Carbon Emissions Based on Transportation for Post-Tensioned Slab Foundation Construction: A Production Home Building Study in the Greater Phoenix Arizona Area

Abstract

There is significant focus on sustainable development of the built environment. Previous studies on sustainable construction have focused primarily upon improving the energy efficiency of buildings during the operational phase, recycling/reuse of building materials, and minimizing wastes. The environmental performance of onsite construction processes is not currently measured or reported as an industry standard practice. Measurement of carbon emissions is one way to understand and improve the environmental performance of onsite construction processes. This study provides an estimation of carbon emissions for transportation in post-tensioned slab foundation construction. Data were collected from a concrete trade contractor and sub-trade contractors in the Greater Phoenix Arizona area. First, carbon emissions are quantified for a typical production home using regional average data. Second, the influence of material and equipment transportation on the relative contribution of trades toward total carbon emissions is quantified. Ready-mix concrete transportation is found to be one of the most significant components and accounts anywhere from 25% to 63% of the total carbon emissions. Third, what-if scenario analysis is presented to study the influence of floor slab size and the travel distance on carbon emissions based on ready-mix concrete supply. Finally, an example is presented to demonstrate the aggregate level impacts.

Keywords:

• carbon emissions
• onsite construction processes
• post-tensioned slab foundation construction
• production home building
• residential construction

Introduction

Environmental Performance of Onsite Construction Operations

The construction industry accounts for significant amounts of material and energy use in different phases of a project's life cycle (Augenbroe, Pearce, & Kibert, 1998; Hendrickson & Horvath, 2000; Horvath, 2004). For example, building construction and operation accounts for 40% of the materials and 33% of the energy used in the world economy (Rees, 1999). Buildings in the United States account for 72% of the electricity consumption, 39% of energy use, 38% of all carbon dioxide emissions, 40% of raw materials use, 30% of waste output, and 14% of potable water consumption (U.S. Green Building Council, 2008). Limitations in the availability of natural resources, environmental impacts at the regional and global level, and increased awareness of carbon emissions, global warming and climate change are leading a paradigm shift in the construction industry: an increased emphasis on environmental performance in addition to traditional project objectives of time, cost, quality, and safety (Augenbroe et al., 1998; Vanegas, DuBose, & Pearce, 1996).

Sustainable construction is defined as the “creation and operation of a healthy built environment based on ecological principles and resource efficiency” (Kibert, 1994). Numerous studies have led to improved environmental performance in the built environment. The resulting improvements have focused primarily upon improving the energy efficiency of buildings during the operational phase, incorporating recycling and reuse of building materials, and minimizing material waste (USGBC, 2005; Kibert, 2007; Jensen & Kouba, 2007; Glavinich & Associated General Contractors, 2008; Centex Homes, 2009). In spite of these dramatic changes, environmental performance of the construction phase is not currently integrated into industry standard practices (Matar, Georgy, & Ibrahim, 2008). This is primarily due to lack of well defined sustainable construction practices (Pearce & Vanegas, 2002a), lack of metrics to measure sustainability at the operational level (Pearce & Vanegas, 2002b) and challenges in collecting accurate construction data.

Construction phase impacts refer to resource and environmental impacts of onsite construction processes. Cole (2000) attributes the inadequate coverage and challenges associated with quantifying the construction phase impacts to the following: 1) perceived lower significance of the construction phase of facility development compared to operational phase; 2) lack of understanding of the complexity of construction processes and associated environmental impacts; and 3) limited information about what actually occurs at the jobsite during construction. Although construction phase impacts are probably small when compared to the entire facility life-cycle, these impacts can be significant at the aggregate level, for example, in the temporal and spatial dimensions (Guggemos & Horvath, 2005; Bilec et al., 2006). As the operational phase of buildings becomes more energy efficient, the next focus of improvement should be the construction phase (Cole & Kernan, 1996; Guggemos & Horvath, 2005). Process-specific quantification of resource and environmental impacts of onsite construction processes is essential to understand and improve the environmental performance of the construction phase.

Carbon Emissions

There is significant emphasis on greenhouse gas emissions due to global warming and climate change. Carbon emissions represent more than 70% of the total global green house gas emissions (U.S. Environmental Protection Agency, 2008a). The transportation sector accounts for a significant portion of green house gas emissions. The Intergovernmental Panel on Climate Change (2007) report on climate change and mitigation, states that the transportation sector accounted for 23% of the world's total green house gas emissions in the year 2004. Several web-based software tools are available to estimate carbon footprint at the individual level (U.S. EPA, 2008b; Berkeley Institute of the Environment, 2009). In addition, software tools that estimate carbon footprint are expected to be used by the construction industry as part of the green building design in the future (Hunter, 2008).

Several initiatives have been taken to reduce carbon emissions. For example, one national objective in the United States is to reduce green house gas emissions by 18% in the 10 year time period 2002 to 2012 (US EPA, 2008c). Initiatives undertaken in the construction industry in support of this national objective have focused upon improving the energy efficiency of buildings during the operational phase. For example, a typical large scale home builder attempts to improve the environmental performance of residential homes by including energy efficient heating, ventilation, and air-conditioning (HVAC) systems, better insulation, high performance windows, tight sealing to avoid air leakage, tight air ducts and Energy Star appliances (Centex Homes, 2009).

The Leadership in Energy and Environmental Design (LEED) rating system developed by U.S. Green Building Council (USGBC) is widely used by builders and contractors to understand and improve the environmental performance of buildings. LEED rating includes several categories such as sustainable sites, water efficiency, energy and atmosphere, materials and resources use and indoor environmental quality (USGBC, 2005). Examples of LEED credits related to construction operations include the following: 1) Construction activity pollution prevention, 2) Brownfield redevelopment, 3) Storm water management, 4) Construction waste management, and 5) Material reuse, recycled content and regional materials. However, the environmental performance of onsite construction processes in terms of energy used for transportation and construction equipment are not currently part of the LEED rating system. Measurement of carbon emissions provides one way to quantify and improve the environmental performance of onsite construction processes. Further, this would enable incorporation of onsite construction operations into green rating systems such as LEED.

Production Home Building

Residential construction represents an important component of the U.S. economy in terms of GDP, annual sales and material consumption. In the U.S. housing market, the total number of permits issued for new single family homes in the years 2004, 2005, and 2006 are 1.61, 1.68, and 1.38 million, respectively (Joint Center for Housing Studies, 2006). The annual sales of new single family homes in the Greater Phoenix Arizona area varied from 35,000 to 45,000 in the past 3 years (Butler, 2008). It is estimated that more than 80% of these residential homes are production homes. Given the fact that large numbers of homes are constructed at the aggregate level, the environmental impacts of onsite construction operations of production homes can be significant.

Production home building involves the construction of 50 to 100 similar homes on a tract of land called a ‘subdivision.’ The vertical portion of production home construction consists of 10 to 12 major phases, 90 to 100 different activities and 25 to 30 specialty trade contractors (Bashford et al., 2003). In addition, code compliance inspections are performed by city-building inspectors and third-party inspectors. Examples of vertical construction phases completed by specialty trade contractors include concreting, plumbing, framing, drywall, siding/stucco, roofing, HVAC, electrical, doors and windows, painting, carpeting, counter tops, and fencing (Engle Homes, 2006). Among these phases, the framing and the concreting phases are the top two cost components as they account for 29% and 14% of the total cost of a production home, respectively (Housing Research Institute, 2006). The concreting phase is the first vertical construction phase of a production home and involves transportation of heavy materials, equipment and work crews. For example, the concreting phase of a typical production home requires the transportation of 30 to 60 tons of Aggregate Base Course (ABC), 0.5 to 1 ton of steel strand which will be post-tensioned, 75 to 100 cubic yards of ready-mix concrete and a concrete pump. Production homes typically built in the Phoenix area consist of a post-tensioned concrete slab-on-ground foundation and wood framed construction.

Objectives and Scope

The following are the objectives of this study:

1. Quantify the carbon emissions for transportation in post-tensioned slab foundation construction using a case study from the Greater Phoenix Arizona area.

2. Identify the production parameters that influence carbon emissions and perform what-if scenario analysis.

3. Suggest guidelines to minimize carbon emissions.

The scope of this study is limited to direct transportation. Indirect transportation based on supply chain processes is excluded. Examples for direct transportation include: transportation of trade contractor crew and equipment from the truck yard to the jobsite and transportation of materials from the production plant to the jobsite. Examples of indirect transportation include transportation of raw materials such as cement, sand and aggregate from the source to the material production plant.

Post-Tensioned Slab Foundation Construction

Post-tensioned slab foundations are typically used in the areas of expansive soils to avoid differential settlement. Greater Phoenix Arizona is such an area, and this foundation type is used for more than 90% of the new production homes. The construction of a post-tensioned foundation slab involves a concrete trade contractor, several sub-trade contractors, material suppliers and equipment suppliers. Figure 1 shows the list of various participants involved in the post-tensioned slab foundation construction in the Greater Phoenix Arizona area. These participants perform construction activities in the following sequence:

1. Layout

2. Spread lumber and set perimeter

3. Underground soils/plumbing—managed by the plumbing trade independently

4. Backfill plumbing trenches

5. Aggregate Base Course (ABC) supply

6. Grading of ABC

7. Set formwork for floor slab and outside flatwork

8. Internal quality control (QC) inspection and fix floor

9. Install post-tension strands

10. QC inspection by city building inspector

11. QC inspection third-party inspector

12. Pre-treat (termite treatment)

13. Transportation of concrete pump

14. Transportation and delivery of ready-mix concrete

15. Pour floor (place concrete into the foundation)

16. Remove formwork after curing and patch floor

17. Stress, cut and patch post tension strands

18. Set formwork for drive and walk and pour concrete

Figure 1 Participants involved in post-tensioned slab foundation construction.

Methodology

The methodology used for this study consists of the following steps: 1) Fuel use calculation for a base case scenario using the regional average data; 2) Carbon emissions calculation; and 3) Sensitivity analysis: Influence of change in significant production parameters on fuel use and associated carbon emissions.

Step-1: Fuel Use Calculation for a Base Case Scenario Using Regional Average Data

1. Transportation data for trades and sub-trades shown in Figure 1: for each trade type, identify the list of activities performed; for every activity, quantify relevant production parameters such as number of trips per lot, total travel distance per trip and number of lots worked on per trip. (Although the plumbing phase overlaps with the concreting phase, it is completed by the plumbing trade contractor independently and is not included in the scope of this study.)

2. Fuel efficiency data: for every activity performed by trades and sub-trades, identify the truck type, fuel type and quantify the fuel efficiency of trucks.

3. Fuel consumption for transportation per lot: for every trade type, fuel use per lot is calculated using the total travel distance per trip, number of lots worked on per trip, fuel efficiency of the truck and number of trips needed per lot. Fuel consumption for transportation in post-tensioned slab foundation construction is the sum of fuel use per lot for all participants listed in Figure 1.

4. Equations (1)–(3) show the fuel use calculation methods for the concrete trade crew, material transportation, and code-compliance inspection, respectively.

   where:

   • F_ct–fuel use for transportation of concrete trade crew per lot

   • F_mt–fuel use for material supply per lot

   • F_insp–fuel use for code-compliance inspection per lot

   • D–total distance traveled per trip

   • fe–fuel efficiency of the truck used

   • n–number of lots worked on per trip

   • p–number of activities performed by the concrete trade per lot

   • Q–total quantity of material needed per lot

   • tc–truck capacity

   • m–number of trips required for material supply

   • mc–number of trips needed for code-compliance inspection

Step-2: Carbon Emissions Calculation

Greenhouse gas emissions consist of the following gases as per the U.S. EPA guidelines (U.S. EPA, 2005a): Carbon dioxide (CO₂), Nitrogen dioxide (N₂O), Methane (CH₄) and Hydro fluoro carbon (HFC). Among this, CO₂ consists of 94–95% of the total green house gas emissions from vehicles, with other gases such as N₂O, CH₄ and HFC account for the remaining 5% to 6% (U.S. EPA, 2005a). While CO₂ emissions depend on the fuel consumption of the vehicle, other emissions such as CH₄ or N₂O depend more on the age of the vehicle than the fuel use. The scope of this study is limited to the quantification of CO₂ emissions for transportation in the post-tensioned slab foundation construction.

Using the guidelines published by the U.S. Environmental Protection Agency, carbon emissions are estimated as follows (U.S. EPA, 2005b):

1. Carbon content per one gallon of gasoline fuel: 2421 grams

2. Carbon content per one gallon diesel fuel: 2778 grams

3. Oxidation factor = 0.99 [assuming that only 99% of the carbon in the fuel is oxidized; 1 percent remains not oxidized]

4. Molecular weight (mw) of CO₂ = 44

5. Molecular weight of carbon = 12

6. CO₂ emissions (grams per gallon) = (Carbon content per gallon) ∗ Oxidation factor ∗ [mw of CO₂/mw of carbon]

7. CO₂ emissions from 1 gallon gasoline = 2421 ∗ 0.99 ∗ [44/12] = 8788.23 grams

8. CO₂ emissions from gasoline = 8.788 kg or 19.4 lb per gallon (1 kg = 2.204 lb)

9. CO₂ emissions from 1 gallon diesel = 2778 ∗ 0.99 ∗ [44/12] = 10,084.14 grams

10. CO₂ emissions from diesel = 10.084 kg or 22.23 lb per gallon

Step-3: Influence of Change in the Significant Production Parameters

The following steps are used to perform what-if scenario analysis for significant parameters:

1. Identify the significant trade type that influences carbon emissions based on transportation in post-tensioned slab foundation construction.

2. Identify production parameters that influence the carbon emissions.

3. Perform what-if scenario analysis to study how the home buyer choices and operational practices of trade contractors influence carbon emissions.

4. Study the impacts at the aggregate level.

5. Suggest guidelines to minimize the carbon emissions.

Data Collection

Computation of carbon emissions for transportation requires fuel consumption estimates. In order to estimate fuel consumption, travel distances and fuel use records of trucks were collected from the concrete trade contractor and all sub-trade contractors involved in the post-tensioned slab foundation construction. This includes: 1) average total distance traveled per trip and average number of lots completed per trip for transportation of crew, material and equipment; and 2) truck type and fuel efficiency of trucks used for all construction activities.

Travel Distances

Travel distances were estimated using the crew time cards of the concrete trade contractor. Crew time cards provide information such as: 1) number of subdivisions visited per trip; 2) number of lots worked on per subdivision; 3) location of each subdivision; 4) time spent at each lot; 5) crew size; and 6) drive time. Crew time cards were collected for every activity performed by the construction crew of the concrete trade. Table 1 presents the average total distance traveled per trip and the average number of lots completed per trip based on five samples. It can be noted that the travel distance per trip varies from activity to activity. This is because the crew visited different lots and subdivisions in each trip while working on a specific activity.

Table 1. Travel data for concrete trade contractor based on five trips

S. No.	Activity	Average total travel distance per trip (miles)	Average number of lots completed per trip
1	Layout	122.2	4
2	Spread lumber/set perimeter	143.2	7
3	Set floor	92.2	1
4	QC floor and fix floor	108.2	4
5	Pour floor	90	1
6	Strip/patch floor	131.8	6
7	Set drive walks	105	4
8	Pour drive walks	90.4	3

Travel data collected from sub-trade contractors, material and equipment suppliers in the Greater Phoenix area are summarized as follows:

1. ABC grading sub-trade:

   1. Average total travel distance per trip: 100 miles

   2. Average number of lots completed per trip: 3

2. Post-tensioning sub-trade (installation crew):

   1. Average total travel distance per trip: 124.7 miles

   2. Average number of lots completed per trip: 4.2

3. Post-tensioning sub-trade (stress/cut/patch crew):

   1. Average total travel distance per trip: 223.8 miles

   2. Average number of lots completed per trip: 15

4. ABC supply:

   1. Average one-way travel distance from the ABC plant to the subdivision: 12 miles

   2. Number of trips is a function of quantity of ABC supplied per lot

5. Ready-mix concrete supply:

   1. Average one-way travel distance from the ready-mix concrete plant to the subdivision: 15 miles

   2. Number of trips is a function of quantity of concrete supplied

6. Concrete pump supply:

   1. Average one-way travel distance from the truck yard to the subdivision: 26 miles

   2. Number of lots completed per trip: 1 (some times concrete pump is shared across two lots in the same trip)

In addition to the transportation of the construction crew, superintendents and inspectors travel to the jobsite regularly to perform supervision and inspections for completed work items. The summary of travel data for supervision and code-compliance inspections are as follows:

1. Superintendent of the concrete trade:

   1. Average total travel distance per trip: 200 miles

   2. Average number of lots supervised per trip: 15

   3. Number of trips: 8

2. Superintendent of the home builder:

   1. Average total travel distance per trip: 75 miles

   2. Average number of lots supervised per trip: 15

   3. Number of trips: 8

3. City building inspector and third-party inspector:

   1. Average total travel distance per trip: 150 miles

   2. Average number of lots supervised per trip: 10

Material Quantity and Number of Trips

The number of trips for material supply is estimated using the total material quantity needed per lot and the truck capacity. The following quantities were determined by averaging the actual amounts used for 50 production homes:

1. ABC supply:

   1. Average quantity of ABC: 44 tons per production home

   2. Truck capacity: 22 tons

   3. Number of trips: 2

2. Ready-mix concrete supply:

   1. Average quantity of concrete for floor slab with 9” slab thickness, outside flatwork and drive/walk: 86 CY per production home

   2. Truck capacity: 10.5 CY

   3. Number of trips: 8½ (assumed that the last trip of the truck is shared across 2 lots)

Fuel Efficiency of Trucks

The concrete trade contractor who participated in this study uniquely identifies each of their trucks with a truck number. This truck number and the type used for every activity performed by the concrete trade contractor were identified. Odometer readings and the actual daily fuel use record were collected for one month period for these trucks. Table 2 presents the average vehicle fuel efficiency values. It can be noted that F-150 trucks, which burn gasoline, were used for simple activities such as layout or quality control inspection where as F-450 trucks, which burn diesel, were used for those construction activities that require transportation of heavy materials and equipment. Table 2 indicates that the fuel efficiency values for the same truck type are influenced by the activity type. This is primarily because each truck is used to transport a variety of materials and equipment depending upon the construction activity. For example, the fuel efficiency of the F-450 truck used for ‘pour floor’ activity is 8.6 miles per gallon (mpg) where as the fuel efficiency of the truck used for ‘strip/patch floor’ activity is 7.6 mpg.

Table 2. Vehicle fuel consumption for concrete trade contractor

S. No.	Activity	Truck type	Fuel type	Fuel mileage (miles per gallon)
1	Layout	F-150	Gasoline	15
2	Spread lumber/set perimeter	F-450	Diesel	7.6
3	Set floor	F-450	Diesel	7
4	QC floor and fix floor	F-150	Gasoline	15
5	Pour floor	F-450	Diesel	8.6
6	Strip/patch floor	F-450	Diesel	7.6
7	Set drive walks	F-450	Diesel	9.9
8	Pour drive walks	F-450	Diesel	8.6

Table 3 presents the vehicle fuel consumption for sub-trades involved in post-tensioned slab foundation construction. In order to perform supervision and inspections, superintendent of the concrete trade, home builder superintendent, city building inspector and third-party inspector typically use a Ford 150 type truck which uses gasoline fuel. The average fuel efficiency value of F-150 truck is 15 miles per gallon.

Table 3. Vehicle fuel consumption for sub-trades

S. No.	Sub-trade	Truck type	Fuel type	Fuel consumption (miles per gallon)
1	ABC supply	Sterling	Diesel	5.5 (average of both fully loaded and empty truck)
				5.2 (loaded truck)
				5.8 (empty truck)
2	ABC grading sub-trade (truck with tractor attached in the trailer)	Ford F-150	Gasoline	11
3	Post-tensioning sub-trade (strand installation)	Ford F-650	Diesel	9
4	Post-tensioning sub-trade (stress/cut/patch)	Chevrolet 2500 Plus	Gasoline	13
5	Concrete pump transportation	Mack	Diesel	3
6	Ready-mix concrete supply	Kenworth 316	Diesel	3.05 (average of both fully loaded and empty truck)
				2.8 (loaded truck)
				3.3 (empty truck)

Analysis and Results

Base Case Scenario

Table 4 presents the estimation of carbon emissions using the travel and the fuel use data presented in the previous section. Metrics presented in Table 4 are based on regional average travel data in the Greater Phoenix Arizona area and represent a base case scenario for one typical production home.

Table 4. Carbon emissions for transportation in post-tensioned slab foundation construction

S. No.	Item	Fuel type	Fuel use per lot (gallons)	CO2 emissions per lot (lb)
1	Concrete trade contractor: (Activities: layout, QC floor)	Gasoline	3.84	74.50
2	Concrete trade contractor: (All activities except layout and QC floor)	Diesel	35.4	786.94
3	Concrete trade contractor: superintendent	Gasoline	7.11	137.93
4	ABC grading sub-trade	Gasoline	3.03	58.78
5	Post-tensioning sub-trade: Installation crew	Diesel	3.3	73.36
6	Post-tensioning sub-trade: Stress/cut/patch crew	Gasoline	1.15	22.31
7	ABC supply	Diesel	8.73	194.07
8	Ready mix concrete supply	Diesel	83.61	1858.65
9	Concrete pump supply	Diesel	17.33	385.25
10	Home builder superintendent	Gasoline	2.67	51.80
11	Inspections: City inspector and third-party inspector	Gasoline	2	38.80
Total fuel use (diesel):			148.4	3682.4
Total fuel use (gasoline):			19.8
Total fuel use (diesel and gasoline):			168.2

The following example demonstrates carbon emissions calculation for activities ‘layout’ and ‘QC floor’ performed by the concrete trade (Table 1):

1. Fuel use for ‘layout’ activity: {(122.2/4)/15}= 2.04 gallons of Gasoline.

2. Fuel use for ‘QC floor’: {(108.2/4)/15}= 1.80 gallons of Gasoline.

3. Carbon emissions for layout and QC floor: (2.04 + 1.80) ∗ 19.4 = 74.5 lb of CO₂.

Figure 2 shows carbon emissions for transportation by trade type using the results presented in Table 4. Figure 2 indicates that the ready-mix concrete supply is one of the most significant components in terms of fuel use and carbon emissions. Figure 3 shows the relative contribution of trades on total carbon emissions. Figure 3 indicates that the ready-mix concrete supply accounts for 50.5% of total carbon emissions. The concrete trade contractor, concrete pump supply and ABC supply accounts for 27.1%, 10.5% and 5.3% of total carbon emissions respectively. Post-tensioning sub-trade, ABC grading sub-trade, builder superintendent, and inspections collectively account for less than 7% of total carbon emissions.

Figure 2 Carbon emissions for transportation in post-tensioned slab foundation construction.

Figure 3 Relative contribution of trades on carbon emissions.

Sensitivity Analysis: Influence of Change in Production Parameters on Carbon Emissions

It is important to note that Figures 2 and 3 represent a base case scenario as defined in the ‘data collection’ section. The travel distances for material and equipment supply for the base case scenario are as follows:

1. Average one-way travel distance from the concrete plant to the subdivision (X): 15 miles

2. Average one-way travel distance for concrete pump (Y): 26 miles

3. Average one-way travel distance for ABC supply (Z): 12 miles

While this base case scenario is derived from regional average data and is most representative of a typical production home in the Greater Phoenix Arizona area, change in the above production parameters will change the carbon emissions for each trade type and their relative contribution on total emissions. Data collected in the Phoenix area indicates the following variation in the travel distance for material and equipment supply:

1. One-way travel distance for ready-mix concrete supply (X): 5 to 25 miles

2. One-way travel distance for concrete pump supply (Y): 10 to 45 miles

3. One-way travel distance for ABC supply (Z): 5 to 20 miles

Sensitivity analysis was performed to understand and quantify the influence of change in production parameters on total carbon emissions by trade type. Table 5 presents the summary of sensitivity analysis. Table 6 presents the relative contribution of trades and sub-trades on total carbon emissions based on Table 5. Results presented in Tables 5 and 6 indicate the following observations:

1. Ready-mix concrete supply: The relative contribution of ready-mix concrete supply on total CO₂ emissions is 25.4% when the one-way travel distance for concrete supply is 5 miles (Figure 4). This relative contribution increases to 63% when the one-way travel distance is increased to 25 miles (Figure 5). CO₂ emissions for concrete transportation per production home increases from 619.6 lb to 3098.9 lb when the one-way travel distance increases from 5 miles to 25 miles.

2. Concrete pump transportation: The relative contribution of concrete pump supply on total carbon emissions increases from 4.3% to 16.8% when the one-way travel distance from the pump truck yard to the subdivision changes from 10 miles to 45 miles. CO₂ emissions for concrete pump transportation increases from 148.3 lb to 666.9 lb per production home.

3. ABC supply: When the one-way travel distance between the ABC production plant and the subdivision changes from 5 to 20 miles, the relative contribution of ABC transportation on total CO₂ emissions increases from 2.3% to 8.5%.

4. The relative contribution of concrete trade contractor on overall emissions varies between 20% and 41% depending upon the variation in the travel distances for concrete supply, ABC supply and concrete pump supply.

Figure 4 Relative contribution of trades on total carbon emissions (X = 5 miles).

Figure 5 Relative contribution of trades on total carbon emissions (X = 25 miles).

Table 5. Sensitivity analysis: Carbon emissions for transportation in post-tensioned slab foundation construction (CO₂ in lb)

		Base case	Variations in the base case due to material and equipment supply
S. No.	Item	X = 15; Y = 26; Z = 12	X = 5	X = 25	Y = 10	Y = 45	Z = 5	Z = 20
1	Ready mix concrete supply	1858.65	619.55	3098.86	1858.65	1858.65	1858.65	1858.65
2	Concrete trade contractor	999.37	999.37	999.37	999.37	999.37	999.37	999.37
3	Concrete pump supply	385.25	385.25	385.25	148.27	666.90	385.25	385.25
4	ABC supply	194.07	194.07	194.07	194.07	194.07	80.92	323.45
5	Post-tensioning sub-trade	95.67	95.67	95.67	95.67	95.67	95.67	95.67
6	ABC grading sub-trade	58.78	58.78	58.78	58.78	58.78	58.78	58.78
7	Home builder superintendent	51.80	51.80	51.80	51.80	51.80	51.80	51.80
8	City building and third-party inspection	38.80	38.80	38.80	38.80	38.80	38.80	38.80
	Total CO2 emissions (lb)	3682.4	2443.3	4922.6	3445.4	3964.0	3569.2	3811.8

Table 6. Sensitivity analysis: Relative contribution of trades on total carbon emissions based on Table 5

		Base case	Variations in the base case due to material and equipment supply
S. No.	Item	X = 15; Y = 26; Z = 12 (%)	X = 5 (%)	X = 25 (%)	Y = 10 (%)	Y = 45 (%)	Z = 5 (%)	Z = 20 (%)
1	Ready mix concrete supply	50.47	25.36	62.95	53.95	46.89	52.07	48.76
2	Concrete trade contractor	27.14	40.90	20.30	29.01	25.21	28.00	26.22
3	Concrete pump supply	10.46	15.77	7.83	4.30	16.82	10.79	10.11
4	ABC supply	5.27	7.94	3.94	5.63	4.90	2.27	8.49
5	Post-tensioning sub-trade	2.60	3.92	1.94	2.78	2.41	2.68	2.51
6	ABC grading sub-trade	1.60	2.41	1.19	1.71	1.48	1.65	1.54
7	Home builder superintendent	1.41	2.12	1.05	1.50	1.31	1.45	1.36
8	City building and third-party inspection	1.05	1.59	0.79	1.13	0.98	1.09	1.02

What-if Scenario Analysis: Ready-Mix Concrete Supply

Ready-mix concrete supply is found to be one of the most significant components in terms of fuel use and carbon emissions in the construction phase of post-tensioned slab foundation system. Two production parameters, namely ‘slab size’ and ‘travel distance between the concrete plant and the subdivision,’ influence the fuel use and carbon emissions. To understand and quantify the influence of these parameters on carbon emissions, the following steps are used:

1. Find the relationship between the concrete volume and the slab size and quantify the number of trips made by concrete trucks as a function of slab size.

2. Develop a histogram of travel distance for concrete supply.

3. Analyze the influence of slab size and travel distance on the fuel use and carbon emissions.

Number of Trips by Ready-Mix Concrete Truck

The actual quantity of concrete placed for 110 production homes was collected from a concrete trade contractor in the Greater Phoenix Arizona area. These data represent production homes with 9” thick post-tensioned slab foundations. Based on these 110 samples, Equation 1 shows the relationship between the slab size and the concrete volume using regression analysis (R² value >0.95).

where, ‘Y’ refers to the actual volume of concrete placed for floor slab and outside flatwork in cubic yards (excluding concrete needed for drive and walk); and ‘X’ refers to the size of the floor slab and outside flatwork in square feet.

The average volume of concrete placed for drive and walk is found to be 9 CY. Total volume of concrete placed per production home is the sum of concrete placed for floor slab, outside flatwork, and drive/walk. The number of trips by the concrete truck is determined using the total concrete volume and the truck capacity. The capacity of a typical concrete truck is 10.5 CY. Table 7 presents the relationship between the slab size and the number of trips made by concrete trucks by varying the size of the floor slab from 1500 square feet to 5000 square feet. When the concrete volume needed per lot in the last trip is less than the truck capacity, concrete supply is typically shared across two lots. Hence, the calculated number of trips is rounded with the resolution of 0.5 to obtain the actual number of trips.

Table 7. Slab size and number of trips by ready-mix concrete truck

	Ready-mix concrete volume (CY)
Size of the floor slab and outside flatwork (square feet) [1]	Floor slab and outside flatwork [2] = 0.02599 ∗ [1] + 9.58	Drive and walk [3]	Total concrete volume per production home (CY) [4] = [2] + [3]	Number of trips [5] = [4]/10.5
1500	48.57	9	57.57	5.5
2000	61.56	9	70.56	7.0
2500	74.56	9	83.56	8.0
3000	87.55	9	96.55	9.5
3500	100.55	9	109.55	10.5
4000	113.54	9	122.54	12
4500	126.54	9	135.54	13
5000	139.53	9	148.53	14.5

Histogram for Travel Distance of the Ready-Mix Concrete Truck

Observations at the jobsite indicate that ready-mix concrete is not always supplied from the closest concrete plant due to factors such as availability of concrete and trucks with a particular supplier and the type of concrete ordered. Also, concrete is often supplied concurrently from multiple plants to the same lot in practice. To model the influence of these scenarios on travel distances, a total of 16 subdivisions and 20 ready-mix concrete plants located in different parts of the Greater Phoenix area were identified. Travel distances were computed for each subdivision based on the assumption that concrete will be supplied on a random basis from any one of the four concrete plants closest to the subdivision. Based on this, Figure 6 shows a histogram of travel distances for ready-mix concrete transportation (Palaniappan et al., 2009). The average one-way travel distance was found to be 15 miles. Figure 7 indicates that the one-way travel distance for concrete supply varies as follows: 1) 5 to 10 miles for 20.3% of the samples; 2) 10 to 15 miles for 26.6% of the samples; 3) 15 to 20 miles for 37.5% of the samples; 4) 20 to 25 miles for 10.9% of the samples; and 5) greater than 25 miles for 4.7% of the samples.

Figure 6 Histogram: Travel distance for concrete supply.

Figure 7 Carbon emissions based on ready-mix concrete transportation. (‘D’– one-way travel distance from the concrete plant to the subdivision)

Influence of Floor Slab Size and the Travel Distance on Carbon Emissions

To study the change in carbon emissions based on ready-mix concrete supply, the following variations in the production parameters are considered: 1) slab size: 1500 to 5000 square feet in the increment of 500 square feet; and 2) one-way travel distance from the concrete plant to the subdivision: 5 to 25 miles in the increment of 5 miles. Table 8 presents the change in carbon emissions for ready-mix concrete transportation as a function of slab size and travel distance. Figure 7 shows a graphical representation of change in carbon emissions.

Table 8. Carbon emissions for ready-mix concrete transportation per lot (CO₂ in lb)

	One-way travel distance ‘D’ from the concrete plant to the subdivision (miles)
Size of the floor slab and outside flatwork (square feet)	5	10	15	20	25
1500	400.9	801.7	1202.6	1603.5	2004.3
2000	510.2	1020.4	1530.6	2040.8	2551.0
2500	583.1	1166.2	1749.2	2332.3	2915.4
3000	692.4	1384.8	2077.2	2769.6	3462.0
3500	765.3	1530.6	2295.9	3061.2	3826.5
4000	874.6	1749.2	2623.9	3498.5	4373.1
4500	947.5	1895.0	2842.5	3790.0	4737.5
5000	1056.8	2113.7	3170.5	4227.3	5284.2

An example for calculating carbon emissions for a particular lot with a slab size of 2500 square feet is presented below:

1. Concrete slab size: 2500 square feet

2. Concrete quantity for floor slab and outside flatwork = 0.02599 ∗ slab size + 9.58

3. Concrete quantity for floor slab = 0.02599 ∗ 2500 + 9.58 = 74.56 CY

4. Concrete quantity for drive and walk: 9 CY

5. Total concrete quantity per home = concrete for floor slab + drive and walk

6. Total concrete quantity per home = 74.56 + 9 = 83.56 CY

7. Truck capacity: 10.5 CY

8. Number of trips: total concrete quantity/truck capacity

9. Number of trips: 83.56/10.5 = 8

10. Distance from the concrete plant to the subdivision: 15 miles

11. Average mileage of the concrete truck: 3.05 miles per gallon

12. Fuel consumption per trip: (2 × 15)/3.05 = 9.84 gallons of diesel

13. Fuel consumption per lot: number of trips X fuel use per trip

14. Fuel consumption per lot: 8 × 9.84 = 78.72 gallons

15. Carbon emissions: 22.23 lb of CO₂ per gallon of diesel (US EPA 2005)

16. Carbon emissions per lot: 78.72 × 22.23 = 1749.95 lb of CO₂

Observations based on Table 8 indicate the following:

1. Fuel use and carbon emissions for ready-mix concrete transportation for a typical production home with 2500 square feet slab size and 15 miles one-way travel distance is determined to be 78.72 gallons of diesel and 1749.95 lb of CO₂ respectively.

2. Fuel consumption for ready-mix concrete transportation is found to vary from 18 gallons (1500 square feet and 5 miles one-way distance) to 237 gallons (5000 square feet and 25 miles one-way distance) per production home depending upon the home size and travel distance between the concrete plant and the subdivision.

3. CO₂ emissions based on ready-mix concrete transportation is found to vary from 400.9 lb (1500 square feet and 5 miles one-way distance) to 5284.2 lb (5000 square feet and 25 miles one-way distance) per production home depending upon the home size and travel distance.

Production Parameters That Influence Carbon Emissions for Ready-Mix Concrete Supply

Based on the data collection and analysis, Figure 8 shows the causal relationship among production parameters that influence carbon emissions for ready-mix concrete transportation.

Figure 8 Production parameters that influence carbon emissions for concrete supply.

Aggregate Level Impacts

To demonstrate the aggregate level resource impacts and emissions, the following test cases are identified:

1. Base case scenario:

   1. One-way travel distance for concrete supply (X) = 15 miles.

   2. One-way travel distance for concrete pump transportation (Y) = 26 miles.

   3. One-way travel distance for ABC supply (Z) = 12 miles.

2. Test case-1: Concrete plant is located at 5 miles distance from the subdivision.

3. Test case-2: Concrete plant is located at 25 miles distance from the subdivision.

Table 9 presents the quantification of fuel consumption and CO₂ emissions for the three test cases mentioned above. Resource impacts and emissions were quantified at three levels: per production home, per subdivision and at the Greater Phoenix area level. The following are some observations based on Table 9:

1. When the ready-mix concrete plant, located at 5 miles distance from the subdivision, is chosen instead of another concrete plant located at 15 miles from the subdivision, the following benefits can be achieved (Base case scenario and Test case-1):

   1. Savings in miles driven: 170 miles per lot, 8500 miles per subdivision and 4,250,175 miles in the Greater Phoenix area.

   2. Savings in fuel use: 55.7 gallons per lot, 2787 gallons per subdivision and 1,393,500 gallons in the Greater Phoenix area.

   3. Reduction in emissions: 1239 lb of CO₂ per lot, 61955 lb of CO₂ per subdivision and 15488.9 tons of CO₂ in the Greater Phoenix area.

2. Reducing the distance between the concrete plant and the subdivision from 25 miles to 15 miles saves 111.5 gallons of fuel per lot and 5575 gallons per subdivision. Further CO₂ emissions are reduced by 2479 lb per home and 62 tons per subdivision.

Table 9. Aggregate level fuel use and CO₂ emissions for transportation in post-tensioned slab foundation construction

		Test cases
		Base case	1	2
Metrics	Scope	X = 15 Y = 26 Z = 12	X = 5 Y = 26 Z = 12	X = 25 Y = 26 Z = 12
Fuel use (in gallons)	per production home	168.17	112.43	223.96
	per subdivision	8,409	5,622	11,198
	Greater Phoenix area per year	4,204,250	2,810,750	–
CO2 Emissions	per production home (lb)	3682.39	2443.28	4922.60
	per subdivision (lb)	184,119	122,164	246,130
	Greater Phoenix area per year (tons)	46,029.9	30,541	–

Summary of Results and Discussion

The relative contribution of trades and sub-trades on carbon emissions based on transportation in post-tensioned slab foundation construction is as follows: Ready-mix concrete supply: 50.5%, Concrete trade contractor: 27.1%, Concrete pump supply: 10.5%, ABC supply: 5.3%, Post-tensioning sub-trade: 2.6%, ABC grading sub-trade: 1.6% and Supervision and inspections: 2.5%. These estimates are based on the regional average data in the Greater Phoenix Arizona area for one typical production home. Depending upon the variation in travel distances for material and equipment supply, the relative contribution of these trades on total carbon emissions is found to vary as follows: Ready-mix concrete supply: 25% to 63%, Concrete trade contractor: 20% to 41%, Concrete pump supply: 4% to 17%, ABC supply: 2% to 9%, Post-tensioning sub-trade: 2% to 4%, ABC grading sub-trade: 1% to 3% and Supervision and inspections (each): 2% to 4%. Ready-mix concrete supply has been identified as one of the most significant components in terms of fuel use and emissions. Depending upon the home size and the travel distance, CO₂ emissions based on ready-mix concrete transportation for one production home can vary anywhere from 401 lb (1500 sq ft slab size and 5 miles one-way travel distance) to 5284 lb (5000 sq ft slab size and 25 miles one-way travel distance).

The trade contractors who participated in this study after reviewing the results, suggested there is potential to reduce the fuel use and carbon emissions based on transportation using the following practices:

1. Residential development in the form of clusters or master-planned communities should be promoted. Such development enables production home building stakeholders to concentrate critical construction resources at one place and reduce the transportation distances. For example, ready-mix concrete suppliers operate multiple concrete plants within 5 to 15 miles distance from the Vistancia master-planned community that consists of several thousand homes in the northwest region of the Greater Phoenix area. On the other hand, the travel distances for concrete supply for homes in the southeast or southwest region of the Greater Phoenix area varies from 5 to 25 miles.

2. Size of the floor slab and outside flatwork could be optimized just to meet the needs of the home buyer. Preference could be given to small/medium size homes over large size homes to reduce material quantity, number of trips by trucks and associated fuel use and carbon emissions. This is primarily driven by the desires of potential home owners regarding the size of their home. Although it is obvious that small slab size would result in less carbon emissions, this study provides quantification on the influence of different slab sizes on carbon emissions based on ready-mix concrete supply (Table 8 and Figure 7).

3. Design of a hybrid post-tensioned slab foundation that has a thinner slab (less thickness) and greater spacing of post-tension strands: The contractors believe that post-tensioned slabs in the Greater Phoenix area are over-engineered because of risk aversion, engineering costs, builders moving a product from one region to another and lack of interest to custom-design slab foundations for each home. Achieving a balance between design optimization and serviceability has important economic implications for a contractor and this could reduce the total concrete quantity, number of trips for concrete supply, associated CO₂ emissions based on transportation as well as the material cost.

4. The fuel efficiency of concrete trucks (which is 3 to 4 miles per gallon) is an important parameter that influences the fuel use for concrete supply (Figure 8). Increasing the fuel efficiency of concrete trucks is outside the realm of construction issues. Initiative toward reducing the fuel use for concrete supply could focus on investigating alternate options in terms of vehicle type, engine power, truck capacity, and fuel type. For example, research initiatives undertaken by concrete truck manufacturers focus upon the use of light weight drums and reinforced single tires in place of dual tires. Potential benefits of these efforts include reduction in the truck weight by 550 pounds, increase in the truck capacity by ¼ to ½ CY and increased fuel mileage due to reduced rolling resistance (Yelton, 2009).

Conclusions

Process-specific quantification of environmental performance of onsite construction processes is important to identify sustainability metrics at the operational level and define sustainable construction processes. This paper presents a methodology for estimating carbon emissions based on transportation in post-tensioned slab foundation construction and provides quantitative analysis on the influence of different production parameters. The construction of a post-tensioned slab foundation requires a large number of participants (Figure 1). The relative contribution of these participants on the total transportation-based fuel use and carbon emissions has been estimated. Field observations on the transportation of construction crews, materials, equipment, supervisors and inspectors indicate that the following production parameters influence fuel use and carbon emissions:

	Concrete trade contractor and sub-trade contractors: Relative location of the trade contractors and the subdivision, fuel efficiency of the trucks used for every activity, number of lots worked on per trip, number of activities performed per lot and number of trips to the same lot on different days.
	Material supply: Number of trips as a function of home size, material volume and the truck capacity. Relative location of the subdivision and the material production plant. Fuel efficiency of the truck used for material supply.
	Supervision and inspections: Total travel distance per trip, number of lots worked on per trip, number of inspections or supervisions completed per lot on different days and the fuel efficiency of the truck.

The following are some conclusions based on the results presented in this study:

1. The ready-mix concrete supply is found to be one of the most significant components in terms of fuel use and carbon emissions (Figure 3).

2. It is important to note that the relative contribution of trade contractors towards total carbon emissions can change significantly depending upon the location of the trade contractor's truck yard and the material production plant. For example, the ready-mix concrete supply for production homes in the Greater Phoenix area represents anywhere from 25% to 63% of the total carbon emissions for transportation in post-tensioned slab foundation construction (Figure 4, Figure 5 and Table 6).

3. Detailed analysis on the fuel use for ready-mix concrete supply indicates the following:

   1. The influence of slab size and the travel distance on the fuel use and carbon emissions is inter-related, i.e., bigger the slab size, higher the impact of travel distance on the fuel use and carbon emissions (Figure 7).

   2. Effort toward minimizing the fuel use for concrete supply should focus on improving the fuel efficiency of concrete trucks, optimizing the home size and choosing a concrete plant located close to the subdivision (Figure 8).

   3. Although the fuel use and emissions are less significant for one production home, the local resource impacts or savings could be significant at the aggregate level, for example, at the subdivision or at the regional level (Table 9).

This study is beneficial to concrete trade contractors and ready-mix concrete suppliers as follows: 1) to understand the influence of their operational practices on fuel use and carbon emissions; 2) to estimate carbon footprint of construction operations in terms of transportation; and 3) to incorporate the environmental performance of onsite construction processes as a planning variable in addition to traditional project objectives such as time, cost, quality, and safety. Trade contractors who participated in this study are excited to discover the results of this study and they have indicated that these results are useful to them in their project planning. This study, along with similar studies completed in non-residential construction domains, could be used to develop a generic framework for representing the environmental performance or greenness of onsite construction processes at the operational level. Further work based on this study can focus on: 1) the analysis of economic advantages and disadvantages of greening onsite construction processes from the contractor's perspective; and 2) the development of a web-based carbon calculator for onsite construction processes in residential construction.

Acknowledgment

This study was supported by Salt River Project (SRP) Grant CWU 0001 and Science Foundation of Arizona Grant CAA 009407. Data collection for this study was supported through a research grant award provided by Graduate and Professional Student Association of Arizona State University, Tempe, AZ. We would like to express our sincere gratitude to Mr. Steven Hay, Mr. Daniele Pagano, Mr. Ray Moyers and Mr. Artie Collins of SCP Construction, LLC who provided timely assistance and valuable feedback during data collection and site visits. Also, we would like to thank all sub-trade contractors who participated in this study. Review comments provided by the four anonymous reviewers and the journal editorial board are gratefully acknowledged.

Notes

Note. Fuel mileage represents average value based on fuel use record for one month.

Note. 1 Gallon Gasoline = 19.4 lb of CO₂; 1 Gallon Diesel = 22.23 lb of CO₂.

Note. X: Average one-way travel distance from the ready-mix concrete plant to the subdivision; Y: Average one-way travel distance from the concrete pump truck yard to the subdivision; Z: Average one-way travel distance from the ABC production plant to the subdivision.

Notes. ‘X’—One-way travel distance from the ready-mix concrete plant to the subdivision in miles; ‘Y’—One-way travel distance from the concrete pump truck yard to the subdivision in miles; ‘Z’—One-way travel distance from the ABC production plant to the subdivision in miles.

Metrics per subdivision is based on the assumption that there are 50 homes in the subdivision.

Metrics at the Greater Phoenix area level is based on the conservative assumption that 25,000 homes are completed per year.