11 th INTERNA TIONAL BRICKJBLOCK MASONRY CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

THE REPAIR OF HISTORIC MASONRY STRUCTURES BY INJECTION ANCHORS

Birger GiglaJ and Fritz Wenzef

1. ABSTRACT

A research project within the SFB 3153 at the University of Karlsruhe is investigating the loadbearing characteristics of supplementary injection anchors as a repair method in his­toric masonry. Such anchors are necessary for carrying additional loadings, or for new connections. They basically consist of a tensile element - usually steel - inserted into the slightly larger borehole and the annulus is grouted with cement. The solid plug of injec­tion material transfers the tensile forces to the masonry.

Because of the present lack of regularized codes dealing with adrnissable bond stresses there is an uncertainty in the design of injection anchors. Complementary standards for concrete should not be applied unless they are considerably reduced. If the anchors relate to structural safety then fie1d pull-out tests are required by the Local Authority.

Prelirninary results of the research are introduced in the text. The end aim is to be able to assess the amount and dispersion of bond stress during gradual loading to failure, and finally the development of a design recommendation. The collapse mode, namely "failure between steel and cement" or "failure between cement and borehole" has to be defined. Other important features are compatibility with the parent material, durability and mini­rnizing the loss of historie substance. The laboratory tests incorporate different types of natural stone, masonry and brick masonry. Key parameters to be considered are bore­hole-geometry, bond length, grouting material and type of bar. Poll-out tests in situ will be carried outto verify the findings.

Keywords: Masonry; Historie Monument; Repair; Injection Anchor; Design

I Dipl.-Ing. Birger Gigla, Research Assistant, Institut für Tragkonstruktionen, Universitiit Karlsruhe

2 Prof. Dr.-Ing. Fritz Wenzel, Professor, Institut für Tragkonstruktionen, Universitiit Karlsruhe

3 Sonderforschungsbereich 315: Erhalten historisch bedeutsamer Bauwerke ("Special Research Area 315: Care and maintenance of historie buildings"), Universitiit Karlsruhe

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2. INDRODUCTION

Supplementary injection anchors are used as a repair-system for historie masonry for transferring additional tensile- and shear forces that can not be sustained by the masonry alone [1] . Such anchors perform as untensioned steel reinforcement or as prestressed tendon. Forces can only be transmitted in axial direction either by bond between the an­chor and surrounding masonry or with anchor plates. The described research concems the transmission by bond.

The first step is drilling the borehole. Usual borehole diameters are 56 mm for unten­sioned anchors and 76 mm for prestressed tendons to realize a sufficient cement-coating for corrosion protection. Stainless steels are used increasingly for the repair of historie masonry and therefore it is one of the intentions of this research to define statical re­quirements for adequate borehole diameters. Borings for untensioned anchors are usually less than 4,0 m, whereas for prestressed tendons they may go up up to 35 m in length. The drilling method will be selected on the basis of cost effectiveness and sympathetic treatment of the historie structure.

Anchor materials are standard reinforcing steel, threaded rods or special prestressing bars with roll-formed sections. After clearing and pre-wetting the drill hole, the anchor element is inserted, centred with spacers and grouted. The admissible grouting pressure has to be adjusted according to the state of the masonry and may vary between 1 to 6 bar. The in­jection grout has to ensure bond and corrosion protection.

a) b) c)

Fig. 1 Samples of injection anchors as a repairing system inside historie masonry (schematic) a) Locking of a stone corbel b) Transmission of tensile forces inside cross section of wall c) Locking of foundation reinforcing

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Injection anchors are particularly suited to the repair of historic monuments because the functlOn and appearance of the masonry will not be affected and the new element will be discemible. The original substance is only replaced within the boring hole with a physical and mechanically similar material, if cement or lime suspensions are grouted. The prece­dent for use of metal based materiais in masonry structures is centuries old. Fig. 1 shows different examples of use of injection anchors in masonry. The shown cases contain the repair of small elements like stone corbels and the reinforcing of walls and foundations that might even require prestressed tendons.

3. THE TYPE ANCHOR UNDER TEST

The injection anchors currently being investigated in this research program transfer their forces through bond shear. The first priority of the research therefore, is to establish the failure criteria across the weakest of the different interfaces. In this respect the behavior of supplementary injection anchors can not be compared directly with process of analysis for concrete [N2], [N7] or reinforced masonry [N4]. The anchor force has to be trans­mitted across three surfaces, namely steel and injected mortar, within the masonry mortar and the parent stone. There are other differences in masonry work which involve other parameters. The most important are the structure of the parent masonry and injection technique. It was found moreover, that rock hnchors, used in geotechnical engineering offer only limited "Technoloy Transfer" benefit as they have to be coated with double corrosion protection systems that are alien to the philosophy of protection of historic monuments. Cement grouted steel rods for rock anchors are only allowed on temporary works. Supplementary anchors inside masonry are usually designed horizontal while rock anchors have to be angled downward by reason of their task to carry earth pressure and also to ensure satisfactory grout flow.

There are only a limited number of publications referring to the loadbearing characteristic of grouted anchors in masonry. However, preliminary results [3] indicate that shear forces can be transferred satisfactorily and in sufficient amount to develop the tensile strength of the steel.

In present practice the design of supplementary masonry injection anchors is carried out in accordance with recommendations given by Wenzel (1988) [1] and Haller (1981) [2]. Where anchor connections rely on bond, the limited shear stress on the steel/mortar inter­face has to be checked. Recommendations are currently based on German Standard DIN 1045 for concrete design [N2] but with the use of quite large reduction factors and grout­ing of pure cement suspension. Other cementing materiais like trass lime are not allowed to inject.

The gap in present knowledge focusses on the joint interface strength between the injected mortar grout and the inside surface of the borehole.

4. THE LOADBEARING OF MASONRY INJECTION ANCHORS FROM THE RESUL TS OF PULL-OUT TESTS

4.1 Laboratory programme

The goal of the laboratory programme is to systematically evaluate the load bearing char­acteristics of injection anchors in the context of repair work in historic masonry and from that, to develop a design recommendation. The particular association with preservation of historic monuments has to be recognised and various parameters require consideration in respect of the diversity of existing masonry structures and the range of the foregoing

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techniques, that can be used for repair. The relative influence of each need to be isolated through the range of laboratory tests . The test program itself is defined into series with each containing a minimum of five single pull~out tests, this being the minimum neces­sary to give a statistic. Fig. 2 gives an schematic overview of the laboratory test arrange­ment and introduces the used terms. It shows an anchor grouted inside blockwork during a pull-out test. The free anchor length is defined between end of bond length and front of cJamp. The truckness of the masonry mortar bed joints in the test prograrnrne wiU be about 20 rnrn.

blockwork

suspension of tensioning jack

vertical bedjoint

I ring dynamometer

free anchor end

/r borehole

anchor

'. ,~.". , .... .:.: . ", ',. : ... ':';'. '" _.' .

injected mortar

bond length

anchor length

free anchor length

Fig. 2 Diagram of supplementary injection anchor in laboratory pull-out tests

loaded anchorend

-cJamp

The tests must simulate the true situation as realistic as possible. Therefore natural stone and brick masonry were chosen as test-medium according to the relation between the loadbearing characteristics of the supplementary injection anchors and the structural prop­erties of the surrounding masonry. The most important influences are sorption, cavity and shear strength of the parent masonry as well as technological characteristics of the drilling process like inner surface of the borehole. The water/cement ratio of the suspension has to be chosen in reliance to the sorption of the parent stone material. Tab. 1 shows stone type, structure and sizes of the chosen specimens.

The selection of injection mortar is important in practice, because corrosion protection and compatibility with the existing materials have to be ensured. Injection of an incompatible grout mix has been known to cause considerable damage. During the investigations de­scribed here trass cement, trass-lime, portland blastfumace slag cement (HOZ) and port­land cement are used [N5].

The borehole geometry is usually govemed by requirements for corrosion protection. Referring to German Standard DIN 1045, Tab. 10 [N2], a grouted annulus of cement with 20 rnrn thickness can be sufficient. For reinforcing steel at 16 mm diameter, 56 mm diameter boreholes would be necessary in normal practice. Any features wruch may promote corrosion or changing humidity leveis have to be avoided.

By reason of the philosophy af monument care, a priority is placed on minimizing the loss of parent material . Moreover the number and size of borings should be reduced, both

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for this reason and for economics. Likewise, durability is sought after, but reduction of the borehole diameter requires altemative corrosion protection methods. Since coatings and steel sleeves are not normally acceptable for monument repair corrosion resistant materiais are recommended. In these investigations therefore cold strained stainless rein­forcing steel and stainless threaded rods are used. The materials are more expensive than ordinary mild steel, but the borehole size (relative to the steel) can be reduced to what is necessary to give sufficient bond.

No. material stone type and structure width height thick minimum strength [cm] [cm] ness referring to [cm] DIN 1053-1 [N3]

LiO CannsUitter Travertin travertine monolithic block 70 100 50 fc.min = 20 MN/m2

L2 Postaer Sandstein soft sandstone monolithic block 100 100 45 L9 fC.min = 30 MN/m2 100 100 25

Li Maulbronner Sand- hard sandstone monolithic block 100 100 45 L3 stein fC.min = 50 MN/m2 100 100 25 LU 100 100 45 Li2 70 100 50 L4 Raumünzacher Granit granite monolithic block 70 100 50

fC.min = 120 MN/m2

L7 Cannstatter Travertin travertine blockwork 70 130 50 fc.min = 20 MN/m2 DIN 1053-1

L6 Postaer Sandstein soft sandstone blockwork 70 130 50 fC.min = 30 MN/m2 DIN 1053-1

L5 Maulbronner Sand- hard sandstone blockwork 70 130 50 Li3 stein (.min = 50 MN/m2 DIN 1053-1 70 130 50

L8 Raumünzacher Granit granite blockwork 70 130 50 fC.min = 120 MN/m2 DIN 1053-1

Tab. 1 Overall test programme

If the masonry has other cracks or cavities which have to be repaired by grouting without reinforcing, different types of boreholes are employed. Normally such borings are 30 mm in diarneter, and one of the features of the test program is the dual usage of bor­ings for the grouting of masonry and injection anchors. With good planning the numbers of boreholes could be noticeable reduced.

4.2 First test series

The first specimen are from sawn monolithic blocks of Maulbronner and Postaer sand­stone (specimen Li, L2 and L3, see Tab. 1). Drilling was by overcoring with diamond matrix and water flush at 30 mm diameter and after completion the anchors are then in­serted into the cJeared and pre-wetted holes. Contrary to practice the anchor bars in labo­ratory tests need an overhang on both sides for fixing the tensioning jack etc. Accordingly the anchor bar has to be fitted through the packers without affecting the seal between steel and stone. The special packer system used in the grout holes of 30 mm diameter was

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developed in the Institut für Tragkonstruktionen. It consists of an elastic medi um that is expanded between an alurninium body and a washer by turning a nut.

The anchor material was 10 mm diameter/1200 mm long ribbed stainless reinforcing steel BSt 500 NR as used in concrete. It is an austenitic steel, material number 1.4571 ref. DIN 17440 [N8]. The mechanical properties were verified within SFB 315 at the Ver­suchsanstalt für Stahl, Holz und Steine of Karlsruhe University. The E-Modulus was certified as 160.000 MN/m2 (secant-modulus at 1/3 tensile strength). The average failure load was about 55,9 kN with an breaking elongation of 26,2 %. The average tensile strength was certified as 702,2 N/mm2

•

Bond lengths varied between 50 and 250 mm. Injection mortar was a trass-cement sus­pension with waterlcement ratios of 0,66 and 1,00. A simple membrane hand pump pro­vided the injection pressure and no additives or chernical agents were added. A colloidal­rnixer was used for rnixing the grout material. Mixing time was a constant two rninutes. The injection pressure varied between 1,5 and 2,5 bar and this was maintained for two minutes. The exact anchor geometry is measured before each pullout-test and afterwards. Parallel with the pullout-testing the mechanical characteristics of the injection material is investi­gated.

4.3 Test arrangement and measurement methods

In the pull-out tests all anchors are loaded to failure after 28 days. The anchor displace­ments at the free and the loaded ends are checked from an independent datum point. The displacement of the injected morter is also measured at the free end. A 250-kN-tension jack with 50 mm travei and a 100 kN-ring-dynamometer are used for loading and force measurement. The tension jack is connected with a spring suspension and fitted into the mouth of the borehole with a guide tube to avoid interference out of the test set-up like shown in Fig. 2. The measurement set-up is shown as schematic diagram in Fig. 3. Five displacements and the load have to be recorded. Ali force and displacement quantities are recorded at I second intervals via a data logger.

free anchor end

blockwork

Fig. 3 Diagram of test arrangement

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loaded anchor end

The photography at Fig. 4 gives an impression of the test series. It shows a specimen of Maulbronner sandstone (No. LI) during the tests: 25 anchors have been built in, row "C" is about to be tested. The tensioning jack connected with the ring-dynamometer is hang­ing at its suspension.

Fig. 4 View of specimen LI with anchors built in

The loading program is in accordance with DIN 4125 [N6]; Fig. 6 and 7 show the re­sulting load versus displacement graphs. Bevor the inductive displacement transducers at the loaded end are zeroed a preload of 20% of the final load is applied. The load is then progressively taken to failure . The displacements of the free end are measured using twin inductive displacement transducers for avoiding influences out of tension jack eccentrics. These are checked with an additional transducer perpendicular to the anchor axis as shown in Fig. 3.

The failure inside the joints is determined from displacements of the free end. A compari­son of the displacements af free and loaded ends provides additional information as to load-transfer characteristics within the bond length. From the differences in displacement between load and unload and the resulting steel-strains an empirical point of support can be calculated by knowledge of E-Modulus and cross sectian of steel. This point moves under raising load empirically to the free end of the bond length and can indicate the state of failure . The understanding of this mechanism af failure can be applied to the altemative field case where the ancharends are nat free .

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5 RESUL TS OF THE FIRST TEST SERIES

5.1 Bond strength

Tab. 2 provides a summary list of the first test series. The used terrns are:

TS specimen, like described in Tab. 1 L'b bond length w/c \vater/cement-ratio fm.c compression strength of the injected mortar (28 d), ref. to Gerrnan standard

DIN 18555-3 [N9] F ma. maximum test load St,free

sm f .. O.01 •d

f,.o.l.d fm.O.Ol.d

anchor displacement at the free end at the end of the test displacement of injected mortar plug at the end of the test calculated shear strength at 0,01 mm displacement of free anchor end calculated shear strength at 0,1 mm displacement of free anchor end calculated shear strength at 0,01 mm displacement of injected mortar plug

The displacements are the absolute measurements. The relative displacement between steel and mortar must be calculated from the differences.

Inspite of low compression strength of the injected mortar fm.c and low injection pres­Sures relatively strong connections have been achieved. The bond length L'b of 150 m m was sufficient to develope the full tensile strength of the anchor at a small displacement s~rree at the free end. The shown shear strengths have been measured during the tests. To indicate that they rely on defined displacements and existing steel surfaces they are called calculated shear strength. The calculated shear strength within the joint between steel and injected mortar in column 9 f,.o.Ol.d is based on a free end displacement of 0,01 mm. Free end displacements up to O, I mm, which defines an upper limit for corrosion protection, were a relatively rare occurence. The resulting calculated shear strength f .. o.1•d is shown in column 10.

In cases of failure in the joint between injected mortar and borehole the calculated shear strength fm.OOl .d is given for a corresponding deflection of 0,0 I mm (see column 11).

5.2 Behavior at failure

For bond lengths of about 250 mm (test series L2E l-I) the tensile strength of the anchor itself was exceeded without bond failure . At bond lengths about 150 mm (test series L2EI-2 and L2EI-5) the anchor strength was also reached in ali cases, but this appeared to correspond to the threshold of bond strength failure. Free end displacements in excess of 0,0 I mm only occured at very high load leveis. A failure in the joint between injected mortar and borehole was observed in two tests (L2EI-B4 and L2EI-E2). In these cases the average calculated shear or bond limits were:

12,2 N/mm2 at the interface between steel and mortar 3,9 N/mm2 at the interface between injection mortar and face of the drill hole

They differ by the factor of 3 and should be considered in relation to the boundary pa­rameters (i.e. ratio of borehole diameter to diameter of steel and bond length).

Presuming constant borehole geometry then the joint between borehole and injected mortar will govem the failure condition at shorter bond lengths.

For bond lengths of about 100 mm failure occured as expected on the mortarlborehole interface except in the case of test L3E I-B5. The break occured at the transitional phase between both mediums. Around the borehole-wall there were remains of a thin grout layer which indicated a shear sufface in the mortar material.

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1 2 3 I 4 5 6 7 8 I 9 10 11 : steel morta-.! steel steel mortar

Test-No. TS L'b i w/c fm.c Fmax s,! ... Sm f O.Old t f'Old fmO.01,d

[mm] 1 [-] [N/mm2 [kN] [mm] [mm] [N/mm21 [N/mm2 [N/mm2

L2El-l L2E1-Al L2 2531 0,66 20,5 58,89 0,002 0,000 L2EI-A2 L2 247 0,66 20,5 59,06 0,002 0,000 I

L2EI-A3 L2 248 , 0,66 20,5 59,13 0,004 0,000 -L2EI-A4 L2 248 ' 0,66 20,5 57,47 0,005 0,002 L2EI-A5 L2 · 251 ' 0,66 20,5 59,39 0,000 0,000

! 1 L2El-2 ! L2EI -Bl L2 149 0,66 19,9 58,72 0,062 0,000 12,39 L2EI-B2 L2 147 1 0,66 19,9 59,46 0,156 0,000 12,54 12,76 L2EI -B3 L2 148 0,66 19,9 57,16 0,009 0,002 12,29 L2E1-B4 L2 146 j 0,66 19,9 59,62 1,089 0,025 12,22 12,78 4,02 L2EI-B5 L2 146 0,66 19,9 55,05 0,002 0,000 average I 12,36 standard deviation 0,14 5-%-fractile 12,13

1 L2El-5 I L2E1-El L2 145 ' 1,00 11,1 56,53 0,026 0,000 11 ,91 L2EI -E2 L2 151 1,00 11,1 56,69 0,018 0,015 3,78 L2EI -E3 L2 143 1,00 11,1 56,97 0,017 0,000 12,28 L2EI-E4 L2 141 1,00 11,1 57,19 0,013 0,000 12,46 L2E1-E5 L2 148 1,00 11 ,1 57,09 0,038 0,000 11 ,53 average I 12,05 standard deviation 0,41 5-%-fractile 11,37

i L3El-l L3E1-A1 L3 52 0,66 24,6 18,20 0,902 0,022 2,07 5,28 2,42 L3E1-A2 L3 41 0,66 24,6 8,24 0;927 0,000 2,54 3,43 L3EI-A3 L3 L3EI-A4 L3 51 0,66 24,6 2,88 0,435 0,435 L3EI-A5 L3 53 0,66 24,6 4,95 1,046 0,957

I L3El-2 L3EI-Bl L3 . 98 0,66 25,8 30,61 1,127 0,039 2,22 3,50 0,45 L3EI-B2 L3 97 0,66 25,8 16,79 1,085 0,638 1,38 3,10 0,27 L3EI-B3 L3 101 0,66 25,8 29,18 1,001 0,400 5,67 6,58 .0]6 L3EI-B4 L3 100 0,66 25,8 6,97 0,910 0,674 1,27 0,13 L3EI-B5 L3 98 0,66 25,8 25,05 1,022 0,000 1,12 1,87

Tab. 2 Summary list of the first test series

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The calculated shear strengths appears to vary along the bond length - like shown in Fig. 5 - and any explanation will need more investigation.

At bond lengths of about 50 mmtwo tests provided no meaningful result. This appeared to be due to difficulty in handling a short length assembly of the test equipment according lO unintentional preloading by pushing the anchor during rem oval of the packers. The tests are noted in Tab. 2 (L3E1-A4 and -A5).

Fig. 6 and 7 show test load as a function of displacement for the tests L2E 1-B4 and L3E1-B3 . The displacements of the loaded and the free anchor end can be compared.

Pull-out tests L2El-2, L2El-5, L3El-l and L3El-2 Bond length and shear strength (failure between the steel and mortar surface)

14,00 ,-------------------------;

~ 12,00 .~ • . ! Na ..ê 10,00 6,

8,00

6,00

4 ,00

2,00 • •

• • ~ 0,00 -f---------------+------,----------'

o 20 40 60 80 100 120 140 160

Bond length [rnrn]

Fig. 5 Shear strength as function of bond length for 10 rnrn diameter stainless steel BSt 500 NR inside sandstone grouted with trass-cement at boreholes of 30 rnrn diameter

At the shown bond length of 146 mm (Fig. 6) the first displacements of the free end are initiated at high loads of about 58 kN. The remaining displacements at the loaded end attribute to irreversible strain of the anchor-steel. The progessive characteristic of failure becomes apparent since the steel is drawn out at the front of the bond length, while the backspace movement during unloading is hindered in part by friction inside the remaining joint.

At the shown bond length of 10 1 rnrn (Fig. 7) displacements of the free end start at low loads. According to the failure between mortar and borehole the displacement of the loaded anchor end referres to movements of the whole anchor. The function of free end displacement of steel and mortar versus load follows the Mohr-Coulomb expression.

5.3 Results

In the case of failure between the steel and mortar interface, and where a minimum bond length of 150 rnrn is available, the results of 15 test (in series L2El-1, L2El-2, L2El-5)

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Pull-out test L2E1-B4, 17.12.1996 Postaer Sandstone, monolithic, L'b = 146 mm

Trass cement CEM IIIB-P 32,5, w/c = 0,66, fm•c = 19,9 N/mm2

Steel BSt 500 NR, DIN 17440-1.4571, d = 10 mm

Test force F, [kN]

0,00 10,00 20,00 30,00 40,00 50,00 60,00

0,00 ~~:':±::;=.:::::;;:j::;;::==.:::±:;:==~~~t::::~~ ---steel(free end)- -

I 0,50 +--~~------------~ ~ '" ] ~ 1, --

] ... o C 1,50 e ~ ê. .~ 2,00 "O

~ 12,50 .:

'" "O c: cu

"O cu

"O

'" .9 ... o ê:

3,00

3,50

e 8 -ã 4,00 '" ;a

~ ~ 4,50

~

+- -

.... -t- "t •

~ --.. -...,..-+--

.• '~ T to - ._+ - .-t--+-

t +.! ~... _ + .---t-... _-t" ._-.,.. r

- . - t- .- .... ~- ..... .- 1" - - t ~ .

. - ..... ~-

• •. -<0 T

--, .,--r- +-.- .

5,00 ..L-______ ....:.-______ ....:.-_--+ __ ...J

Fig.6 Bond length 146 mm (test L2EI-B4),load versus displacement

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Pull-out test L3EI-B3, 11.03.1997 Maulbronner Sandstone, monolithic, L'b = 101 mm

Trass cement CEM II/B-P 32,5, w/c = 0,66, fm•c = 25,8 N/mm2

Steel BSt 500 NR, DIN 17440-1.4571, d = 10 mm

Test force F, [kN]

0,00 10,00 20,00 30,00 40,00 50,00 60,00 0,00 -f--,'<""<:::,.,.........-+-"T"""T--,-..-+-,-..--.--r-r-r-"'T"'"'T--,--t-T-,-T"""'!"'~,......-........,.-i

1050 ~ , til

~ = 11.) 1,00 8

<l:: ...... o ~ 1,50

~ ~ ]o 2,00 ~

1 12,50

til

~ = 3,00 11.)

~ 11.) ~

'" .9 ...... 3,50 o c ~ 8

!:! 4,00 ~ ;a ~ ! 4,50

~~---- --. --

steel,-average ofloaded entl .

~ -

~ -~- - --... - - .....

...... ,... +... +- . --- _+.. ... • + ....

... -,.. --- ... - T ..-

0- T ...

.. -

- ,. -

>----t--... ,... - -t- ----:--- ... ,...-.,. - -

- .-- ..

---n----- - -- . -=-_.~L:-

, ----~ . ---

... +-- ----;--+-+ .... -

.. . t- t---T-" - .. -t ... --- ... -.- . -+---------..,...--

-+ t- _-J- ~-" .- r-L t- t"- --~.....---L -1-----+ - . --

- . ___ .............. _- T t-!.-

5,00 ~------~r_----------------~----~--~-~--~I ~'--~~

Fig.7 Bond length 101 mm (test L3EI-B3), load versus displacement

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showed sufficient consistency to justify an admissable shear stress of f, adm = 5,7 N/mm2

on the basis of a supposed safety factor of 2,0 (within the 5%-fractile). . .

The calculated shear strength on the mortar plug/borehole interface can only be regarded as indicative, because there are being 2 results only. It would follow a limit-value for the shear strength of fm.O.OI.d = 3,9 N/mm2

• This is based on the injection of trass cement in sandstone.

The use of this or any value relies on good quality injection work. If the injection proce­dure is flawed, then the safe working stress cannot be covered by safety factors, nor can it be overcome by substituting longer bond lengths. Fig. 10 shows an example for flawn injection work according to problems with pump-valve timing. The anchor is only half embeded in grout.

,. ~'~ 4 • ... ':': ~'.

Fig. 8 Half embeded injection anchor accordingto unsatisfactory grouting work (problems with pump-valve timing)

6. CONCLUSION

The preliminary findings of an investigation into the pull-out strength of drilled and grouted anchors in historie masonry are reported and function and possibilities of such anchors are explained. The results from 5 test series with 25 pullout-test are described and discussed. Bond lengths of 150 mm were found to be adequate for mobilising the full tensile strength of stainless steel anchors of 10 mm diameter. Within the range of the tests carried out (diameter, required minimum bond length, borehole size, good quality injec­tion work etc.) an admissable shear stress of ft.adm = 5,7 N/mm2 can be transfered at the joint between anchor and injected mortar. For the investigated joint between injected mortar and borehole the preliminary findings indicate a limit value of the calculated shear strength of fm.OOI .d = 3,9 N/mm2 for trass cement groutings in sandstone.

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ACKNOWLEDGEMENTS

The decribed research is carried out within the Sonderforschungsbereich 315 "Erhalten historisch bedeutsamer Bauwerke" at the Institut für Tragkonstruktionen of Karlsruhe University. The research program started at July 1996 and is financed by the Deutsche Forschungsgemeinschaft (DFG). Thanks are due to senior lecturer David Cook from The University Of Bath for viewing this text.

REFERENCES

[ I] Wenzel, Fritz: Verpressen, Vernadeln und Vorspannen von Mauerwerk his­torischer Bauten, Stand der Forschung, Regeln für die Praxis. In: Erhalten his­torisch bedeutsamer Bauwerke, Sonderforschungsbereich 315, l ahrbuch 1987. Berlin: Verlag Wilhelm Ernst & Sohn, 1988, S. 53-72

[2] Haller, Jürgen: Untersuchungen zum Vorspannen von Mauerwerk historischer Bauten. Karlsruhe, Universitat (TH), Institut für Tragkonstruktionen: Aus For­schung und Lehre, Heft 9, Diss. , 1981

[3] Brüggemann, Bernhard: Die Ermittlung der aufnehmbaren Krafte von in das Mauerwerk eingebauten Nadelankern aus Betonstahl. Braunschweig, Technische Universitat, Lehrstuhl für Hochbaustatik, Prof. Dr.-Ing. K. Pieper, Forschungs­bericht, 1976

Referred standards

[N 1] Norm DIN EN 1537, Entwurf, 1994-10, VerpreBanker

[N2] Norm DIN 1045 1988-07 (Juli 1988): Beton und Stahlbeton; Bemessung und Aus­führung

[N3] Norm DIN 1053-1 1990-02 (Fetlruar 1990): Mauerwerk; Rezeptmauerwerk, Berechnung und Ausführung

[N4] Norm DIN 1053-3 1990-02 (Februar 1990): Mauerwerk, Bewehrtes Mauerwerk, Berechnung und Ausführung

[N5] Norm DIN 1164-1 1994-10 (Oktober 1994): Zement; Teil 1: Zusammensetzung, Anforderungen

[N6] Norm DIN 4125 1990-11 (November 1990): VerpreBanker; Kurzzeitanker und Daueranker, Bemessung, Ausführung und Prüfung

[N7] Norm DIN 4227-5 1979-12 (Dezember 1979): Spannbeton; Einpressen von Ze­mentmortel in Spannkanale

[N8] Norm DIN 17440 1996-09 (September 1996): Nichtrostende Stahle; Technische Lieferbedingungen für Blech, Warmband und gewalzte Stabe für Druckbehiilter, gezogenen Draht und Schmiedestücke

[N9] Norm DIN 18555-3 1982-09 (September 1982): Prüfung von Morteln mit miner­alischen Bindemitteln; Festmortel, Bestimmung der Biegezugfestigkeit, Druckfes­tigkeit und Rohdichte

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