Material properties and explosive spalling of ultra-high performance concrete in fire

Abstract

Ultra-High Performance Concretes (UHPC) has high strength (greater than 150 MPa), high durability, and enhanced fracture energies due to its densely packed microstructure and very low permeability. However, these good features make UHPC more vulnerable to explosive spalling in fire condition. Explosive spalling is generally characterized by a forcible removal of pieces or layers of concrete from the heated surface of a structural element. It compromises the load-carrying capacity of structures because it involves loss of integrity of structures, loss of concrete cross section, and exposure of steel reinforcements to fire. Several mechanisms responsible for explosive spalling have been proposed by a number of researchers. However, the exact mechanism for spalling of UHPC is not yet widely understood or accepted. This work was aimed at investigating the behavior of UHPC at elevated temperature, especially explosive spalling of UHPC. The main factors involved are polypropylene fiber, polyethylene fiber, steel fiber, and aggregate size. Firstly, an experimental study was carried out to identify control UHPC matrix by Taguchi method. Secondly, residual mechanical properties of UHPC with PP fibers, steel fibers, and larger aggregates after exposure to elevated temperature (from 300 ℃ to 900 ℃) and after water quenching were investigated. Subsequently, the effects of polyethylene (PE)-steel fiber hybridization, water-to-binder ratio, and aggregate size on flexural performance of UHPC were investigated. The microstructure and phase change were investigated by means of field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). Thereafter, the focus was switched to transport properties of UHPC. The influence of aggregate size and inclusion of PP and steel fibers on the intrinsic permeability of UHPC at hot state was first investigated. Then, the effects of fiber fraction, geometry, and induced microcracks on the intrinsic residual permeability of UHPC were investigated in detail. A model was proposed to predict residual permeability of UHPC based on image processing and quantification of microcracks.

Behaviour of recycled tyre polymer fibre reinforced concrete at elevated temperatures

“…This can be ascribed to the migration of moisture from the heated surface inwards towards the lowtemperature region, where the moisture and vapour accumulate to form a 'moisture clog' that impedes the vapour migration and thereby causes the pore pressure build-up [4]. It is noteworthy that based on the previous studies of plain concrete at elevated temperatures, greater pore pressure gradients were found within 20 mm from the heated surface than in the deeper zone of plain concrete [8,50],…”

“…The most commonly used heating rate ranges from 1 to 10 °C/min, such as 1 °C/min [2,9], 2 °C/min [45], 4 °C/min [46], and 10 °C/min [14,18,47]. A higher heating rate can significantly speed up the pore pressure buildup inside concrete due to a higher induced thermal stress [8]. In this study, the heating rate was set as 10 °C/min to attain the target temperatures of 105, 250, 400 and 600 °C .…”

“…When subjected to elevated temperatures, concrete would experience damage or even spalling with development of cracks and loss of concrete cover, leading to direct exposure of steel reinforcement to high-temperature environment and reduction in the load-carrying capacity of reinforced concrete structures [4][5][6][7]. The damage mechanisms of concrete at high temperatures have been extensively studied and widely accepted as a combination of two aspects: (1) thermo-hygral: accumulation of internal pore pressure due to difficulties to relieve free water and vapour until it exceeds the threshold value [8]; (2) thermomechanical: thermal stresses that develop along the heated surface [9]. The pore pressure induced damage is highly dependent on microcracks and pore structure of concrete, whereas the thermal stress induced damage takes place near the heated surface in the form of concrete compression failure, which is significantly associated with heating rate [10].…”

“…Steel fibres melt at temperatures higher than 1300 °C [18]. Therefore, adding steel fibres can result in an augment of tensile strength of concrete by retaining fibre bridging effect over a large temperature range, so that fire endurance of concrete can be improved [8,18]. Steel fibres can also withstand and mitigate pore pressure by entrapping air bubbles around the fibres to accommodate water and vapour [19].…”

Spalling resistance of fiber embedded ultra-high performance concrete

“…At least two specimens of each concrete mix were tested. Details of the test setup and procedure for evaluating hot permeability are given in (Li andTan 2016, Li et al 2018).…”