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.

 

[frv]

I know higher shrinkage for sure.

 

[dcarr82775]

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.

 

[JedClampett]

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.

 

[TXStructural]

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.

 

[Quanski]

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

 

[DWHA]

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.

 

[Ron]

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.

 

[JedClampett]

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.

 

[Forensic74]

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....

 

[Ron]

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.

 

[pmblair]

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.

 

[Forensic74]

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.

 

[DWHA]

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.

 

[Forensic74]

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.

 

[DWHA]

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.

 

[ATSE]

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.

 

[TXStructural]

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.

 

[Forensic74]

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.

 

[apsix]

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.