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MODIFIED ELASTIC SILICIC -ACID ESTER APPLIED ON NATURAL STONE AND TESTS OF THEIR EFFICIENCY

BOOS,M., GROBE,J., Westfalische Wilhelms - Universitat Munster

HILBERT,G. , Remmers Bauchemie GmbH

MOLLER-ROCHHOLZ J Fachhochschule Munster ' .

1. Introduction For weathered sandstones degradation is often caused by the loss of the binding agent of the granulat quartz particles / 1, 2/. Recompletion of the binding agent by organo-silicic binders leads to a strengthening of the structure. These ,,strengtheners" shall increase stiffness (expressed as Young' s-(E)modulus). A favourable consolidating effect is reached, when the recompletion of binder leads to a strength vs depth profile, which is on the level of the original stone 131. The measurement of these strength vs depth profiles is difficult and especially measurements over longer time periods are problematic because of the high variation of stone properties. So a nondestructive method, which uses the identical specimen, is to be preferred. Nondestructive methods must be validated by other means.

2. Investigations

2.1 Stone material A set of typical German sandstones were chosen for the investigation. Table 1 gives code, names and material properties. The mechanical properties were measured in the laboratory for construction materials of Fachhochschule Mtinster, the other values were taken from literature /4/.

BA BB BC BP OK RG SS

binder % 26 1 4 4 7 10 8

porosity vol.-% 19,08 23 ,01 22,48 21,19 20,56 23,97 19,99

compressive strength N/mm2 47,3 65,4 45,1 74,9 113,8 50,0 75,8

flexural strength N/mm2 9,3 3,5 3,5 4,7 10,0 3,8 7,8

elastic-modulus kN/mm2 15,5 10,8 15, 1 14,8 16,0 12,4 14,6

splitting tensile strength N/mm2 3,25 3,2 1 2,42 4,31 6,4 3,03 4,15

Table 1 a: Characteristic properties of the stones under investigation

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Code Type Age Source of sandstone

BA chemical sediment carbonate chalk Baumberge

BB elastic sediment chalk Bentheim

BC elastic sediment Keuper Buch

BP elastic sediment Keuper Burgpreppach

OK elastic sediment chalk Obernkirchen

RG elastic sediment, green chalk Rtithen

SS elastic sediment Keuper Sander Schilf

Table 1 b: Codes, types, ages and sources of stones

2.2 Silicic ester material An investigation on four types of stone strengtheners based on a silicic acid ethyl ester (SAE) was executed. In order to take the multitude of the different weathering-profiles into account, three of these ,,ready for use" produced strengtheners (Fa. Remmers Bauchemie GmbH, Loningen, Germany) differ in their deposition amount of binder (SiOrgel). Active ingredients of these stone strengtheners are a mixture of monomeric silicic acid ethyl ester (Tetraethoxysilan, Tetraethylorthosilicate; Si(OEt)4)

and its bigger (oligomeric to polymeric) condensation products (e.g. di-, tri-, or tetra-silicic-acidethyl-ester). This kind of stone protective agents reacts with water stored in the pore system or with humidity. During this process, pure cementitious, amorphous, aqueous silicondioxide-(SiOr )gel is deposited as a binder.

Stone strengthener Deposition amount Solvent Code ofSiOrgel

Funcosi I Stone Strengthener I 00 ca IOOg/I white spirit 26

Funcosil Stone Strengthener 300 ca 300g/I free from organic solvents 28

Funcosil Stone Strengthener 510 ~ 400g/I free from organic solvents 30

Table 2: Description of the common stone strengtheners with different SiOrdeposition

Silica gels, formed by ethyl silicate, show a high amount of shrink cracks due to a constant reaction of th~ act_ive ingredients. ~s a conse~uence, silica gels are brittle. In order to minimize the shrinkage, elastic bridges. should be integrated into the ~etwork structure /5/. For this reason a fourth protective agent (Funcos1I Stone Strengthener 510 modified following the instructions published by Dr. E. Wendler, code 22) was used /6, 7/.

2.3 Treatment and testing Prismatic specimen (4 cm x 4 cm x 16 cm) from one stone sample were cut These spec· . . . . 1men were set in a glass dish on glass beads which are covered with the different strengtheners 3 time ~ 2

· · h 30 · ft s or minutes wit a minute pause. A er the treatment the specimen were stored at roo t . . m emperature in the lab (climate close to 20 °C/65 % r. H.).

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After 3 weeks of curing the pulse velocity perpendicular to length axis of the specimen (i.e. parallel to the treated surface) was measured in 1 mm steps close to the surface and 5 mm steps in the middle of the specimen. The dynamic elastic modulus E was calculated. After the nondestructive testing the specimen were cut into slices of 7 mm thickness (representing 10 mm) by hydro-jetting. These slices were tested in a circular flexion test (see schematical figure ) using a PC-controlled electro mechanical testing machine (UTS 10). The maximum load is given as the result.

iF +5+

I • • I + 30 + +

0 40 dimensions in mm

Figure 1: schematical sketch of circular flexion test

Figure 2: Flexural test device

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3. Results A typical resulting graph for BC is given as figure 3 :

JOO

100

10 30 40

Tiefe('""4

so 60 70 80 10 20 30 40 so 60

Tl!fe (mr(

Figure 3: Results of destructive (flexion) and nondestructive (ultra sonic) testing (BC 22)

70 80

In the left part each column of maximum force (,,Kraft") represents one slice of 7 mm tested in flexion, in the right part the E-modulus (,,E-Mod.") measured by ultrasonic pulse on the prisms before cutting is plotted. The values are plotted vs depth in mm (,,Tiefe").

• Penetration depth

BA The low porosity of this material only allows penetration of SAE to a maximum depth of 10 mm. This was also found in /8/.

JOO

~ 200

100

10 20 30 40

T\el•l""1

so 80 70 80 10 20 30 40

Tiele (rmf

so 80

Figure 4: Results of destructive (tlexion) and nondestructive (ultra sonic) testing (BA 22)

BB

70 80

The open porosity of this stone causes a deep penetration and consequently a high increase of the dynamic modulus. Comparing force and stiffness of BB 30 and BB 22 it is evident that a high amount of SAE in BB 30 gives high stiffness and slight increase in forces, while the elastified SAE gives higher strength at lower stiffness.

0 10 20 JO 40 50 60 70 80 90 100 110 120 130 140

Tiefe{rrrrf

Figure 5: Results of destructive (flexion) and nondestructive (ultra sonic) testing (BB 22)

JOO

100 .

~ ~ 8 § 0 ~ 0 ~

T• l• (rmf 0 10 20 JO -«! so 60 70 80 90 100 110

120 130 140 Tieie [fm1

Figure 6: Results of destructive (tlexion) and nondestructive (ultra sonic) testing (BB 26

)

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30L ....... ~ .............. . ==--~ }: __::j

0 10 20 30 40 so 60 70 80 90 100 110 120 130 140

llefo(nnj Tiele !nTT'f


JOO

100

0 0 ~ g ~ ~ i ~ g i ~ 0 ~ ~ 0

Tiefe(rmf

0 10 20 30 40 so 60 70 80 90 100 110 120 130 140

Tiefe (rmf


• Strength/stiffness

BC This sandstone shows another example of the influence of the elastic modification on strength and elastic modulus.

JOO

100

10 20 30 40

Tiefe(rnrt

50 60

}'.:['""" ': .... ~.... l 70 60 0 10 20 30 40 so 60 70 80

T111fe [rmi


10 20 30 40

llefe(nnj

50 60 70 60

rt..____··_····--~==-·. 10 20 30 40 50 60

Tiefe (rmi

70 80


The increase in strength with strengthener ,,22" is combined with a lower elastic modulus measured by ultrasonic pulse velocity.

Local inhomogeneity/NOT-OT

RG The good coincidence of nondestructive (NDT, ultrasonic pulse velocity) and destructive (OT, circular flexion) tests can be taken from the examination of RG 22 (see figure 11). The small weak zone behind the higher strength values can be found in both graphs as the specimen was first tested with NOT and

then with DT.

s 200

100

10 20 30 40

T.ie(omj

so 70 80

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r._c~-~-~· .. _·-·---~---__,_ ._ _____ -~ ~-.. == .. =·=·,____,·~! 10 20 30 40

Till•loml

50 60 70

Figure 11: Results of destructive (flexion) and nondestructive (ultra sonic) testing (RG 22)

• Results example

OK The Obernkirchen sandstone, well known as an exchange material used at the Cologne Cathedral, is described by the following 4 graphs, which again show the increase in strength for the materials ,,28", ,,30" and the lower elastic modulus for ,,22". Material ,,26" shows very little effect, better to be recognized by NOT than by OT-means.

Figure 12: Results of destructive (flexion) and nondestructive (ultra sonic) testing (OK 26)

500

10 20 JO 40

Till• (oml

so "" 70

~ .. _ ....... ~ ...... / !

10 20 30 ·iO so "" 60 70 eo T•l•(rm1


400

~ 300

~ 200

100

10 20 JO 40

Tiefe(rmi

so 60 70 80

N : r:=--------------., ~ JO ~ 25

}~ ll 10

~ s

10 20 JO 40

Tiet•f"'"1

so 60 70 60


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400

~ 300

~ 200

100

~=~ j ~ 20

t:~ ~ .. ·~:·-.. -·~·,___,.,.__,._ .. _,._ .. _,_,_,_,_,,,_...,.,, ___ :·_,,_,,~,-----.i,

10 20 30 50 BO 70 BO 10 20 30 40 50 80 70 80

n.te l ...... "''•! ......

Figure 15: Results of destructive (flex ion) and nondestructive (ultra sonic) testing (OK 22)

4. Conclusion • The non destructive measurement clearly shows the SAE-penetrated and strength/stiffness effected

zone. • Effects found by NDT can be verified by DT, with better local solution and less effort for NDT. • Increasing strength by elastified SAE with lower increase of stiffness is possible.

References / 1/ Sattler, L:

,,Untersuchungen zu Wirkung und Dauerhaftigkeit von Sandsteinfestigung mit Kieselsaureester" (Dissertation), Forschungsbericht 9/92, Bayrisches Landesamt fur Denkrnalpflege (Zentrallabor), Miinchen

/2/ Grimm, W.D.: ,,Naturwerksteine und Denkmalgesteine", ,,Die Geowissenschaften", 9-10/92, Weinheim

131 Snethlage, R./Wendler, E./Klemm, D.D.: ,,Tenside im Gesteinschutz - bisherige Resultate mit einem neuen Konzept zur Erhaltung von Denkmalern aus Naturstein" in ,,Denkmalpflege und Naturwissenschaft - Natursteinkonservierung I" , Verlag Ernst und Sohn GmbH, Berlin, 1995

141 Grimm, W.-D.: ,,Bildatlas wichtiger Denkrnalgesteine der Bundesrepublik Deutschland", Karl M. Lipp Verlag, Miinchen, 1990

151 Wendler, E.: ,,Modifizierung von Steinschutzmitteln" in: ,,Die Geowissenschaften", 10. Jahrg. 1992; Nr. 9 - 10, VCH Verlagsgesellschaft mbH, Weinheim

161 Wendler, E./Klemm, D.D./Snethlage, R.: ,,Consolidation and hydrophobic treatment of natural stone", Fifth international conference on durability of building materials and components, Brighton, 7. - 9. 11. 1990,

171 Snethlage, R./ Wendler, E.: Surfactance and adherent silicon resins - New protective agents for natural stone",

Spring Meeting, Materials Research Society, 16. - 21 .04.1990, San Francisco, California

181 Miiller-Rochholz, J.: ,,Verfestigung und Hydrophobierung von Baumberger Sandstein",

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