Basement Construction and Failures

The most common failures with basement construction are related to;

  • pushing the construction schedule, and
  • the improper installation of materials used for damp-proofing.

Generally in residential construction, foundations are poured, forms stripped, and a week or two later the foundation are often sprayed with a damp-proofing product. Then, about week 3 or 4 the foundations are backfilled.  This is a pretty typical construction process in many residential areas. It is also not uncommon to see this scope of work completed, and then backfilled within a two-three week time frame. Sadly, there are many issues that have occurred in this process, that prevent the foundation construction from meeting either the industry’s ‘Standards of Good Practice’, the Manufacturer’s material installation requirements, and/or the minimal requirements of the building code. Let’s look at some of those issues and how they can be improved.

1) Concrete Durability: Depending on the ambient air temperature and concrete mix design, once a concrete foundation is poured it generally takes two to seven days for the new concrete to cure to a strength that is strong enough to build on top of. Concrete curing is generally defined as the the process of cement hydration, A chemical process that releases heat and moisture from the concrete mix, transforming the cement powder, aggregate and sand mixture (that is in a plastic state) to a hardened state. The result is a dense interlocking crystalline structure of hydrated compounds, which we call concrete. Through the curing process, a percentage of the water in the mix is used for the hydration of the cement. The remainder of the water is released from the concrete by evaporation. The more controlled the curing process (the rate at which moisture is allowed to release from curing concrete), the more cement hydration will occur within and at the surface of the concrete, increasing strength and durability. Too fast of curing can cause cracking and reduced durability. At 28 days after the pour the concrete may be tested for strength. Concrete will generally reach its design strength prior to the 28 days and may continue to cure (often gaining additional strength) up to 60 days or more. Concrete that has not yet reached its designed/intended strength is often referred to as ‘green’ concrete.

Concrete drying, is the process of the remaining un-used moisture evaporating out of the curing concrete until the concrete reaches its final moisture rate, for its environment. Complete drying of the concrete can take up to a year or longer.

In fast track residential construction, the concrete forms are often stripped within 1 to 3 days after a pour. Once the forms are removed the curing process is no longer controlled. Depending on the ambient temperature, this is not necessarily a bad thing. However, we would like to note that when we pour architectural concrete, we leave the forms on for 5 days, and the form joints are tight and do not allow moisture to release/leak from the forms, holding more moisture within the concrete mix. Thus slowing/extending the curing process. Resulting in less cracking and more durable concrete. As a general rule we recommend to leave the forms on for 4 to 5 days for all concrete. Don’t rush it!

An often over looked, but a very important issue is concrete placement. Concrete is often pumped into the forms. When placing concrete we want to avoid segregation of the mix, which is the separation of the aggregate from the cement paste. This weakens the concrete. Segregation will start to occur after 20 feet of pumping through pump lines. This is often not avoidable, so concrete placers should be aware of this and reduce the length of pump lines were possible. The other segregation issue is the actual placement of concrete into the forms. Concrete should be dropped into the formwork as near as possible to its final location. It is recommended that concrete not be dropped more than a few feet, and should not be dropped down through rebar within the form, as the rebar causes segregation, knocking out and separating the aggregate from the cement paste and creates air bubbles/pockets. The recommended procedures is use a tremie hose, drop chute or elephant trunk to get down to the bottom of the formwork and place the concrete.

Another issue is vibrating the concrete to remove honeycombing and other defects. Fresh poured concrete can contain 5% to 20% entrapped air, that needs to be removed through the process of vibration. Honeycombed concrete, bug holes, rock pockets and sand streaks are all a result of improper vibration. Unfortunately, we see a lot of concrete with bug holes, honey combing and other placement defects. Which after a pour, all one can do is patch this weaker, less durable concrete.

Concrete can be made better by leaving forms on longer and providing proper vibration. We also recommend the use of plasticizers and other additives to densify the concrete, making it strong and water tight. Refer to comments below on water tight concrete and our Dry Basement Design.

2) Backfill: Backfill over 4 feet (1.2m) around a foundation requires that the top of the foundation be laterally supported. The top of the foundation is considered laterally supported, once the the floor is framed and sheathed. The sheathing works as a diaphragm to support the top of the foundation. Pushing a construction schedule by backfilling too early, before the floor is framed and sheathed, for easier site access, can load and crack the concrete foundation, and in some instances may simply push the top of the wall inwards (without cracking it), making the concrete foundation wall un-square to the building’s design. Do not backfill before the floor is framed and sheathed.

3) Damp-Proofing: The manufacturers of most of the spray-on damp-proofing products recommend that the damp-proofing is not installed until 28 days after the concrete is poured. The reason for this is that, throughout the first 28 days after the pour, the concrete is vapouring off moisture and it cures. The damp-proofing manufacturers have determined for these products that there is too much vapour emitting from the concrete during this time period for their products to be applied to and bond successfully to the concrete. Some products are designed to be applied over ‘green’ concrete. However, check it out for yourself. Go review a foundation that is a few weeks old and has been sprayed with black bituminous damp-proofing. Within a week or two of being sprayed, you will likely note that the spray-on damp-proofing has developed a number of air bubbles that are caused by the vapour emitting out of the concrete as it cures. Sometimes these bubbles burst on their own, and when it comes time to backfilling, the action of backfilling tears open the bubbles. The result is a damp-proofing product with a bunch of holes in it, much like a cheese grater. Now the damp-proofing will not function as intended by the manufacturer, and cannot adequately retard the penetration of moisture into the concrete foundation wall. With respect to the interior space, there will be a constant wick of moisture through these holes in the damp proofing, providing for a damp basement. 

Unfortunately, in our area this seems to be the common practice in residential construction. Why ? Because the Contractors is pushing the schedule too fast. Faster than following what is actually recommended by the manufacturers that provide the products that the Contractor is using. 

4) Dimple Drainage Sheets: Due to the issue of blistering spray-on products, some Contractors choose to use a dimple drainage sheet product. However, the issues we find with dimple drainage sheet products are again related to installation. Drainage sheets are a good product, but the issue we have noted are;

  • the dimple sheets are either not adequately attached to the concrete foundation wall, or
  • the dimple sheet is not properly lapped at seams or over footings down to the weeping tile.

Often during the backfill process, the dirt pushed into the excavation to fill the hole, falls and catches, and tears the dimple sheet from the foundation wall and is not repaired. Or, as many contractors do not compact their backfill, when the backfill settles it grabs the dimple drainage sheet and pulls it down off of the foundation wall with the settling grade.

The result is the top edge of the dimple drainage sheet is dragged downwards with the settling earth, opening the top edge of the drainage sheet for surface and ground water to drain down into. As most residential contractors do not install any damp proofing behind the drainage sheet, there is nothing there to protect the basement from surface water draining into the drainage sheet. If this occurs and the foundation wall is cracked at that same location, that water runs straight into the basement through cracks, as I unfortunately discovered at my very own home builder grade home.

Both of these products (spray-on damp-proofing and dimple drainage sheets), if installed correctly do a great job of what they are intended to do. However, we are amazed as to how often we come across installation issue that cause a failure. The result of these failures are often a damp/cool, and uncomfortable basement with a musty smell. Or worse yet, a leaky basement and maybe mold.

5) Moisture Wicking Protection: Damp proofing is just damp proofing, it’s not water proofing. Therefore, its function is to simply retard the flow of moisture through the foundation wall. As moisture will move through the foundation wall with only damp-proofing installed, special precautions must be taken inside to prevent the insulation from becoming wet. The are two options;

  • Install a poly moisture barrier sheet between the inside face of the concrete foundation wall and the insulation, to prevent the insulation from wicking moisture from the concrete, or
  • build the interior stud framed wall out far enough from the face of the concrete, so that when the insulation is installed between the studs, the insulation is far enough forward, that the insulation is not in contact with the concrete foundation wall, to provide an air gap prevent the wicking process of moisture into the insulation.

We find that most contractors choose not to install the poly moisture barrier due to added cost, and choose to frame the stud wall out from the foundation wall. A few things can go wrong with this installation. Many contractors choose to frame the wall with only a 1″ gap between the stud and the foundation wall. In our opinion this is not enough, due to the fact that the foundations walls are never straight or exactly vertical. We find that the studs end up 1″ away at the bottom of the wall, but only +/- 1/2″ away at the top or at other locations within the wall.

Then, when it is time to insulate with a 3.5″ batt in a 3.5″ stud, the batt is pushed back too far and becomes in contact with the concrete and moisture wicking can occur. If you review the installation of your insulation within the wall studs, as seen in the photo below, where ever you can see 1″ or more of the side of the wood stud, the insulation will be 1″ or more further back on the back side of the stud, and will be in contact with the concrete foundation wall, where moisture wicking can occur. 

Check it out for yourself. At the bottom interior side of your foundation wall, cut open the vapour barrier, move the insulation aside and feel the back side of the insulation for moisture. If the insulation is wet, the R-value of the insulation is reduced. View the moisture on the interior side of the concrete.  All this moisture can provide for a damp, musty smelling, often uncomfortably cool feeling basement living space. 

What can be done ?

BCS’ Dry Basement Design

 We design our basement systems to avoid these failures. Our design intent is to provide that warm dry basement feeling that makes you want to hang out in your basement. Yes it costs more, but you will enjoy your basement space more as it will be dry, more comfortable and will not smell musty on occasion.

For our DRY Basement Designs, we generally have two methods;

Option 1: Install a sheet waterproofing product on the foundation wall, primed, lapped and rolled. Then we install a drainage sheet with the fabric face, facing the fabric face out from the foundation wall. Then the foundation is back filled in 12-16″ lifts, to prevent tearing the drainage sheet off the foundation wall.

Option 2: We use concrete additives within the concrete mix design that cause the curing process of the concrete to engage a substantially greater percentage of the moisture, provides an increased volume of cement paste, resulting in a much denser, water tight concrete. Moisture simply cannot pass through our cured water tight concrete design. The additives make the concrete stronger, with less shrinkage, so less cracking. We leave the forms on for 5 days for better curing control. The concrete itself becomes water tight, basically waterproof. Then we install a dimple drainage sheet product in combination with the water tight concrete design mix, to assist in directing moisture to the weeping tile below. Our Dry Basement designs may vary from project to project pending site conditions, but this is our general design intent of Option 2.

We use the same additives in the basement floor slabs to reduce slab curl, reduced cracking and provide a moisture tight concrete floor. Your basement is much dryer and much more comfortable with no dampness or odour smells.

Contact us for our design services for Dry Basement Designs for new foundations. If you have an existing foundation that is damp or leaking we can also assist in designing the correct Dry Basement Repair for your home.

We also provide Flood Resistant Basement Designs for homes in areas with high water table, and for homes in flood prone areas. Yes, there are a number of design options that can be incorporated into your home and basement foundation to greatly reduce the chances of basement flooding. 

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