## 3 Sources of Discrepancies in LCA Stages
### Stage 2: Inventory Data  
The second source of uncertainties and unreliability in LCA results after the scope definition is LCA inventory (LCI). This is the data collection stage, in which the input and output factors, including energy, raw materials, products and waste, are analysed for the LCA of concrete. The LCI for a concrete mix mainly include: (a) upstream processes: those involved in the production of each of the constituents and its transportation to the concrete production plant; (b) core processes which involve the energy and emissions required for mixing concrete and transportation to site; and (c) downstream processes needed for the demolition or any other end-of-life scenario [24]. Out of all the inputs/outputs data from these processes, it is primarily important to quantify the emissions and energy-use rather than the oil use, waste generated and the rest [63]. Upon reviewing the necessary literature, it shows that LCI data is a major contributor to the uncertainty in a concrete LCA study due to the following reasons (i-iv):  
(i) The LCI source has no standards as to where and how to get LCI data for a LCA of concrete. [PERSON] and [PERSON] [73] stated that concrete inventory data should come from primary sources for reliability purposes or secondary sources if the former is not available. Primary data could be lab results, governmental reports or EPDs from the building industry to which the user has access. EPDs are standardized documents to communicate the environmental performance of a product that are accredited by local authorities [74]. On the other hand, secondary data could be from accredited environmental databases, such as EcoInvent, GaBi and ELCD database or just using previously published data from the literature. By examining the 107 papers that actually included an LCA study, it was found that more than half, as seen in Figure 9, opted for the use of secondary sources for inventory data, which could not be reliable enough to describe the special scenarios being modelled in the concrete LCA. Although EcoInvent (which was developed by the Ecoinvent Centre, a competence centre of the Swiss Federal Institutes) and GaBi (which was created by Thinkstep Inc.) are updated annually to reflect any changes in the inventory data included, the environmental impact of concrete when modelled using the Ecoinvent database and EPDs has a variability of up to 20% [61]. Hence, it is suggested that the priority in the source of upstream processes of a concrete mix is for EPDs and in the case of several EPDs, an average should be taken. The reason is that EPDs are done in accordance with the same process, an LCA, under the guidance and supervision of local authorities such as the Green Building Council of Australia's concrete [75]. This would contribute to standardized processes and more efficient error tracking of concrete LCAs. (ii) Apart from the reliability issue of the choice of the suitable LCI for the data, the existing data in each of the LCI sources contain large uncertainties. Looking into 25 papers, the inventory impact for OPC was found to vary between 691 kg eq CO\({}_{2}\)/tonne and 1452 eq kg CO\({}_{2}\)/tonne as shown in Figure 10. The reason could be that the OPC production process is different in efficiency between one producer and the other [76]. Additionally, the upstream process for OPC production depends on the electricity mix of the country of origin. For example, in the U.S. about 8% of the OPC used is imported and the upstream inventories of the imported clinker specific to the country of origin, as well as the energy consumed in transporting the OPC to the US, would increase the resulting impact of the OPC over the local alternatives [63]. The electricity mix in China almost has twice the environmental impact as that of Malaysia, Indonesia and Thailand due to, for example, the higher dependency on fossil fuel in electricity generation [31].