By Greg Sitek
Concrete has been important to construction for centuries, going back to 3,000 B.C. Without it, very few construction projects could be started and completed. In the United States, concrete as a construction material made its debut with the construction of the Erie Canal in 1825, the first shipment of portland cement into the country in 1868, and the opening of the first portland cement plant three years later in 1871.
In 1891, the city of Bellefontaine, Ohio, laid the first concrete paved road; it is interesting to note that some of it is still in use. With the advent of the industrial revolution at the turn of the century, the introduction of the horseless carriage and the urbanization of the country, and the demand for better roads, more buildings and more affordable housing, the country leaped forward and found new applications for the magical material. In 1916, the Portland Cement Association (PCA) was founded to help harness the potential that this material offered humanity.
The use of concrete as a building material captured the imagination of some of the country’s industrial geniuses. Thomas A. Edison was among these forward thinkers. He opened and operated his own portland cement plant, but also used concrete to build residential houses and then supplied 160,000 bags of portland cement for the construction of Yankee Stadium in the 1920s.
Hoover Dam and the Grand Coulee Dam are other monuments to the use of cement to create concrete structures that changed the face of the nation and improved its quality of life. Without concrete, we would not have the lifestyle or infrastructure that we enjoy. There is very little construction that doesn’t involve the use of concrete in some phase or application.
As with everything, there are negative aspects that accompany the extensive use of this material. There are environmental concerns related to the production of cement and to the depletion of the stone, sand and aggregate resources. The industry is working to develop better production methods as well as to use other materials that may leave less of a footprint on the environment.
Advances are being made. Stanford professor Brent Constanz says he has invented a “green” cement that could eliminate the huge amounts of carbon dioxide (CO2) spewed into the atmosphere by manufacturers of the everyday cement used in concrete for buildings, roadways and bridges.
His vision of eliminating a large source of the world’s greenhouse CO2 has gained traction with both investors and environmentalists. His company, the Calera Corporation, is operating a pilot factory in Moss Landing (Monterey County, Calif.) that churns out cement in small batches. Constanz claims his new approach not only generates zero CO2, but also has an added benefit of reducing the CO2 power plants emit by sequestering it inside the cement.
Typical cement production requires temperatures over 1,800 degrees Fahrenheit. Constanz takes the exhaust gas and bubbles it through seawater, since his pilot plant is located across the highway from the ocean. The chemical process creates the key ingredient for his green cement and allows him to sequester a half-ton of CO2 from the smokestacks in every ton of cement he makes. The process is secret and pending a patent.
The Portland Cement Association has expressed an interesting in testing a bag of the green cement, and show management for the World of Concrete (WOC) has invited him to speak at next year’s show that will be held in February at the Las Vegas Convention Center. There is little more information available, as the product and technology aren’t ready for mass consumption. It will be interesting to hear Constanz’s remarks if he elects to speak at the 2010 WOC.
Another emerging technology is what has been classified as self-healing concrete. The University of Michigan developed a concrete material that can heal itself when it cracks. No human intervention is necessary – just water and carbon dioxide.
According to the university, a few drizzly days would be enough to mend a damaged bridge made of the new substance. Self-healing is possible because the material is designed to bend and crack in narrow hairlines rather than break and split in wide gaps, as traditional concrete behaves. Victor Li, the E. Benjamin Wylie Collegiate Professor of civil engineering and a professor of materials science and engineering, designed the self-healing concrete.
In Li’s lab, self-healed specimens recovered most if not all of their original strength after researchers subjected them to a 3 percent tensile strain, which means they stretched the specimens to 3 percent beyond their initial size. It’s the equivalent of stretching a 100-foot piece an extra three feet – enough strain to severely deform metal or catastrophically fracture traditional concrete.
Li has published a paper, “Autogenous healing of engineered cementitious composites under wet-dry cycles,” on the exciting new product and says this new substance could make infrastructure safer and more durable. By reversing the typical deterioration process, the concrete could reduce the cost and environmental impacts of making new structures – and repairs would last longer.
The American Society of Civil Engineers (ASCE) recently gave the country’s roads, bridges, water systems and other infrastructure a “D” grade for health. The federal stimulus package includes more than $100 billion for public works projects.
“Our hope is that when we rebuild our roads and bridges, we do it right, so that this transportation infrastructure does not have to undergo the expensive repair and rebuilding process again in another five to 10 years,” Li said in an article published in the University of Michigan Science News. “Also, rebuilding with self-healing bendable concrete would allow a more harmonious relationship between the built and natural environments by reducing the energy and carbon footprints of these infrastructures. As civil and environmental engineers, we are stewards of these mega-systems. Advanced materials technology is one means to keep them healthy.”
In addition to “traditional” portland cement, there are other products currently in use today. The two that are definitely worth looking at are fly ash and recycled concrete.
Fly ash is one of the residues generated in the combustion of coal. It is generally captured from the chimneys of coal-fired power plants, and is one of two types of ash that jointly are known as coal ash; the other, bottom ash, is removed from the bottom of coal furnaces. Depending upon the source and makeup of the coal being burned, the components of fly ash vary considerably, but all fly ash includes substantial amounts of amorphous and crystalline silicon dioxide (SiO2) and calcium oxide (CaO), both endemic ingredients in many coal-bearing rock strata.
Toxic constituents depend upon the specific coal bed makeup, but may include one or more of the following elements or substances in quantities from trace amounts to several percent: arsenic, beryllium, boron, cadmium, chromium, chromium VI, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium and vanadium, along with dioxins and polycyclic aromatic hydrocarbon (PAH) compounds.
In the past, fly ash was generally released into the atmosphere, but pollution-control equipment mandated in recent decades now requires that it be captured prior to release. In the U.S., fly ash is generally stored at coal power plants or placed in landfills. About 43 percent is recycled and often used to supplement portland cement in concrete production. There have been health concerns about this, but there have been no definitive reports to substantiate these concerns.
Fly ash concrete was first used in the U.S. in 1929 for the Hoover Dam and is now used across the country. Consisting mostly of silica, alumina and iron, fly ash is a pozzolan – a substance containing aluminous and silicious material that forms cement in the presence of water. (The Romans used pozzolana, a volcanic ash, in 300 B.C.) When mixed with lime and water it forms a compound similar to portland cement. The spherical shape of the particles reduces internal friction, increasing the concrete’s consistency and mobility and permitting longer pumping distances. Improved workability means less water is needed, resulting in less segregation of the mixture. Although fly ash cement itself is less dense than portland cement, the produced concrete is denser and results in a smoother surface with sharper detail.
As noted above, the composition of fly ash will vary depending on the kind and quality of coal burned in the power plant. Class F fly ash, with particles covered in a kind of melted glass, greatly reduces the risk of expansion due to sulfate attack, which may occur in fertilized soils or near coastal areas. It is produced from Eastern coal.
Class C fly ash, produced from Western coal, is also resistant to expansion from chemical attack, has a higher percentage of calcium oxide, and is more commonly used for structural concrete.
Although the federal government has been using the material for decades, smaller and residential contractors are less familiar with fly ash concrete. Competition from portland cement is one consideration. Because fly ash comes from various operations in different regions, its mineral makeup may not be consistent; this may cause its properties to vary, depending on the quality control of the manufacturer. There are some concerns about freeze/thaw performance and a tendency to effloresce, especially when fly ash is used as a complete replacement for portland cement.
The Clean Air Act of 1990 requires power plants to cut nitric oxide emissions. To do so, plants restrict oxygen, resulting in high-carbon fly ash, which must be reprocessed for cement production. As a result, fly ash could be less available or more costly in the future. Ongoing research is being done on the use of fly ash as a substitute or enhancement for traditional cement.
The use of fly ash cement continues to increase as more research is done and more contractors use it. With the growing “green” emphasis there will be a growing emphasis on the use of fly ash. Currently, typical applications include portland cement and grout, embankments and structural fill, waste stabilization and solidification, raw feed for cement clinkers, mine reclamation, soil stabilization, road sub base, aggregate, flowable fill and mineral filler in asphaltic concrete. Other applications include cellular concrete, geopolymers, roofing tiles, paints, metal castings, and filler in wood and plastic products.
For more information on fly ash cement, the Internet will give you as much or as little information as you want. Google is a good starting place, or Portland Cement Association (www.cement.org) or visit Wikipedia. Any of these will take you on a cyber tour of fly ash.
Construction and demolition (C&D) waste constitutes anywhere from 23 to 33 percent of municipal solid wastes. Based on tonnage, some estimates put concrete, asphalt and rubble at about 50 percent of C&D waste.
Landfill space continues to shrink and becomes increasingly more expensive as availability decreases. Recycling concrete makes sense, and for some contractors, money.
To recycle concrete, it has to be broken, removed and crushed to a specific size. This can be done in a number of ways. Typically, a subcontractor brings recycling equipment on site, although stationary recycling plants are also common in heavily developed areas. Recycling plants can produce any desired gradation. After processing, the crushed concrete makes an acceptable aggregate for use in new concrete, road base material or other applications.
Mobil crushers have become more popular in some areas of the country. I recently had the pleasure of discussing this subject with David Schielein, president and owner of IronHustler Excavating, Inc., Peoria, Ill., and also On-Site Recycling. Schielein had been involved in a number of demolition projects and noticed that “there were tons of concrete and masonry-type debris that had to be removed from the sites, and then tons of aggregate had to be brought back. This process seemed like a tremendous waste of time, energy, natural resources, fuel and money.”
Schielein saw this as an opportunity and also a solution to managing C&D waste. He tested several mobil crushers and couldn’t find one that gave him the results he needed. He finally found one that could do the kind of crushing he wanted, bought a Rubble Master and put it to work.
There Is a Market
Various agencies have specified recycled aggregate for all types of concrete pavement, including jointed plain, jointed reinforced and continuously reinforced, with both regular and epoxy-coated rebar. Though concrete properties differ when using recycled aggregate, no special construction techniques are necessary for paving new concrete made with recycled concrete aggregate.
Other applications for recycled aggregate include sidewalks, curbs, bridge substructures and superstructures, concrete shoulders, median barriers, residential driveways, erosion control, general and structural fills, subbase, and the list continues to grow. Fine aggregate from the crushing operation can also make good fill for subgrade corrections.
One of the advantages of on-site recycling is that you don’t have to transport the materials off site to have them recycled and you don’t have to transport them or other material to the site for use as part of the project.
Illustrating the point, Schielein noted that on his first job with the crusher they produced 11,000 tons of CA-6 gradation that they were able to sell back to the project owner. Schielein said, “This was a win-win situation all the way around because the material was used in the new construction and the project owner didn’t have to pay for transportation costs to haul the material in; there were no costs for hauling the debris away, and it didn’t get dumped into a landfill.” Less than a year later he had the second machine and became the dealer for Rubble Master in Illinois and surrounding states.
The use of recycled C&D continues to grow as it finds acceptance by state and municipal DOTs. Why not? Early on Illinois, Oklahoma, Wisconsin, Oregon and Wyoming, to name only a few, started using recycled concrete in a wide variety of applications. These DOTs not only spared acres of landfill space but also stretched tight highway budgets to get more miles for the money.
Standards For Recycled Aggregate
As with all construction materials, testing of aggregate and fines made from recycled concrete determines how it will perform. As long as the recycled aggregate can meet the requirements of ASTM Standard C 33 for new aggregate, it can be used in concrete.
Most of the same tests performed on virgin aggregate are necessary for recycled concrete aggregate, including gradation, Los Angeles abrasion, freeze-thaw durability and alkali reactivity. Many of these tests have shown recycled aggregate to be of better quality than some virgin material, though specific characteristics must always be considered.
Recycled aggregate often does not require a sulfate soundness test. Typical values for recycled concrete aggregate are 3 or less, far below the maximum loss allowed by ASTM Standard C 88. Many recycled-aggregate concretes also provide better freeze-thaw durability than concrete made with all virgin materials. Using 100 percent recycled coarse aggregate produces acceptable quality concrete; however, use of recycled fines in a new mix requires close examination.
Usually replacement of only 10 to 20 percent virgin sand is acceptable. Recycled fine aggregate is angular, with a high absorption rate and low specific gravity. Concrete produced with recycled aggregate has 80 to 90 percent of the strength of a comparable natural aggregate concrete. Using recycled fines further reduces strength compared with virgin sand, so its use in new concrete mixes should be carefully controlled.
Recycled coarse aggregate water absorption values are typically slightly higher than virgin aggregate, due to old cement mortar attached to the recycled aggregate. The values range from 2 to 6 percent, and increase as particle size decreases. Mix designers will likely increase typical water batch weights.
Guidelines for designing concrete mixes using recycled aggregate are contained in RILEM’s (www.rilem.net) “Recycling of Demolished Concrete and Masonry.”
Contaminants and Other Problems
Contamination is usually not a problem when recycling rural highways or airport pavements. Urban recycling, though, requires more concern for contaminants such as plaster, soil, wood, gypsum, asphalt, plastic, vinyl or rubber. While contaminants are usually not a concern for recycled aggregates used as a base course, strict control must be used for recycled aggregates in concrete to ensure that there are no more contaminants than are allowed for virgin coarse aggregate.
States have developed recommendations for the use of recycled concrete aggregates. If you are going to be using them, it would be a good move to become familiar with these guidelines for both aggregate size and mixture requirements.
Schielein believes in the concept of using recycled materials not only from a business perspective but also from an environmental concern. Schielein has several case studies posted on his companies’ websites, www.ironhustlerexcavating.com and www.on-siterecycling.com, to illustrate the value in recycling concrete.
What lies ahead? I believe everyone would like the answer to that question. The Portland Cement Association’s chief economist has been providing cement forecasts for years and knows the market. PCA publishes quarterly forecasts. The following is from its most current forecast, summer 2009.
Nonresidential Cement Outlook
PCA has expected a dramatic pullback in nonresidential construction for some time. Its projections are based on the Department of Commerce’s “Put-In-Place” construction estimates. Year-to-date, real nonresidential construction activity has declined only 7 percent. PCA’s forecast suggests larger declines will materialize during the remainder of 2009. The current forecast suggests a 14 percent decline in 2009 followed by a 23 percent decline in 2010. This pessimistic outlook may contain considerable downside risks. Dodge contract awards data, for example, suggest a 36 percent decline has materialized this year.
Since the start of the recession, the economy has shed 6.7 million jobs. Of these, office worker job losses total 2.3 million – accounting for roughly one-third of total job losses. As job losses continue, occupancy rates will decline and vacancy rates will increase – peaking during the first half of 2010.
Large inventory excesses in office space must be burned off. PCA estimates full office occupancy equivalent to 32 million jobs – suggesting a current shortfall of 4.5 million jobs, or a 16.5 percent vacancy rate. As job losses continue, vacancy rates will increase, eventually reaching 18.5 percent during the first half of 2010.
As job creation gains traction in 2010, vacancy rates will begin to improve. Leasing rate improvement is expected to lag the improvement in vacancy rates. Vacancy and leasing rates are not expected to improve to sufficient levels to generate high enough ROIs until the second half of 2011. As a result, PCA expects office construction activity will decline nearly 30 percent in 2009 and another 25 percent in 2010. Second half 2011 gains are expected to offset first half 2011 losses.
The harsh economic downturn and the resulting weak demand fundamentals facing the office sector are similar to those expected for manufacturing, retail and hotels. The adverse consequences of the expected downturn in nonresidential construction activity are expected to be compounded by cyclical declines in nonresidential cement intensities.
Public Cement Outlook
Public construction typically accounts for roughly 50 percent of cement consumption. Given the weak outlook for private sector construction activity, public construction activity will play an even more important role in determining the industry’s near-term outlook. Unfortunately, recent data and assessments have dimmed even this sector’s outlook.
Based on assumptions regarding the timing of The American Recovery and Reinvestment Act of 2009 (ARRA) stimulus spending, PCA’s spring and preliminary summer forecasts expected improvements in seasonally adjusted annual rate (SAAR) to materialize in the second half of this year. Recent data suggests this assumption regarding ARRA spending for 2009 is overly optimistic.
Among the public construction projects, highway construction plays the most important role for cement consumption. The near-term highway construction outlook is expected to be dominated by two competing influences: namely, ARRA stimulus spending and deteriorating state fiscal balances.
Compared to the spring forecast, PCA’s current forecast incorporates larger state deficits and longer bureaucratic delays in administering ARRA spending. The larger state deficits translate into additional sustained adverse fundamentals facing public construction during 2009-2010. Bureaucratic ARRA delays, in contrast, translate into a timing issue – pushing expected 2009 spending into 2010.
Discretionary State Highway Spending
More than 90 percent of all highway and street spending is put-in-place by state and local governments. State fiscal conditions impact discretionary spending by the states. Adverse economic conditions and job layoffs impact state income and sales tax revenue collections. Declines in home prices, in combination with higher home vacancy rates, adversely impact property taxes and municipal government spending on roads. Combined, the harsh economic environment facing state and local governments may result in a 15 to 18 percent decline in discretionary highway/street spending during 2009, followed by another 10 to 15 percent decline in 2010. The harsh fiscal picture facing state and local governments will partially sterilize the ARRA impacts for street and highway construction.
As jobs are created and consumer spending returns, state and local government revenues will rebound – but not until 2011. Thereafter, discretionary state/local highway spending is expected to grow, reaching 2006 levels by 2013.
Stimulus Highway Spending
Due to administrative lags, PCA has long maintained that ARRA highway construction activity would not begin to materialize until August. Recently introduced data suggests PCA’s assumption regarding ARRA spending for 2009 is overly optimistic. Bureaucratic delays have hindered the release of ARRA highway funds. Through July, highway contract lettings represent only 12 percent of total apportionments. Outlays represent a meager 2.5 percent of total apportionments. This July level represents a level expected in May.
Allowing for lags between outlays and construction activity and cement usage during the stages of construction suggests that very little second half stimulatory impact from ARRA will materialize during 2009 – contrary to PCA’s previous assumptions.
According to Federal Highway Administration (FHWA) data, 2.5 percent of total ARRA highway outlays have been dispersed through July (this reflects monies actually paid out). Of these outlays, 40 percent has materialized during the last two weeks of July. The letting of ARRA dollars has been slower to develop than expected. Clearly, however, the release of funds is now accelerating (July highway ARRA spending was up 176 percent over June’s levels). This acceleration is expected to continue until winter weather intercedes. Southern states could see a more sustained acceleration. A sustained and dramatic escalation of outlays must continue if a significant increase in highway construction is going to materialize in 2009. Anecdotal evidence suggests that bureaucratic paperwork is moving slowly and may hinder the amount of ARRA construction that materializes this year.
According to PCA assumptions, total ARRA highway obligations will be fully met by the end of the second quarter 2010 – one quarter ahead of the government’s deadline. Total lettings will be awarded by the end of 2010. Given the slow dispersal of outlays that has materialized thus far, a dramatic acceleration in lettings and outlays must occur during the remainder of 2009 and through 2010. This implies that some of the outlays that were previously expected to materialize in 2009 will likely materialize later – boosting the potential stimulatory impact of ARRA in 2010.
The 2010 stimulatory impacts of ARRA could be magnified further by pricing conditions. The harsh economic environment has dramatically increased competition among contractors for public projects. While projects may typically get three to six bids for highway work from contractors, the bids coming into state DOTs for projects are a multiple of typical bid levels. As a result, the bids are reportedly coming in well below expected levels. This implies that state DOTs could use these savings to let more projects. This has the potential of increasing both the effectiveness of the stimulus and the amount of cement consumed by stimulus projects. It is extremely difficult to quantify this phenomenon. PCA has assumed the price impact increases highway spending by 2 percent in 2009 and 1 percent in 2010.
Based on the foregoing assessments, PCA expects sustained year-over-year declines to characterize the full year, with weakness concentrated during the first half of 2009. All market segments and all regions are expected to record significant declines in consumption during 2009. Consider the following assessments:
• Compared to the spring forecast, the economic outlook remains largely unchanged. A gradual and prolonged recovery is expected. Real GDP is expected to decline 3 percent in 2009 and grow at an anemic pace of 1.3 percent in 2010. Growth in 2010 is back-ended.
• PCA’s economic projections are conservative relative to the consensus of economists and should be viewed with the potential of modest upside risk in both 2009 and 2010.
• The residential sector has largely run its course as a significant contributor to cement consumption declines. The expectation of a slow reduction in home inventories suggests that this sector will likely be a neutral contributor to cement consumption growth rates through mid-2010. Thereafter, the residential sector is likely to become a strong contributor to growth in cement consumption.
• Nonresidential cement consumption is expected to be a significant drag on cement consumption during 2009 and 2010. Weak underlying fundamentals and declining nonresidential cement intensities support this conclusion. By 2011, the drag on growth is expected to be milder and become a contributor to growth by the end of 2011.
• State deficits are expected to be larger than previously forecast and will act as a larger drag on public construction activity during 2009-2010 – partially sterilizing ARRA spending impacts.
• Compared to PCA summer preliminary estimates, new data suggests that bureaucratic delays have significantly diminished the level of outlays and hence construction activity that is likely to materialize during 2009 – diminishing the stimulatory impact on this year’s second half impact on cement consumption volumes.
• Delays in the lettings of highway funds suggest depressed stimulus for 2009 that is likely to be recaptured in 2010.
Taken together, these assessments suggest a 21.6 percent decline in portland cement consumption during 2009, followed by a 10.7 percent gain in 2010. Including masonry cement, cement consumption is expected to decline 22 percent in 2009, followed by a 10.9 percent gain in 2010. Sustained gains in consumption are expected from 2011 to 2014.
We all want tomorrow to hurry up and get here so we can start pouring, pumping or spreading more concrete. Even in the worst of times, there are people who find ways to innovate and use the opportunities that are presented to them. Information is the tool that can and does make a difference in how we manage the times, circumstances and conditions we are given. No matter what, concrete will continue to play a part in tomorrow’s world as we build it.
This article appeared in the October 2009 issues of all ACP magazines. For related articles on concrete applications,, techniques and trends visit the ACP publications section of Site-K Construction Zone.