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Block and Backache Part II

August 25, 2006

  concrete-wall-forms.jpg After a previous posting about some of my experience with building a house out of insulated concrete forms, I noted some traffic to my post from searches on these kinds of products.  With the potential to share this experience with other people planning, or underway with this work, I thought I’d do a part II.

The picture above shows how the block appeared after I unloaded the first of 3 tractor trailers.   This particular type of block is 4′ long, 16″ tall and 10″ thick, and varies in weight from 75 to 100 lbs per unit.  It came stacked 24 pieces to a pallet,  22 pallets per truck load.   I used a tracked skid loader in the next frame to unload these blocks.

 1-ton-of-block.jpg A couple points on this.   Unlike a forklift, loaders lift in an arc as the boom is raised.  About halfway through the arc, the load is farthest away from the machine, and the tendency to tip is greatly increased.  These stacks are 7 feet high, and lifting them off a tractor trailer deck adds another four feet.   As a result, you’ll need to ensure that the machine you are using to unload can lift at least 2400 lbs, and probably closer to 3000 for any margin of safety.  I foresaw this, and cast a 500 lb concrete counter weight for this machine, barely visible at the right edge of the photo.  Even with this, it was a definite balancing act getting them off the truck.  A 6000 lb capacity, telescopic boom forklift such as a lull or skytrack would have been better for the job, and if your walls are more than 10′ high, the reach of that type of machine will come in handy during the install.  I owned the machine above, so I just went with it.  As an aside, that’s my fiance’, Leslie driving.

Vertical stobs of rebar should be cast into your foundation slab or walls to align with the cores of the block for your install.  The rebar should extend at least 36 times it’s diameter for maximum strength.  This works out to 18″ for #4 half inch rebar which is the minimum you should use, and 24″ would probably be better, spanning into the second course of block.  Specify grade 60 rebar vs grade 40.  It’s stronger.  If one is in the wrong place, grind it off, then redrill with a hammer drill and epoxy or pound the rebar into the new location.

 custom-cuts.jpg The block should be stacked horizontally, checking the face of the block for plumb and across the top for level as you go.  Shim with scraps of block, or small pieces of cut off rebar.  The blocks are glued together with low expanding urethane foam that is next to impossible to get off your hands and clothes.   The glue is generically available in smaller cans under the name “Great Stuff” and is a Dow product.  It is also available in “pro” formulas in 24 oz and 32 oz cans and is dispensed through a more expensive gun $40 – $80, and is cleaned by pressurized cans of cleaner.  It is imperative to clean the foam quickly (less than 1 min) after removing a can from the gun.  It sets up fast and permanent.   Fill gaps in the blocks with foam, and inject the foam at intervals by inserting the tip or hose from the gun between edges of the block.  Be sure to get a least 1″ into the wall of the block, not just the outer edge where it will do little good and create big blobs that will have to be removed later.  Because wall dimensions are almost never in whole block increments, some cutting will be required.  Usually, only one block in each course for a given area of wall requires cutting to fit. The block can be cut with a sawzall as I did, although an electric chain saw works better.  The problem here is the sawzall blade is flexible and doesn’t have much side area the way the chainsaw guidebar has.  As a result, it wanders during the cut and the cuts are seldom plumb or square, and resulting, your walls won’t be either.  Trust me.  Stucco fixes a lot, but not all.  The chainsaw works better.

first-day-of-block.jpg  You can stack 2-3 courses while standing on the ground, and you should be adding horizontal runs of rebar with each course.  Use short pieces (6″) of rebar at intervals to space the horizontal runs in the cavity, using ties to secure.  You’ll drop the verticals in from the top when you complete the last course.  You want the rebar to wind up roughly centered in the 6″ horizontal and vertical cores where it can best do it’s job.

I was asked by many why I didn’t stagger the seams the way you do with conventional masonry products.  You want the cores to align vertically and horizontally.  While it is possible to offset one whole core to the right or left to accomplish the stagger effect, it adds more blocks that will have to be cut on each course.  In the end, it’s the poured concrete cores that provide the strength, so staggered seams do nothing for the final strength.  After about the 3rd course, (4 feet) you will need to get off the ground. While scaffolds can be used, you have to lift the heavy block up to the scaffold, so unless you have a crew, I recommend some alternatives. (Your knees and back thank you)  I had a flatbed wrecker at my disposal, so I loaded a pallet of block and drove it alongside the house. This gave me a 17′ work platform, the block ready at hand, and another 3 feet of height.   After that, I would load 3 blocks on a pallet, stand on the pallet and have Leslie lift me up with the loader.  This worked well, and while decidedly against all OSHA regs, allowed work to progress quickly.   However, this was limited by the reach of the loader, and while work on a 10 foot first floor wall was possible, 20 feet for the second floor was not.  This is where the shooting boom forklift with 34′-40′ of reach would come in.  Still unsafe, and if you tell the rental place you are going to do this, they won’t rent to you.  What they will rent you is a basket lift.  You could use a scissor lift, but due to rough terrain, and inability to get absolutely against the house, the basket / boom lift is better, even though you will only be able to lift one, possibly two blocks at a time. 

In the last post, I already covered the concrete pour basics – using a pump truck.  I’ll add that one should ensure a 3500 psi mix with pea gravel, pour at a slump of 6 to 7, have plywood patches ready to screw on the walls in case of a blowout, and have lots of help ready to apply patches and clean up what can be large spills.  As the cells are all interconnected, a blowout can pretty well drain a section of wall.  Each 10 X 10 section consumes about 1 yard of concrete.  Pour no more than 2 to 4 feet of concrete into a section at a time, so move around the house several times.  The concrete will have stiffened by the time you return to the same spot and will reduce the tendency to blow out.  Resist the temptation to keep pouring  too much in one spot if things are going well – the tendancy for a blowout seems to be inverse to your confidence in how well the pour is going.   Be sure to use a reducer on the pump hose, and a “S’ bend if they have one to slow the fall of concrete.  Plan ahead – the pump pushes the concrete to the top of the boom, but then it’s all gravity from there.  As such, when you tell the pump operator to stop, there is still a lot of concrete left on the way down that he can do nothing to about.  So, say ‘whoa’ probably about 10-15 seconds ahead of time – less clean up later. 

I’ll plan one more installment on this topic where I’ll cover, cranes, building wood forms in place for arches,  spanning large openings with bond beams, and solid precast panels.  If you read this post, and are planning or doing a project like this, I’d love to hear from you.  Good luck on your project!

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