse strain of steel tube surface are taken as the breakthrough point,then the ultimate failure load,ultimate shear strength,ultimate failure bond slip of the interface of concrete-filled steel tube and interfacial bond stress under different freeze-thaw cycles are obtained.It is indicated that the ultimate failure load and ultimate shear strength are basically unaffected during the first 160 freeze-thaw cycles and then decrease until 240 freeze-thaw cycles.Bond failure slip at the loading end of concrete maintain within 2mm.The change pace is basically consistent with the ultimate bond strength.After a certain number of freeze-thaw cycles,the longitudinal strain and lateral strain begin to fluctuate and the continuity of strain is also destroyed due to the occurrence of partial slip.