With the increased number of catastrophic fires in structures, assessing fire-affected structures is a new area of research in the construction industry. Engineers and architects have difficulties in restoring the strength and appearance with the operation of a structure following fire exposure. Adequate design and material selection are the key factors to ensure the safety of the buildings under fire hazards. Fire protection codes and guidelines must be updated on a regular basis based on research findings [1
]. Designers must fully understand the impact of fire on structural members to overcome failures when they are subjected to elevated temperatures for an extended period [1
The capacity to withstand energy, often known as “toughness”, is a basic design characteristic for structural members susceptible to impact stresses. Impact strength is the capacity of a material to bear an unexpected load and is defined in terms of its energy. Impact loads significantly deteriorate structural strength and result in a loss of functional stability and integrity. Some building components, such as airport runways, and beam-column junctions, when subjected to seismic loads, and abrupt explosions, must withstand the impact forces applied to those building components. Therefore, the impact resistance of concrete must be improved to withstand such frequently applied impact loads. High-performance concrete (HPC) is required for impact-resistant structures [2
]. Its constituents and microstructure strongly influence the hardening characteristics of concrete. The calcium silicate hydrate (C-S-H) gel is vital for the strength development in concrete. Significant changes in strength and morphology can be seen when nanomaterials are employed as supplementary cementitious materials in concrete [3
The use of nanomaterials can resist the alkali-silica reaction, rusting, and freeze-thaw characteristics of concrete, as well as minimize porosity and shrinkage occurrence [4
]. Many researchers have examined the impact of various types of nanomaterials on the hardening characteristics of composites (paste, mortar, and concrete), namely nano-silica [5
], nano-alumina [6
], and nano-titanium di-oxide [8
]. Because of its strong cementitious nature and low cost, NS is the most extensively used nanomaterial. Furthermore, due to its strong pozzolanic activity, NS is combined with calcium hydroxide, which is formed during the hydration reaction process of cement, to form C-S-H gel [9
However, there is no information in the literature on the optimal dose of such nanomaterials and the associated enhancement of the hardened properties of concrete, particularly its compressive strength (CS). Researchers [10
] showed that the use of nano-silica and nano-titanium dioxide to replace cement by 3% could increase the CS by 15.5% and 8.6%, respectively. They also discovered that nano-silica improves the early strength while titanium dioxide has the opposite effect. The highest CS was achieved when nano-silica was employed at a dose of 0.5%, after which the gain in the CS was reduced [11
]. Kanagaraj et al. [12
] reported that incorporating NS into concrete enhanced its CS by up to 6% at all curing ages.
Concrete containing 30% fly ash (FA) and nano-silica showed significant enhancement in its CS at 28 days. Safiuddin et al. [13
] investigated the effects of nano-silica and nano-alumina on the freeze and thaw resistance of concrete. The improvement in the 28-day CS was 30.13% when 5% nano-silica was employed, but the strength development was only 7.9% when 3% nano-alumina was used. Kanagaraj et al. [14
] found that the inclusion of nano-silica increased the tensile strength (TS) and flexural strength (FS) of high-strength concrete (HSC) at 28 days by approximately 45% and 24%, respectively. The addition of fly ash to concrete as a partial replacement for cement increases matrix density, durability, and sustainability [15
]. According to Naji Givi et al. [17
], the optimal substitution of nano-silica was between 1% and 1.5%. According to researcher [12
], the FS of high-performance concrete increased up to 1% with an increase in nano-silica dose, then decreased. Several researchers evaluated the mechanical characteristics of concrete with single nanomaterial while some used two materials. For example, using 6% NM and 0.02% carbon nanotubes together may raise the CS of concrete by 29% [18
]. According to [19
], adding hybrid nano-silica and nano-clay was found to improve the performance of regular cement pastes. Very few studies have explored the strength aspects of single nanomaterial-blended concretes at elevated temperatures and the strength parameters of nano-blended concrete at room temperature [20
At 400 °C, 600 °C, and 800 °C, Bastami et al. [23
] investigated the hardening characteristics of the nano-silica-blended HSC. The results show that nano-silica improves residual CS, reduces spalling, and reduces the mass loss in specimens by creating a denser internal structure. According to Heikal et al. [24
], nano-alumina accelerates the hydration process of cement. For nano-alumina cement pastes without the superplasticizer, an increase in the nano-alumina content from 0 to 2% (with an increment of 1%) by mass was found to increase the CS by 10.89%, 31.03%, and 20.33%. However, at an elevated temperature of 450 °C, the rise in nano-alumina content from 0 to 2% increased the CS by 25.22%, 45.74%, and 28.49%, respectively. Horszczaruk et al. [25
], on the other hand, concluded that nano-silica (up to 3%) improved the thermal resistance of mortars, particularly at temperatures up to 200 °C. However, at elevated temperatures ranging up to 400 °C, the impact is either negligible or not significant. Moreover, they have shown that nano-silica may react with lime to produce more C-S-H gel, thus improving the morphology of the matrix and reducing the risk of fracture propagation when exposed to elevated temperatures. When cement mortar was subjected to elevated temperatures of 200 °C, 400 °C, and 600 °C, Irshidat et al. [26
] demonstrated that including nano-clay significantly decreased the deterioration in the TS and FS of cement mortar. Additionally, scanning electron microscope (SEM) pictures showed that the density and length of hairline fractures that developed along the matrix due to the elevated temperature were less when nano-clay materials were added.
The aforementioned literature revealed that most studies focused on adding nanomaterials to regular cement paste or mortar and measuring its CS. Additionally, limited experiments are conducted with nanomaterials in concrete to examine their TS and FS. Therefore, the primary goal of this study is to determine the impact resistance of concrete blended with NC, NS, NF, and NM nanomaterials after being subjected to temperature exposure. The nanomaterials were used to replace 10%, 20%, and 30% of the cement in four different grades of concrete, ranging from M20 to M50, at various temperatures. This nano-blended matrix was subjected to temperatures ranging from 250 °C to 1000 °C, with increments of 250 °C. A total of 384 new tests were conducted, the results of which have been reported in this paper. Finally, the morphological changes of concrete specimens were determined using SEM.
3. Impact Test Results and Discussion
3.1. Effect of Temperature on the Impact Energy of Concrete with Different Nanomaterials
According to the findings from the experimental study, the energy loss was substantially above 250 °C and most pronounced between 750 °C and 1000 °C. From the results of the experiments, it was found that the heating rate and peak temperature are the two key parameters that significantly affect the impact energy of concrete. As the temperature rises, the impact energy decreases, as can be seen from the graphs (Figure 3
, Figure 4
, Figure 5
and Figure 6
). All grades of concrete that contained various types of nanomaterials exhibited the same behavior. Compared to other materials, concrete with NC and NF was found to have superior impact energy, while NM-blend concrete showed the lowest impact energy. In every instance, the impact energy of the concrete was found to be lower for specimens cooled by water than for specimens cooled by air. Figure 3
shows the impact energy of M20 grade concrete with varying proportions of nanomaterials (NC, NF, NM, and NS) starting from 10% to 30%.
Similarly, Figure 4
, Figure 5
and Figure 6
show the impact energies of M30, M40, and M50 grades of concrete, respectively. Amongst all the concrete grades, the ones blended with NC performed better than the ones blended with NF, NM, and NS for the heating range from 250 °C to 1000 °C. Water-cooled (WC) specimens exhibited poorer performance than the air-cooled specimens; this might be attributed to the quenching effect [24
]. Degradation of the C-S-H gel, thermal incompatibility between the filler and binder medium, and high pore pressure inside the cement paste all occurred when materials were exposed to elevated temperatures. The hardened cement pastes of higher-grade concrete generated higher pore pressure inside the HSC, preventing moisture vapor from evaporating at elevated temperatures. If the thermal stresses exceed the tensile strength of the concrete, micro-cracks will form, causing the concrete to spall, as was observed in the experiments.
, Figure 8
, Figure 9
and Figure 10
illustrate the impact energy of various grades of concrete with respect to the percentage of nanomaterials at 250 °C, 500 °C, 750 °C, and 1000 °C, respectively. From the experimental results, it is found that concrete mix blended with 10% NC suffers an energy loss of 1.21%, 79.54%, 95.77%, and 98.59% when subjected to 250 °C, 500 °C, 750 °C, and 1000 °C, respectively, and cooled under the ambient temperature. In contrast, for water-cooled specimens, the energy loss was found to be 14.62%, 82.36%, 97.88%, and 99.29%, respectively, as illustrated in Table 4
. This shows that the water-cooled specimens had more energy loss than the air-cooled specimens. This might be attributed to a sudden drop in the surface temperature that causes higher thermal incompatibility inside the concrete [31
]. Air-cooled concrete mix with 30% NC had an energy loss percentage between 5.17% and 95.01% (for the temperature range of 250 °C to 1000 °C). However, in case of the water-cooled specimens, the energy loss was found to be 10.11% to 100% (for the temperature ranging from 250 °C to 1000 °C).
In the case of the air-cooled concrete mix blended with 10% NF, the energy loss was found to be 2.07% to 99.27% (for the temperature ranging from 250 °C to 1000 °C). For the water-cooled specimens, the energy loss was found to be between 14.40% and 99.27% (for the temperature ranging from 250 °C to 1000 °C). For an air-cooled concrete mix blended with 30% NF, the energy loss percentage was found to range between 4.69% and 100%, however, for water-cooled specimens, the energy loss percentage was found to range from 43.93% to 100% (as given in Table 5
). In the case of a concrete mix blended with 10% NS, the energy loss was found to range from 2.75% to 100% for air-cooled specimens. In the case of water-cooling, the loss was found to vary between 10.85% and 100% (as given in Table 6
). For the mix with 30% NS, the energy loss was found to be between 5.76% to 100% for the air-cooled specimens and 52.88% to 100% for the water-cooled specimens.
The loss in the impact energy the concrete mix blended with 10% NM was observed to be in the range 5.20% and 100% for air-cooled specimens and from 12.13% to 100% for the water-cooled specimens. The impact energy loss of the concrete mix blended with 30% NM varied from 6.21% to 100% for the air-cooled specimens and from 57.37% to 100% for the water-cooled specimens as given in Table 7
According to Nadeem’s experiments [32
], concrete containing FA performed better than concrete with MK at and above 400 °C. The IS of concrete with MK was also discovered to be nearly zero in the current investigation. This is a caution that MK blends should be used carefully, particularly in construction that may be exposed to temperatures of 400 °C or higher [33
]. Morsy [34
] stated that the CS of specimens reduced significantly at elevated temperatures above 250 °C. According to the findings of Kodur and Agarwal [35
], impact energy decreased for the specimens exposed to 500 °C compared to the reference specimen.
At a temperature of 400 °C to 450 °C, C-S-H deteriorates to CaO with a volume reduction of 44% [36
]. The performance of OPC decreases at elevated temperatures up to 400 °C due to the chemical and physical changes in the hydrated phases of the binder. Dehydration and loss of chemically bonded water are caused by heat exposure, mostly due to the dihydroxylation of CH, which reduces the chemical bonding and strength [35
]. Gel-like hydration products disintegrated at 400 °C. CaCO3
dissociates into CaO and CO2
at 600 °C. Between 600 and 800 °C, re-crystallization of non-binding phases from hydrated cement during re-heating was observed [39
]. The C-S-H gels were fully destroyed at 900 °C after continuing to dehydrate and decompose [40
The C-S-H gel was formed during the hydration of cementitious ingredients and water, which helped in achieving good hardening properties of the concrete. Free water in the concrete mix evaporates at elevated temperatures ranging from 150 to 300 °C. Hydrates get dissolved, and chemically bonded water evaporates as the temperature increases. The breakdown of CaOH begins around 350 °C, while the partial volatilization of C-S-H gel starts at about 500 °C, according to experimental data from [41
]. As a result, the mechanical properties of the hydrates are impaired; also, the pore size and porosity of the hydrated matrix increase [43
]. Additionally, the aggregates expand beyond 600 °C due to low specific heat and a rapid thermal expansion rate, increasing the volume of concrete. With all these modifications, the mechanical properties of heated concrete become temperature dependent. Due to the dissolution of C-S-H gel, all forms of concrete showed serious degradation at 800 °C [44
]. The current experimental tests confirmed these findings.
3.2. Effect of the Percentages of Replacement of Nanomaterials/Powder Content on the IS of Concrete
It is well known that the density of concrete will increase with the concrete grade due to its increased powder content [45
]. The current experimental study demonstrates the impact of powder content on the IS of heated concrete. The data shows that concrete’s impact energy decreases as powder concentration increases. This rate of decline is found to be greater with rising temperatures and better-quality concrete. The type of additive used in the concrete specimens also significantly influences the impact energy of concrete when subjected to elevated temperatures.
The spalling of denser concrete at a moisture level of less than 3% by weight was not observed [46
]. NS-containing concrete was more severely affected than any other nanomaterial. Adding various nanomaterials to cement paste improves the concrete’s compressive strength, lowers permeability, and creates a denser microstructure. However, in heated specimens, additional fine powdered particles enter the pores of the concrete, increasing the pore pressure during heating (beyond 600 °C), which causes the concrete to spall [47
From the experimental investigation, it was found that adding SF significantly densifies the concrete, which can lead to explosive spalling as a result of steam building up inside the pores, as shown in Figure 11
. Furthermore, such concrete may be inferior to standard concrete when subjected to elevated temperatures, as the evaporation of physically absorbed water begins at 80 °C and causes thermal fractures. In addition, the results revealed that the ingredients of concrete, such as cement and admixtures, significantly impacted the fire resistance of concrete.
The impact energy reduction in concrete with SF is noticeably higher than those in concrete with NC, NF, and NM. This is linked to the presence and quantity of SF in concrete, which formed an extremely thick transition zone between aggregates and paste due to its ultra-fine filler particles and pozzolanic reactions. As a result, higher stress concentrations are created in the transition zone during aggregate expansion and paste contraction. Further, the bonding between filler and binder medium containing SF is more sensitive in OPC concrete. Thus, SF concrete has greater strength losses.
When high-strength concrete is made with the silica-fume-dense matrix, it is likely to spall when heated. The high density of the mix prevents the steam produced during heating from escaping, leading to explosive spalling.
Based on the tests conducted in this study, it can be concluded that the presence of SF, when subjected to elevated temperatures, has a greater detrimental influence on the IS of concrete. Concrete with NM has a fairly high impact energy compared to concrete with NS and a lower impact energy than concrete with NC and NF. Unlike other concrete mixes, MK concrete incurred more strength loss and had lower residual strengths. The major reasons for the poor performance of concrete with MK at elevated temperatures are its dense microstructure and low porosity. Compared to its mechanical strength, this concrete demonstrated a greater loss of impermeability.
Through two processes, NM increases the CS and FS of cement mortar [48
]. The first process involves filling of the interstitial spaces inside the hardened cement with NM particles, thus increasing its density and strength. The second process creates more C-S-H gel through a pozzolanic interaction between NM and the free CH released after concrete’s hydration of OPC. The dehydration process of the produced hydrate relates to the release of free water when a denser structure is subjected to high temperatures. The residual fraction of water causes the creation and growth of microcracks. High thermal stresses caused by induced temperature gradients up to 800 °C led to enhanced microcracking [49
]. The calcium oxide changes back into calcium hydroxide if wetted after cooling or the atmosphere is humid. These volume variations can disintegrate the concrete. The increase in peak strain is due to fissures caused by the thermal incompatibility of the aggregates and the cement paste during heating and cooling. Due to the formation of microcracks and brittle microstructure, the concrete mixes were more damaging when the specimens were subjected to tensile stress. Concrete that was cooled in water as opposed to air showed a significant drop in strength, likely owing to micro-cracks development by applying thermal shock to the heated specimens.
According to the current investigation, elevated temperatures may have caused micro- and macro-cracks to form in mortars made of various nanomaterials. The particle size of cement is below 75 µm, and for fly ash, it is less than 100 µm. Whereas in the case of silica fume and metakaolin, the particle sizes are from 0.1 to 0.3 µm and less than 5 µm, respectively. FA, SF, and MK have respective surface areas of 370 m2/kg, 500 m2/kg, 12,000 m2/kg, and 9000 m2/kg. Despite having smaller particles than cement, FA, MK, and SF make the concrete mix more solid, compact, and impenetrable due to their high powder concentration. When the temperature is too high, water vapor cannot escape from the concrete containing SF, which raises the pore pressure and causes explosive spalling.