Bond and reinforcing bar lap length
BS EN 1992-1-1:2004, Eurocode 2: Design of concrete structures — Part 1-1: General rules and rules for Buildings, clause 8.4.2 Ultimate bond stress categorises bond strength as either “good” or “other cases”. The coefficient related to the quality of the bond condition and the position of the bar which influences lap length and is 1 when the bond is considered to be “good”, otherwise 0.7
But what is “good”? In tops of slabs or deep beams the concrete may be prone to poorer bond to the reinforcement. Fib Model code 2010, clause 18.104.22.168 Basic bond strength, has some advice on what is ‘good’ casting position. If differentiates good and ‘not good’ as those bars where the bond "may be impaired by consolidation of the fluid concrete under the bar” i.e. in regions of possible plastic settlement.
So if a contractor can prove (needs supervision) that measures to minimize plastic settlement are being undertaken e.g. by using a less fluid concrete perhaps or one without high levels of cement replacement and is willing to re-vibrate the concrete to reinstate the compaction and minimize the occurrence of settlement, the designer may be confident to use a shorter lap length.
Basically good concreting practice is required. This is covered in BS EN 13670 Execution of concrete structures but also see Good Concrete Guide 8 Concrete practice.