Density of 3500PSI 4.5% Air Entrained 5.5 Sack Concrete 2

[MTUJeeper]

As the title says, I'm trying to calculate the weight of the slab. I've found a standard 145lb/cf number, but I'm looking to be more specific as to the exact concrete that was used. Trying to verify proper design/construction of a 50'x10'x8" pad. Thanks! Christian

 

[JedClampett]

It's not worth refining your number beyond the 145 pcf, no matter what the air content is.

 

[concretemasonry]

The only way to get an exact weight in pcf is to actually measure a sample from the slab because the batch weights are theoretical and placement, moisture content and actual thicknesses can vary.

The calculations provide a good starting point since they reflect the normal aggregate densities. The consolidation of the concrete is a big factor, but eventually in the end, the voids do not usually weigh much if you are considering inplace density of cured concrete. The moisture contents can be a wild-card, but usually decrease with curing and and the exposure factors. A slab on grade, even with a vapor retarder/barrier, will have a higher actual unit pcf than an elevated slab.

If you want a higher density, use denser aggregates or use a lighter aggregates like pumice, expanded shale/slate/clay and similar premium cost materials for a lighter slab.

That is why all refernces give a range of values between 115 pcf and 150 pcf, although special materials and procedures can be used to go beyond the ranges.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hokie66]

MTUJeeper,
I see you are a mechanical engineer. Advice given by Jed and CM is correct. Concrete is not a material like metals which have a known and almost constant specific gravity. There are many variables, and the type aggregates are the most important. I use 150pcf or 2400kg/m^3 for "normal" concrete in dead load calculations.

 

[cvg]

aggregates are a key component to concrete and the exact size gradation and weight of the gravel and sand mixture used in your slab are unknown to you. If you insist on knowing the exact unit weight of the concrete, obtain a core and take it to a lab and you can get the precise number. As indicated, the thickness also varies, so you will need several more cores to determine that...

 

[Ron]

Agree with the above comments. The coarse aggregate is the only variable that makes much difference.

If you are using a calcareous coarse aggregate (limestone for example) then a lower unit weight is appropriate (140 pcf). For most other coarse aggregates, 150 pcf is appropriate.

Find out the coarse aggregate type, then find it's specific gravity. If the specific gravity is less than 2.50 use 140. If greater than 2.60, use 150.

 

[MTUJeeper]

Thanks guys, I'll just stick with 145 to split the difference, I was only 7 days old when the pad was designed (3/7/85) so I don't have a lot of info to go off of as far as aggregate type. The reason for calculating is our ammonia tank skid has a small tweak in it, which makes it not contact the pad, and therefore generates a large moment on one end of the pad. I need to check if this will cause failure of the pad or not. I do have another quick question for you. When calculating 2 way shear, the skid is made from C-Channel running most of the length of the pad, but overhangs on one end. What is done in the calculations for the end that overhangs the pad, as the shear perimeter would fall outside the pad? If you need a picture to understand it better I can upload one.

 

[Ron]

Shear is transmitted only by the contact of the channel with the slab. The calculations are handled as if the channel stopped at the slab edge with all load applied to the channel within the confines of the slab.

 

[hokie66]

I fail to understand why you were asking about the density of the concrete, when you are really trying to estimate something about the strength of the pad. Perhaps a sketch would help, and the drawing showing the pad configuration and reinforcement.

 

[MTUJeeper]

Density X Volume=Weight Weight/Area=Soil Bearing Pressure And the soil bearing pressure is used as the basis for all the other calculations for strength. Bear with me, never EVER designed or have any experience with reinforced concrete...so I'm learning as I go... I'll work on a sketch and upload it if it will be helpful. I'm at the point of calculating the bending strength, just trying to sort through it as everything I can find is for designing the size of the bars, rather than finding the strength from the design.

 

[Ron]

The bearing pressure on the soil will also include other loads on the slab. You have only considered dead load.

 

[MTUJeeper]

Don't worry, I've calculated the factored load for soil pressure including the dead load of a full ammonia tank and also snow load. The slab doesn't see any vehicular or foot traffic so no live load. Have all the calc's done now, just have to make them pretty with some fancy diagrams and write the report tomorrow. Thanks for keeping an eye out though. I followed the ACI 318 manual for my calculations, so I'm fairly confident in my results. Gotta love having an ME doing civil work because it is cheaper than contracting it out, haha.