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Pumping lightweight aggregate concrete is challenging due to the high water absorption of the aggregate in it which results in issues such as segregation, slump loss, inconsistent mix, etc. Therefore, engineers and contractors should consider appropriate strategies to overcome problems associated with pumping lightweight aggregate concrete.
ACI 213R-03 recommends several measures to improve the pumpability of this type of concrete. For instance, adequate and consistent presoaking of the lightweight aggregate to reduce its water absorption capacity is essential to eliminate pumping issues of concrete.
Introducing cementitious materials like fly ash and replacing a small quantity of coarse aggregate with sand enhance the concrete pumpability. The change in the unit weight of concrete should be considered when these techniques are implemented.
How to Improve Pumpability of Lightweight Aggregate Concrete?
- Lightweight aggregate has higher water absorption than conventional aggregate, and pump pressure further increases its water absorption capacity. Therefore, the addition of water is one of the measures to improve pumpability.
- Nonetheless, the above measure is not always effective since some deficiencies of the concrete mix cannot be overcome by adding water to the mixture. The focus should be on preventing water loss in the concrete mixture.
- Saturate the lightweight aggregate before batching by sprinkling, immersion of partially cooled aggregate in water, or vacuum saturation. Adequate saturation minimizes slump loss, maintains mix consistency, and improves pumpability.
- In the case of sprinkling, the minimum time for effective saturation of lightweight aggregate is constant and continuous sprinkling is required for at least three to seven days.
- The concrete mix should contain at least 335 Kg of cement per cubic meter.
- Employ pumping aid or water-reducing admixture. For instance, expanded shale aggregate concrete mix can be improved by increasing air content from 4% to 8% and replacing some sand with fly ash.
- Replace a small quantity of coarse aggregate with sand.
- Use a slump of 100 mm to 150 mm. If required, increase cement in the mixture to maintain this high slump.
- The natural sand in the mix should be well-graded with a fineness modulus ranging from 2.2 to 2.7. If the utilized sand does not meet these conditions, increase the sand content.
- Grade coarse and fine aggregate properly using absolute volume instead of mass to account for differences in relative density of the various particle sizes.
- Regarding pumping equipment, utilize the largest size line available (minimum 125 mm diameter), prevent rapid size reduction, and decline pumping pressure through limiting bend number, employing steel line instead of rubber line, and slowing down placement rate.
The minimum time for effective saturation of lightweight aggregate is constant and continuous sprinkling is required for at least three to seven days.
The aggregates should be tested for 3 to 4 times a day to check whether the aggregates have been saturated adequately or not.
Vacuum saturation is conducted by introducing dry aggregate into a vessel from which the air can be extracted. After that, the vessel is filled with water and returned to atmospheric pressure. This process can be carried out only at the aggregate plant.
Yes, it is necessary to test the designated concrete mixture for pumpability.
Pumping pressure can be reduced by limiting the number of bends, employing steel lines instead of rubber lines, and slowing down the placement rate.
Immersion of partially cooled aggregate in water and vacuum saturation are the best options to saturate aggregate in cold weather conditions. The use of sprinkling may lead to the freezing of aggregate.
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