Water Reducer in Concrete

[SteelPE]

I have a project where we are pouring a large exterior slab. A question came in from the owner with regards to the water reducer being used for the project. His concerns revolved around another project that was constructed a few years ago and the fact that the slab has surface cracking and this surface cracking is being blamed on the water reducer.

I am no where near a concrete additive expert. This is the first time I have ever heard of this issue with regards to a water reducer. I actually thought a water reducer would help with cracking over time (shows what I know).

Is the problem referenced, water reducing additives causing surface cracking over time, a real issue or is this something that the owner is making up? Also, if it is a real issue, how do you prevent this from happening while maintaining a workable concrete mix?

 

[Settingsun]

What strength and what water:binder ratio?

Could easily be that the contractor threw the concrete mix under the bus rather than admit the curing of the other slab was not done properly.

 

[SteelPE]

I don't know the specifics about the project the client was referring to.

Our project is using a 4,500 psi concrete mix. I don't know what you are referring when you reference binder ration.... but our water/cement ratio is 0.42.

 

[STrctPono]

I have never heard of such a thing. Honestly, even if the superplasticizer were overdosed, I don't think this would happen. There are so many other potential culprits. I would write this one off as the owner just making biased and post hoc accusations for something they know nothing about. What is the owner's background and do they have any experience with concrete?

 

[Rabbit12]

I had a situation a couple years ago where we were using a low w/c ratio mix and seeing surface craze/map cracking. Due to the low w/c ratio the ready mix supplier thought the cracking was due to not having enough water to help with the finishing process and the surface was flash drying. I think it's a plausible since they were getting a lot of their workability with chemical vs. water.

We adjusted our specs slightly to allow a little higher w/c ratio. I think it was possible to finish with the low water cement ratio, but there was a very small window before we saw surface cracking.

I wouldn't completely dismiss your client. In my experience the cracking showed up relatively shortly after the placement.

 

[SteelPE]

In this instance, I would give the owner some leeway.

Since the slab is an exterior slab in a freezing area we have requested air entrainment and a broom finish, so I don't know how much "finishing" will be required for this project.

 

[Enable]

I'll preface this with the fact that I am not a concrete chemist, and you'll have to talk to one to get a definitive answer. However, based on my experience as a contractor who has poured 35 mPa air-entrained C1 concrete (w/c = 0.40) with superplasticizer (high range water reducer) several times a week for the last few decades, I would be disinclined to believe cracking resulted from the water reducer. I have not once had such an issue with these types of admixtures.

Surface cracking is almost always the result of A) too much restraint (read: reinforcing steel in SOG or not enough at restrained locations in reinforced slabs), B) ambient conditions (read: wind, temperature, direct sunlight) leading to high evaporation rates, C) improper curing, or D) improper finishing. The cracking Rabbit12 discusses would probably be plastic shrinkage given its occurrence immediately after placement; this is a known phenomenon and is likely not due to the presence of a water reducer (though maybe not, more details would need to be known to assess of course).

Hot weather concreting requires special considerations, and in the North we typically use the following to aid the cause:
1 - Ice water (reduces temperature of concrete)
2 - Retarders

I've certainly had cracking on my pours, but most assuredly it wasn't due to the water reducer.

 

[Rabbit12]

Enable it wasn't plastic shrinkage cracks.

 

[Enable]

On the other hand, we have this paper that provides some evidence otherwise. Though the testing procedure gives me some pause as they are measuring shrinkage cracking in a 24" specimen. Also, the results are variable since in the last comparison F1 vs F2 it's the specimen with the higher admixture content that cracks less by area. Also, I find it odd that D2/F2 are the designation for the higher dosed mixed, but it switches for E1/E2 where E2 is the lower dosed mixed.

I've also found this discussion about shrinkage due to admixtures by a few big wigs (VP at Euclid). He is on the side of not inducing cracking but rather helping.

 

[dik]

I suspect strongly that the cause was something else... quite possibly improper finishing, and if the cracks were shrinkage... maybe poor sawcutting procedure.

You have to be extra careful when the owner recommends something... it opens up a whole new area of liability.

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

-Dik

 

[STrctPono]

Rabbit12, unless your mix was below a 0.40 w/c ratio and you guys were limiting them to below 650 lbs of cement there should be enough cream in the mix for them to finish it. Did they use any finishing aid? Was the aggregate base properly moisture controlled? Was it a hot, dry, and windy day? How soon did they get to curing it? There are so many other more common culprits for why this may have been the case.

 

[dik]

Is it possible to get photos of the type of damage?

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

-Dik

 

[Ron]

Agree with dik...probably something else was the cause.

Water reducing admixtures, in and of themselves, do not contribute to cracking. Improper mix design for application, poor placement and poor finishing techniques are the more likely culprits. Keep in mind contractors will blame issues on anything other than their work.

 

[Enable]

Agree with dik...probably something else was the cause.

Water reducing admixtures, in and of themselves, do not contribute to cracking. Improper mix design for application, poor placement and poor finishing techniques are the more likely culprits. Keep in mind contractors will blame issues on anything other than their work.

Rule No.1: It's always someone else's issue / if there is a problem it is due do something other then your crew (e.g. late concrete, bad batch, bad design)
Rule No.2: See rule No.1

- Contractor Playbook Play No. 23

If play No.23 fails attempt No.24

This is industry standard. Don't know what else to tell you. Honestly, I cannot tell you how the actual standard that measures 1/4" deviation in 10ft doesn't apply here. You must not have industry experience. This is industry standard construction

Did I mention this is industry standard?

- Contractor Playbook Play No. 24

 

[dik]

dik[/s] Ron...probably something else was the cause.]

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

-Dik

 

[dik]

@Enable... I have a torrofecisimeter that cuts through playbook rules pretty quick. That's one of the few times that I would consider coring to get some petrographic information.

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

-Dik

 

[Rabbit12]

Enable, last post on this from me as I'm not going to argue with you. The w/c ratio was below .4; I'm going from memory but I think it was .38.

Did the HRWR "cause" the cracking? No, but if my client heard that using chemical (HRWR) to get workability made finishing/curing very difficult (which turned into cracking) it's a pretty simple correlation for them to think it was caused by HRWR whether that's the case or not.