Lateral Failure in Historic Concrete Opening Headers: Causes & Solutions
The majority of the historic buildings here in Washington DC were built from brick masonry over about 100 years ago at that time, concrete existed and was being used in some cases but it wasn’t really set up yet to be marketed in distributed in a mass production sort of context. The brief history of concrete is really interesting because it was invented and used in large scale thousands of years ago by the Romans but through the Dark Ages was lost together with many different Technologies of the time and then only rediscovered or reinvented more recently about 150 years ago.
So, as a result, many of the brick headers had a lower structural capacity then concrete headers, but they’re more common example follows in the picture below, showing a segmented archway above a window in a historic facade.
Looking at the segmented Archway first is probably a good start to establish a baseline. This type of arch is very common here in historic buildings in Washington dc. Unlike a Roman art or semicircle arch, this shorter Archway allows for a higher degree of fenestration and sash operability in a low ceiling floor and low ceiling floors save money because they’re cheap to build. Many of the very expensive and fancy neighborhoods of Washington DC today were built to be cheap, so workers and horses could live in the building’s to help build the government buildings. Hence, cheaper was better, at the time.
You could even say, to a degree, that the segmented Arch was more efficient or effective because it did allow a greater ratio of ventilation compared to windows with a semicircle or Roman arch.
The next picture below shows the steel reinforced concrete window header. This type of header is actually more common today, even in some types of modern buildings such as CMU block buildings.
CMU block buildings are generally very low cost functional or industrial buildings not necessarily intended for beauty or architectural achievement. In the picture above and the picture below, we showed two different concrete window opening headers. Both have a lateral crack running the length of the header, from side to side. 1.5 in above the bottom of the concrete header and then runs the majority of the length relatively consistently horizontal through the concrete header. Cracks like this often occur in concrete overtime for a few different reasons but one of the main reasons or causes of this type of damage is related to oxidation of the steel or iron reinforcement inside of the concrete.
When metals like steel or iron, which are commonly used as reinforcement within concrete, oxidize, they simply rust. The process of rusting is commonly understood, but it’s an actual chemical reaction that is hard to defend against without special chemical provisions.. Oxidation occurs when ferrous metals are exposed to moisture and oxygen, resulting in formation of iron oxide, or rust. This process can be detrimental to the structural capacity of reinforced concrete because rust expands and as a result begins to move and push the components of concrete apart, as it grows to take more volume than the original metal.
The expansion of rust expands with the application of significant internal pressure on the surrounding concrete. This phenomenon is also referred to as “rust jacking” or “rebar expansion.” Concrete, while strong in compression, is relatively weak in tension, meaning it can easily crack or fracture when subjected to tensile forces. As the rusting steel expands, it generates tensile stress within the concrete. Over time, this stress can cause cracks to form, particularly along the plane where the steel reinforcement is located. These cracks may initially be small, but they can propagate over time, especially as the oxidation process continues and the pressure from the expanding rust increases.
The cracks not only weaken the concrete structurally but also create pathways for more water and oxygen to penetrate, accelerating the corrosion process. This creates a vicious loop where more rust leads to more cracking, which in turn leads to more corrosion. Eventually, this can compromise the load-bearing capacity of the concrete structure, leading to significant structural damage, such as the lateral cracks described in the concrete window headers in the CMU block buildings.
In addition to the mechanical stresses caused by rust expansion, the presence of cracks also exposes the steel reinforcement to more aggressive environmental conditions, like chloride ions in salt-laden environments, which further accelerate the corrosion process. This can lead to localized failures where the concrete will eventually spall or flake off, exposing more of the reinforcement to the elements which exacerbates the problem(s) and hastens further deterioration.
Preventing this type of damage typically involves three main things:
- Building headers so that the concrete is adequately configured with sufficient cover (with concrete) over the reinforcement to protect it from moisture ingress.
- Regular maintenance, including sealing cracks and ensuring proper drainage, can also help prevent water from reaching the reinforcement, thereby reducing the risk of oxidation and subsequent concrete damage.
Additionally, the use of corrosion-resistant materials, such as epoxy-coated or galvanized steel, can mitigate the risk of oxidation.
We can Help
Our company focuses on historic restoration more than modern building upkeep, maintenance, and construction, but our company understands both types of construction very well and a full picture well-rounded approach is needed in any niche in the construction industry. Although we focus on historic restoration, repointing, tuckpointing and historic brick repair, our company also has technical knowledge and competencies in the areas of modern and contemporary construction as well as we become one of the leaders in that area of the market today. Understanding both historic and modern or contemporary construction is useful because both aspects help understand the challenges and potential solutions for challenges in building science and construction.
We can help with a variety of historic masonry restoration needs and upkeep, from modest tuckpointing and or repointing to complicated and extensive historic masonry restoration. Infinity Design Solutions is a historic restoration specialist contractor specializing in both historic masonry restoration such as tuckpointing our repointing, and brick repair. If you have questions about the architectural details or facade of your historic building in Washington DC, reach out and say hello and if we can help we’ll be glad to assist you. You can email us or call us on the telephone at the following link: contact us here.
