Thursday, September 24, 2015

Construction - Concrete Work - Wide Footing for Concrete Walls

As described in the first post on concrete work, there are two kinds of footings -- one for the stick-built walls and another for the concrete walls.  This post visits the wide footing for the latter.

Wide Footings
The footing under the earth contact concrete north wall required a hefty footing, plus five deadmen, because (a) it is very long, (b) it will have only one "T-wall" bracing it, (c) most of it is two stories high and (d) it will be backfilled to the top. Therefore, it is a retaining wall on steroids rather than a typical "basement" wall.  The footing for it is 4' wide and 2' deep and poured directly against the earthen walls of the excavation except for the top few inches that were formed up with lumber.

Wood Forms
Salvaged 2 x 4s were used for the forms but they could not be installed in such a way as to seal off completely the gap between the forms and the adjacent grade. Also I wanted to keep the lumber as clean as possible so it could be used later in construction.  So, in order to kill two birds with one stone, I stapled 30# felt (Craigslist-bought for pennies on the dollar) over the tops of the boards and let it hang down along the walls of the trench.  In this way, the boards did not come into contact with the concrete and the gap between the forms and the grade was sealed off.  (Jamie Schulte, the concrete contractor, said the heavy felt was overkill, that 6 mil plastic would have worked as well).

Horizontal Rebar
The rebar for the footings presented two learning opportunities.  First, the "L-bars" that are tied to the horizontal rebars and protrude upwards to tie the footing to the wall should not be DIYed.  The local supplier, who cut the rebar to size and bent it, charged only the per-foot price for the rebar without adding a surcharge for cutting and bending.  The second learning opportunity was my naive choice of plastic high chairs to support the horizontal rebar in the trench -- they were a joke.  Their selection was based upon trying to avoid steel in contact with the soil to avoid the potential of its rusting, expanding and cracking the concrete. 

After trying to stabilize the rebar on individual plastic high chairs (talk about the domino effect -- a nudge to the rebar at the east end caused the entire rebar assembly to fall off of the chairs clear to
Good view to show the wood forms covered with felt paper
and the configuration of the rebar. Notice the outcroppings
 in the form for the deadmen.  (Click on the photo to 
enlarge it for detail.)
the west end of the trench  over 90' away). When I mentioned to Jamie ahead of time that we were using the plastic high chairs, he was not critical but did say that he used what is known in the trade as "bolsters" but what is called "continuous high uppers" on the supplier's website. They come 5' long and have to be cut to length. Professionals use gas-operated portable abrasive cutters; I gang-cut them with a metal cutting blade in a reciprocal saw.  Then, in order to obviate the potential for steel contacting the soil, I cut rectangles of 6 mil plastic sheeting to slip under each bolster.

Rebar Configuration
For the 4' wide footing three courses of horizontal rebar were necessary.  First the two outer courses were tied to the bolsters then short pieces of rebar were tied at right angles to the tops of the outer courses.  Then the third, i.e., middle course, of horizontal rebar, was tied on top of the short bars.  The bolsters were the tallest available (6") in order to raise the horizontal rebar as high as possible in the exceptionally thick footing. 

Pouring the footing with the help of a conveyor truck
and rotary laser.  The view is from the NE corner.
The vertical "L-bars" were tied to the horizontal bars on 2' centers.  The short end of each "L' rested on one of the outermost horizontal bars, passed under the middle horizontal bar with the vertical portion butting against the middle bar, thus centering it in the wall. Once wired to place, they were secure at the lower ends but were tipsy at the tops. So, another course of horizontal rebar was used to tie the tops of the "L-bars" together and keep them upright. The fact that the rebar turned the corner at the west and east ends helped to keep the entire assembly stable.

The Pour
The pour might have been within DIYer capability but I am glad that I delegated it. The
Finishing up the pour; notice the outcroppings for the
access to the forms was limited to the extent that a conveyor truck was necessary and, instead of screeding off of the form boards, it was poured level without screeding using a rotating laser. Altogether, 37 yards of concrete went into the footing which is slightly more than the wall itself took.

Click on either of the bottom two photos to enlarge it and notice the pink insulation lining a small section of the outside wall of the trench and serving also as the form between the wide footing and what will be the narrow footing for the stick-built walls. The vertical insulation is critical for the shallow frost-protected foundation (post on foundation design) that is being used here and for the rest of the house.  And it should go without saying that the same configuration for the insulation was used at the south end of the footing for the west wall.