The incorporation of recycled brick aggregate in 3D printed concrete

In this paper a more sustainable 3D printable concrete is developed by the replacement of natural aggregate in a concrete mix with recycled brick aggregate (RBA). An existing 3D printable concrete mix is used as reference and adjusted by replacing 64% of its natural aggregate with RBA. The potential for sustainability in the building sector by automation as well as recycling of construction and demolition waste is therefore addressed. To date no research exists on the use of recycled brick aggregate in 3D printed concrete, which has particular relevance in developing countries, where construction with bricks remains a popular option. Two different types of RBA from local demolition projects are considered in three mix designs containing Type 1 RBA, Type 2 RBA and a combination of Type 1 and Type 2 RBA. To arrive at the three mix designs, the water absorption and porosity of each RBA type are first tested to determine the estimated additional water requirement in each concrete mix. The ASTM mini-slump cone test is used as a replacement of a full rheological characterisation to determine the achievement of acceptable printability. After the fresh-state characterisation, the mix design containing the more porous Type 2 RBA is chosen for mechanical characterisation based on availability of materials and relevance of highly porous recycled brick aggregate in future studies. Mechanical characterisation tests are performed on the chosen RBA mix, as well as the reference 3D printable mix for comparison. The mechanical characterisation tests include compressive cube strength tests, direct tensile tests on 3D printed specimens, and uniaxial compressive tests on 3D printed specimens. The results of the mechanical characterisation tests show that the replacement of 64% of the natural aggregate with RBA in the mix reduces the compressive cube strength by 25%, the interlayer tensile 3DPC strength by 20% and the compressive 3DPC strength by 14% and 20% in Direction 1 and Direction 3 respectively.