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The Specifier: Making Concrete More Durable

By Jason Cramp posted 10-02-2019 12:10 PM

  

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By Nithya Caleb

 

Nithya Caleb is the Editor of The Construction Specifier Magazine, the Official Magazine of CSI. This article originally appeared in The Construction Specifier. Subscribe for your own print copy of this must-read magazine for construction professionals.

 

Airport pavements have a limited design life, after which it must be replaced. If it must be replaced quickly, only a few choices are available for a concrete base material. Common Portland cement does not reach structural strength for days. It can be accelerated with organic additives, but this leads to mixes that are difficult to work with and have significant shrinkage and limited life cycle.

Another choice is calcium sulfoaluminate (CSA)-base concrete. CSA cement sets within hours, exhibits low shrinkage, high strength, and sulfate resistance. In his article for The Construction Specifier, John Kim, a senior research and development engineer for CTS Cement Manufacturing Corp., focuses on Belitic CSA (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.

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.

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.

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.

In addition to shortening timelines for construction and improving the sustainability of concrete, BCSA cement creates concrete with superior durability.

 

MasterFormat No.: 03 00 00–Concrete

UniFormat No.: B10–Superstructure

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