Evaluation to enhance the strength and durability of recycled aggregate concrete by using GGBS (Glass Granulated Blast Furnace Slag) and Hybrid Fiber.

Abstract

Recycled aggregate concrete (RAC) presents a structurally sound pathway to circular-economy construction, yet its inherent weaknesses
elevated porosity, compromised interfacial transition zones (ITZ), and diminished mechanical capacity relative to conventional concrete have
constrained its deployment in load-bearing structural members. This study presents a systematic factorial experimental investigation into the
synergistic remediation of these deficiencies through two concurrent strategies: partial replacement of ordinary Portland cement with ground
granulated blast furnace slag (GGBS) at 25%, 30%, and 35% by binder mass, combined with hybrid steel–polypropylene fibre reinforcement
(0.5% steel + 0.15% polypropylene by volume). Four concrete designations were evaluated a conventional concrete control (CC) and three
hybrid RAC mixes (M1: 20% RCA, 25% GGBS; M2: 30% RCA, 30% GGBS; M3: 40% RCA, 35% GGBS) on M25-grade specimens across
compressive strength (150 mm cubes), split tensile strength (150×300 mm cylinders), and flexural strength (150×150×700 mm beams) at 7
and 28 days, alongside durability assessment through water absorption, rapid chloride penetration testing (RCPT), and sulphate attack
resistance. M1 representing the lowest RCA content with balanced GGBS substitution achieved peak 28-day compressive strength of 31.9
MPa, surpassing the conventional concrete control (30.3 MPa) by 5.3% and demonstrating that the pozzolanic densification from GGBS more
than compensates for the ITZ degradation introduced by 20% RCA. The RCPT charge passage for M1 (1658 coulombs) classified it as low
permeability, versus the moderate classification of the control (2217 coulombs), confirming a 25.2% reduction in chloride ingress susceptibility.
M3 exhibited a 5.3% compressive strength reduction relative to the control at 28 days, attributable to the cumulative effect of 40% RCAinduced ITZ weakening outpacing the matrix densification from 35% GGBS under ambient curing. Hybrid fibre reinforcement providing both
microcrack arrest (polypropylene) and macrocrack bridging (steel) elevated the flexural strength of M1 by 8.4% above the control (6.31 vs.
5.82 MPa) and reduced sulphate-induced weight loss to 0.71% against an industry threshold of 1.0%. A mechanistic framework coupling GGBS
secondary hydration kinetics, fibre pull-out bond mechanics, and ITZ fracture energy is developed to interpret the observed performance
landscape. Life-cycle analysis indicates that M1–M3 reduce embodied CO₂ by 29–38% relative to the control while diverting demolition waste
from landfill. These findings establish quantitative mix-design envelopes for hybrid fibre GGBS-RAC and support its adoption in moderate
structural applications.