Making concrete more durable

by sadia_badhon | September 20, 2019 4:00 pm

by John Kim

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Concrete is the most widely used building material in the world, according to the Royal Society of Chemistry (RSC). However in recent years, it has come under scrutiny, with many parties questioning its sustainability. Chatham House, a world-leading policy institute, calculates that the 4 billion tons of cement produced every year (to be used as a binder in concrete) contribute around eight percent of global carbon dioxide (CO₂) emissions. Further, installed concrete has historically suffered from performance issues, notably cracking.

The widespread use of concrete has now spurred extensive research into how it can be improved, leading to numerous innovations in its composition and manufacture. Recent developments have focused heavily on making concrete more ‘green.’ Cement alternatives, in particular, are now available to improve both the sustainability and performance of finished concrete. An example is calcium sulfoaluminate (CSA), a single hydraulic material manufactured with limestone, clay, and gypsum. The amount of limestone required for CSA is relatively small. Since limestone is the primary source of CO₂ during the chemical sintering process, this reduction leads to a decline in the amount of gas released into the atmosphere. CSA has more alumina and less calcium oxide and silica than traditional cement and its clinker phases form at lower kiln temperatures. These factors combine to make the production of CSA cement energy efficient with improved CO₂ emissions.

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CSA cement concrete experiences rapid precipitation of ettringite mineral crystals forming an interlocking matrix. This produces considerable early strength. While CSA cement concrete requires a typical amount of mix water to ensure workability, the ettringite crystals in it bind the free water, thereby avoiding bleed and reducing concrete shrinkage and related cracking.

Belitic CSA cement concrete—a rapid-setting material containing CSA as a minor phase and belite (dicalcium silicate) as the major phase—was developed in the early 1970s in the United States. Following the formation of ettringite during early hydration, the slowly crystallizing belite contributes to long-term strength gain in the concrete.

BCSA concrete exhibits shrinkage as low as 200 microstrains at 28 days, without the use of shrinkage-reducing admixtures. Standard drying shrinkage tests such as ASTM C596, Standard Test Method for Drying Shrinkage of Mortar Containing Hydraulic Cement, have demonstrated BCSA exhibits about a third of the drying shrinkage of Type II Portland cement. Setting times can be as short as 20 minutes and compressive strengths of 31 MPa (4500 psi) can be achieved in an hour. Silicates, which hydrate to form a dimensionally unstable gel-like substance, are also reduced when using BCSA. Most Portland cements contain about 70 to 80 percent by weight of silicates, but BCSA cement contains only 45 percent by weight (Figure 1).

The California Department of Transportation (Caltrans) first used BCSA cement concrete for highway repair in 1994,  the Northridge earthquake. The project was finished weeks before the estimated completion date. The success of the application led Caltrans to use BCSA cement concrete for its individual slab replacement program, which requires the overnight replacement of damaged sections of concrete highway pavement. Since 1994, the state has placed an estimated 1609 lane-km (1000 mi) of BCSA cement concrete.

BCSA cement concrete’s rapid-setting characteristics allowed it to enter the marketplace. However, with the passage of time, it is becoming apparent another advantage is the material’s durability.

Long-term field performance

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Several heavy-use installations are success stories for BCSA cement. The San Bernardino Freeway (Interstate I-10) in California provides a good example of the cement’s in-service performance. The Seattle-Tacoma (SeaTac) Airport in Washington is the 15^th largest one in the United States in terms of aircraft movement, as well as the Lincoln Tunnel, the world’s busiest underground vehicular passage, represent additional field-installed successes of BCSA cement.

San Bernardino Freeway

In October 1999, I-10 launched Caltrans’ longer life pavement rehabilitation quality team program. At that time, the project represented one of the most significant rapid-setting concrete placements ever attempted, being a demonstration project utilizing a 55-hour weekend closure (from 10 p.m. Friday to 5 a.m. Monday) with round-the-clock construction operations.

The freeway averaged 240,000 vehicles daily and suffered from severely distressed pavement. During construction, two of the four eastbound lanes were shut down in a section between the 57/210 interchange and Garey Avenue, Pomona. Contractors used 2676 m³ (3500 cy) of rapid-setting BCSA concrete to replace the existing 229-mm (9-in.) thick pavement. The project rehabilitated a 2.8 lane-km (1.7 mi) section of roadway and was completed in the allotted 55 hours. The BCSA cement concrete allowed heavy-duty traffic on the new lanes in three hours following placement. Due to the fast schedule, Caltrans realized future lane closures could be reduced to 30 hours.

Today, after 20 years of service, the repaired section of the San Bernardino Freeway has demonstrated durability. It has not required concrete replacement and displays no apparent problems.

Lincoln Tunnel approach

The Lincoln Tunnel is a 2.4-km (1.5-mi) passage under the Hudson River. It connects New Jersey with Midtown Manhattan in New York City and accommodates approximately 21 million vehicles annually. In addition to eastbound and westbound lanes located in underwater tubes, a third, center tube contains reversible lanes.

By 2004, concrete approaches to the tunnel on the Weehawken side (New Jersey) needed repair. Due to the high-traffic location, the roadway could not be closed for more than a short period of time. The Port Authority of New York and New Jersey (PANYNJ) wanted to accomplish reduced closure times using rapid-setting concrete. Therefore, in July 2004, contractors performed a test pour using BCSA cement concrete. The concrete achieved 4.8 MPa (690 psi) flexural strength in four hours and 6 MPa (890 psi) in 28 days. The success of the test pour led the team to use the same rapid-setting mix for a panel located directly in front of the tunnel, a critical location. In June 2005, the project was complete, with the placement of approximately 160 m³ (210 cy) of rapid-setting concrete.

The BCSA-containing rapid-setting cement used for the Lincoln Tunnel project has, to date, provided 15 years of service, again demonstrating the building material’s durability.

SeaTac Airport

As with other early applications of BCSA, time constraints were the deciding factor in SeaTac project owners’ choice to use rapid-setting concrete. In the early 1990s, one of the airport’s two runways was determined to need repair. As the airport’s only instrument landing system (ILS) runway, it could not be taken out of service. To accomplish quick repairs, a BCSA concrete mix was selected and, starting in 1995 and continuing for more than 10 years, the Seattle-Tacoma airport used the rapid-setting mix to perform night repairs. An estimated 30,000 m³ (39,239 cy) of BCSA cement concrete were poured, both on runways and taxiways.

REFERENCES

“Belitic Calcium Sulfoaluminate Cement: History, Chemistry, Performance, and Use in the United States” by E. Bescher and John Kim, proceedings of the first international conference on Innovation in Low-carbon Cement and Concrete Technology, London, United Kingdom, from June 24 to 26, 2019. “Opportunities from Alternative Cementitious Materials” by James K. Hicks, Michael A. Caldarone, and Eric Bescher for Concrete International, April 2015. “Calcium Sulfoaluminate Cement” by Robert J. Thomas, Marc Maguire, Andrew D. Sorensen, and Ivan Quezada for Concrete International, April 2018. “Low-carbon Footprint Pavement: History of Use, Performance And New Opportunities For Belitic Calcium Sulfoaluminate” by Eric Bescher, J. Kim, Chris Ramseyer, and J.K. Vallens for the 13th International Symposium on Concrete Roads, in Berlin, Germany, from June 19 to June 22, 2018. ASTM C596, Standard Test Method for Drying Shrinkage of Mortar Containing Hydraulic Cement. ASTM C78/C78M, Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading). ASTM C666-90, Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing. “Performance of Concrete Rehabilitation Using Rapid Setting Calcium SulfoAluminate Cement at the Seattle-Tacoma Airport” by Chris Ramseyer and Eric Bescher for Transportation Research Board’s 93rd annual meeting in Washington, D.C., from January 12 to 16, 2014.

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To develop the mix, concrete beams were tested per ASTM C78, Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading), and freeze-thaw tests were performed in accordance with ASTM C 666-90, Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing. Special cement concrete pavement specifications (P-503) were developed with the help of the Federal Aviation Administration (FAA) for the project. (The new specifications were in addition to other applicable FAA specifications.) The mix was designed to provide a 20-year life pavement, and accommodating 1,150,000 departures of McDonnell Douglas MD-11 aircraft. Specifications required minimum strengths of 4.5 MPa (653 psi) flexural strength at four hours and 5 MPa (740 psi) flexural strength at 28 days.

Researchers from both the University of California, Los Angeles (UCLA) and the University of Oklahoma were interested in documenting the onsite, long-term performance of BCSA cement concrete, so when one of the SeaTac runways was closed for unrelated repairs in August 2012, it presented them with an opportunity to evaluate the condition of the previously placed BCSA-containing panels. The resulting report[5], “Seattle-Tacoma Airport Concrete Rehabilitation Performance Review,” was published in 2013.

At the outset of their research, the team noted that of the 531 rapid-setting concrete panels placed between 1994 and 2005, only 20 (or four percent) had been replaced since that time. Those panels demonstrated a lower failure rate (four versus 35.5 percent) than the original panels. They also demonstrated a much better success rate than accelerated PCC panels placed after 2005. The condition of the BCSA-containing panels in the field was observed to be excellent.

The research team had removed a slab from a pavement section in the center of Runway 16 on August 1, 2012. (Pavement in the center of a runway is exposed to the most loading.) The section had concrete made with rapid-setting cement and placed in 1997. The sample was cored as well as saw cut into beams.

Three full-depth cores of concrete were tested for compressive strength. Beams cut from the slab were tested for flexural strength using third-point loading procedures as specified by ASTM C78. Overall, the findings indicated:

• BCSA cement concrete cores did not show any strength regression;
• compressive strength increased from 34 MPa (4931 psi) at day one to 78.6 MPa (11,400 psi) at 15 years;
• flexural strength had increased from 3.8 MPa (551 psi) at two hours to 8 MPa (1160 psi) at 15 years; and
• fatigue capacity, tested using the 15-year-old slab removed from the runway, was shown to exhibit a lifespan of more than 100 years.

The researchers note actual flexural strengths were likely to exceed the measured results since the test beams and cores were subject to damage during the sawing and handling process, as well as to changes in moisture content after removal.

Conclusion

The long service life of BCSA cement concrete makes it a suitable choice when designing for disaster resilience, an important consideration at a time when natural disasters, such as hurricanes, earthquakes, and fires, are on the rise. The hydration reactions of BCSA cement concrete prevent weakness in surface layers of the finished concrete. Compromised surface layer strength is the primary cause of impact and abrasion damage and spalling, which, in turn, exposes reinforcement to corrosive elements. BCSA cement concrete, therefore, will demonstrate greater impact and abrasion resistance along with the advantages of reduced shrinkage cracking and permeability, allowing it to meet the demanding requirements of disaster-resilient structures.

The performance of BCSA cement concrete is not only demonstrated by independent laboratory tests, but also by successful installation in pavements experiencing extreme loading. In addition to shortening timelines for construction and improving the sustainability of concrete, BCSA cement creates concrete with superior durability.

John Kim is a senior research and development engineer for CTS Cement Manufacturing Corp., Garden Grove, California, and has been a steady force for CTS Cement since he joined more than seven years ago. Active in many industry associations, Kim is involved with the American Concrete Institute (ACI), American Society of Civil Engineers (ASCE), and Transportation Research Board (TRB). He holds both a bachelor of science and master of science degrees in materials science and engineering from the University of California, Los Angeles (UCLA). Kim can be reached at jkim@ctscement.com[6].

Source URL: https://www.constructionspecifier.com/making-concrete-more-durable/

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