In Situ Analysis of Plaster Detachment by Impact Tests
In Situ Analysis of Plaster Detachment by Impact Tests
Grazzini, Alessandro
2019-01-12 00:00:00
applied sciences Article In Situ Analysis of Plaster Detachment by Impact Tests Alessandro Grazzini Department of Structural Geotechnical and Building Engineering, Politecnico di Torino, 10129 Torino, Italy; alessandro.grazzini@polito.it Received: 18 December 2018; Accepted: 9 January 2019; Published: 12 January 2019 Abstract: The frescoed surfaces of historical buildings may be subject to detachment due to various causes of deterioration. A new non-destructive experimental methodology is described to assess in situ the safety against plaster detachments from historical wall surfaces. Through small and punctual impacts exerted with a specific hammer on the plastered surface it is possible to evaluate the level of the plaster ’s detachment. A case study at Palazzo Birago in Turin (Italy) is described to give an example of the application of this innovative technique on frescoed surfaces of historical vaults. The test allows to evaluate the safety of frescoed decorations without affecting the material consistency or creating damage, therefore, making it very suitable in the field of architectural heritage. Keywords: frescoed surfaces; non-destructive test; plaster detachment; impact hammer test; historical masonry building 1. Introduction In the field of historical buildings, the role of monitoring and diagnostics is increasingly important for the purpose of securing the masonry structures and also the decorative apparatuses. Often the normal degradation over time or the external climatic causes can compromise the stability of historical plasters [1,2]. The potential detachment of plaster can be further dangerous if it comes from masonry vaults, with the risk of material inside historical buildings containing residential or public functions falling. The Non-Destructive Testing Laboratory of the Politecnico di Torino introduced an impact method to be applied on the wall surface. By means of an instrumented hammer, the impact of a known mass with predetermined energy against the plaster surface was produced. The force–time diagrams produced by the impact of a mass of known energy against the test surface were analyzed. The knowledge of the evolution over time of the forces between the impact mass and the tested material allows to evaluate the parameters that other impulsive methods would not allow. For example, the concrete sclerometer test was limited to the detection of a single quantity, i.e., the elastic energy returned by the material after the impact, proportional to the rebound length of the mass. In the case of the impact hammer test, in addition to the elastic energy returned by the material, the given energy, the dissipated energy, the duration of the impact, and the maximum force can also be evaluated. The impact method allowed to assess the elastic and anelastic properties of the materials [3]. An experimental analysis of the stability of the decorated plaster covering three masonry vaults of the Birago Palace (16th century, planned by Filippo Juvarra) in the center of Turin was described by the use of the impact method test. The frescoed vaults of the Pelagi, Blu, and Giunta rooms showed some small cracks branched out in a layer of plaster that needed an evaluation regarding the risk of detachment (Figures 1–3). Surveys carried out on several points of the vaulted surfaces allowed mapping of the points of potential detachment of the de-coated plaster. Appl. Sci. 2019, 9, 258; doi:10.3390/app9020258 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 258 2 of 11 Appl. Sci. 2019, 9, x 2 of 11 Appl. Sci. 2019, 9, x 2 of 11 Appl. Sci. 2019, 9, x 2 of 11 (a) (b) (a) (b) (a) (b) Figure 1. (a) Pelagi room at Birago Palace; (b) cracks branched in the decorated plaster of the masonry Figure Figure 1. 1. (a ()aPelagi ) Pelagi r room at Bi oom at Birago rago P Palace; alace; ( (b b) ) cracks branche cracks branche d d in in the the decorated pla decorated plas ster ter of th of e masonry the masonry Figure 1. (a) Pelagi room at Birago Palace; (b) cracks branched in the decorated plaster of the masonry vault in the Pelagi room. vault in the Pelagi room. vault in the Pelagi room. vault in the Pelagi room. (a) (b) (a) (b) (a) (b) Figure 2. (a) Blu room at Birago Palace; (b) cracks branched in the decorated plaster of the masonry Figure 2. (a) Blu room at Birago Palace; (b) cracks branched in the decorated plaster of the masonry Figure Figure 2 2. (a.) (Blu a) Bl ru room at oom at Birago Birago Pala Palace; ce; (b (b ))cracks cracks branched in branched in the the de decorated corated plaster plaster of th ofe masonry the masonry vault in the Blu room. vault in the Blu room. vault in the Blu room. vault in the Blu room. (a) ( b) (a) (b) (a) (b) Figure 3. (a) Giunta room at Birago Palace; (b) cracks branched in the decorated plaster of the masonry Figure 3. (a) Giunta room at Birago Palace; (b) cracks branched in the decorated plaster of the masonry Figure 3. (a) Giunta room at Birago Palace; (b) cracks branched in the decorated plaster of the masonry Figure 3. (a) Giunta room at Birago Palace; (b) cracks branched in the decorated plaster of the masonry vault in the Giunta room. vault in the Giunta room. vault in the Giunta room. vault in the Giunta room. 2. Equipment Setup and Methods 2. Equipment 2. Equipmen Setup t Setup and and Methods Methods 2. Equipment Setup and Methods The instrumentation used to carry out the tests consisted of an impact instrumented hammer The instrumentation used to carry out the tests consisted of an impact instrumented hammer The instrumentation used to carry out the tests consisted of an impact instrumented hammer The instrumentation used to carry out the tests consisted of an impact instrumented hammer and a data analyzer. The electric impact hammer used was: PCB Piezotronics; model 086B09; force and a data analyzer. The electric impact hammer used was: PCB Piezotronics; model 086B09; force and a data analyzer. The electric impact hammer used was: PCB Piezotronics; model 086B09; force and a data analyzer. The electric impact hammer used was: PCB Piezotronics; model 086B09; force variable from 44.48 N to 4448.26 N; 208M51 model PCB force sensor; force sensor sensitivity 2.47 mV/N Appl. Sci. 2019, 9, x 3 of 11 variable from 44.48 N to 4448.26 N; 208M51 model PCB force sensor; force sensor sensitivity 2.47 mV/N (Figure 4a). The electric impact hammer was predetermined energy, characterized by the Appl.p Sci. resence 2019, of 9, x one amplifier level and impedance adapter and a spherical head (10 mm diameter) in 3 of 11 Appl. Sci. 2019, 9, 258 3 of 11 cemented steel rigidly connected to a piezoelectric impulse transducer with a total mass of 207 g. variable from 44.48 N to 4448.26 N; 208M51 model PCB force sensor; force sensor sensitivity 2.47 The LMS Pimento multi-channel signal analyzer, with the “real-time” acquisition and recording function, had the following characteristics: model MSP 424; number of channels 4; input range: ±316 mV/N (Figure 4a). The electric impact hammer was predetermined energy, characterized by the (Figure 4a). The electric impact hammer was predetermined energy, characterized by the presence of mV ÷ ±31.6 mV; 24-bit ADC (analog digital converter); bandwidth greater than 20 kHz (on all presence of one amplifier level and impedance adapter and a spherical head (10 mm diameter) in one amplifier level and impedance adapter and a spherical head (10 mm diameter) in cemented steel channels); signal sampling rate up to 100 ksample/second; and personal computer interface: FireWire cemented steel rigidly connected to a piezoelectric impulse transducer with a total mass of 207 g. rigidly connected to a piezoelectric impulse transducer with a total mass of 207 g. IEEE1394—managed by its own dedicated software (Figure 4b). The LMS Pimento multi-channel signal analyzer, with the “real-time” acquisition and recording function, had the following characteristics: model MSP 424; number of channels 4; input range: ±316 mV ÷ ±31.6 mV; 24-bit ADC (analog digital converter); bandwidth greater than 20 kHz (on all channels); signal sampling rate up to 100 ksample/second; and personal computer interface: FireWire IEEE1394—managed by its own dedicated software (Figure 4b). (a) (b) Figure 4. (a) The electric impact hammer; (b) the LMS Pimento multi-channel signal analyzer. Figure 4. (a) The electric impact hammer; (b) the LMS Pimento multi-channel signal analyzer. The points of the vault were randomly selected according to the logistic possibilities of The LMS Pimento multi-channel signal analyzer, with the “real-time” acquisition and recording movement inside the three rooms through mobile scaffolding. Most of the points chosen were found function, had the following characteristics: model MSP 424; number of channels 4; input range: inside the cracked areas where there was a need to evaluate the adherence of the plaster on the 316 mV 31.6 mV; 24-bit (a) ( ADC (analog digital converter); bandwidth gr beater ) than 20 kHz (on all masonry vault. Several points were also analyzed in non-cracked areas in order to compare the channels); signal sampling rate up to 100 ksample/second; and personal computer interface: FireWire Figure 4. (a) The electric impact hammer; (b) the LMS Pimento multi-channel signal analyzer. experimental results with the cracked points. For each point, at least three acquisitions were made to IEEE1394—managed by its own dedicated software (Figure 4b). improve the statistical data (Figure 5a). For each single point test, the instrumented hammer was The points of the vault were randomly selected according to the logistic possibilities of movement The points of the vault were randomly selected according to the logistic possibilities of positioned with the impact mass perpendicular to the test surface. The perpendicularity was achieved inside the three rooms through mobile scaffolding. Most of the points chosen were found inside the movement inside the three rooms through mobile scaffolding. Most of the points chosen were found by means of the four metallic footsies (Figure 5b). The test consisted in generating a small impact of cracked areas where there was a need to evaluate the adherence of the plaster on the masonry vault. inside the cracked areas where there was a need to evaluate the adherence of the plaster on the the hammer’s mass against the test surface, with an absolutely non-destructive intensity, and Several points were also analyzed in non-cracked areas in order to compare the experimental results masonry therefore, also compatible vault. Several point with the conservation s were also analyzed of th in e fre non-cr scoed acke surfac d are es. The as in o impact was triggered rder to compare t he with the cracked points. For each point, at least three acquisitions were made to improve the statistical experi by a t mental rigge r c resul ontt rol on t s with the cra he electr cic ked poi impactn hammer. ts. For ea ch point, at least three acquisitions were made to data (Figure 5a). For each single point test, the instrumented hammer was positioned with the impact improve the statistical data (Figure 5a). For each single point test, the instrumented hammer was mass perpendicular to the test surface. The perpendicularity was achieved by means of the four positioned with the impact mass perpendicular to the test surface. The perpendicularity was achieved metallic footsies (Figure 5b). The test consisted in generating a small impact of the hammer ’s mass by means of the four metallic footsies (Figure 5b). The test consisted in generating a small impact of against the test surface, with an absolutely non-destructive intensity, and therefore, also compatible the hammer’s mass against the test surface, with an absolutely non-destructive intensity, and with the conservation of the frescoed surfaces. The impact was triggered by a trigger control on the therefore, also compatible with the conservation of the frescoed surfaces. The impact was triggered electric impact hammer. by a trigger control on the electric impact hammer. (a) (b) Figure 5. (a) Use of the electric impact hammer for the adherence test of the frescoed plaster of the masonry vaults at Birago Palace; (b) calibration test where it is possible to see the positioning of the electric impact hammer on the test surface. (a) (b) Figure 5. (a) Use of the electric impact hammer for the adherence test of the frescoed plaster of the Figure 5. (a) Use of the electric impact hammer for the adherence test of the frescoed plaster of the masonry vaults at Birago Palace; (b) calibration test where it is possible to see the positioning of the masonry vaults at Birago Palace; (b) calibration test where it is possible to see the positioning of the electric impact hammer on the test surface. electric impact hammer on the test surface. Appl. Sci. 2019, 9, x 4 of 11 Appl. Sci. 2019, 9, 258 4 of 11 3. Impact Energy Principles The following are some energy considerations to better understand the theory underlying the 3. Impact Energy Principles impact method. Consider the impact of a mass m with a semispherical surface and having a velocity The following are some energy considerations to better understand the theory underlying the v0 on the flat surface of a semi-finished space. The direction of impact is perpendicular to this surface. impact method. Consider the impact of a mass m with a semispherical surface and having a velocity Moreover, the velocity v0 of all points of the mass is equal and coinciding with the velocity v0 of its v on the flat surface of a semi-finished space. The direction of impact is perpendicular to this surface. center of gravity. In this case the kinetic energy of the mass at the moment of impact is given by Moreover, the velocity v of all points of the mass is equal and coinciding with the velocity v of its 0 0 Equation (1): center of gravity. In this case the kinetic energy of the mass at the moment of impact is given by Equation (1): (1) 𝜀 = 𝑚𝑣 # = mv (1) c1 and the momentum is: and the momentum is: 𝑄 = 𝑚𝑣 . (2) Q = mv . (2) 1 0 Considering the instant t0 in which the mass touches the surface and the instant t1 in which the Considering the instant t in which the mass touches the surface and the instant t in which 0 1 maximum contact deformation δ occurs and in which the velocity v0 is canceled (Figure 6a), the the maximum contact deformation d occurs and in which the velocity v is canceled (Figure 6a), the corresponding momentum variation results: corresponding momentum variation results: |𝑚𝑣 −𝑚𝑣 | = Fdt (3) jmv mv j = Fdt (3) 0 1 and therefore: and therefore: 𝑚𝑣 = Fdt (4) mv = Fdt (4) The force impulse is given by the area A1 subtended to the curve (F, t) obtained experimentally The force impulse is given by the area A subtended to the curve (F, t) obtained experimentally as as shown in the Figure 6b. shown in the Figure 6b. (a) (b) Figure 6. (a) Geometry of the mobile mass; (b) force–time curve obtained from an impact test. Figure 6. (a) Geometry of the mobile mass; (b) force–time curve obtained from an impact test. The kinetic energy provided by the mass is: The kinetic energy provided by the mass is: 1𝐹𝑑𝑡 Fdt (5) 1 t 𝜀 = 𝑚𝑣 2 = # = mv = (5) c1 2 2𝑚 2 2m Subsequently from the instant t1, in which the vector displacement of the mass changes direction, Subsequently from the instant t , in which the vector displacement of the mass changes direction, at the instant t2, in which the contact between mass and flat surface ceases, the change in momentum at the instant t , in which the contact between mass and flat surface ceases, the change in momentum of the mass results: of the mass results: mv = Fdt (6) Appl. Sci. 2019, 9, 258 5 of 11 wherein v is the velocity of displacement of the mass from the surface, and results v < v . The value 2 2 0 of the integral (6) is given by the area A . The ratio between initial and final mass momentum provides the return coefficient e that measures the elasticity of the impact: mv v 2 2 = = e (7) mv v 0 0 In the perfectly elastic collision e = 1, in the perfectly inelastic collision e = 0. Figure 6b shows that the return coefficient is given by the following equation: Fdt t 2 = = e (8) Fdt The energy returned in the collision is given by: R 2 Fdt 1 t # = mv = (9) c2 2 2m The ratio between the energy supplied and returned is given by: c2 2 = e (10) c1 The energy dissipated # in the impact due to the elasticity of the materials is given by: # = 1 e # (11) d c1 Therefore, in the case of a perfectly elastic impact: e = 1, i.e., A = A (12) 2 1 4. Experimental Results at Birago Palace Tests In the impact test carried out at Birago Palace in Turin (Italy), every masonry vault was divided into survey areas as shown in Figure 7, labeled with alphabet letters, within which both apparently intact and potentially damaged points were tested. For each point, at least three impacts were performed to obtain a better statistical response, and the return coefficient e was evaluated. The maps of the areas tested and the force–time curves of some tested points are shown, respectively, in the Pelagi (Figures 8 and 9), Blu (Figures 10 and 11), and Giunta rooms (Figures 12 and 13). Tables 1–3 show the coefficient averages for each point. Appl. Sci. 2019, 9, x 6 of 11 Figure 7. Survey area on the Pelagi room vault. Figure 7. Survey area on the Pelagi room vault. Figure 8. Map of the points and areas tested on the vault of the Pelagi room. Table 1. Results of impact test on Pelagi room vault. Area Test Point e = A2/A1 Notes Adhesion Plaster P1 0.77 safe P2 1.21 stuccoing not safe A P3 0.83 safe P4 1.08 not safe P5 1.33 stuccoing not safe P6 0.74 safe B P7 0.73 safe P8 0.71 safe P9 0.66 safe C P10 0.91 safe P11 1.36 not safe Appl. Sci. 2019, 9, x 6 of 11 Appl. Sci. 2019, 9, 258 6 of 11 Figure 7. Survey area on the Pelagi room vault. Figure 8. Map of the points and areas tested on the vault of the Pelagi room. Figure 8. Map of the points and areas tested on the vault of the Pelagi room. Table 1. Results of impact test on Pelagi room vault. Table 1. Results of impact test on Pelagi room vault. Area Test Point e = A /A Notes Adhesion Plaster Area Test Point e = A2/A1 Notes Adhesion Plaster 2 1 P1 0.77 safe P1 0.77 safe P2 1.21 stuccoing not safe P2 1.21 stuccoing not safe P3 0.83 safe A P3 0.83 safe P4 1.08 not safe P4 1.08 not safe P5 1.33 stuccoing not safe P5 1.33 stuccoing not safe P6 0.74 safe P6 0.74 safe B P7 0.73 safe B P7 P8 0. 0.7173 sa safefe P8 0.71 safe P9 0.66 safe P10 0.91 safe P9 0.66 safe P11 1.36 not safe C P10 0.91 safe P12 0.66 safe P11 1.36 not safe P13 0.62 safe P14 0.65 safe P15 0.73 safe P16 0.75 safe E P17 0.91 safe P18 0.69 safe P19 0.73 safe P20 1.43 not safe P21 0.68 safe P22 0.60 safe P23 0.62 safe P24 1.19 not safe P25 0.69 safe Appl. Sci. 2019, 9, x 7 of 11 Appl. Sci. 2019, 9, x 7 of 11 P12 0.66 safe P12 0.66 safe P13 0.62 safe P13 0.62 safe D P14 0.65 safe D P14 0.65 safe P15 0.73 safe P15 0.73 safe P16 0.75 safe P16 0.75 safe E P17 0.91 safe E P17 0.91 safe P18 0.69 safe P18 0.69 safe P19 0.73 safe P19 0.73 safe P20 1.43 not safe P20 1.43 not safe P21 0.68 safe P21 0.68 safe P22 0.60 safe P22 0.60 safe P23 0.62 safe P23 0.62 safe Appl. Sci. 2019, 9, 258 G P24 1.19 not safe 7 of 11 P24 1.19 not safe P25 0.69 safe P25 0.69 safe Appl. Sci. 2019, 9, x 8 of 11 (a) (b) (a) (b) Table 2. Results of impact test on Blu room vault. Figure 9. Impact test on Pelagi room vault: force–time curve of (a) P13 test; (b) P5 test. Figure 9. Impact test on Pelagi room vault: force–time curve of (a) P13 test; (b) P5 test. Figure 9. Impact test on Pelagi room vault: force–time curve of (a) P13 test; (b) P5 test. Area Test Point e = A2/A1 Notes Adhesion Plaster P1 0.68 safe P2 0.75 safe P3 0.71 safe P4 1.98 Stuccoing not safe P5 0.78 safe P6 0.65 safe P7 0.77 safe P8 0.76 safe C P9 0.84 safe P10 0.79 safe P11 0.78 safe P12 0.75 safe P13 1.11 not safe D P14 0.87 safe P15 0.70 safe P16 0.56 safe P17 0.83 safe P18 0.63 safe P19 0.72 safe P20 0.66 safe F P21 0.66 safe P22 0.70 safe Figure 10. Map of the points and areas tested on the vault of the Blu room. Figure 10. Map of the points and areas tested on the vault of the Blu room. Figure 10. MapP2 of3 the 0. points and73 areas tested on the vault ofsathefe Blu room. (a) (b) Figure 11. Impact test on Blu room vault: Force–time curve of (a) P7 test; (b) P4 test. Figure 11. Impact test on Blu room vault: Force–time curve of (a) P7 test; (b) P4 test. Appl. Sci. 2019, 9, 258 8 of 11 Table 2. Results of impact test on Blu room vault. Area Test Point e = A /A Notes Adhesion Plaster 2 1 P1 0.68 safe P2 0.75 safe P3 0.71 safe P4 1.98 Stuccoing not safe P5 0.78 safe P6 0.65 safe P7 0.77 safe P8 0.76 safe P9 0.84 safe P10 0.79 safe P11 0.78 safe P12 0.75 safe P13 1.11 not safe P14 0.87 safe P15 0.70 safe P16 0.56 safe P17 0.83 safe P18 0.63 safe P19 0.72 safe P20 0.66 safe F P21 0.66 safe P22 0.70 safe P23 0.73 safe Appl. Sci. 2019, 9, x 9 of 11 Figure 12. Map of the points and areas tested on the vault of the Giunta room. Figure 12. Map of the points and areas tested on the vault of the Giunta room. Table 3. Results of impact test on Giunta room vault. It is possible to observe that most of the points tested had a return coefficient e lower than 1. This means that the energy returned was lower than that emitted, because part of this energy was Area Test Point e = A2/A1 Notes Adhesion Plaster dissipated by the tested structure through sufficient bonds in the interface between the plaster and P1 0.68 safe the masonry surface. On the contrary, the return coefficient e > 1 showed a returned energy greater P2 0.74 safe than the one emitted: in this case the material was already damaged [4,5] because it was partly or P3 0.70 safe P4 0.65 safe P5 0.84 safe P6 0.73 safe P7 0.72 safe P8 0.77 safe P9 0.93 safe P10 0.70 safe P11 0.80 safe P12 0.72 safe P13 0.65 safe P14 0.78 safe P15 1.07 Wiring channel not safe P16 0.74 safe P17 1.04 not safe E P18 1.26 not safe P19 0.70 Stuccoing safe P20 0.74 safe F P21 0.74 safe P22 0.68 safe P23 0.66 safe G P24 0.62 safe P25 0.77 safe P26 0.75 safe H P27 0.71 safe P28 0.62 safe Appl. Sci. 2019, 9, 258 9 of 11 completely disconnected and returned more energy due to the deformations and the microscopic Appl. Sci. 2019, 9, x 10 of 11 movements active due the non-perfect adherence between plaster and masonry surface. (a) (b) Figure 13. Impact test on Giunta room vault: force–time curve of (a) P4 test; (b) P15 test. Figure 13. Impact test on Giunta room vault: force–time curve of (a) P4 test; (b) P15 test. Table 3. Results of impact test on Giunta room vault. 5. Discussion Area Test Point e = A /A Notes Adhesion Plaster 2 1 The points where the plaster was still adherent to the wall surface showed more symmetrical P1 0.68 safe and regular force–time curves, with higher values than the maximum impact force as the material P2 0.74 safe was more compact (Figures 9a, 11a, and 13a). On the contrary, in the points already covered by P3 0.70 safe previous stuccoing, lower values of the maximum force and more asymmetric curves were recorded P4 0.65 safe in which the area after maximum force was greater than that which preceded it (Figures 9b, 11b, and P5 0.84 safe 13b). P6 0.73 safe Overall, the impact test showed the stability and safety of the adhesion between decorated P7 0.72 safe plaster and masonry surfaces of the vaults examined P8 0.77 in the three rooms. Some p safe oints of lesser safety regarding adherence have emerged. Some of these concerns point to previously stuccoed areas only P9 0.93 safe a few decades old. This point high P10 lighted t0.70 he potential critical stabilitysafe of some of the plaster, which P11 0.80 safe had already been the subject of micro-grouting, and for which restorations had not been perfectly P12 0.72 safe carried out. On the contrary many other previously stuccoed points showed a return coefficient <1. P13 0.65 safe The impact method was therefore also useful to qualify the effectiveness of previous restoration P14 0.78 safe work. Wiring P15 1.07 not safe On the masonry vault of the rooms some points of potential detachment of plaster have been channel found, characterized by a return coefficient e > 1 (Tables 1–3). These results were in agreement with P16 0.74 safe what has been possible to perceive qualitatively with a simple hand knock on the point under P17 1.04 not safe investigation. Some of these points with a high return coefficient were stuccoed previously, a sign E P18 1.26 not safe P19 0.70 Stuccoing safe that some critical issues of potential detachment already existed in the past (Figure 14). In some cases, as for point P15 of the Giunta P20 room, the imp 0.74act method confirmed th safe e presence of an installation P21 0.74 safe channel that feeds the chandelier as a zone with weak adherence of the plaster (Figures 13b and 14b). P22 0.68 safe The decorated surfaces of the vaults therefore appeared to be in a good state of conservation. P23 0.66 safe The ramified cracks present in most of the surface derive from the shrinkage effects of the historical P24 0.62 safe plaster mortar, due to different reasons: climatic conditions, setting of the binders (lime and cement), P25 0.77 safe binder/inert quantity ratio. P26 0.75 safe P27 0.71 safe P28 0.62 safe 5. Discussion The points where the plaster was still adherent to the wall surface showed more symmetrical and regular force–time curves, with higher values than the maximum impact force as the material was more compact (Figures 9a, 11a and 13a). On the contrary, in the points already covered by previous (a) (b) Appl. Sci. 2019, 9, x 10 of 11 (a) (b) Figure 13. Impact test on Giunta room vault: force–time curve of (a) P4 test; (b) P15 test. 5. Discussion The points where the plaster was still adherent to the wall surface showed more symmetrical and regular force–time curves, with higher values than the maximum impact force as the material was more compact (Figures 9a, 11a, and 13a). On the contrary, in the points already covered by previous stuccoing, lower values of the maximum force and more asymmetric curves were recorded in which the area after maximum force was greater than that which preceded it (Figures 9b, 11b, and 13b). Overall, the impact test showed the stability and safety of the adhesion between decorated Appl. Sci. 2019, 9, 258 10 of 11 plaster and masonry surfaces of the vaults examined in the three rooms. Some points of lesser safety regarding adherence have emerged. Some of these concerns point to previously stuccoed areas only stuccoing, lower values of the maximum force and more asymmetric curves were recorded in which a few decades old. This point highlighted the potential critical stability of some of the plaster, which the area after maximum force was greater than that which preceded it (Figures 9b, 11b and 13b). had already been the subject of micro-grouting, and for which restorations had not been perfectly Overall, the impact test showed the stability and safety of the adhesion between decorated plaster carried out. On the contrary many other previously stuccoed points showed a return coefficient <1. and masonry surfaces of the vaults examined in the three rooms. Some points of lesser safety regarding The impact method was therefore also useful to qualify the effectiveness of previous restoration adherence have emerged. Some of these concerns point to previously stuccoed areas only a few work. decades old. This point highlighted the potential critical stability of some of the plaster, which had On the masonry vault of the rooms some points of potential detachment of plaster have been already been the subject of micro-grouting, and for which restorations had not been perfectly carried found, characterized by a return coefficient e > 1 (Tables 1–3). These results were in agreement with out. On the contrary many other previously stuccoed points showed a return coefficient <1. The what has been possible to perceive qualitatively with a simple hand knock on the point under impact method was therefore also useful to qualify the effectiveness of previous restoration work. investigation. Some of these points with a high return coefficient were stuccoed previously, a sign On the masonry vault of the rooms some points of potential detachment of plaster have been that some critical issues of potential detachment already existed in the past (Figure 14). In some cases, found, characterized by a return coefficient e > 1 (Tables 1–3). These results were in agreement as for point P15 of the Giunta room, the impact method confirmed the presence of an installation with what has been possible to perceive qualitatively with a simple hand knock on the point under channel that feeds the chandelier as a zone with weak adherence of the plaster (Figures 13b and 14b). investigation. Some of these points with a high return coefficient were stuccoed previously, a sign that The decorated surfaces of the vaults therefore appeared to be in a good state of conservation. some critical issues of potential detachment already existed in the past (Figure 14). In some cases, as for The ramified cracks present in most of the surface derive from the shrinkage effects of the historical point P15 of the Giunta room, the impact method confirmed the presence of an installation channel plaster mortar, due to different reasons: climatic conditions, setting of the binders (lime and cement), that feeds the chandelier as a zone with weak adherence of the plaster (Figures 13b and 14b). binder/inert quantity ratio. (a) (b) Figure 14. Points of potential plaster detachment: (a) P11 vault Pelagi room; (b) P15 vault Giunta room. The decorated surfaces of the vaults therefore appeared to be in a good state of conservation. The ramified cracks present in most of the surface derive from the shrinkage effects of the historical plaster mortar, due to different reasons: climatic conditions, setting of the binders (lime and cement), binder/inert quantity ratio. 6. Conclusions The impact method was used to evaluate the adherence of the decorated plaster of some masonry vaults. The method confirmed its non-destructive typology and has proved its validity also for the diagnostics of historical buildings. In the campaign tests carried out to evaluate the adherence of the decorated plaster on three masonry vaults of the Birago Palace (Turin, Italy), the impact method clearly highlighted points of critical and potential detachment, as well as confirmed the effectiveness of the consolidation of many previously stuccoed points. Funding: This research was funded by the CAMERA DI COMMERCIO DI TORINO. Acknowledgments: The author wishes to thank Vincenzo Di Vasto for his valuable collaboration during the performance of the tests. Conflicts of Interest: The authors declare no conflict of interest. References 1. Bocca, P.; Valente, S.; Grazzini, A.; Alberto, A. Detachment analysis of dehumidified repair mortars applied to historical masonry walls. Int. J. Arch. Herit. 2014, 8, 336–348. [CrossRef] Appl. Sci. 2019, 9, 258 11 of 11 2. Grazzini, A.; Lacidogna, G.; Valente, S.; Accornero, F. Delamination of plasters applied to historical masonry walls: Analysis by acoustic emission technique an numerical model. IOP Conf. Ser. Mater. Sci. Eng. 2018, 372, 1–7. [CrossRef] 3. Bocca, P.; Scavia, C. The impulse method for the evaluation of concrete elastc characteristics. In Proceedings of the 9th International Conference on Experimental Mechanics, Copenhagen, Denmark, 20–24 August 1990. 4. Bocca, P.; Carpinteri, A.; Valente, S. On the applicability of fracture mechanics to masonry. In Proceedings of the 8th International Brick/Block Masonry Conference, Dublin, Ireland, 19–21 September 1988. 5. Johnson, K.L. Contact Mechanics; Cambridge University Press: Cambridge, UK, 1985. © 2019 by the author. Licensee MDPI, Basel, Switzerland. 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