Early Determination of the Presence of Low Strength Concrete in Reinforced Concrete Beam-Column Joints Using Piezoceramic-Based Transducers

In high-rise concrete buildings, frame columns are usually made of higher strength concrete than that of horizontal beams or slabs to ensure sufficient axial compression ratio. However, relative lower strength concrete used for casting beams or slabs can migrate into the frame column, causing a sandwich zone, which can significantly reduce the column loading capacity and may create potential safety issues. In this paper, we employed a piezoceramic-based active sensing approach to provide an early determination of the presence of low strength concrete in beam-column joints. In the active sensing approach, a pair of embedded piezoceramic transducers function as stress wave transmitter and receiver, respectively. Due to the different wave attenuation ratios in concrete of different strength, the presence of the low strength concrete can be determined in its early curing period. To enable the experimental study, a sandwiched concrete column specimen, which has two high strength concrete sections and a sandwiched low strength concrete section, was fabricated in the laboratory. In each section, a pair of piezoceramic transducers were embedded. During the 28-day concrete curing period, one transducer in each pair, as a sensor, continuously detected the signal response from the corresponding actuator. Furthermore, the wavelet packet-based energy approach was used to analyze the data. Experimental results show that the stress wave in lower strength concrete has a higher wave attenuation, based on which, the presence of the low strength concrete in a beam-column joint can be determined.