Weight of concrete

[htex404040]

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.

 

[ChipB]

sounds like it's using lightweight concrete in it's conversion.

 

[concretemasonry]

It has to be lightweight concrete. Anything below 115 pcf is classified as lightweight concrete.

Dick

 

[dbuzz]

I've always used 2,400 kg/m³ for reinforced concrete, which is 150 lb/ft³ (if my conversion is correct!).

 

[JStephen]

Are you sure that's not the weight of dry cement powder?

The 150 should be the average weight of reinforced concrete- perhaps the plain concrete itself would also be closer to the 111.

 

[Tomfh]

Plain concrete is around 2300 kg/m3 (145 lb/ft3)

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.

 

[swivel63]

density tests i've seen is about 138-143 for normalweight, and i've used 145 in my calcs for dead load.

 

[UcfSE]

Almost all of the concrete mix designs I have reviewed show the unit weight within a pcf or so of 140 pcf (normal weight). Using 150 pcf provides a little breathing room and accounts for the heavier unit weight of the rebar.

 

[Dinosaur]

Unfortunately, it doesn't sound like we are going to get an answer to the question of where the 111 lb/ft^3 value was obtained.

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.

 

[cvg]

to take this one step further, when checking overturning or sliding for a concrete retaining structure, dead weight of the wall and footing is used in the calculation. Using a heavier unit weight may end up giving a (unrealistic) higher safety factor.

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...

 

[hokie66]

Your calculator is defective.

 

[strguy11]

It is obviously lightweight concrete, but my guess is that your calculator probably uses a "metric" base unit as the stored unit weight, then converts it to english.

 

[htex404040]

Hokie66...best answer yet.

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!

 

[jdonville]

Dinosaur & cvg,

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

 

[cvg]

I'm not a structural engineer, but in my experience, dead loads are rarely factored down, however live loads sometimes are. And I have not seen an example where a dead load was reduced for bouyancy or overturning analysis. Most codes require that full hydrostatic pressure be used for uplift, but required FS is not always specified. I don't think the FS should compensate for a known or expected difference in dead load. I think the dead load should be estimated accurately or even a conservative number used and then apply the factor of safety.