Gst works contract clarification

How GST on Govt works shall be calculated. Building works has items eg. Steel, Cement, bricks, sand, garvel etc besides labour and each item has different GST tax rate. Contractor to whom govt has awarded work procures these individual items after paying GST tax. Works contract treat it as single service and rate is fixed at 12%. It means that 12% again shall be levied on all items combined including labour. Of course input credit is available but it shall be available to contractor to whom work is awarded. there aren't mechanisms by which he will refund to Govt the input credit he has got. How can estimates of particular work be prepared. It works fine for Builder who procures material himself and levies 12% WCT to customer under works contract and then gets ITCredit and paases benefit to customer. But how will Govt contracts be executed . How tax under works contract will be levied. Please Reply