Last week we posted a blog article taking a look, and overview, of masonry paving. This week we continue with the same theme, discussing additional details and types of masonry paving. The streets, lots, and walkways of Washington DC are full of a mixed tapestry of various types of masonry paving. Some range from purely modern to true historic types of construction.
The picture below shows an example of a cobblestone paving in the walkway adjacent to an exposed concrete sidewalk. Cobblestone is commonly used for historic walkways in Washington DC. It’s one of the only materials that is true to historic times prior to the 20th century.
In the picture above, the area to the left of the granite cobblestone walkway is a exposed aggregate concrete sidewalk. The image below shows a close up view of a plumbing cleanout set into the concrete walkway. Cleanouts and access portals are commonly set into concrete flat work so that utilities and services can be accessed underground. Access points may be used for valves for plumbing supply or gas supply pipes and or meters for services entering a property or building. Generally shut-off valves and meters are set outside of the building so they can be controlled or accessed by public authorities without necessarily having to interrupt building facility usage. In the image below you can see the rough texture of the exposed aggregate. This rough texture helps to prevent foot traffic slippage during precipitation or icing.
The image below shows a close-up view of an asphalt parking surface. Asphalt is extremely common and used throughout the vast majority of the United states. This particular picture shows a worn surface where the aggregate within the asphalt mixture is visible. The surface has faded to an extent and worn which exposes the top of the embedded gravel material.
The visual similarity between the exposed aggregate concrete and the asphalt is interesting, in comparison, you can see areas of similarities and areas of contrast. For example, the aggregate used in the asphalt has a greater variation in the asphalt than in the concrete. This particular type of concrete is intended to have exposed aggregate and in the case of the asphalt, it’s just an undesired side effect of wear and deterioration. In the case of the concrete,the aggregate was specifically selected to have a relatively low amount of variation.
An area of asphalt gravel follows below for reference and comparison. The stone shown below is comprised of relatively larger crushed stone, larger than the typical crusher run or common #57 stone. Stones such as the larger stone shown below can be possibly used as an alternative to paving. In some cases, for temporary use or for heavy trucks, the stone covering on top of a raw earth substrate is better and more practical than exposed earth. Exposed earth, can become muddy and difficult for maneuvering and driving vehicles when hydrated through precipitation.Exposed earth surfaces, can also create enormous amounts of dust when used as a traffic surface when dry.To avoid the challenge of managing the moisture of a dirt traffic surface, it’s easier, in many cases, to cover the surface with the gravel such as the one shown below.Actual paving is an even better alternative, but paving is much more expensive than a loose gravel surface.
In contrast to the asphalt and exposed aggregate concrete paving areas shown in the pictures above, the picture below shows an example of a broom finish concrete walkway. This area is a combination of a walkway and a garage apron. The garage apron leads into a driving path which goes into the garage.This concrete, a broom finish, has a thin texture which helps to reduce slippage of foot traffic and vehicle traffic as well.
The painted stripes happened to be utility location markings as required before excavation. In this case the yellow markings indicate gas or similar type of piping below grade and the red markings indicate an electrical feed or similar utilities below grade.
The picture below shows a closeup view of one of the control joints in the sidewalk shown above. The picture below though, by comparison shows that this line, unlike the expansion joints, is just a control joint and therefore very thin. This line is not cut into the concrete when the concrete is still wet. Instead, this line is cut into the concrete while the concrete is hard yet not fully hardened, ”green”, and yet remains not yet cured to design strength, yet hard enough to allow the blade to cut without dislodging adjacent aggregate.
Although concrete and masonry paving is built from hard and heavy materials, these materials actually move a tiny bit and provisions need to be built into the overall construct to allow for a little bit of movement over time period building movement for construction movement, even with hard and durable materials of this type happen for a few reasons some of those reasons follow in the list below
- Thermal expansion and contraction
- Slight movements in the substrate
- Vibration from heavy live loads such as vehicles or even foot traffic
- Moisture induced movement, from material swelling or constriction
The picture below is an example of a thick expansion joint adjacent to a thin control joint. The control joint is just a quarter inch in width. By comparison though the expansion joint is much thicker,over 1 inch in width. The expansion joint is created by installation of a homasote or similar expansion board.Then on top, as you can see in the picture below an elastomeric gray sealant is installed on top of the expansion joint. The color of the elastomeric sealant is determined by the architect, but in this case it happens to be a gray color which is possibly a good balance between the color of the concrete and the adjacent granite stone. The granite stone, for reference, is shown in the top left corner of the image below.
For many years, in Washington DC, there have been incentives to increase the percentage of permeable paving on properties, so architects and engineers are always looking for ways to increase the amount of permeable paver space. It’s complicated sometimes because permeable pavers increase groundwater and therefore increase hydrostatic pressure against building foundations which can be problematic without proper systems to divert groundwater. The picture below shows an example of a composite decking material installed in plane with the adjacent concrete sidewalk. It’s not a common type of installation but it’s interesting because the deckboard system has a relatively high amount of permeability considering the spacing between adjacent boards. This type of building assembly is an interesting alternative to typical masonry paving. But installing deck boards at grade will not work in many cases. Generally deck boards require a wood frame. Wood frames are susceptible to groundwater exposure in direct contact through wicking which makes installation of a deck board system problematic at grade. This particular incident happens to be an exception because in this rare case this installation is part of a greater site plan which includes drainage systems directly under the deck board installation.
The image below shows a classical style clay brick installed on a stairwell landing in a herringbone pattern, built by Infinity Design Solutions, IDS, This photo shows the process with work still ongoing in the middle of construction. Later, after the original herringbone pattern is set, the brickwork will be grouted and pointed with appropriate mortar.
Another area of paving, also built by Infinity Design Solutions, IDS, follows with brick and a herringbone pattern. This brick also has an edging set in a shiner position at the sides and in a sailor position at the top and bottom, perpendicular to the herringbone feature.
The next picture below shows the same walkway with the public walkway in the background. In this picture you can see the comparison between these two types of brick. They’re not exactly the same but you can see that there is significant consistency between the aesthetic of the public walkway and the aesthetic of this private walkway. Both are kiln fired clay bricks, similar to a historic sewer brick. However, the public walkway is set without mortar. The lack of a concrete base and lack of mortar between the joints creates higher susceptibility to movement but allows for a degree of permeability which is better for the environment overall.
The image below shows a flagstone or bluestone patio with a consistent, not rectified but plant cut, type of stone tile, set on a running bond. Unlike kiln fired ceramic tiles these stones are not actually rectified in the same process. Ceramic tile, when run through a rectification process, are cut to a level of consistency that there is almost no variation or no significant variation in tile shape and sizes. That superior consistency allows for relatively tight grout lines. With natural stone such as these thick masonry units, larger grout lines are required to accommodate slight variations in tile shape and sizes.
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:
- Shut-off valves
- Foot traffic slippage
- Control joint
- Design strength
- Expansion joints
- Elastomeric sealants
- Permeable paving
- Ground water
- Hydrostatic pressure
- Drainage systems
- Kiln fired ceramic tile
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.