PERFORMANCE OF NDTI MDT AND DT TECHNIQUES IN ASSESSING THE EFFECTIVENESS OF GROUTING

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1 Structural Analysis of Historical Constructions Jerzy Jasieńko (ed) 2012 DWE, Wrocław, Poland, ISSN , ISBN PERFORMANCE OF NDTI MDT AND DT TECHNIQUES IN ASSESSING THE EFFECTIVENESS OF GROUTING rranjek jojmir 1 I Žarnić ooko 2 I Bokan-Bosiljkov sioleta 3 I Bosiljkov slatko 4 ABSTRACT Grout injection is a common and widely used method for seismic strengthening of stone masonry walls with sufficient amount of voids. However, the influence of different types and properties of grout on the mechanical properties of injected walls, still remains the area which requires further research. Consequently, a stone masonry building located in the rural, earthquake-prone area of Posočje in Slovenia, was subjected to several in situ investigation techniques. The objective was to evaluate the effectiveness of different types of injection grout and at the same time to assess the usefulness of different investigation techniques in determining the quality and effectiveness of grouting. Selected parts of the building s walls were strengthened with grout injection technique, using cement and combined cement-lime grouts. Wall specimens were tested in their existing-ungrouted state and 180 days after strengthening. Effectiveness and quality of grout injection, assessed by nondestructive, minor destructive and destructive investigation techniques, showed that the combination of several investigation techniques provides better insight and enables more reliable assessment of strengthening by means of grout injection. It was also found that the mechanical characteristics of strengthened walls significantly depend on the type of grout (cement, cement-lime) used for grouting. heywords: ptone masonryi ptrengtheningi drout injectioni drout typesi fn situ tests NK INTRODUCTION Many historical buildings in Slovenia were built from stone, or as mixed stone and brick assemblages. Stone types such as mixed stone and limestone, sandstone and slate were used. Often brick intrusions were included by constructing the walls. Thicker walls were constructed in three layers (outer layers made of roughly shaped stones and inner core from leftovers and smaller stones), while thinner walls mainly had only two layers. In general low strength lime mortars with the lime:sand volume proportion of 1:3 were used to bind the stone masonry units. Because of weak connections between outer leaves, voids and the low strength of used mortar, the load bearing capacity of such walls, especially towards horizontal loading such as earthquake, is usually insufficient. Among different techniques for the improvement of this type of masonry such as repointing, transversal tying, reinforced-concrete or FRP coating, grout injection may be the most appropriate strengthening technique. This is especially the case when we are faced with the requirements regarding the preservation of the original appearance of stone masonry walls. Although grout injection is a common and widely used method for seismic strengthening of stone masonry walls with sufficient amount of voids, the influence of different types and properties of grout on the mechanical properties of injected walls, still remains the area which requires further research. Most of the authors who studied the strengthening of masonry by means of grout injection [1-3], agree, that the compressive strength of 1 Ph.D., Project Manager, Building and Civil Engineering Institute ZRMK, mojmir.uranjek@gi-zrmk.si 2 Ph.D., Professor, University of Ljubljana, Faculty of Civil and Geodetic Engineering, rzarnic@fgg.uni-lj.si 3 Ph.D., Associate Professor, University of Ljubljana, Faculty of Civil and Geodetic Engineering, vbokan@fgg.uni-lj.si 4 Ph.D., Assistant Professor, University of Ljubljana, Faculty of Civil and Geodetic Engineering, vbosiljk@fgg.uni-lj.si 2544

2 grout injected walls is not directly proportional to the compressive strength of the injection grouts used for grouting. It has also been found that an important parameter for the improvement of the behaviour of stone masonry strengthened by grout injection is the bond achieved between grout and the in situ material, and that bond strength is not necessarily proportional to the compressive or tensile strength of the grout [3]. In order to evaluate the effectiveness (the ability of the grout to establish bonding between the stones and the leaves of the wall) and quality (performance of the grout at filling up the voids) of the grout injection technique by using different types of grouts for the strengthening of and actual stone masonry building, selected parts of the stone masonry building were injected with cement and combined cement-lime grouts. The walls of the building from 1948, located in the rural, earthquakeprone area of Posočje in Slovenia, were constructed with lime mortar and roughly shaped limestone and sandstone, with separate brick intrusions. Based on the criteria presented in [4], four types of grouts were selected for in situ application of grout injection: two cement grouts designated C1 and C2, and two combined cement-lime grouts designated LC1 and LC2. Grout injection was performed on wall specimens 1-C1, 2- LC1, 3-LC2, 4-C2. Wall specimens were tested in their existingungrouted state and 180 days after strengthening by using several non-destructive, minor destructive and destructive tests. Within the scope of preliminary research work, laboratory tests were performed on cylinders representing the inner core of the strengthened walls. OK TESTS ON CYiINDERS In order to simulate the inner core of the multiple leaf stone masonry walls, cylinders of 15 cm diameter and 30 cm height were prepared. The cylinders were gradually filled with limestone and sandstone, i.e. 37% wt. of the fractions 45/63 mm and 32/45 mm, 25% wt. of the fraction 16/32 mm, and 1% wt. of the fraction 8/16 mm. The cylindrical specimens were injected with the grouts LC1, LC2, C1 and C2. For each grout 6 test specimens were prepared: 3 for compressive tests and 3 for tensile splitting tests. At the age of 90 days the cylinders were subjected to compression tests according to [5], whereas the tensile splitting strength tests were performed according to [6]. The results are presented in Fig. 1. FigK N Average compressive and tensile splitting strengths of cylinders and percentage of separate failure modes at tensile splitting strength test After the tensile splitting test had been performed, the cross-sections of the specimens were inspected. On average, the area of stone represented about 68%, and the area of grout 32% of the total crosssection. The results presented in Fig. 1 show that the prevailing mode of failure by tensile splitting test was that of the bond between the stones and the grout, regardless the type of used grout, and that better bonding was achieved in case of cement grouts. The compressive and tensile splitting strengths were greater in case of the cement grouted cylinders compared to the cement-lime specimens. PK EVAiUATION OF QUAiITY OF GROUT INgECTION BY NDT TESTS Three NDT techniques were used for determining the quality of grout injection: GPR measurements, sonic pulse tests and thermo-graphic measurements. GPR measurements are based on the emission and 2545

3 reflection of very short electromagnetic impulses by an antenna system. Reflection of the emitted impulses occurs at the interfaces between materials with different permittivities or conductivities, such as the interface between air and the building material or voids. Because the propagation velocity and the signal penetration depend on the electric and dielectric properties, the boundaries between different materials can be distinguished. Measurements were performed with a vertical ( 1-C1, 4-C2 ) and a horizontal ( 2-LC1, 3-LC2 ) arrangement of GPR profiles at centre-to-centre distances of 30 cm, before and after grout injection. The comparison of GPR profiles before and after grout injection shows that very good penetration of the grout into the voids of the walls was achieved. As an example, the GPR profiles performed on the wall specimens 2-LC1 and 3-LC2 are shown in Figure 2. The blue and pink stripes measured before injection represent the electromagnetic anomalies, i.e. the voids, which become blurred after grout injection. From the Fig. 2, also the position of chimney flue can be distinguished (larger anomaly located at the middle of the GPR profiles which was visible also after grout injection). FigK O Horizontal GPR profiles P3-P6 on the walls 2-LC1 and 3-LC2 at height cm in intervals of 30 cm before (left) and after grout injection (right) Sonic pulse test is based on the generation of sonic impulses at a selected point of the structure. Based on the time that the impulse takes to cover the distance between the transmitter and the receiver, the quality and homogeneity of the tested wall can be determined. Sonic pulse velocity test was used to evaluate the quality of the grout injection technique at the position of flat-jack test. Higher pulse sonic velocities were measured after grout injection (Fig. 3). The mean value of pulse sonic velocity measured before grout injection amounted to 1132 m/s, and increased to 2082 m/s after grout injection. The individual values before grout injection were between 566 and 1636 m/s, whereas the values obtained after grout injection mainly ranged between 1343 and 3000 m/s. Results show, that the voids had been successfully filled. FigK P Sonic test velocities measured at the position of flat-jack test before (left) and after grout injection (right) Thermo-graphic measurements are usually used for detecting hidden mistakes in building envelopes, but nevertheless in the case being studied the method was successfully applied in the monitoring of the quality of the grout injection. Thermo-graphical measurements made it possible to identify the wall areas with an increased surface temperature during a given time interval after injection when, because of the hydration process, heat was released. It has to be emphasized that the measured temperature of the wall depends not only on the hydration but also on the external temperature. This is why the results need to be interpreted with knowledge of the external temperatures at the time of measurement, and 2546

4 a comparison of the measured temperatures between the injected and non-injected parts of the wall. The results of thermo-graphic measurements performed on the wall 1-C1 28 hours and 10 days after grout injection are presented in Fig. 4. FigK Q Thermo-graphic measurements of the wall 1-C1 28 hours (middle) and 10 days (right) after grouting It can be seen that the greatest temperature difference between the injected and non-injected areas of the wall 1-C1 was measured 28 hours after injection and reached ΔT = 4 C, with an outside temperature of 20 C. 10 days after injection the temperature difference between the injected and the non-injected area was ΔT = 2 C. Obtained results showed high potential of this measuring technique for monitoring the quality of grouting by cement injection grouts. QK EVAiUATION OF MORPHOiOGY AND MECHANICAi PROPERTIES OF THE WAiiS BY MDT TESTS Texture and morphology of the walls was evaluated with the help of surface and in-depth probing. Brick linings (Fig. 5 left) which we have sought to avoid when selecting representative parts of wall for shear test, were present at some window and door openings. It was found that the walls had been built with lime mortar and roughly shaped limestone and sandstone, with separate brick intrusions. Although the walls were constructed with two leafs without an explicit inner core, depth probing confirmed the considerable presence of voids (Fig. 5 right) and therefore the relatively high injectability of the walls. FigK R Texture and morphology of tested walls at window opening (left) and at the central part of the wall (right) The morphology of the walls was further examined on cross sections after cutting out the specimens for the shear test. The walls of analysed building had two leaves with simple connections made by occasionally overlapping stones. The analysed walls had approximately 10 % of voids. The latter was calculated for the location 2-LC1, taking into account the consumption of grout during the injection procedure and the injected volume of the wall which was vertically limited by a door opening and a chimney flue, and horizontally by a solid foundation structure underneath and an RC slab on top. In order to determine the deformability characteristics of the tested walls before and after injection, 2547

5 a double flat jack test was performed. The tested part of the wall was injected with cement grout C2. Two parallel cuts were made at a distance of 50 cm. A thin flat-jack was placed in each of the cuts, and the oil pressure operating the jack was gradually increased. LVDT`s placed between the two cuts made it possible to monitor the vertical and lateral deformations during the test. The results of the flat jack tests are presented in Fig. 6. FigK S Set up of the measuring equipment and results of the double flat-jack test before (above) and after grout injection by cement grout C2 (below) An elastic modulus E of 785 MPa was obtained for the ungrouted state. Elastic modulus for grout injected state amounted to 1507 MPa. Considering the course of the slope of the stress-strain curve obtained from flat jack tests, an estimation of the compressive strength of the wall before and after grout injection was made. Consequently values of f co = 1.75 MPa for the non-injected walls and f cc = 2.50 MPa for the walls injected with cement grout C2 were obtained. Compressive strength of the wall in case of using lime-cement grout was roughly estimated at f clc = 2.00 MPa. RK FORMATION OF CRACKS AND DEFORMATION SHAPE OF THE WAii SPECIMENS AT DT SHEAR TEST The procedure and detailed results of the shear characteristics of walls obtained by in situ shear test are presented in [7]. During the execution of shear test, cracks that were formed on wall specimens were monitored. Based on the results obtained by three horizontal and two vertical LVDT`s, also the final deformation shape of wall specimens was determined. Formed cracks by-passed solid built areas (areas where solid stones were present at a major part of the wall s cross-section) and were more likely to be formed on weaker areas where voids or weaker materials such as brick or mortar were present. After the injection of grout, cracks were more likely to be formed on originally solid built areas without voids that were not injectable, rather than in highly injectable areas where good bonding 2548

6 between the stones and the grout was achieved. Formation of the cracks and the deformation shape of the non-injected wall 6 is shown in Fig. 7. FigK T Formation of cracks by shear test and deformation shape of wall 6 Response of the upper and lower part of the wall 6, that is the measured drift and the distribution of the formed cracks, was similar. The first crack was recorded in the tensioned zone by the absolute displacement of 1.5 mm. By increasing the imposed displacement, shear cracks formed simultaneously in lower and upper part. The test was concluded at absolute displacement of 12.0 mm where the shear mechanism was almost completely established and the maximum lateral load was already attained. The final drift amounted to 0.89% in the case of the upper and 0.85% in the case of the lower wall element. Coring and depth probing (Fig. 8) revealed that the wall was mostly built out of stones with separate brick inclusions and considerable amount of mortar and voids. FigK U Borehole and in-depth probing at the lower half of the wall 6 Formation of the cracks and the deformation shape of the wall 2-LC1 injected by cement lime grout LC1 is shown in Fig. 9. FigK V Formation of cracks by shear test and deformation shape of wall 2-LC1 2549

7 The first cracks of the wall 2-LC1 were formed in the central tensioned part at absolute displacement of mm. By increasing the imposed displacement, shear cracks were formed at first on the lower and than also on the upper wall specimen. The test was concluded due to local buckling of weaker area in the lower wall element which could result in fragile failure of the test specimen. At the same time also the maxumum lateral load was attained. The final drift amounted to 0.19% in the case of lower and 0.26% in the case of upper wall element, which is considerably lower value if compared to the non-injected wall specimen. Core driled by the shear crack of the upper wall element revealed that this was a solid built area that was by-passed by the formed crack. Depth probing executed across the shear crack of the lower wall specimen showed that this was the area built with smaller stones and larger amount of mortar without voids (Fig. 10 right). Such composition continued in the grey hatched area (Fig. 9), where the local buckling occured, because of which the test was stopped. FigK NM Core from the upper and in-depth probing at the bottom part of the wall 2-LC1 SK THE INFiUENCE OF GROUT TYPE ON THE MECHANICAi PROPERTIES OF INgECTED CYiINDERS AND WAiiS The results of shear test of injected walls showed, that the behaviour of the wall after grout injection is significantly influenced by the injection s grout ability to establish a good bond between separate stones, the mortar and the outer leaves of the wall. An obvious distinction between related types of grouts (cement/cement-lime grouts) was evident both in the case of compressive and tensile splitting strength tests of injected cylinders, as well as by the shear tests performed on the walls (Fig. 11). FigK NN Comparison of the average compressive and tensile splitting strengths of cylinders and injection grouts with tensile strengths of the walls The main reasons for the considerable improvement of mechanical properties of walls after grout injection and different degree of the improvement (depending on the type of used grout) are initially low mechanical properties of the existent mortar (1.17 MPa) and relatively high percentage of voids (around 10%). The behaviour of the walls after grout injection was no longer dependent solely on the properties of existent materials (mortar, stones, brick intrusions and the adhesion among them) and the morphology of the walls, but also on the ability of the injection grout to assure an adequate bond between separate stones and leaves of the wall. It can be concluded that in the case of walls under consideration (low initial mechanical properties, high percentage of voids), shear characteristics of walls (tensile strength and stiffness) depend significantly on the grout s ability to achieve a solid bond between the stones and the leaves including the strength and stiffness of the grout itself. 2550

8 TK CONCiUSION Presented work aimed to evaluate the efficiency of strengthening by means of grout injection performed on an actual stone masonry building. At the same time, the objective was to assess the usefulness of diferent investigation techniques in determining the quality and effectiveness of this strengthening technique. GPR and sonic test methods have shown to be reliable NDT techniques for evaluation of the quality of grout injection. Another effective method for monitoring of the quality of grout injection, particularly in case of cement grouts, is thermography. Examination of the drilled cores gave a clearer picture of the quality of grout injection and the achieved bond between the stones, the lime mortar and the injected grout. The formation of the cracks by shear test was in good correlation with the morphology (injectability) of the wall areas. Before grouting, the pattern of cracks by-passed solid built areas and was more likely to be formed on weaker areas built with weaker materials or where voids were present. After the injection of grout, cracks were more likely to be formed on originally solid built parts that were not injectable, rather than in highly injectable areas where good bonding between the stones and the grout was achieved. The usage of several NDT, MDT and DT investigation techniques has proven to be a an appropriate aproach, since it enables more reliable assessment of the quality and effectiveness of grout injection. The compressive and tensile strengths of cylinders representing the inner core of strengthened stone masonry were higher in case of the cement grouted cylinders compared to the cement-lime grouted ones. Regardless of the type of grout used for grouting, the prevailing mode of failure in the tensile splitting test was always that of the bond between the stones and the grout. The adhesive strength achieved between stones and injection grout obviously had the most important influence on tensile splitting strength of cylinders. Results of in situ shear test showed that by analysed type of walls which exhibited rather low mechanical properties and quite high injectability in their existent, unstrengthened state, shear characteristics (tensile strength and stiffness) depend significantly on the type and properties of injection grout used for strengthening. ACKNOWiEDGEMENTS The research was partially supported by the Ministry of Science and Technology and the Ministry of the Environment and Spatial Planning, and was also partly financed by the European Union's Cohesion Fund through the Slovenian Technology Agency TIA and PERPETUATE GA REFERENCES [1] Toumbakari EE., Van Gemert D., Tassios TP., Vintzile, E. (2004) Experimental investigation and analytical modeling of the effect of injection grouts on the structural behaviour of three-leaf masonry walls. In: ptructural Analysis of eistorical Constructions, Padova, Italy: [2] Valuzzi MR., da Porto F., Modena C. (2004) Behavior and modeling of strengthened three-leaf stone masonry walls. ofiej jater ptruct, 37: [3] Vinzileou E. (2006) Grouting of three-leaf stone masonry: Types of grouts, mechanical properties of masonry before and after grouting. In: ptructural Analysis of eistorical Constructions, New Delhi, India: [4] Uranjek M., Žarnić R., Bokan-Bosiljkov V., Bosiljkov V. (2010) Problems related to grout injection of heritage buildings walls. In: mrock of the U th fnternational jasonry Conference, Dresden, Germany: [5] SIST EN (2002) Testing hardened concrete Part 3: Compressive strength of test specimens. Slovenian Institute for Standardization, Ljubljana, Slovenia, p. 15. [6] SIST EN (2001) Testing hardened concrete Part 6: Tensile splitting strength of test specimens. Slovenian Institute for Standardization, Ljubljana, Slovenia, p. 10. [7] Uranjek M., Bosiljkov V., Žarnić R., Bokan-Bosiljkov V. (2011) In situ tests and seismic assessment of a stone-masonry building. ofiej jater ptructi DOI: /s z: p