NATIONAL BUREAU OF STANDARDS REPORT

2825

TRANSVERSE STRENGTH OF MASONRY WALLS

WITH FACINGS OF REINFORCED GUNITE

by

D. Wat stein and E. J. McCamley, Jr.

Report to

Federal Civil Defense Administration

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeka, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT MBS REPORT

1001-10-1000 September 28, 1953 2825

TRANSVERSE STRENGTH OF MASONRY WALLS

WITH FACINGS OF REINFORCED GUNITE

D, Watsteln and E. J. McCamley, Jr.

To

Federal A drain! stration

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, re

unless permission ii

25, D, C, Such pei

cal ly prepared If t

Approved for public release by the

ln pa,t ' 18 prohiblted

Director of the National Institute of

andi'' d*’ Wa* hingl0n

iport has been speclfl- Standards and Technology (NIST) 8port for lt8 own U80 , on October 9, 2015.

Table of Contents

Page

1. Introduction 1

2. Materials and Test Specimens 2

3. Testing Procedure 3

3.1 Transverse Tests of Masonry Walls 3

3.2 Tests of Auxiliary Specimens 4

4. Results 4

5. Summary 7

TRANSVERSE STRENGTH OP MASONRY WALLS

WITH PACINGS OP REINFORCED GUNITE

by

D. Watstein and E. J. McCamley, Jr.

Abstract

The transverse strength of masonry

walls having a lightly reinforced

gunite facing was determined for

two types of masonry construction.

These were a wall of hollow concrete masonry units and a wall of

brick facing backed with concrete

blocks. The walls were tested as

slabs having a span of 8 ft and the

load was applied at quarter points

.

A marked increase in the transverse

strength was observed for the walls

faced with gunite as compared with

conventional masonry construction.

1. INTRODUCTION

The degree of protection against blast loads afforded

by a masonry structure to its occupants is determined largely

by the resistance of the masonry walls to transverse forces.

In an effort to determine to what extent existing masonry

buildings can be strengthened to resist transverse loads, the

Federal Civil Defense Administration sponsored a limited number of tests of masonry walls at the Structural Engineering

Section of the Division of Building Technology. Three test

walls of conventional masonry construction were faced on both

sides with pneumatically applied portland cement mortar, popularly known as "gunite” and the mortar facings were suitably

reinforced in two directions. The walls were subjected to

transverse tests in a horizontal position.
2. MATERIALS AND TEST SPECIMENS

The test walls used inthis study are illustrated in

figure 1. All the walls were approximately 8-in. thick,

8 ft 8-in. high and 4 ft long. Two of the walls, Nos. 1

and 3, were constructed of 8- in. hollow concrete masonry

units, while wall No, 2 consisted of 4 in. of brick

facing backed with 4“i n * hollow concrete blocks. The

walls, as shown in figure 2 and 3, were constructed within

a timber frame which extended 1,5 in. beyond each face of

the walls to provide a guide in applying the required

thickness of gunite to the test specimens. A view of the

finished walls with a gunite facing is shown in figure 4*

The physical properties of the hollow concrete

masonry units and the brick are given in table 1. It will

be noted that the 8-in. hollow masonry units met the requirements of Federal Specification SS-C-621 for loadbearing concrete units, while the 4"in. block failed to

meet the requirement of 1000 psi for compressive strength

by a slight margin . The brick met the requirements of

Federal Specification SS-B-656 for clay building brick of

medium grade

The masonry mortar containing a port land masonry

cement was proportioned by volume approximately in the

ratio of 1:3. The compressive strength of the mortar obtained with ten 2-in. cubes was 1240 psi.

The gunite mortar consisted of portland cement of

type I and concrete sand, the proportions of the mix being

roughly 1:3, by volume. The average compressive strength

of the gunite mortar determined with four 2-in. cubes was

5010 psi . The average flexural strength of four 1- by 6-

by l8-in. slabs of gunite mortar loaded at quarter points

of the l8-in. span was ll40 psi, and the sonic modulus was

4,880,000 psi. The sieve analysis of the sand used in

this mortar is given in the following tables

Sieve No. io passing

No. 4 100

8 84

16 72

30 53

50 9

100 4

As can be seen in figure 1, the gunite facing was applied

to each wall surface. The thickness of the gunite facing was

2 .
about 1.5 in. Each facing of gunite was reinforced with vertical and horizontal steel bars. Wall No. 1 was reinforced

vertically with 1/4- in. diameter bars spaced 3 in. apart, and

walls Nos. 2 and 3 had vertical bars 6 in. apart. The reinforcing bars had a yield point of I+IjOOC) psi and a tensile

strength of 68,600 psi.

Mats of 1/4- in. reinforcing bars were fabricated by

wiring together the vertical and horizontal bars at the specified spacing. The mats were then wired to 1/4-in. diameter

spacer bars attached to the surface of the walls with fluted

hardened steel nails driven into the masonry joints. The vertical reinforcing bars were on the outside of the mats and

were accurately positioned to receive a 3/U in. cover of

guni te

.

The walls were constructed during the period of February

13 through February 17, 1953. The gunite was applied on April

29 when the walls were a little over 60 days old. The walls

were then left outdoors to age until the last week in June,

when they were transported to the laboratory. The walls were

tested during the period of July 2 through July 7, when they

were nearly four months old and the gunite was about two

months old.

3. TESTING PROCEDURE

3.1 Transverse Tests of Masonry- Walls

The walls were transported from the construction site

to the testing laboratory in an upright position. A. view of

the wall being transported with an electric fork lift truck

is shown in figure 5. The walls were tested in a horizontal

position in a 600,000 lb capacity hydraulic testing machine

and a view of the test wall in the machine is shown in figure

6 . The walls were tested as simply supported slabs having

a span of 8 ft* Wall No. 2 was placed in the testing machine

with the brick facing down. The load was applied to the walls

at quarter points through two 5-in. I-beams, which received

the load from two bridge beams spanning the distance between

the quarter points. The two bridge beams were loaded at the

center with a transverse beam which received the load from

the testing machine through a spherically seated bearing

block

.

- 3-
The bearing surfaces of the supports and the loading

beams were coated with neat plaster of paris to assure uniformity of contact between the metal and the gunite facings

of the test walls.

The deflection of the walls under load was measured

at two points along the center line of the specimens. The

dial gages measuring the center deflections were supported

by tripods resting on the lower platen of the testing

machine. The average of these two dial gages was corrected

by the readings of another pair of dial gages which measured

the movement of the supports with reference to the platen

of the testing machine.

For the first wall tested, No. 2, only the compressive

strain was measured at two points along the center line of

the specimen. For walls Nos. 1 and 3, the compressive

strain was measured at three points and the tensile strain

at two points, at the mid-se6tion of the specimens.

3.2 Tests of Auxiliary Specimens

The compressive strengths of the masonry and gunite

mortars were determined with 2-in. cubes which were tested

in a 60,000 lb capacity hydraulic testing machine.

The flexural strength and the modulus of elasticity

of the gunite mortar were determined with 1- by 6- by l8-in.

slabs. The slabs were cast in a single gang mold which was

held in vertical position while the slabs were fabricated in

order to obtain specimens representative of the vertical

gunite facings on the test walls. The gunite slabs were

tested transversely as simply supported slabs with the load

applied at the quarter points. The modulus of elasticity

was determined by the sonic method which employed the longitudinal resonant frequency in the long direction of the

specimens

4. RESULTS

The results of the tests are summarized in tables 2 and

3. Wall No. 1 which consisted of 8-in. concrete masonry

units first started cracking at a uniformly distributed load

of 635 psf and failed at a load of 15>60 psf„ Wall No. 3

which was identical with wall No'. 1, except that No. 3

had only half as much reinforcement as No. 1, started cracking at 628 psf and failed at a load of 1094 psf. Wall No. 2

which consisted of 4“in * brick facing backed with 4

- i n '- concrete masonry units, started cracking at 569 psf and failed

-4-
at 860 psf . It is noted that walls 1 and 3 failed by tension

of the longitudinal reinforcement, while wall No. 2 failed in

horizontal shear. The horizontal crack in wall No. 2 developed

in the plane between the brick facing and the concrete block

backing, as can be seen in the view showing the crack pattern

(figure 8 ). It is believed that the shear failure of wall

No. 2 occurred prematurely and that it might have been averted

had the wall terminated at the top with a course of header

brick

The load-deflection diagrams for the three walls are

shown in figure 9. It is noted that walls 1 and 3 which were

identical except as to the amount of reinforcement, exhibited

essentially the same relationship between the load and deflection up to about 800 psf. Wall No. 2 deflected with load

more rapidly than wall No. 3, although both of them contained

the same amount of longitudinal reinforcement. It is believed

that the greater deflection of wall No. 2 can be attributed

to the imperfect bond between the brick facing and the concrete unit backing which finally resulted in shear failure in

that plane.

The bending moments observed in the test walls at first

crack and at failure are given in table. 3 . It will be noted

that the bending moments at first crack were substantially

the same for all three test walls, the range of bending moments

being 223,000 to 2l| 9,000 lb-in.

The resisting moments of the walls developed at the yield

point and the tensile failure of the reinforcement were computed using two different methods. The method identified as V* in table 3 was based on the assumption that the compressive stress block of a section of a wall was a rectangle having

a depth equal to the thickness of gunite and that the resultant

of the compressive stresses lay at the centroid of the gunite

facing. The values of resisting moments listed under method "B"

were computed by the conventional method of analysis of reinforced concrete, except that the value of n j" was assumed to be

0.875 and. the presence of compressive reinforcement in all

slabs was disregarded. The values of ultimate resisting

moments, Mu , computed by both methods are given in table 3

along with the ratios of Mu

to Mra , the observed bending moments

at failure. The ratios of Mu to Mm were somewhat greater for

method m A

h than for method "B n

, the values for walls 1 and 3,

respectively, being O. 8J4.6 and 0.620 by method "A" and 0.795

and 0.582 by method "B" . It is noted that the agreement between the computed and observed values of maximum bending

moments was considerably better for wall No. 1 than No. 3 which

had only half as much reinforcement as wall No. 1.

- 5

-
The moduli of rupture of the test walls given in table

3 were computed as the tensile stresses in the gunite facing

at loads causing the first crack in the specimens. It was

assumed for the purpose of computations that the value of

the moment of inertia of the cross-section was that of a section consisting of the two gunite facings spaced 8-in.

apart. The moduli of rupture computed on this basis were

[|_37,381| and I4.O8 psi for walls 1, 2, and 3? respectively.

It is of interest to compare these moduli of rupture with

the values obtained from the transverse tests of auxiliary

1- by 6- by l8-in. slabs of gunite mortar. The tests of

four auxiliary slabs gave strengths ranging from 1030 to

1260 psi, the average being III4.O psi. Thus, the ratio of

the average modulus of rupture observed with the test walls

to the value obtained for the auxiliary slabs was 0 . 3 &. The

low value of this ratio may be attributed to two factors:

a real difference in the strengths of the mortars on the

faces of the test walls and in the auxiliary slabs, and the

difference in the stress distribution in the two types of

specimens. While an effort was made to secure thin slabs

of gunite mortar representative of the facings on the walls,

it is possible that the auxiliary thin slabs received more

compaction both in the deposition of the mortar and the

screeding operation than the mortar on the test walls. The

difference between the distributions of stresses in the two

types of specimens probably accounts for the greater part

of the apparent difference between their strengths. It is

noted that the mortar in the thin auxiliary slabs was subjected to a flexural test whereas the mortar in the facings

of the walls could be regarded as undergoing a direct tensile test. The apparent difference between their strengths

ins , _ .ported that the ratio of

tensile strength to the flexural strength of concrete was

about 0.55.

The load-strain diagrams for the three test walls are

given in figure 10 . The strains were measured at the midsection of the walls and represent the average values given

by two to three bonded wire strain gages. In specimens 1

and 3 both compressive and tensile strains were ja&asured but

in specimen 2 only the compressive strain was measured.

1/ H . P . Gonnerman and E. C. Shuman, Compression, flexure

and tension tests of plain concrete, Proc. ASTM , part 2 , 1928.

is other observers. For

- 6 -
Comparison of the data in figures 9 and 10 reveals

general similarity between the curves for like specimens.

The more heavily reinforced specimen No. 1 exhibited

greater rigidity than the other two walls both in regard to

deflection and the flexural strain. Wall No. 2 which ultimately failed in horizontal shear showed considerably greater

deflection and strain than the wall No. 3 having an equal

amount of longitudinal reinforcement

5. SUMMARY

The tests of the three masonry walls showed that the

transverse strength of existing masonry may be markedly increased by the addition of lightly reinforced "gunite" to

the faces of the walls. The transverse strengths of two

8-in. walls of concrete masonry units, Nos. 1 and 3, tested

as simply supported slabs with an 8 ft span, were 1560 and

1094 psf for reinforcement ratios of 0.00157 and 0.00077,

respectively. The third wall, No. 2, consisting of 4“in « brick facing backed with l±-in. concrete units failed in

shear in the plane of bond between the facing and backing

at a load of 860 psf.

The computed value of the maximum resisting moment

based on the observed tensile properties of the steel, was

15 percent less than the observed value for wall No. 1 and

38 percent less for wall No. 3. It was also observed that

the moduli of rupture computed as the stresses in the walls

at the appearance of the first crack differed markedly from

the values determined in the tests of auxiliary thin slabs

of gunite mortar. On the average, the ratio of the moduli

of rupture observed in the test walls to those obtained with

the thin specimens, was 0.3&.

The data obtained in these tests are insufficient to

provide a basis for calculating the transverse strengths of

masonry walls with thicknesses of gunite facing and amounts

of reinforcement materially different from those used in

these tests. It is felt that additional tests of masonry

walls are needed In order to establish adequate criteria for

strengthening existing masonry buildings.

- 7-
' •

*
Table 1. Physical Properties of Hollow Concrete

Masonry Units and Brick,

Concrete Masonry Units

Dimensions, in.

: Absorption [ Compressive *

; strength

; lb/sq in. of”

* gross area

Thi cknes s

: Height

•

•

: Length : Shell

thickness

: lb/cu ft

: of concrete

7.65 7.70 15.65 1.50 13.3 1360

3.65 7.70 11.65 1.05 15.5 940

Brick

Dimensions

• in. :

•c Absorption ;

• Compressive

strength

•

•

. Transverse

strength

percent psi psi

2.30 x 3.75 x 7.90 2.3 11,910 570

Note: The data in this table are average values based

on tests of 3 to 5 specimens of each kind.
- -

' ' ; : -

; :

;

ri:

.
Table

2,

Summary

of

Data,

Transverse

Tests

of

Walls \

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CRACK PATTERN ON

MASONRY WALLS WITH

REINFORCED GUNITE FACINGS

1 LjLIf

^

WALL NO 3

\

L_J LJ \

UNIFORMLY

DISTRIBUTED

LOAD,

LB/SQ

FT

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6 <5

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S WITH

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9

200

UNIFORMLY

DISTRIBUTED

LOAD,

LB/SQ

FT

CENTER DEFLECTION, INCHES

LOAD- DEFLECTION DIAGRAMS

FOR MASONRY WALLS WITH

REINFORCED GUNITE FACINGS

“TJNITOTRMC^

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STRAIN FIG. 10

THE NATIONAL BUREAU OF STANDARDS

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3, 1901, as amended by Congress in Public 'Law 619, 1950. These include the development and

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