Sunday, November 7, 2010

Cages 'n' Crete, Blocks 'n' Bond

This past week has seen the completion of 80% of the foundation for the home and the starts of some CMU (Concrete Masonry Unit, aka cinder blocks) wall stems.  Because of strict seismic codes and being in a residential area, the construction thus far has been pretty traditional with the concrete, rebar, and sealing agents, but it is quite literally impossible to lay a foundation any differently in this time and area.  But this coming week Jimmy Cotter will be coming in with his crew from Down To Earth and raising 2 sections of load bearing rammed earth wall.  But lets not get too ahead of ourselves, first let us quickly wrap the work week.

Because of our wicked pace and need for the rammed earth to go up before continuing, this week was pretty mellow.  Monday we finished up all of the rebar cages for the footings of the upper level.  Tying up cages is pretty easy work, but will definitely toughen up your fingers in starting the tie around the rebar connection joint.  Producing these cages would be much more difficult if we didn't have the help of our friend 'Canada' (a nickname given to the beast of a tool after one worker pointed out that the cost of the apparatus was equal to a flight to Canada from New Zealand).  It looks like this:

       Da Beast (aka Canada)

  After the cages are done, we placed them in the footing trenches with the help of chairs (pictured below) so that they will sit or 'float' properly as the concrete is poured and set.  In the deeper trenches these chairs were also screwed into the shutters/sides to space the cage off the ground and centered within the form/shutters (pictured below & right). 

The 'block-ies' then came in and laid a few courses of the CMU's in a brick and mortar like fashion.  Because the CMU's are hollow, after the final course was laid the interior gap was filled with concrete and bolts were positioned in the un-set mix at the top of the walls to allow for a solid connection to wood to be affixed to for the continuation of the wall later on in the construction process (picutred below).

To prepare a proper base for the rammed earth wall sections, the footings were laid in a special way with more substantial square cages.  Also, instead of using timber shutters along the sides of the footing trench for forms, thick (75 mm/3 in.) insulative and rigid XPS foam was employed to create a 'thermal break' between the ground and the base for the rammed earth.  This is done to prevent conductive heat loss from the thermal mass of the rammed earth (which, with solar gain and proximity to the stove will help keep the home nice and warm) wall into the colder ground.  An additional measure taken to prevent this heat transfer to the ground from the mass, is the placement of Hebel block directly beneath the rammed earth (pictured below ).

This Hebel block is an aerated and specially treated concrete chunk which creates thousands of mini air pockets within the unit and increases its insulation value.  The Hebel and the rebar ties will connect the section to the footing, and with its 450mm/18" thickness the wall should be stable as you like it.  One of the last things to be arranged for the arrival of the rammed earth process, was the laying of a mini (3.5m/12 ft.) concrete slab for the mixing of clay, earth, and cement for the wall pouring

So ended the work week with the final laying of the slab and Hebel blocks.  To finish of the post we will close with an example of a soil test from the site to see if the earth on the lot could be used for a reasonable portion of the rammed earth or other earth based material such as plaster, earthbag fill, or floor.  Unfortunately the test showed that our particular area was very sandy with some deposits of silt, but very lacking in actual clay.  Here is the breakdown of the components.

  Resting on the bottom will always be the bigger particles- in this case sand and very small pebbles.  Above that is the fine sands/silt, and at the top will be the MVP clay.  This photo isn't the greatest as I unsettled it when I moved it to a better location and the lighting isn't mint.  This sample is very silty (approx. 60%) and has almost no clay.  The red mark on the jar was the original/dry height of the sample before water was added.  If you would like to conduct your very own basic soil composition test here are simple steps:

(1)  Obtain a ~ 1 liter glass (or extremely clear plastic) vessel with a tightly secured lid & flat sides to view the results with greater ease.

(2)  Dig a 400mm/1.5 ft. hole to snag a sample without too much organic matter in it, and fill the jar half full.

(3)  Fill the soiled (haha) jar with water until the water level is about 3/4 capacity of the vessel.

(4)  A little trick that seems to help in making the layers settle faster is to add a drop of dish-washing soap and a pinch of salt to the water.

(5)  Close lid and shake violently to your heart's content (if your heart is only content to shake for 20 seconds make sure to triple it up-a.k.a shake for at least a minute)

(6)  Place the container with the suspended sample somewhere out of the way where it won't be disturbed.

(7)  Depending on how much clay your soil has, the settling process will take up to 3 days (the more clay the longer the fine particles will take to drop).

(8)  To find exact proportions of sand/silt/clay, use a measuring stick to determine total sample depth in the jar and then measure each individual layer and make ratios in relation with the total ammount.

Depending on what you would like to do with the soil/earth you will want various ratios of clay to sand, but for more structural elements like earthbags, adobe, rammed earth, the desired mix is usually 75% sand & 25% clay (give or take 5%).  For plastering applications you will start with a similar ratio add more and more sand as you get nearer the finishing layer.

Thanks for reading and get excited for some rammed earth action next week!! Peace!