By Peter Taylor, Contributing Author
Transportation is the backbone of our current lifestyle. The necessity for effective transportation is clear when natural disasters occur, and transportation infrastructure fails. An integral part of this system is the 4-million-mile U.S. public roadway network.
Pavements that facilitate this system are therefore critical to the well-being of the nation. Technologies that allow construction and maintenance of pavements that are sustainable, reliable, long lasting, and cost effective are needed.
The Past
The Eisenhower highway system opened the U.S. to rapid civilian transportation in the 1960s. The trip between Washington D.C. and San Francisco took 62 days in 1919, while it can be done in 41 hours today. Before surfaces were paved, many rural roadways were dusty when dry and muddy when wet, making travel slow and sometimes impassable. Communities had to be reasonably self-sufficient and close together, hence the tendency for towns to be no more than 10 miles apart in the Midwest.
The first known concrete pavement in the U.S. was built in 1893 in Bellefontaine, Ohio, and is still in use at present. The 6-inch concrete was hand placed in two layers and grooved to provide traction for horses.
Following this, concrete pavements were constructed using forms and hand placed concrete up until 1949, when the first slipform paver was used for a roadway. Hand placing could achieve about 1,000 to 2,000 feet per day, while slipforming increased productivity by up to five times. By 1955, several manufacturers had begun marketing slipform pavers.
Smooth, firm, paved surfaces meant that vehicles could move further in a given time. This meant that people could live in ever larger cities, because the resources they need do not have to be locally derived.
Present
Slipform technology has changed steadily over time. Current machines can pave up to 50 feet wide and over 20-inches thick. Other innovations in these machines include:
- Stringless control systems, removing the risk of errors incurred by people bumping or tripping over the stringline used to guide the paving machine. Smoothness can be significantly improved, so resulting in improved fuel economy of vehicles using the pavements.
- Electronic control systems, making it simpler for operators to control the machine.
- Steerable tracks, refining set-up and control of the machine.
- Adjustable pans to allow placement of varying crossfall sections.
- Augurs or plows that help control the head of concrete in front of the machine.
- Vibrator monitors to indicate whether vibrators are operating as intended.
- Real-time smoothness sensors, allowing the crew to monitor smoothness on the fly and make rapid adjustments.
- Dowel bar inserters, removing the need for baskets in front of the paver, facilitating delivery of concrete to the front of the machine.
- Wider machines allow construction of multiple lanes and/or shoulders in one pass, shortening construction times.
In terms of the materials and mixtures used in slipform concrete paving, there have been many changes over the last few years. Early on, proportions were based on volumetric ratios that made it difficult to control properties, because changes in the moisture content can change sand volumes by up to 20%. Adoption of weigh-batching improved batch to batch uniformity.
Like structural mixtures, concretes for paving used to be specified based on slump, strength, and air content. However, these parameters were found to be poor indicators of concrete durability.
There was a period in the 1980s when high strengths were provided, yet the mixtures proved to be non-durable because they were highly permeable. Considerable work has since been conducted to identify what the critical parameters should be, how to measure them, and how specifications can be written to control them.
A standard practice published by the American Association of State Highways and Transportation Officials, R101, has recommended that mixtures delivered to the front of the paver should be assessed by monitoring six critical properties: workability, permeability, cold weather resistance, aggregate stability, strength, and shrinkage. Test methods have been developed and standardized that allow rapid measurement of these properties.
Another suggestion in R101 is that many properties of a potential mixture should be assessed in the laboratory early on, while acceptance testing should be based on confirming that the mixture delivered is similar to the prequalified mixture. Acceptance testing also needs to evaluate whether factors demonstrating variance within the construction process, such as water and air contents, are within limits.
Early concrete paving mixtures contained four ingredients: rock, sand, cement, and water. Mixture proportioning was relatively simple, and rules of thumb based on a strong correlation between cement content and performance were valid. However, current systems add intermediate aggregates, supplementary cementing materials, and chemical admixtures that makes proportioning more complex.
Work is ongoing to develop tools to proportion mixtures that can meet several additional demands, including reduced cement content to limit carbon footprint, workability appropriate for the equipment in use, sufficient durability for the environment and mechanical properties to carry the loads.
Controlling the combined gradation of the aggregate system using tools has been found to have a positive impact on workability, allowing reduction in binder contents while maintaining desired mixture properties. An example is two test sections built at the Minnesota Road Research Facility (also known as MnRoad) in 2022.
One used a conventional mixture and another where gradations of the same materials were adjusted. The binder content was reduced by 12% (70 per cubic yard). Test data from both sections are similar, indicating that just adding cement to a mixture may not solve problems.
There is increasing use of recycled concrete (RCA) as aggregate in the base and in the pavement. This reduces disposal needs and can allow the old pavements to be fully recyclable. Equipment is available that crushes and classifies the recycled concrete on the grade, thus reducing haul costs and fuel consumption. Some quality controls are needed, particularly if the RCA is used in concrete, primarily to control the dust content in the system.
Non-steel dowels are finding application in new pavements. Advantages include zero risk of corrosion and lighter baskets reducing injury risks to the site crew. They have been used successfully at times when local supplies of steel bars have been constrained.
Design practices for pavements have also changed. Tools are available to help designers make more efficient use of the materials while delivering longer performing pavements.
Concrete overlays placed over all types of existing pavements have proven to be effective at making use of equity already in the structure, reducing disposal needs, reducing the time to upgrade a system, and providing long lasting surfaces.
The Federal Highway Administration’s EDC6 program is promoting the use of overlays for pavements. Careful planning and scheduling recently allowed 9 miles of concrete overlay to be placed in Iowa in 25 days from closure to re-opening, while residents only experienced property access issues for a maximum of three days.
Other materials also allow overlays to be effective. Fibers permit the construction of thinner sections, or larger panels, keeping saw cuts outside of wheel paths. Non-woven geotextiles installed between layers in unbonded overlays simplify construction and provide effective drainage below the overlay. Internal curing provided by inclusion of small amounts of lightweight fine aggregate is effective in reducing warping as well as reducing permeability in overlays and bridge decks.
Tools for the contractor include maturity testing to provide guidance on when the new pavement can be opened to construction traffic, real time smoothness sensors, ultrasonic pulse velocity sensors for assessing when sawing is needed, and resistivity-based devices that signal when sawing and curing are needed.
The biggest change is that the typical design life has been extended from 20 to 40 years (or beyond). While this does not impact embodied carbon at the time of placement, it does have a significant effect of reducing lifecycle environmental impact.
Future
There is a large impetus to reduce carbon footprint — at the time of construction and throughout the life of the pavement. It is critical that while cement clinker content in the mixtures can be reduced, long term performance must not be compromised. It is being observed that leaner mixtures are less forgiving of mistakes, meaning that quality systems must be tighter, and field staff must be adequately trained.
While strengths may be satisfactory using low-clinker cements, other properties, such as setting times, bleed rates, shrinkage, and modulus of elasticity, may be changing. These changes require that designs and construction practices are modified to accommodate. New tools are needed to help field staff make decisions about sawing, finishing, and curing; these are tools that go beyond the old rules of thumb and may not be valid for future cement systems.
In the future, effective sensing devices and communications tools will allow paving machines to be more autonomous, but considerable work is still needed to get us there.
This is an exciting time to be involved in the concrete pavement industry. There are many challenges and rewards ahead as we continue to aid the backbone of civilization. RB
Peter Taylor is the director of the National Concrete Pavement Technology Center at the Institute for Transportation at Iowa State University.
