Functional Design : Concrete slabs for driveways, sidewalks and patios
A concrete slab feature for a building typically serves several purposes. It is both a structural element in terms of supporting loads and an aesthetic enhancement that completes the overall appearance of a building.
When you typically think of a driveway or a sidewalk, it’s easy to forget the versatility and functionality it offers. The concrete material is also quite unique, given that it’s extremely resistant to water, can take on any shape or contour and generally requires minimal maintenance.
The concrete is poured into place to form a durable, hard surface that supports vehicular and pedestrian traffic, and heavy loads such as planters, trailers, stairs, railings and many other things we can place on, or attach to the surface. This surface provides a smooth transition from inside a building to the streets or yards connected to it. A clean and level concrete surface will also enhance the visual appeal of a building structure which is of value, especially when it comes time to sell a property.
Another significant benefit is the ability to channel water from rain or melting snow away from the building and out to a surface water collection system such as storm drains etc.
Based on the above, it becomes easy to understand the value a concrete feature provides to the home or building owner.
When A Slab Sinks or Becomes Uneven
Because of the inherent value as described above, a sunken concrete driveway, sidewalk, patio, or basement floor is typically worth fixing. Even if the concrete surface is cracked, this does not necessarily mean the concrete is in bad condition. Some cracks are formed along control or expansion joints as a controlled method of relieving stress on the concrete surface after it has cured.
Safety – Once concrete slabs become uneven, the safety of the surface is now compromised. Tripping hazards from protruding edges appear, and the risk of injury increases.
Runoff water – One of the primary problems many customers notice is that water that would normally run off the concrete surface in a controlled manner may now begin to collect in unwanted locations, like around the foundation of the building. Once this water collects in these locations, it begins to damage the subgrade below the concrete or in other areas where it is flowing. This can also contribute to flooding issues in basements or garages.. Once this process begins, the damage typically worsens over time as the affected soil begins to wash away and create an uneven surface or depression in the ground for even more water to collect.
Ice – Often, an uneven slab will allow water to collect in unwanted locations around the building or on the slab itself during freeze and thaw cycles in the winter. This water can penetrate the cracks in the concrete or void spaces underneath, freeze, expand, and cause even further damage to the concrete slab. If the water collects in the space above a sunken slab, it can freeze and create a slippery surface to walk on for pedestrian traffic.
Why do concrete slabs sink or crack?
When a concrete driveway or sidewalk or any other concrete feature is poured, it is usually poured on top of a mix of compacted sand, gravel and soil. These materials in their respective layers make up what is typically referred to as the subgrade. The idea behind a compacted subgrade is to force the sand, gravel and soil mixture into a solid base that does not allow movement below the concrete which was poured above. Over time, if the compaction was not enough, the materials (sand, gravel, and soil) compact even further under the weight of the loads above (including the concrete itself) and the subgrade begins to fall. Additionally, any water that flows under the concrete and into the subgrade will aid in this compaction and can also begin to wash out the materials under the concrete. (Void spaces) The concrete on top will do one of two things; it will fall with the gravel and sand below; or it will temporarily remain in place until the void space underneath becomes too large and eventually the concrete cracks under the weight of vehicles or other loads, including the concrete itself. Improperly placed downspouts can allow water to wash away sand and soil under a concrete slab instead of harmlessly flowing over it. Over time, this can also lead to a sunken or cracked concrete driveway, patio or walkway.
If a concrete slab has sunk or cracked, what are the options?
- Remove and Replace the Concrete
Sometimes, this option is the only one available to the building owner due to excessive damage to the existing concrete slab. If a slab is to be replaced, the old concrete will need to be jackhammered out and hauled away for disposal. A new driveway or sidewalk can then be framed, steel rebar is installed and concrete is then poured. This is generally the most expensive option, with a typical new driveway costing in excess of $10,000. Once the size and any design patterns are considered, the costs can escalate even further. This process will also take several days or longer once things like demolition, pouring time, and concrete curing are factored into consideration. The attributes to consider for this option are:
- Disturbance to the surrounding area
- Amount of time the slab will be in the “demolition and construction” phase
- High energy consumption (Manufacturing of new cement is heat intensive)
- Landfill or recycling costs for the concrete that was removed
- Concrete Lifting
Another option is to have the sunken concrete lifted back into place by injecting material below the concrete surface. In general, holes are strategically located and drilled into the sunken concrete slab. Once the holes are drilled, pumping equipment is fastened to the holes and material is pumped into them. The material being pumped is usually one of two types:
- Cement grout (“mud” and commonly referred to as “Mud jacking”)
- Polyurethane foam
- In the case of “Mud jacking”, the cement grout is pumped into 2 inch holes. As the cement grout or “mud” is pumped into the holes, the mud will begin to displace the slab upwards. The grout will harden and begin to cure over the next 24 hours. The 2 inch diameter holes are patched with a cement filler mix. Because of the size of the patch, the repair will likely remain very visible over time.
Polyurethane Foam Injection
- In the case of Polyurethane foam injection, the Polyurethane starts out as two separate components (A and B) that are stored in the service vehicle. The A and B components are pumped through a heated hose to an injection gun. The injection gun is placed on a special tip that is inserted into a 5/8th inch hole drilled into the slab. The A and B components that were pumped through the hose are mixed together in the gun just before they are injected under pressure below the concrete slab. Once injected, they quickly begin to form a new material; the Polyurethane foam. The Polyurethane foam expands quickly. It is this expansion force that lifts the concrete into place in seconds. Very quickly, the Polyurethane foam begins to cure after it has finished expanding to form a very strong but lightweight support for the concrete above. Because of its’ light weight, the Polyurethane foam will add minimal load to the subgrade below. The process is repeated until the entire slab is lifted back into position and is level. Once the slab lifting is complete, the small 5/8th inch holes are filled in and sealed using a concrete filler mix to patch the hole. Because the holes are small, and the patch uses a concrete filler, over time, the patches should blend into the surrounding slab due to weathering and exposure.
Typically, the Polyurethane foam injection process has multiple advantages over a cement grout process or “Mud Jacking”.
- Smaller holes– 5/8th inch for Polyurethane versus 2 inches or greater for cement grout (Mud Jacking). The significantly smaller size of the hole, (roughly the diameter of a dime) means the injection locations are far less visible than a Mud Jacking repair.
- Light weight –The Polyurethane material is extremely lightweight yet very strong. The cement grout, (or “Mud”), is very heavy (like more concrete) which adds to the load burden which the subgrade needs to support. Because the subgrade has already sunk, any additional loading could cause the subgrade to fall further, even after the repair.
- Hydrophobic –Polyurethane does not deteriorate in the presence of water. Water cannot wash it away, dissolve it, or weaken the material over time. Water cannot penetrate the material and then affect it during freeze and thaw cycles.
- Set-up and cure time –Due to the reaction between the A&B components of the Polyurethane during the injection process, the material expands and sets up literally in seconds. It cures very quickly and is ready to support vehicular or other types of loads almost immediately.
- Non-Toxic – Once the two components mix together to form the Polyurethane foam material, the resultant foam that supports the slab above is non-soluble, inert, and does not release any chemicals. Artisan Teknik uses a high quality Polyurethane system designed specifically for lifting concrete slabs. The components of the Polyurethane are manufactured in the United States and shipped to us in Canada. As a result, we can utilize these chemicals with the highest confidence in both the quality and safety for their intended applications.
- As noted above, Artisan Teknik utilizes one manufacturer for all Polyurethane specific chemicals and the equipment used in their application. This manufacturer conducts extensive testing, research and design in the development of their Polyurethane foams and the equipment they supply. Providing outstanding support to contractors like ourselves, we believe we are using the most advanced system from the best supplier in North America for lifting concrete.
- Eco friendly materials –The Polyurethane foam which Artisan Teknik uses is made from up to 49% recycled material. While helping to reduce the volume of material that would otherwise be destined for landfills, overall energy consumption decreases through the reduction in the manufacture of base feedstock chemicals.