The RadProgressBar is designed to track a process by manually updating the value of the progress bar on the client when the state of the process changes. This means that the progress bar control does not automatically track the process, so when you set a value for the control, it directly displays it.
To demonstrate a scenario, in which the progress bar is utilized I attached a simple example that uses a progress bar to track changes to a server-side integer variable and communicate back the current value. This is a simulation of a task progress. Please, note that the progress bar should be updated on the client-side using the set_value method (this is why your script did not update the progress bar, you need to use the client-side API of the control for this purpose).
Also, for every new run of the provided sample page, you need to reset IIS, because the tracking integer is static and will not be reset until the web application gets restarted.
If automatic updating of the progress is what you are after, you can use RadProgressArea instead. I would suggest checking the following comparison between RadProgressBar and RadProgressArea in order to choose the control that best suits your requirements: http://www.telerik.com/help/aspnet-ajax/progress-bar-vs-progress-area.html