Cobblestone Paving Part II: Style & Strength Combined
Today, we pick up where we left off from last week’s blog article looking at an example of a rough but relatively typical cobblestone paving used on some historic streets, as we often find here in Washington DC. As we touched on this past week, even though this aggregate is filling the joints between the stones, normally it is not set with a cement, but sometimes polymer sands are used to hold these individual pieces together. Here this is a loose type fill between the stones which remains permeable. Different types of masonry paving have different amounts of permeability.
In some cases the paving is made with large intentional voids and or omissions which allow grass or vegetation to grow directly through the stone. Those areas remain permeable and may have permeabilities that are as high as 40%. That 40%, in this case would be compared to a purely grass or unpaved area which would have near 100% permeability. By comparison, Cobblestone like this may only have 5% permeability, but it’s a lot better than a fully cement or concrete paved area which may have almost 0% permeability.
In the next picture below, you can see the area, more closely, where DDOT or a utility company opened the street to access the underground piping. They put the stone back in a quick and rough type of application. DDOT often will tear the street open and then put a temporary patch back together because it takes time to procure and arrange a contractor to make or install the final application of repaired street or walkway areas. This doesn’t just apply to cobblestone, obviously, it also applies to brick sidewalks. Brick sidewalks are common here in Washington DC, even along concrete or asphalt streets as well.
While we are talking about cobblestone, we are most often referring to stone made of granite. It might sound just like every other type of stone and there are several different types of stone here on our planet, but the main types used for building construction fall into a couple of large categories. One of them is granite like this cobblestone, granite happens to be very hard from a compressive strength, yet it’s not entirely that difficult to cut, with zirconium blades and power tools.
Sandstone, by comparison, is also very abundant in common, but it’s a little bit softer, doesn’t have quite the same structural strength or weather and freeze type resistances, yet it’s easy to work with from a masonry construction perspective, and is also abundant.
Like granite though, sandstone is a sedimentary stone. Limestone is another type of common stone, also sedimentary and found in abundant quantities. Limestone is also a little bit more susceptible to aging and deterioration from weathering, but also relatively easy to work with from a masonry construction perspective.
It’s definitely an advantage, a benefit, to using cobblestone because when utility companies need to open the street up, it’s a lot easier for them to do a halfway decent makeshift type repair like this then leaving steel plates in the middle of the street for prolonged periods of time. Also, hot patch type asphalt paving is seasonally limited. It can’t really be applied in the dead middle of our freezing cold winter here in Washington DC. By comparison, cobblestone applications are largely weather independent and don’t have to be applied during decent weather. You could essentially set cobblestone in the middle of a deep freeze and it would be okay from a performance perspective.
Here, below, a particular area is shown in the picture below, where somebody went and installed a cementitious mortar between the stones. It’s different than almost every other location in this particular city block. For whatever reason, the rest of the city block maintains the degree of permeability but this particular patch area was lifted up in reset later. It’s not clear why the particular contractor used a cementitious type mortar but it helps in some ways and hurts in other ways.
It hurts because it detracts from permeability and makes it so that it’s only possible to access the area below this page portion of the street by destructive force, later, in the future. The benefit though is that the cobblestones remain a little bit more rigid. Sometimes the flexibility inherent in sand filled masonry joints can also be a benefit though because it allows for a tolerance to flex instead of breaking. Generally, flexural type resistance is a significant benefit in engineering and resistant type of structural construction. Materials that are too hard are prone to fracture when the universe of forces around them sometimes requires at least micro movement.
Kind of switching subjects but also kind of related, the picture below shows the area of depression. This is a defect or a type of damage that has happened to this limited area of the street. We’re not sure what caused this damage, but it could be either from a concentrated force such as a crane outrigger, maybe used to lift elements for the nearby building construction or the suppression could be related to improper subgrade construction.
Subgrade design and building is extremely important for all types of Paving applications. We don’t really think about this with masonry construction because masonry construction, above grade in walls and building construction is self supporting, at least at the portions above the foundation footing. At the footing, this is extremely important though as well. Here, in the case of paving, there are an assembly of materials that together have a collective compressive strength resistance value. Together, they essentially resist compression from forces above that bears downward.
When looking downward, at the same area, you can sense the overall with and size of the area of depression. It’s roughly 5 ft x 4 ft, overall.
The stones here have no cementitious mortar apply between them, but it’s not actually certain that just a mortar application can really provides the tensile strength required to unify the stones in a way that would resist applying a compressive force to the subgrade below. In other words, putting it in simple terms, it’s not even clear that having a cementitious mortar between these stones would prevent the failure related to a concentrated area of depression.
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.
