Concrete Unit Weight Control

[jheidt2543]

In general, for the same local aggregates & sand, is the unit weight of the concrete increased by going from 1" stone to a pea gravel size stone in the mix?

We have a project that requires 4,500 psi @ 148 pcf concrete and the "standard mix" designs are producing 145 pcf concrete at the site. It seems that a shift to pea gravel produce a more dense concrete with little or no affect to the strength. Comments?

 

[Ron]

Reducing the aggregate size causes the need to increase the volume of cement paste because the total surface area of aggregate than needs to be coated by cement paste increases. This means that you need more cement and water. Cement has a specific gravity of about 1.5 times the aggregate specific gravity, so the combination will increase the unit weight.

Now, requiring a specific unit weight for concrete is absurd. At best, you can only hit a range of unit weights given the variation in aggregates and mixes. I would suggest that a range of 145 to 155 pcf would be appropriate and would give the mix supplier the necessary latitude to provide you with a mix responsive to the needs of the project.

Also keep in mind that when you reduce the coarse aggregate size and adjust the cement content accordingly, you will also increase the amount of plastic and drying shrinkage you will get, so there is even more criticality to the timing of finishing and sawcut joints.

 

[jheidt2543]

Thanks for your comments. This project has a minimum unit weight requirement to achieve the necessary shielding for a medical device. The shielding is a function of the concrete thickness and unit weight. The wall thickness is fixed at 4'-6" which drives the need for the minimum unit weight of 147 pcf of the cured concrete. I've been told to achieve this, the as placed unit weight must be 151 pcf or above since upon reaching full cure, the concrete will loose 2-3 pcf.

 

[concretemasonry]

jheidt -

In an earlier life, I was involved in providing CMUs (concrete block) for shielding in nuclear power plants and later for X-ray facilities.

Because of the specifications for pre-construction certification of the products and not mix designs they required up to 6 wythes of very dense (somewhat higher than your requirements) 6" solid block for walls that were straight and curved and installed in the middle stages of construction. Since block were required by the turn-key contractor and engineers you could not play with adding cement to a mix and we had no choice but to switch from a very good, dense local aggregates, we used a non-local iron ore bearing aggregates (hematite, I recall) and had it crushed to an acceptable gradation for manufacture and had to up the mix cement and sand content (minor cost) because of the manufacturing process. The cost increase was due to the shipping of the aggregate, but it had impact to the owner or CM since they knew how to meet the government requirements.

In short, the aggregate was costly, but the results made for a flawless jobs because we had tested and certified approved cured units to be used in construction instead of relying on comparing mix designs with field samples. If it is critical, look to other types of aggregate that a good local ready-mix supplier should be able to develop.

Dick

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

 

[hokie66]

Ron is correct. The specification is absurd. But if you want the concrete to be heavier, find some aggregate with a higher specific gravity rather than relying on adjusting the size.

 

[BigH]

I can agree that the unit weight specification might be absurd in many cases but, say, in the case of a concrete gravity structure (dam, for instance), there is a need for the unit weight to be considered. UK used to have a 2400 kg/m3 mix - and this seems, at least here, to be continued. Our aggregates won't reach it - about 50 kg short to just about 2400. If the design is that critical then something is wrong with the design. Never had that problem on another job as our aggregates specific gravity was something like 3.

just wondering if the use of silica fume might help in the shielding aspects . . . gives a much lower "permeability" which, in my view, means a denser mix . . .? Any thoughts on this?

 

[Ron]

BigH...no problem with considering and specifying a unit weight range, it was just that in this case they seemed to specifying an exact unit weight that would impracticable to meet and maintain with consistency.