Negative effects of high water to cement ratio 3

[DWHA]

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.

 

[Ron]

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.

 

[S275]

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.

 

[chopnhack]

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.

 

[DWHA]

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.

 

[Forensic74]

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!

 

[dcarr82775]

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.

 

[DWHA]

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.

 

[TXStructural]

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.

 

[B16A2]

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.

 

[DWHA]

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.

 

[hawkaz]

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.

 

[B16A2]

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.

 

[TXStructural]

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.

 

[dcarr82775]

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.