Carbon Footprint Reduction for Major Transit Projects

By Contributing Author July 23, 2024

By Verya Nasri, Medhi Bakhshi and Pegah Jarast, AECOM

The Montreal metro Blue Line Extension toward the northeast consists of the construction of five new underground stations. The project includes 6 kilometers of the tunnel which will be constructed using a Tunnel Boring Machine (TBM). In line with the commitment to integrating sustainability best practices, Envision verification is pursued on this project. The Envision reference framework was developed to cover all the sustainable development aspects of an infrastructure project and each phase of its life cycle (planning, design, construction, operations and maintenance, and end-of-life). As a part of this endeavor, low-carbon concrete and shotcrete mixtures are proposed for the linings of TBM and SEM tunnels as well as stations and auxiliary structures. The CO2 emission of the low-carbon concrete and shotcrete mixtures were compared with their typical mixtures and the total CO2 emissions reduction related to the tunnel lining is determined. The study is of strategic significance for achieving emission reduction in the tunnel industry.

1. Introduction

Carbon footprint analysis is becoming more and more popular in every industry due to increasing concerns about global warming and greenhouse gas (GHG) emissions. The construction industry is a major producer of CO₂ emission. Huang et al (2018) reported that in 2009, the total CO2 emission of the global construction sector (which includes building and infrastructures) was 5.7 billion tons, contributing 23% of the total CO2 emissions produced by global economic activities. In the U.S., the construction processes generate the third highest greenhouse gas (GHG) emissions among industrial sectors. It is therefore important that this sector significantly reduces its consumption of energy and materials. One of the most relevant activities within this sector is tunneling construction. The significant impact of the construction industry largely arises from the embodied carbon in the primary construction materials – cement and steel and utilizing various types of high energy-consuming equipment. Research shows that more than 80% of the CO₂ emissions in the construction phase of a tunnel are attributable to the construction materials cement and steel. Thus, reducing the need for cement and steel can make a signiﬁcant contribution to reducing CO2 emissions. This comes along with a signiﬁcant reduction in construction costs.

Several studies have focused on the calculation of emissions related to tunnel construction (Li et al. 2011, Miliutenko et al. 2012, Huang et al. 2013, Fremo 2015, Huang et al 2020, Lee et al 2016, Xu et al. 2019).

This paper estimates the portion of produced carbon emission of the project related to tunnel lining. Using a low-carbon concrete mixture instead of a typical one, the saving in CO2 emission is evaluated. The comparison includes a baseline concrete mixture with ordinary Portland cement (OPC) and rebar reinforcement with a low-carbon concrete mixture with fiber reinforcement. The results of this study can be used as a reference for CO2 emission calculation of tunnel projects. It also helps to have a better quantitative idea about the carbon emission of the tunnel liner.

2. Project Information

The Blue Line Extension project consists of 5 stations and a TBM tunnel. The tunnel is approximately 6 km long. It is 15m deep on average and will be excavated through rock.

In the baseline design an arc shape section with Cast in Place (CIP) concrete lining using Ordinary Portland Cement (OPC) concrete mixture was proposed. To reduce carbon emissions, an alternative case was suggested. In this case, while a similar section was assumed, CIP lining was replaced with low-carbon shotcrete lining.

TBM bored tunnel is another option instead of arc shape CIP section. For this case, an external diameter of 9.3 m with 7+1 precast segmental lining system and 1.8 m width was proposed. For this case, changing from an OPC concrete mixture into a low-carbon concrete mixture for prefabricated segments is suggested.

In the following, details of tunnel geometry and concrete mixtures for baseline and low-carbon cases are presented.

2.1 Baseline design

2.1.1 Tunnel geometry

Figure 1 shows the tunnel geometry in the baseline design. It consists of three components. 75 mm temporary shotcrete, 40 cm CIP lining, and 32 cm thick invert.

2.1.2 Concrete mixture designs

The baseline concrete mixture consists of ordinary Portland cement (OPC) with no slag and silica fume. Table 1 presents the mixture design for temporary shotcrete, CIP arc, and invert.

Table 1 Baseline concrete mixture design

	Cementitious material (kg/m3)	Portland Cement (kg/m3)	Aggregate (kg/m3)	Water/ Cement	Rebar (kg/m3)	Fiber- (kg/m3)	Admixtures (kg/m3)	Total (kg/m3)
Temporary shotcrete	475	475	1430	0.38	–	20	4.5	2108.6
CIP arc	415	415	1800	0.41	62	–	3.94	2451.1
Invert	415	415	1786	0.41	76	–	3.94	2451.1

2.1.3 CO2 emission calculation

The Co₂ emission of a material can be calculated using the following formula:

Mass of CO_2eq = CO_2eq Factor X Material quantities(1)

CO2eq Factor is the amount of equivalent CO2 per kg of material from Environmental Product Declarations (EPDs). In this paper, EPA report to Congress EPA 530-R-08-007, Life 365 version 2.0, and Bath University’s Inventory of Carbon & Energy (ICE) was used as the reference.

Table 2 summarizes the detail of CO2 emission calculation for the unit volume of baseline concrete mixture designs. The quantities of materials are picked from Table 1.

Table 2 CO2 emission of baseline concrete mixture designs

		Baseline temporary shotcrete		Baseline-CIP arc concrete			Baseline-Invert concrete
Components	CO2 eq factor*	Mass/m3 (kg)	CO2 eq/m3 (kg)	Mass/m3 (kg)	CO2 eq/m3 (kg)	Mass/m3 (kg)		CO2 eq/m3 (kg)
Portland Cement	0.92	475	437	415	382	415		368
Admixtures	1.67	4.5	7.5	3.94	6.6	3.94		6.3
Aggregate	0.006	1430	8.6	1800		1786
Steel bar	1.85	–	–	62	114.7	76		140.6
Fiber	0.86	20	17.2	–	–	–		–
		Total:	470.3		513.9			539.7

Based on the geometry of the tunnel presented in Figure 1, the volume of shotcrete, CIP arc, and invert per 1m of the tunnel can be calculated. Having these quantities together with the information presented in Table 2, the total CO2 emission per 1 m length of the tunnel can be estimated. Table 3 summarizes this calculation.

In the calculation for the shotcrete volume, the thickness of the shotcrete is adjusted for over-excavation tolerance. Considering drill and blast as the excavation method, the over-excavation tolerance would be 0.5m and so the average adjusted thickness can be calculated as:

t_adjusted=75+ 500/2= 325 mm

Table 3 Total embodied carbon footprint per 1 m length of the tunnel-baseline design

	Thickness (mm)	Volume (m3) /1m tunnel	CO2 eq/m3 (kg)	CO2 eq/1 m length (ton)
Temporary shotcrete	325	5.9	470.3	2.8
CIP arc concrete	400	7.03	513.9	3.61
Invert concrete	320	2.854	539.7	1.54
		Total =		7.9

2.2 Low-carbon concrete design

As an alternative solution, the temporary shotcrete and CIP arc were replaced with reinforced shotcrete. In the following, similar calculations presented for baseline design in section 2.1 is presented.

2.2.1 Tunnel geometry

Figure 2 shows the tunnel geometry for low-carbon. In this design, the CIP is replaced with reinforced shotcrete. The components consist of 50 mm temporary shotcrete, 100 mm permanent shotcrete lining, and 32 cm thick invert. The shotcrete was reinforced with 3D 65/35BG fiber and the invert has the same mixture design as reinforced shotcrete.

Figure 2 Tunnel geometry-Shotcrete lining

2.2.2 Concrete mixture designs

To reduce the carbon emission while having the same strength properties, Portland cement was relaced with limestone Portland cement and 27% of the Cementous material was replaced with slag and silica fume (22% slag and 5% Silica fume which is considered moderate Supplementary Cementitious Materials (SCM)). This mixture was proposed for both shotcrete and invert. Table 4 presents this concrete mixture design.

Table 4 Shotcrete mixture design-SCM

	Cementitious material (kg/m3)	Portland Cement (kg/m3)	Slag (kg/m3)	Silica Fume (kg/m3)	Aggregate (kg/m3)	Water/ Cement	Rebar (kg/m3)	Fiber- Fiber-3D 65/35BG (kg/m3)	Admixtures (kg/m3)	Total (kg/m3)
Shotcrete-SCM	475	346.75	104.5	23.75	1430	0.38	–	40	4.51	2128.6

2.2.3 CO2 emission calculation

Using the same methodology as presented in section 2.1.3, the embodied carbon footprint for the unit volume of SCM concrete mixture is calculated. Table 5 summarizes the detail of this calculation. The quantities of materials are picked from Table 4.

Table 5 CO2 emission of SCM concrete mixture design

	Shotcrete Mixture-SCM
Components	CO2 eq factor	Mass/m3 (kg)	CO2 eq/m3 (kg)	% Replacement by mass
Portland Limestone Cement	0.85	346.75	295
Slag	0.1466	104.5	15	22%
Fly Ash	0.093	0.0	–	0%
Silica Fume	0.014	23.75	0.3	5%
Admixtures	1.67	4.51	7.5	1%
Aggregate	0.006	1430	8.6
Steel bar	1.85	–	–
Fiber-3D 65/35BG	0.86	40	34.4
	Total		361

Based on the geometry of the tunnel presented in Figure 2, the volume of shotcrete, CIP arc, and invert per 1m of the tunnel can be calculated. Having these quantities together with the information presented in Table, the total CO2 emission per 1 m length of the tunnel can be estimated. Table 6 summarizes this calculation.

In the calculation for the shotcrete volume, the thickness of the shotcrete is adjusted for over-excavation tolerance. Using the road header for excavation together with robotic shotcrete and real-time in-situ 3D laser scan, the over-excavation tolerance would be 0.02 m and so the average adjusted thickness can be calculated as:

t_adjusted = 150 + 20/2 = 160 mm

Table 6 Total embodied carbon footprint per 1 m length of the tunnel-Low carbon design

	Thickness (mm)	Volume (m3) /1m tunnel	CO2 eq/m3 (kg)	CO2 eq/1 m length (ton)
Temporary shotcrete +Shotcrete lining	160	1.8	361	0.65
Invert concrete	32	2.854	361	1.03
		Total =		1.68

Comparing the total CO2 emission of this case with the baseline, the total saving is 79%.

2.3 TBM tunnel with prefabricated segmental lining design

2.3.1 Tunnel geometry

Figure 3 shows the tunnel geometry of the TBM segmental lining design. It consists of 7+1 segments 1.8 wide.

Figure 3 Tunnel geometry-prefabricated segmental lining

2.3.2 Concrete and grout mixture designs

(a) Concrete design

The baseline concrete mixture consists of ordinary Portland cement (OPC) with no slag and silica fume. To reduce the carbon emission while having the same strength properties, Portland cement was replaced with limestone Portland cement and 27% of the Cementous material was replaced with slag and silica fume (22% slag and 5% Silica fume). Table 7 presents these concrete mixture designs.

Table 7 OPC and SCM concrete mixture designs

	Cementitious material (kg/m3)	Portland Cement (kg/m3)	Slag (kg/m3)	Silica Fume (kg/m3)	Aggregate (kg/m3)	Water/ Cement	Rebar (kg/m3)	Fiber- 4D 80/60BGP (kg/m3)	Admixtures (kg/m3)	Total (kg/m3)
Segment-OPC	475	475	–	–	1430	0.38	80	–	4.5125	2168.59
Segment-SCM	475	346.75	104.5	23.75	1430	0.38	–	40	4.5125	2128.59

(b) Backfill/Infill concrete

The baseline backfill/infill concrete consists of Ordinary Portland Cement (OPC) with no slag and silica fume. In the carbon-reduced case, 50% of the Portland cement was replaced with slag. Table 8 presents the backfill/infill concrete mixtures for these two cases.

Table 8 OPC and SCM backfill/infill concrete mixture designs

	Cementitious material (kg/m3)	Portland Cement (kg/m3)	Slag (kg/m3)	Aggregate (kg/m3)	Water/ Cement	Water (kg/m3)	Admixtures (kg/m3)	Total (kg/m3)
Baseline Concrete (OPC)	220	220	–	1950	0.8	176	2.09	2348.09
Carbon reduced Concrete	220	110	110	1950	0.8	176	2.09	2348.09

The tail-void backfill grout design is presented in Table 9. The same design is used both for the baseline case and the carbon-reduced case.

Table 9 Tail-Void grout mixture design

	Cement + Bentonite+ Stabilizer (kg/m3)	Portland Cement (kg/m3)	Bentonite (kg/m3)	Stabilizer (kg/m3)	Water/ Cement	Rebar (kg/m3)	Admixtures (kg/m3)	Total (kg/m3)
Baseline-Grout	325	325	42.5	5	2.55	0	3.0875	1205.59

2.3.3 CO2 emission Calculation

Using the same methodology as presented in section 2.1.3, the embodied carbon footprint for the unit volume of OPC and SCM concrete mixtures and grout mixture is calculated. Table 10 to Table 12 summarizes the detail of these calculations. The quantities of materials are picked from Table 7 to Table 9.

Table 10 CO2 emission of OPC and SCM concrete mixture designs

	Baseline Concrete Mixture (OPC)				Moderate SCM Concrete Mixture
Components	CO2 eq factor*	Mass/m3 (kg)	CO2eq/m3 (kg)	% Replacement by mass	Mass/m3 (kg)	CO2eq/m3 (kg)	% Replacement by mass
Portland Cement	0.92	475.0	437		346.8	319
Slag	0.1466	0.0	–	0%	104.5	15.3	22%
Fly Ash	0.093	0.0	–	0%	0	0	0%
Silica Fume	0.014	0.0	–	0%	23.8	0.3	5%
Admixtures	1.67	4.5	7.5	1%	4.5	7.5	1%
Aggregate	0.06	1430	8.6		1430	8.6
Steel bar	1.85	80.0	148.00		–	–
4D 80/60BGP	0.88	–	–		40	36.4
		Total	601			387

Table 11 CO2 emission of OPC and SCM backfill/infill concrete mixture designs

	Backfill/infill (inside the tunnel) OPC concrete mixture-Baseline			Backfill/infill (inside the tunnel) OPC concrete mixture- 50% slag
Binder	CO2 eq factor*	Mass/m3 (kg)	CO2 eq/m3 (kg)	Mass/m3 (kg)	CO2 eq/m3 (kg)
Portland Cement	0.92	220.0	202	110.0	101.2
Slag	0.15	0.0	0	110.0	16.2
Admixtures	1.67	2.1	3.5	2.1	3.5
Aggregate	0.006	1950	11.7	1950	11.7
		Total (kg/m3)	218		133

Table 12 CO2 emission of tail-void concrete mixture design

	Tail-Void backfill OPC grout-Baseline
Binder	CO2eq factor*	Mass/m3 (kg)	CO2 eq/m3 (kg)	% Replacement by mass
Portland Cement	0.92	325	299
Bentonite	0.093	42.5	4	9%
Stabilizer	1.31	5	6.6	1%
Admixtures	1.67	3.1	5	1%
		Total (kg/m3)	315

Table 13 and Table 14 summarize the detail of the total embodied carbon footprint of the tunnel based on the geometry of the tunnel and unit CO2 emission presented in the previous tables for baseline mixture design and SCM mixture design.

Table 13 Total embodied carbon footprint of the tunnel- Segmental lining

	Baseline Concrete Mixture (OPC)
	Ring width (m)	Tunnel length (m)	D-ex (m)	D-in (m)	Ring Volume (m3)	Total concrete volume (m3)	CO2 eq/m3 (kg)	CO2 eq/1 m tunnel (ton)	Total CO2 eq (ton)
40 cm thickness Rings-OPC	1.8	4086	9.3	8.5	20.1	45698.1	601	6.7	27,469.9
35 cm thickness Rings-SCM	1.8	4086	9.3	8.6	17.7	40210.5	387	3.8	15,568.6

Table 14 Total embodied carbon footprint of the tunnel- Backfill/infill and tail void backfill grout

	Volume (m3/1m tunnel length)	Tunnel length (m)	Total volume (m3)	CO2 eq/m3 (kg)	CO2 eq/1m tunnel (ton)	Total CO2 eq (ton)
Backfill/infill with OPC concrete mixture	10.25	4086	41881.5	218	2.2	9,113
Backfill/infill with OPC concrete mixture- 50% slag	10.25	4086	41881.5	133	1.4	5,550
Tail-Void backfill grout-Baseline	4.45	4086	18195.8	315	1.4	5,725.5

Based on Table 14, the total carbon footprint of the two cases, baseline design, and carbon reduced design is calculated as below:

As is seen, by using the carbon-reduced concrete mixture for segments and backfill/infill concrete, the total carbon footprint per 1 m length of the tunnel is reduced by 36%.

3. Conclusion

This paper presents the detail of embodied CO2 emission calculation. As is shown, in CIP tunnel lining by improving the mix design (using limestone Portland cement and 27% supplementary cementitious material) together with using real-time in-situ 3D laser scan, CO2 emission can be reduced by 80% (from 7.9 CO2 kg per 1 m length of the tunnel to 1.7 CO2 kg per 1 m length of the tunnel). For prefabricated segmental lining, by improving the mix design and using steel fibers instead of steel rebars, the CO2 emission can be reduced by 36% (from 10.3 CO2 kg per 1 m length of the tunnel to 6.4 CO2 kg per 1 m length of the tunnel).

Considering the scale of tunnel projects, this reduction in CO2 emission is huge and should not be ignored or underestimated. It should be noted that this emission estimation is a part of the total CO2 emission during the construction phase and produced CO2 by construction equipment and transportation should be added to it to have a complete estimate.

4 References

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