Overview
This project aimed at evaluating the feasibility of reintroducing recycled aggregates (RA) sourced from concrete into the precast industry. A technical specification complementing the LNEC E471 specification was proposed after evaluating the properties of concretes produced with the incorporation of fine (FRA) and coarse (CRA) recycled aggregates.
The precast industry is an area of the construction sector in clear worldwide expansion; not only the construction times are shorter, but also a higher quality level is ensured Since precast production is a highly controlled environment, in its quality control processes substantial quantities of construction waste are generated. These wasted elements, made in high quality concrete, have a tremendous potential for incorporation on new concrete as recycled aggregates.
To ensure that the RA produced meet the demands of structural concrete, this project intended to optimize the crushing process used to obtain the RA. As such, the performance (herein always related to durability and mechanical behaviour) of concrete produced with two crushing processes was compared: a traditional process, with single primary crushing, and a two-step primary/secondary crushing process, analogous to the process currently used to obtain natural primary stone aggregates.
Using the two-step crushing process, the ability to produce concretes with a pre-determined mechanical strength capacity and different FRA and CRA incorporation ratios was also evaluated. The concretes that were crushed to produce the FRA and CRA of this study (source concrete - SC) had the mechanical properties desired for the recycled concrete. This auto-reproduction capacity was studied for three ranges of strength, from low mechanical performance concrete (20 - 30 MPa) to high performance concrete (60 - 70 MPa).
From these results, one of the tested SC was chosen and a complementary experimental campaign was conducted in order to determine the potential of its respective FRA and CRA in the production of concretes with different strength characteristics, with particular focus on high performance concrete (HPC).
Precast concrete elements are frequently produced with self-compacting concrete (SCC), relinquishing the need of fallible and costly mechanical vibration systems. The performance of SCC produced with different incorporation ratios of FRA and CRA was also evaluated.
A considerable share of structural precast elements is made with HPC allowing the reduction of weight and dimensions of the structural elements, easing transport and handling. The project included a research aimed at the execution and characterization of HPC (not exclusively with RA incorporation) with a compressive strength over 90 MPa. In this phase, the maximum ratio of FRA and CRA that could be incorporated on HPC of this strength level without increasing the environmental impact (due to the incorporation of admixtures, special cements or higher binder ratios) of the HPC production, when compared to conventional concrete production, was studied.
Finally, in order to allow the use of RA by the precast industry without theoretical restrictions related to possible rheological differences between RAC and conventional concrete, various recommendations regarding RA incorporation on concrete were established, complementing the scarce standards and specifications presently in practice.
The main objective of this project was to provide to the precast industry in particular, and to the structural concrete industry in general, an internationally innovative mean to use and value the waste produced, minimizing the use of natural resources and significantly reducing the environmental impact of the concrete industry.
The precast industry is an area of the construction sector in clear worldwide expansion; not only the construction times are shorter, but also a higher quality level is ensured Since precast production is a highly controlled environment, in its quality control processes substantial quantities of construction waste are generated. These wasted elements, made in high quality concrete, have a tremendous potential for incorporation on new concrete as recycled aggregates.
To ensure that the RA produced meet the demands of structural concrete, this project intended to optimize the crushing process used to obtain the RA. As such, the performance (herein always related to durability and mechanical behaviour) of concrete produced with two crushing processes was compared: a traditional process, with single primary crushing, and a two-step primary/secondary crushing process, analogous to the process currently used to obtain natural primary stone aggregates.
Using the two-step crushing process, the ability to produce concretes with a pre-determined mechanical strength capacity and different FRA and CRA incorporation ratios was also evaluated. The concretes that were crushed to produce the FRA and CRA of this study (source concrete - SC) had the mechanical properties desired for the recycled concrete. This auto-reproduction capacity was studied for three ranges of strength, from low mechanical performance concrete (20 - 30 MPa) to high performance concrete (60 - 70 MPa).
From these results, one of the tested SC was chosen and a complementary experimental campaign was conducted in order to determine the potential of its respective FRA and CRA in the production of concretes with different strength characteristics, with particular focus on high performance concrete (HPC).
Precast concrete elements are frequently produced with self-compacting concrete (SCC), relinquishing the need of fallible and costly mechanical vibration systems. The performance of SCC produced with different incorporation ratios of FRA and CRA was also evaluated.
A considerable share of structural precast elements is made with HPC allowing the reduction of weight and dimensions of the structural elements, easing transport and handling. The project included a research aimed at the execution and characterization of HPC (not exclusively with RA incorporation) with a compressive strength over 90 MPa. In this phase, the maximum ratio of FRA and CRA that could be incorporated on HPC of this strength level without increasing the environmental impact (due to the incorporation of admixtures, special cements or higher binder ratios) of the HPC production, when compared to conventional concrete production, was studied.
Finally, in order to allow the use of RA by the precast industry without theoretical restrictions related to possible rheological differences between RAC and conventional concrete, various recommendations regarding RA incorporation on concrete were established, complementing the scarce standards and specifications presently in practice.
The main objective of this project was to provide to the precast industry in particular, and to the structural concrete industry in general, an internationally innovative mean to use and value the waste produced, minimizing the use of natural resources and significantly reducing the environmental impact of the concrete industry.