ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2023 26 such as a compressive strength above 30 MPa and a modulus of elasticity above 30,000 MPa can be achieved using building demolition concretes with appropriate mix design, without unnecessarily increasing the cement content and by relatively high substitution of natural aggregates up to 50% by weight with recycled concrete aggregates[14]. On the contrary, to achieve the main durability requirements, such as resistance to freeze and thaw, resistance to carbonation and chloride penetration to limit the steel rebar corrosion, and low porosity and water permeability, it is necessary to use high-quality recycled concrete aggregates. Often these derive from the demolition of infrastructures such as bridges[15], tunnel elements, and highway supporting walls. Recycled material homogeneity and selective demolition of structures are key issues for enhancing the recycling process. Concretes are also produced with mixed granulates consisting of rocks and excavation material, demolished concrete, and ceramic waste coming from tiles and bricks (Fig. 2c). Nevertheless, the wide difference in material properties makes this type of concrete questionable for some applications[16]. These types of cementitious blends are used for low grade applications or as filling material along pipelines. The coarse ceramic component generally exhibits a high porosity. This component significantly reduces mechanical performance and durability. Therefore, it is recommended to use the fine-grained ceramic component within the cementitious systems. However, the use of recycled concretes with mixed granulates is no longer suggested[17]. In this situation, a complete separation of the component, such as the excavation material and the ceramic components, is necessary. DURABILITY The long-term field behavior of structures is a concern. The large amount of infrastructure present worldwide and the need to repair, restore, and replace these structures, significantly increase the maintenance costs of public and private institutions. Fresh and hardened state concrete parameters are tested within laboratories and several limits are set for the compressive strength, water/cement ratio, and cement content. Limits are also set for some durability tests, such as the resistance to freeze and thaw in the presence of deicing salts, to chloride penetration, and to carbonation. The durability requirements imposed tend to increase the early strength and the modulus of elasticity, i.e., the stiffness of concrete, and to close the porosity of the hardened blends. Consequently, premature cracking is often observed in the field. This fact adversely affects the durability of reinforced concrete infrastructures (Fig. 3). Durability is a main component of sustainability. Thus, the relationship between lab durability tests and real-field behavior is not always adequate. The separation of the durability tests in single detrimental agents and mechanisms, such as chloride penetration, carbonation, and freeze and thaw simplifies the interpretation of the results, but does not correspond to real field attack. In fact, several parts of a reinforced concrete infrastructure and its deterioration are largely asymmetrical and subjected to combined environmental detrimental actions[18-20]. Therefore, it is necessary to partially review the testing mode and simulate in laboratory the combined degradation mechanisms occurring on site. This will make interpretation of the single action degradation contribution more difficult, but will improve the laboratory- field relationship. In addition, the concrete required parameters, such as water/cement ratio and the cement content may be substituted by measurable parameters, such as compressive strength classes, modulus of elasticity ranges, new durability limits under combined detrimental actions, and type of cement used. This will set a new basis toward performance-based durability tests and enable a broader use of supplementary cementitious and waste materials with hydraulic or filler properties and slowly reduce the clinker of cements. A further step forward in the world’s sustainability. CONCLUSIONS Concrete is a versatile material with strong potential as a sustainable material. The increased use of mineral additions to lower the clinker component is a feasible option. Adequate recycled material quality monitoring and mix designs enable a wide use of concrete recycled aggregates for the built environment. In addition, new performance-based durability tests that better simulate the real field exposure with the combined environmental actions will promote more resilient, durable, and sustainable reinforced concrete infrastructures. ~AM&P Fig. 3 – (a) Reinforced concrete bridge across a highway exposed for more than 40 years, (b) cracks along the support shoulder region. (a) (b)
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