*3.3.3.3 Porosity*

*Compressive Strength of Concrete*

those with less super plasticizer dosage.

volume increase and damage [18, 19].

ture are schematized as shown in **Figure 5**.

*3.3.3.2 Influence of silica fume on the mechanical strength*

• The concretes with superplasticizer have high resistance. The role of the super plasticizer in the distribution of cement particles in improving the compactness of concrete is highlighted. The concrete mixes containing superplasticizer are affected by high temperatures especially at 600°C and above compared to

• The decrease of resistance estimated at 78% for HPC with 2% super plasticizer exposed to a temperature of 900°C, on the other hand, is zero for ordinary concrete. During these processes, some cracks occur, and concrete is crumbled and becomes a porous material [48]. Aggregate's effect on concrete at high temperatures is related to their mineral structures. This process results in

In this part of the study, we want to highlight the influence of silica fume on the mechanical behavior of HPC. After the passage in the oven, the concrete specimens were cooled for 24 h in the laboratory, at a temperature of 20 ± 5°C before submitting to compressive strength test. The specimens of HPC exposed at high tempera-

As expected, the replacement of cement by 5% of silica fume increased the compressive strength approximately 30% at 28 days. This is due to the reaction of the silica fume with calcium hydroxide formed during the hydration of cement that caused the formation of calcium silicate hydrate (C-S-H) as well as filler role of very fine particles of silica fume. In general, it can be concluded that concretes containing silica fume had significantly higher strength than that of CR concretes at room temperature. After exposure to 200°C, significant reductions occurred in the compressive strength of concretes without SF. Results showed the strength recovery of 18% for the concretes HPC2.5 after heating to 400°C when compared to 200°C. The compressive strength gains at 400°C are attributed to the increase in the forces between gel

particles (van der Waals forces) due to the removal of water content [48].

*Compressive strength of (HPC2.5) and (CR2.5) with temperatures [18].*

In the range of 400–600°C, severe strength losses occurred in two concretes, HPC2.5 and CR2.5. During exposure to high temperatures, cement paste contracts, whereas aggregates expand. Thus, the transition zone and bonding between aggregates and paste are weakened. After heating to 600°C, the compressive strengths of CR were lower than those of the concretes HPC2.5. This is attributed to the presence and amount of silica fume in concretes that produced very denser transition zone between aggregates and paste due to its ultrafine particles as filler and its pozzolanic reactions.

**116**

**Figure 5.**

The results of the porosity for the HPC after various heat treatments ranging from 20 to 900°C are shown in **Figure 6**.


Between 20 and 200°C, the porosity increases very little. The HPC shows a decrease of 0.26%. Kalifa explains that the decrease in the porosity of HPC is associated with the densification due to the complementary hydration of HSC.

**Figure 6.** *Porosity as a function of temperature [18].*

Between 200 and 400°C, the porosity increases by 4.5% for the HPC. This growth is associated with the discharge of water, whether present in the water network or chemically bonded. Kalifa explains that the decrease in porosity of CR between 200 and 300°C compared to HPC is associated with densification due to complementary hydration and carbonation of portlandite under internal autoclaving conditions, that is to say, under a pressure higher than atmospheric pressure. On the other hand, this densification is not observable in the HPC which contains very little portlandite, thanks to the presence of silica fume.

At 600°C, the porosity value has substantially increased by 6.1% compared with that of 20°C. For the concrete CR2.5 and is almost 6.33% for the HPC. The evaluation of the porosity at 900°C is practically impossible as the test tubes that have undergone severe damage and have disintegrated.

Concerning the porosity of cement paste at high temperature, Piasta has shown that the porosity increases in a parabolic manner according to the temperature [49]. This increase, also noted in other works by Bazant et al., is accompanied by an increase in the average pore size and total pore volume. This is due in part to the internal fracture of the C-S-H gel structure during the dehydration process [49].

### **3.4 Internal structure, ATG, and ATD of concrete**

Dehydration of the cement paste, thermal expansion and cracking, crystal processing, and mineral decomposition of aggregates are important reasons for the deterioration of concrete at high temperature.

Internal structure XRD patterns, ATG, and ATD of concrete subjected at high temperature are shown in **Figures 7** and **8**.

The analysis by X-ray diffraction is carried out in the physics laboratory of the University of M'sila by an X-ray diffractometer (X'Pert) coupled to a computer system. The essential purpose of this analysis is to identify the different crystalline phases present in a sample.

The analysis of the spectrum in **Figure 7** is used to report the following findings:

The diffractogram of a heated concrete (20°C) reveals the presence of portlandite, calcite, or α-quartz.

To a more severe heat treatment (400 and 600°C), all the peaks relative to the portlandite disappear (dehydroxylation between 400 and 600°C). This transformation was observed on ATD and ATG. Also, it does not detect the allotropic transformations of α-quartz in β-quartz between the diffractograms of 400 and 600°C since it is about a reversible transformation.

At higher temperatures (900°C), crystalline transformation of aggregates occurs, e.g., α- to β-quartz transformation in siliceous aggregates. Decarbonation of carbonates plays a prominent part if the concrete contains limestone aggregates.

**Figure 7**C shows that the peak intensity at the position that 2θ is 55°C increases. Therefore, it can be concluded that a new production was formed.

The results of the ATG and ATD powder of HPC at 20, 600, and 900°C are shown in **Figure 8**. A test sample of 200 mg of the concrete was analyzed according to linear heating from ambient temperature to 1400°C, with a speed of 10°C/min.

Six endothermic peaks were observed: 110–130, 180, 400, 450–550, 573, and 800°C. These thermal flux peaks are essentially related to the phase exchange temperatures of the different hydrates of the cement paste.

At a double peak of 110 and 130°C, the free water starts evaporating rapidly. In the temperature range from 80 to 150°C, dehydration of ettringite takes place followed by the decomposition of gypsum between 150 and 170°C [50].

In contrast, a small endothermic peak is observed at a temperature of 180°C; this peak indicates the dehydration of the calcium monocarboaluminate hydrate [51].

**119**

pastes [52].

**Figure 7.**

*Br, brucite) [18].*

brucite [Mg(OH)2].

limestone Ca(OH)2 CaO [54].

*Mechanical Behavior of High-Performance Concrete under Thermal Effect*

Between 200 and 300°C, a so-called water plug develops in concrete pores, and

*XRD patterns of powder of HPC at (A) 20°C, (B) 600°C, and (C) 900°C (notes: C, calcite; Q, quartz; and* 

These authors attribute this change of crystalline state or dehydration of a solid solution of Fe2O3. But other sources [53] attribute this peak to the decomposition of

Between 450 and 550°C, the peak corresponds to the decomposition of the free

At 573°C, the allotropic transformation of the quartz-α and quartz-β is accompanied by a phenomenon of expansion (cracking of the siliceous aggregates) [54]. Between 600 and 700°C, C-S-H decomposes and transforms into a new form of

there are slight variations in flux to the continuous dehydration of C-S-H [51]. At 400°C, a small peak was observed which we could not identify clearly the phase. A similar transformation was observed by Sha et al., on cement

*DOI: http://dx.doi.org/10.5772/intechopen.89916*

*Mechanical Behavior of High-Performance Concrete under Thermal Effect DOI: http://dx.doi.org/10.5772/intechopen.89916*

#### **Figure 7.**

*Compressive Strength of Concrete*

Between 200 and 400°C, the porosity increases by 4.5% for the HPC. This growth is associated with the discharge of water, whether present in the water network or chemically bonded. Kalifa explains that the decrease in porosity of CR between 200 and 300°C compared to HPC is associated with densification due to complementary hydration and carbonation of portlandite under internal autoclaving conditions, that is to say, under a pressure higher than atmospheric pressure. On the other hand, this densification is not observable in the HPC which contains very

At 600°C, the porosity value has substantially increased by 6.1% compared with that of 20°C. For the concrete CR2.5 and is almost 6.33% for the HPC. The evaluation of the porosity at 900°C is practically impossible as the test tubes that have under-

Concerning the porosity of cement paste at high temperature, Piasta has shown

that the porosity increases in a parabolic manner according to the temperature [49]. This increase, also noted in other works by Bazant et al., is accompanied by an increase in the average pore size and total pore volume. This is due in part to the internal fracture of the C-S-H gel structure during the dehydration process [49].

Dehydration of the cement paste, thermal expansion and cracking, crystal processing, and mineral decomposition of aggregates are important reasons for the

Internal structure XRD patterns, ATG, and ATD of concrete subjected at high

The analysis by X-ray diffraction is carried out in the physics laboratory of the University of M'sila by an X-ray diffractometer (X'Pert) coupled to a computer system. The essential purpose of this analysis is to identify the different crystalline

The analysis of the spectrum in **Figure 7** is used to report the following findings: The diffractogram of a heated concrete (20°C) reveals the presence of portland-

To a more severe heat treatment (400 and 600°C), all the peaks relative to the portlandite disappear (dehydroxylation between 400 and 600°C). This transformation was observed on ATD and ATG. Also, it does not detect the allotropic transformations of α-quartz in β-quartz between the diffractograms of 400 and 600°C since

At higher temperatures (900°C), crystalline transformation of aggregates occurs, e.g., α- to β-quartz transformation in siliceous aggregates. Decarbonation of carbonates plays a prominent part if the concrete contains limestone aggregates. **Figure 7**C shows that the peak intensity at the position that 2θ is 55°C increases.

The results of the ATG and ATD powder of HPC at 20, 600, and 900°C are shown in **Figure 8**. A test sample of 200 mg of the concrete was analyzed according to linear heating from ambient temperature to 1400°C, with a speed of 10°C/min. Six endothermic peaks were observed: 110–130, 180, 400, 450–550, 573, and 800°C. These thermal flux peaks are essentially related to the phase exchange

At a double peak of 110 and 130°C, the free water starts evaporating rapidly. In the temperature range from 80 to 150°C, dehydration of ettringite takes place

In contrast, a small endothermic peak is observed at a temperature of 180°C; this peak indicates the dehydration of the calcium monocarboaluminate hydrate [51].

followed by the decomposition of gypsum between 150 and 170°C [50].

Therefore, it can be concluded that a new production was formed.

temperatures of the different hydrates of the cement paste.

little portlandite, thanks to the presence of silica fume.

**3.4 Internal structure, ATG, and ATD of concrete**

deterioration of concrete at high temperature.

temperature are shown in **Figures 7** and **8**.

it is about a reversible transformation.

phases present in a sample.

ite, calcite, or α-quartz.

gone severe damage and have disintegrated.

**118**

*XRD patterns of powder of HPC at (A) 20°C, (B) 600°C, and (C) 900°C (notes: C, calcite; Q, quartz; and Br, brucite) [18].*

Between 200 and 300°C, a so-called water plug develops in concrete pores, and there are slight variations in flux to the continuous dehydration of C-S-H [51].

At 400°C, a small peak was observed which we could not identify clearly the phase. A similar transformation was observed by Sha et al., on cement pastes [52].

These authors attribute this change of crystalline state or dehydration of a solid solution of Fe2O3. But other sources [53] attribute this peak to the decomposition of brucite [Mg(OH)2].

Between 450 and 550°C, the peak corresponds to the decomposition of the free limestone Ca(OH)2 CaO [54].

At 573°C, the allotropic transformation of the quartz-α and quartz-β is accompanied by a phenomenon of expansion (cracking of the siliceous aggregates) [54]. Between 600 and 700°C, C-S-H decomposes and transforms into a new form of

**Figure 8.** *ATG and ATD powder of concrete HPC at (a) 20°C, (b) 600°C, and (c) 900°C [18].*

hydrates less rich in water and donation without it being formed of anhydrous compounds. These are mainly di-calcium silicates (β-C2S) and β-wollastonite (β-CS) [53].

The last peak coincides with the temperature of 800°C. It is well defined that in the temperature range from 700–900°C, the limestone decomposes, so this peak indicates the decomposition of calcium carbonates (CaCO3), also known as "calcite," by releasing lime accompanied by a release of CO2 [5] according to the highly endothermic reaction which is as follows:

$$\text{CaCO}\_3\text{ CaO} + \text{CO}\_2.$$

A quasi-linear decrease is observed up to 800°C, and the concrete exhibits a severe decrease in the density above 800°C. This decrease in density is related to two phenomena, i.e., complete dehydration and anhydrous formation, which take place only at temperatures in the region of 900°C.

**121**

*Mechanical Behavior of High-Performance Concrete under Thermal Effect*

From the study, one can conclude that, when using HPC specimen, the speed of

• Beyond 600°C the concrete may lose the majority of these properties, i.e., there are properties that can cancel out; one can say that the concrete has become

• The concretes with super plasticizer are affected by high temperatures especially at 600°C and above compared with less super plasticizer dosage.

• For a more resistant concrete, the addition of silica fume leads to lower resistance (24%) in the temperature range tested, between 400 and 600°C.

• Color changes were observed on concrete under the effect of high temperature.

• The HPC specimens containing silica fume have high compressive stress

\* and Nadia Tebbal<sup>2</sup>

2 Geomaterials Development Laboratory, Institute of Technical Urban

\*Address all correspondence to: zineelabidine.rahmouni@univ-msila.dz

1 Geomaterials Development Laboratory, Civil Engineering Department, Faculty of

© 2020 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,

compared to HPC specimen without silica fume (CR).

• The critical temperature, which causes maximum attenuation of different properties (compressive strength, mass loss), is between 400 and 600°C.

temperature rising influences the drop in strength between 400 and 600°C.

*DOI: http://dx.doi.org/10.5772/intechopen.89916*

**4. Conclusions**

weak.

**Author details**

Zine El Abidine Rahmouni1

Technology, M'sila University, M'sila, Algeria

provided the original work is properly cited.

Management, University of M'sila, M'sila, Algeria

*Mechanical Behavior of High-Performance Concrete under Thermal Effect DOI: http://dx.doi.org/10.5772/intechopen.89916*
