Table 312 Slump test classes to European Standard EN 2061 2000

Slump class Slump in mm

51 10-40

52 50-90

53 100-150

54 160-210

55 >220

before it begins to stiffen. Air voids weaken the concrete, increase its permeability, and therefore reduce durability. In reinforced concrete, lack of compaction reduces the bond to the steel, and on exposed visual concrete, blemishes such as blowholes and honeycombing on the surface are aesthetically unacceptable and difficult to make look good successfully. Vibration, to assist compaction, may be manual by rodding or tamping for small works, but normally poker vibrators and beam vibrators are used for mass and slab concrete respectively. Vibrators which clamp on to the form-work are sometimes used when the reinforcement is too congested to allow access for poker vibrators.

The degree of compaction achieved by a standard quantity of work may be measured by the compacting factor test. In this test a fresh concrete sample is allowed to fall from one hopper into another. The weight of concrete contained in the lower hopper, when struck off flush, compared with a fully compacted sample gives the compacting factor. The compacting factor for a medium-workability concrete is usually about 0.9.

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