Concrete

[Smitash]

Hi, I am actually preparing for registration with the council of engineers and will be happy if anyone can help me clarify some queries..I wanted to know how can i ensure that the whole batch of concrete passes the test (slump test or cube test)?..thanks

 

[fegenbush]

The concrete plant should be able to provide you with a mix report. This will detail many items such as the compressive strength, sieve analysis, and slump from several batches. That in combination with your test cylinders and field slump tests will confirm that the mix is proper. The last mix report I reviewed showed the maximum, minimum, and average values from about 20 batches.

 

[Ron]

Smitash...
Your question is a bit open ended, but I'll give you a procedural guide.

The concrete mix design is to be responsive to the structural design. For instance, the structural engineer will specify that he wants a certain compressive strength for structural purposes.

The mix design is then developed to be responsive to the compressive strength requirement and other specifications limits as may be dictated, such as a slump ( usually in a range from 3" to 5" or 5" to 7" ,etc.) It can also be specified as "4" +/- 1" or similar). A maximum water-cement ratio may also be specified and sometimes a maximum or minimum nominal coarse aggregate size is given.

Other than for compressive strength, the other parameters are for ease of construction and durability.

How do you test to "ensure" that a whole batch of concrete passes "the" test? Well, you don't! The process is one of statistical validation of the mix (as noted by fegenbush), followed by taking representative samples of concrete from a batch of concrete for testing, then applying both to the acceptance or rejection of the concrete.

As an example, let's look at placing a floor slab that has 100 cubic yards of concrete in it, all to be placed in one day.....

100 cubic yards of concrete will require the delivery of 10 trucks, each carrying 10 cubic yards of concrete. For typical specifications, the rate of sampling/testing for compressive strength will be one test set of specimens for each 50, 100 or 150 cubic yards placed in a day. That means that out of your 10 trucks, a maximum of 2 would be sampled for compressive strength testing. Each of them could be sampled for slump compliance as this is a field test and results are obtained rather quickly.

So a testing laboratory would dispatch a technician to obtain the samples, mold them properly in the field, protect them from damage, retrieve them from the field within 24 to 48 hours and transport them to a laboratory, where they would be cured under standardized conditions for testing at 7 and 28 days, typically. For US projects under the American Concrete Institute guidelines and ASTM standards, a compressive strength test is the average of two specimens taken from the same batch and tested at 28 days.

Other locales and jurisdictions will have varying requirements but they will be similar to what I've described in context and intent.

 

[Smitash]

Thanks Fegenbush and Ron..You mean to say that the concrete will be delivered to site i will get a compliance certificate from the supplier which will give information on the slump test, compressive strength, cement fineness, water cement ratio?

 

[bridgebuster]

You specify a slump. The slump test is performed as the concrete is being discharged from the truck.

You specify a minimum f'c. The actual compressive strength is determined from cylinder breaks.

Cement fineness and w/c ratio might be on the delivery ticket; if not you'd find it in the mix design.

 

[Ron]

Smitash...no. When the concrete is delivered to the site, you will get a delivery ticket. Depending on the specification being met (in the US it would typically be ASTM C94 "Standard Specification for Ready Mix Concrete"), you will get the minimum requirements of that specification reflected on the delivery ticket. You should, beforehand, have a copy of the approved mix design and its unique identification (typically a mix designation or number), which will be on the delivery ticket and on the mix design. This allows you to check the delivery against the approved mix design. If you want the batch ticket printout, you'll typically have to request that unless your specification already requires that it be provided.

Cement fineness is not likely to be found on the delivery ticket or the mix design. It is found on the mill certificate for the cement, which should accompany the submitted mix design.

 

[Smitash]

Ok Ron..thanks a lot..