Early strength testing method of Shotcrete! (urgently need help) 1

[lamy2781]

Hello all,

I have to carried out the 8hrs strength test of the shocrete as the contract specification. As the time is tight, it is not possible to send the sample to laboratory for testing.

As such, I could like to ask is there any insitu testing method can do for that?

The specification requirement on the 8hrs compressive strength should be min. of 5MPa.

I was think of Schmidt Hammer Testing before, however, I try to search in the internet and noted that it seems only applicable for min. of 10MPa.

I am not sure whether Schmidt Hammer Testing really can't working for the 5MPa concrete. Does any of you have the idea?

Many thanks!!!

 

[BigH]

Schmidt hammer would not be acceptable - shotcrete doesn't go on smooth and the Schmidt hammer test, unless calibrated to the actual mix is rather useless except to give a general idea that the concrete (shotcrete) is consistent. What did the mix design strengths show for 8 hr, 1 day, etc.? How far are you from a lab? If only an hour or so away - I'd take the core, say at 6 hours, transport and then test. it may not be perfect but it is reasonable. Did you shoot panels as a backup or are you taking cores from the shotcrete? Our specs ask for 20 MPa at 3 days and 40 MPa at 28 days. Panels are to be left on site for 3 days before transport . . . Would be interesting to see where your specs came from and how they anticipated that a contractor could do an 8 hour core. . .

 

[Ron]

BigH is exactly right. Don't use the Schmidt hammer for this application. There are several methods that can be used, but each of them should be correlated to the compressive strength specimens done in the lab. You can use LocTest (a cast-in-place pullout method) or Windsor Probe, although if your shotcrete section thickness is low, this will yield erratic results.

It is best if you just do the testing properly, then you'll know.

Use the "basket method" to fabricate specimens as you shoot the shotcrete. Carefully transport to the lab or have the lab bring a portable machine to the site for testing, since you will not have any appreciable curing to replicate.

 

[lamy2781]

Thanks for BigH and Ron's kindly reply.

Actually, I am working on a tunnelling project, and 24hr working with 2 shifts will be carried out. As such some shotcrete may applied in the evening time and 8hrs after will be in the mid-night which result in really difficult to test at the laboratory.

The 8hrs smaple may not have to cored. The contract simply stated testing method "AS APPROVED" and that is why I am finding the "correct" way to test it.

yes, shoot panels will be used for the testing purpose and sample will be cored from the panels for 3/7/28days testing.

Ron, I think Windsor Probe is the similar method as "Hilti bolt peneation method" (not really remember the name), but as it is consumable and we have to carry out a lot, the implication on the cost will be a lot. As such, it could be prefer to adopt the cost in "one-go" e.g. Schmidt Hammer Testing.

do you have any idea?

Thanks!!!!

 

[BigH]

If you still wish to pursue the Schmidt Hammer, which I don't suggest, (and check other threads on the site - for this has been discussed in detail previously - several times previously) you better have a good statistical correlation between your shotcrete mix and compressive strengths - and I am not suggesting 3 or 4 tests - but upwards to 25 to 30 - and provide a statistical criteria such that only x% may fall below the specified strength . . .

 

[Ron]

lamy2781...the Schmidt hammer,as BigH has noted twice, is just not an appropriate test for the shotcrete. It is not a good method to determine actual compressive strength on any concrete, particularly shotcrete. In the US, the standard we use for the Schmidt Hammer testing states that it is not to be used for compressive strength determination.

It only determines a rebound number, from which you can judge the consistency of the concrete, but not its strength. In fact, if you use it to test the roof of the tunnel, that is the worst possible position for it (vertical, up).

It's not a big deal for a laboratory to have a technician on site at midnight...I've done it many times.

 

[rowingengineer]

I think you need to know the purpose of the 8hr test. Maybe you can do a trial mix test to show how your concrete gains strength to teh requried amount at 8hrs and then only do onsite testing for 3 day, 7 day and 25 days.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[Zambo]

Whay not buy a concrete compression test machine and do the tests yourself?