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Petrophysical and mechanical properties of the Struganik limestone(Vardar Zone, western Serbia)

VIOLETA GAJIĆ1, VESNA MATOVIĆ1,NEBOJŠA VASIĆ1 & DANICA SREĆKOVIĆ-BATOĆANIN1

Abstract. The Struganik limestone has been increasingly popular in recent years for interior and exteriorbuilding applications, due to easy workability, low cost and multi purpose suitability. The quality of limestoneis determined by its mineralogical and textural characteristics and physico-mechanical properties. TheStruganik stone corresponds to marl, clayey limestone and limestone (micritic and allochemical). The lime-stone is commonly layered, either thinly bedded or banked. Chert concretions are present in all varieties. TheUpper and the Lower Campanian age were deduced by the abundant foraminifers. Micrite limestone is anautochthonous rock generated in a deep marine environment, whereas allochemical limestone is related to ashallow marine environment but subsequently brought into a deep-water system.

The mass quality of the Struganik limestone is controlled by variability of lithology and layering. The val-ues of physico-mechanical properties, such as density, porosity, water absorption and strength, were statisti-cally analyzed and the obtained data were used to assess the rock quality in the quarry. The relationship amongthe quantified properties is described by regression analyses and the equations of the best-fit line.

The Struganik limestone was qualified by its petrological and engineering properties coupled with statisti-cal analysis. It satisfies the majority of the requirements of the Main National Standard as a decorative stone,but with a limiting factor regarding abrasive resistance.

Key words: Struganik limestone, western Serbia, petrology, physico-mechanical properties, correlation,rock quality.

Апстракт. Интересовање и примена струганичких кречњака како у екстеријеру, тако и у ентеријерује у порасту последњих година, посебно у западној Србији, због њихове лаке обраде, ниске цене имогућности различите примене. Квалитет кречњака зависи од његових минералошких и текстурних,као и физичко-механичких својстава. Петролошки, струганички кречњаци одговарају лапорцима, гли-новитим кречњацима и кречњацима (микритски и алохемијски). Обично су слојевити, танкоплочастиили банковити. У свим варијететима јављају се конкрециони рожнаци. На основу бројних форами-нифера ови седименти су сврстани у доњи и горњи кампан. Микритски кречњаци су аутохтоне стененастале у дубоководној маринској средини, док је порекло алохемијских кречњака везано за плиткуморску средину, одакле су принети у дубоководни систем.

Квалитет струганичких кречњака је условљен литолошким саставом и типом слојевитости.Вредности физичко-механичких својстава, као што су густина, порозност, упијање влаге и чврстоћа,статистички су анализирани, а добијени резултати су коришћени као показатељи квалитета стенскемасе каменолома. Односи између испитиваних својстава су испитани регресионом анализом и израже-ни једначином “best-fit line”. Петрографска и техничка својства заједно са резултатима статистичкерегресионе анализе која је примењена у овом раду, указују да струганички кречњаци по свом квали-тету испуњавају већину захтева сходно главном националном стандарду за примену архитектонскогкамена, а да је ограничавајући фактор њихова отпорност према хабању.

Кључне речи: струганички кречњак, западна Србија, петрологија, физичко-механичке особине,корелација, квалитет кречњака.

GEOLO[KI ANALI BALKANSKOGA POLUOSTRVA

ANNALES GÉOLOGIQUES DE LA PÉNINSULE BALKANIQUE72 87–100 BEOGRAD, decembar 2011

BELGRADE, December 2011

1 Department of Petrology and Geochemistry, Faculty of Mining and Geology, University of Belgrade, Djušina 7, 11000Belgrade, Serbia. E-mails: [email protected]; [email protected]; [email protected];[email protected]

DOI: 10.2298/GABP1172087G

Introduction

Natural stone is one of the most common construc-tion materials. Dimension stone is used in areas wherethe aesthetical properties (color, design), soundness ofthe deposit and market demand are crucial factors(LUODES et al. 2000). To be used for the interior andexterior of buildings, a natural stone must satisfy stan-dardized physical and mechanical requirements strict-ly (YAGIZ 2010). Petrophysical and mechanical prop-erties of sedimentary rocks are influenced by the size,shape, packing of grains, porosity, cement and matrixcontent, depending strongly on depositional fabricand post-depositional processes (ANDRIANI & WALSH

2002). These properties are measured in laboratoriesby standardized, relatively simple methods but theyare time consuming and expensive and require a well-prepared rock sample. For this reason, the modernworld trend in geological and civil engineering prac-

tice is the investigation of the relationship betweencomplex mechanical and simple physical properties.The equations for estimating mechanical properties onthe base of simpler, faster and more economical indi-rect tests have been presented by a great number ofauthors (HANDLIN & HAGER 1957; D’ANDREA et. al.1964; DEER & MILLER 1966; BROCH & FRANKLIN

1972; OLLSON 1974; BIENIAWSKI 1967, 1975; HUG-MAN & FREIDMAN 1979; GUNSALLUS & KULHAWY

1984; FREDRICH et al. 1990; WONG et. al. 1996;HOFFMAN & NIESEL 1996; HATZOR et al. 1997, 1998;TURGUL & ZARIF 1999; PALCHIK et al. 2000, 2002).

Carbonate deposits with different lithology, age andquality are widespread in Serbia, making limestonethe most excavated stone for building purposes. In thewestern part of Serbia, particularly in the Mionicaarea (Fig. 1), there are more than 70 quarries excavat-ing limestone known as Struganik stone. The firstwritten data related to the exploitation of this stonedates back to 1737. In 1861, Struganik stone was usedfor the building of the Vienna Opera House.

In the last two decades, the popularity of this lime-stone and its use as an architectural stone has increasedrapidly due to its cost coupled with aesthetic value,quality and available reserves. It is still the main build-ing material in this area, used for exterior and interior

cladding but with limited ap-plication mainly due to insuf-ficient investigations from geo-logical and engineering as-pects. Many authors investigat-ed the stratigraphy of this area(e.g., ANĐELKOVIĆ 1978), pale-ontological assemblages (e.g.,FILIPOVIĆ et al. 1978) and tec-tonic features (KARAMATA et al.1994, 2000; DIMITRIJEVIĆ 2001;GERZINA 2002; ROBERTSON etal. 2009). Only a small numberof recently published papersprovide general information onthe stratigraphy and lithologyof stone in the Struganik area(VASIĆ et al. 2001; RABRENOVIĆ

et al. 2002; VASIĆ et al. 2005;GAJIĆ 2007; DJERIĆ et al. 2009;GAJIĆ & VASIĆ 2011). Unfor-tunately, petrological and tech-nical features of the Struganiklimestone, data of the depositi-onal processes and problems ofthe quality and assortment ofuse have not yet been studied indetail. Insufficient data raised anecessity for an improvementof the exploration methodologyof the Struganik limestone.Therefore, studies concerning

the fabric of the Struganik limestone and its quality as abuilding stone have not only theoretical significance,but also practical importance for civil engineering.

The object of the present investigation was the old-est and largest quarry in the Struganik area, with thesame name (Fig. 2), located about 100 km southwestof Belgrade (Fig. 1).

VIOLETA GAJIĆ, VESNA MATOVIĆ, NEBOJŠA VASIĆ & DANICA SREĆKOVIĆ-BATOĆANIN88

Fig. 1. Location of the Struganik limestone in western Serbia, central part of the VardarZone. Right: Terranes of the central part of the Balkan Peninsula (According toKARAMATA & KRSTIĆ 1996; KARAMATA 2006 and ROBERTSON et al. 2009). Key (inalphabetic order): CBES, Eastern Serbian Carpatho–Balkanides; DIU, Drina–IvanjicaUnit; DOB, Dinaride Ophiolite Belt; EVZ, Eastern Vardar Ophiolitic Zone; JU, JadarUnit; KU, Kopaonik Unit; SMM, Serbo–Macedonian Massif; WVZ, Western VardarOphiolitic Zone.

This quarry is irregular in shape, elongated in theNW–SE direction and opened at approximately 0.32km2 (KIJANOVIĆ 2007). The extraction in it, 30 yearsold, is constrained by bedding planes in limestone(Fig. 2). The extraction comprises the removal oflarge tiles and their cutting into natural-faced slabsusually 2–3 cm thick using a circular blade saw orhand tools. These products, with natural appearance,are packed on a palette and ready for use as externaland interior cladding. Their limited application ismainly caused by insufficient investigations of theirproperties needed to meet the requirements of archi-tectural stone.

The purpose of this study was a detail determina-tion of the lithological, structural and textural proper-ties, and the depositional processes and their link withphysical and mechanical properties. This work pro-vides new geological data for the Struganik limestonebased on petrological field observations, optical andchemical analysis of the collected samples, as well ason results of a statistical analysis of the geomechani-cal characteristics and, finally, an assessment of thelimestone quality as a building stone.

Geological setting

Geological setting of the Struganik area consists ofMesozoic formations of Triassic, Jurassic and Creta-ceous age, and of Quaternary sediments to a lesser ex-tent. According to ANĐELKOVIĆ (1978), the shallowTriassic sediments were deposited over the Permian.Complex and geotectonic processes during the Jurassicled to the formation of the ophiolite mélange (SCHMID

et al., 2008). The Struganik area is part of the northernrim of the Maljen ophiolitic massif. This region is in theVardar Zone Western Belt (Fig. 1; KARAMATA & KRSTIĆ

1996; KARAMATA 2006; ROBERTSON et al. 2009).

In the wider area, Cretaceous sedimentation com-menced with the Albian transgression. Terrigenous-carbonate and carbonate sedimentation continuedfrom the Albian to the Maastrichtian, while flysch(“Ljig flysch”) sediments were deposited in the latestSenonian (FILIPOVIĆ et al. 1978).

According to data obtained during geological stud-ies in the villages Osečenica, Brežđe and Struganik,MARKOVIĆ & ANĐELKOVIĆ (1953) considered theStruganik limestones as Senonian, i.e., as the latestCretaceous products in this area. The Cretaceous sed-iments in the broader Struganik area were studiedrecently by VASIĆ et al. (2001), RABRENOVIĆ et al.(2002), GERZINA (2002), VASIĆ et al. (2005), GAJIĆ

(2007), DJERIĆ et al. (2009), GAJIĆ & VASIĆ (2011).FILIPOVIĆ et al. (1978) distinguished within the Creta-ceous sediments the following members: detrital lime-stones, claystones and sandy marlstones of Albianage, conglomerates and limestones of Albian–Ceno-manian age, Cenomanian conglomerates, limestones,marlstones and sandstones, organogenic-detrital lime-stones and marlstones of the Cenomanian–Turonian, alimestone and limestone–marlstone series of Turoni-an–Senonian age and Campanian–Maastrichtian fly-sch (Fig. 3).

Petrophysical and mechanical properties of the Struganik limestone (Vardar Zone, western Serbia) 89

Fig. 2. The “Struganik” quarry in the Struganik area.

Fig. 3. Simplified geological map of the Struganik area ba-sed on the Basic Geological map sheet Gornji Milanovac1:100000 (FILIPOVIĆ et al. 1976). Legend: 1, Jurassic ophi-olite mélange; 2, Triassic shallow sediments; 3, Conglo-merate and limestone of Albian–Cenomanian age; 4, Ce-nomanian sediments; 5, Limestone and limestone–marl-stone series of Turonian–Senonian age; 6, Campanian–Ma-astrichtian flysch.

The Turonian–Senonian age of the Struganik lime-stone is documented by the microfossils: Praeglo-botruncana helvetica, Rotalipora sp., Globotruncanalapparenti coronata, etc. (FILIPOVIĆ et al. 1978). Arecent micropaleontological analysis, i.e., the associa-tion of pelagic globotruncanas, indicates that thesesediments are of Campanian age (GAJIĆ 2007). Theage of radiolarians from the layer of smectite claywithin the micrite limestone of the Struganik wasdefined as the Coniacian (VASIĆ et al. 2005; DJERIĆ etal. 2009).

Material and methods

Determination of petrological properties of theStruganik limestone commenced with field observa-tions in the quarry and preparation of a detailed litho-logical column. A total of 33 samples were collectedfrom the lowest level of the quarry for further labora-tory testing in order to assess the structural and chem-ical composition of the limestone (Fig. 2). A micro-scopic analysis of thin sections by optical microscopywas used to characterize the mineral composition,microtextural features (size and kind of allochem,grain orientation, etc.), and content and type of micro-fossils. Based on the field inspection and the results ofa petrographic (optical) investigation, the rock sam-ples were classified according to the FOLK (1959) andDUNHAM (1962) classification scheme. In the samesamples, the contents of CaO and MgO were deter-mined by the complexometric method (EDTA titra-tion) using a standardized solution of 1 M EDTA asthe titrant.

In order to examine the quality of the Struganiklimestone as a building stone, the physical (bulk andapparent density, porosity, water absorption, frost re-sistance) and mechanical properties (compressivestrength, abrasive resistance) of a total of 18 samplesfrom the quarry were determined. The laboratory testswere performed according to the national standards(Unification of Serbian Standards) at the Laboratoryof the Highway Institute of in Belgrade. The bulk den-sity is given as mass per unit of apparent volume. Thevolume was determined by hydrostatic weighing ofthe specimens soaked and suspended in water underatmospheric pressure (according to SRPS B.B8.032).On powdery samples, the real densities were deter-mined using a pycnometer with water as the displace-ment fluid. From the values for real and bulk density,the total porosity was calculated (according to SRPSB.B8.032).

The specimens that were used for the determinationof the bulk density were also used for the determinationof the water absorption, which was determined bymeasurement of the mass of water absorbed by thesample after immersion for 48 h at atmospheric pres-sure. It is expressed as a percentage of the initial mass

of the sample previously dried at 105 °C to constantweight (SRPS B.B8.010). The resistance to frost wasdetermined by 25 cycles of freezing to –25 °C and thaw-ing according to SRPS B.B8.001. The freeze–thaw testswere performed on cubic samples to assess the durabil-ity of the limestone samples by determining theirstrength and mass loss after 25 freeze–thaw cycles. Thetest procedure involved repeating of cycles where onecycle involved placing a water-saturated sample in afreezing chamber for 4 h, after which the sample wastotally immersed in water in a thawing tank for 2 h.

The limestone samples were tested for their me-chanical properties (compressive strength and abra-sion resistance). The uniaxial compressive strengthtests (SRPS B.B8.012) were performed on both dryand water-saturated limestone samples (5×5×5 cmcube) and on samples that had been subjected to thefreeze–thaw test. According to SRPS B.B8.015, cubicsamples of 7 cm in size were used for determinationof the Böhme abrasion resistance. The test procedureinvolved 440 cycles of sample abrasion using abra-sion dust (crystalline corundum) on a rotating steeldisc under an applied stress of 300 N.

In order to evaluate the relationships between thephysical and mechanical properties, different statisti-cal analysis techniques (basic descriptive statistics,correlation, bivariant regression analysis, etc.) wereperformed using the SPSS computer program. All themeasured values of the technical properties were sta-tistical analyzed at the level of significance (α) of0.05. The descriptive statistical parameters includedthe mean, minimum and maximum value, standarddeviation, median, standard error, range, quartiles,standardized skewness, kurtosis, etc. The Kolmogo-rov-Smirnov (non-parametric test for normality) wasused with the level of significance set at 0.2.

Subsequently, the results of the laboratory measure-ments were correlated using the method of leastsquares regression. Linear regression (y = ax+b) wereperformed and the equations of the best-fit line, theregression coefficient (R2) and the 95 % confidencelimit were determined for the regressions.

Validation of the regression analyses was confirm-ed by the Student t-test and by the test of variantanalysis (the F-test).

Petrological propertiesof the Struganik limestones

Struganik limestones are the latest Upper Creta-ceous sediments in this part of western Serbia. Thelimestones are best uncovered in the village Stru-ganik. Exploration were performed in the “Struganik”quarry and a lithological column about 10 m tall wasconstructed (Fig. 4).

The limestones are massive and compact. Theiruniform grayish color throughout the entire deposit is



Fig. 4. Local lithological column of the lowest level in the “Struganik”quarry.

sporadically disturbed by the presence of dark gray,grayish blue or whitish chert concretions.

The main lithological member is micrite limestone,which represents the autochthonous rock of deepwater, marine carbonate sedimentation (Fig. 5).Micrite limestones are layered in thick beds (Fig. 5a).The distinctive beds display notable horizontal lami-

nation. Allochemical limestone, the second lithologi-cal member, is related to a shallow marine environ-ment. Subsequently, the shallow water allochemswere transported into a deep-water environment byturbidity currents to produce either beds or banks(Fig. 5a) or sporadic packages composed of two orthree beds. Allochem limestone carries textures that


Fig. 5. The appearance of the Struganik limestone. a, Beds of Struganik limestone. Thick bed of the allochemical-sparitelimestone with gradation (A). Beds of the micrite limestone (1, 2, 3, 4, 5, 6 and 7). Stylolitic boundary surface between beds2 and 3. Concretional interlayer of black chert in bed 4; b, Bank of the allochemical limestone with well-developed Boumaintervals a & b. Interval “a” is calcrudite with fragments of the micrite limestone (caught by turbidity flow). Interval “b” iscalcarenite. Along the boundary between a & b whitish concretional chert is developed; c, Upper bedding plane with ripplemarks in fine-grained calcarenite; d, Grouped tracks of life activities on the lower bedding plane in the Struganik limestone(obscured origin); e, Test of Inoceramus in the micrite limestone.

are characteristic for the turbulent transport (particu-lar intervals of the Bouma Sequence within the layers,Fig. 5b).

Mechanical and biogenic textures are common onthe bedding planes in the Struganik limestone (Fig.5c, Fig. 5d). Bedding planes in both of the mentionedlimestones are often stylolitic (Fig. 5a). On the upperbedding planes in micrite limestones, Inoceramus canbe found abundantly (Fig. 5e).

The third lithological member, concretional cherts,was produced by silification during diagenesis. Thechert concretions are notable textural, as well as beinga petrological feature of the Struganik limestone. Thechert concretions are uniformly distributed through-out the whole column in both micrite and allochem-sparite limestone (Fig. 5a, Fig. 5b).

The paleontological study of the microfauna (manyforaminifers) indicates to Campanian age of the Stru-ganik limestone (GAJIĆ 2007). According to the ob-served association, the first three meters of the columncorrespond to the Lower Campanian, whereas theremaining 7 m of the column corresponds to theUpper Campanian.

Micrite (orthochemical) limestone

The micrite limestone is mainly composed ofmicrocrystalline calcite – micrite. Non-carbonaceouscompounds are clay minerals, organic matter and sub-ordinated silty material. According to the CaCO3 con-tent, micrite limestone refers to pure limestone, clayeylimestone and marlstone (Fig. 4). The least amountsof CaCO3 were determined inside the Lower Campa-nian rocks (marlstone - 55–60 % and clayey limestone- 70–85 %). This is, beside their micro paleontologi-cal characteristics, identifiable feature in respect tothe Upper Campanian rocks (86–94 %). The allochem(biogenic) component is represented by well-preserv-ed moulds of microorganisms. Pelagic foraminifers(globotruncana) are prevailing, but some calcispheresand radiolarians were also registered. The micrite li-mestone could be defined by the amount of biogeniccomponent, defined either as micrite and fossiliferousmicrite (less than 10 % of biogenic allochem, Fig. 6a),or as biomicrite (biogenic allochem exceeding 10 %,Fig. 6b). According to the DUNHAM classification(1962), the micrite limestone belongs to Mudstoneand Wackestone.

Allochemical limestone

The allochemical limestones are represented byalternation of very coarse-grained rudites to fine-grained calcarenites, regarding their textural features,i.e., the coarseness of the particles and the allochemcomponent.

The allochem and orthochem types, the bio-intra-spar varieties are the commonest (Fig. 7.). Among thesparry varieties, intrasparrudite, biosparrudite, intra-biosparrudite, biosparite and intrabiosparite may bedistinguished.

The biogenic allochem compounds include frag-mented moulds of shallow-water organisms (mol-lusks), moulds of benthos and planktonic microorgan-isms as well as algal fragments (Fig. 7.а.). Intraclastsmight be simple or composite. The simple intraclastsusually occur within the medium and fine-grained cal-carenite varieties (Fig. 7.b), while the latter were regis-tered in the coarse-grained varieties of calcarenite ofthe allochemical limestone. The amount of CaCO3 (cal-cite) in these limestones is in general higher than in themicrite varieties (Fig. 4), attaining from 67 to 96 %.

Depositional environmental

The Pelagic sediments, represented by marlstone,clayey limestone and limestone, were deposited in thedeeper parts of an Upper Cretaceous sea. All the men-tioned sediments contain chert concretions, just likethe Struganik limestone. The autochthonous sedi-


Fig. 6. Photomicrographs of the micrite limestone: a, Bio-micrite; b, Fossiliferous micrite with globotruncana in themiddle.

ments are marly limestone and marlstone of micriteand biomicrite composition. The presence of weaklypreserved radiolarians and silica sponges in theserocks indicates to their formation at depths deeperthan 200 m, while the partial replacement of thesiliceous skeletons with calcite implies an alkalineenvironment, most probably with pH 8 during diage-nesis. The presence of concretional cherts indicatesthe presence of organogenic siliceous remains. Theassociation of rocks and fossil remains is indicative ofsedimentation related to a deep marine environment.The presence of layers and banks of allochemsparite,calcarenite and calcrudite with characteristic texturesfrom the Bouma sequence suggests an intermittentsupply of shallow-water material by turbid flowsalong the continental slope, i.e., through the BathyalZone.

It could be concluded that the Struganik limestonewas derived in a deep marine environment, throughdeposition on a continental slope.

Physico-mechanical propertiesof the Struganik limestone

The results obtained from the statistical analyses ofthe physico-mechanical values of the Struganik lime-stone are given in Table 1. The real density of a rockis one of its basic properties. It is influenced primari-ly by the density of the minerals, their content andamount of void space inside the rock (BELL & LIND-SAY 1999). In the investigated samples of Struganiklimestone, the real density displayed little variationand range from 2700 to 2720 kg/m3, with a meanvalue of 2710 kg/m3. The bulk density also variedslightly due to the low amount of void space and wasin range 2610 to 2680 kg/m3, with a mean value of2650 kg/m3. All of the tested samples had a moderatedensity according to BILBIJA & MATOVIĆ (2009). Thevalues of the absolute porosity varied from 1.10 to3.30 %. Central tendency parameters indicated a neg-ative asymmetrical unimodal distribution of theporosity, with the most frequency class (1.0–1.5 %) of43 %. According to the mean value of 2.0 %, theselimestones generally have a low porosity, although insome samples a moderate porosity was detected (BIL-BIJA & MATOVIĆ 2009). The obtained values of waterabsorption (from 0.17 to 0.67 % with an average valueof 0.4 %) indicate a very low water absorption by theStruganik limestone. All the investigated limestonesamples were resistance to frost. The mass loss afterfreezing and thawing was low due to the low porosityof the Struganik limestone.

The values of the central tendency for all the phys-ical parameters were similar and indicate to a sym-metric, unimodal distribution conforming closely to aGauss curve. The statistical parameters of the Kolmo-gorov-Smirnov test confirmed that all the tested sam-ples had a normal distribution of the measured valuesof the physical properties (Dmax > Dcr, Table 1).

The dry unconfined compressive strength values ofthe limestones varied between 48 and 189 MPa, witha mean value of 134 MPa (Table 2). Most of the testedsamples were very strong (strength > 150 MPa), whileover a quarter of the samples were moderately strongaccording to the proposed strength classification ofBILBIJA & MATOVIĆ (2009). The values of the saturatedcompressive compression strength were between 41 to177 MPa, with an average value of 120 MPa. The meanof the water saturated samples was 10 % lower thanthat in the dry state, while after 25 freeze–thaw cycles,it had decreased by 4 % to 129 MPa (mean value),indicating that the Struganik limestone is durable tofreezing and thawing. The negligible decrease in theunconfined compressive strength reflects that theamount of saturated water is not an important factorfor the strength reduction. A significant reduction inthe strength appeared when the porosity of the lime-stone exceeds 3 % (the reduction was about 30 %).The values of volume loss on wear (Böhme abrasion


Fig. 7. Photomicrographs of the allochemical limestone: а,Fine-grained intrabiosparite. In the middle of the photo is astylolite seam marked with the accumulation of a sandycomponent; b, Coarse-grained biosparite.

value) varied from 13.5 to 23.2 cm3 per 50 cm2. Interms of the mean value, this limestone is a hard tomoderately hard rock (BILBIJA & MATOVIĆ 2009).

Regression analysis

Regression analysis was applied to obtain relation-ships between the mechanical and physical propertiesof the Struganik limestone. Some of these relation-ships and the best-fit curves between the differentparameters are shown in Fig. 8. The relationshipsshow fairly higher correlations between the bulk den-sity (ZSP) and the porosity (P). The relationship be-tween them is highly significant with a correlationcoefficient of –0.929 and a Students t-value of –5.598(p < 0.05; Fig. 8a). The cross plot of porosity and bulkdensity shows a general trend of increasing bulk den-sity with decreasing absolute porosity. The plot sug-gests that the bulk density may be used to obtain thevalue of the porosity.

The results of regression analysis indicate to a pro-minent relationship between the dry compressivestrength (σs) and the water equivalent (σv). The trend

is positive and clearly linear with a regression coeffi-cient R2 = 93 % and a Students t-value of 14.717 (Fig.8b). Comparing the value of the dry strength (σs) andthat after the freeze–thaw cycles (σm), the correlationis positive and high statistically significant with aregression coefficient R2 = 78 % (t = 4.159; Fig. 8c).Low correlations with no significant Pearson coeffi-cients were obtained for all the other physico-mechan-ical properties.

Validation of this model was confirmed by the t-testand analysis of the variance (Table 2). As can be seenin Table 2, all the regression coefficients are signifi-cant at the 95 % confidence level. The computed t-va-lues are greater than the tabulated critical t-values of±2.36 and ±2.10 for the obtained models. The analy-sis of variance follows an F-distribution with degrees


Table 1. Statistical parameters of physico-mechanical properties of the Struganik limestone. Legend: n, number of samples; X, mean; Vmin, Minimum values; Vmax, maximum value; PX-95%, confidence intervals for the mean; M, median; Md, Mod;R, range; Qi, quartile range; Q1, lower quartile; Q2, upper quartile; S2, variance; S, standard deviation; As, Skewness; Kt,Kurtosis; SE, standard error; Dmax, value of the largest absolute difference between the observed and the expected cumula-tive distributions; p, confidence interval; Dcr, critical value; α, probability level.

of freedom df1 = 1 and df2 = 5 for models with samplesnumber of 7, therefore the critical region consists ofvalues exceeding 6.61. For the model with a samplenumber of 18 (df1 = 1 and df2 = 16), the correspondingcritical F value is 4.49. All the computed F values weregreater than those tabulated; thus, the null hypothesiscan be rejected and it can be concluded that all equa-tions are significant according to the F-test.

Discussion and conclusions

The presence of two lithological types of limestone,i.e., micrite and allochemical, was emphasized. Mi-crite limestone is the autochthonous rock of a deepwater, marine environment. In the original terminolo-gy (FOLK 1959), these limestones are considered asorthochemical. The allochemical limestones werederived from shallow-water material that was broughtby turbidity currents into the system. The Struganiklimestone represents a series of marlstone, clayey li-mestone and limestone. Concretional cherts and manymicrofauna are present in all the distinguished vari-eties. Among the macrofaunal remains, inoceramus isthe most frequent.

Generally, the values of physico-mechanical prop-erties indicate the good quality of Struganik limestoneas a dimensional stone. Their quality and way of ex-ploitation are directly governed by their petrologicalproperties. Pronounced layering in the micrite vari-eties results in their exploitation in the form of thinplates and simultaneously determines the dimensionsof the final products. As Struganik limestone is usedas natural-faced slabs, this texture is undoubtedly asignificant economic factor in that it simplifies theprocess of final shaping. Allochemical, banked lime-stone, is used to a lesser extent due to its mode ofappearance and economic interests. The petrologicalheterogeneity of the Struganik limestone, including itstextural and structural characteristics, has an impacton the numerical values of its physico-mechanicalproperties. Statistical analysis showed notable varia-tion in important mechanical parameters, i.e., in majorcriteria, for the application of the stone. This is theconsequence of not only the natural heterogeneity ofthe rock mass, but also probably partially of the fabricand structural differences between the investigatedsamples. The results of the regression analysis pre-sented in this paper imply that the porosity could bededuced from the bulk density values. Similarly, usingthe regression equation, the values of the strength of awater-saturated sample or a sample after freezingcould be accurately determined using values of thestrength in the dry state that was estimated in the lab-oratory.

However, other properties cannot be estimate withany confidence due to low regression coefficient. Itshould be emphasized that the analysis was performedon a statistically small number of sample, which addi-tionally affected the obtained results. The real densityof limestone is relatively constant as was to be expect-ed due to the uniform mineral composition. The bulkdensity had a value range of 70 kg/m3, which is prob-ably related to small range in the porosity of this lime-stone. However, the inverse relationship between theporosity and the bulk density was predicted. Gene-rally, these limestones have low interparticle andintraframe (intraskeletal) porosity, which results from


Fig. 8. Relationship between physico-mechanical proper-ties: bulk density vs. porosity (a); compressive strength dryvs. water equivalent (b); compressive strength dry vs. theafter freezing equivalent (c).

the depositional process. However, some higher val-ues of porosity (secondary type: stylolite and microfissures) are probably caused by post depositionalprocesses. The very low value of water absorption isin agreement with that of their porosity. Even in sam-ples with a porosity value of 3 % or higher, the waterabsorption was very low (under 0.4 %). This meansthat these rocks contain isolated, unconnected poresor micro-cracks unable to receive and retain water.The low values of water absorption indirectly implylimestone resistance to frost.

The mechanical properties data of the Struganiklimestone point to its variable quality, which is inaccordance with the petrological heterogeneity of therock mass. The same part of the rock mass is strong tomoderately strong regarding the unconfined compres-sive strength. Simultaneously, it is hard to moderatelyhard rock concerning its wear abrasive. Samples withhigh values of strength (over 150 MPa) and lower val-ues of abrasive resistance (below 18 cm3 per 50 cm2)probably correspond to micrite, fossiliferous micriteor biomicrite. On the other hand, layers/banks of bio-intra-spar varieties as well as layers with a higher claycontent (marly limestone and marlstone) are presentin the quarry. This may be the main cause for the verylow strength values (below 70 MPa) and poor resist-ance to wear (over 22 cm3 per 50 cm2). Additionally,the presence of chert concretions, variable layer thick-ness, lamination, stylolite and other textural forms arereasons for the variable technical properties.

All the mentioned parameters are limiting factors forthe use of the Struganik limestone as a dimension stone.Namely, the thickness of the final slabs is naturallydefined by the layering, whereas the pronounced petro-logical heterogeneity requires selective exploitation.The statistically analyzed data confirmed that the aver-age values of the technical properties satisfy the major-ity of the requirements of the national standard forstone slabs (SRPS B.B3.200 national standard withoutobligatory use). Unfortunately, the average value of theabrasive wear is higher than the limit for some applica-tions of building stone and actually represents the mainlimiting factor. According to this standard, in the casethat the tensile strength has been previously deter-mined, the Struganik limestone may be used for:

– Pavement of interior vertical surfaces and hori-zontal surfaces with intensive and moderate trailertraffic;

– Pavement of exterior ver-tical surfaces up to 30 m inheight in objects;

– Pavement of exterior hori-zontal surfaces with moderatetrailer traffic (if the value of thewear abrasive is max. 18 cm3

per 50 cm2 or lower).Compact, thinly bedded mi-

crite varieties of chemicallypure limestones entirely fulfill the requirements forthe above-listed purposes, whereas the banked allo-chemsparite varieties or beds with an elevated contentof clayey components should be avoid as dimension-al stone.

Acknowledgements

Authors would like to thank ELENA KOLEVA-REKALOVA

(Bulgaria) and LADISLAV PALINKAŠ (Croatia) for their veryhelpful and much appreciated comments and suggestionsfor the improvement of the manuscript. We are grateful tothe “GABP” Editor in Chief VLADAN RADULOVIĆ (Serbia)for his help. This work was supported by the Ministry ofEducation and Science of the Republic of Serbia, ProjectNo. 176019 and 176016.

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Pезимe

Петрофизичке и механичке особинеструганичких кречњака (Вардарсказоне, западна Србија)

Природни камен је највише коришћен грађевин-ски материјал на свету, како за унутрашња тако иза спољашња зидања. Да би се користио у гра-ђевинарству, камен треба да поседује добар ква-литет односно физичко-механичка својства којазадовољавају захтеве стандарда у грађевинарству.Карбонатне наслаге различите литологије, старо-сти и квалитета су широко распрострањене уСрбији па су и кречњаци најчешће коришћен гра-ђевински камен. У индустрији архитектонског ка-мена значајно место заузима струганички креч-њак, најпознатији украсни камен западне Србијечија је експлоатација започела још у 18. веку. Да-нас, више од 70 каменолома обавља активнуексплоатацију кречњака у области Струганика ишире, али услед недовољне геотехничке истра-

жености ове стене имају још увек ограниченупримену у грађевинарству. Рад представља синте-зу резултата детаљних петролошких и техничкииспитивања кречњака из најстаријег и највећегповршинског копа „Струганик“, интерперетацијудепозиционе средине стварања кречњака, као ко-релацију физичко-механичких карактеристика тј.утицај петролошких карактеристика на квалитетстенске масе.

Према петрлошким карактеристикама, мине-ралном и хемијском саставу, стенску масу повр-шинског копа „Струганик“ граде лапорци, глино-вити кречњаци и кречњаци. Стратиграфски ониприпадају доњем и горњем кампану, а нова от-крића асоцијације пелашких глоботрунканидапотврђују њихову кампанску старост (ГАјИЋ 2007).Кречњаци су једри, компактни са хомогеном си-вом бојом коју делимично нарушава присуствотамносивих конкреција рожнаца. Основни тек-стурни облик појављивања кречњака је слојеви-тост са доминантном дебљином од 5–15 cm. Слој-не површи су равне или стилолитске, а на горњимповршинама се налазе и бројни биогени текстурниоблици и трагови богате животне активности. Одинтерних текстурних облика јављају се хоризон-тална ламинација, стилолитски шавови и веомакарактерстична конкрециона тела рожнаца. Пале-oнтолошка анализа микрофауне показала су дадоњи део литолошког стуба стратиграфски при-пада доњем кампану а преосталих седам метара –горњем кампану. Према структурним карактери-стикама издвајују се два основна литотипа кре-чњака: микритски – ортохемијски, аутохтони кре-чњаци (Mudstone, Wackestone) и алохемијскикречњаци, био-интра-спар варијетети (Grainstone,ретко Rudstone). Основу микритских кречњакагради микрокристаласти калцит док некарбонатнидео чине минерали глина, органска материја и але-вритска компонента. Биогена компонента је пред-стављена добро очуваним љуштурицама пелаш-ких фораминифера (Globotrunkanide) и у зависно-сти од њиховог садржаја разликују се микрити,фосилиферни микрити и биомикрити. СадржајCaCO3 у овим стенама варира од 55 до 94 %. Бан-ковити био-интра-спар варијетети (интраспарру-дити, биоспаррудити, биоспарити, интрабиоспа-рити) су изграђени од разноврсног биогеног ало-хема (фрагменти љуштура плитководних органи-зама, љуштурице бентоских и планктонских ми-кроорганизама, фрагменти алги) и простих до сло-жених интракласта. Природа транспорта којим јематеријал принешен у басен омогућила је развојинтерних текстура карактерстичних за Боуминусеквенцу. Садржај CaCO3 у овим стенама варираод 86 до 98 %.

На основу петролошких показатеља утврђено једа су аутохтони микритски варијетети пелашкиседименти таложени на континенталној падини у


дубоководној средини горњокредног мора, докприсуство слојева и банака алохемоспаритскихкалкаренита и калкрудита указују на повременипринос плитководног материјала турбидитнимтоковима низ континенталну падину.

Квалитет струганичког кречњака као украсногкамена у функцији је његовог минералног састава,склопа и физичко-механичких својстава. Премастатистичким показатељима испитивани кречњакприпада категорији умерено тешког, компактног итврдог камена са врло малим упијањем воде,средње до високе чврстоће према притиску. Ква-литет и начин експлоатације предодређени суекстерним и интерним текстурним облицима.Изражена слојевитост микритских варијететаусловила је: експлоатацију у облику танких плоча,димензије готових производа; равни слојевитостису природне, али и финалне површи ове врстеархитектонског камена. Петролошка хетерогеностстенске масе условила је велику варијабилностнумеричких вредности физичко-механичких ка-рактеристика тј. главних критеријума за применукамена. Густина кречњака је релативно константнашто је и очекивано с обзирома на уједначени ми-нерални састав. Запреминска маса показује малиопсег варирања – последица релативно уједначенепорозности. Струганички кречњак има нискуинтерпартикуларну и интерскелетну порозносткоја је резултат депозиционих процеса. Нискевредности упијања воде индиректно указују на

отпорност кречњака на агресивно дејство мразашто је потврђено и лабораторијским анализама.Чврстоћа на притисак и отпорност на хабање сутехнички параметри са највећим варирањемвредности. Узорци кречњака са високим вредно-стима чврстоће на притисак (изнад 150 МРа) ивисоком отпорношћу према хабању брушењем(<18 cm3/50cm2) одговарају микритским варијете-тима док био-интра-спар варијетети и варијететиса високим садржајем глиновите компонентеодговарају стенама умерене чврстоће, а њихованижа абразивна отпорност је и главни огранича-вајући фактор примене. Резултати регресионе ана-лизе приказани у раду, показују да су порозност изапреминска маса у директној линераној зави-сности са високим статистички поузданим коефи-цијентом регресије. Високу међусобну линеарнузависност показују и параметри чврстоће напритисак у сувом, водозасићеном стању у последејства мраза. Остала физичко-механичка свој-ства, услед ниског регресионог коефицијентанемају статистички значајне функције зависности.

На основу резултата статистичке анализе фи-зичко-механичких својстава може се закључити даквалитет струганичког кречњака задовољава ве-ћину захтева националног стандарда за применуплоча за унутрашња и спољашња, вертикална ихоризонтална облагања. Отпорност на хабање јеједини фактор који лимитира примену кречњакакао украсног камена.


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