Durability of recycled aggregate concrete using pozzolanic materials

Concrete made with recycled aggregates often shows reduced strength and durability compared to traditional mixes using natural aggregates. This limitation has raised concerns about the long-term performance of structures incorporating recycled materials. Prior research has shown that natural aggregates provide better mechanical properties and resistance to environmental factors like chloride ingress. However, the environmental benefits of using recycled aggregates remain a strong motivation for their adoption. No prior work had resolved how to maintain durability while using recycled materials. This gap motivated the investigation into supplementary cementitious materials like PFA and GGBS. Their potential to enhance concrete properties had been suggested in earlier studies. Yet, the specific effects on chloride resistance and corrosion behavior remained unclear. This paper addresses the need for a clearer understanding of how these materials influence concrete durability when recycled aggregates are used.

The study aimed to assess whether adding pozzolanic materials could restore the strength and durability of concrete made with recycled aggregates. The specific problem is the reduced performance of concrete containing recycled materials compared to conventional mixes. The motivation comes from the need to sustainably use construction waste without compromising structural integrity. The researchers focused on compressive and tensile strength, as well as chloride ion penetration resistance. They wanted to determine if these materials could delay corrosion initiation in steel-reinforced concrete. The study also sought to quantify the effectiveness of PFA and GGBS in mitigating chloride-induced corrosion. By comparing results with control specimens, the goal was to evaluate the practical viability of these materials. The findings could guide the use of recycled aggregates in structures exposed to chloride environments.

The study involved replacing portions of cement with PFA and GGBS in concrete mixtures containing recycled aggregates. The replacement levels were set at 30% for PFA and 65% for GGBS. Compressive and tensile strength were measured at 28 days using standard testing procedures. Chloride ion penetration was assessed using the rapid chloride penetration test (ASTM C 1202-91). The test evaluates how easily chloride ions move through the concrete matrix. Corrosion behavior was analyzed by measuring the corrosion rate after exposure to chloride solutions. The study compared results from the modified mixes to those of control specimens made with natural granite gravel. Data collection focused on mechanical properties and chloride resistance metrics. The analysis aimed to determine whether the pozzolanic materials could restore or enhance concrete durability.

Concrete with 30% PFA and 65% GGBS achieved compressive strength levels comparable to control specimens made with natural aggregates. However, tensile strength remained lower in the modified mixes at 28 days. The chloride ion penetration test showed improved resistance in the PFA and GGBS concretes. The rapid chloride penetration test indicated a lower chloride penetration index compared to controls. Corrosion rate measurements revealed that PFA and GGBS concretes maintained lower corrosion rates after initiation. This suggests reduced access to oxygen and water, which are necessary for corrosion processes. The chloride threshold level for steel corrosion was not significantly increased by the pozzolanic materials. As a result, the expected corrosion-free life of the modified concrete matched that of control specimens.

The study found that adding PFA and GGBS can restore compressive strength in concrete with recycled aggregates. Tensile strength remained lower, but chloride resistance improved based on penetration tests. The researchers propose that the reduced corrosion rate is due to restricted oxygen and water access. However, the chloride threshold level for steel corrosion was not significantly raised. The authors suggest that the corrosion-free life of the modified concrete equates to that of control specimens. The findings indicate that PFA and GGBS can help maintain durability in recycled aggregate concrete. The study highlights the importance of balancing mechanical and durability properties. The results support the use of pozzolanic materials to enhance the performance of sustainable concrete mixes.