Concrete Unit Weight Control
Concrete Unit Weight Control
(OP)
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?
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?





RE: Concrete Unit Weight Control
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.
RE: Concrete Unit Weight Control
RE: Concrete Unit Weight Control
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.
RE: Concrete Unit Weight Control
RE: Concrete Unit Weight Control
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?
RE: Concrete Unit Weight Control