[plt-scheme] Require and a path

I have expressions like:

(require "string-utils.scm")

Is there a way to use a full or relative path here? I have tried 
(require (build-path 'up "string-utils.scm")) and 
(require (file (build-path 'up "string-utils.scm"))) etc. 
but it all fails with "bad module path ..."


Thanks,

Alex