By recreating field conditions (including reduced slab thickness and recreating ambient humidity conditions), the study suggests the ambient environment poses a greater role in slab drying. Placed in a warehouse which was not climate controlled, neither the lightweight nor the normal-weight slabs consistently held the MVER limit. This implies rewetting from rainwater or relative humidity (RH) in unsealed buildings has more influence on drying timelines than initial water content. Until a structure is enclosed, a concrete slab of any type cannot begin to dry. Therefore, one can expect both lightweight and normal weight concretes will require flooring adhesives that are capable of working on a higher moisture substrate or some mitigation technique to address the potential for moisture contents in concrete slabs greater than the industry limits.
In short, volume to volume comparisons of normal-weight and lightweight concrete in laboratory conditions indicate normal weight will dry more quickly. However, when reduced slab thickness and ambient humidity of a site are taken into account, the drying times between concretes becomes less significant.
Is lightweight concrete more expensive to use?
Since lightweight concrete made with ESCS aggregate incurs additional production and shipping costs, many assume this material is not an economic solution for a multi-story or high-rise building. While this is certainly true if only the cost of materials were compared, the use of lightweight concrete reduces costs in other areas of the building design. It satisfies fire ratings with thinner slabs, provides longer spans due to increased tensile strength,6 and reduces load weight for beams, columns, and foundations (for instance the 55-story Bank of America building reduced floor weight by 32.5 percent by using structural lightweight concrete instead of normal- weight concrete). Though the increased tensile strength impacts bridge and pavement projects most, its benefits extend to building construction as well. With all the above qualities, lightweight concrete can not only offset its increased per cubic meter cost but can also lead to significant overall savings regardless of geographical location.
| UNDERSTANDING EMBODIED ENERGY AND EMISSIONS OF A MATERIAL |
| While many construction specifiers understand the performance and cost benefits of structural lightweight concrete, these are not the only aspects to consider when deciding between building materials. Understanding the embodied energy and emissions of a material can help professionals reach a project’s sustainability and net zero emission goals.
Since lightweight coarse aggregate needs to be produced in a high heat rotary kiln, its material energy can be nearly 30 times greater than normal-weight aggregates. This would cause many to consider lightweight concrete a less sustainable option than normal-weight concrete, precluding its use in sustainability-minded buildings. However, lightweight concrete can be poured in thinner slabs with lower densities and thus reduce the material needs of other systems. When looking at a five-story building completed and studied by Walter P. Moore and Associates for the Expanded Shale, Clay and Slate Institute (ESCSI), this quality created a 1.4 percent reduction of embodied energy when compared to the assembly requirements of normal-weight concrete. For the same reason, using lightweight concrete also contributed 5.3 percent fewer overall emissions than normal-weight concrete—further accentuating this material’s utility in sustainable construction.1 While not completely quantifiable, it is also important to note a building’s resiliency when discussing sustainable building practices. If materials present initially sustainable metrics but need to be replaced or repaired often, due to their inability to withstand natural or manmade weathering, then they may not be the most ecologically conscious material to specify. Lightweight concrete not only resists chloride attack and premature cracking but also helps provide substantial resilience to seismic activity and other natural disasters. This means buildings made from lightweight concrete have a greater chance of withstanding cataclysmic events, as well as normal weathering without the need for extensive repairs and replacements. Accordingly, buildings made more resilient with lightweight concrete can mitigate the ecological impact of future repairs, contributing to the long-term sustainability of a building. Notes |
A five-story building completed in Salt Lake City provides an excellent example. The Utelite Corporation completed the concrete flooring assemblies in 2017 and produced a cost comparison report on the building. The report summarizes data for building designs with the following basic parameters: a design 133.4-mm (5.25-in.) thick lightweight concrete 1762 kg/cm3 (110 pcf) floors with a two-hour fire rating and a design for 165-mm (6.5-in.) thick normal-weight concrete 2323 kg/cm3 (145 pcf) floors which achieve the same two-hour fire rating. As assumed, the per m3 (cf) cost of lightweight concrete was greater than normal-weight concrete by 7.5 percent. Additionally, the lightweight concrete design required more shear studs, further increasing the cost.7
