Structural Performance of Lightweight Concrete

Structural Performance of Lightweight Concrete

The Pantheon, built 18 centuries ago, demonstrates the structural performance of lightweight concrete to be superior to many contemporary building materials. In time, the use of lightweight concrete has spread across the U.S., the UK, Sweden and a number of other countries. The performance of the muti-faceted building material, nevertheless, depends on a myriad of components which range from and including the materials used to create the lightweight concrete to the manufacturing process also contribute to the characteristics and durability the finished product/s.

Based on the literature the author reviewed for the report, contemporary builders and developers could benefit from an intensive review of the way the Romans used lightweight concrete to construct the Parthenon eighteen centuries ago. Perhaps then, when future builders and develops examine contemporary projects; they will be compelled to study and replicate these projects.

STRUCTURAL PERFORMANCE of LIGHTWEIGHT CONCRETE

"As with all site installed materials, the quality of the finished product is based on the skill level of the applicator"

John a. D'Annunzio (2007, p. 2).

Introduction

Reports Regarding Lightweight Concrete

The Romans reportedly used lightweight concrete 18 centuries ago. "The application on the 'The Pantheon' where it uses pumice aggregate in the construction of cast in-situ concrete," according to Hjh Kamsiah Mohd Ismail, Mohamad Shazli Fathi and Norpadzlihatun bte Manaf (2003), all with the Universiti Teknologi Malaysia Institutional Repository, confirms the Romans use of lightweight concrete. In the journal article, "Study of lightweight concrete behavior," Ismail, Fathi and Manaf recount that during the late nineteenth century, American and English builders used clinker, a form of lightweight concrete in their construction projects like the British Museum as well as in low cost housing. During the research paper which investigates the structural performance of lightweight concrete, the author asserts the hypothesis: When the builder or developer uses lightweight concrete to construct contemporary projects, then the structural performance of the muti-faceted building material will simultaneously fortify the completed project.

In the article, "The Pantheon," David Moore, P.E. (1995) explains that although much of the Pantheon was constructed18 centuries ago, it still stands tall in Rome's business district. The Pantheon, with parts constructed with lightweight concrete, has remarkably withstood the negative effects of the elements and war "permitting a firsthand view of a unique product constructed by Roman hands. Now, it is exposed to acid rain and fumes from passing automobiles and overshadowed by buildings of inferior taste…" (Moore, ¶ 6). The Jutland Archaeological Society investigations report that builders constructed the lower section of thePantheon's dome with concrete, as they alternated layers of bricks and tufa.

[B]oth [bricks and tufa] have good affinity with the lime-pozzolan mortar which filled the voids. The upper dome above the step-rings (the top 30 feet/9.1 m) is concrete comprising about 9-inch lumps of light tufa and porous volcanic slag in alternating layers bonded with mortar.18 it was customary for the Romans to use larger stones in the dome concrete than in the walls. Selecting light stones for the aggregate is another case of gradation to get light-weight concrete, a process that seems to have been evolved about the middle of the first century B.C. (Moore, 1995, Dome Section, ¶ 6).

The Pantheon's design reveals numerous features unparalleled in contemporary design standards. Even though several major cracks have appeared in the dome, it continues to function unimpaired. Most consider it incredible that the expansive concrete dome of the ancient structure, which was built entirely without steel reinforcing rods, today deemed necessary in concrete members to resist tensile cracking, could last for centuries. In contemporary construction, an engineer would reinforce such a structure with steel rods.

The Emperor Hadrian rebuilt the Panthenon during 118 to 128 a.D., according to a Ward-Perkins. Lugli, however, disputes time estimation by Ward-Perkins and contends the building began after 123 a.D. And that Emperor Pius comploeted it about 140 a.D. Most of the bricks, nevertheless, were made and positioned in the Pantheon during 123 a.D., confirmed by a date the maker stamped on his bricks. George Chedanne, the French archaeologist discovered this in 1892. "It appears the construction of the rotunda walls took a period of 4 to 5 years, and the dome required a like period because of its height and the meager tools the Romans used" (Moore, 1995, ¶ 6). The extended construction period permitted th pozzolan concrete used in construcing the Panthenon ample time to cure and increase strength. The following figure portrays a photo of the Panthenon.

The Panthenon (Moore, 1995).

During the First World War in the United States (U.S.), builders and developers used lightweight concrete in numerous construction projects, primarily for making concrete blocks and for shipbuilding. "The foamed blast furnace-slag and pumice aggregate for block making were introduced in England and Sweden around 1930s" (Ismail, Fathi and Manaf, 2003, ¶ 10). Contemporary technology, albeit, has advanced lightweight concrete and expanded its uses; including applications for use in roof decks, vessels, and numerous other applications. Builders and developers have extensively used lightweight concrete in the form of perlite. They use perlite, for example, with its outstanding insulating characteristics as loose-fill insulation in masonry construction. Used in this area, it improves fire ratings and decreases noise transmission. Perlite proves to be termite resistant as it does not rot.

Since the late 1990s, builders, developers, and roofers have increasingly used lightweight concrete as a roof decking and as a component of the insulation system. In the article, "New Lightweight Concrete Technology," D'Annunzio (2003) asserts: "Lightweight concrete can achieve similar strengths as standard concrete, and it produces a more efficient strength-to-weight ratio in structural elements" (p. 2). Although the growing use of lightweight concrete in roof decking, insulation systems and a myriad of other building venues may be attributed to the recent industry-wide delamination deficiencies and insulation shortages, this increase may also be ascribed to the numerous economic and environmental advantages lightweight insulating concrete (LWIC) offers in roof assemblies.

Although lightweight concrete may initially appear to be more expensive than traditional concrete, the reduced volume of lightweight concrete offsets this expense as it allows designers to use less, consequently adding less cost. In addition to construction costs totaling lower when builders and developers choose to use structural lightweight concrete, the project proves ultimately more durable. In determining the validity of the hypothesis for the report, the writer presents the following subsections:

Components of Lightweight Concrete

Advantages and disadvantages of lightweight concrete;

High Performance Fiber Reinforced Lightweight Concrete;

Proper Mixing Methods;

Volcanic Pumice;

Conclusion.

Lightweight Concrete

Components of Lightweight Concrete

Lightweight concrete may be produced by injecting air into the composition, by leaving out the finer sizes of the aggregate, or by replacing the aggregate with hollow or porous aggregate. Ismail, Fathi, and Manaf (2003). Lightweight concrete comprises as a particular kind of concrete which includes an "expanding agent in that it increases the volume of the mixture while giving additional qualities such as nailbility and lessened the dead weight. It is lighter than the conventional concrete" (Preface Section, ¶ 1). In time, the use of lightweight concrete has spread across the U.S., the UK, Sweden and a number of other countries. Ismail, Fathi, and Manaf explain that lightweight concrete may be grouped into one of the following three catagories:

1. No-fines concrete

2. Lightweight aggregate concrete

3. Aerated/Foamed concrete (Ismail, Fathi, and Manaf, 2003, ¶ 5-7).

Builders and developers have used structural lightweight concrete, made with accumulation of lightweitght concrete aggregate, in the U.S. For more than 50 years. The article, "Concrete in practice, what, why and how?," (2003) explains "structural lightweight concrete has an in-place density (unit weight) on the order of 90 to 115lb/ft3 (1440 to 1840 kg/m3) compared to normal weight concrete with a density in the range of 140 lb to 150lb/ft3 (2240 to 2400kg/m3)" (p. 1). Manufactures typically use lightweight aggregates, like clay, shale or slate materials to make structural lightweight concrete. The firing of these lightweight aggregates in a rotary kiln causes this type concrete to have a porous structure.

Manufactures may also use air-cooled blast furnace slag may to create lightweight concrete aggregates. "There are other classes of non-structural lightweight concretes with lower density made with other aggregate materials and higher air voids in the cement paste matrix, such as in cellular concrete" (Concrete in practice…, 2003, p. 1). Builders and developers generally use this type of concrete only for its insulation properties.

Builders and Developers primarily use strucural lightweight concrete to minimize the dead load of a structure constructed with concrete. This construction practice permits the designer to decrease the size of columns and footings or other load bearing essential features. "Structural lightweight concrete mixtures can be designed to achieve similar strengths as normal weight concrete. The same is true for other mechanical and durability performance requirements" (Concrete in practice…, 2003, p. 1). Strucutral lightweight concrete also produces a better strength to weight ratio for structural materials.

Advantages of Lightweight Concrete

Two distincitive features of lightweight concrete include its low density and thermal conductivity. Ismail, Fathi and Manaf (2003), explain "advantages are that there is a reduction of dead load, faster building rates in construction and lower haulage and handling costs. Lightweight concrete maintains its large voids and not forming laitance layers or cement films when placed on the wall" (p. 1). As noted at the start of the report, "The Pantheon" in Rome, built more than 18 centuries ago depicts an explemaenary example of the durability of lightweight concrete.

In contemporary construction projects, sructural lightweight concrete proves to be in high because of its lower density. The use of smaller load bearing elements or cross sections results in the builder or designer being abile to construct a smaller foundation. In the journal article, "The effect of high temperature on compressive strength and splitting tensile strength of structural lightweight concrete containing fly ash," Harun Tanyildizi and Ahmet Coskun (2008), both with the Department of construction education, Firat University Elazig, Turkey, identify a number of advantages to using structural lightweight concrete . These include the project possessing increased strength and more flexibility, with less coefficient of thermal expansion.

Disadvantages of Lightweight Concrete

Lightweight concrete applications may also present particular disadvantages and liabilities. These typically relate, however, to the cabability of the contractor istalling light concrete product/s. D'Annunzio (2003) warns that lightweight concrete "has additional constraints because the success of the system is based on the proper mix ratio" (p. 2). If the lightweight concrete is not mixed properly, this could present a major problem with lightweight concrete as it could create numerous empty spaces that could, in turn lead to deficient strength.

The compressive strength of lightweight concrete evolves from a foam additive. When mixed correctly, this additive molds around the cement which serves as an aggregtae. "If the foam additive is not properly mixed, there is a probability of foam collapse, which weakens the product's compressive strength" (D'Annunzio, 2003, p. 2). One factor, evolving from human errors, that could contribute to lightweight concrete failing involves the mixing process, typically done at a jobsite. The use of pumping equipment or other technology to percisley weigh the ingredients and accurately mixes the foam and cement, however, helps elimante the problem of human error. The following table depicts the advantages and disadvantages of lightweight concrete.

Lightweight Concrete Advantages/Disadvantages (Ismail, Fathi & Manaf, 2003, p. 8).

Advantages of Lightweight Concrete

Disadvantages of Lightweight Concrete

Quick and relatively simple construction

Very sensitive with water content in the mixtures

Economical in terms of transportation as well as reduction in manpower

Difficult to place and finish because of the porosity and angularity of the aggregate. In some mixes the cement mortar may separate the aggregate and float towards the surface.

Significant reduction of overall weight in saving structural frames, footing or piles

High Performance Fiber Reinforced Lightweight Concrete

As typical lightweight concrete is weaker than traditional weight concrete, improving the strength of lightweight concrete to promote it for use for structural applications proves critical. Bengi Arisoy, Faculty of Engineering, Ege University, Bornova, Turkey and Hwai-Chung Wu (2008), Department of Civil and Environmental Engineering, Wayne State University, Milwakee, address numerous concerns in the journal article, "Material characteristics of high performance lightweight concrete reinforced with PVA." "With a much higher ductility high performance fiber reinforced lightweight concrete (HPFRLWC) becomes superior to regular concrete because of elimination of sudden catastrophic failure of otherwise brittle concrete. Ductility results from imposed crack resistance due to bridging fibers" (Arisoy and Wu, Theoretical background section, ¶ 1). From their study, Arisoy and Wu found that when made with lightweigh aggregates and air entraining agent, fiber reinforced lightweight concrete displays strain hardening by the addition of 1.5% fiber volume fraction. They explain:

By adding about 10-20% fine cement substitute such as fly ash and silica fume, it improves both ductility and flexural strength. Improvement of high performance FRLWC may be summarized as follows: 50-150 times (5000-15000%) increase in flexural displacement (ductility) at ultimate load than plain lightweight concrete, 50-250% increase in ultimate flexural strength than plain lightweight concrete, 30-65% decrease in weight than normal weight concrete. (Arisoy and Wu, 2003, Conclusion section, ¶ 1)

Proper Mixing Methods

In contemporary building considerations, the concrete's compressive strength and durability prove vital. Chao-Lung Hwang, Department of Construction Engineering, National Taiwan University of Science and Technology, Taiwan, and Meng-Feng Hung (2005), Department of Civil Engineering, National Taiwan University of Science and Technology, Taiwan, compare lightweight concrete's performance under various w/cm ratio and diverse cement paste content in the journal article, "Durability design and performance of self-consolidating lightweight concrete." Designing lightweight aggregate (LWC) with "high strength, flow-ability and excellent durability is a challenge [as] the porous feature of (LWA), its compressive strength is relatively low and adsorption capacity is high" (Hwang and Hung, Introduction Section, 3). As a result, attaining suitable workability and designed compressive strength requires a large amount of cement paste be used in LWA. This, however, complicates challenges as it could contravene the durability requirement of normal weight concrete as the porous aggregate will reduce the lightweight aggregate concrete's and thermal conductivity.

During batching, another concern arises as LWA fractures easily. This causes hefty water absorption and elevated workability loss. The porous feature of lightweight aggregate contributes to its compressive strength typically being low and the capacity for absorpution fairly high. "Hence, it needs large amount of cement paste to achieve suitable workability and designed compressive strength" (Hwang and Hung, 2005, ¶ 4).

If challenges in creating lightweight concrete are not overcome, the end concrete structure cracks and/or becomes porous. It also becomes more susceptible to harsh outside elements, like acid rain and seawater. The decreased quality of lightweight concrete in a structure may lead to the structure's deterioration (Hwang and Hung, 2005).

In regard to carbonation performance, in various field conditions, lightweight concretes have typically performed adequately. T.Y. Lo, W.C. Tang and a. Nadeem (2008), all with the Department of Building and Construction, City University of Hong Kong, report results of their tests of the performance of lightweight concrete in the journal article, "Comparison of carbonation of lightweight concrete with normal weight concrete at similar strength levels." "Some field investigations on the carbonation performance of LWC in ships and bridges at exposure age from 15 to 43 years, compressive strength from 23 to 35 MPa and density from 1650 to 1820 kg/m3 have been reported" (Lo, W.C. Tang and Nadeem, Carbonation of lightweight…section, ¶ 1). Findings indicated that the depth of carbonation in these structures varied in regards to exposure conditions, density and strength, and was typically less than 10 mm.

A number of researchers, including Swenson and Sereda, Bilodeau et al., Swamy and Jiang, and Gunduz and Ugur have studied the effects moisture content, porosity and cement to water ratio have on the limits of carbonation. Lo, Tang and Nadeem (2008) explain:

Carbonation is one of the most common causes of deterioration in reinforced concrete. With the growing use of structural lightweight concrete for prefabrication of precast modules in high rise building construction, it is important to investigate the carbonation performance of lightweight concrete (LWC). Carbonation is regarded as a physiochemical reaction that takes place between carbon dioxide (CO2) and alkalinity of concrete due to calcium hydroxide (CH) and calcium silicate hydrate (CSH). The C[O.sub.2] gas is present in the atmosphere at approximately 0.03% by volume of air; it could penetrate in concrete and react with CH and CSH in the presence of moisture forming CaC[O.sub.3]. Generally, the relative humidity, the concentration of C[O.sub.2], the temperature, the permeability and alkalinity of concrete are the influencing factors for carbonation in concrete. (Lo, Tang and Nadeem (2008, Introducton, ¶ 1)