5.3 Thermally induced damage in concrete

Thermal stresses are a further cause of damage formation in concrete. To analyze the efficiency of the SSM in monitoring evolution of damage due to heating, mortar samples of size 25 <sup>25</sup> 100 mm<sup>3</sup> were prepared in [8]. Ordinary Portland cement (CEM I 42.5N) was used with aggregates from alluvial sand, with wellrounded particles and dry density of 2659 kg m<sup>3</sup> . The amount of aggregates in the mortar was about 40% by volume, corresponding to a cement-to-aggregate ratio (c/a) by a mass fraction of 0.917. Water-to-cement ratio (w/c) was chosen as 0.3 by mass. Samples with larger and smaller grains were prepared.

Samples were heated in an oven at different temperature levels and ultrasonically monitored at each temperature level after having removed from oven and let cool down to room temperature. A linear analysis was applied measuring time of

#### Figure 6.

Relative variations of the nonlinear indicator (with respect to the uncorroded initial state) as a function of the corrosion step. Squares and circles correspond to data recorded by receivers 1 and 2, respectively.

Time Domain Analysis of Elastic Nonlinearity in Concrete Using Continuous Waves DOI: http://dx.doi.org/10.5772/intechopen.82621

flight, but it proved to be only slightly sensitive to the increase of damage and without any difference in the behavior of samples with small and large aggregates sizes.

Thus, in order to appreciate the different behaviors expected for the different samples, the SSM technique was applied as discussed previously, with ultrasonic sensors attached on the bases of the sample prisms. Results are reported in Figure 7, where the nonlinear parameter is shown vs. the output amplitude. It is possible to observe that damage starts earlier in the sample with large aggregates, as expected. Macrocracks are more rapidly formed and at the largest thermal excitation, the increase in nonlinearity is noticeable.

Furthermore, it is also possible to observe a change in slope (nonlinear indicator b) when macrocracks start appearing (large aggregates case only). This is similar to what observed for the case of quasi-static loadings. Indeed, in both quasi-static and thermal cases, the situation is similar: microcracking (first) and coalescence into macrocracks (later) is due to the presence of localized mechanical stresses (due to load or local gradients of thermal expansion), without the formation of any reaction products, as in the cases of corrosion and salt expansion (see next subsection).

#### 5.4 Degradation by expansive salts in masonry systems

The presence of soluble salts into capillary water is one of the major problems affecting masonry structures. Numerous works pointed out the potential noxiousness of water-transported salts (which might happen during repeated wet-dry cycles). Indeed, due to crystallization of some salts in the form of expansive compounds, progressive cracking and detachment phenomena happen especially at the interfaces between different material layers. In that sense, there is a clear need to develop an effective and reliable diagnosis of the onset of such damage phenomena in order to make corrective actions, in terms of repair and maintenance optimization, possible [49–53]. The close interaction between the ultrasonic wave and the material mechanical/elastic properties made ultrasonic methods widely used for

#### Figure 7.

Nonlinear parameter vs. output amplitude for samples with small and large aggregates at different levels of thermal damage.

At the time when linear indicators, namely ultrasonic velocity and attenuation,

nonlinear indicator extracted from data of receivers 1 and 2 was almost the same at the first two stages of corrosion. For the last two corrosion states, a difference can be noticed between both results, but the increase of sensitivity to corrosion is clearly

Thermal stresses are a further cause of damage formation in concrete. To analyze the efficiency of the SSM in monitoring evolution of damage due to heating, mortar samples of size 25 <sup>25</sup> 100 mm<sup>3</sup> were prepared in [8]. Ordinary Portland cement (CEM I 42.5N) was used with aggregates from alluvial sand, with well-

mortar was about 40% by volume, corresponding to a cement-to-aggregate ratio (c/a) by a mass fraction of 0.917. Water-to-cement ratio (w/c) was chosen as 0.3 by

Samples were heated in an oven at different temperature levels and ultrasonically monitored at each temperature level after having removed from oven and let cool down to room temperature. A linear analysis was applied measuring time of

Relative variations of the nonlinear indicator (with respect to the uncorroded initial state) as a function of the

corrosion step. Squares and circles correspond to data recorded by receivers 1 and 2, respectively.

. The amount of aggregates in the

visible for both sensors.

Acoustics of Materials

Figure 6.

150

5.3 Thermally induced damage in concrete

rounded particles and dry density of 2659 kg m<sup>3</sup>

mass. Samples with larger and smaller grains were prepared.

manifested a weak sensitivity to corrosion (the velocity regularly but slightly decreases and the attenuation increases), the nonlinear indicator showed an important evolution. Indeed, the relative variations corresponding to velocity and attenuation as a function of the corrosion step were determined as 4 and 70% at most, respectively. However, larger effects were observed for the relative variation of the nonlinear indicator by changing up to 350%. Here, it should be pointed out that when comparing the first and the last corrosion steps (instead of considering as reference the intact sample), the relative change of the velocity and attenuation were 1.5 and 30%, respectively, while the nonlinear indicator was 250%. This evolution shows the high sensitivity of nonlinear methods to the microstructure modifications due to corrosion. Finally, the nonlinear indicator remains sensitive to corrosion creation and evolution even when the access to the opposite side of the corroded concrete samples is not possible. Indeed, Figure 6 shows that the

damage characterization in civil engineering, in particular when dealing with cement-based materials. However, it has been remarked that ultrasonic pulse velocity, dispersion, and attenuation phenomena in concrete and multiphase materials in general are only considered in the linear regime at the time when without taking into account any possible dependence on the excitation amplitude [54–56]. Indeed, degradation of material mechanical properties can be more easily detected using nonlinear ultrasounds, which is significantly more sensitive at the very early damage stages than traditional linear ultrasonic techniques.

In [36], the sensitivity of the SSM was investigated, with respect to its application to the characterization of damage induced by the crystallization of expansive salts in masonry systems. Salt crystallization in coupled brick-mortar specimens was carried out through a damage protocol, which consisted of repeating cycles of imbibition in a salt solution followed by a drying phase. Specimens were manufactured by coupling a clay brick with two external layers of dehumidifying mortar using commercially available bricks in order to be representative of most common building products. A hand-molding procedure was used with natural drying and subsequent baking at the temperature of 1030°C. Irregular prismatic shape sizing 240 <sup>115</sup> 63 mm<sup>3</sup> , lightly sanded on the outside surface, was then obtained. In analogy with typical restoration systems, a dehumidifying mortar was used for the external layers of the specimens in order to protect the underlying masonry from capillary rising dump, in the presence of soluble salts.

Recommendations from Standard UNI EN 12370 were used as a starting point for producing an accelerated decay protocol on the lab specimens. Then, the following damage protocol is applied.


Direct transmission of longitudinal ultrasonic waves was performed using two identical piezoelectric transducers with a central frequency of 55.5 kHz and 40 mm diameter (see scheme in Figure 8). Input bursts are composed of 10 sine cycles with a frequency of 55.5 kHz and fixed amplitude. Then, the excitation amplitude was progressively increased for successive bursts: for each damage step, the excitation amplitudes ranged between 1 and 20 V, before amplification. A time window of 200 ms was used for the acquisition of the received waveforms, and only first arrivals were considered in the analysis.

parameter without changing the power law behavior. At the end of the damage steps (fully cracked sample), a clear change of the slope can be noticed, since it goes

Evolution of the nonlinear parameter vs. normalized energy at increasing degradation steps by expansive salts.

from 2 to 1, again in accordance with the mass variation measurements.

In this chapter, we analyzed effects of different types of damage on the nonlinear properties of concrete using longitudinal direct transmission mode measurements. Our results suggest that, despite the damage origin (mechanical stress, thermal actions, corrosion, etc.), nonlinear ultrasonic measurements could be a useful tool for monitoring damage. In particular, the analysis performed in the time domain using the scaling subtraction method allows obtaining a sufficiently large signal-to-noise ratio, which is not always the case with standard analysis in the frequency domain. Furthermore, the high sensitivity to the appearance of degradation allows to provide precursory indications with approaching rupture. Further

6. Conclusion

153

Figure 9.

Figure 8.

Experimental set-up for ultrasonic characterization tests.

DOI: http://dx.doi.org/10.5772/intechopen.82621

Time Domain Analysis of Elastic Nonlinearity in Concrete Using Continuous Waves

Figure 9 shows the evolution of the nonlinear effects caused by damage accumulation. Indeed, through the latter, it is possible to see the change in the rise of the nonlinear parameter as a function of the excitation energy when damage increases. Indeed, the power law evolution shows that the necessary amount of the excitation energy to trigger the nonlinear behavior decreases as a function of damage. Note that during the same damage steps, almost no mass variation occurred. This means that the brick-mortar adherence of the tested specimen ensured a very good resistance to damage growth during the first imbibition-drying cycles. On the other hand, the SSM analysis is in accordance with the mass variation measurements. Indeed, the semi-log plot of the same curve shows that the slope remains unchanged (approximately 2) during the first damage steps (partially cracked specimen), and the presence of first cracks (or microcracks) increases the sensitivity of the SSM

Time Domain Analysis of Elastic Nonlinearity in Concrete Using Continuous Waves DOI: http://dx.doi.org/10.5772/intechopen.82621

Figure 8.

damage characterization in civil engineering, in particular when dealing with cement-based materials. However, it has been remarked that ultrasonic pulse velocity, dispersion, and attenuation phenomena in concrete and multiphase materials in general are only considered in the linear regime at the time when without taking into account any possible dependence on the excitation amplitude [54–56]. Indeed, degradation of material mechanical properties can be more easily detected using nonlinear ultrasounds, which is significantly more sensitive at the very early

In [36], the sensitivity of the SSM was investigated, with respect to its application to the characterization of damage induced by the crystallization of expansive salts in masonry systems. Salt crystallization in coupled brick-mortar specimens was carried out through a damage protocol, which consisted of repeating cycles of imbibition in a salt solution followed by a drying phase. Specimens were

manufactured by coupling a clay brick with two external layers of dehumidifying mortar using commercially available bricks in order to be representative of most common building products. A hand-molding procedure was used with natural drying and subsequent baking at the temperature of 1030°C. Irregular prismatic shape

obtained. In analogy with typical restoration systems, a dehumidifying mortar was used for the external layers of the specimens in order to protect the underlying

Recommendations from Standard UNI EN 12370 were used as a starting point for producing an accelerated decay protocol on the lab specimens. Then, the fol-

• Immersion in a solution (14% of Na2SO4, by weight), for 2 h at 20 <sup>0</sup>:5°

Direct transmission of longitudinal ultrasonic waves was performed using two identical piezoelectric transducers with a central frequency of 55.5 kHz and 40 mm diameter (see scheme in Figure 8). Input bursts are composed of 10 sine cycles with a frequency of 55.5 kHz and fixed amplitude. Then, the excitation amplitude was progressively increased for successive bursts: for each damage step, the excitation amplitudes ranged between 1 and 20 V, before amplification. A time window of 200 ms was used for the acquisition of the received waveforms, and only first

Figure 9 shows the evolution of the nonlinear effects caused by damage accumulation. Indeed, through the latter, it is possible to see the change in the rise of the nonlinear parameter as a function of the excitation energy when damage increases. Indeed, the power law evolution shows that the necessary amount of the excitation energy to trigger the nonlinear behavior decreases as a function of damage. Note that during the same damage steps, almost no mass variation occurred. This means that the brick-mortar adherence of the tested specimen ensured a very good resistance to damage growth during the first imbibition-drying cycles. On the other hand, the SSM analysis is in accordance with the mass variation measurements. Indeed, the semi-log plot of the same curve shows that the slope remains unchanged (approximately 2) during the first damage steps (partially cracked specimen), and the presence of first cracks (or microcracks) increases the sensitivity of the SSM

masonry from capillary rising dump, in the presence of soluble salts.

C during 14 h.

C during 4 h.

• Cooling at room temperature during 2 h.

arrivals were considered in the analysis.

, lightly sanded on the outside surface, was then

C .

damage stages than traditional linear ultrasonic techniques.

sizing 240 <sup>115</sup> 63 mm<sup>3</sup>

Acoustics of Materials

• Heating at 85 <sup>5</sup>°

• Heating at 105 5°

152

lowing damage protocol is applied.

Experimental set-up for ultrasonic characterization tests.

Figure 9. Evolution of the nonlinear parameter vs. normalized energy at increasing degradation steps by expansive salts.

parameter without changing the power law behavior. At the end of the damage steps (fully cracked sample), a clear change of the slope can be noticed, since it goes from 2 to 1, again in accordance with the mass variation measurements.
