The effect of recycled fine aggregates treated as washed, less washed and unwashed on the mechanical and durability characteristics of concrete under MgSO4 and freeze-thaw cycles

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The objective of this work was to study the impact of recycled fine aggregates (RFA) treated as washed (W-RFA), less washed (L-RFA) and unwashed (U-RFA) on the transport, mechanical and durability characteristics of concrete under freeze-thaw (F-T) cycles and MgSO4 attack. Totally three groups of concrete mixtures were produced by replacing natural fine aggregates (NFA) with W-RFA, L-RFA and U-RFA at the percentages of 10, 20, 40 and 80% by weight of NFA. 12 mixtures incorporating RFA and reference mixture containing only NFA were fabricated. Compressive strength and splitting tensile strength were determined after curing age of 7 and 28 days. Nondestructive test of Schmidt rebound hammer after 28 days was also performed to assess the mechanical performance of the concretes. Physical and transport properties including dry density, porosity, water absorption and sorptivity of 28-day concrete specimens were explored too. Evaluation of the durability characteristics was determined under different F-T cycles and sulfate attack. Assessment of abrasion resistance of the concretes was done by abrasion test. Microstructural analysis, mass loss, compressive strength, splitting tensile strength and relative dynamic elasticity modulus of the concretes were measured after conducting durability tests. The results indicated that 80%W-RFA incorporated concrete showed the compressive strength improvement considerably and achieved compressive strength of 51 MPa. 10% L-RFA and 10% U-RFA incorporated concretes exhibited compressive strength of above 52 MPa after 28 days curing. 40% W-RFA incorporated concrete with presented the best abrasion resistance after 7 and 28 days. 20%W-RFA incorporated concrete exhibited the best resistance under after 150 F-T cycles considering compressive strength.