Workability & Related Mix Properties

There is little choice in methods of delivery and placement of high-performance concrete. Due to the nature of this special concrete and the small market that it commands, it is normally not economical for a batching plant to buy new equipment specifically for high-strength or otherwise high-performance concrete. It is also not economical to purchase special delivery or finishing equipment. Therefore, plants producing high-strength concrete also produce normal concrete in order to get the best return on their investments. The result is that high-strength concrete is batched, delivered, and finished in much the same way as normal concrete.

The workability of the mixture is directly related to the slump. The lower water/cm ratio for high-strength concrete yields a very low slump and thus poor workability. However, since fly ash or blast furnace slag is included in the mixture, the slump is increased, reducing the effects of the low w/cm ratio. Rheological tests can be performed at the mixing site and superplasticizers added as required. Aggregate shapes and grain size distributions are examined carefully, since the materials control is an important factor in the quality control. Limiting aggregate size increases workability. Adjustments made to improve rheological properties must not change the proportioning of materials completed in the design phase or the entire mixture will change. Workability is important to obtain a dense, void-free mass in full contact with the steel reinforcement. The mixture should be easy to vibrate without causing segregation.