Carlton Masters Thesis Report on Hypar Roof Construction

UNIVERSITY OF OKLAHOMA
GRADUATE COLLEGE
MATERIAL BEHAVIOR OF LATEX-MODIFIED CONCRETE
IN THIN HYPERBOLIC PARABOLOID SHELLS
A THESIS
SUBMITTED TO THE GRADUATE FACULTY
in partial fulfillment of the requirements for the
Degree of
MASTER OF SCIENCE
CIVIL ENGINEERING
By
WILLIAM SETH CARLTON
Norman, Oklahoma
2013

MATERIAL BEHAVIOR OF LATEX-MODIFIED CONCRETE
IN THIN HYPERBOLIC PARABOLOID SHELLS
A THESIS APPROVED FOR THE
SCHOOL OF CIVIL ENGINEERING AND ENVIRONMENTAL SCIENCE
BY
______________________________
Dr. Chris Ramseyer, Chair
______________________________
Dr. Kianoosh Hatami
______________________________
Dr. Royce Floyd

“Your people will rebuild the ancient ruins and will raise up the age-old foundations;
you will be called Repairer of Broken Walls, Restorer of Streets with Homes.”
Isaiah 58:12

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ACKNOWLEDGMENTS
I would like to thank everyone who has been a part of this research. It is
amazing what can be accomplished in a few years with a good opportunity, the Lord’s
favor, and a motivation to work hard. If this work should succeed in advancing the use
of HyPar roofs, then it is because HyPars were already a great housing solution. I am
thankful mostly for the Lord’s continued provision and favor over my life. His grace
changes everything, making all things possible.
Thank you to my family, who has supported me in all of my endeavors.
Wherever life has taken me, you have been a guiding and reassuring light. Thank you
to my closest friends, who have been a constant encouragement. We have studied many
different things over the past six years, and it has been a joy to learn and grow alongside
you. Thank you to my bride, Ashleigh. We have been busy this year, and you have
seen me at my worst and most stressed, but you have steadfastly supported and
encouraged me to finish. For everything else, I can’t thank you enough.
Thank you to my advisor, Dr. Chris Ramseyer. Ever since taking structural
analysis, you have taught me well, given me opportunity, and challenged me often.
Your support is one of the main reasons I returned to the University of Oklahoma to do
this research. I will always count you as a significant influence on my education and
development as an engineer. Thank you to Mike Schmitz, who was able to answer
every question I had while working in Fears Lab. You are invaluable. Thank you to the
rest of my committee, Dr. Kianoosh Hatami and Dr. Royce Floyd. Your expertise and
investment in this university and its students is what will continue to make this place a
great learning environment.

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Thank you to Engineering Ministries International, to Craig Hoffman, Brad
Crawford, and Rex Barber. Those few months of working with you were some of the
richest of my life. It is truly amazing to see how a thing, these HyPar roofs, can
develop when passion and opportunity follow after each other. I look forward to
continuing to work with EMI, whether it’s with HyPar roofs or other volunteer work.
Thank you to TSC Global, to George Nez, Brad Wells, Steve Riley, and the rest of
the gang. I became enamored with HyPar roofs and have tried to contribute to their
bright future as much as possible. From feverishly taking notes in Colorado as George
elaborated on the design and testing of HyPars, to working closely with Steve in
Thailand as we taught a group of Burmese medics how to build the roof, I have
cherished every opportunity.
Thank you to Cambridge University, to Dr. Matt DeJong and Dan Balding. It has
been a great pleasure collaborating with you this past year. I truly believe that this work
on HyPar roofs may propel them to faster and wider spread adoption. Thank you, Dan,
for putting me up, or maybe more appropriately, putting up with me while I was in
Cambridge. Thank you to the University of Oklahoma, the CEES Department, the
Engineering Department, and the Graduate College, who provided the funding for my
trip to Cambridge.
Finally, I would like to take a moment to draw attention to the great global need for
safe and sustainable housing and infrastructure. Every effort I have given to this
research and thesis has been out of a motivation to provide something better to those
people in need of the “restorer of homes.”

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TABLE OF CONTENTS
Acknowledgments ............................................................................................................ii
Table of Contents ............................................................................................................ iv
List of Figures................................................................................................................... v
List of Tables.................................................................................................................. vii
Abstract..........................................................................................................................viii
1 Introduction ................................................................................................................ 1
1.1 Summary of problem........................................................................................... 1
1.2 Objective of research........................................................................................... 2
1.3 Thesis Format...................................................................................................... 4
2 Background................................................................................................................. 5
2.1 Concrete .............................................................................................................. 5
2.2 Latex modification ............................................................................................ 11
2.3 Shell structures.................................................................................................. 20
2.4 Hypar shells....................................................................................................... 27
2.5 Ultra-thin HyPar roofs....................................................................................... 34
2.6 HyPar construction............................................................................................ 36
3 Journal Article .......................................................................................................... 45
3.1 Introduction....................................................................................................... 47
3.2 Objectives.......................................................................................................... 48
3.3 Research Significance ....................................................................................... 48
3.4 Background ....................................................................................................... 49
3.5 Experimental Research...................................................................................... 58
3.6 Experimental Results......................................................................................... 69
3.7 Conclusions and Recommendations.................................................................. 87
Combined References..................................................................................................... 90
Appendices ..................................................................................................................... 92

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LIST OF FIGURES
Figure 1.1: Half-scale HyPar roof, Cambridge ............................................................... 3
Figure 2.1: Acrylate polymer structures (EA and MMA) ............................................ 13
Figure 2.2: Adhesion versus years of exposure of acrylic mortars ............................... 17
Figure 2.3: Elastic and plastic response ........................................................................ 19
Figure 2.4: Pantheon dome, Rome ................................................................................ 21
Figure 2.5: Anticlastic and synclastic shells.................................................................. 25
Figure 2.6: Hypar roof at railway station, Poland .......................................................... 27
Figure 2.7: Hypar formwork, Candela .......................................................................... 29
Figure 2.8: Hypar reinforcement installation, Candela ................................................. 29
Figure 2.9: Umbrella hypars, Candela........................................................................... 30
Figure 2.10: Broadmoor Hotel hypar, Ketchum............................................................. 31
Figure 2.11: Hypar failure at Tucker High School........................................................ 33
Figure 2.12: HyPar school project in Kenya ................................................................. 34
Figure 2.13: HyPar frame made of lumber in England ................................................. 37
Figure 2.14: Hypar frame made of bamboo in Thailand............................................... 38
Figure 2.15: Installation of fiberglass mesh reinforcement........................................... 40
Figure 2.16: Installation of chicken-wire reinforcement............................................... 40
Figure 2.17: HyPar shell after first layer....................................................................... 42
Figure 2.18: Mixing latex-modified concrete................................................................ 43
Figure 2.19: Application of latex-modified concrete .................................................... 44
Figure 3.1: Typical HyPar frame................................................................................... 50
Figure 3.2: Typical CMU wall or concrete column support structure .......................... 50
Figure 3.3: Reinforcing fiberglass mesh ....................................................................... 51
Figure 3.4: Finished HyPar roof.................................................................................... 51
Figure 3.5: Flow table.................................................................................................... 60
Figure 3.6: LMC cubes.................................................................................................. 61
Figure 3.7: Hydraulic compression machine................................................................. 62
Figure 3.8: LMC prisms, HyPar shell panel................................................................... 64
Figure 3.9: Flexure testing machine .............................................................................. 64

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Figure 3.10: Franktown HyPars .................................................................................... 67
Figure 3.11: Franktown HyPar panel location .............................................................. 68
Figure 3.12: Franktown HyPar specimen in flexure ..................................................... 68
Figure 3.13: Compressive Strength versus Latex Content ............................................ 70
Figure 3.14: Flexure Strength versus Latex Content..................................................... 71
Figure 3.15: Compressive Strength versus Flexure Strength (l/c) ................................ 73
Figure 3.16: Flow versus Latex Content ....................................................................... 74
Figure 3.17: Compressive Strength versus Water Content ........................................... 75
Figure 3.18: Flexure Strength versus Water Content .................................................... 76
Figure 3.19: Compressive Strength versus Flexure Strength (w/c)............................... 77
Figure 3.20: Flow versus Water Content....................................................................... 78
Figure 3.21: Bad Franktown HyPar Sample, 2SL......................................................... 80
Figure 3.22: Good Franktown HyPar Sample, 1SH...................................................... 81
Figure 3.23: Flexure Strength of Franktown HyPar Specimens.................................... 83
Figure 3.24: Common failure mechanisms of first HyPar shell (1SH) ......................... 84
Figure 3.25: Common failure mechanisms of second HyPar shell (2NWH)................ 85
Figure 3.26: Second common failure mechanisms of second HyPar shell ................... 86

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LIST OF TABLES
Table 2.1: Typical constituents of Portland cement ...................................................... 10
Table 2.2: Portland cement composition....................................................................... 11
Table 2.3: Properties of polymethacrylates................................................................... 14
Table 2.4: Drycryl physical properties .......................................................................... 18
Table 2.5: Drycryl chemical composition ..................................................................... 18
Table 2.6: HyPar concrete mix design .......................................................................... 41
Table 3.1: Properties of acrylate polymers.................................................................... 53
Table 3.2: Flexure strength of LMC.............................................................................. 72
Table 3.3: Flexure strength of Franktown HyPar specimens ........................................ 82

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ABSTRACT
Safe and sustainable housing is a global need, as nearly one-quarter of the
world’s population lives in substandard housing. HyPar roofs, which are hat-shaped
concrete shell roofs, are one solution to this need. Utilizing the world’s most common
construction material, HyPar roofs employ concrete in an innovative way. By using
latex-modified concrete over a doubly-curved tensile fabric form, HyPar roofs can
achieve a shell thickness of about 0.4 inches, resulting in a lightweight structure that
exhibits impressive strength and durability. These benefits are commonly met with
disbelief, as many potential clients and non-profit investors do not understand how a
concrete roof could be so thin. To address this need for better understanding and
engineering proof of HyPar strength and durability, this research will investigate and
present important characteristics of the material science and mechanical behavior of the
latex-modified concrete used in HyPar roofs.
In order to appeal to the diverse audience that may be interested in innovative
housing solutions, and to progress the understanding and adoption of HyPar roofs, this
research covers a broad scope. To first understand the current research and
understanding of shell structures and latex-modified concrete, an in-depth history and
literature review was conducted. Building on that foundation, laboratory investigations
were made into the compressive and flexural strength of latex-modified concrete, as
well as the material’s workability. The specific focus of these tests were on concrete
that is modified with Drycryl, which is the most common latex product used in HyPar
roofs today. Finally, existing HyPar roof samples were tested for flexure strength,

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making an investigation into the durability of the roof, as well as the importance of
quality control during construction.
The research presented in this report concludes that latex-modification
significantly increases the flexural strength of the concrete, improving its performance
in thin shell applications. Additionally, latex improves the water performance and
workability of the concrete. Using quality and well-preserved latex is vitally important
to the strength and durability of the HyPar shell, as degraded latex has shown to have an
adverse effect on the flexure strength of the concrete. These findings should inform and
support the adoption, design, and future use of HyPar roofs.

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1 INTRODUCTION
1.1 Summary of problem
There is an enormous need for sustainable shelter across the world, especially
following disasters and in developing regions. According to Habitat for Humanity,
about 1.6 billion people, approximately 23% of the world’s population, live in
substandard housing and 100 million are homeless (Habitat, 2010). These substandard
conditions are apparent in Port au Prince, Haiti, especially after the devastating 2010
earthquake. The New York Times published an article on August 16, of 2012,
explaining “Two and a half years after the earthquake [in Haiti], despite billions of
dollars in reconstruction aid, the most obvious, pressing need — safe, stable housing for
all displaced people — remains unmet” (Sontag, 2012). Extreme poverty is perpetuated
when there isn’t a sustainable and lasting solution for the housing crisis.
HyPar roofs are a safe and cost-effective solution to this pressing need. They are
hat-shaped roofs made of four hyperbolic paraboloid sides, constructed by building a
wood frame and installing strips of fiberglass mesh in orthogonal directions. A latex-
modified concrete is applied in thin layers over the fiberglass mesh until a final
thickness of approximately 10 mm (0.40 in.) is reached. The potential of these roofs is
far-reaching, but there has been little scientific testing to prove their effectiveness. TSC
Global is the leading advocate of HyPar roofs, and has branded the name “HyPar”.
They have constructed these roofs in a number of developing countries with great
success, but their proof of HyPar strength is mainly allegorical. In order for the HyPar
roof to be accepted on a larger scale it must first be scientifically investigated.

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1.2 Objective of research
In the Fall of 2011, TSC Global partnered with EMI, a non-profit ministry,
which assembled a team of engineers and architects to analyze and improve the design
of the roofs. During preliminary analyses many assumptions were made because there
was very little engineering data available for thin HyPar roofs. Recognizing the need
for thorough investigation into the HyPar’s material properties, research soon began at
the University of Oklahoma.
A unique mixture of latex-modified concrete (LMC) is used to create the thin
HyPar shell. The LMC may be understood as a mortar, because it excludes large
aggregates, but for the purpose of this research and report it will be referred to as
concrete. The primary goal of this research is investigating the mechanical behavior of
the concrete mixture by testing varying latex contents in the mix design. Latex is the
most unique and expensive ingredient in the mixture, so it is important to understand its
contribution to the shell strength. Achieving an optimal ratio of latex to cement is a
desired outcome of this research also, as it may decrease the total cost of the HyPar
roof. The latex-modified concrete is applied in very thin layers, usually around 0.10
inches thick, so achieving a highly workable concrete is necessary. Water is added
liberally to the mix during construction to increase the mixture’s flow and workability,
but this may decrease the concrete’s strength. Another goal of this research is to
achieve the best mix design for strength and workability by testing different water
contents in the mix design. Finally, field samples of existing HyPar roofs will be tested
for flexure strength, and examined for the common modes of failure. This research will
promote better design and construction of HyPar roofs.

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While research at the University of Oklahoma investigates the material science
of the HyPar shell, Cambridge University has begun research that will investigate the
HyPar roof’s performance during an earthquake. The University of Oklahoma has
partnered with Cambridge University to assist with the construction of a half-scale
HyPar model, shown in Figure 1.1, which was tested to assess the seismic performance
of the roof. As academic interest in the HyPar roof continues to grow, the body of
knowledge relevant to the roof will grow as well, hopefully contributing to its increased
acceptance worldwide.
Figure 1.1: Half-scale HyPar roof, Cambridge

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1.3 Thesis Format
This thesis is formatted in a way as to include an extensive literature review and
also a self-contained, unpublished journal article. The literature review provides a
suitable background for the technologies that are employed in a typical HyPar roof.
Understanding that there is not a large body of information specific to HyPar roofs, a
journal article may become useful to the future of the structure if it is published. The
article is formatted in a way that is can be submitted for publishing without additional
formatting and editing. Formatting requirements for publishing in ACI journals can be
found at http://www.concrete.org/PUBS/pubs_authorguidelines.htm.
THESIS FORMAT
Chapter 1) Background
Chapter 2) Journal Article
Combined References
Appendices

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2 BACKGROUND
This section provides the body of information that is helpful in understanding the
specific goals and scope of this HyPar research. HyPar roofs are the synthesis of
several technologies, each of which will be discussed individually. The first topic of
discussion will be concrete and its history, chemistry, and modern applications. The
next topic will be shell structures. After discussing principles that govern their design, a
history of shell structures will be explored from their earliest use as domes to their
modern use as HyPar roofs. These topics combine to form the background for HyPar
roofs, which will be discussed last in this section.
2.1 Concrete
Concrete has high compressive strength but low tensile strength. It is generally
weak in adhesion as well. Modifying concrete with materials of higher tensile strength
and adhesion can yield a stronger and more durable product. Latex-modified concrete
is a popular composite material used today and is an integral part of the HyPar roof,
which is the subject of this research.
In order to best understand concrete as it applies to this research, it is important
to discuss the chemistry behind hydration and latex modification. Before that
discussion though, it will be beneficial to explore the history of concrete.
2.1.1 History of concrete
As early as 3,000 BC, Egyptians mixed mud with straw to form dried bricks.
Although they weren’t making concrete, they clearly understood the process of using a

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paste and aggregate mix. They also pioneered the use of a lime-gypsum mortar as a
cementitious paste in the construction of the Pyramids. As the same time, cementitious
materials were being used in other places around the world.
Moving forward, the Romans advanced concrete with the use of aggregates,
calcination, and even admixtures. The cementitious paste in Roman concrete was
typically made of quicklime and pozzolana, a volcanic ash found in Pozzuoli. Pumice
was used commonly as an aggregate in the concrete mixture. Even though Roman
concrete was invented more than 2,000 years ago, it resembles concrete used today in
many ways. For example, Vitruvius specified a mix of one part lime to two parts
pozzolana for concrete to be used underwater. That is essentially the same mix ratio
used for hydraulic concrete today (Lechman, 1986).
The chemistry behind Roman concrete is very similar to the chemistry behind
modern concrete. When limestone is burned at its calcination temperature of 1,500
degrees Fahrenheit, it becomes quicklime, or calcium oxide. Limestone contains
calcium carbonate (CaCO3), and burning it liberates the carbon dioxide, leaving only
calcium oxide. When mixed with water, calcium oxide becomes calcium hydroxide.
Pozzolana is a volcanic material composed of silica and aluminum. In the presence of
water, it reacts with the quicklime to form calcium silica hydrate (CSH). This reaction
sequence will be discussed in further detail later. CSH is the most important product in
concrete, primarily responsible for the concrete’s strength. Pumice, a vesicular volcanic
rock with a high silica content, was often used as an aggregate in Roman concrete, as
seen in the Pantheon. The art of concrete was lost after the fall of the Roman Empire in
the fifth century AD. More than 1,000 years passed until it was rediscovered.

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In 1756, John Smeaton rediscovered how to make hydraulic cement. For his
discovery, he is considered today as the father of engineering. What made his hydraulic
cement different from common mortars was the introduction of clay during the
calcining process. When heated, calcium in the limestone reacts with the clay,
producing silicates that enable the lime to set without exposure to air. This innovated
process allowed for an earlier initial set in the concrete, allowing it to be used even at
sea. Smeaton used his hydraulic concrete in the Eddy Stone Lighthouse, which was
constructed in 1759, rising to a height of 59 feet.
The next great milestone in the history of concrete is the invention of Portland
cement. Joseph Aspin, a British brick-layer, patented Portland cement in 1824. His
method of manufacturing the cement was to produce a CSH clinker product through
calcination, and then pulverizing into a cementitious powder. The calcining
temperature was approximately 2,650 degrees Fahrenheit, much hotter than the process
used to create Roman cement. Today, Portland cement is the most commonly used
cement in the world.
2.1.2 Concrete hydration
Hydration is the chemical process that occurs in concrete when cementitious
material reacts with water, bonding all elements in the concrete matrix together and
hardening over time. Calcium silicate hydrates, which are crystallized during the
hydration process, are responsible for the strength gain in the concrete. The creation of
CSH crystals is expressed in Equation 2.1.

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Equation 2.1: Formation of CSH
CSH crystals are formed when calcium hydroxide reacts with a silicate. Calcium
hydroxide may be formed by the reaction of calcium carbonate and water. Although
these are the fundamental elements necessary for CSH formation, cement often contains
other elements. Table 2.1 outlines the major constituents of Portland cement. The three
most important constituents of cement are the aluminates and silicates, as they account
for most of the weight and reactive elements in the cement. Each aluminate and silicate
hydrates at a different rate. Hydration of tricalcium aluminate (C3A) and tricalcium
silicates (C3S) is responsible for the initial set and strength of the concrete. During this
stage, the first calcium silicate hydrates are created (CaH2SiO4), bonding sand and
aggregate to the cement. Early strength gain is continued by the tricalcium silicates,
because they hydrate slightly slower than the aluminates. Dicalcium silicate (C2S)
hydrates the slowest, making it responsible for long-term hardening and strength gain.
The reactions continue with the remaining water until ultimate strength is reached.

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Table 2.1: Typical constituents of Portland cement
Name Chemical Formula Notation % by Weight
Tricalcium silicate 3CaO SiO2 C3S 50%
Dicalcium silicate 2CaO SiO2 C2S 25%
Tricalcium aluminate 3CaO Al2 O3 C3A 10%
Tetracalcium aluminoferrite 4CaO Al2 Fe2 O3 C4AF 10%
Gypsum CaSO4 H2O -- 5%
Portland cement is available worldwide. In 2010, cement production in the world
reached 3.64 billion tons (USGS, 2011). Depending on the desired properties of the
concrete, different types of Portland cement may be obtained.
Table 2.2 shows the composition of three different types of Portland cement.
Type I is general use cement and the most commonly used type. Type II is designed to
have moderate sulfate resistance, making it useful for concrete in places where it is in
contact with soils or ground water that may have high sulfate content. Type III is
designed to have higher early strength. Typically, concrete made with Type III cement
exhibits three-day compressive strengths similar to the seven-day compressive strengths
of Type I and II cements. This is due to the finer grinding and higher concentration of
tricalcium silicates found in Type III cement. There is a trade-off however, because
Type III cement may exhibit lower long-term strength gain due to the reduced
concentration of dicalcium silicates. These three types of Portland cement are the most
commonly used products on the market today.

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Table 2.2: Portland cement composition
Portland Cement Composition
Chemical Name Notation Type I Type II Type III
Tricalcium Silicate C3S 55.0% 51.0% 57.0%
Dicalcium Silicate C2S 19.0% 24.0% 19.0%
Tricalcium Aluminate C3A 10.0% 6.0% 10.0%
Tetracalcium alumioferrite C4AF 7.0% 11.0% 7.0%
Magnesium Oxide MgO 2.8% 2.9% 3.0%
Sulfate SO3 2.9% 2.5% 3.1%
Ignition loss Q (Heat) 1.0% 0.8% 0.9%
Calcium Oxide CaO 1.0% 1.0% 1.3%
2.2 Latex modification
Using latex to modify concrete is nothing new in construction, but it still remains
a topic of research and debate. Manufacturers of latex modifiers boast about a wide
range of benefits and it is generally accepted that latex-modification may increase the
exterior durability of the concrete and its flexural strength. Such benefits are extremely
attractive for thin-section concrete, so latex is most commonly used in patchwork and
thin concrete overlays. Considering that HyPar roofs are usually not thicker than half
an inch, latex is considered an indispensible part of the concrete mix. This section will
discuss various types of latex modifiers and their benefits.

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2.2.1 Types of latex modifiers
There are several types of latex modifiers that have been historically used in
mortars and concrete. The oldest of these latexes is polyvinyl acetate (PVA), which has
been commonly used in tile grouts. PVA increased the workability of the tile grout but
decreased the grout’s water performance. When cement was hydrated in the grout
mixture, the PVA latex would encapsulate sand particles, preventing the cement from
adequately bonding to the sand. After the initial hydration, the PVA could rehydrate
and release from the sand, causing the grout to fail quickly. Today PVA is more
commonly used in Elmer’s and other water-soluble glue products.
Another type of latex that merits discussion is styrene-butadiene (SBR). SBR is a
synthetic rubber product created by the copolymerization of approximately 25% styrene
and 75% butadiene (Britannica, 2013). It has been used as a sealing and binding agent
in concrete renders. SBR reduces shrinkage and increases the flexibility of the
concrete, but it has poor aging characteristics and low UV resistance. As SBR ages it
hardens and becomes brittle due to oxidation and UV exposure. Today, SBR is used in
nearly 50% of tires, but not for tires that experience heavy use like those on buses or
airplanes. Because of its poor UV performance, SBR it is a poor additive for concrete
renders.
The latex modifiers of particular interest to this study are acrylic polymers.
Where PVA and SBR fail, acrylic polymers perform well. Aside from improved water
performance and UV resistance, acrylic polymers have other benefits like improved
flexure strength, impact strength, and adhesion. Acrylic polymers are especially useful
in thin sections and are commonly used in concrete overlays, patchwork, and renders.

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An acrylate polymer is a mixture of monomers which is based on the structure
of acrylic acid. Monomers are selected from the C1 to C8 acrylate esters, which are
organic compounds that are combined during polymerization. The three most relevant
acrylate polymers are methyl methacrylate (MMA), ethyl acrylate (EA), and butyl
acrylate (BA). Their structures are shown in Figure 2.1. Each of these acrylates is the
combination of a fundamental carbon chain and a side chain. Methyl acrylates (CH3)
are created by the substitution of a hydrogen atom in the carbon chain with CH3. Ethyl
acrylates (C2H5) are created by the substitution of a hydrogen atom in the carbon chain
with CH2 – CH3. Butyl acrylates (C4H9) are created by the substitution of a hydrogen
atom in the carbon chain with CH2 – CH3 – CH4. By observing the engineering
properties of these polymers, presented in Table 2.3, an important conclusion can be
made. As the side chain becomes longer, the tensile strength of the polymer decreases
and the elongation before rupture increases (Lavelle, 1988). In summary, methyl
acrylates are much stronger polymers than butyl acrylates.
Ethyl acrylate Methyl methacrylate Butyl acrylate
Figure 2.1: Acrylate polymer structures (EA and MMA)
side chain -

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Table 2.3: Properties of polymethacrylates
Polymethacrylate Tensile strength, psi Elongation, %
Methyl 9000 4
Ethyl 5000 7
Butyl 1000 230
There are many forms of polymerization, but the most common for latex
additives is emulsion polymerization. The emulsion usually incorporates monomers,
water, and a surfactant. During emulsion polymerization latex particles are
spontaneously formed when individual polymer chains attach themselves to the free
radicals of other chains. Each latex particle is surrounded by surfactant, which acts as
the emulsifier by repelling other particles electrostatically. Water provides the lubricant
that allows deflocculating of the latex particles. Most latex additives are packaged and
sold as an emulsion.
2.2.2 Acrylic latex modifiers
Acrylic latex has been used in concrete most commonly for patchwork,
overlays, and renders. It is typically sold as an emulsion of 50% solids and has a milky
appearance. The concrete mix must be formulated to account for the water that is
already present in the latex emulsion. When acrylic latex is used to modify concrete,
two other important considerations must be made.

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First, using latex increases air entrainment, especially during mechanical
mixing. This lowers the concrete density, ultimately compromising the concrete’s
compressive strength. Traditional concrete has a density of about two grams per cubic
centimeter (145 lb/ft3
) or greater. Latex-modified concrete theoretically lowers the
amount of water necessary, so its density should be equal or greater than typical
concrete.
Possible solutions to lower air entrainment in the latex-modified concrete are as
follows. The most common solution is to add a defoaming agent to the concrete
mixture. When done properly, this will limit air entrainment and ensure a dense
concrete mix. If a defoaming agent is unavailable or undesirable, then care should be
taken to mix the concrete gently. Hand mixing smaller batches allows for more control
and less agitation. If measures are taken to reduce air entrainment, then the resulting
concrete will be denser and exhibit increased performance in all areas.
The second consideration has to do with the curing process. For traditional
concrete, wet curing provides the optimal conditions for hydration because water needs
to be readily available for CSH reactions. For latex-modified concrete, ambient curing
is necessary. This requirement is a favorable one, considering that concrete is almost
never properly wet cured in practice.
When latex-modified concrete hydrates, the acrylic latex retains water necessary
for long-term hydration. This occurs because of film formation in the latex. When
water first evaporates during curing, a film of coalesced latex particles forms around the
cement and sand particles. In order for the film to form properly, the spherical acrylic
polymers must be sufficiently deflocculated during manufacturing of the latex.

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Flocculated particles create a spongy film and introduce voids into the matrix (Lavelle,
1988). Proper film formation prevents further water loss through evaporation, allowing
the concrete to be optimally cured in ambient conditions.
Acrylic latex-modified concrete, when mixed and cured properly, increases the
performance of the final product in a number of ways. Water resistance is improved,
increasing the concrete’s performance during freeze-thaw cycles. This material won’t
absorb UV radiation because acrylics are mostly transparent to natural sunlight. This
increases UV durability and the concrete’s lifespan. Acrylics are also mostly
chemically inert, so they don’t easily react with many acids or bases. Dense latex-
modified concrete exhibits improved impact and flexure strength as well as improved
adhesion. Many of these benefits are especially useful for thin-section concrete.
In a study of shear bond adhesion, the latex-modified system performed
significantly better than traditional concrete (Figure 2.2, Lavelle). All adhesive tests in
this study showed cohesive failure in the latex-modified concrete and adhesive failure in
the traditional unmodified concrete (Lavelle, 1988). This means that latex-modified
concrete is suited especially well for overlays and construction where successive thin
layers of concrete are applied on top of each other, such as HyPar roofs.

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Figure 2.2: Adhesion versus years of exposure of acrylic mortars
2.2.3 Drycryl
Drycryl is the latex of particular interest to this study. It is an acrylic polymer
manufactured by DOW Chemical that comes in the form of a dry, dispersible powder.
Drycryl is cheaper to ship and easier to store because it comes as a powder and not a
liquid. TSC Global takes advantage of this benefit because they have imported and
used Drycryl in HyPar construction around the world.
Being an acrylic polymer modifier, Drycryl offers the benefits discussed earlier.
According to the manufacturer, “incorporating this powder allows compounders to
attain the dramatic improvements in adhesion, abrasion resistance, flexural strength, and

17
exterior durability that are associated with acrylics.” DOW Chemical recommends
using a ratio of 10-20% latex to cement for best results. They also recommend using a
defoamer to reduce air entrainment, claiming that the density of concrete modified with
Drycryl is very similar to unmodified concrete.
Due to the proprietary nature of this product, the best information available for
the composition of the Drycryl is found in the material safety data sheet (MSDS).
Further data has been collected from representatives and the best available information
is presented in Table 2.4 and 2.5.
Table 2.4: Drycryl physical properties
Appearance Free-flowing, white powder
Polymer type 100% acrylic
Bulk density, lb/ft3
25.0
Glass transistion, Tg, °C 17
Average particle size, microns ~60
Anti-caking agent, % ~5.0
Table 2.5: Drycryl chemical composition
Component CAS-no. Concentration
Acrylic Polymer(s) Trade Secret 94.0 - 96.0%
Methyl methacrylates
Butyl acrylates
Individual residual monomers Not Required < 0.1%
Calcium Carbonate 471-34-1 1.0 - 3.0%
Water 7732-18-5 0.5 - 3.0%

18
Drycryl is a unique blend of butyl acrylate and methyl methacrylate polymers.
As discussed earlier, methyl acrylates are tough and have high tensile strength while
butyl acrylates are softer and have a higher modulus of rupture. These polymers
account for the 95% of the acrylic latex modifier and their presence in the concrete
matrix may increase the tensile and flexure strength of the concrete.
The glass transition temperature (Tg) is the temperature at which a polymer
transitions between elastic and plastic behavior. In engineering, the transition between
elastic and plastic response identifies the material’s yield strength (Figure 2.3: Elastic
and plastic response). Polymers become more pliable and moldable above their glass
transition temperature. When a polymer is cooled below its Tg, it becomes hard and
brittle. To illustrate this transition, think of a plastic bucket that is left outside year-
round. The plastic bucket will be brittle in the winter months and then become softer in
the summer months.
Figure 2.3: Elastic and plastic response

19
For engineering purposes, rising above the glass transition temperature is
recognized by a sharp decline in the material’s stiffness and an increase in its impact
strength. Polymers with a Tg above the ambient temperature are brittle and have low
impact strength, while polymers with a Tg below the ambient temperature are soft and
flexible. Polymers with a Tg that is similar to the ambient temperature will exhibit
plastic behavior, being tough and having good impact strength. Drycryl has a glass
transition temperature of 17°C (63°F) which may be considered as similar to ambient
temperatures.
2.3 Shell structures
Shell structures are desirable for a number of reasons. They possess an
impressive aesthetic but they also serve the important function of spanning large
distances without obstruction. When the Romans built the Pantheon it was a part of a
large construction campaign meant to convince the world that their empire was
supreme. After the Romans, domes became an integral part of the most impressive
cathedrals. Domes are only one form of literally thousands of possible shell structures.
As engineering and construction advanced over time designers began experimenting
with new types of shells.

20
2.3.1 History of shell structures
The Pantheon was the earliest shell structure constructed out of concrete. After
fires destroyed two previous temples, the Pantheon we know today was built in Rome in
AD 125. Its most impressive feature is the large dome that measures 142 feet in
diameter, shown in Figure 2.4.
Figure 2.4: Pantheon dome, Rome
The shell of the Pantheon dome is twenty-one feet thick at its base but only four
feet thick at the oculus, a skylight measuring thirty feet in diameter. Roman builders
ingeniously built the dome with denser concrete at the bottom than at the top by using
progressively more lightweight pumice in the concrete mix as they created the thinning
shell. This practice, combined with the honeycomb structure of the dome, reduced the
weight of the structure. The 5,000-ton dead weight of the dome is carried by eight
barrel-vaults that distribute the load to the Pantheon’s outer walls, which are twenty-one
feet thick.

21
Even though this structure was built almost 2,000 years ago, the technology
required to build it is impressive. Compressive strengths of the concrete have been
estimated at 2,800 psi; not far off from the strength of some concrete used today.
Tensile strengths have been estimated at 210 psi. Although the dome was not
reinforced with elements of higher tensile strength, modern finite element analysis has
determined that the Pantheon’s dome experiences a maximum tensile stress of only 18.5
psi (Mark et al., 1986), and that occurs at the point where the dome joins to the outer
walls. Ingeniously, the thickest section of the Pantheon, measuring 21 feet, was built
where the highest tensile stress occurred. The Pantheon is an impressive structure that
still stands today. After it was built over 1,000 years passed until the reemergence of
concrete shells in the modern era.
The first concrete dome of the modern age is the Jena-Zeiss Planetarium, which
opened in 1926 and is still in operation today. Shortly after in the 1930s, the Roberts
and Schafer Company of Chicago was the first firm to build thin concrete shells in the
United States. Their predominant use of concrete shells was for industrial buildings.
The next major use of concrete shells was during World War II.
2.3.2 Design of Shell Structures
Material science has improved substantially as it applies to shell structures. As
discussed earlier, thin section concrete is possible with the latex modification.
Traditional concrete or masonry domes could typically achieve a radius to thickness
ratio of 50, but modern domes can attain a ratio of 800 (Denny, 2010). Because of this
larger areas are being spanned with less material and shells are only becoming thinner.

22
Each shell presents a unique challenge of design and analysis. While there are
only a few structural systems for basic post and beam design, there are thousands of
structural systems for shells, because each shell requires its own approach to design.
This being said, there is always a simple method of analysis that can be used to check
more precise analysis. Instead of relying on design procedures, shells require thorough
knowledge of design principles.
Most shells can be understood simplistically as a set of beams, arches, and
catenaries. This is a simplistic view, but it is useful during preliminary design when the
most important task is to gain an understanding of how the structural system behaves.
Typical post and beam structures rely on the strength of materials, but this is not true for
most shells. Shell structures get their strength primarily from their shape.
The fundamental purpose of a shell is to evenly distribute applied loads and
transfer them to the supporting members and finally the ground. Distributed loads are
transferred to the supports by tangential shearing and tensile or compressive forces that
act along the shell. These internal forces acting in the shell are generally of small
magnitude, except in the region near each column support. It’s in these regions that
critical tensile forces and bending moments are developed. For this reason, the supports
for the shell are more important that the shell itself.
Creating a rigid frame is one of the most important considerations during the
design of a shell structure. The shell supports must be capable of taking the shell
reactions without appreciable deformations. When the supports are designed and built
as a rigid frame, the shell may transfer loads directly as tensile and compressive

23
stresses. For most spans, the internal stresses in the shell will be less than the allowable
stress.
Another important consideration when designing a shell is determining its size.
For most spans, the load carrying capacity of the shell is greater than required.
Compressive stresses are usually a fraction of the allowable stresses. Considering this,
the size of the shell is not usually determined by its strength. Construction stability and
serviceability requirements usually dictate shell size and thickness.
The final, and most important consideration for shell structures is their shape.
Thin shell structures are “characterized by their three dimensional load carrying
behavior which is determined by their geometrical shape” (ACI, 2002). Shells are
categorized by their curvature. For this study, we will only discuss shells of double-
curvature.
Shells of double-curvature may be categorized as either synclastic or anticlastic
surfaces. A synclastic surface in one in which the two principal directions of curvature
have the same sign. An anticlastic surface is one in which the two principal directions
of curvature have opposite signs. These surfaces are depicted in Figure 2.5. Domes are
synclastic surfaces, behaving as compression structures. Anticlastic surfaces perform
better than synclastic surfaces because of their opposing curvature. Anticlastic shells,
like the hyperbolic paraboloid (hypar), will have the combined benefits of an arch and
catenary structure. Within this report, “hypar” shall be used as a general term for such
shell structures, and “HyPar” shall be used to indicate the specific shell structure being
researched.

24
ANTICLASTIC SYNCLASTIC
Figure 2.5: Anticlastic and synclastic shells
Before the discussion continues onto hypar structures, a few more design
considerations are worth mentioning. The American Concrete Institute (ACI) has
published a paper (ACI 334.1R-92) on thin concrete shell design and analysis. This
section will briefly outline some of the design requirements.
According to ACI, three-dimensional elastic analysis is permitted. Elastic
behavior assumes the concrete shell is uncracked, homogeneous, and isotropic.
Poisson’s ratio may be assumed as equal to zero. To simplify design, a rigid frame of
supporting members should be used. Flexible frames are permitted with accompanying
design documentation, but the analysis becomes much more difficult and deflections
become larger, so flexible frames are discouraged.
The concrete compressive strength (f’c) shall not be less than 3,000 psi. Any
contribution of tensile strength from the concrete should be neglected, meaning the
tensile stresses in the shell should be resisted completely by reinforcement. The
maximum percentage of reinforcement allowed is 5% for reinforcement that has a

25
tensile yield strength (fs) of 25,000 psi. Fiberglass has tensile strengths ranging from 15
ksi to 25 ksi. The maximum aggregate size shall be smaller than half the shell thickness
and smaller than the reinforcement spacing. Considering these specifications, shell
thickness is not always dictated by strength requirements, but by construction and
serviceability requirements.
Stability of the shell should always be examined. Buckling in thin shells is the
most important stability consideration. The buckling load depends on shell geometry,
rigidity of the supporting members, material properties, and the type of load exerted on
the shell. As a thin shell deforms under load, membrane forces develop. Tensile
membrane forces, which exist in anticlastic shells, tend to return the shell back to its
original shape. A hypar shell is a great example of this. It is often possible to use the
linear buckling theory for shells that exhibit this behavior.
Now that the general history and design of shells has been discussed, it should
be obvious that hypar shells are superior to single-curvature shells, such as a dome. The
next sections will discuss hypar shells in depth.

26
2.4 Hypar shells
A hypar shell combines an arch and a catenary to form a three-dimensional
surface. The arch carries loads in compression while the catenary carries loads in
tension. Edge members of the hypar must be larger than the cross-sectional area of the
shell because they collect forces and distribute them to vertical supports that carry the
forces to the ground. Another interesting feature of hypar shells is that they can be
formed with completely straight lines. This phenomenon is highlighted in the hypar
roof shown in Figure 2.6.
Figure 2.6: Hypar roof at railway station, Poland
2.4.1 History of hypar shells
The first hypar roofs were built during the mid-twentieth century. They were
made possible by the reemergence of concrete shell structures and the advancement of

27
construction techniques and engineering design. Their popularity increased as
designers and engineers became more creative with their use of shells. Two pioneers of
hypar roofs that merit discussion are Felix Candela and Milo Ketchum.
Felix Candela constructed many concrete shell structures of varying sizes in
Mexico in the 1950s and 60s. He admired the shells for their beauty and function, as
they are able to span large distances while remaining thin. A lifelong builder, Candela
was educated as an architect, but he is also regarded as a self-taught engineer. He had a
keen understanding of his buildings, their design and construction, and he was able to
see every part of the project form start to finish. Mexico offered a great working
climate to experiment with new and strangely shaped structures because of low labor
costs. Each of these reasons contributes to Candela’s success with hypar roofs and
other shell structures.
Felix has an imaginative mind that created lots of interesting hypar shapes, but
he was also talented at overseeing their construction. Candela’s method of construction
illustrates perfectly how hyperbolic curves are created by straight lines. The
construction of each project was initiated by building incredibly complex scaffolding, as
seen in Figure 2.7. Once the formwork was finished construction would proceed with
the installation of a tensile reinforcement, as seen in Figure 2.8. Candela’s most
popular choice of reinforcement was thin welded wire mesh. This is a suitable
reinforcing material because it easily takes the shape of its form. Most of Candela’s
hypar roofs had an average thickness of three inches (Draper 2008).

28
Figure 2.7: Hypar formwork, Candela
Figure 2.8: Hypar reinforcement installation, Candela

29
Candela’s most economical use of hypar roofs was in industrial buildings. As
seen in Figure 2.9, umbrella hypars were used modularly in a grid layout. Through an
iterative process, Candela was able to optimize the shape for his larger hypar roofs. For
these structures he settled on an optimal thickness of four centimeters and an optimal
length to width ratio between one and two (Draper 2008).
Figure 2.9: Umbrella hypars, Candela
Milo Ketchum was a contemporary of Felix Candela. He also appreciated the
aesthetically beautiful and cost-effective nature of hypar roofs. While speaking about
the industrial hypars Candela built, Ketchum once remarked “Felix told me that he
could not charge owners what they cost. They were so inexpensive that it would
undermine the industrial building market.”

30
Milo’s first hypar project was for the First Methodist Church of Boulder,
Colorado. The project included a relatively small use of hypar roofs, with short spans
of 26 feet. This project allowed Ketchum to experiment with the hypar shape and grow
more comfortable with it. He later wrote in his memoirs “do not throw away all your
structural intuition when you design shell structures.”
Ketchum’s next hypar roof really pushed the envelope. He designed a four-
gabled hypar for the Broadmoor Hotel in Colorado that spans 260 feet diagonally. As
depicted in Figure 2.10, this hypar covers an area 185 feet by 185 feet, rising to a height
of 50 feet at its center. Milo was fond of calling this roof his “three inch shell spanning
260 feet.” It truly is an impressive structure.
Figure 2.10: Broadmoor Hotel hypar, Ketchum

31
Before construction began, the hotel suggested that they would hang a large
curtain down the middle of the structure in order to separate spaces beneath the roof.
When Ketchum was asked if the shell would carry the weight, he went back to the
drawing board. His solution was to prestress the members of the roof’s frame,
especially the top rib. All of the ribs were prestressed with steel cables. Doing this
helped manage deflections, stiffened the roof against torsional forces, and ultimately
may have saved the roof from collapsing (Ketchum 1999).
Thin concrete shells are very good at spanning long distances without column
interruption, but as the spans grow larger the risk of failure increases. Proper design
becomes more important and there is less room for error. Ketchum’s roof at the
Broadmoor has remained structurally sound because of good design and construction,
most notably the proper use of prestressed members.
In 1970, a large hypar roof at Tucker High School, in Richmond, Virginia, failed
catastrophically. The four-gabled hypar roof housed the school’s gym, covering an area
of 155 feet by 162 feet. Three other similar roofs had been built on the school’s
campus, and although only one of them failed, all four were demolished as a
consequence. When Milo Ketchum was consulted about the failure of the roof, he made
a site visit before the remaining roofs were torn down. While on site he observed an 18-
inch deflection at the center of the remaining roofs. Such a high deflection is an
obvious indicator that the ridges in the structure should have been cambered.
Prestressing the members, as was done to the Broadmoor hypar, could have prevented
the failure (Shaaban 1976).

32
Figure 2.11: Hypar failure at Tucker High School
2.4.2 Decline of hypar roofs
Hypar roofs experienced a decline in the 1970s for a number of reasons. Steel
post and beam structures are much easier to design and they can be more cost effective
for structures with shorter spans. The cost of concrete shells became more prohibitive
when the concrete industry experienced a tough financial downturn at the end of the
1960s. Increasing labor costs during and after the Vietnam War also contributed to the
decline of shell structure construction. Ultimately, shell structures require ingenuity
and take a longer time to design, so they didn’t stand a chance against the growing
popularity of rapid or prefabricated design and construction in America.

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2.5 Ultra-thin HyPar roofs
Although hypar roofs had declined in popularity by the 1970s, they weren’t
gone completely. Another man, Geroge Nez, had become interested in the technology
during the 1960s. Over the course of a few decades he developed an ultra-thin HyPar
roof which he was fond of using for residential housing in a number of developing
regions around the world. These HyPar roofs, as seen in Figure 2.12, are the subject of
this research. This section will discuss their development and construction.
Figure 2.12: HyPar school project in Kenya
George Nez pioneered thin HyPar roofs in the 1960s. In 1962, he worked for the
United Nations on an emergency relocation project in Ghana that required 14,000 new
homes be constructed in less than 18 months (Nez, 2011). His plan was to utilize
‘roofs-first’ construction. By putting up the roofs first and allowing the walls to be built
in later, shelter was made available quicker than a traditionally constructed home. Later
in his career, George was inspired by the hypar shape and realized it could be coupled

34
perfectly with thin-shell latex-concrete construction. George Nez co-authored the book
“Latex Concrete Habitat” with Albert Knott advocating ultrathin HyPar roofs as
permanent shelter solutions in low-income and developing regions (Nez, 2003). This
book inspired a man named Steve Riley, who became a pupil of Nez as he began
building HyPar roofs in a number of developing countries.
In March of 2010, Steve Riley partnered with an entrepreneur named Brad Wells
and others to found TSC Global. TSC has built these roofs in many different countries,
advocating their suitability for disaster relief and developing regions. Their attention
turned towards Haiti after the devastating 2010 earthquake. Although HyPar roofs are
an excellent solution to the housing crisis in Haiti, their adoption is stifled by the
uneducated beliefs of local Haitians and humanitarian organizations. There is a general
disbelief in the strength and durability of HyPar roofs, because their concrete shell is
less than ½ inch thick. In order to overcome this disbelief, two universities have begun
research programs that focus on the material strength and seismic performance of HyPar
roofs. The research presented in this paper investigates the material strength of the
latex-modified concrete that makes up each HyPar shell.

35
2.6 HyPar construction
HyPar roofs are built all over the world. Since the beginning of this research at
the end of 2011, HyPars have been built in Thailand, Burma, Bangladesh, and England.
Although each roof is unique, there is a basic method of construction that can be taught
and used regardless of the project’s location.
Construction of a HyPar roof can be broken down into three stages. The first
stage is the construction of the frame. Second is the installation of the fabric
reinforcement, which creates the curvature in the HyPar shape. The third and final
stage is the mixing and application of latex-modified concrete. Depending on the
availability of materials and labor, a HyPar roof large enough for a single-family
residence can be built in five days. This section will describe the construction process
in more detail. For more photos of HyPar roofs that were constructed in Thailand and
at Cambridge University, please refer to Appendix – D.
2.6.1 Frame construction
The frame of each HyPar roof is important for several reasons. The first and
most important reason is shape. A proper HyPar shell will be impossible to build if care
isn’t taken to build the frame correctly. The second reason is added strength. Although
the concrete shell is shown to carry all of the structural loads in simple analysis, the
frame also provides a significant amount of strength in the roof.
A HyPar roof with a base measuring twenty feet by twenty feet (6 m x 6 m) is
the most commonly built size, suitable for a single-family residence. A picture of a
finished lumber frame is shown in Figure 2.13. The roof shown was built at half-scale

36
in order to fit on the shake table in the structures laboratory at Cambridge University in
England. In full-size construction it is common to use 2x6 dimensional lumber.
Figure 2.13: HyPar frame made of lumber in England
In order to build the frame properly, first construct the base and take care to
build it square. As shown in Figure 2.14, measuring the exact distance between corners
and midpoints is important. Notice that this frame is built out of bamboo, since the roof
was being built in Thailand. Many different types of material may be used to build the
frame, as long as the frame remains rigid and square. If the frame is not perfectly
square it will create inaccuracies in the hypar shape that may distribute loads unevenly.

37
Figure 2.14: Hypar frame made of bamboo in Thailand
Once the base has been built the next step is to install the ridges. The ridges
should rise at a 45° angle and meet in the center of the roof. The most important
connections in the frame are located at the midpoints and corners of the base. Of these,
the connection at the midpoints should be the sturdiest, because it is the location that
collects forces in the shell and transfers them to columns and into the ground.

38
2.6.2 Fiberglass mesh installation
After the frame has been built the next stage of construction is the installation of
a fabric reinforcement. Aside from providing the primary tensile reinforcement in the
shell, the fabric also produces the HyPar shape. During this stage the HyPar will take
on its true shape because a hyperbolic paraboloid will form when the fabric is pulled
taut over the frame.
Install strips of fiberglass mesh in orthogonal directions, as shown in Figure
2.15. Using a stapler, first attach the fabric strip to the ridge member. Once attached,
pull the fabric taut across the edge member and staple it to that member. Achieve a
uniform tautness by pulling small sections of the strip “finger-tight” and then stapling
them to the frame. Using staples liberally is recommended because it is better to use
too many than too few. Once the first strip is installed, the rest of the strips are installed
in similar fashion but in overlapping orthogonal directions. Depending on the amount
of reinforcing desired, layers may be longitudinally overlapped. A typical overlap at
the top is about half the width of a strip. As shown in Figure 2.15, there will be more
overlap at the bottom of the roof than at the top. This is because the length of the ridge
member is longer than the length of the edge member. Gaps between layers of fabric
reinforcement may occur due to small errors in its installation. If this occurs simply
stitch the gaps together using a fine thread.
Other than providing tensile reinforcement, the main job of fiberglass mesh is to
create the hyperbolic paraboloid shape. As shown in Figure 2.15, the arch and catenary
curves of a hyperbolic paraboloid are formed during installation of the fiberglass mesh.

39
Figure 2.15: Installation of fiberglass mesh reinforcement
Fiberglass mesh is a relatively costly material. Its cost may be prohibitive in
some places or it may not be available at all. Alternative reinforcement, such as
chicken wire or window screening, may be used if fiberglass mesh is unavailable. After
constructing one roof with fiberglass mesh in Thailand, a second roof was constructed
with chicken wire (Figure 2.16). Before the chicken-wire was stitched together and
pulled across the frame a cotton sheet was installed. The purpose of the cotton sheet is
to hold the first layer of latex-modified cement as an integral fabric formwork.
Figure 2.16: Installation of chicken-wire reinforcement

40
2.6.3 Mixing and applying latex-modified concrete
After the frame has been built and the reinforcement has been installed, the final
stage of HyPar construction is to mix and apply the latex-modified concrete. This is
done in thin layers until the desired thickness is achieved. Using the right concrete mix
is important, and the mix changes depending on which layer is applied. Table 2.6
presents a general mix design, with proportions given by weight of material (Nez 2005).
Table 2.6: HyPar concrete mix design
Cement Sand Latex Water
First Layer 1 part 0 parts 0.1 parts 0.5 parts
Middle Layers 1 part 1 part 0.1 parts 0.5 parts
Last Layer 1 part 0 parts 0.1 parts 0.5 parts
For every layer, the latex-modified concrete is mixed the same way. A general
mix procedure is as follows: Cement and sand, the dry products, should be mixed
together in one bucket while a second bucket is used to combine the latex and mix
water. Redispersible powders, like Drycryl, may be incorporated into either the dry or
wet mix. Typically, Drycryl is mixed with water first in order to disperse it more
evenly into the latex-modified concrete.
It is best to mix the latex-modified concrete is small batches by adding the dry
mix into the bucket where the latex and water were combined. Most mixes are done by
hand or with a stirrer connected to a power drill. The latex-modified concrete should be
thoroughly mixed before application.
The first layer excludes sand from the mix in order to create a concrete slurry
with a larger proportion of cementitious material. This is important for the first layer,

41
when the main objective is to create a layer that covers the fabric reinforcement and
begins to give hardness to the hypar shape. During the first application of the concrete
slurry up to half of it may fall through the gaps in the fiberglass mesh. Care should be
taken to prevent this from happening as much as possible, but it is common that gaps in
the concrete layer will still exist after the first layer has hardened, as seen in Figure
2.17. Any gaps that remain will be easily covered during the application of the second
layer.
Sand is added to the concrete mix as additional layers are applied. The sand
should be fine, without any large aggregates. Large aggregates, up to half the thickness
of the final shell, will cause voids that weaken the final shell. So when sand is added,
care should be taken to use it properly.
Figure 2.17: HyPar shell after first layer

42
The final layer of the HyPar shell again excludes sand from the concrete mix.
This creates a finer concrete slurry, producing a smoother surface when it hardens. By
excluding sand the overall latex content in the final layer is increased as well. This
helps with waterproofing the roof, because the latex naturally resists water penetration.
For every layer, the latex-modified concrete should be mixed in small batches,
as seen in Figure 2.18. This is done for two reasons. First, latex-modified concrete
tends to set up faster than unmodified concrete, so a small batch may be realistically
applied before the initial set occurs. This will lead to less wasted product. The other
main reason for mixing in small batches is to have greater control over the product as it
is mixed. Latex in the concrete mix tends to foam because of the mechanical agitation
during mixing. Mixing small batches by hand reduces the foam, thereby reducing the
air entrainment in the concrete slurry.
Figure 2.18: Mixing latex-modified concrete

43
Once the small batch of latex-modified concrete is mixed it should be applied
quickly to the roof. Depending on the mix and the ambient conditions at the site, the
concrete may begin its initial set within fifteen or twenty minutes of mixing. The best
way to apply the concrete to the roof is using brushes and paint rollers, as seen in Figure
2.19. For the first layer, one person should be inside the roof to brush the concrete
slurry onto the reinforcing fabric, as it will naturally want to fall through. As the
concrete begins to harden it will become easier to brush and create a smoother surface.
Every layer should be applied in similar fashion, and extra care should be taken to
create a smooth surface when the last layer is applied.
Figure 2.19: Application of latex-modified concrete
The method of construction described herein is good practice, regardless of
where the HyPar roof is built. For a more comprehensive understanding of the HyPar
roofs that were constructed in Thailand and England, please refer to Appendix – D.

44
3 JOURNAL ARTICLE
This chapter of the thesis is an unpublished journal article to be submitted to the
American Concrete Institute journal publications. ACI publishes two journals,
“Materials” and “Structural.” These journals are published in the same format, so this
article will be formatted in similar fashion.
Abstract
There are an estimated 1.6 billion people living in substandard housing around
the world, according to Habitat for Humanity. With nearly one-quarter of the world
population living in these conditions, many of them in developing regions, providing
safe and sustainable housing is a global need. HyPar roofs, which are hat-shaped
concrete shell roofs, are one solution to this need. Utilizing the world’s most common
construction material, HyPar roofs employ concrete in an innovative way. By using
latex-modified concrete over a doubly-curved tensile fabric form, HyPar roofs can
achieve a shell thickness of about 0.4 inches, resulting in a lightweight structure that
exhibits impressive strength and durability. These benefits are commonly met with
disbelief, as many potential clients and non-profit investors do not understand how a
concrete roof could be so thin. To address this need for better understanding and
engineering proof of HyPar strength and durability, this research will investigate and
present important characteristics of the material science and mechanical behavior of the
latex-modified concrete used in HyPar roofs.

45
In order to appeal to the diverse audience that may be interested in innovative
housing solutions, and to progress the understanding and adoption of HyPar roofs, this
research covers a broad scope. To first understand the current research and
understanding of shell structures and latex-modified concrete, an in-depth history and
literature review was conducted. Building on that foundation, laboratory investigations
were made into the compressive and flexural strength of latex-modified concrete, as
well as the material’s workability. The specific focus of these tests were on concrete
that is modified with Drycryl, which is the most common latex product used in HyPar
roofs today. Finally, existing HyPar roof samples were tested for flexure strength,
making an investigation into the durability of the roof, as well as the importance of
quality control during construction.
The research presented in this report concludes that latex-modification
significantly increases the flexural strength of the concrete, improving its performance
in thin shell applications. Additionally, latex improves the water performance and
workability of the concrete. Using quality and well-preserved latex is vitally important
to the strength and durability of the HyPar shell, as degraded latex has shown to have an
adverse effect on the flexure strength of the concrete. These findings should inform and
support the adoption, design, and future use of HyPar roofs.

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3.1 Introduction
There is an enormous need for safe and stable shelter across the world. An
estimated 1.6 billion people, approximately 23% of the world population, live in
substandard housing (Habitat 2010). The greatest needs are found in impoverished,
developing regions and areas that are recovering from disaster. Even in the most
impoverished regions, concrete is a common construction material, although it is often
of poor quality. Concrete performance can be improved in a number of ways, but latex
modification is one of the most common methods. Endeavoring to improve housing
conditions and bring shelter to more people, HyPar roofs have been built in a number of
developing regions.
HyPar roofs are thin concrete shell structures that derive their name from the
hyperbolic paraboloid. The roof consists of a rigid frame, usually of lumber, fabric
reinforcement, usually of fiberglass mesh, and a HyPar shell of latex-modified concrete
(LMC). The thin HyPar shell is a surface with double curvature that is typically 1
centimeter (0.4 inches) thick. Performance of the thin concrete section is enhanced by
polymer modification, tensile reinforcement, and double curvature of the HyPar shell.
The resulting product is a LMC shell that is stronger and more durable than traditional
unmodified concrete.
Evidence of the strength and durability of HyPar roofs is primarily allegorical.
Although roofs built more than two decades ago remain strong and durable, without
significant degradation, the general absence of research specific to this roof system
stifles its possible adoption by prudent humanitarian organizations. Such organizations
are more willing to fund technologies that have an existing body of research and

47
engineering knowledge. New research into HyPar roofs investigates the material
science of the LMC shell and the seismic performance of the entire roof.
3.2 Objectives
The research discussed in this article was conducted at the University of
Oklahoma. Objectives of the present study were: 1) to investigate the compressive and
flexure strength of the most common LMC mix; 2) to investigate the relationship
between latex content and the performance of the LMC, including density, workability,
compressive strength, and flexure strength; 3) to investigate the plausibility of a natural
latex alternative, specifically for HyPar applications in Haiti; 4) to investigate the
relationship between water content and the performance of the LMC, including density,
workability, compressive strength, and flexure strength; and 5) to examine the effect
that latex quality control has on the performance of the LMC.
3.3 Research Significance
By studying the mechanical behavior of the LMC in the HyPar shell, a body of
knowledge may be broadened for HyPar roofs. In addition to this study, other research
was conducted to assess the lateral stability and seismic performance of the HyPar roof
system. This research was conducted at the University of Cambridge, England. It is not
within the scope of this research, but it will be referenced, as it is beneficial to the
advancement and greater adoption of HyPar roofs. Practically, this research also aims
to provide recommendations for better HyPar design and construction.

48
3.4 Background
HyPar roofs are essentially the combination of three different technologies: a
hyperbolic paraboloid shell, fiberglass reinforcement, and latex-modified concrete.
Each technology is interesting and beneficial in its own right, but it is their synthesis
that makes HyPar roofs truly unique.
Hypar roofs first became popular during the 1950s among a niche of designers
who were interested by the form and function. Felix Candela utilized hypar roofs and
other shell structures in central Mexico during the 1950s. His contemporary, Milo
Ketchum, is a notable pioneer of hypar roofs in the United States. Both designers
appreciated the roofs for their cost-effectiveness and their ability to span large distances
in stylish fashion. George Nez, pioneer of the ultra-thin HyPar roof, saw a different
benefit of hypar shells. In 1962, Nez worked on a large UN relocation project in Ghana
that required the construction of 14,000 homes in less than two years. It was then that
he adopted his “roofs first” ideology. Since hypar shells only need to be supported in a
few locations, as shown in Figure 3.2, they can be built rapidly, allowing walls to be
constructed after the roof has already provided shelter for the family.
Shell structures possess an impressive aesthetic, but they also serve the important
function of spanning large distances without obstruction. Concrete shells have been
built for centuries, even millennia, the earliest being domes. Traditional concrete or
masonry domes could achieve a radius to thickness ratio of 50 (Denny 2010), but the
shell of the HyPar roof achieves ratios greater than 500. Measuring only 1 centimeter
thick, the HyPar roof obtains its strength from two structural elements: a rigid frame
(Figure 3.1) and a reinforced hypar shell of LMC (Figure 3.3; 3.4).

49
Figure 3.1: Typical HyPar frame
Figure 3.2: Typical CMU wall or concrete column support structure
HyPar roof may be
supported by concrete
columns at the four
locations shown,

50
Figure 3.3: Reinforcing fiberglass mesh
Figure 3.4: Finished HyPar roof
Catenary curve
Arch curve

51
3.4.1 Latex-modified Concrete
Polymeric modification is nothing new to construction and it is not reserved for
only the technologically advanced and developed regions of the world. The
Babylonians used bitumen, a natural polymer, in mortars used to construct the walls of
Jericho and other structures as early as the third millennium B.C. Other natural
polymers, like blood and rice paste, were used in ancient mortars too. During the
modern era, natural rubber was used in patching concrete for roads beginning in the
1920s. Synthetic polymers were invented during World War II, in response to the
growing scarcity of natural rubber (Chandra et al. 1994).
Since World War II, many different synthetic polymers have been used in
polymer modified concrete (PMC). Polyvinyl acetate (PVA) was first used in tile
grouts. It increased the mortar’s workability, but it decreased its water performance,
because PVA can rehydrate. Today, PVA is commonly used in water-soluble
adhesives, like Elmer’s glue. Another polymer, styrene-butadiene (SBR), has been used
in concrete patchwork. It was better suited for thin-section concrete because SBR
reduces shrinkage and increases the flexibility of the concrete, but it has poor aging
characteristics and low UV resistance. As SBR ages, it hardens and becomes brittle due
to UV exposure. Today, SBR is commonly used in automobile tires. Weaknesses of
these two types of polymers disqualify them from use in thin-section concrete.
The present research focuses on PMC modified with acrylic polymers. Where
PVA and SBR fail, acrylic polymers perform well. Aside from improved water
performance and UV resistance, acrylic polymers have other benefits like improved
flexure strength, workability, and adhesion. Although improved performance is

52
generally true of PMC compared to traditional concrete, each polymer is unique, and
therefore deserves its own research (Soroushian 1993).
An acrylic polymer is a chain of carbon-based monomers, attached end to end
by their free radicals. The three most relevant acrylic polymers, in decreasing chain
length: methyl methacrylate (MMA), ethyl acrylate (EA), and butyl acrylate (BA). Of
these, MMA has the highest tensile strength and elastic modulus, while BA has the
lowest (Table 3.1, Lavelle). In summary, MMA is a brittle polymer and BA behaves
more like an elastomer (Lavelle 1988).
Table 3.1: Properties of acrylate polymers
Polymethacrylate Tensile strength, psi Elongation, %
Methyl 9000 4
Ethyl 5000 7
Butyl 1000 230
Acrylic polymers are commonly manufactured as a latex emulsion. During the
emulsification process, latex particles are spontaneously formed when individual
polymer chains attach themselves to the free radicals of other chains. These latex
particles remain suspended in their lubricant, usually water, and can be introduced into
the concrete directly during mixing. Acrylic polymers are also manufactured and sold
in a dry form, as a redispersible powder. Using a dry powder simplifies shipment and
storage of the latex.
During the curing process, concrete gains strength when the alkalis and silicates
in Portland cement react in the presence of water, forming calcium silicate hydrates
(CSH). These CSH crystals provide the primary strength in concrete. For unmodified

53
concrete, wet-curing is necessary to achieve the best performance, but for LMC, air-
curing at ambient conditions leads to better performance.
When LMC hydrates during the curing process, a film of coalesced latex
particles forms around the cement and sand particles. This film prevents further water
loss through evaporation, meaning that LMC may cure in ambient conditions and still
retain water necessary for long-term hydration and CSH formation (Lavelle 1988).
Considering that wet-curing is rarely practical or achievable on the job site, LMC has an
advantage over unmodified concrete when it comes to curing conditions.
An important consideration of concrete mix design is the water content, which is
given as the water-cement ratio (w/c). Higher water content in unmodified concrete
yields a more workable mix, but adversely affects the final strength of the concrete.
Adding excess water to the concrete mix is a poor practice, but is especially common in
developing regions due to a lack of understanding. LMC has improved workability at
low water-cement ratios, which also leads to improved strength and durability
(Kuhlman 1991).
The polymer content of LMC, given in this research as the latex-cement ratio
(l/c), is an important factor that affects the concrete’s performance in several ways.
Dow Chemical, manufacture of the acrylic polymer Drycryl, recommends using a latex-
cement ratio between 0.10 and 0.20 to achieve the best results. This amount is typical
of most manufacturer recommendations. Low polymer content may actually decrease
the compressive strength of the LMC compared to unmodified concrete, but higher
polymer contents yield improved compressive and flexure strengths (Bayasi 1996).

54
LMC also exhibits improved adhesion strength. In a study of shear bond
adhesion, Joseph Lavelle observed that LMC performed significantly better than
unmodified concrete. All adhesive tests in the study showed cohesive failure in the
latex-modified concrete and adhesive failure in the traditional unmodified concrete.
Consequently, LMC is suited especially well for overlays and construction where
successive thin layers of concrete are applied on top of each other (Lavelle, 1988).
As concrete is a permeable material, it will deteriorate more quickly in thinner
sections. LMC has better impermeability than unmodified concrete, giving it an
advantage in thin sections. Traditional concrete has a density of about two grams per
cubic centimeter (145 lb/ft3
) or greater. LMC theoretically lowers the amount of water
necessary for hydration and creates a more compact concrete matrix, so its density
should be equal or greater than typical concrete. Increased impermeability improves the
durability of LMC, especially in thin sections (Gerwick 1978).
Drycryl is the acrylic polymer of interest to this research, as it is the latex of
choice in most HyPar roofs. Dow Chemical, Drycryl’s manufacture, states that,
“incorporating this powder allows compounders to attain the dramatic improvements in
adhesion, abrasion resistance, flexural strength, and exterior durability that are
associated with acrylics.”
Drycryl is a proprietary blend of BA and MMA polymers. These polymers
account for the 95% of the Drycryl product. As discussed earlier, MMA polymers are
tough and have high tensile strength while BA polymers are softer and more ductile. It
is plausible that LMC that employs Drycryl will exhibit increased strength and
durability.

55
Drycryl has a glass transition temperature (Tg) of 17°C (63°F). The glass
transition is unique to polymers, and is the temperature at which a polymer transitions
between elastic and plastic behavior. Polymers with glass transitions close to ambient
temperatures, like Drycryl, exhibit plastic behavior, characterized by toughness and
good impact strength.
3.4.2 Reinforced HyPar Shell
A hypar shell, as it relates to this research, is an anticlastic surface. Anticlastic
surfaces may be described as shells of double curvature, with a concave curve about one
axis and a convex curve about the other. The concave curve behaves as an arch and the
convex curve behaves as a catenary. Hypar shells handle loads through membrane
stresses, as the arch carries loads in compression while the catenary carries loads in
tension. As with most shells, bending moments are minimized, allowing for a much
thinner structural element.
Distributed loads are transferred to the supports by tangential shearing and
normal forces that act along the shell. These internal forces acting in the shell are
generally of small magnitude, except in the region near each column support. It’s in
these regions, in areas where point loads are applied, that critical tensile forces and
bending moments are developed. For this reason, the supports for the shell are more
important that the shell itself (Ketchum 1976).
The American Concrete Institute (ACI) has published a paper, ACI 334.1R-92,
on thin concrete shell design and analysis. For most shells, a simplified approach is
possible. Assuming that the concrete shell is uncracked, homogeneous, and isotropic,

56
elastic analysis is permitted and Poisson’s ratio may be assumed as equal to zero. To
simplify design, a rigid frame of supporting members is recommended. Flexible frames
are permitted with accompanying design documentation, but the analysis becomes
much more difficult and deflections become larger, so flexible frames are discouraged
(ACI 1992).
For typical spans, compressive stresses are usually a fraction of the allowable
stresses. Considering this, the size of the shell is not usually determined by its strength,
but by construction and serviceability requirements. Although this is true, the concrete
yield strength (f’c) shall not be less than 3,000 psi. Any contribution of tensile strength
from the concrete should be neglected; meaning the tensile stresses in the shell should
be resisted completely by reinforcement. The maximum percentage of reinforcement
allowed by ACI is 5% for reinforcement that has a tensile yield strength (fs) of 25 ksi
(ACI 1992).
Fiberglass mesh is the most common type of reinforcement used in HyPar roofs
because of its strength, flexibility, and it can be easily found in many places around the
world. Fiberglass mesh is a composite material, made of fiberglass strands coated in an
acrylic copolymer. It is acid-resistant, alkali-resistant, and has good durability.
Fiberglass strands have tensile strengths ranging from 15 ksi to 25 ksi. For a 1.0
centimeter thick shell, two layers of fiberglass mesh (5 mm x 5 mm grid) may be used
to achieve 5% tensile reinforcement.

57
3.5 Experimental Research
The previous section described HyPar roofs and reviewed some of the body of
research belonging to LMC. In this section, the experimental research into the roof
material will be presented. The primary objectives of this study were to investigate the
mechanical behavior of the LMC used in HyPar roofs.
3.5.1 Specimen Preparation
Preparing laboratory specimens is vastly different from building a HyPar roof in
the field. Most HyPar roofs are built in developing regions, where construction must be
adapted to fit the needs of the location. This section will briefly discuss the efforts
taken to prepare laboratory specimens that abide by accepted research practices while
also accurately reflecting field conditions of HyPar construction.
In the field, LMC is almost always mixed with hand tools, such as a power drill
and mixing paddle. This practice is not appropriate for research, because ASTM C305
dictates that, “the mixer shall be an electrically driven mechanical mixer of the epicyclic
type, which imparts both a planetary and a revolving motion to the mixer paddle.” All
specimens in this research were prepared by a mixer that meets these ASTM
specifications.
Latex in concrete tends to foam during mechanical mixing, increasing the air
voids in the final concrete matrix and thus decreasing its strength. Most manufactures
of latex modifiers recommend using a defoaming agent, but this is done infrequently in
actual HyPar construction. To remain true to actual practice, the LMC mix for
specimens in this research did not include a defoaming agent. Instead, to minimize

58
foam during mixing, small batches were mixed at a low speed. This is the same
practice used in HyPar construction.
For a typical roof, measuring 1.0 centimeter thick and 6.0 meters by 6.0 meters
in plan (0.4 in., 19.8 ft. x 19.8 ft.), requires only 0.46 cubic meters (16.3 ft3
) of LMC. A
more realistic estimate, that takes wasted concrete into account, would be closer to
20ft3
. This is one-tenth the amount of concrete required for a flat concrete roof, 5
inches thick, covering the same area. Considering the low material requirement of
HyPar roofs, LMC is always mixed in small batches, usually less than one cubic foot.
This practice has been adopted in the research. Each LMC batch was approximately 1.2
ft3
, yielding between 25 and 30 specimens for compressive and flexural tests.
3.5.2 Specimen Properties
Three types of specimens were prepared for this research: 1) LMC cubes,
measuring 2.0 inches square; 2) LMC prisms, measuring 1.0 inch thick; 3) Reinforced
LMC shell, measuring 0.4 inches thick. Additionally, HyPar shell specimens have been
taken from two adjacent roofs located in Castle Rock, Colorado. These specimens are
referred to herein as the Franktown HyPar samples.
The LMC cubes were prepared in accordance with specifications for
compressive strength tests, as presented in ASTM C109. The LMC prisms were
prepared in custom-built forms to accommodate the specifications of third-point flexure
tests, as presented in ASTM C78. The shell specimens were prepared in a way that
accurately reflects HyPar roof construction.

59
3.5.3 Test Procedures
Three types of tests were performed on the LMC samples. The objective of
these tests was to investigate the strength and workability of LMC modified with
different latex and water contents. Each test was performed in accordance with the
American Society of Testing and Materials Specifications. The tests are as follows:
3.5.3.1 Flow of Hydraulic Cement Mortar (ASTM C1437)
The flow of each batch of LMC was measured immediately after mixing. The
apparatus used for this test is a flow table (Figure 3.5), as specified in ASTM C230.
The basic procedure of this test is filling and tamping the flow cone with freshly mixed
LMC, dropping the flow table 25 times in 15 seconds, and measuring the average
diameter of the LMC puddle. Performing this test provides the basis for understanding
the varying workability of different LMC mixes.
Figure 3.5: Flow table

60
3.5.3.2 Compressive Strength of Hydraulic Cement Mortars (ASTM C109)
This test was performed on 2-inch LMC cubes after 3, 7, and 28 days of curing.
The LMC specimens were cured in an environmental chamber that was kept at a
temperature and relative humidity of 73.4°F and 50% respectively. They were de-
molded after 24 hours of curing (Figure 3.6). All tests were performed with a hydraulic
compression machine (Figure 3.7), as specified in ASTM C109.
Figure 3.6: LMC cubes
Figure 3.7: Hydraulic compression machine

61
3.5.3.3 Flexure Strength of Concrete Using Third-Point Loading (ASTM C78)
This test was performed on two different types of specimen: 1) Unreinforced
LMC prisms, specimens prepared in the lab; 2) Reinforced LMC shells, specimens
taken from Franktown HyPars. The lab-prepared LMC prisms were tested at 3, 7, and
28 days of curing. All LMC specimens were cured in the same conditions as the LMC
cubes used in the compression tests. The LMC prisms were de-molded after three days
of curing (Figure 3.8). The field specimens were taken from two Franktown HyPar
roofs in Castle Rock, Colorado. These specimens were 20 years old at the time of
testing. All tests were performed on a hand operated testing machine that provides a
continuous load for each stroke (Figure 3.9), as specified in ASTM C78.
The span of the testing rig measured 12.5 inches, resulting in a span-thickness
ratio of 12.5 for the LMC prisms, and 25 or greater for the Franktown LMC shell
specimens. This is greater than the ASTM specified ratio of 3.0, but considering the
thin-layer application of LMC in HyPar roofs, choosing a higher span-thickness ratio
was desirable.
A maximum deflection of 3 inches across the 12.5 inch span was allowed during
testing. All of the lab prepared specimens failed before this limit, but some of the field
specimens reached this limit before total failure. When this was the case, it was noted
and the peak load at maximum deflection was recorded.

62
Figure 3.8: LMC prisms, HyPar shell panel
Figure 3.9: Flexure testing machine

63
3.5.4 Latex Content Investigation
An experimental investigation of more than 120 LMC specimens of four
different latex-cement contents (l/c) was conducted at the University of Oklahoma’s
Fears Structural Engineering Laboratory. One third of these specimens were 2-inch
cubes, tested in compression, and the remaining specimens were prisms, tested in
flexure. Other tests in this investigation include: 1) measure of LMC flow/ workability,
as specified in ASTM C1437; 2) measure of LMC density, as specified in ASTM C138.
As mentioned earlier, LMC usually contains 0.10 to 0.20 latex-cement ratios.
For this investigation, the four latex-cement ratios studied were: 0.00, 0.10, 0.15, 0.20
l/c. The sand-cement ratio (s/c) was kept constant at 3.0 for all specimens. The water-
cement ratio (w/c) was kept constant at 0.5 for these specimens.
Lewis and Lewis (1990) conducted research on PMC using constant water-
cement and aggregate-cement ratios. Their research criticized the practice of altering
the water-cement ratio in order to achieve a similar workability between specimens.
Keeping these ratios constant would yield a better representation of the effect that
Drycryl latex content has on the LMC strength.

64
3.5.5 Water Content Investigation
An experimental investigation of more than 220 LMC specimens of four
different water-cement ratios (w/c) was also conducted at the University of Oklahoma’s
Fears Structural Engineering Laboratory. One third of these specimens were 2-inch
cubes, tested in compression, and the remaining specimens were prisms, tested in
flexure. Other tests in this investigation include: 1) measure of LMC flow, as specified
in ASTM C1437; 2) measure of LMC density, as specified in ASTM C138.
Measuring the flow of the LMC provides an understanding of the workability of
the mix. While LMC theoretically improves workability at lower w/c ratios, HyPar
LMC is generally made at a w/c ratio of 0.6 or greater. Such a high water content is
perceived as necessary in order to apply layers of LMC that are only 1-2 millimeters
thick. For this investigation, four water-cement ratios were studied: 0.48, 0.54, 0.58,
and 0.62 w/c. The sand-cement ratio was kept constant at 3.0 s/c for all specimens.
The latex-cement ratio was kept constant at 0.10 l/c for these specimens. This latex
content is the most common ratio in HyPar construction.
3.5.6 HyPar Shell Investigation
An experimental investigation of 27 shell specimens from two different HyPar
roofs was also conducted at the University of Oklahoma’s Fears Structural Engineering
Laboratory. The specimens were cut from a total of twelve panels, which were cut from
the roofs as shown in Figure 3.10 and 3.11. All specimens were tested in flexure. Two
loads were investigated: 1) the load that induced initial cracking in the specimen; 2) the
peak load, which indicates either total failure or the load that induced 3.0 inch
deflection over the 12.5 inch span (Figure 3.12).

65
Figure 3.10: Franktown HyPars
Figure 3.11: Franktown HyPar panel location
1NWL
1NWH
1NEL
1NEH
1SH
1SL
2NWL
2NWH
2NEL
2NEH
2SH
2SL

66
Figure 3.12: Franktown HyPar specimen in flexure
The Franktown HyPar roofs are identical in shape and design, but they were built
a year apart from each other. The first roof was constructed in 1992 with fresh,
undisturbed latex. The second roof was constructed the following year with the same
latex emulsion, which had not been stored properly. Over the course of a year between
the construction projects, the latex was severely degraded by the freeze-thaw cycles of a
typical Colorado year. Liquid latex emulsions are known to be sensitive to freezing.
After 20 years of service, the Franktown HyPar roofs were demolished due to the poor
condition of the second roof, which included severe spalling of the top LMC surface
and delamination between the layers within the shell. It was hypothesized that poor
quality control of the latex led to the accelerated deterioration of the second HyPar roof.

67
3.6 Experimental Results
This section presents the results from each aforementioned investigation. As the
results are presented, basic observations are made and later developed into conclusions
and recommendations. The rest of this page is intentionally left blank.

68
3.6.1 Latex Content Investigation
Figure 3.13 presents the development of compressive strength over a span of 28
days for LMC modified with varying latex contents. The unmodified concrete
exhibited a strength gain curve typical for Portland cement concrete, as specified in ACI
318. By day 7, these specimens had developed 86% of their 28-day strength. Latex-
modified specimens had only developed between 63% and 71% of their 28-day strength
by day 7. Also, an increase in latex content yielded a decrease of compressive strength.
This being said, the worst performing latex-modified specimens had still developed a
compressive strength in excess of 3,000 psi by day 28.
Figure 3.13: Compressive Strength versus Latex Content
0
1,000
2,000
3,000
4,000
5,000
6,000
0 7 14 21 28
CompressiveStrength(psi)
Time (days)
Compressive Strength vs. Latex Content (l/c)
w/c = 0.50; s/c = 3.00
l/c = 0.00
l/c = 0.10
l/c = 0.15
l/c = 0.20

69
Figure 3.14 presents the development of flexure strength over a span of 28 days
for LMC modified with four different latex contents. The unmodified specimens
developed 94% of their average 28-day flexure strength by day 7. Latex-modified
specimens continued to develop significant flexure strength between days 7 and 28. On
average, the latex-modified specimens had developed between 54% and 63% of their
28-day strength by day 7. Also, an increase in latex content yielded an increase in
flexure strength. The best performing specimens in this investigation, modified with a
latex content of 0.20 l/c, performed more than twice as well as unmodified specimens.
Figure 3.14: Flexure Strength versus Latex Content
0
200
400
600
800
1,000
1,200
0 7 14 21 28
FlexureStrenght(psi)
Time (days)
Flexure Strength vs. Latex Content (l/c)
w/c = 0.50; s/c = 3.00
l/c = 0.00
l/c = 0.10
l/c = 0.15
l/c = 0.20

Carlton Masters Thesis Report on Hypar Roof Construction

Carlton Masters Thesis Report on Hypar Roof Construction

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