This is the sixth in the Engineer’s Corner series of articles in ESCSI Lightweight Design eNews that addresses issues related to concrete made using expanded shale, clay, or slate (ESCS) lightweight aggregate that has been manufactured using the rotary kiln process at temperatures around 2000 deg. F. When the particles reach these high temperatures, the material softens and bubbles are formed within the aggregate particles as gases are released from within. These bubbles become permanent pores in the aggregate when it cools and hardens. The pores in the vitrified ceramic aggregate particles give lightweight aggregate its reduced density and other unique properties.
The previous article in this series discussed design advantages offered by the common and efficient use of structural lightweight concrete on composite metal decks for floors and roof slabs in buildings. The article included a discussion of the comparative cost between normalweight and lightweight concrete for several buildings. This article extends the comparison between normalweight and lightweight concrete to the embodied energy and CO2 emissions involved in the construction of a typical building. Such a comparison is significant as sustainability is becoming a part of the design process.
Continuing the theme of the early articles in the Engineer’s Corner series, this article will address the following common myth or misconception about the sustainability of lightweight concrete buildings:
Myth — Because lightweight aggregate is produced using a high temperature process that requires a significant energy input and produces emissions, the use of lightweight concrete in a building cannot provide a more sustainable solution compared to a building using normalweight concrete.
A study was prepared by Walter P. Moore and Associates, Inc. for ESCSI comparing the embodied energy for a steel-framed building with normalweight and lightweight concrete floor slabs1. The study was developed in 2012 and was revised slightly in 2022, but the values used in the tables discussed below have not been changed. The purpose of the study was to compare the embodied energy in the structural system of a steel framed building with lightweight concrete floor slabs on composite steel deck to the same system utilizing normalweight concrete.
The study considered a representative 5-story office building that was designed in an area of moderate-seismicity in suburban Charlotte, North Carolina. The building provides approximately 115,000 SF of office space. Each floor has a footprint of 210’ x 110’ with seven 30 ft bays in the long direction and 3 bays in the short direction with 40 ft outer bays and a 30 ft inner bay. Floor to floor heights are 14 ft. The building is shown in the figure below. Detailed structural framing plans are included in the study.
The main building structural system is structural steel framing supporting a composite metal deck. The lateral load resisting system is concentric braced frames. The foundations are spread footings and a 5” slab on grade. Roof construction consists of 1 ½” metal deck over steel joists.
The study provided evaluations using four sets of design assumptions for the floor system in the building.
Based on the analysis presented in the study, the first design option using a lightweight concrete deck provided the most economical solution. Therefore, for this article, the discussion will be limited to the first two design options, which are designated as NWC and LWC A in the tables that follow. Properties of the floor system designs for these two options are summarized here:
Both floor slabs used a concrete mix with a design compressive strength of 3500psi. The lightweight concrete mixture used lightweight coarse aggregate and normalweight sand that probably had a design equilibrium unit weight of 115 pcf, although the report does not provide a value. Other details for the building components are described in the study.
A summary of material quantities (weights) for the two designs is presented in the following table, which is taken from the study:
The reduction in weight of materials required to construct the building option using lightweight concrete floor slabs is significant; the total weight is reduced by 26%, which would translate into significant cost reductions in transportation of materials to the project site, but such reductions are not considered in this analysis. The total weight of the building above the foundations is reduced by over 30% when lightweight concrete floor slabs are used, which would significantly reduce foundation loads and quantities. This would be particularly important for designs at sites where seismic effects are significant. In this case with only moderate seismic design requirements, the total quantity of material in the foundations was reduced by about 14%. The report makes a number of other conclusions based on the comparison of weights that are not repeated here due to space limitations.
The study includes the following table of material energy intensities:
For comparison purposes, the energy intensities in the table above can be multiplied by the concrete mix proportions provided in the report to obtain an index of relative energy intensity for the normalweight and lightweight concrete mixtures, as shown below.
While the material energy intensity (based on weight) for lightweight coarse aggregate is nearly 30 times the value for normalweight coarse aggregate, it is remarkable that the energy intensity of the lightweight concrete mixture is only double the intensity for the normalweight concrete mixture. While there are several factors that cause this, much of the reduction is the result of the weight of lightweight aggregate in a cubic yard of concrete being less than half the weight of the normalweight aggregate. However, more cement is used for the lightweight concrete mixture. The energy intensity for fly ash was not provided in the report; however, the quantities of fly ash used for the different mixtures does not vary much, so neglecting the embodied energy in fly ash would not have a noticeable effect on the comparison. The quantity of water and admixtures is also not provided for the concrete mixtures.
By multiplying the material energy intensities in the second table by the material quantities in the first table, a comparison of the embodied energy for the two building systems can be made, as shown in the table below.
From this analysis, the design using lightweight concrete floor slabs has slightly less embodied energy, when only considering the contribution of the construction materials. There are other factors that should be considered to obtain a more comprehensive comparison, including transportation of materials, as mentioned earlier, and the handling of the concrete, since the handling and placing of lightweight concrete will require less energy, although this is not expected to be a large component of the embodied energy.
The embodied energy value for lightweight aggregate used in the comparisons above was 1180 Btu/lb, which represents an average value obtained from an industry-wide study conducted by ESCSI in 2000. Another study of the industry was conducted in 2006, which resulted in a slightly lower average embodied energy value of 1080 Btu/lb. Using the embodied energy value from the 2006 study provides a slight improvement in the total embodied energy. Unpublished data from the 2006 study indicates that embodied energy values for individual plants that manufacture expanded lightweight aggregate were as low as 730 Btu/lb. Using the minimum value from the measured industry range in the above calculations, the total embodied energy for the building is further reduced. The effect of using the different values for the embodied energy for lightweight aggregate is summarized in the following table.
While the difference between the analysis using the different values of embodied energy for the lightweight aggregate are not large, the change may be significant for the project. As mentioned previously, the difference in total embodied energy from a design with a normalweight concrete floor slab to a design with a lightweight concrete floor slab could be widened if the evaluation included other aspects of construction. The difference would be expected to widen further if the full life-cycle cost of the building, including operating costs were considered.
Another aspect of sustainability that is now being considered is the emissions that are released during the construction and operation of a facility. The report prepared by Walter P. Moore in 2012 does not address the issue of emissions. However, the material quantity data presented in the report can be combined with emission data for materials to provide an estimate of the expected emissions from the construction of the building. From this analysis, the results are limited to material production and do not including transportation or construction site activities.
A table of emission intensities for the materials for which material energy intensities were provided in the 2012 Report is provided here. Values shown were obtained from various industry association or corporate sources.
As was done for the embodied energy comparisons discussed above, multiplying the material emission intensities by the material quantities in the first table provides a comparison of emissions for the two building systems, as shown in the table below.
This comparison indicates that the design with lightweight concrete floor slabs also has reduced emissions compared to a design with normalweight concrete floor slabs.
In addition to embodied energy and emissions, the impact of a building construction process on water use and waste production can also be considered. Furthermore, the entire life cycle of the building, including operations, should be evaluated to fully assess differences in costs and impacts on the environment over the life of the structure. In the future, ESCSI plans to provide such an example of a comprehensive comparison for a building.
The embodied energy data on which the analysis in this article is based are taken from studies of the lightweight aggregate industry conducted in 2000 and 2006. Work is currently underway to collect new data that can be used to update the analyses in this article. An article to update the analysis in this article will be written when the new information becomes available.
More current data on the other materials used in this evaluation are becoming more widely available, so these can be assembled and used in any future analysis. Other approaches that can be used in performing more comprehensive embodied energy and emissions evaluations are available and will be considered in future work.