A Smoother Ride



Concrete and Asphalt Standards Support Innovation and Infrastructure
Jack Maxwell

If you drive a car, you’re probably all too familiar with the problem of poorly maintained transportation infrastructure. You feel it with every rut and pothole. You are inconvenienced by it every time a local bridge suddenly closes for urgent repairs. And you’re frustrated by the seemingly slow pace of improvement.

With millions of kilometers of paved roads around the world, it should be no surprise that things are a little bumpy here and there. A look at the cumulative estimated length of paved roads in major countries around the world hints at the massive scope of this issue:

  • Brazil: 212,798 km
  • China: 4,046,300 km
  • France: 1,028,446 km
  • South Africa: 158,952 km
  • United States: 6,500,000 km

And as more roads are laid down each day to keep up with population growth, the ever-increasing backlog of road and bridge construction and maintenance has become a pressing worldwide challenge. Whether you’re talking about a two-lane farm road in Provence, France, or a 16-lane interstate highway in California, repairing and replacing endless ribbons of aging roads will require massive and sustained commitments by governments.

And then there are the bridges. In the United States alone, the American Society of Civil Engineers estimates that nearly 40 percent of the country’s 614,000-plus bridges are at least 50 years old. More than 55,000, or just over 9 percent, were deemed to be structurally deficient in 2016.

Concrete and asphalt are key elements of road and bridge construction and repair projects. ASTM International committees on cement (C01), concrete and concrete aggregates (C09), and road and paving materials (D04) have played — and continue to play — important roles in the evolution of infrastructure.
Innovation in concrete and asphalt technology, such as advanced material formulations, are being supported by regular updates to globally accepted standards at places like ASTM International. Together, new standards and new infrastructure approaches are creating a path forward for transportation and infrastructure.

A Few Hard Facts about Concrete

Concrete is generally acknowledged to be the world’s most widely used man-made material. It is deceptively simple, with just a few basic ingredients, water and portland cement (which are mixed to create a paste), and coarse and fine aggregates (which are reinforcing particulates). A chemical reaction between the water and cement causes the paste to harden into concrete.

Additional cementitious materials, typically byproducts from other manufacturing processes, are commonly used in concrete. These include fly ash from burning coal, slag cement from the manufacture of iron, and silica fume from the manufacture of silicon or ferro-silicon metal. The amorphous (non-crystalline) silica in these materials provides concrete with long-term strength and durability.

Typical proportions by volume are approximately 10 to 15 percent cementitious materials, 15 to 20 percent water, and 60 to 75 percent aggregates. Tiny air bubbles (about 5 to 8 percent) are incorporated to improve resistance to cycles of freezing and thawing.

Concrete performance is essentially determined at the microscopic level. Attributes like strength and durability are traceable to the properties of the calcium-silicate-hydrate paste — formed by the reaction of water and portland cement — that binds components of concrete together. Materials for concrete are continuously being tweaked to refine its microscopic structure and to improve its long-term performance.
According to Colin Lobo, Ph.D., P.E., executive vice president of engineering at the National Ready Mixed Concrete Association, about 340 million cubic meters of concrete were produced in the United States in 2016, with about 15 percent of this total used for road and bridge infrastructure projects. Paul Tennis, Ph.D., director of standards and technology for the Portland Cement Association, notes that about 30 percent of portland cement produced annually in the United States becomes a component of the concrete used in these projects.

An increased focus on the environmental impacts of materials used for road and bridge construction, as well as the difficulty of financing the massive amount of work that needs to be done, has created a strong incentive to develop alternatives to standard cement and concrete that cost less, perform better, and are more sustainable. Fortunately, several promising options are emerging.

Colloidal Silica

One such option is colloidal silica, a supplemental material (like fly ash and silica fume) that reacts to form compounds that improve the properties of concrete. Colloidal silica is a recent evolution that allows for adjusting properties at the microscopic level.

Colloidal silica is provided as a liquid suspension containing extremely tiny silica particles so small that they’re measured in nanometers, one-billionths of a meter. A 2012 study published in the international journal Construction and Building Materials found that “significant improvement was observed in mixtures incorporating nano-silica in terms of reactivity, strength development, refinement of pore structure, and densification of interfacial transition zone.”

The material has been used as an admixture in concrete densifiers that fill pores, increase surface density, and polish concrete floors and countertops. Jon Belkowitz, director of research and development for Intelligent Concrete (a consulting firm), points out that it is also a common additive for “shotcrete,” a type of concrete that is sprayed from a hose onto a construction surface at high velocity.

However, Belkowitz believes a lack of standards is preventing more widespread use of this additive. “Despite the rise in the popularity of using colloidal silica in concrete, the main obstacle that prohibits the entry of colloidal silica into day-to-day concrete is the lack of an ASTM International standard specification,” Belkowitz says. “Engineers, superintendents, architects, and concrete producers are uneasy using a concrete product unless there is an ASTM International specification governing its entry into the concrete arena.”
The good news is that such a standard — a proposed specification for colloidal silica (WK53768) — is being developed that will support its use.

Embedded Energy and Recycling

Making cement is an energy-intensive undertaking. Calcium carbonate, mostly from mined limestone, is the primary raw material. Components including limestone, clay, iron, and aluminum are heated to over 1500°C to create “clinker,” a rock-like substance that is ground to a fine powder an used as cement in a concrete mixture.

The calcination process breaks down the calcium carbonate in the cement mixture, generating carbon dioxide. Fuel used in the manufacturing process also contributes to CO2 emissions.  That said, Colin Lobo of NRMCA points out that portland cement only represents about 10 percent of the weight of concrete used in construction.

For many construction materials, recycling products that have already been through the manufacturing process requires less energy than making them from scratch. Richard Szecsy, Ph.D., P.E., is president and CEO of the Texas Aggregates and Concrete Association, and has served the ASTM International concrete committee in a number of capacities over 15 years. He notes that one particular development will be crucial in the future: the introduction of “returned” concrete and a new standard (C1798/C1798M) that could help broaden its use. This has the potential to reduce waste associated with concrete production and construction as well as the energy needed to produce concrete and its ingredients.

“The development of C1798 has the potential to have a huge impact,” says Szecsy. “The standard allows for the use of returned concrete, in an unhardened state, in the production of new concrete. It considers the residual or unused concrete in the mixer drum as a potential material source just like aggregates, cementitious materials, or water. Prior to this standard, that material was generally disposed of in a landfill. With this specification, returned concrete can be used rather than ending up as a waste material.”

“We started work on C1798 about 12 years or so ago because we knew that there was concrete coming back to the producer and that it was being loaded on top of and sent back out the gate,” adds Chris Slate, technical services director for Alamo Concrete Products Co. and a member of the committee. “We wanted to put something in place that would protect both the purchaser and the producer, as well as give guidance to a producer that wanted to start a program of dealing with returned fresh concrete.”

Slate points out that C1798 is in use, but that the specification for ready mix concrete (C94) still needs modifications that would allow returned fresh concrete to be used in the production of fresh concrete. He hopes that work will be completed this year.

Recycling Asphalt

Any discussion of road and bridge infrastructure must take asphalt into account.

In the United States, for example, figures from the country’s Federal Highway Administration indicate that of the 1,530,000 km of paved roads, only 254,000 km are concrete; the rest – nearly 1,300,000 km — are asphalt.

Who decides which material to use? According to NRMCA’s Colin Lobo, “Generally, the U.S. state highway agency establishes a preference for asphalt or concrete. In some Midwestern states, concrete is the primary material used for road pavements, while in other states concrete will be used on heavily trafficked interstates with larger volumes of truck traffic.” Other factors, such as initial and life cycle cost, maintenance expenses, and the speed with which a project can be completed, are also taken into consideration.

Hybrid approaches — asphalt used as an overlay applied atop older concrete pavement that has started to deteriorate or, conversely, concrete overlays placed on asphalt pavement when traffic loads have changed — are quite common as well.

“Superpave” is a common asphalt mix design method used in countries like the United States.  It is also known as hot mix asphalt, asphalt mixture, and asphalt concrete.

As technical director of the North Central Superpave Center and vice chairman of the committee on road and paving materials (D04), Rebecca McDaniel, Ph.D., P.E., has worked in the asphalt industry for over 20 years. “About 350 million tons of asphalt pavement is produced each year for roads, airfields, parking lots, etc.,” she notes. And, “when those pavements need rehabilitation or replacement, about 99 percent of the material removed (known as reclaimed asphalt pavement, or RAP) is recycled, with the vast majority being reused in new asphalt mixes.”

RAP is far and away the most common form of recycled asphalt, but another approach is also gaining traction in the industry. In “full-depth reclamation” deteriorated asphalt road and underlying granular base material are incorporated into a new, stronger base material. FDR with cement base is stronger and more durable as well as faster and less expensive to build than traditional removal and replacement techniques. It’s also very sustainable, since most of the existing material is recycled in place. The subcommittee on stabilization with admixtures (D18.15) develops and maintains standards related to FDR.

Other materials are finding their way into asphalt used for road construction as well. “Tire rubber is available in large quantities, at least in some areas, and is increasingly being used in asphalt. The rubber imparts some added flexibility to the mix. Some U.S. states, like California, Arizona, and Florida, among others, use ground tire rubber extensively,” says McDaniel.

The Chicken or the Egg?

From ultra-high performance and self-consolidating concrete, to new blended cements, to even more widespread use of reclaimed asphalt pavement, there are many exciting developments in the concrete and asphalt industries that could have a positive impact on future road and bridge infrastructure rehabilitation efforts. The key word in that statement is “could,” and the big question is what will it take for these nascent technologies to expand their market share.

Several of the industry experts contacted for this article cited the crucial importance of consensus standards and test methods that support existing, new, and evolving technologies. For example, Richard Szecsy says, “It’s a little like the ‘chicken and the egg’ analogy in terms of which came first. As far as the engineering and specification community goes, no new technology exists before there is an ASTM International standard or specification to recognize that technology. But to develop the standard or specification, someone has to develop a technology that is not being recognized by the current standards or specifications. In my experience it has always been the development of the technology that precedes the development of the standards.”

Szecsy also points to the time lag between the development of standards and their implementation. “Standards development is occurring faster, but the adoption of those standards into actual construction documents is moving at the same historical pace — that is, too slow. We have new ASTM documents that can and do improve the quality or the materials used in infrastructure projects, but the construction specifications are still referencing ASTM documents from a decade ago.”

Perhaps Whitney Belkowitz, president and CEO of Intelligent Concrete, said it best in a comment she once made that, while specifically focused on the concrete industry, could certainly be applied to the broader issue of infrastructure rehabilitation: “We are solving today’s problems with yesterday’s technologies.” The many dedicated professionals serving on ASTM International concrete and asphalt committees are working hard to close this gap.

Sidebar
The Nuts and Bolts of... Nuts and Bolts

Structural bolts may not be as impressive as a suspension bridge’s soaring towers, but the humble fasteners are as essential as steel and concrete to bridge building and reconstruction projects.

ASTM International’s committee on fasteners (F16), which oversees more than 60 standards, provides the construction industry with test methods, product specifications and performance parameters related to nuts, bolts, and washers. Now, thanks to the recent consolidation of multiple bolt standards, it is easier than ever for architects and engineers to specify this crucial item.

Steel, stainless steel, and nickel-based alloy bolts are used in conjunction with nuts and washers to create structural fasteners that hold bridge components together. “In the case of a steel bridge,” says Salim Brahimi, director of engineering technology at the Industrial Fasteners Institute, “all of the structural steel members must be fastened. Some parts are welded, but all high-strength steel joints are fastened with bolt/nut/washer assemblies in bolted connections.”

Prior to 2016, there were no less than six specifications pertaining to structural fasteners such as steel and steel alloy heat-treated bolts, and twist-off tension control bolt/nut/washer assemblies. The new consolidated standard is ASTM F3125.

“F3125 did not add anything new in terms of structural bolt products,” Brahimi says. “What it did was establish a single, consistent and easily maintained standard for all structural bolts.” He also notes that F3125 is a work in progress and will continue evolving to cover new products like 144 ksi (kilopounds/square inch) bolts, which could one day supersede both 125 and 150 ksi alternatives.

Jack Maxwell is a freelance writer based in Westmont, New Jersey.

July/August
2017
Industry Sectors: 
Construction
Safety
Transportation