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15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

SHEAR TRANSFER CAPACITY OF ALTERNATIVE MATERIALS FOR HORIZONTAL SLIP JOINTS IN MASONRY

Totoev, Yuri Zarevich1; Simundic, Goran2. 1 PhD, Senior Lecturer, University of Newcastle, Centre for Infrastructure Performance and Reliability, [email protected] 2 MEng, Laboratory Manager, University of Newcastle, Centre for Infrastructure Performance and Reliability, [email protected]

Masonry structures contain slip joints between concrete slabs and their supporting masonry walls to accommodate differential movements due to concrete slab shrinkage or thermal effects. Traditionally slip joints consist of one or two layers of a membrane type material placed between the masonry and the concrete. These materials are usually an embossed polythene, greased steel or bitumen-coated aluminium. According to Australian Standards all structures must be designed for earthquake loading. Therefore the slip joints must satisfy two apparently conflicting requirements - slip under long-term loads and transmit short-term earthquake loads. Tests at the Universities of Newcastle and Adelaide have indicated that these types of joints exhibit substantial shear capacity under short-term loads. The performance of these joints under induced long-term strains (i.e. differential movement effects) and their potential to behave as slip joints have also been established in thermal expansion tests developed at the University of Newcastle. It was found that, of all currently used slip joint materials, embossed polythene satisfies both major structural requirements best. It is, however, desirable for the design of shear wall systems to have a wider range of materials available. The purpose of current tests is to check long term shear transfer capacity of alternative materials. Test results will be presented.

Keywords: Masonry, slip joint, differential movement, embossed polythene INTRODUCTION The use of unreinforced masonry as load bearing walls in the construction of low-rise residential buildings is highly popular in Australia due to their favourable properties such aslongevity, good thermal and sound insulation, and good fire resistance. Load bearing masonry walls commonly support, or are supported by, a reinforced concrete slab floor. During the life of the structure, differential movements between masonry and concrete have the potential to pose significant serviceability issues to the structure. These differential movements may be caused by thermal actions, by shrinking of concrete or by long-term expansion of bricks. In an effort to minimise the effects caused by these actions, a slip joint is utilised between the masonry and concrete to prevent cracking from any differential movement that may arise, and to provide a discontinuity between the two materials. The most commonly used slip joint material is damp-proof-course membrane of embossed polythene (see Figure 1).



a) Embossed polythene DPC placement b) Surface texture

Figure 1: Most common Australian membrane material The frictional resistance of slip joints has been studied after introduction of the Australian Earthquake Loading Code AS1170.4 (2007). In the majority of cases the slip joints were found to possess significant shear capacity under short-term loading and therefore able to transfer seismic forces. Unless the same joints are capable of accommodating long-term differential movements, their use as slip joints (as in common practice) is very much open to discussion. Therefore a slip joint has to satisfy two apparently conflicting requirements: (i) the ability to allow slip under long-term differential movements between the materials and hence to alleviate any build-up of stresses, particularly in the masonry; (ii) the ability to transfer the short-term earthquake induced forces by friction in the joint. The difference between these two types of internal effects lies in the time scale involved and thus the corresponding strain rates. To be able to satisfy the two requirements the shear stresses developed in the joint should be high if the strain rate is fast and, in contrast, when the strain rate is slow, the induced shear stresses should be low. The dependence of a material response on strain rate is typical for viscoelastic materials. Similar behaviour is required in a slip joint, which can be referred to as “the pseudo viscosity of a joint”. A number of researchers have studied the behaviour of slip joints previously. Schubert (1983) studied the effect of adhesion, friction, and mechanical interlock on the drag resistance of a number of different slip joints. He conducted his tests at different strain rates (some as slow as ~10mm/hour) and reported that the strain rate did influence the drag resistance.



Page et al. (1998) and Griffith et al. (1998) performed a series of monotonic unidirectional shear tests, cyclic shear tests and dynamic shaking table shear tests for a selected range of slip joints. Two sets of friction coefficients (static and dynamic) were determined. They also reported a noticeable difference between the two coefficients for some joints. Importantly, the difference between the respective coefficients was quite variable: for some types of joints the dynamic coefficient was lower than the slow strain rate coefficient; for other types of joints the dynamic coefficient was higher; and for some joints there was no difference between the two. Suter et al. (1992), Rajakaruna (1997) and Zhuge et al. (1998) performed a number of monotonic shear tests of masonry containing joints with damp-proof-course membranes and reported variable friction coefficients. Simundic et al. (2000) reported results of long-term shear tests for several slip joints. In these tests the shear force on the joint was maintained at a constant level and the differential movement (slip) was monitored. These results demonstrate that slip joints do have the potential to exhibit some creep. Trajkovski and Totoev (2002) performed shear tests on masonry panels with damp proof courses varying the sliding velocity. They confirmed that the friction between masonry and a damp proof course depends on sliding velocity. Therefore the shear strength of a joint varies at different rate of load application. Totoev et al. (2001, 2003, 2005) developed two different long-term test to measure the frictional forces generated between a concrete slab and a masonry wall. They found that some types of slip joints exhibit the pseudo viscosity and are able to satisfy two apparently conflicting requirements – to slip under long-term loads for adequate serviceability performance and to transmit short-term dynamic loads from earthquake in order to create effective load paths through the structure. It is clear from the literature that while embossed polythene joints exhibit some pseudo viscosity, other types such as greased galvanised steel or bitumen-coated aluminium do not. It is not clear, however, whether this property should be attributed to the simple presence of plastic in the joint, to the amount of plastic in the joint, to the type of plastic in the joint or to the combination of these factors. The main objective of this project is to investigate the long-term behaviour of joints with different amounts of polythene and with the joint made of different plastics. TESTING APPARATUS A purpose built testing rig was developed for the low velocity slip tests (Totoev 2005). The schematic testing arrangement is shown in Fig. 2 and the instrumentation set up in Fig. 3. The slip driving force is provided by longitudinal thermal expansion of water-filled insulated aluminium closed pipe. It is 3m long, 63.5mm in external diameter and has 6.35mm thick walls. An automatic heater controls water temperature. The pipe has several supports uniformly distributed along its length in order to alleviate bending. As can be seen from Fig. 2, connecting the far end of the pipe to the concrete pad created a “closed system”.



Figure 2: Schematic rig configuration for low velocity slip tests

a) Testing rig b) Load cell and LVPT

Figure 3: Instrumentation set up

A half of a standard dry pressed clay brick (114mm X 110mm X 70mm) was used to simulate the underside of a load-bearing masonry wall. A steel cap was placed over the brick to ensure that the brick remained horizontal during the test and also to ensure the uniform distribution of vertical load. A slip joint was formed between the brick and the concrete pad by inserting plastic membrane material. The shear transfer area of the joint of 0.0125m2 was equal to the area of the bed face of the half brick. Metal weights were placed on top of the steel cap to simulate vertical load in the masonry wall. The total weight of 644kg was constant in these tests. The resulting vertical compression of 0.5MPa is typical for a 3-4 storey residential building.



The differential movement between the brick and the beam was measured by a linearly varying potentiometric transducer (LVPT of 10mm travel capacity). A load cell of 10kN capacity has been incorporated into the “closed system” to measure the shear force induced in the slip joint by elongation of the aluminium pipe. Output electric signals from the LVPT and the load cell were monitored constantly and recorded by a data-logger. TEST SPECIMENS Two types of plastic material was tested:

• High density polythene (HDPE), • Polypropylene (PP).

Polythene is a commonly used polymer throughout the world. It is created through the process of polymerisation of the ethylene molecule (C2H4). This process causes the double bond between the two CH2 groups to brake and form new bonds with other ethylene molecules to form a long polymer chain. Polythene is available in a number of varieties, with the specimen chosen for this investigation being High Density Polythene (HDPE). HDPE possesses a more of a tightly packed structure than Low Density Polythene (LDPE) used in embossed polythene tape, due to its lack of side branching. HDPE also possesses a higher crystallinity than does LDPE, making it more rigid. The thickness of tested specimens was 1.5mm, 2mm, and 3mm. HDPE specimens and molecular structure are shown in Figure 4.

a) Specimens of different thickness b) Molecular structure

Figure 4: Polythene (HDPE) The second material used for testing was polypropylene (PP). PP is created from the propylene (C3H6) molecule to form a long chain polymer through the polymerisation process. Due to the –CH3 branch in PP, the polymer chains do not pack together as tightly as they are in HDPE, making it weaker. PP also has less crystallinity than does HDPE, making it less rigid. The PP investigated in this experiment has a flat, smooth surface. PP possesses a similar



structure to that of LDPE of embossed polythene tape. The thickness of tested specimens was 2mm and 3mm. PP specimens and their molecular structure are shown in Figure 5.

a) Specimens of different thickness b) Molecular structure

Figure 5: Polypropylene (PP) EXPERIMENTAL RESULTS AND DISCUSSION Experimental program consisted of twelve tests (Doman 2010). In the first three tests slip joints were made of 1.5 mm thick HDPE plastic. Graphs of the shear force evolution with time are presented in Figure 6. In previous tests on embossed polythene (about 1mm thick) at

Figure 6: Shear force evolution curves for 1.5 mm HDPE

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Shear F

orce (k

N)

Time (hours)

HDPE 1.5 mm, Test 1

HDPE 1.5 mm, Test 2

HDPE 1.5 mm, Test 3



the vertical compression of 0.3 MPa (Totoev et al. 2003) the maximum shear force of almost 2 kN was recorded. It was also observed that higher vertical compression results in higher maximum shear force. It appears that the shear transfer capacity of a joint depends more on the thickness of the plastic in the joint than on the type of the plastic. A similar trend is

Figure 7: Shear force evolution curves for 2 mm HDPE and PP

Figure 8: Shear force evolution curves for 3 mm HDPE and PP

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HDPE 2 mm, Test 2

PP 2 mm, Test 1

PP 2 mm, Test 2

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HDPE 3 mm, Test 1

HDPE 3 mm, Test 2

HDPE 3 mm, Test 3

PP 3 mm, Test 1

PP 3 mm, Test 2



found in the average shear transfer capacity of joint (regardless of the plastic type) in Figures 6-8. The average maximum shear force for 1.5 mm joint is 1.47 kN; for 2 mm joint - 1.36 kN; and for 3 mm joint – 1.33 kN. This limited experimental study is inconclusive about the influence of the type of material used in the joint. While results for HDPE appear distinct from PP results for 2 mm joints (Figure 7), there is no clear distinction between HDPE and PP results for 3mm joints (Figure 8). Only in seven of twelve tests performed the clear point of slip with following shear relaxation was observed during the first 200 hours. It was observed (as expected) in tests on HDPE and PP, both of which are viscoelastic materials. This indicates that both plastics could be used to create a pseudo viscous structural slip joint. To understand why not all joints exhibited shear relaxation during the first 200 hours would require additional investigation. CONCLUSIONS A small series of low velocity slip tests was performed to check long term shear transfer capacity of HDPE and PP plastics as an alternative to a currently common slip joint material - embossed polythene (LDPE). The following conclusions could be drawn from these limited tests:

• Both HDPE and PP could be used in pseudo viscous slip joints; • A pseudo viscosity of 0.03 MPa⋅sec was calculated for PP and 0.051 MPa⋅sec for

HDPE; • It appears that the long-term shear transfer capacity of joints depends on the thickness

of plastic tape in a slip joint. Thinner plastic transfers higher shear; • It appears that the long-term shear transfer capacity of joints depends less on the type

of plastic in a slip joint (however, this is not fully conclusive). REFERENCES Doman, T., Shear Relaxation in Masonry Slip Joints, The University of Newcastle, Final Year Research Project Report, 2010. Griffith, M.C. and Page, A.W., On the Seismic Capacity of Typical DPC and Slip Joints in Unreinforced Masonry Buildings, Australian Journal of Structural Engineering, The Institution of Engineers, Australia, Vol.1, No.2, pp. 133-140, 1998. Page, A.W. and Griffith, M.C., A Preliminary Study of the Seismic Behaviour of Slip Joints Containing Membranes in Masonry Structures, The University of Newcastle and University of Adelaide, Research Report 160.02.1998, 1998. Rajakaruna, M.P., Effect of the Damp Proof Course on the Shear Strength of Masonry. Proceedings of the 15th Australasian Conference on the Mechanics of Structures and Materials, Melbourne, Australia, pp. 633-637, 1997. Shubert, T.J., Slip Joints Under Concrete Roof Slabs, Experimental Building Station, Department of Housing and Construction, Chatswood, NSW, Technical Record 502, 1983.



Simundic, G., Page, A.W., and Chen, Q., The Cyclic and Long Term Behaviour of Slip Joints in Loadbearing Masonry Construction, Proceedings of the 12th International Brick/Block Masonry Conference, Madrid, Spain, Vol.2, pp. 1409-1420, 2000. Standards Association of Australia, AS1170.4 Minimum Design Loads on Structures, Part 4: Earthquake Loads, Standards Australia, Sydney, 2007. Suter, G.T. and Ibrahim, K.S., Shear Resistance of Damp-Proof-Course Materials in Brick-Mortar Joints. Proceedings of the 6th Canadian Masonry Symposium, Saskatoon, Canada, pp. 119-130, 1992. Trajkovski S. and Totoev Y.Z., Shear Strength of Masonry Including Damp Proof Course: Experimental Determination at Different Strain Rates, Proceedings of 6th International Masonry Conference, London, UK, pp. 487-492, 2002. Totoev, Y.Z., Page, A.W. and Simundic G., Long-Term Shear Transfer Properties of Horizontal Slip Joints in Load-Bearing Masonry, Proceedings of the 9th Canadian Masonry Symposium (on CD-ROM), Fredericton, New Brunswick, Canada, 2001. Totoev Y.Z., Page A.W. and Simundic G., Shear Transfer Capacities of Horizontal Slip Joints in Masonry at Different Levels of Vertical Load, Proceedings of the 9th North American Masonry Conference, Clemson, South Carolina, June 2003, pp. 791-800, 2003. Totoev, Y.Z. and Simundic, G, New Test for the Shear Transfer Capacity of Horizontal Slip Joints in Loadbearing Masonry, 10th Canadian Masonry Symposium, Banff, Alberta, 2005. Zhuge, Y. and Mills, J., The Behaviour of Masonry Walls Containing a Damp Proof Course under Cyclic Loads, Proceedings of the 2nd Australasian Structural Engineering Conference, Auckland, New Zealand, Vol.2, pp. 655-661, 1998.

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