Rammed Earth incorporating Recycled Concrete Aggregate: a sustainable, resistant and breathable construction solution

Construction and demolition debris, mainly concrete and masonry rubble, represent a significant share of municipal waste. Recycling crushed concrete aggregates and using them as substitutes for natural ones might therefore be determinant in reducing landfilling and mineral resource depletion. An innovative way to give new value to Recycled Concrete Aggregates (RCAs) is to ram them in layers to form load-bearing walls for stabilised Rammed Earth (RE) applications. However, the success of those few existing RE projects using RCA is mainly due to the knowledge and experience of the contractors rather than official standards or guidelines or scientific literature. The objective of this study was to further the knowledge of this building technique by determining the effect of different RCA replacements on the material's mechanical resistance, sustainability and hygroscopic properties: indicative of the structure's structural, environmental and hygrothermal performance. Mechanical resistance was assessed by means of the Unconfined Compressive Strength (UCS, commonly used for rammed earth-like materials), hygroscopic properties via Moisture Buffer Value (MBV) and sorption isotherms while the sustainability was assessed via consequential Life Cycle Assessment (LCA). Microstructural investigations via mercury intrusion porosimetry, nitrogen adsorption-desorption isotherms, scanning electron microscopy and X-ray diffraction were performed to understand and explain material mechanical and hygroscopic behaviour. The building technique, already proven to be durable, was demonstrated to be resistant (from 4 to 12 MPa at 28 days depending on the RCA replacement and cement content), sustainable (down to 25 kg CO2-eq. of embodied carbon per square meter of load-bearing wall) and to have good moisture buffering abilities (0.88 g/(m2%RH) for mixtures containing only RCA). Strength appeared to be more related to the particle size distribution of the mix rather than to the percentage of RCA added. The amount and type of stabiliser added to the mix and the distance covered by the RCA during its lifetime strongly affected the environmental sustainability of the mixture; to maximise the potential of this building technique, reducing the amount of cement in the mixture by using alternative stabilisers should be the main priority.