In Japan the pumping of concrete is generally in practice in the building projects to save labor because of manpower shortage in the country. But there are some problems in pumping the concrete, especially of the lightweight aggregate concrete.
It is the aim of this report to present information on the above mentioned problems which has been obtained through the investigations into the practices adopted in several projects.
The summaries are as follows:
(1) The mix proportion of concrete is revised to some degree to make the pumping easier, although there has not yet been found a general standard.
(2) In many cases, the water requirement in concrete mix is increased from 5 to 10kg per cubic meter of concrete and also is the fine/combined aggregate ratio increased from 3 to 5%.
(3) The mix proportion of cement paste in concrete which is pumped up to comparatively high places (e. g. higher than 20m) has tendency to be increased.
(4) By receiving high pressure during the pumping process, the lightweight aggregate absorbs much more water than when it is under atmospheric pressure, therefore, the workability of concrete becomes lower than what it was before the pumping. The decrease of slump is in many cases not more than 3cm but in some cases exceeds 10cm.
(5) The entrained air increases or decreases, as the case may be, but the variation shows in many cases not more than 1%.
(6) The compressive strength of concrete is also affected by pumping. In certain circumstances, there is decrease of strength, even at times at the rate of more than 10%. This phenomenon may be ascribed, though not thoroughly analyzed yet, to the water which has found its way into the lightweight aggregates during the process of the pumping, due to its pressure, and which oozes out into the bond between the aggregate and the cement paste and damages it, after the concrete is moulded.
(7) Thorough irrigation of the aggregates in stock yard is effective in decreasing the amount of water to be pumped into the lightweight aggregates.
