Problems with spalled or cracked and broken bricks are prolific in Washington DC and historic areas such as Capitol Hill, Georgetown and Dupont Circle neighborhoods. One of the most common points of broken bricks are at retaining walls and bordering edges of stairways where handrails have been installed. Handrails resist lateral force. Essentially they are installed to create a strong point which someone can grip when walking down the stairs to steady and balance themselves as they walk along inclined walkways or climb up and down stairways. Since they need to support somebody’s weight, they need to be strong. Generally they’re installed in masonry with metal anchors. These locations are problematic though because it is one of the highest frequency areas of broken bricks on historic homes and buildings. The issue occurs both in historic masonry and modern bricks. Generally modern masonry has a higher compressive strength but in tensile resistance, even modern masonry is relatively weak.
The other main reason for failure at mounting locations hunting locations is due to ferrous expansion. Essentially, when ferrous metal such as steel or iron is installed for mounting lags, shields or bolts, when ferrous metal rust it expands in size. That expansion can create significant lateral force on the surrounding masonry. That masonry then will fail and crack as a result of the expanding rusting metal.
In the picture below you can see an example of a security panel mounting clip where it was installed into a modern hollow fascia brick. The brick cracked because it could not resist the expansion force of the rusting metal.
Oxidized ferrous metals can expand up to 3 times the size of the same non rusted ferrous metal. Oxidation can happen as a result of exposure to water and moisture, both in the form of precipitation or even condensation and wicking of adjacent moisture and groundwater. When clay bricks are fired to an extremely high temperature, they become vitreous similar to impermeable porcelain. Their properties to resist moisture increase as they are fired at extremely high temperatures towards the scale of vitreousness. However most bricks are permeable and not vitreous, and therefore they will wick water. Particularly, historic bricks are permeable and will absorb and wick water. Wicking of water is a process where water is drawn into an otherwise dry material. In such circumstances, water can even defy gravity and travel upwards vertically. In cases of groundwater moving upwards through masonry, we call that rising damp.
In the picture below, you can see the remaining cracked bricks at the other side of the security panel. The expansive force of the oxidized ferrous metal was so extreme that it even created a propagation crack which traveled down through the adjacent perpend joint below the mounting area. Mortar joints, whether historic masonry or modern bricks, offer very low tensile resistance. Particularly, historic brick mortars have a lower compressive strength.
In the photo below, you can see the broken bricks, a result of spalling from the expansion of the oxidized fastener.
The image below shows both bearing points in one image side-by-side. You can see that this particular security guard has lost all integral bearing connection to the adjacent masonry wall.
This type of tensile strength failure does not only occur from oxidized ferrous fasteners installed in bricks, it also occurs in materials such as heavy stone work and concrete. In the image below you can see an example where an example where a sandstone historic front porch had a more modern handrail installed and the handrail, mounted with short lags, broke out of the stone stairway.
Handrails at a bearing connection, at the base of the handrail, are in a critical point of lateral resistance. Where a handrail bears against retaining wall or edge supporting element at a stairway, it is generally, when installed in the linear form, weak to resist lateral forces. The joint is at a fulcrum point. A fulcrum point, in a lever provides a movement joint. Handrails are intended to maintain stability and resist movement but in a configuration such as the one below, in the photo, the handrail experiences extreme pressures at the fulcrum point which in this case have caused the fracture of the substrate sandstone stairwell wall.
A closer look at the fastener holes in the photo below shows that they only had an embedment of less than 3″. This shallow embedment created a condition where an intense amount of pressure is applied to a very small area which lacks the size, mass, and tensile strength to support the load.
Across-the-board, consistently, we recommend that in embedments be cordial and set with a non shrink non magnetic cementitious fill at a much deeper mounting depth.
From a conservation and preservation perspective, several approaches can be taken to improve conditions and avoid failures related to metal fasteners in masonry and brick substrates. These problems are prolific in Washington DC but they can be avoided by understanding and implementing proper construction techniques for building with fundamentals of historic preservation and quality control. Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings.
In this article we talked about the following terminology and concepts, follow the links below for more related information from the IDS website:
- Spalling masonry
- Lateral force resistance
- Compressive force resistance
- Tensile force resistance
- Oxidation and ferrous expansion
- Hollow fascia brick
- Vitreous materials
- Msyetial permeability
- Moisture wicking
- Rising damp.
- Propagation crack
- Perpend joint
- Stone work
- Non shrink non magnetic cement
These concepts are part of the fundamentals of historic masonry restoration, tuckpointing, and brick repair.
The links in the list above will take you to other articles with more information on defects, failures, preservation and repair of historic masonry. You can learn a lot more on our blog. Feel free to check it out. If you have questions about the historic masonry of your building in Washington DC, fill out the webform below and drop us a line. We will be in touch if we can help.