Freeze thaw is the whole story. Water absorbed into mortar or into a brick face freezes, expands by roughly nine percent, and pushes. Then it thaws and does it again. Chicago runs through this cycle dozens of times every winter, not once. Each cycle is microscopic damage. Across a hundred winters it opens joints, delaminates brick faces and breaks concrete apart. Every spalled brick on a Chicago building was broken by ice, one freeze at a time.

The city is built almost entirely of masonry. After the 1871 fire, Chicago’s building ordinances pushed construction toward brick and stone. The bungalow belt, the two flats, the greystones, the courtyard buildings, the warehouses. That means the majority of the housing stock has exposed masonry taking direct weather on every elevation, and most of it was laid between 1890 and 1940.

The original mortar was lime based and soft. Pre war Chicago masonry was laid with lime mortar, which is flexible and breathable by design. Repointing that wall with modern Portland cement mortar, which is harder than the brick, traps moisture and forces the expansion into the brick faces instead of the joints. This is one of the most common and most damaging mistakes made on Chicago buildings, and it accelerates deterioration rather than stopping it.

Lake effect and wind exposure. Wind driven rain off Lake Michigan hits north and west elevations hard, and those faces almost always deteriorate first. On a corner property or an exposed elevation the difference between the weathered side and the sheltered side is often dramatic.

Road salt attacks concrete. Steps, stoops, walks and driveways take de icing salt every winter, which accelerates surface scaling and corrodes the reinforcement inside. This is why Chicago front steps spall so badly compared with concrete just a few hundred miles south.