A chain is only as strong as its weakest link. The Titanic, history’s most infamous vessel, may have sank to the bottom of the ocean in part because many of its tiny rivets were made of lower-grade iron. That iron grew weak and brittle in the frigid Atlantic waters, permitting rivets to shear off, resulting in more substantial damage.
Masonry is not exempt from this weak-link concept, and even the broadest roads and longest sidewalks can be undone by the lack of a product people never see, much like the rivets that were likely invisible to passengers in such a glamorous sea-setting
Due to the expansion and contraction of cement in the elements, it is impossible to pour large sections of concrete with the expectation that it won’t fracture. To counter this, seams are included in masonry so breaks can at least be anticipated in a predictable pattern. But to keep moisture and ice from getting in these joints and accelerating damage, sealant must be applied to the joint.
This is where the magic material comes in. Sealant is expensive, and would also form a three-point bond, with both sides of concrete and the ground, damaging the concrete during expansion. To prevent this, bond-breaker material needs to be introduced ensure the seal only between the two sections of concrete. Typically made from thin closed-cell foam tubing, backer rod is squeezed into these joints, filling the gap. The sealant does not stick to the foam, and is able to form a two-point bond across the joint, allowing for expansion and contraction. Backer rod is never seen in finished construction, but is the little part of the build that keeps the whole project afloat.