A non-destructive evaluation technique using piezoceramic (PZT) as an actuator-sensor has an ability to efficiently detect structural damage. In this technique, a PZT actuator-sensor patch is bonded on a structure. Through the measurement of its electrical impedance, which is related to mechanical impedance of the structure being bonded, the change in structure properties due to damage can be detected. This paper presents the use of PZT in structural health monitoring to quantitatively detect damage of bolted joints. The structure used in this study consists of two aluminum beams connected by a bolted joint. The damage is simulated by loosening of the bolts. To quantitatively monitor the damage, a numerical model of the structure is formulated. Spectral element method (SEM) based on wave propagation approach is used to model the structure. A bonded-PZT beam and a bolted joint element are developed by using SEM. The equations of motion are derived by using Hamilton's principle subsequently, the spectral element matrices are formulated. Experimental results show the ability of this method to detect the damage. By using the proposed model, the loosening of bolts can be quantitatively identified as the change in stiffness and damping at the bolted joint. Therefore, this method has high potential to quantitatively monitor damage of bolted joints.