Superplastcisers and shrinkage cracking 2

[Kyle85]

Good evening all!

Lately I’ve been working on precast wall sections for a series of fertiliser sheds that are in high demand.

As part of my role, I batch the concrete for my own work And lift the panels out of the form by crane, and due to high demand I have to do this with limited curing time to get the next one out. They’re placed horizontally, internal vibration, screeded, floated and surface finished.

In an effort to achieve this successfully, the boss has got me onto a high range water reducer (superplasticiser). It would be seen as beneficial to time critical production due to its “accelerant” characteristics and short term compressive strength. The HRWR is Sika Viscocrete PC hrf2, if that narrows it down. Otherwise, it’s a composition of modified carboxylates.

Batch design would be 32mPa, various aggregate diameter, overall batch water average of 250L/m3 and implied admixture of 350ml/100kg Portland cement.
The admixture is added at approx 1/3 initial batch water, and cement gradually introduced at approx 1/2. Additional water is introduced gradually and in small quantities (I can’t indicate how much as I can’t see the water scale whilst manually adding). But allowed to mix through for 60sec/m3. This will achieve a low slump, with enough plasticity to work with it without it flowing like molten lava.

In this time, I’ve had no end of drama. Rapid surface drying a shrinkage cracks like the gates of hell have opened up to name the most problematic. And more often than not, I’m finding that I’m fighting to an inch of my life just to achieve an average aesthetic result. (My main reason for reaching out. I love this job! It’s who I am. 14 proud years and no plan on stopping. But some days, when I start dropping rocks I feel sick to my stomach knowing that this could be the one that I can’t save).

Am I doing something wrong? Is there something I can do differently? Any feedback, links, resources, learning material or expert advice would be forever appreciated! I take the utmost pride in the end result, but coming to work with no more than hope that it’ll go well is becoming exhausting.

(In addition to contributing factors are a northwesterly that turn my work space into a wind tunnel, and being in the onset of the Australian summer climate).

 

[SwinnyGG]

Your questions yield more questions

What size and type of aggregate are you using in your mix?

What is your total final w/c ratio?

Are you accounting for retained water due to aggregate washing?

What range of air temps are you casting this concrete under?

What range of slump values does your mix yield after the addition of the Sika HRWR?

What exactly (outside of surface cracking) are the cosmetic problems you are dealing with (ie is it trouble with getting a smooth enough finish via trowel/fresno, or something else)?

Does this mixture have fly ash, silica fume, or any other supplementary cementitious material in it?

How did you arrive at 350ml/100kg of cement?

Prior to adding HRWR to your mix, did you evaluate the use of an accelerator instead (For example, SikaSet NC or similar)?

 

[Kyle85]

I’ll try to answer as best I can. I should also add that it’s not a typical workplace. Our intelligence is downplayed and we don’t get much information given to us.

Aggregate is typical granite. Can’t say what size for certain, I just know it’s various from a delivery docket. About 10mm to 30mm.

It gets a bit nonsensical with w/c ratio. It’s a .6 average. It makes no sense to me at all. It’s about as dry as workable can be.

Aggregate washing is accomodated for.

Air temperature would be in the low to mid 30°c.

Slump tests are only done on request, I’ve never been required to do one. But after using mid range WR for most of my career, with slump often being between 50mm to 70mm and observing SP slump tests that colleagues have done, which are around 150mm, I don’t need a slump test to see that mine are far less.

Aside from premature surface drying and shrinkage cracking, the surface finishes turn out great in cooler and still days. In fact, in some ways better than standard MRWR concrete. But other days, and lately more frequent days have been a nightmare.

I think one contributing factor could be that it’s a “one diet fits all” approach. Batch design doesn’t change according to any environmental factors. It’s kept the same because we’re all stupid.

 

[Ron]

It sounds like you're doing a lot of "seat of the pants" mix design and not understanding the impact of some of your variables.

You don't say when you are adding the HRWR admixture. If you are putting it in at the time of initial batching, your slump seems awfully low.

Your placement temperature is fairly high. If you have any wind blowing across your forms, you're likely to get plastic shrinkage cracks very early after placement.

I suggest you get a copy of several publications of the American Concrete Institute (ACI). I suggest you start with ACI 301, ACI 211 and ACI 305. All of these are applicable your task.

 

[epoxybot]

You don't say whether you are using a curing compound but it sounds like this is happening during or just after the finishing. In a Precast Yard, it might be worth it over the long run to look at a fogging system but have you tried an Evaporation Retarder? You would have to be done with finishing because putting a trowel to the Evaporation Retarder would break the surface of the film.

 

[ATSE]

In addition to Ron's suggestions, I would move this to the front of the stack:
PCA Design and Control of Concrete Mixtures. Get the 16th edition. Read 1/4 of that document and you will be an expert.

 

[cvg]

.6 is a high water cement ratio and combining that with superplasticizer normally would result in a very fluid mixture and prone to cracking. but you say it is very dry mix... something doesn't seem quite right

 

[Settingsun]

CCAA's Guide to Concrete Construction is free and probably covers Australian concretes better than ACI guides would. 30+ degrees in a wind tunnel is a major no-no though. A precasting yard should have wind shelters sorted.

If you have to move the panel out of the form and can't do accelerated curing (steam), you should be using a curing compound or continue wet curing wherever you move the panel to.

420kg cement for 32MPa also doesn't seem to stack up. You could ask Boral or someone else for a mix design sheet for their standard 32MPa mix. They give the basic quantities out.

 

[Enable]

[ul]
[li]10mm to 30mm aggregate size difference is monumental in terms of shrinkage (also in terms of aggregate absorption of water)[/li]
[li]w/c = 0.6 and calling it a dry mix is incompatible unless you're taking 4 hours to pour from batch or 0 water is added[/li]
[li]super-p mix with 50 to 70mm slump means A) it's all super-p and 0 water or B) you haven't measured the slump correctly or C) literally no super-p[/li]
[li]if ambient conditions are 30 degrees + where is the ice water?[/li]
[/ul]

You need a concrete chemist to review. There are so many things needing attention on the surface here that I find it doubtful that we can be of any real help other than to say your concrete sounds like it's being batched improperly in a lot of ways.

As an side, we (Canadians) generally don't use super-p as an accelerator. Is that actually common practice? Here we increase surface area of the cement particulates to induce a quicker reaction (or use chloride accelerators if under code max limit).

 

[dik]

...and this Canadian engineer never uses chloride accelerators. There are too many horror stories about this.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik