## 4 Results and Discussions
### Effects of Fire on Mechanical Properties  
#### 4.2.1 Compressive Strength Development  
For evaluating the quality of concrete and strength development characteristics of all mixes, cube compressive strength was determined at 3, 28, and 90 days, which is shown in Figure 10.  
Figure 10: Concrete cube compressive strength measured at 3, 28, and 90 days. Figure 9: Temperature measurements of internal thermocouples installed at different depth of the slab at the end of 60 min of fire exposure (275 mm thermocouple is nearest to the fire). As shown in the figure, it is observed that with the addition of the fibers of either basalt or PVA, the compressive strength of concrete dropped by 4% to 14% at all ages in comparison to the compressive strength of the control mix. Similar observations are reported in the literature [7,8]. The higher reduction (more than 10%) is observed with higher fiber content, i.e., 1.5 and 2% basalt fibers caused 12 and 14% reduction in the compressive strength, respectively. Whereas, the addition of 1.5 and 2% PVA fibers caused 9 and 12% reduction, respectively. The reasons reported in the literature include the formation of the coarser interfacial transition zone, ITZ around the fibers as well as balling effects of fibers during mixing [7,8]. (1) Effects of 120 min of fire exposure on the compressive strength  
In this segment of the study, effects of 120 min of a simulated hydrocarbon fire exposure were investigated on the compressive strength of all concrete mixes. For this, 100 mm diameter and 200 mm high cores were taken out from the slabs after 90 days of ambient curing. Three cores were taken from each pair of the slab (the slab not exposed to fire, and the slab exposed to fire). From the slabs exposed to fire, cores were taken from the fire exposure side. A sample of cores taken out from different slabs is shown in Figure 11.  
Compressive strength was conducted according to the requirement of BS-EN 12504-1 [24]. The test was performed using a Universal Testing Machine (UTM) of 1000 kN capacity at a constant loading rate of 2.4 N/mm/s. As suggested by the code [20], the equivalent in-situ compressive strength, \(f_{csq}\) was determined using the following equation:  
\[f_{c,cq}=\frac{D}{\left(1.5+\frac{1}{\lambda}\right)}f_{c,m} \tag{1}\]  
where \(f_{c,m}\) is the measured compressive strength estimated as the crushing load divided by the core surface area, \(D\) is taken as 2.3 for vertically drilled core, and \(\lambda\) is the length to diameter ratio of the core, which is 2 for this case. Figure 12 showed the compressive strength test (\(f_{c,cq}\)) of all concrete samples exposed to fire and not expose to fire. It is observed that for all concrete cores, exposure to fire after 120 min has caused a reduction of strength between 21% to 35%. Among all concrete mixes, basalt fiber concrete cores showed the lowest strength. As stated earlier, all slabs were cured for 90 days in the ambient conditions. Basalt fiber (BF) concrete cores (1, 1.5, and 2% fiber content) unexposed to fire showed the compressive strength of 55, 52, and 46 MPa, respectively and the fired samples showed a value of compressive  
Figure 11: The sample cores, 100 mm in diameter, taken out from slabs. strength of 37, 33, and 32 MPa, whereas the controlled concrete M-0F showed 68 MPa and 54 MPa of strength without and with the exposure of fire, respectively. Table 3 showed the average reduction in the compressive strength after 120 min of exposure to fire. The lowest reduction in strength (21%) is observed in the controlled concrete mix. Basalt fibers (1, 1.5, and 2% addition) showed an average reduction of 30% in the compressive strength after 120 min of fire exposure. Whereas, PVA fibers (1, 1.5, and 2% addition) showed an average reduction of 25% in the compressive strength after 120 min of fire exposures.