Fracture Line in Concrete Molds

Have you tried a mold release agent?

http://www.smooth-on.com/Release-Agents-and/c9_1123/index.html

Hi, Thanks for your eply, Yes, I have tried mould release agent. I use this to coat the moulds to aid removal. The first batch I tried I didnt use any, and the concequence was a nightmare trying to release! I dont think this has any effect with the fracture line though as it is in the same point everytime.

In the thread title you say 'Concrete', in the post you say 'Stone'. Clarify, please.

How long are you allowing to cure before opening the mold? Is this enough time?

How long are you allowing the part to rest after demolding?

Are you casting individual balusters, or are you truly casting a balustrade?

When on the vibrator table, is the part vertical or horizontal? Neck up or down?

A few pictures may be worth 3 of 4 hundred words in this thread.

Hi Thanks for your reply,

The balustrades are based on a limestone mix with cement in to alow it to bond. My choce of words probable didnt help my origional query! The cure time has been varied in order to try to irradicate the fracture. I have tried times from 12 hrs to three days before opening, there seams to be no difference. The frustrating party is that some are fine, however other are not. There is no difference in the way I manufacture each one which is what i cant understand. Once the mould has been split then the balustrades are left to stand for a period weeks before being sent out. A Pair of moulds are placed in a stand, in a vertical position, so that they can be filled, then left to vibtate for as long as possible in order to try and remove all the air. They are then placed into another holder in order to start the drying process. Once these have started to set, approx two - three hours later, then they are turned and layed on their side till the following day. I have found this helps a bit but it may just be luck! Below you can see a photo of the issue.

This is an obvious tension fracture where the concrete bottom is 'hung by the neck until broken'

Solution 1? less water in the mix, followed by later wetting to provide enough water for the full cure.

Solution 2? cure them upside down so there will not be a problem.

The upside down is easy to test as is the dryer mix

Concrete is very fragile in the uncured state. Remember, it typically takes 28 days for it to reach near full strength, without additives.

It also sounds like you have a built-in stress riser in the area that is cracking.

Doorman's suggestions all sound valid.

....fully agreed, also should it not be kept wet for as long as possible to achieve maximum strength?

(Remember the way concrete boat hulls were built many years ago?)

Does "good" concrete "shrink"? I must admit I am only a hobbyist, but I never saw shrinkage......(Rank amateur actually)

I think you may be running the vibrating table too long.

Check this out. It's similar to what you're doing. After filling the top, I don't think you want the table vibrating for very long at all.

are you using cement/sand or mixed size pit run gravel, sized aggregate?

For a use like yours, a mix with aggregate with find numerous failure paths across the narrow points and bad adhesion to the aggregate.

I think you need to make these balustrades with only a fine gravel, like aquarium gravel for max strength

Prestressed concrete works in every case. But, it is an overkill for a balustrade.

It is much simpler to think about what happens to a rubber form liner after you take it off the shaking table. The rubber rebounds. It then pushes down at the bottom pedestal against the end of the form. It pushes up agains the top pedestal end of the concrete, neatly breaking and lifting at the neck. Solution is to fill it up to its top neck, shake it, fill up to top without further shaking. carefully set it aside for curing. Or clamp the bottom and top plates together hard and cure. That clamping pressure overcomes the tendency of the rubber form to rebound. I would get some heavy rebar into a C shape for the custom made spring clamps. Inexpensive, exactly what you need. When the endplates are under pressure, the tendency to break at the neck is much diminished or eliminated.

Maybe a single, thin rebar along the axis for good luck, belt and suspender wise.

Consider using a different cement; I don't know if it is still offered, but the one I recall was "Chemcomp", which was shortened from Chemically Compensated. The company we bought from was Southwest Portland Cement, if memory serves. The concrete made with this would expand initially by some tiny percentage, and shrink during cure by the same amount, giving a net zero change. Ordinary cements would shrink by a couple of tenths of a percent, if I recall the numbers correctly - it HAS been 25 years, now. A mold with a necked-down stretch between larger features causes the break because the portion between those large features tries to shrink, but the mold holds them at a constant distance. Concrete has lousy tensile properties, especially prior to full cure, unless very carefully reinforced with fibers (and one could argue that it is the fibers which have tensile strength, NOT the concrete).

Look at the info at http://rapidset.com/Articles/Literature_ChemComp_PT.pdf. The first document they reference looks like it could also be useful to you. Another is at http://trid.trb.org/view.aspx?id=105881. The beginning of an article can be read at http://pubs.acs.org/doi/abs/10.1021/cen-v042n032.p038. You apparently need to be properly credentialed to retrieve the rest, though. One warning: water content needed to be precise; going over the spec cut strength quickly. We also controlled temperature of the water at 80 - 84 degrees F; this provided a quick initial set but left sufficient time to cast it.

Use Polycarboxylate type Super Plasticizer 0.5%>1% of/WITH CEMENT and Stick to minimum water :cement Ratio i.e. 0.3 by mass.

And you do not Vibrate. Mold fills complete without vibration.

And you can release from mold in 48Hrs.

BASF Glenium 6520 is the latest -highest performing Brand.

No better Solution possible

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Pretend for a moment that you are still a mechanical engineer. If you can, saw the balustrade down its length, so you can examine the concrete composition. Try crushing samples, say by belting with a lump hammer. You will very likely find that the answer/s has/have been given already, this will confirm it.

As pointed out by various comment:

1) you are vibrating too long: the top of the mould will be shaking with a bigger amplitude than the base, when in vertical position at the vibrating table, thus dislodging the top part from the rest, when partially drying and towards the end of the too long period of vibration. --> Make the mix more fluid (more water) to start: this will promote the movement of the air bubbles towards the top when vibrating, and will keep the mixture fluid to better settle down.It will also allow a shorter vibration time.

2) pour the mix while vibrating or start vibrating halfway during pouring of the mix into the mould.

The mixture should still be fluid when you stop vibrating. if it starts to cure or dry, the vibartion will cause fractures in the mix and they will not heal back.

Your good results maybe because the mixture was more fluid than the ones that proved bad with a fracture at the neck (Most shaken part and the one to dry slower than the base.

The local people make these as a sandstone lookalike product. To do this they use a dry mix ( very little water ) and fill by handfulls at a time tamping down as they go. They can achieve nice colours and a smooth sand type finish. Obviously not a cheap version. I have given a GA to Passingtongreen and Aurizon as i believe they have hit the nail on the head.

I made 500 x 350 x 2400 limestone mix lintels for my house and i saw both shrinkage and a lot of water expelled and then reabsorbed.

You may have to consider a Low Vertically Induced Vibrations as oppose to the traditional horizontally induced way of removing trapped air. This approach will minimize any unnecessary horizontal shaking forces which will also be magnified as exerted onto the top end of the free standing balustrade during the curing process.

If I were you, I would contact the PCA (Portland Cement Association) for their expert advice, as they are intimately familiar with virtually every conceivable use of Portland Cement. You should also contact Master Builders for the appropriate add-mixture(s) for your application. They formulate many very speciallized products for various Portland Cement mixtures used for difficult processes.

I remember having done some site work for an expansion at a CMU (Concrete Masonry Unit) plant. Their system involved the use of a machine that held several molds which were filled with a very dry mix, vibrated for a fraction of a minute, followed by the withdrawal of the molds. The molded blocks were then transported to steam-curing rooms where they were left for 24 hours, I think. The aggregates used were no larger than 3/8" and the rest of the mix was angular sand and cement.

You absolutely must limit the water content in the mix and the earlier suggestion to use a super plasticizer is an excellent one. I would guess that you need to experiment with timing of the mold removal and that it should occur within a short period of time after filling, provided that you control the water/cement ratio appropriately. The higher the water/cement ratio is, the greater the shrinkage that will occur during curing. No concrete mix volume will ever be greater than at the time that it is placed, period.

I think the answer to your predicament has been found somewhere already and the above resources can help you identify them.

I am not a concrete guy.

You may already be using the correct type of sand in your concrete so this comment may not apply.

I have been told that sharp angular sand must be used in the concrete mix not polished river type sand. The polished river sand hurts the strength of the concrete.

Doug