Saturday, February 18, 2012

Blockwork, round #1

As planned, an early start this morning saw a start on the first course of blockwork for the retaining wall. I have to admit a little trepidation at first - I’ve never mixed mortar before, let alone laid bricks or concrete blocks. The fact that this wall is a crucially important structural member in the building had me a little .. not nervous, but let’s say cautious.

I’ve always held the belief that “I can do that!” when it comes to hands-on stuff, and this was no exception. All it takes is a little careful research and learning beforehand, which in the case of this masonry wall involved the Boral Masonry Design Guide and the CMAA Concrete Masonry Handbook. What’s interesting (and something I didn’t know) is that clay bricks and concrete blocks have different properties in that clay bricks will tend to expand after manufacture, while concrete blocks will tend to shrink. Therefore brick mortar is different to block mortar - brick mortar made with “brickie’s loam” will shrink as it dries, matching the expansion of the bricks. If this is used with concrete blocks, it’s likely to crack.

Just as well I did the research ;)

With recipe in hand (or mind) I mixed up 1 part cement to 4.5 parts concrete sand with 0.5 parts lime in a wheelbarrow, being careful to mix the ingredients thoroughly dry before adding water. To my mild surprise, I did a pretty good job at it and the result was a pretty great mortar, if I do say so myself :) I was worried it was going to go off quickly (memories fresh of the concrete slab and the more recent chem-set epoxy..) but I got over an hour out of the first batch before it started to go stiff. Just as well, because my first few blocks were slow going until I learned the tools and the technique :)

Progress was pretty good, my technique improving all the time. It took about two hours from the first ingredients hitting the wheelbarrow until I used the last of the batch, and about a third of the first course was complete!

Sadly, from there things started to slow down a little. Each block is precisely 400mm in length, and the reo bars are set more-or-less 200mm apart (some are slightly more, some slightly less). However, it’s pretty impossible to place each block in its allotted 400 ± 0mm space, so over the length of the wall the clearance between the bars and the webs in the blocks started to shrink until a little creative problem solving was required. This, coupled with the increasing heat of the day meant my second batch of mortar went off after laying fewer blocks, so half a barrow went to waste. It was always going to be a short day anyway with the boys’ swimming comp to attend, but it meant that I’m about half a dozen blocks short of a full course (if there’s a better euphemism for my state of sanity, I don’t know of it :D )

No matter, it’ll be there waiting for me next weekend…

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