Measure of a Limestone Weathering Degree Using Laser Scanner

International Journal of Architectural Heritage, 7: 591–607, 2013Copyright © Taylor & Francis Group, LLCISSN: 1558-3058 print / 1558-3066 onlineDOI: 10.1080/15583058.2012.654893

MEASURE OF A LIMESTONE WEATHERING DEGREEUSING LASER SCANNER

Laura Ercoli,1 Bartolomeo Megna,1 Alessandra Nocilla,2and Margherita Zimbardo1

1Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale e dei Materiali,Università di Palermo, Palermo, Italy2Dipartimento di Ingegneria Civile, Architettura, Territorio e Ambiente, Universitàdi Brescia, Brescia, Italy

The weathering degree of the building materials and natural stones is generally quantified asthe decrement of some mechanical features that can be measured experimentally by meansof compressive tests or point load tests in the laboratory or Schmidt hammer tests carriedout in situ. Such destructive or damaging tests are unacceptable in case of cultural heritagesince even small amounts of damage must be avoided. This work shows a correlation betweenSchmidt hammer rebound values and the reflectivity that is detected by means of terrestrialscanner laser; therefore it allows assessing the weathering degree of buildings or stones insitu. The results demonstrate that such an investigation could be a significant alternativerepresenting an innovative and non-destructive technique.

KEY WORDS: nodular limestone, weathering degree, Schmidt hammer, terrestrial scannerlaser, mechanical decay

1. INTRODUCTION

When, during the restoration process of an historical building, the weathering degreehas to be quantified, many difficulties might occur. First is the difficulty to carry out repeat-able, quick, and non-destructive tests on samples of the building materials. The weatheringdegree of the building materials and natural stones is generally quantified as the decrementof some mechanical features that can be measured experimentally by means of a set of labo-ratory destructive tests (e.g., uniaxial compressive test, point load tests). Therefore for manybuildings such as the case of cultural heritage, samples cannot be taken, avoiding damageto the integrity of the building or of the artifact aesthetic. For example, in case the Schmidthammer, significant damage cannot be prevented, and it might cause microcracks or thedetachment of flakes. Furthermore, if a decay map for large walls is required, scaffoldingwould be necessary in order to perform direct tests, which would be extremely expensivein terms of time and money as reported in Ercoli and Speciale (1988), that employed theSchmidt hammer to mapping the alteration of the rock of the walls of ancient Greek quarries(Latomie del Paradiso in Syrcause, Italy).

Received July 14, 2011; accepted January 2, 2012.Address correspondence to Laura Ercoli, Dipartimento di Ingegneria Civile, Ambientale e Aerospaziale,

Università di Palermo, Viale delle Scienze 90128 Palermo, Italy. E-mail: [email protected]

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592 L. ERCOLI ET AL.

In order to solve these issues, terrestrial laser scanner detection can be enormouslyhelpful. It correlates the decrease of the rebound values r to the reflectivity index I, bymeans of a non-destructive technique. As is well known, laser-scanning technology allowsthe acquisition of the three-dimensional (3D) coordinates X, Y, and Z for a large number ofpoints on the surface of the object that must be detected. Therefore, the instrument recordsthe RGB values and measures the incident and reflected ray energy, providing the reflec-tivity index I of the micro-portion surface, which is one of the discretized parts of the wallthat has to be characterized.

By means of the laser scanner technique, a very high accuracy in localizing rockdiscontinuities can be achieved (Nocilla, 2002; Voyat, 2005) and it is worth mentioning,moreover, that the technique is more reliable and less expensive if compared to the terres-trial photogrammetry. As for the 3D roughness of the discontinuity surface, it is importantto say that limitations associated with the profilometer technique can be overcome (Re andScavia, 1999; Grasselli et al, 2002).

Many features might play a significant role for reflectivity. In some recent appli-cations of terrestrial scanner lasers to rock mechanics (Evangelista et al., 2007), theinfluence has been highlighted of many parameters such as the distance between therock surface (Mesozoic limestones, Neapolitans tufs, and Sicilian calcarenites) and thelaser scanner, the light signal wavelength, the incident ray inclination. and, above all,the material features that constitute the surface such as its water content (Bornaz andRinaudo, 2004; Lichti, Harvey, 2002a; 2002b). Hence, using the same laser scanner, ifthe same boundary conditions are imposed, important information on the textural charac-teristics (e.g. micro-fractures, micro-roughness, rock porosity) can be obtained (Lanaro,1998).

In the literature, different studies of the use of terrestrial laser scanners as a detec-tion technique have been presented. In order to estimate the state of conservation ofthe stone or other materials, the ultrasonic longitudinal pulse velocity and the frequencyspectra have been correlated to the reflectivity (Casula et al., 2009). Digital image pro-cessing has been combined with intensity data from terrestrial laser scanners (Gonzálezet al., 2010) and it has been possible to estimate the reflectance which is linked to theclay mineral content in rocks through an inverse linear relationship where the correlationcoefficient r = 0.85 (Franceschi et al., 2009) and the mineral content has been eval-uated through gas chromatography on rock samples taken from the same stratigraphicsection.

The aim of this work is to verify that the terrestrial laser scanner can be a significantalternative in situ detection technique for the case of materials for monumental buildings.A correlation will be presented between the reflectivity values I and the Schmidt hammerrebound values r, which are traditionally used for the estimation of the compressive strengthin situ.

2. TESTED MATERIAL, EXPERIMENTAL ANALYSIS, AND PROCEDURES

2.1. Material

The research presented here is from the case study of the two main entrance columnsin the Jung Palace in Palermo, Sicily. At the end of the 18th century and the beginning of the19th century, Mr. Rosario Sciarrino, a rich bourgeois who bought the title of Lord, built thePalace with the purpose of celebrating his entrance into the blueblood class. Abandoned in

INTERNATIONAL JOURNAL OF ARCHITECTURAL HERITAGE 7(5): 591–607

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MEASURE OF LIMESTONE WEATHERING 593

the 1980s, it was then restored between 2000 and 2003 to become one of the most attractivevenues for artistic and cultural expositions. It is located at few meters above sea level andis 700 meters from the shoreline. Its total area is 1190 m2, with a rectangular plan spreadover two elevations. The main entrance is located on the southern side with a monumentalgate, which is framed by two columns that support a wide stone balcony.

The columns, which are 0.6 m diameter and 2.8m high, are two monolithic blocksof ammonitic red limestone and are each placed, on a large square block of Billiemi stone,which is a dark grey breccia of the Mesozoic age. The ammonitic red is a Jurassic redlimestone that, during the 17th, 18th, and 19th centuries, was usually used indoors andoutdoors as a marble for decorative and structural purposes in the monumental architectureof Western Sicily. The more precious and uniform varieties of this “marble”, e.g. the Redof Piana degli Albanesi, was used also in well known monumental buildings of Rome, suchas the Montecitorio Palace, the seat of the Italian chamber of deputies. This lithotype, thatis characterized by a nodular and sometimes flaser texture, if used outdoors, undergoes toa selective weathering that increases the inhomogeneities and in the worse cases, causessuperficial fractures.

The study of the two columns has been carried out to highlight the influence of theweathering degree due to exposure marine spray. In order to show the orientation andlocation of the coastline, a map shown in Figure 1 has been included. The northern sideis more exposed to the marine spray and shows signs of a selective degradation, whichaffects not only the chromatic aspect but also the aggregation state. In contrast, the south-ern side is more intact (Figure 2), and so it is possible to notice the different weatheringdegree between the two columns, the right one being exposed to the marine spray. Despitethis difference, no evidence of salt crystallization is present in either columns, nor sur-face black crusts or oxalate patinas. It is also shown that the column surface has beendivided into four horizontal levels and four vertical strips (16-cell grid) as will be describedlater.

Figure 1. Photograph of the location of Jung Palace (ortophoto 1:5000 from Regione Sicilia) (color figureavailable online).


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Figure 2. Photograph of the Palazzo Jung monumental main entrance and the 16-cell grid (color figure availableonline).

2.2. Testing Equipment and Analysis

In order to verify if the terrestrial laser scanner can be a significant alternativedetection technique in the case of materials for monumental buildings, it was first nec-essary to perform an accurate investigation of the materials involved and their mechanicaland physical characteristics. Direct inspection of the columns has been then carried outby means of direct observations, microscope observations, Schmidt Hammer tests, andlaser scanner tests. These measurements were carried out on dried surfaces because theywere performed after a dry climate period. Transmitted light optical microscope investiga-tions on thin sections, x-ray diffractometry (XRD), simultaneous thermal analysis (STA)and microraman spectroscopy of samples have been performed in order to analyze themineralogical composition of the rocks.

2.2.1. In situ observation To describe the physical characteristics of the column sur-faces, visual observation the first investigation. The column rock surfaces, which showedno evidences of pollutant particles attached to the surface, have been then observed undera direct microscope “in contact” with them. The textural and compositional characteristicsof the rock have also been observed in thin sections by means of a transmitted light opticalmicroscope.

2.2.2. Physical and chemical analysis XRD was used to identify the ammonitic redlimestone mineralogical characteristics. In addition, simultaneous thermal analysis, STA,and microraman spectroscopy were performed to quantify calcite percentage in the stoneand estimate the mineralogical characteristics at specific points respectively. The XRD wasperformed by means of a Philips PW 1130/1050 () equipped with CuKa radiation, and thetube conditions were 40 kV and 30 mA, in the 2! range from 4 to 60. In order to analyzeclayey characteristics, the analysis was also performed in an enriched sample, obtainedby grinding a representative piece of stone and separating by gravimetric precipitation theparticles with diameters less than 2 mm that represent the clay minerals within the rock


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sample. Samples of orientated aggregates, obtained by precipitation of clay particles withthe addition of magnesium chloride, were analyzed before and after a treatment with ethy-lene glycol and after heating at 550! C. The comparison between the three different XRDpatterns reveals the actual clay minerals.

The STA refers to the simultaneous application of thermogravimetric analysis (TGA)and differential thermal analysis (DTA) to the same sample in a single instrument. It wasperformed by means of a Netzsch STA 9 analyzer (Netzsch, Germany), in the range 30! C"1000! C and in static air, and heating rate of 10! C/min.

Microraman spectroscopy was performed to detect point characteristics by means ofa Renishaw Invia Raman microscope (Renishaw, United Kingdom), equipped with 633-nmlaser, notch filter, grating 2400 lines/mm and a CCD detector.

2.2.3. Mechanical analysis In order to measure the rebound hardness of the rock sur-faces of the columns, a type-L Schmidt hammer was used (original Schmidt Type L-9;impact energy: 0.735Nm; piston diameter: 15 mm). The column surface was divided intofour horizontal levels and four vertical strips in order to create a rectangular 16-cell grid(mesh 15 cm # 20 cm). Placing the hammer in a horizontal position, on average 22 mea-surements were carried out for each cell randomly (Figure 2). The dimensions of the gridwere chosen in order to consider the rock as isotropic.

2.2.4. Laser scanner analysis The measurements of reflectivity I were carried out withthe laser scanner LMS-Z210 RIEGL, which has a field of scanning of 360! # 80! and iscapable of measuring angles up to a distance of 486 m with an accuracy of 14 mm. Thespeed of acquisition can reach up to 9387 points/s. The wavelength of the emitted ray isbetween 0.8 and 1.1 microns. The “targets” were installed on square blocks at the base ofeach column. The laser scanner was positioned at a distance of approximately 10 m and thescan was performed with a resolution of 0.018!. The instrument was placed in two differentpositions, both perpendicular to the façade of the building crossing through the axis of eachcolumn. The reflectivity indices I, obtained dividing the surface of the grid into the 300 cm2

cells, have been acquired by means of the software Riscan–Pro (RIEGL, Austria) and havebeen recorded in Excel (Microsoft Excel, 2007) files.

3. RESULTS AND DISCUSSION

3.1. Ammonitic Red Composition and Texture

Optical microscopy shows that ammonitic red limestone from Piana degli Albanesiis made of elements, with diameters of approximately 1 cm, in which grains are welded inpressure-solution contacts that originated stylolithic interfaces with a residual concentrationof clays and oxides. The elements are made of two different grainstones:

1) a grainstone with 70% sparitic bioclasts and 30% of micritic botroids and peloids, witha grain size varying between 30 and 450 µm;

2) a grainstone containing more than 80% micritic botroids and peloids and about 20% ofbioclasts, mainly lamellibrancs fragments, characterized by a grain size varying between30 and 350 µm.


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Figure 3. Microphotos of the ammonitic red limestone in thin sections showing the different textures of the twograinstones indicated as A and B (color figure available online).

The elements are streaked by very thin sparitic veins. In some locations the veins areforked, with calcite crystals between them. The thickness of patinas of the residual prod-ucts, wrapping the nodular elements, ranges from 25 to 150 µm. The dark red areas appearto be cement concretations, and, indeed, are made up of smaller grainstone lithoclasts veryclose to each other and enclosed in the residual material, i.e., clay minerals and iron oxides(Figure 3).

The XRD identified that the whole sample consists mostly of calcite, quantified bySTA as the 95.8% of the total mass. The remaining mass (4.2%) consists of clay minerals,iron oxides, and other components. The quantification of the calcite content is made bythe conversion of the percentage weight loss between 600! C and 900! C, according to thestoichiometry of calcium carbonate thermal decomposition:

CaCO3 = CaO + CO2 (EQ1)

The weight loss of 42.15% related to CO2 development leads to a weight percentage ofcalcite within the sample of 95.8% (Figure 4).

As for the weight loss, the DTA curve shows an endothermic peak revealing that thereaction occurs with a high-energy consumption, as expected for calcite decomposition.

Traces of quartz and feldspar (Figure 5) are however present. In Figure 4, five XRDtraces for ammonitic red limestone are shown. Figure 5A shows the results obtained fora whole sample, where the calcite predominance is evident, with no other mineralogicalphases. Figure 5B shows the results obtained for the clay enriched sample, that showedthree peaks related to clay minerals, at low angle values, corresponding to d0 spacing of6.5!, 9! and 12!. Figure 5C shows the results obtained for an enriched clayey sample aftersaturation with ethylene glycol vapor. The saturation of swelling clays with glycol vaporcauses the partial shift of the 6.5! peak to a lower angle. Figure 5D shows the resultsobtained for an enriched clayey sample after a treatment at 550! C, the peak remainingat 9! and that at 12! absent. The comparison of the three XRD patterns of the enrichedsample allows the presence of two main minerals to be identified: kaolinite and swellingchlorite.

Microraman spectroscopy was then performed to detect point characteristics. TheRaman spectra revealed the presence of iron oxide, i.e., hematite, as a coloring agent of


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Figure 4. Graph showing simultaneous thermal analysis (STA) of the ammonitic red limestone.

Figure 5. X-ray diffractometry (XRD) pattern of the ammonitic red limestone: A) whole sample; B) clay-enrichedsample; C) clay-enriched sample after saturation with ethylene glycol vapor; D) clayey-enriched sample aftertreatment at 550! C (color figure available online).

the red veins, and identified in several red particles as shown in Figure 6, both after andbefore background subtraction (Figures 6A and 6B, respectively). The three main peaks ofhematite, partially obscured by background fluorescence, are clear in the curve obtainedafter background subtraction.


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Figure 6. Graph showing raman spectra of red particles identified as hematite: A) before background subtraction;B) after background subtraction.

3.2. Weathering

The surfaces of the columns are largely affected by fading which increases from theareas towards the wall, that are always shadowed and protected from rain, to those areasexposed to sunshine and to rain drops that can rebound in the presence of wind or to rainthat can be only occasionally direct. A stronger decay is visible in column areas that arecloser to the street. This decay is stronger as much of the areas are exposed to rainfall andto marine spray. As a proof of that, both columns show the maximum alteration processesin correspondence to the vertical band exposed to the east, towards the direction of the sea.This phenomenon is less evident in the left column since this is protected, being shelteredfrom the wind by the right column.

As shown in Figure 7, the microphotographs taken in situ, by means of a portablemicroscope in direct contact, highlight the following state levels of alteration:

1) Fading: the surface is of a very pale red and still smooth, but it has lost its originalsmoothness and brightness (Figure 7A).

2) Corrosion of bigger nodules: their surface becomes very rough and with lower reliefcompared to the cement veins (Figure 7B).

3) Collapse of cement veins: irregular eroded micro-channels corresponding to the cementveins are present in the surface and the nodules are in a slightly protruding position(Figure 7C).


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Figure 7. Photographs of the Rosso di Piana in contact microscopy: (A) fading; (B) corrosion of grainstoneelements; (C) collapse of the red veins; (D) microcrack development (color figure available online).

4) Micro-crack development all around the nodules that leads to the formation of a rockingflakes that, as in case of milk teeth, causes cavities when detachment occurs (Figure 7D).

It is worth mentioning that the fading, in the laser scanner classification of the degree ofdecay, is the zero level, since there do not exist areas of the surfaces that are in the originalstate of smoothness or brightness.

Since the textural characteristics play a major role in the evolution of mechanismsof decay, it is worth recalling briefly the results of accelerated laboratory weathering per-formed on samples of ammonitic red limestone from Piana degli Albanesi in a previousstudy. The effects of weathering were reproduced successfully in the laboratory on virginstone slabs accurately polished. This was achieved by means of many treatments such asultraviolet radiation plus dew, thermal treatment, washout or exposure to oxidizing andacidic agents. The results were then compared to natural ageing in an urban polluted envi-ronment (Rizzo et al., 2006; 2007). Thermal ageing performed between +70! C and "13!

C in isostatic conditions, i.e., without rigid bonds around the samples, did not produce anysignificant effect even after 50 cycles, as no new cracks seemed to grow or propagate.

As an effect of the washout process, a significant differential dissolution phenomenaof lithoclasts occurs with respect to the welding contacts and micro-crack growth. Thedissolution of the lithoclasts proceeds uniformly but at different rates for the two kindsof grainstone of which the elements are made, as described previously. As a first step,dissolution is more severe in grains, which are in lower relief with respect to the stylolitic


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joints, so that the red veins appear in relief and show a new micro-porosity due to thedissolution of the calcite microcrystals. As a second step the red veins collapse and styloliticjoints are in lower relief compared with the grains. This second step of decay occurs whenthe first step has progressed sufficiently to allow mechanical removal of the clayey particlespresent in the veins, as the effect of the passage of flowing water, and the vein becomes afurrow. As a consequence of these steps the red veins appear to be alternatively in reliefor in a lower relief with the consequence of a generalized increase of the roughness of theexposed surfaces.

3.3. Schmidt hammer

In order to measure the rebound hardness of the rock surfaces of the columns a type-LSchmidt hammer was used on a rectangular 16-cell grid. All the rebound values r have beeninitially represented in a cumulative frequency diagram in which the results of each columnare presented separately, as shown in Figure 8.

The r values for the left column, which is more intact, are between 50 and 82. Thevalues for the right column are spread over a larger interval between 20 and 71. If the ratiobetween the 90th percentile and the 10th percentile (U) is assumed to be a measure of thedispersion, it is clear that a surface of a rock, which has a value of U close to 1 can bedeclared uniform in its mechanical behavior. The left column has a value very close to 1(U(90th/10th) = 1.19) and so it is almost uniform. In contrast, for the right column, whichis less intact and more involved in the weathering phenomena, a much higher value of U(U(90th/10th) = 2.01) has been reached. This finding highlights a heterogeneity in the data,which is due to the presence of intact and decayed areas in the surface. From this point ofview, after this first estimation, it is clear that the rebound values are strongly influenced bythe weathering of the surfaces.

In order to estimate if the Schmidt hammer is suitable and sensitive enough to mea-sure the weathering degree, the measured values of r for the right column have beenprocessed. This choice has been done since, all around the column surface, the weatheringphenomena ascend in vertical bands from the left to the right, or, it is possible to say, fromthe southwest exposure to the northeast exposure towards the sea.

Figure 8. Graphs of cumulative frequency curves: rebound values r measured on the altered column and intactcolumn. Cumulative frequency curves: rebound values r measured on the altered column and processed for thedifferent vertical bands (A, B, C, D) (see Figure 2) (color figure available online).


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Cumulative frequency diagrams have been processed for the vertical bands of differ-ent areas. At first the vertical bands that are characterized by a minimum value (U(90th/10th) =1.19) were processed, so that the left vertical bands were considered, being protected fromthe weathering factors. The results are the same as those for the intact column. Successivevertical bands towards the right have then been taken into consideration. These bands arecharacterized by increasing values of U up to 1.89 where a greater exposure occurs (UB =1.56; UC = 1.83; UD = 1.89) (Figure 8).

In Figure 9, eight histograms represent the r values measured on the altered columnsand processed for both vertical and horizontal bands. Four of these histograms for the fourhorizontal bands of cells (1, 2, 3 and 4) and the other four for the four vertical ones (A,B, C and D). It is possible to note that for the first four (the histograms for levels), thedispersion of values it is similar to that recorded for the cumulative frequency diagram forthe whole set of measurements, because in each level, every state of decay of the rock ispresent, from fading to severe weathering. This circumstance is highlighted by the two orthree-modal distributions. For the other four histograms (for vertical cell bands), it is clearthat the dispersion is minimum for vertical band A, which has a south-west exposure inthe direction opposite to the sea. The dispersion then increases as B, C, and D bands areprogressively considered. The difference Dr between the bigger and the smaller reboundvalue rmax and rmin respectively (Dr = rmax"rmin) also increases from band A to D (DrA =24; DrB = 42; DrC = 51; DrD = 45).

A leptokurtic distribution characterizes band A, where a high frequency of the modalvalue (rAmoda = 69,5;f = 33,6%) is present. Platikurtic distributions are then present forbands C and D, with a reduced frequency of the modal value (f = 10%–16%) and a splitof the modal values. Therefore, the detailed examination of the investigated surfaces and ofthe hammer piston support points has highlighted that the lowest r values and the greatestdispersion are always relative to rocking parts of rock. Instead bigger values are relative tomore stable parts of the rock, which can be affected by weathering of the surface but arestill strongly linked to the intact core.

3.4. Laser scanner

The reflectivity values, measured by means of the laser scanner, have been pro-cessed following the same procedure used for the Schmidt hammer tests. Figure 10ashows the cumulative frequency diagrams obtained for the complete set of measurementsin each column. It is possible to notice, also in this case, the differences between theleft column, which is almost intact, and the right one, which is affected by significantweathering.

In the intact column, the reflectivity has values between 51% and 55%. Insteadin the weathered one, the values vary between 42% and 55%. Unlike the measurementsobtained with the Schmidt hammer, the uniformity index U(90th/10th) for both columns is,however, quite high, close to one and in both cases it has a leptokurtic frequency distri-bution with a single modal value for each curve, only slightly higher than for the intactcolumn (Uweathered = 1.09; Uintact = 1.04).

Reflectivity measurements that have been obtained by means of laser scanner on theweathered column have been taken on the same reference grid cut in levels and bands asalready described, (see Figure 11). Leptokurtic curves with Gaussian distributions havebeen achieved from the processed data, for both horizontal levels and vertical bands. Thishighlights clearly the surface weathering phenomena.


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Figure 9. Graphs of rebound values r measured on the altered column and processed in histograms relative tovertical bands and horizontal levels (color figure available online).

The lowest level is characterized by data that are slightly more scattered. This indi-cates a higher weathering degree toward the street plane, where air pollution derived fromvehicular traffic is associated with other weathering factors. Greater differences can benoted for the modal values of the histograms in which measurements have been processedfor the vertical bands. The reflectivity modal values here decrease from band A, where therock is more intact, to band D, where the weathering degree is highest (IAmodal = 50.3%;IBmodal = 49.7%; ICmodal = 48.6%; ICmodal = 48.3% ). The scatter of reflectivity data of DI,which is minimal in the band A, increases progressively in the more weathered bands (B,C and D) and the trend is clear (DIA = 7.5%; DIB = 8%; DIC = 9.0%; DID = 9.5%). The


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Figure 10. Graphs of cumulative frequency curves: a) reflectivity values I measured on the altered and intactcolumns, b) reflectivity values I measured on the altered column and processed for the different vertical bands (A,B, C, D) (color figure available online).

cumulative frequency diagrams processed for the vertical bands, as shown in Figure 10b,highlight a reflectivity value change that is smaller when the weathering degree changes ifcompared to the curves of Figure 8.

Correlating reflectivity modal values (Imodal) to the modal rebound values (rmodal), twostraight lines can be obtained, for which equations a) and b) can be given as:

I mod e = 0, 105 # r mod e + 42, 94 (EQ2a)

I mod e = 0, 058 # r mod e + 46, 4 (EQ2b)

The straight lines obtained are drawn in figure 12. The straight line a), for the highest rmodal values characterized by a very high correlation coefficient R2 which is close to one(R2 = 0.992). This is the clear sign of a high dependency between I and r. The straight lineb) is for lower modal values (rocking flakes) and is characterised by a smaller correlationcoefficient.

Taking into account rebound and reflectivity value scatters, it is possible to con-clude that the laser scanner is more suitable to measure with a greater precision thesurface weathering. When the weathering degree of rocking flakes has to be considered,the Schmidt hammer gives instead more scattered data that are less directly linked to thereflectivity I. The use of scanner laser avoids the flakes detachment. It is worth to men-tion that results can be affected by a relevant presence of pollutant particles attached to thesurface but this is not the case of this investigation.

4. CONCLUDING REMARKS

The aim of this work was to verify that the terrestrial laser scanner can be a significantalternative detection technique in order to estimate the conservation state for the case ofmaterials for monumental buildings. A correlation between the reflectivity values I and theSchmidt hammer rebound values r was presented.

The increasing intensity of the weathering degree of the column surface has provideda good and appropriate sample to estimate the possible relation between the weathering


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Figure 11. Graph of reflectivity values measured on the altered column and processed in histograms relative tothe vertical bands and horizontal levels (color figure available online).

degree measured by means of direct measures, such as the Schmidt hammer rebound values,or by means of indirect measures, as reflectivity values obtained with the laser scanner tech-nique. An appropriate and proper knowledge of textural features have made experimentalresults easier to understand.

The decay of the ammonitic red limestone of Piana is characterized by different lev-els of weathering, from the simple fading to the corrosion of the bigger nodules, fromthe collapse of the cement veins that make the surface more rough, to an increase of the


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Figure 12. Graph of correlations for reflectivity against rebound data (modal values).

micro-roughness up to a final micro-cracking and fissure development with the formationof rocking flakes. Such an alteration is due to a complex dissolution process, the red veinsare less soluble than the nodules so that, in the first step, the latter corrodes and becomesvery rough and lowered if compared to the cement veins. In the second step opening of thestylolitic contacts occurs and the red veins are mechanically removed after dissolution orerosion of the calcitic silt between them.

After the description of the different weathering degrees on the areas affected, thelaser scanner technique has been used and it has been demonstrated that it represents anadequate procedure to measure superficial weathering phenomena. Indeed the techniquemeasures the reflectivity indices of each rock grain or particle, as shown in the modalvalues obtained for the vertical bands and in the related Gaussian leptokurtic distributionsof the I values.

Diagrams have been obtained by means of the same procedure of processing theSchmidt hammer rebound data. Histograms with a very small scatter have been found forthe rock band that is almost intact. The scatter increases when more weathered bands aretaken into consideration and the modal values distribution passes from a leptokurtic toplatikurtic one, which is characterized by a reduced frequency of the modal value and,where a higher weathering degree occurs, by a split of the modal values.

Hence, it is possible to conclude that the laser scanner technique is even more accu-rate than the procedure with the Schmidt hammer, because the reflectivity is measured overan area as small as a single grain surface, which is smaller than the larger area that theSchmidt hammer piston hits. Therefore the rebound values, in the mechanical procedure,are highly influenced by any part detachments in the rock volume close to the surface that ishit. As for the possibility of expressing all these values in a unique equation, the reflectivitymodal values agree with the rebound modal values so that the laser scanner technique can


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be a useful alternative and represents an innovative and non-destructive technique. Theprocedure is indeed able to detect in detail the weathering degree of this rock material thatis used for buildings and is more efficient and less expensive than direct measurementsthat are potentially detrimental for weathered surfaces. In the literature the technique waseffectively applied to natural formation of other lytothipes (Mesozoic limestones) and tolaboratory specimens of yellow Neapolitans tufs and Sicilian calcarenites (Nocilla et al,2007).

ACKNOWLEDGEMENT

We acknowledge Professor Aldo Evangelista of the University of Naples who loanedus the laser scanner. This research was supported by Università degli Studi di Palermo withits share of research Funds assigned by MIUR-Ministrero Istruzione,Università e Ricerca(Ercoli–Miur ex 60%). Regrettably, this and other research works could be not furtherdeveloped due to the drastic cuts, made from 2010 onwards, by Government and Parliamentof the Repubblica Italiana to funds for University, Public Education and Scientific Research.

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Documents

Measure of a Limestone Weathering Degree Using Laser Scanner