Cracks in masonry
No, this is not an article about an additive drug, a quick glib joke or that exposed skin area of the refrigerator repairman's lower back, as he crouches down in front of you. This article is about masonry cracks; the little fissures that appear in walls, ceilings and floors and cause distress in general for many homeowners and real estate purchasers.
To understand cracks, let us understand some basic material properties of typical masonry materials (i.e. bricks, mortar and concrete). In general, these materials are very strong if under compression or forces that push the material together, however they are very weak in tension or forces that pull apart the material. Steel, on the other hand, has similar compressive and tensile strength. As tension is relieved in masonry materials, cracks are formed.
Portland cement is the basic component of concrete that bonds the various ingredients together in a hydration process. Water is required to cause a chemical reaction that ultimately produces the hard stone like final product. This leads us into the cause of one type of cracking, which produces surface cracks in a random pattern that sometimes appears like the skin of an alligator. These types of surface cracks are shrinkage cracks and are commonly seen on the surface of a wall that has been covered with a stucco-like material. During the curing process of the stucco, the water is consumed by the Portland cement. It evaporates from the stucco due to heat produced during the chemical reaction until the stucco dries out and causes tensile forces in the surface of the material. The drying and curing process for stucco is dependent on exposure of the surface to sunlight, ventilation and the relative humidity of the air. The slower and moister the curing process is, fewer and smaller cracks form in the final surface.
The curing process takes a long time but, the first three days are the most critical to minimize the surface cracks. The process of drying of concrete usually is 90% to 95% complete after 28 days. After these 28 days, further minor surface cracks can develop, however usually the stucco surface has been sealed and painted and the minor cracks can be more apparent on the painted surface. Some surface treatment such as plaster and marble dust ( marmolina) will help minimize the appearance of minor cracks in the future after the stucco material has had a long time to cure and dry.
Now cracks that run at an angle away from a corner (i.e. wall corner, window/door corner, etc) may be indicating movement in the wall due to expansion, contraction or movement of a supporting element. In one case, I noted a long angle crack in a masonry wall due to movement of the beam below the wall. The beam was strong enough, however the designer of the wall had not accounted for the natural deflection of the steel beam due to dead load of the masonry wall above the beam combined with other live loads.
The beam was an end-supported cantilevered beam. When designing a structure, the engineer will consider not only ultimate structural strength, but also consider serviceability criteria that account for the deflection of the beam under all anticipated loads and the materials that are being supported. Of course, masonry walls will not tolerate very much movement, which will cause cracks in the wall. Although the beam was strong enough to support the loads, the natural heating and cooling caused some movement in the wall, and simple minor movement caused by people walking through the room resulted in a long crack.
Sometimes the masonry crack is caused by the settlement of a foundation and, in one case I studied, was due to overloading of the structure by the placement of a large gas tank on an area of the roof that did not have sufficient structural strength. The resulting load of the full tank, caused the dramatic cracks in the corner of the room below the tank. The gas tank was emptied and moved to a more appropriate location that was able to support the weight of the full tank.
Cracks sometimes are simply the indication of another related problem that needs to be addressed rather than only patching the cracks. Resolving the root cause will help prevent future cracks in the masonry wall.