NBS

PUBLICATIONS

NBSIR 86-3467

A111D5 L313QE

NATL INST OF STANDARDS & TECH R.I.C.

All 102631 302

Gaynor, Patricia/Influence of mortar bed QC100 .1156 NO.86-3467 1987 V19 C.1 NBS-P

;e of Mortar Bedding on

y Prism Behavior

Patricia Gaynor

Kyle Woodward

Charles Scribner

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

February 1987

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

-QC

100

.1156

86-3467

1987

c.2

NBSIR 86-3467

INFLUENCE OF MORTAR BEDDING ON

MASONRY PRISM BEHAVIOR

nbs

RESEARCH

INFORMATION CENTER

Qcioo

Patricia Gaynor

Kyle Woodward

Charles Scribner

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

February 1987

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director
.

•;

At v 
ABSTRACT

The results from compression tests of seventy ungrouted, hollow

concrete block masonry prisms are presented. The prisms are

three-unit high, stack-bonded assemblages. The parameters varied

in the investigation include block strength, mortar type, and

mortar bedding type (area) . The resulting data include the

compressive strengths of the prisms and strains measured ac

various locations on each prism. Major observations of prism

behavior are that mortar type has a negligible influence on prism

behavior, block strength affects masonry prism strength in

proportion to its own variation, and mortar bedding type

significantly affects masonry prism strength, variability of

strength, strain distributions, and mode of failure. It is shown

that disturbed strain fields occur in faceshell-bedded prisms and

that the variability of data obtained from tests of faceshellbedded prisms is high. For these reasons, it is recommended that

fully-bedded prisms be used as quality control samples for

masonry construction.

11

TABLE OF CONTENTS

Page

ABSTRACT ii

LIST OF TABLES iv

LIST OF FIGURES v

1 . INTRODUCTION . 1

2 . TEST SPECIMENS . 2

2 . 1 MATERIALS . . 2

2 . 2 PRISM DETAILS AND TESTING . . 2

3 . MASONRY PRISM STRENGTH ANALYSIS . . 12

3 . 1 MODES OF FAILURE 12

3 . 2 EFFECT OF BEDDING TYPE ........... . . 16

3 .

EFFECT OF MORTAR STRENGTH ......... ....... 19

3.4 EFFECT OF BLOCK STRENGTH 19

4. PRISM STRAIN DISTRIBUTION 21

4 . 1 UNIFORMITY OF STRAIN 21

4.1.1 Full-Area-Bedded Prisms . 21

4.1.2 Faceshell-Bedded Prisms 21

4.2 MODE OF FAILURE AND WEB STRAINS 2 6

4.2.1 Faceshell-Bedded Prisms 26

4.2.2 Full-Area-Bedded Prisms 33

4.2.3 Influence of Mortar Bedding 33

4.3 STRAIN REDISTRIBUTION DUE TO WEB SPLITTING ... 36

5. IMPLICATIONS OF RESULTS 3 8

5.1 COMPRESSIVE STRENGTH AND ITS VARIABILITY 38

5.1.1 Compressive Strength 38

5.1.2 Variability of Compressive Strength ... 39

5.2 STRAIN DISTRIBUTION 41

5.3 RECOMMENDATION 41

6 . SUMMARY AND CONCLUSIONS 4 3 6.1 SUMMARY 43

6.2 CONCLUSIONS 4 3

7 . ACKNOWLEDGMENTS . 44

8 . REFERENCES . _ 45

APPENDICES

A. FINITE ELEMENT MODEL 46

A . 1 MODEL 4 6

A. 2 ANALYSIS 4 6

A. 2.1 Full-Area-Bedded Prisms 46

A. 2. 2 Faceshell-Bedded Prisms 50

B. PRISM UNIAXIAL STRESS-STRAIN CURVES 53

iii
LIST OF TABLES

Page

Table 2.1 Details of Masonry Units 4 Table 2.2 Mortar 28-day Compressive Strengths

Table 3.1 Faceshell-Bedded Masonry Prism Compressive

Strength 13

Table 3.2 Full-Area-Bedded Masonry Prism Compressive

Strength 14

Table 3.3 Ratio of Average Compressive Strengths

Faceshell to Full-Area Bedding 18

Table 3.4 Average Compressive Strengths

by Group 18

Table 3.5 Coefficient of Variation of Compressive

Strengths 2 0

Table 3.6 Ratio of Average Compressive Strengths

Low to High Strength Mortar 2 0

Table 3.7 Ratio of Average Compressive Strengths

Low to High Strength Block 2 0

Table 5.1 Average Compressive Strength Based on

Mortared Area 4 0

IV
LIST OF FIGURES

Page

Figure 2 . 1 Concrete block . Figure 2.2 Concrete block prism

Figure 2 . 3 Mortar bedding types

Figure 2 . 4 LVDT instrumentation 8

Figure 2 . 5 Leaf spring transducer instrumentation .... 9

Figure 2.6 Locations of LST's and LVDT's 10

Figure 3.1 Failure of a full-area-bedded prism 15

Figure 3 . 2 Failure of a faceshell-bedded prism 17

Figure 4.1 Web vertical strain by block for full-areabedded prisms 22

Figure 4.2 Web lateral strain by block for full-areabedded prisms 23

Figure 4.3 Comparison of vertical strains - faceshell

to web for full-area-bedded prisms 2 4

Figure 4.4 Comparison of faceshell vertical strain by

block for full-area-bedded prisms ......... 25

Figure 4 . 5 Comparison of web vertical strain by block

for faceshell-bedded prisms 27

Figure 4.6 Influence of web stiffness on vertical

strain distribution for faceshell-bedded

prisms 2 8

Figure 4 . 7 Comparison of faceshell vertical strains

by block for faceshell-bedded prisms 29

Figure 4.8 Stress trajectories in webs of a faceshellbedded prism 31

Figure 4.9 Support conditions for webs of faceshellbedded prisms 3 2

Figure 4.10 Web lateral strains by block for faceshellbedded prisms 3 4

Figure 4.11 Stress trajectories in webs of a full-areabedded prism . 3 5

Figure 4 . 12 Strain redistribution after web cracking

in faceshell-bedded prisms 37

Figure A.l Finite element mesh 47

Figure A. 2 Finite element prism dimensions (inches) .. 48

Figure A. 3 Loading on finite element prism 49

Figure A. 4 Stress trajectories in faceshell of a

full-area-bedded prism 51

Figure A. 5 Stress trajectories in faceshell of a

faceshell-bedded prism 52

v
LIST OF FIGURES (continued)

Page

Figure B. 1 Stress-strain curves for prisms with

strength mortar, low-strength block,

faceshell bedding

lowand

54

Figure B. 2 Stress-strain curves for prisms with

strength mortar, low-strength block,

full-area bedding

lowand

55

Figure B. 3 Stress-strain curves for prisms with

strength mortar, low-strength block,

faceshell bedding

highand

56

Figure B.4 Stress-strain curves for prisms with

strength mortar, low-strength block,

full-area bedding

highland

57

Figure B . 5 Stress-strain curves for prisms with

strength mortar, high-strength block,

faceshell bedding

lowand

58

Figure B.6 Stress-strain curves for prisms with

strength mortar, high-strength block,

faceshell bedding

highland

59

Figure B. 7 Stress-strain curves for prisms with

strength mortar, high-strength block,

full-area bedding

highland

60

vi
1 . INTRODUCTION

Results from uniaxial compression tests of hollow concrete block

masonry prisms were investigated for this report. The prisms were

constructed as companion specimens to wall panels and were tested

to obtain a measure of the uniaxial compressive strength of the

wall panels. The wall panels were the primary test specimens

under study in the overall research program and, consequently,

the prisms were not originally built with the intention of

conducting a research study on their behavior. However, the

instrumentation that was to be used on the wall panels was placed

on the prisms to verify its operation and as a result, an

extensive amount of data on the strain distributions in masonry

prisms were obtained. The analysis of the data and an examination of conditions at failure produced a comprehensive description of the behavior of these masonry prisms in uniaxial

compression. The analysis of the experimental data was aided by

a simple but effective finite element analysis.

Compression testing of masonry prisms is typically governed by

ASTM Standard E447[l], Such tests yield a measure of the

compressive strength of a masonry assemblage, typically expressed

as a stress. One use of compression testing of prisms is to

assure that the masonry materials used to build a structure

achieve a specified design strength. The compressive strength of

masonry is used in the design process to calculate a number of

different allowable stresses in structural members constructed of

masonry. Allowable compressive and flexural stresses, allowable

bearing stress, allowable shearing stress in masonry, and the

moduli of elasticity and rigidity are typically expressed as

functions of the compressive strength of masonry prisms, as in

ACI 531-79, Building Code Requirements for Concrete Masonry

Structures [2 ]

In the experimental study reported herein, seventy prisms were

constructed, instrumented and tested. Mortar strength, block

strength and type of mortar bedding were the varied parameters.

There were two types of mortar, two block unit strengths, and two

types of mortar bedding. The principal data obtained from the

tests were histories of the applied load and the resulting

displacements (strains) at various locations on the prisms'

faceshells and webs. In addition, crack patterns and failure

modes were recorded.

Chapter 2 details the materials, construction and testing

procedures. Experimental conditions at failure are described in

Chapter 3. Chapter 4 presents a description of strain

distributions based on finite element analyses and experimental

results. Implications of the data are discussed in Chapter 5. A

brief overview of the finite element analyses is presented in

Appendix A. The uniaxial stress-strain curves for all of the

prisms are shown in Appendix B.

1
2 . TEST SPECIMENS

2 . 1 MATERIALS

All materials used in constructing the prisms were commercially

available and were representative of those commonly used in

masonry construction. The masonry units were hollow, two-core

concrete blocks, as shown in figure 2.1 and described in Table

2.1. The thickness of the faceshells and the webs of the units

varied along the height of the units. The concrete blocks were

from one of two lots. One lot contained blocks having a grossarea unit compressive strength of 1813 psi and the other lot

contained blocks with a gross-area unit compressive strength of

13 04 psi. For the purposes of this report, the blocks that had

the strength of 1813 psi were termed "high" strength while the

blocks that had a strength of 1304 psi were termed "low"

strength. These terms were used as descriptions of relative

strengths and were not intended to be a classification of the

absolute strengths of the blocks.

The mortar used in constructing the prisms was proportioned as

either Type S, which was termed "high" strength, or Type N, which

was designated "low" strength. Both mortars were proportioned in

accordance with ASTM C270[3], with the Type S mortar having a ratio of cement: lime: sand of 1:3/8: 4 by volume. Type N mortar

was proportioned as 1:1:5 of cement: lime: sand. One wall, three

prisms, and six mortar cubes were made from each batch of

mortar. Mortar cubes were air cured to match the curing

conditions of the prisms. After 28 days of curing, three cubes

from each mortar batch were tested and their compressive

strengths were averaged to obtain a measure of the mortar's

compressive strength. The average measured strengths of mortars

are listed in Table 2.2. The overall compressive strength was

2571 psi for the high-strength mortar and 172 6 psi for the lowstrength mortar. These mortars were batched and tested over a period of eight months.

2.2 PRISM DETAILS AND TESTING

One use of compression testing of prisms is to assure that the

specified design strength of masonry is achieved by the masonry

materials used to build a structure. When used to this end,

prisms are normally constructed with the same materials and

workmanship as those in the structure. Workmanship includes

mortar bedding type and mortar joint tooling. However, prisms

are typically laid in stack bond regardless of the bond pattern

used in the structure. The usual practice is to cap prisms fully

on both bearing surfaces. The strength of prisms constructed of

hollow blocks is calculated by dividing the maximum load

sustained by a prism by the net cross-sectional area of the

blocks

.

2
WEBS

Figure 2.1 Concrete block

3
Table 2.1 Details of Masonry Units

Low-Strength

Unit

High-Strength

Unit

Width (inches) 7.65 7.63

Height (inches) 7.56 7.59

Length (inches) 15.66 15.62

Minimum Faceshell

Thickness (inches) 1.30 1.30

Gross Area (sq. in.) 119.8 119 .

Net Cross-sectional

Area (sq. in.) 60.4 61.5

Gross Area Compressive

Strength (psi) 1304 1813

Net Area Compressive

Strength (psi) 2586 3514

Density (Ib/cu. ft.) 98.1 102.4

Absorption (Ib/cu. ft.) 14 .

0

10.8

Measurements were made in accordance with ASTM C140[4] and

represent the average for six high-strength units and nine lowstrength units.
Table 2.2 Mortar 28-day Compressive Strengths

Hiah-Strenath Mortar Low-Strength Mortar

Prism

Identifier

Compressive (psi)* Strength * Prism

Identifier

Compressive

Strength (psi)

3 - 1 1825 5 - 1 1826

3 - 2 2139 5 - 2 1809

3 - 3 2095 5 - 3 1761

3 - 4 2237 5 - 4 1490

3 5 2055 6 - 6 1841

3 - 6 1847 6 - 7 1987

4 - 1 3254 6 - 8 1591

4 - 2 2746 6 - 9 1505

4 - 3 3067

4 - 4 2425

6 - 1 2646

6 - 2 2657

6 - 3 2772

6 - 4 3127

6 - 10 2985

6 - 11 2892

6 - 12 2700

6 13 2810

Average Stress3 2571 psi Average Stress= 1726

Std. Deviation= 444 psi Std. Deviation= 178

Coefficient of Coefficient of

Variation3 17.3% Variation3 10.3%

psi

psi

* Each strength entry is the average of the compressive strength

of three mortar cubes.

5
Prisms were constructed by stack bonding three stretcher units

(fig. 2.2). Blocks were placed with the thicker cross-section

towards the top. This convention made mortaring easier and was

consistent with typical masonry construction practice. Mortar

bedding was either faceshell or full-area bedding (fig. 2.3).

Mortar joints were struck flush with a trowel, but were not

tooled. The mason used a four-foot level to maintain the level

of each block and to plumb the prism. Prisms were cured in air

for at least 28 days and then fully capped. Prisms were from 90

to 185 days old when tested.

Prisms were tested in a uniaxial testing machine having a capacity of 400,000 pounds force. A spherically-seated upper

bearing block covered the entire bearing area of the prism under

test. The load was applied to the prism at any convenient rate

for the first 40,000 pounds force while the remaining load was

applied at a rate of 40,000 pounds force per minute until failure

occurred. The maximum load sustained by a prism was used in

computing its compressive strength.

Prisms were instrumented with two different types of displacement

measuring devices. Every prism was instrumented with two linear

variable differential transformers (LVDT's) oriented vertically,

one on each faceshell at mid-length (fig. 2.4). The gage length

of the LVDT's was sixteen inches measured between the approximate

vertical centers of the top and bottom blocks. Twelve leaf

spring transducers (LST's) (fig. 2.5) were placed on each prism

over a gage length of either 3/4 inch or 1 inch. The mounting

location of the LST's varied between groups of prisms. Thus,

while all of the mounting locations and orientations identified

in figure 2.6 were used during the test program, LST's were not

placed at all of the locations during the test of each prism. It

was not possible to place LST's both vertically and horizontally

over the same gage area. Vertical and horizontal displacement

data at the same location were obtained by using separate, but

comparable prisms, one instrumented to measure vertical strains

and the other to measure horizontal strains. It was not possible

to place LST's across LVDT gage lengths.

The displacement data and the load data from the testing machine

were read simultaneously at regular intervals during a prism

test. The displacement data were subsequently divided by

appropriate gage lengths to produce strain data. The combination

of the load and deflection data into a plot of stress vs. strain

produced a strain history that described the response at the gage

location for the duration of the test.

The two types of displacement sensors provided two descriptions

of prism behavior. LVDT's measured deflections over a gage

length that encompassed a large percentage of the height of a three-unit-high masonry prism. As a result, the LVDT data were

good indicators of average prism behavior. LVDT's were placed in

6
3 unit high

stretcher units

Figure 2 . 2 Concrete block prism

NET CROSS-SECTIONAL AREA FACESHELL AREA

Figure 2.3 Mortar bedding types

7
Figure 2.4 LVDT instrumentation

8
Figure 2.5 Leaf spring transducer instrumentation

9
Represents

LVDT

locations

03G

O

03 •r-t 4-1 4J c 03 <u a

03 o

<U rH

3la h

QJ cn PS hJ *

<33 u

cOS

•HX

o

Clcl

CL

CO

a)

CO

03co

+

It*

CN

o

o

+ +

n

-e-1 OSoo

OSo

o

03 LJ

4J C

3- -H o o Z n

Figure 2 .

6

Locations of LST's and LVDT's

10

FACESHELL

(typical)

END

WEB

(typical)
the same position on all of the prisms. Thus, comparisons among

the various types of prisms were made using LVDT data. The short

gage length of an LST rendered LST data sensitive to local

effects. This had advantages and disadvantages. The principal

advantage was that this made possible the measurement of strain

distribution in a prism. The major disadvantage was that adverse

local flaws such as voids could dramatically affect the measured

data. In general, LST data was less consistent numerically than

was LVDT data but was valuable in the identification of general

trends

11
3. MASONRY PRISM STRENGTH ANALYSIS

This section examines the influence of block strength, mortar

strength, and mortar bedding configuration on masonry prism

compressive strength. Also included in this section is a description of the modes of failure of the prisms. In all tables

in this section, the following codes are used for simplicity:

HM = high-strength mortar LM = low-strength mortar

HB = high-strength block LB = low-strength block

FS = faceshell bedding FB = full-area bedding

Masonry prism strengths were calculated in accordance with ASTM

E447 [ 1 ] , i.e., as maximum load sustained divided by the net

cross-sectional area of the prism. The computed compressive

strengths and elastic moduli of prisms are listed in Tables 3.1

and 3.2 for faceshell-bedded prisms and full-area-bedded prisms.

Moduli for these prisms were computed as the slope of the leastsquares linear regression for stresses (based on net crosssectional area of prisms) on strains (computed by dividing

deformations measured by LVDT's by gage length) for stresses

between zero and 0.33 f' m . Also listed in Tables 3.1 and 3.2 are

the average compressive strengths, the standard deviations of the

strengths, and the coefficients of variation of strength for each

group of prisms having the same block strength, mortar type, and

configuration of mortar bedding.

3 . 1 MODES OF FAILURE

Failure was defined as the condition where the specimen under

test could no longer sustain the applied load. Prisms cracked

prior to reaching their failure loads. The crack patterns were

indicative of the mode of failure of the specimen. Results from

this study indicate that the mode of failure was strongly dependent on the type of mortar bedding used in the prism.

Prisms that were mortared on the net cross-sectional area

(full-area bedding) exhibited compressive shear failures across

one or more blocks of the prism. A typical failure is pictured

in figure 3.1. Compressive shear failure was shown by diagonal

cracking through the thickness of a faceshell or web of the prism

element, such that the masonry bounded by cracks fell off.

Partial splitting of the faceshells along a plane parallel to the

faceshell was common in full-area-bedded prisms. This type or splitting was more common in faceshells than in webs of

full-area-bedded prisms.

12
Table 3.1 Faceshell-Bedded Masonry Prism Compressive Strength

High-Strength Block

Net Cross- Sectional Area = 61 .5 sq. in.

Low-Strength Mortar High-Strength Mortar

Prism Strength Em Prism Strength ^m

Identifier (psi) (ksi) Identifier (psi) (ksi)

5-1-1 2159 1181 3-1-2 1911 1005

5-1-2 1992 820 3-1-3 1874 1017

5-1-3 1951 813 3-1-4 1675 1181

5-1-4 2094 1026 3-3-1 2093 973

5-4-1 1976 1080 3-3-2 2059 1079

5-4-2 1870 829 3-3-3 2133 1069

5-4-3 1894 1032 3-4-1 2146 1181

5-4-4 1951 1059 3-4-2 2187 1068

3-4-3 2069 1057

4-1-1 1930 1056'

4-1-2 2041 1037

4-1-3 2179 1121

4-3-0. 1801 872

4-3-2 2024 996

4-3-3 1927 *

Average Stress = 1986 psi Average Stress = 2003 psi

Std. Deviation = 97 psi Std. Deviation = 147 psi

Coef. of Var. = 4.9% Coef. of Var. = 7.4%

* Data not Available

Low-Strength Block

Net Cross-Sectional Area = 60.4 sq. in.

Low-Strength Mortar High-Strength Mortar

Prism Strength Em Prism Strength Em

Identifier (psi) (ksi) Identifier (psi) (ksi)

6-7-1 1589 851 6-1-1 1598 959

6-7-2 1474 651 6-1-2 1672 990

6-7-3 1502 787 6-1-3 1623 964

6-9-1 1474 896 6-3-1 1474 510

6-9-2 1416 723 6-3-2 1556 952

6-3-3 1536 960

6-10-1 1395 *1042

6 -*10-3 1391 758

6-12-1 1631 1036

6-12-2 1507 669

6-12-3 1477 907

Average Stress = 1491 psi Average Stress = 1533 psi

Std. Deviation = 63 psi Std. Deviation = 94 psi

Coef. of Var. = 4.2 % Coef. of Var. = 6.1 %

13
Table 3.2 Full-Area-Bedded Masonry Prism Compressive Strengths

High-Strength Block

Net Cross-Sectional Area = 61. 5 sq. in.

Low-Strenath Mortar Hiah-Strenath Mortar

Prism Strength ^m

Identifier (psi) (ksi)

3-2-1 2825 1339

No prisms of this type were 3-2-2 2650 1918

tested. 3-2-3 2967 1465

3-5-1 2431 1498

3-5-2 2854 1561

3-5-3 2650 1084

3-6-1 2675 1273

3-6-2 2691 1371

3-6-3 2618 1208

4-2-1 2528 1159

4-2-2 2764 1318

4-2-3 2553 1072

4-4-1 2894 * 4-4-2 2675 1346

4-4-3 2780 1399

Average Stress = 2704 psi

Std. Deviation = 145 psi

Coef. of Var. = 5.4%

* Data not Available

Low-Strength Block

Net Cross-Sectional Area = 60. 4 sq. in.

Low-Strencrth Mortar Hiah-Strenath Mortar

Prism Strength Ejp Prism Strength Ejp

Identifier (psi) (ksi) Identifier (psi) (ksi)

6-6-1 1999 1193 6-2-1 2045 1106

6-6-2 1954 1236 6-2-2 1921 990

6-6-3 1912 919 6-2-3 2136 1125

6-8-1 2078 1215 6-4-1 2119 1122

6-8-2 1921 978 6-4-2 2161 1184

6-8-3 1950 1196 6-4-3 2119 1250

6-11-1 1995 .1176

6-11-2 2119 1215

6-11-3 2169 1021

6-13-1 2103 1322

6-13-2 1958 1257

6-13-3 2016 1332

Average Stress = 1969 psi Average Stress = 2072 psi

Std. Deviation = 62 psi Std. Deviation = 82 psi

Coef. of Var. = 3.1% Coef. of Var. = 4.0%

14
Figure 3.1 Failure of a full-area-bedded prism

15
Webs cracked along an essentially vertical line at about one-half

(40 to 60 percent) of the failure load in prisms that were

mortared on only the faceshells. As will be discussed later, web

cracking was due to bending introduced by the web loading

condition. This cracking occurred in the webs of the middle

blocks of all prisms and in most of the webs of the top and

bottom blocks of the faceshell-bedded prisms in this study. A

typical faceshell-bedded prism at failure is pictured in figure

3.2. Once the web cracks formed, they extended rapidly with

increasing load. Web cracking produced two columns consisting of

faceshells, whose degree of interaction changed with increasing

load. This configuration was apparently stable up to failure for

three-unit-high prisms. The faceshells sometimes cracked through

their thickness as described for full-area-bedded prisms.

3.2 EFFECT OF BEDDING TYPE

Bedding type had a significant effect on the average ultimate

load capacity of the prisms tested as part of this study.

Prisms constructed with faceshell bedding failed at lower applied

loads than did prisms constructed with full-area bedding. For a single combination of block strength and mortar strength, the

ratio of average ultimate load capacity of prisms constructed

with faceshell bedding to that of prisms constructed with

full-area bedding was about 0.75. A listing of this ratio for

all mortar and block strength combinations is contained in Table

3.3. The uniformity of this ratio indicated that the influence

of bedding type on strength was unaffected by the properties of

the materials used to construct the prisms.

Table 3.4 contains the average prism compressive strengths for

all prisms constructed with the same mortar strength, block

strength and mortar bedding type for all combinations tested.

The compressive strengths were based on net cross-sectional

area. The data in Table 3.4 suggests that the ultimate stresses

developed in faceshell-bedded prisms were less than the ultimate

stresses developed in full-area-bedded prisms constructed from

comparable materials. However, this apparent difference was due

to the use of a common area to calculate stresses when, in fact,

faceshell-bedded prisms had a different bearing area than did

full-area-bedded prisms. Mortared area was the actual effective

bearing area between blocks of a prism. Stresses calculated on

the basis of mortar-bedded area can be expected to be similar for

equivalent materials.

Bedding type had a significant effect on the amount of scatter of

the values of ultimate strength of prisms constructed of

comparable materials. The coefficient of variation of the

ultimate stresses for the faceshell-bedded prisms was higher than

for full-area-bedded prisms for all material strength combinations. Table 3.5 contains the coefficients of variation for all

combinations of material strengths and bedding types.

16
Figure 3.2 Failure of a f aceshell-bedded prism

17
Table 3.3 Ratio of Average Compressive Strengths

Faceshell to Full-Area Bedding

HM - HB HM - LB LM - HB LM - LB

Faceshell-Bedded 2003 1533 1986 1491 (psi)

Full-Area-Bedded 2704 2072 — 1969 (psi)

Ratio (FS / FB) 0.74 0.74 — 0.76

Table 3.4 Average Compressive Strengths by Group

Prism Group

Characteristics

HM - HB - FS

HM - HB - FB

LM - HB - FS

LM - HB - FB

HM - LB - FS

HM - LB - FB

LM - LB - FS

LM - LB - FB

Compressive

Strength (psi)

2003

2704

1986

1533

2072

1491

1969

18
It also appears that, as overall prism strength increased, the

coefficient of variation of the strengths also increased.

However, this trend may be due in part to the fact that fewer

tests were conducted using prisms having lower strengths. For

the typical number of tests, scatter of data could be expected to

increase as sample size increased because of the variable nature

of masonry prisms.

3 . 3 EFFECT OF MORTAR STRENGTH

The effect of variations in mortar strength on the ultimate load

sustained by a prism was small. This observation was based on a comparison of the ultimate stresses achieved by prisms

constructed with low-strength mortar to the ultimate stresses

achieved by prisms made with high-strength mortar. The

comparison was most conveniently made by comparing the ratios of

ultimate stresses developed in prisms which were similar except

for mortar strength. The values of the ratios of average

compressive strengths for the different combinations were all

nearly equal to one, as shown in Table 3.6. This fact suggests

that the effect of mortar strength on masonry prism strength was

negligible. Such a conclusion is reinforced by noting that the

ratio of the average mortar compressive strengths for the two

mortar types was 0.66. Clearly, if mortar strength had been a significant determinant of prism strength, the ratio of masonry

prism strengths would not have been unity, but some value more

nearly equal to 0.66. Thus, it is reasonable to state that

variations in mortar strength had a negligible effect on a prism's compressive strength.

3.4 EFFECT OF BLOCK STRENGTH

The effect of block strength on overall prism strength can be

determined by comparing average ultimate stress among prisms

constructed with the same mortar type and mortar bedding, but

with blocks of different strengths. This comparison was most

easily made by using ratios of average compressive strength of

prisms made with low-strength block to the average compressive

strength of prisms made with high-strength block. The ratios,

listed in Table 3.7, were all approximately 0.75, regardless of

the combination of mortar type and mortar bedding. Thus, the

effect of changes in block strength on prism strength was

significant. The ratio of the unit compressive strength of the

low-strength block to the compressive strength of the highstrength block was approximately 0.72. The strength ratio for

the block approximated the strength ratio for the prisms. Not

only was the effect of block strength on prism strength

significant, but the effect seemed to be predicted by the simple

ratio of block unit strengths. This result also demonstrated

that block strength was significantly more influential than

mortar strength in its effect on the load carrying capacity of a prism.

19
Table 3.5 Coefficient of Variation of Compressive Strengths

HM - HB HM - LB LM - HB LM - LB

Faceshell-Bedded 7.4% 6.1% 4.9% 4.2%

Full -Area-Bedded 5.4% 4.0% 3.1%

Table 3 . 6 Ratio of Average Compressive Strengths

Low to High Strength Mortar

HB - FS HB - FB LB - FS LB - FB

Low-Str Mortar 1986 1491 1969 (psi)

High-Str Mortar 2003 2704 1533 2072 (psi)

Ratio (LM / HM) 0.99 0.97 0.95

Table 3.7 Ratio of Average Compressive Strengths

Low to High Strength Block

LM - FS HM - FS LM - FB HM - FB

Low-Str Block 1491 1533 1969 2072 (psi)

High-Str Block 1986 2003 2704 (psi)

Ratio (LB / HB) 0.75 0.77 0.77

20
4. PRISM STRAIN DISTRIBUTION

In this chapter, data are presented which describe the distribution of strain in the prisms. Strain measurements are reported

which support the observed difference in behavior between

faceshell-bedded and full-area-bedded prisms. Results of a simple, linear elastic finite element analysis are shown which

graphically illustrate prism behavior. Measured strain data are

presented which support the predicted behavior.

The strain data used in this chapter and elsewhere in this report

are computed values taken as the displacement measured by an LST

or LVDT divided by the appropriate gage length. The strain data

are from LST's unless otherwise specified. The sign convention

is that tensile strains are positive and compressive strains are

negative.

4.1 UNIFORMITY OF STRAIN

This section describes the nature of the strain distributions in

full-area-bedded and faceshell-bedded prisms. The measured

strain distributions indicated that mortar bedding type

significantly affected the uniformity of strain distribution and

the strain magnitude within a prism.

4.1.1 Full-Area-Bedded Prisms

Strain in the webs of full-area-bedded prisms was uniform in

magnitude over the web. Strain data indicated that the webs

carried load predominantly by vertical compression: lateral

tensile strains were small. The magnitude of lateral strains in

webs was comparable among blocks, as was the magnitude of

vertical strains in webs (figs. 4.1 and 4.2).

The strain data taken from faceshells of full-area-bedded prisms

indicated that faceshell strains were uniform in distribution and

in magnitude. Faceshells, like webs, resisted applied load

predominately by vertical compression. In addition, the

faceshell vertical strains were numerically similar to vertical

web strains in material-equivalent prisms (fig. 4.3). Values of

vertical strains were similar for all blocks of prisms having

full-area bedding. A typical comparison of vertical strains in

webs of the blocks at all levels of load may be found in figure

4.4. The strain history illustrated in the figure is regular and

linear.

4.1.2 Faceshell-Bedded Prisms

Strains measured during tests of faceshell-bedded prisms varied

significantly in magnitude and distribution within both webs and

faceshells of prisms. The magnitudes of vertical strains in webs

were different among vertically aligned webs of a prism as shown

21
MET

CROSS-SECT

I

CiMRL

RREfl

STRESS

coa

2000 .

STRfl I M, mi crostra i

Figure 4.1 Web vertical strain by block for

full-area-bedded prisms.

22
MET

CROSS-SECTIONAL

AREA

STRESS,

ps

2000 .

STRAIN, microstrain

Figure 4.2 Web lateral strain by block for

full-area bedded prisms.

23

o o
MET

CROSS

-SECT

I

OMflL

flRER

STRESS,

ps

1 bO 0 .

1200 .

30 0 .

60 0 .

30 0 .

STRRIM, m i crosira i

Figure 4.3 Comparison of vertical strains - faceshell to veb

for full-area-bedded prisms.

24
MET

CROSS-SECT

IONRL

RRER

STRESS,

ps

2000 .

1600. -

1200 . -

80 0 . -

400 . -

0 . -2S00

-2000. -1S00. -1000.

STRRIM, m i crostra i

u

Figure 4.4 Comparison of faceshell vertical strain by block

for full-area-bedded prisms.

25
in figure 4.5. Vertical strains in the webs of the middle block

were smaller than vertical strains in the webs of the end blocks

(fig. 4.6). The larger vertical strains in the end blocks were

most likely due to the fact that the end bearing surfaces of the

prisms were fully capped. While this was in accordance with

typical practice, it caused the end block webs to be directly

loaded by the testing machine platens. However, the middle block

webs were not directly loaded because no mortar was present

between the webs of the end and middle blocks of the faceshellbedded prisms.

The webs of the middle block of the faceshell-bedded prisms were

loaded by shearing forces transmitted from faceshells at the

intersections of the webs and the faceshells. As a result,

vertical compressive strains could have been expected to be large

in the faceshells and small in the webs of the middle block

relative to the strains in corresponding portions of the end

blocks, which were more uniformly loaded in direct vertical

compression. This behavior was verified in these tests.

Measured strains in the faceshells of the middle block were

larger than the strains measured in faceshells of the end blocks,

as shown in figure 4.7.

4.2 MODE OF FAILURE AND WEB STRAINS

As previously stated, the mode of failure of faceshell-bedded

prisms was very different from that of full-area-bedded prisms.

The most distinguishable difference was the premature vertical

web splitting which occurred at approximately 50 percent of the

ultimate load for faceshell-bedded prisms. The full-area-bedded

prisms typically did not crack until load was near the ultimate

load. Clearly, the response to the applied load was different

and, as a result, a significant difference in measured strains

should have been observed. Measured differences in response

supported the concept that premature web splitting in faceshellbedded prisms was the result of bending induced in the web. Web

bending occurred because of the support conditions at the prism

bed joints. Because the response of webs appeared to be the

aspect of behavior which differed most between faceshell-bedded

and full -area-bedded prisms, this section will consider observed

and predicted behavior of webs.

4.2.1 Faceshell-Bedded Prisms

The strain data acquired from the faceshell-bedded prisms

indicated a strain distribution in the webs that was markedly

different from that expected for a uniaxial compression test.

Strain data were measured at too few locations to permit detailed

mapping of the strain distribution. Thus, it was necessary to

develop an analytical prediction of the strain distribution and

confirm its validity with the strain data available. Finite

element analysis techniques provided the analytical solution.

26
MET

CROSS-SECT

I

OMFIL

RRER

STRESS,

ps

i

STRRIM, microstrain

Figure 4.5 Comparison of web vertical strain by block

for faceshell-bedded prisms.

27
MET

CROSS-SECTIONAL

AREA

STRESS,

ps

- 500 . - 300 . - 100 . 100 . 300 .

STRAIN, microstrain

Figure 4.6 Influence of web stiffness on vertical

strain distribution for faceshell-bedded

prisms

.

28
MET

CROSS-SECT

IOMRL

RREfl

STRESS,

ps

i

Figure 4.7 Comparison of faceshell vertical strains by

block for f aceshell-bedded prisms.

29
Details of the analysis may be found in Appendix A. Only the

most pertinent results are noted in the subsequent discussions.

The finite element analysis was useful as a predictor of stress

(or strain) flow, but was not intended or used to quantitatively

predict behavior. The graphical output of the analysis was in

terms of stress flow, but because the analysis was linearly

elastic and the results were compared to strains measured in

uncracked prisms, a direct correspondence between stress and

strain was assumed.

The predicted stress flow in a typical vertical line of webs in a

f aceshell-bedded prism is shown in figure 4.8. The deflected

shape, although exaggerated for effect, indicates that bending

takes place in all of the webs. As shown in figure 4.9, the webs

could be considered to be an assemblage of beams, the top and

bottom members of which had characteristics representative of a deep beam while the middle was something of a hybrid. The stress

flow associated with deep beam action was similar to the stress

flow predicted by the finite element analysis. Although the

stress flow from the analysis was plausible, it was necessary to

verify that the measured strain data qualitatively agreed with

the analysis.

The first confirmation that the analysis had predicted a reasonable stress flow within a prism was provided by the fact

that webs first cracked at locations at which high tension

stresses were predicted (fig. 4.8). The distribution of the

measured lateral strains in the web of the middle block also

confirmed the appropriateness of the analysis. The predicted

lateral stress/strain condition at the center of the middle web

was compressive, which contradicted the expected behavior in a uniaxial compression test where compressive vertical strains

should have produced tensile lateral strains. However, the

measured lateral strain at the center of the middle web was

consistently compressive until web cracking occurred. In fact,

the measured lateral strains at the center of each web were

compressive until the web cracked. Typical histories of lateral

strain measured in the blocks of a faceshell-bedded prism are

shown in figure 4.10.

Other confirmation of the predicted stress flow was provided by

the measured vertical strains at the web centers. As illustrated

in figure 4.6 for a typical prism test, the histories of vertical

strain at the centers of the webs indicated compression until web

cracking. More interestingly, the magnitude of strain in the

middle web was much smaller than that in the end webs. Again,

this was predicted by the analytical solution, which indicated a severely disturbed stress flow in the webs and vertical strains

in the end webs larger than in the middle web. It was concluded

that stress conditions in faceshell-bedded prisms were not

comparable to the stress conditions assumed in uniaxial

compression tests.

30
top zones in which the

major principal stress

is tensile

middle

bottom

/ / / f 1 I \ \

/ / / 1 1 \ \ \

/ / / < l \ \ \

/ / / / \ \ \ \

/ / / * % \ \ \

/ / /

**\\ \ \

/

fy^ • - - • \

/y - — - - - f\

\

X

s . /

*7

/

\ \ // t /

\ V - • 1 i /

i i * • 1

/ t \/ \

/ J • N \ / /

\ - — - -

\

v\ • • • /* /

\ \ X s • // / /

\ \ \ % # / / /

\ \ V 4 / / /

\ l i 4 / / /

\ » » * » i J /

i » » 1 1 i 1 i

Figure 4.8 Stress trajectories in webs of

bedded prism.

f aceshell31
SUPPORT CONDITIONS REACTION STRESSES

Figure 4.9 Support conditions for webs of faceshelbedded prisms.

32
4.2.2 Full-Area-Bedded Prisms

Full-area-bedded prisms behaved much like prismatic members

subjected to uniaxial compression. The measured strains in webs

were consistently more uniform than those measured in webs of

faceshell-bedded prisms. In addition, the vertical strains

measured in the webs of full-area-bedded prisms were compressive,

uniform and similar in magnitude to the vertical strains in

faceshells. Small lateral tensile strains in webs resulted from

unrestrained lateral expansion. To complement the analytical

solution obtained for the faceshell-bedded prisms, the results of

a similar finite element analysis for a full-area-bedded prism

are presented.

The stress flow in the webs of a full-area-bedded prism predicted

by the finite element analysis (figure 4.11) was very much as

expected, with the stress flow being essentially undisturbed and

the vertical stress being uniform. The measured web strains

support the predicted stress flow by exhibiting essentially

uniform vertical strain in all of the webs and small lateral

tensile strains. The uniformity of vertical strain in a typical

full-area-bedded prism is illustrated in figure 4.1, where

histories of vertical strains measured in the end webs of all

three blocks of a prism are shown. The lateral strains measured

at the same locations are presented in figure 4.2. It is

interesting to note that a typical ratio of lateral strain to

vertical strain is approximately 0.15, which is a reasonable

value for the poisson's ratio of concrete masonry. Clearly, the

distribution of strains in full-area-bedded prisms was much more

uniform and predictable than that in faceshell-bedded prisms.

4.2.3 Influence of Mortar Bedding

It has been demonstrated that reasonable predictions of

stress/ strain flow prior to prism cracking have been obtained for

faceshell-bedded and full-area-bedded prisms. These stress flows

suggested that webs split in faceshell-bedded prisms because of

induced bending. In a brittle material such as concrete, whose

capacity in tension is low, stress conditions which induce

tensile stresses or strains are not desirable in a compression

test. The bending introduced in the webs of faceshell-bedded

prisms produced these unwanted tensile stresses. The finite

element solution indicated that bending magnified tension

stresses by at least three times over the stresses that would

have resulted from the poisson effect alone in the zone of high

tension stresses in the webs.

33
NET

CROSS-SECTIONAL

AREA

STRESS,

ps

-400. -240. -80. 80. 24C . 4..;

STRAIN, microsirain

Figure 4.10 Web lateral strains by block for

f aceshell-bedded prisms.

34
Figure 4.11 Stress trajectories in webs of a full-areabedded prism.

35
4 . 3 STRAIN REDISTRIBUTION DUE TO WEB SPLITTING

As described in the previous section, webs in faceshell-bedded

prisms split as a result of bending at approximately 50 percent

of the prism's eventual ultimate load. As noted in Section 3.2,

the ultimate load sustained by faceshell-bedded prisms was

markedly lower than the ultimate load sustained by full-areabedded prisms. It may be argued that the premature web cracking

did eventually lead to early failure because the strain

distribution in the prism changed when the webs split.

The strains measured in faceshell-bedded prisms indicated that

strain redistribution in the prism began at the onset of web

cracking and continued until failure. The redistribution of

strain in faceshell-bedded prisms is illustrated by typical

measured strain histories shown in figures 4.7 and 4.12. In

figure 4.12, representative vertical faceshell strain histories

are shown together with lateral web strains. The histories are

all for the same prism. At the onset of web splitting, as the

web strains abruptly became tensile, the faceshell vertical

strains also changed abruptly. The change occurred with

essentially no change in applied load, suggesting sudden strain

redistribution. The change in vertical strain was of a different

sense for the two histories. This change in sense of the strains

was more important because it supported the previously described

analytical solution.

The history of vertical strains in faceshells of all three blocks

of a typical faceshell-bedded prism is shown in figure 4.7. The

strains were measured along a vertical line on the prism and at

the center of each block. The vertical strain in the middle

block was consistently larger than the vertical strains in the

end blocks prior to web cracking (strain shift)

.

This was

expected, as the web of the middle block supported very little of

the applied load in direct vertical compression. Thus, the

faceshells of the middle block carried more load than the

faceshells of end blocks. When the webs split, the amount of

load supported by the webs in the end blocks decreased and the

amount of load supported by the faceshells in the end blocks

increased.

The sudden decrease in compressive strain measured in the

faceshells of the middle block was difficult to explain. The

analytical solution suggested that the decrease resulted from a change in the amount of bending imposed on the middle block by

the end blocks. The uncracked webs in the end blocks induced

concave bending in the faceshells of the middle block. When the

webs split, their ability to induce bending was diminished.

Bending in faceshells of the middle block decreased, resulting in a decrease in the measured compressive strain.

36
NET

CROSS-SECTIONAL

AREA

STRESS,

ps

STRAIN, microstrain

Figure 4.12 Strain redistribution after web cracking in

f aceshell-bedded prisms.

37
5. IMPLICATIONS OF RESULTS

The data presented in Chapters 3 and 4 demonstrate that mortar

bedding configuration has a pronounced effect on prism behavior.

Mortar bedding configuration affected prism compressive strength,

the variability of strength, mode of failure, strain (stress)

distribution, and strain distribution variability. The effects

are interdependent and not easily isolated. The obvious

differences in observed behavior between faceshell-bedded and

full-area-bedded prisms suggest that the use of faceshell-bedded

prisms to determine uniaxial masonry compressive strength is

questionable, especially for quality control purposes.

5.1 COMPRESSIVE STRENGTH AND ITS VARIABILITY

The measured load capacities of the faceshell-bedded prisms were

uniformly lower than those of the full-area-bedded prisms. For

all combinations of materials, faceshell-bedded prisms could

carry only about 72 percent of the ultimate load carried by

full-area-bedded prisms. In addition, the variability of the

strengths of faceshell-bedded prisms was larger than that of

full-area-bedded prisms. The larger variability suggests that

faceshell-bedded prisms are a less-suitable test specimen than

are full-area-bedded prisms for obtaining a statistical measure

of masonry compressive strength. The difference in ultimate load

may be attributable to a reduced bearing area (faceshell mortar

joint) or to the severely cracked condition of the prism due to

premature web splitting.

5.1.1 Compressive Strength

In computing masonry prism compressive strength for a hollow

block prism, the net cross-sectional area of the unit is

typically used as the effective area. This area is easily

obtained and normally reported as part of the unit properties.

The use of the net cross-sectional area as the effective area is

reasonable for full-area-bedded prisms because the mortar bedding

area is nearly the same as the net cross-sectional area of the

unit. However, the use of the net cross-sectional area of the

unit is clearly not directly applicable to faceshell-bedded

prisms. The effective area of such a prism is the area of mortar

bedding, which is much less than the net cross-sectional area.

However, the practicality of using the area of mortar bedding in

computation of stress is questionable. The determination of

actual mortar bedded area is tedious and would significantly

increase the cost of conducting prism tests. An alternative to

using the actual mortar bedded area is to assume an effective

area that can be quickly calculated and is a realistic

approximation of the faceshell-bedded prism's effective area.

One possibility is to use the minimum faceshell area of the unit,

i.e, the minimum faceshell thickness multiplied by twice the

length of the faceshell. The faceshell thickness and length are

38
normally reported as unit properties. The use of faceshell area

in calculating prism strength for faceshell-bedded prisms does

not result in convergence between the compressive strengths

reported for all prisms. The lack of comparability is evident in

the values listed in Table 5.1. In this table, the average prism

strengths have been computed on the basis of faceshell area for

faceshell-bedded prisms and net cross-sectional area for

full-area-bedded prisms.

In summary, the selection of an appropriate area to use when

computing masonry compressive strength for faceshell-bedded

prisms is somewhat arbitrary. In addition, while it may be

possible to select an effective area for a faceshell-bedded prism

which produces computed masonry strengths equal in magnitude to

those obtained from full-area-bedded prisms, the variability of

the data will not be decreased.

5.1.2 Variability of Compressive Strength

Prisms are used to obtain a measure of the uniaxial compressive

strength of a masonry assemblage. The variability of the data

from prism tests is an indicator of the confidence to be placed

in the data. As the variability of data increases, the

confidence we may have in the measured strengths decreases. As a result, lower values of masonry strength must be assumed for

design purposes. It is desirable, therefore, to use a test

procedure which minimizes the variability of the data.

On this basis alone, faceshell-bedded prisms are less desirable

than are full-area-bedded prisms for determining masonry

strength. Estimates of masonry compressive strength based on

tests of faceshell-bedded prisms show large variability. It is

inconsistent with the intent of a standard test to measure

compressive strength using a test that permits alternate

procedures for conducting the test, particularly when those

alternate procedures result in different variabilities of

measured strengths. Such a "standard" test, in which variability

of estimated strength is not uniform, may be difficult to

incorporate into masonry codes based on probabilistic design

procedures that assume uniform factors of safety. However, even

if the higher variability of measured masonry strengths inherent

in the use of faceshell-bedded prisms is taken into

consideration, the fact that failure modes of such prisms are not

consistent with uniaxial compression can not be overlooked. A

strain distribution other than that assumed and web splitting

prior to prism failure make questionable the validity of tests

using faceshell-bedded prisms to measure masonry compressive

strength.

39
Table 5.1 Average Compressive Strength Based on Mortared

Area

CONSTRUCTION STRENGTH fpsi)

HM HB FS 3107

HM HB FB 2690

LM HB FS 3072

LM HB FB N/A

HM LB FS 2299

HM LB FB 2072

LM LB FS 2236

LM LB FB 1969

40
5.2 STRAIN DISTRIBUTION

Measured and analytically predicted strains indicated that

full-area-bedded prisms behaved as uniaxially-compressed,

prismatic members. The predominant strain (stress) was vertical,

compressive, and relatively uniform through both cross-section

and height. Cracking and/or spalling occurred at a load only

slightly less than that at failure and the failure was typically

sudden. It is reasonable to conclude that the maximum load

carried by the prism divided by the solid cross-sectional area of

the prism was a representative and consistent measure of the

compressive strength of the masonry assemblage.

In sharp contrast to the strain distribution exhibited by

full-area-bedded prisms, the strain distribution exhibited by

faceshell-bedded prisms was strongly influenced by bending. The

magnitudes of vertical strains in faceshells varied markedly

along the prism height. The distribution of strains changed

during a test due to premature web splitting resulting from the

effects of bending. The web splitting divided the previously

whole prism into two interacting columns whose principal vertical

load resistance was provided by the faceshells. The effective

bearing area changed during the test, making selection of an

appropriate area for use in stress calculations difficult. The

fact that the distribution of strains changed dramatically when

the faceshell-bedded prisms split raises questions as to what is

actually being measured when a faceshell-bedded prism fails.

Because the distribution of strain changes as load is increased,

it is not realistic to say that it is uniaxial compressive

strength that is actually measured.

5.3 RECOMMENDATION

The data obtained from uniaxial compression tests of faceshellbedded prisms indicated that such prisms should not be used to

determine the compressive strength of masonry. Although the

strengths of these prisms may adequately approximate the uniaxial

compressive strength of faceshell-bedded walls, tests of

faceshell-bedded prisms do not permit the calculation of a meaningful compressive strength. Because typical masonry design

codes use masonry prism compressive strength as the basis for the

determination of the strength of the masonry under other actions,

such as shear, it is imperative that the measured masonry prism

compressive strength be a well defined, repeatable quantity.

The data for faceshell-bedded prisms demonstrates that the loadcarrying capacity of such prisms is strongly dependent on

conditions which are not representative of uniaxial compression.

Faceshell-bedded prisms exhibit strain distributions inconsistent

with uniaxial compression. The compressive strength of

faceshell-bedded prisms is more variable than that of prisms with

full-area bedding and the effective bearing area of a faceshell41
bedded prism is difficult to determine. Both of these factors

argue against the use of prisms with faceshell bedding as a standard of comparison.

It is suggested that only full-area-bedded prisms be used for

determination of masonry compressive strength, regardless of the

configuration of mortar bedding used in the actual masonry member

the prisms are intended to represent. Such prisms have test

conditions which can be more precisely controlled, have

distributions of strain which are closer to an assumed condition

of uniform strain, and have lower variability of strength than do

faceshell-bedded prisms. The adoption of such a procedure will

provide a material test method better suited to quality control

applications than is the current procedure. As masonry design

codes begin to be based on probabilistic estimates of material

strength, they can use as their basic strength a single,

well-defined compressive strength. It is recognized that the

adoption of a single test procedure requires reformulation of

design procedures to account for the new basis strength and the

adoption of rational effective areas to be used in computing

specific element stresses. However, these actions are best left

to voluntary design standards development groups and are outside

the scope of this report.

42
6 . SUMMARY AND CONCLUSIONS

6.1 SUMMARY

Experimental data were presented from compression tests of

seventy hollow, ungrouted, three-unit-high masonry prisms. The

data included both load and deflection (strain) data and ultimate

strengths. Block strength, mortar strength and mortar bedding

configuration were varied. Block strength strongly influenced

ultimate strength, but mortar strength did not. Mortar bedding

configuration significantly affected the magnitude and

variability of masonry prism compressive strength, strain

distribution in prisms under uniaxial compression, and mode of

failure of prisms. Analytical evaluations of strains in prisms

supported the observed and measured behavior of prisms in the

elastic range. The recommendation was made that only full -areabedded prisms be used as quality control test specimens

regardless of the mortar bedding configuration used in the

masonry structure.

6.2 CONCLUSIONS

Several conclusions can be drawn on the basis of the strain and

ultimate load data for the prisms.

* The strain distributions in three-unit-high, hollow concrete

masonry prisms are significantly affected by the

configuration of mortar bedding.

* The strain distributions in prisms under uniaxial

compression are unaffected by changes in the strength of the

materials used to construct the prisms.

* Full-area-bedded prisms exhibit uniform stress-strain

behavior that indicates uniaxial compression in all

portions of a prism.

* Faceshell-bedded prisms exhibit erratic stress-strain

behavior that indicates compression in faceshells and

bending in webs.

* Faceshell-bedded prisms have ultimate loads which are lower

than ultimate loads of full-area-bedded prisms.

* Compressive strengths measured for faceshell-bedded prisms

are less repeatable than are strengths measured for

full-area-bedded prisms.

43
7 . ACKNOWLEDGMENTS

The National Concrete Masonry Association provided the concrete

block and the mason to fabricate the prisms. Mr. Frank Rankin

prepared the prisms for testing and carried out the tests. Mr.

Mark Hogan provided assistance in many ways during this project.

Mr. Stuart Foltz assisted in preparation of the final report.

44
REFERENCES

"Compressive Strength of Masonry Prisms," (ASTM C447-74)

,

ASTM Specifications for Concrete Masonry Units, American

Society for Testing and Materials, Philadelphia, PA,

pp. 29-32.

"Allowable Stresses," Building Code Requirements for

Concrete Masonry Structures (ACI 531-79)

,

American Concrete

Institute, Detroit, MI, p. 30.

"Mortar for Unit Masonry," (ASTM C270-82)

,

1983 Annual Book

of ASTM Standards, Vol. 4.01, American Society for Testing

and Materials, Philadelphia, PA, pp. 250-254.

"Sampling and Testing Concrete Masonry Units," (ASTM

C140-75) , 1983 Annual Book of ASTM Standards, Vol. 4.05,

American Society for Testing and Materials, Philadelphia,

PA, pp. 117-120.
APPENDIX A. FINITE ELEMENT MODEL

A. 1 MODEL

A simple finite element model was developed to aid in

interpreting the experimental strain data. The results of this

model were used as a predictor of trends in the strain data, but

not as a numerical predictor of stress and/or strain. A

commercial computer program having planar finite elements was

used to model and analyze the prisms. Although results of the

analyses were expressed in terms of stress, which could be graphically displayed, because the program was based on linear elastic

behavior, the resulting stress flows were taken as direct

representations of strain distribution.

The prism was modeled as a three-dimensional assemblage of

two-dimensional plate (membrane) elements. The analytical model

of the prism was composed of five planes which represented the

two faceshells and three webs. The faceshell plates were

orthogonal to the web plates. Since the plates were discretized

into membrane elements (fig. A.l), only stresses in the plane of

the plate were computed. While the vertical stresses from all of

the elements were parallel, the lateral (horizontal) stresses in

the faceshells were oriented orthogonal to the lateral stresses

in the webs. Thus, lateral stresses assumed the directions of

their corresponding plates.

Principal stresses and their directions were also calculated.

The term "major principal stress" described the maximum normal

stress in an element and the term "minor principal stress"

described the minimum normal stress in an element. The

dimensions of the prism elements were taken as centerline

dimensions (fig. A. 2) . The taper of the walls of the masonry

unit along its height was disregarded. It was assumed that

friction between the platens and the prism was sufficient to

prevent any lateral expansion at those interfaces. Thus, lateral

displacements were suppressed at the top and bottom end surfaces

of the model prism. A uniform vertical applied stress equal to

loO psi was applied to the top surface of the model prism as

illustrated in figure A. 3. The vertical translation of the lower

surface was suppressed to simulate lower platen support.

A. 2 ANALYSIS

A. 2.1 Full-Area-Bedded Prisms

According to the finite element analysis, webs of full-areabedded prisms exhibit stresses typical of an element in uniaxial

compression. The major principal stress orientation corresponds

to the direction of loading in all webs of a prism. That is,

stresses are largest in the vertical direction and

compressive (fig. 4.11). The magnitude of vertical stresses

46
u

o

rH

42

a.o

4J

4«SU

o

rH42

<U

r-H T313

4c: ao

rH

43s

O

O

43 Figure A.l Finite element mesh.

47

typical

web

mesh

typical

faceshell

me
Figure A. 2 Finite element prism dimensions (inches).

48
Figure A. 3 Loading on finite element prism.

49
in webs is comparable among blocks of a fully-bedded prism.

Faceshells of full-area-bedded prisms behave in a manner similar

to the webs. Vertical stresses are compressive at all points in

a faceshell. The major principal stress orientation corresponds

to the direction of loading (fig. A. 4). The vertical stresses

are numerically equivalent among the three blocks of a prism.

A. 2.

Faceshell-Bedded Prisms

Webs of faceshell-bedded prisms have loading configurations which

vary depending on whether the block is a middle or an end block.

This results in stress distributions that also vary, according to

the finite element analysis. Top and bottom blocks have webs

that behave like deep beams in bending. Compressive stresses

flow from the line of contact with the platen outward to mortar

joint supports, with major principal stress orientation ranging

from vertical to about forty-five degrees at the mortar joint

support. The area bounded by the neutral axis and unsupported

edge (unmortared joint between blocks) is in tension and the

major principal stress orientation is parallel to the unsupported

edge in regions near the edge. At the geometric center of the

end block web, both vertical and lateral stresses are

compressive.

Middle block webs of faceshell-bedded prisms are stressed

indirectly through middle block faceshells. The vertical

stresses can not be transmitted directly from the end block webs

because there is no mortar between webs of an end and middle

block. The analysis predicts compressive stresses that flow from

upper faceshell mortar joint out into the plane of the center

block web, then to the lower faceshell mortar joint (fig. 4.8).

At midheight of a middle block web, from faceshell to mid-web,

major principal stress orientation varies from vertical to near

horizontal. In the web regions near the unsupported edges,

lateral stresses are tensile with major principal stress

orientation parallel to the unsupported edge.

In faceshell-bedded prisms, end block faceshells have vertical

stresses that differ from those in the middle block faceshell in

magnitude and the stress flow is disturbed near the web-faceshell

intersection (fig. A. 5). These differences were directly

accountable to web loading condition. End block webs and

faceshells transferred all load into middle block faceshells.

Some portion of this stress flowed into the middle block webs,

but the result was that middle block faceshells were more highly

stressed than were end block faceshells. In end block

faceshells, major principal stresses were compressive. In middle

block faceshells, the major principal stress orientation was

vertical over most of the faceshell. These vertical stresses

were compressive and were much larger than lateral stresses,

which were tensile.

50
1 l 1 1 1 1 l 1 i 1 1 1 \ 1 1 1

1 1 1 1 1 1 1 1

1 i 1 1 1 1 1 1

1 1 1 1 1 1 1 I

1 1 1 1 1 1 1 1

1 1 1 1 I 1 1 1

1 1 1 1 1 1 1 1

1 1 1 ! 1 I i /

1 1 1 1 If 1 1 1

1 1 1 1 1 1 1 1

1 1 1 1 1 f 1 1

1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1

~! |

I }—l—|—|—

“i—

i—H——

i—i—

i i i i i i i i

i i i i i i i i

! 1 1 1 1 1 1 1

i i i i i i i i

i i i i i i i i

i i i i i t i i

i i t i i i ii

i i ii i i i i

i i i i i i i i

i i i i i i i i

i i i i i i i i

i i i i i i i i

i i i i i i i i

1 1 |I

| 1 1 1 “1—( l i i p I" r1 j 1 _ I l 1

1—f1 1 1 1 1 ! 1 1

i i i i i i i i

i i i i i i i i

i i i i i i i i

i t i i i i i i

i i i i i i i i

i i i i i i i i

l l l 1 1 1 l l

I I I i I l l 1

1 l I 1 i i i I

1 l I l l l l i

I l I i i l I I

i i i i i i 1 1

i i i i i i i i

I I I I I i \ i

Figure A. 4 Stress trajectories in faceshell of a full-area-bedded prism.

51
\ 1 \ I II

\ \ \ I I I

l \ \ I I I

\ \ I I I

I

1 II I II

I II I I I

I /

I I

\ II HI II

II//// III///

I I I I / /

I I I I / /

I I I f I / 111 III

III III

I I I I I I I I

Mill/

I I II

I I I I

I I I

i

I I I i

/III

I III

l III

/III

/ l l I

i l I l

i l i i

I I \

I I \

I I

I l

I I i

l I l

/ / I

/ f i

t

I 1 \

I I \

I \ \

I \ \

\ \ \

I \ \ \

i \ \ \

Figure A. 5 Stress trajectories in faceshell of a faceshell-bedded prism.

52
APPENDIX B. PRISM UNIAXIAL STRESS-STRAIN CURVES

Figures B.l through B.7 contain the uniaxial stress-strain curves

for all of the prisms and are grouped by common parameters. The

stress is computed by dividing the applied load by the net

cross-sectional area of the unit used in the prism. The strain

is the average computed strain derived from the two LVDTs ! displacement readings. The displacement of each LVDT is divided

by the gage length, 16 in., and the two strains are averaged to

produce the uniaxial strain value.

53
MET

CROSS-SECT

I

ONRL

RREfl

STRESS,

ps

3000 ,

2400 . -

130 0 *

1200 ,

600

0 . -3000 - 2400 . - 1300 . - 1200 . -600

STRAIN, microstrain

Figure B.l Stress-strain curves for prisms with

low-strength mortar, low-strength block

and faceshell bedding.

54
MET

CROSS-SECT

I

OMRL

RREf)

STRESS,

ps

i

-3000. -2400. ' -1800. -1200. -SCO.

STRAIN, m I crostra I

Figure B.2 Stress-strain curves for prisms with

low-strength mortar, low-strength block,

and full-area bedding.

55
NET

CROSS-SECTIONAL

AREA

STRESS,

ps

3000 .

240 0 . -

1800. -

1200 . -

600 . -

0. 4—

-3000.

Figure

2400. -1800. -1200. -SCO.

STRAIN, m l crosira I

B . 3 Stress-strain curves for prisms with

high-strength mortar, low-strength block,

and faceshell bedding.

56
MET

CROSS-SECT

I

OMflL

AREA

STRESS,

ps

-30 0 Q . -2400. -1800. -1200. -SCO.

STRfl IN, m i crostra I

Figure B.4 Stress-strain curves for prisms with

high-strength mortar, low-strength block,

and full-area bedding.

57
MET

CROSS-SECT

IOMAL

AREA

STRESS,

psi

STRftIM, m I crosira I

Figure B.5 Stress-strain curves for prisms with

low-strength mortar, high-strength block,

and faceshell bedding.

58
NET

CROSS-SECTIONAL

AREA

STRESS,

ps

i

3000 .

2400 . -

1800 . -

1200 . -

600 . -

0. -I - 3000 . STRAIN, m I crosira I

n

Figure B.6 Stress-strain curves for prisms with

high-strength mortar, high-strength block,

and faceshell bedding.

59
MET

CROSS-SECTIONAL

AREA

STRESS,

ps

3000

2400 . -

1800 . -

1200 . -

600 . -

0 . -3000 -2400 - 1800 . -1200 -600 .

STRfl IN , m I crostra I

Figure B.7 Stress-strain curves for prisms with

high-strength mortar, high-strength block,

and full-area bedding.

60
N BS-114A irev. 2 -ac) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instruction s)

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report NoJ 3. Publication Date

4.

TITLE AND SUBTITLE

Influence of Mortar Bedding on Masonry Prism Behavior

5. AUTHOR(S)

Patricia Gaynor and Kyle Woodward and Charles Scribner

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7„ Contract/Grant No. NATIONAL BUREAU OF STANDARDS __

DEPARTMENT OF COMMERCE S. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

National Bureau of Standards

Department of Commerce

Washington s D.C. 20234

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here)

The results from compression tests of seventy ungrouted, hollow, concrete block

masonry prisms are presented. The prisms are three-high, stack-bonded assemblages.

The varied parameters in the investigation include block strength, mortar type,

and mortar bedding type (area) . The resulting data include the ultimate strength

of the prisms and strains measured at various locations on each prims. Major

observations of prism behavior are that mortar type has a negligible influence

on prism behavior, block strength affects prism ultimate strength in proportion

to its own ultimate strength variation, and mortar bedding type significantly

affects ultimate strength, its variability, strain distributions, and mode of

failure. It is recommended that faceshell bedded prisms not be used as quality

control samples for masonry construction because of their disturbed strain fields

and higher variability of test data.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s)

compression; concrete block; masonry; mortar; prism; ultimate strength

13. AVAILABILITY

| X| Unl imited

| 1 For Official Distribution. Do Not Release to NTIS

H Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

j^T Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

15. Price

USCOMM-OC S043-PS0


I