Is Rusty Rebar OK for RCC?...How much rust by volume allowed for casting RCC structure as per standards/codes?

In civil engineering construction, we often face this issue of corrosion/rust in reinforcement steel while casting RCC structures due to weather condition. Is this enhance the bonding of steel with the concrete??.. How much rust by volume allowed for casting??.. How can we measure it??.. Can any one brief in detail as per standards/code??