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Adding mixing water in the field

Adding mixing water in the field

Adding mixing water in the field

(OP)
I am wondering how to control the addition of water in the field. Contractors often want to add water to make a mix more workable, but doesn't this compromise the w/c ratio? Neither ACI 318 nor ACI 117 seem to indicate a tolerance for w/c ratio. Any comments?

RE: Adding mixing water in the field

The short answer is that adding water in the field should not be allowed. In place strength is very sensitive to the amount of water added in the field. A rule of thumb I have heard is that every gallon added per yard of concrete will reduce the 28 day compressive strenth by 250 psi. This is not an issue to be taken lightly, and most concrete specifications do not allow water added in the field for that reason.

ACI does not have a tolerance on water-cement ratio because acceptance is based on the 28 day strength, not the water cement ratio. As long as the concrete meets the performance criteria (in this case strength), it is acceptable.

Workability should be considered in the mix design. If workability is a concern, the ready mix supplier may well need to add mid or high range water reducing admixtures to meet the contractor's workability needs while maintaing the w/c ratio to meet your strength needs.

RE: Adding mixing water in the field

The supplier should batch the concrete with less than than the maximum w/c required so that their is water available for slump adjustment. If this is not possible consider the use of a superplasticizer to increase slump. ACI-305 and ACI-306 should be of some help as far as slump adjustments are concerned.

RE: Adding mixing water in the field

If water is added in the field, it should only be done by the concrete producer, not the finishers or placers. The producer knows his mix design (or should) and can adjust as necessary to produce workability, preferably with admixtures, not water. Adding water at the site does compromise the strength of the concrete, but rarely does it fall below the design compressive strength because of overdesign factors in the mix design. You are much more likely to compromise the durability of the concrete by water addition. This can be critical for exposed concrete or concrete subjected to traffic (interior or exterior)

RE: Adding mixing water in the field

The above are excellent points. Don't be afraid to simply reject a load for being too "dry". It will quickly get the point across to the producer to do better QC at the plant. The rejected loads will immediately be attended to by the producer who is more knowledgable in tempering the mix with add water while staying within the acceptance limits for strength.
We are typically geared to look for high slumps as rejects, but dry mix can be problematic also. In formed walls, voids are likely due to inadaquate consolidation or perhaps segregation due to over vibrating the dry mix into place. In pavements or other flat work, a dry mix usually finishes poorly.

RE: Adding mixing water in the field

i disagee with the second statement above concerning " rarely does it fall below the design compression strength"

my background 17 years indepent testing lab field technician. concrete,soils, asphalt

with the high price of fossill fuels,cement ,labor, equipment concrete producer are doing everything to cut cost. while two producer in the same town are turning out a good product there is alway a third producer that is cutting back on cement and over doing he fly ash and forgetting about the admixture. so he is producing a 3000 that will barely hold a 4 slump. then the unskill, uneducated, uncontrolled concrete finisher start dumping the water to it. contractor will not argue because afrraid they will walk. producer are scare stiff. cold joint are more problem than returning loads. so i must step in and argue and look like an ass. must fight with drivers, then finisher, then contractor just to turn a good product. if the sample do not break its alway the lab. never the stupid un skilled i do not give a damn finisher. think about it the total srength of that skyscaper is in the hands of a six grade dropout finisher.

RE: Adding mixing water in the field

I agree with brownbagg!!! You have to watch the concrete supplier like a hawk! Sometimes the the mix delivered to the site does not remotely resemble the design mix. Since there are so many other variables affecting the concrete, reject any truck that does not meet specifications WITHOUT THE ADDITION OF WATER AT THE SITE.

RE: Adding mixing water in the field

Interesting observations, I must admit that in 15 years of owning and operating my own practice here in Sydney I have only had a few occassions where the quality of the supplied concrete is suspect. We have a few rules that assist this process.

1. Our specification requires that the builder must provide the mix design to us for our approval. We then get to see who the supplier is. If there is any doubts as to the suppliers credibility and expersience then it is rejected and another supplier/mix design is requested. You get to know the names of the shonky suppliers in town and hence steer clear of them.

2. For most large construction jobs, it is a QA requirement (in Australia) that the builder holds what is called "toolbox meetings" whereby all the sub-contrcators for a particular aspect of the job are put in the one room and the construction process discussed in detail. These are veryuseful meetings and I normally try to get one of my engineers to attend this. It gives us a good chance to table our expectations for things like reo, PT and concrete supply.

3. For the larger concrete pours we would normally have at least two engineers on site on the day. One checking the reo (and any changes you requested) and keeping an eye on the laying and compacting techniques and the other keeping an eye on the concerete supply. As one is linked to the other I have both engineers communicate with each other via two way radio.

4. If there are any doubts as to the concerete supply then it is rejected and the truck sent away. A good tip is to note the rego of the truck as they have been known to switch drivers and simply drive around the block.

RE: Adding mixing water in the field

Adding mix water in the field is the first indication that something has gone wrong.  The addition of mix water at the site may be made on "add water" allowable to meet the W/CM limit of the mix design and shown on the batch ticket provided a batch ticket is required, however there is are unknowns in this paper reliance system.  What is the amount of water already in the ready-mix truck drum or, what is the mix characteristic in the drum that was rejuvenated overnight from a leftover load remainder?  40 gallons of wash water left in the drum wouldn't show much if there was a visual inspection at the plant before batching and production for the new day.  Also, the mix batch is based on the last SSD/moisture content measurement made, whenever that was done.  Once the mix has been batched and put in the ready-mix truck, the money has been spent and there is no way to improve the concrete durability from there to placement and finishing, only degrade it  (however, the addition of superplasticizers at the site rather than during batching is recommended to adjust for delays in delivery).  

Tom Bryan
bryantc@mail.icongrp.com

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