How to obtain beams for tensile testing of shotcrete 3

[BigH]

G'day all,

There are several places I could post but figured this might see the most traffic.

In obtaining beams for testing shotcrete in quality control, do you use:

1. beams cast in moulds (as one does with cylinders)to determine the "potential" tensile strength.
2. test panels and then cut the beams to determine the "actual" shotcrete tensile strength

I, of course, favour 2. The contractor likes 1.

I would be pleased to see your opinions and if possible provide industry standards for 1 and 2.

 

[Ron]

For cast in place concrete, I prefer the beams in molds (moulds), since the testing is to check the mix design. For shotcrete, I prefer removal from in place panels since the mix design is somewhat controlled by the placement in shotcrete. I still believe the old method of making hardware cloth cylinders against a solid panel is valid for shotcrete, but a sawcut beam from a panel would be better for flexural strength and you and drill cores from the beam for splitting tensile correlation.

 

[BigH]

Thanks Ron. I am not worried about the mix design but, seeing the contractor's shotcreting techniques, am a bit worried about "in place".

 

[STrctPono]

BigH,

Not sure why you are talking about tensile strength....

First things first... Get a hold of these documents: ASTM C42, ACI 506.2, ASTM C1140. If you read these, you will get most of your questions answered.

The standard way is to have the Contractor shoot preproduction test panels. One panel will be dedicated to grading the workmanship and should be installed with rebar similar to the project rebar. The second panel should be unreinforced. Let it cure, core both panels, the cores from the unreinforced panels should have an aspect ratio close to 2:1 and get sent off for compressive strength tests to verify the mix design and strength. The cores from the reinforced panel will then be visually graded by the EOR or CM to verify that the nozzelman is skilled and can get proper consolidation of the shotcrete around the bars. Once that nozzelman passes, he is permitted to shoot on the project. If a new nozzelman comes on then he too needs to be qualified.

During the project, the standard way of testing shotcrete strength is to shoot shoot test panels every so often during production work, core, and break. This is the more correct than testing the mix straight out of the truck like you do with concrete. They can take cylinders too but for our State projects the only official acceptance is through cores. One thing to note is that cores typically break at lower strengths than cylinders and this is accounted for in the testing procedures. There is the provision in ASTM C42 for sawing beams from the panels to perform flexural testing. I have never required it. Flexural strength is not a criteria that I care about for shotcrete.

 

[BigH]

STrctPono - many thanks for your contribution. I have worked several hydro projects where shotcrete is used extensively for slope support and temporary tunnel lining - or permanent. It has always been in the previous jobs - using production panels to determine the actual strength characteristics of the shotcrete. Of couse, cores are taken but also beams were always cut. Not sure why you have little interest in the tensile strength but with slope support, movement could put the shotcrete in tension/flexural - hence tensile strength is important. This is borne out in the European Specification for Sprayed Concrete (

http://www.efnarc.org/pdf/EuroConcreteSpecEng.pdf

) where, depending on the type of control, specific frequencies are mentioned. These do require flexural strength (tensile), residual strength, toughness and others. For thin linings tensile and flexural strengths are important - probably far more than compressive strength. (See:

https://books.googleusercontent.com...PBjkPIRpNzRsllYScaj9mC3nm2ejQ-h9QK_mjFcXi7QJg

I have a contractor that casts in a mould just as he would for taking cubes/cylinders - which no way reflects the in situ values. He is getting 6 and 7 MPa whereas I know from experienc on other projects that the panel beams given about 3 MPa sometimes a tad lower. My main issue is I want to push the Contractor to do panel beams but need a "standard" that says so. Usually it is a specification call or the Contractors just know to do it!

Again - appreciate you response.

 

[STrctPono]

Well the "standard" for setting up the test panels, sawing the beams, and testing them is in the referenced documents that I mentioned above so now you have that information to hold the Contractor's feet to the fire. However, this project doesn't sound very well thought out since this should have all been thoroughly vetted out in the project specifications. Without it in writing at the time of the bid, the Contractor is well within their rights to tell you to go kick rocks. The correct way of obtaining in situ flexural strengths is through sawed beams not casting molds. Of course the Contractor is going to want to cast molds but that is not the correct way.

I don't care about flexural strength or tensile strength for shotcrete projects. Period. For all my shotcrete projects, they are designed through conventional reinforced concrete theory and it is assumed that the shotcrete is going to crack on the tensile face and not exceed 0.003 strain on the compression face. The only time that I ever care about flexural strength of concrete is when I am designing PCC pavement for a highway or airport runway. This is because they are conventionally unreinforced panels and are designed to remain ELASTIC with subgrade modulus support. You need to guarantee the flexural strength to ensure the panels do not crack under the concentrated weight of the truck/airplane load. Cracked PCC panels on an airport runway = you're fu@%ed!

Let me turn the question around on you and ask you.... what will you (the designer) do with the information from the flexural tests results when you get them? Does the flexural strength or tensile strength actually factor into your design? If you (as the designer) feel the need to check the toughness of the material through a residual strength check for a fiber reinforced mix then I can see why you may want to do the test. For me, I usually accept the fiber manufacturer's reported residual strength data based on the different dosage rates and call that good. But for extra quality control, you could have the contractor perform the tests. I'm actually curious what kind of projects you do that require this, because perhaps I am just not working on the high highfalutin type of shotcrete projects that necessitate this type of testing...

For thin linings tensile and flexural strengths are important

I don't have experience with thin linings so I cannot speak to this point. What thickness do you consider to be a thin lining? Is it reinforced with steel bar?

I was not able to access your 2nd link.

 

[BigH]

STrctPono - again - thanks. This is an EPC contract. There were global Employer's Requirements but the detailed specificaitons, in the EPC contract, are the purview of the Contactor. So there you go. Looks like I will be starting another battle with the Contractor. Thin typically means, in these types of works, 50 to 100 mm thickness; 6 mm wire mesh included; rock bolts involved.

 

[miningman]

Oh oh. This sounds like a jackpot in the making. Sounds like the specs were put together by someone within the EPC group who has minimal practical experience with shotcrete. Wire mesh is a real no no in shotcrete application. The mesh cannot be installed with 100% contact with the rock which leads to shadows behind the mesh with zero shotcrete adhesion. No real objection to rockbolts but are these installed before or after shotcrete application??

If it were me , I would spec an allowable / unallowable rebound loss. This is a good predictor of the competacy or otherwise of the guy at the end of the hose.

 

[BigH]

Yes minimingman - I would prefer fibre reinforced. Bolts before spraying. I am not part of the EPC group.