▁bar ▁supports ▁▁ ▁Pl astic ▁spac ers ▁and ▁bar ▁supports ▁do ▁not ▁bond ▁well ▁with ▁concrete , ▁therefore ▁they ▁are ▁NOT ▁compatible ▁materials . ▁Much ▁of ▁this ▁compatibility ▁is ▁explained ▁through ▁a ▁single ▁phrase ▁: ▁Pl astic ▁has ▁mechanical ▁( holds ▁the ▁bar ▁in ▁position ) ▁but ▁NO ▁struct ural ▁properties ▁( always ▁will ▁remain ▁a ▁foreign ▁element ▁within ▁the ▁construction ). ▁But ▁why ▁is ▁that ? ▁Well ▁here ▁is ▁a ▁detailed ▁explanation ▁of ▁this ▁aff irm ation : ▁ ▁When ▁the ▁concrete ▁is ▁pou red ▁into ▁the ▁form , ▁a ▁small ▁gap ▁is ▁created ▁between ▁it ▁and ▁the ▁pl astic . ▁Pl astic ▁has ▁a ▁coefficient ▁of ▁thermal ▁expansion ▁and ▁contra ction ▁ 1 0 ▁to ▁ 1 5 ▁times ▁that ▁of ▁concrete , ▁and ▁when ▁subject ed ▁to ▁temperature ▁variations , ▁the ▁pl astic ▁continues ▁to ▁expand ▁and ▁contract ▁at ▁that ▁higher ▁coefficient . ▁At ▁high ▁temper atures , ▁pl astic ▁can ▁even ▁m elt . ▁The ▁result ▁is ▁a ▁separation ▁between ▁the ▁spac ers ▁and ▁the ▁pou red ▁concrete , ▁creating ▁a ▁free ▁passage ▁for ▁cor ros ive ▁agents ▁to ▁reach ▁the ▁steel ▁rein for cement ▁from ▁the ▁exterior ▁of ▁the ▁concrete ▁product . ▁This ▁will ▁cause ▁the ▁steel ▁– ▁and ▁eventually ▁the ▁concrete ▁– ▁to ▁cor ro de . ▁▁ ▁If ▁steam ▁cur ing ▁is ▁applied ▁to ▁the ▁concrete , ▁the ▁heat ▁in ▁the ▁cur ing ▁process ▁causes ▁the ▁pl astic ▁to ▁expand ▁while ▁the ▁concrete ▁is ▁relatively ▁fresh ▁and ▁weak . ▁After ▁reaching ▁the ▁maximum ▁cur ing ▁temperature