Weight of concrete
Weight of concrete
(OP)
Simple question with probably a simple answer.
I have always used the weight of reinforced concrete to be 150lb/ft^3. Although I am confident that this is a good density to use, I have a calculator that does automatic conversions and when doing a volume to density conversion it uses 111.11 lb/ft^3 for the weight of concrete.
Does anyone know where they are getting this number from??
I have an old concrete book that states reinforced concrete is 150lb/ft^3 and plain concrete is 145lb/ft^3.
I have always used the weight of reinforced concrete to be 150lb/ft^3. Although I am confident that this is a good density to use, I have a calculator that does automatic conversions and when doing a volume to density conversion it uses 111.11 lb/ft^3 for the weight of concrete.
Does anyone know where they are getting this number from??
I have an old concrete book that states reinforced concrete is 150lb/ft^3 and plain concrete is 145lb/ft^3.






RE: Weight of concrete
RE: Weight of concrete
Dick
RE: Weight of concrete
RE: Weight of concrete
The 150 should be the average weight of reinforced concrete- perhaps the plain concrete itself would also be closer to the 111.
RE: Weight of concrete
Steel is 7800 kg/m3 (490 lb/ft3)
Assuming 2% steel then RC density = 2300*.98+7800*0.02 = 2410 kg/m3
115lb/ft3 is definately light.
RE: Weight of concrete
RE: Weight of concrete
RE: Weight of concrete
However, I would like to "hijack" this thread for a second and ask a related question; but first, my two cents ...
As a designer, I always used 150 pcf for reinforced concrete even though I was told it is often less than 145 pcf. For most calculations this is reasonable and conservative. However, there are occasions when 150 pcf will not be a conservative assumption, although it may still be reasonable. For example, when checking some conditions in continuous beams, the 145 pcf concrete can give you a more severe condition. Another example, and one I am dealing with at the moment, is the prediction of camber in P/S girders subject to creep. If you use the 150 pcf value, you start with a lower initial camber. Then when this value is magnified due to the effects of creep, you again have a value that is lower than may reasonably occur. Do any of you folks use a unit weight below 150 pcf when determining the camber in a P/S girder?
Also, if the precaster sees the cambers rising beyond the predicted camber plus tolerance, what is normally done to mitigate the problem?
Thanks in advance.
RE: Weight of concrete
Bouyancy of an underwater concrete structure would also be affected by unrealistic values for the unit weight. And with safety factors approaching 1.0 for floatation, this could be problematic if lighter aggregate were used in the actual concrete mix supplied to the jobsite...
RE: Weight of concrete
RE: Weight of concrete
RE: Weight of concrete
Well I guess this 111.11lb/ft^3 number will remain a mystery, since the post, I looked in the "Weights of Building Materials" chart my ASD book and it says lightweight (reinforced) concrete is anywhere from 72lb/ft^3 to 120lb/ft^3.
From that I suppose 111.11lb/ft^3 would be considered lightweight, but where they pulled this random 111.11lb/ft^3 is still a mystery, like strguy11 says, it may be taken from a metris base unit.
Thanks for all the replies!
RE: Weight of concrete
I thought that the load factor for dead load in certain load combinations was less than 1.0 in order to compensate for the beneficial effect of dead loads and to correct for those situations where the unit load of concrete is actually less than the normal design value of 150 pcf.
I would think, too, that the FS for buoyancy checks (and I run into this problem very occaisionally) should be larger than the uncertainty in the material unit weights.
Jeff
RE: Weight of concrete