Plastic Shrinkage Cracking

Concrete is made by mixing cement, gravel and sand with water and various chemical and physical additives. After concrete has been placed at the construction site, it begins to hydrate, reacting to form a strong chemically-bound matrix. Keeping the concrete from drying is called “curing”. However, in many circumstances, the concrete begins to dry out. This drying occurs on the surface of the concrete, leading to a dry outer layer, and a moist inner layer. This differential drying leads to cracking; since concrete is so heavy it acts to anchor itself, and when the dry concrete shrinks it cracks. This cracking, referred to as “plastic shrinkage” cracking, is a major problem in the multi-billion dollar concrete construction industry.

Plastic Shrinkage Cracking vs. the Environment

Concrete production consists of both direct and indirect sources of greenhouse gases (GHGs); where direct sources are described by the production of concrete and cement, and indirect sources include fuel, placement, quarrying, suppliers, etc. According to the Portland Cement Association (PCA), direct sources of CO2 account for ~400 kg of CO2e per cubic meter of concrete (1). As indirect sources account for another ~400 kg of CO2e per cubic meter of concrete(2), the total complete lifecycle CO2e contribution per cubic meter of concrete is ~800 kg CO2e.

As Canadians use ~28.1 million cubic meters of concrete annually(3), concrete consumption can be estimated as ~0.8 cubic meters per Canadian (based on current census numbers).(4) Using this value in conjunction with the Albertan population, it is estimated that million cubic meters of concrete are used in Alberta annually.(4) Using the current industry as the base-case, 2.6 million tonnes of CO2e are emitted by the Alberta concrete industry every year.

400,000,000 cubic meters

The total annual concrete use in Canada and the United States. Equivalent to 500,000,000 cubic yards.

Currently, cracking in concrete is the most significant cause for concrete repair or replacement. Of the 500 million cubic meters of concrete used annually in North America, ~10% is used for repair/replacement of cracked concrete. Very little concrete contains plastic fibers (current height of industry crack mitigation technology), as performance is lacking. As cracking is the primary mode of concrete failure, the addition of fiber reinforcement for crack mitigation will lead to significant reduction in concrete needing repair and replacement; a savings of 0.8 T of CO2e per cubic meter of concrete. Assuming eventual market penetration to be 10% of concrete, hemp fiber reinforcement could increase the longevity of 5,000,000 cubic meters of concrete, leading to an increased energy efficiency in the concrete industry, and a reduced annual requirement of annual concrete.

Baseline for Calculations – Current Concrete Industry

  • Canada uses 28.1 million m3 of concrete annually(3), and Population of Canada is 35.2 million people(4), therefore, Canadians use 0.8 m3 of concrete, per capita, per year.
  • The Albertan population is 4.1 million people(4), using 0.8 m3 of concrete annually, becomes 3.3 million m3 of concrete used annually in Alberta.
  • Accounting for direct and indirect sources of CO2e, 800 kg (0.800 T) of CO2e is emitted per m3 of concrete.(1,2) 3.3 million m3 of concrete, at 0.8 T CO2e per m3 concrete, accounts for the production of 2.6 million T of CO2e produced annually.
  • These values include all current crack mitigation technologies and strategies.
  • The current industry condition described is an ideal baseline, as there have been no significant changes to the status quo.
Hemp Fiber Processing

Reductions from Baseline – Addition of Hemp Fiber

  • These calculations exhibit energy efficiencies in concrete use through reductions in CO2e (increasing the longevity of concrete, and therefore reducing the amount required per year).
  • All achieved GHG reductions can be considered permanent.
  • Production of the hemp fiber feedstock is estimated to produce 4.05 kg of CO2e per tonne of fiber
  • The CO2e embodied by hemp fiber upon sequestration in concrete is 1.60 tonnes of CO2e per 1 tonne of fiber. (Determined by weighted addition of mass and carbon content in cellulose (72% dry weight, 45% carbon), hemicelluloses (19% dry weight, 48% carbon), and lignin (5% dry weight, 40% carbon), yielding 0.44 tonnes of absorbed carbon per tonne of fiber.
  • The molar masses of carbon and CO2 are 12 g/mole and 44 g/mole, respectively, 1.60 tonnes of CO2 are absorbed per tonne of agricultural fiber (production steps are a negligible 0.004 tonnes of CO2e per tonne agricultural fiber).
*Stats are provided by Canadian Greenfield Technologies (the only provider of hemp reinforcement for concrete)

Hemp Advantage

Reinforcing fiber for controlling plastic shrinkage cracking in concrete is a constantly growing trend, responding to ever increasing demand for a higher quality of concrete infrastructure with a longer life-span. Conventional synthetic fiber products are often imported and produced from unsustainable and non-renewable feedstock, which has limited performance due to poor bonding and dispersion in concrete and often comes with high costs.

NForce-Fiber®, the only hemp concrete reinforcement fiber, is produced from domestic sustainable grown hemp feedstock, is cost efficient and offers high performance. NForce-Fiber® has also been proven to give the competitive advantage in oil & gas cementing as it is more resistant to elevated temperatures in comparison to most synthetic fibers. NForce-Fiber® has been technically developed, tested, verified and commercially implemented by independent experts in the concrete industry.

Why is hemp fiber a viable solution for concrete reinforcement?

  1. Separate single fibers are discrete, allowing the fiber to be dispersed in concrete mix.
  2. The fiber is intact and continuous, which allows the fiber to reinforce the concrete matrix.
  3. Longer hemp fibers have a higher probability of bridging crack gap.
  4. When dried, hemp fiber still remains a discrete and not stuck together.

Impacts

  • Reduction in Greenhouse Gases

    Cement is a major component of greenhouse gas (GHG) emissions.  Globally, on the order of 3.3B tonnes of cement is consumed annually with the vast majority (over 90%) being consumed by the concrete industry.  Typically, the US and Canada produce approximately 100M and 13M tonnes of cement, respectively, under healthy economic conditions.

    The cement manufacturing industry emits approximately 0.9 tonnes of CO2e per tonne of cement manufactured, making cement manufacturing one of the largest anthropogenic sources of CO2 in the world (5 to 7% of global anthropogenic GHG emissions). According to Environment Canada, in 2008 the Canadian cement manufacturing industries emitted 10.9M tonnes of CO2e.

    The cement manufacturing process directly generates CO2 as a chemical by-product of cement production (called calcination).  During calcination, the cement raw materials (mainly limestone – calcium carbonate) are thermally roasted in large kilns at temperatures on the order of 1,300 to 1,500 deg C.  CO2 gas is directly liberated from the limestone and emitted from the kiln.  The majority of the industrial GHG emissions (on the order of 50-70%) is a result of calcination and cannot be reduced through the use of alternative fuels or energy efficiency.

    Cement manufacturing is highly energy intensive. Approximately 3 to 6 GJ of thermal energy per metric tonne of cement is required during cement manufacture (global range).  The Canadian cement manufacturing industry averages 3.9 GJ of energy consumed per tonne of cement manufactured, according to the Cement Association of Canada.  Approximately 85% of the Canadian cement manufacturing industry process fuel consumption is based on coal or petroleum coke combustion.   In addition to large amounts of energy consumed, process fuel combustion accounts for approximately 30 to 40% of the cement industry’s GHG emissions.

  • Natural Resource Conservation

    Two major natural resources are consumed during cement and concrete manufacture as follows:

    1) Natural Minerals:

    Cement manufacture requires substantial quarrying of natural limestone, the main ingredient in cement manufacturing.  Each tonne of cement manufactured requires approximately 1.15 tonnes of mined limestone, consumed as a raw material.  The Canadian cement manufacturing industry annually consumes on the order of 15M tonnes of limestone for cement manufacturing.  Limestone is mined from local deposits, typically requiring removal of overburden, explosive blasting, quarrying and transportation to the cement plant.

    By volume, concrete is typically comprised of 60 to 75% natural sand and gravel, from natural mineral deposits.  In 2007, approximately 13B tonnes or 5.3B cubic meters (m3) of concrete was consumed globally.  On a mass tonnage basis, more concrete is consumed globally than steel, coal and crude oil combined.  The global concrete industry consumes on the order of 10B tonnes of sand and gravel annually.  These are natural minerals that must be strip-mined from the earth and washed with freshwater (in order to remove impurities), prior to utilization in concrete manufacture.

    The Canadian concrete industry produces approximately 33M cubic meters of concrete on an annual basis.  Canadian concrete manufacturing consumes on the order of 60M tonnes of natural, mined sand and gravel.  Sand and gravel utilized by the Canadian concrete manufacturing industries are mined/quarried locally and typically require removal of vegetation and topsoil overburden.

    2) Fresh Water:

    Concrete manufacturing requires consumption of substantial volumes of fresh water in order to wash sand and gravel (impurity removal) and to mix the concrete itself.  Globally, the concrete industry consumes over 1 trillion liters of freshwater annually (approximately 200 L per cubic meter of concrete – including sand and gravel wash water and concrete mix water.)   The Canadian concrete industry consumes on the order of 6.6B liters of fresh water on an annual basis.

    By reducing the amount of cracking in concrete, hemp reinforcement products could extend the life of concrete under cracking conditions and thus economize the production and consumption of cement, natural aggregates and fresh water that would have otherwise been consumed in the manufacture of the replacement concrete.

  • Long Term Benefits

    Additionally, over the long-term, hemp concrete fiber can displace conventional synthetic concrete fiber products derived from thermoplastic, polypropylene plastics or steel.   Polypropylene fiber is derived from non-renewable petrochemicals and utilizes a thermal energy intensive manufacturing process involving high temperature, melt processing of polypropylene in to concrete fiber. Steel fibers are derived from mined mineral raw materials and utilize very high temperature, thermal energy intensive process involving melt processing of steel alloy into concrete fiber.  It is conservatively estimated that hemp fiber concrete reinforcement products could displace on the order of 3,000 tonnes per year of polypropylene concrete fiber and 15,000 tonnes of steel concrete fiber, mainly imported into Canada from Asia.

    From the sustainability and environmental impact perspectives, hemp-base concrete reinforcing fiber would result in a technology with substantial benefit to the public including: GHG emission reduction, energy conservation, natural resource conservation and replacement of synthetic products, with high embodied manufacturing energy and/or derived from non-renewable resources.

    The bellow table provides estimates of the amount of Canadian GHG emissions, energy and natural resources conserved by the national cement and concrete industries if the agri-fiber technology was eventually implemented and utilized across Canada over the long-term (5 to 10 years):

    Savings in GHG, Energy, and Natural Resources


    Savings in GHG, Energy, and Natural Resources with Long-Term Implementation

    ‘* conservation calculations assume 20% of national concrete consumption (33M m3/yr) replaces concrete that has reached the end of its service life due to cracking.  It is also assumed the implemented technology could save 30% of this replaced concrete volume (approximately 2M m3 of concrete /yr).  Calculations are also based on typical concrete consumptions of 275 kg of cement per m3 of concrete, 1,850 kg of natural aggregate per m3 of concrete and fresh water consumption of 200 liters per cubic meter of concrete.  Calculations are based on energy consumption of 3.9 GJ/tonne of cement manufactured, 0.9 tonnes of GHG per tonne of cement manufactured and 1.15 tonnes of limestone consumed per tonne of cement manufactured.

References

  1. Portland Cement Association. Green in Practice 102 – Concrete, Cement, and CO2 Technical Brief. http://www.concretethinker.com/technicalbrief/Concrete-Cement-CO2.aspx (accessed June 20, 2015).
  2. Chester, Mikhail V. (2008) Life-cycle Environmental Inventory of Passenger Transportation in the United States. Institute of Transportation Studies. University of California, Bekeley, USA. Retrieved from: http://escholarship.org/uc/item/7n29n303 (accessed June 20, 2015).
  3. Cement Association of Canada. Economic Contribution. http://www.cement.ca/en/Economic-Contribution.htmp (accessed June 21, 2015).
  4. Statistics Canada (January 30, 2013) Population and dwelling counts, for Canada, provinces and territoties, 2011 and 2006 censuses. http://www12.statcan.gc.ca/census-recensement/2011/dp-pd/hlt-fst/pd-pl/Table-Tableau.cfm?LANG=Eng&T=101&S-=50&oA (accessed June 21, 2005).
  5. Cherrett, N.; Barret, J.; Clemmett, A; Chadwick, M.; Chadwick, M.J. Ecologiecal Footprint and Water Analysis of Cotton, Hemp, and Polyester; Stockholm, 2005,pg12.

Background

Every year, more than 50 million tons of asphalt are produced in Canada, with ten times that amount being produced in the United States (~500 million tons). In Canada alone, asphalt is a $5 billion industry, While the industry is dominated by small companies (90% of asphalt companies have fewer than 20 employees), large centers produce their own asphalt, as they are tasked with construction, repair, and replacement of all roads(1).

Asphalt Failure

Asphalt is used for the majority of all road construction. Under duress, stripping of the asphalt binder from the aggregate occurs (called sheer), leading to rutting and cracking.(2,3) Rutting and cracking form settlement points for moisture retention which, upon freezing and thawing, become more and more damaged. This damage can lead to further disintegration, worsening rutting and cracking, and creating potholes.(2,3)

Current Fiber Reinforcement

Current fiber reinforcement consists of glass, polyester and polyaramid fibers added to the asphalt matrix. These fibers have only a minimal effect on sheer reduction, due to their low surface area, high elasticity, and low softening point.(4) Polyester and polyaramid fibers are quite expensive, while glass fibers have brittleness and compatibility issues in the asphalt matrix.

The Hemp Fiber Advantage

NForce-Fiber®, the only hemp reinforcement fiber, does not melt and the temperatures that NForce degrade are higher than those of asphalt. In addition, the surface area is far greater for NForce-Fiber® than competition; strongly adhering to asphalt matrix. This increase in surface area leads to an increase in friction between the fiber, matrix, and aggregate, leading to a decrease in rutting and potholes.

Most importantly, successful reinforcement of asphalt leads to extended lifetimes of roadway infrastructure. Extended life-cycle means reducing the greenhouse gases (GHG) produced during heating, placing, repairing, recycling, and replacing asphalt.

Sustainability

Asphalt, like concrete, can be considered a permanent structure. As such, adding hemp fiber to asphalt is akin to permanently sequestering CO2 that has been pulled from the air. With the low environmental impact of production, hemp fiber reinforcement can be considered to have a negative carbon footprint (meaning that it uses up more atmospheric CO2 than it produces!).

References
  1. Natural Resources Canada. Road Rehabilitation Energy Reduction Guide for Canadian Road Builders. (2005)
  2. Q-S., Zhang; Y-L., Chen; X-L., Li; “Rutting in Asphalt Pavement under Heavy Load and High Temperature” Asphalt Material Characterization, Accelerated Testing, and Highway Management – Geotechnical Special Publication 190 (2009) 39-48
  3. Ohio Asphalt. Preventing And Correcting Rutting In Asphalt Pavements. Summer (2004) 12-16
  4. National Cooperative Highway Research Program. Synthesis 475: Fiber Additives in Asphalt Mixtures. (2015)