concrete sampling 1

[Steel Inspector]

Hello

If i have a slab concrete pouring, say a total amount of 150 cubic meters. concrete will be delivered to site through trucks, each of a capacity ranging from 6 - 8 m3 on average. which means that around 20 trucks will come.

my question is, on what basis should depend when choosing cars for slump and compressive strength test spcimens sample collection. should i check slump of each car of all the 20 cars? or there is a process by which i choose a certain number of cars for sample collection? and how may compressive strength test spcimens should i do? 6 is a minimum right?

thank you.

 

[Enable]

Depends on your local code. For the one I am familiar with (Ontario Building Code) the provisions are as follows:

A) A minimum of 1 test set for every 100m3 of identical mix
B) If pour is less than 100m3, at least one test set per day where concrete is placed

Each test set to be comprised of a 7, and two 28s.

Now...I think the above is wholly inadequate given the variable nature of concrete. Perhaps it is my training in restoration (where we test every other truck). But i'd like to see way more cylinders being cast.

In addition, I always cast a 3,7,28,28 and a spare (3-day for me for form stripping and spare because shit happens and cylinders are cheap).

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[Ron]

Check the slump and document the trip ticket info for every truck. Sample at least every 50 m^3 for air content and compressive strength. Let the slump results lead you to the trucks to sample for cylinders. If you are using 6x12 cylinders, you need at least a 7 day and 2-28 day samples. If you are using 4x8 cylinders, you need 3 cylinders at 28 days for proper test.

 

[Steel Inspector]

Mr Enable & Mr Ron,
Thank you both, but don't you think that sampling every truck or every other truck is too much? i mean trying to be almost 100% sure that every thing is perfect is good, but also efficiency is important.

what about eye judgment? can't i check slump of a number of cars and observe for any change in consistency?

 

[Ron]

You are not sampling every truck for strength, only slump. This helps you determine the consistency of the plant production so that if there is an issue later (during curing, after curing, in service, etc.) you have better documentation to troubleshoot the issues.

It takes only a few minutes to do the slump and you can stage the trucks so there is no production slowdown.

Eyeballing slump is not accurate. You might be able to tell within 2 inches, but not much better than that.

 

[Steel Inspector]

thank mr Ron,
really appreciate your opinion regarding "eyeballing" slump.
have been wondering about it for sometime.

 

[Enable]

While I do not disagree with Ron, I will offer a slightly different perspective: in the world of superplasticizers and water reducing admixtures the slump is a relatively unimportant value that tells you very little about the important properties of the mix delivered to site. It's prime importance is that of workability, which should not be your concern as an engineer.

I've discussed this previously on this site, but the slump test made a great deal of sense when it was highly correlated with the water to cement ratio (and hence strength). However, in today's world where slabs are poured with mixes dosed with high water reducing admixtures the slump alone can rarely tell you much. If the slump is low you dont know why that is the case (is it because the admixture dose is low or is it because w/c was low) or if the slump was high you also dont know (is it because the admixture dose was too high or is it because the w/c was too high)?

At best, a variability in slump can tell you there is variability from batch to batch at the same plant or as it often turns out between plants for the same mix...well, I have never been on a pour where that is not the case. Ready mix suppliers are awful and variability all over the map is to be expected. So if that is all you can extract from such tests well you already only confirmed what you knew to be true.

As a contractor I care not about slump other than if the slump to site is unworkable for me (e.g. if I am pouring stairs and slump comes in at 200mm). Otherwise, I am pouring and will tell the engineer to core in the field if they have concerns. The more important test by far is air content as that can really destroy strength and durability.

If I had my own standard, the Enable standard (lol), we would be testing every 3rd truck for all items and every truck for air.

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/