Its the contractor's responsibility to follow spec according what stated in the engineering drawing in the first place. If they think the drawing is over spec, they have to consult u upfront and suggest their method to reduce cost and than submit to architect which normally architect will accept. If they change the method after the tender process, its not according what agreed. No of steel rods, footing size, beam size, water proofing spec, cement grade ( make sure u know how to monitor what grade their are using) all are important.