PRACTICAL ASPECTS OF TESTING COMPOSITE ACTION IN MASONRY …

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

PRACTICAL ASPECTS OF TESTING COMPOSITE ACTION IN MASONRY WALLS

Vermeltfoort, A.T.1 1 PhD, Ass. Prof., Eindhoven University of Technology, Dept. of the Built Environment, [email protected]

In the past decade, over 100 masonry walls were experimentally tested in the laboratory of Eindhoven University of Technology to investigate composite action between concrete lintels and masonry. Most of them were in plane loaded to failure. A few test-walls were heated and others were loaded for a considerable time to investigate creep behaviour. In all these tests the detailing of the support conditions and load introduction were of importance, as well as measurements of loads, deflections and strains. This paper describes the experiences with different methods to apply load, to support masonry walls and to heat masonry. Alternatives based on the experiences with the performed masonry wall tests are discussed and compared. For some cases suggestions for practical improvements are given. The ideas described in this paper can be helpful for the design of new experiments. Keywords: Load-cell, water filled hose, measuring instruments, composite action. INTRODUCTION In the past decade over 100 masonry walls were tested in the Pieter van Musschen-broek laboratory of our university, Vermeltfoort (2009, 2008, 1992). The main goal was to investigate the composite action between lintel and masonry. Therefore, the test walls were loaded in plane by hydraulic jacks and in other cases by dead load. The effect of varying temperatures was also investigated. This paper is an inventory of practical details of these tests. To study composite action, test walls were built including concrete lintels. The lintel-masonry test walls represented the masonry structure above openings in walls and were supported either by point supports or by wall-pieces that represented piers. Other test specimens represented walls above an open floor space, like a parking garage, and rested on columns (Sterrenburg, 2007), (Ramezani, 2010). In all tests a piece of a structure was loaded in plane and the test structure was supported either at its corner points or on piers. The load was usually applied by hydraulic jacks (forces) and in a smaller number of cases walls were locally heated or cooled, i.e. temperature loaded (Welters, 2004), (Bertrand, 2001). After a few test-preparation aspects, the characteristics of the tests are discussed, subdivided in: a) the supports and boundary conditions and b) load introduction. PREPARATION Building small specimens and test walls. The brick test walls were built in the laboratory by an experienced mason in series of three in a ‘practical’ manner. Together with building walls, small specimens like mortar prisms and

mailto:[email protected]



shear triplets were made to be tested simultaneously with the walls. The goal was to obtain masonry with uniform properties. Soft mud bricks, brand Rijswaard (mean compressive strength ±27 N/mm2) were used in combination with a factory made mortar. More details on mortar and masonry shear testing small specimens are given in (Vermeltfoort 2009). To get the quality as constant as possible, a factory made mortar was used with a mean compressive strength of f’mor = 9.5 N/mm2. This brick-mortar combination results in masonry with a compressive strength of f’rep = 7.1 N/mm2. Prefabricated lintels (60x100 mm2) were used. After building, the walls were wrapped in plastic to prevent extreme drying and stored under laboratory conditions, Vermeltfoort (2009) and (2002). Calcium silicate walls were built in a similar way as the clay brick walls. Main difference was the joint thickness, 3 mm instead of 12-15 mm. The calcium silicate test walls were built using brick sized units, in building practice elements are usually used. The size ratio was approximately 1:3, brick sized units are 210x100x54 mm3, elements are 900 x 600 x 300 mm3. Verdel (2011) used the bricks on their side and consequently wall thickness was 54 mm. Transport of test walls was necessary because walls were built in a special corner of the laboratory. After curing they were moved to the test set-up at another place in the laboratory. By doing so, building could be done independently from testing, without interference (moisture) with the test equipment. Consequently, building became less critical in time planning. Test Frames Some test walls were built directly on the relatively flexible laboratory floor. This was allowed while the expected loads were relatively small. In other cases, when test walls were to be loaded to failure, a closed frame of HE300B (300x300 mm2) profiles was used, Figure 1. Hydraulic jacks and load distribution beams were used to apply the load, Figures 3a and 3b.

Figure 1 Test set-up for walls. Left: Schematic front view of test set-up, wall supported on a roller and in a swing (right). Middle: side view of an additional frame to strengthen the main frame, oblique projection. Right: specimen after testing. Vertical bars are part of the additional strengthening frame. The force in each jack was measured with a load cell. The displacement of the bottom of the lintel was measured with five LVDTs mounted on a separate frame. The height and length of the test walls varied, however, this could easily be taken into consideration in the design of the test frames. The main issue was to become a closed system of forces. In some cases an extra frame was required to give the set-up sufficient stiffness when large loads were expected.



Instrumenting test walls and measurements. For short term deformation digital measuring instruments, like Mitutoyo clocks, LVDTs and strain gauges were used. The long term deformations were recorded by using a demec instrument every few days. Because the expected deformations were relatively small the Demec instrument was elongated to increase the accuracy. Figure 2 shows the adaption. The measuring length corresponded with the dimensions of the stretcher bond masonry, the length of the bricks used and the height of the layers. Detailed measurements over 40 mm were done with a “clip on gauge” (Figure 2) using strain gauges. The connection of sensors was done by gluing nuts on the surface. In these nuts, bolts were screwed to form the connection with a specially made connector, Figure 2. Temperatures were measured by thermo couples and an infra red camera.

From left to right and top to bottom: Schematic section over LVDT connectors, Three LVDTs at one measuring unit, Elongated Demec gauge, Clip on gauge with strain gauges, LVDTs on a concrete lintel

Figure 2 LVDT Connections, Clip on gauge, Elongated Demec gauge

Figure 3 a) test wall set-up supported on steel cylinders and b) supported by swings,



SUPPORTS AND SUPPORT-CONDITIONS Point support on a steel block with interface material. Contact between the test specimen and the load introduction point may cause unintended peak stresses by irregularities in the concrete-lintel interface (grains in the concrete or at the brick surface. A kind of interface material may be used to smoothen load transfer, Figure 4. However, the support material often reduces the horizontal confinement in an unknown magnitude. In practice, special support materials are available, like felt or rubber. In tests, either soft board, card board, felt or rubber was used.

plate

bearing

Roller

Figure 4 Supports and load introduction. Left: wall supported on cardboard, position measuring devices. Middle: Load introduction via a steel plate and cardboard. Right: Wall supported on a steel roller via a steel plate to distribute the load over a part of the lintel and so preventing peak stresses. Deformation measurements near the support in a series of tests showed a linear relationship between the compressive deformation of the support material and the load. The rotation of the bottom of the lintel varied with the type (span and height) of the tested lintel. The load-deformation relationships showed initially relatively large displacements, probably caused by small voids and incomplete contact between support material and lintel. Once these voids are closed by the increasing pressure, the stiffness increases relative rapidly, Vermeltfoort (2009). Free rolling point support To obtain data for numerical simulations accurate boundary conditions are required. Point supports are schematized as roller of hinged. It is much easier to simulate a roller support than to build it. Roller supports (free movement in a direction perpendicular to the load) can be made by: a) a swing, b) steel roller plates; c) special sliding material. In a swing. In some cases it is desired that the supports only take vertical reactions, i.e. a roller support condition. Such a support was simulated by suspending a steel block from the roof beam of the test frame using two threaded rods, 1500 mm in length, called a swing. In this way, the steel support block rotated easily and moved freely in the horizontal direction. On steel rollers. Another way of reducing horizontal reactions is by using a steel roller of certain dimensions. If the diameter is too small, friction will increase when loads increase. The roller is pushed into the supporting material (Herz contact stresses). The effect of the surface roughness may not be neglected. Further, in practice, it is often easier to make small welding connections but



then free movement is no longer possible. So be careful in practice and check details in the set up. Special sliding material To reduce horizontal reaction forces due to friction, the use of Teflon sheets may be considered. Van Mier (1997) indicates a coefficient of friction of Teflon starting at 0.3 reducing to 0.01 for larger displacements. The larger coefficient is due to the observed “Stick and slip” behavior. The supplier of the so called Schrumpf sliding material claims a friction coefficient of 0.032. The system consists of two layers of 3 mm thick rubber and two layers of 0.5 mm thick polyethylene sheets. The maximum allowed stress is 60 MPa. Horizontal reactions also can be reduced by using a “roller bearing plate”, a 3 mm thick copper plate with a large number of steel pellets, Figure 4. Point support as in practice. Another support condition, often used in practice, was simulated by a layer of felt on a brick. This brick was positioned in a two mm thick layer of gypsum on the same steel block as used for the roller support. However, this block was bolted to the bottom beam of the test frame. Stiffening the test-wall with two M16 threaded rods, (Figure 5, middle) did not work satisfactory and complicated the testing process, Vermeltfoort (2008).

Figure 5 Point supports. Left: roller (soft board on a steel block that is suspended from the roof beam of the frame, see also Figure 3. Middle: Lintel, stiffened with steel ties, 2 x M16. Right: special felt on a brick in a bed of lime.

A short column used to simulate building practice in a wall test on a 1:3 scale

Safety aspect. Be aware of out of plane deformations, consider the use of lateral supports as shown. Load is introduced by a hose.

Figure 6. Support via a column. Figure 7 Lateral support.



Wide supports. To study the behaviour of masonry around a window opening due to temperature effects, specimens representing a part of the wall, including the piers, were used. Like in the other cases, the support conditions were important. Two kinds of supports were simulated: a rolling support and a ridged support. The roller support was built as follows. Pieces of steel pipe were positioned at 100 mm centre to center distances on the floor(beam). Steel plates were placed on these pipes. They were temporarily connected by clamps. After building the wall, the temporary connection was removed. In this way, the piers of the test wall were supposed to be able to move freely in the length direction of the wall. The ridged support was formed by starting the brick laying with a mortar joint directly onto the floor (Figure 8) or in another case directly onto the bottom beam of the test frame. As an alternative for the rolling support the use of a double layer of greased polyethylene sheets, to allow free movement by sliding, was considered but not used because rollers were more easily available and guaranteed free movement.

Figure 8 Piers supported on rollers and on a mortar-bed. Horizontal supports against out of plane movement. The experiments discussed in this paper all concern in plane behavior. It was assumed that the deformations were in vertical downward direction. However, depending on the thickness of and height of the test wall some out of plane deformations occurred. This out of plane action is controlled by the vertical supports and the load introduction by the jacks. For safety, vertical bars or beams were used (see Figures 1, 3 and 7). In some cases the supports could affect the behavior. If the supports bend, they give some unintended vertical reaction. To prevent this, a friction reducing double layer of greased polyethylene sheets was used between the wooden supports and de wall, Figure 6. LOADING Load introduction: Point loads or uniformly distributed ? When a (hydraulic) jack is used the load is applied on a relatively small area and may be considered for design purposes as a point load. However, locally peak stresses occur. Similar to supports, interface materials can be used. According to EN 846-9:2000 either one point load (shear testing) or four point loads are to be applied. With four loads on a beam almost the same bending moments as in a uniformly distributed beam are obtained but locally near the



point loads stress distribution is affected, Figures 9 and 10. In relatively low test walls this may affect the behavior of the wall and the ultimate load.

Figure 9 Loading schemes. Four point loads or one point load and their lines of thrust.

Figure 10 Comparison of results of numerical simulations by Ramezani (2010) of load introduction as four point loads or as a uniformly distributed loading. Point loads Loading with hydraulic jacks In our laboratory jacks of various capacities are available. Jacks are usually combined with load cells. Loading of jacks can be done by a) a manually operated pump, b) a mechanical pump; motor driven, c) via parallel system in a standard testing machine. The smoothest load path is obtained by using method c). By hand speed control is more difficult, but it is also easier to change the speed and to react on things happening while testing. A combination of two jacks, one in a Schenck compression testing machine coupled with other jacks in the test frame allows for the use of the control equipment of the Schenck machine and a smoother loading increase on the test wall compared with the use of a hand pump. A pumping machine allows for a relatively smooth load increase in time, however, the possibilities to control the test are less. Loading via a Spindle. Tightening means applying load by means of a screw, a manually operated, simple and relatively cheap method. However, the friction in the connection could be a problem. The use of e.g. a thrust ball bearing could be considered, Figure 11. Due to complications during the first test, in the subsequent tests a hydraulic jack was used (Sterrenburg 2007). Loading using dead weight The advantage of using dead weight as loading is the clear way of action. Problem is that a structure or part of a structure is usually so strong that a relatively large volume of material has to be used to load the system to failure. Steel is, in practice, the heaviest material; water is easy to apply but has the risk of leakage resulting in a water ballet. Concrete beams as used



for the tests shown in Figure 11 are a good alternative. Positioning two walls parallel to each other makes it easy to introduce the load by means of small lateral beams, (Bertrand, 2001).

Figure 11 Wall, lintel and piers; long term loading by dead weight (creep), Right: thrust ball bearing, exploded view and attached to the end of a tie. Uniformly distributed loading via a water filled hose

Figure 12 U shaped beam detail

When a true uniformly distributed load is required the use of a water filled hose can be considered using the method described in (Mauerwerk 2004). The hose rests on the edge of the wall and is loaded via two jacks on a steel beam. The hose is kept in position by the flanges of the used beam. The test wall can however move in the out of plane direction. Therefore additional supports are required, Figure 7. The hose used can withstand a maximum pressure of 18 bar (1.8 MPa). A weak spot, as experienced in our experiments, is the end-stop, which broke out once. Besides safety aspects, the water that came out gave quite a mess. The hose was confined in a U-shaped profile to give some lateral stability, Figure 12

Temperature load effects In composite structures it is expected that stresses will develop in the interface between two materials. These stresses may increase the stresses already present due to loading e.g. by dead load and own weight. To investigate the thermal effects on expansion of masonry and lintels the test walls were heated and cooled (shortly), followed by long term loading using additional dead weight. Finally, the walls were loaded to failure. Two methods were used to vary the temperature of the lintel and masonry close to the lintel, to increase the effect of the difference in linear expansion coefficient. In method a) it was decided to only heat the concrete lintel. A cover with pipes and insulation was made. Water pipes ran parallel to the length of the lintel. The pipes and lintel were covered by polystyrene isolation material. The cover fitted closely around the lintel. Figure 13 shows a section of lintel, pipes and insulation material. In method b) the whole structure around the lintel was heated by lamps. Both methods performed well, lintel temperatures between 60ο C and 10o C were reached. The use of water allowed both for a temperature increase (heated water) as for a decrease by using ice water. Advantage of method a) is that also cooling is possible by pumping ice-water through the system. Method b) is more realistic compared to the real situation i.e. heating by the sun.

Floria


nópolis – Brazil – 2012

Figure 13 Water system to heat (or cool) the lintel with insulation to prevent heat loss.

Figure 14 Temperature test: a) heating with light bulbs, b) measuring with an infra red camera. CONCLUDING REMARKS Roller supports at both sides require temporary horizontal support during instrumenting the test. Once a small load is applied the jacks give sufficient horizontal support. In most cases the test wall and loading system were symmetric. The roller supports allowed for free horizontal movement and elongation of the tensile side of the test wall while horizontal reactions partly prevented shortening at the compressed side which was considered to affect the results negligible. The hose used to load the walls uniformly distributed performed satisfactory, however, the maximum pressure of 1.8 MPa limited its use. The hot water system allowed for locally heating of the composite structure, maximum temperatures between 10 o and 60o C were reached. Positive experiences were gained by testing walls on a 1:3 scale. This allowed testing with a water filled hose. By using dimensional scale reduction, loads became manageable.



REFERENCES Mauerwerk-Kalender, 2004, Taschenbuch für Mauerwerk, Wandbaustoffe, Brand-, Schall-,

ditions on lintel-masonry interaction. In W.W. l-dakhakhni & R.G. Drysdale (Eds.), Proc. 11th Canadian Masonry Symposium, Ontario,

ermeltfoort, A.T. and Schijndel, A.W.M. van, 2008, Modeling of lintel-masonry interaction

R.W., 2002, Strains in masonry near the support of lintels explorative research of a test set-up. In G. Thompson (Ed.), Proceedings of the British

ermeltfoort, A.T., 1992, Experimental research on prefabricated lintels and walls, Proc. 6th

and a short description per title are given.

or of composite masonry due to temperature effects studied by y using light bulbs,

Ramezani, A., 2010, The behavior of the masonry-lintel bed joint in composite masonry. Verdel, N., 2011, Calcium silicate deep beams. Loading via a water filled hose.

Wärme- und Feuchtigkeitsschutz, Ernst, Berlin. Van Mier, J.G.M., 1997, Fracture processes of concrete, CRC Press, Boca raton. Vermeltfoort, A.T. 2009. Effects of support conEJune 2009 (pp. 1-10). McMaster University. Vusing COMSOL. In European COMSOL Conference Hannover (pp. 1-8). Vermeltfoort, A.T. and Martens, D.- Masonry Society (pp. 531-535). VCanadian masonry symposium, Saskatoon, Canada, 1992. pp. 531..537. Experimental work done as part of a master’s study done at Eindhoven University of Technology (In Dutch). Below, the titles Bertrand, T., 2001, The way forces act around a support in composite masonry, Creep testing on walls, using loading by dead weight, Welters, A., 2004, The behaviHolonite, 2007, Temperature effects in composite lintels masonry busing central heating system, Sterrenburg, R., 2007, Deep beams made of calcium silicate units, Hoekstra, E.M.A, 2008, Composite Lintels (Samengestelde lateien),

PRACTICAL ASPECTS OF TESTING COMPOSITE ACTION IN MASONRY …

Documents

Transcript of PRACTICAL ASPECTS OF TESTING COMPOSITE ACTION IN MASONRY …