CSA A23.3 / S6: dv and f´c 1

[Dominik P.]

Hey guys,
I´m currently designing a bridge (shear problem) with the canadian standard (CSA S6:19 or A23.3:19). I have already read a lot of literature and other background reports on the Canadian standard, but unfourtunatly I have some problems where I cant find any solution. Therefore i would like to try it this way and already say thank you in advance!

1. Question:
I´m trying to figure out what the definiton of the concrete strength in Canada is? (according CSA A23.2 ??)

I have already researched the following informations (according CSA S6:19):
- fcm = fc´+ 8Mpa (with fcm.. avarrage concrete compressive strength)
- fc´... specified strength of concrete --> 28-day compressive strength

So my final question is: What is the exact strength definiton of fc´?
Is it an cylinder strength? And what are the dimensions of the cylinder or cube?

In Germany we use the 5%-quantile value for the specified strength and a cylinder with dimenisons of height= 300mm and diameter= 150mm
I read that according to ACI 318 f´c is a 9%-fractile value.

2. Question:
Is there any equation for the calculation of the effective shear depth dv or effective depth d under consideration of a tendon (post) and reinforcing bars?
is that equation correct: d= (ds*As*Es+dp*Ap*Ep)/(As*Es+Ap*Ep)
with ds...effective depth for reinfrocing bars
dp...effective depth for tendon

Thanks a lot!
Best regards from Germany

 

[jayrod12]

Regarding strength, it's essentially whatever you need it to be, subject to the exposure conditions.

I don't work in bridges, but I'd have to assume the S6 code outlines the exposure class. My bet is you'd be looking at C-1 or C-XL class concrete meaning 35 MPa minimum strength but can go much higher.

It is a percentile of cylinder tests. Generally the cylinders are 100mm diameter by 200mm height. Most redi-mix plants have a whole bunch of break data on each of their mixes.

Regarding dv. Check clause 11 of CSA 23.3/23.4. I believe that's the shear clause and in there it discusses shear depth. Off the top of my head it's the larger of 0.72h or 0.9d where d is the depth from top of member to centroid of tension steel, h is full member depth.

 

[ggcdn]

Hi Dominik, I'm not familiar with S6, but in A23.3, they abstract away all the statistics about 28-day strength (f'c), and it becomes a term that is specified in the design documents and used in the calculations. With regards to d and dv: d is the distance from extreme compression fibre to the resultant of the tension force provided by reinforcing. dv is the shear depth taken as max(0.9d, 0.72h). They don't provide an equation for d as far as I know, because there are so many possible scenarios. But I believe you should calculate the tension force resultant (something like sum(Asi*Fyi*di)/sum(Asi*Fyi) )

-JA
try [link calcs.app]Calcs.app[/url] and let me know what you think

 

[Dominik P.]

Thank you so much for your answer.
I´m still wondering why there is no clear definition for the value of f´c.

So as I understand you, f´c is a percentile value/strength and definetly not a average strength.

In Germany we also use the system with the exposure conditions. So f.e. we select the strength 40MPa which is an specified/characteristic strength which means for us that there is a propability of failure of 5% that the actual strength might be below 40MPa. On top we have saftey factor for concrete strength when we do shear design f.e..

So when you do concrete design in canada and select a concrete strength what do you know about the strength according "saftey"?

 

[jayrod12]

In 14 years, I have seen only one concrete pour come back under specified strength that I can remember. So it's extremely rare.

Then, we use a factor of 0.65 on that to provide some material safety factors. Plus other modification factors for shear, bearing, etc. The loads are also factored up to provide additional reduction in risk.

 

[Enable]

You're looking for A23.1 not A23.3. A23.3 is the design code whereas A23.1 is the concrete mix / testing / whatever part of the code.

In A23.3 they reference A23.1 about f`c in Section 4: Concrete quality, mixing, and placement

The compressive strength of concrete f`c shall be determined by testing as specified in CSA A23.1

Unless otherwise specified, f`c shall be based on 28 day tests

In A23.1 they spell this out

The strength level of each class of concrete shall be considered satisfactory if the average of all sets of 3 consecutive strength tets for that class at one age equal or exceed the specified strength, and no individual strength test is more than 3.5 mPa below the specified strength. These requirements shall not apply to field cured specimens

I'm sure they back-calculated that using a statistical procedure of sorts but unfortunately I don't have that kind of info for you.

In practice, we cast a minimum of 3 cylinders per pour (or per 100m3 depending) a 7 day, and two 28 days. We tend to take one test at each age as a sufficient indicator.

I actually dislike that practice since shit tends to happen. So I usually cast a 3 day (for my forms), 7 day, two 28 days, and a spare. Cylinders are cheap to cast and you dont have to break them if you dont need them. But I think I've been burned more times than Jayrod that it's worth it for me (though it is pretty rare). Invariably the readymix supplier's response to understrength concrete is: wait longer lol

 

[jayrod12]

We get that "wait longer" response from one of the suppliers here. They are an international company, whose name rhymes with their size, large. The rest of the redi-mix guys around here usually send concrete that well exceeds design strengths. However we aren't governed by seismic here so extra strong concrete doesn't make us cringe.