The finished bench looks like this. I wanted it to have a fairly massive
appearance to fit into the decor of my large patio. The base blocks are 1
cubic foot of concrete, each. The finished weight of each bench is about
300 pounds. Two people can move them around with some effort, but they aren't
likely to be carted off to someone else's place .
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Here is some more detail of the finished bench. The seat is planked with
2X4's on edge, and skirted with 5/4" X 6" deck planking. All lumber is
treated, and finished with an outdoor deck stain (Behr brand). Spaces between
the planks allow rainwater to run off, and the wider skirt gives it a heavier look,
as well as concealing the end grain of the planks and the hardware used to hold
everything together. Mitre joints to join the skirt at the corners gives a bit more
refined appearance. The deck planking has a rounded nose on each edge, and the round
part is left just proud of the seat surface.
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An exploded view of the parts used in assembling the bench. The threaded rods are
inserted in countersunk holes in the planking. A detailed view of the blocks and
the embedded anchor brackets is shown below.
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Here, the 1/4" threaded rod is visible, extending out one side. The gaps are
created by inserting a stack of galvanized flat washes over the threaded rod between
each of the planks. The holes for each of the four threaded rods are
counterbored to conceal the nuts and washers on the outside planks.Put a washer
and nut on one end of each rod, and insert it into the predrilled countersunk hole.
Leave the rod longer than necessary, and when everything is all tightened up,
cut off the excess with a hacksaw or a ferrous metal cutting blade in a circular
saw. Don't worry too much about small nicks on the side on the planking, as
it will be concealed by the skirting. I used 6' segments of threaded
1/4"X20TPI rod, cut into 4 pieces. The locations for the holes was chosen
to create a uniform spacing between the cross-pieces underneath the planks, and
between the cross-pieces and the ends of the bench. Use a framing square
to keep the ends of the planks square and evenly lined up across the ends.
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It's WAY EASIER to thread the planking over the rods like this, as opposed to
laying the planks down in their ultimate orientation and pushing the rods through
the holes horizontally. I can't believe I couldn't have figured that out intuitively.
Notice the stacks of washers all lined up ready to drop over the threaded rods. It
is surprising how much time it takes to count out all of these little things both
at assembly time and in the hardware store. All of the lumber should be cut to size
and stained before assembly. Once the planks are assembled onto the threaded rods,
the last washer and nuts are tightened up, into the countersunk holes. Adjust the
tension on the nuts so that the bench top is uniformly wide down its full length.
This design uses seven planks, each spaced about 3/8" from the neighbouring plank
using stacks of 6 flatwashers. I chose to use flat washers because I wanted the
round surface that would shed water, and to avoid having to cut and drill a lot of
small pieces. I used galvanized washers to prevent rust, and also to help deter
rotting of the adjacent lumber.
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Some details of the fastening system used to hold the planking together. The fasteners
are completely concealed by the skirt. Holes are countersunk to prevent interference with
the skirts at final assembly time.
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To cast the concrete base-blocks, I made a simple mold out of 1/2" sheathing and 2X4's.
My project required a total of 8 blocks, so I cast them in 2 batches of 4. The blocks
were to have beveled edges and corners to refine the appearance a little, and to help
prevent the sharp edges from getting chipped. The bevels were ripped from a 2X4, and
fastened to the plywood with drywall screws. Disassembley and reassembly was fairly easy
to do by removing screws as necessary with a cordless drill/driver. Some thought had to
be put into where to put screws so they were acessible after the mold was filled with
concrete.
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Here is a view of the inside of the mold. Getting the angles on the bevels is a bit
tricky, especially where the third piece mates with the other two at the bottom
corners. Not shown in this photo are the 6" square pieces of 3/4" melamine particle
board that were used to make the hollows that will be filled with the exposed
aggregate detail. They were screwed to two opposing inside faces of each block mold.
Although I used about 6 or 8 degrees of draft on the edges of the squares, they
were still very difficult to extract from the solidified concrete blocks. If I
were to repeat the process, I would either bevel them with a draft angle of 15
or so degrees, and/or apply a coat of some kind of mold-release to the edges of
the exposed particle board. I chose the melamine particle board because I wanted
something that would be easy to release from the cured concrete, and I didn't
want to apply any substance that might prevent the exposed aggregate infill from
adhering.
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A more detailed illustration of the base block. The anchor brackets are standard
products used in construction. They have holes punched in the vertical flanges,
to allow nailing to dimensional lumber. I enlarged the holes to accept 1/4" X 2"
lag screws; 2 in each bracket.
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This is a block extracted from the mold, and layed on its side. The 6" square
indentation will be filled with exposed aggregate, to match the exposed aggregate
of the existing patio. Almost invsible in these photos are the galvanized steel
anchor brackets that were cast into the tops of the blocks, and which will later
be used to attach the wooden bench top to the concrete blocks. The brackets are
the shape of U-channels, wide enough and deep enough to hold a 2X4 on edge. There
are two of them in each block at opposite corners, with the channels running
parallel to each other. The channels will accept the horizontal cross braces that
are attached to the bottoms of the seat planking. The minor bits of flash at the
edges and corners is easily cleaned up with a hammer.
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The final stages of the exposed aggregate finish on the decorative panel.
At left, the aggregate has been seeded and pressed into the surface of the wet
mortar. The center image shows the exposed aggregate after having been allowed
to set, almost to the point of becoming dry, and then the top layer of cream
has been carefully brushed away, clearly exposing the embedded pea gravel.
The right image is of the final gentle washing of the nearly hardened surface.
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Some more assembly information. This is the last step before cutting and attaching the
skirt sections.
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