In spite of advances in materials and construction technologies, our nation's civil infrastructure remains in a state of serious decay. The 2013 American Society of Civil Engineers infrastructure report card graded our roads, transit, and drinking water distribution system with a D and our nation's bridges with only a C+. Because the rate of infrastructure turnover is relatively low, it is the repair of existing structures that is critical to optimizing the expenditure of existing resources and assuring a sustainable infrastructure. In 2008, the National Academy of Engineering identified the restoration and improvement of urban infrastructure as one of the 14 grand challenges in engineering . However, most materials research remains focused on new (and greener) materials and technologies, with few active research programs being dedicated to repair materials, despite the fact that repair/maintenance actually has a much higher environmental impact per dollar spent than new construction . The repair market for concrete alone is currently a $20 billion industry, but it is estimated that 50 % of the materials do not perform satisfactorily for their intended lifetime . This project will provide a focused effort on advancing the state-of-the-art in measurement science for repair material and procedure evaluation and selection. Particular focus will be devoted to characterization and optimization of the bonding/adhesion and ensuring physical and chemical compatibility between repair and existing materials. This research will be performed in partnership with other federal agencies, including the Federal Highway Administration, and universities.
Objective - Develop the measurement science tools (metrologies, standards, and guidance documents) for quantitatively evaluating the critical material properties and ensuring the desired field performance of repair materials for our nation's concrete infrastructure.
What is the new technical idea? The technical idea is to develop field-ready measurement science tools to assess the integrity and robustness of the interfacial region, between repair and substrate by employing two new approaches to improving performance: internal curing and controlled carbonation. Even when a material to repair concrete is designed and selected to provide equivalent properties (modulus, strength, coefficient of thermal expansion, chloride ion diffusion coefficient, etc.) to those of the existing substrate, the success/failure of the repair will often be controlled by the nature and integrity of the interfacial bond and the volumetric stability of the repair material throughout its hardening and initial aging. Moisture, ion, and stress transfer across the interface will impact the performance and service life of the repair. However, beyond the current practice's perfunctory recommendation that an existing concrete substrate be saturated-surface-dry (SSD) prior to the repair, little attention is paid to assuring/maximizing the bonding of the repair material to the existing concrete substrate. Failure of this bond is one of the primary reasons for repair failures. NIST will exploit its expertise in concrete materials to investigate the use of internal curing to supply extra "internal" curing water to replace that lost due to transport into the existing concrete substrate, drying and evaporation, etc. Previous NIST research has indicated that while cement hydration products readily precipitate on calcite surfaces, they do not do so on the aragonite form of CaCO3 . Since natural carbonation of existing concrete in the field can produce all three polymorphs of calcium carbonate (including vaterite), as well as an amorphous form , the research will first quantify the carbonates formed under different environmental conditions and then evaluate their impact on the bonding of cement-based repair materials to existing (carbonated) concrete.
What is the research plan? Controlled carbonation will be facilitated by understanding the formation of carbonate species as a function of environmental conditions at the molecular level. Initially, thin cement paste specimens will be prepared and cured in a carbon dioxide-free environment and then later exposed to different temperatures, relative humidities, and carbonation levels in the X-ray diffractometer's environmental cell, while monitoring the quantity and form(s) of CaCO3 that are formed. Both natural carbonation and accelerated carbonation, obtained by increasing the concentration of CO2 in the flowing gas stream, will be examined. Total carbonate will also be evaluated using thermogravimetric analysis (TGA) of paste specimens, so that an indication of the amount of amorphous CaCO3 can be obtained by difference. This will provide key insight into the expected form of CaCO3 in field-exposed specimens and may lead to controlled carbonation strategies to maximize the formation of calcite and thus enhance the subsequent bonding of cement-based repair materials to carbonated concrete.
Both small scale and near full size specimens will be employed in investigating the bonding of various repair materials to existing concrete. Controlled exposure environments for larger scale specimens, with respect to temperature and relative humidity, will be provided via the group's suite of walk-in environmental chambers. Initially, bond strength will be measured using the existing ASTM C1583 pull-off test method following coring; additionally, shear slant test specimens may be evaluated to provide supplemental information on the nature and strength of the bond between repair and existing materials.
NIST is recognized as a world leader in the field of internal curing, which will be leveraged for this research. The performance of cement-based repair materials with and without internal curing will be evaluated with respect to autogenous deformation, drying shrinkage, stress development under restrained conditions, and bond strength using standardized test methods. Isothermal calorimetry will be used to quantify the reaction rates in each system. Internal curing will be provided using pre-wetted fine lightweight aggregates and also potentially via the incorporation of salt-insensitive superabsorbent polymers (SISA). If time is awarded at the NIST Center for Neutron Research (Dr. Daniel Hussey and Dr. David Jacobson), neutron radiography and imaging will be employed to quantify water movement between repair materials and existing substrates to further demonstrate the efficacy of internal curing.
1) "Introduction to the Grand Challenges for Engineering," National Academy of Engineering, February 2008.
2) Kneifel, J., "Sustainability," EL Net Zero Energy Program Brown Bag Lunch Seminar, November, 2013.
3) Khan, L., Presentation on repair materials at ACI-SDC meeting, Atlanta, Feb. 2014.
4) Bentz, D.P., Ardani, A., Barrett, T., Jones, S.Z., Lootens, D., Peltz, M.A., Sato, T., Stutzman, P.E., Tanesi, J., and Weiss, W.J., "Multi-Scale Investigation of the Performance of Limestone in Concrete," Construction and Building Materials, Vol. 75, 1-10, 2015.
5) Morandeau, A., Thiery, M., and Dangla, P., "Investigation of the Carbonation Mechanism of CH and C-S-H in Terms of Kinetics, Microstructure Changes, and Moisture Properties," Cement and Concrete Research, 56, 153-170, 2014.