Journal of Building Engineering 7 (2016) 292–299

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Journal of Building Engineering

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Diagnostic approach for quantifying the soiling level and the aging oflimestone façade

Iqbal MarieDepartment of Civil Engineering, Faculty of Engineering, The Hashemite University, Jordan

a r t i c l e i n f o

Article history:Received 30 March 2016Received in revised form30 June 2016Accepted 18 July 2016Available online 19 July 2016

Keywords:Rebound numberLimestone façadeBlackeningTransition Impact Number

x.doi.org/10.1016/j.jobe.2016.07.00802/& 2016 Elsevier Ltd. All rights reserved.

ail addresses: iqbal@hu.edu.jo, iamarie2002@y

a b s t r a c t

This paper addresses a new approach for quantifying soiling and contamination of limestone façadesbased on contaminated surface hardness following multiple impacts of the Schmidt hammer plunger at agiven point. The Schmidt rebound hammer test is a quick and inexpensive test for determining surfacehardness. Accordingly, a number of residential buildings, in Amman, incorporating limestone façades ofdifferent ages and levels of soiling have been selected for testing. Finally, correlations between limestonehardness and Environmental impacts are drawn based on the soiling level or age of the façade. Theresults obtained demonstrate that it is possible to thoroughly characterize the level of contamination andcleaning requirements depending on the successive impact of Schmidt hammer on the same location toget a Transition Impact Number (TIN). Consequently, this could be significant in evaluating buildingcladding quality for cleaning or maintenance decision-making as it reflects the degree of contaminationand age of the façade.

& 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Wall cladding is a nonstructural material used for the exteriorsurface of buildings, which protects the other building compo-nents against moisture or other severe environmental factors. Itadds an aesthetic appeal to buildings [1]. Thus appropriately, de-graded and unappealing buildings leave poor impressions oncustomers, visitors and the majority of people. Subsequently, abuilding façades appearance sets the tone; allowing for first im-pressions, and further affecting the aesthetic appeal of the wholebuilt environment. Therefore, periodic façade cleaning is a vitaltask which will lengthen the life of a building, preserve the ex-terior surface and improve its value [2].

Natural white limestone is widely utilized in buildings as anexterior cladding of façades in Amman, the capital of the Ha-shemite Kingdom of Jordan. It is considered as one of the mostimportant cladding materials giving Amman its individual archi-tectural identity. The quality of limestone should fulfill the ne-cessary requirements in terms of strength, hardness, durability,color and porosity. Further, limestone has the advantage of beingreusable, in such a way that it can be used in road construction oras new aggregate material in concrete construction once its use asa stone cladding has been reached. Limestone cladding is an en-vironmentally friendly material; it has a cradle to cradle life cycle.Reusing limestone reduces the need for virgin natural resources,

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hence preserving the environment. A flow diagram depicting thelife cycle of limestone building cladding is shown in Fig. 1.

This study will concentrate on the environmental impacts ef-fecting limestone cladding during its service life phase. During thisphase, a building façades cladding may become contaminated byair pollution, evidently effecting it through depositing dust andloose materials thus staining it with an unpleasant dark color.Furthermore, air pollution may cause the rain to become acidic.The acidic nature of the rain can cause major damage to limestonefaçades over time. Air and rain in addition to soiling processesincluding biological and non-biological deposits, may be con-sidered as main contributing factors in limestone façade dete-rioration. A combination of physical and biological processes sig-nificantly increase the degree of decay [3]. The rate of deteriora-tion or soiling can be related also to the type of voids and surfacetexture. Soiling also depends on the rainfall conditions, whichcould influence both particle deposition and removal from stonesurfaces [4].

The level of contamination may affect the visual appearance ofthe building and its durability. Evidently, assistance in cleaning itis necessary, whether that be to increase aesthetic appeal or formaintenance purposes.

The accumulation of contaminants on buildings is not ne-cessarily evenly spread out rather produce different patterns.Different blackening patterns on buildings create different levels ofacceptability and could be relevant in regards to making a decisionabout cleaning [4]. The built environment and its appearance havea visual impact on visitors and local people. That is clearly evident

Fig. 1. Limestone life-cycle.

Fig. 2. Factors affecting stone façade cleaning and maintenance requirements.

I. Marie / Journal of Building Engineering 7 (2016) 292–299 293

through the statements of an American resident, stating: “Ammanis truly unique in that the majority of the buildings are constructedwith a white stone façade”. And said: “It does not take long for thedust and pollution to muddy up a beautiful white façade, turning itto a grungy, streaky gray. If the city developers want to continue tobuild using white stone construction, fine. I just wish the muni-cipality would enforce some sort of system to maintain a cleanappearance” [5]. A cleaning decision-making diagram for aesthe-tical requirements was developed by Marie [6], which involves theidentification of contamination patterns and the degree of col-oration in relation to the type of building. Although this representsthe first step towards the decision and application of the optimalcleaning procedure, Marie [6] revealed many divergent opinionsamong the assessors in regards to the acceptable level of con-tamination, depending on the type of the building.

The factors that may control the stone façades cleaning andmaintenance requirements are: building function, stone texture,stone quality, environmental conditions and aesthetic requirementsas shown in Fig. 2. Proper evaluation of these factors is often theinitial step towards cleaning and maintaining any building facade.The building function and the aesthetic requirement factors wereaddressed by Marie [6], revealing that each building should betreated as a separate case according to its type. As an example,heritage buildings should reflect the history and time of exposureand thus it is preferable they not be cleaned, unless maintenance isrequired to preserve the buildings structural stability. Dissimilar toheritage building, for religious and commercial buildings it is pre-ferable that they maintain their original appearance over time [6].Therefore, strategic cleaning schedule should be established for thewhole service life of the stone façade. Necessarily, the façades ofcommercial buildings in Jordan are heading towards new materialssuch as glass and aluminum, as natural stone façades have a rapidsoiling process. Therefore, the researcher will address the stonetexture and the stone quality under uniform environmental condi-tions for residential buildings that preserve the Jordanian identity oflimestone façade.

The stone texture factor plays a major role in the rate of soiling.Therefore, this study will be conducted on the most famouslimestone textures used in residential building façades in Amman.Table 1 presents these textures with their local names.

The level of stone deterioration can be quantified as a decline ofsome mechanical properties that can be measured experimentallyby means of nondestructive testing. The Schmidt hammer is anondestructive test that has been used in rock mechanic practicessince the early 1960s as an index test for a quick rock strength anddeformability characterization. Therefore, the Schmidt Type Lhammer will be implemented as an assessment tool for perfor-mance monitoring of limestone exterior claddings as part of a

maintenance management decision-making process based ontheir visual and physical properties under standard service con-ditions. Understanding the rate of deterioration mechanisms oflimestone cladding permits the criteria for proper conservationaltreatment to be established. Tests on both fresh and contaminatedsamples of limestone cladding of existing buildings at differentages were conducted using the L-type Schmidt hammer test. TheIndex value detected for each surface is a Rebound number (R),which is an indicator of the rocks surface hardness. The value of Rwill be used as an estimating tool for the surface contaminationlevel and its age. It has been used for estimating the effect of en-vironmental controls characteristic of rock weathering [7]. The Rvalues have been widely used to determine rock surface hardnessand the degree of surface weathering; hence length of exposureand relative age [8]. In fact, the instrument should be viewed as atechnique for preliminary assessment of age [9].

2. Methodology

In order to achieve the aim of this study, the following se-quential steps were carried out:

1. Existing residential façades in Amman were visually classifiedand selected according to the texture of the cladding. Four

Table 1The most famous stone textures used in Amman residential buildings with theirlocal names.

Rock name Description Texture

Musamsam A series of short, fine parallelline dressings done with atooth chisel.

Polished (Flat) Either a smoothly polishedsurface achieved with specialpolishing machines or aroughly polished surface thatcan be achieved throughsand blasting.

Mufajjar A medium dressing which isdone with a point chiselhammered on the stone sur-face with single strokes,creating a speckled surface.

Tubzeh A roughly dressed surfacedone by having relativelythick stone with a portion ofabout 5 cm from the surface.Small pieces are then split orchipped out from the surfacewith a pitching tool.

I. Marie / Journal of Building Engineering 7 (2016) 292–299294

different common texture types were selected as described inTable 1.

2. Further classification has been done according to the age andlevel of darkness of the façades for each type of texture rangingfrom new facades up to 25 years old.

3. The degree of soiling of the façade was quantified for eachtexture by collecting the amount of accumulated particles onthe measured area of each façade. This has been done over fivedifferent areas per each facade under assessment.

4. Since the Mufajjar texture is the most common among the othertextures in residential buildings in Amman, the hardness testwere conducted on the Mufajjar texture for all selected facades.

A flow chart describing the sequence of this study built fromtop to bottom and the hierarchies within each group is shown inFig. 3.

12

3. Facade properties and environmental conditions

3.1. Limestone properties

Limestone is a sedimentary rock composed primarily of cal-cium carbonate with the occasional presence of magnesium.Limestone in Jordan is classified into three categories according tocompressive strength, specific gravity, absorption and surfaceabrasion as shown in Table 2 [10].

The type of limestone plays a great part in the deteriorationlevel due to variation in its properties. The most famous types ofnatural limestone used in Jordan as building facade are commer-cially named according to the area from which it was extracted.Their quality are also commercially given [11]. For example, Ajlounlimestone is commercially specified as very solid, Hayyan stone is

the most common and sought after stone due to its moderate priceand its acceptable physical specifications, while Ma'an stone is thebest type according to its high commercial physical specifications.Ruweished stone has different varieties of quality. Commercialspecifications are unreliable and should not depend on. Customersshould refer to the scientific mechanical/chemical classificationand engineering tests in order to assess their suitability as buildingstones.

From communication with key actors in the construction andstone production field, it has been determined that the majority ofresidential building façades were made from category B limestonewith the quality described in Table 2.

Limestone was classified into three quality categories accordingto specific mechanical properties as shown in Table 2. The con-trolling factor in the classification process is the uniaxial un-confined compressive stress [10].

3.2. Testing conditions and façades samples

Jordan has a Mediterranean climate which is characterized bywarm to hot, dry summers and moderate to cool, wet winter. Themovement of moisture and variation of temperature plays animportant role in the decaying process of limestone façades.Therefore, the test was conducted in June during the summerweather where the cladding surface is dry. This will minimize theeffect that moisture and or a wet surface has over the results. Thefaçades selected for testing have not been subjected to cleaningduring their service lives.

Different types of buildings in Amman were selected withemphasis on residential façades. They were classified according tothe age and texture of the cladding and according to the darknesslevel of soiling assuming category B limestone with properties asdescribed in Table 2. The darker the color the more contaminatedthe façade. However, there is no clear visual distinctive line be-tween different contamination levels. The color of the surfaceranges from light - medium gray or medium brown to dark brownor black. Buildings with different level of darkening are shown inTable 3. Highly affected surfaces which appear to be have majordeteriorating features such as small voids and minor fissures havenot been considered. Limestone façades with an accumulation ofbiological colony have also not been considered. Moreover, thetested areas were selected away from driving rain that reach its'highest values at the edges and top corners of a building, as a wayof negating the effect that rain flow patterns have on façades.Further, as rain is responsible for the occurrence of stains andstreaks [12], the areas required for testing are those with evenlydistributed soiling, thus areas which had signs of streaking anduneven stains were not tested.

4. Testing

Field and laboratory testing were conducted on the selectedfaçades to determine:

) Soiling degree) Surface hardness

4.1. Soiling

Soiling of façades is one of the most apparent characteristics ofthe environmental impact on buildings [13]. It can be a potentialsource of facade decay and even structural damage. As the accu-mulation of dust depends critically on the texture of the facade onwhich it is deposited, therefore different textures and ages of stonefaçades, exposed to the same environmental and climate

Fig. 3. Flow chart for evaluating soiling degree on selected limestone façade textures.

Table 2Quality classification of limestone into three categories [10].

Qualitycategory

Absorption (%) Compressivestrength (MPa)

Specificgravity

Surface abra-sion (mm)

A o3.0 455 2.56 o33B 3.0–4.2 28–55 2.45 33–37C 4.2–7.5 12–28 2.16 37–44

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conditions, were studied. The mass of accumulated material perunit area was used as a means of quantifying the amount of dustpresent for each façades age. This was performed by brushing a300 mm�300 mm surface area with a brush and collecting all theaccumulated particles until the original color of the rock wasreached. Subsequently the collected material was weighted. Thetest was repeated on five different areas all with evenly distributedsoiling on the same façade as specified in Section 3.2. Five areaswere selected to minimize the effect of any variation in the soilingaccumulation. The selection protocol used in this study is shown inFig. 4, in which a grid of nine squares was drawn on the selectedsurface and the crossed squares represent the areas tested. Fig. 5shows the accumulation for the five selected areas for Mufajjarstone and the average for different ages. The error bars indicatethat the results showed an acceptable spread both above and be-low the mean. Therefore, the mean of five results will be takeninto consideration.

A Soiling Factor (SF) will be considered as the mass of accu-mulated dust per unit area. Fig. 6 presents the relationship be-tween the SF and the age of the façades with different textures.

4.2. Surface hardness

The Schmidt hammer L-type was used in this study to provide amean of rapid assessment for rock hardness. Schmidt reboundhammer number (R) value is on a scale from 0 to 100. According toASTM-D5873-14, this test method is suitable for rock materialwith uniaxial compressive strengths ranging between approxi-mately 1 and 100 MPa. Our tested limestone samples are of cate-gory B and within this range as indicated in Table 2.

There are a number of factors that influence rock surface hard-ness. For example, water content, loose particles layers which may beseveral millimeters and the presence of microbial colonies. These

factors can dampen the impact of the rebound test indenter. All thesamples were tested under dry surface conditions whilst sampleswith microbial colonies were not included in this test. The surfacehardness test was limited to the Mufajjar texture in this study.

The following steps were followed:

1. Limestone façades of residential buildings with the Mufajjarsurface dressing texture were selected varying between dif-ferent levels of contamination and age.

2. Prior to testing, each stone sample was visually inspected for sur-face defects and rock fabric/structure to avoid testing near fissures.

3. The surface was polished gently to some extent as the degreeof surface smoothness significantly affects the rebound values.Any surface irregularities are often crushed under the plunger,resulting in a loss of impact energy and therefore the wrongrebound number [14].

4. The surface should be totally dry to eliminate the effect ofmoisture on the results.

5. A total of twenty impacts of Schmidt hammer were conductedfor each tested façade surface. The test locations along the rocksurface were separated by at least the diameter of the hammerpiston. The hammer was held in normal position against thetested surface [15].

6. The surface of the limestone was visually inspected after eachplunger impact. Readings were rejected if any individual im-pact resulted in noticeable damage such as cracking or anyother visible failure [15].

7. Rebound number readings were recorded and the mean valueswere calculated. Any deviated readings within 77 units fromthe average were discarded and the average was calculated forthe remaining readings [14].

8. The R values along with their corresponding average and cal-culated standard deviation for each tested façade are presentedin Fig. 7.

9. A plot of the R values as a function of the age of façade is shownin Fig. 8.

10. Ten successive impacts at the same location have been carriedout on an unweathered limestone. The R values were recorded.The test was repeated on 10 different points with the resultsshown in Fig. 9.

11. The same procedure used in step 10 has been implemented forthe selected contaminated facades.

Table 3Buildings with different level of blackening.

Age (years) Description Sample

0–5 Clean surface with loose soilparticles. light gray or lightbrown color

5–10 Medium gray or mediumbrown color

10–15 Dark brown or black color.Some contains white stainsdue to rain wash out

15–20 Very dark brown or black

Any age Stones with biological colony

Fig. 4. A grid showing the selected areas on a façade for soiling accumulationmeasurements.

0

20

40

60

80

100

120

0 1 2 3 4 5 6

Acc

omul

atio

n ( g

m/m

2 )

Area Number

5 years 10 years 15 years 20 years avg

Fig. 5. The accumulation for the five selected areas for Mufajjar stone and theaverage for different ages.

I. Marie / Journal of Building Engineering 7 (2016) 292–299296

5. Results and discussion

5.1. Phases of blackening levels

The damage process in terms of blackening of building's lime-stone façade can be divided into three phases:

1. Phase 1: Light accumulation of soil or loosed particles. Goodsurface quality.

2. Phases 2: An increase in the darkening of the stone with anincrease in the accumulation of loose particles along withfragmentation in the stone surface through partial loss of theprojected texture.

3. Phase 3: The final deterioration level, having high levels ofblackening and a thick layer of deposited materials.

5.2. Hardness

The experimental data was statistically analyzed to determinethe best-fit correlation between the Schmidt hammer reboundnumber and the age of limestone façade as shown in Fig. 8. Thereis a noticeable relationship between the rebound number and age.

Fig. 6. Soiling Factor (SF) against time for different types of stone texture.

Fig. 8. R value as a function of the age of the façade due to a single impact on thesurface.

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As the rebound number decreases the age of façade increases. AnExponential model was suggested to represent this relation with ahigh regression coefficient R2 value. The reduction of the R valueswith age indicates that the higher the level of contamination orsoiling accumulation, the lower the surface hardness. Environ-mental impact on limestone surface significantly affects theSchmidt rebound hammer results. A low rebound number indicatesthat the surface of the stone is losing its hardness due to environ-mental impact when compared to a fresher surface. Unfortunately,there is no theoretical relationship between surface hardness andlevel of deterioration. A single impact of Schmidt hammer on astone surface cannot indicate the soiling degree, although it isshown in Fig. 8 that it does decrease with age. It can be used as a

Fig. 7. Rebound number per each data lo

quality control for the limestone façade. Therefore, the tests wereperformed on the same spot and the results are plotted per thenumber of the test. Variations in the R-value reflects the degree ofweathering of the rock surfaces [16]. The changes in the reboundvalues during multiple impacts at a given point produced a betterindication of the weathering grade than a single impact value [14].When an individual surface displays a decrease in the reboundvalue from the first or second impact at the same point, this may be

cation for facades of different ages.

Fig. 9. Rebound readings for successive impacts on the same location for freshlimestone at 10 different points.

Fig. 10. Relationship between the R values and the number of impacts at the samelocation.

Fig. 11. Relation between TIN and age of façade.

0

5

10

15

20

25

30

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5 6 7 8 9

Age

(yea

rs)

SF

TIN

SF age

Fig. 12. Correlation curves relating the TIN to the SF and the age of the façade.

I. Marie / Journal of Building Engineering 7 (2016) 292–299298

taken as a mechanical index of weathering [17]. Fig. 9 reveals thatsuccessive impacts on the same location of fresh limestone de-creases while increasing the impact number. All data points ofsuccessive impacts on the same location are within 71 SD from themean, with a reduction in the R value generated by increasing theimpact number on the same location. However, successive impactson the same location for contaminated limestone at different agesdisplays data results of two distinct linear trends that change slopeat a certain impact number which differs according to the age andcontamination grade of the stone. This number will be referred hereas the Transition Impact Number (TIN). It is the point where therebound value against the impact number changes its slope. It isclear from Fig. 10 that the TIN increases with an increase in agefollowing an exponential trend as shown in Fig. 11.

It reveals a strong correspondence between the TIN and the ageof the limestone façade. This is supported by the high value of thecoefficient of determination R2¼0.9953.

The proposed TIN is a reliable property since it varies con-sistently throughout the soiling process. It is a reliable indicator ofthe age of the stone cladding and the degree of contamination.Fig. 12 illustrates correlation curves relating the TIN to the SF andthe age of the façade.

The results indicated that each soiling state was associated witha specific TIN, as the soiling degree increased, the TIN increased.

This observation can be attributed to the depth and properties ofthe accumulated loose materials.

Reduction in the R value with successive impacts on the samelocation is due to the reduction in the surface hardness for highlycontaminated limestone due to greater levels of dust accumulatedand low cohesiveness between the loose deposited particles andthe surface. At the Transition Impact Number some of the accu-mulated material was removed and the surface quality was mov-ing towards the original surface taking into consideration the re-duction in the R values due to the successive impacts as revealedin Fig. 9.

The phase of blackening levels, the soiling factor, the age of thefaçade and the TIN were related in Fig. 13. The proposed TINhardness estimator is a unique expression for estimating thesoiling degree in terms of the age of the façade and may be con-sidered as a significant improvement in the rebound number testfor measuring surface contamination.

6. Conclusion

In this paper, environmental impacts on limestone façades andthe prospects for detection were studied while also evaluating

Fig. 13. Phases of contamination level and its relation to age of the façade, the SF and the TIN.

I. Marie / Journal of Building Engineering 7 (2016) 292–299 299

these impacts through nondestructive methods. The concept ofchange in the surface hardness of limestone façade due to soilingthrough its long service life was discussed, with emphasis on fa-cades' aesthetical values. As a final point, correlations betweenlimestone hardness and the environmental impact as a con-tamination level or age of the façade are drawn. This study adviceson measures which must guide a cleaning or maintenance judg-ment of the building façade. The predicted Transition ImpactNumber (TIN) can be used to reflect the degree of contaminationas well as the age of the façade to help with maintenance orcleaning decision- making.

Although this study was limited to facades of 20 years old usingthe nondestructive Schmidt hammer test, it has been widely usedto determine rock surface weathering, and its relative age [8], andtherefore it may be used for detecting the soiling level of the ex-terior façade of historical buildings and therefore their age.

Based on the results described in this paper, the followingconclusions can be drawn:

� Hardness of rock surface can be used as a tool for mappingsoiling progression as well as dating exposure of rock surfaces.Applying successive impact on the same location results in a TINcontamination and aging estimator. The TIN estimator is a un-ique expression for estimating the soiling level in terms of theage of the façade and may be considered as a significant im-provement in the rebound number test for measuring surfacecontamination.

� This research was intended to be useful to the future estab-lishment of rules for evaluating the level of soiling and the ageof building estimation.

7. Further study

The proposed TIN hardness estimator does not consider severalfactors such as water content of limestone which requires furtherstudy. Moreover, this estimator is only compared with soiling anddoes not take into account other important decay mechanisms.Further research into a wider range of rock textures and durabilitytests are needed in order to be able to fully apply the proposedestimator to other types of stones.

Acknowledgement

The author acknowledges Mrs Maha Nayfeh, a Bachelor of Artsand Education, University of Queensland, Australia for her effortsin cross checking this article.

References

[1] L.G.W. Verhoef, Soiling and cleaning of building facades. RILEM Report. NewYork, NY Chapman and Hall, 2008.

[2] Kaizen, Cladding Cleaning and Refurbishment, 2014 [cited 2016 March];(Available from): ⟨http://www.kaizengroup.co.uk/building-cleaning-services/cladding-%20cleaning/⟩.

[3] S. Papida, W. Murphy, E. May, Enhancement of physical weathering of buildingstones by microbial populations, Int. Biodeterior. Biodegrad. 46 (4) (2000)305–317.

[4] C. Grossi, R. Esbert, F. Dıaz-pache, F. Alonso, Soiling of building stones in urbanenvironments, Build. Environ. (2003) 147–159.

[5] Dave Amman: The Gray City, 2006.[6] I. Marie, Perception of darkening of stone façades and the need for cleaning,

Int. J. Sustain. Built Environ. 2 (1) (2013) 65–72.[7] C.D. Hansen, et al., Aspect-controlled weathering observed on a blockfield in

Dronning Maud Land, Antarctica, Geogr. Ann.: Ser. A Phys. Geogr. 95 (4) (2013)305–313.

[8] R.A. Shakesby, J.A. Matthews, G. Owen, The Schmidt hammer as a relative-agedating tool and its potential for calibrated-age dating in Holocene glaciatedenvironments, Quat. Sci. Rev. 25 (21) (2006) 2846–2867.

[9] D. McCarroll, Potential and limitations of the Schmidt hammer for relative-agedating: Field tests on Neoglacial moraines, Jotunheimen, southern Norway,Arct. Alp. Res. (1989) 268–275.

[10] K. Tarawneh, S. Al-Thyabat, M. Al-Harahsheh, Mineralogical, physical andmechanical properties of limestone rocks in Ma’an area, South Jordan, Geologyand Geophysics 50 (1) (2007) 123–128.

[11] M. Tarrad, O. Al-Omari, A. Mohammed, Natural stone in Jordan: characteristicsand specifications and its importance in interior architecture, J. Environ. Sci.Eng. B 1 (6B) (2012) 720.

[12] B. Blocken, D. Derome, J. Carmeliet, Rainwater runoff from building facades: Areview, Build. Environ. 60 (2013) 339–361.

[13] M.-N. Terrat, R. Joumard, The measurement of soiling, Sci. Total Environ. 93(1990) 131–138.

[14] A. Aydin, A. Basu, The Schmidt hammer in rock material characterization, Eng.Geol. 81 (1) (2005) 1–14.

[15] R. Atkinson, et al., Suggested methods for determining hardness and abra-siveness of rocks, Int. J. Rock Mech. Min. Sci. 15 (3) (1978) 89–97.

[16] V. Gupta, R. Sharma, M.P. Sah, An evaluation of surface hardness of natural andmodified rocks using Schmidt hammer: study from northwestern Himalaya,India, Geogr. Ann.: Ser. A Phys. Geogr. 91 (3) (2009) 179–188.

[17] R. Ulusay, The ISRM Suggested Methods for Rock Characterization, Testing andMonitoring: 2007–2014, Springer, 2015.