I layer the heck out of concrete and tufa of all sorts that is often weeks old with no problems.
The 4 day 'rule' (I don't know where it came from or why 4 days and not five) is interesting in that bonding agent isn't used and the concrete layers apear to 'fuse' as one if proper cure methods are used.
I use admix and bonding agents willy nilly but I have never been able to break my layers apart. Even smashing stuff on the ground has never separated the layers. I've never tried to test the limits to find a point at which layers fail.

Tango does ferro cement in layers so he would be the expert in these matters.

You can add layers to ordinery concrete forever, but after curing about a week, a bonding agent is highly recommended. So-called "neat cement" is another story. For the record..."neat cement" is just that. Cement with nothing added. "Neat cement paste" is what you get when you mix nothing but cement and water. However, most "neat cement" sculpting formulas are anything but neat and are most often quite exotic blends of microaggregates and admixtures. But no matter how you blend your "neat" mix, it will be dense. Extremely dense. So much so that adding layers can be next to impossible if it has cured more than a couple of days without extreme scarification to form a mechanical bond. In order to form a good bond between layers, the new one has to penetrate into the old one...and once a neat mixture has set, very little will penetrate the surface. Not water, not bonding agent and certainly not the new mixture. You can lay it on, but it will generally form a seperate shell over the old. Sometimes it stays put, but if impacted, it usually breaks away as you might expect an outer shell to do. I have some test samples of "neat" sculpting mixes that aren't even affected by acid. I poured some acid etch color on the surface of a stepping stone I had formed and left it overnight. Next morning, there were simply dried mini-puddles that could be dusted of. No etching at all and no color. If you want to build up many layers of neat cement paste...add to it before the previous layer cures more than about 48 hours and has been kept moist the whole time. Otherwise you wind up with a very hard, but fragile onion. Oh yeah...and curing these mixtures...tha's a whole 'nuther story.

"Oh yeah...and curing these mixtures...tha's a whole 'nuther story."

Well, what's the story, Morning Glory? I've used neat cement paste, highly polished, over cement paste (over tufa). When I dropped a piece, it chipped on the edge but did not delaminate. Of course, the paste layer wasn't solid, but used to even out the surface as per Charles Grasley (I hope I spelled his name correctly) the furniture guy.

Howdy all!
And THANKS for your input. I appreciate you all cutting thru the "crap" that is out there and showing me the way. It makes this hobby just that much more enjoyable, knowing there is experienced help out there.

Big Smiles!

Rockyn & Daisy --- Regarding the curing processes for neat Portland pastes...

Very thin layers used to fill voids and bugholes set quite rapidly and once that initial set has occurred should simply be kept moist and protected from any breezes for about a day. It is rarely problematic simply because there is not enough mass to pull itself apart as the moisture goes away.

Thicker sculpting layers of neat pastes ( basically anything more than 1/8th an inch )get more demanding. While they vary depending on the formulation, all that I am aware of require very close monitoring to avoid self-destuction from severe cracking. There are multiple forms of shrinkage that may occur in concrete. These include plastic shrinkage, drying shrinkage, autogenous shrinkage and carbonation shrinkage. Early onset plastic shrinkage is one of the main culprits and is due to excessive moisture loss. Just as big cracks will form as a mudhole dries up, cement paste can also shrink and crack. The only real solution is to carefully maintain the mixes moisture content until it has achieved enough strength to resist the forces that are trying to tear it apart. And unfortunately it's not as simple as just hosing it down every hour. As with all things cementious, it requires adapting your practices to the mix and situation. Temperature, humidity, wind conditions and the specific formulas properties all conspire to complicate matters and unless you take them all into account and respond appropriately...it will fail. Too much water too soon and you are simply weakening the formulation the same as if you had added it into the mixer. Wait too long and the cracks that form will be too severe to ever repair. The only way I know to judge "how much & when"...is to work with it and familiarize yourself with what it wants under different conditions. The basics of the environmental considerations are fairly straightforward. The higher the temperature...the more rapidly cement sets and the more rapidly water evaporates. The lower...the slower. But even that can be tricky. Example:
I am currently using a formula that I developed last Spring. It worked amazingly well considering the temperatures were consistently above 100 degrees when I used it last Summer on a county project. Come Winter it still works well, but sets & cures even slower ( to be expected, right? ). What I didn't expect was HOW MUCH SLOWER. Stages that lasted for a couple of hours in the heat are taking days to complete. While it's great to be able to "work" the mix in infinite detail, the downtime between phases is killing me. I am now more than 12 weeks into what was supposed to ba a "six to eight week project". Another lesson learned. Even after all these years I am still amazed by the "dicoveries" that await us. On the bright side...I believe I now know how to balance this particular formula to different environmental conditions and achieve the detail I want within the time frame desired. So...that's what it all omes down to. Work with it. Play with it. Observe and record. Once you get the hang of the basics, anyone can learn to pretty much control the different properties to meet their own individual requirements.

As for "Charlie"...his last name is Streshley and he is indeed a fine sculptor. Love his concrete Moderne furniture. Take a look at his website if you have not seen it. He also has a fairly lengthly and informative tutorial on his site that all should find informative. I hope to have my own website up and running pretty soon and it too will have tutorials. Lots of them. Some very in-depth. 'Til then, see the link below and as always...my best to all.

Here is a link that might be useful: Sculptor Charles Streshley's Concrete Furniture

thanks.

I've been working with thin layers of 1:1 and detail mesh.I've added to it by painting the previous surface with admix.I've seen no delamination,shrinkage or cracking so far on the balls I've created,with thicknesses at about 1/4 to 3/8 of an inch.On the flip side of that,I've had problems with cracking when I try to get too thin(1/4 inch or less)on some bowls that I've created and tried to sand the surfaces mechanically or handle too roughly.I'm going away from that as it seems to be a labour of futility.
But,that's what concrete is all about!