An investigation into the properties of concrete made from concrete debris and effects of the three-step mixing procedure on it

Using by-product materials in making concrete is the recent development in the advanced concrete technology. The products obtained from experimental crushed concrete can be used in constructing the new concrete. Recent research revealed that using crushed concrete as partial replacement of sand and coarse aggregate, using traditional mixing procedure, reduced the compressive strength. In the present research, the special procedure of mixing concrete in three steps and its effect on concrete strength parameters have been evaluated.  The aim of this study is to use crushed tested concrete as partial replacement 25% (mix A) and 50% (mix B) of conventional sand and aggregate. Also for experimental comparison the control mixes were casted with traditional sand and aggregate.  Superplasticizer based on carboxylate was used in the mixes.  This procedure causes proper efficiency, proper coating on the aggregate and protection of the alkaline reaction of the aggregate. The method of mixing was carried out in three steps 1: Coarse aggregate + 50% water + 50% cement: mixing time 30 seconds to 1 minute, step 2: Adding 50% cement + 25% water + superplasticizer + sand: mixing time 2 minutes, step 3: Adding 25% water: mixing time 3 minutes. All the mixing time is about 6 minutes. Six specimens of 15 cm concrete cubes for each designed mixes were casted. Densities, water absorption, electric resistance (an indication of permeability and durability of concrete) and compressive strength tests were carried out. Tests were performed at 7 and 28 days. As a result, the designed mix (A) presented the higher electric resistance and compressive strength at 28 days. According to the obtained appropriate resistance, it can be deduced that with a special mixing method, crushed concrete can be used in the composition of concrete that is cost-effective in terms of environmental and resistance.