SEVERE CORROSION OF REINFORCEMENT IN CHANNEL SLABS OF A ROOF STRUCTURE WAS REPRODUCED BY CYCLIC EXPOSURE OF LABORATORY MOLDED PRISMS AND CYCLINDERS CONTAINING STEEL INSERTS. EARLIER STUDIES USING SMALL CYLINDERS SHOWED THAT THE FIRST INDICATIONS OF CORROSION WOULD BE EVIDENT AFTER CYCLES OF WETTING AND DRYING. SPECIMENS EXPOSED TO DAILY CYCLES INVOLVING DIRECTIONAL EXPOSURE TO CONDENSATION AND SATURATION AT ELEVATED TEMPERATURES SHOW 100 CYCLES ADEQUATE TO DETERMINE THE ABSENCE OF ANY CORROSIVE TENDENCY. WHILE THE PRESENCE OF TWO PERCENT CALCIUM CHLORIDE MAY NOT BE CORROSIVE IN MORE FAVORABLE ENVIRONMENTS, THE RESULTS INDICATE THAT FOR SEVERE EXPOSURE TO MOISTURE AND TEMPERATURE CHANGES THE SAFE PRACTICE IS TO USE MATERIALS KNOWN TO BE FREE OF CHLORIDES. INSERTS IN CONTROL SPECIMENS CONTAINING NO CHLORIDE ACCELERATOR WERE FREE OF CORROSION, WITH EITHER OF TWO TYPES OF LIGHTWEIGHT AGGREGATES, EVEN AFTER 150 CYCLES OF EXPOSURE. /AUTHOR/