What is the degree of compactability?

The degree of compactability (in Dutch: verdichtingsmaat) is a number that expresses the consistence, or stiffness, of fresh concrete. It is found with the Walz test: how far does loosely filled concrete settle when you compact it fully? The stiffer the concrete, the more it settles and the higher the value. The test is meant mainly for stiff and earth-moist concrete, where the slump and flow table tests say little. It is described in EN 12350-4.

How the test works

1. Wipe an open-topped container of known height (h1) with a damp cloth and stand it on a firm, level base.
2. Fill it loosely with concrete, using a trowel and tipping alternately from each of the four edges, without pressing it down.
3. Strike off the surplus.
4. Compact the concrete, for example on a vibrating table or with a poker vibrator, until the volume no longer drops.
5. Measure the distance from the rim to the concrete surface at the middle of each side and take the average (s).

The degree of compactability is c = h1 / (h1 – s), rounded to two decimals.

Classes

Under EN 206, the degree of compactability is linked to these consistence classes:

• C0: 1.46 or more (very stiff)
• C1: 1.45 to 1.26 (stiff)
• C2: 1.25 to 1.11 (plastic)
• C3: 1.10 to 1.04 (soft)

For more fluid concrete the test is unsuitable; use the flow table test instead.

Compactability or degree of compaction?

In earthworks, something different is measured: the degree of compaction (relative compaction). That is the dry density of the compacted soil on site as a percentage of the Proctor density, the maximum density reached in the laboratory with a set compaction effort. The specification sets the requirement, often as an average and as a minimum per test point. Check the specification for your job.

Application

The degree of compactability is used for stiff mixes, such as earth-moist concrete for tamped concrete, precast concrete products, bedding kerbs and other work with stiff concrete.

Tips and points of attention

• A lower number means a wetter mix: a lower degree of compactability usually means more water and so less strength. Do not add water on site just to make the concrete easier to place.
• Test straight away: stiff concrete changes quickly. Test it when the truck arrives.
• Do not mix up the terms: degree of compactability is about fresh concrete, degree of compaction about soil.

Related terms

• Consistency class
• Fresh concrete
• Concrete compaction
• Soil compaction
• Tamped concrete

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