Progress in pumping structural lightweight concrete has reached the point where a lightweight project can be approached with the same confidence that would be justified for any pumping project. Most pumping contractors and ready mix producers are in agreement on two points:
- By far the most important single factor in producing pumpable concrete is the adequacy of presoaking the aggregate.
- If aggregate is adequately and consistently presoaked, the problems of pumping lightweight are little different from those of pumping normal weight concrete.
Some of the problems in pumping are: inconsistent mixes, slump loss, segregation, and need for excessive pump pressures. For each problem it would be possible to respond by adding water. Excessive water demand, however, means that there is some other deficiency to correct. The major problem is not in getting enough water into the mix, but in preventing the water required for workability from being lost to absorption. If the aggregate is not presoaked, so much water can be lost through absorption during mixing and under the pressure of pumping that slump will be uncontrollable. For all practical purposes pumping becomes impossible.
PRESOAKING AGGREGATE BY SPRINKLING
Presoaking by the concrete producer is usually done by sprinkling. Depending on the aggregate's rate of absorption, the minimum time would be 3 to 7 days of uninterrupted sprinkling. To be effective, the sprinkling of aggregate piles should be constant. Frequent testing, at least 3 or 4 times a day, should be done to determine moisture content.
THE LIGHTWEIGHT CONCRETE MIX
The first word of caution is that the concrete mix, no matter how well it is designed, should be field tested to confirm its pumpability before the project starts. One difference between the mix proportions of lightweight and normal weight concrete for pumping is in the amount of fine aggregate used. The proportion of fine aggregate is higher in lightweight than for normal weight mixes. The fine aggregate passing the No. 50 sieve is of greatest importance. As in normal weight concrete, air entrainment improves pumpability of lightweight mixes. It also reduces the tendency for the mix to segregate or to bleed. Water reducers, water reducing retarders and superplasticizers may be used separately or in combination in the same way they are used in normal weight mixes.