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Negative effects of high water to cement ratio
3

Negative effects of high water to cement ratio

Negative effects of high water to cement ratio

(OP)
Other than reduction in strength, are there other negative effects to having a high water to cement ratio? Such as additional shrinkage, cracking, durability, etc.

The reason I ask is because in my experience, most of the issues I see with concrete is poor consolidation.  Additionally, I have rarely seen low break strenght results.  Based on this, I would rather see a higher ratio to achieve proper consolidation.   

RE: Negative effects of high water to cement ratio

I know higher shrinkage for sure.

RE: Negative effects of high water to cement ratio

Shrinkage is the one that comes to mind.  Lower w/c ratios tend to perform better in freeze/thaw cycles with salt (in my experience anyway). You can spec a lower w/c ration and a larger slump an the supplier will use a mix with water reducing admixtures.

RE: Negative effects of high water to cement ratio

For one, you could add so much water that there's too much to react with the cement and the free water is trapped in the concrete.
Higher w/c also increases the porosity of the concrete. That's why it doesn't perform well with melting compounds. They absorb into the concrete and the chlorides wreak havoc.
There's other ways to improve consolidation of concrete. Increasing w/c should be the last resort.

RE: Negative effects of high water to cement ratio

3
Shrinkage and durability are the big two, as mentioned above.  It also encourages segregation of aggregates.
Increasing water above optimum does not help consolidation of aggregates; it increases paste volume so the mortar fills voids which encourages shrinkage.  Use a water reducing admix and if you need really fluid concrete, also use a viscosity modifier to keep aggregates from segregating as readily.

RE: Negative effects of high water to cement ratio

High w/c ratio has a major effect on concrete durability and strength.  I would not recommend increasing w/c ratio for better consolidation.  I suggest using chemical admixture (i.e. plastizers) to get the workability and consolidation you need.  Concrete with high w/c ratio are more susceptible to freeze/thaw damage (i.e. scaling, cracking), chloride ingress (that leads to corrosion/spalling), sulphate attack, carbonation, etc

RE: Negative effects of high water to cement ratio

(OP)
I was just wondering bacause too many times I have seen other engineers want the concrete that is coming out of the shoot almost so dry you need to shovel it down the shoot of the truck.  I am in no way advocating that the w/c ratio be overly high, but I am not one of those guys that wants super stiff concrete on a structural pour with lots of rebar.  

And by allowing a more workable concrete, I was just wondering what side effects it was inducing.  

I feel there needs to be some flexability in the field.  I don't want soup, but I also don't want a 1" slump.

RE: Negative effects of high water to cement ratio

Concrete mixes can and should be designed for the application.  As noted by others, a high w-c ratio is detrimental to strength and durability.  

Ease of placement can be designed into the mix while maintaining a low w-c ratio.  This can be done with high range water reducing admixtures (plasticizers/super plasticizers).

Concrete placement and finishing crews typically know very little about the properties and effects of their work on the performance of concrete.  They know their experience with placement, what makes it easier and faster for them, and little else.

Have the mix designed for the application, using the largest coarse aggregate practicable for the application and then don't let them screw it up in the field by their desire to make life easier for themselves.

RE: Negative effects of high water to cement ratio

There are also code requirements on w/c ratio. In wastewater treatment plants, w/c needs to be at .4. For water treatment, it needs to be less than .45. In corrosive soils, the w/c is restricted, depending on the aggressiveness of the soil.
So we're not only limiting the water to make the concrete hard to handle.

RE: Negative effects of high water to cement ratio

DHWA, why are you blaming this on the engineer and not the cheap contractor who's not using an adequate dosage of super P?  During my grad work I did full scale testing with w/c=0.3.  It came out of the truck just fine.  Autogenous shrinkage was another story....

RE: Negative effects of high water to cement ratio

MainMan10...a w-c ratio of 0.30 will not typically come out of the truck "just fine" without admixture enhancement.  If your graduate study was based on this alone, it did not adequately cover the issues of placement of low slump concrete.

RE: Negative effects of high water to cement ratio

For interior Slab on Grade, you can run into issues with finishes if the w/c ratio is high, adhesives wont bond well and it can take longer to get the humidity out of building if the envelope is closed. Thats all dependent on when the slab is poured obviously.


Other then that the typical shrinkage as mentioned by others.  

RE: Negative effects of high water to cement ratio

Ron, you do know what super P is right?  Maybe I should have said High-Range Water Reducer to keep it old school. :)  If 0.3 isn't coming out of the truck just fine it's because the supplier doesn't know what he's doing, didn't bid the job right to include the correct dosages/proportions, or is trying to force you to adjust your specs so he can supply a cheaper mix.... take your pick.

I was the one designing the mixes, paying for them, leading the crew to cast and finish, to ultimately control shrinkage cracking, and permeability...it wasn't an online masters degree.

It all boils down to the lack of knowledge at the concrete plant to stray outside typical mixtures and the obvious drive to keep the admixture and paste contents as low as possible.     

RE: Negative effects of high water to cement ratio

(OP)
MainMan - Ron stated "will not typically come out of the truck "just fine" without admixture enhancement"  by super is an admixture enhancement.

Additionally, I am not blaming an engineer for anything.  From my field experience I have seen some engineers push back on the construction company very hard when it comes to adding water to the mix in the field.  And it was evident when the forms were stripped, a lot of honeycome was noticed.  

I on the other hand would rather allow some water on site to prevent this from occuring.  

The purpose of this post was to gain additional insite to know what negative effects are more likley to occur when water is added in the field.   

RE: Negative effects of high water to cement ratio

Yea that's what I said twice now. I dunno, you said engineers wanted such a dry mix that it couldn't be placed.  Thats not the engineer's fault that the supplier couldn't dose his mix right.  You shouldn't be making that call in the field anyway because it allows the supplier to absolve his liability because you're changing his mix design.  Its like modifying a brand new PEMB in the field....if something happens down the line they're off the hook.  

And btw, they will usually try to cast cylinders before adding water, so if crushing is really that big of a concern to you, then you may want to consider when they pull from the truck.  

RE: Negative effects of high water to cement ratio

(OP)
It is not like modifying a PEMB in the field.  You can stop and start the construction of a PEMB at about any point which you would like.  Not a concrete pour.  Additionally the mix designs typically have a W/C ratio range.  If the mix comes out to the site at the lower end of the range, and the contractor would like to add some water to bring it up to the middle or upper end of the range it is not absolving the supplier of their responsibility.  The mix still meets specs.  

I always require cylinder, air and slump be from the mix after everything is added.   

RE: Negative effects of high water to cement ratio

MainMan - w/cm=0.3 may very well be possible, but certainly not common.  Even a relatively "standard" 0.40 w/cm mix has a much higher tendency to have field problems with an experienced team (engineers, contractors and producers).
You might be on to something big, but I would hesitate to say that the ready-mix plant (or engineers, or contractors) doesn't know what they're doing.  They might simply be acting cautiously and prudently, which is good.  Most ready-mix plants stay within known mix proportion bounds for obvious reasons.
I would agree that most ready-mix suppliers can provide w/cm < 0.45 that has both adequate workability and reliable durability and strength - you don't have to choose.  Below 0.40, you're doing research.

RE: Negative effects of high water to cement ratio

I think the problem centers around the contractor or ready mix guys wanting to add water but not knowing how much is ok, and how much takes you over the limit.  If the trip/batch ticket says how much was in the original mix, it is fairly easy to calc how much can be added - but in general, if it left the plant on spec, I wouldn't let them add.  And if it is modified with water reducers and viscosity modifiers, it should come out of the truck right without adding extra water or they simply don't know how to make concrete with the materials they are using.  The aggregates may have been dry, but then they might have messed up the batch... you won't know until the breaks come back.

Process control by the supplier is the only way to assure you get what you need, and sloppy control will get you wet and dry mixes with similar batch tickets.

RE: Negative effects of high water to cement ratio

Exactly.  FWIW, bridge decks and roadways are now frequently cast with 0.36.  My point being that really any w/c is possible at whatever workability you want.

A concrete mix is a pre-engineered product.  If you change it, you take responsibility for it.  Doesn't matter what your spec says, it's the mix design put together at the plant that holds any water in court.  Since OP doesn't have a good handle on what adding water does, and probably doesn't know how to design mixes, I dont recommend taking the risk, but hey he can do what he likes.

 

RE: Negative effects of high water to cement ratio

Specify the required strength, cement content, w/c ratio and slump for your project and there should be no need to add water on site, provided the concretors are competent.
Adding water should not be entertained unless there are specific instructions from the batch plant.

RE: Negative effects of high water to cement ratio

Quote:

My point being that really any w/c is possible at whatever workability you want.

MainMan10....this is where we agree; however, most ready-mix suppliers know a lot more about mix designs than most engineers.  They are the suppliers.  They are supposed to deliver concrete to the site within the specifications....but they need to know the parameters at the site before they design the mixes.  They are responsible for the mix design, characteristics at delivery, and performance, assuming no adulteration of the mix at the site.  Water addition, if done by anyone other than the supplier's QC representative, is adulteration of the mix and relieves the supplier of his responsibility.

RE: Negative effects of high water to cement ratio

I remember a colleague who told me about a ready-mix concrete slump test with slump of 150 mm (acceptable and used for a multi-million project) and there were no shrinkage cracks.

I assume that very high workability was due to admixtures rather than higher w/c ratio! Or maybe balancing between both.

RE: Negative effects of high water to cement ratio

From my limited experience with concrete, is there a reason pozzolan is not being used instead of more water to improve workability? In cold weather it might retard curing, but in warmer climates the gain in workability may offset the slower onset of final strength and ultimate strength should be higher.

RE: Negative effects of high water to cement ratio

(OP)
MainMan - You are right, I do not design concrete mixes.  I had an inkling of what adding excessive amounts of water would do.  That is why I asked the question.  

I have never changed the mix design, nor would I ever.  What I have been saying is that I will not stop the contractor (QC representative) from adding water if they felt inclinded to do so.  They are the ones taking responsibility for the mix, not myself.  

You are getting off the point.  The question was WHAT NEGATIVE EFFECTS", not who's fault, or who is going to take responsibility, or who's liability is absolved,  etc.  

RE: Negative effects of high water to cement ratio

Sometimes when you don't know enough on a subject you don't know the correct question to ask.  Just trying to make you think.  Obviously you're bent on adding water....risky considering you have to come on here to ask the fundamental question of w/c.  Good luck!

RE: Negative effects of high water to cement ratio

DWHA,

If you knowingly allow them add water and do nothing you can pretty easily be caught in the net should something go wrong. Many suppliers keep some of the batch water out of the mix for adding at the site if needed (essentially delivering a lower than designed w/c ratio to the site).  The amount that may be added at the site is printed out on the ticket.  I never let them add water beyond that amount.  If the contractor has concerns about workability/placement I work with them and the supplier ahead of time to get a mix everybody is happy with before the truck gets on site.

RE: Negative effects of high water to cement ratio

(OP)
Ya, that is exactly what I typically see.  There is an amount of water allowed in the field (either printed on the batch ticket or documented by the QC person).  

I generally do not object to allowing the contractor to add water up to the amount allowed on the batch ticket or QC paperwork.  I have seen others push back on the contractor preventing them from adding any of such water to the mix.

RE: Negative effects of high water to cement ratio

1) increased risk of segregation of aggregates, reducing homogeneity.
2) increases porosity/permeability
3) decreased resistance to chlorides and such
4) increased risk of shrinkage
5) increased surface defects, depends on finishing
6) decreased strength

Segregation can result in non-uniform strength and shrinkage.
The way to reduce shrinkage is to design a mix which, when placed, results in maximum rock-to-rock contact.  In addition to other benefits like workability, small aggregate/sand acts as a filler to reduce paste.  The paste will shrink as it cures and dries, so any excess paste can result in shrinkage.  Adding water increases the amount of paste AND the the amount of excess water which will evaporate rather than react.

In the reaction/curing/drying process, wetter mixes end up with more and larger pores in the matrix. As the water fraction increases above optimum, the voids remain connected into pathways, rather than becoming individual pore. This is a reason for the increased permeability.  The increased porosity can also mean less strength and durability since the optimum pores size for strength and durability is roughly 10 nm.

You might take a gander at:
http://ciks.cbt.nist.gov/~garbocz/appendix1/adler.html
for some more info.  As the article says, it is difficult to related nano-scale reaction products with the mass of concrete in front of you when they represent nine orders of magnitude in size.

RE: Negative effects of high water to cement ratio

OP you sound confused, is this a question of adding an excessive amount of water or not?  You go back and forth depending on who you're arguing with.  The few gallons a supplier MIGHT hold back at the plant is never an excessive amount.

I only comment because you could actually have greater shrinkage if an excessive amount of water is held back and you dont allow them to add the remaining mix water.  The mix could self desiccate early on and your capillary stresses would be huge causing a lot of shrinkage.  This is a big problem with bridge decks and high performance mixes where they're using sub 0.39 w/c.

If you're really talking about letting them add water just to increase workability above and beyond the approved mix (like most contractors will do without even asking you), I would avoid doing that because of all the bad things listed above.

RE: Negative effects of high water to cement ratio

(OP)
To clarify.  Excessive is a realative term.  Let me try to further explain.  

Concrete truck arrives on site, batch ticket/QC doc allows up to 4 gallon per yard to be allowed and still meet mix criteria.  Concrete rolls out of truck stiff (1" - 2" slump).  

What I have seen is some field engineers push back on the contractor telling them not to put any additional water in the mix.  I on the other hand would not object to adding up to the 4 gallons per yard.  Obviously, I would suggest adding 2 gal per yard and mix it up to see what that gives you.  

I do not advocate adding more water than the mix design allows.

I was just trying to get a feeling why some want the mix as dry as possible - Even when the mix design allows more water to be added.

RE: Negative effects of high water to cement ratio

I specify that no water is to be added after the concrete has been batched. It is up to the supplier to get the mix right so it is workable before it leaves the plant.

If you add water in the field, there are very specific requirements about how many rotations need to be done to get it to mix properly. As the old joke goes: "What is dumber than a concrete truck driver... 2 concrete truck drivers". I have little confidence that this will be mixed properly in the field.

By the way- adding 4 gallons per yard is a lot of water. You could easily lose a couple hundred psi based on that.

RE: Negative effects of high water to cement ratio

I hope he means 4 gal/truck.  Either way, methinks some other engineer should make the call.  4 gal/yard roughly takes a 0.42 and makes it 0.56.

RE: Negative effects of high water to cement ratio

It is folly for engineers to specify slump, yet we have not yet eliminated it from the ACI specification requirements.  Slump functions only as an indicator that a mix might roughly comply with the mix design requirements.  If the supplier's mix design includes an amount of water, the only reason that water is withheld is to prevent too high a slump at the time of delivery, or maybe to offset the possibility that a mistake was made somewhere.  Also, admixtures should be added at the time of batching, and slump is not useful once a superplasticizer/HWRA has been added.

Aside from preventing a deviation from required slump, there should be no reason to withhold the allowed water.  Drier concrete mix is not necessarily stronger or more durable.  The supplier will have QA for a history of the mix, upon which we should rely, since we, as the engineer, really have no idea how that particular mix will look or behave.

Adding only the allowed 4 gallons of water is not a problem.  The mix will almost always decrease slump between the plant and the job site, so some adjustment should be allowed using this water.

RE: Negative effects of high water to cement ratio

The batch tickets I see give an amount that can be added to a truck, not per yard.  Averages 6-10 gallons for a full truck.   

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