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  #1  
Old 06-25-2005, 03:23 PM
Carnac the Magnificent! Carnac the Magnificent! is offline
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How much does one cubic FOOT of concrete weigh?

A contractor is delivering freshly mixed concrete to a friend's house for repair work.

This friend is curious re: how much 1 cubic FOOT on liquid concrete weighs, allowing for variances in mixes.

I guessed somewhere in the 100-pound range. Any ideas?
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  #2  
Old 06-25-2005, 03:36 PM
scr4 scr4 is online now
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Some numbers here. You can use Google to convert between units (example).
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Old 06-25-2005, 03:37 PM
Tapioca Dextrin Tapioca Dextrin is offline
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Not a bad guess. 140 - 150 would be better.
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  #4  
Old 06-25-2005, 04:30 PM
Carnac the Magnificent! Carnac the Magnificent! is offline
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Quote:
Originally Posted by Tapioca Dextrin
Not a bad guess. 140 - 150 would be better.

whoops. I should note it will be poured from a cement mixer. The chart you link speaks of cement in a "slurry" form, giving a weight of 90 pounds per sq ft.

Would that be the correct term/weight?
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  #5  
Old 06-25-2005, 08:29 PM
justwannano justwannano is offline
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From my concrete mixer driving days I was told that a cubic yard of concrete weighs just short of a ton.
Just divide a ton by 27 for an aproximate amswer.
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  #6  
Old 06-26-2005, 12:09 AM
RM Mentock RM Mentock is offline
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Quote:
Originally Posted by justwannano
From my concrete mixer driving days I was told that a cubic yard of concrete weighs just short of a ton.
Just divide a ton by 27 for an aproximate amswer.
That's just a little more than water weighs--you put a lot of rocks in it, it's going to weigh more
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  #7  
Old 06-26-2005, 12:23 AM
Triskadecamus Triskadecamus is offline
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Solid rock runs 140 to 180 lbs. per cubic foot, depending on what type of rock. (Excluding the rare ones like pure gypsum, or Galena) While concrete will be variable, depending on what you use as an aggregate, it should be in the same range. So, 150 lbs. for a cubic foot seems entirely reasonable.

Tris
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"It is when I struggle to be brief that I become obscure." ~ Horace
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  #8  
Old 06-26-2005, 01:32 AM
danceswithcats danceswithcats is offline
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Quote:
Originally Posted by RM Mentock
That's just a little more than water weighs--you put a lot of rocks in it, it's going to weigh more
No. Water is extremely dense and non compressible. If you substitute volume x of water with an equal volume of rock, the weight will likely go down, as rock is not as dense as water. Put a brick in a pan of water and watch how much water it absorbs via capillary action.
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  #9  
Old 06-26-2005, 01:38 AM
Jman Jman is offline
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Quote:
Originally Posted by danceswithcats
No. Water is extremely dense and non compressible. If you substitute volume x of water with an equal volume of rock, the weight will likely go down, as rock is not as dense as water. Put a brick in a pan of water and watch how much water it absorbs via capillary action.
Rock is not as dense as water? Oh....so that explains all the floating rocks in my lake.
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  #10  
Old 06-26-2005, 01:48 AM
RM Mentock RM Mentock is offline
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Quote:
Originally Posted by danceswithcats
rock is not as dense as water.
?? your average sex rock is two and a half times more dense than water
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  #11  
Old 06-26-2005, 02:17 AM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Jman
Rock is not as dense as water? Oh....so that explains all the floating rocks in my lake.
Well, maybe all the rocks around his place are pumice.

I took my son and some friends up to reservoir on the Los Angeles aqueduct near Bishop one time. One of them idly picked up a rock and threw it in the lake and the rock floated. They had a great time throwing the floating rocks in the water.
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  #12  
Old 06-26-2005, 02:36 PM
Can Handle the Truth Can Handle the Truth is offline
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Architectural designer/engineer weighing in.

In my structural calculations I use 150 lbs per cubic foot for concrete.
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  #13  
Old 06-27-2005, 12:25 AM
justwannano justwannano is offline
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Quote:
Originally Posted by RM Mentock
That's just a little more than water weighs--you put a lot of rocks in it, it's going to weigh more
Water weighs 8 pounds per gallon.
I think you have the decimal in the wrong place.
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  #14  
Old 06-27-2005, 12:46 AM
RM Mentock RM Mentock is offline
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Quote:
Originally Posted by justwannano
Water weighs 8 pounds per gallon.
I think you have the decimal in the wrong place.
And there's a factor of about 8 gallons per cubic foot
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  #15  
Old 06-27-2005, 01:37 AM
Waterman Waterman is offline
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Water weighs 8.34 pounds per gallon and there are 7.48 gallons per cubic foot, therefore water weighs 62.4 pounds per cubic foot at normal temperatures (~60 - 80 degrees F).

An individual rock (or other mineral matter) has a density greater than that of water, however, it's bulk density will be less because of void volume.

If the previously stated number of one ton per cubic yard then concrete would weight ~ 74.7 pounds per cubic foot which is too low.

I checked it out and here's a link to site where the value is given as 135 - 142 pounds per cubic foot depending on the admixture and aggregates used.

Weight of a Cubic Foot of Concrete
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  #16  
Old 06-27-2005, 01:39 AM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Can Handle the Truth
Architectural designer/engineer weighing in.

In my structural calculations I use 150 lbs per cubic foot for concrete.
Sounds about right. Fresh water weights 62.4 lb./ft3 and 2.5 times that is 156.
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  #17  
Old 06-27-2005, 03:45 AM
NillyWilly NillyWilly is offline
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danceswithcats

If rock is not as dense as water, all rock would float.
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  #18  
Old 06-27-2005, 12:27 PM
justwannano justwannano is offline
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Well it appears I'm wrong.


From Truck Mixer Driver"s Manual
May 1995
National Ready Mixed Concrete Association


The purpose of the unit weight is to find the weight of 1 cubic foot of concrete. Unit weight is measured in pounds of concrete per cubic foot,which is abberviated lb./cu.ft. Expect concrete to have a unit weight of 140 to 150 lb./cu.ft. Comcrete made with lightweight aggreates will have a unit weight of 115 to 125 lb./cu.ft.

I always thought that guy was blowing smoke most of the time anyway.
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  #19  
Old 06-27-2005, 01:02 PM
scr4 scr4 is online now
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Quote:
Originally Posted by RM Mentock
?? your average sex rock is two and a half times more dense than water
A what rock?
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  #20  
Old 06-27-2005, 02:00 PM
RM Mentock RM Mentock is offline
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any old fucking rock
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  #21  
Old 06-27-2005, 07:52 PM
Valgard Valgard is offline
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Quote:
Originally Posted by Can Handle the Truth
Architectural designer/engineer weighing in.

In my structural calculations I use 150 lbs per cubic foot for concrete.
Second this value from former civil/structural engineer. That's what we used in every single concrete design class I took.

And before you ask that's a good value for both unreinforced and reinforced concrete :-)
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  #22  
Old 06-27-2005, 08:44 PM
lostronin lostronin is offline
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165lbs approx.

7.4805 gallons per cubic foot
8.33 pound per gallon
thus a cubic foot of water weighs 62.31 lbs

aggregate producers typically mine and crush limestone for use in concrete
and asphaltic concrete. Specific gravity of the raw mineral (sand and gravel)
is typically 2.65, give or take. Cement is a little lighter but there isn't much compared to aggregate content.

7.4805x8.33x2.65 is as close as you'll get without factoring in dry cement SG

that of course does not take into consideration the weight of rebar if used
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  #23  
Old 06-27-2005, 09:03 PM
FordPrefect FordPrefect is offline
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Quote:
Originally Posted by lostronin
165lbs approx.

7.4805 gallons per cubic foot
8.33 pound per gallon
thus a cubic foot of water weighs 62.31 lbs

aggregate producers typically mine and crush limestone for use in concrete
and asphaltic concrete. Specific gravity of the raw mineral (sand and gravel)
is typically 2.65, give or take. Cement is a little lighter but there isn't much compared to aggregate content.

7.4805x8.33x2.65 is as close as you'll get without factoring in dry cement SG

that of course does not take into consideration the weight of rebar if used
Don't forget the embedded air, and also that most (I think) of the water will evaporate from the concrete after catalyzing the cement powder
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  #24  
Old 06-27-2005, 09:38 PM
tremorviolet tremorviolet is offline
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Quote:
Originally Posted by FordPrefect
Don't forget the embedded air, and also that most (I think) of the water will evaporate from the concrete after catalyzing the cement powder
Not really. The water doesn't "catalyze" the cement powder; through hydration, water forms chemical bonds with the cement. (here's a little CE101 page if you're interested, this page also has the concrete professors' favorite refrain: Under no circumstances should the word "cement" be used to refer to the composite product "concrete". ) So there isn't a lot of water to evaporate as it's used during hydration.

Wet concrete is essentially the same volume as cured concrete with shrinkage of usually much less than 1%. Which is one reason why it's such a great building material. So using the standard weight of 150 pounds/cf (which is the weight most engineers use for their calcs, it's the highest and, therefore, most conservative) for both weight and cured concrete is fine.

And enineers usually don't bother to calculate the weight of the rebar. It's true steel is 490 lbs/cf compared to concrete's 150 lbs/cf but you usually use such small amounts of rebar that it's not worth the time to calculate.
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  #25  
Old 06-28-2005, 07:13 AM
lostronin lostronin is offline
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Quote:
Originally Posted by FordPrefect
Don't forget the embedded air, and also that most (I think) of the water will evaporate from the concrete after catalyzing the cement powder
You're right, didn't consider entrained air! That accounts for the 150 rule of thumb most engineers use I presume.
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  #26  
Old 06-28-2005, 03:32 PM
FordPrefect FordPrefect is offline
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Quote:
Originally Posted by tremorviolet
Not really. The water doesn't "catalyze" the cement powder; through hydration, water forms chemical bonds with the cement. (here's a little CE101 page if you're interested, this page also has the concrete professors' favorite refrain: Under no circumstances should the word "cement" be used to refer to the composite product "concrete". ) So there isn't a lot of water to evaporate as it's used during hydration.
Interesting. So you are referring to the barest, minimum amount of water required to hydrate (not catalyze LSNT) My use of "evaporation" which isn't quite the right term either for what I meant comes from my experience in concrete construction. After pouring a floor or beam there would be water that would rise to the surface. Admittedly this water would be there because we were using more than the minimum amount of water. (It didn't really matter since the concrete would still cure to a strength well beyond the engineer's specs.)
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  #27  
Old 06-28-2005, 03:54 PM
tremorviolet tremorviolet is offline
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Quote:
Originally Posted by FordPrefect
Interesting. So you are referring to the barest, minimum amount of water required to hydrate (not catalyze LSNT) My use of "evaporation" which isn't quite the right term either for what I meant comes from my experience in concrete construction. After pouring a floor or beam there would be water that would rise to the surface. Admittedly this water would be there because we were using more than the minimum amount of water. (It didn't really matter since the concrete would still cure to a strength well beyond the engineer's specs.)
Yeah, you have to add a littlemore water than absolutely necessary for workability (particularly with things like beams where you need to make sure the concrete works down around the rebar cage). And it's true we usually use a pretty conservative concrete strength for our calcs. But you really don't want to use a whole lot of excess water 'cause it'll actually decrease the concrete strength. Usually the concrete guys carefully calculate the water to cement ratio (and check it with a slump test which is excatly like it sounds) so you get sufficient workability with the required strength. Here's a good paragraph from that page I linked above:

Quote:
Strength of Concrete

The strength of concrete is very much dependent upon the hydration reaction just discussed. Water plays a critical role, particularly the amount used. The strength of concrete increases when less water is used to make concrete. The hydration reaction itself consumes a specific amount of water. Concrete is actually mixed with more water than is needed for the hydration reactions. This extra water is added to give concrete sufficient workability. Flowing concrete is desired to achieve proper filling and composition of the forms. The water not consumed in the hydration reaction will remain in the microstructure pore space. These pores make the concrete weaker due to the lack of strength-forming calcium silicate hydrate bonds. Some pores will remain no matter how well the concrete has been compacted. The relationship between the water/cement ratio and porosity is illustrated in Figure 3.
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  #28  
Old 06-29-2005, 08:40 AM
Carnac the Magnificent! Carnac the Magnificent! is offline
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Mant thanks for all the responses. Never thought I'd get so many responses regarding concrete.

Imagine if I'd inquired about silicon...
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  #29  
Old 06-29-2005, 11:57 AM
UncleBeer UncleBeer is offline
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Quote:
Originally Posted by RM Mentock
any old fucking rock
LoVeRoCk?
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