Monitering water added to concrete on-site 2

[HeavyCivil]

I have a few years of field experience around concrete but never wearing the engineer / inspector hat. I understand the implications of water added to concrete on-site.

I calculate the amount of water that can be added to the mix based on the ticket and target or max w/c that we have specified. The problem is that many of the trucks I've been dealing with have broken meters or inaccurate ones due to pressure in the tank. The redimix company is well respected and very professional - not a cobb job outfit.

I cannot justifiably send a truck back because I don't know how much water was added, but I cant justifiably be comfortable with the mix either when all I have is the operator's word that he "added 10 gallons".

Any solutions to this?

 

[cntw1953]

I am not familiar with field testing practice, but I guess one would maintain a record for such, and I might make notes on number of truck load with w/c ratio uncertain, notify and hold the redimix company responsible for the end result, if the compressive strength falls below.

 

[Splitrings]

VTEIT,
Here is a link to our concrete construction spec. I follow this as best I can. After 19 years of watching concrete coming down a chute, I have little faith in concrete truck drivers! I will allow the addition of just enough water to increase the slump by one inch. If they jerk you around send the load back. They usually catch on pretty quick. Just watch out if the plant isn't too far away, you will get the same load back with a few extra bags of cement added.

I also have been requiring a water reducing or plastizer be added for most flat work. Then the contractor can place (pour) the higher slump concrete they like too.

 

[abusementpark]

We do not allow and it is written in bold in our General Notes.

 

[msquared48]

That's what a slump cone is for and the slump range specified in the specs. It can be measured in the field at the truck if push comes to shove.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

VTEIT...
A couple of points..

Assuming you are in the US.

If the water meter or sight glass are not functional, then the transit mixer does not meet the requirements of ASTM C94, "Standard Specification for Ready-Mixed Concrete", paragraph 8.3. Further, if water is added at the site, the revolution counter on the drum must be working as well, as you are required to run at mixing speed for at least 30 revolutions after water addition, but to remain within the maximum number of revolutions allowed for the mix.

If, as the "inspector", you are a party to the water addition at the site, you have change the path of responsibility for the performance of the concrete and you have assumed liability you probably don't want. If water is added at the site, it should only be done by the Quality Assurance personnel of the ready mix supplier...not the engineer, not the contractor, not the finisher...or anyone. As soon as you or anyone else on site directs the driver to add water, you have taken the liability for concrete performance away from the supplier, where it should be.

As for calculating the actual w-c ratio, you have to have all the batching information including moisture content of the aggregates, absorption of the aggregates, water added at the plant, admixture volumes, and water added at the site. Many ready mix suppliers do not measure the moisture content of the aggregates on a per batch basis...they generally will check it twice a day for record purposes, even if they have a moisture indicator in their batching system.

If you have a project that has a lot of concrete on it, I would require a batch plant inspection, including an inspection of the transit mixers. All should meet the requirements of the National Ready Mix Concrete Association (NRMCA). I would further require that the supplier have a QA representative on site to make mix adjustments within the allowable criteria of ASTM C94.

Finally, the slump of the mix should always be taken for record at the back of the truck...not then end of the pump line if pumped. You can check it there for modification purposes, but for the record to check the mix design, it should be taken at the truck.

 

[Drumchaser]

Agree with Ron that the "certified" plant must comply with ACI, and the spec.s for mixer trucks are pretty tight: fins, site glass, rev. counter, overall condition of the unit. Your Concrete Plant may be well respected, but in order to comply with DOT's ragged mixers will not cut it.

For structural concrete-

The DOT's in my area require a batch ticket (in addition to the delivery ticket)that states "water withheld" at the plant, which is an indication of how much water can be added at the job site.

Water added to mixer trucks around here is usually done by those who are paying for it. Then,if too much is added, your slump requirements allow it to be rejected.

 

[HeavyCivil]

Thanks for the responses

Splitrings:
We use a Super-P in all highstrength mixes which gets the slump from 3.5-4 to 6.5-7.5 much easier to work and pump.

Abusementpark:
this on many firm's plans in bold and in spec. In any high-strength stiff mix I have ever seen poured water is added on site whether your plans say to or not. If you're there you can send a truck back but I dont see the need to do that if they dont exceed our max w/c or max slump. Some contractors take liberties wherever possible, some are pretty responsible, but very few will hesitate in adding water to a stiff mix.

msquared48:
1. So THATS why its called a slump cone! we measure slump with a slump cone at the truck on the first truck and at regular intervals thereafter.


Ron:
1. Thanks for that ASTM number and the info on revs. This will help in getting them to send trucks with functioning meters.
2. I have never directed anyone to add water. I will permit the addition if slump and w/c is maintained meeting spec. It is this liability, however, that I am worried about when I cannot determine exactly how much water has been added. Hopefully in citing the spec you mentioned we can get good trucks...
3. I take a ticket the second the truck pulls in. Water from sand and stone is accounted for so the w/c I'm working with is accurate, assuming plants numbers are correct.
4. We check slumps and air at regular intervals on every pour at the truck after the addition of water.

Drumchaser:
I will look into all the mix truck specs and bring them up.

Thanks again

 

[TDAA]

I have never allowed it unless it was withheld at the plant, and shown on the ticket.

 

[Bobber1]

What does your Contract say you can do? Generally, as an inspector, you cannot tell or direct the Contractor to do anythng. You can only report what is happening and pass it on to your chain of command. If the trucks don't meet C-94, write it down and pass it on. If they add a gazillion gallons of water in the field, write it down and pass it on.

Depending on your Contract, your job is to observe and report, and not to manage the Contractor in any way. If the Contractor is not following the Contract, it is your duty to advise the Owner or Contract Manager (or whoever), and their job to take Contractual action. The Contract has (should have) provisions as to what is required when the work is nonconforming.

So relax, stand back, watch silently and record relentlessly. Don't get excited when you see wrong-work; write it down, pass it on, and grin at the Contractor all the while.

 

[hokie66]

Bobber1,
An inspector with such limited authority is a useless inspector.

 

[DRC1]

You should be able to look at the mix, pick up and check the consitancy. If it looks good it probably is. A good mix will have a slump of 6 inches. The biggest problem with concrete is that it is placed to dry to meet some arbitary ASTM or ACI specifications. If you have a critical pour such as a suspended slab or CIP beam or high retaining wall, The ASTM and ACI specs are not so arbitary, but some experience is needed in their application. For example a standard4,000 mix will often at the right w/c break at 5,000+ adding water will lower the strength, but if you are careful, not below the required.
Pouring with a 3 inch slump as is common will lead to honeycombing, excessiv vibration, and poor rebar bond in congested forms.
Water reducing agents are often seen as a cure all, but they do not give as good a finished product as a higher natural slump (by the addition of water). Granted excessive water added to the surface durring finishing, or excessive water bleeding to the top will lead to cracking, but as long as the work is finished slowly on a cool afternoon and properly cured with out excessive water, the results will be a quality finish.
Although my point is that a little extra water is not a bad thing, too much is. If you are doing small work, such as sidewalks, low walls, etc. I would complain about the sight glasses.
If you are doing critical work, I would have a more serious dicussion with the owner and the contractor. You do have a right to verify how much water is in the mix.

 

[cntw1953]

Inspector in this case is similar to the gate keeper, for suspicious visitors/deliveries, you stop them, question them, report to person in the upper chain, obtain and excute instructions received. Done.

 

[apsix]

"Generally, as an inspector, you cannot tell or direct the Contractor to do anythng."

That may be true, but it is in everyone's best interest to advise the Contractor if something is wrong, and the consequence of proceeding against that advice.

 

[hokie66]

DRC1,
You and I will have to agree to disagree. A slump of 6" does not indicate a "good mix". Placing 3" slump concrete need not lead to honeycombing. Honeycombing is generally due to inadequate consolidation. The term "excessive vibration" is foreign to me, as I have never experienced it.

 

[msquared48]

Excessive vibration, Hokie, can and will cause aggregate consolidation and settling, causing an uneven concrete mix, affecting the overall reated strength.


Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

Agree with hokie66...a 6-inch slump mix is not necessarily a good mix. Overvibration is relatively rare and usually occurs when the contractor is trying to move the concrete with a vibrator, rather than using rakes or other devices. The result is usually segregation. It has been a long time since I've seen overvibration...usually I see the opposite...insufficient vibration resulting in lack of consolidation and voids.

I've worked with concrete from zero slump to 10"+ slump. Each has its place for a properly designed application. The problem comes in when the concrete is designed one way and someone in the field takes it upon themselves to think better of it and adulterate the mix to suit their purposes. Simple solution is reject the concrete and get their attention.

 

[msquared48]

Remember that any vibration in concrete causes a quick condition in the matrix similar to quicksand, causing heavier solids to settle relative to the rest. The more vibration, the more settling. There is an optimum limit.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[cntw1953]

In VTEIT's response to Ron:

"2. I have never directed anyone to add water. I will permit the addition if slump and w/c is maintained meeting spec. It is this liability, however, that I am worried about when I cannot determine exactly how much water has been added...."

No one seem directly resolved his issue with truck driver adding water, but the sample is still meeting specified slump range. Turn the truck? At what base? Accept, take the responsibility?

 

[cntw1953]

Simply put this way - does inspector need to worry w/c ratio if all samples passing field tests?