Seismic Analysis of Partially-Grouted Reinforced Masonry Walls Constructed Using Masonry Cement Mortar

(PCA Index No. 03-12)

by

A.A. Hamid and F.L. Moon

Drexel University Department of Civil, Architectural and Environmental Engineering

3141 Chestnut St. Philadelphia, PA 19104

12 April 2005

ii

EXECUTIVE SUMMARY

Since the establishment of ASTM standard C91, masonry cement mortars have become

widely used for masonry construction in low to moderate seismic regions. However, the

Masonry Standards Joint Committee Code prohibits the use of masonry cement mortars

in lateral force resisting systems for structures that fall into Seismic Design Categories D,

E, and F. The general objective of this study is to examine the appropriateness of this

restriction in light of past research and, if necessary, propose additional research required

to fill any existing knowledge gap.

Since information about the specific impact of the physical and mechanical properties of

masonry cement mortars on the seismic response of masonry shear walls is limited, this

report primarily focuses on research that has identified the influence of mortar on the

behavior of masonry assemblages. In particular, the influence of mortar type is examined

in terms of in-plane pier response, out-of-plane wall response, and the response of

masonry assemblages under axial compression, flexural tension, bed joint shear and

diagonal tension. The primary gap identified through this literature review was the lack of

experimental research that addressed the response of reinforced masonry shear walls

constructed with masonry cement mortar. To establish a comprehensive and efficient

research program to fill this gap, the available literature related to the behavior of

partially grouted reinforced masonry shear walls was also reviewed and key factors that

influence response were established.

Based on this literature survey, it is the authors’ opinion that the use of masonry cement

mortar instead of portland cement lime mortar will not have a detrimental effect on the

strength and deformation capacity of grouted and partially grouted (up to a spacing of 48

in.) reinforced masonry walls subjected to in-plane and out-of-plane loadings. However,

it must be emphasized that the evidence supporting this conclusion for in-plane loading is

limited to investigations of assemblage behavior and a single in-plane pier experimental

study. As a result, the report concludes with an outline of a proposed in-plane testing

program aimed at filling this knowledge gap.

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TABLE OF CONTENTS

EXECUTIVE SUMMARY…………………………………………………………….. ii 1. INTRODUCTION…………………………………………………………………... 1

1.1 Background…………………………………………………………………... 1

1.2 Reinforced Masonry Walls……………………………………………........... 2

1.3 Development of Mortar…………………………………………………..….. 4

1.4 Seismic Provisions Related to Mortar Type………………………................. 5

1.5 Objective and Scope…………………………………………………………. 6

1.6 Outline of Report…………………………………………………………….. 6

2. EFFECT OF MORTAR TYPE ON THE PROPERTIES MASONRY

ASSEMBLAGES…………………………………………………………………… 7

2.1 Introduction………………………………………………………………….. 7

2.2 Compressive Strength………………………………………………………... 7

2.3 Bond Strength………………………………………..………………………. 9

2.4 Diagonal Tensile Strength…………………………..……………………… 11

2.5 Summary………………………………………….………………………... 12

3. EFFECT OF MORTAR TYPE ON THE PROPERTIES OF MASONRY

WALLS………………..…………………………………………………………….. 14

3.1 Introduction………………………………………………………………… 14

3.2 In-plane Behavior………………………………..…………………………. 14

3.3 Out-of-plane Behavior………………………………………………………17

3.4 Summary…………………………………………………………………… 20

4. EFFECT OF GROUTING ON THE PROPERTIES OF MASONRY

ASSEMBLAGES……..…………………………………………………………….. 22

4.1 Introduction………………………………………………………………… 22

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4.2 Compressive Strength…………………………………………………….... 22

4.3 Bed-joint Shear Strength…………………………………………………… 24

4.4 Tensile Strength…………………………………………………………… 25

4.5 Summary…………………………………………………………………… 27

5. IN-PLANE BEHAVIOR OF PARTIALLY GROUTED MASONRY SHEAR

WALLS…………………………………………………………………………….. 29

5.1 Introduction………………………………………………………………… 29

5.2 Review of Past Research…………………………………………………… 29

5.3 Summary…………………………………………………………………… 45

6. CONCLUSIONS……………………………………………………………………. 47 7. REFERENCES……………………………………………………………………... 48

1

1. INTRODUCTION

1.1 Background

Masonry is one of the oldest construction materials employed by man as evident from the

historic remains of the Egyptians and the Greeks. The first masonry was a crude stack of

selected natural stones often with earthen mortar packed between them. This type of

massive masonry could resist large compressive forces and was quite durable, although

its tensile strength was poor. As a result, traditional masonry buildings exploited the

weight of the floors and the massive walls to offset tensile stresses that arose due to

eccentric vertical and lateral loads. Due to its constructability and substantial durability,

construction using this type of masonry (termed UnReinforced Masonry [URM]) was

widespread throughout the 19th Century in the United States. However, a series of

earthquakes around the turn of the century, including the 1886 Charleston, 1906 San

Francisco, 1925 Santa Barbara, and the 1933 Long Beach earthquakes, clearly illustrated

the seismic vulnerability of URM structures. These events prompted the 1933 passage of

the California Field Act, which banned the use of URM for public buildings in California.

This ban was subsequently adopted by other western states; thus, URM construction was

effectively halted west of the Rocky Mountains.

This restriction has led engineers and builders to seek a more ductile, earthquake resistant

form of masonry construction for high seismic regions. Along with economic concerns,

this impetus resulted in the development of reinforced masonry, which typically

combines high strength manufactured concrete and clay masonry units along with

grouting and reinforcing steel (Figure 1.1) to more efficiently resist tensile stresses and

provide a more ductile and reliable system. Throughout the last 70 years, this type of

construction has been used extensively in high seismic regions throughout the United

States and, based on past performance, is widely considered as one of the most

earthquake resistant structural systems.

2

1.2 Reinforced Masonry Walls

In general, both clay and concrete masonry construction have been widely used in the

past; although, in current practice concrete masonry is more common. Depending on the

distribution of vertical and horizontal reinforcement (location and spacing) the MSJC

code (2005) classifies reinforced masonry into the following three categories:

• Ordinary reinforced masonry shear walls

• Intermediate reinforced masonry shear walls

• Special reinforced masonry shear walls

Figure 1.2 shows reinforcement details for these three designations.

In addition to these MSJC definitions, a more general way to classify masonry shear

walls exists. Walls with reinforcement spaced at large intervals (greater than 48 in.

vertically and horizontally) are referred to as partially, lightly or nominally reinforced

masonry, whereas walls with more closely spaced reinforcement are known simply as

reinforced masonry. A partially reinforced masonry wall is considered to consist of

reinforced strips of masonry with URM spanning between them (Figure 1.3), similar to

“confined masonry” commonly used in Europe and South America.

Figure 1.1 Reinforced Fully Grouted Concrete Block Masonry Wall (Drysdale et al. 1999)

3

(c) Special Reinforced Masonry Wall

Figure 1.2 MSJC code (2005) reinforcement details for masonry shear walls

(a) Ordinary Reinforced Masonry Wall

(b) Intermediate Reinforced Masonry Wall

4

In addition to the distribution of reinforcing steel, reinforced masonry walls can also be

distinguished based on the extent of grouting. Partially grouted masonry walls typically

only have grout placed where reinforcement is located whereas fully grouted masonry

walls have grout placed in every cell. From a construction standpoint, partially grouted

masonry (with grouted cells spaced at larger than 24 in.) is more efficient. For example, it

enables easier installation of services (in ungrouted cells), faster construction, material

savings, and reduced weight (resulting in reduced seismic loads) compared with fully

grouted walls. However, from a behavior standpoint, fully grouted walls are superior

since grouting has been shown to reduce the inherent variability in masonry and also

improve the tensile and shear strength.

1.3 Development of Mortar

Early mortars were primarily used to fill cracks and provide uniform bedding for

masonry units. Such mortars were typically composed of clay, bitumen, or clay-straw

mixtures (Dysdale et al. 1999). Following centuries of lime mortar use in masonry

construction, portland cement-lime mortars were developed to be suitable for particular

applications in the late 1800’s and early 1900’s (Speweik 1995). More recently, masonry

cement mortars have been introduced in the market. Masonry cement is primarily

composed of portland cement or blended hydraulic cement and plasticizing materials. In

addition, other materials are often added to improve properties such as workability,

Figure 1.3 Partially reinforced masonry walls (Drysdale et al. 1999)

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setting time, and durability. Aside from these improved properties, the primary advantage

of masonry cement is that it is proportioned in controlled conditions which greatly

enhance its uniformity. This alleviates the need for on-site mixing of portland cement and

lime and results in savings in construction time as well as more reliable mix proportions.

Since the establishment of the ASTM standard C91 for these products in 1932, masonry

cement mortars have become widely used in masonry construction in regions of low to

moderate seismicity (Speweik 1995).

1.4 Seismic Provisions Related to Mortar Type

Currently, the Masonry Standards Joint Committee (MSJC) Code (2005) prohibits the use

of MC mortars in the construction of lateral force resisting systems for structures that fall

into Seismic Design Categories (SDC) D, E, and F. Two factors have likely contributed

to establishing and maintaining this ban: (1) MC mortars were not common in high

seismic regions where seismic codes were developed and (2) research into the behaviour

of ungrouted (solid or hollow) masonry assemblages has shown that MC mortars

typically display lower bond strength than PCL mortars. Clearly the first factor is social

in nature rather than related to the actual seismic performance of shear walls constructed

with masonry cements. The second factor, while well established, may not have as large

an influence on seismic performance as expected. Consider that for SDC D, E, and F, all

masonry is required to be reinforced and either partially or fully grouted (the MSJC Code

requires a maximum horizontal and vertical reinforcement spacing of 1220 mm [48 in.]).

According to past research, as the extent of grouting and reinforcement increases, the

influence of mortar type and mortar unit bond diminishes (Drysdale et al. 1999). This

phenomenon is attributed to the continuity across the weak bed-joint plane, and the

additional load path provided by the grouted cells and reinforcement. As a result, it is

generally accepted that for fully grouted masonry this provision is overly restrictive;

however, it is unclear at what level, if any, of partially grouted reinforced masonry this

restriction is appropriate.

6

1.5 Objective and Scope

The study reported herein had two primary objectives. The first was to summarize the

available literature that addresses the influence of mortar type and grouting on the

behavior of masonry elements. This included investigations on masonry assemblages

under axial compression, diagonal tension, and flexural tension, and masonry

components (piers and walls) under in-plane and out-of-plane loads. Close attention was

paid to how the mortar type and extent of grouting affect the properties and the response

of masonry to load. The second objective involved evaluating the validity of the code

restriction (in light of past research) and recommending future research required to fill in

any identified knowledge gaps that may be preserving this ban. The primary gap

identified throughout the literature review was the lack of experimental research that

addressed the response of partially grouted, reinforced masonry shear walls constructed

with MC mortar. To establish a comprehensive and efficient research program to fill this

gap, the available literature related to the behavior of partially grouted reinforced

masonry shear walls was reviewed and key factors that influence response were

established.

1.6 Outline of Report

The report is organized into six sections. Section 2 outlines past research into the effect of

mortar type (MC versus PCL) on the mechanical properties of masonry assemblages. The

literature available on the effect of mortar type on component response is summarized in

Section 3. Section 4 summarizes the available literature on the effect of grouting on the

response of masonry assemblages and Section 5 reviews the literature available on the

response of partially grouted reinforced masonry shear walls. The conclusions of the

study are summarized in Section 6.

7

2. EFFECT OF MORTAR TYPE ON THE PROPERTIES OF MASONRY

ASSEMBLAGES

2.1 Introduction

This section briefly outlines past research that addressed the difference between MC and

PCL lime mortar in regards to the response of masonry assemblages under compression,

flexural tension, and diagonal tension. The large size required to investigate the response

of partially grouted masonry typically precludes the use of masonry assemblages.

Therefore the literature regarding the effect of mortar type on the properties of hollow

and fully grouted assemblages was summarized. The behavior of partially grouted

masonry, depending on spacing of grouted cells, falls between that of hollow and fully

grouted masonry. The MSJC code allows linear interpolation between these two extreme

conditions.

2.2 Compressive Strength

Drysdale et al. (1999) discussed the effect of mortar type on the compressive strength of

ungrouted clay and concrete masonry prisms. The experimental results showed that for

both clay and concrete masonry the mortar properties influenced the compressive

strength; however, this influence was far more pronounced for clay masonry (Figure 2.1).

The effect was attributed to the lateral expansion of the mortar under uniaxial

compression which places the units in biaxial tension transverse to the applied load

(Figure 2.2). Two critical factors that affect the interaction of masonry units and mortar

joints, and explain the increased sensitivity of clay masonry, were identified: relative

unit-joint strength and relative unit height to joint thickness. In the case of clay masonry,

the large differences in unit and mortar strength (i.e. a high unit-to-joint strength ratio)

resulted in a larger differential lateral expansion between the unit and mortar, and in turn

increased the biaxial tension stress in the unit. In addition, in clay masonry the unit

height-to-joint thickness ratio is typically small and thus larger biaxial tension stresses

develop within a given prism height. The lower sensitivity of concrete masonry

compressive strength to mortar properties was attributed to the fact that concrete unit

8

strength is comparable to that of the mortar and the unit height-to-joint thickness ratio is

much larger than that of clay masonry.

Figure 2.1 Effect of mortar strength on prism compressive strength (Drysdale et al.

1999)

Figure 2.2 Behavior of solid prisms under axial compression (Drysdale et al. 1999)

(a) Clay masonry (b) Concrete Masonry

9

2.3 Bond Strength

The influence of mortar on the bond strength of masonry has been investigated by several

researchers. Matthys (1992) investigated the difference between the bond strength of

ungrouted clay masonry constructed with MC and PCL mortars. This investigation tested

prisms constructed of both cored and solid clay masonry units according to ASTM

C1072-86 to determine flexural tensile strength. Based on these results, Matthys

concluded that prisms constructed with MC mortar displayed lower tensile strength than

prisms constructed with the corresponding PCL mortar. In addition, the results showed

that the specimens constructed with PCL mortar displayed lower variability than those

constructed with MC mortar.

Ghosh (1990) reviewed the results from three experimental studies that investigated the

bond strength of MC and PCL mortars. The experiments that were included in this review

were conducted at the University of Texas at Arlington (UTA) (Matthys 1992) and the

Construction Technologies Laboratories (CTL) (Dubovoy and Ribar 1989). These

investigations employed both 4-point bending tests (ASTM E518) and bond wrench tests

(ASTM C1072) to investigate bond strength. Several types of clay brick and concrete

units were incorporated in the testing along with 20 different MC mortars selected from a

wide geographical area. Figure 2.3 summarizes the results of the bond wrench tests

conducted at UTA and CTL. This figure clearly illustrates that MC mortars display lower

bond strengths than the corresponding PCL mortars. However, considering the large

differences in bond strength observed for different units (Texas Brick versus US Brick),

and the same units in different labs (UTA versus CTL), it is clear that unit-mortar bond is

not simply a property of mortar, but rather a property of unit-mortar combination and is

highly variable (perhaps due to variations in construction conditions). This is supported

by other research that has found interface bond is significantly affected by the physical

properties of units such as absorption and surface texture as well as workmanship and

curing conditions (Drysdale et al. 1999, Borchelt et al. 1999, and Drysdale and Hamid

1979).

10

Figure 2.3. Flexural bond strength of clay masonry (Ghosh 1990)

Using the experimental studies discussed by Ghosh as well as additional testing

conducted at the National Concrete Masonry Association’s (NCMA) Laboratory;

Melander et al. (1993) reported an investigation aimed at establishing the characteristic

bond strength of MC mortar relative to bond strength of PCL mortar established in a

previous study (Hedstrom et al. 1991). Based on the data sets obtained from UTA, CTL,

and NCMA, the authors concluded that the characteristic bond strength of MC mortar

was approximately 0.48 MPa (69 psi) for Type S and 0.30 MPa (44 psi) for Type N.

Comparing with the bond strengths of PCL mortars, it was concluded that reduction

factors of 0.6 and 0.5 be applied to the allowable bond strengths of Type S and Type N

PCL mortars, respectively, when MC mortar is used. These factors were later adopted by

the UBC (ICBO 1991) and are currently provided in the MSJC code (2005).

While the majority of the investigations into the different bond strengths displayed by

MC and PCL mortars have focused on ungrouted masonry, Brown and Melander (1999)

studied this issue for fully grouted masonry. This investigation subjected 40 fully grouted

11

masonry prisms (both clay and concrete) to four-point bending test as per ASTM C1390.

Four types of mortar were investigated: MC Type N, MC Type S, PCL Type N, and PCL

Type S. Figure 2.4 provides a summary of the test results. The authors note that

negligible difference in the flexural tension strength was observed regardless of unit type,

mortar type or whether the mortar was MC or PCL. Based on these results it was

concluded that the flexural tensile strength of grouted prisms normal to the bed joint is

dominated by the grout strength. This study prompted the MSJC code (2005) to increase

the allowable tensile strength perpendicular to the bed-joint for fully grouted masonry

constructed with MC to be more consistent with the allowable tensile strength of PCL

mortar.

2.4 Diagonal Tension Strength

Matthys (1989) investigated the difference in diagonal tension strength for clay masonry

constructed with MC and PCL mortars. The experimental program subjected 10 clay

brick shear panels to diagonal compression loading in accordance with ASTM E-519.

Both type S and N mortars were included in the study and MC manufacturers from each

region of the US were represented. Figure 2.5 provides a summary of the experimental

results. The walls constructed with Type S and N MC mortars displayed shear capacities

of 0.94 MPa and 0.73 MPa, respectively, compared to 1.56 MPa and 0.95 MPa for the

walls constructed with Type S and N PCL mortars, respectively. As a result it was

concluded that MC mortars do not provide shear capacity equivalent to PCL mortars.

Figure 2.4 Effect of mortar type on the flexural tensile strength of grouted masonry (Brown and Melander 1999).

12

However, it was also pointed out that both MC and PCL mortars provided shear

capacities that greatly exceeded the UBC (ICBO 1988) allowables (0.110 MPa and 0.094

MPa for Type S and N, respectively).

Figure 2.5 Shear strength of (a) Type N mortar panels and (b) Type S mortar panels

(Matthys 1989).

2.5 Summary

The research described in this section focused primarily on the effect of mortar type on

the properties of grouted and ungrouted masonry assemblages. The available literature

showed that the compressive strength of masonry is affected by the properties of mortar.

The primary factors that influence the sensitivity of this effect are the relative unit-joint

strength and the relative unit height to joint thickness. In addition, the literature clearly

demonstrated that MC displays lower bond strength than the corresponding PCL mortar

for ungrouted masonry. In the case of grouted masonry, research has indicated that

negligible differences between the bond strengths of MC and PCL mortar exist due to the

continuity provided by grouted columns. Finally, research has shown that assemblages

(a) (b)

13

built with MC mortar had lower diagonal tension strength than the corresponding

specimens built with PCL mortar for ungrouted clay masonry.

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3. EFFECT OF MORTAR TYPE ON THE PROPERTIES OF MASONRY WALLS

3.1 Introduction

While masonry assemblages are ideal for providing information about the interactions

between the constituent masonry materials, they have difficulty providing insight into the

response of components, such as walls and piers. This is particularly true for cases of

partially grouted masonry, since it is difficult to obtain the dimensions required to

properly represent partial grouting with assemblages (typically assemblages do not

exceed 4 ft. by 4 ft.). In addition, the behavior of assemblages may not properly simulate

component behavior due to size effects and differences in boundary conditions, among

other factors. The following sections outline the available literature that addressed the

effect of mortar type on the properties of partially grouted reinforced masonry

components under both in-plane shear and out-of-plane flexure. Due to the limited

amount of research conducted in this area, an attempt was made to ascertain the effect of

mortar type by comparing the results from different testing programs; however, the large

variation of other factors hampered this effort.

3.2 In-Plane Behavior

The only research located that specifically addressed the influence of using MC mortar

instead of PCL mortar on the in-plane response of reinforced masonry components was

reported by Johal and Anderson (1986). This study subjected 32 partially grouted

reinforced masonry piers to in-plane loads applied through diagonal compression. Both

clay (16 specimens) and concrete (16 specimens) masonry were investigated along with

various brands of masonry cement (both Type M and S). Each specimen consisted of a

square pier section (36 in. by 36 in. [914 mm by 914 mm] for clay masonry specimens

and 32 in. by 32 in. [813 mm by 813 mm] for concrete masonry specimens) and spandrel

beams along the top and bottom. All of the walls were reinforced and grouted at

approximately 24 in (610 mm) vertically and contained no horizontal reinforcement or

grout within the pier section. Figure 3.1 and 3.2 show schematics of the test specimens

and test setup, respectively.

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Figure 3.1 Schematics of (a) clay masonry pier specimens and (b) concrete masonry pier

specimens (Johal and Anderson 1988).

(b)

(a)

16

Figure 3.2 Schematics of loading system (Johal and Anderson 1988).

The specimens were loaded diagonally through the use of hydraulic rams, high strength

rods, and loading shoes (Figure 3.2). To prevent rotation of the spandrel beams during

loading, additional struts were provided between each side of the spandrel beams (Figure

3.1). As the specimens were loaded through the diagonals, the jacks located along the

struts were controlled to balance the vertical component of the diagonal force. As a result,

the pier sections were only subjected to flexure and shear forces during testing. The

loading protocol subjected the specimens to increasing cyclic shear strains (applied by

alternating the diagonal forces) until failure.

The failure mode of all piers consisted of diagonal cracking with the cracking load

ranging from 40% to 90% of the ultimate load of the pier. Initial cracking occurred

through mortar joints; however, at failure cracks also propagated through the units. Table

3.1 provides a summary of the average shear strengths, which were calculated as the

maximum shear force divided by the net shear area. In addition, representative shear

stress versus shear strain response for both clay and concrete masonry specimens are

shown in Figure 3.3.

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Table 3.1 Summary of measured shear strengths (Johal and Anderson 1988)

PCL Mortar MC Mortar Specimen Type

Mortar Type

Average Shear Strength

COV Average Shear Strength

COV

M 139 psi 0.8 % 141 psi 9 % Clay Masonry S 137 psi 8 % 122 psi 15 %

M 154 psi 5 % 150 psi 6 % Concrete Masonry S 144 psi 2 % 154 psi 2 %

Based on the results, the authors concluded that behavior, in terms of cracking strength,

ultimate strength, and failure modes, of the piers constructed with MC mortar were

comparable to those constructed with PCL mortar. In addition, the authors also pointed

out that the measured shear strengths ranged from two to four times the allowable shear

strengths given by UBC (ICBO 1988). Finally it was concluded that the UBC’s

restriction on the use of MC mortars for lateral force resisting systems was unfounded

and should be lifted.

Figure 3.3 Representative shear stress versus shear strains for (a) clay masonry

specimens and (b) concrete masonry specimens (Johal and Anderson 1988).

3.3 Out-of-Plane Behavior

Abboud et al. (1993) investigated the effect of mortar-type on the out-of-plane response

of partially grouted masonry walls. The experimental program consisted of three four-

point bending tests on vertically spanning partially grouted masonry walls. Figure 3.1

shows schematics of a typical specimen and the test setup. The three walls were

constructed of face-shell mortar bedded 6 in. 2-core hollow CMU and Type S MC mortar

(a) (b)

18

meeting the requirements of ASTM C270-82. The walls were reinforced with grade 60

steel, and grout conforming to ASTM C476-83 was placed in the cells containing

reinforcement, all other cells were left ungrouted. To assess the impact of mortar type,

the results of these tests were compared with the response of two similar walls

constructed of Type S PCL mortar and tested during Task 3.2(a) of the US-Japan Joint

Technical Committee on Masonry Research (TCCMAR) (Hamid et al. 1989).

\

The test walls were 48 in. wide and 104 in. high (see Figure 3.1a). The walls spanned

vertically and were pin-supported at the base and roller-supported along the top. The

walls were reinforced with #5 bars vertically at 24 in. on center and #3 bars horizontally

at 32 in. on center. All testing was performed in displacement control with the first wall

being loaded monotonically and the other two being subjected to TCCMAR sequential

phase cyclic loading (Porter 1987).

(a) (b)

Figure 3.1 Schematic of (a) typical test specimen and (b) test setup (Abboud et al. 1993)

19

The failure of the walls was a typical flexural mode with flexural cracks located at the

mortar-block interface of the bed-joints and spalling of the face shell in compression. The

authors note that this failure was nearly identical to the failure of the walls constructed

with PCL mortar during previous tests. Table 3.1 summarizes the results for both the

walls constructed with MC mortar and the walls constructed with PCL mortar. Figure 3.2

shows the monotonic force-displacement response and the cyclic force-displacement

response for the masonry cement walls and the portland cement/lime walls.

Table 3.1 Summary of test results (taken from Abboud et al. 1993)

Based on the results summarized in Table 3.1, it is apparent that the cracking moment

capacity and the ultimate moment capacity of the MC walls were nearly identical to those

of the PCL walls. Of particular note is the maximum flexural tensile stress (ft’) calculated

using the value of cracking moment. Although the bond strength of hollow masonry

prisms built with MC mortar has been shown to be lower than that of PCL mortar (Ghosh

1990), this values remained essentially unchanged for all walls. This suggests that the

effect of mortar type on the out-of-plane response of partially grouted walls is not as

large as reported for hollow masonry assemblages. In addition, the force-displacement

responses shown in Figure 3.2 are very similar for both types of walls in terms of

stiffness, ductility, and energy dissipation.

20

Based on these results, the authors concluded that walls constructed with MC mortar and

walls constructed with PCL mortar will behave in a similar manner when loaded out-of-

plane in terms of failure mode, cracking moment, force-displacement response, flexural

strength, and ductility.

3.4 Summary

This section outlined research that addressed the effect of mortar type on the behavior of

partially grouted masonry components subjected to in-plane and out-of-plane loading.

Regarding in-plane response, the research indicated that behavior, in terms of cracking

strength, ultimate strength, and failure modes, of the piers constructed with MC mortar

(a)

(b)

(c)

(d)

Figure 3.2 Force displacement response of (a) masonry cement mortar wall during a monotonic test, (b) portland cement/lime mortar wall during a monotonic test, (c) masonry cement mortar wall during a cyclic test, and (d) portland cement/lime mortar wall during a

cyclic test (Abboud et al. 1993).

21

were comparable to those constructed with PCL mortar. In terms of out-of-plane

response, the research suggested that the specimens constructed with MC mortar and

specimens constructed with PCL mortar behaved in a similar manner when loaded out-of-

plane in terms of failure mode, cracking moment, force-displacement response, flexural

strength, and ductility.

However, while the research results presented are consistent, they are also somewhat

limited. For example, the located studies investigated only reinforcement and grout

spacing of 24 in. As a result, it is not possible to simply extend these conclusions to cases

where the maximum reinforcement and grout spacing of 48 in. (for SDC D, E, and F

[MSJC 2005]) is used. In addition, the studies outlined employed a single test setup and

specimen configuration which does not allow these results to be generalized. For

example, the study which focused on the in-plane response of masonry piers used a single

aspect ratio and one level of vertical stress, although in-plane response of partially

grouted reinforced masonry shear walls has been shown to be sensitive to variations in

these factors (see Section 5). While this past research is extremely valuable, it is clearly

limited and, thus, a more comprehensive program is required to fully establish the impact

of MC on the response of masonry components.

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4. EFFECT OF GROUTING ON THE PROPERTIES OF MASONRY

ASSEMBLAGES

4.1 Introduction

It is well established that grouted masonry assemblages behave considerably different

than ungrouted or hollow masonry assemblages. Since the objective of this study focuses

on partially grouted masonry, it is important to understand how the behavior mechanisms

of masonry assemblages are affected by grouting in order to gain further insight into the

possible influence of using MC mortar instead of PCL mortars for partially grouted shear

walls. The following sections discuss the effect of grouting on the behavior mechanisms

of masonry assemblages subjected to compression, bed-joint shear, diagonal tension and

flexural tension.

4.2 Compressive Strength

Drysdale and Hamid (1979) studied the effect of grouting on the compressive strength of

masonry prisms. The experimental results indicated that grouted masonry prisms have

failure loads lower than those predicted using superposition to combine the capacity of

the grouted area and the capacity of the unit area (Figure 4.1). This was largely attributed

to transverse tensile stresses that develop in the unit due to wedging action of the grout as

well as the lateral expansion of the grout due to the Poisson’s ratio effect. This implies

that the compression strength of grouted masonry can be expected to be less influenced

by mortar properties than that of ungrouted masonry due to the dominance of the grout

with respect to the failure mechanism. It was also noted that fully grouted specimens

typically display more uniform behavior with less scatter than ungrouted specimens.

23

Figure 4.1 Effect of grout strength on compressive strength of masonry (Drysdale and Hamid 1979)

Using 1/3 scale 6 in. concrete masonry units (Figure 4.2a) Hamid and Chandrakeerthy

(1992) studied the effect of partial grouting on compressive strength of concrete masonry.

Specimens with grouted cells at 8 in., 16 in., 24 in. and 32 in. full-scale spacing were

used, see Figure 4.2b. This study demonstrated that ultimate compression load per unit

length of partially grouted walls increases and the variability reduces as the spacing of

grouted cells decreases, see Figure 4.2c. The authors recommended the use of

compressive strength based on gross area rather than net area since stresses are not

distributed uniformly over the net area. Based on best fit of experimental results, an

empirical equation was proposed to predict the compressive strength of partially grouted

concrete masonry, f’pg, in terms of compressive strength of fully grouted masonry, f’g,

and grouted cells spacing in inches, S (Eq. 4.1).

f’pg = f’g(1.08- 0.01S) (4.1)

Note that the validity of the above equation is limited to grout spacing not to exceed 32

in.

24

4.3 Bed-joint Shear Strength

Hamid et al. (1979) investigated the influence of grouting on the bed-joint shear strength

of masonry. Figure 4.3 gives the results generated for the shear strength of both

ungrouted and grouted masonry as a function of vertical stress. Based on these results, the

authors concluded that grouted cells significantly increase the shear strength along the

mortar bed joints due to “dowel action” of the grouted columns. Additional analysis of

the results indicated that the magnitude of this increased strength is influenced by the

tensile strength of the grout and the percent solid of the units.

(b)

(a)

(c) Figure 4.2 (a) 1/3 scale concrete masonry units, (b) 1/3 scale partially grouted specimens

for compression testing, and (c) effect of grout spacing on axial compression capacity (Hamid and Chandrakeerthy 1992)

25

Figure 4.3 Effect of grouting on bed joint shear strength of concrete masonry (Hamid et

al. 1979)

4.4 Tensile Strength

The influence of grout on the response of masonry subjected to flexural tension (normal

to bed joints), diagonal tension (45o from bed joints), and tension parallel to bed joints

was studied by Drysdale and Hamid (1982). Figure 4.4 summarizes the experimental

results and provides schematics illustrating the different loading configurations. These

results show that grouting had a significant influence on the capacities of both flexural

tension and diagonal tension, since in these cases the failure plane crosses the columns of

grout. Similar to the study on bed-joint shear capacity (Hamid et al. 1979), it was

concluded that the increase in tensile strength depends on the percent solid of the units

and the tensile strength of the grout. For the case where tension was parallel to bed joints,

negligible increase in capacity was observed, which was attributed to the failure planes

occurring in between the grouted columns.

26

Figure 4.4 Effect of Grouting on Splitting Tensile Strength of Concrete Masonry (Drysdale and Hamid 1982)

Hamid et al. (1992) employed 1/3 scale concrete masonry units to investigate the tensile

strength of partially grouted masonry. The test program subjected 15 wall elements with

grouted cells at varying spacing (from 8 in. to 32 in.) to out-of-plane bending using the

ASTM C1072 bond wrench. Figure 4.5 shows the test specimens and the test results.

These results showed that the flexural strength was reduced with larger grout spacing and

that this relationship was nonlinear. Based on best fit of the experimental results, the

following empirical equation (Eq. 4.2) was proposed to predict the flexural tensile

strength of partially grouted masonry, ftpg, in terms of the tensile strength of fully grouted

masonry, ftg , and grout spacing.

ftpg = 4.4 ftg S –0.75 (4.2)

Based on the experimental results, the authors proposed analytical expressions to express

the flexural tensile strength of partially grouted masonry, ftpg, in terms of the strength of

ungrouted, ftug, and fully grouted, ftg , masonry (Figure 4.5c). Based on this study it was

shown that the flexural strength of partially grouted masonry is highly sensitive to grout

27

spacing and that grout spacing in excess of 48 in. (1220 mm) has no significant influence

on improving flexural tensile strength. This suggests that for grout spacing greater than

1220 mm (48 in.) the effect of grout cells on flexural strength can be ignored and the wall

can be treated as ungrouted from a flexural strength standpoint.

4.5 Summary

The research described in this section addressed the effect of grouting on the properties of

masonry assemblages. Overall, the research described herein indicated that as the extent

(a) Test Specimens

Figure 4.5 Flexural tensile strength of hollow, partially and fully grouted concrete masonry (from Hamid et al. 1992)

Experimental results Average

(b) Experimental Results (c) Analytical Results

Grout Spacing (in)

Flex

ural

Ten

sile

Stre

ngth

(psi

)

Grout Spacing (in)

Rat

io f t

pg /

f tug

Lower limit – Ungrouted Masonry

28

of grouting increases, behavior becomes more uniform with a lower degree of scatter. In

terms of compression behavior, the literature established that grouted masonry prisms

have failure loads lower than those predicted using superposition due to different failure

mechanisms. In addition, past research suggested that grouted cells significantly increase

the shear strength along mortar bed joints due to “dowel action” of the grouted columns.

The tensile strength of masonry was also shown to be improved by grouting for tension

perpendicular to the bed-joints and diagonal tension. Tension parallel to the bed-joint is

not affected by grouting since the failure plane occurs between the grouted columns.

29

5. IN-PLANE BEHAVIOR OF PARTIALLY GROUTED MASONRY SHEAR

WALLS

5.1 Introduction

As mentioned in the previous section, very few studies have been identified that

addressed the effect of mortar type on the in-plane response of either partially grouted or

fully grouted reinforced masonry walls. Bridging this knowledge gap will require

additional experimental research. To develop the most efficient and effective program to

fill this need, the behavior of partially grouted masonry walls constructed of PCL mortar

must be clearly identified and understood. To that end, the following sections summarize

the available research in this area. Particular attention is paid to the behavior mechanisms

as well as key parameters that influence wall response.

5.2 Review of Past Research

Ghanem et al. (1992) investigated the effect of the distribution of vertical and horizontal

reinforcement on the in-plane response of partially reinforced masonry shears walls. The

experimental program tested three, 1/3 scale shear walls constructed of one wythe of

scaled 6 in. CMU, face-shell bedded with scaled Type S mortar joints. Scaled grout and

grade 60 #3, #4, and #5 reinforcing bars were also used in construction. The scaled walls

were approximately 3 ft by 3 ft representing a 9 ft by 9 ft prototype wall. For the three

test walls, three different distributions of reinforcement were used (Figure 5.1); however,

the horizontal and vertical reinforcement ratios were kept constant at 0.12%.

The test setup is shown in Figure 5.2. Using this configuration, the walls were subjected

to monotonic in-plane displacements and a constant vertical compressive stress of 100 psi

(6.9 MPa). The walls were loaded as cantilevers and were braced against out-of-plane

displacement.

30

Figure 5.2 Test setup (Ghanem et al. 1992)

Figure 5.1 Reinforcement configuration for (a) Wall SWA, (b) Wall SWB, and (c) Wall SWC (Ghanem et al. 1992)

(a) (b) (c)

31

The three walls tested displaced three distinctly different failure modes. Wall SWA

displayed essentially a shear failure with very little flexural deformation. Diagonal

cracking initiated at 4.8 kip of lateral load (87% of ultimate) and 0.08 in. of lateral

displacement (0.2% drift) followed by severe toe crushing at 5.5 kip of lateral load and

0.2 in. (0.5% drift) displacement. Wall SWB failed due to combined flexure/shear.

Flexural cracking was observed at a lateral load of 3 kip (50% of ultimate) and

displacement of 0.13 in. (0.3% drift). This was followed by diagonal cracking at a lateral

load of 6 kip and displacement of 0.22 in. (0.6% drift). Wall SWC failed primarily in a

flexural mode and exhibited a somewhat ductile behavior. Flexural cracking initiated at a

lateral load of 5 kip (64% of ultimate) and a displacement of 0.13 in. (0.3% drift).

Following this point the resistance of the wall continued to increase up to the peak

resistance of 7.8 kip and a displacement of 0.4 in. (1.1% drift).

Table 5.1 provides a summary of the salient results and Figure 5.3 shows the force-

displacement response of each of the three wall specimens. These figures are annotated to

show the occurrence of significant events and clearly illustrate the different post-peak

behavior for each of the reinforcement distributions investigated.

Table 5.1 Summary of test results (Ghanem et al. 1992).

Based on these results, Ghanem et al. (1992) concluded that the behavior of partially

reinforced masonry is strongly dependent on the distribution of reinforcement. As the

reinforcement is more uniformly distributed, both the strength and deformation capacity

of the walls increased. In addition, the distribution of reinforcement had a marked effect

of wall failure mode. As the reinforcement went from being concentrated in local areas

(SWA) to being more uniformly distributed (SWC), the failure modes switched from

32

shear to shear/flexure to flexure. Finally, it was concluded that in order to avoid brittle

shear failure the horizontal reinforcement should be distributed; however, to enhance

flexural strength the vertical steel should be concentrated at the ends of the wall.

Ghanem et al. (1993) investigated the effect of vertical stress on the in-plane response of

partially reinforced masonry walls. This investigation was essentially an extension of the

above research program reported by Ghanem et al. (1992) and utilized the same

materials, test setup (Figure 5.2), and monotonic loading protocol. The experimental

program subjected three identical masonry shear walls to lateral displacements under

different levels of axial load: Wall SWA-1 was test with no axial load; Wall SWA-2 was

tested with 100 psi of compressive stress; and Wall SWA-3 was test with 200 psi of

(a) (b)

(c)

Figure 5.3 Monotonic force-displacement response of (a) Wall SWA, (b) Wall SWB, and (c) Wall SWC (Ghanem et al. 1992).

33

compressive axial stress. Each wall had vertical and horizontal reinforcement ratios of

0.12% located in the same configuration of Wall SWB shown in Figure 5.1.

The three walls tested displayed three distinctly different failure modes. Wall SWA-1 (no

axial load) displayed a flexure failure with some sliding along the base due to the lack of

vertical stress. Flexural cracking initiated at 1.8 kip of lateral load (42% of ultimate) and

0.04 in. of lateral displacement (0.1% drift) followed by diagonal cracking below the

center bond beam. The ultimate load was 4.3 kip which was achieved at a lateral

displacement of 0.25 in. (0.7% drift). Wall SWA-2 (100 psi of axial stress) displayed a

combined flexure/shear failure. Initial flexural cracking began at the lateral load of 2.8

kip (48% of ultimate) and a displacement of 0.07 in. (0.2% drift). As the displacement

increased, diagonal cracking initiated and propagated significantly at a lateral load of 5.8

kip and lateral displacement of 0.28 in. (0.8% drift). Wall SWA-3 (axial stress of 200 psi)

displayed a shear failure. At a lateral load of 6.8 kip (87% of ultimate) and displacement

of 0.12 in. (0.3% drift), a diagonal crack formed and propagated as the load increased to

7.8 kip. The wall’s post-peak behavior was reasonably stable due to the high level of

vertical stress; however, as the displacement reach 0.25 in. (0.7% drift) the resistance of

the wall diminished rapidly.

Table 5.2 provides a summary of the salient results and Figure 5.4 shows the force-

displacement response of each of the wall specimens. This figure clearly shows the

significant effect of the level of axial load in increasing wall post-cracking stiffness and

lateral load resistance. Specifically, increases in lateral resistance of 35% and 79% were

obtained by applying 100 psi and 200 psi of axial stress, respectively. Figure 5.5

illustrates the effect of vertical stress on both cracking strength and wall ductility.

34

Table 5.2 Summary of test results (taken from Ghanem et al. 1993).

Figure 5.4 Monotonic force-displacement response of partially grouted shear walls with

different levels of axial stress (Ghanem et al. 1993).

Figure 5.5 Effect of vertical stress on (a) cracking strength and (b) ductility of partially grouted shear walls (Ghanem et al. 1993).

(a) (b)

35

Based on these results Ghanem et al. (1993) concluded that the behavior of partially

reinforced masonry is strongly dependent on the level of axial stress. As the axial stress

increases, the lateral resistance of masonry shear walls increases; however, the

deformation capacity (wall displacement ductility) decreases. The level of axial stress

also has a significant influence on wall failure mode. As the axial compressive stress

went from 0 psi to 100 psi to 200 psi, the wall failure modes went from flexure to

flexure/shear to shear. Based on this trend, the authors concluded that in order to avoid

brittle failures the level of axial stress should be kept below 5% of the masonry

compressive strength.

Fattal (1993a) evaluated the capability of an analytical expression proposed by

Matsumura (1986) (Eq. 5.1) to predict the lateral strength of partially grouted masonry

shear walls that exhibit a shear failure mode. To evaluate this expression, the results of 72

past tests of partially grouted shear walls of both concrete block and brick masonry were

assembled and used for comparison. Specifically, 51 tests reported by Matsumura (1986)

(denoted Set M), 11 tests reported by Chen et al. (1978) (denoted Set B), and 10 tests

reported by Yancey and Scribner (1989) (denoted Set N) were used. Figure 5.6 shows

schematics of the two test setups (denoted wall-type and beam-type) employed in these

investigations. All walls were subjected to quasi-static cyclic displacements of increasing

magnitude. The results of walls which failed in flexure were neglected in this study.

qsmp VVVV ++= (5.1)

( ) ( )( ) ( )( ) ⎟⎠⎞

⎜⎝⎛

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+

+=

Ldfk

rV mtuve

dm

5.0'3.004.4012.07.0

76.0 ρ

( ) ( )( )( )[ ] ⎟⎠⎞

⎜⎝⎛⋅=

LdffV mtyhhs

5.0'5.0157.0 δγρ

( )[ ] ⎟⎠⎞

⎜⎝⎛=

LdqVq 175.0

where,

Vp = predicted ultimate lateral shear strength (MPa)

Vm = shear strength attributed to masonry and vertical reinforcement (MPa)

36

Vs = shear capacity attributed to horizontal reinforcement (MPa)

Vq = shear strength attributed to axial load (MPa)

rd = aspect ratio (H/d) based on effective depth, d

ρve = ratio of vertical reinforcement in one end cell

f’mt = compressive strength of masonry prisms (MPa)

d = effective depth (mm)

L = length of wall (mm)

ρh = horizontal reinforcement ratio

fyk = yield strength of horizontal reinforcement

q = nominal axial stress (MPa)

ku = 1.0 for fully-grouted concrete and brick masonry and partially grouted beam-type

brick masonry

ku = 0.8 for partially-grouted wall-type brick masonry and partially-grouted beam-type

concrete masonry

ku = 0.64 for partially-grouted wall-type concrete masonry

γ = 1.0 for fully-grouted concrete and brick masonry and partially-grouted brick

masonry

γ = 0.6 for partially grouted concrete masonry

δ = 1.0 for fixed-fixed boundary conditions

δ = 0.6 for cantilever boundary conditions

Based on data manipulation and regression analyses, Fattal highlighted several

deficiencies in Eq. 5.1. First, for walls without vertical reinforcement, Eq. 5.1 provides

poor strength estimates. This was attributed to the relative weights given to the Vm and Vs

terms. Second, Eq. 5.1 cannot accurately predict the shear strength of unreinforced

masonry walls since when Vm and Vs are zero, the lateral strength is provided exclusively

by a linear function of vertical stress. Finally, the analysis of data also pointed to

deficiencies in the functional forms of r, ρve, and ρh.

37

To improve the model proposed by Matsumura (1986), the functional forms of each

component of shear resistance was modified, based on engineering judgment, to more

closely match the experimental data sets. The resulting expression is given in Eq. 5.2. Of

particular interest in Eq. 5.2, is the modifier ko which is taken as 0.8 for partially-grouted

(a) Wall-type loading

(b) Beam-type loading

Figure 5.6 Typical test setups used to determine the lateral strength of masonry shear walls (Fattal 1993a).

38

masonry and 1.0 for fully-grouted masonry. These values were selected to minimize the

standard deviation of the error function and suggest that a uniform decrease in shear

capacity can be expected for partially-grouted compared to fully-grouted walls.

qsmp VVVV ++= (5.2)

( ) ( ) ( ) ( ) 7.05.05.0'18.08.0

5.0vyvmt

duom ff

rkkV ρ⎟⎟

⎠

⎞⎜⎜⎝

⎛+

+=

( )( )( )( )( ) 31.0011.0 hyhos fkV ρδγ=

( )( ) ( )( )qfkV mtoq 2.0012.0 ' +=

where,

fyv = yield strength of vertical reinforcement

ρv = vertical reinforcement ratio

ko = 1.0 for fully-grouted masonry

ko = 0.8 for partially-grouted masonry

Based on this research, the author concluded that Eq. 5.2 more closely approximated the

experimental data set than Eq. 5.1. The predictions obtained by Eq. 5.1 varied from 23%

to 180% of the measured value, and less than half of the experimental results fell within

+/-20% of the predicted value. In contrast, the predictions of Eq. 5.2 varied between 41%

and 143% of the measured value, and 68% of the experimental results fell within +/-20%

of the predicted value. Finally, it was stated that Eq. 5.2 should be viewed as a first step

and that the functional forms represented in this expression must be tested for consistency

against new data sets.

In a parallel study, Fattal (1993b) investigated the influence of critical parameters on the

response of partially grouted masonry shear walls. The critical parameters studied were

horizontal reinforcement ratio (ρh), vertical reinforcement ratio (ρv), aspect ratio (r), axial

load (q), and masonry compression strength (fmt’). Figure 5.7 shows a schematic

illustrating the six behavior events used for comparison purposes. The approach taken

39

used both the data set and strength expression (Eq. 5.2) developed by Fattal (1993a) to

establish the desired trends.

The limitations of the test data notwithstanding, the following conclusions about the

influence of the identified critical parameters were drawn:

• Within the range of 0%-0.2% horizontal reinforcement, the ultimate strength

and ultimate deformation of partially grouted masonry walls increases with an

increase in ρh. Above this level of reinforcement, ρh had little influence on

either of these properties. Although to a lesser degree, this conclusion also

applies to the cracking deformation and cracking strength.

• Above 2.0, the aspect ratio has a negligible effect on the strength of masonry

walls. As the aspect ratio is reduced, both the ultimate strength and cracking

strength of masonry walls increased. This increase was most pronounced for

aspect ratios in the range of 0.75-1.5. For the range of aspect ratios investigated

(0.75–3.0) no correlation between deformation and aspect ratio was observed.

• For the range of axial stress included in the study (0-260 psi), the lateral

strength of partially grouted masonry shear walls increased linearly within

increasing vertical stress. This trend was also reported for ungrouted,

unreinforced masonry walls; however, the author points out that partially

grouted masonry walls are approximately half as sensitive to axial stress.

40

Although the author also employs Eq. 5.2 to establish trends for ranges of the critical

parameters outside those provided by experimental data, these results are not included

here. Given the empirical nature of Eq. 5.2, attempting to garner information about shear

wall response outside the scope of the “training data” is questionable.

Given the conclusions about the impact of critical parameters on the response of partially

grouted masonry walls, the author developed recommendations for future research. The

experimental portion of the recommended research consists of a series of reversed cyclic

shear wall tests in which the critical parameters identified are varied. Table 5.3

Figure 5.7 Schematic illustrating six major events in the response of a masonry

shear wall (Fattal 1993b)

41

summarizes the recommended test program. The author also points out the need for

analytical research focusing on both simple formulations as well as linear finite element

analysis.

Table 5.3 Fattal’s (1993b) proposed test program.

Critical Parameter Recommended Values

Horizontal reinforcement ratio Four different ratios should be used in the range of 0-

0.2%, with one being the default value of 0%.

Aspect ratio Three different aspect ratios in the range of 0.5-2.0

should be investigated

Axial Stress Two levels of axial stress corresponding to the

default value of 0 psi and a typical axial stress in

bearing wall buildings should be included

Masonry compressive strength Two values

To study the seismic performance of partially grouted masonry walls, Schultz (1996)

tested six concrete masonry shear walls under in-plane loads. The wall specimens were

constructed of 8 in., two-cell CMU conforming to ASTM C90 and a Type S mortar

meeting the requirements of ASTM C270. Except for the outside edges, all masonry was

face-shell bedded. The test walls were composed of seven courses giving a height of 56

in. and had lengths of 56 in., 80 in., and 112 in. to allow aspect ratios (H/L) of 1.0, 0.7,

and 0.5 to be investigated (Figure 5.8). All six walls had two #6 bars (grade 60) as

vertical reinforcement in each exterior vertical cell; all other cells remained ungrouted.

Two levels of horizontal reinforcement corresponding to 0.05% and 0.12% (based on

gross area) were investigated. All horizontal reinforcement was placed in the bond beam

located at mid-height of the wall. A test matrix summarizing the experimental program is

shown in Table 5.4.

The specimens were subjected to quasi-static cyclic lateral displacements modeled after

the TCCMAR phased-sequential displacement procedure. The walls were loaded with

fixed-fixed boundary conditions and were subjected to near constant vertical compressive

42

stress. However, since the testing fixture was exclusively displacement-controlled, the

vertical stresses did vary during testing with a maximum coefficient of variation of 5.2%.

Table 5.4 Schedule for shear wall specimens with bond beams (Schultz 1996)

Figure 5.8 Test setup for partially-grouted masonry walls (Schultz 1996)

For all walls except Wall 3, vertical cracks initiated at the top of the wall along the

interface between the grouted vertical cell and the ungrouted masonry on the “leading

jamb” (e.g. for loading in the east direction these cracks form adjacent to the east grouted

cell). As the cyclic displacements increased, these cracks opened in excess of 0.25 in. and

propagated downward to the horizontal bond beam. As the cracks reached the bond

beam, the anchorage of the horizontal reinforcement was “disturbed” and thus its

effectiveness was limited. In addition, bed-joint sliding deformation was observed along

43

the top and bottom of the masonry panels. The failure mechanism of Wall 3 differed

considerably from the other walls and consisted of distributed cracking much like the

response of reinforced fully grouted masonry. According to the author, this discrepancy is

likely caused by differences in the actual anchorage conditions of the vertical

reinforcement bars. Whatever the cause, this anomaly clearly illustrated that the cracking

pattern exerts significant influence over the response of the wall.

All walls displayed fairly stable, well-behaved force-displacement responses (see Figure

5.9). Wall response was initially linear followed by gradual stiffness/strength degradation

and moderate pinching as the peak amplitude of the drift levels increased. As illustrated

in Figure 5.9, both degradation and pinching increased with increasing aspect ratio.

Figure 5.10 shows envelopes of the force-displacement response for each wall smoothed

by an eleven-point moving average. Based on these figures, the author concluded that

shear walls displayed essentially bi-linear behavior similar to a yielding system although

no widespread yielding was observed.

Figure 5.9 Force-displacement response for (a) Wall 7 (H/L=0.5, ρh = 0.05%) and (b)

Wall 11 (H/L =1.0, ρh = 0.12%) (Schultz 1996)

Further analyses of the measured responses for each wall are given in Tables 5.5 and 5.6.

Table 5.5 shows a comparison between the “inferred” and calculated (based on elastic

(a) (b)

44

mechanics) stiffness as well as a normalized measure of energy dissipation for each

specimen. Table 5.6 gives a comparison between the measured and calculated (based on

Fattal 1993b) strength and displacement capacity (taken at 75% peak resistance) for each

wall.

Table 5.5 Initial stiffness and normalized energy dissipation (taken from Schultz 1996).

Figure 5.10 Smoothed force-displacement envelopes for (a) walls with ρh = 0.05% and

(b) walls with ρh = 0.12% (taken from Schultz 1996).

(a) (b)

45

Table 5.6 Summary of strength and deformation estimates (taken from Schultz 1996).

Based on these results Schultz (1996) concluded the following:

• The lateral resisting mechanism for reinforced partially grouted masonry is

vastly different than reinforced masonry

• As the aspect ratio increases, the ultimate shear stress increases but the

toughness decreases

• Increasing the level of horizontal reinforcement will slightly increase the

ultimate shear stress but has a negligible effect on toughness.

It should be pointed out that when interpreting the first conclusion drawn, caution should

be exercised. Consider that the test specimens contained unconfined regions of ungrouted

masonry, which is not common or advisable for partially grouted masonry structures. In

addition, this configuration likely caused the failure modes observed, which in turn fueled

the conclusion regarding behavior mechanisms.

5.3 Summary

The literature summarized has identified three principal variables that exert substantial

influence over the response of reinforced partially grouted masonry walls: vertical stress,

aspect ratio, and the amount and distribution of vertical and horizontal steel

reinforcement. The following conclusions summarize the findings:

• For the range of axial stress included in the studies outlined (0-260 psi), the

lateral strength of partially grouted masonry shear walls increased linearly and

46

the displacement ductility decreased within increasing vertical stress. In

addition, the level of vertical stress had a marked effect on failure mode. As the

level of vertical stress increased wall failures were altered from flexure to

flexure/shear to shear.

• As the aspect ratio increased, the ultimate shear strength and cracking strength

of masonry walls decreased. However, above an aspect ratio 2.0, there was a

negligible effect on the strength of masonry walls. For the range of aspect ratios

investigated in the studies outlined (0.75–3.0) no correlation between

deformation and aspect ratio was observed.

• For horizontal reinforcement ratios in the range of 0%-0.2%, the ultimate

strength and ultimate deformation of partially grouted masonry walls increased

with an increase in ρh. In addition, for a constant reinforcement ratio, the

distribution of the reinforcement had a significant effect on wall failure mode

with failures progressing from shear to shear/flexure to flexure as the

reinforcement becomes more distributed.

Although none of the investigations identified examined the influence of masonry cement

mortar on the response of partially grouted masonry shear walls, none of the

investigations suggested that mortar type had any influence on wall response. Of

particular note are the studies that focused on manipulation of test data from numerous

studies. The models developed through these studies did not identify mortar properties or

mortar unit bond as factors affecting response. While this line of reasoning may not be

scientifically valid, it does at least suggest that mortar type has a negligible influence on

the response of partially grouted masonry shear walls.

47

6. CONCLUSIONS Based on the review of available literature presented in this report, the following

conclusions are drawn:

(1) The impact of mortar physical properties on compressive strength of hollow

concrete masonry is much less than that for clay brick masonry. This is

attributed to two main reasons: the low joint thickness to unit height and the

comparable unit to mortar compressive strength.

(2) For ungrouted masonry, MC mortars tend to display a lower bond strength and

a lower assemblage diagonal tension strength than the corresponding PCL

mortars. However, for fully grouted masonry, the diminished mortar bond

strength becomes negligible due to the influence of continuous grouted

columns.

(3) For masonry piers with reinforcement and grout at approximately 24 in. (610

mm) on center and aspect ratios of 1.0, negligible difference in cracking

strength, ultimate strength, and failure modes were observed between walls

constructed with MC mortars and those constructed with PCL mortars under

diagonal compression loading.

(4) For a grout and reinforcement spacing of 24 in. (610 mm), walls constructed

with MC mortar and walls constructed with PCL mortar behave in a similar

manner when loaded out-of-plane in terms of failure mode, cracking moment,

force-displacement response, flexural strength, and ductility. The conclusion

regarding cracking moment was also supported by bond wrench tests on

prisms.

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(5) Grouting the cells of masonry units provides continuity across the weak bed

joints, reduces variability, and significantly diminishes the impact of mortar

joint properties on strength and deformation capacity of grouted masonry.

(6) The extent of grouting affects the strength of partially grouted masonry in a

nonlinear manner. That is, as the grout spacing is increased from 406 mm (16

in.) to 813 mm (32 in.) the contribution of grout does not diminish by 50%.

Based on the above findings, the authors find little evidence to support the contention that

the use of MC mortar instead of PCL mortar will have a detrimental effect on the strength

and deformation capacity of grouted and partially grouted (up to a spacing of 48 in. [1220

mm]) reinforced masonry shear walls. This conclusion is largely based on the

extrapolation of assemblage data that suggests a diminished dependence on mortar

properties when grout is introduced into the system. While this conclusion is strongly

supported by a component study of reinforced partially grouted piers, several factors

make the generalization of this study questionable including: the exclusion of influential

variables such as aspect ratio and vertical stress, the highly idealized loading condition,

and the limited grout spacing of 24 in (610 mm). (which is smaller than the maximum

allowed for structures in SDC D, E, and F). As a result, it is also recommended that a

series of shear wall tests be carried out to supplement past assemblage tests and validate

this conclusion in terms of shear wall response. To address this issue in a comprehensive

manner, the study should take into account critical parameters, in addition to mortar type,

such as wall aspect ratio, extent of grouting, level of vertical compressive stress, and the

amount and distribution of vertical and horizontal reinforcement.

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7. REFERENCES

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Drysdale, R. and Hamid, A. (1982) "In-Plane Tensile Strength of Block Masonry," Journal of the Canadian Society of Civil Engineering, Proc. Vol. 9, No. 3, pp.413-421. Drysdale, R., and Hamid, A. (1984) "Effect of Grouting on the Flexural Tensile Strength of Concrete Block Masonry," The Masonry Society J. Proc., Vol. 3, No. 2, July-Dec., pp. T10-T19. Drysdale, R.G., A.A. Hamid, and L.R. Baker (1999) “Masonry structures behavior and design,” The Masonry Society, 2nd Edition, Boulder, CO. Ghanem, G. M., A. S. Essawy, and A. A. Hamid (1992) “Effect of steel distribution on the behavior of partially reinforced masonry shear walls,” Proceedings of the 6th Canadian Masonry Symposium, University of Saskatchewan, Saskatoon, Canada. Ghanem, G. M., A. E. Salama, S. A. Elmagd, and A. A. Hamid (1993) “Effect of axial compression on the behavior of partially reinforced masonry shear walls,” Proceedings of the 6th North American Masonry Conference, Philadelphia, PA. Ghosh, S. K. (1990) “ Flexural Bond Strength of Masonry: An Experimental Review”, in Proceedings of the Fifth North American Masonry Conference, University of Illinios at Urbana-Champaign.

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Fattal, S. G. (1993a) “Strength of partially-grouted masonry shear walls under lateral loads,” NISTIR 93-5147, National Institute of Standards and Technology, Gaithersburg, MD. Fattal, S. G. (1993b) “The effect of critical parameters on the behavior of partially-grouted masonry shear walls under lateral loads,” NISTIR 93-5147, National Institute of Standards and Technology, Gaithersburg, MD. Hamid, A., Drysdale, R., and Heidebrecht, A. (1979) "Shear Strength of Concrete Masonry Joints," Journal of Structural Engineering, ASCE, Vol. 105, ST7, pp. 1227-1240.

Hamid, A. A., B. E. Abboud, M. W. Farah, M. K. Hatem, and H. G. Harris (1989) “Response of reinforced block masonry walls to out-of-plane static loads,” US-Japan Coordinated Program for Masonry Building Research, Report No. 3.2(a), 97 pp. Hamid, A., Chaderakeerthy, S. and Elnawawy, O. (1992) "Flexural Tensile Strength of Partially Grouted Concrete Masonry", Journal of Structural Engineering, ASCE, Vol.118, No. 12, Dec.1992

Hamid, A., Chandrakeerthy, S. (1992) "Compressive Strength of Partially Grouted Concrete Masonry Using Small Scale Wall Elements," TMS Journal, Vol. 11, No.1 Hedstrom, E.G., Tarhini, K.M., Thomas, R.D., Dubovoy, V.S., Klingner, R.E., and Cook, R.A. (1991) “Flexural bond strength of concrete masonry prisms using portland cement and hydrated lime mortars,” The Masonry Society Journal, Vol. 9, No. 2, pp.8-23. International Conference of Building Officials (1988) “Uniform Building Code,” Whittier CA. International Conference of Building Officials (1991) “Uniform Building Code,” Whittier CA. Johal, L.S.P. and Anderson, E.D. (1988) “Shear strength of masonry piers under cyclic loading,” Masonry: Materials, Design, Construction, and Maintenance, ASTM STP 992. Philadelphia, PA. pp. 18-32. Masonry Standards Joint Committee (2005) “Building code requirements for masonry structures,” ACI 530/ASCE 5, TMS 402, American Concrete Institute, American Society of Civil Engineers, and The Masonry Society, Detroit, New York, Boulder. Matsumura, A. (1986) “Shear strength of reinforced hollow unit masonry walls,” Second Meeting of the U.S.-Japan Joint Technical Coordinating Committee on Masonry Research, Keystone, CO.

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