Our website and blog focus a lot on repointing and tuckpointing of historic masonry, in fact we have posted some of the most extensive information on historic masonry restoration of the entire internet. As a company, Infinity Design Solutions focuses on historic masonry restoration and facade restoration but we actually will do a varied array of different types of work. Today, instead of typical historic masonry restoration and repointing of historic brick, we are going to look at a type of masonry that is much newer: Interlocking CMU retaining walls. These types of walls are newer looking and they even look a bit cheap and boring, compared to traditional historic masonry. Unlike historic bricks, these interlocking masonry retaining wall units are actually made of cement, like cinder blocks. By comparison, historic bricks and historic masonry are made of kiln fired clay, often directly from the earth. The correct name for retaining wall CMU (Concrete Masonry Unit) blocks that fit together in an integral way without mortar is often referred to as “interlocking concrete blocks” or “interlocking retaining wall blocks.” These blocks are designed with features that allow them to fit together seamlessly without the need for mortar, creating a stable and durable retaining wall. The interlocking design typically involves protrusions and recesses on the blocks that interconnect, providing structural stability and resisting lateral forces. Interlocking retaining wall blocks are commonly used in landscaping, garden walls, and other applications where a cheap and easy retaining structure is needed. The outline of today’s article follows:
Lateral Deflection and Differential SettlementUnlike traditional brick construction, the interlocking block retaining walls are generally set without mortar, they’re actually often made to be dry-set, and will often include intentional cavities or voids within the blocks that allow for installation of vertical rods and/or filling with mortar or cement / concrete, but in many cases these blocks are completely dry-set walls that interlock based on the shape of the block itself. Although block walls of this type are formed with integral lips and ridges to interlock and resist the lateral load of shifting soils retained behind the wall, some shifting soils and associated forces are too great for the wall to resist. Stair-step cracking is often associated with differential settlements. Differential settlement is caused by below-ground instability. Often people erroneously believe that stair-step cracking, when associated with differential settlement, is because a portion of the subgrade or sub soils were improperly compacted. That may be true. The bigger point in understanding the principles at work is to understand that differential settlement is more often related to inconsistent compaction of subgrade ground or subsoils: more often inconsistent vs. improper. When a building foundation is built on soils that are compacted relatively consistently throughout, stair-step cracking or differential settlement for that matter are relatively uncommon. The problem more often lies in subgrade compaction that is inconsistent or different from one area of a footing foundation to another. Historic masonry buildings, for example, like the ones in the historic neighborhoods of Washington DC such as Capitol Hill, Dupont circle, and Georgetown are often built with heavy stone or brick masonry footing. Often in historic buildings, the footing is relatively indesternable from the foundation. Often in historic brick buildings in Washington DC the footing itself is just a corbeled portion of the lowest part of the foundation, effectively working as a footing. In most cases though, particularly modern or contemporary construction a footing will be built with steel reinforced concrete and then the foundation will be built on top of the footing and continue to above grade. In cases of a concrete footing, particularly when built with steel reinforcement, the footing has significantly higher tensile strength and therefore can resist the weakening forces of differential settlement. Capstone and CopingsCapstones and copings are the masonry elements at the top of masonry walls. In many cases capstones and copings will be a different shape and use, different functional purpose than the rest of the masonry elements of the wall. The main elements of the field of masonry walls are made to interlock to one another, to a degree. Bricks, for example, by comparison to capstone are different because they have a simple rectilinear shape, a typical rectangle. However, particularly in double wythe or triple wythe walls where bricks are laid in a rowlock position, the integral cohesion of the bricks can be very strong even in large or expansive walls. Copings and capstones can be much longer / wider than typical brick units because they do not necessarily need to have the same structural capacity to support a large walls above. After all, they are placed at the very top of a wall. Their functional purpose, more than anything else, is to protect the wall below from entry of elements of weather and precipitation. It serves this purpose for these elements to be longer, because longer units have a lower proportional amount of joints. Joints deteriorate faster than brick or masonry units and therefore over time, if a capstone or coping is built with very short or thin masonry units it will have more joints and as those joints wear out and weather will enter into those joints and then down into the remainder of the wall below causing deterioration and will shorten the lifespan of the wall. These areas of capstone or wall copings still require upkeep, maintenance, and pointing or repointing In timelines of 25+ to 50+ years. Pointing or repointing, as it applies to capstones or coping masonry is the process of removing the deteriorated mortar at the top of the wall at the joint between the brick or masonry units. At a horizontal area such as a wall coping, the mortar joints will deteriorate faster than almost any other area of the wall, maybe with the exception of only the base of the wall. This is an example of why, here, even in a non-historic masonry wall, preservation and restoration and pointing and repointing can be important. Small incremental amounts of maintenance and restoration, such as repointing, can extend the life of masonry for decades and even centuries. Most coping and capstone units are laid in a stretcher position, as shown in the photo above. In that stretcher position the height is generally less than units in the remainder of the structural wall below. This lower thickness / height in the structural unit means that the stone, brick, or CMU masonry element is not as strong to resist compressive forces, but that compressive strength resistance is hardly needed because there is generally no wall built on top of coping or capstones. Compressive and Tensile StrengthAlso, while it is technically true that the compressive strength of thinner unit is less than a thicker unit, compressive strength is rarely the factor that leads to failure. Tensile strength resistance, is a much greater factor in typical masonry failure than compressive strength resistance or the lack of either type of resistance. Stone, brick, concrete, all types of cement, often have very high compressive strength. It’s one of the inherent characteristics of concrete, stone and masonry. Compressive strength is the measure of a material’s ability to withstand axial loads or forces applied in a direction toward its center without permanent deformation. It is a crucial parameter in assessing the stability and load-bearing capacity of construction materials like stone, brick, and concrete. A typical retaining wall of this type, compressive strength is an important factor because each unit, especially towards the bottom, is loaded with the weight of every unit above. However, when a wall is built with units set on a staggered running pattern, the forces at the higher parts of the wall are distributed widely among several units below and typically the compressive strength of those lower units is SO high that the load above them Is small in comparison to the load of what the lower units can support. Tensile strength refers to a material’s ability to resist forces trying to pull it apart or elongate it. In the context of masonry, tensile strength is essential in evaluating how well a material can withstand stretching or pulling forces. While compressive strength measures a material’s resistance to compression or squeezing forces, tensile strength assesses its resistance to tension or pulling forces. Typically, brick, stone, and concrete have very low tensile resistance strength. The key difference between tensile and compressive forces is usually in the direction of the applied force. Compressive strength focuses on resisting forces pushing inward, while tensile strength assesses resistance to forces pulling outward. In a retaining wall, compressive strength is critical for withstanding the upper portions of tge wall it supports above, preventing crushing forces. Tensile strength becomes relevant in situations where external forces, such as ground movement or seismic activity, induce tension forces that the wall must resist. Reinforcements, like steel bars, are often added to enhance tensile strength in masonry structures, ensuring overall stability and durability. Weeps and Hydrostatic Pressure ReleifIt is wasn’t for hydrostatic pressure, retaining walls would basically almost never collapse, and fail. As explained, blocks, brick, and stone masonry can generally support the compressive strength of the wall itself without any problem at all. They are generally able to support much more than the overall load which they are engaged to support, in most cases. Tensile strength becomes a factor when there is significant differential settlement which pulls masonry units out of the regular shape and/or applies pressure to masonry units in different directions. Specifically though for retaining walls, the biggest factor or problem that leads to their overall demise, failure, or collapse, is almost always issues related to hydrostatic pressure, pointing or repointing alone will not afressd the root issues. Hydrostatic pressure stems from the presence of significant groundwater without low resistant pathways built into the wall as part of the design. As soil absorbs water, especially during periods of heavy rainfall or snowmelt, the subsoils become saturated and hydrostatic pressure builds up against the wall. This pressure increases with the depth of groundwater, becoming a substantial force that retaining walls must counteract. In comparison, the same elements of precipitation, freezing rains and/or whether with freezing temperatures are also a significant problem for brick and/or related issues of pointing anf repointing and or brick mortar joint deterioration. As those joints deteriorate, the brick facade must be protected from weather on the outside. However, although it’s essential to provide historic restoration or preservation when needed, These forces on the exterior side of a building facade are much different than the issues of hydrostatic pressure as they affect retaining walls. Repointing or pointing in mortar joint restoration is also a very tedious and laborious job, but it’s actually a little bit easier to defend against the exterior elements at a facade wall than at a retaining wall. When a retaining wall is built properly, from the beginning, there should be hydrostatic pressure relief systems behind the wall itself which allows water to flow from the upper portion of the retained ground out through the wall. The impact of hydrostatic pressure is most pronounced in situations where the groundwater level rises significantly (in storms or prolonged rainfall periods, for example), leading to a surge in lateral pressure against the wall. This force intensifies as the wall’s height increases, meaning taller walls generally bear a much higher coefficient of exposure to hydrostatic pressure. Inadequate drainage and poor relief design exacerbate the problem, heightening the risk of structural failure. The overarching consequence of hydrostatic pressure is the potential for wall destabilization and structural damage. The force exerted by the water-saturated soil can result in wall tilting, cracking, or even catastrophic failure if the pressure exceeds the wall’s load-bearing capacity. Additionally, hydrostatic pressure can induce lateral soil movement, causing erosion at the backfill and compromising the wall’s stability. Hydrostatic pressure may be difficult to visualize because we can’t see it, it’s underground and behind a wall. However imagine a lage jar full of dry sand. The pressure against the walls of the jar is relatively low because the weight and mass of the sand is generally a downward force. Some sands have a angle of repose greater than 45-degrees. This jar appears full, completely full. However, you can still add a lot of water to the jar. That water than increases the pressure against the walls of the jar significantly. That is how hydrostatic pressure works, in a jar and against a retaining wall, except the retaining wall is comparatively massive and much more susceptible. To mitigate the impact of hydrostatic pressure, effective drainage systems are crucial in retaining wall design. Properly designed weep holes or drainage pipes allow excess water to escape, reducing the build-up of hydrostatic pressure. This prevents waterlogged soil behind the wall, minimizing the lateral forces exerted on the structure. The choice of materials and construction techniques also plays a pivotal role in addressing hydrostatic pressure. Reinforced concrete walls, for instance, provide enhanced resistance to lateral forces, while retaining wall designs incorporating geogrid reinforcements help distribute loads more effectively. Additionally, backfill materials with good drainage properties can contribute to minimizing hydrostatic pressure. Regular inspections and maintenance are essential for retaining walls subjected to hydrostatic pressure. Identifying signs of distress, such as cracks, bulging, or tilting, allows for timely interventions to reinforce the structure and prevent further damage. Implementing preventive measures, such as proper drainage and stabilization methods, is a proactive approach to safeguarding retaining walls against the debilitating effects of hydrostatic pressure. The Cohesive Nature of Interlocking Concrete BlocksInterlocking concrete blocks derive their cohesive nature from specifically designed interlocking top and bottom faces. Each block is crafted with features such as interlocking lips, tongues, or grooves that facilitate a secure, in resting position, connection between adjacent blocks. These shapes create a geometric puzzle-like fit, allowing the blocks to interlock horizontally and vertically. The most common design involves a tongue-and-groove system, where one block has a protruding tongue that fits into a corresponding groove on the adjacent block. This interlocking mechanism allows for alignment, creating a cohesive and stable wall structure without the need for mortar. Additionally, some blocks incorporate setback features, where the blocks are slightly angled backward as they rise. This setback contributes to the structural integrity of the wall by providing resistance against the pressure exerted by retained soil. The cohesive nature of interlocking concrete blocks not only simplifies installation but also enhances the overall stability of the retaining wall. It minimizes the likelihood of misalignment or shifting, creating a reliable and visually pleasing solution for various landscaping and construction applications. Historic masonry upkeep and preservationTo properly maintain, repair, and care for these historic buildings, a knowledge, interest and understanding of historic building principles is required. Here in Washington DC, historic masonry buildings are extremely expensive and the amount of financial loss caused by improper repointing and low quality construction is staggering. However, in addition to the direct financial value of the property, there is also a cultural loss when historic buildings are damaged. By comparison, consider neighboring poor cities, when historic buildings are damaged, it’s not just the loss of value to the property owner, there’s also a loss to all inhabitants and visitors of a city, present and future, who care about architecture, history, and culture. We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the historic built environment of Washington DC and neighborhoods such as Capitol Hill, Dupont Circle, and Georgetown and become educated on on the difference between proper historic preservation versus improper work which leads to significant damage to the historic fabric of a building. 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. 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, contact us or fill out the webform below and drop us a line. We will be in touch if we can help. <p>The post Interlocking CMU Retaining Walls first appeared on Infinity Design Solutions.</p> Via https://www.ids-dmv.com/masonry/interlocking-cmu-retaining-walls/
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About UsInfinity Design Solutions LLC (IDS) is a full service general contracting company in the heart of the Dupont Circle neighborhood of Washington, DC. We focus on repair and renovation of buildings and facilities in both historic designated neighborhoods and the commercial-zoned central business district of the city. Follow Us
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