Development of concrete composites using blended cements for repair of heat damaged concrete

Abstract

Fire incidents pose a significant threat to life and property, with residential and commercial structures being particularly vulnerable. Recent reports highlight an alarming rise in fire-related fatalities and structural failures, emphasizing the urgent need for effective, high-performance, fire-resistant repair materials. Concrete, a widely used construction material, degrades severely under fire due to thermal expansion, spalling, and reduced load-bearing capacity. While concrete outperforms steel in fire resistance, its mechanical properties deteriorate rapidly beyond 400°C exposures, necessitating advanced repair solutions.

This study aims to bridge this gap by developing a sustainable, thermally resilient concrete repair material. The research optimizes cementitious blends of Ordinary Portland Cement (OPC), Limestone Calcined Clay Cement (LC3), Calcium Aluminate Cement (CAC), and hybrid fibers (Polypropylene and Steel Fibers). Through an extensive experimental program, the study evaluates these composites' thermal, mechanical, durability, and bond behavior under varying heating-cooling regimes. Key objectives include. (1) Analyzing workability, color change, crack propagation, and mass loss; (2) Assessing residual compressive strength using destructive and non-destructive testing methods (3) Examining durability properties, including chloride penetrability, sorptivity, thermal conductivity, and microstructural changes via X-ray diffraction (XRD) and thermogravimetric analysis (TGA); (4) Evaluating the bond strength between the newly developed mix and heat-affected substrate concrete.

The research adopts a two-phase experimental approach. In Phase 1, forty-five concrete mixes (1350 samples) were subjected to elevated temperatures (200°C, 400°C, 600°C & 800°C) under air cooling and water quenching. Their performance was assessed using non-destructive (rebound hammer, ultrasonic pulse velocity) and destructive (compressive strength) tests, supplemented by microstructural analysis (XRD and TGA) to identify phase transformations and degradation mechanisms. Phase 2 (85 samples per mix) focused on the top-performing mix (selected from Phase 1), evaluating its durability (chloride penetrability, sorptivity, void spaces, and thermal conductivity) and bond strength with fire-damaged substrate concrete.

The results indicate that replacing 20% of OPC with LC3 or CAC individually, or 15% in combination, optimally enhances residual strength. Hybrid fibre-reinforced LC3 mixes (Mix-3) demonstrated superior performance, retaining 8.42% higher compressive strength at 200°C and 44.56% higher splitting tensile strength at ambient temperatures than conventional OPC. Ambient cooling proved less detrimental than water quenching, while non-destructive tests (NDT) correlated well with mechanical outcomes, albeit with a 20–30% underestimation. At higher temperatures (600°C–800°C), the LC3 mix with hybrid fibers performed exceptionally well. Microstructural analysis confirmed that LC3 densifies concrete, reduces permeability, and improves chloride resistance up to 400°C. The proposed repair mix also exhibited low thermal conductivity and strong substrate bonding, making it a viable solution for rehabilitating fire-damaged structures.

In conclusion, this study presents a sustainable, high-performance composite that addresses fire-induced concrete degradation, offering enhanced durability, thermal resistance, and mechanical strength restoration. The findings advocate for adopting limestone calcined clay cement-fibre blends in fire-prone constructions. Future refinements through broader statistical modelling and field validations are recommended to optimize performance further. The practical applications of this research are extensive, providing valuable insights for producing fire-resistant concrete in construction sites, precast members, concrete blocks, power plants, and other critical infrastructure.