Adding water to concrete onsite 3

[chubbyone]

I do work for a company that strictly enforces global practices. Their practice for placing concrete is that their contractor is not allowed to add water onsite. Obviously, they are just following the industry standard. I’m looking for supporting documentation to get an amendment to the global practice when it comes to minor structural uses of concrete, for instance a small water treatment plant foundation and slabs I’m involved with now. I believe, based on many years of working with concrete, that there is a certain amount of water that can be added without compromising the design strength. I’m looking for this amendment for extenuating circumstances only. My utmost priority is to follow safe work practices and industry guidelines. I’m not looking for the easy way out. I just feel that there has to be a way to handle this. I understand that there are those who will say under no circumstances should water be added but I’m looking for an experienced factually based opinion(s) to aid me on my mission to change G.P. I can give examples of why I'm looking for the amendment, to anyone interested, that will better explain the issue at hand. Any takers?

 

[Ron]

The issue is not whether water can be added to the concrete without compromising its properties, but who might be responsible for that concrete in the event of a failure. Suppose 10 gallons of water is added to a full 10 cy transit mixer. It makes a difference who adds that water.

Let's further suppose that the water was added at the your request. You have now taken on the responsibility of the performance of the concrete and relieve the ready-mix supplier of his responsibility.

The ready mix supplier knows, or should know, more about his mix design than anyone else on the job. Only the ready mix supplier should add water or change the mix in any other way.

Your authorization of additional water might will change the water-cement ratio of the mix. If the supplier did not hold back any water at the time of mixing, then you might have authorized a change that puts the mix out of compliance with the specifications. You don't want to assume the liability for that. Not only do you then bear the responsibility for the direct cost of the concrete if it if fails to meet the project requirements, but also the cost of removal and replacement of the concrete. If it is in a foundation, that can get really expensive!

Water can successfully be added at the site to accommodate some conditions. Convenience of placement is not one of those conditions. The mix design should be prepared to accommodate placement conditions, not changing the mix at the jobsite to satisfy the concrete crew.

Only the supplier should add water to the concrete. If you are having problems with too little slump or other placement problems, have the supplier put a QA representative on the site to make those changes. Don't do it yourself or let the GC or concrete sub do it!

Remember, good concrete is made from cement, aggregate and water. Bad concrete is made from the same materials! Compressive strength is not the only measure of concrete performance. Durability is often more important than gross strength.

 

[TXStructural]

Don't do it. The best way to ensure concrete problems and durability problems is to add excess water. I am not sure what a "minor structural use" is, but if it needs to be concrete, don't intentionally allow it to be unpredictably weak.

Allow proper admixtures, but not additional water, especially for structures requiring dense, durable concrete, such as water/waste treatment facilities, and those in coastal areas or otherwise exposed to chlorides.

Require proper, proven mix designs. If water reducers are added with the initial water, conventional slump will not really work, so rely on other indicators (batch tickets plus consistency of mix, flow cone, etc.)

 

[wannabeSE]

See ACI 318-05 Commentary R5.10 for a breif discussion on adding water. But, heed Ron's advice concerning liability.

 

[asixth]

Thanks Ron, good advice

 

[asixth]

I was thinking today, how would placing concrete on water fair i.e. such as when footings accumulate water from rain. I had a builder who told me that the concrete will push the water away. Would this water be taken up into the concrete creating a weaker and less durable concrete?

 

[hokie66]

Some of it will contaminate the concrete. Builders will tell you anything to save themselves some work and avoid compliance with good practice.

 

[slickdeals]

I am curious about the response to using concrete to displace water during a foundation concrete pour as opposed to dewatering till it is completely dry.

There are instances, depending on the water table, where no matter how hard you try, you cannot get the bottom of the foundation dry and you might have to resort to placing concrete while displacing the water. What do you sage folks think?

We are Virginia Tech
Go HOKIES

 

[dik]

Often used Tremie concrete including some pours of >1000 yds (continuous day and night operation). You have to look at each application and determine if the (generally) slight reduction in properties is OK. Also, it's not so much a matter of dumping concrete in water, but pumping concrete into the initial mass of concrete that initiated the tremie pour.

Dik

 

[TXStructural]

Concrete should be pumped through a tremie to the bottom of the water-filled hole. As the column of concrete rises above the end of the tremie, it is withdrawn, keeping the end submerged in good concrete. Keep pumping until the water-concrete mix at the top is expelled from the forms.

 

[Ron]

I treat wet footing bottoms in one of two ways. First, you can overexcavate the footing and bring back to bearing level with #57 stone (stone graded between about 1/2" and 1") or similar. Second, you can place a sacrificial layer of concrete (say 2" thick) to create a placement mat. Allow the placement mat to sit for a couple of days to get any initial compression of the sloppy wet soils out of the way...then place the footing.

As hokie66 said, some of the concrete will be contaminated by the excess water.

 

[hokie66]

Just to clarify my answer, it was for high level footings. Concrete can be placed UNDER water by tremie, but it should never be dumped INTO water.

 

[msucog]

most of my comments have already been covered above but i'll post them to simply support the previous posts.

experience here placing lots of concrete and testing lots of concrete...never dump concrete in to water. if water has collected in excavation, remove it then place concrete. and beward overexcavating and placing thick layers of #57 stone where water is present. #57 stone must be compacted/densified and soil may migrate in to the stone layer over time.

if water has collected on top of footing and within wall forms, pump/siphon it off. if the concrete arrives too stiff to effectively place, then i don't have a problem with the contractor adding a minimal amount of water for workability. however, the contractor is responsible for this. the batch should be remixed in accordance with ACI and the slump and air content should remain within specification ranges. also, hot weather placement practices according to ACI must be followed.

in other words, a minimal amount of water may be added once the truck arrives onsite to get the mix more workable but must still satisfy the project requirements. if workability is a huge issue, there are plenty of admixes that will do the trick. the contractor should address such concerns with the project team in advance and adjust the mix beforehand as well as receiving approval to use said mix during construction. the process is really not all that difficult to follow...that being said as both a previous contractor for large projects and now a construction materials testing engineer.

by the way, i would oppose any and all remotely vague language that allows the contractor to use their own judgement as to whether they can vary from the specifications without prior approval from the engineer of record.

 

[rscassar]

I have seen concrete specifications which state the ready-mix supplier is to submit the maximum amount of water which can be added at site and to submit information on how much water was added on-site.