Would it be all right to mix a load of concrete that is too wet (7-inch slump or wetter) with a dry load of the same mix design to obtain concrete of allowable slump (under 6 inches)?
The specifications would probably not allow it. If the mixing were attempted in the forms by means of vibrators it would not produce a homogeneous mixture unless unusual methods were worked out. It is unlikely that methods could be devised that were adequate because the need to mix the two truckloads would probably be an emergency for which no preparations had been made. To mix the two truckloads adequately would require at least one extra mixer truck into which the other two could discharge and would require mixing for perhaps a minimum of a couple of minutes to do the job adequately. Altogether it seems as though it would be a tremendous amount of work to achieve results that would still be questionable.