Cast Concrete Mouldings

[tghattaq]

For decades I was fascinated by the concept of using decorative concrete mouldings cast in situ. At last given now the time and the possibility of having made up in my own workshop the complex shuttering that is required to cast classic mouldings onto concrete slabs I have partially achieved success. To give an example - as this generalised description is a little nebulous - where you cast a concrete floor out into a short cantilevered balcony, you can apply to the flying edge an attractive moulded shape by the use of a complex shuttering. This is primarily a work of joinery and there is a lot of planning into how the shuttering progressively dismounts.

I have been very successful in creating the shapes and achieving a clean release of the shuttering. In fact I am more than pleased with results. BUT one problem is an occasional hairline cracking in the cast surface.

Can anyone with a better knowledge of concrete technology help me?

I am using a very fine aggregate of about 1/4" hard limestone and a mixture of ground hard limestone and marble dust obtained from the poishing of marble. Silica sand is not available to us. I use a mixture of light coloured Russian grey cement and the common white cement. To this mixture I add a waterproofing preparation which acts as a plasticiser and greatly inhibits drying so that it cures to become extremely hard and dense. The mix is made in a standard cement mixer and kept very dry. When I need reinfocement I use glass fibre strands laid in the concrete. It is well vibrated with a very effective small wand. These mouldings have been between 150mm and 350mm in height and a projection of

50mm up to 150mm from the surface to which they are bonded.

The external finish is most satisfactory - it has an appearance similar to the grey hardstone used in central Italy since the Renaiassance for stairs and architectural mouldings. However hairline cracks do appear in the surface on some of the mouldings. I can repair with a slurry of cement and marble dust which sands out quite well and maintains the shaping very crisply, but the delightful sheen that direct contact with shutter confers is lost and the surface becomes somewhat dull and lifeless.

Can anyone advise me how to avoid this occasional surface cracking on the profile?

[Richard J Kinch]

Cracking comes from stress. Is this induced in the casting itself, such as from shrinkage during curing, or is it from being bonded to another structure which moves?

Do you use a polymer admix?

I'm curious how you make these molds. What patterns from what source, what materials, what process.

[DanG]

Surface cracking sounds like a moisture problem to me.

Possibilities: The forms are dry (absorb water) or allow water to escape rather than hydrate and cure the concrete. Are you using some type of form release? You might try wetting the form work to aid in concrete placement and to prevent form material absorbing moisture.

You mention applying fiber reinforcement. It would be normal here to blend the fiber throughout the concrete mixture.

You mention keeping the mixture quite dry and plasticizing with a waterproofing agent. I assume this is to avoid shrinkage cracking and to maximize ultimate strength. You have no aggregate to speak of and are using dust rather than sand. Many powder type products demand mixing time and a resting time to absorb water and a retempering. Are you bulking by volume or by weight? It still sounds like loss of moisture, insufficient moisture, or absorption. I would not depend on a waterproofing additive to provide curing. I assume your mixture is a hydraulic concrete that will set under water. Can you provide wet cure? - flood, submerge, steam, tented moist cure.

(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) [e-mail]

[Goedjn]

No, but I'll bet that if you don't stop at sanding, and go to polishing the patches with 600-1000 grit wet sanding, you can get back to a shiny surface again.

[tghattaq]

Thanks for your reply Richard.

To give details of the form work in chapter and verse. I build up the form in good, clean timber like first grade red deal. It has to be pretty good to successfully router to shape. As I stated, it is a good mental exercise to devise how this will demount leaving support underneath whilst freeing the face to dry after a couple of days. To the mould I have grain filler applied and sanded, then several coats of the sort of commercial epoxy that hobbyists use on furniture. When this is fully dried and sanded I have several coats of classic beeswax polished applied and polished with plenty of time in-between each coat. The release is as near perfect as I could wish for.

My primary source for design is Isaac Ware's 'Vitruvius Britannicus" (know I should be using Piranesi) but I never copy directly as I wish to add a slightly more Mediterranean feeling. An exaggeration. Guess I generally close my eyes and think of the Amalfi coast rather than Colchester, drink a large glass of Grappa and get stuck in. I have just cast a moulding 43cm x 380cm which came out perfectly without blemish. Cors its sod's law that this is one that I want to paint over anyway.

The surfaces that I am casting onto are totally stable. There is no movement or shrinkage what-so-ever.

I think that the answer may be to add short strands of glass fiber to the mix. We leave it a half hour running in the mixer for the moisture to be absorbed. I have found that any wetter mixture leaves nasty bubbles in the upper surfaces where excess water has accumulataed and then dried out. I do add a polymer in the form of the old-fashioned compound Pudlo which greatly increases the slump of the concrete and makes it settle very well into the complex mouldings.

Sometimes it works and at othertimes ......

I more than welcome you comments. Giles

[tghattaq]

refer you to the answer that I gave Richard.

the concrete is much slower than you might envisage. I think we get

complete hydration because when 'set' it rings like a bell and

hard. If there is any sun exposure I have the casting

which are kept moist for days. The shuttering

an iota of moisture.

layer is no more than 100mm thick and averages 50mm. It is

fascia on a very stable base. I simply cannot see why

few hair line cracks after continuous vibration

perfect from the crysallis.

[Goedjn]

I don't understand this. WHy would you want to dry the surface?

[tghattaq]

Sorry slipping into local terminology. Simply if I do not release the shutter from the complex face of the casting after three or four days there is so much adhesion when the mixture has become cured that the face is prone to damage and slithers come away attached to the form work in prising it off.

Giles

[DanG]

It is not unusual to use compressed air to blow the forms loose. This method is typically used on large pan forms when used to make "waffle" decks.

(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) [e-mail]

[tghattaq]

Could you be a little more explicit ? How do you you use compressed air and what are waffle decks ? G.

DanG Wrote:

[DanG]

you be a little more explicit ? How do you you use compressed air

Here is a good, technical explanation of one way concrete. This same system can be used as two way concrete. The resultant look of the bottom of the deck strongly resembles a waffle.

here is another with some pictures: Here is a source/pictures of the forms The forms are a bit like big bread pans turned upside down and once they have wet concrete vibrated into location, hardened, and cured they can have quite a tenacious hold of those "bread pans". Prying and crow bars will damage either the concrete or the forms, so high pressure compressed air is usually used to blast the pans loose.

(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) [e-mail]

you be a little more explicit ? How do you you use compressed air