Understanding the Density of Concrete

The density of concrete is a measurement of concrete’s solidity. The process of mixing concrete can be modified to form a higher or lower density of concrete end product.

A very high density of concrete is that made around steel cables that have been stretched by hydraulic jacks. The concrete is allowed to harden and then the jacks are released. As the cables contract, they compress the concrete. Compressed concrete is the strongest concrete there is and is used for bridges, roofs, and floors. On the other hand, concrete with air entrained in it works well in harsh weather and is used in roads and airport runways. Lightweight concrete uses pumice, a very lightweight mineral, as aggregate.

One way to determine the density of concrete is to first determine the density of the materials that go into it. Cement weighs 830 to 1,650 kilograms per cubic meter, equal to a range of 52 lbs. per cubic foot to 103 lbs. per cubic foot. Cement that is pneumatically loaded into cement silos is less dense, while cement that has been stored and exposed to vibration (as it would be if transported by truck) is more dense. The rule of thumb is to consider that a 94 lb. bag of cement will make one cubic foot when it is freshly packed. It will naturally compress during transport. Clearly, because of the large difference in bulk volume, cement should be measured by mass rather than volume.

As for concrete itself, the density of concrete of normal weight is about 2,400 kg per cubic meter, or 145 lbs. per cubic foot. The concrete density varies depending on the amount and density of the aggregate, how much air is entrapped or purposely entrained, the cement concentration, and the maximum size of aggregate used. Lightweight concrete has a density of 115 lbs. per square foot.

Perhaps the easiest and most accurate way to calculate the concrete density is to measure some into a container of known volume and weighing it. Density is simply a mass to volume ratio.

Some people considering concrete counter tops may wonder if lightweight concrete should be used to minimize stress on the cabinetry and flooring below. But with standard counter top thickness of 1.5 inches, normal weight concrete weighs about 18 lbs. per square foot, while lightweight concrete weighs about 14.5 lbs. per square foot. The 3.5 lb weight savings is not enough to make a significant difference, and normal density of concrete for counter tops is easier to polish.

For those who need a less dense concrete, some or all normal-weight aggregate can be replaced with lightweight aggregate. Half a pound of lightweight aggregate takes up as much space as one pound of normal aggregate. In other words, same volume, but lighter weight. There are important considerations to account for if you convert regular concrete to lightweight. Lightweight aggregate in concrete mix affects the appearance, performance, and workability of concrete. The lightweight aggregates are usually brown, orange, or dark gray, and because of their porosity they don’t polish well.

Compressive strength, elasticity, and tensile strength are also affected by the use of lightweight aggregates. Lightweight concrete that uses Vermiuclite, Styrofoam, and Perlite as aggregates are not suitable for structural purposes. However for use as lightweight filler or insulation, such a lightweight density of concrete is a good choice.

Another consideration with lightweight concrete is that the aggregates are more porous and therefore hold more moisture. Lightweight aggregates will continue to soak up moisture for weeks after being wetted for the curing process. Therefore, concrete losing its mix water to the “thirsty” aggregates during the curing process can cause color variation, mottling, and “surface crazing,” small networks of surface cracks that occur during the early drying process. It’s important to keep track of all the mix water added and keep it in the proper range so as not to adversely affect the density of concrete as well as its performance and appearance.

Source:www.bukisa.com

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