Make a Concrete Garden Bench

Oil the Forms

I take the forms apart before I coat them with oil. I also disassemble the bolt block to get oil on all surfaces and seams.
I had some mineral oil left over from making the garden box forms.

coating the form with mineral oil to keep the concrete from sticking — Applying mineral oil to seal wood

This oil is sold as Butcher’s Block or Cutting Board Oil. It’s fast and easy to apply and seems to work well. Mineral oil can be bough from Animal Feed suppliers or in drug stores or pharmacies.
I apply it to all faces then leave overnight to soak in and dry to the touch.
I reassemble the main box of the form and run a bead of latex caulk on the inside edges.

Make it Watertight

latex caulk rounds the corners and seals the form to make it water tight — Latex calking to form

This makes the form watertight, gives a small radius on the outside corners, and it’s easy to remove after casting. Latex works well as it’s not overly strong and will allow you to disassemble the mold once the concrete sets up.
When the caulking is dry I spray the inside of the form with vegetable cooking oil to keep the concrete from sticking.

vegetable cooking spray as form release agent — Cooking spray as form releasant

Now I’ll insert the three bolts. I line them up with the outside face of the bolt block. I don’t want the ends to stick out past the outside edge as this will make the bolts stick up on the bench seat. I add a nut to the ends of these bolts and snug it finger tight. The counterbore allows room for my fingers or a socket.
As this is a prototype, I wasn’t sure how well the bolt block would hold the bolts in place during assembly, pouring, and vibrating the form. The nut is just a bit of added insurance.

galvanized carriage bolts are cast in the CSA concrete — Carriage bolts used

Now I secure the bolt to the bridge with a washer and nut then set the bridge in place and screw the supports to the form walls.

Final Form Assembly

a garden bench concrete leg form ready for concrete casting — Form is now ready for concrete

The form is now ready for concrete.
I set out saw horses and use some shim strips to level them front to back and side to side. This is an important step as the form has to be level.

the forms need to sit level — Levelling form on saw horses

Concrete Mix to pour the legs

So here’s the ingredients for the concrete. CSA Mortar Mix, glass fibre, plasticizer, colour, and Citric Acid powder.

The csa concrete ingredients to make the garden bench legs — Ingredients for Concrete Mix

I use 55 lbs of CSA Mortar Mix to 5 quarts of cold water, a packet of plasticizer, 1.3 ounces of citric acid, and 4 and a half ounces of colour. I also will add a bit of glass fibre for extra strength. One 55lb bag of this Mortar Mix is required per leg.

Concrete Recipe

The csa concrete formula to make the garden bench legs — Mix recipe I used

I measure the water then add it to a pail. I then add the other ingredients and dissolve them in the water.

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mixing the concrete in a pail — Mixing concrete in a pail

I have an egg beater style mixer on my drill that works well for mixing this concrete.
I use a plasticizer to make the concrete flow and pour like water.
And I add the citric acid to give more working time.
I then slowly add the mortar mix, about a quarter of a bag at a time. In North America, the Rapid Set brand can be found at Home Depot. I add a third of an ounce of glass fibre for more strength. This is the manufacturers recommended amount.
The CSA mortar mix has a light beige colour. And that’s fine but I wanted to make the legs grey so I added black concrete dye to the mix. I was thinking that it would blend better if I added it to the water first. I’ve been asked a lot about colour additives for concrete so I added some for these castings and will try other colours in the future.
I used a length of wooden dowel to stir the mix to check for any clumps on the sides or the bottom of the pail.

Pouring Concrete into the Mold

pouring the concrete into a form — Pouring concrete in form

When it’s blended smooth I pour it into the form, filling it right to the top.

pouring the concrete into a form — Almost full

I really like this CSA mortar mix and I used it in Part 4 of the garden box series so I decided to use it again for these forms. It’s really easy to mix, pours like water, sets up fast, and requires little or no troweling. After about 10 minutes I vibrated the form to bring any bubbles to the surface. For this I use a reciprocating saw without the blade.

vibrating and settling the concrete mix — Vibrating form with recip saw

While the first pour was setting up I mixed another bucket of concrete. And filled the second form. Then vibrated it after about 10 minutes.
In about 45 minutes the concrete began to solidify. Then shortly after that it began to dry and show a white haze.

Curing Concrete Heats Up

CSA concrete solidifies fast! — Concrete drying – showing haze

I sprinkled water on the surface every 10 minutes or so for about an hour to keep it wet and allow it to cure.

Water curing the CSA concrete — Water curing Mortar Mix

Strip the concrete legs from the forms

After an hour of this, the concrete can be removed from the forms. It was the end of the day so I covered the forms with plastic and left them overnight.
Now for what I think is the best part of all of this: stripping the forms to reveal the casting.

stripping the forms from the concrete casting — Stripping forms

I wasn’t sure how well the split bolt block would work or if it would be difficult to remove but it went just fine.

stripping the forms from the concrete casting — The base is freed from the casting

The newly cast legs should now be at around half their full strength. They will reach their full strength if you keep wet to cure for four weeks. For this project I’m not going to worry about that here and I’m going to build the bench right away.

tape added to the exposed bolts for safety — Painters tape applied to exposed bolts for safety

Make the bench seat

For safety, I put a few wraps of tape around the exposed bolts while I was handling the leg castings.

cutting the red cedar slats to make the seat of the outdoor concrete garden bench — Cutting cedar seat planks to length

I used a small block of concrete from a previous pour to round off any sharp corners on the casting while it was still relatively soft.
I’ll make the seat for the bench from red cedar 2×6’s. I’ll cut them to length on the mitre saw then lay out the hole locations on each end.

Laying out the Parts

laying out the hole locations on the bench seat slats — Marking hole locations in seat planks

I’ll use the drill guide to first counterbore for the nut and washers, then drill through for the bolt.

using a drill guide — Counterbore seat planks

I use a square as a gauge to get a consistent bore depth.

using a drill guide — Using a drill guide ant forstner bit

The stretcher is made from a 2×4. I had some cedar 2×6 left from the seat planks so I ripped it down on the table saw.

using a drill press for this step — Drilling stretcher

I lay out and mark the location for the holes on each end and drill them from both sides with a forstner bit. I drilled a pilot hole first to help guide the larger bit. And this worked pretty well.
To make drilling into end grain for the stretcher bolt easier I first make a drill guide from a scrap piece of 2×2.

a DIY drill guide to help drilling into the end of the spreader — Drilling into end grain of stretcher with a drill guide

Then I clamp the stretcher and guide to my bench.

seat slat ready for finishing — Stretcher holes intersect

The drill will follow the guide and give a centered and true hole.
I’ll attach ties under the seat to connect the planks and prevent any side to side racking .

seat slats will be held together with cross members underneath the bench — Seat ties

These are made from a 1×4. And they have a 45 degree bevel on each end. I’ll drill pilot holes now to make assembly of the bench easier.
I sand the ends of the planks to round over the corners. And sand off any rough spots on the other parts.

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