The behavior of reinforced-concrete structural elements under moving loads involves many complex parameters related to material properties, vehicle loading, environmental changes and structural response. It is difficult to generate the real conditions of loading when performing laboratory tests or numerical simulations. Due to these difficulties, many uncertainties and non-conclusive results prevail. This paper presents a fatigue-monitoring concept to evaluate the cracking activity of a reinforced-concrete slab of an existing road viaduct under moving loads. Both steel rebars and concrete of the deck slab were instrumented and are being monitored continuously. The monitoring system comprises strain gauges to measure the strain in the rebars and acoustic emission (AE) system to evaluate the AE activity of the concrete under traffic loading. Parametric study of AE features and b-value analysis were performed to quantify six weeks of acoustic emission data. Results show that the AE activity is due to the opening and closing of microcracks. The monitoring system indicates successfully on-site the activity of microcracks in concrete when subjected to stress reversals due to passing vehicles.