Simultaneous utilization of fine and coarse recycled concrete aggregate for the fabrication and experimental analysis of the structural performance of precast reinforced and prestressed concrete elements

Abstract

his doctoral thesis aims at taking a step forward in concrete recycling, since it is proposed to replace simultaneously the fine and coarse fraction of the aggregate without having to previously sieve the raw recycled aggregate. The structural behaviour of both precast reinforced and prestressed concrete elements was assessed, compared to conventional concretes with similar compressive strength. A first experimental campaign was performed with self-compacting recycled concrete. Different percentages of the total amount of natural aggregates (0, 20, 35 and 50%, including fine and coarse fraction) were replaced with recycled aggregates from rejected members of a precast plant. Fresh and hardened concrete properties were compared in laboratory for the different replacement levels. The mechanical properties were not significantly affected. So, the resulting recycled concrete was utilised for the fabrication of reinforced concrete beams in a precast plant with replacement levels of 0, 10, 20, 35 and 50%. Reinforced beams were subjected to quality control shear and flexural failure tests in the precast plant showing a similar ultimate bending moment and shear strength regardless of the replacement ratio. However, the ductility factor of flexural beams and cracking moment decreased for higher replacement levels. Flexural tests were repeated with displacement control and accurate monitoring in the CITEEC at the University of A Coruña, including strain gauges on concrete surface to obtain the flexural behaviour up to failure, the ductility factors, and moment vs. curvature relationships. The results obtained showed a lower cracking moment and ductility for the higher replacement levels but similar ultimate moments. A second experimental campaign was performed with vibrated concrete and prestressed precast concrete beams. In this case, the source of the recycled aggregate was old concrete sleepers. Different percentages of the total amount of natural aggregates (0, 8, 20 and 31%, including fine and coarse fraction) were replaced by recycled aggregates and fresh and hardened properties were compared in laboratory for the different replacement levels. The mechanical properties did not experience considerable decrease depending on the replacement percentage. So, the resulting recycled concrete was utilised for the fabrication of prestressed concrete beams, as well as Pull-Out test samples with replacement levels of 0 and 8%. The aim was to evaluate the loss of bond performance between prestressed strands and surrounding concrete when recycled aggregates are introduced in the mix. Pull-Out tests showed that the bond stress at which the first slip occurs is approximately 24% lower when 8% of the total aggregate is replaced with recycled aggregate. Prestressed beams were monitored to evaluate transfer and development length. Transfer length was evaluated measuring the strain values with DEMEC points at different ages. It was determined to be 717 mm in natural concrete beams and 957 mm in recycled beams. That is, there is an increase of 33% in transfer length when 8% recycled aggregate is incorporated in the concrete mix. Development length was evaluated performing flexural tests at different embedment lengths and measuring the strand end slips during the tests. It was determined to be 1475 mm in natural concrete beams and 1850 mm in recycled concrete beams. That is, there is an increase of 25% in development length when 8% recycled aggregate is incorporated in the concrete mix. From this research it may be concluded that it is feasible to replace up to 50% of the total aggregate (fine and coarse) in reinforced concrete beams. However, the incorporation of recycled aggregate in prestressed concrete significantly reduces the bond between prestressed strands and surrounding concrete in beams fabricated with 8% replacement of the total aggregate. Consequently, further research is recommended in this subject.