Masonry wallettes with damp-proof course membrane subjected to cyclic shear: An experimental study

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Abstract

Cyclic load tests were performed on two series of unreinforced masonry wallettes with a damp-proof course placed either between the first two masonry courses or between the concrete base and the wallette. Two types of failure were observed, namely sliding and compression (toe crushing) failure. Wallettes that failed in compression exhibited limited energy dissipation. Wallettes which failed through sliding displayed considerable energy dissipation and behaved in a quasi ductile manner. Greater ductility was observed in the wallettes with the DPC in the bed joint rather than at the wallette–slab interface, indicating that the former detail would be more desirable for enhanced seismic performance. Simple analytical models for predictions of failure shear load are proposed and discussed.

Introduction

A joint research project by the University of Newcastle and the ETH Zurich on the structural behaviour of unreinforced masonry elements subjected to cyclic shear is underway at the University of Newcastle. The main goal of the research project is to investigate the influence of a damp-proof course (DPC) on the structural behaviour of masonry walls subjected to shear when the DPC is placed in the bed joint or at the interface of the masonry and its supporting concrete slab.

A damp-proof course (DPC) is often placed at the base of masonry walls as a moisture barrier and/or to act a slip joint to allow for differential movements (see Fig. 1). Although it is desirable for the DPC to be sandwiched in the mortar joint, in reality it is usually placed in the joint above or below the mortar. In some cases, the DPC alone is used, particularly if it is serving as a slip joint at the interface between a masonry wall and a concrete slab. Thus a DPC membrane in a joint has the potential to act as a plane of weakness due to the resulting lower shear and tensile capacities of the joint. From a structural design perspective it is therefore important to understand the influence of the DPC on the overall wall behaviour, particularly in relation to the in-plane behaviour of shear walls.

Cyclic load tests were performed on two series of masonry elements with a DPC placed into one of the bed joints. Each series consisted of nine 110 mm thick clay brick masonry wallettes with nominal dimensions of 1200 × 1200 mm. The DPC was placed either between the first two courses (Series A) or between the concrete base and first masonry course (Series B). In addition, three control specimens with the same dimensions and without a DPC were tested (Series C). The specimens were at first subjected to a vertical pre-compression load which was kept constant during the test and then subjected to a cyclic shear load applied in time steps with prescribed horizontal displacements. Three different levels of pre-compression were considered, see Table 1. For each level of pre-compression, three replicates were tested for Series A and B, resulting in a total of 21 tests being performed.

Section snippets

Previous investigation

The previous research activities related to the masonry behaviour under cyclic shear loading have been focused both on theoretical and experimental aspects. A substantial amount of theoretical work has been invested into modelling of structural masonry under cyclic shear. Both analytical and numerical solutions are reported in the literature, e.g. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Regardless of the mechanical model applied, in order to gain a deeper insight into the

Masonry materials and strength

A comprehensive overview of the results of different material tests performed on masonry and its components has been given in the research report [21]. In following, only the summary of these results is presented.

Wallette test procedure and measurements

The wallette test specimens were built by a skilled mason in the laboratory on reinforced concrete beams. The specimen length lw and height hw were nominally 1200 mm (five units long and 14 courses high). The wallette thickness tw was 110 mm. The wallettes were built in running bond and both the bed and the head joints were 10 mm thick and fully filled. All wallettes were cured in air in the laboratory for 28 days before testing. Prior to testing the specimens were painted white in order to follow

Wallette test results and structural behaviour

Table 3 shows the values of the extreme (maximum and minimum) horizontal forces, H, recorded during the testing and their ratio to the applied vertical force, V. The total number of the load cycles as well as maximum horizontal displacement (jack travel), vmax, is given. The failure mode for each wallette together with applied level of pre-compression, σpc, are also shown. Rocking failure (R) denotes tensile flexural cracking at the corners of the wallette which afterwards begins to behave as a

Sliding failure and friction coefficient in the bed joint with DPC

Sliding failure of the specimens can be described by classical Mohr–Coulomb’s failure criterionτ=c+σtanφwhere c denotes the cohesion and φ is the angle of internal friction, i.e. tan φ is a friction coefficient. In the previous investigation on small specimens [15] the friction coefficient for embossed polythene subjected to static-cyclic loading was found to be 0.329 for the membrane placed between two masonry courses and 0.267 for the membrane placed between masonry element and concrete. Note

Conclusions

From the results of this experimental study on the shear behaviour of masonry wallettes with incorporated DPC subjected to static-cyclic shear loading the following conclusions can be drawn:

  • The behaviour of the wallettes was highly influenced by the pre-compression level. Furthermore, the presence and position of the DPC had a considerable influence on the behaviour of the wallettes, especially on the failure mode. Two types of failure were observed, namely sliding along the bed joint

Acknowledgements

This was a joint project involving the University of Newcastle and the ETH Zurich, and the support of both those organisations is acknowledged. Funding and support for the program was also provided by Think Brick Australia and its member companies and the Centre for Infrastructure Performance and Reliability in the School of Engineering at the University of Newcastle. The assistance of the Civil Engineering laboratory staff is gratefully acknowledged, particularly that of Mr. Ian Jeans in

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