Summary:
An experimental vibrator, operating on the top surface, has been used to compact freshly placed concrete (mix 1:2½:5; water-cement ratio 0·60) of low to very low workability. A study has been made of the effects on the compaction of slabs 18 in. thick of varying the frequency of vibration within the range 1,500-6,000 v.p.m., the acceleration within the range up to 12g, the amplitude within the range 0·004 to 0·064 in., and forward speeds of travel within the range 1-8ft per min. The density of the concrete was measured after test, while measurements were made during the experiments of the vibration occurring at various depths in the concrete.
A small variation in the workability of the concrete was found to have a considerable efIect on the surcharge required and the depth of compaction achieved. Consequently the effect of the method of compaction has only been studied so far for concrete of one workability, only those tests where the compacting factor was within a narrow range around 0·80 being considered.
When the vibrator operated at constant acceleration and constant number of vibrations per foot of travel, increase of amplitude of the beam increased the depth of compaction. At constant acceleration and amplitude of the beam, slower speeds of travel gave increased depth of compaction. When both the amplitude of the beam and number of vibrations per foot run were constant, the depth of compaction was unaffected by change of acceleration.
The total work done on a particle at a given depth was found to be related to the density attained at that depth. A connexion has been found between equivalent depth of compaction and the product of the amplitude of the beam and the number of vibrations per foot run. The depth of compaction increases with this factor but attains a limiting value, probably dependent on the concrete mix and the characteristics of the vibrator. It seems desirable, with vibrators designed to operate at constant acceleration, to operate them with a large amplitude and low frequency.
