Verifying quantity of concrete 2

[bazaka]

I have suspicions that my concrete supplier may be leaving us short changed on concrete. How much tolerence should we allow before we get annoyed. We have never been given too much.

 

[Ron]

Have your testing laboratory do a yield calculation on the concrete. Then compare your form dimensions with you load yield and you should be very close.

 

[cvg]

you should be checking grades/dimensions on both the subgrade and the forms. How are they able to short you if dimensions are checked? Tolerances should be specified and will vary depending on what type of structure you are building.

 

[Dinosaur]

Before you get too legal on them, remember to subtract for the displacement of the reinforcement.

 

[Skull9]

First off ask for the batch weights from the plant to make sure enough material is going in for what the batch calls for. See when the last time your supplier had his scales certified. Get your testing lab to run a yield test. Anything below 26.0 cu ft per yard and I would be concerned. That basically means you are giving up a yard for every 27 that your pour. Most normal yield tests run in the 26.5 to 27.3 range. Also remember the 5 percent truck hold-back as you will not be able to get all the concrete off the truck.

 

[Zambo]

Dinosaur - the allowance for rebar should mean less concrete is required.

If you are using good quality forms and the concrete is not for a slab at grade then it should be easy to accurately calculate the concrete required compared agaibst delivered. Why not send the truck to a weigh bridge.

 

[Ron]

Zambo...there's a time limit between first water introduction and discharge of the concrete. To send the truck to a scale would likely breach that time. Good idea, just probably not workable with the time constraints.

Do a batch plant inspection for compliance to the National Ready Mix Concrete Association (NRMCA). This is a detailed review of their storage, batching, and transit mixing operations. It gets their attention. In almost every case where I've done a batch plant review inspection, the consistency of the concrete immediately improved (for that project at least!). Keep in mind that whomever does the inspection will have to crawl inside a transit mixer! (I've done it in Florida in the summer...not fun!) It will require confined space entry procedures.

Secondly, have a yield done at the site by the testing laboratory (third recommendation on this one!!). Make sure the technician is ACI certified (assuming you are in the US).

Third...make sure everyone in the process is aware of the requirements and the standards. A pre-placement conference with all parties is invaluable.

 

[Dinosaur]

Zambo,

Yes, the rebar displaces concrete and reduces the total volume of concrete to complete the work. My point was, along with another post indicating the method of obtaining quantities in-place, the displacement of steel needs to be accounted for before you accuse the contractor of shorting you on the concrete.

Some folks may be surprised but we have had bridge projects where the concrete in the bolster above the beams was significant. Contractors have challenged our quantities in the past because they believed the bolster had not been included. In some cases they were correct.

My agency has plant inspectors and field inspectors for major concrete pours. If there is a disagreement, we will shoot elevations and verify the in-place quantities. It is routine for our bridge folks to account for rebar; they use much more than the other folks. We have to remind the other designers to address this before accusing the contractor of shorting the order. The real problem is what to do when it is found that he has. There is usually an adjustment for payment but does this reflect the consequence of getting a short order?

 

[DRC1]

If you are discharging directly from the truck to the forms, there is still a little that will not discharge. The 5% mentioned by Skull9 is not unreasonable. If you are pumping or bucketing, you will have additional material lost. Generally for estimating prices or orders I figure a 10% loss which is a little conservative. This is generally one yard per truck.

 

[Zambo]

where does all this "spare" concrete go! 10% is a worrying amount it wouldn't take long for the washing out area to be a small concrete mountain.