The vast majority of ICF foundations are poured using a two stage method. But, by coupling an innovative fabric form with adjustable supports, it's now possible to pour footings that are level and square at the same time as the first few courses. Besides eliminating a cold joint, it saves time, and eliminates one of the pumper's trips to the jobsite.
What are the risks and benefits of monopoured foundations and how can the ICF industry compete in this growing market?
Two-Stage vs. Monopour
A two-stage foundation is one where wall forms are constructed on a prepoured concrete footing. A monopour foundation is one where concrete in the footing and wall forms is placed at the same time.
The biggest disadvantage of a two-stage pour is that it is expensive. Calling out the pump twice costs big money, as does the double return of concrete to the ready mix plant.
The second disadvantage is that any footing inaccuracies are magnified by the ICF wall. To correct these, the contractor must scribe blocks to high points, wedge the low points and fill the gaps with foam glue; a time consuming and expensive process. Rebar stubbed in the footing sometimes has to be moved to align with the ICF wall cavity.
The third disadvantage is time: a two-stage pour takes about two days longer than a monopour. For time-sensitive projects, this is a big concern.
Finally the cold joint between the footing and wall is a weak point in the foundation.
On the other hand, two-stage pours have a number of advantages: first, since the footing is already cured, there is no danger that the wall forms and bracing will shift or settle during the wall pour, as they are supported by the pre-poured footing. This reduces contractor risk and is very important. Second, the contractor has two kicks at the "accuracy can."
Concrete forming is the minimization of three constraints: risk, cost, and time. As will be shown, monopouring is less expensive and faster than a two-stage pour. The real question for the ICF contractor is whether risk of wall settlement can be eliminated when monopouring.
There are two types of monopours used with ICFs. Conventional lumber systems use wooden footing forms, stakes, cleats made of 1x4s or hat-track, and U-channels to support the bottom edge of the first row of ICF.
This method has some serious drawbacks, not the least of which is that the wall nearly always experiences significant movement. Trying to correct for this movement once the wall is full of concrete is virtually impossible. Additionally, the 1x4 cleats form a moisture pathway through the wall, which many jurisdictions deem inacceptable. This method will not be considered further in this article.
The Fastfoot ICF monopour system eliminates these drawbacks. The system consists of four elements: First, the bottom two courses of ICF blocks are glued into panels three blocks long and two blocks high with a two web offset. Second, two pairs of side supports are screw-attached to each component to suspend it accurately above the ground at the 'footing' height. These supports can be quickly and accurately adjusted for height with an electric drill. Third, the Fastfoot fabric membrane is attached with screws to the bottom edge of each large forming panel to form the footing. Fourth, an adjustable bracing system is needed to align the wall.
Fastfoot Monopour Installation
To pour an ICF wall using the Fastfoot monopour system, the foundation components are installed clockwise around the excavation, with each interlock glued to the adjacent component. Each component is aligned one inch away from a string line marking the outside edge of the wall, and leveled using an electric drill on the side supports. After building up to the fourth course, bracing is installed.
With traditional vertical bracing, this is as high as the wall can go with the monopour system. That's because the base of the brace is not fixed to the ground but rather 'floats' above the ground at the footing height. It is essential that the base of the ICF brace be locked to the ground to prevent lateral movement. To address this problem, Fab-Form developed Zont bracing which uses horizontal walers in conjunction with vertical strongbacks. If Zont bracing is used, the installer can build to the finished wall height before pouring the footing.
In any case, before pouring, check the wall elevation and alignment. Adjust the footing side supports and turnbuckle kickers until the wall is plumb, level, and square.
The first lift of concrete should fill the footing and first block only. (I recommend a 5" slump.) Do not over-vibrate. Allow sufficient time for the first lift to "go off" before filling the balance of wall.
Once the wall has been poured, use the string line to check
wall alignment. It may be necessary to laser-screed the top of concrete wall.
The Fast-Foot Monopour method has significant benefits to the ICF contractor.
First, time is reduced by about two days. Second, it achieves a more accurate foundation, as the contractor can adjust and align his foundation even on pour day. Third, the monopour provides a stronger foundation without any cold joint. Fourth, the monopour eliminates the second pump and second concrete return, saving money. Fifth, monopour eliminates all forming lumber, stakes and associated labor to build and strip footing forms. Finally, the Fastfoot® membrane prevents ground moisture from entering the concrete footing.
With monopour, uplift and settlement can occur if the pour is not done carefully.
The top of footing can produce uplift if the footing concrete is not allowed to set sufficiently. This uplift can lead to form uplift. To eliminate this risk, the contractor must ensure the footing concrete has set sufficiently before continuing with the second lift.
Settlement can occur in three different ways: failure of the side support, failure of the attachment of the support to the ICF block, and settlement of side support pad in the ground. Most of these risks can be eliminated with Zont bracing, as all scaffolding loads are taken through the strongback.
Side supports are designed to carry 300 pounds of vertical load, or 100 pounds per running foot of wall. Even if the structural engineer asks for #7 horizontal rebar, 16" on center, that's only 23% of the support's capacity. Similarly, each side support is attached through the ICF web with three screws. The shear capacity of each screw through the web is approximately 150 pounds, so the attachment is incurring only 15% of its potential. Assuming the soil has a load-bearing capacity of 1,500 pounds per square foot, each pad can support 226 pounds; 30% of its' load capacity.
We've all been on enough jobs to know that in the real world, unevenness in the soil or the weight of the concrete, may cause settlement to occur. The solution is to do a final top-of-wall leveling on pour day to ensure exact elevations.
For projects with deep footings, step footings and/or complex corners, a two-stage pour is still the best option.
That said, the Fastfoot monopour works exceedingly well with normal basements, producing a more accurate foundation than a two-stage pour. With Zont bracing, perfect wall alignment can be achieved.