analysis of bonded and unbonded capping materials … · load at a constant rate in accordance with...

Analysis of Bonded and Unbonded Capping Materials Used in

Determining the Compressive Strength

of Concrete Masonry Units and Prisms

Hector Mexia Grivel

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

Kyle M. Rollins

Department of Civil and Environmental Engineering

Brigham Young University

April 2014

Copyright © 2014 Hector Mexia Grivel

All Rights Reserved

ABSTRACT

Analysis of Bonded and Unbonded Capping Materials Used in Determining the Compressive Strength of Concrete Masonry Units and Prisms

Hector Mexia Grivel Department of Civil and Environmental Engineering, BYU

Master of Science

The most common test used for quality control of concrete masonry units (CMU) is the compressive strength test. The main purpose of the compressive strength test is to simulate the actual strength that masonry will withstand in the field.

Capping is the preparation of the ends of the specimens in order to ensure that the specimens have smooth, parallel, uniform bearing surfaces that are perpendicular to the applied axial load during compressive strength testing. The American Society of Testing and Materials (ASTM) C1552 sets the guidelines for capping concrete masonry units, related units and masonry prisms for compression testing. The specified capping material to be used is high strength gypsum cement. This method is the only one accepted for concrete masonry capping, therefore gypsum was chosen as the control material in this research. In addition, for this research other types of capping materials were tested for grouted prisms neoprene pads, fiberboard and cement paste were used while for concrete masonry units cement paste and fiberboard were used.

A steel confinement ring was used to try to restrain the lateral expansion of the neoprene pads. Unbonded capping performed poorly in general, obtaining mean compressive strengths much smaller than the control. Different pad durometers were used for neoprene samples because of the unexpected results and abnormal behavior. Previous researchers had recommended durometer hardness for neoprene pads between 65-70; however, these hardnesses were too stiff and could not conform to the surface of the specimens. Cement paste, the only bonded material used besides gypsum, did not achieve strengths similar to that of the control group for grouted prisms; results were scattered and the average capacity of the prisms was 13.73% smaller than that of the control. With these results cement paste is not likely to be a reliable alternative to gypsum. On the other hand, cement paste performed well when used as a capping material for concrete masonry units with a mean strength just 5% smaller than that of the gypsum. Fiberboard exhibited the lowest strength for both tests: 31.71% and 23.09% smaller than that of the control for grouted prisms and CMUs, respectively. Specimens tested with fiberboard failed earlier than expected due to stress concentrations.

Keywords: Soft capping, hard capping, gypsum, neoprene, fiberboard, cement paste, prism, CMU, prism, grout, compressive strength.

v

TABLE OF CONTENTS

LIST OF TABLES …………………………………………………...……………………….. vii

LIST OF FIGURES ………………………………………………...…………………………. ix

1 INTRODUCTION………………………………………………………………………….. 1

1.1 UNBONDED CAPPING………………………..…………………………………… 2

2 METODOLOGY…….…………………………...……………………..………………….. 7

2.1 MATERIALS AND CONSTRUCTION……………………….….………………… 7

2.1.1 CONCRETE MASONRY UNITS (CMU)………………………………. 7

2.1.2 MORTAR……………………………………………………………...… 8

2.1.3 GROUT…………………………………………………………………. 10

2.2 TEST PROCEDURES ……………………………………………………………... 14

3 RESULTS………………………………………………………………………………….. 16

3.1 GROUTED PRISMS……………………………………………………………….. 16

3.2 CONCRETE MASONRY UNITS………………………………………………….. 17

4 DISCUSSION…………………………………………………………………………….... 19

4.1 GROUTED MASONRY PRISMS……………………………………………….… 19

4.1.1 HARD CAPPING………………………………………………………. 19

4.1.2 SOFT CAPPING………………………………………………………... 21

4.2 CONCRETE MASONRY UNITS………………………………………………….. 26

5 CONCLUSION……………………………………………………………………………. 29

REFERENCES……………………………………………………………………..………….. 31

APPENDIX…………………………………………………………………..………………… 33

vii

LIST OF TABLES

Table 1-1: Fiberboard - Technical Data……………………………………………………..…… 6

Table 2-1: Mortar Flow Test………………………………………………………………...…… 9

Table 2-2: Mortar Compressive Strength Results………………………………………...……… 9

Table 2-3: Grout Compressive Strength Results…………………………………………..…….13

Table 2-4: Summary of Compressive Strength Results……………………………………….... 13

Table 3-1: Summary of Strength Results for Prisms…………………………………………… 16

Table 3-2: Concrete Masonry Units Compressive Strength Results……………………...……. 17

Table 3-3: Summary of Compressive Strength for Concrete Masonry Units…………….…….. 18

Table A-1: Gypsum Compressive Strength Results for Grouted Prisms……………..………… 33

Table A-2: Cement Paste Compressive Strength Results for Grouted Prisms…………………. 34

Table A-3: Fiberboard Compressive Strength Results for Grouted Prisms………………….…. 35

Table A-4: Neoprene Compressive Strength Results for Grouted Prisms……………………… 36

ix

LIST OF FIGURES

Figure 2-1: Picture of single Concrete Masonry Units used to construct the prisms……………. 7

Figure 2-2: A) Mortar Flow Test Picture……………………………………………………….. 10

Figure 2-2: B) Picture of mason laying CMUs with mortar……………………………………. 10

Figure 2-3: Picture of a finished grouted prism sealed in a plastic bag………………...………. 11

Figure 2-4: Grout sampling ASTM C1019 procedure………………………………….………. 12

Figure 4-1: Gypsum. Compressive Strength vs Strain………………………………………….. 19

Figure 4-2: Cement Paste. Compressive Strength vs Strain……………………………………. 20

Figure 4-3: Compressive Strength vs Grout Batch for all materials……………………………. 20

Figure 4-4: Face-shell separation example…………………………………………………...… 21

Figure 4-5: Mechanism triggering the confinement when soft neoprene pads are used [13]…... 22 Figure 4-6: Neoprene. Compressive Strength vs Strain………………………………………… 23

Figure 4-7: Face-shell separation fail…………………………………………………………... 24

Figure 4-8: Compressive Strength vs Durometer………………………………………………. 24

Figure 4-9: Fiberboard. Compressive Strength vs Strain………………………………………. 25

Figure 4-10: CMU’s imperfections………………………………………………………….….. 26

Figure 4-11: Gypsum. CMU’s C. Strength …………………………………………………… 26

Figure 4-12: Cement Paste. CMU’s C. Strength …………………………………………..…… 26

Figure 4-13: Fiberboard. CMU’s Comp. Strength………………………………………..…….. 27

Figure 4-14: CMU’s Failure Mechanism Using Fiberboard……………………………………. 28

1

I INTRODUCTION

This research project was divided in two parts. The first part is a continuation of the

project “Analysis of Unbounded Capping Materials Used in Determining the Compressive

Strength of Concrete Masonry Prisms" [13]. The main objective of this part of the research is to

investigate alternative capping methods to determine the compressive strength of masonry

prisms. The compressive strength of masonry, which is used to calculate the design capacity of

masonry elements, is that of concrete masonry prisms. The second part of this research is to

investigate alternative capping methods to determine the compressive strength of concrete

masonry units. Instead of testing concrete prisms to determine the compressive strength of the

masonry, the masonry code provides an alternative method to determine the compressive

strength. The alternative method uses the compressive strength of concrete masonry units to

determine that of the masonry. Specimens were tested in compression by applying a uniaxial

load at a constant rate in accordance with the ASTM C1314 [14] for masonry prisms and ASTM

C140 [15] for masonry units.

All prisms, were built and tested in the Structural Laboratory of Brigham Young

University. Prisms were two unit high and laid on stack bond. Prisms were constructed in one

day and grouted the next day. After 28 days, twelve prisms were capped with gypsum and twelve

were capped using cement paste; the remaining prisms, twenty four of them were tested using

soft capping methods, either neoprene pads or fiberboard. The CMUs were also tested under the

same circumstances using gypsum, cement paste and fiberboard, twelve CMUs for each material.

ASTM C1552 [16] specifies gypsum and molten sulfur as the only capping materials to

be used for determining masonry compressive strength. Neoprene caps (unbonded caps) have

become a reliable alternative to hard capping in concrete cylinders and an ASTM standard,

2

ASTM C1231 [17] had been adopted approving the method. The advantages associated

with soft capping can be translated into cost savings. However, ASTM does not recognize soft

capping as a standard practice for concrete masonry prisms. Hopefully, with more research, soft

capping can be adopted which could yield benefits similar to those observed in concrete cylinder

testing.

1.1 UNBONDED CAPPING

NEOPRENE

ASTM C1231 [17] describes alternate procedures for using unbonded caps for concrete

cylinders. It has been determined that the use of unbonded pads is a convenient and efficient

capping method that gives test results comparable to those obtained with bonded caps in concrete

cylinders. Unbonded caps are currently not permitted to determine the compressive strength of

masonry. But, hopefully, it will be permitted in the future [1]. In the last 25 years, very little

research has been done on unbonded capping methods for CMU prisms [2].

There are some disadvantages on the use of bonded capping. The use of bonded capping

such as gypsum or molten sulfur requires intensive labor, time and skilled labor. Researchers

have found that the use of unbonded pads is a convenient and efficient capping method that

produces test results comparable to those obtained with bonded caps, for some materials [3]. The

unbonded capping method is considered to be economical and sustainable because neoprene pads

can be used multiple times and still can attenuate the imperfections on the surface of the test

specimen. The elastomeric pads can be reused until physical damage is observed [3]. ASTM

C1231 [17] requires replacing pads which have cracks or splits exceeding 3/8 in [4].

According to Gaynor [4] the strength obtained with worn pads was still very similar to

the ones obtained with new pads. There was also no evidence of any detrimental effect on the

strength of concrete cylinder specimens, regarding the shape of the pads [4]. Ideally, the elastic

modulus of the capping material should be similar to that of the material being tested. The

capping material must have a high elastic modulus to distribute the applied load uniformly to the

ends of the specimen. Research has also shown that the measured cylinder strength of a given

3

concrete is related more closely to the elastic modulus of the capping material than to its strength

[3].

Neoprene pads deform considerably laterally as the compressive load is applied due to its

high Poisson ratio. When the lateral strain of the cap is greater than the lateral strain of the

specimen, tensile stress on the interface between the specimen and the cap occur. As a result of

the tensile stress present, there is an apparent decrease in ultimate strength of the specimen. If the

lateral strain is lower than the strain of the specimen, the opposite happens, confinement occurs

and the strength of the prism is apparently higher than the actual strength [5]. Apparently, the

grouted masonry prisms tested by Ballard [13] experienced confinement and as a result higher

strengths were obtained, 13% greater than the specimens capped with gypsum.

The detrimental effect caused by the end caps can be mitigated. In order to avoid such

behavior, retaining steel rings are be used essentially to restrict lateral flow of the neoprene pads

that would otherwise induce lateral tension in the specimen ends [1]. Favorable results for

neoprene pads has been obtained. The effectiveness of the use of neoprene pads confined in steel

ring bases at both sides of the cylinders has been reported by Carrasquillo and Carrasquillo [6]. It

seems to have given satisfactory results for concrete strengths up to 11,000 psi, yielding average

test results within 3% of those obtained using sulfur mortar. However, for concrete cylinders of

higher strengths, specimens capped with neoprene pads presented higher strengths compared to

specimens capped with gypsum, either because the gypsum capping failed earlier, because the

strength of concrete was higher, or because the neoprene was not hard enough to confine the

cylinders hence inducing lateral compression, thus the compressive strength of the specimens

appeared to be higher than the actual compressive strength [6]. This last effect apparently

increments the capacity of the sample compared to the ones capped with gypsum or molten

sulfur. For that reason, neoprene pads must be changed constantly while testing high strength

concrete cylinders, making the method uneconomical for high performance concrete cylinder [7].

Richardson conducted similar research and concluded that for a range of compressive

strengths between 3500 and 5700 psi neoprene caps restrained in aluminum can be used instead

4

of traditional methods. However, more research has to be done for low and high-strength

concretes to understand better the relation between the strength of concrete relative to the

hardness of the neoprene pad to be used [8].

Crouch, has also investigated neoprene pads with steel confinement rings. However, the

masonry prism geometry induces nonlinear stresses at the prism ends, which increases from the

center to the corners of the prisms. Accumulation of these stresses around the corners is expected

to reduce the measured strength of the prism [5].

For the research presented herein, neoprene pads with steel restraining bases on both

sides were tested on grouted CMU prisms, starting with a neoprene pad with Durometer of 75

(Hard neoprene). Based on previous tests [13], soft pads produce higher strengths on concrete

prisms due to its confining effect. Therefore, a harder pad was used in order to try to reduce the

confinement effect previously mentioned and obtain more realistic results. In spite of all the

evidence of the behavior of neoprene as a capping material from previous research, it is not

known yet if the same observations applies to CMU prisms, thus a prediction of what is going to

happen cannot be made.

FIBERBOARD

One of the unbonded capping materials that will be evaluated in this project is

Fiberboard. Grouted masonry prisms and hollow single concrete units were tested using this

material. The intention of using this material is to provide a flat surface to distribute the load

evenly through the specimen with the advantages of low cost and fast preparation of the samples.

Fiberboard is simple to use and inexpensive and samples can be tested quickly. Fiberboard has

been commonly used to test concrete units for quality control purposes [9].

5

This capping method has been successfully proved to be an accurate method to obtain the

compressive strength of concrete CMUs, with values just slightly less than the control material.

Maurenbrecher [9] used a 11 mm thick fiberboard and reported very similar mean strengths for

hollow prisms and bricks. Mean strengths recorded were less than 8% compared to the control

capping material which was dental plaster. Although good results were obtained, Maurenbrecher

recommended that prisms with irregular surfaces still need to be capped with plaster or mortar in

addition to fiberboard. Such procedures appear to negate some of the benefits of using fiberboard

as capping method.

Roberts [10] reported results that suggested that there is no reason why board capping

should not be used. Roberts also pointed out that changing the thickness of mortar joints within

the range of 3 to 25 mm has little effect on the overall strength of the prisms [10]. None of the

previously mentioned research projects, however explained with detail the type of board used

and failed to provide mechanical properties of the board.

Fiberboard is a material that varies widely from manufacturer to manufacturer, and it is

unlikely materials that are carefully regulated having properties that are the same anywhere. The

properties of the Fiberboard used for this research is presented in the Table 1-1.

6

Table 1-1: Fiberboard - Technical Data

Properties Terminology Standard* Unit Standard Values

(2.5 mm) Thicknesses(3.0 mm)

Specific Weight Ratio between the mass

and volume of the assessed body

ABNT 10024 Kg/m3 Minimum 800

Moisture Content

Quantity of water which the assessed body

eliminates after drying at the temperature of 105°C ±

2°C ‐3 hours

AHA A 135.4/95

% Minimum 2; Maximum 9

Water Absorption

Moisture content of the assessed body after immersion in distilled

water at the temperature of 20ºC ± 1ºC, during 24 hours, for about 15

minutes

AHA A 135.4/95

% Maximum 35

Modulus of Rupture

Resistance which the assessed body supported features on its ends, when force is applied to its center

AHA A 135.4/95

Kgc/cm2 Minimum 315

Resistance to perpendicular tensile strength

Resistance which the assessed body features when tensile strength is

applied perpendicularly to its surface

AHA A 135.4/95

Kgc/cm2 Minimum 6,2

Resistance to Parallel tensile

strength

Resistance which the assessed features when tensile strength is applied parallel to its surface

AHA A 135.4/95

Kgc/cm2 Minimum 152

*Data obtained from manufacturer’s webpage. http://www.eucatex.com.br/en/Hardboard/Product.aspx?id=29

7

2 METHODOLOGY

2.1 MATERIALS AND PRISM CONSTRUCTION

2.1.1 CONCRETE MASONRY UNITS (CMU)

The masonry units used in this project were donated by a local CMU manufacturer

company. All the units used were produced from the same batch. All CMUs were subjected to

the same circumstances and fabrication method. Therefore their properties can be considered

identical.

Concrete masonry units come in various shapes, sizes, color and textures. For this

research the masonry prisms were made out of two nominal 8”x8”x8” sash concrete units.

Masonry prisms were built following the ASTM C1314 [14] procedures. The prisms were built

in the Structural Laboratory at Brigham Young University by a professional mason under the

supervision of the research team. The prisms were set on a flat surface above an open, moisture-

tight plastic bag large enough to enclose and seal the prisms completely. The prisms were laid in

stack bond with full mortar beds as shown in Figure 2-1.

Figure 2-1: Picture of single Concrete Masonry Units used to construct the prisms.

8

The prism´s height-to-thickness was 2. The failure mechanism that is observed in prisms

with height-to-thickness ratio of 2 usually doesn’t represent the failure mode observed in the

field. However, for quality control purposes, the use of a two unit height prism is an acceptable

and efficient way to determine the compressive strength. Such prism configuration is also easy to

test because of transportation convenience and because most of the commercial test machines

can test such an arrangement. ASTM C1314 [14] provides correction factors for different height-

to-thickness ratios for comparison purposes. The failure mechanisms of masonry are more

similar to the failure that occurs in prisms with three or four unit heights. Two unit height prisms

exhibit apparently higher strength, roughly 15 percent more than prisms with height-to-thickness

ratio of 3 or greater [11].

2.1.2 MORTAR

Mortar is a mix of Portland cement, sand, lime, water and admixtures. Mortar is used in

masonry primarily as a bonding agent to provide uniform bearing between the units, but it also

provides leveling and sealing of units irregularities. There are several types of mortar in the

market [12]. For this project “SPEC MIX® Portland Lime & Sand mortar” Type N was used. It

is a commercial high performance dry pre blended mortar mix. The manufacture procedure for

this mortar meets the ASTM C270 [19] and ASTM 1714 [20].

An average mortar compressive strength of 750 psi was provided by the manufacturer.

However, the actual strength of the mortar used for this project measured an average

compressive strength of 1234 psi exceeding the minimum compressive strength provided by the

manufacturer. Table 2-1 shows the results of the flow test and Table 2-2 shows the results of the

compressive tests for mortar cubes.

9

Table 2-1 Mortar Flow Test

Mortar Flow Testing Batch # 1

SPEC MIX® Portland Lime & Sand

mortar (lb) 80

Mortar Type N

Weight of water (lb) 18.2

Final Diameter 1 of Flow Specimen (in) 9




Final Diameter Average (in) 10

Table 2-2: Mortar Compressive Strength Results

Compressive Strength of Mortar Cubes

# Cubes P lb Comp. Strength psi 1 4905 1226.25 2 4370 1092.5 3 4640 1160 4 4680 1170 5 4945 1236.25 6 4940 1235 7 5275 1318.75 8 5080 1270 9 5135 1283.75 10 5055 1263.75 11 5180 1295 12 5005 1251.25 13 5030 1257.5 14 5305 1326.25 15 4480 1120

Average 4935.00 1233.75

10

As the mortar was being prepared, samples were taken in order to perform the Mortar

Flow Test, conforming to the requirements of specifications C230/C230M [21]. Only one batch

of mortar was done for this project, which consisted of one 80 lb bag of “Spec Mix” Type N

mortar mixed manually on a wheelbarrow with 18.2 pounds of water. All processes, such as

mixing and sampling were carefully supervised by the research team. Figure 2-2A shows a

sample of mortar used on the flow test and Figure 2-2B shows the mason while laying the CMUs

on stack bond.

A B

Figure 2-2: A) Mortar Flow Test Picture B) Picture of mason laying CMUs with mortar

A total of 60 grouted prisms was constructed. In order to be consistent with all the

samples, a professional mason was hired to assemble all specimens. The mortar was mixed on

the wheelbarrow and no spills of water or material occurred. Mortar joints were cut flush and

were 3/8” thick. Mortar fins that protruded into the grouting space were removed.

2.1.3 GROUT

Grout is a material that is widely used for masonry construction, it is a high-slump

concrete (slump between 8 and 11 in), that is employed typically to fill the cells of hollow CMUs

or hollow bricks. The primary function of grout is to bind reinforcing steel and to increase the

capacity of the masonry assemblage. Grout is a mixture of cementitious materials, such as

11

Portland cement and lime, aggregates and water. Furthermore, materials are proportioned in a

broad range of amounts depending on the grout’s purpose and applications [12].

For this research, six identical batches of grout were necessary to cast all the samples.

The prisms were grouted 24 hours after the prisms were built; the same mason that built the

prisms, grouted them all. Each batch consisted of three bags of “Quikrete Ready mix”, a pre

blended commercially available concrete, and 33.4 lb of water. This mix registered a slump of 8

inches, which is within the parameters for grout. All the batches were carefully measured and

mixed in the drum mixer.

The grout was placed in the units in two layers of equal depth; each layer was tamped 15

times with a rod. This action ensured a good distribution and consolidation of the grout over the

whole cross section of the prim´s core. Approximately five to ten minutes after the grout was

placed, it was noticed how the grout set because of water absorption by the units and its

consolidation. Slightly more grout had to be placed on the top to make the grout even with the

prism´s surface. After the prisms were grouted they were sealed using plastic bags in order to

retain the moisture and make the curing process more consistent for all the samples. Figure 2-3

shows some prisms after the grouting procedure and in sealed plastic bags.

Figure 2-3: Picture of a finished grouted prism sealed in a plastic bag

12

The average compressive strength of grout used for this research was obtained by

performing the standard test method for sampling and test grout as described in the ASTM

C1019 [18]. Grout samples for each batch were taken before pouring it inside the prisms, after 24

hours the grout samples were removed from their mold, labeled and taken to the fog room where

they remained undisturbed before the testing. Figure 2-4 shows the grout specimens right after

their casting. After the specified 28 days, the grout samples were taken out from the fog room

and capped with gypsum. The average compressive strength was obtained by dividing the

maximum axial load applied to the specimen by the average cross-sectional area as ASTM

C1019 [18] procedure specifies. Table 2-3 shows the results obtained from grout samples.

Figure 2-4: Grout sampling ASTM C1019 procedure

13

Table 2-3: Grout Compressive Strength Results

Dimensions Compressive Strength of Grout

Batch a b Area in2Load

lb Com. Strength psi 1-1 4.5 4.25 19.13 77360 4045 1-2 4.1875 3.875 16.23 53015 3267 2-1 4 4 16.00 89505 5594 2-2 4.1875 4.375 18.32 86590 4726 2-3 4 4 16.00 75305 4707 3-1 4.1875 4.1875 17.54 48055 2740 3-2 4.375 4.25 18.59 71965 3870 3-3 4.25 4.25 18.06 89745 4969 4-1 4.375 4.375 19.14 74200 3877 4-2 4.0625 3.875 15.74 67320 4276 5-1 4.1875 4.3125 18.06 80180 4440 5-2 4 4.0625 16.25 77250 4754

Average Compressive Strength psi 4526

Table 2-4 summarizes the average compressive strength of grout and mortar, materials

employed for the construction of the prisms. Specimens 1-2 and 3-1 were discarded because they

did not represent the real strength of grout. Specimen 1-2, had a very irregular surface and shape

and Specimen 3-1 was suddenly loaded and failed prematurely.

Table 2-4: Summary of Compressive Strength Results

Strength of Materials

Material Mean Strength psi Grout 4526

Mortar 1234

14

2.2 TEST PROCEDURES

Grouted Prisms:

Twelve grouted prisms were capped with gypsum according to ASTM C1552 [16]. This

material was considered the control for this research; the remaining prisms, capped with different

materials, were compared to the average compressive strength of samples capped with gypsum.

Sixteen grouted prisms were capped with cement paste. Even though there is no ASTM

procedure specifically for cement paste to be used as a capping material, the process used for this

research was similar to that outlined on ASTM C1552 [16]. Cement paste capped specimens take

several days to be ready to test due to the low gain of strength of the cement paste. It takes

between two to three days for the cap on one side of the prism to be hard enough so that the

specimen can be flipped; then the same process has to be done for the other side of the prism. For

this reason, the accelerant additive “Duraset” was added to the cement paste mix, allowing the

samples to be ready in only two days.

The remaining prisms were tested using unbonded methods, the surface of these prisms

were not pretreated. Fifteen prisms were tested using fiberboard. Two fiberboard pieces slightly

bigger than the masonry prisms were placed on both sides of the prism covering the whole

surface of the sample. Finally, eleven prisms were tested with neoprene pads on both sides of the

sample. The neoprene pads were placed inside steel rings to prevent excessive lateral expansion

while they were loaded in compression. It is important to know that the function of the steel ring

is to confine the neoprene pad, not the prism. However, confinement pressures on the sample’s

surface might occur if the neoprene pad used is too soft, this mechanism is illustrated on Figure

4-5.

All prisms were tested on a testing machine with a load capacity of 300 kips. Before

placing the prisms, the bearing surface was cleaned up using a brush, then the two centroidal

axes of the samples were aligned with the center of the testing machine. The upper platen was

15

brought down carefully touching the surface of the specimen and slightly preloading the

specimen; the preload was approximately 100 lbs, which is about 0.1% of the load capacity of

the specimen. The loads were applied at a uniform rate and adjusted depending on the capping

material employed for each prism, thus the specimen would fail after one minute but no more

than two minutes after the load was applied, as ASTM C140 [14] requires.

Single Concrete Masonry Units:

A total of 36 hollow concrete masonry units with nominal dimension of 8”x8”x8” was

tested under compression. Bonded capped CMU’s were pretreated and capped first. Twelve

specimens were capped with gypsum and twelve specimens were capped with cement paste. The

remaining twelve CMU’s were tested using fiberboard (unbonded capping). Gypsum was the

control material and results were compared relatively to specimens capped with this material. For

each capping material, the capping procedure and testing was done the same way as it was

explained for grouted prisms.

16

3 RESULTS

3.1 GROUTED PRISMS

The results for compressive strength, pictures and compressive strength vs strain graphs

for all capping materials and samples are displayed in Tables A-1 through Table A-4 and Figure

4-1 to Figure 4-10. Some of the samples were discarded because by mistake the load was

suddenly applied and other prisms failed because of the defective capping procedure. These

tables can be found in the Appendix. The previous results have been summarized and presented

in Table 3-1 in this section. From this table; mean strength, standard deviation, relative strength

to that of the control group and coefficient of variation were obtained.

Grouted prisms capped with gypsum, the control material, had a mean strength of 3153

psi and it was the highest, followed by cement paste with 2720 psi. Unbounded materials such as

Neoprene and Fiberboard performed poorly with a mean strength of 2284 psi and 2153 psi,

respectively. The closest one to the control group was the Cement Paste which had a capacity

13% smaller. Table 3-1 shows the results obtained for each capping method used on grouted

prisms.

Table 3-1: Summary of Strength Results for Prisms

Capping Method

# of Samples

Mean Strength psi

Standard Deviation psi

Relative Strength %

Coefficient of Variation %

Gypsum 12 3153.36 150.25 100 4.76 Neoprene 11 2284.18 294.34 72.44 12.89

Fiberboard 15 2153.47 394.86 68.29 18.34 Cement 16 2720.4 259.35 86.27 9.53

17

GYPSUM FIBERBOARD CEMENT PASTE

1 4535.28 4305.55 4696.86

2 4832.36 2948.31 4649.42

3 4965.37 4296.52 4284.21

4 4541.46 3855.80 4587.65

5 4804.22 3278.99 4517.60

6 4438.97 3643.48 4334.26

7 4633.18 3444.07 4417.68

8 4741.59 3528.92 4718.75

9 4849.97 3800.43 4397.70

10 4767.21 3965.51 4456.05

11 5042.48 3143.74 4467.45

12 4513.54 3372.28 4714.90

Average 4722.14 3631.97 4520.21

St. Deviation 190.36 429.47 151.50

3.2 CONCRETE MASONRY UNITS

Similarly, the average compressive strengths obtained for each capping method and CMU

sample are presented in Table 3-2. The behavior of CMUs when they were loaded in

compression are located in section 4 from Figure-4-11 to Figure 4-13. None of the samples were

discarded this time. The summarized version of Table 3-2 is presented in Table 3-3, where the

mean strength, standard deviation, relative strength to that of the control group and coefficient of

variability values are presented.

Again, gypsum, the control was the highest of all compressive strengths with 4722 psi.

Cement paste capping had an acceptable performance based on CMUs results, an improvement

in the difference of strengths between the control material and cement paste was noticed. It had a

mean strength of 4520 psi, which is only 4.25% smaller the control. On the other hand,

Fiberboard with a mean strength 3631.96 psi, performed again poorly with an apparently

unacceptable difference of 23.09% smaller in capacity compared to the control material. Table 3-

2 shows the results for compressive strength obtained for all the CMUs tested with different

capping methods.

Table 3-2: Concrete Masonry Units Compressive Strength Results

18

Table 3-3 shows a summary of the results obtained for each capping method used on CMUs.

Table 3-3: Summary of Compressive Strength for Concrete Masonry Units

Capping Method

Mean Strength (psi) Standard

Deviation (psi)

Relative Strength of Control (%)

Coefficient of

Variability (%)

Gypsum 4722.13 190.35 100.00 4.03 Fiberboard 3631.96 429.47 76.91 11.82

Cement 4520.21 151.49 95.72 3.35

19

4 DISCUSSION

4.1 GROUTED MASONRY PRISMS

4.1.1 HARD CAPPING

In this section, the results obtained for grouted prisms will be discussed and compared to

each other. Gypsum was used as the control capping material for this research. Gypsum capping

results are very uniform as shown in Figure 4-1, with an average compressive strength of 3153

psi. Prisms capped with cement paste experienced a decrease of 13.73% in capacity compared

with the control. Results of the prisms capped with cement paste are shown in Figure 4-2.

Figure 4-1: Gypsum. Compressive Strength vs Strain

20

Figure 4-2: Cement Paste. Compressive Strength vs Strain

An abnormal is behavior observed in Figure 4-2 which reduced the strength of cement

paste capped specimens. The reduction happened because cement paste cannot fill all the voids,

because the paste is not fluid enough. The uneven loading caused stress concentrations on prisms

capped with cement paste. In contrast, gypsum is very fluid and watery. Gypsum penetrates the

imperfections of the prisms better and spread the load more uniformly, for this reason, the

samples capped with cement paste presented lower compressive strength. The average

compressive strength in relation to the batch of grout of the samples is presented in Figure 4-3

for each capping material.

Figure 4-3: Compressive Strength vs Grout Batch for all materials

21

Capping materials (Gypsum and Cement Paste) performed much better than unbonded

materials, yielding more consistent strengths. From these results it can be inferred that grout’s

strength is very similar from batch to batch. Assuming that specimens capped with gypsum

reached its full capacity, it can be considered that the strength of grout is very consistent and

therefore the different batches are not likely to be a variable that could have caused the failure of

the prisms with soft capping.

Results for unbonded capping materials (Neoprene and Fiberboard) were significantly

different, there is no relationship between the strength of the grout and the strength of the

specimens. The grout didn’t reach its entire compressive capacity, due to the early failure of the

prisms.

4.1.2 SOFT CAPPING

Neoprene and Fiberboard samples presented early failure. “Face Shell Separation” was

the failure observed for all neoprene samples (See Figure 4-4). Due to the high compressive

forces, the different materials on the interfaces began to expand laterally at different rates,

inducing tensile stresses on the top and bottom of the prism.

Figure 4-4: Face-shell separation example

22

Neoprene prisms failed in two stages, first stage: Face Shell Separation occurs, as an

abrupt reduction in the load capacity, (See Figure 4-6) then the prism accommodates itself and

start to resist load again until the final failure occurs. For Neoprene Capping, the average

compressive strength reported (2284 psi) is not representative of the prism strength, because after

the 1st failure, the net area is reduced. Therefore, the actual average strength should be slightly

higher.

The prism strengths recorded using neoprene as capping material were lower than the

expected values. The average strength was 27.56% less than the control. Based on previous

research by Ballard [13], the average compressive strength for specimens capped with Neoprene

pads was 13% higher than the control (gypsum). Nevertheless, some inconsistencies were found

in that research. It seems that neoprene pads with a durometer of 30 were used, instead of 60

durometer, as the research specified. The softness of the neoprene pad (30 D) permitted the

prism to sink into the neoprene pad and then the prism’s strain that was greater than the steel

ring’s strain caused confinement effects on the sample. Figure 4-4 illustrates the mechanism of

the development of the confinement effect in specimens tested with soft neoprene pads.

Figure 4-5: Mechanism triggering the confinement when soft neoprene pads are used [13].

The use of a harder neoprene pad (75 D) will not wrap around the specimen and cause

confinement of the specimen. Instead of confining the sample, the lateral expansion of the hard

neoprene, even though small, induces tensile stresses causing the prisms to fail earlier.

23

Figure 4-6: Neoprene. Compressive Strength vs Strain

Failure Mechanism of Neoprene:

• After the grout sets, it shrinks, this effect result in a small depression on the

prism’s surface. Therefore the load applied is not uniform. Face shells receive

more load than grout does.

• Materials with different Poisson’s ratio expand laterally inducing tensile stresses

as Figure 4-7 describes. The grout pushes out the faces of the CMU.

• Face shells cannot resist the force induced by the other elements and fail in shear.

• Because the thickness for shells of the CMU are different, the rupture takes place

at the thinnest part of the face shell. This mechanism is depicted in Figure 4-7.

1st Failure

Final Failure

24

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

50 55 60 65 70 75

Compressive Strength psi

Durometer

COMPRESSIVE STRENGTH vs NEOPRENE DUROMETER

Figure 4-7: Face-shell separation fail

Alternatively, In order to avoid this behavior, prisms were tested using softer neoprene

pads. Figure 4-8 shows the results of this testing. Neoprene pads with durometer ranging from 50

to 75 were used. Even the softer of the neoprene pads used (50 D) was not flexible enough to

mold into the imperfect surface of the prism. The prisms failed the same way, before reaching

their full capacity again. More research is needed to determine the best neoprene durometer

value.

Figure 4-8: Compressive Strength vs Durometer

Tension

Shear

Compression

Shear

25

FIBERBOARD:

Fiberboard is the most inappropriate material to use as unbounded capping. There is too

much scatter in the results, as can be seen in Figure 4-9. Based on the results, prisms capped with

Fiberboard presented the weakest compressive strength at 2153 psi, 31.71% smaller than the

control.

Figure 4-9: Fiberboard. Compressive Strength vs Strain

The strength of the prism rely on the quality of the surface of the specimen. It was

observed that prisms with smooth surfaces on both sides of the sample presented higher strengths

and different failure mechanisms.

Concentrated loads due to the uneven surface of the prism and the inability of the

fiberboard to conform to the surface would make the prisms fail in shear from the defective spot

through its bottom. Even though fiberboard also could have induced some lateral tensile stresses

on the prisms, the failure observed was not like neoprene samples. Fiberboard prisms failed due

to stress concentrations on the surface. Concrete masonry unit (CMU) manufacturing processes

create rough and uneven surfaces as shown in Figure 4-10 which produce stress concentrations

and decrease the measured compressive strength. All the CMUs used in this project presented

considerable depressions and a very rough surface, on one side.

26

Figure 4-10: CMU’s imperfections

4.2 CONCRETE MASONRY UNITS

Single hollow concrete masonry units were also tested for compressive strength with

different capping materials for this research. Gypsum was used again as the control capping

material. The results for the samples capped with gypsum were very uniform and the ones with

higher capacity, with an average compressive strength of 4722 psi. The results are shown in

Figure 4-11 for the gypsum group and in Figure 4-12 for the cement paste group.

Figure 4-11: Gypsum. CMU’s C. Strength Figure 4-12: Cement Paste. CMU’s C. Strength

27

Even though Cement Paste was considered as a deficient capping material for grouted

prisms, it performed remarkably well for hollow CMUs. With an average compressive strength

of 4520 psi. The reason of the improvement is because there were less variations on the

specimen’s surface and voids were easier to fill. Furthermore, major surface defects were

pretreated before capping with cement. Prisms capped with cement reported 4.28% less capacity

than the specimens capped with gypsum. Results for the Fiberboard group are shown in Figure 4-

13.

Figure 4-13: Fiberboard. CMU’s Comp. Strength

Fiberboard samples performed poorly again because of the same reasons as stated above.

The specimens tested had an average compressive strength of 3631 psi. With a capacity of 23%

smaller than specimens capped with gypsum. Based on these results, fiberboard is not a reliable

material to use as a capping material. In addition to the low resistance obtained, the results

scattered significantly. Figure 4-14 shows how a concrete masonry unit capped with Fiberboard

failed due to stress concentration on the surface.

28

Figure 4-14: CMU’s Failure Mechanism Using Fiberboard

Manufacturing defect on Shell’s surface

29

5 CONCLUSION

Based on results obtained through this research, recommendations and conclusions for

different types of capping used to determine the compressive strength of the samples are listed

below for both grouted prisms and hollow CMUs.

Even though capping with gypsum is time consuming and labor intensive, gypsum is the

capping material that represents best the compressive strength of grouted prisms as well

as that of hollow CMUs.

Cement paste could be considered an alternative capping material to gypsum, exclusively

for hollow CMUs. However, it takes more time for the cement paste to set and gain

strength in comparison to gypsum. High strength cement is not commercial, therefore is

seldom in stock. Even though the use of an accelerant can solve this issue, the capping

procedure will be more expensive.

The use of unbounded capping materials like neoprene is not a reliable way to obtain the

compressive strength for grouted prisms. Because Prisms differ from concrete cylinders

in the following:

o Concrete cylinders are made out of just one material.

o Smoother surface compared with CMU prisms

o Equal stresses due to its circular geometry

Different types of neoprene pads should be tested in the future, especially softer

neoprene pads. With the data obtained from these experiments, an optimum durometer

30

that could not induce lateral tensile stress nor confinement effects could be determined

then. Therefore, a more accurate and realistic results of the compressive strength for

masonry could be determined.

Another possibility is to prevent lateral expansion of the hard neoprene pad.

Since fitting of the pad into the steel ring is not exact, shims cold be used between the

edges of the pad and the steel ring in an attempt to restrain the lateral expansion.

Two important features make gypsum the best capping material: First, its high fluidity.

Gypsum is very fluid, it molds very well into the imperfections and porous surface of the

CMU. Secondly, its strength. Gypsum sets very fast and gains strength quickly, which is

an advantage compared to cement paste. Good candidate materials to be analyzed in the

future should have these two characteristics.

Fiberboard is a material that varies too much from supplier to supplier. Results obtained

were not satisfactory for grouted prisms or hollow CMUs. Unsatisfactory results are

attribute to the irregular surface of the concrete units used. The surface was not pretreated

as Maurenbrecher recommended for this case in particular. Further research should be

done trying different thicknesses, stiffness, and density.

31

REFERENCES

[1] NRMCA, Technology in Practice (TIP). What, Why & How? Page 4

[2] Osama A. Abaza, Ameed Abu Salameh, “The Effect of Capping Condition on the

Compressive Strength of Concrete Hollow Blocks” An-Najah Univ. J. Res. (N. Sc), Vol. 17(1),

2003. Page 76.

[3] CELIK OZYILDIRIM AND NICHOLAS J. CARINO.” Significance of tests and properties

of Concrete”. ASTM International. April 2006. Page 129,130.

[4] Richard D. Gaynor, Gary M. Mullings and Fernando Rodriguez. “Test of Used Pad Caps”,

January 2001. Page 2-4.

[5] Jacob Richard Ballard. “Analysis of Unbonded Capping Materials Used in Determining the

Compressive Strength of Concrete Masonry Prisms”. Brigham Young University. April 2012.

Page 3-4.

[6] P.M Carrasquillo and R.L Carrasquillo. “Effect of Using Unbonded Capping System on the

Compressive Strength of Concrete Cylinders” ACI Materials Journal. May – June 1988. Page

146.

[7] S. Ali Mirza and Claude D. Johnson. “Compressive strength testing of high performance

concrete cylinders using confined caps” Department of Civil Engineering, Lakehead University,

Thunder Bay, On, Canada. July 1996. Page 590.

[8] David N. Richardson. “Effects of testing variables on the comparison of neoprene pad and

sulfur mortar-capped concrete test cylinders” ACI Materials Journal. September – October 1990.

Page 494.

[9] A.H.P Maurenbrecher. “Effect of test procedure on compressive strength of masonry prims”.

National Research Council Canada. 2nd Canadian Masonry Symposium. June 1980. Page 121.

[10] J.J Roberts. “The effect upon the indicated strength of concrete blocks in compression of

replacing mortar with board capping” Cement and concrete association. Wexham Springs,

England. June 1976. Page 26.

32

[11] Robert G. Drysdale. Ahmad A. Hamid. “Masonry Structures Behavior and Design” 3rd

Edition. The Masonty Society. Boulder, Colorado. 2008. Page 161,163.

[12] Shan Somayaji. “Civil Engineering Materials” 2nd Edition. California Polytechnic State

University, San Luis Obispo. December 2000. Page 245-249.

[13] Ballard, Jacob R. “Analysis of Unbonded Capping Materials Used in Determining the

Compressive Strength of Concrete Masonry Prisms” Brigham Young University. Provo, Utah.

April 2012. Page 17-20.

[14] ASTM Standard C1314-09 “Test Method for Compressive Strength of Masonry Prisms”,

ASTM International Vol. 4.05, West Conshohocken, PA, 2010.

[15] ASTM Standard C140-09a “Test Methods for Sampling and Testing Concrete Masonry

Units and Related Units”, ASTM International Vol. 4.05, West Conshohocken, PA, 2010.

[16] ASTM Standard C1552-09a “Practice for Capping Concrete Masonry Units, Related Units

and Masonry Prisms for Compression Testing”, ASTM International Vol. 4.05, West

Conshohocken, PA, 2010.

[17] ASTM Standard C1231 /C1231M-10 “Practice for Use of Unbonded Caps in Determination

of Compressive Strength of Hardened Concrete Cylinders”, ASTM International Vol. 4.02, West

Conshohocken, PA, 2010.

[18] ASTM Standard C1019-09 “Test Method for Sampling and Testing Grout”, ASTM

International Vol. 4.05, West Conshohocken, PA, 2010.

[19] ASTM Standard C270-08a “Specification for Mortar Unit Masonry”, ASTM International

Vol. 4.05, West Conshohocken, PA, 2010.

[20] ASTM Standard C1714/C1714M-09 “Specification for Preblended Dry Mortar Mix for Unit

Masonry”, ASTM International Vol. 4.05, West Conshohocken, PA, 2010.

[21] ASTM Standard C230/C230M-08 “Specification for Flow Table for Use Tests of Hydraulic

Cement”, ASTM International Vol. 4.01, West Conshohocken, PA, 2010.

33

Gypsum #4 3231.49414

Gypsum #5 2945.72406

Gypsum #2 2967.9208

Gypsum #6 2967.29864

Gypsum #5‐2 3065.04172

Gypsum #4‐2 3180.63227

Gypsum #1 3326.76091

Gypsum #3 3310.08621

Gypsum #6‐2 2730.47305

Gypsum #2‐2 3107.74608

Gypsum #3‐2 3248.75923

Gypsum#1‐2 3335.49741

Average 3153.36013

Standard Deviation 150.251361

GYPSUM (psi)

APPENDIX

Table A-1. Shows the results of compressive strength for grouted prisms using gypsum as

capping method. Sample #6-2 was discarded because of its defective capping procedure.

Table A-1: Gypsum Compressive Strength Results for Grouted Prisms

34

Cement #4* 2123.60749

Cement #3 2449.12858

Cement #2* 2704.88387

Cement #6‐3 2464.36401

Cement #1* 2929.53056

Cement #6‐1 2068.05194

Cement #3* 2718.6865

Cement #1*‐ 2 2444.43584

Cement #4‐2 2953.7255

Cement #4‐3 2614.95159

Cement #5‐1 2880.35688

Cement #1‐3 3056.09609

Cement #2‐1 2936.42894

Cement #2‐2 2435.41556

Cement #5‐2 2634.60447

Cement #1‐4 3286.42028

Cement #6‐2 2524.87095

Cement #2‐3 2492.4386

Average 2720.39614


CEMENT (psi)

Table A-2. Shows the results of compressive strength for grouted prisms using cement

paste as capping method. Sample #4* was discarded because of its defective capping procedure

and sample #6-1 because the load was suddenly applied causing the prism to fail early.

Table A-2: Cement Paste Compressive Strength Results for Grouted Prisms

35

Fiber Board #5 1805.10986



Fiber Board #5‐2 1581.46366



Fiber Board #4‐2 1977.48053


Fiber Board #2‐2 1833.60703

Fiber Board #1‐2 2754.32806

Fiber board #2‐3 1709.2163

Fiber board #4‐3 2850.72318

Fiber board #3‐2 2101.88253

Fiber board #3‐3 1818.39397

Fiber board #6‐2 2702.79251

Average 2153.46985


FIBERBOARD (psi)

Table A-3. Shows the results of compressive strength for grouted prisms using fiberboard

as capping method.

Table A-3: Fiberboard Compressive Strength Results for Grouted Prisms

36

Table A-4. Shows the results of compressive strength for grouted prisms using neoprene

as capping method.

Table A-4: Neoprene Compressive Strength Results for Grouted Prisms

Noeprene 50‐3 2496.75827

Neoprene 60‐2 2613.74428

Neoprene 60‐3 2484.94766

Neoprene 65‐5 1794.33953

Neoprene 75‐1* 2272.82119

Neoprene 75‐2 2165.55072

Neoprene 60‐1‐2 2352.12997

Neoprene 60‐1 2008.35871

Neoprene 60‐6 2658.01839

Neoprene 65‐4 1860.49128

Neoprene 65‐5‐2 2418.83771

Average 2284.18161


NEOPRENE (psi)

analysis of bonded and unbonded capping materials … · load at a constant rate in accordance with...

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