Avaliação da substituição parcial de agregado miúdo por resíduos cinza de classe a em concretos de ultra-alto desempenho reforçado com fibras metálicas

Abstract

The high generation of construction and demolition waste (CDW) and the increasing pressure on natural aggregate extraction represent a significant environmental challenge associated with concrete production. At the same time, ultra-high-performance fiber-reinforced concrete (UHPFRC), although presenting superior mechanical properties and high durability, requires large amounts of fine materials and natural aggregates, which intensifies its environmental impact. In this context, the incorporation of CDW as a partial replacement for natural fine aggregate emerges as a promising alternative to reconcile technical performance and sustainability. This study evaluated the technical feasibility of using gray Class A CDW as a partial replacement for natural sand in UHPFRC cementitious matrices. Four experimental series were produced: a reference mixture (REF) and mixtures with 10%, 20%, and 30% replacement of sand by CDW. The constituent materials were characterized through physical, granulometric, and microstructural tests, including scanning electron microscopy (SEM) and X-ray diffraction (XRD). In the fresh state, consistency index, apparent density, entrapped air content, and isothermal calorimetry were evaluated. In the hardened state, density, compressive strength, flexural tensile strength, limit of proportionality (LOP), toughness, and stress–strain behavior were analyzed at 7, 28, and 90 days of curing. The results indicated that the incorporation of CDW did not compromise the mechanical performance of UHPFRC. At moderate replacement levels (20%), an increase in the LOP and an improvement in the pre-cracking behavior were observed. At higher replacement levels (30%), an increase in toughness and greater load-carrying capacity in the post-peak regime were observed. At 90 days, although a slight reduction in maximum flexural strength was verified at higher replacement levels, the mixtures containing CDW maintained satisfactory mechanical performance and a high energy dissipation capacity. Microstructural analyses indicated that the initial effects associated with increased heterogeneity and the dilution effect caused by CDW were mitigated with the advancement of hydration reactions, due to the late hydration of the residual adhered paste, microstructural refinement, and the evolution of the interfacial transition zone. Therefore, it can be concluded that the partial replacement of natural fine aggregate by Class A CDW in UHPFRC matrices is technically feasible, allowing the maintenance or even improvement of properties related to post-cracking behavior, while contributing to the reduction of natural resource extraction and to the proper management of construction waste, promoting more sustainable practices in the construction industry.