Aggregate and cement concrete play a vital role in the civil engineering where the construction of structures both for domestic and commercial purpose is dependant upon the quality and scientific characteristics of the concrete used as argued by Leif Berntsson Satish Chandra (2002) . This is evident from the fact that concrete is used in many applications apart from structural applications including insulation, filling etc…
In this report a critical analysis on the lightweight aggregate concrete (LAC) is presented to the reader. The research will provide a comprehensive insight on the scientific aspects surrounding LAC and the need for using LAC in structural applications.
1.2: Aim and Objectives
The aim of this report is to present a critical analysis on the Lightweight Aggregate Concrete (LAC) and research on its structural applications and further developments.
The above aim is achieved by embracing the report on the following objectives
1. To conduct a comprehensive overview on the Lightweight Aggregate Concrete (LAC) by providing the history, definitions and economic factors surrounding the applications and use of LAC in civil structures.
2. To provide a critical overview on the production of cement and its properties in the light of LAC and its application in civil structures.
3. To present a scientific analysis on the properties of LAC using different composition materials including both the mechanical and chemical properties.
4. To investigate on the regulations pertaining to LAC and their effects on a specific composition of LAC in a given geographical region.
5. To investigate on the LAC production and use in Greece.
1.3: Research Scope
The presence of concrete in civil engineering is exhaustive in nature and hence the research scope is limited to the composition of the LAC and its application in civil structure applications. The key aspects of the LAC and its manufacture in Greece is also included in the scope of the research although a global research on the LAC manufacture and regulations pertaining to LAC is out of scope of this project.
The research scope also includes the investigation of the general regulations adhered in the Europe including the ACI-318 and design considerations in the light of Seismic Design.
1.4: Research methodology
The nature of the research is dependant on the analysis and findings surrounding the LAC which is used in civil structures. Since building a civil structure is not part of the project due to cost and resource constraints, the research methodology is purely dependant on qualitative analysis using secondary research data. The qualitative approach to research in cases of the engineering analysis is advised as a reliable approach as the findings from the research on the secondary resources are already published thus providing a validated source of information for analysis. This is further justified by John W. Creswell (2002) .
Hence the research methodology in this report is qualitative research using published resources including journals, text books and scientific papers. The Internet is used as the main search space for collecting information to perform the qualitative analysis.
1.5: Chapter Overview
Chapter 1: Introduction
This is the current chapter where the reader is provided with a brief introduction on the topic, research aim and objectives, scope and methodology. The chapter sets the stage for the overall research presented in the report.
Chapter 2: Literature Review
In this chapter a historic overview on the concrete and the use of Lightweight Aggregate Concrete (LAC) is presented to the reader. This is then followed by the definition of the LAC and its application in the civil structural applications from a historic perspective. The chapter is concluded with an overview on the economic factors and benefits realised through the use of the LAC in the civil structure applications with examples. The economic overview also throws light on the key aspects of LAC that benefit the overall concrete composition in structural applications as well as provide a detailed review of the various LAC compositions used in the past during the initial stages of LAC’s use in the construction industry.
Chapter 3: Concrete Production
This chapter presents a detailed overview on the modern concrete production techniques and the evolution of the production techniques over the years.
The two popular techniques used in the production namely the rotary kiln and the sintering process with insight on the variations is presented to the reader in this chapter. Furthermore, the lightweight aggregate production and the key production methods used in the commercial applications including the advantages associated are presented to the reader.
Chapter 4: LAC – Properties, Regulations and composition analysis based on geography.
This chapter presents a critical analysis on the properties of LAC and the various combination of lightweight aggregate that is used in different categories of the construction. The research throws light on the various compositions of lightweight aggregate and their distinct features that help achieve the desired benefits in a structural application. The chapter then presents a critical overview on the regulations pertaining to the LAC followed by the composition analysis based on the materials that are available locally to a given geographical location. The chosen geography for this research is Greece.
Chapter 5: Conclusion
This chapter reviews the objectives of the research followed by providing the conclusion to the report.
Chapter 2: Literature Review
2.1: Historical overview
Concrete, typically a mixture of sand, gravel and cement dates back to ancient history when red lime was used as a cementing component in making concrete (Leif Berntsson Satish Chandra, 2002). This makes it clear that the current mixtures of concrete have evolved over the historic periods to cater various engineering requirements with the developments in science and the innovations in engineering as argued by Leif Berntsson Satish Chandra (2002). A classical example for the evolution of concrete since the ancient history is the use of concrete by Romans in 300 BC when they found that mixing a pink sand-like material which was volcanic ash they obtained from Pozzuoli with their normal lime-based concretes resulted in a far stronger material.
The history behind the use of aggregate to make concrete mix dates back to as early as the early the Roman period when the Romans used innovative methods in preparing concrete mixes with different aggregate materials to suit the structural requirements and strength. The classical examples for the above statement include the use of lightweight aggregates as in the roof of the Pantheon, and embedded reinforcement in the form of bronze bars as argued by Leif Berntsson Satish Chandra (2002). The technology in concrete using different aggregates as well as accounting for thermal and other physical qualities of re-enforcing materials to make concrete mixes that provide the desired strength is evident throughout history although the industrial revolution and the increase in the engineering and the role of steel in the 20th century have increased the innovation as well as developments around the technology of making the right concrete mix.
Concrete is not only a critical element in the civil structural applications but also a key element in many other applications surrounding the construction business making it one of the important and most sought after product in the engineering business itself as argued by Fu-Tung Cheng and Eric Olsen (2002) . This is naturally because of the fact that concrete is not only a component in the construction of civil structures but also a design ingredient in deciding upon the strength, truss and other physical elements that govern the stability of a given building. This is also justified in the arguments of Leif Berntsson Satish Chandra (2002).
The definition of cement in engineering terms refers to powdered materials which develop strong adhesive qualities when combined with water. This makes it clear that the cementing action of volcanic ash that was used to make concrete by the Romans fall under the cement. It is further evident that concrete is referred to as a composite building material made from the combination of aggregate and cement binder.
From the above it is clear that the developments in the quality of cement and the invention of Portland cement, gypsum plaster, etc… have a direct influence on the development of the concrete technology although the aggregate component of the concrete composition plays an equally important role in various mixes of concrete that serve a desired purpose as argued by Leif Berntsson Satish Chandra (2002).
Another element of greater significance to the development of concrete technology in the recent years as early as the 1900s is the development of concrete boats during the second world war where the lightweight aggregate concrete played a vital role in the design and construction of the ships itself as argued by Glenn A. Black (2004) . It is also interesting to note that the importance of concrete has increased with the need for refined and purpose specific concrete mixes where the role of concrete has been not only to provide the structural support but also the desired strength at the required physical conditions that is set in the given geographical location as argued by Glenn A . Black (2002). The growth of the expanded clay and shale industry since the dawn of the 20th century and the developments during the Second World War when the lightweight aggregate concrete using clay and shale was used to construct the war ships marked the accelerated growth of the use of lightweight aggregate concrete as argued by Glenn A. Black (2004).
2.2: Definition of lightweight aggregate
In order to define the lightweight aggregate – the topic under research in this thesis, it is essential to present the fundamentals surrounding the lightweight aggregate. Hence this section first presents a brief research on aggregate, its role in construction as part of the concrete mix and then move towards the core topic (i.e.) the definition of lightweight aggregate.
Aggregate is the terms used to collectively refer to the ingredients in making a concrete mix that gives strength and texture to the overall concrete composition made of sand, cement and aggregate as argued by Glenn A. Black (2004). Aggregate is the composite material of the concrete that is aimed to resist compressive stress making it clear that the size, strength and weight of the aggregate materials are critical components for the overall efficiency of the concrete to manage the compressive stress as argued by Glenn A. Black (2004).
The modern day concrete uses Portland cement as the cementing element and the aggregate that is held together by the cement and water to design concrete for different degrees of strengths, durability, heat & sound insulation, and water tightness as argued by Glenn A. Black (2004). This makes it clear that the aggregate is the critical component of the concrete that attributes to note only the strength and quality of the concrete but also dictates the nature of the applications and the extent to which innovation in engineering can be taken to. The key physical quality of the aggregate is the compressive strength that it can support for a given composition.
The lightweight aggregates that are researched in this report typically attribute to up to 80 pounds per square inch which is used mainly applications that demand lightweight concrete by virtue of the positioning or for the reinforcement, insulation etc. specific gravity is another critical element in describing the quality of aggregate as the specific gravity off the substance is directly proportional to its stress and compressibility factors as argued by Glenn A. Black (2004). A typical combination and most popular in the construction industry for the aggregate is the gravel and sand mix at different sizes and compressibility that are used in high demand construction structural applications as argued by Glenn A. Black.
Hence aggregate in concrete is defined as the component of concrete that attributes to the strength, durability, compressibility and insulation attributes to support the desired construction application.
Lightweight Aggregate
Glenn A. Black (2004) says “The term "Lightweight Aggregate" describes a range of special use aggregates that have an apparent specific gravity considerably below normal sand and gravel which were at one time used in almost all concrete”.
From the above it is clear that the lightweight aggregate is one of the critical elements that makes concrete flexible and versatile to make the overall structural design and specifications as to meet the construction requirements as argued by Leif Berntsson Satish Chandra (2002). It is also interesting to note that the lightweight aggregate in the concrete that is made using light weight materials also provide an appreciable level of compressibility as well as possess strength that can be defined based on the composition thus making it a versatile and cost effective process in the production process itself.
The range of lightweight aggregate is extensive in nature from extremely light materials used for insulation and non-structural concrete all the way to expanded clays and shales used for structural concrete. This makes it clear that the lightweight aggregate in the concrete is mainly aimed to achieve high level of physical stability and compressibility through effectively utilising the physical qualities of the aggregate materials. This is further justified in the arguments of Leif Berntsson Satish Chandra (2002) who argues that the lightweight aggregate in the concrete is a major step towards innovation in the field of engineering itself.
The strength and the air trapped in each individual particle of the components of the aggregate materials are inversely proportional to each other thus making it clear that in order to gain lightweight aggregate the amount of air trapped in the individual particles must be high thus making it clear that the strength of the concrete thus obtained is low. The above relationship stated provides the guidance to ensure the balance between the air trapped and the strength required in the concrete mix thus making the overall lightweight aggregate concrete customisable to meet the structural requirements of the application on hand.
Lightweight Aggregate Concrete Spectrum
The concrete spectrum resulting from the use of the lightweight aggregate is extremely diverse in nature ranging from very lightweight aggregate concrete up to high strength aggregates dedicated for specific bespoke applications as argued by Glenn A. Black (2004).
The super lightweights range of aggregate concrete that are derived from Vermiculite and Perlite are the capable of delivering weights as low as 15 to 20 pounds per cubic foot thus making it clear the application of lightweight aggregates in the engineering business provides a diverse range of applications for concrete.
The natural aggregates, Pumice and Scoria for example can be used to make concrete weighing at about 25 to 30 pounds per cubic foot and extended as high as 65 pounds per cubic foot as argued by Glenn A. Black (2004). Furthermore, the use of coal cinders and expanded shale, clay and slate aggregates produced using rotary kiln method can deliver weights in a varying range from 75 to 120 pounds per cubic foot.
Another popular production method for this range of aggregates includes the sintering where the weights are delivered typically ranging from 90 to 120 pounds per cubic foot.
The high end applications of aggregate concrete include the production of aggregates capable of delivering weights up to 150 pounds per cubic foot using the air-cooled slag aggregates and the hard-rock aggregates such as sand and gravel and crushed stone, which produce conventional concretes as mentioned by Glenn A. Black (2004).
From the above it is clear that the aggregates that lie in the lower end of the weight that have lower compressive strength are used primarily for insulation purposes whilst those in the middle spectrum are used for insulation and filling. The high end of the lightweight aggregate concrete spectrum are used in a wide range of structural applications that demand high compressive strengths as well as efficient management of weight as argued by Glenn A. Black. The concrete spectrum for the lightweight aggregate concrete is presented in the figure below.
Fig 1: Lightweight Aggregate Concrete Spectrum
(Source: Glenn A. Black (2004), Lightweight Concrete history, Applications and Economics, Indiana University)
2.3: Economics surrounding lightweight Aggregate Concrete
The key aspects of Lightweight Aggregate Concrete that attribute to the economical and structural benefits derived through the structural applications using LAC include the following
2.3.1: Fire resistance – Resistance to fire is one of the critical elements that is expected in concrete to ensure that the fire resistance and the structural stability of the civil structure is maintained through the use of aggregate concrete as argued by John P. Ries and Thomas A. Holm (2006) . The fire resistance of lightweight aggregate is higher compared to the typical concrete aggregate mainly because of the fact that the aggregate materials composing the lightweight aggregate have lower thermal conductivity, lower coefficient of thermal expansion as argued by John P. Ries and Thomas A. Holm (2006). The fact that the aggregate materials possess inherent fire resistant properties is the fundamental element that is emphasised and strengthened in case of the lightweight aggregates where the aforementioned heat resistance properties help achieve higher fire resistance. It is also interesting to note that the inherent fire stability of aggregate is high and in case of the lightweight aggregate it is at a heat of over 2000 degrees Fahrenheit.
As it is stated in the “ACI 216 "Standard Method for Determining fire Resistance of
Concrete and Masonry Construction Assemblies", when slab thickness is determined by fire resistance and not by structural criteria (Goists, waffle slabs e.g.), the superior performance of lightweight concrete, will reduce the thickness of slabs resulting in significantly lower concrete volumes”, (John P. Ries and Thomas A. Holm, 2006).
From the above it is clear that the fire resistance properties of the lightweight aggregate directly contributes to the overall structural stability and the reduction in the volume occupied by the concrete in the structural applications. This justifies the versatile nature of the lightweight aggregate thus enabling it to be used in innovative structural applications as argued by John P. Ries and Thomas A. Holm (2006).
2.3.2: Service Life of the Structure – The service life of the structure is another critical element that is used as measure of economic use in case of assessing the concrete and the aggregate efficiency against the capital invested as argued by John P. Ries and Thomas A. Holm (2006). Glenn A. Black (2004) further states that the durability of lightweight aggregate is high and the life of the structures constructed using lightweight aggregate prove to have higher life durability. The historical evidence to justify the aforementioned include popular structures like The Port of Cosa – built about 273 B.C. where the builders used lightweight concrete made out of natural volcanic materials, The Pantheon that was finished in 27 B.C that incorporates concrete varying in density from bottom to top of the dome and the most popular Coliseum, built in 75 to 80 A.D. where the foundations were cast as lightweight concrete using crushed volcanic lava as argued by John P. Ries and Thomas A. Holm (2006). From the above arguments it is evident that the durability of the structures designed using the lightweight concrete is extensive in nature.
Looking into the more modern examples to justify the service life of the lightweight aggregate used in concrete for construction include the lightweight concrete ships built by the American Emergency Fleet Corporation during the First World War. The compressive strengths of the concrete used were in the range of 5000 psi (35 MPa) obtaining a unit weight of 110 lb/ftJ (1760 kg/mJ) or less using the rotary kiln produced expanded shale and clay aggregate as identified by John P. Ries and Thomas A. Holm (2006). The service of these lightweight concrete boats during the world wars and their subsequent in the merchant ships justify the durability and service life of the lightweight aggregate used in concrete construction applications. Furthermore, the fact that the higher level of air trapped in the particles make the submerging efficient in case of the marine applications makes lightweight aggregate as a natural choice for the marine applications although the use of lightweight concrete extends to commercial structural applications in many bridges across the United States of America where the structural efficiency and stability on bridges that were deemed unusable due to poor load bearing capabilities was improved through the use of lightweight aggregate concrete as argued by John P. Ries and Thomas A. Holm (2006). Furthermore, the critical element that attributes to the service life of the lightweight concrete is the heat resistance, resistance to environmental corrosion and its lightweight that reduces the load on the structure making its service life longer than the typical concrete applications.
2.3.3: Economic sustainability
John P. Ries and Thomas A. Holm (2006) argue that the structural applications in the modern days are judged against the cost, functionality, aesthetics or a combination of these as argued by John P. Ries and Thomas A. Holm (2006). This makes it clear that the costs associated with the construction of the structure as well as the running costs associated with maintenance, space and repair are the critical elements that attribute to the choice of a given concrete mix over another. The lightweight aggregate that is used in the LAC is higher in costs compared to the typical concrete mix as argued by John P. Ries and Thomas A. Holm (2006). This is naturally because of the need to produce the concrete mix using materials of unique physical properties and the extent of research and development involved with the overall design of the construction application. Alongside, the cost is treated as the key element in case of commercial implementation of construction applications predominantly because of the fact that the measure on the returns in terms of return on investment is attributable when compared against the costs associated with the construction of the structure.
John P. Ries and Thomas A. Holm (2006) say that although the capital involved with the construction of lightweight structures is high, the fact that the low maintenance costs and costs associated with other supporting structures during the constructions like the reduction in steel, girders and also the reduction in the slab thickness will balance the costs with the production of the LAC concrete mix itself. This further justifies that the economic sustainability where the return on the investment and the optimum choice for construction is achievable using lightweight aggregate as argued by John P. Ries and Thomas A. Holm (2006)
The arguments of Glenn A. Black (2004) that the lightweight aggregate also has the benefit of lower level of maintenance and negligible repairs associated due to its durability features further justify that the effectiveness of the lightweight aggregate in achieving economic sustainability is high. Hence the lightweight aggregate is highly recommended in the construction of critical structural applications like bridges and commercial building where the load bearing is high and the space is a critical element to save costs.
2.3.3: Energy consumption and energy savings – The Energy Performance of Buildings Directive of the European Union is a classical example for the justification that the composition of the concrete and the properties of the components comprising the structure of building commercial and domestic contribute directly to the overall energy consumption as argued by John P. Ries and Thomas A. Holm (2006). This is further justified in the arguments of Sarah Gaventa (2006) where the author has justified that the concrete mix and the design of the overall structure to suit the structural requirements have a direct impact on the energy consumption. Alongside, the heat resistant properties of the lightweight aggregate and the ability to trap higher amount of air within the particles comprising the aggregate further make the lightweight aggregate to be able to meet the heating and cooling requirements in a given structure as argued by Sarah Gaventa (2006). It is also interesting to note that the energy performance efficiency in the building especially in the west where a major portion of the energy is used for heating purposes justify that the concrete mix and the aggregate composition to make the concrete mix are critical for the successful energy savings in the buildings as argued by Sarah Gaventa (2006).
The durability, stability and other physical properties including the compressibility of an aggregate material attribute to the ability of the concrete used in the building to retain heat thus reducing the consumption of energy for interior heating purposes as argued by Sarah Gaventa (2006).
It is also a well known fact that the reduction in the concrete density increases the thermal resistance thus making it clear that the lightweight aggregate will increase the thermal resistance due to the lower specific gravity of the aggregate composition that reduces the density of the concrete used in the construction of the structure. A typical example is the concrete density of 90lb/cubic foot will have a resistance (R value) of 0.26/inch whilst the R value for a density of 135lb/cubic foot is approximately 0.10/inc thus making it clear the energy efficiency is greatly increased through the use of lightweight aggregates as argued by John P. Ries and Thomas A. Holm (2006).
Chapter 3: Concrete Production
3.1: Overview
The production of concrete mix using the aggregate is achieved through the mixing of the aggregate, sand and cement with right amounts of water to produce the concrete mix of the necessary strength. The concrete mixing is dependant upon the quality of the aggregate as well as the cement used to achieve the desired density, strength and compressibility of the concrete for the structural application. In this chapter a critical overview on the cement production followed by the production methods for lightweight aggregates is presented to the reader.
3.2: Cement Production
The main ingredient for the production of cement is limestone of varying chemical compositions that are freely available in the quarries as argued by Sarah Gaventa (2006). The lime stone is processed and further chemicals are added to gain the cement of the necessary strength and compressibility. The following explains the production process briefly
The raw limestone of varying chemical combinations is first collected to prepare the raw mix where the limestone is mixed with minerals of minerals containing calcium oxide, silicon oxide, aluminium oxide, ferric oxide, and magnesium oxide. This mixture is prepared to a fine mixture which forms the raw mix for a typical Portland cement. This is then blended to form the raw blend where the raw mix is formulated to a very tight chemical formulation to gain the desired strength from the finished produce of the cement as argued by Sarah Gaventa (2006).
The raw blending process is conducted in a fashion where the relative content of each oxide in the chemical composition is kept constant throughout the production process in order to ensure that the properties of the final product is not altered. It is also argued by Sarah Gaventa (2006) very small changes to the calcium content in the raw mix may lead to large changes in the ratio of alite to belite in the clinker, and to corresponding changes in the cement’s strength-growth characteristics (Sarah Gaventa, 2006). This makes it clear that the effective control of the raw mix is critical for the production of consistent quality cement to meet the demands of the structural application.
The next stage is the formulation of the clinker where the blend raw mixture is put through a complex chemical reaction process in a large cement kiln with temperature increasing over the length of the cylinder as argued by Sarah Gaventa (2006). The final product of the process is called clinker which is the final product of the cement produced in the solid form at the desired chemical combination. This is then put through a cement grinding process where the clinker that is produced is mixed with small amounts of calcium sulphate to grind the cement to the desired granularity in order to support the structural application.
Sarah Gaventa (2006) further argues that the major components that decide on the strength and quality of cement include the following
• Clinker
• Gypsum
• Limestone
• Blast Furnace Slag
The Blast Furnace Slag is one of the critical elements in contributing to the stability of the chemical reaction in the cement kiln as argued by Sarah Gaventa (2006). Another interesting element with the blast furnace slag is the fact that the effective use of the slag in the cement production process also allows to control the specific gravity of the cement when mixed with aggregate and water to form concrete as well as the ability to reach the desired strength of the cement concrete through the right combination of limestone and gypsum.
The schematic of the cement production process is presented in the fig 2 below.
Fig 2: Cement production Schematic
(Source: http://www.cimnat.com.lb/Production/Model.gif)
3.3: Aggregate production
The aggregate production is the next critical element in the lightweight concrete preparation as the lightweight aggregate is one of the major elements that must be produced at a higher level of precision in order to ensure the desired level of strength and specific gravity are achieved as argued by John P. Ries and Thomas A. Holm (2006).
The rotary kiln method is a traditional method of production which is popular since 1946 as argued by Glenn A. Black (2004). The process of the production involves the application of heat to shale, clay and slate under controlled conditions. The conditions include the pressure and other characteristics that trigger chemical reactions in order to achieve the preferred specific gravity and density of the aggregate component which is then ground to the required granularity as argued by Glenn A. Black (2004).
The sintering method as well as the rotary kiln method typically use the similar base raw material that comprise of a highly siliceous clay or shale that exhibits a bloating characteristic which is achieve through gas-forming minerals which release gas on exposure to the desired level of heat as argued by Glenn B. Black (2004). It is also interesting to note that the preparation of the aggregate is dependant upon the extent to which the pressure and the external temperature is controlled that set the temperature-based chemical reactions as argued by Glenn A. Black (2004).