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May/June 2009
Feature

Building to Specification

Cement and Concrete in Modern Construction

Cement and concrete form the foundations of the modern world. Without them, no buildings could soar high into the sky, no miles-spanning bridges could be built, and no city skyline would rise more than a few stories.

Many types of cement and concrete exist, but before they can reliably and safely be used in construction, proper standards and test methods must be developed. In existence since 1914, ASTM International Committee C09 on Concrete and Concrete Aggregates has worked hard in recent years to develop test methods for pervious and self-consolidating concrete, two of the newest concrete systems. ASTM Committee C01 on Cement, which celebrated its centennial in 2002, has modified time-proven standards and developed new ones that have helped green the industry and improve material quality.

Cement and concrete are often confused. If one starts with the dry powder that is hydraulic cement — usually the particular class of hydraulic cement known as portland cement — and adds water, what results, depending on the amount of water added, is cement paste or grout. Grout can be poured like gravy. If fine aggregate is added, the result is mortar or sanded grout. If both fine aggregate and coarse aggregate are added, the result is concrete. As the Supreme Court of Pennsylvania once wrote in a decision dealing with cement-manufacturing plants, “cement is to concrete as flour is to fruitcake.”1

Concrete Flows Around It

Self-consolidating concrete is “a concrete that’s so ‘flowable’ you don’t have to use vibrators to compact it in forms,” says Nicholas Carino, a consultant in concrete technology in Chagrin Falls, Ohio, and the former chair of ASTM Committee C09. SCC fills distinct needs in the marketplace. One such need is in locations with limited access.

For example, when multiple steel reinforcing bars crowd a small, hard-to-reach space, conventional concrete may not flow completely around each bar, leaving unseen pockets of air. Such voids leave the steel susceptible to corrosion. That’s not the case with SCC, which “flows in and around the steel better with less compaction,” says Steven Kosmatka, vice president of research and technical services at the Portland Cement Association in Skokie, Ill.

SCC’s ability to encase steel reinforcement and protect it from corrosion is significant for infrastructure projects, such as those enabled by the new stimulus package. “The corrosion of steel in bridge decks came about from the demand for snow-free roads,” Carino says. “We used salt to do that, but the concrete used to build bridges and pavements wasn’t resistant to salt penetration. Now we know how to provide adequate protection to the embedded steel. If we use the latest technologies and knowledge, the replacement infrastructure should have a longer lifetime than what we’re replacing.”

Because SCC is so new, ASTM Committee C09 has worked hard and fast to develop tests for its use. According to Carino, three are complete: the slump flow test, C1611/C1611M, Test Method for Slump Flow of Self-Consolidating Concrete; the J-ring test, C1621/1621M, Test Method for Passing Ability of Self-Consolidating Concrete by J-Ring; and the test for segregation tendency, C1610/C1610M, Test Method for Static Segregation of Self-Consolidating Concrete Using Column Technique. The first two determine how far SCC flows, with and without obstacles. The third checks uniformity of aggregate distribution in a fresh SCC sample.

These SCC tests, like all tests, essentially look at the performance of the material to check whether it matches the specifications. In this sense, tests such as these support a new breed of standards that differs from the historical focus of C09 and C01’s standard specifications. That focus was always on the composition of concrete and cement. The new standards focus more on performance.

With prescriptive standards, “The producer is given a recipe for the mix and must follow it,” says Anthony Fiorato, a consultant in Glenview, Ill., and chair of Committee C09. “A performance specification addresses what I want the concrete to do.” Performance standards for concrete can address durability — resistance to freezing and thawing — and strength in terms of the amount of compression needed to crush a sample.

“Prescriptive standards are easy to judge,” Fiorato says. “Performance is trickier, because it’s based on tests that must reflect long-term behavior. Part of the standards process is to come up with reliable performance tests. Virtually all new C09 standards include both prescriptive and performance elements.”

Solid Concrete Drains

Pervious concrete is an owner’s dream, since it lets water flow right through it into the ground. Therefore, the U.S. Environmental Protection Agency includes it among best practices for managing run-off from buildings and parking lots. “It’s seen as a sustainable technology,” says Matthew Offenberg, Southeast U.S. technical service manager for W.R. Grace and Co. in Atlanta, Ga., the chair of the American Concrete Institute’s Committee 522 on Pervious Concrete and secretary of ASTM Subcommittee C09.49 on Pervious Concrete.

“Pervious concrete is very significant to sustainable construction,” says Colin Lobo, senior vice president of engineering at the National Ready-Mixed Concrete Association in Silver Spring, Md. “With impervious concrete, you need a place for storm water run-off, like a detention pond. Pervious concrete minimizes runoff of pollutants into streams and rivers. Storm water flows through the concrete into the ground to replenish ground water, where it undergoes natural purification. Pervious concrete can minimize the capacity and possibly the need for stormwater drains.”

Builders would like to use pervious concrete for all locales, including those that sustain heavyweight traffic. To date, however, the concrete has only been used in the United States for light-traffic areas. Because there is little risk to consumers or liability for builders and owners, “The industry doesn’t usually test concrete for sidewalks, residential roads and parking lots for ordinary commercial use,” Offenberg says. “We are working on ASTM test methods to expand the technology into heavier uses.”

Committee C09 has completed one test method, C1688/C1688M, Test Method for Density and Void Content of Freshly Mixed Pervious Concrete, which details how to measure the density of freshly mixed pervious concrete, and is also working on a proposed method for determining how fast water drains through pervious concrete. Other tests for durability and strength are still in development. ACI has performance specifications for light-duty uses, but the tests were adopted from other concrete types. “When ASTM develops these first test methods for light-duty pervious concrete, then ACI can move forward with performance specifications for heavier uses,” Offenberg says.

Like SCC, pervious concrete can contribute to the quality of infrastructure replacement. “Committee C09’s work provides the standards and test methods to make sure that materials and construction practices can be properly monitored for compliance with specifications,” Fiorato says. “All of those aspects — strength, permeability, durability, workability — directly impact performance. Without standards, there would be no way to determine the quality of materials and construction.”

Reducing Cement’s Carbon Footprint

C150 is a specification for portland cement, one of the primary materials in the construction industry,” says Stephen Lane, chair of Committee C01 and associate principal research scientist in materials at the Virginia Transportation Research Council, located at the University of Virginia in Charlottesville. C150 is a specification for five different types of hydraulic cement, which harden by chemical reaction with water. It is prescriptive regarding composition, but also includes performance requirements for strength and durability.

In 2004, Committee C01 voted to allow limestone to replace up to five percent of the portland cement used to make concrete. “It was a marketing issue,” Lane says. Limestone was already being used in Canada, Europe and other parts of the world, so the modification of C150 made it “easier for Canadian producers to ship into the U.S. and meet C150 specifications and vice versa,” Lane says. That’s also true for shipments to and from other countries.

Substituting some portland cement with limestone also has significant environmental benefits. “About one ton of carbon dioxide is produced per ton of portland cement,” says Kosmatka. The carbon dioxide reduction from using 5 percent limestone isn’t one to one, however. “There are technical and quality control issues,” Kosmatka says, “so practically, the reduction is more like 3 percent.” By modifying C150, ASTM enabled the elimination of some three million tons of carbon dioxide from portland cement production every year.

One-Upping the Ancient Romans

Fly ash and blast furnace slag belong to a class of materials known as pozzolans that can be added to cement mixes. Pozzolans react with lime and water to form calcium silicate hydrate, the primary binding agent in hydraulic cements. The name comes from the Italian town of Pozzoli, where ash flows from Mt. Vesuvius are found. Ancient Romans mixed the ash from the volcano with lime to make cement, and modern pozzolans are similar in chemistry and function to that original volcanic ash.

Fly ash and slag are waste by-products, the first from coal-fired power plants and the second from blast furnaces that produce iron from iron ore. Adding either or both during cement production uses waste products that would otherwise go to landfills. Their use in cement production reduces both the amount of carbon dioxide that would otherwise be released and the energy needed to make the cement.

“Slag, fly ash and pozzolanic cements are included in C595 [Specification for Blended Hydraulic Cements], which is a specification for blended hydraulic cements,” Lane says. “C595 has both prescriptive and performance requirements.” Those performance requirements include compressive strength and durability, such as resistance to chemical compounds in soil that could eat away and damage the cement.

The addition of limestone to portland cement and fly ash and slag to hydraulic cements can improve both sustainability and quality of the final products.

Standards Around the World

“We’ve had a lot of use of Committee C09 standards internationally,” Fiorato says. “It tends to be driven by the use of the ACI 318 building code, because 318 references about 60 ASTM standards and test methods.”

Even countries that don’t use ACI 318 are interested in ASTM standards, because of their technical strengths and because the voluntary consensus method of approval includes all stakeholders. Fiorato says Saudi Arabia and Qatar both use ASTM standards, though some nations, such as Qatar, mix the use of standards, including those from ASTM and other sources.

“It’s the same story in Latin America,” says Carino. “They use ACI 318, so they use ASTM standards.” Carino adds that Committee C09 has strong Latin American participation.

This international interest shows the worldwide importance of standard specifications and test methods. “Standards are referenced in project specifications,” says Lobo. “They form the basis of the contract between the owner, contractor and materials suppliers. Without standards for materials and acceptance methods, every specifier would have to devise his own ways to determine acceptance of materials and methods used to build structures. Reference standards in project contracts establish an understandable basis of compliance and associated risk between the owner and the builder. If there were no standards, owners and producers would have no way of determining what’s important.”

A Unique View of Standards and Tests

Financially supported by ASTM and housed by the National Institute of Standards and Technology, the Cement and Concrete Reference Laboratory’s mission “is to improve the quality of testing of construction materials through laboratory inspections, proficiency samples and guidance on the use of ASTM standards in the field and offices,” says Steven Lenker, director of the Construction Materials Reference Lab, a joint activity between CCRL and the American Association of State Highway and Transportation Officials Materials Reference Laboratory.

Inspections reach more than 1,300 laboratories in North America. Laboratories in the Americas, Asia and the Middle East evaluate proficiency samples. Both programs ensure that laboratories adhere to ASTM standards and test methods.

When a specifying agent requests a laboratory inspection, “The inspectors go out and examine equipment and how procedures are performed compared to a standard,” Lenker says. Reports don’t just go back to the specifier. “Reports also give the laboratory management a good picture of how the laboratory is doing and what needs improvement,” Lenker says. Specifying agencies use reports to ensure that materials used in highways and construction are tested by the standard methods. AASHTO, among other organizations, also uses the reports to accredit laboratories.

With proficiency testing, CCRL prepares large quantities of homogeneous cement or concrete, divides them up and sends the samples to participating laboratories. Each laboratory tests the samples and reports results to CCRL, which analyzes results and sends confidential reports to each laboratory. CCRL’s analysis comes up with the statistical averages of all test results. “It’s a way to compare yourself to another 1,500 laboratories running the same tests,” Lenker says. “If you’re not near the middle of the pack, then you might have a problem.”

CCRL’s inspectors have a view of standards no one else has. “We can influence the way standards are written and cause them to be modified,” Lenker says. “If we go to a thousand laboratories and 60 percent of them can’t meet part of the standard, that suggests there may be a problem with the standard. We are the only ones equipped to tell people that.”

Reference
1. www.concrete.org/PUBS/JOURNALS/AbstractDetails.asp?ID=13505

 

Bridget Mintz Testa has B.S. degrees in physics and psychology. She worked for five years at NASA-Johnson Space Center, first in lunar and planetary exploration and then on space station robotics. Since 1993, she has been a full-time freelance writer, writing for such magazines as Discover, Popular Mechanics, Telephony, and Workforce Management.