Influence of Powder and Paste on SCC Using Recycled Concrete Aggregate

The use of recycled concrete aggregate is an important issue for preserving the natural resources, protecting the environment, saving of energy and for sustainable development. In this study, a unique simple mixture proportioning method was successfully adopted for developing SCC based on the absolute volume concept, starting with a volume of paste with maximum aggregate size of 20 mm. The method involved very few trials with different types of aggregate that can be advantageously used in field practice. The volume of paste chosen was 0.38 to 0.43 fraction of the total mix. Natural crushed angular granite and recycled demolished concrete waste were used as aggregate for making SCC with fly ash as filler. SCC and vibrated normal concrete mixes were made for varying cement contents of 300 kg/m3 to 450kg/m3 and for a constant water content of 185 l/m3. Cubes of 150 mm size were cast and tested for compressive strength at different ages of 7, 28 and 60 days. The results indicate that higher paste content not only makes the mix more cohesive with better lubrication of aggregates, but also increases the slump flow (reduces yield stress) and compressive strength. The 28 day compressive strength of SCC with both natural and recycled concrete aggregate showed higher compressive strength than vibrated normal concrete and significant increase at later age, possibly due to pozzolanic activity of the filler material (fly ash). However, the compressive strength of SCC mixes with recycled concrete aggregate has a lower value, when compared with SCC mixes of natural crushed angular granite aggregate at the same age. The study clearly shows the importance of paste in SCC and the advantage of using recycled concrete aggregate with better performance than the vibrated normal concrete with natural crushed aggregate and can go a long way in conserving the natural resources by the use of recycled concrete aggregate for sustainable development.