concrete details

As we've noted before, our concrete foundation and slab have some interesting sustainability aspects. Half the cement portion of the concrete mix has been replaced by blast furnace slag, which means an industrial waste product has been substituted for virgin material (in our case, that means more than seven cubic yards of recycled material). The slag will make the concrete denser and harder once it has fully cured.

The steel anchor bolts will hold the steel columns in place. The bolts are embedded deeply enough (20" - 27") to secure the house, even during a category 5 hurricane. Some of the bolts moved a bit during the concrete pour, so they'll need to be "adjusted" with a sledge hammer before the columns can be erected.

slab!

The concrete subcontractor poured the slab Monday. We'll blog more about this in the coming days, but for now it's a big, tangible step forward.

Catching Up on Construction: anchor bolts

The anchor bolt templates have been made to place the anchor bolts on site. They will join the steel columns with the foundation. Our steel erectors from Litecrete came out last Thursday to verify the anchor bolt locations. A few of them will need to be reset slightly to be ready for the steel. We are double checking with the foundation engineers to be sure that the eccentricities placed on the column bases with the reset anchor bolts will be within a reasonable tolerance range.

Catching Up on Construction: vapor barrier

Our vapor barrier went down easily. It laps up the sides of the slab to help protect us from termites and moisture. While plastic may be a material we would like to have avoided using, in this condition, its negative characteristics will be quite helpful. Hopefully, others will have an alternative to use down the road.

The rebar and wire mesh create a marvelous texture that will be hidden in the slab but we get to record it here in photos.

rebar

We're getting close to the big concrete pour. Now that the utilities are all in place and have been inspected and approved by the city, the concrete subcontractor can finish setting the steel rebar in place. Sort of.

formwork, p.2

Two weeks ago, our team started putting up wood formwork for the concrete slab and grade beam. The first round of formwork was necessary for the plumber, electrician and air conditioning contractor to use as a point of reference when laying the underslab utilities. This week, they've been completing the formwork in preparation for the big concrete pour next week.

formwork

Our contractor's concrete guys are setting up the formwork for the concrete slab (the floor for the whole house) and the reinforced edge beam around the perimeter of the house. They won't pour the slab until the week after next, but they needed to set up the formwork in order for the plumber, electrician and air conditioning contractor to lay their conduit and pipes under the slab next week.

footings

Our concrete subcontractor poured the foundation footings today. "Footings" is an ambiguous term, since it can mean both the full foundation piece supporting the steel column, or it can mean just the broad pad that supports the vertical piece of the foundation (the pedestal). In our case, we just poured the bottom half, which takes the weight of the column and spreads it out while transferring it to the ground.

The cage of steel rebar inside the concrete handles the tensile forces within the footing, while the concrete takes care of the compressive forces. From a sustainability standpoint, the steel is almost entirely recycled, and the concrete mix includes steel blast furnace slag as a replacement for half the cement that would ordinarily be used. The aggregate and sand that comprise most of the concrete mix are produced locally, to minimize the embodied energy that comes from transporting materials.

slag

An interesting turn of events. We had planned to use fly ash - a waste product of coal-burning power plants - as a replacement for 25% of the cement in our concrete foundations and slab. Our concrete supplier, however, recommends using slag (ground granulated blast-furnace slag) to replace half the cement in the concrete mix. Slag is a waste product from steel production, which, magically, works a lot like Portland cement, so using it in the foundations will obviate the need for more virgin cement and find a useful home for an industrial waste product.

coordination

Saturday morning's coordination meeting went well. We met with our general contractor and foreman, our plumber, our electrician and our architect-of-record to talk through the next couple of weeks of work. We plan to start excavation on Thursday, then assemble the wood formwork for the concrete footings that will support the house's steel frame. The footing need to be poured before we can lay the utilities (plumbing and electrical lines), which in turn need to be set in place before we can pour the concrete slab. All of this work needs to be done before the steel frame can go up in April.

Precision

The structural steel for the house should arrive the week of April 4, around six weeks from now. The steel pieces arrive pre-drilled so that they can be quickly bolted together on site. Part of the reason we're leaving the steel frame exposed is to show off the precision of the prefabrication system.

But before the steel can go up, a number of things have to happen in sequence: