Hybrid concretes: solutions for better bond and splitting tensile strength under elevated heat exposure

PurposeThis paper presents the second part of the investigation on resistance to elevated temperatures of a proposed hybrid composite concrete (NCSF-Crete) mix. The composite including nano metakaolin (NC) and steel fibers (SF) in addition to regular concrete components has proven -in the first published part-earlier promoted fresh concrete behavior, and to have reduced loss in compressive strength after exposure to a wide range of elevated temperatures. This presented work evaluates another two critical mechanical characteristics for the proposed composite -namely- splitting and bond strengths.Design/methodology/approachA modified formula correlating splitting and compressive strength (28 days) based on experiments results for NCSF is proposed and compared to formulas derived for regular concrete in different design codes. Finally, both spitting and bond strengths are evaluated pre- and post-exposure to elevated temperatures reaching 600 °C for two hours.FindingsThe proposed NCSF-Crete shows remarkable fire endurance, especially in promoting bond strength as after 600 °C heat exposure tests, it maintained strength equivalent to 70% of a regular concrete control mix at room temperature. Improving residual splitting strength was very significant up to 450 °C exposure.Research limitations/implicationsObvious deterioration is monitored in splitting resistance for all concretes at 600 °C.Practical implicationsThis proposed composite improved elevated heats resistance of the most significant concrete mechanical properties.Social implicationsUsing a more green and sustainable constituents in the composite.Originality/valueThe proposed composite gathers the merits of using NC and SF, each has been investigated separately as an addition to concrete mixes.

Preparation of Intumescent Fire Protective Coating for Fire Rated Timber Door

Intumescent flame-retardant coating (IFRC) provides a protective barrier to heat and mass transfer for the most efficient utilization of a wide variety of passive fire protection systems at the recent development. This article highlights the fire-resistance, physical, chemical, mechanical, and thermal properties of the IFRC using a Bunsen burner, furnace, Scanning Electron Microscope, freeze-thaw stability test, Instron Micro Tester, and thermogravimetric analysis (TGA) test. The five IFRC formulations were mixed with vermiculite and perlite for the fabrication of fire-resistant timber door prototypes in this research project. Additionally, the best fire-resistance performance of the fire-rated door prototype was selected and compared with a commercial prototype under the fire endurance test. An inventive fire-rated door prototype (P2), with a low density of 636.45 kg/m3, showed an outstanding fire-resistance rating performance, resulting in temperature reduction by up to 54.9 °C, as compared with that of the commercial prototype. Significantly, a novel fire-rated timber door prototype with the addition of formulating intumescent coating has proven to be efficient in preventing fires and maintaining its integrity by surviving a fire resistance period of 2 h.

Buckling of Radially Loaded Concrete Cylinders in Fire Condition

Concrete cylinders are commonly used in water treatment and sewerage plants, in the form of wells or basins. They are mainly subjected to axial compression resulting from soil lateral pressure and aqueous hydrostatic pressure, in case of the presence of a groundwater table; that is why they are mostly designed in the form of a circular hollow section. Concrete cylinders face a complicated case of loading in fire condition, as a result of material degradation in addition to thermally induced stresses. This paper studies buckling stability of that case where, a concrete cylinder is subjected to an internal fire load in addition to superimposed structural loads from the surrounding environment. The main objective of the research is to study buckling stability of concrete cylinders through identifying various structural and thermal parameters, controlling that behaviour. Finite element modelling using "Ansys 18.1" has been chosen as an approach to deal with the research problem. Twenty-five solid elements models have been prepared to study both thermal and structural behaviour of concrete cylinders in fire condition. Cylinder thickness, slenderness ratio, load ratio, and groundwater presence have been adopted as main research parameters to identify their effect on well's fire buckling endurance, in accordance with ISO 834 standard fire curve. A parametric study has been designed to study fire endurance vulnerability to cylinder thickness ranging from 50 mm up to 800 mm; diameter to thickness ratio [D/t] ranging from "10" up to "160"; full spectrum of structural load ratios; in addition to the presence of a surrounding groundwater. Outputs of the parametric study have been introduced in the form of figures, which could be used as preliminary design aids to identify buckling fire endurance as function of load ratio for various spectrums of thickness and slenderness ratios. Moreover, critical thicknesses and load ratios have been revealed.

Structure behavior of concrete filled double-steel-plate composite walls under fire

Concrete filled double-steel-plate composite walls with shear studs, one type of steel–concrete–steel walls, are recently developed and have been used in high-rise buildings, for which fire safety is a big concern. In order to investigate the fire endurance of this new type of concrete filled double-steel-plate composite walls, three specimens with different axial compression ratios and different lengths and intervals of shear studs were tested under one-side ISO-834 standard fire to obtain the temperature distribution, deformation, and detailed failure modes. Each specimen consisted of a concrete filled double-steel-plate composite wall-body and two boundary columns. Moreover, finite-element-based numerical investigations were conducted to confirm and extend experimental findings. All the concrete filled double-steel-plate composite walls failed in compression–flexure mode with the local buckling at the compressive steel plate. The results indicate that the fire endurance of concrete filled double-steel-plate composite walls is significantly affected by the axial compression ratio, the eccentricity of the axial load, and the bond strength between shear studs and concrete. Axial compression ratio, defined as the ratio of axial compression to the nominal compressive capacity of concrete filled double-steel-plate composite walls, has both positive and negative effects on the fire endurance of concrete filled double-steel-plate composite walls. The axial load eccentricity toward the unexposed side is much more detrimental to the fire endurance of concrete filled double-steel-plate composite walls than the one toward the exposed side. In engineering practice, it is recommended that proper intervals (not greater than 300 mm) and lengths (not less than 40 mm) of the shear studs should be used to ensure the bond between concrete and steel plates.

Modeling the Thermo-Structural Response of Railcar Floor Assemblies During Standard Fire Resistance Tests

Performing fire endurance tests of railcar floor assemblies in accordance with NFPA 130 is expensive given the minimum size requirements of 3.7 m (12 ft) in length and the full vehicle width. Often it is not financially viable to conduct such tests on several iterations of designs for the purpose of design optimization. Simulations of the fire endurance tests can be performed in place of experiments to provide predictions of floor assembly response of multiple designs at much lower cost. However, capturing the thermo-structural response of the floor assembly requires the ability to model the relevant physical phenomena including softening and weakening of the steel frame, degradation of the fire insulation, and failure of the composite floor. A methodology for performing such simulations was developed under this research addressing each of these phenomena. Temperature dependent thermal and mechanical properties of all modeled materials captures material softening and weakening. Degradation of the insulation was handled through a novel temperature dependent shrinkage approach. Failure models for the sandwich composite floor panels were obtained from literature to predict shear fracture of the core based on a maximum principal shear stress approach and delamination of the core/facesheet based on a maximum strain energy approach. The developed methodology was applied to the simulation of a fire endurance test of an exemplar railcar floor assembly using the commercial finite element solver Abaqus. The assembly was known to hold a passing rating for a 30-minute fire endurance test according to NFPA 130. The floor assembly consisted of a stainless-steel frame, fiberglass insulation, and a ply-metal composite floor. Sequentially coupled thermal and structural models were developed to predict the thermostructural response of the floor assembly for a 30-minute exposure to the ASTM E119 prescriptive fire curve. User-subroutines were utilized to implement the sandwich composite failure models developed for predicted core shear fracture and core/facesheet delamination. The predicted temperature rise on the unexposed surface of the floor assembly after a 30-minute exposure ranged from 50°C to 90°C. The floor assembly was also predicted to maintain structural integrity with the applied crush load, having a center-point vertical deflection of 161 mm after the 30-minute exposure. This resulted in a predicted pass rating for a 30-minute exposure which agrees with the floor assembly’s actual fire rating.

Intumescent coatings with improved properties for high-rise construction

The paper overviews the way of creating intumescent fire-protective compositions with improved properties by adding nano-and micro-sized supplements into them. Intumescent paints are inert at low temperatures, and at higher temperatures they expand and degrade to provide a charred layer of low conductivity materials. The modified intumescent paints are able to form a more stable charred layer than the classical paints. The stability of a charred layer is crucial if the fire safety in high-rise construction must be secured, because a weak charred layer will not provide a required fire endurance for steel bearing structures and they will break down in case of fire. The fire-protective properties of modified intumescent paints were estimated using an electrical furnace. Also the way of thermal decomposition of the paints was studied with thermogravimetric analysis. Results show that modified intumescent paints form a charred layer with improved fire-protective properties; it can serve as a thermal barrier for a longer period of time. Thermogravimetric analysis confirms this fact showing that the temperatures of full thermal decay in case of modified paints are higher than those of non-modified paints.

EXPERIMENTAL STUDY ON STRUCTURAL BEHAVIOR OF FIRE PROTECTED DOUBLE CFT COLUMN SUBJECTED TO FIRE LOAD

New concrete filled double-tube (CFDT) sections consist of an inner and outer tube with fire protection mortar (FPM) filling the cavity between them and the inner tube also filled with concrete or not. An investigation into the fire performance of CFDT during the standard fire test is reported. Six full size FPM filled CFDT columns were designed for the fire tests. Detail failure modes of overall specimens and each component in the columns as well as temperature, deformation and fire endurance were presented. It showed that the fire resistance in the CFDT columns is significantly higher than that in concrete filled steel tubular (CFT) columns. Investigation into the fire performance of the columns reveals possible solutions to improve the fire resistance of CFT members.