Compactness Monitoring of Compound Concrete Filled with Demolished Concrete Lumps Using PZT-Based Smart Aggregates

Concrete recycling is an important element of sustainable development. Demolished concrete lumps (DCLs) can be blended with fresh concrete as a filler, but achieving good compactness of the compound concrete can be a problem. An effective compactness-monitoring method for DCL compound concrete is needed urgently. In this study, a simple and rugged lead zirconate titanate (PZT)-based smart aggregate (SA) for monitoring compactness using transient excitation was tested in the laboratory. The same type of sensor was used as both the excitation and the receiver transducer. The level of compactness was measured by directly propagating longitudinal stress waves in a specimen of concrete $600\times 600\times \mathrm{1,000}\mathrm{mm}$. Twenty-four transducers were deployed at four heights to create 12 parallel monitoring paths. The excitation signal is a smoothed tone burst with an amplitude of 90 V and frequency of 70 kHz. Reliabilities of velocity, amplitude, and central frequency of the received stress waves as the compactness-monitoring index were evaluated by comparing with core sample tests. The technique demonstrated great potential in monitoring the compactness of DCL compound concrete.