*3.9. Stress-Strain Relationships*

The response of the concrete against stresses is different for the four mixes; when the strength of the concrete is higher, the strains were smaller for the same load level. Stress-strain relationships for the concrete mixes were shown in Figure 5. Two cylinders were tested up to 70–90% of the failure load for each of the mixes. The results are showing that the higher strength concrete mixes, especially for HPSCC and HPVC mixes were going in a straight path up to 80% of the load, whereas the case is not similar for NSVC, as it was starting deviation from linearity in the earlier stage of about 40% of the applied load. In 30 MPa stress level, and when comparing other mixes with HPSCC, the strain was

higher by 26% and 100% for HSSCC and NSVC respectively, while the strain was less in HPVC by 12%.

**Figure 5.** Stress-strain relationships for the four concrete mixes.

#### *3.10. Modulus of Elasticity*

Determination of modulus of elasticity was based on the 40% of ultimate load and 0.000050 strain level. The obtained results were arranged in Table 7. When comparing the values in the table, it can be noted that the elastic modulus is not only the function of the compressive strength, since the composition of the mixtures plays a great role. The elastic moduli of the HPVC and HPSCC were close in value despite their different compressive strengths. The ratio of E/√*<sup>f</sup>* c was 4.49 and 4.27 for HPSCC and NSVC mixes respectively. The recorded ratio by [6] was 4.20 and 4.33 for an SCC and NSC, respectively. As it is known, the modulus of elasticity depends on the proportion of Young's modulus of the individual components and their ratio by volume; thus, the modulus of elasticity of concrete increases with a high content of aggregates of high rigidity, whereas it decreases with increasing hardened paste content, and increasing porosity [7]. On the other side, packing of the particles and optimization of the mix composition leads to a higher elastic modulus, even with fewer or no coarse particles as in the case of UHPC [16]. Research showed that the modulus of elasticity of SCC seems to be very similar to that of VC, with an important but similar scatter present on the results for both types of concrete [59]; other authors concluded that the reduction in the elastic modulus of SCC compared to VC is 5% for SCC with high compressive strength (100 MPa) and up to 40% for those with the lowest strength (20 MPa) [2]. Meanwhile, the modulus of elasticity of SCC specimens with SF is increased with SF content increase [25], while it sensitively decreases with an increase in the FA replacement ratio [33].

#### *3.11. Oven Heating Test*

Concrete is a composite material that derives properties from its multiphase and multi-scale ingredients. These ingredients are thermally inconsistent and during fire conditions, start to dissociate, leading to degradation in its strength and durability. Although, the behavior of HPSCC subjected to fire has not been extensively studied and thus remains largely unknown [60]. The heating of concrete may be advantageous or causing a reduction in strength. Research showed that concrete specimens exposed to 300 ◦C might have an increased compressive strength by 18–22%, especially in earlier ages [2,16]. This increase refers mainly to the acceleration of the hydration process at an increased temperature. The limit at which transfers the heating of specimens changes from a useful to destructive factor depends on many parameters; however, in general, temperatures higher than 400 ◦C are regarded as destructive. In this study, the temperature of the oven was 700 ◦C, to prevent the explosion of the specimens. Results the Table 8 show that the loss of compressive strength was 79%, 63%, 52%, and 49%, respectively for NSVC, HSSCC, HPSCC, and HPVC. Thus, when a NSC exposed to +700 ◦C for 2 h it can resist only one fifth of its designed

load, but HPC can still resist half of the load. The results of residual compressive strength were within the range of the database presented graphically in [47] which includes limits of codes and researchers' data. After initial heating to 400 ◦C in [5], the compressive strength decreased by 41–48% for an HSC containing 12.5% of silica fume. At 600 ◦C and 800 ◦C, the loss in strength was up to 44% and 79%, respectively. The strength loss at 400 ◦C was up to 18% whereas, at 600 ◦C and 800 ◦C, the strength loss was around 44% and 76%, respectively [19].


**Table 8.** Resistance of specimens exposed to high temperature, and specimens exposed to direct fire.

When concrete is exposed to a gradually increased temperature, the heat was transferred from the outer face of the concrete to its core, this process requiring less time for NSC and more time for HPC. The weak point of HSC classes is in that the heat was restricted by the dense microstructure, which leads to an explosion and spalling of concrete corners due to pore pressure, but the root cause of the failure was the cracking of concrete due to thermal tensile stresses, and the specimens with higher tensile strength can resist more pore pressure and spalling stresses. The relatively loose microstructure of NSC leads to absorption of heat to the concrete core and disintegrating its structure mainly due to the pore pressure build-up and the development of thermal stresses. Gravel particles can easily pull out and the burned paste is similar to dust, crushable with fingers, as shown in Figure 6, while in the case of HPC, the core of the cube is safer and the bond is still strong. The weight loss of 5–8% is recorded in this study, which is similar to that found by other authors. A mass loss of 4–6% was observed in [46] for 9 different mixes subjected to 1000 ◦C and last in the furnace for 90 min, and the mass loss of 2–9% has been reported in [19].

**Figure 6.** The cleaned core of broken cubes heated to 700 ◦C.

#### *3.12. Fire Resistance Test*

Subjecting of concrete specimens directly to the fire is different from oven heating, regarding the distribution of the heat around concrete faces. Fire test results on +500 ◦C for 45 min were shown in Table 8. The loss of compressive strength for NSVC was 34% and it was 22% for HPSCC. Reduction in the strength of the cubes under fire was less than onehalf when compared to the heating of the cubes in the oven; however, the main reason for

these smaller reductions was the lower level of heating. Generally, the temperature which makes an NSC have a poor strength is in the range of 600 ◦C. The strength degradation is primarily ascribed to decomposition of hydration products, such as, calcium silicate hydrates, calcium hydroxide, and carbonates. Le et al. reported that HPC loses up to 50% of its ambient temperature strength at 600 ◦C [46]. The pore pressure development in HPC samples is much faster than in SCC samples. The moisture content, the dense microstructure, and the tensile strength are the main influencing factors that determine the spalling of HPSCC. Research showed that the critical pore diameter of SCC is bigger than HPC; therefore, SCC will have larger damage once exposed to fire. When exposed to the fire of 200 ◦C for 18 min, the highest pore pressure at 10 mm depth of HPC was 2.52 MPa; while in SCC it was 1.27 MPa [46].

#### *3.13. Freezing and Thawing Cycles*

The test results of the freeze-thaw cycles were evaluated through the mass loss of concrete and the residual compressive strength, as presented in Table 9. HPCs showed negligible mass loss of 0.02%, while NSVC exhibited a drastic loss of 83%. HPSCC had lost 9.7% in the compressive strength, whereas, NSVC was almost damaged by losing 86% in compressive strength. HSSCC had lost 6% of its weight and 37% of its strength (the mass loss of this type of specimen was primarily in the top surface, which had less relative density).


**Table 9.** Loss in mass and compressive strength for 100 mm concrete cubes due to freeze-thaw cycles, Mass losses of concrete specimens the scaling test, and Wearing of the concrete surface due to freeze-thaw cycles.

> The changes in concrete surface and the corresponding number of freeze-thaw cycles were shown in Figure 7. Deterioration processes typically begin when; aggressive fluids penetrate through capillary pore structure to the reaction sites where they trigger chemical or physical deterioration mechanisms [61]. When the test was running, in the first 10 cycles, the NSVC corners were subjected to the internal tensile stress. Later when the frozen salty water was causing volumetric internal pressure on the concrete surface, gravel particles started appearing and then got pulled out. If the pores are critically saturated, water will begin to flow to make room for the increased ice volume. The concrete will rupture if the hydraulic pressure exceeds its tensile strength. The cumulative effect of successive freeze-thaw cycles is the disruption of paste and aggregate eventually causing deterioration of the concrete. HPCs were resisting pore pressure due to their high tensile strength. In this

test, loss in dimensions or lose of the concrete cover seems to be logically more acceptable when considering large structural members. The thickness loss was 25–30 mm in NSVC, 2–3 mm for HSSCC and the rest of the cubes almost had no thickness loss as in Figure 7. Similar deteriorations of NSVC and HSSCC concrete cubes have been observed in [30].

**Figure 7.** Freeze-thaw specimens after 25 and 50 cycles for NSVC and HSSCC and 50 cycles for HPSCC and HPVC.

Water absorption is a key parameter in the investigation of the durability of concrete. Because of its low w/b ratio of 0.20–0.45, it is widely believed that HPSCC should be highly resistant to both scaling and physical breakup due to freezing and thawing. Research showed that non-air-entrained HPC with w/b 0.22–0.31 could be extremely resistant to freeze-thaw damage and it was suggested that air-entrainment and supplementary cementitious materials are not needed. Among six mixtures tested; only the silica fume concrete with w/b 0.22 was frost resistant [8]. The weight change is an indication of the deterioration of the concrete specimen. Weight change of 0.3–5.3% recorded in [35] and 2.0–56.5% is recorded by [30] for 13 SCC mixes.

#### *3.14. Scaling Test*

The concrete specimens were exposed to 50 cycles of freezing and thawing. NSVC lose weight of 372.6 kg/m<sup>3</sup> and the aggregate particles appeared in early stages on the entire surface of the concrete; in HSSCC, initially, the first layer of concrete surface wore at earlier stages, but later, the degradation of the concrete surface was almost stopped or it was wearing very slowly so that the total weight loss after 50 cycles reached 27.2 kg/m3; both HPSCC and HPVC mixes were durable, showed no scaling, and a negligible loss of weight by having 0.387 and 0.286 kg/m3 respectively, as shown in Table 9. Mass loss of 0–0.5 kg/m3 after 50 cycles is recommended for HPC. Rating of specimens was performed as in ASTM C672 [55]. Scale rating of 0–1 after 50 cycles is recommended for HPC. The rating results for important checkpoints were shown in Table 10. The test is qualitative, and the rating was decided with a visual examination based on the surface of two specimens. For HSSCC, For HPCs, the top thin paste layer or was resisting the wearing and was not removed until the end of the test; only several small dark spots appeared as shown in

Figure 8. Rating of the concrete surface can be evaluated by loss of concrete mass and visibility of gravel particles, whereas the interesting parameter in a practical point of view is the thickness of the deteriorated concrete, therefore it is better to determine the loss in thickness of the concrete which exposed to freezing and thawing cycles. Table 9 is also showing the concrete depth lost by the action of thermal stresses.


**Table 10.** Rating of concrete specimens in scaling test ASTM C672.

**Figure 8.** Scaling resistance test of concrete surface exposed to de-icing salts after 50 cycles.

When comparing the scaling test results and the results of freezing and thawing test cubes, NSVC had lost 15.8% of its weight in the scaling test, but the loss was 82.6% in concrete cubes. This difference can be justified by the that the cubes were entirely submerged in water and attacked all sides, but the scaling pans were exposed to freezethaw cycles only at the top surface. Gagne et al. tested 27 mixes using silica fume with w/b of 0.23, 0.26, and 0.30, and a wide range of air–void systems. All specimens performed exceptionally well in salt scaling, confirming the durability of HPC. Also in [8] the weight loss of 0.1–4.5 kg/m2 is obtained after 40 cycles of scaling, for 3 concrete types with an air content of 2, 4, and 6% and w/c ratio of 0.25–0.50.
