Mechanical Properties and Microstructural Characterization of Concrete with Recycled Aggregates from 20-Year Marine-Exposed Structures.

This study evaluated the mechanical properties and microstructural characteristics of concrete made with recycled concrete aggregates (RCAs) derived from a pier exposed to a tropical marine environment for more than 20 years. The RCA was characterized physically, chemically, and mechanically to assess porosity, inherited chemical contamination, and potential degradation mechanisms associated with prolonged marine exposure. New concretes were produced with 50% replacement of natural coarse aggregates by RCA and water-cement (W/C) ratios of 0.67, 0.61, and 0.47. Compressive strength was measured at 3, 7, and 28 days. The mixture with 50% RCA and W/C = 0.47 achieved a 28-day compressive strength of ≈22.8 MPa, exceeding the design strength of 21 MPa. The source concrete and derived RCA exhibited a thin-section air-void content of 3.3%, acid-soluble chloride content of 0.074% by mass of concrete, and SO3 content of 1.15%. Petrographic examination identified no evidence of harmful alkali-silica reactivity in the examined material. Statistical analysis of the experimental results indicated increasing compressive strength with decreasing W/C ratio and increasing curing age. These findings demonstrate that, under the investigated mixture proportions and curing conditions, 50% RCA replacement can achieve the specified compressive strength. They also indicate an association between the source concrete's inherited microstructural characteristics and the strength development of the recycled aggregate mixes.