rb, You can use hypertufa but depending on what recipe you use it could be weak or very pockmarked. Concrete would be the best bet for a bench top. No matter which mix you use add reinforcement. Are you making the bench top right-side up or upside down? If right side up, just edge it right in the mold after it's set up for 30 or 40 minutes. Set time depends on your temperatures, how wet your mix was. This is what masons do when putting the finish edge on a concrete patio or walkway. You can still have sharp edges so use a brick or small piece of concrete as a sanding block to gently sand off the edges.
I don't know if the posts are still here but do a search on this forum. There was tons of information for doing mosaic or tile tops and how to make molds for legs. Billie

Thanks for the prompt reply, billie ann. It will be cast upside-down, with a tile design in the bottom, and three transverse re-bars in the middle of the mix. I wanted to use a Hypertufa recipe because of the "aged" look it can give. Is there a particular recipe that has greater strength than others? If not, I'll have to be satisfied with concrete.

Since the mold will screw together I guess I will have to wait for it to to set and then turn it over on a flat surface, and take the mold apart.

One thing about casting concrete: On the History Stones how-to video, the concrete bench top set for only 24 hours before being un-molded. This seems awfully fast. After 24 hours is concrete too hard to edge easily?

Sorry for all the detail, but I haven't done nearly enough hands-on projects to have learned all of this.

RB Whale

rb, 24 hours is about normal to unmold something that size. Same goes for almost everything made on this forum.
I haven't seen the molds from History Stones in a while but people that use to post here used them. I believe the edge is formed in the mold so you didn't have much cleanup on your piece. The concrete or hypertufa will be soft enough that you can round the edges. Just don't wait past the 24 hours or you'll need the jackhammer.
Oh, and that's a smart move "screwing the mold together". Good luck and let us know how it turns out.
What are you using for legs?
Almost forgot regarding the recipe, which one are you looking at? If it's got peat moss make sure you sift the peat to get out sticks and clumps. Stay away from too many additives like peat moss, perlite and vermiculite. Maybe just use one of those and make sure your recipe has sand. Billie

Ah...thanks, billie ann, for reminding me about sifting the peat moss. I intend to used one part cement, one part, sand and one part peat moss, with some strengthening admix in the water. The legs will be made from a plastic waste can I got at Target...it has an interesting shape, and is tall enough to make the bench tall enough for me (I'm 6' 4").I I will definitely post photos, however it turns out.

RB Whale