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.

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
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