The Benefits of Foundation Mono-Pours
In concrete foundations, typically the footing is first poured, followed by the foundation stemwall. In this two-step process, a “key” and rebar are used to lock the two concrete sections together. A mono-pour, in contrast, is where the footing and stemwall are formed together, in a single pour. At first glance, there does not seem to be much difference between the two methods, other than construction method. However, upon closer inspection, mono-pours actually offer several fabrication benefits.
First, a mono-pour foundation is stronger. Rather than relying on mechanical forces, such as friction, keys or rebar, a mono-pour is a monolithic structure that works together without any joints. Secondly, the idea of a mono-pour is that you get as much prep work done as possible, such that the foundation can be better integrated. Part of that work happens where the foundation meets framing. The prep work there, in conjunction with mono-pours, result in better framing as well.
Consider anchor bolts, which are embedded in concrete and connect the wood mud sill to the foundation. The placement of anchor bolts must be precise to meet engineering and code requirements. This is often difficult when you do things in stages. Traditionally, little wood bridges can be used to hold the anchor bolt in place, or they can be attached to the rebar or formwork with clips. Whatever the method, this process is time consuming and often inaccurate (measuring errors), resulting in anchor bolts that are all over the place, causing headaches down the road in the attachment of the wood sill.
Rather than this tedious process, the mudsill can be pre-attached, with all the anchor bolts already in place. This way, when the concrete is poured, you end up with a single foundation footing and wall, with the mud sill already in place and anchor bolts precisely located.
This technique is being used by John Chung (Associated Builders) for the 147 Bronte project. The mud sills are already in place, with anchor bolts fastened, awaiting the concrete pour.
The same method can be used for other framing elements that interface with the foundation. Here, we see the W-section that supports the cantilevered deck, placed directly on top of the form. Anchor bolts are similarly fastened pre-pour. The beam is leveled pre-pour as well, eliminating the need for any leveling grout that would otherwise be required if the steel beam were installed after the concrete work. These small techniques, when added up, result in significant cost savings and improved construction over the long-run.
