The National Ready Mixed Concrete Association studied non-admixtured concrete to find out whether blending old and freshly batched concrete had harmful effects on setting time or compressive strength.
NRMCA data and results of further tests show that re-use of returned concrete in cool weather, under conditions where delivery and discharge are possible without excessive delay, doesn't significantly affect compressive strength. However, almost all the blended concretes in the study set faster than freshly batched concrete. Thus, blended concrete should be used in applications where setting characteristics are less critical.
Also, take care when developing your own blending procedures. Results will vary with the temperature and with the cement and admixtures used. Take special precautions in hot weather. Finally, on jobs requiring many truckloads of concrete, NRMCA researchers don't advise sending some trucks with blended and some with unblended concretes because the resulting variations in setting time could cause finishing and placement problems.