[Twelfth progress report]: investigation of prestressed rei-nforced concrete for highway bridges / prepared as a part o-f an investigation conducted by the Engineering Experiment -Station University of Illinois ; in cooperation with the Di-vision of Highway State of Illinois and U.S. Department of -Commerce, Bureau of Public Roads 

 - Urbana, Ill. : University of Illinois 
 - [106] p. 
 - Progress report ; no.12 .