Use of electrical impedance spectroscopy for online monitoring of concrete compaction

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Insinööritieteiden korkeakoulu | Master's thesis
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Master's Programme in Building Technology (CIV)
75 + 4
Compaction of concrete is a crucial on-site step that enables concrete to reach design strength and density. This process is generally carried out using vibrators that pack the aggregates together and expel the entrapped air from the concrete element. Proper compaction ensures that all the entrapped air escapes the concrete without causing segregation. Based on the duration of vibration concrete can be under-vibrated, well-vibrated, or over-vibrated. Under-vibrated concrete causes many durability problems, such as: honey-combing, cold joints, and subsidence cracking. On the other hand, over-vibration of concrete leads to segregation and probable failure of the concrete element. Typically, compaction is subjectively monitored by personnel on site, which is insufficient to obtain a well-vibrated concrete. This thesis investigates the use of Electrical Impedance Spectroscopy (EIS) to monitor concrete compaction during vibration. Concrete can be divided into a resistive part and a conductive part, where the first is the cement paste and water-filled voids, while the latter is the air voids as well as the aggregates. An electrode panel was designed to investigate top, middle, and bottom layers of tested concretes. This panel was fitted inside a plexiglass mould where seven samples of the same concrete mix were tested. Three vibration dura-tions were chosen to obtain under-, over-, and well-vibrated concretes. Alternating current was sent at different frequencies using an impedance analyzer, and the resistance of each layer was obtained through analyzing the measured reactance and resistance of concrete. Followingly, three cores were drilled form each hardened concrete sample and the densities of these cores were statically analyzed to check for segregation. Additionally, vertical sections were cut to visually inspect the distribution of aggregates and air voids inside the concrete. Analysis of the data showed that segregation could be detected by studying the variance of the resistance values for different layers during compaction. The top and middle layer showed a significant resistance decrease, whereas an increase was noted in the bottom layer. This change was linked to the observed segregation in the cut vertical segments, where the bottom layer had a higher amount of accumulated aggregates compared with the other two layers. Additionally, statistical analysis was carried out on the drilled cores densities, which showed a large variance in values denoting segregation. Further analysis was carried out comparing the difference between the resistance of top and bottom layers over the vibration duration. It was found that all segregated samples passed a limit of dif-ference in those EIS obtained resistances compared with the unsegregated sample.
Punkki, Jouni
Thesis advisor
Ojala, Teemu
concrete, vibration, compaction, EIS, monitoring, segregation
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