Half-joints in concrete bridges are known to exhibit an increased rate of degradation. Their main vulnerability is situated in the re-entrant corner, buried deep inside the joint. This makes visual inspection nearly impossible. A Structural Health Monitoring (SHM) system offers a promising alternative, but interpretation of the half-joint status from SHM data is not straightforward. The study presented in this paper is a case study of the SHM system on the Naardertrekvaart bridge in the Netherlands. Analysis of SHM data revealed a dependence of the movement of the bridge on a seasonal temperature cycle, presumably caused by hindered thermal contraction of the half-joints. This phenomenon offered no reliable estimation of the half-joint status. In addition, from the movement of the lower half-joint nibs under traffic loads, a stiffness parameter was devised, used as an estimation of the half-joint status. The study indicated that a high-frequency approach can increase effectiveness of the SHM system.