Concrete Road Performance
In Canada, however, governments typically award highway pavement construction contracts based only upon initial costs. Asphalt pavements are often selected because they are perceived to be less expensive than concrete. But planners are now beginning to recognize that tenders for road infrastructure projects should include a life cycle cost analysis (LCCA) component, based on the estimated costs of a project over its entire service life. When this concept is applied to maintenance, rehabilitation, reconstruction and salvage value of pavements, life cycle costs are evaluated, as well as initial costs, revealing the full expense of the selected material. Over the life of a highway, concrete performs better on all counts.
Some Canadian provincial and municipal governments have already begun to choose rigid concrete pavements for selected projects. Many cities are also using concrete pavements at hightraffic and high-wear intersections and bus stops, where asphalt has a tendency to rut and shove. As Canada’s transportation infrastructure comes under growing stress from escalating traffic, informed decision-makers who base their pavement selection on life cycle cost analysis, safety, environmental and social benefits will increasingly secure the future in concrete.
Concrete: An Excellent Choice
Concrete is hard, rigid and durable—it is the traditional material of choice for constructing buildings, bridges and other types of infrastructure. For the same reasons, concrete is also an excellent choice for roads.
There are essentially two types of pavements – rigid and flexible. A rigid concrete pavement distributes heavy loads over a relatively wide area, with minimal pressure on the subgrade – the natural soil under the roadway. On the other hand, flexible asphalt material concentrates weight and transmits it deeper into the roadbed. As a result, asphalt pavements require a thicker gravel base and subbase (material between the subgrade and the gravel base) for equivalent highway designs,2 as illustrated in Figure 1.
Concrete pavement acts as a bridge over the subgrade.
There are many different technical and economic reasons to choose rigid concrete pavement, rather than flexible asphalt.
Concrete pavement lasts longer
Concrete pavement lasts longer and requires less maintenance over its lifetime than asphalt. Its rigid nature provides a stable surface that will not rut, washboard or shove, and also minimizes the potential for potholes. This translates into safe highways that require less maintenance, with less disruption for the traveling public and commercial truckers.
The cost-effective advantage
Life Cycle Cost Analysis (LCCA)3 is a forward-looking decision framework that helps governments assess the lifetime costs of a roadway, rather than merely considering the initial construction costs. When LCCA is applied, concrete pavement is, in many cases, less expensive than an asphalt surface of equivalent design.
LCCA is emerging as a policy component used in decision-making in some jurisdictions in Canada. The Cement Association of Canada urges governments at all levels to adopt LCCA in the tendering process, as a means of ensuring accountability in public infrastructure investments and assessing the true cost of a highway over time.
A true alternative leading to better prices
A long-term commitment from governments to the use of rigid concrete pavements will lead to lower costs through increased competition between concrete and asphalt road builders.
Data from the American Concrete Pavement Association confirms that American states truly committed to building concrete highways create competition between the concrete and asphalt paving industries resulting in lower unit costs for both concrete and asphalt highways.4 This results in more roads paved for the same cost. Figure 2 illustrates the benefits of competition between pavement types on construction costs.
5-year average data (2000-2004) for 14 American states
Concrete is produced across Canada, using both locally available aggregates and regionally manufactured materials including cement that are not dependent on the supply of oil. While the pricing of asphalt highways fluctuates with the world price of oil, the pricing of concrete highways remains relatively stable.
Stands up to seasonal stresses
Concrete highways have a strong track record in some of the coldest parts of Canada and the United States. Concrete mixes are tailored to specific applications and conditions – including cold weather and de-icing agents – to ensure strength and durability throughout the life of the highway. Thus concrete is not damaged by anti-icing agents normally used in winter. Furthermore, due to the way concrete pavements distribute vehicle loads, it is virtually unaffected by the seasonal weakening of the underlying granular bases and subgrade.
Concrete’s durability is most evident during Canada’s spring thaw season. Unlike some asphalt highways, springtime weight restrictions are not necessary on concrete highways. A study by the American Association of State Highway Officials (AASHO) showed that 61 percent of asphalt roads tested failed during spring conditions.5
Widespread use of concrete roadways would produce a significant positive economic impact on the movement of goods in Canada.
Reduced lighting cost
An industry report notes that concrete pavement reflects light in a diffuse manner, compared to the slightly spectral (somewhat mirror-like) reflectiveness of asphalt pavement. As a result, a concrete highway requires fewer lights per unit length of pavement to achieve the same level of illumination. The report concludes that the light colour of concrete results in a cost-saving of up to 31 percent for night-lighting, including construction, energy consumption and maintenance, when compared with the lighting requirements of asphalt pavement.6
Easy, fast-track maintenance
The cement and concrete industries have developed methods for rapid maintenance and restoration of highways. Roadway owners can use both fast-track reconstruction techniques and pre-cast concrete pavement panels to minimize user delays.
Uses less energy
The Athena Sustainable Materials Institute7 compared equivalent concrete and asphalt pavement designs for a typical Canadian high-volume highway from a Life Cycle Analysis perspective. The report revealed that over a 50-year period, the embodied primary energy required to construct, maintain and rehabilitate a highway is three times higher for the asphalt design than for its typical concrete equivalent (pavement structure B). However, if one uses concrete shoulders and concrete restoration with no overlay as part of the M/R schedule, as mentioned in pavement structure A, the primary embodied energy is 5.6 times higher for the asphalt option. Embodied primary energy is all of the energy required in bringing a material to its final product, including transportation.
In this example, the feedstock energy component, i.e. bitumen, is the largest contributor to embodied primary energy used. Feedstock energy is the energy of raw material inputs that are not being used as energy sources. Even when feedstock energy is excluded from the analysis, the asphalt pavement design still uses 2 ?3 more net energy than its equivalent concrete pavement design. Figure 3 below illustrates these findings.
Note: Concrete pavement option has feedstock component due to asphalt shoulders and overlay as part of the maintenance and rehabilitation schedule.
Heavy truck fuel savings and reductions in emissions
Extensive studies by the National Research Council of Canada8 confirm previous findings9 showing that fully loaded tractor-trailers consume less fuel traveling on concrete pavements than on asphalt pavements over a wide temperature range.10,11 When compared to asphalt, under actual road test conditions, reductions in fuel consumption on concrete pavement ranged from 0.8 to 6.9 percent. These results are statistically significant. CO2 and other harmful emissions are also reduced.
Table 1 illustrates the potential savings in litres of fuel, dollars of fuel and reductions in CO2 , NOx and SO2 for a typical major urban highway 100 km long carrying 20,000 vehicles a day (of which 3,000 are trucks) if it was built in concrete.
Reusable and recyclable
Concrete pavement is a versatile construction material that can be reused through restoration techniques that minimize the amount of new aggregate required to construct highways, and eliminate the need for disposal of the old material. Concrete pavement is also 100 percent recyclable and provides cost-effective reconstruction options, such as use as a road base or aggregate for new concrete pavements.
Reduces road base material required
Compared to flexible asphalt pavement, rigid concrete pavement requires thinner granular bases because it distributes the weight of vehicles more evenly over a larger area. Typically, in most cases, up to 50 percent less granular material is required for a concrete highway road base, reducing both costs and the use of scarce resources. Additionally, less hauling of granular material results in significant fuel savings and associated emissions reductions.
Makes use of industrial by-products
Some of the cement in concrete is commonly replaced by industrial by-products that would otherwise use up space in a landfill. The three most commonly used supplementary cementitious materials are fly ash (a by-product of burning coal), blast furnace slag (from steel manufacturing) and silica fume (from making silicon or ferrosilicon alloy). Using these materials in the appropriate quantities can improve the durability, permeability and strength of concrete pavement. Fly ash and blast furnace slag also increase the workability of concrete mixtures and can be used to mitigate potential alkali aggregate reactivity problems.
Reduces urban heat island effect
Large cities can be several degrees warmer than outlying areas in the summer, due to the heat absorbed by dark surfaces such as asphalt pavements. This temperature increase, known as the urban heat island effect, can be reduced by using concrete pavement because of its light colour and reflective properties.
A smooth and comfortable ride
Pavement smoothness affects both quietness and comfort on highways. Improvements in concrete paving equipment and construction techniques have helped contractors to construct smoother and quieter concrete pavements in recent years.
A Nova Scotia Department of Transportation and Public Works five-year study,12 on adjoining sections of asphalt and concrete pavement built in 1994 on their Highway 104, found that the concrete pavement section outperformed the adjoining asphalt pavement in both smoothness and riding comfort.
Data from the comparative study shows that although new asphalt pavement had a higher initial riding comfort index (RCI), it deteriorated over time to a lower level than adjoining concrete pavement. Figure 4 illustrates how the RCI values changed over the five-year evaluation period. Note that at year five, the RCI reading of 7.4 for concrete pavement was superior to the asphalt pavement reading of 6.9.
Note: higher values correspond to higher comfort levels.
The study also demonstrated that the concrete pavement section maintained a smoother ride than the asphalt pavement over the same evaluation period. Figure 5 provides the profile ride index (PRI) which is a measure of pavement smoothness. The values show that the concrete pavement section retained most of its original smoothness, while the asphalt section showed increased roughness.
Note: Lower PRI corresponds to a smoother ride.
In a more recent study that evaluates pavement roughness in the Unites States, similar results were obtained, supporting the Nova Scotia findings described above. 13 Long-term pavement performance data for Kansas roads show that roughness increased more quickly on asphalt roads than on concrete roads (69.9 percent increase on asphalt sections over 8 years vs. 3.7 percent increase on concrete sections over 9 years). Significantly, this accelerated deterioration occurred on the asphalt sections although the concrete roads carried 3.5 times more truck traffic.
A quiet ride
The Cement Association of Canada and the Portland Cement Association, in partnership with the American Concrete Pavement Association, are conducting ongoing research to develop quieter concrete pavements to better serve the transportation community. To examine the acoustic performance of different pavement types and textured surface finishes properly, it is important to consider performance throughout the pavement’s service life.
A report by the U.S. Department of Transportation in 1996 concluded, “Properly constructed PCCP (Portland cement concrete pavement) with a transversely tined surface, matches the performance of dense-graded asphalt considering both safety and noise factors.”14 Tines are small grooves cut into the pavement either transversely (at right angles to the highway direction) or longitudinally (parallel to the highway direction). In the previously noted Nova Scotia Department of Transportation and Public Works study, it was also found that concrete road noise averaged only two decibels higher than the asphalt section after five years of operation.15
Research has shown that longitudinally tined concrete is even quieter than the conventional transversely tined pavement. See Figure 6 below which identifies the ranges of sound intensity levels for various surfaces types and texturing.
Anthony Henday Drive in Edmonton, constructed in 2005–2006, is the first longitudinally tined concrete pavement road in Canada. Many people have commented on how quiet this pavement is.
Less potential for hydroplaning
The non-rutting quality of concrete pavement reduces the potential for hydroplaning. Concrete pavement can be textured to create a roughened surface for good skid resistance, and grooved to help carry rainwater away from the road surface for improved traction as compared to asphalt pavement, thus providing safer roadways.
Better night-time visibility
The light colour of concrete results in better night-time visibility for motorists. It reflects light from vehicles and street lamps better than asphalt pavement, thus illuminating potential hazards.
- US Department of Transportation Federal Highways Administration web site, Office of Highway Policy Information, Highway Statistics 2005. http://www.fhwa.dot.gov/policy/ohim/hs05/xls/hm12.xls
- Tighe, S., Smith, T., Fung, R., Concrete Pavements in Canada: A Review of their Usage and Performance, Paper for Transportation Association of Canada Annual Conference, September 2001.
- American Concrete Pavement Association, Life Cycle Cost Analysis: A Guide for Comparing Alternate Pavement Analysis, EB 220P, 2002.
- Southeast Chapter American Concrete Pavement Association, “Who says...”Concrete Pavement Costs Too Much?” Count on Concrete Pavement, 2005.
- American Concrete Pavement Association, Whitetopping - State of the Practice, Engineering Bulletin 210P
- Gajda, J.W., Van Geem, M.G., A Comparison of Six Environmental Impacts of Portland Cement Concrete and Asphalt Cement Concrete Pavement, PCA R&D Serial No. 2068, Portland Cement Association, 1997.
- The Athena Sustainable Materials Institute, A Life Cycle Perspective on Concrete and Asphalt Roadways: Embodied Primary Energy and Global Warming Potential, Ottawa, September 2006.
- Taylor G.W., Patten, J.D., Effects of Pavement Structure on Vehicle Fuel Consumption – Phase III, prepared for Natural Resources Canada Action Plan 2000 on Climate Change and Cement Association of Canada, January 2006.
- Taylor G.W., Dr. Farrell, P. and Woodside A., Additional Analysis of the Effect of Pavement Structure on Truck Fuel Consumption, prepared for Government of Canada Action Plan 2000 on Climate Change, Concrete Roads Advisory Committee, August 2002.
- See above, note 8.
- See above, note 9.
- Nova Scotia Transportation and Public Works, Asphalt Concrete Pavement and Portland Cement Concrete Pavement, Highway 104, Cumberland County, Year 5 of 5 Year Study, October 1999.
- The Transtec Group, Pavement Roughness Progression Case Study, July 2006.
- Federal Highway Administration, Tire-Pavement Noise and Safety Performance, Publication No. FHWA-SA-96-068, 1996.
- See above, note 12.