Better Roads Staff
X-Seed has innovated production of precast-concrete components by serving as a curing accelerator, which the company says not only allows precast concrete units to be produced more rapidly and in better quality, but considerably reduces energy consumption and the associated emission of carbon dioxide (CO2) greenhouse gas.
Cement is produced by pyroprocessing limestone, clay and minerals at high temperatures to produce cement clinker. Pyroprocessing consumes enormous amounts of energy, and releases large amounts of CO2 from combusting the fuel (principally coal or natural gas) and from the chemical reaction the combustion enables. Finally, the coarse-grained clinker is ground into a fine, gray cement powder that hydrates after mixing with water. Calcium silicate hydrate and other compounds crystallize out of the cement during this process to form a compact stone matrix in which aggregates and sand are embedded.
Concrete products are manufactured by placing the uncured concrete mix into forms. Only when the concrete has cured sufficiently can the mold be opened and the component removed. At ambient temperatures (68 degrees F) it can take up to 12 hours to cure, which is valuable production time, during which the formwork cannot be reused. To speed production, the mold often is heated with steam. Although this accelerates production, it also demands much additional energy. Moreover, this treatment can lead to internal thermal stresses, discolorations and a coarser surface of the finished concrete part.
“X-Seed makes heat curing, with all its disadvantages, largely superfluous,” says Dr. Michael Kompatscher, responsible for BASF’s European precast concrete component market. “With this additive, concrete hardens just as fast at 20 degrees C (68 degrees F) as it otherwise does at 60 degrees C (140 degrees F), by a adding something that’s already present in the concrete anyway – calcium silicate hydrate (CSH).”
Countless millions of tiny CSH crystals with a diameter of several nanometers are suspended in liquid in X-Seed, BASF says. Because of their nanosize, more very homogeneously distributed crystallization seeds can be accommodated in the same mass, and thereby promote faster growth. When the concrete cures, further molecules from the cement can attach themselves to these CSH “seeds.” The resulting crystals grow more densely and finally interlock to form the compact cement stone.
When conventional cement hydrates, the CSH seeds first have to form spontaneously from several molecules released from the cement, which accidentally come into contact with each other. X-Seed negates this first barrier to crystallization by providing an excess of these tiny crystal seeds. Another factor is that the CSH crystals form in a more homogeneously distributed manner.
Both these effects of the synthetic crystal seeds halve the time to formwork removal at 68 degrees F from about 12 to six hours, without any detectable differences in the final product, BASF says.
Zeolites and WMA
Warm-mix asphalt (WMA) technologies are a family of processes that produce low-energy asphalt mixes that can be placed at significantly lower temperatures than conventional hot-mix asphalt. At least one additive for warm-mix asphalt operates at the nanoscale, Aspha-Min.
Zeolites are nanoporous crystalline alumino-silicates with important attributes. A zeolite is a constituent of a group of commercially valuable minerals – metamorphosed crystals of hydrated aluminum silicates – of interest to industry for a variety of applications. Zeolites have large vacant spaces or cages in their structures that allow space for large, positively charged ions such as sodium and potassium, and even entire molecules such as water.
Aspha-Min is a synthetic zeolite compound, which releases water (H20) into the asphalt mix to improve workability at lower temperatures. Aspha-Min is available as a very fine, white-powdered form in bags or in bulk for silos. The percentage of water held by the zeolite is 21 percent by mass and is released in the temperature range of 212 to 392 degrees F.
By adding 0.3-percent Aspha-Min to the preheated mixture of sand and stone at the same time liquid asphalt is being introduced, a water-based vapor is created. The water released from the crystal causes the binder to expand to a kind of foam, permitting better workability and coating of aggregates at lower temperatures. Tests indicate that 54 degrees F reduction in temperature equates to a 30-percent reduction in fuel energy consumption.
Thanks for the MEMS
Nanotechnology for transportation infrastructure goes beyond engineered materials, into appliances manufactured at the nanoscale. These nanotechnology-driven sensors and instruments – microelectronic and mechanical systems, also called microelectromechanical systems (MEMS) – have the ability to detect motion and monitor corrosion, cracking and performance of structures and pavements under service loads and conditions.
In their 2008 paper, Applicability of Microelectronic and Mechanical Systems (MEMS) for Transportation Infrastructure Management, investigators Kelvin C.P. Wang and Qiang Li, Department of Civil Engineering, University of Arkansas-Fayetteville, describe the application of MEMS for pavements and bridges.
“With the tremendous advancement in technology, it is possible to employ devices embedded in structural members for real-time monitoring of infrastructure health,” they say. “Micro-electromechanical systems are miniature sensing or actuating devices [that] can interact with their environment to either obtain information or alter it. With remote query capability, it appears such devices can therefore be embedded in structures to monitor distresses such as cracking.”
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