Keeping Concrete Forever Solid
Given that concrete is composed of few ingredients, manufacturing it should be simple, yet mix designs vary depending on their application. According to the Portland Cement Association (based in Skokie, Illinois; portland cement is named for an English island), a typical concrete mixture starts with 10 to 15 percent cement and 15 to 20 percent water that creates a paste that coats 60 to 75 percent aggregate. Around 5 to 8 percent entrained air (purposely created air bubbles) is added during the mixing process to make concrete more workable during placement and increase its durability when hardened in climates with high freeze-thaw cycles.
Scott Davis, president of Davis Block & Concrete (one of approximately twenty concrete manufacturers in Alaska), explains there are five steps to the concrete manufacturing process. The first involves batching, where materials are measured for the concrete mix. Once the materials are measured, they’re mixed to the desired level of consistency. Depending on the project, concrete can be mixed by hand, by stationary mixer, or during transport. Once mixed, concrete is transported and placed using a variety of methods that range from manual application to large pump trucks equipped with booms. While still wet, the concrete must be compacted before it can be cured, often with additional chemicals, and finished.
Concrete is used in many ways in commercial and residential construction projects. As building material, concrete is used for foundations, superstructures, columns, driveways, floor construction, and exterior surfaces. For infrastructure, concrete is used to build bridges, dams, wastewater treatment facilities, traffic guides/barriers, sidewalks and curbs, and parking lots/structures. Concrete roads and highways are rarely found in Alaska, though they are more common in the Lower 48.
Davis Block & Concrete supplies precast items, landscape pavers, retaining walls, masonry units, and bulk bags of materials blended to specification. Davis says his company relies on experience and technology to assist clients with their ready-mix concrete services. In addition to a computerized batching system, it also provides concrete pump trucking services for quicker, less labor-intensive pours at a job site. The company also developed a Remote Concrete Division with an in-house trucking fleet, a mobile concrete batch plant, and materials storage that can be set up at job sites much farther away from the home office in Kenai.
“Technology has played a role in increasing portability, mix accuracy, and application,” says Davis.
Along with technology, Davis says materials testing protocols have increased, particularly with state and federal projects. Different testing methods are used to measure the air, slump, and unit weight prior to application, as well as strength after it has dried.
Given the competitive nature of the industry, Davis says manufacturers generally don’t share how much concrete they typically produce in a year, but he did reference a single large project for which Davis completed up to 40,000 cubic yards in one year.
Room for Improvement
Richard Giessel is the quality assurance engineer for the Alaska Department of Transportation & Public Facilities (DOT&PF); his mission is to eliminate defects in state roads, bridges, ferry terminals, airfields, and buildings by improving design and construction methods. He also co-chairs the Alaska Concrete Alliance, a collaborative effort between contractors and the DOT&PF to increase both the quality of concrete products and the understanding of practitioners statewide. The alliance does this by sharing industry news and discussing improved design mixes, technologies, and methods of application.
Giessel says the general approach to concrete changed little between the time of ancient Rome until the ‘30s, when one of the most significant innovations in concrete technology occurred. Engineers had noted that certain concrete pavements better withstood freeze and thaw cycles than others. After investigation, they discovered that the most durable pavements were less dense, and the cement came from a mill that used beef tallow as a grinding aid. The tallow acted as an air-entrained admixture, creating tiny air bubbles that gave the concrete more room for expansion and contraction and was therefore less susceptible to cracking from freeze/thaw. Until the mid-’90s, the most-used air-entrained admixture was neutralized wood resin. Now manufacturers lean toward synthetic detergents like salts of organic acid and sulfonated hydrocarbons, which have improved stability since the ‘30s.
More recently, Giessel says engineers are looking at the benefits of using fly ash in concrete. Fly ash is a fine powder that is a byproduct of burning pulverized coal. As a pozzolan, fly ash contains aluminous and siliceous material that forms cement in the presence of water. Mixed with lime and water, the compound is similar to portland cement. When used in concrete, fly ash improves its ability to resist water infiltration and its strength while making it easier to pump.
Giessel says aside from increased strength, there are many reasons to incorporate fly ash. Since it reduces CO2 emissions and uses a lower water/cement ratio for similar slumps, fly ash is considered environmentally friendly. It is cold weather resistant, which is extremely important in Alaska; has a high workability; is considered a non-shrink material; and produces dense concrete with a smooth surface and sharp detail.
There are drawbacks, though. Fly ash concrete has a longer setting and curing time, which means slower strength gain; it has seasonal limitations as to when it can be poured; and it’s more difficult to control the color variability. Despite this, Giessel says fly ash concrete may provide the durability and strength that increases the longevity of finished products.
“Durability is a lot more important than strength,” says Giessel. “At times people will focus on enhancing the strength of concrete by adding more cement, which creates more shrinkage that eventually reduces the longevity of the final product. Durability is not an easy problem to address with the prescriptive specifications we use now.”
The Alaska Concrete Alliance is currently discussing the difference between prescriptive-based and performance-based concrete specifications. Engineers wrote prescriptive-based specifications decades ago when quality control standards were just being developed. Prescriptive specifications include clauses for the methods of construction and impose restrictions on the compositional parameters of concrete mix. For instance, a specified minimum amount of cement content, requiring the use of brands or sources of materials, or restrictions on adjustments to mix proportions as needed.
As quality control standards evolve, many in the engineering community are advocating for a move to performance-based concrete specifications. Performance specifications don’t rely on parameters like components or proportion of the mixture. Instead, they are based on performance indicators like strength, permeability, shrinkage, sulfate resistance, et cetera. Indicators are measured by standard test methods with defined acceptance criteria.
“It’s something I hope we can get changed,” says Giessel.
Design mix is only one factor that affects the longevity of a finished concrete product, Giessel says. Workmanship is at least 15 percent of the equation. Nick Scott, Alaska business agent for Cement Masons & Plasterers Local Union 528, says it takes at least four years to complete the 288 hours of classroom and skill development and 4,000 hours of on-the-job training in the union’s apprenticeship program. By the definition of the program, “Cement masons are responsible for all concrete construction, including pouring and finishing of slabs, steps, wall tops, curbs and gutters, sidewalks, paving, and other concrete construction. They also handle epoxy, polymer, and other plastic materials for topping repair and injection. Cement masons are responsible for all preparation and repairing of concrete. They also set forms and screed pins for slabs, steps, curbs and gutters, and paving.”
“Concrete construction is something everyone thinks they can do until they get to the actual job site,” says Scott.
He explains that finishing concrete is more than just waiting for it to dry once it’s poured. Cement masons smooth and compact it and then apply curing compounds to prevent moisture loss while the concrete is gaining strength. There are many ways to place and finish concrete depending on the desired look. In addition to hand trowel, broom, and exposed aggregate, cement masons can make concrete look like any natural material, such as slate, brick, wood, or stone. They can also color, stain, polish, and stencil concrete or apply decorative and commercial toppings of epoxy, acrylic, or polymer-based floor installations.
“You only have so much time to complete the surfacing texture and detail,” says Scott. “I tell people concrete will not wait for you. In the event you don’t finish it in time, it ends up costing a lot to try to resurface it and could end up being a liability.”
Scott says the 135 members in Local 528 Alaska work on hundreds of projects every year statewide. Most of them are public infrastructure projects like curb and gutter, building foundations, airport taxiways, and concrete utility facilities. Like Giessel, Scott sees future mix designs and admixture chemicals resulting in more environmentally friendly concrete.
How concrete is poured is also changing in ways that ancient Romans could never have imagined. Computer-driven 3D printers can squirt concrete into precise shapes, even at the scale of whole buildings. However, Scott believes the size and amount of equipment needed limits the feasibility of 3D printing concrete.
“You’ll always need some element of manual labor to finish concrete,” says Scott. “It’s not a job that’s going to be lost to a computer.”