Read on to learn the history of masonry paving and walkways
Even just within the historic districts of Washington DC there are many many miles of sidewalks and walkways in our outdoor urban environments. There are many variations in aesthetic design and also best practices to consider when designing and building masonry or concrete walkways. In this week’s IDS blog, we take a closer look at walkways and the materials and methods of their construction.
Concrete was used in United States, en masse, hundreds of years after we first started using kiln fired clay brick. So by all standards, concrete is much newer. However, oddly, in Washington DC some historic preservation boards consider concrete more in keeping with original construction and historic standards than stone masonry. Therefore, stone paving would fit as a third choice, within those options. There are strong arguments on both sides, but our discussion will stay out of that debate on which surface is most in keeping with historic standards, since the guidelines are muddled in this area.
Concrete walkways are much less expensive. The materials are cheaper and the methods of installation are much faster than all types of masonry. By comparison, brick has to be laid unit per unit and therefore requires a higher amount of labor for installation. As well, stone masonry also has a higher cost of installation, and materials are the most expensive of these main 3 options.
The picture below shows an example of a concrete walkway in Washington DC. The concrete walkway blends in relatively well with the adjacent granite stone retaining walls and the adjacent gray painted stone or exposed concrete steps to the front yards of the adjacent houses. The concrete, although foreign in the vernacular of true historic construction, still almost melds appropriately from an aesthetic design perspective in this case.
Concrete performs very well, from a strictly functional perspective. In the case of the walkway shown above, control joints are installed at a 2.5 X 2.5′ (30″ X 30″) square pattern. This control joint layout allows for natural breakage and cracking in the concrete to be controlled so that it follows the general layout of the lines troweled into the concrete at the time of installation. A closer look at the photo below shows that pattern.
One of the biggest potential failure points of walkways are the roots of the beautiful trees which line DC streets. In the picture below you can see a box-out (sometimes referred to as a block-out in concrete) or a niche which was built into the original concrete configuration or layout to allow the tree to have a little bit of space to grow. This box-out is pragmatic and an intelligent part of good planning, but the box-out alone does not prevent paving destabilization from the tree roots.
The picture below shows another example of a box-out or niche built into the layout of the concrete sidewalk; however, in this case the niche has also been dressed with 4 inch X 6 inch wood garden tie to create a somewhat decorative border and a somewhat functional separation of space. Effectively, installation of the wooden garden ties slightly converts this niche or box-out in the concrete to a tree box or planter box. DC refers to these constructs as “tree fences” in the “tree space“. Dogs and pets commonly use the tree areas along outer walkways or any available greenspace in the city as their ideal place to relieve themselves. Some nearby homeowners may see these garden ties as a way to deter animals from taking a bathroom break in the public space near their home. Throughout the city there are numerous interesting designs for planter space demarcation, in the public space. However, homeowners should be aware that the city can require them to remove decorative planter Box borders if the borders do not meet certain configuration requirements. For example borders must be below a height which will conflict with a typical car door swing height and radius, for example. You can read more about some of the rules at the following link to the DC Municipal Regulations section 24-09. However, this reference is not complete, as other standards, rules, and laws apply.
In the case of the image below a large and beautiful maple tree grows slowly year after year and sways in the winds from time to time. As the tree grows and moves the roots grow in size and until the sidewalk finally gives way under pressure and starts to move and pop up above the adjacent areas of the walkway. This terrific maple tree towers over 50 foot in height.
In the photo below you can see a large root from this Maple tree pushes the sidewalk upwards, out the sidewalk’s original planar form.
We talk about the planar form of walkways. But we should distinguish that a walkway is not intended to be flat. They may resemble a flat shape, but walkways should always have some amount of intended unlevelness. They should always have grade or slope to at least a small extent, at minimum. Without angle or slope the walkway will pond water. Ponding water means that water will build-up in puddles and sit in a still position without flowing. If water sits without flowing in a puddle or build-up area, that same area will consistently form an ice slick which is dangerous for pedestrians in the winter. As well, even in the heat of the summer months, build-up areas of ponding lead to concentrated water infiltration through the masonry. Concentrated water infiltration is bad for the masonry because it leads to slow and insidious deterioration.
In the case of the image below you can see that a kiln fired red clay brick alley pathway is built with a swale or valley (the opposite of a berm). In the middle of the pathway. This engineered or planned unlevelness from the outer edge to the center is built at a rate of approximately 3/8 inch of fall per linear foot. That amount is enough for water to flow smoothly and naturally towards the center and then down the alley to the main street where it enters into a gutter inlet box and into the public storm sewer system.
At a bend in the pathway, a central areaway drain is installed to collect the majority of rainwater that runs down through the walkway. This happens to be a large pathway and a typical similar sidewalk would not include a central areaway drain. As an alternative, sidewalks are usually sloped towards the street. That slope, as explained above, allows water to flow without puddling on top of the sidewalk.
This particular sidewalk is in the public space and therefore built by the city of Washington DC or DC Department of Public Works (DPW). Government agencies in Washington DC, when building streets, and pathways associated with the roadways, will often build per a DDOT specification. In this case a modern kiln fired clay brick has been used. The modernness of this brick is an important distinction. This brick is very different from a historic brick. The surface looks similar, but the sides have a integral spacer, monolithic and and integral with the remainder of the brick. That spacer automatically sets bricks at a quarter inch spacing, so the joint will have an opening of roughly 1/4 inch from brick to brick, between the bricks. As well, modern bricks are generally much harder than historic bricks, both significantly lack tensile strength, but modern bricks have a much higher compressive strength. The photo below shows a close up view looking down from above to the surface of the bricks. You can see that since a locust tree is growing nearby, the joints between the bricks have been filled and covered with organic debris from the nearby tree.
Brick joints of this type are not typically installed with a filler such as a polymeric sand or an even more solid material such as grout or mortar. Instead they remain open, filled with loose masonry sand. Where in proximity to a tree, joints of this type will become filled with organic materials, over time, which provides a perfect breeding ground for weeds, baby saplings, and grasses. The picture below shows an area of the brick paving, directly under a tree, with the joints of the brick paving filled with weeds, and other plant growth.
This article just talks about some of the most common types of walkways, but there are a great number of different styles and varieties for walkways and paved pathways. The photo below shows an example of a brick walkway with a motor installed between each brick unit. This walkway happens to be installed in a completely different pattern. The image above shows a running bond in the brick. The photos below, in contrast, show a very unique 5×8 shiner facing basketweave brick bond.
Also, as another example of an alternative in types or styles of walkway construction, the image below shows a wide walkway with intermittent control joint space at about 10 linear feet per joint. By comparison, the sidewalk shown in the photos at the top of this article shows a concrete walkway with control joints at just 30″ of spacing. Both walkways look aesthetically pleasing, for concrete walkways, but they have different functional purposes for the pattern of control joints. The walkway in the photo below has less potential impact from large tree roots and therefore may be potentially safe with greater spacing between control joints.
From a conservation and preservation perspective, several approaches can be taken to improve conditions related to deteriorated historic brick masonry. Primarily, lime mortar brick joints and low temperature fired soft red clay bricks should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually. If brick masonry is kept in good condition, the life of embedded wood elements can be significantly extended. 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:
- Historic districts
- Urban environments
- Kiln fired clay brick
- Historic preservation boards
- Stone masonry
- Aesthetic design
- Control joints
- Paving destabilization
- Tree box
- Planter box
- Tree fences
- Tree space
- Public space
- Configuration requirements
- Maple trees
- Planar form
- Intended unlevelness
- Water ponding
- Concentrated water infiltration
- Valley Fall per linear foot
- Inlet box
- Public storm sewer
- Areaway drain
- Department of Public Works (DPW).
- DDOT specifications
- Historic brick masonry
- Tensile strength
- Compressive strength
- Organic debris
- Polymeric sand
- Masonry sand
- Organic materials
- Shiner shiner facing
- Basketweave brick bond
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.