Table 3 summarises the moisture content and compressive strength of each of the test specimens, as well as their spalling test results. The “Spalling time” refers to the time in heating at which a specimen initially experienced explosive fire-induced spalling. “Max spalling depth” is the depth of spalling at the most severely spalled position. “Total weight loss” is the difference between the weights of the specimen before and after the test, and “concrete loss” is the weight of the spalled concrete pieces collected after testing. It should be noted that PF2-1 failed during a trial room-temperature loading test, and that the spalling depth and concrete loss of Series 1 tests were unavailable.
Table 3 Results of the Explosive Spalling Tests and Corresponding Measurements of Compressive Strength and Moisture Content Moisture Content
Moisture content is an important factor affecting the occurrence of fire-induced spalling. Eurocode 2 (EC2) states that explosive spalling is unlikely to occur when the moisture content of concrete is less than 3% by weight for concrete grades below C80, although it is recognised that the in-service moisture content of concrete in structures is not well known. The specimens’ moisture content was measured at the time of testing. The average moisture content of specimens was between 2.12% and 3.33% by mass. However, most of the spalled specimens had moisture contents below 3%, indicating that the EC2 threshold might lead to unsafe design.
The relative moisture content profiles for specimens of Series 2 are shown in Fig. 5. The values presented are normalised as proportions of the moisture content on Day 0 to quantitatively compare the rate of moisture loss of the various concrete slices. As expected, the greatest moisture loss was experienced by the slice nearest to the surface. Furthermore, for any given depth, more moisture was lost by the specimens with fibres than by the plain concrete specimens. This confirms that these fibres increase the connectivity of pores in concrete, facilitating the escape of vapour.
Compressive Strength
The average cube compressive strengths of the plain concrete specimens Plain1 and Plain2 were found to be 69 MPa and 70 MPa, respectively (see Table 3). Previous research [32] had reported that the addition of PPF could affect the workability of fresh concrete, introduce voids and lower the strength of the hardened concrete. However, the mix adopted in this research was designed to be self-compacting, and so the influence of RTPF on the fresh concrete’s workability was low. The average compressive strength of the RTPF concrete, for Series 1, ranged from 65 MPa to 67 MPa. For the highest fibre dosage (7 kg/m3), the compressive strength was around 6% lower than that of the corresponding plain concrete (Plain1).
Specimen SFC, containing just 40 kg/m3 of RTSF, had a slightly higher average compressive strength of 73 MPa, 4.5% higher than that of the corresponding plain concrete (Plain2). An increase in strength due to the addition of steel fibres could be caused by the lateral restraint which they provide, which tends to increase the ductility of concrete [33, 34]. RTSF concrete specimens with the addition of 2 kg/m3 and 5 kg/m3 of RTPF (SF40PF2 and SF40PF5) show a small reduction (< 5%) in compressive strength, probably due to an increase in the air entrapped during RTPF integration.
The above-mentioned compressive strength data is the mean value of the three compressive test cubes for each spalling test specimen. The standard deviation of each set of three cubes ranges from 0.5 to 3 MPa.
Spalling Test Results
The applied load (250 kN) increased only slightly (up to 5%) during the tests, as exemplified in Fig. 6 for specimen FP7, due to the restraint to thermal expansion provided by the reaction frame.
Images of the exposed surfaces for Series 1 and 2 of the spalling tests are shown in Tables 4 and 5, respectively. For Series 1, two (Plain1-1 and Plain1-3) of the three plain concrete specimens and two (PF1-2 and PF1-3) of the three specimens with low RTPF dosage (1 kg/m3) experienced explosive spalling. In particular, specimens Plain1-1, PF1-2 and PF1-3 spalled severely. None of the specimens with RTPF doses higher than 1 kg/m3 (PF2 and PF7) spalled. PF2-1 failed at ambient temperature in a trial loading test. For Series 2, two (Plain2-1 and Plain2-2) of the three plain concrete specimens experienced severe spalling, with a maximum spalling depth of 14 mm. The other plain concrete specimen (Plain2-3) did not spall, but a large splitting crack occurred on the top face (Fig. 7). This may have caused a reduction in pore pressure and prevented the explosive spalling from happening. None of the specimens with RTSF, except SF40PF2-1, experienced spalling, showing that RTSF may also contribute to the reduction of the risk of fire spalling; however, more research is required to confirm this tentative conclusion. RTSF, which is composed of finer fibres (of 0.1 mm to 0.2 mm diameter and 20 mm average length) is distinctly different from the typical manufactured fibres (of 0.4 mm to 0.5 mm diameter and 35 mm length) used for tunnel applications, leading to larger numbers of fibres per unit volume of concrete, and hence a denser fibre distribution. Moreover, the irregular geometry of RTSF enhances the bond between concrete and fibres. SF40PF2-1, one of the three specimens with RTSF and 2 kg/m3 of RTPF spalled, but the spalled concrete was held in place by the steel fibres and remained attached to the specimen surface. This shows that RTSF might also contribute by keeping the spalled concrete on the heated surface and retaining its thermal insulation to the steel reinforcement.
Table 4 Samples Plain1, PF1, PF2 and PF7 After Testing and Cooling Table 5 Samples Plain2, SFC, SF40PF2 and SF40PF5 After Testing and Cooling Series 1: Temperature Measurement
Figure 8 shows a typical infrared (IR) camera image and the specific points and areas used for data analysis. The location, measuring angle and focus of the IR camera are identical for all tests. The accuracy of the IR camera is ± 1% of reading.
Figure 9 shows the maximum surface temperature plotted against time, determined from one of the three areas AR01, AR02 and AR03 (see Fig. 8) directly heated by the blowtorch for the three specimens of PF1. The specimens experienced a high initial heating rate, which then reduced after a few minutes. This maximum temperature is used to calculate the heating rate, which is then compared with the heating rate of the PD 7974-1 large hydrocarbon pool fire curve [31]. The comparison indicates a reasonable match between the two. It is worth noting that the maximum surface temperature of the severely spalled specimen (PF1-2) dropped suddenly when spalling occurred, revealing the temperature on the now-exposed cooler inner layer of concrete.
The temperatures measured by the internal thermocouples (see Fig. 10) show smooth curves of increasing temperature with time. These smooth curves indicate a steady heating rate, without large variations in temperature. As the surface heating of the specimens was consistent between tests this means that each mix has a similar thermal gradient.
Series 2: Thermocouples
The thermocouple measurements are plotted for Plain2 and SFC in Fig. 11, and for SF40PF2 and SF40PF5 in Fig. 12. For Plain2-1, Fig. 11a shows clearly that the concrete temperatures at depths of 1 mm and 10 mm from the heated surface suddenly increase when spalling occurs. This is because both thermocouples are then directly exposed to the fire due to the spalling, and so they no longer measure the inner concrete temperature, but that of the fire. A similar phenomenon can be observed in specimen Plain2-2, which also spalled. The fire temperatures for these two spalled specimens (Fig. 11a, b), measured by a thermocouple (Fig. 4d) placed at the heated surface, are higher than those of the other tests. This is because the impact due to the explosive spalling dislocated the surface thermocouple and pushed it towards the part of the flame which is at a higher temperature. It is worth noting that for all concrete mixes a short temperature plateau could be observed at around 200°C at 10 mm below the heated surface. The phase change from water to gas (vaporization) is an endothermic transformation, so that the system absorbs energy from its surroundings; in this case using some of the heating energy. The effect is to temporarily reduce the heat transfer into the concrete, causing the short temperature plateau observed. The temperature at which this plateau occurs depends on the concrete compaction. Previous researchers [35] have reported a similar phenomenon and speculated that it may be caused by the capillary forces that exist in pores at the interface between liquid water, gas phase and solid.
Although none of the specimens SFC and SF40PF2, with RTSF, experienced explosive spalling, the temperatures at 10 mm below the heated surface of the SFC specimens also show a short plateau at around 200°C.
It can be seen that the thermocouple measurements at common depths vary between the three repeated specimens. This is rather expected since, although pre-fixed, the thermocouples might have moved during casting and so their locations might vary; also, any concrete cracking could affect the thermocouple measurements, especially for those close to the heated surface.