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<p>This study investigates the thermomechanical behavior of eight distinct granitic rock types from northern Pakistan under elevated temperatures ranging from 25&#xa0;°C to 1000&#xa0;°C. Through a series of standardized laboratory tests, key physical and mechanical properties—including water absorption, specific gravity, porosity, unconfined compressive strength (UCS), tensile strength (UTS), point load index (PLI), Schmidt hammer rebound value (SHT), and ultrasonic pulse velocity (UPV)—were systematically evaluated across seven thermal intervals to understand the full thermal evolution of the rocks. The results reveal a consistent trend of thermal degradation, with a critical transition occurring at 400&#xa0;°C. Below this threshold, thermal effects were minimal (&lt; 5%), but between 200&#xa0;°C and 400&#xa0;°C, granodiorites such as Kesu (KeG) and Warai (WG) exhibited thermal hardening, with UCS increasing by 12.5% and 38.7%, respectively. Above 400&#xa0;°C, all samples showed significant deterioration: UCS decreased by over 90% in Ambela granite (AG) (from 146 to 15&#xa0;MPa), and porosity in altered Malakand granite (MG) reached 10.42%, compared to 2.56% in fresh Kesu granodiorite. UPV dropped 15–17% by 400&#xa0;°C and up to 60% at 1000&#xa0;°C, correlating with the quartz α–β phase transition and widespread microcrack propagation. Microstructural analysis indicated substantial mineral instability with increasing temperature, marked by thermally induced fractures and widening of pre-existing cracks, particularly in feldspar and quartz. Altered rocks were more prone to texture modification at lower temperatures, highlighting their increased thermal vulnerability. Quantitative thermal damage indices were developed for five parameters (UCS, UTS, PLI, SHT, UPV), with granodiorites showing negative damage values up to 400&#xa0;°C (e.g., UCS DT = − 0.13 to − 0.39), reflecting strength enhancement before subsequent degradation. Predictive thermal response models demonstrated strong correlation (R<sup>2</sup> = 0.72–0.99), providing practical tools for engineering design. This study delivers novel insights into the behavior-specific thermal thresholds of granitic rocks and offers valuable parameters for geotechnical applications such as geothermal energy systems, nuclear waste disposal, and deep underground excavations. It represents the first in-depth, quantitative evaluation of thermally induced degradation in granitic rocks from northern Pakistan, contributing significant benchmarks for global rock engineering practices in high-temperature environments.</p>

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Thermal Thresholds and Mechanical Behavior of Granitic Rocks from Northern Pakistan: Implications for Rock Engineering

  • 期刊论文

  • 发表日期：

    2025年8月21日

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  • Abdul Rahim Asif,
  • Muhammad Sajid,
  • Waqas Ahmed,
  • Ihtisham Islam,
  • Abid Nawaz,
  • Salman Ahmed Khattak

摘要

This study investigates the thermomechanical behavior of eight distinct granitic rock types from northern Pakistan under elevated temperatures ranging from 25 °C to 1000 °C. Through a series of standardized laboratory tests, key physical and mechanical properties—including water absorption, specific gravity, porosity, unconfined compressive strength (UCS), tensile strength (UTS), point load index (PLI), Schmidt hammer rebound value (SHT), and ultrasonic pulse velocity (UPV)—were systematically evaluated across seven thermal intervals to understand the full thermal evolution of the rocks. The results reveal a consistent trend of thermal degradation, with a critical transition occurring at 400 °C. Below this threshold, thermal effects were minimal (< 5%), but between 200 °C and 400 °C, granodiorites such as Kesu (KeG) and Warai (WG) exhibited thermal hardening, with UCS increasing by 12.5% and 38.7%, respectively. Above 400 °C, all samples showed significant deterioration: UCS decreased by over 90% in Ambela granite (AG) (from 146 to 15 MPa), and porosity in altered Malakand granite (MG) reached 10.42%, compared to 2.56% in fresh Kesu granodiorite. UPV dropped 15–17% by 400 °C and up to 60% at 1000 °C, correlating with the quartz α–β phase transition and widespread microcrack propagation. Microstructural analysis indicated substantial mineral instability with increasing temperature, marked by thermally induced fractures and widening of pre-existing cracks, particularly in feldspar and quartz. Altered rocks were more prone to texture modification at lower temperatures, highlighting their increased thermal vulnerability. Quantitative thermal damage indices were developed for five parameters (UCS, UTS, PLI, SHT, UPV), with granodiorites showing negative damage values up to 400 °C (e.g., UCS DT = − 0.13 to − 0.39), reflecting strength enhancement before subsequent degradation. Predictive thermal response models demonstrated strong correlation (R2 = 0.72–0.99), providing practical tools for engineering design. This study delivers novel insights into the behavior-specific thermal thresholds of granitic rocks and offers valuable parameters for geotechnical applications such as geothermal energy systems, nuclear waste disposal, and deep underground excavations. It represents the first in-depth, quantitative evaluation of thermally induced degradation in granitic rocks from northern Pakistan, contributing significant benchmarks for global rock engineering practices in high-temperature environments.