Preparation and Testing of Concretes
The fresh concretes were prepared using a pan-type mixer. Constituent materials were batched and
mixed to produce the concretes. The total initial mixing time was 10 minutes. At first, fine and coarse aggregates were mixed for 1 minute with first quarter of the mixing water. Mixing of aggregate blend was continued for another 1 minute with the addition of AEA dispensed in second quarter of the mixing water. After 2-minute mixing, the mixer was stopped to add the binding material (cement alone or cement with RHA). Immediately, the mixer was restarted, third quarter of mixing water was added, and the mixing was continued for 2 additional minutes. Then the mixer was stopped again, the pan was covered with wet burlap and the aggregate-binder mixture was allowed for 3-minute rest. Thereafter, the mixer was restarted and the concrete materials were mixed for 3 more minutes with the gradual addition of HRWR dispensed in fourth quarter of mixing water. The saturation dosage of HRWR was used to produce the required flowing ability. For all concretes, the saturation dosage was split into four parts such as 70, 10, 10 and 10%, and added sequentially to maintain similar slump and slump flow over the whole testing period. The initial dosage of HRWR was equivalent to 70% of the saturation dosage. The remaining three parts of the saturation dosage were added to the concrete mixture at subsequent mixing stages, as shown in Table 2.
Table 4: Dosages of HRWR and AEA for Various Self-consolidating Concretes
The air-void stability in fresh concretes was investigated with respect to post mixing and agitation. The entire testing task was comprised of four stages, as shown in Table 2. In both series of concrete, the primary mixtures were tested after 15 minutes from the start of mixing to determine the air content. Then the concrete mixtures were covered with wet burlap and allowed for rest followed by further mixing and testing. The rest period varied from 10 to 25 minutes depending on testing time shown in Table 2. The post-mixing was conducted for 2 minutes for each test stage. Similar flowing ability was maintained in fresh concretes during the entire test period. For this, an additional HRWR by 10% of the saturation dosage was added during post-mixing. The subsequent measurements for air content were taken after 30, 45 and 60 minutes for series 1. In case of series 2, these measurements were carried out after 30, 60 and 90 minutes. The air content of fresh concretes was determined in accordance with ASTM C231 (2004) with an exception that concrete sample was placed in the measuring bowl without any consolidation. The readings obtained from the air meter were corrected based on aggregate correction factors. The flowing ability of the fresh concretes was also examined during each test stage. For this, the slump and slump flow were determined using the Abram’s slump cone specified in ASTM C143/C143M (2004). The freshly mixed concrete was placed in the slump cone in one layer and without any consolidation. Then the slump cone was raised vertically and the concrete was allowed to subside. The fall in height of initial concrete sample and the average diameter of the deformed concrete were measured and recorded as the slump and slump flow, respectively.