facts about concrete density

Important facts about concrete density

What does concrete density have to do with anything you might be asking yourself? Actually, it’s quite important and it’s one of the defining qualities of good concrete. Density is a measurement that tells us how much there is object x (mass) in size y (volume)so it can help us measure structures and their qualities.

So what is concrete density about? Concrete consists of cement and fine and coarse aggregates (sand and gravel) and it’s mixed with water for the hardening/curing process to start. Low water to cement ratio makes the hardest concrete while too much water to cement will make it weaker.

Normal weight concrete density per cubic foot is reported to be from 143 lb/ft³ (2300 kg/m³) to 150 lb/ft³ (2400 kg/m³). The real density varies depending on mixing conditions and materials used.

So if you’re wondering how much does concrete weight, you just need to calculate how much concrete you need and use the information above to multiply it.

How about wet concrete, is density of fresh concrete different from the hardened one?

Concrete loses around 5% of its weight when water evaporates. So the density of wet concrete can be calculated with that in mind. One of the concrete properties is also that it shrinks when it hardens/cures. For more concrete math, here is about concrete yardage calculations.

Lightweight concrete density

Lightweight concrete is different from “normal” because the aggregates used are of lighter density. Lightweight concrete aggregates are materials like shale, clay, volcanite pumice, scoria, tuff, pellite and slate.

Lightweight concrete density is around 90 to 115 lb / ft³ (1440 to 1840 kg/m³). There is actually more than one way to make lightweight concrete besides lighter aggregates. It can also be made by doing voids in the concrete, resulting in aerated concrete. The third type is leaving fine aggregates like sand away.

The density of sand can actually be higher than the lightest lightweight concrete (from 90 lb/ft³(1442 kg/m³) to 100 lb/ft³ (1602 kg/m³)) so that isn’t all that surprising. Normal weight concrete aggregates density is around 75-110 lb / ft³ (1200-1750 kg / m³)

Lightweight concrete mixes that can be done at home would depend on what aggregate is used and the amount of sand. Glass fiber could also serve to strengthen its structure. One way to test it at home could be a 1:1:1 ratio of cement, sand, lightweight aggregate, and mix it with a little over 0.5 water.

When it comes to lightweight concrete vs normal-weight concrete I guess it’s really about everything has its uses. Lightweight will help to lessen structure weight load, but on the other hand, the structure will make it less resistant to moisture. For the same reason, it takes longer for lightweight concrete to dry.

Reinforced concrete density

Concrete can take a lot of pressure but is weak when it comes to pulling force. That’s why the concrete structures are almost always reinforced with steel. Reinforced concrete density would depend on how much steel has been used on the structure. Steel density is multiple times higher, 484 to 503 lb/ft³ (7750 and 8050 kg/m³).

Concrete testing equiptment doesn’t have to be too expensive

How is concrete testing done

Concrete testing is one way of controlling quality and a good way to check if the building material is appropriate. I listed a few of them here. Density is actually one of the points.

Some ways to test fresh concrete are slump test, density (unit weight), the air content of fresh concrete by pressure. For hardened concrete there is Schmidt Rebound Hammer, pull out, ultrasonic pulse velocity, and compression test.

Fresh concrete measuring

The slump test is done with fresh concrete to test its flow. The tool used is an inverted cone that has no top or bottom. It’s held against platform-wide side down and filled from the top with concrete. The concrete is pushed with tools now and then so it’s as dense as possible. When the cone is full it’s lifted up and the concrete “slumps” due to gravity. Then it’s measured how much the concrete sunk from the top, 4-5 inches is ideal.

Density is easy to measure as we know how much it should ideally weight. We take a bucket, calculate how much it should weigh full of concrete. Then we fill it and weigh it and after we take off the container weight we know how close we are to ideal.

Air pressure test can be done with fresh concrete on the construction site as well. Concrete is placed on a circular container, tamped with something like in the slump test to be as dense as possible. Then the container is closed with a device that has a dial and air is hand-pumped into it. The result can be read from the dial.

Hardened concrete measuring

Concrete can be measured when it has hardened as well. Rebound hammer test is one way to measure it. To be short, the device launches a mass on springs and measures the rebound from concrete. The best thing is that it’s non-destructive.

The pull-out test is done with a metal rod that’s placed within the concrete structure during the cast. Then it’s pulled with straight pull force and it can be seen how much the concrete can handle. The bad side is that concrete is damaged during the process.

Ultrasonic pulse velocity test is to measure density and elastic properties. The device sends acoustic waves through material that can tell about those qualities. Here the problem might be metal in the structure and other problems with the material like cracks.

A compression test is done by extracting concrete samples of the existing structure. Then it’s put on compression to see how much pressure it can take. This is a reliable method to measure strength, but the downside here is that it is also destructive.

Conclusion

Density might not seem too important at first, but it’s really one of the most important factors of concrete. It defines its strength (with other factors of course) and so we know how to use it.

The weakness of concrete is how it can’t handle pulling forces well. That can be taken care of with proper reinforcement. It’s not only building houses, but it’s useful to know even with smaller structures and hobby projects.

Testing of concrete in fresh form and hardened is good for knowing how well we succeeded. Testing is also an easy way to oversee concrete projects and usually, we aim for the best results on the casting phase.

The easiest way is to follow ready mix instructions and remember that amount of water needed varies a little from bag to bag. If you’re working the concrete materials separately it’s best to try taking even measures so the quality stays the same.

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