tensile strenght comparison: #3 vs #4 rebar
tensile strenght comparison: #3 vs #4 rebar
(OP)
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
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





RE: tensile strenght comparison: #3 vs #4 rebar
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.
RE: tensile strenght comparison: #3 vs #4 rebar
RE: tensile strenght comparison: #3 vs #4 rebar
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.
RE: tensile strenght comparison: #3 vs #4 rebar
RE: tensile strenght comparison: #3 vs #4 rebar
"Mea maxima culpa"
RE: tensile strenght comparison: #3 vs #4 rebar
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
www.dtware.com
RE: tensile strenght comparison: #3 vs #4 rebar