STRUCTURAL DESIGN OF SUPPLEMENTARY INJECTION ANCHORS INSIDE MASONRY

Transcription

1 STRUCTURAL DESIGN OF SUPPLEMENTARY INJECTION ANCHORS INSIDE MASONRY Gigla, Birger 1 1 Dr.-Ing., Professor, University of Applied Sciences Lübeck, Germany, gigla@fh-luebeck.de Supplementary injection anchors are used as a repairing-system inside historic masonry for transferring tensile-forces that cannot be transmitted by the masonry itself or for new connections. They basically consist of a tensile element - usually steel - inserted into a slightly larger borehole and the annulus is grouted with cement. The solid plug of injection material transfers the tensile forces to the masonry. Assuming successful grouting, considerable tensile forces can be transferred at short bond lengths. Supplementary Injection Anchors have been utilised since the 1920s and applied within a core philosophy of minimum intervention. The field of application extends from the repair of small elements like stone corbels up to the reinforcing of walls and foundations that might require prestressed tendons. In certain practical situations, e.g. when connecting two shells of masonry, extremely short bond lengths are required. Connections are not only carried out inside masonry structures but also between masonry and reinforced concrete slabs or frameworks. The paper summarizes the latest development of design recommendations for supplementary injection anchors inside masonry, based on more than 600 tests in laboratory and in situ. Main parameters to obtain the characteristic value of bond strength are compressive strength of grout, grouting technology and capillarity of the surrounding stone. The paper takes aspects of the application inside low strength masonry into special consideration. Keywords: Restoration, repair, rehabilitation and refurbishment, injection anchor INTRODUCTION Supplementary injection anchors (figure 1) are used as a repair-system for masonry to cover tensile forces that cannot be sustained by the masonry alone. In Germany, such anchors have been utilized since the 1920s and frequently applied to repair monuments within a core philosophy of minimum intervention. A new field of application is the reinforcement of masonry against dynamic loads. They are applied in the form of untensioned steel reinforcement or as prestressed tendon. While new tensile elements like glass fibre reinforced plastic bars did not prove successful in practice, stainless steels are used increasingly for the repair of historic masonry. Forces are transmitted by bond between the anchor and the surrounding masonry in axial direction. Standard borehole diameters are between 30 mm and 76 mm. Ordinary steels require a sufficient cement-coating for corrosion protection.

2 Figure 1: Diagram of a supplementary injection anchor [Gigla 2008] Borings for untensioned anchors are usually less than 4.0 m, whereas for prestressed tendons they may go up to 35 m in length. The drilling method is regularly selected on the basis of cost effectiveness and aspects of minimum intervention in the case of monuments. After clearing and prewetting the drill hole, the anchor bar is inserted, centred with spacers, sealed 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. Figure 2 gives examples for the utilisation of injection anchors in masonry. Figure 2: Utilisation of injection anchors [Gigla 2004] a), d), j) Anchors stabilising cracked or deformed masonry leaves b), c), f) Anchors fixing cracked structures h), k) Strengthening against dynamic loads e, i) Joints or keys between old and new work g) Reinforcing of walls or foundations against uneven settlement

3 Recommendations for the design of injection anchors have been given by Gigla and Wenzel (2000, 2002) and Gigla (2004, 2008, 2010), based on a research project at Sonderforschungsbereich 315 (Collaborative Research Centre 315: Care and maintenance of historic buildings, Karlsruhe Institute of Technology, ). Aspects of force transmission of injection anchors are part of ongoing research at University of Applied Sciences, Luebeck. Overall 600 pull-out tests have been performed to establish design recommendations. PULL-OUT TESTING OF INJECTION ANCHORS Pull-Out tests of injection anchors should be performed as series with a minimum of five identical anchors each to provide a statistic evaluation. Figure 3 shows grouting works in the laboratory with five series of total 25 injection anchors inside monolithic sandstone (variety Maulbronn). Current parameter under investigation is the type of anchor bar at a bond length of 200 mm and with a nominal diameter of 16 mm constantly. Anchor bars are stainless reinforcing bars (left), ordinary reinforcing bars (second left), and three different types of threaded rods (right). In all five series the boreholes are 56 mm in diameter. Grout is a trass-cementsuspension with a water/cement-ratio 0.6. The compressive strength of the grout of the five series varies between 28.5 and 30.1 MPa, tested in accordance with standard DIN EN Figure 3: Grouting works in laboratory The test set-up is shown in figure 4. During testing a loaded and a free anchor end have to be distinguished. Usually, the free end is not accessible in field tests and therefore, free end displacements are only to be explored in laboratory. Anchor displacements at both ends are checked while the anchors are loaded to failure in a cyclic loading programme. The basic test load required to ensure transfer of forces has been chosen equivalent to 20% of working

4 stress. Loading sequences are covering 50%, 75%, 100% and 125% of estimated working load. The final loading to failure proceeds in 25% increments of estimated working load. Observing intervals are minimum 1 min at load and unload. The displacements of steel and injected mortar plug at the free end and twin steel displacements and test force at the loaded end are recorded at 1 to 10 Hz frequency via a data logger. Inductive displacement transducers and a ring-dynamometer are used for measurement. Construction of injection anchors, test set-up, evaluated parameters, test results and evaluation are discussed in greater scale by Gigla/Wenzel (2000) and Gigla (2004). Figure 4: Test set-up in laboratory [Gigla 2008] In the laboratory, the bond strength is nominally defined as average bond stress at a displacement level of 0.1 mm at the free steel end. Loaded end displacements of steel contain elastic and plastic anchor bar deformations. A comparison between free end and loaded end displacements during a cyclic loading programme provides additional information concerning the current state of failure. For analysis, one diagram of structural failure of each test series is numerically evaluated from the five curves of test force versus free end steel-displacements of every pull-out test by skipping the cyclic parts. The diagrams of structural failure are the average of five tests and useful for the understanding of the influence of different parameters. Design recommendations are referring to characteristic values of bond strength (5%-fractiles), nominally defined at free-end steel displacements of 0.1 mm. BASIC PRINCIPLES OF ANCHOR FORCE TRANSFER Deflections inside the masonry structure are transferred to the anchor bar depending on the specific conditions in the interfaces between masonry and grout and also between grout and anchor bar. The most effective load transfer requires limited deflections in both interfaces. In this case, force transfer causes cracked cone shaped segments inside the grout plug between masonry and lengthened anchor bar (figure 5). Basic parameters for the failure load are the bond between anchor bar and grout, the compressive strength of the grout plug, the tensile strength of the surrounding material and the bond between masonry and grout.

5 Figure 5: Force transmission during failure, diagrammatical (left) and picture of a grout plug in monolithic sandstone with cone-shaped cracks after pull-out testing (right) BOND BETWEEN ANCHOR AND INJECTED GROUT PLUG The bond between anchor and injected grout plug depends basically on the rib-spacing and rib-sections of the anchor bar. The load transfer is comparable with the mechanisms between reinforcing bar and concrete. Assuming injected grout with lower compressive strength than the surrounding stone or masonry, the lengthening of a steel bar with appropriate rib-design induces cone shaped cracking inside the grout plug, like documented in figure 5. The angle of those segments depends on the properties of the surrounding type of stone or masonry. Inside stone with higher water absorptive capacity like sandstone angles of around 50 have been observed. Inside stone with low water absorptive capacity like granite the angles tend to be around 60. Anchor bars without ribbing should not be used for injection anchors. Threaded rods with nominal diameters up to 16 mm are suitable for injection anchors inside masonry. They provide an effective bond with small deflections. Remaining oil from the production process has to be removed. Additional screws and washers will not improve force transmission, because at higher loads the tensile strength of the grout plug will be locally exceeded underneath the washer. Threaded rods provide a less ductile force transmission compared to reinforcing bars, because the gaps between the threads are smaller than the gaps between the ribs of reinforcing bars. Therefore, the smaller cone-shaped segments are easily torn off with increasing deflections (compare fig. 6). Similar problems have to be expected during application of glass fibre reinforced plastic bars. Dependent on the production process, the ribs of those bars are made of plastic or coiled fibre-rings on the surface of the bar. During different test series glass fibre reinforced plastic bars did not develop a sufficient bond to cover their high tensile strength. Figure 6 shows the diagram of structural failure of the test series for the investigation of the influence of different types of anchor bars, inside the specimen documented in figure 3. The diagram shows avarage failure-lines from five pull-out tests each with cyclic loading.

6 The force transmission of reinforcing bars and threaded rods is similar with nominal diameters of 16 mm. The deflections of the threaded rods are slightly lower at the same level of test force. Accordingly, injection anchors with threaded rods tend to be stiffer, but do not offer additional load capacity at growing deflections. Reinforcing bars provide an increase of load at growing deflections according to load redistribution inside the cone-shaped segements. The difference in the diagrams of structural failure of stainless and standard reinforcing bars is small. Standard reinforcing bars provide better bond at deflections below 50 μm according to the flash rust on the surface. Stainless reinforcing bars gain a higher test force conforming with their higher tensile strength. Additional washers with a head screw at the free end of the threaded rods provide no additional benefit: The wheelspin of the screw is bigger then the deflections between anchor bar and grout plug. Furthermore, the compressive strength of the grout underneath the washer is exceeded. This is documented in the diagram of structural failure of the threaded rod covered with coating. Though washer and nut have been added at the free end the diagram shows plastic deformations referring to local exceedance of grout compressive strength underneath the washer. Figure 6: Diagram of structural failure of five series with 25 pull-out tests focussing on the influence of the type of anchor bar. Anchor grouting shown in figure 3. COMPRESSIVE STRENGTH OF INJECTED GROUT PLUG The compressive strength of the grout plug is an important factor for the description of the load transfer of injection anchors and the basic parameter of the design recommendations. Successful grouting has to be ensured, because cavity inside the grout plug will significantly decrease bond strength. In this context the effect of water absorption of the surrounding stone or masonry has to be taken into account. Water absorption will increase the compressive strength of the grout plug, if the absorbed water is replaced with sufficient grout, see Gigla/Wenzel (2000) and Gigla (2004). The characteristic value of bond stress is described by equation (1):

7 2 fg, c f A, b, k (1) 500 f A,b,k : characteristic value of bond strength of the injection anchor f G,c : compressive strength of grout tested in accordance with standard DIN EN The characteristic value of bond strength of the injection anchor f A,b,k is defined as test force divided by the lateral surface of the anchor bar (nominal diameter) at 0.1 mm deflection at the free end, based on 5%-fractiles. Terms to describe the increase of bond strength inside water absorptive stone, the calculation of the design value of bond strength and a discussion of the equations in greater scale can be found in Gigla (2004). BOND BETWEEN INJECTED GROUT AND MASONRY The bond between grout plug and surrounding stone or masonry depends on the macro porosity of the stone surface and is described by the water absorptive capacity of the material. Figure 7 shows the basic difference between water absorptive material like sandstone (left) and non water absorptive material like granite (right). The particular interface between grout and stone is depicted in thin layer microscopic views of 3.62 x 3.62 mm size. Figure 7: Thin layer microscopic views of an 3.62 x 3.62 mm section of the interface between grout plug and sandstone (left) and grout plug and granite (right), Gigla 2004 The macro-porosity of the sandstone (fig. 7, left) results in an interlocking between borehole surface and grout plug. This mechanism prevents relative displacements between grout plug and stone and provides an effective transmission of tensile forces. The resulting angles of the cone shaped segments during failure are around 50. In tight and smooth stone material like limestone or granite (fig. 7, right) the cone-shaped segments are not developing. In case of shorter bond lengths, the whole injection anchor with grout plug will be drawn out. Therefore higher anchor loads require long bond lengths or greater borehole diameters, if possible. TENSILE STRENGTH OF SURROUNDING STONE OR MASONRY Inside stone, the anchor tensile force has to be limited to avoid cracking. Based on the theory of elasticity expression (2) describes the maximum anchor tensile force that might be sustained by surrounding stone sections, Gigla (2004): 1, 9 f B,t L b d B h 2 2 S d B F m tan d 2 2 B h S (2)

8 F: anchor tensile force d B : borehole diameter h S : stone height in relevant section L b : bond length φ: angle between cracked cone shaped segments and axis of anchor bar, approximately: φ = 50 in water absorptive stone (comp. fig. 5) and φ = 60 in non water absorptive stone f B,t : stone tensile strength γ m : Partial factor of safety for stone tensile strength, recommendation: γ m = 1,5 Inside masonry the maximum tensile force depends on the local ratio of joints to stonework and on the vertical stress (see Gigla 2004). This aspect is part of ongoing research. The basic system of load transfer is shown diagrammatically in figure 8. Figure 8: Load transfer between anchor bar and surrounding stone or masonry f A,b : bond strength of the injection anchor; f G,c : compressive strength of grout; f B,b : bond strength of the grout plug; f B,t : stone tensile strength; d B : borehole diameter; φ: angle between cracked cone shaped segments and axis of anchor bar APPLICATION OF INJECTION ANCHORS INSIDE LOW STRENGTH MASONRY Current approaches strive to utilise injection anchors for the reinforcement of low strength masonry. One example is the strengthening of clay masonry against dynamic loads from earthquakes (figure 9). Before the application of injection anchors inside low masonry it should be generally considered to improve the loadbearing structure, like to add and interconnect framework elements. The minimum grout compressive strength that has been investigated during the described research program was f G,c = 16,6 MPa. With lower compressive strength, e. g. with grout made from trass-lime suspensions with less than 20 mass-% of cement no effective transmission of tensile forces could be established. One reason is the aspect of the particular water absorption capacity of the surrounding stone material that has been discussed in Gigla/Wenzel (2000) und in greater scale in Gigla (2008). Figure 10 shows the result of a grouting test in sandstone, variety Posta. This type of sandstone is high-absorptive (absorption of 14 kg/m² water in 1h). The borehole in the picture (diameter: 48 mm) has been completely filled with grout. Significant cavity occurred due to water absorption. Because there is a slower increase of compressive strength after grouting, suspensions with low content of cement are sensitive for cracking, when water is absorbed by the surrounding material. Therefore suspensions for grouting injection anchors inside low strength masonry have to be specially designed. Special additives are required to compensate the effect of water absorption and also the effect of shrinkage. Furthermore, anchor bars with small diameters around 10 mm and adequate borehole diameters of 30 to 56 mm diameter are recommended.

9 Figure 9: Grouting of a clay wall in laboratory at University of Applied Sciences Lübeck Figure 10: Grouting test in sandstone, variety Posta. High water absorption leads to significant cavity, if the absorbed water is not replaced with fresh grout sufficiently The application of injection anchors inside low strength masonry is part of on-going research. To assess the possible transmission of tensile forces, equation (2) is evaluated in figure 11. The diagram on the left shows the maximum anchor tensile force as a function of the tensile strength of the surrounding masonry, assuming bond length L b = 190 mm, borehole diameter d B = 30 mm, stone height in relevant section h S = 240 mm and φ = 50. Under those conditions in masonry with a tensile strength of f B,t = 0,5 MPa a maximum anchor force of F = 10 kn could be expected. In the diagram in figure 11 on the right the maximum anchor tensile force is a function of the stone height in relevant section, assuming bond length L b = 190 mm, borehole diameter d B = 30 mm, tensile strength of the surrounding masonry f B,t = 1.0 MPa and φ = 50. Both diagrams are extrapolated from pull-out tests in masonry with higher compressive and tensile strength and have to be evaluated by means of additional test series in clay masonry. Based on the current state of research it appears safe to assume that in the case of clay masonry, the admissible maximum anchor tensile forces are limited. It has to be further checked, if they cover estimated earthquake loads.

10 Figure 11: Evaluation of equation (2) for the application of injection anchors inside low strength masonry CONCLUSIONS Proceeding from basic principles of force transfer, main aspects of the application of injection anchors inside masonry have been summarised, based on the results of more than 600 tested anchors. One focus is the latest development of design recommendations for injection anchors to choose the appropriate anchor bar for individual applications. It is pointed out, that additional nuts and washers at the free end of the anchor are not improving the maximum anchor tensile force. An actual demand is the utilisation of injection anchors inside low strength masonry, e. g. to strengthen clay walls against earthquake loads. To assess the underlying admissible anchor tensile forces, the design-equation for the limitation of the maximum anchor tensile force to avoid cracking of the surrounding stone or masonry was evaluated. It is indicated, that in the case of clay masonry, only limited anchor tensile forces can be transferred. Though the paper provides a detailed approach for the design of supplementary injection anchors, individual pull-out tests should be performed, if the anchors relate to structural safety. REFERENCES Gigla, B., Wenzel, F., Design Recommendations for Injection Anchors as supplementary Reinforcement of Historic Masonry, Proc. 12th Int. Brick/Block Masonry Conf., Madrid, Spain, 2000, pp Gigla, B., Wenzel, F. Instandsetzung von denkmalgeschützten Bauwerken, Verpressanker im Mauerwerk, Mauerwerk-Kalender 2002, Ernst & Sohn, Berlin, 2002, pp Gigla, B., Bond Strength of Injection Achors as Supplementary Reinforcement inside Historic Masonry, Proc. 13th Int. Brick/Block Masonry Conf., Amsterdam, The Netherlands, 2004, pp Gigla, B. Instandsetzung und Ertüchtigung von Mauerwerk, Teil 5: Vernadeln, Verankern (Berechnung), Mauerwerk-Kalender 2008, Ernst & Sohn, Berlin, 2008, pp Gigla, B. Comparison of failure of injection anchors as supplementary reinforcement inside masonry and concrete, Proc. 8th International Dresden, 2010 Standard DIN EN Methods of testing cement Part 1: Determination of strength, May, 2005