{"id":96516,"date":"2019-09-20T16:00:33","date_gmt":"2019-09-20T20:00:33","guid":{"rendered":"https:\/\/www.constructionspecifier.com\/?p=96516"},"modified":"2019-09-20T15:54:59","modified_gmt":"2019-09-20T19:54:59","slug":"making-concrete-more-durable","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/making-concrete-more-durable\/","title":{"rendered":"Making concrete more durable"},"content":{"rendered":"

by John Kim<\/strong><\/p>\n

\"Images<\/a>
Images courtesy CTS Cement Manufacturing Corp.<\/span><\/em><\/figcaption><\/figure>\n

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<\/sub>) emissions. Further, installed concrete has historically suffered from performance issues, notably cracking.<\/p>\n

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 \u2018green.\u2019 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<\/sub> 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<\/sub> emissions.<\/p>\n

\"Figure<\/a>
Figure 1<\/strong>: Using a standard drying shrinkage test, such as ASTM C596, Standard Test Method for Drying Shrinkage of Mortar Containing Hydraulic Cement, the reduced drying shrinkage of belitic calcium sulfoaluminate (BCSA) is exhibited.<\/span><\/figcaption><\/figure>\n

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.<\/p>\n

Belitic CSA cement concrete\u2014a rapid-setting material containing CSA as a minor phase and belite (dicalcium silicate) as the major phase\u2014was 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.<\/p>\n

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<\/em>, 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).<\/p>\n

The California Department of Transportation (Caltrans) first used BCSA cement concrete for highway repair in 1994,\u00a0 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.<\/p>\n

BCSA cement concrete\u2019s rapid-setting characteristics allowed it to enter the marketplace. However, with the passage of time, it is becoming apparent another advantage is the material\u2019s durability.<\/p>\n

Long-term field performance<\/strong><\/p>\n

\"The<\/a>
The I-10 San Bernardino Freeway (California) repairs required a 55-hour weekend closure with round-the-clock construction operations, so traffic could resume for the workweek.<\/span><\/figcaption><\/figure>\n

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\u2019s in-service performance. The Seattle-Tacoma (SeaTac) Airport in Washington is the 15th<\/sup> largest one in the United States in terms of aircraft movement, as well as the Lincoln Tunnel, the world\u2019s busiest underground vehicular passage, represent additional field-installed successes of BCSA cement.<\/p>\n

San Bernardino Freeway<\/em><\/p>\n

In October 1999, I-10 launched Caltrans\u2019 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.<\/p>\n

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<\/sup> (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.<\/p>\n

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.<\/p>\n

Lincoln Tunnel approach<\/em><\/p>\n

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.<\/p>\n

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<\/sup> (210 cy) of rapid-setting concrete.<\/p>\n

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\u2019s durability.<\/p>\n

SeaTac Airport<\/em><\/p>\n

As with other early applications of BCSA, time constraints were the deciding factor in SeaTac project owners\u2019 choice to use rapid-setting concrete. In the early 1990s, one of the airport\u2019s two runways was determined to need repair. As the airport\u2019s 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<\/sup> (39,239 cy) of BCSA cement concrete were poured, both on runways and taxiways.<\/p>\n\n\n\n\n
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