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, “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.
