Where to dump concrete before testing from truck 3

[canwesteng]

Where do you have the truck dump concrete before testing? Basically the truck needs to unload 10% before samples can be taken for testing. If they dump in the forms, and the concrete doesn't have the correct air/slump, you end up with out of spec concrete in place.

 

[jgailla]

You dump it in place in the forms. Nobody will want to waste 10% of each truckload.

 

[canwesteng]

The issue would be that then I could end up with an entire pile cap out of spec

 

[JStephen]

That can happen anyway, can't it?

 

[jayrod12]

I agree, never seen it wasted before. first concrete out of the truck hits the testers wheelbarrow. Then straight to the job at hand no wasting or waiting. Rarely around here is the concrete below spec. Extremely rare in fact.

Although on the odd residential job I've received testing for there's always a tester's comment "Water was added after the tests were complete". Scary.

 

[TehMightyEngineer]

Agreed with above, I'd much rather have to hash out with the concrete company how to demo and replace a pile cap than add 10% to the concrete cost for no reason. I'd only ever consider doing this if the job was on such a critical timetable that running over the deadlines was a penalty more than money.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[BigH]

What I would do is to take it first out of transit mixer; do slump - if okay - then start placement. Then take, for the record, the slump as per ASTM requirements. This way - you get an early idea if the concrete will be good; then you have the "as per specification" record. What I find, though, is that there are times that the first out appears dry and as more is discharged, it gets "wetter" - which is why you need to have on record the slump/cylinders as per sampling specification requirements. So you must be able to keep your eye on the mix as it is discharged from the transit mixer.

 

[Ron]

For US and Canadian project, ASTM standards are commonly used. For sampling fresh concrete, ASTM C172 is the reference standard. The standard is very specific about sampling. The concrete should be sampled in the middle third of the discharge of a transit mixer. The sampling must be done within a 15 minute period and the compressive strength samples must be made within 15 minutes of the testing for slump and air content. All of this contemplates that material will be placed in the forms in the interim.

These tests and the sampling are being performed for Quality Control/Quality Assurance compliance. There is a statistical expectation that the concrete delivered to the jobsite will comply with the specifications and, therefore, the testing is expected to be confirmatory. In that regard, there is no wasting of the concrete prior to testing. Further, the only "tests" that can be done at the jobsite on the wet concrete are slump and air content. While they might be specified and rejectable parameters, they are not directly indicative of the ultimate compressive strength, which is the common design parameter the structural engineer will be looking for, although the durability of the concrete (which can be related to both slump and air content) should also be considered.

 

[JStructsteel]

Is air/slump a big rejection issue? IF it still gets achieved strength, it would be hard to justify rejection of a poured item

 

[TehMightyEngineer]

I agree with you jrisebo. Unless an entire job had the wrong slump and/or air, I would just let a single truck be considered acceptable as long as it hit the design strength.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

I've rarely concerned myself with slump. I've taken a beating on this forum for this very stance. I don't typically specify it at all. With all the different blends the concrete companies create as long as I'm getting the air content on site I'm after I have no reason to reject the concrete on site. Then my only other concern is strength once the cylinder tests start coming through.

 

[canwesteng]

Yeah I'm not real concerned about slump. I guess making strength with too much air wouldn't be too bad either. But too little air is a concern