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. 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.
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)
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)
Swenson and Sereda, two prominent researchers, found that no matter if high of low, the moisture content in lightweight concrete was not favorable to rapid carbonation. "Swamy and Jiang found that carbonation was higher for concrete with higher total porosity at a given water to cement ratio. Bilodeau et al. attributed the low carbonation in high strength LWC to low water to cement ratio" (Lo, Tang and Nadeem, 2008, Carbonation of lightweight…section, ¶ 2). Gunduz and Ugur analysed the carbonation of pumice aggregate lightweight concrete and expressed that the carbonation was lessened when the aggregate to cement ratio of lowered.
Pumice, a natural material, comes from volcanos when gases are released and the lava solidifies. Khandaker M.A. Hossain, Associate Professor in the Department of Civil Engineering at Ryerson University, Toronto, Canada and Mohamed Lachemi (2007), Professor in the Department of Civil Engineering at Ryerson University, investigate lightweight pumice concrete in the journal publication, "Mixture Design, Strength, Durability, and Fire Resistance of Lightweight Pumice Concrete." Pumice, these authors explains, is mainly used an aggregate in lightweight building block and other building products. Volcanic pumice (VP) has been utlizied as an aggregate in producing lightweight concrete.
Pumice has been used for builing over 2000 years, especially in Rome and Europe where many pumice structures are still standing to this day. "Lightweight concrete made with pumice and pozzolanic cement with volcanic ash/lime (developed…
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.
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