Whenever grouting stops, such as when masons have breaks during the day, a cold joint will be created. Doesn't this form a weak plane in the wall?

The Uniform Building Code (UBC) in section 2104.5.1 requires that when a cold joint is to be formed, grout must stop a minimum of 11/2 inches below a mortar joint. This forms a key that makes the shear strength at the cold joint comparable to that of the rest of the grouted cavity. The bending strength is not affected because the steel reinforcement is continuous through the cold joint.

The UBC also requires that the grouting of any section of wall be completed in one day with no interruptions greater than 1 hour. Therefore, the grouting of a section of wall should be scheduled so that it doesn't stop for longer than an hour. If the work is stopped for longer, the grout must be stopped below the mortar joint to form a shear key.