Mortar joint cracking is most commonly caused by minor to significant movement of a home’s concrete foundation. The concrete foundation is made of two main parts: the slab and the beam. The slab is the flat part that you walk on, which is about 6 inches in thickness. The beam is located at the perimeter. It’s normally 12 inches wide and 18 inches thick on a single story and 24 inches thick on a multi-level home. The slab and beam are connected as one continuous pour. If you’ve ever driven by a new home construction site and seen form boards around the perimeter with trenches dug around the inside perimeter of the boards, those trenches are the beams. Most new homes also have a grid of beams throughout the center. The slab sits on top of the beams, but everything gets poured together. They are not separate.
The concrete is reinforced with either rebar or post tension cables. If it’s rebar, then a grid of steel rebar is placed throughout before concrete is poured and all intersections are tied together. The purpose of this is to ensure if stress occurs and a fracture in the concrete appears, the concrete cannot pull apart in the future. In order to pull one strand of rebar out, it would require pulling the perpendicular rebar strands sideways through concrete. This cannot occur. This is known as a mechanical bond. Rebar is better, stronger, and unfortunately, more expensive than cables. Therefore, it has fallen out of favor among builders. If post-tension cables are used, which is the most common construction practice in the last 30 years, the cables are still placed in a grid; however, they’re not connected at the intersections. Instead, they are individually stranded across the foundation, and a cap washer with a bolt is placed at the edges. After the concrete cures, the bolt is tightened, creating tension on that particular cable. If you walk around your house and look at the foundation, you will occasionally see a 3 inch circle in the concrete. This is the location of the cap.
Now that you understand how your foundation is constructed, let’s talk about why it moves. No matter where a person is on the earth, the soil, to a particular depth, will be affected by outside weather conditions like a freeze, rain, or a drought. Although each will affect the soil differently, what really matters is that they are either expanding or contracting the soil. (Note: In Texas, the total weather-affected zone is a depth of about 15 feet; however, approximately 95 percent of the expansion and contraction of the soil occurs within the first 4 feet.) As previously mentioned, in a single story home, the perimeter beam is 18 inches deep, and on a multi-level home, the perimeter beam is 24 inches deep. Regardless of how strong the concrete is constructed, it’s still floating within the 4 feet. As the soil dehydrates, gravity brings the elevation of the house down. When the soil rehydrates, the soil begins to struggle to expand because the soil is not accustomed to the weight of the house. The soil tries to expand but can’t until the hydraulic pressure beneath the home is greater than the weight of the home. Once the amount of water in the soil beneath the home reaches a necessary amount, then the pressure of expanding soil becomes greater than the weight of the home, and the home will lift. How much lift depends on how much contraction occurred prior and how much water was available to fuel the expansion. Remember, water has mass and takes up space. Therefore, when it’s available in the soil, the soil will be larger. When the water is not available, the soil will be smaller. Anything sitting on that soil will be affected accordingly.
Lastly, this expansion and contraction never occurs evenly around the circumference of the home. One side of the house may receive more sunlight, another side may have a big tree, or perhaps the rain is running downhill from a neighbor’s house and dumping along one side of the home, etc. The end result is that the home contorts as it rises and falls with the soil. Having a stronger foundation will mitigate the damage but may not eliminate it.
All the energy that’s created as a result of movement, whether up or down, must transfer through the brick and continue an upward journey until it escapes and neutralizes. It will always follow a path of least resistance when available. In brick, the path of least resistance is a straight line. This is why Advantage Masonry installs expansion control joints in brickwork. The expansion control joint is an intended weak spot within the wall. Its job is to crack. Without the expansion control joints, cracks are most commonly seen because the stress energy must find another way out. As a result, a person will often notice cracks near a door or window because the straight line along the door or window acts as expansion control. Somehow, mother nature knows where that window is and will break a zigzag line from the beginning point of the stress in the foundation diagonally upward toward a window, because that stress energy is looking for the straight line along the side of the window. Once it finds the window, the energy simply travels vertically upward along the seam between the brick and the edge of the window until it reaches the top and quietly neutralizes.
When no door or window is located near the area of stress, that stress makes its own expansion control. It will just fracture right through brick and mortar. It has no choice because the laws of nature dictate that this energy finds neutral balance. Since brick and mortar don’t stretch, they must fracture anywhere this energy passes through brick and mortar.
Sometimes for various reasons, the expansion or contraction is so great that the home will require foundation piers to relieve the contortion of the concrete. The stress of re leveling the foundation, via piers, has a similar effect as the soil on the brick. Sometimes cracks close up with piers (although they don’t go away) and other times, they get worse. Either way, once the home is stable, brick and mortar cracks must be repaired to ensure the structural integrity of the home.