The Life Cycle Energy Consumption and Emissions of Asphalt Pavement Incorporating Basic Oxygen Furnace Slag by Comparative Study

Round 1

Reviewer 1 Report

This paper has clearly studied the life cycle energy consumption and carbon emissions of pavement with BOF through five scenarios by considering the effect of rainy weather, porosity and dust content, which finally showed some positive results that the use of BOF instead of crushed stone is appropriate. This study has performed for a useful topic. However, several comments are listed below for the authors to consider:

-The abstract and conclusion are repeated partly, so it is suggested to revise them. The abstract could be simplified accordingly.

-The exhausted energy type of involved equipment in the stage of manufacture for crushed stones and BOF aggregate is electricity, and why listed the “Energy consumed per kg Diesel” in the 4th column. Please explain it detailedly in paper.

-Figure 6 and 7 are not clear enough visually, please check them.

Author Response

This paper has clearly studied the life cycle energy consumption and carbon emissions of pavement with BOF through five scenarios by considering the effect of rainy weather, porosity and dust content, which finally showed some positive results that the use of BOF instead of crushed stone is appropriate. This study has performed for a useful topic. However, several comments are listed below for the authors to consider:

-The abstract and conclusion are repeated partly, so it is suggested to revise them. The abstract could be simplified accordingly.

 Reply: We have revised the abstract to make it brief and concise. The new abstract was put below, please check.

Abstract: Basic Oxygen Furnace Slag (BOF), as alternatives for aggregate in asphalt pavement construction, is beneficial to environment by reducing land occupation and resource consumption. However, the quantitative effects on energy consumption and emissions reduction remains poorly understood due to the unavailability of local life cycle inventory. Therefore, its LCI needs to be built by accounting for the properties of BOF aggregate in terms of high porosity and dust content in BOF, the rainy interference condition that reducing efficiency in production, and transportation distance. Here we investigated the life cycle energy consumption and global warming potential (CO2-eq emission) of asphalt pavement incorporating BOF aggregate by per-forming a case study with uncertainty analysis. Five scenarios were elaborated and performed in the case study. The results show that the energy required for BOF production is 0.024 MJ/kg, approximately half the energy required for crushed stone of 0.044 MJ/kg. The pavements with BOF can reduce up to 12% of emission compared to ordinary pavement. Considerably more negative impact of rainy weather on energy consumption of BOF than natural crushed stone can be concluded. Monte Carlo simulation indicates that the order of magnitudes of the energy values were varied, from materials extraction as the maximum contributor to transportation. The benefits for BOF utilization are gradually offset by increased transport distances and the displacement ratios to fine crushed stones, due to the increase in fuel and resource consumption for mixing, construction, and transportation.

-The exhausted energy type of involved equipment in the stage of manufacture for crushed stones and BOF aggregate is electricity, and why listed the “Energy consumed per kg Diesel” in the 4th column. Please explain it detailedly in paper.

Reply: Thank you for your careful review. The type of energy used for equipment in the stage of manufacture is definitely electricity. We checked the results again and found that it was just put in the wrong place. Now we move the related results to the column of Electricity(kwh/kg). Here we present the revised Table.3.

Table 3. Life cycle inventory for crushed stone and BOF aggregate.

-Figure 6 and 7 are not clear enough visually, please check them.

Reply: We have redrawn Figs. 6 and 7 by scaling up the fonts in it and adding some highlights. Now we think the figures are more clear visually.

Reviewer 2 Report

This is an interesting work. Some comments:

1. The title is too long, please shorten it.
2. The abstract is a little bit too long, please shorten it without losing essential information.
3. Please include the influence factors in keywords.
4. How the authors consider the effect expansion of steel slag on LCA.
5. How do the authors consider the BOF based filler?

Author Response

This is an interesting work. Some comments:

1.The title is too long, please shorten it.

Reply: We have shortened the title as good as we can. The new title is ‘The Life Cycle Energy Consumption and Emissions of Asphalt Pavement Incorporating Basic Oxygen Furnace Slag by Comparative Study’. 

2.The abstract is a little bit too long, please shorten it without losing essential information.

   Reply: We have revised the abstract to make it brief and concise. The new abstract consists of 237 words. It was put below.

Abstract: Basic Oxygen Furnace Slag (BOF), as alternatives for aggregate in asphalt pavement construction, is beneficial to environment by reducing land occupation and resource consumption. However, the quantitative effects on energy consumption and emissions reduction remains poorly understood due to the unavailability of local life cycle inventory. Therefore, its LCI needs to be built by accounting for the properties of BOF aggregate in terms of high porosity and dust content in BOF, the rainy interference condition that reducing efficiency in production, and transportation distance. Here we investigated the life cycle energy consumption and global warming potential (CO2-eq emission) of asphalt pavement incorporating BOF aggregate by per-forming a case study with uncertainty analysis. Five scenarios were elaborated and performed in the case study. The results show that the energy required for BOF production is 0.024 MJ/kg, approximately half the energy required for crushed stone of 0.044 MJ/kg. The pavements with BOF can reduce up to 12% of emission compared to ordinary pavement. Considerably more negative impact of rainy weather on energy consumption of BOF than natural crushed stone can be concluded. Monte Carlo simulation indicates that the order of magnitudes of the energy values were varied, from materials extraction as the maximum contributor to transportation. The benefits for BOF utilization are gradually offset by increased transport distances and the displacement ratios to fine crushed stones, due to the increase in fuel and resource consumption for mixing, construction, and transportation.

3.Please include the influence factors in keywords.

 Reply: The factors ‘Rainy interference condition’, and ‘Production efficiency ‘that influence the energy consumption were included in keywords.

4.How the authors consider the effect expansion of steel slag on LCA.

 Reply: Volumetric expansion is potentially harmful for the concrete with steel slag. Currently the most energy-efficient and cost-efficient way is to make the steel slag exposed to air for at least half year. During the weathering time, the free calcium oxygen would hydrate with water.

So, on one hand, the weathering condition for steel slag has limited influence on environmental impact regarding the energy consumption and emission in system boundary. On the other hand, the steel slag that has been appropriated treated has positive effect to improve the performance of asphalt pavement. The authors have published related research to investigate it (DOI:http://dx.doi.org/10.1016/j.jclepro. 2016.05.106). A pavement with BOF serves for 15 years without rehabilitation and maintenance (R&M)     

(DOI:https://doi.org/10.1016/j.conbuildmat.2019,117630).

However, for the steel slag that has not been appropriated treated, the asphalt pavement has high possibility to damage. Therefore, the R&M project should be carried out in advance than schedule. It definitely has negative impact on the life cycle energy consumption and emissions.

5.How do the authors consider the BOF based filler?

 Reply: BOF based filler is a good anti-stripping agent for asphalt pavement, as our previous research revealed (DOI: https://doi.org/10.3390/ma12193211). The addition of BOF filler could improve the moisture resistance of asphalt mixture, then prolong its service span. The use of BOF filler instead of cement could definitely decrease the life cycle energy consumption and emissions.

However, there is no related specification that can guide the use of BOF filler in highway asphalt pavement in China. Subsequently there is no case project for us to investigate. The effects of BOF filler on the life cycle environmental impact is an interesting topic. The authors have plan to involve it in the further research. Thanks for the suggestion.