**3. Results**

### *3.1. Mechanical Performances*

Table 5 shows the mechanical properties of the UHP-FRCC used in the jackets. The compressive strength and the elastic modulus both do not change with age because of the acceleration of hydration due to the steam curing. Table 5 summarizes the results of the uniaxial compression tests on the jacketed specimens. In this table, the average values of the maximum load (Pmax)—and of the compressive strength (σmax) as well, the Young Modulus (*E*cm) and the Poisson's Ratio (ν) of the composite specimens are reported. *E*cm and ν were both calculated at one-third of the maximum load, according to the Japanese Standard JIS A 1149 [22], which complies with ISO 6784 [23]. In Table 6, the strength is also normalized with respect to the strength of the unconfined concrete cylinders (i.e., fc\_CORE). *E*cm increased with the thickness of the jacket due to the confined effect. In the same manner, ν decreased as the jacket thickness increased. The tests on the specimen that is shown in Figure 1 were performed until the failure, which is generally produced by the formation of a large tensile crack in the UHP-FRCC jacket (see the right edge photo of Figure 3). During the first stage of loading, the jacked was uncracked and a linear relationship between the stress (calculated by dividing the load by the cross-sectional area of the core concrete) and the strain can be observed (see Figure 4). When the first crack appeared in the jacket, the slope of this relationship drastically reduces in all of the specimens. This is due to the fact that multiple fine cracks, having a width lower than 0.1 mm, formed in the jacket. Despite the growing number of cracks, the stress continuously increased up to peak, where the tensile strains localised in a single crack of the jacket and the failure occurred.




**Table 6.** The average values of the parameters measured in the uniaxial compression tests.

**Figure 4.** Stress-strain curves of the jacketed cylinders: (**a**) behaviour of different UHP-FRCC jackets having a constant of thickness *t*<sup>i</sup> = 25 mm; (**b**) behaviour of the same UHP-FRCC jacket (FA20) having different thickness.

The composition of the UHP-FRCC binder considerably influences the mechanical performance of the column. In particular, the effect of the substitution strategy on the mechanical properties is evident in Figure 4a, which reports the stress-strain curves of the concrete cylinders that were confined with a jacket of 25 mm, but with 0%, 20%, 50%, and 70% of cement being replaced by fly ash. The results that were obtained in the case of the FA20 mixture (σmax = 59.06 MPa) are close to those that were achieved from the FA0 mixture (σmax = 61.30 MPa). This is the same tendency of the results of UHP-FRCC material itself, as shown in Table 5. In other words, the replacement of 20% in weight of cement content is paid with a loss of 4% of the maximum compressive strength. When considering the cylinder confined with a jacked of FA0 and *t*<sup>i</sup> = 25 mm as a reference, the decrement of the compressive strength is about 14% and 18% when 50% and 70% of cement is replaced by fly ash, respectively.

On the other hand, Figure 4b reports the average stress-strain curves of the cylinders that were reinforced with the same type of jacket (FA20), but of different thickness (i.e., 25, 50, 37.5, and 70 mm). In this case, the strength increases with the thickness of the reinforcing UHP-FRCC layer.

For each UHP-FRCC series, Figure 5a shows the relationship between the thickness of the jacket and the strength of the reinforced cylinders. In all of the cases, the following linear relationship can be used to predict the compressive strength:

$$
\sigma\_{\text{max}} = \mathbf{s} \cdot t\_i + f\_{\text{c\\_CORE}} \tag{1}
$$

where *s* = slope of the linear relationship. The coefficient *s* can be separately computed for each series, as shown in Figure 5b, and the values can be plotted as a function of the replacement rate of cement with fly ash (see Figure 6). The slope gradually reduces as the percentage of substitution of cement increases (*Csub*). Thus, the following linear correlation can be introduced:

$$s = -0.004 \cdot C\_{sub} + 0.584\tag{2}$$

where *s* is measured in MPa/mm.

**Figure 5.** Compressive strength vs. thickness of jacket in the four ultra high performance fiber reinforced concrete (UHP-FRC) series investigated herein: (**a**) results from the tests; and, (**b**) the trend lines of the experimental data.

**Figure 6.** Formula for predicting the slope of the linear approximation of Equation (1).

#### *3.2. Ecological Performances*

The parameter considered evaluating the ecological performances is the amount of CO2 related to the production of 1 m<sup>3</sup> of UHP-FRCC. The amount of CO2 emitted per unit volume of each mixture is calculated in accordance with the inventory analysis [24] while using the values that were provided by the Japan Concrete Institute (JCI) [25], which listed the main materials used in cementitious composites and the relative carbon footprint. Such values are reported in Table 7 in terms of kg of CO2 released in the atmosphere for the production of one-ton of material.


**Table 7.** CO2 emissions of UHP-FRCC components [25].

Figure 7 compares the environmental impacts of the various jackets, which were obtained by multiplying the values reported in Table 7 and the mass of materials used to cast the four series of UHP-FRCC. For the sake of completeness, Figure 7 also shows the values of some specimens that has not been tested. In this Figure, when the amount of cement replaced with fly ash is quite high (>50%), the environmental impact is considerably reduced with respect to FA0 (it is halved for FA70). Moreover, it can be noticed that a reduced environmental impact is attained by decreasing the thickness of the jacket and, in parallel, by increasing the percentage of fly ash in the mixture. Here, the CO2 emission due to the steam curing is not taken into account, because all of the series were subjected to the same procedures and, subsequently, a comparative analysis among all of the specimens is performed.

**Figure 7.** The ecological performances of the UHP-FRCC jackets used to reinforce concrete cores.

#### *3.3. Eco-Mechanical Analysis and Design Procedure*

By means of the approach that was proposed by Fantilli and Chiaia [19], ecological and mechanical analyses can be combined to define the best material. Compressive strength (σmax) is considered as the functional unit, herein called the mechanical index (MI), whereas the ecological impact is evaluated through the carbon footprint (ecological index—EI). The reference values MIinf (i.e., the minimum mechanical performance) and EIsup (i.e., the maximum impact) are those of the concrete cylinder reinforced with the UHP-FRCC jacket without the substitution of cement with fly ash (i.e., FA0) and with a thickness *t*<sup>i</sup> = 25 mm.

Figure 8 shows the non-dimensional chart that was used to perform the comparative analysis among the concrete specimens reinforced with the UHP-FRCC layer. In this diagram, the ratios MI/MIinf and EIsup/EI are the abscissa and the ordinate, respectively. In other words, the following formula are used to define the non-dimensional axes in Figure 8:

$$\frac{\text{MI}}{\text{MI}\_{\text{inf}}} = \frac{\sigma\_{\text{max}}}{\sigma\_{\text{max}} \text{ of } FA0 \text{ with } \mathfrak{t}\_{\text{i}} = 25 \text{ mm}} \tag{3}$$

$$\frac{\text{EI}\_{\text{sup}}}{\text{EI}} = \frac{\text{kg CO}\_2 \text{ of } FA0 \text{ with } t\_{\text{i}} = 25 \text{ mm}}{\text{kg CO}\_2} \tag{4}$$

Most of the experimental results fall in Zone 2, where the mechanical performances are increased at the expense of an environmental impact higher than that of FA0 with *t*<sup>i</sup> = 25 mm (for which MI/MIinf = EIsup/EI = 1).

**Figure 8.** Eco-Mechanical analysis of the UHP-FRCC jackets [19].

However, a group of experimental values falls within Zone 4, which shows ecological performances greater than those shown by the reference specimen, but lower mechanical performances. Although none of the test results fall within Zone 3, where the ecological and mechanical performances are both improved, an area of the possible best solutions can be defined in Figure 8. More precisely, the UHP-FRCC jackets with a thickness between 37.5–50 mm and made with FA70 series, and those of the FA50 series with thickness being included in the range 25–50 mm (see the dashed lines), might perform better than the reference cylinders (i.e., part of the dashed lines falls within Zone 3). The same is also valid for some thickness (within the range 25~37.5 mm) of the series FA20, as evidenced by the dashed line that is reported in Figure 8.

Nevertheless, Equations (1) and (2) can be used to relate the compressive stress, the thickness of the jacket, and the type of fiber-reinforced concrete to design the exact UHP-FRCC layer. In particular, the design procedure that is shown in Figure 9 can be introduced with the aim of increasing the strength of the UHP-FRCC jacketing system, and reducing the CO2 emissions as well.

**Figure 9.** The design procedure used to optimize the UHP-FRCC jacket of the concrete columns.

As a result, Figure 10 shows the curves that relate the CO2 emissions with the percentage of cement replaced by fly ash for three values of the compressive strength σmax (i.e., 55, 65, and 75 MPa). For each load carrying capacity, four thicknesses of the jackets have been obtained referring to the mixtures FA0, FA20, FA50, and FA70.

**Figure 10.** The ecological impact of UHP-FRCC jackets made with different mixtures (having different σmax).

If σmax ≤ 55 MPa is required, the CO2 emission does not change with the thickness or the percentage of cement replacement with fly ash. Conversely, as the required strength increases, the corresponding CO2 emission has a minimum in correspondence of a specific thickness. For instance, the optimal replacement of cement with fly ash to achieve a compressive stress of 75 MPa is 50% and the corresponding thickness is 67 mm. In the three curves that are shown in Figure 10, the best substation rate is at 50%, as obtained by Fantilli et al. [26] in the reinforced concrete beam. Indeed, for lower substitution rates, thickness reduces, but the CO2 emissions are higher.

When the substitution strategy is forced to higher percentages, the same mechanical performances can only be obtained through solutions with a high environmental impact, as the thickness of the jacket increases. In other words, the UHP-FRCC jacket with a low fly ash content shows mechanical performances that are not compensated by the environmental impact. In the same way, the jacketing system is so thick to produce a large environmental impact for large substitutions of cement.
