Better Roads Staff
• Drying the concrete will work if it’s possible, but in most outdoor structures, getting the moisture level low enough (below 80 percent relative humidity) is impossible.
• Impregnation of the concrete with sufficient lithium ions will control the reaction. SHRP researchers demonstrated this with lithium hydroxide solutions, but they observe that penetration is minimal and the caustic nature of hydroxide solutions make the procedure problematic. Also, it was found that hydroxide solutions added alkalis to the concrete mix.
Instead, the Renew lithium nitrate solution can be sprayed on concrete surfaces, ponded on surfaces, or pressure-injected into concrete. Demonstrations suggest surface treatment quantities ranging from 3 to 9 gallons per 1,000 square feet, with 6 gallons per 1,000 square feet being the norm. If the solution is sprayed on, repeat applications over time should be considered, FMC reports.
ASR at the Nanoscale
FHWA’s Advanced Infrastructure Research program has been conducting research on ASR, including research in Colloidal Chemistry of Alkali-Silicate Reaction (ASR) Gels.
This involves fundamental research into the chemical and physical processes that cause ASR gel damage. The ASR gel expansion mechanism appears to involve a phase transformation from amorphous gel to layered structure on the nanoscale, FHWA reports. The research includes the application of neutron scattering and positron annihilation spectroscopy to measure nano and sub-nanoscale changes in gel microstructure as a function of gel chemistry, temperature and relative humidity.
Other FHWA research has included:
• Fly Ash Reactivity Characterization. This FHWA-funded research is a fundamental look into the interactions between fly ash, and the portland cement gel nanostructure, that affect the strength and durability of concrete, including ASR reactivity. It includes the use of small angle neutron scattering to quantify the changes on a nanoscale as a function of time and fly ash composition. A unique vibrational spectroscopy also is being employed to nondestructively measure the reactivity of fly ashes.
• Aggregate ASR Potential Tests. ASR in concrete can be precluded by using nonreactive aggregates. This FHWA research involves fundamental research into the formation of ASR gels by reaction with different types of aggregates, using solid state nuclear magnetic resonance (NMR) to measure the formation of silicate chains on the nanoscale.
• Delayed Ettringite Formation Damage. Delayed ettringite is an internal sulfate attack on concrete. The FHWA research was exploring how delayed ettringite forms and causes damage in concrete, in transforming from an amorphous ettringite gel to nanoscale crystals. The research involves the application of synchrotron radiation to study the relationship between ettringite crystal formation and concrete expansion.
Lightweight aggregates comprise a ceramic material produced by expanding and vitrifying select shales, clays and slates in a rotary kiln. This pyroprocessing produces an aggregate that is structurally strong, durable, environmentally inert and low in density, according to the Expanded Shale, Clay and Slate Institute. Such aggregates are ideal for use in applications where weight is an issue, for example, as aggregates in mixes for bridge decks.
Synthetic aggregates can be made from recycled materials. For example, in the SYNAG process of the Western Research Institute, coal combustion fly ash is cold-bonded chemically to produce a hardened product that can be crushed and sized for construction applications, with properties specified by ASTM and AASHTO.
Lightweight aggregates refer to a class of building materials that weigh less than 70 pounds per cubic foot, but more than 55 pounds per cubic foot (lightweight aggregates less than 55 pounds per cubic foot are used in insulation, agriculture or horticulture, but are too weak for use in robust applications). They generally exhibit a porous structure, with the weaker, lighter aggregates exhibiting high porosity, and the stronger displaying a finer, more evenly distributed porosity.
Aggregates produced by altering both physical and chemical properties of a parent material may be considered synthetic or artificial aggregates. Some are produced and processed specifically for use as aggregates; others are the byproduct of manufacturing and a final burning process, such as ground granulated blast furnace slag.
While there are naturally occurring lightweight aggregates — such as pumice and other volcanics — these tend to be too light and weak for construction use. Instead, engineers prefer “pyroprocessed” natural materials, that is, those that have been chemically and physically altered by the heat of a rotary kiln.
Pyroprocessed lightweight aggregates include those made from shale, clay and slate, which expand into lightweight aggregates when heated to temperatures in excess of 1000 degrees C (1800 to 2100 degrees F). This synthetic lightweight aggregate, according to ESCSI is a ceramic material produced by expanding and vitrifying select shales, clays, and slates in a rotary kiln, not unlike cement manufacture, but at a lower temperature.
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