Analysis of Unbonded Capping Materials Used in

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Analysis of Unbonded Capping Materials Used in

Determining the Compressive Strength

of Concrete Masonry Prisms




Jacob
Richard
Ballard





A
project

submitted to the faculty of

Brigham Young University

in partial fulfillment of the requirements for the
degree of


Master of Science





Fernando
S
. Fonseca
, Chair

Paul
William
Richards

Richard J
ack

Balling






Department
of
Civil and Environmental

Engineering

Brigham Young University

April 201
2



Copyright
© 20
1
2

Jacob
Richard
Ballard

All Rights Reserved


v

ABSTRACT


Analysis of Unbonded Capping Materials Used in

Determining the Compressive Strength

of Concrete Masonry Prisms


Jacob Ballard

Department of
Civil and Environmental

Engineering
, BYU

Master of

Science



Compression testing is often used as a qual
ity control measure in concrete masonry
production and construction.
Caps are used on masonry prisms to eliminate surface
imperfections and provide uniform load distribution. Currently
bonded caps, specifically
gypsum
and sulfur
,

are the only accepted meth
od

for Concrete Masonry Unit (CMU)
prism
capping.
Preparing bonded caps is time consuming and labor intensive. The use of reusable or
unbo
nded

caps could dramatically re
duce specimen
preparation

time and

labor while still providing
accurate results.
This p
roject researches the viability of several unbonded caps as suitable for
compressive strength testing. Selection of cap material was based on multiple criteria, including
compressive strength, surface bonding, thickness, Poisson’s ratio and hardness. Hydro
cal
Gypsum cement was ch
osen as the control method and was compared against recycled rubber,
neoprene, fiberboard and laminated foam. The rubber and neoprene pads were restrained against
lateral expansion using a steel confining apparatus to avoid developm
ent of lateral tensile forces
at the prism
-
cap interface. Results indicate that the rubber caps provide nearly identical strengths
compared to the control, with a 3% reduction in observed strength. The fiberboard caps also
produced comparable strengths to
the control with a 5% increase in observed strength. The
repeatability of these methods
is evident from the calculated coefficient of variation
. The
neoprene cap
results had a small coefficient of variation (3.18)
, but
strengths were 13% greater
than the c
ontrol
’s
. The significant increase in strength was likely a product of high confining
stresses at the prism ends. The laminated foam exhibited the lowest strength with a 27%
reduction compared to the gypsum. Based on results, rubber caps of a durometer har
dness of 65
with proper confinement could be adopted as a suitable alternative to hard capping.













Keywords:

Jacob Ballard
,
gypsum capping
,
soft capping
,
unbounded capping, neoprene cap,
board cap, reusable cap, masonry compression test, masonry
prisms, rubber cap.

v

TABLE OF CONTENTS


LIST OF TABLES

................................
................................
................................
......................

vii

LIST OF FIGURES

................................
................................
................................
.....................

ix

1

Introduction

................................
................................
................................
...........................

1

1.1

Background

................................
................................
................................
.....................

2

1.2

Unbonded Capping

................................
................................
................................
.........

3

1.2.1

Neoprene and Recycled Rubber

................................
................................
..................

3

1.2.2

Board

................................
................................
................................
...........................

5

1.2.3

Laminated Expanded Polystyrene

................................
................................
...............

5

2

Methodology

................................
................................
................................
..........................

7

2.1

Physical Properties

................................
................................
................................
..........

7

2.1.1

Masonry Prisms

................................
................................
................................
..........

7

2.1.2

Capping Materials

................................
................................
................................
.......

9

2.2

Prism Fabrication

................................
................................
................................
..........

10

2.3

Test Procedure

................................
................................
................................
..............

11

3

Results

................................
................................
................................
................................
..

13

4

Discussion

................................
................................
................................
.............................

17

5

Conclusions

................................
................................
................................
..........................

2
1

REFERENCES

................................
................................
................................
............................

2
3



vi



vii

LIST O
F TABLES


Table 2
-
1:


Average and Provided Absorption, Density and Moisture Content for Test
Concrete Masonry Units

................................
................................
................................
.........

8

Table 2
-
2:

Grout Mix Proportions

................................
................................
................................
.

8

Table 3
-
1:


Prism Compressive Strength Results

................................
................................
.........

14

Table 3
-
2
:


Summary of
Prism Compressi
on

Results

................................
................................
...

14



viii



ix

LIST OF FIGURES


Figure
2
-
1:

A
P
ic
t
ure of the
F
our
U
nbonded
C
ap
M
aterials
U
sed. Specifically
L
aminated
F
oam,
F
iberboard,
N
eoprene,
&

R
ecycled
R
ubber
P
icture from
L
eft to
R
ight
.

.....................

9

Figure
2
-
2
:

The
R
etainer
U
sed to
C
onfine the
N
eoprene and
R
ubber
a
gainst
E
xpansion
.

.........

10

Figure
3
-
1
:

A
B
ox
P
lot of the
F
ive
T
ested
C
ap
R
esults
.

................................
.............................

1
5

Figure
4
-
1
:

An
I
llustration of the
M
echanism
B
ehind the
D
evelopment of
A
dditional
C
onfining
P
ressures at the
P
rism
-
cap
I
nterface.

................................
................................
...

1
7

Figure
4
-
2
:

A
P
icutre of a
T
ypical CMU
F
ailure
P
attern
u
sing
F
iberboard
C
aps
.

......................

1
9

Figure
4
-
3
:

A
P
icutre of a
T
ypical CMU
F
ailure
P
attern
u
sing
N
eoprene

C
aps
.

........................

20



x






1

1

INTRODUCTION

Concrete masonry compressive strength is commonly used as a quality
-
control measure
and thus requires reliability and accuracy. Masonry compressive strength is typically determined
by applying a uniaxial load on double stack prisms. Concrete

masonry unit (CMU)
manufacturing processes create rough and uneven surfaces which produce stress concentrations
and decrease the measured compressive strength. Prism capping thus becomes a necessary, as it
eliminates these imperfections and produces a pla
ne surface that uniformly distributes the load
[1]. ASTM C1552 (Standard Practice for Capping Concrete Masonry Units) specifies bonded
caps, specifically gypsum plaster or molten sulfur, as suitable capping materials for determining
masonry compressive str
ength. Unfortunately, the process of hard capping is time consuming,
and
labor intensive
. In addition, it

requires experience
d technicians

and exposes
workers

to
hazardous materials. In concrete compression testing
,

soft or un
-
bonded caps, more specificall
y
neoprene pads, have become a suitable alternative to hard capping [2
-
7]. Soft capping has the
advantage of reducing specimen preparation time and effort which can also translate into cost
savings [8]. Though soft capping has yet to become a standard prac
tice for determining
compressive strength in masonry, its implementation would provide advantages similar to those
observed in concrete cylinder testing [8,9]. The scope of this project is to research the viability of
several unbonded capping systems as ac
ceptable methods for determining the compressive
strength of masonry.

2

1.1

Background

Ideally masonry samples should be tested under pure compression when determining
compressive strength. The cap provides a flat surface that evenly transfers compressive forces

from the test machine to the masonry prism. Unfortunately, most caps also tend to introduce
additional forces on the prism. To achieve true uniaxial compression the cap and prism must
have an equal Poisson’s ratio or there must be a frictionless cap
-
prism

interface to avoid
unintended lateral forces from developing. Both of these requirements are difficult and
impractical to achieve. Instead, the selection of capping material is based on several criteria
which include a minimum compressive strength, bondin
g capacity, thickness [10], strain
compatibility and stiffness.

Failure of caps during testing can
cause

lower apparent strengths and larger variations in
the results. To avoid premature failure of the caps a minimum compressive strength of the
material is

typically specified. Common practice indicates that specifying a minimum material
compressive strength is typically adequate for most applications. However, because of the small
cap thickness relative to its other dimensions, material failure is more like
ly to occur in tension
or flexure. Thus selecting a capping material based on flexural, tensile and compressive strength
capacities would be more appropriate.

The contact between the cap and the prism provides a path to transfer compressive forces.
Voids b
etween the cap and prism decrease the effective bearing area and introduce stress
concentrations, which decreases the measured strength. It is critical, therefore, that capping
materials easily conform to rough surfaces and fill voids to achieve a uniform
load distribution.

Research in concrete cylinder testing has shown that in cases where the capacity of a
specimen is affected by the capping material, the apparent strength relates to the elastic modulus
3

rather than the strength of the cap [11]. In additio
n the rates of lateral deformation under
compression, or Poisson’s effect, can have significant effect on the measured strength [12].
Friction at the cap
-
to
-
prism interface restricts strain deformation of the material with the higher
rate of expansion. The

differing rate of lateral strain between the two materials induces stresses
at the prism ends and produces a tri
-
axial stress state. When the lateral strain of the prism is
greater than the lateral strain of the cap, the ends become confined under compres
sive stresses.
There is an increase in axial compressive strength as axial compressive strain decreases, which
results in

a higher apparent strength. Th
e
s
e

confining pressures

are

a portion of the mechanism
behind the conical failure pattern typically obse
rved in specimens capped with gypsum and
sulfur. If the cap strain is greater than the prism strain, the ends are subjected to lateral tensile
stresses and the apparent compressive strength is reduced. Soft caps are often composed of
materials that undergo

large strain deformations relative to the prism. This is one reason
unbonded methods using
soft caps produce lower apparent compressive strength th
an bonded
methods [8]. However,
soft
unbonded caps, if used correctly,

could also have the advantage of
redu
cing confining pressures commonly observed in bonded capping.

1.2

Unbonded Capping

1.2.1

Neoprene and Recycled Rubber

Neoprene has become a common capping material for testing concrete cylinder
compressive strength, performed under ASTM C1231 / C1231M (Standard Prac
tice for Use of
Unbonded Caps in Determination of Compressive Strength of Hardened Concrete Cylinders).
Research

on

concrete testing indicates that the use of confined neoprene caps produces similar
compressive strength results observed in sulfur capping [
2,

5
,

8,13,14]. Consequently, neoprene
4

capping has been adopted as an acceptable method for determining concrete cylinder
compressive strength. Neoprene caps are durable, reusable and readily deform to accommodate
imperfections and surface irregularities a
t the prism ends.

Both the hardness and thickness of the neoprene pad are critical in assuring this method is
effective. The strength and elastic modulus of a specimen correlate directly with the required
strength and stiffness of the neoprene cap. The sho
re durometer test is a method of measuring the
resistance to inelastic deformation of rubber, polymers and elastomers. Specimen with higher
strength and elastic modulus require pads with higher durometer hardness. However, pads that
are too hard are unable

to deform to surface imperfections, resulting in areas of higher stress
concentrations. Conversely, pads with low durometer values used on higher strength concrete
experience excessive wear and damage after a few uses. The durometer hardness values should

be chosen according to the
expected

strength and elastic modulus of the
specimen
. This assures
both a good distribution of compressive force and acceptable durability of the pad.

Neoprene has a relatively high Poisson’s ratio which introduces significant

lateral tensile
stress at the platen
-
prism interface, potentially reducing the apparent ultimate strength of the
specimen. In concrete compression testing the neoprene cap is confined with a steel ring to
minimize lateral displacements. Steel confinement
has also been investigated in CMU neoprene
capping tests [8,14]. However, the masonry prisms geometry induces nonlinear stresses at the
prism ends, which increases from the center to the corners [11]. Accumulation of these stresses
around the corners is ex
pected to reduce the observed strength of the prism.


5

1.2.2

Board

Though not recognized as a standard test method, board capping has been used for
quality control by many for decades. Cap tests have been performed on a wide variety of board
materials, including
Oriented
-
Strand
-
Board (OSB), fiberboard, particleboard and even ceiling
tiles. The main advantages of using a board over other unbonded materials are cost and
availability. Smaller testing labs can perform routine compression tests without having to invest

in a more costly neoprene system or devote a significant amount of labor for gypsum capping.

The use of board over hard capping has been shown to reduce the apparent compressive
strength by roughly 10%. Roberts reported ratios of 0.99 and 0.92 for two ser
ies of soft to hard
capped units [15,16
]
. Other studies using various types of board also show a reduction in the
observed compressive strength [12,13]. Board materials are more rigid than rubber and
therefore

less prone to conform to surface imperfections
. This translates into areas of higher stress and
early failure. Board capping is also dependent on variables such as thickness, wood species,
manufacturing process and hardness. Consequently the repeatability of
this method

is reduced
,

making board cappin
g difficult to adopt as a standard.

1.2.3

Laminated Expanded Polystyrene

The use of EPS as a potential capping material has only recently been investigated. EPS
is a rigid closed
-
cell foam. It’s commonly used for thermal insulation in buildings, but has
numerous

other applications. EPS deforms well under compression, making it ideal for filling in
voids and imperfections on the masonry surface. EPS gains compressive resistance at 10% of
yield. EPS has many of the same advantages as board but is less prone to comp
ositional
variability.

6


7

2

METHODOLOGY

2.1

Physical Properties

2.1.1

Masonry Prisms

Masonry prisms were constructed using 8x8
x8

in (nominal) half blocks. All block were
donated and from a single source. The use of half blocks versus full blocks increases the prism
as
pect ratio, reducing the effect of platen restraint. Half blocks prisms tests are also easier and
more economical to perform. Upon arrival blocks were visually examined for defects before
prism assembly, and defective blocks were rejected. The concrete mas
onry units used in this
study were manufactured by Oldcastle
,

compli
ed

to ASTM C90 standards

and

were

produced
from the same batch using standard fabrication methods. All units were mold
-
formed and
consequently have tapered cells. The tapering creates face

shells/webs that vary slightly i
n
dimension from top to bottom.

Following ASTM C140 six representative units were selected for determination of
absorption, in situ moisture content before sample construction, density, and measurement of
dimensions. The di
mensions of a grouted prism were utilized to calculate a bearing area of 58.1
sq. in. The average absorption, density and moisture content of the CMUs were
determined
accordingly

and are presented in
T
able
2
-
1

along with corresponding measurements provided

by
the manufacturer.

8

Table
2
-
1
:

Average and Provided Absorption, Density and

Moisture Content for
Test
Concrete Masonry Units

Average Absorption
(%)

Average Density
(pcf)

Average Moisture
Content (%)

Measured

Provided

Measured

Pr
ovided

Measured

Provided

6.6

8.76

128.21

110.39

43.01

56.51


Commercial grade Quikcrete Mason Mix Type S Mortar was used throughout all
samples. The mix is a dry pre
-
blended mixture of sand and cement meeting ASTM C270, C387
and C1714. An average mo
rtar
flow of 4.4

in was determined from four measurements. Th
e
temperature during mixing was

measure
d

as 70 degrees Fahrenheit. An average mortar
compressive strength of 2500 psi was provided by the manufacturer. Mortar and grout were both
prepared in drum mixe
rs.


Mix proportions for the grout are presented in Table
2
-
2
.
A slump of 9 in was measured
prior to pouring the grout following ASTM C172 standards. The temperature of the grout was
also monitored according to ASTM C1064/C1064M guidelines, with a recorded

temperature of
68 degrees Fahrenheit. Grout specimens were produced to determine the average compressive
strength in accordance with ASTM C1019. The 28
-
day compressive strength of the grout was
3150 psi.

Table
2
-
2
:

Grout Mix Propor
tions

Material

Weight
(lb)

Percent Weight
(%)

Sand

1923.5

49.7

Gravel

813.8

21.0

Free Water

540.7

14.0

Portland Cement

591.8

15.3

9

2.1.2

Capping Materials

Five capping materials were used for compressive strength test comparisons. Gypsum
-
cement was used as t
he control. Gypsum caps were made using Hydrocal White Gypsum
Cement which has a compressive strength of 5 ksi. Bonded caps were prepared according to
ASTM C1552 (Standard Practice for Capping Concrete Masonry Units), with a thickness less
than 0.118

in.

F
our unbonded capping materials were tested against the control

and are shown in Figure
2
-
1
. The first was a ½

in x 7.85

in x 7.85

in recycled rubber pad with a durometer
-
shore A
hardness of 65, chosen based on previous research [8]. The recycled rubber is
composed of
rubber buffings and EDPM virgin rubber flecks. The pad was placed into a welded steel retainer
to reduce tensile forces induced by lateral expansion. The steel retainer was comprised of 4 bars
welded to a plate with
¼

in welds, as seen in Figur
e
2
-
2
. The retainer had inside dimensions of
1.5

in x 7.9

in x 7.9

in and outside dimensions of 1.5

in x 8.25

in x 8.25

in.





Figure 2
-
1
: A
P
icture
o
f the
F
our
U
nbonded
C
ap
M
aterials
U
sed,
S
pecifically
L
aminated
F
oam,
F
iberboard,
N
eoprene,
&

R
ecycled
R
ubber
P
ictured from
L
eft to
R
ight.

The second unbonded material was a ½ in neoprene rubber pad with a durometer
-
shore A
hardness of 50. A lower durometer was chosen based on research by correlating the typical
masonry strength of 2500 psi [17]. The neoprene rubber was also confined with the same steel
retainer used for the recycled rubber. The neoprene rubber was cut to 7.8
7 in x 7.87 in
.

10

Figure 2
-
2: The
R
etainer
U
sed to
C
onfine the
N
eoprene
&

R
ubber
against

E
xpansion
.

The
two other

capping materials were ½ in EPS (expanded polystyrene) foam laminated
with a
0.12 in

plastic acrylic sheet,

and ½ in fiberboard. Both products
were saw cut to 8 in x 8 in
dimensions.

2.2

Prism Fabrication

65 masonry prisms were constructed by experienced masonry professionals over one day.
Prisms were set in an opened, moisture
-
tight bag, large enough to enclose and seal them once
completed. Units w
ere laid in stack bond with full mortar beds and were free of surface moisture
during construction. The mortar consistency was proportioned by weight. The amount of mortar
mixed per batch was determined by the maximum work time before retempering was requi
red.
The mortar joints were 0.375 in and flush cut. Within the following 25 hours grout was mixed
and poured into the prism cells. Prior to grouting all mortar fins and droppings were removed.
The gout was consolidated with a low force vibrator. Excess gro
ut was screeded from the surface
and prisms were then sealed in plastic bags and cured for 30 days.

11

2.3

Test Procedure

All prisms surfaces were cleaned and lightly sanded prior to capping to create a more
uniform surface. Eleven gypsum capped specimens were pr
epared according to ASTM 1552.
Gypsum
-
capped prisms were inspected prior to testing to ensure they met ASTM standards. The
remaining 54 prisms were grouped by capping method. 11 recycled rubber, 8 neoprene, 10
laminated
-
EPS, and 11 fiberboard
samples were
tested.

Prisms were placed into the load bearing
apparatus with their respective capping material. All specimens were tested on a Baldwin
-
Tate
-
Emery Testing Machine with a load capacity of 300,000
lbs
, as shown in Figure 2
-
3
. The loading
platens were spher
ically seated and met dimension requirements under ASTM C1314 Section
10.1 through 10.1.2. The loading rates were adjusted for each material so that prism failure
occurred within approximately 240 seconds. Failed samples were examined and the failure
patte
rns documented. Reusable caps were also examined to assess wear.

12


13

3

RESULTS


The results for all five capping methods are displayed in Table
3
-
1
. The fiberboard,
rubber and neoprene capping methods produced mean compressive strengths of 3328 psi, 3077
psi
and 3581 psi
,

respectively. These strengths vary from the gypsum cap by 5% for the
fiberboard, 3% for the rubber and 13% for the neoprene. The foam mean compressive strength
was 2322 psi, which was only 73 percent of the control.

Table
3
-
2

summarizes the
results from the previous table. The coefficient of variability
was calculated to determine the repeatability of the test method compared to the control. All
unbonded methods, with exception of the foam, had favorable coefficients.

Figure 2 shows the box

plots of each cap type. As illustrated, the spread for the 2nd and
3rd quartiles is tighter than the gypsum. The data spread of the rubber and foam specimen are
slightly greater than the control. The neoprene has the smallest spread followed by the
fiberb
oard. Neoprene showed the highest repeatability, i.e. lowest COV, but also produced
significantly greater apparent strengths.

Neoprene sample 1 was excluded from the mean, standard deviation and COV results as
the results were affected by failure of the co
nfinement ring welds. The remaining 7 neoprene
samples were tested with a modified confi
ne
ment ring that had been reinforced to prevent weld
fracture.


14

Table
3
-
1: Prism Compressive Strength Results

Specimen
Rubber
Foam
Fiberboard
Neoprene
Gypsum
1
2626
2090
3443
2848
3292
2
3003
2178
3172
3540
3536
3
2700
2086
3440
3410
3390
4
3125
2506
3531
3702
3661
5
3104
2087
3353
3609
2988
6
3064
2620
3559
3741
2841
7
3305
2494
3515
3558
3152
8
3184
2141
3132
3508
2958
9
3099
2797
2927
-
3102
10
3048
2217
3259
-
3011
11
3588
-
3283
-
2920
Mean
3077
2322
3328
3581
3168
Std Dev
261
260
196
114
268
Compressive Strength (psi)

Table 3
-
2: Summary of Prism Com
pression Results

Capping Method
Mean Strength
(psi)
Standard
Deviation (psi)
Relative Strength of
Control (%)
Coefficient of
Variability (%)
Gypsum
3168
268
100
8.45
Rubber
3077
261
97
8.47
Foam
2322
260
73
11.2
Fiberboard
3328
196
105
5.90
Neoprene
3581
114
113
3.18

The hardness and durabil
i
ty of the neoprene and recycled rubber were compared during
testing. The neoprene wore out significantly faster than the rubber and only 8 neoprene samples
were tested due to over wearing of the neoprene pad. Tes
ting of the eleventh foam specimen was
excused as the first several samples indicated that this material would be an unsuitable
alternative cap.


15




Figure 3
-
2
: A Box Plot of the Five Tested Cap Results.


2000
2500
3000
3500
4000
Compressive Strength (psi)
Mean
16


17

4

DISCUSSION

The prism s
trengths recorded using neoprene caps were significantly higher than previous
tests by Crouch [8]. One hypothesized reason for the increase in strength was the introduction of
additional confining stresses produced by the retainer. The steel retainer was w
elded to a base
plate, which produced a fillet where the sidewall and plate were joined. As the neoprene
expanded
,

it was pushed away from the prisms bearing surface toward the sides, as illustrated in
Figure
4
-
1
. The additional confinement decreased the
prisms compressive strain resulting in
higher apparent strength. Proper seating of the neoprene or rubber would reduce confining
pressures and produce a more accurate strength.


Figure
4
-
1
: An
I
llustration of the
M
echanism
b
ehind the
D
evelopment of


A
ddi
tional
C
onfining
P
ressures at the
P
rism
-
cap
I
nterface.

The neoprene had a low coefficient of variation suggesting that neoprene capping may
improve accuracy versus bonded methods. A lower CV was also observed during testing done by
Crouch [8]. The low CV w
as likely a product of the durometer
-
shore hardness of the neoprene.
18

Compared to other unbonded capping methods the neoprene more readily deformed to
imperfections to create a uniform load distribution. However, the lower hardness also reduced
the durabili
ty of the pad and there were significant signs of wear after 8 tests.

The foam capping system performed poorly against the control with a relative strength of
73% and a CV of 11.2. Inspection of post
-
test caps suggests areas of higher stress concentration
s
on the bearing surface. Though inspection of the cap showed excellent deformation and void
filling, the foam was not rigid enough to distribute the load. Instead the foam compressed until
the plastic laminate made contact with the prism surface. These co
ntact points provided
pathways for stress transmission to the prism and created areas of high stress concentration,
resulting in low apparent strengths.

The fiberboard produced promising results though roughly 20% higher than strengths
reported by Knight a
nd the National Concrete Masonry Association [9,13]. The board readily
deformed but was rigid and resisted compression. Inspection of the failure pattern
, as shown in
Figure 4
-
2,

and the higher compressive strength suggest the development of confining pres
sure
due to decrease
d

late
ral expansion
.

Theoretically confining pressures could be eliminated if the capping material was
allowed to expand until it neared the lateral elongation of the masonry at failure and then
confined against further strain. This how
ever, would be difficult to achieve.

The hypothesized reason behind the differences in strength between the neoprene and
rubber capping methods is twofold. First, the neoprene fit more snugly into the confining platen
than the rubber did. The lower Poisso
n’s ratio of the rubber compared to the neoprene translated
into lower confining pressures and consequent strength. Second, the rubber had greater hardness
and was more prone to develop stress concentrations.

19


Figure 4
-
2
:
A
P
icture of a

T
ypical CMU
F
ailur
e
P
attern using
F
iberboard
C
aps
.

Based on the results
,

the recycled rubber cap is the most viable alternative to bonded
caps. The durometer hardness of 60
-
65 was a good balance between deformation and durability.
Minor wear was observed on the pad after 11

tests. The CV and mean strength indicate that
rubber capping could produce similar results as the control. The rubber was also less confined
when compared to the neoprene as the pad dimensions were slightly smaller. This allowed the
rubber to expand furth
er and reduce the effects on confining pressure.


Though observed strengths were higher than previous testing done by Crouch [8], the
neoprene capping method
ha
s potential to be a viable alternative to hard capping. A low
coefficient of variation suggests
that neoprene capping may improve accuracy versus bonded
methods. The low CV was likely a product of the durometer
-
shore hardness of the neoprene.
Compared to other capping methods the neoprene more readily deformed to imperfections to
create a uniform loa
d distribution. However, the lower hardness also reduced the durability of the
pad and there were significant signs of wear after 8 tests. Additionally, higher compressive
20

strengths were observed on the neoprene capped specimens. This was likely a product
of the
steel retainer used to confine the neoprene. The retainer was welded to a base plate, which
produced a fillet where the sidewall and plate were joined. As the neoprene expanded it was
pushed away from the prisms bearing surface toward the sides. The

larger confining area
decreased the prisms compressive strain resulting in higher apparent strength

and conical failure
patterns, as seen in Figure 4
-
3
. Proper seating of the neoprene or rubber would reduce these
additional confining pressures and result
in more typica
l

strengths.





Figure 4
-
3: A Picture of a Typical CMU Failure Pattern using Neoprene Caps
.

21

5

CONCLUSIONS

A summary of findings and recommendations for this investigation of various capping
methods used to determine of masonry compressive st
rength is listed below.


1.

Material properties such as strength, elastic modulus, bonding capacity and strain
compatibility are crucial in selecting a suitable cap.

2.

Gypsum capping requires significantly greater time and effort over unbonded capping.

3.

Rubber w
ith a shore A durometer hardness between 60
-
65 produced compressive
strength results most comparable to gypsum, with almost equal variation coefficients.

4.

Plastic laminated foam (EPS) generated the lowest compressive strengths and the highest
variation. Thi
s method was determined to be inacceptable based on the results.

5.

Fiberboard yielded slightly higher compressive strengths compared to gypsum, but
produced less strength variation. This capping method was the most inexpensive.

6.

Neoprene exhibited the lowest
variation, but also produced the highest strengths. It was
suggested that correct manufacture of the steel confinement ring and neoprene pad could
reduce the measured strengths. The durometer hardness of 50 was too low for this
particular research resultin
g in excessive wear on the pad. A suggested hardness of 65
-
70
would likely result in improved durability without a significant increase in variation.


22


23

REFERENCES


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24

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-
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