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Carlton Masters Thesis Report on Hypar Roof Construction

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Carlton Masters Thesis Report on Hypar Roof Construction

  1. 1. 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
  2. 2. 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
  3. 3. © Copyright by WILLIAM SETH CARLTON 2013 All rights reserved.
  4. 4. “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
  5. 5. ii 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.
  6. 6. iii 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.”
  7. 7. iv 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
  8. 8. v 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
  9. 9. vi 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
  10. 10. vii 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
  11. 11. viii 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,
  12. 12. ix 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.
  13. 13. 1 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.
  14. 14. 2 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.
  15. 15. 3 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
  16. 16. 4 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
  17. 17. 5 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
  18. 18. 6 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.
  19. 19. 7 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.
  20. 20. 8 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.
  21. 21. 9 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.
  22. 22. 10 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.
  23. 23. 11 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.
  24. 24. 12 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 -
  25. 25. 13 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.
  26. 26. 14 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.
  27. 27. 15 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.
  28. 28. 16 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
  29. 29. 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%
  30. 30. 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
  31. 31. 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.
  32. 32. 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.
  33. 33. 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.
  34. 34. 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
  35. 35. 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.
  36. 36. 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
  37. 37. 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.
  38. 38. 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
  39. 39. 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).
  40. 40. 28 Figure 2.7: Hypar formwork, Candela Figure 2.8: Hypar reinforcement installation, Candela
  41. 41. 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.”
  42. 42. 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
  43. 43. 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).
  44. 44. 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.
  45. 45. 33 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
  46. 46. 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.
  47. 47. 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
  48. 48. 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.
  49. 49. 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.
  50. 50. 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.
  51. 51. 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
  52. 52. 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,
  53. 53. 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
  54. 54. 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
  55. 55. 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.
  56. 56. 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.
  57. 57. 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.
  58. 58. 46 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
  59. 59. 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.
  60. 60. 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).
  61. 61. 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,
  62. 62. 50 Figure 3.3: Reinforcing fiberglass mesh Figure 3.4: Finished HyPar roof Catenary curve Arch curve
  63. 63. 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
  64. 64. 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
  65. 65. 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).
  66. 66. 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.
  67. 67. 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,
  68. 68. 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.
  69. 69. 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
  70. 70. 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.
  71. 71. 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
  72. 72. 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
  73. 73. 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.
  74. 74. 62 Figure 3.8: LMC prisms, HyPar shell panel Figure 3.9: Flexure testing machine
  75. 75. 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.
  76. 76. 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).
  77. 77. 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
  78. 78. 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.
  79. 79. 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.
  80. 80. 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
  81. 81. 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

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