1. Introduction
A self-compacting concrete (SCC) is a very fluid, homogeneous, and stable concrete; implemented without vibration; and givesthe structure a quality that is at least equivalent to that corresponding to conventional concrete that is implemented by vibration [
1]. SCCs are distinguished from OC mainly by their properties in the fresh state. The criteria characterizing an SCC are [
2]:
Slump flow test values must be between 60 cm and 75 cm;
The rate of passage to the L-box must be greater than 0.8;
The concrete must be stable under the effect of gravity (no segregation) and have a limited bleeding capacity. The absence of visual segregation during the Abrams test is not sufficient.
The constituents of SCCs can be quite different from those of OC. They can differ both in their proportions and in their choice. Given the mode of its implementation, the constituents used in the manufacture of SCCs, according to their use, are grouped into three categories: base materials (cement, aggregates, and mixing water), mineral additives, and chemical additives [
3]. Several studies have been carried out to analyze the behavior of concrete at high temperatures [
4,
5,
6,
7].
Ahsan et al. [
8] studied the effect of adding seashell powder (0, 10, 20, and 30%) in cementitious composites to improve their fire resistance. The prepared specimens were exposed to temperatures of 200 °C, 400 °C, 600 °C, and 800 °C. They noticed a decrease in cracks at the higher temperatures compared to the conventional concrete and an improvement of some characteristics such as compressive strength, elastic modulus, and compressive toughness.
In the experimental study that was carried out byAbolhasani et al. [
9], the microstructural, mechanical, and fracture features of calcium aluminate cement concrete (CACC) were assessed under the effect of exposure to high temperatures. These authors noted that the concrete became more ductile and the number of pores in the concrete structure increased. The residual fracture toughness and flexural strength decreased as the temperatures increased.
Abed et al. [
10] carried out a laboratory study toinvestigate the effect of elevated temperatures on both the residual compressive and flexural strengths of self-compacting high-performance concrete (SCHPC) that was produced by incorporating recycled coarse aggregate (RCA) as a partial replacement of natural aggregate (NA) and unprocessed waste powder materials as cement-replacing material (CRMs). The unprocessed waste powder materials that were used were waste fly ash (WFA) and waste perlite powder (WPP); the fire resistance of SCHPC that is produced by incorporating unprocessed waste materials has hardly been discussed in the literature.These authors observed that using an RCA of up to 50% enhanced the residual mechanical properties of SCHPC after exposure to the elevated temperature due to the strong aggregate–mortar contact zone and the similarity of thermal expansion between them. The fire resistance of SCHPC has been enhanced by replacing the cement up to 15% of WPP; meanwhile, WFA did not affect the fire resistance of SCHPC significantly.
JelčićRukavina et al. [
11] studied the effect of mineral additives on the compressive behavior of high-strength self-compacting concrete that was exposed to temperatures up to 600 °C. A total of ten different concrete compositions were tested, in which part of the cement (by weight) was replaced by three different mineral additives (5–15% metakaolin, 20–40% fly ash, and 5–15% limestone). The damage that was caused by the high temperatures was assessed using scanning electron microscope micrographs and the stress-strain curves, compressive strength, modulus of elasticity, and the strain at peak stress were evaluated from uniaxial compression tests. After 200 °C, the authors observed a decrease in the mechanical properties and an increase in the peak strain for all the mixes that were tested. The different mineral additives that were used in this study affected the variations of the residual compressive strength by 24% and the peak strain by 38%, while the variations of the residual modulus elasticity were 14%.
In the study by Abed et al. [
12], the residual density, compressive strength, flexural strength, and ultrasonic pulse velocity (UPV) of high-performance self-compacting concrete (HPSCC) were evaluated after exposure to elevated temperatures. A total of 21 HPSCC mixes were prepared by incorporating coarse recycled concrete aggregate (RA) and alternative waste materials (waste fly ash, perlite, and cellular concrete powders) as a partial replacement of coarse natural aggregate (NA) and cement, respectively. The mixes were conducted to check the correlation between the relative residual UPV and other properties of concrete after exposure to elevated temperatures ranging from 20 °C to 800 °C. The results of this study showed that the incorporation of RA as replacement of NA as well as alternative sustainable materials as cement replacing materials not only increased its sustainability but also improved its performance after exposure to elevated temperatures. It was also found that the relative residual UPV is correlated to the strength and density of HPSCC after fire exposure.
In the study of Sideris [
13], the compressive strength of four SCCs and four conventional concretes (CC) of different strengths was determined. A speed of 5 °C/min was adopted for heating for temperatures at 100, 300, 500, and 700 °C. The results showed that for the same resistance class, the residual compressive strength of CC was lower than that of SCC mixtures. The two concretes SCC and CC presented the same spalling behavior and depended only on the category of resistance.
Noumowé et al. [
14] examined the effect of adding of polypropylene fibers on the thermal stability of high performance SCCs during slow and fast heating. They noticed the instability of high performance SCCs during slow heating, while these same concretes to which they have incorporated polypropylene fibers did not present any disorder or instability.
Sideris [
13] observed explosive bursts for water/binder (W/B) ratios of 0.45 and 0.46 corresponding to high performance SCCs (75 and 55 MPa respectively) and high performance concretes (65 and 45 MPa and W/B of 0.43 and 0.46, respectively). In this study, the SCC presented a higher residual resistance than that of the OC and spalling was observed for all the concretes that were tested.
Janson et al. [
15] measured the thermal conductivity and temperature fields in three different types of concrete: OC (W/C = 0.70, Rc28 = 38.5MPa), HPC (W/C = 0.28, Rc28 = 114.2 MPa), and SCC (W/C = 0.38, Rc28 = 92.3 MPa) up to a temperature of 600 °C. They noticed that the thermal conductivity of OC was lower than that of the HPC and SCC. On the other hand, the HPC and the SCC have very close conductivities.
Persson [
16] studied the evolution of porosity in SCC mixed with glass or limestone that was subjected to the following temperatures: 105, 200, 400, and 600 °C. He found that the porosity of SCC with glass fillers was greater than the other concretes that were tested. Between 400 °C and 600 °C, Persson noted a significant increase in the porosity for all the concretes due to the transformation of quartzites and the departure of water from cement hydrates during the dihydroxylation of portlandite.
In the study by Liu et al. [
17], a comparison of the pore structure of SCC containing polypropylene fibers at high temperatures was performed. After heating to 500 °C, the authors observed a decrease in the porosity with the increase in fiber dosage due to the decomposition of hydrates and the increase in pores.
The impact of natural aggregates on concrete residual mechanical properties varies with their mineral composition. Tufail et al. [
18] noted that natural granitic aggregate concrete retained the highest residual compressive strength at all tests elevated temperatures up to 800 °C compared to the natural limestone and quartzite. Likewise, Khaliq [
19] compared the residual compressive strength of concrete that was made from natural crushed limestone and river gravel coarse aggregate during exposure to temperatures that rangedfrom 200–1200 °C. He observed a significant difference between the relative compressive strength in concrete with limestone (90%) and river gravel (50%) of unheated concrete strength after being exposed to temperatures up to 600 C.
In this study, the two types of concrete SCC and OC were tested at temperatures of 20, 150, 300, 450, and 600 °C. These values have been chosen according to the main transformations of the cementitious matrix that was observed by most researchers during the rise in temperature and which are [
20]:
- -
20–120 °C: departure from the open air and first signs of ettringite decomposition
- -
130–170 °C: double endothermic reaction during the decomposition of gypsum
- -
450–550 °C: decomposition of calcium hydroxide (CH) into free lime and water
- -
600–750 °C: decomposition of calcium carbonate C-S-H.
The novelty of the study is the comparison between the physical and mechanical properties of the two types of concrete, SCCs and OC, at the same value of compressive strength. Several tests were carried out on the concrete specimens and the results are presented as a function of the temperature for each of the formulations that was studied.
Given the large volume of paste that is present in self-compacting concretes due to the large amounts of fines, we seek to understand the behavior of SCC under the action of high temperatures. From a durability point of view and following a heat treatment, are SCC as efficient as OC for identical resistance. In addition, we want to know if the regulations that are established for OC are applicable for SCC regarding the risk of bursting during heating.
4. Conclusions
This paper presented the behavior of self-compacting concrete under the effect of high temperatures and their effects on the physical and mechanical properties. The following results can be deduced:
For temperatures between 150 and 300 °C, the relative compressive strength of SCC35 significantly increases.
At 600 °C, the relative compressive strength values are very low (lower than 5 MPa) due to a coupled degradation of the cementitious matrix and the beginning of disintegration of the aggregates.
The variation in the elastic modulus is identical for the different types of concrete between temperatures of 20 and 150 °C.
Up to a temperature of 300 °C, the SCC35 has an elastic modulus that is greater than 50% of the initial modulus.On the other hand, the elastic modulus of the OC35 is lower due to the appearance of micro-cracks on the specimens that were tested.
The residual flexural strength of the mixtures decreases progressively with the increase in temperature. During the tensile test, the rupture mechanisms of the specimens are different at 300 °C and 600 °C. Indeed, at 300 °C, concrete exhibits ductile behavior, while at 600 °C the concrete is rather brittle.
The thermal conductivity of concrete decreases with increasing temperature. This behavior is due to the degradation of the microstructure of the concrete and the appearance of micro-cracks which limit heat transfer.
UPV continuously decreases as the temperature increases due to changes in the structure of the concrete.
During heating, cylindrical specimens of dimensions 160 × 320 mm made of SCC35 present a greater risk of bursting than those that are made of OC35.
This study has made it possible to understand the behavior of SCC under the effect of high temperatures. Physico-chemical transformations have created significant repercussions on the physical properties, especially the appearance of cracks that are visible to the naked eye due to the decrease in the elastic modulus of the heated concrete.
This experimental study dealt with only a few mechanical and physical properties of mixtures of SCC under the effect of high temperatures. Further research can be done to study other properties such as water absorption and fire resistance. It is also important to determine the acoustic characteristics of the material to see the possibility of its use as a sound insulator.