Maximum Bar Spacing for Crack Control - Eurocode 2

[techiestruc]

Hi,

Does any of you have an idea how the values from Table 7.3N: Maximum bar spacing for crack control found in Eurocode 2 were derived?

As shown on the table, the assumption for concrete cover is 25mm but I would like to create a new table for 50mm.

The parameters used and the assumptions given were not enough to solve the values using the crack width calculation in Sec. 7.3.4.

Thanks in advance!

 

[rapt]

The tables give 2 possible solutions, either maximum bar size or maximum spacing. So they are really an envelope of solutions based on the crack width formula rather than an exact solution of the equation (as far a crack width calculation can be called exact).

If you want to use different cover, make an assumption on bar size and the formula will give the maximum spacing you need.

If you did this for 25 cover, you would get a less conservative answer than the tables as they are conservative to allow any bar size to be used for the spacing table or any spacing to be used for the bar size table.

 

[techiestruc]

Thanks rapt!

I think it is more preferable to use the "exact" solution to compute crack width instead of developing envelope solutions to produce such tables.

 

[rapt]

Remember the calculated value is actually a mean or average crack width and is based on an assumed bond strength. Actual maximum crack width could be + 50% or more.

And on top of that there are a lot of restraint stresses that are normally not included in the calculations. So most designers take the easy way out and adopt the more conservative figure.

Make sure you do not guarantee a maximum crack width based on the code calculations.

 

[techiestruc]

The result on crack width calculation (BS EN 1992-1-1 Sec. 7.3.4) will be compared to wmax given in Table 7.1N (in my case 0.3mm).

Is this not enough? Where can I find the provisions stating maximum crack width instead of the average crack width?

 

[rapt]

There are not any provisions. I am just letting you know what the logic behind the formula is.

The code is limiting the estimated average crack width, not the maximum.

if you look in 10 different codes you will find 10 different methods of limiting crack width, just about all of them using a different basis for the calculations. There is no exact science on it.

The Eurocode tables gives you a crack width based on bar size and bar spacing. If you look at how it is derived, I disagree that bar size is a big a factor on the calculations as they say it is. They start with a tension force that is controlling it and end up converting that to a bar diameter at a spacing. Yes, bar diameter controls development length and that comes into it as well as it affects crack spacing, but the formula puts a lot more of the effect onto bar diameter than I think it should in the tables they give you in the code.

And the crack spacing used in the calculations is then a "mean" value. It could vary significantly up or down and that would affect the crack width.

But that is my opinion.

 

[techiestruc]

Thanks for your insights rapt!

Since I'm used to ACI code provisions (ACI 318 and 350), I've never thought that there are much complex provisions regarding crack width. Never too late to learn!

 

[rapt]

I would put the ACI318 basic provision up there with tossing as coin compared to the Eurocode provisions