RC concrete crack control with bundled bars

[yakpol]

Curios about your opinion about ACI/AASHTO or any other code provisions about bar spacing. E.g. would it be equal to reinforce with #8 spaced at 4" vs bundled (2-#8) spaced at 8" ?
The bond area of 2-bar bundle is the same as for 2 single bars. Would it matter? Older ACI/AASHTO provisions were based on the 'bar density' within concrete, spacing did not have to be specified.

Thanks!

 

[le99]

"The bond area of 2-bar bundle is the same as for 2 single bars. ", I doubt the correctness of it. But, in concern of crack control, ACI always prefers the practice of smaller bars placed in close space instead of the bundled bars, which tend to have higher local stress.

 

[HTURKAK]

I agree with le99 (Civil/Environmental).. The bond area of 2-bar bundle is NOT the same as for 2 single bars. The bond perimeter for two seperate bars = 2* Π*D and The bond area of 2-bar bundle may be approximated with ( Π*D + 2*D )..

The following clauses are copy and paste from ACI 318

- 25.6.1.5 Development length for individual bars within a bundle, in tension or compression, shall be that of the individual
bar, increased 20 percent for a three-bar bundle, and 33 percent for a four-bar bundle.

25.6.1.7 Lap splices of bars in a bundle shall be based on the lap splice length required for individual bars within the
bundle, increased in accordance with 25.6.1.5. Individual bar splices within a bundle shall not overlap. Entire bundles
shall not be lap spliced.


There are similar restrictions in EC-2 . But EC-2 allows lap splicing of entire bundle of two bars as long as dia. of individual bar is less than 22 mm and splice length L0 is calculated for nominal size ( φn = φ √nb ≤ 55 mm )

8.9.3 Lapping bundles of bars
(1) The lap length should be calculated in accordance with 8.7.3 using φn (from 8.9.1 (2)) as the equivalent diameter of bar.
(2) For bundles which consist of two bars with an equivalent diameter < 32 mm the bars may be lapped without staggering individual bars. In this case the equivalent bar size should be used to calculate l0.

 

[yakpol]

HTURKAK,

ACI does not require to increase development length for 2-bar bundles. So, the difference in bond with a single bar must be small and safe to ignore.