The BRE IMPACT dataset only comprises a limited number of options for concrete when it comes to strength, precast/cast in-situ, reinforcement and fly ash (or blast furnace slag) content. This means that sometimes the real world material included in (or proposed for) a project may not be available in the materials library. This post is designed to act as a guide for selecting the most appropriate alternative when the optimum material isn’t available.
When it comes to selecting a concrete to include in an LCA model, there are 4 variables to consider:
- Whether it is reinforced or not (and the level of reinforcement)
- The strength of the concrete (RC or MPa)
- The percentage of any replacement material (e.g. fly ash, or blast furnace slag)
- Whether it is precast, or cast in-situ
Concrete in the IMPACT material library would state ‘excl. reinforcement‘ if there is none included. Therefore all concrete materials in the IMPACT library that does not state this (usually precast) we would assume has already included reinforcement.
It is important to note that each of these items influence the environmental impact of the concrete, however not equally. In general, you can use the previous list as a hierarchy, so in order of importance it is usually best to prioritise the variables as:
- Reinforced/not reinforced
- Strength (RC or MPa) *
- % fly ash/blast furnace slag
Unfortunately, the guideline above doesn’t always work, or the result may be unclear, so in many cases it can be worth adding a few different materials to the model so that they can be directly compared. You can then make a more informed decision about which is the most appropriate one to keep in the model.
In the following example a proposed improvement strategy recommended replacing 30% of the concrete content of a 40MPa precast concrete panel with fly ash, however there is no ‘Concrete (precast, RC40, 30% PFA, excl. reinforcement)‘ material in the library.
The original material was the ‘Concrete (precast, RC40)’, which has a GWP of 190,000 kg CO2 eq, and the initially proposed replacement was the ‘Concrete (in-situ, GEN1, 30% PFA, exc. Reinforcement)’, which has a GWP of 75,000 kg CO2 eq. It can be seen however, that the equivalent GEN1 concrete without the fly ash content – ‘Concrete (in-situ, GEN1, excl. reinforcement)’ has a GWP of 94,000 kg CO2 eq, which is already 50.5% lower than the original material. In this case a better replacement option is the ‘Concrete (in-situ, RC35, 30% PFA, excl. reinforcement)’, which has a GWP of 110,000 kg CO2 eq. Although this is not the same as a precast 40MPa concrete with 30% fly ash, it is probably the closest option within the library.
Ultimately, the goal is to select the closest match to the real world material used or proposed for the project.
* Note that GEN1 is a versatile, but low grade of concrete generally suitable for strip footings or non-structural applications.