Essay: Reclaimed Asphalt Concrete Aggregate Substitution and Energy Consumption

Introduction

Portland cement is heavily used around the world for infrastructure. 4,200 million metric tons are produced globally per year (Statista). Although Portland cement is a valuable building material, as it has a long life span and is incredibly strong, the environmental impact of it is less than desirable. Portland cement creates over 5% of the world’s greenhouse gas emissions because cement requires a lot of energy and material to produce (Rubenstein 2012). The amount of emissions produced continues to rise as cement production increases at a rate of 2.5% annually (Rubenstein 2012). Along with Portland cement concrete, asphalt concrete is another type of concrete that is heavily used.

Concrete is made up of three components: water, aggregate, and a binder (Concrete Network). The difference between Portland cement concrete and asphalt concrete is the aggregates and binders. Asphalt uses petroleum as the binder while Portland cement uses limestone (Barber-Greene 1992). Asphalt concrete, also known as asphalt or blacktop, is used mainly for roads, parking lots, airports, and driveways. Nearly 83% of all roads in the United States have an asphalt surface (Zapata and Gambatese 2005). Given its frequent use, it is also produced in large quantities. In the United States alone, 400 million tons of asphalt are produced a year, which is over $30 billion worth of asphalt concrete (National Asphalt Pavement Association). However, asphalt concrete’s environmental impact is extensive as well – about 320,000,000J of energy is needed per ton of asphalt mixture, and an extra 13,400,000J are needed per ton of asphalt pavement placement (Zapata and Gambatese 2005). To put it in scale, the energy needed for one ton of asphalt can power over 42 million homes for a month in the United States (EIA). With 400 million tons of asphalt produced a year in the United States, the asphalt production industry is the second most energy-intensive industry in the United States following the cement industry (Zapata and Gambatese 2005 & EIA). See Figure 1 for an energy break down.

Table 1. Chou and Lee 2015

Given the magnitude at which asphalt cement is produced and its environmental impact, it is important to look for environmentally friendly cement alternatives, or green cement. There are multiple ways to decrease the environmental impact of cement, including adjusting the cement components of alite and belite ratios or using recycled aggregates. Alite and belite are minerals that makes up a large percent of a cement clinker, and clinker is used as a binder. However, alite uses more energy than belite, so being able to decrease the amount of alite and increase the amount of belite used can help conserve energy as well (MIT). In this experiment, we will be focusing on green asphalt concrete with varying percentages of reclaimed asphalt pavement (RAP) used as aggregate.

When looking at energy consumption of producing concrete there are additional considerations to make when  recycled aggregates. These additional energy costs come from breaking down the recycled material to the desired size required for the concrete mix. We use the data from Chou and Lee (2015) to create a model to predict  the trend for mixtures of 0-100% of RAP. This will allow us to estimate what percentage of RAP has the lowest energy consumption.

Methods

To begin, we gathered data and information from previous studies including the work of Chia-Pei Chou and Ning Lee (2015). From Chou and Lee (2015) we know the following:

• Energy consumption in MJ per ton of specific component of concrete (virgin aggregate, recycled aggregate, virgin binder)
• The amount of energy used per ton of material for varying percentages of RAP from 0 to 40%
• The equation the study used to calculate energy consumption in varying percentages

Using the above data provided by Chou and Lee, we imported the numbers into Matlab by creating different vectors for each component. Then, we extrapolated this data beyond 40% RAP content by using a polynomial best fit line. In order to find the degree of polynomial that produced the best line of fit, we started at a degree of one and kept increasing it until the values produced by the Polyfit matched the data exactly as provided in the study. Once we fit an equation to the data, we extrapolated the values to 100% RAP.  See Table 2 for the degree of fit determined for each vector.

Next, the data was converted from m3 to tons because the energy breakdowns of each component (binder, aggregate, and virgin aggregate) were provided to us in the unit of tons. Then, the energy consumption was calculated using the following equation at intervals of 10% RAP: ECi% = ECi%VA + ECi%VB + ECi%PP. In this equation ECVA is the energy consumed by the virgin aggregate used, ECVB is the energy consumed by the virgin binder used, and ECPP is the energy consumed by the plant processes for a mixture of i% RAP.

From evaluating the energy consumption equation we determined the range where energy consumption was the lowest. Using this range, a new range vector of percentages was created that contained a specific range of values with an interval of 1 rather than 10 to find the specific percent in this range where energy consumption was the lowest. Following the creation of this vector, the energy consumption equation was repeated. After obtaining the energy consumption values with the new interval, that specific interval data was graphed on top of the original graph as seen in Figure 1. Finally, we looked at the energy consumption values within the new specific interval to determine which percentage yielded the lowest energy consumption.

Results

When we ran our program the model showed that mixtures using somewhere between 30% and 50% of recycled aggregate consumed the least amount of energy. With this narrowed down range we determined that a mixture of asphalt concrete that uses 45% recycled aggregate is the most environmentally friendly as seen in table 3. Per ton of this mixture, 1708.921 MJ of energy is consumed which is equivalent to 475.08 kWh. Our model also shows that a mixture using 100% recycled aggregate would consume 15,436.56 MJ or 4291.36 kWh and a mixture using no recycled aggregate would use 2177.53 MJ or 605.35 kWh.

Figure 1. Trend of energy consumption with respect to RAP percentages. The blue circles are the data points calculated per every 10% of RAP, and the orange “x”s are the data points every 1% from 40 to 50%. The line connecting the data points are the predicted energy consumption values between each evaluated data point.

Table 3. Trend of energy consumption with respect to RAP percentages.

Discussion

Our results support replacing all virgin aggregate with recycled aggregate would result in a 709% increase in energy consumption for the production of asphalt cement. To give an idea of how much energy this represents,  the average household uses 897 kWh per month (U.S. EIA 2017). Therefore the extra energy used by a 100% recycled aggregate mix could power over four homes for a month per ton of concrete asphalt produced. On the other hand, if the optimal mixture of 45% recycled aggregate is used it would save 130.25 kWh per ton produced. This is equivalent to 15% of the energy consumed by a house per month.

According to the Asphalt Paving Association (APA) worldwide 350 million tons of asphalt concrete is consumed each year (Magorka 2014). With a savings of 130.25 kWh per ton produced that would result in an energy savings of 4.56*10^10 kWh. This energy savings is enough to power 4.24 million homes each for one year. Also because the asphalt consumption data is from 2013 (Newest publication from APA)  we can only assume that this value has increased since then as more areas have continued to become more urbanized.

Conclusion

While we assume that using recycled aggregate in concrete has similar properties to that of standard concrete, we do not have data to show that the mixtures have similar lifespans. If all the different mixtures have similar lifespans, than a mixture with 45% recycled aggregate would be the most environmentally friendly since its production consumes the least amount of energy. If another mixture such as 30% or 50% had a significantly longer lifespan, than the increased energy consumption to produce it would cancel out over time as that pavement would not need to be replaced as often. To dive further  into the eco friendliness of  different variations of recycled asphalt concrete there would need to be a substantial analysis on both material strength over time and lifespan to make sure that the structural integrity of roads would not be compromised by incorporating recycled asphalt into the new mixture. Once the mixture of concrete we used to model energy consumption has been tested for strength and lifespan we can then directly compare energy consumption to lifespan to determine the most environmentally friendly mixture of concrete utilizing recycled concrete.

References

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