Analysis of the thermomechanical behavior of different concretes with vermiculite and submitted to elevated temperatures

Abstract

At high temperatures, concrete may exhibit adverse mechanical behavior that compromises the safety of structures. Thus, it is necessary to study and develop concrete that can obtain greater performance in situations of exposure to high temperatures. In this regard, the present study investigates the replacement of 25% of sand by expanded vermiculite in different types of concrete and its performance when subjected to room temperature and punctual temperatures, of little content in literature, such as 200°C, 400°C and 800°C. This study also innovates by proposing the evaluation of prototypes exposed to high temperatures exclusively according to the perspectives of Brazilian technical standards, without resorting to international guidelines. Thus, the consistencies of the mixtures were determined and analyzed, as well as physical and mechanical tests that were carried out with the produced concrete. Among the results found are: vermiculite reduces the workability of the concrete mixture; vermiculite is not recommended for high-performance concrete, as it can significantly reduce its mechanical properties; in general terms, concrete that has a design strength of 50 MPa benefits from replacing the fines aggregate with vermiculite. From 400°C of exposure, all of the concrete samples showed cracks. The spalling phenomenon was also noticed in some specimens. The developed concrete samples cannot be characterized as lightweight concrete according to Brazilian standards, but o can be recommended as structural concrete given that their apparent specific mass is greater than 2000 kg/m³.

Keywords:
concrete; light concrete; expanded vermiculite; vermiculite; thermomechanical properties; high temperature.

1. Introduction

Concrete is the second most used material in the world (Carvalho, Calderón-Morales, et al., 2023). Its application ranges from civil construction to urban infrastructure, where about 97.52% of Brazilian bridges and viaducts are concrete (Moscoso, 2017). The wide use of this material is due to its ease of application, high cost-effectiveness, and durability compatible with design needs. Among the factors that influence the durability of concrete, there is: the water/cement ratio; compressive strength; cover thickness for the reinforcements; crack opening limit and resistance to fire and high temperatures (Medeiros, Andrade e Helene, 2011; Dyer, 2015; Bolina, Gil, et al., 2020).

At high temperatures, there occurs a disintegration of hydrated cement products, which causes the rupture of bonds in the microstructure of the cement paste and aggregates (Kodur, Garlock E Iwankiw, 2012; Bolina, Tutikian e Helene, 2019), and compromises the mechanical properties of concrete (Bolina, Tutikian e Helene, 2019; Dyer, 2015), generating loss of strength and displacement (Dwaikat e Kodur, 2010; Ergün, Kürklü, et al., 2013). The loss of strength occurs through new pores and microcracks resulting from the disintegration of cement paste products (Bolina, Tutikian e Helene, 2019). In this process, there is also a reduction in the transversal section of the concrete structure, which compromises its mechanical properties.

In recent Brazilian history, there have been major structural collapses of concrete buildings due to high temperatures caused by fires. On September 2, 2018, a fire occurred at the National Museum in Rio de Janeiro, whose building was composed of concrete and wood and had its structure destroyed, along with 20 million national historical items (Passarinho, 2018) (Souza, 2021). Another tragedy occurred in 2018 in a building with 24 floors in the city of São Paulo that was set on fire and temperatures reached 700°C, leading to the collapse of the entire building and causing the death of Brazilian citizens (G1, 2019).

Given the mechanical behavior presented by conventional concrete at high temperatures, the performance of different types of concrete was studied, including lightweight concrete. This concrete is porous and has low thermal conductivity, which makes it less susceptible to damage at high temperatures compared to conventional concrete (Morales, Campos e Faganello, 2011). The composition of lightweight concrete distinguishes it from conventional concrete by the adopted aggregates, which are lighter than crushed stone (Bogas, Nogueira e Almeida, 2014; Huang, Yu, et al., 2019), or by the addition of pozzolanic materials, which are expansive and have lower density (Bogas, Nogueira e Almeida, 2014).

In this regard, the incorporation of expanded vermiculite can be a viable alternative for the production of lightweight concrete that is also resistant to high temperatures. Expanded vermiculite is a lightweight, hydrated phyllosilicate mineral with excellent fire resistance, low thermal conductivity (0.04-0.14 W/m.K), low density (80-120 kg/m³), chemically inert and non-toxic (Rashad, 2016). This material, however, is not widely used in the construction industry, especially in Brazil.

So far, vermiculite has been used for the production of fire-resistant vertical sealing panels, containing glass, mica, and gypsum in its composition (Martias, Joliff e Favotto, 2014); for the development of insulating mortars and to evaluate their physical, chemical, and mechanical properties at room temperature (Cintra, Paiva e Baldo, 2014; Passos e Carasek, 2018) and when subjected to high temperatures (Mo, Lee, et all., 2018; Benli, Karatas e Toprak, 2020; Koksal, Nazli, et al., 2021). For mortars with vermiculite at high temperatures, a lower rate of decrease in strength was noted when compared to the reference mixture (Benli, Karatas e Toprak, 2020; Koksal, Nazli, et al., 2021). In the literature studied, only 1 study was detected that uses vermiculite as a fines aggregate to replace, in 10%, 15%, and 20%, sand in the production of lightweight concrete that is subjected to 300°C, 600°C and 900° C (Liu, Zhuge, et al., 2022). Meanwhile, this study aims to determine and analyze the thermomechanical characteristics of different types of concrete, at room temperature, at 200°C, 400°C, and 800°C, with 25% replacement of sand by vermiculite from the perspective of the guidelines in Brazilian regulations. Thus, this study breaks new ground by analyzing the thermomechanical properties of concrete with different design strengths (concrete typification), at specific temperatures not previously explored and with a replacement level of fines aggregate not yet investigated.

2. Materials and characterization procedures

2.1 Materials

Natural sand from Rio Preto in the State of Minas Gerais (MG) and crushed limestone sand extracted from the Pedra Sul Quarry in Juiz de Fora/MG were adopted as fine aggregates. The coarse aggregates used were basaltic gravel 0, from the Quibrita quarry in Campinas in the State of São Paulo (SP), and gravel 1 from the Pedra Sul quarry in Juiz de Fora/MG. The granulometry of fine and coarse aggregates, as per NBR NM 248/2003 (ABNT, 2003), can be checked in Figure 1 and Figure 2.

Figure 1
Granulometric curve of fine aggregates.

Figure 2
Granulometric curve of coarse aggregates.

Figure 3 informs the actual unit-specific mass and the powdery content of each aggregate, according to the guidelines of NBR 52/2009 (ABNT, 2009), NBR 53/2009 (ABNT, 2009), NBR NM 45/2006 (ABNT, 2006) and NBR NM 46/2003 (ABNT, 2003).

Figure 3
Real specific mass and puoverulent material of aggregates.

The cement was chosen according to its purity level, so the Portland CPV ARI Plus (high initial resistance) type was used, with a minimum compressive strength of 51 MPa at 28 days of age. The adopted industrial expanded vermiculite was of the Standard Izo-Flok type, in the granulated solid state, with a density of 80 to 130 kg/m³ and melting point at 1350°C. Silica fume with an average diameter of 0.1 μm was used for the high-performance concrete mixes. To increase the workability of the mixtures, the plasticizer and superplasticizer additives of normal setting time were adopted. Finally, the water used came from the public supply network.

2.2 Concrete production

The preparation of the concrete mixtures complied with NBR 8953/2015 (ABNT, 2015), which determines that lightweight concrete must have a real specific mass of less than 2000 kg/m³. The replacement of sand by vermiculite was 25% of the reference concrete mixtures. The choice to replace sand with vermiculite was due to an environmental appeal. Currently, the extraction of this resource occurs in an exacerbated way and causes the suppression of the riparian forest, instability of the slopes on the banks of riverbeds, and silting up of watercourses (Carvalho, Júnior, et al., 2023).

The adopted mixtures, by mass, are detailed in Table 1, and their nomenclature relates to the design resistance for room temperature, that is, 35, 50, and 70 MPa. The reference concrete is presented by the acronym RMXX, where RM indicates “reference mixture” and XX is the design strength, in MPa. Concrete with vermiculite is identified by VMXX, with “VM” representing the presence of vermiculite in the mixture.

Table 1
Concrete mixtures.

Mixture	Cement	Artificial Sand	Natural Sand	Vermiculite	Gravel 0	Gravel 1	Water/ Cement Ratio	Plasticizer additive/ Superplasticizer (%)	Silica Fume (%)
RM35	1	0.644	1.933	0	0.914*	2.133	0.52	0.50 / 0.15	0
VM35	1	0.644	1.450	0.083	0.914*	2.133	0.52	0.50 / 0.15	0
RM50	1	0.436	1.307	0.000	0.701*	1.635	0.38	0.50 / 0.25	0
VM50	1	0.436	0.980	0.056	0.701*	1.635	0.38	0.50 / 0.25	0
RM70	1	0	1.175	0.000	1.819**	0	0.40	0.50/0.25	12
VM70	1	0	0.881	0.050	1.819**	0	0.40	0.50/0.25	12

• ^*
  Gravel 0 of calcareous origin
• ^**
  Gravel 0 of basaltic origin

In Brazil, there is no standardized methodology for mixing concrete components, but literature indicates that the order of materials influences the characteristics of the final product (Schaefer, Wang, et al., 2006). Thus, the materials were placed in a stationary concrete mixer at intervals in the following order: coarse aggregates, cement, part of the water, fine aggregates, vermiculite (when used), and the remaining volume of water.

The cylindrical specimens were molded according to NBR 5738/2015 (ABNT, 2015). To ensure the statistical reliability of the results of the destructive tests, 52 concrete specimens were molded for each concrete mixture, in a total of 312 specimens, according to the distribution in Table 2. Then, the specimens were placed in a humid chamber for 28 days until the characterization tests of the concrete in the hardened state (ABNT, 2015), as shown in Figure 4.

Table 2
Distribution of specimens by test and temperature for each concrete mixture.

Test	Temperatures (°C)
	RT	200	400	800
Compressive Strength	4	4	4	4
Tensile Strength	6	4	4	2
Modulus of elasticity	5	5	5	5
Total per Concrete Mixture	52

Figure 4
Specimens test of type VM35 in the humid chamber.

2.3 Concrete characterization

Figure 5 presents a flowchart of the methodological processes adopted to characterize the concrete produced.

Figure 5
Presents a flowchart of the methodological processes adopted to characterize the concrete produced.

The characterization starts with the concrete in the fresh state, through tactile-visual analysis of the mixture. Fresh concrete must have a shiny appearance and its particles must be fully coated with cement paste (Batezini, 2013). The tactile-visual analyses are procedures to analyze the disaggregation of the material when manually compressing a sample of the mixture, in a format close to spherical, in which the particles must remain agglomerated.

Then, the consistency of the material is checked according to the guidelines of NBR 16889/2020 (ABNT, 2020). For this, the slump test was carried out in batches, requiring two batches to determine the slump and consistency of each type of concrete. The consistency is related to the workability of the mixture (Metha e Monteiro, 2014). The low workability of the material implies the difficulty of launching and compacting this mixture, which reduces its mechanical properties and its durability in the solid state (Metha e Monteiro, 2014).

After 28 days of molding the specimens, the concrete characterization tests were carried out in the hardened state, starting with the water absorption test according to NBR 9778/2009 (ABNT, 2009). The value of the result, expressed in %, presents the mass of water in the specimen in relation to the value of the dry mass. The specific mass was determined through NBR 9778/2009 and has the result expressed in g/cm³.

To meet the proposed objectives, thermomechanical tests were performed with the specimens. For this, the specimens were subjected to a controlled increase in temperature in a muffle, with a heating rate of 2°C per minute. The fast-heating rate (greater than 1°C per minute) was chosen to simulate a fire situation and evaluate the behavior of the developed material. In concrete with a lower water/cement ratio, the development of cracks is greater due to the low permeability of the material, which limits water vapor from escaping through the pores. After reaching room temperature (RT °C), 200°C, 400°C and 800°C, the temperature was maintained for 4 hours before the muffle was turned off. These temperatures were adopted so that it was possible to observe differences in the behavior of the specimens at different levels to identify a temperature at which the specimen would deteriorate.

The specimens were cooled inside the muffle until it was safe to remove them, which took approximately 12 hours. Then, the specimens were kept in the laboratory to reach room temperature. Therefore, the results obtained at 200°C, 400°C, and 800°C should be considered as residual results.

To establish a comparison of the heated materials with the reference specimens, a visual analysis test of the deterioration of the material by temperature was carried out. The mass loss of the specimens was determined, and the ultrasonic pulse velocity test was performed, following NBR 8802/2019 (ABNT, 2019). The mechanical properties were measured by the axial compressive strength test according to NBR 5739/2018 (ABNT, 2018), followed by the tensile strength test by diametral compression according to the guidelines of NBR 7222/2011 (ABNT, 2011), and finally, the modulus of elasticity of the specimens was determined with the aid of NBR 8522/2021 (ABNT, 2021).

3. Results and discussions

The reference concretes produced had a glossy appearance and adequate consistency in the tactile test. Concrete with vermiculite replacement had an opaque color with a matte shine and more grainy consistency in the tactile test. Concrete containing vermiculite, when analyzed in conjunction with the data in Table 3, requires a higher amount of superplasticizer additive compared to reference concrete to achieve similar workability. Such behavior is consistent with literature on lightweight and structural concrete (Rossignolo, 2009). It can be seen in Table 3 that in the mixtures in which the design strength was 35 and 70, a greater amount of water was required.

Table 3
Slump of concrete and related data.

Mixture	Water (L)		Plasticizer additive (Kg)		Superplasticizer additive (Kg)		Slump (mm)
	Batch 1	Batch 2	Batch 1	Batch 2	Batch 1	Batch 2	Batch 1	Batch 2
RM35	8.00	8.000	0.000084	0.000084	0.000005	0.000005	165	140
VM35	8.75	8.750	0.000084	0.000084	0.000025	0.000025	140	105
RM50	8.46	8.460	0.000110	0.000110	0.000029	0.000029	195	205
VM50	8.46	8.460	0.000110	0.000110	0.000056	0.000056	190	200
RM70	10.80	10.80	0.000135	0.000135	0.000067	0.000067	100	140
VM70	11.20	11.20	0.000135	0.000135	0.000077	0.000077	115	105

Figure 6 presents the water absorption index and the specific mass of the concrete mixtures. Note that concrete of the same design strength has similar water absorption rates. Also, the addition of vermiculite influenced the actual specific mass of the design strength concrete of 35 and 70 MPa, which presented mean values of this property lower than those obtained by the reference concrete. None of the mixtures produced can be classified as lightweight concrete, according to NBR 8953/2015 (ABNT, 2015), which establishes the maximum actual specific mass of 2000 kg/m³ for concrete in this category. Although lightweight concrete is composed of expansive aggregates, the use of expanded vermiculite in this study only occurred as a partial replacement of fine aggregate, which is why concrete is not considered lightweight. However, this result makes it possible to characterize this concrete for structural purposes, since the Brazilian standard recommends that structural concrete has a real specific mass greater than 2000 kg/m³.

Figure 6
Real specific mass (Kg/m³) and water absorption index (%) of mixtures.

During the methodological stages of this study, none of the specimens showed color changes, even when exposed to temperature increases, which is in line with literature (Akca e Zihnioglu, 2013) (Hager, 2014). The visual analysis of the thermomechanical test made it possible to identify the occurrence of spalling in the specimens of the VM35 concrete, at a temperature of 478°C (Figure 7), and in the RM70 concrete, at 623°C (Figure 8), both still inside the muffle. Small cracks were identified on the surface of all specimens that were subjected to temperatures above 400°C, which is consistent with literature and is presented in Figure 9 (Hager, 2014). None of the specimens showed a change in color during the study, which is in line with what was expected by the studies of Akca (2013) and Hager (2014).

Figure 7
Deteriorated specimen test VM35 after heating in the muffle.

Figure 8
Fragments of the RM70 cylindrical specimen.

Figure 9
Cracks in the RM50 specimen after heating up to 400°C.

The thermal tensions caused by the increase in temperature make it possible to break the chemical bonds of HCS (Hydrated Calcium Silicate), releasing the dehydroxylated water that tends to evaporate and deteriorate the specimen (Bolina, Tutikian e Helene, 2019; Ferreira, 2011). Concrete that has a lower water/cement ratio is more susceptible to cracking due to its low permeability, which limits the exit of water vapor through the pores (Komonen e Penttala, 2003). Finally, it is necessary to consider the thermal expansion coefficient of the concrete components; the cement paste contracts with the increase in temperature while the vermiculite expands, resulting in internal stresses in the material that can cause cracks.

The results of the average mass loss of the concrete mixes are presented in Figure 10. It can be noted that the mass of the specimens decreases as the temperature rises, which can be caused by the exit of free and capillary water from the pores of the concrete, as well as by the dehydroxylation reactions that occur in the cement paste. In both cases, the concretes have an increase in the number of voids (pores).

Figure 10
Loss on fire data of concrete mixtures.

The residual velocity of the ultrasonic pulse can be seen in Figure 11. Notice that there is a tendency for velocity to drop with increasing exposure temperature. This phenomenon is consistent with the mass loss analysis (Figure 10), since the concrete becomes more porous, and therefore, the ultrasound propagation velocity decreases.

Figure 11
Ultrasonic wave propagation speed in concrete mixtures.

Concrete composed of vermiculite obtained a propagation velocity of ultrasound waves with higher values at RT and 200°C. After this temperature, the difference between the ultrasound velocity in the reference concrete and the concrete with vermiculite replacement is of little relevance close to the magnitude scale that these waves propagate. Finally, it is noted that the concrete with the highest design resistance (RM70 and VM70) presented lower ultrasonic pulse velocity than the other concrete at room temperature, but with the increase in temperature, it presented the highest residual values of the ultrasonic pulse velocity. The minimal reduction in the residual ultrasound velocity in the RM70 and VM70 concrete can be explained by the smaller number of pores present in the material and by the greater resistance to abrasion, characteristics that are a result of the use of basaltic aggregate in the composition of the concrete.

The residual compressive strength of the concretes is presented in Figure 12. The VM50 obtained greater resistance than the reference concrete, RM50, in all the studied temperatures. On the other hand, the VM70 concrete showed a compressive strength performance at room temperature of 11.36% lower than its reference concrete, RM70. In this context, when exclusively analyzing this property, it is noted that the addition of vermiculite in concrete may not be beneficial when the objective is to obtain high-performance concrete.

Figure 12
Compressive strength of concrete mixtures.

It can be seen in Figure 12 that concretes with design strengths of 35 and 50MPa have an increase in compressive strength at 200°C compared to room temperature. Concrete, when subjected to temperatures of 400°C and 800°C, shows a high decrease in compressive strength. This behavior is consistent with the results found in the mass loss and ultrasonic pulse velocity tests, since, as the material becomes more porous with increasing temperature, there are larger and more numerous zones of fragility where the body of the test may break. These fragile zones originate from the dehydroxylation of the hydrated phase of the cement paste, as evidenced in literature (Bodnárová, Válek, et al., 2013) (Sadrmomtazi, Gashti e Tahmouresi, 2020). Thus, regardless of vermiculite replacement, it can be considered that the temperature range between 400°C and 800°C is a critical range in which there will necessarily be considerable damage to the material.

Figure 13 shows the results of the residual tensile strength by diametral compression of the concrete, except for RM70 and VM70, as it broke at the beginning of the test, making it impossible to measure the tensile strength obtained by the adopted equipment. This phenomenon probably occurred because concrete is not a high-performance material in terms of tensile stresses, and no components were added to improve this property.

Figure 13
Tensile strength of concrete mixtures.

Except for RM70, which did not show a drop in residual strength at 200°C, the other tested concrete showed significant losses in tensile strength by diametral compression as the specimens were subjected to higher temperatures. For the design strength concretes of 35 MPa, no significant difference was noted in tensile strength between the concrete with vermiculite, VM35, and the reference concrete, RM35. Therefore, vermiculite did not exert any relevant influence on the tensile strength of concrete whose design strength is 35MPa.

The residual static modulus of elasticity is shown in Figure 14. It can be seen that this property decreases in all concrete considered as the exposure temperature increases. Concrete with a projected strength of 70 MPa follows the highest resistance, not resisting until the end of the test when exposed to 800°C, as predicted in literature for this class of concrete (Kakae, Miyamoto, et al., 2017).

Figure 14
Static modulus of elasticity of concrete mixtures.

The addition of vermiculite to concrete with a design strength of 35 MPa resulted in a less significant reduction in the value of the static modulus of elasticity between room temperature and 200°C, which differs from the behavior observed in the other studied concretes. As the temperature increases, this difference decreases and the performance of VM35 concrete is equal to other concrete, regardless of the type of projected resistance or the addition of vermiculite.

4. Conclusions

Concrete is widely used in many applications from civil construction to infrastructure works, such as bridges and viaducts. At high temperatures, the behavior of this material is modified due to the physicochemical reactions that occur and compromise its mechanical properties. Recently, Brazil has suffered from the loss of historic buildings and citizens due to structural collapse caused by fires. In this regard, the present study innovated by investigating different types of concrete, at different specific temperatures not explored until then (200°C, 400°C and 800°C), and with a replacement content of 25% of the fine aggregate by vermiculite; a content that has not been previously addressed in literature. The exclusive use of Brazilian technical norms is also an innovative element, as it allowed for effective testing, analysis, and discussion of the results, without the need to resort to international normative guidelines. In addition, it was unnecessary to carry out advanced microscopy and thermogravimetry tests, demonstrating the full appreciation and application of national standards.

Among the main results achieved are:

• Concrete with the addition of vermiculite has lower workability than the reference concrete;

• From 400°C of exposure on, all the studied concrete showed small cracks;

• Exposure to high temperatures did not influence the color change of concrete;

• According to Brazilian norms, concrete with the addition of 25% vermiculite cannot be considered lightweight concrete because its actual specific mass is greater than 2000 kg/m³. For the same reason, the Brazilian standard recognizes this type of concrete as susceptible to be applied for structural purposes;

• Increased exposure to high temperatures allows the evaporation of water from the interior of the material, as well as the dehydroxylation of the cement paste, resulting in porous concrete. This condition affects the specific mass, the velocity of the ultrasonic pulse, as well as the compressive and tensile strength of the concrete, regardless of the presence of vermiculite as a substitute for the fine aggregate.

• When analyzing the compressive strength in isolation, it can be observed that the incorporation of vermiculite is not recommended for high-performance concrete since, it can result in a significant reduction of this property.

• Only the high-performance concrete showed loss of compressive strength at 200°C;

• Temperatures close to 400°C represent a critical point for concretes with a design strength of 70 MPa, which results in a significant reduction in their mechanical properties. These concretes become brittle when exposed to 800°C and cannot be handled, easily collapsing.

• Among the different mixtures studied, concrete with a projected strength of 50 MPa and using vermiculite showed the best overall performance, standing out mainly in compressive and tensile strength tests, especially in room temperature conditions and at 200° C.

From these results, it is proved that the partial replacement of fine aggregate by vermiculite is efficient in different types of concrete, especially in an exposure condition of up to 400°C. The replacement of sand by vermiculite can have environmental benefits, and contribute to the sustainable use of natural resources, as it would reduce dependence on the application of natural sand. However, this hypothesis should be further investigated in future research for a more accurate analysis. Based on the analysis of the results in the available literature, it can be stated that the Brazilian technical standards are effective in evaluating the mechanical and physical properties of concrete incorporating additions and subjected to high temperatures. Finally, we highlight the desire that this research contributes to the production of buildings with better performance at high temperatures, aiming at increasing the protection and safety of users, as well as national and international historical heritage.

Acknowledgment

We acknowledge the UFOP (Universidade Federal de Ouro Preto) and the UFJF (Universidade Federal de Juiz de Fora) for the structure and supplies made available. We would like to express our gratitude to the institutions that offered financial support for this work: CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), through the doctoral scholarship awarded to Aldo Ribeiro de Carvalho under financial code 001; CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico); and PROPPI/UFOP (Pró-Reitoria de Pesquisa e Inovação da UFOP).

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Publication Dates

• Publication in this collection
  15 July 2024
• Date of issue
  2024

History

• Received
  04 July 2023
• Accepted
  28 Nov 2023