AN EXPERIMENTAL STUDY OF THE FLEXURAL BEHA VIOUR OF POST-TENSIONED HOLLOW CLA Y MASONRY

Kelvin J. Graham1 and Adrian W. Page2

1. ABSTRACT

Hollow clay masonry units are relatively new in Australia, but they are now being used for reinforced and prestressed masonry, particularly in high wind areas. Hollow concrete masonry is also used in a similar fashion. Since there are no design roles for prestressed masonry in the current Australian Masonry Code, there is a need for a fundamental study of the flexural behaviour of prestressed masonry at both the serviceability and the ultimate strength limit states to allow these roles to be developed. This paper describes a study of the flexural behaviour of lightly post-tensioned hollow clay masonry. The system which was assessed consisted of a single skin of hollow clay masonry lightly prestressed to offset the effects of face loading from wind or earthquak:e. Prestressing was induced by stressing threaded steel rods located in selected cores of the hollow units. A locally produced bar ("Threadlock") was used for this purpose. A range of stress leveIs and reinforcement ratios were used to allow a complete study of the serviceability (cracking) performance as well as the ultimate behaviour for under-reinforced, balanced, and over­reinforced sections. Since the ultimate behaviour is governed by the cracked behaviour of the cross section, three different scenarios were considered for the reinforcement: the rod ungrouted and free to move in the core as the wall deflected; the rod ungrouted and held in position in the core; and the rod fulJy grouted in the core. To date a total of fifteen walls have been tested. The results of the tests are presented, and the flexural behaviour of the masonry walls discussed.

Keywords: masonry; hollow; prestressed; post-tensioned; flexure.

lPostgraduate Student, Department of Civil Engineering & Surveying, The University of Newcastle, University Drive, Callaghan NSW 2308, Australia.

2CBPI Professor in Structural Clay Brickwork and Head, Department of Civil Engineering & Surveying, The University of Newcastle, University Drive, Callaghan NSW 2308, Australia.

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2. INTRODUCTION

Single skin reinforced or prestressed hollow clay masonry is now being used in Australia for the construction of low-rise buildings. One of the main functions of the masonry walls in these buildings is to resist out-of-plane wind loads. The flexural capacity of a wall is governed by the tensile strength (flexural bond strength) of the masonry. Since the tensile strength is generally low, prestressing can be used as a means of overcoming this lack of tensile strength by inducing a precompression into the masonry, thus improving its serviceability (cracking) performance. The leveI of prestress does not greatly influence the behaviour of the wall at the ultimate strength limit state, as this is governed more by the reinforcement ratio of the section and the cracked section properties of the masonry.

This paper describes an experimental study of the flexural behaviour of prestressed hollow clay masonry for both the serviceability and the ultimate strength limit states. A series of flexural tests were performed on post-tensioned hollow clay masonry walls with consideration of several variables which affect the flexural behaviour. These included the level of prestress, the reinforcement ratio, the restraint of the prestressing rod in the cores of the hollow units, and the presence of grout.

3. PREVIOUS RESEARCH

The concept of prestressing rnasonry is not new, and is now accepted in many countries. Although it has been generally established that the same principIes as for prestressed concrete can be used, there are some significant differences. The technique is certainly not as common as reinforced masonry, and the codification of prestressed masonry has lagged behind that of its reinforced counterpart. Important fundamental research on prestressed masonry has been carried out by Phipps (1), and some previous tests have been performed on post-tensioned hollow concrete masonry walls (2-5). This research confirmed that post-tensioned walls had an improved serviceability performance and behaved elastically in the pre-cracking range. Review papers have also been produced by Schultz & Scolforo (6-8), and design principIes for post-tensioned masonry recently presented by Hendry (9).

4. EXPERIMENTAL PROGRAMME

A total of 15 walls were tested. The walls were constructed from hollow clay blocks (310 mm long x 76 mm high x 150 mm wide) and a 1:1:6 mortar (cement:lime:sand by volume). The walls were laid in running bond and face-shell bedded throughout (face-shell width 33 mm). Each wall panel was 20 courses high and 2.5 units long, resulting in overall dimensions of 1700 mm high x 800 mm wide. The walls were constructed on channel sections for ease of handling and cleaning of cavities and to provide end anchorage for the rods in lieu of a footing. One prestressing rod was centrally located in the central core of each panel. A schematic arrangement of the panel is shown in Figure 1. Stack bonded prisms were constructed for each mortar batch to allow the masonry compressive strength and the flexural tensile strength of the joints to be determined (using a bond wrench). The masonry had a mean compressive strength of 15.6 MPa and a mean flexural tensile strength ofO.21 MPa.

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4.1 Post-Tensioning

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Wall Details

Masonry Wall

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When the walls were at an age of greater than 28 days they were post-tensioned. The prestress was induced by stressing the steel rod in the central core. Two bar types were used, a 16 mm diameter conventional Y16 deformed bar (400 MPa nominal yield strength), and a 20 mm diameter TL20, "Threadlock" bar (nominal yield strength 500 MPa). This latter bar is produced with deformations which can also double as a thread, so that the rod can be readily joined or anchored using appropriate couplers.

Two different bar sizes were used to produce different reinforcement ratios resulting in ultimate behaviour which ranged from under-reinforced to over-reinforced. To allow use of the same anchorage system in ali cases, sections of the TL20 bar were welded to the ends of the Y16 bar. The prestress was applied by a hydraulic jack through an extended end block to allow access to the rod so that the steel strain could be monitored. The prestressing load was monitored with a load cell during jacking. Immediate anchorage losses could therefore be eliminated during the jacking processo Resistance gauges were also glued to the rods at mid-length to monitor the steel strains during the test

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4.2 Restraint Of Prestressing Rod

One of the important practical variables is the restraint of the prestressing rod in the core of the hollow masonry units as this has a direct influence on ultimate behaviour. Three conditions were investigated : the core containing the rod was fully grouted after stressing ("Grouted (GR)" in Table 1); the core was left ungrouted, but the rod restrained by clay brick inserts pre-glued in some of the hollow units to prevent any transverse movement of the rod during testing ("Guided (G)" in Table 1); and the rod was left unrestrained in the core ("Unguided (UG)" in Table 1). For the grouted cases, the mean compressive strength of the grout was 11.8 MPa.

4.3 Stressing LeveI

Three leveIs of prestress were considered : zero, 1 MPa and 2 MPa, with the imposed force in the rod being chosen accordingly. The main influence of the levei of prestress is on the cracking behaviour of the masonry, rather than the ultimate strength. A summary of the properties of the 15 panels tested is given in Table 1.

4.4 Flexural Tests

Since the basic flexural behaviour of the wall panels was being studied, each wall was tested in 4-point bending in the horizontal position with appropriate allowance being made for self weight effects. A typical arrangement of the test is shown in Figure 2. The Ioad was appIied under dispIacement controI to allow better control of the post-cracking behaviour.

Table 1 - Summary ofWalIs Tested

Masonry Stress Tendon Test Number Nomenclature BarType Restraint LeveI (MPa) Condition

1 1 GR Y16 O Grouted

2 2GR Y16 1 Grouted

3 3GR 1L20 O Grouted

4 4GR 1L20 1 Grouted

5 5G Y16 O Guided

6 6G Y16 1 Guided

7 7G 1L20 O Guided

8 8G 1L20 1 Guided

9 9G 1L20 2 Guided

10 lOUG Y16 O Unguided

11 l1UG Y16 1 Unguided

12 12UG 1L20 O Unguided

13 13UG 1L20 1 Unguided

14 14UG 1L20 2 Unguided

15 15G 1L20 1 Guided

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Fig. 2 - Testing Arrangement

During each test, the longitudinal strain distribution over the depth of the wall at centre span, the strain in the prestressing rod, the location of the rod in the hollow core, and the deflection of the wall were monitored (see Fig. 2). Longitudinal strains were also monitored across the joints where failure was lilcely to occur to detect the cracking load. From these readings the neutral axis location, the cracking load, the wall curvature, and the conditions of the prestressing rod could be monitored throughout each test.

5. RESULTS

A summary of the cracking and ultimate moments is given in Table 2. As for conventional reinforced concrete theory, under-reinforced failure was assumed to occur when the reinforcement yielded before the masonry crushed; over-reinforced failure occurred when the steel was below yield when the masonry crushed; and balanced failure occurred when the steel yielded as the masonry failed.

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Table 2 - Summary of Test ResuIts

Test Bar Prestress Moment (kNm) Failure Mode Type Type LeveI (MPa) Cracking Ultimate

lGR Y16 O 1.18 4.91 U nder -reinforced

2GR Y16 1 3.41 5.72 Under-reinforced

3GR 1L20 O 1.21 9.63 Balanced

4GR 1L20 1 3.38 9.89 Under-reinforced

5G Y16 O 0.57 5.10 Under-reinforced

6G Y16 1 3.32 5.61 Under-reinforced

7G 1L20 O 0.97 7.93 Over-reinforced

8G 1L20 1 3.20 8.88 Balanced

9G 1L20 2 6.20 9.53 Balanced

lOUG Y16 O 0.87 2.37 Over-reinforced

llUG Y16 1 3.32 4.32 U nder -reinforced

12UG TL20 O 0.54 2.68 Over-reinforced

13UG 1L20 1 2.89 4.46 Over-reinforced

14UG 1L20 2 5.24 6.59 Over-reinforced

15G 1L20 1 3.81 7.70 Balanced

5.1 Grouted Walls

Moment-central deflection curves for the grouted walls are shown in Fig. 3. All four curves confirm that the walls behaved as expected, with a change in slope at moment leveIs corresponding to the cracking load, and then reaching a plateau as the steel yielded. The cracking loads for panels lGR and 3GR (with no prestress) were quite low and similar in each case. The cracking loads for 2GR and 4GR were high and almost identical, confmning the direct influence that the prestress has on cracking load levei (and hence serviceability performance).

Once initial cracking had occurred, the steel strains began to increase, and the rod size and properties then became significant, with a higher ultimate moment being achieved for the larger rod (see Fig. 3). As the load increased in the post-cracking range, more cracks appeared in the joints along the length of the wall, with numerous joints being affected. The walls with the Y16 rod exhibited an under-reinforced failure and failed by steel yielding followed by large rotations at the point of failure, with final secondary crushing of the masonry occurring Oil the compression face . The walls with the TL20 rod were at or just below the point of balanced failure . At ultimate load the rod had begun to yield, but at the same time masonry spalling and mortar joint cracking was occurring. Both walls failed suddenly by the forrnation of a crack in the face-shell along the length of the wall adjacent to the grouted cavity, probably due to the difference in stiffness between the grouted and ungrouted cores.

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Fig. 3 - Flexural Behaviour of Grouted Walls

5.2 Ouided Walls

Figure 4 shows the response of the guided walls to out-of-plane loading. As the applied load was increased and the wall deflected, the steel rod moved towards the compression face until the movement was restrained by the guides. The walls then cracked, and with further loading these cracks became visible. The cracking in this case was more localised than for the equivalent grouted cases, and the mortar joint at which most of the cracking and rotation occurred was between rather than at the guide locations. Once cracking occurred, the strain in the rod then began to increase until final failure. Walls 50 and 60, with the Y16 rod, exhibited under-reinforced failure, with the rod yielding first followed by mortar joint crushing (the bulk of the rotation was confined to one joint). By contrast, walls 70, 80, 90 and 150, with the TL20 rod, exhibited over-reinforced or balanced failures with the masonry failing in compression at the same time or before the yielding of the rodo

It is apparent from the curves in the Figure 4 that the prestress leveIs control the cracking load, and the area of reinforcement in the section controls the ultimate strength of the prestressed wall. Comparison of the cracked stiffness for the corresponding grouted and guided walls (Figures 3 & 4) shows that the cracked stiffness of the grouted wall was greater than that of the corresponding guided wall. This is because the elongation of the rod occurs over its full length for the guided wall and is more localised for the grouted wall. From the curves for the over-reinforced ar balanced sections represented by 70, 80 and 90 it can be seen that the prestress leveI affects the ultimate strength of the section with the ultimate strength increasing with prestress leveI. This effect, though not marked, is probably due to the lower rotations that occur in the failed joint when lúgher leveIs of effective prestress are present.

The location of the guides along the length of the wall could also have an influence on the ultimate strength, as after cracking, the movement of the rod witlún the cavity is governed by the change of wall geometry wlúch in turn is influenced by the guide locations. This may account for the lower value of ultimate moment for 150 compared to 80 (replicates of the same test) . The guides in 150 were further apart, and the movement of the rod towards the compression face after cracking is greater, resulting in a smaller internal lever arm and moment of resistance. The wall behaviour is therefore sensitive to this parameter, and the location of the guides should be norninated at the design stage if the rod is to be restrained.

645

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Fig. 4 - Flexural Behaviour of Guided Walls

5.3 Unguided Walls

The effect of lack of restraint of the prestressing rods was significant, particularly with regard to the ultimate strength limit state. Up until cracking the behaviour of the guided and unguided cases was the same, with the cracking moments all being similar. However the cracked behaviour was affected by the movement of the rod in the core. As the cracked section deflected, the distance between the compression face of the wall and the steel decreased significantly. In the limit, for very large deflections, the rod actually carne in contact with the compression face-shell . This movement of the rod within the core reduced the lever arm between the compression and tensile resultants of the cross section, with an accompanying reduction in the moment of resistance. This effect can be seen from Fig. 5 which shows the changing steel depth with applied moment. Failure was quite local, with the large deflection being accompanied by large rotations in the region of one cracked joint. This was accompanied by progressive compression failure at the corresponding location on the compression face.

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Fig. 5 - Flexural Behaviour of Unguided Walls (Movement of rod in the core, waIl 14UG)

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Fig. 6 - Flexural Behaviour of Unguided Walls

The response of each of the walls to om-of-plane loading is shown in Figure 6. The walls with zero prestress levei cracked under their own weight when the walls were placed in the horizontal position for testing. For ali the tests only a limited number of joints cracked, (between one and three), and all subsequent rotation and distress was confmed to one of these cracked joints. It was found that the movement of the rod within the core was approximately equal to the deflection of the wall at mid span (see Figure 5).

As can be seen from Figure 6, the cracking moment depended upon the levei of prestress, with the values being consistent with those obtained from the other tests (see TabJe 2). After cracking the movement of the steel rod towards the compression face reduced the internai lever arm, resulting in a lower ultimate moment than for the grouted or guided cases. When the behaviour of the guided and unguided specimens is compared, it is clear that transverse restraint of the bar in the core of the hollow units is a desirable design feature. With the exception of 11 UG (with a Y 16 rod), ali unguided walls exhibited an over-reinforced failure, with the steel remaining elastic and local compression failure occurring in the masonry (largely confined to the mortar joints).

6. CONCLUSIONS

The paper has described an experimental investigation of the flexural behaviour of post-tensioned hollow masonry . In particular, the significance of prestress levei, reinforcement ratio and prestressing rod restraint conditions has been investigated. Although replicates of each of the tests have not been performed, the trends are dear. As for prestressed concrete, the levei of prestress has a direct influence on the serviceability behaviour, in particular the cracking moment. The degree of restraint of the prestressing rod in the core of the hollow unit is not criticai at this stage. However, it does have a marked influence on the ultimate behaviour, as if the rod is free to move in the cavity, the

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geomenic effects are significant, the capacity of the wall is reduced, and the failure mode can change from under-reinforced to over-reinforced (an undesirable feature). It is therefore desirable to restrain the rod within the core of the hollow block either by grouõng the core or by using inserts to hold the rod in place.

This work is continuing with further tests and invesúgations of the suitability of various analytical formulations to predict the flexural behaviour of post-tensioned masonry at both the serviceability and ultimate strength lirnit states.

7. ACKNOWLEDGMENTS

This work was funded by the Wide Bay Brickworks and the Rod and Bar Products Division of BHP Steel. The support of these companies and the Clay Brick and Paver Institute is gratefully acknowledged as is the assistance of the laboratory staff of the Departrnent of Civil Engineering & Surveying . .

8. REFERENCES

1. Phipps M.E., 'The PrincipIes of Post-Tensioned Masonry Design", Proc . 6th North American Masonry Conference, Phil. , June 1993, pp. 621-632.

2. AI-Manaseer, A.A., and Neis, V.V., "Load Tests on Post-tensioned Masonry Wall Panels", ACI Structural Joumal, November - December, 1987, pp. 467-472.

3. Geschwindner, L.F., and Ostag, W.P., "Post-Tensioned Single Wythe Concrete Masonry Walls", Proc. 5th North American Masonry Conference, Urbana­Champaign, June 3-6, 1990, pp. 1123-1134.

4. Ungstad, D.G., Hatzinikolas, M.A., and Warwaruk, J., "Prestressed Concrete Masonry Walls", Proc. 5th North American Masonry Conference, Urbana­Champaign, June 3-6, 1990, pp. 1147-1160.

5. Dawe, J.L., and Aridru, G.G., "Post-Tensioned Concrete Masonry Walls Subjected to Uniform Lateral Loading", Proc. 6th Canadian Masonry Symposium, Saskatoon, Saskatchewan, June 15-17, 1992, pp. 201-212.

6. Schultz, A.E., and Scolforo, M.J., "An Overview of Prestressed Masonry", The Masonry Society Joumal, Vol. 10, No. 1, August 1991, pp. 6-21.

7. Schultz, A.E., and Scolforo, M.J., "Engineering Design Previsions for Prestressed Masonry Part 1 : Masonry Stresses", The Masonry Society Joumal, Vol. 10, No. 2, February 1992, pp. 29-47.

8. Schultz, A.E. and Scolforo, M.J., "Engineering Design Previsions for Prestressed Masonry Part 2 : Steel Stresses and Other Considerations", The Masonry Society Joumal, Vol. 10, No. 2, February 1992, pp. 48-64.

9. Hendry, A.W., "Reinforced and Prestressed Masonry", Longman Scientific & Technical/John Wiley & Sons, 1991, pp. 125-159 and 78-98.

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