New Road Materials
Efforts to find the perfect road surface and structural components that will withstand Canada's extreme climate have seen some exciting developments in the last decade....
Efforts to find the perfect road surface and structural components that will withstand Canada’s extreme climate have seen some exciting developments in the last decade.
According to John Emery, P.Eng., a geotechnical engineer and paving expert of JEGL in Toronto, the goal is to develop road surfaces that will last 20 years or more, something that could soon be possible now that asphalt concrete technologies such as “Superpave,” “Microsurfacing” and “Long Life Pavements” have arrived on the scene. The frictional characteristics and smoothness of the surface are important factors for safety, he says, and noise characteristics are becoming more important.
There is also the perennial quest to find surfaces that will withstand cracking and rutting, and to find bridge structures that won’t corrode under the onslaught of winter road salts. Environmental concerns have led to the incorporation of various recycled materials within the concrete and the substrata.
Among the latest technologies being tested in Ontario is Cold-in-Place Recycled Expanded Asphalt Mix (CIREAM), a method which shortens the curing period and eases the weather conditions required for laying this type of surface. The province’s transportation ministry is also working with new cathodic protection for bridge decks. Alberta Transportation is testing inventions such as Frostwick, which is a sub-surface insulation layer for reducing frost-heave. Meanwhile, the federally funded agency ISIS at the University of Manitoba continues to lead efforts to design and monitor bridges built with non-corroding, high-strength, fibre-reinforced-polymer (FRP) reinforcement.
Below, in more detail, are three other recent developments.
ECOSMART, OR SCM, CONCRETE
To produce EcoSmart concrete, the Portland cement content is partially replaced with supplementary cementing materials — SCMs (hence its alternative name). The SCMs might be industrial by-products such as fly ash, blast furnace slag or silica fume. The idea is to reduce the greenhouse gas emissions produced in making Portland cement.
EcoSmart concrete has been used in both roads and sidewalks in eastern Canada. In Montreal, for example, sections of road have been built using concrete made with TerC3 cement from St. Lawrence Cement. TerC3 is a ternary blend of Portland cement with 27% (total by mass) of fly ash and silica fume.
In December, Levelton Consultants began working on a project in Edmonton to develop mixtures that will help EcoSmart concrete to better withstand de-icing salts. High volumes of fly ash in concrete mixtures have been known to show scaling problems (cement paste from the surface chips away and exposes the aggregates), but the mechanisms involved are not well understood. It is suspected that in addition to the quality of the mix design, the finishing and curing methods, and timing, play a crucial role in the final quality of the surface.
Funded in part by the Government of Canada Action Plan 2000 on Climate Change, the tests by Levelton use concrete with SCMs produced in Alberta. The concrete is intended for the Henday Drive project being built by Pieter Kiewit. The mixture uses up to 40% fly ash (while 10% has been the effective limit) to partially replace the Portland cement. In parallel tests, Hard-Cem, an integral hardener from Teck Cominco/Cementec that is produced from slag as a by-product of lead-zinc smelting, will be incorporated to improve the concrete’s abrasion resistance and durability. The flyash/Hard-Cem combination has successfully been used in the Metro Skate Park project in Vancouver designed by space2place, and Hard-Cem was also used in highway 1-90 in Spokane, Washington, U.S.
Cost-wise, EcoSmart concrete is typically comparable to conventional concrete. The costs of each constituent material may vary the price of the material. For example, fly ash costs less than Portland cement, but more admixtures may be required with high volumes of fly ash.
Ductal is a proprietary fibre-reinforced cement composite developed by Lafarge, North America. In Canada so far it has been used for a pedestrian bridge in Sherbrooke, Quebec and most recently for the dramatic thin-shell canopy structure of Calgary’s LRT Shawnessy Station, designed by CPV Group Architects and Engineers.
The material has such strength and ductility it has been compared to metal. Its typical compressive strengths are 120-220 MPa (compared to 15-50 MPa for normal concrete). Its flexural strength ranges between 20-50 MPa (compared to 3-7 MPa for normal concrete). Ductal does not require passive reinforcing steel such as rebar.
Formulations are supplied to the precast industry in a three-component premix of powders, superplasticizer and fibres. The powders include cement, silica fume, ground quartz and sand. The fibres can be steel or organic.
Thanks to its dense micro-structure, Ductal has an extremely low porosity (chloride ion diffusion of 0.02 x 10E-12 sq. m./second), a quality that can be enhanced further by the heating process during precasting. Curing reduces the creep coefficient to 0.2 and virtually eliminates further shrinkage.
The U.S. Federal Highway Administration (FHWA) is doing research on Ductal as a solution to replace deteriorating bridges. In 2001, they engaged the Massachusetts Institute of Technology (MIT) to develop an optimized bridge profile based on the technology.
MIT designed a pi-girder, or bulb double-tee, with a 75-mm unreinforced deck designed to H30 highway loading. Manufactured by Prestress Services, two of these girders measuring 22 metres were installed in abutments at FHWA’s research campus in Virginia. Two more girders are now being tested to confirm the ultimate load capacity. Parallel research is being done at other Canadian and U.S. universities to develop designs for punching shear and prestress bond development lengths.
ASPHALT RUBBER CONCRETE
EBA Engineering Consultants of Edmonton have been acting as project manager and technical advisor for a pilot project in Alberta that is testing recycled rubber tire material in road paving on a major scale. Between 2002 and 2003, around 75 kilometres of roads in the province were paved with asphalt rubber concrete, or “ARC.”
Asphalt rubber concrete has been used in the U.S. since the 1960s, but has only recently been tested in Canada, on stretches of road in B.C. and Ontario, and in Alberta. EBA’s task for the Alberta Recycling Management Authority is to adjust the technology for the province’s extreme climate.
The material constituents of ARC are asphalt oil and aggregate, with ground tire rubber crumb at a #10 minus gradation, i.e. the maximum particle size is 2 mm. Lyle Treleaven, P.Eng., the project manager with EBA, says the mixture resembles wet black popcorn when unloaded from trucks. Once placed and compacted it resembles a Superpave mix i.e. it has an open texture and a “bony” structure.
Producing ARC requires higher temperatures than conventional asphalt mixes as well as special blending equipment. Another difference is that a binder design has to be done before the actual mix design to select the appropriate amount of crumb rubber and asphalt oil. More binder is usually needed compared to conventional mixes (generally minimum 7.5% by weight of the total mix, compared to 4-6% in conventional asphalt).
Because the material is so sticky, hauling trucks have to be treated and only steel-wheeled packers can be used to compact the surface. Also it can only be placed when the ambient temperature is more than 12 degrees C.
Despite some setbacks, overall the pilot project in Alberta has shown many advantages to using ARC. Results to date show less rutting and cracking compared to conventional asphalt. Road noise is considerably reduced. In Alberta, tests have found an average reduction of 4-5 decibels — equivalent to the listener moving at least 150 metres away from the road.
Costs for ARC are up to 50% higher than convention
al mixes, but the surface can be laid in thinner layers, potentially up to 66% of standard thickness. Because it can be placed in a thinner layer, it reduces the demand for a non-renewable resource such as aggregate.
The big environmental benefit of ARC is that it makes use of old tires. To date the Alberta project has used almost 190,000 passenger tires, at about 6 kg. of rubber per tire. Those tires would otherwise be piled in dumps, which are a blight on the landscape, can be a fire hazard, and attract insects like mosquitoes.