Joints, Joints, and more Joints...

The split in CMU, concrete masonry unit, that you see above is a separation joint. Separation joints are used to allow movement within the building. These joints need to be placed every 250' or less in order to meet code. In this scenario, where the joint is near an opening, it is likely that the contractor or architect wants to keep the opening from moving with the other interior wall. The adjacent wall may have more load than wall with the opening, thus, you would want to keep these walls separate when the building starts to move.

Keep in mind, when I talk about movements, these aren't 4' or 5' shifts in the ground. These movements are hopefully unnoticeable to the eye and are only micro-movements within the soil.

To connect the gap in the wall, a backer rod is shoved in the gap no more than half the distance of the width of the gap. IE- if the gap is 2" wide, you would place the baker rod 1" into the gap. The easiest way to explain the backer rod is a miniature "noodle" that you would play with in the pool.

This is a picture of someone placing a backer rod in a joint on the ground. Once the backer rod is placed in the wall, you need to take in mind adhesion and cohesion. Adhesion is the ability for materials to bond to a foreign substance and cohesion is the ability for materials to stick or stay together. You ultimately want a good balance of adhesion and cohesion so the joint won't have problems with cracking. If you have too much adhesion or cohesion the bond will end up breaking or cracking and the joint will be weak.

The last image was taken from - http://www.move.com/home-garden/do-it-yourself/masonry-concrete/replacing-expansion-joints.aspx