Recommended materials for molding concrete tiles?

Question

I've made a few test silicone molds to cast custom pentagonal concrete tiles (each about the same area as an ~18" square) for a ~200 sq. ft. residential patio; I've had good results using 'Mold Star 30' silicone to make molds from sanded blanks and then using 5,000 PSI concrete 'QUIKRETE ProFinish 5000' for the tiles, at ~2-3/8" thickness.

I'd like to adapt the process, since it relies on $300/gallon silicone for the molds, expensive mold releases, and what seems a high concrete volume per tile. Those of you who've been doing this regularly likely have found a way to make things more practical.

Any comments are welcome, but to be specific, does anyone have experience to validate or reject these points?

• For pedestrian traffic only, this paver manufacturer suggest 40mm (~1.6") thickness for each paver/tile is suitable, rather than the 2-3/8" thickness I've been using for ~18" square tiles, again with 5000 PSI concrete. I can test, but can anyone suggest a minimum thickness for this small residential patio well underlaid with >6" layers each of mixed gravel, compacted sand and DCG?

• Instead of first casting a mold from an MDF positive, I'd like to make a negative mold from a cheap, smooth material (melamine, or even plywood with contractor bags stapled to it). Has anyone had success casting concrete tiles directly from such a form, or is a silicone mold necessary to (easily) pop the cast tile out after curing?

• Any suggestions for an easy, inexpensive release? Soap?

I'm using a recently discovered pentagonal tiling approach from which uses identical tiles to pave a plane as shown in the following illustration from this paper:

Answer 1

The thickness of your pavers will be determined by how well supported they are. 1.5 inches is plenty (2 3/8" seems excessive)- as long as they are not rocking in their setting bed.

You can simply make your molds out of wood or melamine but as the comment from @solar mike says you must have angled edges or you will break either the pavers or the molds getting them out. Even the slightest undercut will lock the casting into the mold. This is one of the advantage of silicon molds- their elasticity.

As far as release goes, there are commercially available form releases. These are available from building supply houses that deal with concrete. These are made for plywood or wood forms used in concrete pours. These will be way less expensive than typical "mold release agents" and come in 5 gallon buckets. We have also used diesel fuel (not recommended) as a form release- probably even a coating of cooking oil would work. I am not sure about the plastic bag stapled to the form- it would release well but also likely leave an impression in the paver (from folds, etc.).

If you are going to be making wood forms, I would suggest caulking all the corners and letting it dry well prior to the release agent and filling with concrete. This can prevent any undercuts or places where the concrete can lock into the form.

Answer 2

You have (some unknown number) of forms. You only need about 88 pavers for 200 square feet, so just use the ones you have, repeatedly, and arrange a screed board that will level them somewhat below the tops of the forms (or trim the forms) to make them not so thick.

If you are "making them yourself to save money" bagged concrete mix is not the ideal way - get a pile of sharp sand and a pile of small aggregate gravel (or crushed rock, depending on what's commonly used in your area) and buy bags of cement - it should cost a lot less than bagged premix. Tell your aggregate supplier what you are doing and they can supply a pile or two of suitable material, locally sourced. Aggregate can be stored for years (just allow for it being damp [use less water,] and remove any acquired plant material) but cement is best bought only fresh as you need it (I repackage into sealed plastic buckets if only using part of a bag for slightly better storage life.)

Concrete has an initial swell, followed by shrinkage. It also has vastly increasing strength with time (starting very weak) on the short term, both of which lead to not de-molding too soon being ideal. 36-48 hours is a good timeframe. Concrete requires moisture to cure and develop full strength (but mixing concrete too wet makes it weak.) After the initial set the surface should be covered with plastic or damp cloth until demolding, and the demolded pavers should be kept damp or wet (you can submerge them, if you have somewhere handy - or just wrap the pile of them in plastic and hose them down occasionally) for a month. Handle the freshly demolded pavers with care, they are still much weaker than they will be after a few weeks of damp curing.

Form release is a standard product any masonry supply will have. Or you can apparently pay far more at the blue or orange box stores if you look for it there (not shopping very hard, a gallon elsewhere and a quart at the boxes was about the same cost...)

A masonry supply would also have reinforcing fibers, which you might use in the lower part of your pavers to make them more crack resistant (you probably don't want them in the face layer as they can poke out and be unsightly.) This works out fine if you are doing something like coloring the face layer with concrete dye - you can pull out enough for the faces of however many forms you have, color it and pour it, and then mix fiber into the rest of the mix before pouring the rest, (assuming you are using the form side as the face of the pavers per usual practice.)

Other answers have covered making forms more normally pretty well, I think, so I'll not rehash that. For high production (which your one patio is not) polished (and oiled with form release) steel forms are quite usual, if that makes something in your junk pile suddenly look interestingly applicable.

Answer 3

You may be able to find plastic storage containers that will be a close size. Although most are rectangular and have some degree of rounded corners. Maybe change your plan to rectangular shape pavers. I have used various plastic bowels to make concrete sprinkler head protectors ; they work well as molds ,easy to remove the concrete.