Strength evaluation of Recycled Aggregate Concrete

by Thanh-Tung Nguyen, Leutrim Zeka, Danièle Waldmann - University of Luxembourg

The University of Luxembourg is actively working to design a new mixture for precast elements which involve a total/partial substitution of secondary raw materials. The recycled material is supplied by Contern and is sourced from the crushing of unused drainage pipes. 

In the first step, the classification of aggregates to different grades based on particle size was performed. For instance: (i) G1 for the aggregates with a sieve size from 1 to 8 mm, and (ii) G2 for the aggregates with a sieve size from 4 to 16 mm (included the remaining aggregates in a sieve size of 16 mm­­­). Then, the physical and mechanical properties of the recycled aggregates were characterized through experimental testing on the defined grades. The obtained results for the bulk density, aggregate crushing value, and abrasion value, showed comparable properties with respect to the virgin aggregate.

Figures 1, 2 and 3: Characterization of the physical/mechanical properties of the recycled aggregates

In the second step, new recycled aggregate concrete mixes were designed based on the classified grades for three different required aims: (i) G1 - high-strength concrete, (ii) G1 - self-compacting concrete, and (iii) G2 - concrete for open structure. The mechanical properties, such as elastic modulus, compressive strength, flexural strength, and tensile splitting strength were experimentally determined. The preliminary results were very promising. The designed recycled concretes exhibited similar mechanical performance compared to classical concrete (using the same formulation). 

Figures 4, 5 and 6: Evaluation of the mechanical properties of the designed concretes

The third step was dedicated to getting an in-depth understanding of the microstructure impacts that control the mechanical behaviors the most. In particular, the effects of the interfacial transition zone were investigated in detail. The numerical approach was developed to capture these effects. Moreover, the predictive simulations provided useful information which can be used in the designed procedures allowing us to improve the mechanical performance of the recycled concrete.

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