Recycling of construction and demolition waste as natural coarse aggregates in concrete

(Julia García-González and Desirée Rodríguez Robles, joint PhD with University of Leon)

The role of industry in the growth of modern society and economy is indisputable, as is its contribution to the degradation of the environment. The activities comprising the construction sector usually result in environmental impacts of considerable magnitude, with natural resource consumption and waste generation being two of the major negative effects due to the great volumes used/generated. Thus, the idea of closing the production cycles is particularity useful to alleviate the pressure on the carrying capacity of the environment. Therefore, the use of recycled aggregates from construction and demolition wastes (CDW) is investigated, as partial replacement of the conventional coarse aggregates in the concrete manufacture. While the vast majority of previous studies have been focused on the use of aggregates recovered from concrete rubble, the current situation in Mediterranean countries, such as Spain, is that up to 70% of the processed CDW contains varying percentages of ceramic wastes (mixed and ceramic recycled aggregates). This conveys the significance of studies researching the suitability and performance levels of these residues in high-level recycled applications, i.e. structural concrete.

Recycled aggregates from CDW showing varying amounts of ceramic materials.
Recycled aggregates from CDW showing varying amounts of ceramic materials.

 

 The feasibility of using recycled aggregate as replacement of natural aggregate has been shown by evaluation of different physical and chemical properties:

Characterization of recycled aggregates.
Characterization of recycled aggregates.

However, the greater affinity of recycled aggregate for water affects the amount of fluid available for mixing and hinders the hydration of cement. The short-term result is dryer consistency and hence less workable materials. Thus, when required by the application, this problem can be solved by modifying some parameters of the mix design. Therefore, this research developed three techniques to reduce the effect of high recycled aggregate water absorption on concrete characteristics:

  • Pre-saturation of recycled aggregates, to lower aggregate water capture and obtain sufficient workability, by moistening the aggregate prior to batching (soaking time: 3 minutes).
  • Use of superplasticizer, thereby improving the properties of the end product without modifying the water/cement ratio.
  • Biodeposition by calcium carbonate-producing bacteria (Bacillus sphaericus), a biological solution for the high water absorption. The CaCO3 deposited forms a surface layer that renders recycled aggregate less permeable.

All recycled concrete mixtures were evaluated both in fresh and hardened state, focusing on the effect that the use of each recycled aggregate type or correction technique has on the microstructural, mechanical and durability properties of the resulting recycled concrete.

Characterization of properties of fresh recycled concrete.
Characterization of properties of fresh recycled concrete.

 

Characterization of properties of hardened recycled concrete.
Characterization of properties of hardened recycled concrete.