Concrete Unit Weight

[dcarr82775]

Concrete mix designs typically give a dry and plastic/wet unit weight. Is there a reference for what the final in-situ unit weight would be since some of the water evaporates? More or a curiosity on my part, but I would think the final weight would be somewhere in between.

 

[concretemasonry]

It all depends on the conditions and placement methods. High percentages of reinforcement will usually reduce the unit weight of the concrete itself.

When you are dealing with mix designs and testing for results, you could be in "never-never land".

A guess that definitely, a figure between the two densities and much closer to the dry weight since a significant amount of moisture is removed during the hydration and curing process, but should be good enough for curiosity. this could shift depending on whether it is lightweight concrete (less than 115 pcf) or normal concrete (140-150) using most natural aggregates.

Dick

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

 

[JAE]

High percentages of reinforcement will usually reduce the unit weight of the concrete itself.

I'm not sure I follow that statement. Steel has a density of 490 pcf and concrete is 150 pcf. More steel does not mean lower unit weight of a reinforced concrete element.

 

[concretemasonry]

JAE - My statement was somewhat clumsy. The density of the concrete itself may be less for heavily reinforced structures because of the difficulty in placing and vibration that is caused by more steel than normal. - The composite density of the concrete and rebar will be higher than unreinforced concrete.

dcarr82775 - To split hairs, the density of the aggregate used has an effect on the density. Common aggregates (igneous and sedimentary) do have a range of densities. In some cases, a mixture of "lightweight" aggregates (natural and manufactured) can produce densities of less than 100 pcf where lower weights, higher fire resistance or greater insulation are required. For an extreme example, very heavy aggregates (hematite and others) have been used for containment in plants the need radioactive protection where the wall thicknesses are limited.

Dick

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

 

[Ron]

There is not a great deal of difference between the wet unit weight of concrete and the "dry" unit weight. The reaction of hydration binds about 1/2 to 2/3 of the water used in the mix. The remaining water is interstitial and can remain so for years. Yes, some evaporation takes place from the surface as concrete dries and cures. For beams and columns, the surface area to volume ratio is small. For slabs, the surface area to volume is larger and will have more effect.

Depending on the type of coarse aggregate used, the unit weight of normal weight concrete will generally range from about 140 pcf to about 155 pcf, the values typically used in design calcs.

 

[TXStructural]

Careful with a statement like "The density of the concrete itself may be less for heavily reinforced structures because of the difficulty in placing and vibration that is caused by more steel than normal." Since it only applies in poorly-constructed projects and indicates that the structure will not have the design strength. If you allow this, it's your license on the line.

Hydration of concrete requires somewhere around 0.25 w/c ratio (although this is debated, it is the theoretical requirement.) As noted, the trapped moisture in voids also sequesters a fair amount of moisture. Do not count on evaporation of any of the moisture in the concrete, since properly cured concrete retains most of the mix water. Also, aggregates are processed into ready mix at a saturated, surface dry (SSD) condition, so the water added to the mix is not all that is there or all that remains for a long period after placement.

 

[BigH]

. . . and what if you have poor vibration techniques leading to vugs and excessive voids/honeycombs - or concrete without consistent aggregate gradation . . .? and even in cylinders cast on site, there is a variation between cylinders of the same sample - so are we splitting hairs? and what is the underlying reason for the OP?

 

[dcarr82775]

Thanks for the input.