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Experimental shear testing of unreinforced masonry wall panels 2016 NZSEE Conference H. Qiu, R. Chin, J. Ingham, and D. Dizhur The University of Auckland, Auckland, New Zealand ABSTRACT: An experimental program was undertaken with the principal aim of determining the transition point between the stair-step failure mode and diagonal tension failure mode of eight 1200 mm x 1200 mm URM wall panels when subjected to simulated earthquake lateral loads. Preparations included the formulation of mortar compositions and then subsequent pairing with bricks of varying strengths to replicate the range of material characteristics of existing URM structures found throughout New Zealand. Diagonal shear tests were conducted with experimental results indicating two distinct failure mechanisms. It was concluded that the transition between failure modes occurs when the mortar to brick compressive strength ratio is approximately 0.4. In addition, following the failure of the wall panels, three panels were repaired using 8 mm steel wire rope placed in differing orientations and quantities in order to investigate the feasibility and performance of this repair technique. Steel wire rope proved to be a simple and cost effective remediation method with improvements in diagonal shear strength and displacement capacity of up to double and fifty times respectively that of the as-built counterparts. 1 INTRODUCTION According to Magenes and Calvi (1997) the in-plane failure modes of unreinforced masonry (URM) piers can be categorised as either: rocking, diagonal shear, or bed joint sliding. Rocking and bed joint sliding types of failure modes typically allow for the dispersion of energy in cycles through displacement. Comparatively, diagonal shear failures are typically more critical and may be relatively more brittle in nature. Diagonal shear failure may develop as one of two failure mechanisms (see Figure 1), as cracks may either develop through both brick units and mortar joints or through the mortar joints alone in a stepping pattern depending on the ratio between mortar and brick strengths (Dizhur & Ingham, 2013). Failure through both brick units and mortar joints is recognised as being relatively brittle in nature, as the shear strength capacity of the wall deteriorates heavily after the maximum shear stress has been achieved. In the NZSEE (2015) assessment guidelines, URM pier diagonal tension failure modes that are dominated by brick splitting correspond to a force reduction factor, K R of 1.0. In contrast, stair- stepped failure though the mortar bed and head joints creates multiple sliding planes analogous to the bed joint shear sliding failure mode, where additional energy from seismic forces can be subsequently dispersed through sliding. In the NZSEE (2015) assessment guidelines, the stair-step failure mode in URM piers corresponds to a force reduction factor, K R of 3.0. When calculating the URM spandrel capacity in Section 10.8.6.3 (NZSEE, 2015), it is also important to be able to distinguish between stair-step failure modes and diagonal tension failure modes that are dominated by brick splitting, see Figure 1. Consequently, determining the transition point between the two mechanisms of diagonal shear failure is paramount for understanding the behaviour of URM piers and spandrels subjected to in-plane loading. Currently limited guidance is provided in the NZSEE (2015) document on distinguishing the occurrence of the two failure mechanisms. This information would allow engineers to more accurately assess the seismic vulnerability and shear capacity URM buildings. The experimental program reported herein was undertaken to address and attempt to provide such valuable information for

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Experimental shear testing of unreinforced masonry

wall panels

2016 NZSEE Conference

H. Qiu, R. Chin, J. Ingham, and D. Dizhur

The University of Auckland, Auckland, New Zealand

ABSTRACT: An experimental program was undertaken with the principal aim of

determining the transition point between the stair-step failure mode and diagonal tension

failure mode of eight 1200 mm x 1200 mm URM wall panels when subjected to

simulated earthquake lateral loads. Preparations included the formulation of mortar

compositions and then subsequent pairing with bricks of varying strengths to replicate the

range of material characteristics of existing URM structures found throughout New

Zealand. Diagonal shear tests were conducted with experimental results indicating two

distinct failure mechanisms. It was concluded that the transition between failure modes

occurs when the mortar to brick compressive strength ratio is approximately 0.4. In

addition, following the failure of the wall panels, three panels were repaired using 8 mm

steel wire rope placed in differing orientations and quantities in order to investigate the

feasibility and performance of this repair technique. Steel wire rope proved to be a simple

and cost effective remediation method with improvements in diagonal shear strength and

displacement capacity of up to double and fifty times respectively that of the as-built

counterparts.

1 INTRODUCTION

According to Magenes and Calvi (1997) the in-plane failure modes of unreinforced masonry (URM)

piers can be categorised as either: rocking, diagonal shear, or bed joint sliding. Rocking and bed joint

sliding types of failure modes typically allow for the dispersion of energy in cycles through

displacement. Comparatively, diagonal shear failures are typically more critical and may be relatively

more brittle in nature. Diagonal shear failure may develop as one of two failure mechanisms (see

Figure 1), as cracks may either develop through both brick units and mortar joints or through the

mortar joints alone in a stepping pattern depending on the ratio between mortar and brick strengths

(Dizhur & Ingham, 2013).

Failure through both brick units and mortar joints is recognised as being relatively brittle in nature, as

the shear strength capacity of the wall deteriorates heavily after the maximum shear stress has been

achieved. In the NZSEE (2015) assessment guidelines, URM pier diagonal tension failure modes that

are dominated by brick splitting correspond to a force reduction factor, KR of 1.0. In contrast, stair-

stepped failure though the mortar bed and head joints creates multiple sliding planes analogous to the

bed joint shear sliding failure mode, where additional energy from seismic forces can be subsequently

dispersed through sliding. In the NZSEE (2015) assessment guidelines, the stair-step failure mode in

URM piers corresponds to a force reduction factor, KR of 3.0. When calculating the URM spandrel

capacity in Section 10.8.6.3 (NZSEE, 2015), it is also important to be able to distinguish between

stair-step failure modes and diagonal tension failure modes that are dominated by brick splitting, see

Figure 1. Consequently, determining the transition point between the two mechanisms of diagonal

shear failure is paramount for understanding the behaviour of URM piers and spandrels subjected to

in-plane loading.

Currently limited guidance is provided in the NZSEE (2015) document on distinguishing the

occurrence of the two failure mechanisms. This information would allow engineers to more accurately

assess the seismic vulnerability and shear capacity URM buildings. The experimental program

reported herein was undertaken to address and attempt to provide such valuable information for

inclusion in a future revision of the NZSEE (2015) document.

(a) Cracking through

mortar joints only - stair-

step failure

(Kathmandu, Nepal 2015)

(b) Cracking through bricks

and mortar joints

(Christchurch 2011)

(c) Cracking mainly through

bricks - diagonal tension

failure

(Christchurch 2011)

Figure 1. Diagonal shear cracking observed following earthquakes for different wall piers with

different brick to mortar strength ratios

Many URM buildings that were damaged in past earthquakes have been demolished due to a lack of

viable repair options being available, resulting in the loss of building heritage (Moon et al., 2013).

Repair techniques must ensure that the strength of the original building is restored, and that the

remediated building is able to resist forces which may arise from future earthquakes. Near Surface

Mounted (NSM) techniques have previously been proven to be cost-effective with minimal visual

impact, whilst providing protection from environmental impacts (Dizhur et al. 2013). Given the

opportunity to test the viability of remediated wall panels, an additional pilot study was conducted

where the use of NSM Steel Wire Rope (SWR) as a repair technique was explored. As a relatively

inexpensive material with a large surface area for adhesion and a high tensile capacity, SWR has

strength properties comparable to those of Carbon Fibre Reinforced Polymer (CFRP) strips, which

have previously been demonstrated to be successful as a retrofit and/or repair technique for URM

walls (Dizhur et al. 2013).

2 EXPERIMENTAL SET UP

2.1 Materials

Eight URM wall panels were constructed using mortar and solid clay brick units of varying

compressive strengths in order to replicate the material characteristics of vintage URM buildings

found throughout New Zealand. Based on Almesfer et al. (2014), where a wide range of mortar

compressive strengths found in URM buildings was reported, eight different compositions of mortar

were fabricated by changing the ratio of cement, lime and sand as shown in Table 1. Recycled vintage

solid clay bricks were sourced from multiple demolition sites throughout Auckland, with the strengths

estimated on-site by performing a scratch test [as reported in Almesfer et al. (2014)] before being

paired with an appropriate mortar composition in order to construct wall panels with a wide spectrum

of brick to mortar strength ratios.

The compressive strength of individual bricks (f’b) was tested using the half brick compression test

according to ASTM (2003a), while the compressive strength of each mortar composition (f’j) was

determined by loading 50 mm cubes in compression as per ASTM (2008). Masonry prisms (f’m) were

also tested according to ASTM (2003b).

Two-leaf thick URM wall panels with approximate dimensions of 1200 mm x 1200 mm were

constructed using the common bond pattern as per ASTM (2010). Once construction was completed,

the mortar was allowed to cure for a minimum of 28 days before tests were conducted. Once the wall

panels were tested in the as-built condition, repairs were carried out using NSM SWR (see Figure 2a).

The diameter of wire was chosen to be 8 mm based on the thickness of the mortar joints and

preliminary pull-out test results. Preliminary pull-out tests (Figure 2b) were conducted to test the bond

strength between SWR and masonry with the results being compared to NSM CFRP retrofits and

repairs that were reported by Dizhur et al. (2013). The NSM SWR reached 30 kN in direct pull-out

(85% of the capacity of CFRP) and exhibited a higher nominal ductility when compared to the

behaviour of CFRP strips (Figure 2c).

Table 1. Material properties

Wall Mortar Mix Mortar Strength Brick Strength Masonry Strength

(cement:lime:sand) f'j (MPa) Samples COV f'b(MPa) Samples COV f'm(MPa) Samples COV

W1 1:2:15 1.19 5 0.05 9.59 3 0.16 4.05 1 n/a

W2 1:3:15 2.19 5 0.13 8.41 4 0.17 4.73 2 0.13

W3 1:2:9 3.35 5 0.08 8.34 5 0.15 4.09 3 0.25

W4 1:1:10 2.11 5 0.14 7.23 4 0.21 6.49 3 0.77

W5 1:1:8 4.07 5 0.07 9.59 4 0.17 6.88 2 0.24

W6 1:0:6 5.91 5 0.12 13.02 3 0.35 7.50 2 0.55

W7 1:1:6 6.55 5 0.06 13.02 3 0.35 8.76 1 n/a

W8 1:1:6 6.55 5 0.10 8.81 4 0.45 4.71 2 0.08

Note: COV = coefficient of variance, n/a = not applicable

(a) Steel wire rope (SWR)

(b) Pull-out test set-up

(c) Pull-out test results

Figure 2. SWR used in repair and preliminary pull-out test results

2.2 Diagonal Shear Test Set-up

The test method utilised in this series of experiments was a reapplication of that adopted by Dizhur &

Ingham (2013), which is a variation of the test set-up outlined in ASTM (2010). The wall panels were

tested horizontally instead of at a 45o angle, in order to prevent the premature cracking of weaker

mortar compositions while the wall panels were being rotated. In this variation of the standard test,

custom designed and fabricated loading shoes were placed on diagonally opposite corners of each

specimen and connected by two 30 mm steel tension rods. A hydraulic jack, load cell and a

rectangular steel channel were placed on the top loading shoe, as shown in Figure 3. A diagonal

compression force was applied through the hydraulic jack and continuously increased until a

significant drop in load was registered by a load cell, indicating that the wall panel had failed. Two

portal strain gauges were attached diagonally on both sides of each wall panel to measure

displacements arising from tension and compression, which were then converted to a percentage of

lateral drift.

0

10

20

30

40

0 5 10 15

Lo

ad

(k

N)

Displacement (mm)

CFRP

SWR

(a) Schematic of the test set-up showing

individual components

(b) Photograph of a typical test set-up

Figure 3. Test set-up

2.3 Repair Procedure

Once the wall panels had undergone diagonal shear failure, wall panels W1, W3 and W5 were repaired

using SWR. Grooves measuring 15 mm x 15 mm were cut into the faces of the wall panels using a

circular saw. Each wall panel was repaired using a configuration of the SWR, see Figure 4. Grooves

and the SWR were thoroughly cleaned with compressed air and acetone to remove any dust and oil

based lubricants. Two-part epoxy was then applied into the groves followed by the insertion of the

SWR. The epoxy was left to cure for at least 3 days prior to testing in order to reach a minimum of

90% of full strength (Sika, 2015).

W1

W3

W5

Figure 4. Layout of NSM SWR in repaired wall panels (also showing cracking in light grey that was

observed following testing of repaired wall panels)

3 RESULTS

3.1 Failure Mechanisms of As-Built Wall Panels

The diagonal shear failure mechanisms observed from the series of eight diagonal shear tests were

consistent with those reported previously (Dizhur & Ingham, 2013; Russell, 2010 and others).

Generally, wall panels constructed with low mortar to brick strength ratios (f’j/f’b) failed through

mortar joints only, whereas wall panels constructed with relatively higher mortar to brick strength

ratios underwent failure by cracking through both mortar joints and brick units.

It was observed that wall panels W1, W2 and W3, which were constructed with relatively low mortar

to brick compressive strength ratios (f’j/f’b), failed as a result of cracking through the mortar joints as

shown in Figure 5a. The cracking occurred in a stepping pattern around the brick units, with the

bricks remaining visually undamaged. When the peak shear stress for each wall panel was actualised,

the wall panels experienced a reduction in load carrying capacity while cracks began to form in the

stepping pattern. Upon further loading, sliding failure was induced as the interface between cracks

began to slide past one another. Similar behaviour was reported by Dizhur & Ingham (2013).

Due to suspected poor construction, the behaviour of W4 was predominately governed by bed-joint

sliding failure as opposed to diagonal shear, as shown by Figure 5b. The wall panel was constructed

with a slight out-of-plane curvature which resulted in an uneven stress distribution when diagonal

compression was applied. Therefore, upon loading in diagonal compression, the cracked upper section

of the wall panel began to slide horizontally, as exemplified in Table 2. The maximum load occurred

at 1.28% drift, which was substantially higher than for the wall panels which failed in a diagonal shear

mode.

Wall panels W5-W8 had significantly higher ratios of mortar to brick compressive strength (f’j/f’b)

than W1-W4. As a result, diagonal shear failure occurred through a combination of both mortar joints

and brick units, with the number of cracked bricks increasing with an increasing mortar to brick

compressive strength ratio, as shown in Figure 6. This particular failure mechanism was of an

explosive nature, as energy was rapidly released in the form of sound and displacement. Upon failure,

wall panels W5-W8 lost almost all of their lateral load carrying capacity. Figure 5c shows the failure

which occurred for W7, where the upper half of the wall panel was dislodged upon cracking.

(a) Cracking through

mortar (W1)

(b) Sliding failure (W4)

(c) Cracking through

bricks and mortar,

explosive in nature

(W7)

(d) Lack of adhesion

between mortar and

brick interface

(W6)

Figure 5. Observed failure mechanisms (cracks outlined for clarity)

W1

W2

W3

W4

W5

W6

W7

W8

Figure 6. Crack patterns of as-built wall panels

Apart from cracking failure through mortar and brick units, wall panels W6 and W8 also experienced

failure due to a lack of adhesion between brick units and mortar joints at several interfaces, as shown

in Figure 5d. This behaviour was exhibited by clean surfaces on bricks where mortar had been

completely separated. As the walls were built using recycled bricks, the pores on numerous bricks

were filled from previous use. Because the bricks for W6 and W8 were not cleaned adequately, mortar

was unable to adhere sufficiently which resulted in premature failure.

3.2 Failure of Repaired Walls

In wall panels W1, W3 and W5, cracks propagated predominantly through mortar, whilst

simultaneously enlarging pre-existing cracks. For the wall panels (W1 and W5) repaired with

vertically oriented SWR, the load appeared to be resisted by shear friction between the faces of the

original cracks as the cracks began to slowly dilate. New diagonally stepped cracks formed shortly

thereafter (see Figure 7). When the load-time graph began to plateau, the SWR and epoxy were

engaged. Upon the onset of strength loss, the epoxy began to crack, thus exhibiting sequential failure

modes. Hence, the vertically repaired wall panels initially failed in diagonal shear followed by sliding

shear. W3 showed minimal change in crack pattern as it experienced localized crushing of the bricks

directly adjacent to the loading shoes. The epoxy and SWR that resisted the shear stress assisted in

restricting further dilation along the as-built failure planes.

(a) Significant lateral

deformation of repaired W1

following testing

(b) Close up of NSM SWR

repair following partial

demolition of W5

Figure 7. Crack pattern observations for repaired wall panels

3.3 Shear Stress – Drift Response

3.3.1 As-built wall panels

According to ASTM (2010), URM walls fail through diagonal cracking when the applied diagonal

shear stress (τs) exceeds the diagonal tension strength of masonry (fdt). Under the assumption that the

specimen experiences pure shear and that shear stress is uniformly distributed throughout the section,

ASTM (2010) implies that the diagonal tension strength (fdt) can be calculated by the following

equation, where P is the force applied in diagonal compression and An is the area of the mortared

section:

(1)

The diagonal shear capacity (Vdt) of URM walls has been provided by NZSEE (2015) as an important

check for practicing structural engineers, where it represents a horizontal force which acts at the

top of the wall. As a comparison with the ultimate shear stress, the shear force capacity of URM walls

is calculated using Equation 2 below, where β is a factor for correcting the nonlinear stress distribution

and fa is the axial compression due to gravity loads.

(2)

0 50 100

0

100

200

0

0.2

0.4

0.6

0 0.5 1

0 5 10

0

100

200

0

0.2

0.4

0.6

0 0.5 1

0 5 10

0

100

200

0

0.2

0.4

0.6

0 0.5 1

Table 2. Experimental results for as-built wall panels

Wall Mortar

(C:L:S) f'j/f'b P (kN)

τs

(MPa)

Vdt

(KN) Drift (%)

G

(GPa)

E

(GPa) Failure Type

W1 1:2:15 0.14 62.9 0.16 37.4 0.14 0.29 0.72 Mortar

W2 1:3:12 0.26 69.9 0.16 36.7 0.45 0.07 0.17 Mortar

W3 1:2:9 0.40 95.0 0.24 58.3 0.42 0.11 0.27 Mortar

W4 1:1:10 0.29 56.9 0.15 35.7 1.28 0.06 0.14 Sliding

W5 1:1:8 0.42 115.9 0.31 71.2 0.47 0.15 0.38 Mortar/Brick

W6 1:0:6 0.45 188.5 0.49 113.5 0.26 0.52 1.31 Mortar/Brick

W7 1:1:6 0.50 257.7 0.67 154.4 0.43 0.33 0.84 Mortar/Brick

W8 1:1:6 0.74 158.5 0.39 96.0 0.34 0.42 1.04 Mortar/Brick

.

3.3.2 Repaired Wall Panels

Wall panels W1 and W3, which were both repaired using six steel ropes, experienced increased load

carrying capacities. Ultimate loads increased from 63 kN to 86 kN and from 46 kN to 96 kN

respectively, while the maximum drift for both walls was significantly increased. This improved

capacity was due to the dissipation of energy through friction in the mortar bed joints being resisted by

the SWR in tension. Shear stress (τs) and drift were plotted to a maximum of 3% drift to emphasize the

0 5 10

0

100

200

0

0.2

0.4

0.6

0 0.5 1

0 5 10

0

100

200

0

0.2

0.4

0.6

0 0.5 1

0 5 10

0

100

200

0

0.2

0.4

0.6

0 0.5 1

0 5 10

0

100

200

0

0.2

0.4

0.6

0 0.5 1

0 5 10

0

100

200

0

0.2

0.4

0.6

0 0.5 1

Figure 8. Shear stress - drift response of tested as-built wall panels

Wall 1

Shear Strain (γx1000)

Str

ess

(MP

a)

Lo

ad (k

N)

Drift (%)

Wall 2

Shear Strain (γx1000)

Str

ess

(MP

a)

Lo

ad (k

N)

Drift (%)

Wall 3

Shear Strain (γx1000)

Str

ess

(MP

a)

Lo

ad (k

N)

Drift (%)

Wall 5

Str

ess

(MP

a)

Lo

ad (k

N)

Drift (%)

Shear Strain (γx1000) Wall 4

Str

ess

(MP

a)

Lo

ad (k

N)

Drift (%)

Shear Strain (γx1000) Wall 6

Shear Strain (γx1000)

Str

ess

(MP

a)

Lo

ad (k

N)

Drift (%) Wall 7

Shear Strain (γx1000)

Str

ess

(MP

a)

Lo

ad (k

N)

Drift (%)

Wall 8

Str

ess

(MP

a)

Lo

ad (k

N)

Drift (%)

Shear Strain (γx1000)

increase in the initial loading stages relative to the as-built counterparts, as shown in Figure 9.

However, the portal gauges for W3 malfunctioned mid-test resulting in an unreliable result for drift.

For W1 and W5, drift responses of the repaired walls showed a 50 times and 2 times increase

compared to their as-built state. W5, despite showing a drift increase 2 times that of its original,

exhibited a strength reduction of approximately 50%. It is recommended that more tests be conducted

to establish whether there is possibly a linear relationship relating the number of vertical SWR and

shear strength.

Table 3. Experimental results for repaired wall panels

Wall P (kN) τs (MPa) Drift (%) G (GPa) E (GPa)

W1 85.8 0.23 6.87 0.01 0.02

W3 45.7 0.12 1.97 0.02 0.04

W5 226.6 0.61 0.18 1.68 4.21

Figure 9. Shear stress - drift response of tested repaired wall panels

3.4 Distinction Between Diagonal Shear Failure Mechanisms

A transition point was established between cracking through mortar and cracking through both bricks

and mortar joints for the tested wall panels. By examining the failure mechanisms in the series of

diagonal shear tests as well as the tests conducted by previous researchers as shown in Table 4, a

demarcation was identified when the mortar to brick strength ratio (f’j/f’b) was equal to approximately

0.4, as shown in Figure 10. It was observed that as-built walls constructed using mortar to brick

strength ratios of less than 0.4 failed through mortar joints alone. Comparatively, walls built with

mortar to brick strength ratios of above 0.4 experienced failures through both bricks and mortar joints.

Aside from the premature failure of W4 and W8 due to lack of adhesion, it was observed that more

bricks were cracked when the ratio was increased. This diagonal tension failure mode was more

explosive as the upper failure plane displaced significantly after the ultimate shear stress was reached.

Based on the reported results, it is suggested that Table 10.14: Recommended force reduction factors

for linear static method of NZSEE (2015) can be updated where a distinction between stair-step failure

modes (force reduction factor, KR equals to 3 when f’j/f’b ≤ 0.4) and pier diagonal tension failure

modes (dominated by brick splitting, KR equals to 1 when f’j/f’b ≥ 0.4) can now be quantified.

Section 10.8 of NZSEE (2015) can be updated in regards to the peak shear strength of URM spandrels.

It is recommended that Equation 10.47 is to be used when f’j/f’b ≤ 0.4, and otherwise Equation 10.48

should be used.

0 10 20 30

0

100

200

0

0.3

0.6

0 1 2 3

0 10 20 30

0

100

200

0

0.3

0.6

0 1 2 3

As-built

Repaired

Wall 1

Shear Strain (γx1000)

Str

ess

(MP

a) L

oad

(kN

) Drift (%)

Wall 3

Shear Strain (γx1000)

Str

ess

(MP

a) L

oad

(kN

)

Drift (%)

Wall 5

Shear Strain (γx1000)

Str

ess

(MP

a) L

oad

(kN

)

Drift (%)

0 10 20 30

0

100

200

0

0.3

0.6

0 1 2 3

Table 4. Previously conducted diagonal shear tests of wall panels

* - Compressive strength recommended by ASTM C 270 - 08a (2008a), Mortar mix - cement:lime:sand ratio by

volume; f’j – mortar compression strength; f'b – brick compression strength; νmax – maximum shear stress;

Figure 10. Demarcation between diagonal shear failure mechanisms (data from Table 2 and

Table 4 with W4 and W8 excluded)

3.5 Relationship between Maximum Shear Stress and Mortar Strength

It has previously been reported that as mortar compressive strength increases, the diagonal shear

strength of URM walls also increases proportionally. As reported by Dizhur and Ingham (2013),

results obtained by previous researchers as shown in Table 4 were found to have a relationship where

the maximum diagonal shear stress of URM wall panels is estimated to be 0.1 of the mortar

compressive strength with a statistical R2 value of 0.76. When the results from this experiment in

Tables 1 and 2 are plotted alongside the results obtained by previous researchers as shown in Table 4,

it is evident that there is an approximately linear correlation between the maximum diagonal shear

stress and the mortar compressive strength. As shown by Figure 11, the maximum diagonal shear

0

0.2

0.4

0.6

0.8

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

Shea

r S

tres

s τ

s (M

Pa)

f'j/f'b

Failure through mortar Failure through brick/mortar

Wall

panel

Mortar

mix

f’j

(MPa)

f’b

(MPa) f'j/f'b

νmax

(MPa)

Failure

Type

Russell (2010) AP1 2:1:9 12.4* 23.3 0.53 0.71 Brick/mortar

AP3 0:1:3 1.1 24.3 0.05 0.06 Mortar

AP6 1:2:9 2.6 23.6 0.11 0.14 Mortar

AP7 1:2:9 2.3 20.4 0.11 0.13 Mortar

AP8 1:2:9 2.2 20.1 0.11 0.14 Mortar

AP9 1:2:9 2.6 23.4 0.11 0.12 Mortar

Dizhur et al.

(2013)

A2L - 5.3 17.1 0.31 0.21 Mortar

A3L - 5.3 17.1 0.31 0.24

Lin et al. (2013) S0-2L-1

- 1.0 26.5

0.04

0.08

Mortar

S0-2L-2 0.09

S0-3L-1 0.09

S0-3L-2 0.09

S0-4L-1 0.07

S0-4L-2 0.08

Ismail (2012)

W2C-3

-

1.4 39.4 0.04 0.14

Mortar ABI-01 1.4 16.4 0.09 0.15

ABI-02 0.7 18.8 0.04 0.098

stress of URM wall panels can be estimated as 0.09 of the mortar compressive strength, with a

relatively higher statistical R2 value of 0.84. The results gained from W4 and W8 have not been

included in the analysis due to these wall panels failing prematurely due to the lack of adhesion.

Figure 11. Relationship between maximum diagonal shear stress and mortar compression

strength

3.6 Stiffness

In order to determine the stiffness of each tested wall panel, the shear modulus (G) was determined as

the ratio of shear stress (τs) to shear strain (γ) between 0.05 and 0.33 of the ultimate shear strength as

per ASTM (2004). As shown in Table 3, the shear modulus for each wall panel varied significantly

from 0.06 GPa to 0.52 GPa with no identifiable trend. Previous researchers (Dizhur & Ingham, 2013)

also observed large variations in shear modulus without any clear relationships during their

experiments. Similarly, as the modulus of elasticity (E) is a direct function of the shear modulus, the

modulus of elasticity also exhibits a large range of values with no identifiable trend.

4 CONCLUSIONS

Experimental testing was conducted on eight URM wall panels with the aim of determining the

demarcation between diagonal shear failure mechanisms occurring through mortar joints only and

occurring through a combination of brick/mortar. Also, three wall panels were repaired in order to

undertake a pilot study investigating the feasibility of using NSM SWR as a possible repair technique.

The following conclusions were drawn -

Significant load carrying capacity is sustained after the peak shear stress is reached when wall

panels undergo diagonal shear failure through mortar joints in a stair-step pattern.

Diagonal shear failure of wall panels occurring through brick/mortar joints resulted in a sud-

den and brittle failure with substantial reductions in load carrying capacity once the peak shear

stress was reached.

The transition point between the two distinctive diagonal shear failure mechanisms (cracking

through mortar joints alone and cracking through both bricks and mortar) was established as

occurring at a ratio of mortar to brick compressive strength of 0.4. Wall panels with a ratio be-

low 0.4 resulted in cracking through mortar joints, while wall panels with a ratio greater than

0.4 resulted in cracking through both brick and mortar joints.

Based on the attained results herein, Table 10.14: Recommended force reduction factors for

linear static method of NZSEE (2015) can be updated where a distinction between stair-step

failure modes (force reduction factor, KR equals to 3 when f’j/f’b ≤ 0.4) and pier diagonal ten-

sion failure modes (dominated by brick splitting, KR equals to 1 when f’j/f’b ≥ 0.4) can now be

quantified.

Section 10.8 of NZSEE (2015) can be updated in regards to the peak shear strength of URM

spandrels. It is recommended that Equation 10.47 be used when f’j/f’b ≤ 0.4, otherwise Equa-

tion 10.48 should be used.

y = 0.09x

R² = 0.84

0.0

0.2

0.4

0.6

0.8

0 1 2 3 4 5 6 7

Ma

xim

um

Dia

go

na

l S

hea

r

Str

ess,

τs

(MP

a)

Mortar Compression Strength, f'j (MPa)

The maximum diagonal shear stress of URM wall panels is able to be estimated as 0.09 of the

mortar compression strength.

Wall panels W1 and W3 which were repaired using SWR had increases in both load carrying

capacity and maximum drift compared to their as-built counterparts. Further experimentation

is required in order to assess effectiveness of this repair/retrofit technique.

5 ACKNOWLEDGEMENTS

The authors would like to thank Marta Giaretton, Peter Inman, Melissa Brisacque and the engineering

lab technicians at the University of Auckland. The authors would also like to acknowledge our

sponsors Sika Ltd. and D. M. Standen for supplying materials for this experiment.

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