This past week, we had an opportunity to take a look at a modern type of simple residential concrete foundation for a new home. Although the principles and layout of this particular foundation are relatively simple and straightforward, it’s not something we often get to see right here in Washington DC and this particular building is actually out in the suburbs but it provides a lot of good visual information for us to discuss and for our readers to use to learn about the same principles that apply to buildings here in Washington DC. In last week’s blog, we particularly discussed some of the principles related to building layout on site, site grading, drainage as affected by the site grade, and soil composition and soil mechanics.
Today we are looking at some of the structural conditions of the building foundation. The choice of foundation type is influenced by several factors, including soil conditions, structural loads, environmental considerations, and site constraints which can be particularly important here in Washington DC. Shallow foundations, such as spread footings and mat foundations, are suitable for structures with relatively light loads and good soil conditions. Spread footings are individual, isolated foundations that transfer the load from columns or walls to the soil, while mat foundations are continuous, reinforced concrete slabs that distribute the load over a larger area.
Deep foundations, for the larger buildings often found in the DC CBD, on the other hand, are designed to transfer loads to deeper, more competent soil strata or bedrock. Piles and caissons are examples of deep foundations, which are suitable for structures with heavy loads or poor soil conditions at shallow depths. Piles are long, slender structural elements that are driven or cast into the ground, while caissons are large-diameter, cast-in-place concrete shafts that extend deep into the soil or bedrock. This particular building was built with a strip footing, similar to a spread footing but without a taper or significant bevel.
Concrete is a pretty versatile and highly durable construction material, widely used, today, in foundation construction. Concrete is mixed to combine cement, aggregates such as sand and gravel (just like ice cream with toppings added and mixed throughout), water, and admixtures (if required) in specific proportions to achieve the desired strength, workability, and durability. Concrete subcomponent proportions are specifically designed for the particular use. Often this detail will be engineered prior to construction and then checked on site at the time of the pouring of the concrete to verify the details of the mixed specification have been followed appropriately.
The quality of the concrete mix is needed for the long-term performance of the foundation. Factors such as the water-cement ratio, aggregate gradation, and the use of admixtures (e.g., plasticizers, air-entraining agents) can significantly influence the concrete’s properties, including its strength, workability, and resistance to environmental conditions.
Concrete placement in itself, has details that effect the quality of foundation construction. Pumping is a common method for placing concrete in congested or hard-to-reach areas, while chuting and bucket placement are suitable for more accessible locations. Pumping is generally a bit wasteful though because the pump itself will need to have a initial slurry run through the machine before can flow and discharge a production level of concrete. The initial concrete will often be over-hydrated to prime the pump and then discarded and disposed of which creates waste. A similar process is repeated at the end when the last of the concrete is flushed from the machine. Proper consolidation of the concrete using mechanical vibrators or other means is needed to create a dense and uniform mixture, free from air pockets or voids. As the price of construction materials have spiked since about the year 2020, wastefulness is avoided more now, throughout the industry, where possible.
Curing is the process of controlling the moisture content and temperature of freshly placed concrete to ensure it achieves its desired strength and durability. Proper curing methods, such as moisture curing or the application of curing compounds, are needed to prevent premature drying and cracking of the concrete. In some cases, slabs on grade will be covered with plastic sheeting for the first day or two after the concrete has been placed on site. The plastic tarps may be lifted to spray the concrete with water a few times during the first days of curing to deter overly rapid curing.
Often, a variety of tests are performed throughout the concreting process to evaluate the quality and performance of the concrete. Slump tests assess the hydration, workability, and consistency of the concrete mix. Compressive strength tests are later performed on hardened concrete samples (often in the form of cylinders) to ensure the required strength is achieved. Test to help me taken at a specific number of days after the concrete is poured.
Waterproofing measures are needed to protect the foundation from moisture penetration and the potential consequences of water damage, such as corrosion of reinforcement, deterioration of concrete, and soil erosion or settlement. In a future blog this coming week, we will talk about waterproofing techniques that are available, including the use of membranes, coatings, or integral waterproofing admixtures in the concrete mix.
Adequate drainage systems are equally important in foundation construction. These systems are designed to divert water away from the foundation and prevent soil erosion or saturation, which can compromise the stability of the structure. Drainage systems may include surface drains, french drains, or subsurface drainage pipes, depending on the site conditions and the overall drainage strategy.
Proper maintenance and regular inspections of the waterproofing and drainage systems are crucial to ensure their ongoing effectiveness and to identify and address any potential issues promptly.
The picture below shows a series of mudsill anchors along the top of the building foundation.
These mudsill anchors, also known as foundation anchors or hold-down anchors, are structural connectors designed to secure the building frame to the foundation. They resist uplift forces and lateral loads, supporting and reinforcing the structural integrity and stability of the building.
The primary function of mudsill anchors is to provide a positive connection between the mudsill (the bottom horizontal framing member) and the concrete foundation. This connection is typically made through the use of anchor bolts or straps, shown here, that are embedded in the foundation during construction. The mudsill is then securely fastened to these anchors, creating a continuous load path from the superstructure to the foundation. These particular type of mudsill anchors have a portion that you cannot see in the photograph which is below the level of the top of the concrete and cast and set into that concrete when the concrete is still wet.
Uplift forces are a primary structural concern in building design, particularly in areas prone to high winds, seismic activity, or other extreme environmental conditions. These forces can cause the building to lift off its foundation, potentially leading to catastrophic structural failure. Mudsill anchors are designed to resist these uplift forces by mechanically tying the building frame to the foundation, preventing the superstructure from separating from the base. Historic buildings like those here where we are in Washington DC have similar concerns but the uplift concerns generally are mostly related to the roof only.
In addition to uplift forces, mudsill anchors also help resist lateral loads, such as those resulting from wind or seismic events. These lateral forces can cause the building to sway or shift horizontally, potentially causing damage to the structural elements or even complete collapse. By anchoring the mudsill to the foundation, the lateral loads are effectively transferred from the superstructure to the foundation, which is designed to withstand these forces.
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.
