tensile strenght comparison: #3 vs #4 rebar 4

[waxster]

I'm having a garage built and the contractor just put in #3 rebar, 24" on center. I want #4 at 12" on center. Contractor says no real difference. What is tensile strength comparison between what I have (#3, 24"oc) and what I want (#4, 12" oc)?

Ground is very wet after 4 days of rain. Temp is in mid 60s (California, no problems with freezing). Pour will happen 4 days from now, assuming no more rain.

Should I insist on an upgrade? Should I add some #4 to what's there already? Thanks for your help.

Mark Wax

 

[jheidt2543]

Here are a few things to consider:

1. Assuming that the both sizes of rebar are of the same grade of steel, their strength is proportional to their cross sectional area, so .500/.375 = 1.333 Therefore, the #4 bars are 33% stronger in tension.

2. How did the contractor bid the work? If the original specs called for #4 you should get #4 or a credit for the difference.

3. For a slab-on-grade, the subgrade is actually more important than the amount of reinforcing steel under most of the common loading conditions. Heavy storage racks, forktruck traffic etc. cause bending moments in the slab and require a fair amount of reinforcing.

4. Most residential garage slabs-on-grade don't have very heavy loads and could be poured on a good subgrade without rebar of any kind.

5. My suggestions would be:
A) Be sure the subgrade is a well compacted granular fill of sand & gravel or crushed stone at least 4" thick.
B) Leave the #3 rebar, but ask for a credit for the #4 based on the weight difference between the two bars, about 0.3 lbs/lf at about $0.40/lf.
C) The concrete should be at least 3,000 psi mix design with a water:cement ratio of .45 by weight (the ready-mix supplier would know this) and add no water at the site.
D) Saw cut control joints on roughly a 12' x 12' grid as soon as the concrete can support the saw (or use a "Soft-Cut" saw).
E) Wet cure the concrete for at least three days, then apply a high solids, water soluble curing agent that does not "wheel track". Master Builders, R.W. Meadows and Sonoborne all make good ones.

 

[jjeng2]

Forget about the bar diameter. The spacing is the major difference. Your bars are smaller and your area of steel is cut in half by doubling the spacing. You didnt specify if this was for the slab on grade or for footings or foundation walls. If it for a slab on grade for a typical residential garage what you have is probably more than enough.

 

[waxster]

Thanks VERY much for the info and quick replies, jheidt and jjeng.

Garage is residential, slab on grade, and will be 3-1/2" thick. Original contract called for wire mesh but I wanted upgrade to rebar. Contractor recommended #4 but installed #3 and is charging for #3, saying #4 was not necessary.

 

[JAE]

jheidt2543 - you mentioned area as the correct comparison between bar strengths, but then you used diameter to calculate the ratio. The areas are 0.2 and 0.11 so the ratio is 1.82, not 1.33 (just being picky...sorry)

 

[jheidt2543]

JAE: Thanks, I had a senior moment I guess. You are very correct it is the area of the bar, not the diameter. I also missed the fact that the #3's are at 24" and the #4's were at 12". As my second grade teacher used to say, "slow down and read what the words really say".

"Mea maxima culpa"

 

[DanTex]

I don't know anything much about slabs on grade like this and what kind of reinforcement is considered as the norm. However, one thing to keep in mind to control cracking..

For a given cross-sectional area of steel across the width of your slab... you're better off having more smaller bars with a small c-c distance than you are having large bars with a greater c-c distance. In general... a "finer" mesh of steel will do a better job of keeping cracking down in the concrete. So.. from that perspective... you're better off with the #3 bars spaced closer together.

Dan

http://www.dtware.com

 

[SacreBleu]

Most slab-on-grade doesn't need a lot of reinforcing. It is more important to use good concrete mix, applied and cured properly. Tooled control joints are essential. I am not a believer in #3 bars in slab construction because they are easily bent down to the subgrade by the weight of the concrete workers stepping on them during slab placement.