Concrete density testing

How is the density of hardened concrete measured?

Density is the mass of the specimen divided by its volume, so the test is a careful determination of both. The specimen is weighed, and its volume is found either by water displacement or by measuring its dimensions. The test does not damage the specimen and is normally carried out on the same cube or core immediately before a compression test, so a strength result and a density result can come from one specimen.

Which standard covers hardened concrete density?

Hardened density is usually determined to BS EN 12390-7, which covers the methods, the condition of the specimen and how the result is reported. It gives water displacement as the reference method for volume, with calculation from measured dimensions as an alternative. The result is expressed to the nearest 10 kg/m³. Where your specification names a different method, we work to that instead.

What is the difference between fresh concrete density and hardened density?

Fresh density is measured on the concrete as delivered, in a container of known volume and usually to BS EN 12350-6, and is used as a yield and compaction check at the point of placing. Hardened density is measured on a cured specimen days or years later and reflects the concrete as it actually set, including any voids left by poor compaction. The two are related but are not the same number, and a specification will normally say which it wants.

Does the specimen need to be oven dried, saturated or tested as it arrives?

BS EN 12390-7 allows density to be determined in any of three conditions: as received, water saturated or oven dried. The condition changes the answer, because water held in the pores adds mass, so a specification quoting a density limit should say which condition it applies to. Oven drying is the one that affects the programme, since the specimen has to be dried to constant mass before it is weighed, and that adds days.

Which method of measuring the volume should we ask for?

Water displacement is the reference method and the most precise, and it is the sensible choice for cores, trimmed pieces and anything that is not a regular shape. Volume calculated from the actual measured dimensions is a reasonable alternative for a sound cube. Calculating from the designated mould size is the least precise and only works where the moulds themselves are checked. If the figure is going into a conformity decision, use displacement.

What density should normal-weight concrete come out at?

Most normal-weight structural concrete comes out somewhere around 2300 to 2500 kg/m³, though the figure moves with the aggregate used. BS EN 206 treats concrete with an oven-dry density above 2000 and up to 2600 kg/m³ as normal weight, below 2000 as lightweight aggregate concrete and above 2600 as heavyweight. A result well outside the expected range for the mix is worth investigating rather than accepting.

Can a density result tell us whether the concrete was properly compacted?

It can, when there is something to compare it with. Poorly compacted concrete contains voids, and voids show up as a lower density than a well compacted specimen of the same mix, which makes density a useful check on compaction. On its own a single figure says very little. It is most valuable read alongside the mix design, a companion specimen or a run of results from the same concrete.

How do we confirm that a lightweight concrete meets its specified density?

Lightweight concrete is normally specified by oven-dry density, so the specimen is dried to constant mass before the determination and the result is compared with the density class in the specification. Getting the condition right is the whole exercise, because an as-received figure on a lightweight concrete will read higher than the oven-dry value it is being judged against. Tell us the density class you are working to when the specimens are booked in.

What sort of sample do you need for a density test?

A cube, a cylinder or a core is the usual specimen, and density can be taken from specimens already coming to us for strength testing. The piece has to be intact, sound and large enough to be representative of the concrete, which in practice means comfortably larger than the biggest aggregate particles in it. An irregular trimmed piece can be tested by water displacement where a regular specimen is not available.

Why has our density come out lower than the mix design?

The most common reasons are air voids from incomplete compaction, a difference in the moisture condition at test and an aggregate with a different particle density from the one assumed. Entrained air deliberately added for freeze-thaw durability also lowers the figure, as does a higher water content. A low density alongside a low strength usually points at compaction. A low density with an acceptable strength more often points at the mix or the condition the specimen was tested in.

Is the density method we need within your UKAS scope?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope, and density can be determined in several conditions and by more than one method, so this is a question worth asking precisely. Tell us which method and condition your specification calls for and we will confirm the accreditation status before you book.

What should we send you for a density testing quote?

Send the number and type of specimens, the condition and method your specification requires, the site postcode and your preferred dates. Say whether density is wanted on its own or alongside cube or core strength testing, since determining both on the same specimen is the efficient route. Cost is driven by the number of specimens, the preparation involved and whether oven drying is needed, which takes longer than testing as received.