by Elaina Adams | January 1, 2010 3:35 pm
By Sherry Sutherland, M.A.Sc, P.Eng., LEED AP
Concrete pavements are known for their strength, durability, and longevity. In the past, they have also been associated with a high initial price. However, in a number of lifecycle cost studies, concrete pavements prevail due to the significantly lower maintenance and rehabilitation needs. (See Becca Lane and Tom Kazmierowski’s “Guidelines for the Use of Life Cycle Cost Analysis on MTO Freeway Projects,” Ontario Ministry of Transportation (March 2005). Also see ACPA’s 2002 “Life Cycle Cost Analysis: A Guide for Comparing Alternate Pavement Designs.”)
The initial price still remains at the forefront of owners’ minds, but many do not realize concrete pavements are now competitive on a first-cost basis. The new paving realities are that comparative initial bid cost and lifecycle cost assessments increasingly favour concrete over asphalt in the foreseeable future, essentially allowing municipal owners to eat their cake and have it too. (See “Paving: The New Realities” in Portland Cement Association’s Flash Report [October 14, 2008].)
Public officials across the country are faced with the increasing challenge of managing road assets on a stagnant—or in some cases, decreasing—budget. It is essential that an agency critically evaluates its paving material options and determines the best course of action when spending taxpayer dollars.
Concrete pavement basics
Due to its versatility and durability, concrete is the most widely used construction material in the world. In fact, it is second only to water as the most consumed substance on earth. (Refer to P.C. Aitken’s “Cements of yesterday and today; concretes of tomorrow,” Cement and Concrete Research 1349-1359 .)
Concrete is an inorganic and inert material produced by mixing cement, supplementary cementitious materials (SCMs), water, fine aggregate (i.e. sand), and coarse aggregate (i.e. gravel or crushed stone) with or without various admixtures, reinforcement, fibres, or pigments.
Types of concrete pavements
There are three basic design types and each can provide long-lasting concrete pavements that meet or exceed specific project requirements. Jointed plain, jointed reinforced, and continuously reinforced concrete pavements are all suitable for new construction, reconstruction, and overlays of existing roads (Figure 1).
Concrete pavement design involves the development and selection of slab thickness, joint spacing, reinforcement and load transfer requirements, and other pavement features. A pavement designer’s objective is to be economical, while meeting a particular project’s specific needs and conditions (Figure 2).
How pavements carry loads
Concrete’s rigidity spreads wheel loads over a large area through the slab and keeps pressures on the sub-grade low. In contrast, asphalt pavements transmit loads deeper, necessitating a thicker base and sub-base. As they require less sub-base material, concrete pavements offer another cost benefit (Figure 3).
Concrete paving mixes are designed to produce the desired flexural strength and give durability under conditions the pavement will face. Typically, a 28-day modulus of rupture between 3.8 and 4.8 MPa (551 to 696 psi) resists cracking from flexural fatigue.
Determination of load transfer at joints
Load transfer is the slab’s ability to share its load with the neighbouring slab, and aggregate interlock refers to the interlocking action between aggregate particles at the face of a joint. Aggregate interlock relies on the shear interaction between aggregate particles at the irregular crack faces that form below the saw cut or formed groove at control joints.
For jointed concrete pavement (plain or dowelled) to perform adequately, traffic loadings must be transferred effectively from one side to another. Adequate load transfer results in lower deflections, which reduces faulting, spalling, and corner breaks, while increasing pavement life. Where aggregate interlock may be insufficient by itself for good pavement performance—like in the case of heavy truck traffic—dowels should be used. Repeated joint deflections caused by truckloads may gradually wear away the aggregate interlock, until it is no longer effective; using dowels ensures long-term load transfer effectiveness. (Figure 4).
Technology in today’s road design and placement equipment allows owners to specify the ride characteristics of their concrete road. Concrete pavement surfaces are generally textured to provide adequate friction and skid resistance. New concrete surfaces may be textured in many different ways, including various forms of dragged and tined surfaces, and several newer techniques and materials. Each technique can be designed to provide safe, durable, high-friction concrete surfaces. (See ACPA’s “Pavement Surface Characteristics: A Synthesis and Guide,” by Mark B. Snyder, PhD, PE ). Pavement surface texture influences many different tire-pavement interactions, including wet-weather friction, tire-pavement noise, splash and spray, rolling resistance, and tire wear. (See J.J. Henry’s “Evaluation of Pavement Friction Characteristics,” NCHRP Synthesis 291, Transportation Research Board ).
Drag and shallow surface textures are attained by dragging artificial turf or moistened, coarse burlap across the surface of plastic concrete. Tining textures are created by moving a device like a metal rake across plastic concrete. Moving the device parallel to the centreline produces longitudinal tining, whereas movement perpendicular to the centreline is used for transverse.
The combined results from a number of studies completed on pavement noise and/or friction suggest longitudinally tined concrete pavements offer the best mix of consistently low noise, good surface friction, durability, and low maintenance. Asphalt-based pavements are often slightly quieter (initially), but do not consistently provide high friction values. Further, they are subject to rutting, and typically require more maintenance.
Opening pavement to traffic
In accord with Ontario Provincial Standard Specifications (OPSS) 350, Concrete Pavement and Concrete Base, traffic is not permitted on concrete pavement until the material has attained 20 MPa (3000 psi). (Not all provincial regulations are the same; the design team must work with the authority having jurisdiction [AHJ] to remain knowledgeable of the requirements.)
Typically, cylinders are cast and then broken at three days to confirm opening strength. If the project requires the pavement to open sooner than standard concrete allows, ‘fast-track’ concrete mixes can be employed to open traffic in as little as five hours.
Pavements and the economy
For decades, it has always been accepted the initial construction costs of asphalt pavements are lower than that of concrete pavements. However, this perception is changing. For one thing, developments in refining practices will lead to a significant reduction in the future liquid asphalt supply.
New coker refinery processes take the ‘left-over product’ and further refine it into higher-priced items such as diesel, fuel oil, gasoline, and motor oil. Consequently, fewer products are available for bitumen, and its price is increasing globally. The projected rise in costs associated with oil, asphalt, and bitumen, combined with expected shortages, indicates concrete pavements will continue to be more competitive in the future.
Additionally, asphalt paving costs have risen 148 per cent during the past four-and-a-half years, and fluctuated between an increase of 45 and 113 per cent during the last 20 months (Figure 5). Concrete has not been as severely impacted by the current state of the economy, and continues to provide a more stable price point.
Pavements and the environment
On March 10, 2008, the Government of Canada announced details of their “Turning the Corner Plan,” which is one of the toughest regulatory regimes in the world to cut greenhouse gas (GHG) emissions. Concrete pavements reduce emissions and absolute primary energy, which could significantly assist the government in reaching its target to lower emissions by an absolute 20 per cent from 2006 levels by 2020.
Local municipalities across the country have their own strategies in place to strengthen and protect the environment. Responsible material procurement is a direct way for municipalities to have a great impact and gain taxpayer support.
Concrete’s lower embodied primary energy—131 to 425 per cent, depending on road type—is recognized. A study by the Athena Institute compared embodied primary energy and global warming potential on both asphalt and concrete pavements. The flexible asphalt concrete alternatives were clearly shown to embody more primary energy from a lifecycle assessment perspective than their rigid portland cement concrete counterparts. (Refer to Athena Institute’s “A Life Cycle Perspective on Concrete and Asphalt Roadways: Embodied Primary Energy and Global Warming Potential” [September 2006].)
Recycling and natural resources
As an inert material, concrete is ideal for using industrial byproducts that would otherwise be deposited in landfills—such as slag, which is developed in a molten condition simultaneously with iron in a blast furnace. These industrial byproducts are known as supplementary cementitious materials, and have been used in concrete since the 1970s. They not only divert products from landfill sites, but also enhance concrete’s performance properties.
SCMs are added to concrete as part of the total cementing system. They may be added to, or partially replace, portland or blended cement in concrete—depending on the cementing materials’ properties and the desired effect on concrete. Adding SCMs also reduces concrete’s carbon footprint.
Not all SCMs suitable for concrete are easily accessible in all parts of the country. For example, Canadian sources of fly ash are more readily available in provinces that have coal-burning, thermal-power-generating plants, whereas ground granulated slags are primarily sold and employed in Ontario, which has a significant steel industry.
Concrete itself is 100 per cent recyclable. Crushed concrete can be used as granular fill, base under new pavement, or as an aggregate to strengthen new concrete pavement—all of which reduces the use of non-renewable resources.
Another consideration is the volume of granular base/sub-base materials needed to provide structural support for the pavement. Due to concrete’s rigidity and stiffness, the slab itself supplies a major portion of its structural capacity and distributes heavy vehicle loads over a relatively wide area of sub-grade.
An asphalt pavement is not as rigid and does not spread loads as widely. Therefore, these pavements usually require more layers of base granular material at a greater thickness, when compared to an equivalent concrete pavement design. (Refer to ACPA’s EB204P, “Subgrade and Subbases for Concrete Pavements” .) Based on an analysis performed on equivalent pavement designs for asphalt and concrete pavements for an arterial road on a low-strength sub-grade, about 50 per cent more granular material can be needed for an asphalt pavement structure than for a concrete structure. (See Applied Research Associates’ Pavement Engineering Technical Services Equivalent Pavement Designs: Flexible and Rigid Alternatives .) Environmental effects of this increased demand on granular material may amplify if suitable aggregate sources are not locally available.
Reduced energy demand
Concrete surfaces readily reflect light. This characteristic of concrete—generally referred to as albedo—is advantageous for several reasons. It can significantly improve both pedestrian and vehicular safety by enhancing nighttime visibility on and along concrete roadways. Consequently, concrete’s albedo reduces the amount of energy needed for artificial roadway illumination during the night. It also lowers the energy necessary to cool urban environments, and diminishes potential for smog formation.
Paving urban roadways with concrete is an effective strategy to help mitigate urban heat island (UHI) effects. Due to their higher albedo, concrete pavements will reflect significantly more sunlight and are cooler than asphalt pavements. In the United States, Lawrence Berkeley National Laboratory (LBNL) research suggests on exposure to sunlight, most lighter-coloured concrete pavements will have surface temperatures approximately 12 C (22 F) lower than darker-coloured asphalt pavements. (Refer to M. Pomerantz et al’s “The Effect of Pavements’ Temperatures on Air Temperatures in Large Cities,” Lawrence Berkeley National Laboratory Report LBL-43442 .)
A report comparing the environmental impacts of concrete pavements to asphalt pavements indicates the latter require more light per unit length to achieve the same illumination as the former. (See J.W. Gadja and Martha Van Geem’s “A Comparison of Six Environmental Impacts of Portland Cement Concrete and Asphalt Cement Concrete Pavement,” Portland Cement Association, PCA R&D Serial No. 2068 .) The results suggest cost savings of as much as 31 per cent in initial energy and maintenance costs for lighting concrete pavements versus lighting asphalt pavements.
Reduced vehicle fuel consumption and emissions
Fuel consumption is partly a function of the degree to which pavement deflects in response to the load applied as the wheels of vehicles traverse the surface. Any deflection absorbs energy that would otherwise be available to propel the vehicle forward (Figure 6).
Several studies to date suggest resistance (i.e. amount of deflection) encountered by heavy-vehicle wheels on asphalt pavements is measurably greater than the rolling resistance on concrete pavements. Thus, more energy and fuel are required to move heavy vehicles on asphalt pavements. (Refer to Taylor Consulting’s “Additional Analysis of the Effect of Pavement Structure on Truck Fuel Consumption,” Action Plan 2000 on Climate Change, Concrete Roads Advisory Committee [Government of Canada, 2002].)
In 2007, the Cement Association of Canada (CAC) completed a case study to determine potential fuel savings and emission reductions achievable if a 183-km (114-mi) stretch of Ontario’s Highway 401 between Toronto and London was paved in concrete. The research used findings from a 2002/2006 National Research Council Canada (NRC) study and 2005 Ontario Ministry of Transportation traffic data. According to the results, more than 70 million L (18 million gal) per year of diesel would be saved, which would avoid 193,132 tonnes of carbon dioxide entering the environment annually. (See Cement Association of Canada’s “Concrete Thinking in Transportation Solutions” [March 2007].)
A number of new studies on the effect of lighter vehicles are also being completed. One study, now available from Sweden, demonstrates concrete pavements save passenger car and light vehicle fuel. The measurements show 1.1 per cent less fuel consumption on concrete pavement compared to asphalt. (See Per Jonsson and Bengt-Ake’s Measurement of Fuel Consumption on Asphalt and Concrete Pavements North of Uppsala [Swedish National Road and Transport Research Institute, 2008].)
Therefore, differences in fuel consumption as a function of pavement type should be an important consideration for government agencies analyzing potential structures for new or reconstructed pavements. Significant reductions in greenhouse gases and fuel cost savings can be realized by choosing carefully.
Paving the Way: Resources and Tools
Produced by the American Concrete Pavement Association (ACPA), ‘StreetPave’ is the latest in concrete thickness design software for streets and local road pavements. The software utilizes proven engineering analysis to produce optimized concrete pavement thicknesses for municipal streets and roads (i.e. collector, minor, or major arterial). A ‘life cycle cost analysis’ module enables design/construction professionals to perform detailed analyses, helping them make informed decisions on pavement design projects.
‘CANPav’ is another concrete pavement tool designed to offer the specifier and owner an easy initial cost comparison through material cost and design inputs. Developed by the Ready Mixed Concrete Association of Ontario (RMCAO) in collaboration with Cement Association of Canada (CAC), this program is online starting this month, and available to anyone interested in comparing the initial construction costs of different pavement alternatives. Visit www.canpav.com.
Concrete pavements last longer than any other type of pavement. They also reduce emissions and greenhouse gases. In terms of initial and long-term costs, concrete pavements are again the best choice. Essentially, in terms of longevity, sustainability, and economics, concrete pavements outperform their counterparts.
Sherry Sutherland, M.A.Sc, P.Eng., LEED AP, is a technical engineer with the Ready Mixed Concrete Association of Ontario (RMCAO) and represents the province’s concrete industry in advocacy, technical, marketing, environment, and education. Prior to joining RMCAO, she was a technical services representative for St. Marys Cement where she gained extensive experience in the cement and concrete industries. Sutherland is currently past-president of the American Concrete Institute’s (ACI’s) Ontario Chapter. She can be contacted via e-mail at email@example.com.
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