*3.1. Compressive Strength*

The residual compressive strength values after exposure to 100, 200, 300, 400, 500 and 600 ◦C in addition to those at ambient temperature are shown in Figure 6. The figure also shows the percentage reduction in strength due to temperature exposure. It is obvious that a slight strength gain of approximately 1.5% was recorded at 100 ◦C, where the compressive strength after exposure to this temperature was 43.9 MPa, which is higher than that before heating (43.2 MPa). This initial strength gain was reported by previous researchers [25] and is attributed to the increase in the material density due to the evaporation of free pore water and the increase in hydration products owing to the accelerated pozzolanic reaction [72,73]. The strength gain is represented as a negative percentage reduction in Figure 6. After exposure to 200 ◦C, the compressive strength recorded 35.1 MPa, with a percentage reduction of approximately 18.8%, while a noticeable percentage strength recovery was recorded at 300 ◦C. The residual compressive strength at 300 ◦C was 42.3 MPa, and the percentage reduction was only approximately 2.1%. Hence, more than 16% of strength recovery was gained as the temperature was increased from 200 to 300 ◦C. As the temperature increased beyond 200 ◦C, the removal of water from the surfaces of the cement gel particles induced higher attraction surface forces (van der Waals forces), which might increase the ability of the microstructure to absorb higher compression stresses [18,73]. Beyond 300 ◦C, the compressive strength exhibited a continuous decrease in strength, with an increase in temperature, where the percentage strength reductions were 18.8, 22.4 and 50.0% after exposure to 400, 500 and 600 ◦C. The significant strength drop after 500 ◦C is attributed to the volume changes that took place due to the shrinkage of the cement paste and expansion of the aggregate particles, which deteriorate the bond between the two materials [25,74,75]. The dehydration of cement and decomposition of calcium hydroxide also lead to destructive effects [76,77].

**Figure 6.** Compressive strength and its percentage reduction at different temperatures.

Figure 7 shows the relationship between the compressive strength and percentage weight loss of the same cube specimens after exposure to high temperatures of 100 to 600 ◦C. The percentage weight loss was calculated by dividing the weight loss (weight before heating-weight after heating) by the original specimen weight before heating, with the result multiplied by 100. The figure shows that three stages can be recognized in the percentage weight loss relation with temperature. In the first stage, the slope of the percentage weight loss was high after exposure to the sub-high temperature range (100 to 300 ◦C), which indicates early high weight loss. This weight loss can be attributed to the evaporation of the free pore water before 200 ◦C and the absorbed water in the cement gel particles. The second stage, which is a semi-stabilization stage with a very small positive slope, is related to the strength recovery and low reduction between 300 and 500 ◦C, where the microstructure is still not very affected by the chemical and physical changes due to temperature exposure. Finally, the weight loss starts another high-slope reduction region after 500 ◦C, where the cement matrix was cracked owing to the chemical changes, and the bond between the cement and aggregate was almost lost due to the different thermal movements [78,79]. Excluding the initial strength gain at 100 ◦C and the strength recovery at 300 ◦C, it can be said that the strength loss can be related to the loss in weight. The differences between strength loss and weight loss at these regions are attributed to the different behaviors of concrete under the different applied stresses, where it is stated that the residual tensile strength of concrete, for example, has a different behavior with temperature than that of the compressive strength. As weight loss is a stress-free measurement, there would be some expected differences with load tests.

#### *3.2. Flexural Strength*

The residual flexural strength records (modulus of rupture) of the tested prisms are visualized in Figure 8, which also visualizes the percentage reduction in flexural strength as a ratio of the unheated strength. The figure explicitly shows that a similar strength gain to that of the compressive strength was recorded for the flexural strength at 100 ◦C. However, the percentage increase was higher, where the unheated flexural strength was 3.7 MPa, while that after exposure to 100 ◦C was approximately 4.1 MPa, with a percentage increase of approximately 10.1%, which is depicted in Figure 8 as a negative percentage reduction. The same reason discussed in the previous section for the compressive strength could be the source of this increase in the flexural strength. After, a continuous strength deterioration was recorded as the temperature increased beyond 100 ◦C, where the residual modulus of rupture values were approximately 2.9, 2.4, 2.2, 1.6 and 0.3 MPa with respective percentage reductions of approximately 22.5, 35.1, 41.6, 57.3 and 91.2 % after exposure to 200, 300, 400, 500 and 600 ◦C. It can be noticed that the strength recovery recorded for the compressive strength at 300 ◦C was not recorded for the flexural strength, which

reflects the positive effect of physical attraction forces at this temperature to sustain higher compressive stresses, while such effect was insignificant under the flexural tensile stresses. Another result that was noticed is the faster deterioration of the flexural strength compared to the compressive strength, which is directly related to the weak microstructural response of concrete to tensile stresses. A similar flexural strength reduction behavior was reported by many previous works [25,72,80].

**Figure 7.** The relation of compressive strength and percentage weight loss of the cube specimens at different temperatures.

**Figure 8.** Flexural strength and its percentage reduction at different temperatures.

The weight loss for the same prisms was recorded, from which the percentage weight loss was calculated, which is depicted in Figure 9 against the residual modulus of rupture. The figure shows that a similar three-stage behavior can be recognized for the prism specimens to that recorded for the cube specimens, where the slope of the percentage weight loss was higher before 300 ◦C and beyond 500 ◦C, while it was lower between them. Similarly, excluding the high residual flexural strength recorded at 100 ◦C, the strength reduction curve exhibited a similar three-slope behavior to that of weight loss, where the percentage reduction slope was high from 100 to 300 ◦C and from 500 to 600 ◦C, while it was semi-stabilized between 300 and 400 ◦C for both the flexural strength and weight loss, as shown in Figure 9. This similar trend confirms the strong relation between weight loss and strength reduction after high temperature exposure.

**Figure 9.** The relation of flexural strength and percentage weight loss of the prism specimens at different temperatures.

#### *3.3. Repeated Impact Strength*

This section presents and discusses the obtained results from the conducted drop weight repeated impact tests that were carried out at ambient temperature and after exposure to different levels of high temperatures. The results of all specimens are listed in Table 2 together with the mean, standard deviation (SD) and coefficient of variation (COV) of each of the six disc samples. It is obvious that for all specimens, before and after exposure to high temperature, the recorded failure number was slightly higher than the cracking number, which reveals the brittle nature of normal concrete under impact tests [81–83].


**Table 2.** Results of repeated impact test.

3.3.1. Cracking and Failure Impact Numbers

The detailed impact numbers are listed in Table 2, while Figures 10 and 11 show the post-high temperature exposure response of the cracking (Ncr) and failure (Nf) impact numbers, respectively. Figure 10 shows that after exposure to 100 ◦C, the cracking number was not noticeably affected, where this number was decreased from 55 to 53.7, with a percentage decrease of only 2.4%. Similarly, Nf was decreased by no more than 3.5%, as illustrated in Figure 11. Considering the known high variation in the ACI 544-2R repeated impact test [6], it can be said that the impact resistance was not affected by heating to 100 ◦C, which is also confirmed by the comparison of the individual impact numbers in Table 2, where the retained impact numbers for some specimens where higher after exposure to 100 ◦C than before heating. For instance, a cracking impact number of 90 was recorded for specimens heated to 100 ◦C, which is higher than all recorded numbers before heating. As discussed in the previous sections, the compressive strength and flexural strength were increased after exposure to a temperature of 100 ◦C. The strength gain is reported to be due to the shrinkage of the concrete pore holes after the evaporation of the pore water, which resulted in denser media. The slow preheating of specimens in an electrical oven at 100 ◦C might also help to minimize the reduction in the impact strength at this temperature, where the free pore water was partially evaporated during the initial heating phase, resulting in the stress being relieved during the second (furnace) quick heating phase, which maintained the microstructural deterioration at a minimum. It should be mentioned that the oven's slow preheating is essential to prevent any type of thermal explosive failure during heating, which would be harmful to the furnace and other specimens in the furnace. The preheating process was a typical procedure followed by many experimental studies in the literature, where it was reported that thermal explosive spalling is probable at temperatures in the range of 300 to 650 ◦C [25].

**Figure 10.** Cracking impact number and its percentage reduction at different temperatures.

**Figure 11.** Failure impact number and its percentage reduction at different temperatures.

The strength of the material to resist impact forces dropped sharply after exposure to 200 ◦C, where the loss in impact resistance in terms of Ncr and Nf reached 74.2 and 73.5%, respectively. This reduction is dramatic and is much higher than the losses in the compressive, flexural and tensile strengths according to the literature and the current study results, where previous studies showed that the normal compressive strength range of normal concrete after exposure to 200 ◦C falls between 70 and 110% of the unheated strength [18,21,72]. Similarly, it is widely addressed in the literature that the modulus of elasticity of normal concrete would be higher than 70% of the unheated values [73–75].

Phan and Carino [25] reported that, for the case of an unstressed residual compressive strength test, which was the heating procedure followed in this research, an initial strength gain or minor loss is the usual trend of normal concrete up to 200 ◦C. The residual impact strength of the higher temperatures followed the same excessive strength drop, as shown in Figures 10 and 11. The residual cracking impact strength after exposure to 300, 400, 500 and 600 ◦C was, respectively, 11.8, 5.5, 3.9 and 1.8% of the original unheated strength. Similarly, the residual failure impact strengths were 13.1, 7.0, 5.2 and 3.8% after exposure to 300, 400, 500 and 600 ◦C, respectively. Two points should be discussed here, namely, the strength reduction behavior with temperature, and the high drop in strength. It is obvious that the impact strength follows a similar trend of reduction with temperature to the flexural strength, where both the flexural and impact strengths showed stable responses at 100 ◦C, followed by a continuous decrease until the approximate fading of the strength at 600 ◦C. This might be attributed to the type of stresses caused by the repeated impacts on the concrete, where the received impact forces tend to cause a fracture surface and then transfer this into tensile stresses that try to open the cracks until breakage failure. The higher strength reduction can also be attributed to the nature of the impact loads, where a sudden concentrated loading is induced within a very short time, leading to a higher stress concentration and hence a faster deterioration. As soon as the microstructure is internally fractured by the initiation of microcracks, only a few further concentrated drops are required to induce the surface cracking and failure of the specimen. Since the microstructure of the cement paste, the aggregate particles and the bond between them are negatively affected by the preceding heating phase of the specimens, a quick fracture of these specimens is expected under impact drops.

Figure 12 compares the behaviors of the residual impact strength in terms of the impact numbers of the disc specimens and the percentage weight loss of the same specimens. It can be said that the trend of the impact strength reduction is related to that of the percentage weight loss, where the same minor reduction is clear in the figure after exposure to 100 ◦C for both the strength and the weight loss, followed by a steep drop from 100 to 200 ◦C. After, a continuous decrease in the strength and an increase in weight loss are obvious from 300 to 600 ◦C. Indeed, the percentage strength is much higher than the percentage weight loss at each temperature; however, the behavior of the reduction with the temperature increase is quite similar.

**Figure 12.** The relation of the impact number and percentage weight loss of the prism specimens at different temperatures.

As disclosed in the previous sections, the ACI 544-2R repeated impact test is known for its high variation in the obtained impact results. Figure 13 shows the effect of high temperature exposure on the variation in the cracking and failure impact numbers in terms of the coefficient of variation (COV). It is clear in the figure that the COV was not so high (22.8) at ambient temperature, which is a good indication of the better control on the loading parameters using the automatic machine compared to the standard manual apparatus. The COV increased at 100 and 200 ◦C, recording values in the range of 41.4 and 44.8% for both the cracking and failure numbers. This increase in the result variation can be attributed to the dramatic behaviors at these temperatures, where at 100 ◦C, the specimens exhibited an increase and a decrease in strength compared to those tested at ambient temperature, as listed in Table 2. Similarly, the high drop in the impact number records at 200 ◦C resulted in high percentage variations, although there were limited numeral variations. The extremely low records of the impact numbers at the higher temperatures reduced the differences between the tested specimens, which, in turn, reduced the COV, as shown in Figure 13.

**Figure 13.** Coefficient of variation of the cracking and failure impact numbers at different temperatures.

#### 3.3.2. Failure Patterns

Pictures for the disc specimens at room ambient temperature (R) and after exposure to 100, 200, 300, 400, 500 and 600 ◦C are shown in Figure 14a–g, both before and after impact testing. The fracture and failure of the reference unheated specimens align with what has been reported in previous studies [84–88] for plain concrete, where after a number of repeated blows, a small-diameter central fracture zone was created under the concentrated compression impacts via the top surface's steel ball. After a few more blows, the internal cracks propagated to the surface, forming a surface cracking of two or three radial cracks from the central fracture zone, which formed the failure shape that occurred after a few additional blows, as shown in Figure 14a.

Figure 14b–d show that the specimens heated up to 300 ◦C exhibited a similar cracking and fracture behavior to that of the unheated specimens. However, at 200 and 300 ◦C, the cracking and failure tended to be softer, where the fracture started at much lower impact numbers, and the central fracture zone was smaller. The fracture of the specimens heated to temperatures above 400 ◦C is different. The material became so weak and absorbed the applied impact energy after being heated, where the cement paste became softer, the crushing strength of the aggregate particle reduced and the bond between them was almost lost. Due to these effects, internal thermal cracks were formed in the whole volume of the specimens. Therefore, for the specimens exposed to 400, 500 and 600 ◦C, only three impacts, two impacts and one impact were enough to cause the already existing cracks to appear at the surface. As a result, the central fracture zone was barely formed, and a higher number of major cracks (four or five) were formed accompanied by more hair cracks, as shown in Figure 14e–g.

**Figure 14.** Failure patterns of the impact disc specimens at different temperatures (**a**) ambient temperature; (**b**): 100 ◦C; (**c**): 200 ◦C; (**d**) 300 ◦C; (**e**) 400 ◦C; (**f**) 500 ◦C; (**g**): 600 ◦C.
