As the name suggests, reinforced concrete (RC) is composed of primarily two parts- concrete and reinforcement. Therefore, the deterioration of RC structure is mainly depended upon (a) concrete, (b) reinforcement and (c) physio-chemical interaction between them.
Concrete is mixture of aggregates (sand, gravels) and binder (cement) and some admixtures (mineral or chemicals). And there are many many ways that will initiate deterioration of concrete- most of them are due to chemical reaction that undergoes when cementatious material is exposed to external environment. Meanwhile, concrete also deteriorates due to continuous application of static and dynamic loads e.g. concrete shows time depended properties such as creep and fatigue.
On the other hand, reinforcement (here reinforcement means steel) has its own way of deterioration- the most common one being corrosion. Similar to concrete, steel also has time dependent proprieties such as creep and fatigue.
In broad sense, the deterioration of reinforced concrete thus depends on its chemical constituents the and its time dependent parameters.