Backfilling a Foundation Wall
DEAR TIM: I am building a new home and the poured concrete foundation was installed 17 days ago. The weather has been cool with temperatures averaging about 51 F. I was told to let the concrete cure for 25-30 days before backfilling it. However, the contractor is applying pressure to backfill so the rough carpentry subs can get to work. Should I cave in and let them backfill against the walls or should I suffer the consequences of delaying the project? Shawn S., Pittsburgh, PA
DEAR SHAWN: In my opinion, concrete is a mystical building material. It often arrives at a jobsite in a truck pre-mixed and ready to pour. Typically it has the consistency of cooked oatmeal, but hours later it resembles damp solid rock. This transformation happens because of a chemical reaction that starts to happen the instant water is added to the Portland cement powder. The hydration chemical reaction causes microscopic crystals to grow that connect all of the sand and gravel to one another.
The trouble is, this chemical reaction continues for weeks, and even months, after the concrete truck leaves the jobsite. In fact, standard concrete that is moist cured under ideal conditions often only achieves 40-45 percent of its design strength after three days. After seven days, it should hit 75 percent of its final design strength. To get to 100% of it's design strength, you often have to wait 28 days or more. These numbers assume temperatures at or near 70F.
In cold weather, the strength-building process can slow considerably and even stop completely if the temperature gets very cold. Once the weather warms up, the concrete starts to strengthen again on its own so long as it still retains moisture.
If you had told me that you had vertical steel reinforcing bars in the poured concrete, then maybe I would say to move ahead and backfill. But vertical reinforcing steel is rarely placed in residential concrete foundation walls. Foundation walls often have two rows of horizontal reinforcing steel placed near the bottom and top of the walls. This steel helps prevent vertical cracks in the foundation walls should the soil beneath the foundation shift or move. This movement is an up and down movement.
Backfill dirt that is placed against the foundation wall causes a lateral or sideways movement. Vertical steel bars add enormous strength to poured concrete and help prevent horizontal cracks that are caused by soil pressures. Lateral pressure can also be caused by the heavy machinery that places the dirt against the walls. An inexperienced heavy equipment operator can crack a new foundation wall by pushing dirt into the void space against the wall instead of gently dropping it in from above.
Backfilling a poured concrete wall or even a masonry concrete block wall can be very risky if two other tradespeople are not finished with their work. The basement slab and the first floor that sits on top of the foundation should be complete before backfilling proceeds. These two building systems help lock the bottom and top of the foundation walls in place as the soil pressure pushes against the walls. I have seen poured concrete foundations that were backfilled before a basement slab was poured. In certain instances the wall cracked and slid off the footer from the intense soil pressure.
Keep in mind that wet soil is very heavy. It is not uncommon for this material to weigh nearly 100 pounds per cubic foot. When you calculate the weight of all of the soil being dumped against the foundation you quickly discover it is tens of thousands of pounds.
My advice to you is to wait. Your builder and many others will scoff at you and will put pressure on you to move ahead. But this pressure pales in comparison to the weight of the dirt that is placed against the foundation walls. Furthermore, if the foundation does crack because of premature backfilling, this structural scar will haunt you until you eventually sell your home. In fact, you will probably have to explain what happened to the professional home inspector that is hired by the buyers after they see the ugly cracked foundation walls.
The vertical steel bars that should be placed in poured concrete walls are not hard to install. It often only takes an experienced crew maybe one extra day, perhaps two, to install this valuable structural component. The reason steel is needed is simple. Concrete has fantastic compressive strength. This means if you squeeze or compress it, it takes enormous pressure to crack concrete. But if you try to bend or pull concrete, it only has one-tenth the strength it exhibits when you compress it.
Steel, on the other hand has enormous tensile strength. The steel bars a contractor would often buy have a 40,000 pound tensile strength rating. These steel bars, once placed inside the wet concrete, transfer their strength to the concrete. Imagine how strong a foundation wall might be that has vertical steel bars placed two-feet on center. I can tell you that a wall built this way will stand the test of time.