Recycling of concrete in new structural concrete
Abstract: Concrete waste as crushed concrete aggregates (CCA) in structural concrete gives a new purpose and prolongs the technical life of the reference concrete accomplishing closed loop recycling. This research investigates CCA as aggregate replacement in an industrial reference concrete recipe as fine aggregate fractions and overall aggregate replacement. Experimental study of CCA concrete is conducted by testing compressive strength and workability. Results show that CCA concrete has inferior compressive strength and workability than reference concrete due to the adhered mortar and flakiness index of CCA, properties which differentiate CCA from reference concrete aggregates. These properties influence the aggregate packing density and water absorption properties of CCA, crucial to concrete workability and compressive strength. To overcome the challenges with determining water absorption of fine CCA, the standard pycnometer method is modified in an innovative way to test a combined fine and coarse aggregate fraction. The water absorption is measured at 15 minutes to estimate the water absorbed by CCA during the concrete mixing. Workability corresponding to reference is achieved when CCA fractions are momentarily pre-soaked with water corresponding to 50% of the 15-minute water absorption value just before concrete mixing. To improve concrete properties, this research investigates two modifications: enhancing aggregate quality by adhered mortar removal and enhancing cement paste quality by adding secondary cementitious materials (SCM). Firstly, CCA is modified by a fraction-wise mechanical pre-processing in a horizontal rotating drum for 15 minutes to abrade adhered mortar which is then removed by washing. The abrasive nature of pre-processing results in the loss of aggregate material along with the adhered mortar accounted for by a mass-balance; resolved by adjustments in CCA particle grading. The loss of adhered mortar leads to reduction of CCA water absorption, influencing pre-soaking water content. Pre-processing also influences properties such as flakiness index, void-content, unit-weight and density, jointly seen as an increase in CCA packing density. After pre-processing, mixes with CCA as fine aggregates (CCA50) show mean compressive strength exceeding reference concrete. Mixes with overall CCA replacement (CCA100) show same compressive strength as reference concrete. The flow diameters of both mixes correspond to the same flow class F2 as reference concrete. Secondly, modifications of cement paste are investigated by replacing 30% of the reference cement, CEM II/A-LL with granulated blast furnace slag (GGBS). Mixes investigated are CCA with/without mechanical pre-processing at both 50% and 100% replacements. Among the GGBS mixes, CCA100 achieves reference concrete compressive strength while CCA50 reaches the reference concrete strength only when combined with mechanically pre-processing. Addressing early-age strength, an improved mixing method with pre-soaked GGBS is investigated on CCA100 mix. The resulting mean compressive strength at seven days fulfils 93% of the corresponding reference concrete strength. Addition of GGBS causes the concrete workability to resemble a mix with increased mixing water content. Therefore, CCA flow diameter values of reference concrete flow class are achieved at a lower water/binder ratio. The results are investigated with regard to statistical significance and sustainability. For concrete CCA100, GGBS addition results in statistically significant improvements of the compressive strength and a nearly 30% reduction of carbon dioxide-related emissions implying a green concrete. For CCA50 statistically significant improvements in compressive strength are realized for the combination of mechanical pre-processing and GGBS addition.
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