Saturday, September 26, 2015

Construction - Concrete Work - Narrow Footing Under Stick-Built Walls

This is the third post on the concrete phase of construction.  The first post covered the excavations for all of the footings.  The second post covered the wide footing under the concrete walls.  This post deals with the footing under the stick-built walls. (Reminder:  Click on any photo to enlarge it.)

Pouring the Concrete Walls
Here's where I had to stand aside and watch.  Jamie Schulte and his crew of +/-8
Forms in place; ready for the pour.
strongbacks (sorry for the pun -- "strongbacks" are integral to the bracing of concrete forms) took two days to set up the forms, a half-day to do the pour and another day to dismantle the forms and pack them off.  The wall took 36 yards of concrete, including one yard to prime the pump on the truck.

Once the concrete guys were gone, I dismantled the forms for the footings and turned my attention to the footings under the stick-built walls.  
Pouring the wall from a pump truck

Narrow Footing for the Stick-Built Walls
Unlike the wide footing under the concrete walls that was poured largely without forms, the narrow footing was poured in wood forms entirely.  The choice of wood forms was based upon the low tolerances specified for the insulated concrete forms  that will be used on top of the footing for the foundation walls.  The specs for level call for no more than 1/4" variance.  However, we were later to find out that our best efforts at forming up and pouring a level footing still yielded a variance of 7/8" over the span of 164 linear feet of footing.

For an 8" thick concrete wall, the code minimum for the footing is 8" thick and 16" wide. The forms were set for 8" thick and slightly more than 16" wide using salvaged 1 x 8s lubricated with diesel oil.  For the +/-20' section over the conduits for the solar collector, the thickness was increased to 16". The design of the forms was taken from Chapter 7 in the book, "Carpentry & Building Construction; A Do-It-Yourself Guide" by William P Spence.  It took several days to get the forms ready for the pour.  My step-son and a friend, a journeyman carpenter, contributed 2 1/2 man-days towards building the forms.  Surprisingly, it took me as much time to wire in the rebar as three of us took to build the forms.

Rebar Configuration
As with the wide footing, bolsters with plastic sheeting under them were used to support

the horizontal rebars at a height of 4" and at intervals of 4'. The horizontal rebars were wired to the bolsters 8" apart. The latter dimension was easily attained from the 8" distance between two of the vertical elements of the bolsters.

In earlier times, a rhomboidal-shaped two-by-four was typically nestled into the top of the fresh footing concrete to create a "keyway" depression into which the base of the concrete wall locked.  That practice seems to have been abandoned in more recent times in favor of "L"-shaped rebars projecting upward from the footing to be incorporated in the wall.  In our case, the plans called for vertical rebar on 2' centers (which matched the situation in the wide footing for the concrete walls as described in the 
Since the 20' section of the footing over the conduits for
 the AGS system is essentially a beam, the forms provided
for twice as much concrete and twice as much rebar as
 the rest of narrow footing.  Notice the rebar (painted 
orange) protruding from piers
second post).  All rebar was #4, i.e., 4/8" = 1/2" in diameter.

The vertical "L-bars" could not be plunged into the fresh concrete randomly but had to be
The poured wall and the forms for the narrow footing
ready to pour; notice the deadmen behind the wall.
positioned accurately in order not to interfere with the webs in the insulated concrete forms (specs called for them to be situated on multiples of 6"). So-called "spreaders" were used on top of the form boards on 2' centers but not to brace the forms as typically done -- the forms were rigid enough without them.  Instead, the L-bars were wired to them for support in a vertical direction while being wired to the horizontal rebar for support in the other direction.  As soon as the concrete was poured, the spreaders were

removed so they did not interfere with screeding off of the fore and aft form boards as Pat is doing in the photo below. As the concrete gained stiffness, the L-bars were straightened as much as possible with the intention of making them perfectly vertical by bending them later if necessary.  Later, when the forms were set up, a few rebars had to be redirected but none had to be cut off and repositioned.

Pouring the narrow footing was definitely within the grasp of the DIYer. Five of us amateurs were able to off-load the concrete and get it screeded before it began to set -- but
Friend, Pat, and family volunteer, Archie, screed;
off-loading concrete utilizing the extra 14' of chute
barely.  And, unlike the pours for the wide footing and the wall, the forms could be reached by the ready-mix truck with a conventional chute plus, in one area, a 14' chute extension.

The bottom photo is included to show the relationship of the narrow and wide footings.  The insulation was cut away and shallow holes were made in the wide footing into which the horizontal rebar in the narrow footing fitted.  The concrete in the narrow footing cold-jointed with that of the wide footing.  

Fortunately, for the first time this year, rainfall has been, for over a month, less than normal providing the perfect weather for the concrete work.  We were able to get the narrow footings poured while the dry weather persisted.  My hope is that we can get the foundation walls up while it is still dry.

Parenthetically, the Building Director who inspected the forms before the pour, was very complimentary, saying something like "this is the way it is supposed to be done and rarely is".

Foundation French Drain
If I am not mistaken the code that Collinsville is using, requires a footing/foundation French drain -- as well it should.  In
Junction between wide and narrow footings.  Notice he
loose end of the French drain and the fact that it is
bedded in sand instead of rock.
our situation however, it is probably overkill. With the final grade behind the concrete earth contact wall severely sloped northward, an insulation/watershed umbrella under grade extending 20' from the wall plus seven French drains 10' below the floor level, it is highly unlikely that water will ever reach the level of the footing once the house is completed.  If I thought otherwise, I would not have compromised on its installation.

The one thing we did that was not a compromise was to trench for the drain next to the footing so as to keep it below floor level and slope it gradually to daylight.  (Apparently, it is not uncommon for the drain to be laid on top of the footing against the wall.)  But, that said, there were a couple of compromises.  I used corrugated and perforated drain pipe with a sock on it from the local big box home center knowing full well that the sock will not filter out our wind-blown loess soil forever, that it will eventually allow the drain to clog with silt. Moreover, instead of bedding it in clean stone, I used sand, which will work as well and almost as long as stone before clogging with silt.  

Since I plan to backfill in 5' layers and let nature do the compaction, as opposed to shallow lifts and mechanical compaction, the umbrella and final grade will be months away. Therefore, an atypical rationale for the drain was advocated by the concrete contractor. Even if it functions only temporarily, it will serve to keep the backfill drier and reduce the pressures against the wall as the backfill compacts over time.  In this context, the compromises become acceptable.