Examination of Mortars Exposed to High Temperatures Containing Different Types of Aggregates

*Kenan Toklu, Osman Şimşek and Can Demirel* 

#### **Abstract**

Although the concrete containing Portland cement is resistant to high temperatures, its structure shows some physical and chemical changes due to fire or other temperature increases caused by different reasons. The effect of high temperatures on the concrete should be firstly examined for the repair of fire-damaged concrete. This study was carried out to investigate microstructure and strength changes in concretes exposed to high temperatures. For this purpose, mortars containing normal Portland cement and two types of aggregates were prepared. Quartzite and pumice were used as aggregates in the mixture in this study. The prepared mortar samples for compressive strength tests were subjected to a temperature of 100, 200, 500, 700, and 850°C for 4 h, including the temperature rise period. The same region of the same sample was examined before being subjected to temperature and after being subjected to temperature for SEM analysis. It is an advantage of this study to show the effects of temperature on microstructure in the same region of the same sample.

**Keywords:** different types of aggregates, mortar, high temperature, compressive strength, SEM

#### **1. Introduction**

Concrete is known as a material that is more resistant to high temperature and fire effects compared to many building materials. Some characteristics of components of the concrete such as thermal expansion coefficient and thermal conductivity play an important role for resistance to high temperatures. The performance of concrete exposed to high temperatures depends mostly on changes of the mechanical and physical properties of concrete [1]. Portland cement pastes, different types of aggregates, and water in the concrete exhibit completely different behaviors against high temperatures. When the cement paste is heated, it first shows normal expansion; however, meanwhile, hydrates containing water lose the water in their structure and these cause shrinkage in the structure. Thus, the opposite interaction creates stresses in the concrete. One of the hydrates, calcium hydroxide, loses its water at 400–450°C and calcium oxide is increased. When calcium oxide is rehydrated by the moisture, calcium hydroxide is increased again; this can lead to crack formation by creating expansion in the hardened cement paste [2]. Also, calcium silicate hydrate gels are broken down partially at 400–600°C [3]. High temperatures around 600°C induce water loss in the pore structure, and temperature around 700°C causes the deterioration of C-S-H structure and degradation of hydration products [4].

 Different types of aggregates exhibit different behaviors at high temperatures. Quartz is a major component of sand, gravel, and some volcanic rocks, and expands regularly up to 573°C; but at the higher temperatures, α-quartz transforms to β-quartz and the result of this transformation causes a sudden increase in structure volume (2%) [5]. This expansion is thought to be responsible for the formation of severe cracks in the concrete containing quartz aggregates, especially at temperatures above 573°C [6]. As a result of the rapid developments in the construction sector in recent years, there have been rapid developments in the field of building materials. New building materials having technically superior parameters have been searched in the construction sector. The use of light natural aggregates in the construction area gained speed and some new regulations were made in our country. After these arrangements, lightweight aggregate masonry units have started to gain importance and the fire resistance of these structural elements is considered as an important parameter [7]. Pumice aggregate mortar keeps the same compressive strength up to 500°C [8, 9]. The pumice used in this study is highly porous, has a glassy structure, is natural, and is one of the lightweight aggregates.

When the concrete is considered as a whole, it is generally known that the thermal expansion of the components (aggregate and cement paste) of concrete is different for each other [10]. For this reason, temperature changes in concrete cause different volume changes in the components, crack formation, and decrease in the durability of concrete. This event is called as thermal discordance between hardened cement pastes and aggregates [11].

The purpose of this study can be summarized in the light of the literature described above as follows:


#### **2. Material and method**

#### **2.1 Aggregate**

In this study, two different well-known types of aggregates, which are quartzite and pumice in the concrete industry in Turkey, were used. The mineralogical composition of these aggregates is given in **Table 1**.


**Table 1.** 

*Mineralogical composition of aggregates.* 

*Examination of Mortars Exposed to High Temperatures Containing Different Types of Aggregates DOI: http://dx.doi.org/10.5772/intechopen.87836* 

#### **2.2 Cement**

CEM I 42.5N cement was used in this study. The chemical properties of the cement, whose specific weight is 3.14 g/cm3 , are given in **Table 2**.

#### **2.3 Mixing water**

In this study, tap water in Ankara was used as mixing water. The chemical properties of the mixing water [12] are given in **Table 3**.

#### **2.4 Method**

For compressive strength measurements, mortar samples which have 0.5 w/c ratio were prepared according to the mixing, molding, and curing methods given in TS EN 196-1 [13]. Each aggregate was broken and all passed through a 4-mm sieve. A total of 18 samples, which have the size of 50 × 50 × 50 mm, were prepared for compressive strength tests. Mortar samples were cured for 28 days at 20 ± 1°C temperature and 90% relative humidity (**Figure 1**).

 The compressive strengths of the samples cured at 20°C and during 28 days were determined and these strength values were taken as reference. Other samples were exposed to 200, 500, 700, and 850°C with a heating rate of 6°C/min for 4 h. This 4 h of heating also included the time reaching the desired temperature. After heating, the mortars were cooled slowly to the room temperature in the oven for 24 h.


#### **Table 2.**

*The chemical properties of the cement.* 


#### **Table 3.**

*The chemical properties of the mixing water.* 

 **Figure 1.**  *Testing apparatus for compressive strength test.* 

In order to observe the effect of high temperatures for microstructure investigations, the same regions before and after heating in the same mortar sample were examined at similar magnification values. Samples of 20 × 20 mm size by sectioning from the original mortar and then smoothed by abrading the surfaces for microstructure analysis were prepared. Samples after drying were examined as a reference in the scanning electron microscope. The same samples were exposed to a temperature of 200°C and slowly cooled in the oven for 24 h and then examined again under microscope. These processes were repeated to the same sample at temperatures of 500, 700, and 850°C.

The following investigations were carried out in the scanning microscope:


#### **3. Research findings and discussion**

The compressive strength of mortars exposed to different temperatures is given in **Table 4**. In order to better observe the strength changes after heat treatment, the results are expressed according to the strength percentage of the reference mortar for the same aggregate.

 As can be seen from **Table 4**, the compressive strength of the mortars was greatly influenced by the temperature increase regardless of the aggregate type. It was observed that the strength of the mortar containing quartzite decreased slightly at 100°C according to reference mortar containing quartzite. On the other hand, the strength of the mortar containing quartzite exposed to the temperature of 200°C increased slightly according to reference mortar containing quartzite. The strength drop between 50 and 120°C may be due to the weakening of the bonding forces due to the expansion of the water layers in the cement paste at these temperatures. At temperatures up to 200°C, the reason for the recovery of strength is the fact that Van der Waals forces become more effective as a result of the removal of water and the particles are strongly bonded with each other. However, as stated in the literature, a significant decrease in the strength of mortars containing quartzite aggregates occurred at temperatures higher than 200°C [14].

Mortars containing pumice aggregates in terms of strength did not show a similar trend with mortars containing quartzite. As can be seen from **Table 4**, the mortar containing pumice aggregates showed an increase in strength at both 100 and


*Examination of Mortars Exposed to High Temperatures Containing Different Types of Aggregates DOI: http://dx.doi.org/10.5772/intechopen.87836* 

#### **Table 4.**

*Compressive strength of mortars exposed to different temperatures.* 

200°C temperatures. This different situation may result from the different interface structure of the mortar containing pumice aggregates. As is known, pumice is a pozzolanic substance with quite a porous surface. This has resulted in improved mechanical locking in the form of chemical bonding with the cement paste at both the interface and the gaps. This difference in the interfacial structure caused the mortar containing pumice aggregates to exhibit an increased strength at 100°C temperature according to the mortar containing quartzite aggregates. In the case of weakening of the bonds due to swelling of water layers around 100°C temperature, it has been thought that the porous structure on the pumice reduces the applied stress (as the air-entrained concrete in the freeze-thaw cycle). The strength of the concrete depends on the strength of the aggregate and the adhesion forces of the aggregate-cement paste interface as well as the cohesion forces in the cement paste. It is generally thought that the decrease in the strength of concrete is caused by the crack development caused by different reasons. In the case of all mortars exposed to the temperature of 200°C, the similar strength increases in the mortars can be more easily understood by the microstructure examination results.

The following results were obtained by comparing the SEM of both the reference mortar samples and the same samples after exposed to 200°C temperature:


#### **4. Conclusion**

It was concluded that a significant decrease in the compressive strength of mortars containing quartzite aggregates occurred at temperatures higher than 200°C. Also, a significant decrease in the compressive strength of mortars containing pumice aggregates occurred at temperatures higher than 500°C. Lastly, SEM analyses of the samples at 200°C show that there are no changes in hydration products and significant crack formation.

#### **Author details**

Kenan Toklu1 \*, Osman Şimşek1 and Can Demirel<sup>2</sup>

1 Department of Civil Engineering, Gazi University, Ankara, Turkey

2 Department of Construction, Kirklareli University, Kirklareli, Turkey

\*Address all correspondence to: kenantoklu@gazi.edu.tr

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Examination of Mortars Exposed to High Temperatures Containing Different Types of Aggregates DOI: http://dx.doi.org/10.5772/intechopen.87836* 

#### **References**

[1] Topcu IB, Demir A. Effect of high temperature application time on mortar properties. In: 7. National Concrete Congress; 28-30 November 2007; Istanbul. 2007. pp. 455-463 (in Turkish)

[2] Husem M. The effects of high temperature on compressive and flexural strengths of ordinary and high-performance concrete. Fire Safety Journal. 2006;**41**(2):155-163

 [3] Lin WM, Lin TD, Powers-Couche LJ. Microstructures of fire-damaged concrete. ACI Materials Journal. 1996;**93**(3):199-205

[4] Heo Y, Lee G, Leeb G. Effect of elevated temperatures on chemical properties, microstructure and carbonation of cement paste. Journal of Ceramic Processing Research. 2016;**17**(6):648-652

[5] Akman NG. Investigation of Possibilities of Use of rice Husks Ash as Quartz and Effect on Original Form Application. Vol. 2017. Ankara: Hacettepe University, Institute of Fine Arts, Department of Ceramic, Master's Arts Study Report; in Turkish

[6] Poole AB, Sims I. Concrete Petrography: A Handbook of Investigative Techniques. 2nd ed. Vol. 816. Boca Raton, FL, USA: CRC Press; 2015

 [7] Ceylan H, Sapcı N. Fire resistance analysis of pumice aggregated Bims concretes and investigation of their effect on strength. SDU Journal of Technical Sciences. 2016;**6**(2):13-20 (in Turkish)

 [8] Turker P, Erdogdu K, Erdogan B. Influence of marble powder on microstructure and hydration of cements. In: Cement and Concrete World Journal. Turkey: TÇMB; 2002. pp. 38-89

[9] Degirmenci N, Yilmaz A. Use of pumice fine aggregate as an alternative to standard sand in production of lightweight cement mortar. Indian Journal of Engineering and Materials Science. 2011;**8**(1):61-68

[10] Smith GM. The effect of size and thermal expansion of aggregates on the durability of concrete [M.Sc. thesis]. Kansas: Kansas State College; 1951

[11] Shui ZH, Zhang R, Chen W, Xuan DX. Effects of mineral admixtures on the thermal expansion properties of hardened cement paste. Construction and Building Materials. 2010;**24**(9):1761-1767

[12] [Internet]. 1999. Available from: http://www.aski.gov.tr/TR/ SuAnalizSonuclari.aspx (in Turkish)

[13] TS EN 196-1. Methods of Testing Cement—Part 1: Determination of Strength: Turkish Standards Institution; 2016 (in Turkish)

[14] Castillo C, Durrani AJ. Effect of transient high temperature on highstrength concrete. ACI Materials Journal. 1990;**87**(1):47-53

Chapter 17

Abstract

sustainable material

1. Introduction

211

Tubular Props

Experimental Compression Tests

on the Stability of Structural Steel

Ibrahim Al-Jumaili, Samer Barakat and Zaid A. Al-Sadoon

The stability of structural steel props was investigated experimentally. Specimens were of steel tubular props with a total length of 4.5 m and had two hollow steel tubes with the ability to slide into each other. The effect of inserted length between the two steel tubes on the prop buckling capacity was investigated. Four full-scale specimens were tested using a 100-kN hydraulic actuator to determine the buckling strengths exhibited by the props. The steel specimens were of 2 mm and 3 mm thicknesses, while the inserted lengths were of 250 and 1000 mm. The effects of the different parameters on the buckling capacity and modes of failure were discussed. It was concluded that the buckling strengths were sensitive to the inserted length and wall thickness of the inner tube. The prop buckling capacity increased with the increase of the inserted length, and this increase is more pronounced if combined with the increase in the wall thickness. Such increase in the prop buckling capacity that leads to reduction in the required number of steel props and lateral supports for the same working area also contributes to the sustainable solutions in terms of reducing the material use, the construction time, and the costs

while taking into consideration the safety requirements.

Keywords: structural steel, prop, scaffold, stability, experimental research,

Nowadays, the design philosophies and trends focus on sustainability that is decreasing the resource use while reducing the impacts on human health and the environment during the building's lifecycle [1] and as meeting the human needs without compromising needs of the future generations [2]. Steel is considered more efficient in reducing the use of natural resources accompanied with less emissions and energy usage due to its high recyclability that results in less wastes and emissions and its durability that results in many sustainability favors. Many ways could be adopted to limit the environmental-related issues associated with the production of steel, starting from the design optimization, material recyclability, and reuse [3]. The construction methods being used in the construction industry contain shoring systems used to avoid slab deflection and buckling to limit deflections and slab failures. These systems consist of steel props which are evenly distributed under the slab whereby each slab shares the formwork weight as well as the weight of the dead

#### Chapter 17
