Construction - Concrete Work - Slinging the Gravel Sub-base and Pouring the Slab Floor

This is the sixth and final post on the concrete phase of construction (of the living quarters of the house). The slab floors for the garage and the screened porch will follow eventually. (Click on any photo to enlarge it for detail.)

Preparing for the Gravel Sub-base
The plumbing rough-in bunched the conduits together so tightly, especially those going to the bathrooms, that air space existed under them and any downward pressure on many caused them to flex.  Since the 3/4" clean gravel of the sub-base would not find its way

Ready for the gravel sub-base; the conduits and
waste water pipes have been packed with sand, the
grade beams and piers have been marked with paint
and the grade stakes are in place; black plastic covers
the PEX tubing to protect against ultraviolet radiation
for several months until the roof is in place

under and between most of the conduits, the concrete floor would not be properly supported.  In order to remedy the situation, I hand-packed, as much as possible, sand into

the air spaces and left extra sand on top of the conduits.  As much as I hated to see rain a few months ago, I eagerly awaited a downpour to compact the sand and carry the extra into the crevices below and between the conduits.  Mother Nature could not have been more cooperative -- no sooner than step-son Keith, handyman Pat and I finished distributing the sand, she dumped 3/4" of rain overnight that seemed to do the job just fine.

Under load-bearing walls and posts, the thickness of the slab had to be doubled in order

A wooden form blocks off for the bathtub drain; marking
paint delineates a grade beam on the left and a pier
adjacent to the wood form; tall grade stake in the fore-
ground, shorter spikes mark the corners of the grade
beams and a pier

to form grade beams and piers.  These were laid out and delineated first with 6" outdoor spikes driven into the soil then with marking paint on the surface of the soil.  In order to preserve access for connecting the drain to the tub eventually, the area under the bathtub drain had to be blocked out.  A circumscribed, tall wooden form was securely anchored in place to block out the gravel and the concrete.

Finally, the operator of the slinger truck needed reference points for spreading an even thickness of gravel at the proper depth.  Accordingly, grade stakes in the form of 12" spikes were pushed into the soil to form a grid that more or less positioned them 15' apart.  The rotating laser and a hammer was used to position the tops of the spikes at the +/-5" fill level for the gravel.  The spikes were then sprayed with marking paint to give them visibility.   The spikes that delineated the grade beams and piers were driven much deeper so as not to be confused with the grade stakes.

Slinging the Gravel

It took four truckloads of gravel and a long half-day to sling the gravel into place.

The slinger truck in action

The driver/operator skillfully laid down an even layer without getting much more than a sprinkling in the grade beam and pier areas.  Three of us with garden rakes cleaned out the grade beam and pier areas and finished leveling and smoothing the rest of the gravel.


Compacting the  Gravel Sub-base?
Our plans called for 5+" of 3/4" clean gravel under the concrete.  Apparently, usual protocol calls for pouring concrete over the the slung gravel without compacting it first.  Our

Compacted gravel sub-base; burlap covers the black
plastic over the PEX tubing to protect it from wind damage

undisturbed soil under the gravel was more than dense enough to have supported the concrete without the gravel so it made no sense to top it off with gravel that was not compacted.  And the slinger truck driver confirmed our conviction by saying that the amount of shrinkage that could be expected from compaction would be 3/4".   I immediately had him top off the original layer with another 1" of gravel to give a finagle factor for compaction . Thank goodness there was a plate compactor in the family and it immediately saw some serious use.

Pre-pour Tasks
The designated pour day was too windy due to an approaching cold front and had to be postponed for a couple of days in the span of which rain was forecast.  In order to keep the soil dry for the concrete trucks, we spread 6 mil plastic sheeting and weighted it down with enough junk to withstand the stiff winds . Unfortunately, the effort proved fruitless because the rain was minimal. 

Two of Jamie Schulte's crew appeared the day before the pour and used a laser to snap chalk lines on the concrete wall and set grade pins designating the height of the pour.  As soon as Jamie and his crew had unloaded their equipment on the day of the pour, they used the plastic sheeting to fit a

The inaccessible areas were poured using a conveyor
truck; the accessible areas were poured directly from
the chute; notice the plastic moisture barrier in place

moisture barrier over the entire gravel sub-base.  There were three floor drains that also needed to be suspended at the correct height over the roughed-in PVC pipes.  By using metal drains for two of them, I was able to drill, tap and insert three set-screws in each one for securing them to the PVC.  For the shower stall, it was only necessary to wrap foam around the PVC as a space holder for the brass drain to be installed later.

Pouring the Floor
The depth of pour was targeted to be 5" and level with the top of the foundation wall. The square footage was 1,680.  In lieu of metal reinforcing mesh, the mix was

Beginning the final finish 6 hours after beginning the pour

strengthened by the addition of fiberglass fibers and using 8 bags of cement per yard of concrete for a crushing strength 5,000 psi.  Some of the concrete was poured from a conveyor truck, the rest directly from the chutes on regular ready-mix trucks. The amount of concrete needed -- 29 cubic yards -- surprised even Jamie.  The considerable square footage of grade beams and piers, which double the thickness of the concrete, was probably the major reason for underestimating the volume.  In any case, the cost of the concrete, along with the surcharge for the conveyor truck, was about double what I estimated originally.

Wrapping Up the Floor
Concrete shrinks as it sets, causing random cracks.  Control joints are used in an attempt to direct the cracks to where they will be least problematic.  They are sawed into the concrete to a depth of about 1/4 of the floor thickness after the the concrete has had time to set but before cracking.  Jamie and a helper snapped chalk lines and used both a walk-behind and portable concrete saw to make the cuts.  Fortunately, we were able to hide most of the control joints under future partitions and a couple could be partially hidden under the future stairway and kitchen island.  Since it will be quite a while before the finished floor goes in, I am hoping all the cracking will have occurred, whether in the control joints or not, and the ceramic or porcelain floor tile will hide them without the crack expanding more later and cracking the tile.

The pour was done in mid-November and would be susceptible to cold weather damage if not protected.  Accordingly, Jamie recommended a product called "Cure & Seal" as a temporary coating lasting about three months before letting loose, which will get us through the winter, and one that does not compromise the bond of the thin set to the concrete when the tile is laid.  I tried applying it with a cheap garden sprayer but found it faster and easier to roll it on.  Five gallons cost $120 and was only a tad more than we needed for 1,680 sq ft.

Insulated Slab Edge
A final note.  The advantage of pouring the floor flush with the tops of the insulated concrete forms is that the slab edge is separated from the exterior environment by 5" of expanded polystyrene insulation.  Without the slab edge insulation, heat would be conducted in and out through the exposed concrete (thermal bridging) and alter the floor temperature adjacent the wall accordingly.  All energy certification programs require slab edge insulation (see first post on certification and second post on certification  (the latter is more germane to slab edge insulation)).