1. Introduction
Under normal room temperature conditions, the general building structure can maintain its intended functionality for an extended period. However, in instances where there exists a substantial temperature differential within the building environment, the structural load-bearing capacity may diminish, leading to a decline in performance and ultimately resulting in structural failure. The incidence of solar radiation on the sun-facing facade during summer months increases the temperature of the exterior surface of the building. However, this temperature generally does not exceed 60 °C, and it will also cause cracks on the surface of the building, affecting the normal use of the building. In addition, some workshops are in a high-temperature environment for a long time, such as some chemical and metallurgical enterprises’ high-temperature workshops, and their structural surface temperature can reach 200 °C or higher. When the chimney emits high-temperature gas, the internal temperature can reach 500~600 °C. In some accidental cases, there will be a short-term high-temperature impact on the building, such as the temperature of the building reaching 1000 °C or higher within one hour during a fire. At high temperatures, the pore pressure and porosity of concrete will change, and phenomena such as thermal expansion, thermal cracking, and thermal creep will occur. These phenomena will destroy the mesoscopic structure of concrete, leading to a gradual decline in its mechanical performance and problems such as cracking and spalling on the surface. In addition, concrete can also cause damage due to bending or stiffness and mass loss, etc. Various studies indicate that the main effect of fire (high temperature) on concrete is a change in its properties, leading to spalling, while the stress–strain behavior under high-temperature exposure is complex.
In recent years, the research on high-temperature damage of concrete at home and abroad has mainly focused on the change law of mechanical properties of concrete under high temperatures [
1,
2,
3,
4,
5,
6,
7] and the establishment of a series of high-temperature damage models [
8,
9,
10,
11,
12,
13]. In addition, there are few systematic studies on the damage constitutive relationship of nano-titanium dioxide-modified concrete under high temperatures. Gao et al. [
14] prepared the specimens using nano silica and calcium carbonate. Each group was treated at a temperature of 25~800 °C, and the influence of nanoparticles on the compressive characteristics of concrete at high temperatures was studied through uniaxial compression tests. The results showed that the peak stress of nano-silica modified concrete with a content of 1.5% increased by 29.4%, 24%, and 38.7% at room temperature, 400 °C and 800 °C compared with ordinary concrete, respectively. Fu et al. [
15] studied the effects of nanomaterials on the residual compressive strength and residual flexural strength of mortar after high-temperature treatment, and the research results showed that after curing at 800 °C, the residual stress of 1.5% nano-silica modified mortar was higher than that of nano-silica modified mortar mixed with 3%. Elkady et al. [
16] studied the effect of heating on the mechanical properties of nano-silica concrete. Amounts of 1.5%, 3%, and 4.5% nano-silica were added to the concrete mixture, and after 28 days of natural curing, it was treated at a high temperature of 200~600 °C. After high-temperature treatment at 600 °C, the strength loss of concrete incorporated with 1.5% was the lowest, and the residual compressive strength and bending strength were 73% and 35%, respectively. In comparison to the control mixture that lacked nano-silica, the compressive strength and bending strength exhibited a significant increase of 43% and 38.5%, respectively.
Composite materials along with nanoconcrete have been the subject of extensive research by numerous scholars, with a focus on their mechanical properties [
17]. Rawat et al. [
18] present a comprehensive review of prior investigations concerning the impact of nano-titanium dioxide on various characteristics of plain or blended cement systems, including workability, setting time, mechanical strengths, water absorption, and porosity. Alobaidi et al. [
19] discussed the effects of nano-fly ash particles on the room temperature and high-temperature properties of self-compacting concrete. Under the same displacement rate, the size, shape and consistency of nano fly ash particles and fly ash particles were characterized by scanning electron microscopy, and the differences between nano fly ash particles and fly ash particles were compared. Zhang [
20] experimentally studied the splitting tensile, compressive, and flexural strength of ordinary concrete and basalt fiber concrete doped with nano-silica at different temperatures. The results show that the compressive strength, splitting tensile strength and flexural strength of basalt fiber concrete doped with nano silica are higher than those of ordinary concrete at various temperatures. Nikbin et al. [
21] cured the heavy concrete containing magnetite aggregate under different temperatures (25, 200, 400, and 600 °C), and replaced part of the cement with 0%, 2%, 4%, and 6% nano-titanium dioxide particles by cement weight, and carried out compressive strength, γ ray shielding test, and scanning electron microscopy analysis of the concrete specimen. Evaluation parameters for radiative attenuation tests include linear attenuation coefficient, half-value layer, tenth value layer, and mean free path. The results show that the ultrasonic wave velocity and compressive strength of the specimen mixed with nano-titanium dioxide first increase and then decrease with the increase in temperature. Bastami et al. [
22] studied the effects of compressive strength, tensile strength, spalling, and mass loss of nano-silica modified high-strength concrete at high temperatures. The mechanical properties of the modified high-strength concrete were tested by heating the concrete specimen to four temperatures of 400 °C, 600 °C, and 800 °C at a rate of 20 °C/min. The results show that nano-silica is effectively used in high-strength concrete and can improve its high-temperature mechanical properties. The presence of nano-silica improves the residual compressive strength and tensile strength of the material, and with the increase of permeability, the peeling and mass loss of the material decrease.
This study investigates the effects of various high temperatures on both ordinary concrete and concrete modified with nano-titanium dioxide. Subsequently, an investigation was conducted on the compressive strength of the two distinct categories of concrete subsequent to exposure to elevated temperatures. The damage mechanics approach was utilized to establish the constitutive model for ordinary concrete. In this study, a constitutive model was developed for nano-titanium dioxide-modified concrete to assess its damage. The model utilized the peak strengthening coefficient, and the test data presented in this paper were subjected to comparative and analytical scrutiny. The aforementioned offers a precise point of citation pertaining to the efficacy of concrete constructions subsequent to exposure to elevated temperatures, specifically those resulting from conflagrations.