Mechanical Properties and Explosive Spalling Behavior of ...

2.1. Residual Compressive Strength

The residual compressive properties of concrete exposed to elevated temperatures are particularly important not only for the fire protection design of the structure, but also for the evaluation and repair of the structure. A considerable number of experimental studies regarding the residual compressive strength of SFRC have been conducted. Some experimental data regarding the residual compressive strength of SFRC are presented in Figure 1 . As shown, the relationship between the maximum exposure temperature and the residual compressive strength can be classified into three different stages:(1) In the primary stage, the residual compressive strength of SFRC decreased slightly and may even increase slightly as the high temperature caused the cement hydration reaction to occur more fully when the temperature range was between the room temperature and 400 °C. (2) In the second stage, the residual compressive strength of SFRC decreased significantly as the temperature increased from 400 to 800 °C. (3) In the final stage, the residual compressive strength of SFRC was almost completely lost when the temperature was beyond 800 °C.

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As shown in the comparative test data, the attenuation of the residual compressive strength of concrete is effectively suppressed because of the utilization of steel fibers. Lau and Anson [ 33 ] observed that in the temperature range of 105&#; °C, the residual compressive strength of concrete could increase by 5%&#;15% by adding steel fibers with a volume content of 1%. Xie et al. [ 37 ] compared specimens with 1% volumetric quantity of steel fibers with specimens without steel fiber and discovered that the compressive strength of the former improved by 19.4% at 200 °C, 24.3% at 400 °C, 92.9% at 600 °C, and 123.2% at 800 °C. The residual compressive strength of 28 d samples with 1.0% steel fiber additive was 48% higher than that of the sample without a steel fiber additive, when the maximum exposure temperatures were 900 °C and °C [ 40 ]. Ismail et al. [ 36 ] described that the compressive strength of concrete reinforced with 0.5% steel fiber was higher than that of the control concrete when the temperature was increased from 20 to 800 °C. As the concrete was exposed to high temperatures, the C&#;S&#;H in the matrix became hydrated and dehydrated, and thermal disconformity occurred inside the cement when the aggregate was heated; furthermore, the pore pressure generated by water accumulated in the pores of cement was a crucial factor for the volume expansion of the concrete [ 33 45 ]. With these expansions and shrinkages occurring inside the concrete, cracks began to appear and develop gradually, which caused the compressive strength of the concrete to decline [ 40 46 ]. Owing to the bridging crack effect of the steel fibers, after the steel fibers were applied to the concrete, the volume change behavior of the concrete under a rapid temperature change or high temperature environment was limited, and the initiation and expansion of the defects that occurred inside the concrete reduced [ 37 47 ]. Meanwhile, its inherently high melting temperature guaranteed its good performance under high-temperature conditions. Therefore, the deterioration of the residual compressive strength of concrete can be reduced using steel fibers.

3) at different maximum temperatures through experiments. As illustrated in 3, the compressive strength of SFRC increased with the steel fiber content; furthermore, when the steel fiber dosage continued increasing to 160 kg/m3, the compressive strength of SFRC decreased with the increase in steel fiber dosage. This changing trend is supported by the conclusion of Zheng et al. [

The residual compressive strength of concrete is affected by the volume content of steel fibers. Chen et al. [ 39 ] observed the variation law of residual compressive strength of samples prepared with different steel fiber contents (0, 40, 80, 120, and 160 kg/m) at different maximum temperatures through experiments. As illustrated in Figure 2 , with the increase in steel fiber dosage, the residual compressive strength of SFRC increased between the room temperature and 300 °C. However, from 500 to 800 °C, when the steel fiber content was less than 80 kg/m, the compressive strength of SFRC increased with the steel fiber content; furthermore, when the steel fiber dosage continued increasing to 160 kg/m, the compressive strength of SFRC decreased with the increase in steel fiber dosage. This changing trend is supported by the conclusion of Zheng et al. [ 27 48 ]. When the concrete was exposed to a temperature beyond 900 °C, the variation trend of the residual compressive strength of SFRC as shown in Figure 3 , showing that when the steel fiber content is less than 1%, the residual compressive strength of SFRC increases with the steel fiber content; however, when the steel fiber content surpasses 1%, the residual compressive strength decreases with the increase in steel fiber dosage, and the age of concrete exerted little effect on this trend [ 40 ]. When the temperature increased beyond 500 °C, the effect of the steel fibers began to become more significant; however, a higher steel fiber content caused coagulation in the steel fibers, resulting in reduced usable area. Moreover, a greater thermal expansion between the cement and steel fiber generated more cracks, which was considered to be the reason for the decrease in the compressive strength of concrete with the increase in steel fiber dosage [ 27 40 ]. In addition, Scheinherrová et al. [ 49 ] discovered that the residual compressive behavior of concrete exposed to high temperatures was affected by the type of steel fiber coating. Under high temperatures, the compressive strength of reactive powder concrete with Cu&#;Zn-coated steel fibers was significantly higher than that reported in [ 43 ].

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The initial saturation percentages of the sample and its temperature history, such as the heating rate and cooling regime, may be the main parameters used to determine the effect of steel fiber actions. After being exposed to heating at different high temperatures, the residual compressive strengths of concrete reinforced with 1% steel fiber of water curing with three different water saturation percentages (20%, 60%, and 100%) were investigated by Lau and Anson [ 31 ]. They discovered that increased saturation percentages reduced the strength of SFRC, regardless of the maximum heating temperature, which ranged from 105 to °C. This may be because SFRC with high saturation percentages in the heating process resulted in a greater pore pressure, and hence more internal defects. The existence of steel fibers can reduce the pore pressure more effectively under relatively high pressures in deep areas of concrete during a relatively fast heating process rather than at a slow heating rate (5 °C/min) [ 50 ], which results in a higher retention of the original mechanical properties of SFRC. It has been reported that the residual compressive strength of SFRC decreases more than that of naturally cooled concrete owing to severe thermal shock after water-cooling treatments [ 51 53 ]. This may be due to the different thermal expansion characteristics between the concrete matrix and steel fibers, in which water cooling caused more severe interface mismatches between the concrete matrix and steel fiber [ 54 ]. In addition, the mechanical properties of steel fibers may be altered [ 55 ].

Melt Extracted Stainless Steel Fiber

Chemical composition

Steel Grade C Si S P Mn Ni Cr Mo V Cu Ti Co B 446 <0.30 <3.0 <0.03 <0.04 1.0~1.5 0.45 24-27 - - <0.25 - -

<0.003

The Specification Of Melt-extraction SSF

Specification Cross-section areas(mm

2

) Equivalent diameter(mm) Cross-section size(mm) Length(mm) Configuration & shape  0.5*20    0.2    0.5    0.2*1  20   Straight  0.5*25  25  0.5*35  35

Technical Properties And Performance

Alloy 446 Melt temperature range ºC ~ Thermal conductivity @ 540ºC Kcal/m

2

 · h · ºC 69.8 Modulus of elasticity @ 870ºC × 10

6

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kg/cm ² 0.984 Fiber tensile strength @ 870ºC kg/cm ² 540 Coef. of thermal expansion @870ºC × 10

-6

/ºC 13.14 Critical oxidation temperature ºC

Increased flexural strength of 60% alumina

Refractory fired at 870ºC(ºF)for 8hrs 1.3.V% fiber

  1.15 Increased energy absorption 60% alumina ref. fired at 870ºC(ºF) for 8 hrs   10-12 Carburization increase in carbon content% in 25 hrs @982ºC(ºF)   0.07 Corrosion rate in Coke oven gas at 982ºC(ºF) Mils/yr 14 Corrosion rate nitriding Mils/yr Oxidation cycling coditions hrs wt % loss @ 982ºC(ºF)   4%

The properties and performance of refractory materials applying to various beating furnaces kilns and coke ovens should be greatly improved if SSF will be mixed with.

Today SSF reinforcement is finding its way into every parts of modern industries. They can be concluded as follow:

1)Metallurgical industry

blast-heating cupola, iron smelting furnaces, open hearth furnaces, convertors electrical furnaces, spray gun for refining of metal out of furnace, sintering furnaces and kilns. heaters for steel rolling, including lining, furnace-door and wrapping covers around water cycling pipes.

2)Oil oven, and kilns.

3)Furnaces and kilns in ceramic,glass and enamel industries.

4)Rotary and standing kilns in cement industry.

5)Forging heaters and boilers for power plant.

6)Furnaces and kilns in other industries.





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