Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 15363799

An experimental study on the construction materiaIs of the Ankara Citadel

O. Klrca ÇimSA Cement Production and Trade Company, Mersin, Turkey

T.K. Erdem Middle East Technical University, Dept. ofCivil Engineering, Ankara, Turkey

ABSTRACT: Ankara has been an important city throughout its history. Due to the geo-politic position ofthe city, Ankara was subject to many attacks by various civilizations. Therefore, the Citadel experienced many repairs by various people with different construction materiais and techniques. Nevertheless, a great portion ofthe walls oftoday was reported to belong to Byzantine period. Tn this study, some mechanical and physical properties of the cut-stone taken from the inner and outer walls ofthe Citadel were determined. Compressive strength, point load, water absorption, density, and abrasion resistance tests were performed on stones. Tn addition, Schmidt Hammer and ultrasonic pulse velocity values were obtained. Compressive strength test values were correlated to the other data obtained to estimate the compressive strength in the further studies. Besides, the compositions of the two different types of mortar used in the fortification walls were also questioned through X-ray diffraction and X-ray fluorescence.

TNTRODUCTION

Ankara was founded in the centre of Anatolia on the two main axes of transport running East-West and North-South. Thus, the city has been not only an important region for its geographic location through­out the history but also a transfer point for trade as well as military campaigns (Sagdlç 1994).

Ankara Citadel, which is the oldest portion of the city, had been utilised as a military garrison control­ling the rod passing through the plain in the rather old times. Unfortunately, it is not possible to give a certain date for the first construction of the Citadel (Sevgen 1959). Nevertheless, it can be said from the excava­tions that the Hittites founded a military garrison in içkale (inner Citadel), the oldest portion of the city, in 4000- 1200 B.C. This citadel had been constructed with mud brick on large rock blocks (BakIrer 200 I, Baklrer 1987).

Due to the geo-politic position ofthe city discussed above, Ankara (and thus the Citadel) was subject to many attacks by various civilizations. Accordingly, the city was occupied by Hittites, Phrygians, Lydi­ans, Persians, Galatians, Romans, Byzantines, Ara­bians, Seljucks, I1hanids, and Ottomans. Therefore, the Citadel experienced many damages, repairs and restorations by various people with different construc­tion materiais and techniques. It is reported that the

fortifications oftoday belong to the Byzantine period (mainly in 7th and 9th centuries after Persian and Arabian attacks) (Baklrer 2001).

Andesite type of stone (Ankara stone) was used dominantly in the construction ofthe citadel fortifica­tions. Marbles which belong to the ruins ofthe Roman structures, such as statues, entablatures, epigraphs, relief sculptures, marble blocks, colurnn bases and cap­itais, etc. were also utilised very extensively. Rubble stone is used especially at the top of the walls with courses ofbricks. The arches ofthe gates ofthe Citadel were also constructed with bricks.

Although there are many studies about Ankara Citadel, the authors of this report have not met any investigation about the mechanical properties of its construction materiais. Actually, such a research is not easy beca use the structure have experienced many repairs by various people with several materiais in dif­ferent periods throughout its history. In this study, only a limited number of samples were tested but the sam­pies were chosen carefully so that they characterise -to a certain extent - the materiais used in the Castle.

The scope of this study is to determine the com­pressive strength, Schmidt Hammer values, ultrasonic pulse velocity values, abrasion resistance and water absorption capacity ofthe andesite, which is the dom­inant stone type used in the construction ofthe Castle. Besides, the X-ray diffragtograms and chemical

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Figure 1. Plan view of the Citadel (Bakuer 2001).

analysis of the two different types of mortar used in the fortification walls are also questioned.

2 MATERIALS AND TECHNIQUES USED FOR THE CITADEL

It is reported that the Citadel was constructed mainly during the Roman and the Byzantine periods, and the walls of today belong to the Byzantine period (mainly in 7th and 9th centuries afier Persian and Arabian attacks) (Baklrer 2001). The fortifications are the greatest work of Byzantine construction in Ankara (Foss 1989). Although there are various repairs and additions by the Seljuks, Ilhanids, Ahis, and Ottomans, the fundamental structure constructed dur­ing the Byzantine period has been conserved till now (Baklrer 2001).

The inner castle encloses a rectangular area of about 350 by 150 meters. The plan view of the Citadel is shown in Figure I.

The ma in entrance to the Citadel is from the south. In the west, south and east walls, there are very closely spaced towers (only about 20 meters) (Fig. I). Most of these towers have a pentagonal plano Besides these, there are one square and two semi-circular towers. The inner walls were constructed over rubble stone by using cut-stones and huge blocks taken from ancient monuments up to a height of 8- 1 O meters. The upper portions are composed of courses of brick (Baklrer 200 I). At the upper portions of the walls, generally,

Figure 2. A wall from içkale (Sim~ek, et aI. , unpubl.).

brick and stone were used alternatively. (For example, in a wall at the south-side, stonelbrick pattern is as fol­lows: 5 brick / 3 stone / 5 brick / 3 stone / 4 brick / 3 stone / 5 brick / 3 stone.) The alternative use of brick and stone is shown in Figure 2. In some places, bricks were used randomly within the wall together wi th the cut-stones and rubble stones.

The materiais used and the type ofthe construction of the outer walls are very similar to the inner ones, however, the workmanship is coarser in the former. The outer walls have a thickness of approximately 3.20 m and enclose the inner walls. There are 15 towers which are forty meters apart. 12 of them are square and the remaining 3 towers have semi-circular plans (Baklrer 2001).

Besides the use of brick in the fortifica tions, it is used in the arches of the gates of the Citadel. In Genç Kale Gate, the relieving arches are made of brick and lime mortar as the binding material. The vault coming just behind the arch is also made of brick. However, the arch of the Hisar Gate is built by using stones.

There are mainly three types of stones in the Ankara Citadel: andesite, white marble and limestone. Rub­ble stone and brick were also used in the fortification walls.

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Figure 3. Two types of mortars.

The Citadel is dominated by andesite, however, marble is the most striking component ofthe walls.

Marbles used in the construction of the walls belong to the ruins of the Roman structures, such as statues, entablatures, epigraphs, relief sculptures, marble blocks, column bases and capitais, etc (Sagdlç 1994, Baklrer 200 I, Baklrer 200 I). This type of stones can be seen especially at the east and south walls of the Citadel (Fig. 2).

[n addition, there are some stones with a hole at the middle, which had been used as water pipes in the Roman Period. After the failure of the water-supply system in the 4th century, the stone pipes ofthis system were utilised as a construction material in the Byzan­tine period (Baklrer 1987). These stone pipes are made of andesite and they exist especially at the east walls of the içkale.

According to the observations at the lower parts of the fortifications, there are two types ofbinding mate­rial. The first one was nearly white in color, which was probably a lime mortar. The second type was used as flush jointing between the stones on top of the first binding material , which is thought to be Horasan mor­tar. Its color was pinky, and made of pebbles in di fferent colors, tile chips and lime. These two types of mortar are shown in Figure 3.

3 EXPERlMENTAL PROGRAM ANO TEST RESULTS

In this study, experimental program consists of two parts: a) Field work and b) Laboratory work .

In the field, the first step was the survey ofthe Cas­tle. The types of the construction materiais and their usage in the structure were determined. Then proper locations were detected to take samples which would characterise the materiais used in the Castle. Andesite samples were taken from içkale (inner Citadel) and

from Ol~kale (ou ter Citadel). Two types of mortar sam­pies were taken from the wall shown in Figure 3 for chemical and mineralogical analysis. Then, ali of the samples were brought to the laboratory for testing.

Second part ofthe experimental program is the lab­oratory work, which is also composed of two parts: i) Tests on stone and ii) Tests on mortar.

3.1 Tests per(ormed 0/1 cut-stone

Compressive strength, rebound Schmidt Hammer, ultrasonic pulse velocity, abrasion resistance and water absorption capacity tests were performed on andesite, which is the dominant stone type used in the con­struction ofthe Castle. Compressive strength, rebound Schmidt Hammer and ultrasonic pulse velocity can be used to estimate the mechanical properties ofthe mate­riais such as compressive strength (Klrca et aI. 200 I).

Two andesite cut-stones were taken from içkale. One cut-stone (andesite) was taken from Ol~kale . In order to make tests on these stones, core specimens ali having 50.3 mm diameter were taken from them.

After the water absorption and density tests, Schmidt hammer and ultrasonic pulse velocity tests were performed. Since it was not pro per to apply ham­mer and ultrasonic tests on the core specimens with 5 em height, these tests were performed only on the specimens with 10 cm heighl. Three rebound values were taken from each core specimen sue h that the dis­tance between two data points is not less than 2 em which is equal to two times the diameter of the impact plunger of the hammer. Then an average value was calculated for each location and this value was used in the related chart to obtain approximate compressive strength. Pulse time values taken from the ends ofthe core specimens were calculated from the formula:

V =(LlT) X 106, m/s. (1)

where V = pulse velocity (rnIs) ; L = length ofthe spec­imen (m); and T = calibrated average pulse time ( j..l.s).

Then, the core specimens were subjected to com­pression until failure by using universal testing machine. Compressive strength values (O") were deter­mined by dividing the ultimate load (P) to the cross­sectional area ofthe specimens (A), that is

(J = PIA (2)

The results for water absorption, density, rebound Schmidt hammer, pulse velocity, and compressive strength tests are given in Tables 1-4.

Moreover, point load strength tests were performed on the cut-stone taken from içkale. The samples for this test were also taken from the same cut-stone whose test results are given in Tables I and 2.

The point load strength test is widely used to esti­mate the uniaxial compressive strength of the stones

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Table I. Test results for the first stone taken from içkale .

Sample Length Diameter Weight Density No (mm) (mm) (mm) (g/cm3)

I 100.4 50.3 463.3 2.32 2 101.0 50.3 455.5 2.27 3 100.5 50.3 456.0 2.28 4 100. 1 50.3 461.9 2.32 5 100.9 50.3 453.5 2.26 6 100.8 50.3 464.6 2.32

Table 2. Test reslllts for the first stone taken from içkale.

Average Schmidt Pulse Schmidt hammer Comp.

Sample velocity hammer comp. str. strength No (m/s) value (MPa) (MPa)

I 2960 45 50 42.8 2 30 12 45 50 46.8 3 2872 43 46 38.3 4 2897 44 48 39.2 5 3087 47 53 52.2 6 3032 48 55 5 1.9

Table 3. Test results for the second stone taken from içkale.

Dia- Water Comp. Sample Length meter Weight Density abs. str. No (mm) (mm) (g) (g/cm3) (%) (MPa)

7 50.4 50.3 228.6 2.28 5.34 73.0 8 51.3 50.3 236.2 2.32 5.42 67.5 9 50.9 50.3 232.4 2.30 5.38 68.5

lO 51.1 50.3 234.0 2.30 5.45 57.9 l i 50.4 50.3 235.7 2.35 5.34 66.5 12 50.3 50.3 230.0 2.30 5.32 70.0

Table 4. Test results for the stone taken from DI~ka l e.

Dia- Water Comp. Sample Length meter Weight Density abs. str. No (mm) (mm) (g) (glcm3) (%) (MPa)

13 50.1 50.3 227 .2 2.28 5.71 42.8 14 50.4 50.3 233.3 2.33 5.8 1 39.8 15 50.3 50.3 237.7 2.38 5.74 39.3 16 50.5 50.3 22 1.8 2.2 1 5.70 37.8 17 50.2 50.3 222 .0 2.23 5.64 4 1. 8

in f ields and laboratories (Karpuz 1982). Point load test can be applied by 3 ways: diametrically, axially, and on irregular lumps . In this study, test direction was diametrical, that is, point load was applied in the direction perpendicular to its longitudinal axis. The

Table 5. Results for the point load tesl.

Sample Diameter, Failure load, Point load index, No D(mm) P (kN) 1, (50) (MPa)

18 50.3 8.0 3. 16 19 50.3 7.9 3. 12 20 50.3 7.5 2.96 21 50.3 8.0 3.16 22 50.3 8.0 3. 16

Table 6. Results Los Ange les abrasion tesl.

Amount retained on # 12 sieve after

lnitial weight, 200 1000 Abrasion A revolutions, B revollltions, C ((A-C)/A) x 100 (gr) (gr) (gr) (%)

10000 9410 74 10 25 .9

Table 7. Chemical analyses of the mortars.

Amollnt(%)

lnner morta r Outer mortar

Si02 60.21 44.4 1 AI20 3 10.60 7.88 Fe203 4.60 3.33 CaO 13.12 23.21 MgO 2.11 1.92 K20 1.49 0.86 Na20 1.07 1.0 I S03 0.13 1.44 LOl 12.6 1 20.33

results obtained according to the following formula are shown in Table 5:

1,(50) = (P/D/ )x F (3)

where 1,(50) = point load index (MPa); P = fai lure load (N); De = equivalent diameter (mm), (De = diameter of the specimen for diametral test direc­tion); F = correction factor = (De/50)OA5, (F = I for this study).

Finally, as an additional work, Los Angeles abrasion test was performed 0 11 the cut stone takel1 from içkale. This test is important from durability point of view. The results are given in Table 6.

3.2 Tests on mortars

As shown in Figure 3, two types ofmortar were present in a wall ofDl~kale . The first one was used in the inner

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'"

o r TI

Figure 4. XRD pattern of inner mortar (Q: Quartz, C: Ca\cite, R: Cristobalite, A: Albite).

[ li: )

Figure 5. XRD pattern of outer mortar (Q: Quartz, C: Ca\cite, A: Albite).

portions ofthe wall and it was nearly white in colour, which was probably a lime mortar. The second type of mortar was pinky and used as flush jointing between the stones on top of the first binding material.

These mortars were powdered for X-ray diffrac­tion (XRD) test to obtain mineralogical analysis. The chemical analyses of these mortars were obtained by X-ray fluorescence (XRF).

The chemical analyses of the inner and outer mortars are shown in Table 7.

X-ray diffractograms belonging to the mortars are given in Figures 4 and 5.

4 ESTIMATION OF UNIAXIAL COMPRESSIVE STRENGTH PROPERTY

4.1 Estimation 01 compressive strength from ullrasound pulse velocity test

A statistical study was performed to be able to find out an empirical formula that will give the relation­ship between compressive strength and pulse velocity values ofthe dimension stone core specimens. For this purpose, the experimental data ofthese core specimens tested in laboratory were used to form a data file in Microsoft Excel software. Trying to find the highest correlation factor, it was decided to make a non-linear regression analysis by using a Power function:

(J = A X Vb (4)

where a = compressive strength value found in labora­tory (MPa); V = ultrasonic pulse velocity value found in laboratory (m/s); A and B = constant numerical coefficients.

The formula, with a rather high correlation coeffi­cient R2 = 0.95 (R = 0.97), was originated as:

(J = 2xlO-15 X V 4.7354 (5)

4.2 Estimation 01 compressive strength from Schmidt hammer rebound lest

Similar to the procedure given for the estimation of uniaxial compressive strength fiom ultrasound pulse velocities, another statistical study was performed by using the rebound Schrnidt hammer values (SHV). Highest correlation factor was obtained when the rela­tion between the compressive strength and SHV was logarithrnic, that is

(J = A x In(SHV) + B (6)

where a = compressive strength value found in labo­ratory (MPa); SHV = Schrnidt hammer values found in laboratory; and A and B = constant numerical coefficients.

The formula , with a rather high correlation coeffi­cient R2 = 0.9038 (R = 0.95), was originated as:

(J = 141.49 x Ln (SHV) - 494.37 (7)

4.3 Estimation olcompressive strength lrom point load strength indexes

It is not possible to perform both compressive strength test and point load test on the same specimen since both of these tests depend on the failure of the spec­imen. Therefore, a one to one relation between the results of these tests can not be made for the same specimen.

In this study, compressive strength test and point load test were performed on the different core spec­imens of the same stone. Average value of the com­pressive strength values was found as 45.19 MPa and the average value for the point load strength index were calculated as 3.11 MPa. Then, a linear relation­ship was assumed and the following empirical formula was obtained:

(J = 15 x 1,(50) (8)

where a = compressive strength (MPa); 1,(50) = point load strength index (MPa).

5 DISCUSSION OF RESULTS

The results ofthe destructive and non-destructive tests performed on andesite cut-stone (dominant stone type

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used in the Citadel) have been given in Tables between I and 6. These results lead to the fo llowing diseussions:

The densities of the stone speeimens were approx­imately 2.3 g/em3. Abrasion of the stone was found to be 25.9% at the end of 1000 revolution. Sinee, the stones having an abrasion greater than 35% were said to be weak, the sample tested in the laboratory ean be aeeepted as moderate from the abrasion resistanee point ofview.

The 5 x 5 x 5 em cube samples, taken from içkale, had water absorption values of approximately 5.4%. The uniaxial compressive strength values were found as between 58 MPa and 73 MPa. The water absorption eapacity ofthe samples ofDI~kale was approximately 5.8%, and their compressive strength values were between 38 MPa and 43 MPa. Therefore, as expeeted, it can be concluded that compressive strength deereases as their water absorption capacity, in other words porosity of the eut-stone, increases. So, the compres­sive strength property is inversely proportional to the absorption capacity was observed.

Non-destruetive tests (Schmidt Hammer Rebound, and Ul trasound Pulse Veloeity) and uniaxial com­pressive strength tesls were performed on the core speeimens having 5 em diameter and 10 em length. The compressive strength values of these samples were between 38 MPa and 52 MPa at the end of lhe tests. Ultrasound pulse velocity values lied between 2960 m/s and 3087 m/s. Sehmidt hammer rebound values were between 43 and 48 at the end of the tests.

Point load strength test is widely used to estimate the compressive strength of stones in f ield and labo­ratories. In this study, this test was performed on the core specimens having 50.3 mm diameter. The average value of the point load strength index values, Is(5 0), was found as 3. II MPa.

Table 8 gives a c\ass ification Ankara andesites. Aceording to this classification, the results obtained from Sehmidt hammer rebound values, ultrasound pulse veloeity and point load tests suggest that the stones are slightly weathered.

A statistieal study was performed to be able to obtain three empirical formulas giving the correlations between ultrasound pulse velocity, Sehmidt hammer rebound, point load strength index Is(50) and uni­axial compressive strength property, respeetively. These empirical formulas are given below as:

crv = 2xlO- '5 X V 4.7354

crSHV = 141.49 x Ln (SHV) - 494.37

crls = 15 x Is(50)

(9}

(10)

(11 )

The results of the Schmidt hammer rebound tests were a li ttle bi t less reliable, when compared with the ultrasound pulse velocity test as expeeted, since, the

Table 8. The proposed sonic velocity, Schmidt hammer va lue (SHV) and point load strength (PLS) classification for Ankara andesi tes (Karpuz 1982).

Material Sonic description velocity (m/s) SHV PLS (MPa)

Fresh 3900 54-61 3.16-6.63

Slightly weathered 3900- 3050 39- 54 2.24- 3.16

Moderately weathered 3050- 2250 28- 39 1.22- 2.24

Highly weathered 2250- 1350 18- 28 0.41-1.22

Completely weathered <1 350 <18 <0.41

rebound hammer value is affected by smoothness, sur­face hardness (earbonation effect), moisture content, size and age of the material.

The chemical analyses of the mortars have been given in Table 7. Although, it was not proved from the chemical analysis and X-ray diffractograms, a pozzolan is thought to be present in the mortars. Sinee, the relatively high amounts ofAl20 3 and Fe20 3, which were found at the end ofthe analyses, were the strong indicators of a pozzolanic material. Moreover, high background trends in the X-ray diffractograms showed the presence of an amorphous material, and this amorphous material is most probably due to bind­ing material produeed from the pozzolanie reactions with lime.

The results of the ehemical analysis pointed out that, Si02 content of the mortar taken from the inner part was higher than that of from the outer par!. This was partly due to higher amount of sand in the forme r (actually this was also observed by the visual inspee­tion.). The case was reversed, when CaO eontents were compared. The CaO content was higher in the outer part mortar. As a result of these comparisons, it ean be said that, the binding material in the outer morta r was used relatively in higher amounts than that in the inner mortar. This result is quite logieal and shows the indications ofsome advantages: The porosity (perme­ability) and the deleterious effects of weathering ean be deereased by using mortars having high amounts of binding materiaIs at the outer parts of the walls.

6 CONCLUSION

Any ofthese relations, determined experimentally and mentioned above, ean be used to estimate the uni­axial eompressive strength property of the andesite type cut-stones for the future rehabilitation and/or restoration works of the Citadel, when the correspond­ing tests are performed at site. However, it should not

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be forgotten that, these rock formed cut-stones are not homogeneous materiais. There may be some discon­tinuities due to the rock formation , and some defects throughout the surfaces because of externai, weather­ing (abrasive) effects. Therefore, it should be noted that the estimated uniaxial compressive strength values by using the empirical formulas, ali found out and given above, yield approximate results so they can differ a little from the exact strength values, determined at the end of destructive tests performed on core samples.

The binding material in the outer mortar was found to be used relatively in higher amounts than that in the inner mortar. This result is quite logical since the porosity (permeability) and the deleterious effects of weathering was tried to be decreased by using mortars having high amounts ofbinding materiais at the outer part ofthe walls ofthe Citadel.

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Baklrer, O. 2001. Tarih içinde Ankara lI. Ankara: ODTÜ MimarlJk Fakültesi .

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Karpuz, C. 1982. Roek Meehanies Charae/erisries 0/ Ankara Andesires in re/arion /0 Their Degree 0/ Wea/hering, Ph. D. Thesis. Ankara: Middle East Technical University.

Klrca , O., Erdem, T.K., Uslu, 8. , & Baklrer, O. 200 I. Estima­tion ofthe In-Situ Mechanical Properties ofthe Construc­tion MateriaIs in a Medieval Anatolian Building, SahipAta Hanikah in Konya. 2nd Inrerna/iona/ Congress on Srudies in Ancien/ S/rue/ures, Proe. Inrern. Cong, istanbu/ Yi/diz Teknik Universi/esi , July 9-132001. 691 - 701.

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Unpublished study in rhe Deparrmenr of Res/ora/ion in Micld/e Eas/ Teehnica/ University, Turkey.