Effect of fire on concrete mechanical properties and concrete columns

This study includes a review of prior investigations on the effects of exposure to fire and high temperatures on recycled concrete components and concrete columns. Although considerable information has been collected on both phenomena, there is still a need for a fuller knowledge of the performance of recycled concrete and the performance of concrete poles under increased temperatures. There is a scarcity of data from large-scale experiments on concrete buildings. In this study, the compressive and tensile strengths, modulus of elasticity, and stress of recycled materials in concrete, such as bricks and glass, are compared to the findings of regular concrete formed of natural coarse aggregate subjected to high temperatures. Also, it explored the variables that impact how resistant concrete columns are to fire and high temperatures, including the kind of aggregate, relative humidity, length of fire exposure, concrete permeability, concrete cover, and longitudinal reinforcing.


Introduction
Fires are a significant threat to the reinforced concrete column, and their failure has catastrophic consequences for the buildings.Many building constructions may be exposed to difficult conditions such as fire during their tenure of duty.The increased temperature caused by fire may deteriorate the material and mechanical qualities of structural components as shown in figure 1, leading to the eventual collapse and failure of the entire building.Construction fires do severe damage to civilian infrastructure.In the United States, fires claimed 3,010 lives, injured 17,050 civilians, and caused $12.5 billion in property damage in 2009 [1].The north and south World Trade Center building collapses also killed 2,749 people and cost $100 billion in economic damage.Fire factors will steadily increase in the future as a result of high-rise and large-scale development and complicated applications, and the fire scale will also expand.The present fire protection design and testing of structural elements are based on ISO-834 standard fire tests, that is shown in figure 2. In this scenario, the structure under consideration is assessed for structural behavior and fire resistance duration using ISO 834 fire curve.This technique ensures the comparability and reproducibility of trials to explore the fire behavior of different structural elements.Kodur et al. [2] studied how reinforced concrete columns encased in fiber-reinforced polymer responded to a typical fire.Three large-scale reinforced concrete columns, two of which had circular cross-sections and the third of which had a square crosssection.The square column was 406 mm wide; the round columns had a 400 (mm) diameter.The length of three columns was 3810 mm.In the furnace chamber, these specimens were heated by 32 propane gas burners organized into eight columns of four burners each.The findings of the tests revealed that the fiber-reinforced polymer materials utilized as externally bonded reinforcement for concrete buildings were susceptible to the impacts of high temperatures.They also discovered that installing acceptable fire insulation.
Figure 1.Fire effect on the reinforced concrete members.Sustainability concerns have arisen recently due to Climate change and the use of non-renewable resources.Globally, the building and construction sectors produce a significant amount of trash, which includes debris from concrete and brick demolition.For example, the United States produced approximately 170 million tons of removed buildings in 2003.In Canada, construction, remodeling, and demolition trash created 15.5 million tons of rubbish in 2004 and 17.3 million tons of garbage in 2008, demonstrating that waste creation has been steadily increasing [3].One of the environmentally friendly options for concrete work is recycled aggregate (RA).In field technology, the use of RA rather than natural aggregate (NA) was an effective technique to lessen the demand for NA and resolve the problem of concrete pollution.Repurposed coarse aggregate is typically created by crushing, screening, and cleaning impurities from demolition waste.As a result, numerical research has indicated that the RA's quality is typically lower than the NA, which they attribute to the residual mortar particles, micro-cracking, and the RA's greater water absorption and porosity, which were confirmed by some studies [4].The microstructure of concrete incorporating RA will be significantly more complicated, considering RA is a heterogeneous material.Topçu et al. [5] studied the mechanical and physical qualities of concrete, including reused materials.According to the results of this study, the workability, Schmidt's hardness, density, and compressive strength decrease when the amount of RA grows.In 2001, Sageo-Crentsail et al. [6] examined the effectiveness of reused coarse concrete aggregate and showed that, when produced commercially, its performance is on par with that of regular concrete.Many research studies discussing the RCA's impact on concrete production have been conducted in the last few years.The results of those studies indicate that RA generally has a lower quality than NA due to recycled concrete aggregate's higher water absorption and porosity, surface cracks, and residual mortar particles [7].
There are many procedures developed by researchers to test the standard temperature of concrete.Most of them can be classified into two types: the first type is a test at a high temperature, and the second type is a residual test after being subjected to rising temperatures.The first kind is suitable for revealing the characteristics of concrete throughout the fire period in order to calculate structural resistance.The other type can only be used to define the strength of the structure after the fire and the possibility of its repair.However, the terms are rarely defined explicitly in the literary texts, and the remaining test is occasionally confused with "raised temperature testing."In this paper, the term "effect of fire and high temperatures" will be used on concrete, especially recycled concrete aggregates.The current review shows that only a limited number of researchers have investigated the behavior of fire resistance of the columns and recycled materials with concrete under fire and elevated temperatures.The literature indicates that a lot of work must be done on this subject.This paper determines the majority of studies that have been conducted in the field of recycling materials, the performance of concrete columns, and the factors affecting their resistance to fire and high temperatures.Important research gaps and research recommendations were identified in terms of the fire resistance of these alternative materials.

Effect of temperature and fire on recycled concrete materials
The majority of concrete buildings are exposed to temperatures no higher than those imposed by the environmental circumstances under typical settings.nevertheless, there are significant scenarios in which These constructions may be subjected to greater temperatures.(for example, jet engine explosions, building fires, chemical, and mineral industrialized operations in which concrete close proximity to furnaces, some assumptions about nuclear power, and so these have Accidents seriously affect concrete, so what if the concrete is made of concrete materials that have been recycled, and what is the influence of temperature and fire on these materials and the structure in general?Civil engineers, architects, and contractors have recognized extra cementing materials as cement alternatives.Although these industrial products are utilized in substantial quantities, a bigger proportion of them is now disposed of in landfills.In regard to aggregates, based on the gravel's position, using materials that have been recycled from construction trash can help save on transportation expenses.Moreover, RA is a more environmentally friendly option.for prolonging the life of existing gravel quarries.Generally speaking, increasing the proportion of recycled elements in concrete will result in a more substance that is environmentally friendly.Given the potential for extensive use of recycled components, further research on the efficiency of these environmentally friendly concretes in fire circumstances is required. Before displaying the experiments on concrete manufactured with RCA, this part outlines the fundamental principles concerning the rising degree of heat behavior of concretes created with normal aggregates.After heating the concrete, many physicochemical effects take place, resulting in changes to the material's thermo-mechanical characteristics.The first reactions are connected with a slow capillary water loss, from 20 to 80 degrees Celsius [19].Ettringite dehydrates and decomposes between 80 and 100 degrees Celsius [8].At this point, water that is physically bound to the cement matrix and aggregates dries out, causing micro cracking and capillary porosity [9].Between 100 and 200 degrees Celsius, concrete starts to lose water, and the silicate calcium hydrate in the cement paste starts to dry out and degrade, posing C2S [8].Additionally, from 120 and 300 degrees Celsius, cement gel layers begin to move toward one another, increasing van der Waals forces.When C-S-H continues to break down at 350 degrees Celsius, water loss intensifies and micro cracks and porosity grow [8].Above 300 C, concrete begins to lose strength and stiffness significantly [10].Micro cracking in the cement paste results from the CH (portlandite) decomposing after 400 C and continuing up to 600 C [8], [9].Additionally, quartz-containing siliceous aggregates change at 573 C [8], [11].The C-S-H breakdown proceeds through a second phase between 600 and 800 degrees Celsius [9], [11].Most of the compressive force has perhaps been lost at this stage [18].The dehydrated phases begin to melt between 800 and 1200 C, and around 1200 C, there is a significant amount of micro cracking.Mechanical characteristics are the fundamental factors for comprehending and forecasting how concrete will behave at high temperatures.Compressive force, elastic modulus, tensile force, and strain-stress curves are all factors considered in RCA behavior studies.

Compressive strength
The heat and remaining mechanical performance of conventional concrete built with natural aggregates has attracted more attention than that of reused aggregate concrete made utilizing reused concrete aggregates after being exposed to high temperatures.Numerous investigations have been carried out to measure the compressive strength of recycled coarse aggregate concrete after being subjected to high temperatures ranging from 20 to 800 °C.Zhao [12] tested five compositions with differing substitution ratios (0, 25, 50, 75, and 100%) that were produced from RCA. Figure 3 depicts the compressive strength absolute values with standard deviation at increased temperatures.Xiao and Zhang [13] discovered a clear difference in the compression resistance of recycled concrete at low and high temperatures after preparing 160 cubes of recycled concrete and heating them in a single thermal cycle ranging from 20 to 800 C. and different replacement ratios.An upward trend was observed from 300 and 500 degrees Celsius, but above 500 C the compression resistance decreases as the temperature continues to increase to 800°C.He found that when the substitution ratios are equal to 30%, the compressive force is less than the compressive force of ordinary concrete, but when the RCA replacement ratios are more than 50%, Recycled concrete has greater compressive strength than regular concrete.

Elastic modulus
Concrete's modulus of elasticity is resistance to deformation or measure of its hardness.It is commonly used in concrete structure analysis.The variables influencing the elasticity modulus are the ratio of water to cement, the age, type, and gradation of aggregates, and moisture content and substitution ratios.The elasticity modulus declines rapidly with increasing temperatures [14].Authors noticed that the concrete's modulus of elasticity rose when basalt shards or riverbed pebbles were added to it while it was still at room temperature.Figure 4 shows the effect of the percent of coarse RCA substitution on the relative modulus of elasticity after heat exposure.The figure illustrates that following high-temperature exposure, the coarse RCA concentration had no influence on the relative modulus of elasticity.Figure 4 depicts the average relative moduli of elasticity of coarse RCA concretes following high-temperature exposures.Following exposure to 200, 400, 600, and 800, the average values were approximately 81, 34, 15, and 3% of the levels before exposure, respectively.

Tensile strength
Tensile strength for normal-strength concrete is typically about ten percent of compressive strength.This characteristic is frequently ignored.However, in extreme temperatures to which the concrete is subjected, it becomes critical, because cracks in the concrete caused by heat exposure are linked to tensile stresses [16].The tensile characteristics of concrete have been studied in the literature.are often determined by split tests of ordinary concrete, but tensile tests have not been reported in recycled concrete at elevated temperatures.Analyses regarding Tensile strength should be used with caution as there is little accessible data and deterioration in tensile force is more important than the compressive force due to the high tensile strength sensitivity of thermal cracks.Among the results reached by the researchers by collecting previous data is that the tensile force of concrete made of RCA exceeds that of ordinary concrete [17].
The influence of recycled coarse aggregate on the normal and high performance of concrete at increasing temperatures was investigated.The primary factors were the type of aggregate and the concrete strength.There were three types of coarse aggregate utilized.The first was silico-calcareous SCCA natural coarse aggregate, and the second was laboratory-made concrete LRCA.In the case of the third, IRCA, the samples were subjected to temperatures ranging from 20 °C to 750 °C.The tensile behavior was substantially linked to the contact strength.Except for IRC concretes, high-performance concretes had a less reduction in relative tensile strength with temperature as compared to normal strength concretes due to improved bonding at interfaces when water content declined as shown in figure 5.In comparison to compressive strength, the temperature development of tensile strength revealed significant variances between reference and recycled concretes.The increased number of interfaces in recycled concretes accelerated crack formation and lower tensile strength.Again, the IRC concretes performed the worst due to non-cementitious impurities that caused voids and flaws when melted or burnt.

Strain -stress relationship in compression
The concrete's mechanical reaction to an increasing compression load is expressed by the strain-stress relationship.To assess the flame resistance of concrete structures, the generated curves are frequently utilized as input for mathematical models.Typically, stress-strain curves enable the determination of peak strain, elasticity modulus, and compressive strength [18].In general, both the RAC and concretes created with NA have extremely comparable shapes and temperature-related variations [10], [16].Strain-stress curves flatten when the temperature rises, peak stresses decline, slopes flatten, and strains at peak stresses rise.By displaying the strain-stress curves produced by Yang et al. [18], figure 6 illustrates this behavior.Galess et al. [19] were among the initial studies to show a hot strain-stress curve for RCA concrete.With digital image correlation (DIC), strain qualities were assessed, Nevertheless, only the ascending branch was documented.The ascending branch's temperature sensitivity was comparable to that of regular concrete.According to this, with increasing temperatures, concrete with a greater replacement rate had a lower slope and a bigger peak strain.

Recycled materials in normal concrete
The amounts of recycled cement and recycled aggregates represent important elements on which the final strength of concrete depends, as coarse aggregates constitute the largest amount of concrete.

Brick aggregates
Lea [20] began to study fine and crushed recycled brick aggregates.The outcomes demonstrated that there was no decrease in the remaining strength of RBA concrete exposed to 650 °C, whereas a loss of strength observed for normal concrete when exposed to 400 °C.RBA concrete when exposed to high temperatures above 1000 °C.It loses 55% of its strength; unlike normal concrete, it loses 91% after exposure to approximately 800 degrees Celsius.Adding RBA to traditional concrete gives it an advantage after exposure to high temperatures.After two decades of preliminary research at RBA, Newman [21] for fire resistance, add broken bricks to concrete.RBA concrete loses around 6% of its compressive strength when crashed bricks are added, compared to 34% for flint concrete.Similarly to this, the force decreased by 22% and 77%, respectively, after being subjected to 600 °C.Further testing on columns and floor slabs were done under fire conditions.OPC will be replaced in part with finely ground Hamra brick in the building of [22].After exposure temperatures at 300°C and 600°C, paste that contain 10% and 20% fine RBA had greater compressive strength than ordinary putty, which was attributed to the pozzolanic characteristics of RBA.This was done to investigate the influence of cement putty on the behavior of fine and coarse aggregates.The residual force remained steady until 400 °C with the addition of fine RBA and increased at 600 °C after increasing the amount of soft RBA to 20% and 30%.

Concrete aggregates
When subjected to high temperatures, concrete undergoes internal structural changes that cause it to lose strength and bend more readily, thereby endangering the building's useful life.Zega and Maio [23] contrasted the outcomes of traditional concrete built with normal coarse aggregates and different water /cement ratios that were subjected to high temperatures with those obtained using recycled concrete that was prepared using 75% by volume of aggregate and had similar qualities (Crushed waste concrete that has been recycled).A 500 °C temperature was applied to the samples for a period of one to four hours, and then cooled to room temperature.The remaining capacities of the two kinds of concrete declined by 16% and 10%, respectively, after an hour of holding and by around 26% after four hours for both conventional and RCA concrete.Xiao and Zang [13] evaluated earlier studies on lightweight and normal concrete, high-performance and high-strength concrete, which were carried out at increased temperatures.The studies demonstrated that different varieties of concrete performed in various ways at high temperatures.Based on this finding, RCA concrete was studied using destroyed concrete, with replacement rates ranging from 0 to 100% by weight.Even though spalling was not documented for the control samples, the RCA concrete did not exhibit spalling up to the maximum test temperature of 800 °C, suggesting that it had strong spalling resistance.After curing, concrete with 50% or more RCA had a residual compressive strength that was higher than that of normal concrete and specimens with 30% RCA.

Glass aggregates
The use of several industrial byproducts in the building sector is currently highly established since it contributes to sustainability improvement in two ways.First, recycling things that would otherwise pollute the environment and take up valuable land.Due to relatively little digging, it reduces the deterioration of the ecosystem and the soil.Recycling is a method that is widely used nowadays since it helps preserve the resources of the earth.Terro [24] studied the effect of replacing sand and gravel with crushed glass on the hardening and fresh characteristics of Portland cement concrete at elevated temperatures and at room temperature was studied.The replacement percentages ranged from 0-100 of aggregates with fine glass FWG, fine and coarse glass FCWG, and coarse waste glass CWG.The test results showed that concrete made of RG reduces its strength by up to 20 percent of its original value with rising temperatures of 700 C. General concrete made of RG using FWG showed better strength than that made using CWG and FCWG at elevated and ambient temperatures.

Temperature and fire's Effect on columns
Design criteria for determining the resistance of concrete columns exposed to fire and high temperatures are unsafe due to the complexity of failure resulting from the fragmentation of concrete, which generated a large discrepancy in determining the resistance of columns exposed to fire.RC Columns' fire resistance is impacted by a number of factors such as physical, mechanical, and structural l factors, they are important factors and play a key role in the resistance of concrete columns to fire.According to the calculation methodologies, structural behavior at high temperatures is often extrapolated from the material behavior range.The Euro-Code mentions the 500° Isotherm approach and the Zone method as two calculation techniques that may be used to estimate the resistance to bending moments and axial forces.The 500°-Isotherm approach is predicated on the idea that concrete strength exceeding 500° should be disregarded when determining load-bearing strength.Both of them are then utilized to assess the RC Column's fire resistance.The effects of eccentricity and slenderness are included in the American Concrete Institute's (ACI) Method.Also, it should be noted that the "Fire Design" of a compartment is used to assess the fire resistance of the RC column in actual conditions.Also, a big part of this behavior is influenced by the style of architecture and compartments.Yet, many building and design regulations for RC columns are based on typical and unique circumstances.The fire resistance of RC Columns is significantly influenced by a number of factors.

Aggregate
The type of aggregate has an important effect on the resistance of concrete columns to fire and high temperatures.Where the researchers mentioned [25] that the carbon aggregates have greater fire resistance than the siliceous aggregates, it is likely that this is due to the endothermic reactions and the high specific heat of the carbonic aggregates compared to the siliceous aggregates, and thermal conductivity also plays a significant role, as crystalline materials are thought to have higher conductivity than non-crystallized materials.Also, siliceous aggregate leads to increased fragmentation, and cracks are more visible because concrete has a high density, low permeability, and high moisture content [26].

Relative humidity
When the moisture level of high-strength concrete columns exceeds 90%, they begin to split within 10-20 minutes of the firing process [27].The reason for this is that their permeability is lower than that of typical concrete, resulting in the building of pore pressure exceeding 100 C and then breaking [28].Spalling causes a decline in the strength of high-strength concrete columns due to excessive moisture content.

Fire exposure
Because of the floor arrangement, the concrete columns are sometimes exposed to fire or partial heating instead of the four sides.This causes asymmetric behavior in the columns, which may lead to increased structural weakening [29].Raut [30] investigated the behavior of concrete columns subjected to partial fire using the ISO-834 standard heat curve and determined that asymmetric heating reduced the remaining strength of the columns.Furthermore, when two neighboring faces are exposed to fire, the RC fire resistance is lower and greater when the two faces are opposite each other.

Concrete permeability
The permeability of concrete is an essential component that influences its fire resistance, and just as the water-to-cement ratio influences permeability, so does moisture content.Fragmentation is more common in concrete with lower permeability, fragmentation is occurring within 40 to 33 minutes [28].
In columns made of low-permeability, high-strength concrete, the pressure at 300 °C is 8 MPa, which is significantly more than the concrete's 5 MPa tensile force [27].

Concrete cover
At 540 C, the edges and corners of the concrete cover have a tendency to crack and spall, which causes the concrete core to be crushed.A concrete cover of at least 40 mm is required by several regulations for improved fire resistance.However, it has also been shown that thinner cover thickness results in fewer cracks, while higher cover thickness causes the cover to come off more quickly [31].

Longitudinal bar
Previous studies [32] have determined that increasing the quantity of longitudinal reinforcing steel in concrete columns boosts their resilience to fire or high temperatures.Furthermore, the aforesaid researcher recommends that the use of large-diameter reinforcing steel bars cancels out the good effect on resistance caused by raising the amount of longitudinal iron.As a result, it is advised to utilize smaller-diameter bars that are evenly dispersed across the column.

Conclusion
In formal, further study is needed in practically every area of this discipline, including the behavior of concrete in fire, where it is currently not well understood.In particular, it is unknown how much compressive strength remains in concrete made using recycled aggregates after being exposed to high temperatures.Testing at various temperatures is also required in order to accurately assess the performance of recycled concrete since conversion will be hampered by a lack of knowledge about these materials.Materials recycled into conventional concrete.The data and conclusions for the columns also reveal a clear variance in fire resistance, as spalling is a limiting element in determining the resistance and is mainly determined by the permeability because the lower the permeability, the higher the spalling strength.Aggregates, in addition to the longitudinal reinforcement, play an essential role in the resistance, which increases the resistance of the columns.
Results of previous studies conducted on normal concrete and recycled concrete under different levels of temperature showed that normal concrete and recycled concrete showed lower compressive strength than that left at 20°C, a fact more noticeable the longer the heat exposure time.Also, recycled concrete had similar behavior to regular concrete.Under higher temperatures, plain concrete showed lower compressive strength than that left at 20°C, a fact more noticeable the longer the heat exposure time.Also, recycled concrete had similar behavior to regular concrete.When the temperature was raised to about 500°C for an hour, for normal concrete with a weight/water ratio of 0.40, the compressive strength was about 22% lower than that of samples kept at 20°C.As for concrete with a w/c ratio of 0.55 or 0.70, the compressive strength decreases by 10.2% and 15.2%, respectively.The stress-strain curves of the recycled concrete revealed that it grew larger with the increase in temperature and that the failure stress increased with the increase in the w/c ratio, indicating that the recycled concrete is less ductile than the normal concrete.The residual behavior of the recycled concrete members after exposure to fire can be assessed using the stress-strain model.As for the columns, the normal and high-strength concrete columns showed similar brittleness under load tests only.In addition, the effect of low permeability on the fragmentation capacity of high-strength concrete can be counterbalanced or overcome by the effect of high cleavage tensile strength.The authors recommend future studies that can be taken into account by conducting further studies on the behavior of structural columns under fire intensity that is more closely simulated to reality and for longer periods of time with different cooling mechanisms, as well as verifying the thinness of the columns by changing the dimensions of the column and considering the effect that occurs at different lengths, in addition to checking the effect of reinforcing steel and its relationship to the appearance of cracks.As for the use of recycled concrete, it is recommended to conduct greater research on recycled materials with temperatures exceeding 650 C, and compare them with ordinary concrete.

Figure 2 .
Figure 2. (a) Standard fire curve in various countries, (b) ISO 834 standard fire curve.

Figure 4 .
Figure 4. Effect of replacement of the coarse RCA on the modulus of elasticity after exposure to elevated temperatures [15].