Investigation of Fire-Affected Concrete’s Residual Properties and the Link to Petrographic Data

Summary

Fire events are a threat to vulnerable communities and their infrastructure, compounded by climatic heating that is increasing their severity and reach. Cementitious materials are a cornerstone of American infrastructure and feel the brunt of fire damage yet are naturally heat resistant, creating opportunities to respond to destructive effects. Increased protection, calculated repair, and a greater understanding of concrete performance can be achieved by addressing knowledge gaps identified by experts in the field. Petrography is uniquely suited to this task as it can describe the temperature history of a material. Using petrographic examination and companion laboratory techniques, a protocol can be developed, which industry can utilize to identify the severity of fire exposure, correlate it to residual mechanical properties, and make informed decisions regarding repair versus demolition. The developed protocol will be wide-ranging yet can provide regionally specific guidance. Fire events affect a finite area and those environments and their materials may respond in unique ways. Study of concrete mixes and constituents local to states or regions can provide focused guidance on fire response. Through investigation of concrete mixes affected by fire, the research will generate datasets that can be used to advance computational efforts, shortening future studies that implement modeling and helping standards stay relevant to an evolving industry. Through systematic investigation of concrete elements exposed to real fire conditions, answers can be provided to industry and stakeholders allowing more informed decisions when responding to fire events that are expected to consistently threaten the nation’s infrastructure.

Description

Objective
To systematically investigate concrete elements exposed to real fire conditions and provide guidance to citizens, industry, and stakeholders when presented with responding to fire events. Specifically, a methodology, protocol, and/or guidance that supports decisions on when to repair or replace concrete affected by fire.

Technical Idea
Study on fire-affected concrete has been active for decades with damage mechanisms and temperature dependent transitions well documented. Damage inflicted to concretes initiate a complex array of chemo-mechanical-hygro effects primarily due to the thermal input imparted upon the materials surface. Studies of concrete exposed to elevated temperatures reveal a consistent, temperature dependent sequence of physiochemical reactions that begin with evaporation of free water at 100°C, followed by the loss of all chemically bound water at ~270°C. Microcracking begins shortly thereafter as the thermal input begins to degrade the microstructure and cause the dehydrated paste to decompose into calcium oxide. These reactions continue over 300-600°C where spalling occurs which can contribute to the rapid loss of material. Spallation within concretes is the separation of a fragment(s) driven by elevated temperature. This effect is well documented in studies that examine fire damage to concrete structures and is explained by the buildup of pore pressure within the cement paste and aggregates as the heated water cannot easily escape. This is especially problematic within high strength concrete varieties as they have a lower permeability contributing to a higher material loss. Heating rate and pore pressure buildup are inherently related, with fast heating leading to higher pore pressures in deeper regions and higher vapor transport speed, contributing further to the damage. This is classified as explosive spalling, which is exclusive to environments with rapidly increasing temperature, common in high-heat, large area fires.

Concrete as a composite has low thermal conductivity which generally decreases with increased temperature as the moisture leaves the system. When exposed to high enough temperatures, the material effectively produces a refractory insulating layer of lower thermal conductivity, reducing the ingress of heat and preventing further damage. This “baked rim” also contributes to concretes potential for rehabilitation in comparison with other building materials. Due to extensive study, there exists a wealth of information to implement when crossed with petrographic methods. Petrography is a science used by geologists due to their learned skills of identifying textural, mineralogical, and other visual clues to identify a material and its inherent properties. Concrete is essentially a man-made rock with minerals (aggregate) and a binding matrix (cement) forming a hardened conglomerate enabling petrographic techniques and concepts to be applied to concretes. Through this conceit, a core geological concept of metamorphism can be applied to concrete, where altered temperatures or conditions cause a thermodynamic shift, inciting specific phases to equilibrate with that exposure condition, potentially causing a change in a phase’s composition, crystallographic structure, and physical properties. This is exhibited readily by visible color change, but also can be seen through use of microscopy to identify changes on the microstructural level using a polarizing microscope. Geological formations that have experienced metamorphism can often trace an isograd – a linear representation of how far dynamic conditions have penetrated a body – that can reveal a temperature history experienced by a material. This also syncs nicely with concrete, as it has been shown heat penetrates a concrete body at a slow rate due to its low thermal conductivity, effectively creating a traceable front where one can delineate how much of the “formation” is affected. 

Research Plan
To help industry combat an increasing fire threat to infrastructure, knowledge gaps identified by the “International R&D Roadmap for Fire Resistance of Structures, Summary of NIST/CIB Workshop” published by NIST in 2014 need to be addressed. Using petrographic methods and associated techniques, these deficiencies can begin to be rectified. To effectively leverage these methods to the engineering community, generated data must correlate to systems of measurement commonly used by industry that gauge concrete health and structural soundness. Fortunately, well-researched correlations exist between temperature conditions and loss of mechanical strength in concrete, thus allowing petrographic findings to be linked to residual properties. Furthermore, concrete’s ability to insulate itself provides an opportunity for repair if the unaffected material can be delineated from material that has lost an unacceptable portion of its structural capability. The Engineering Laboratory (EL) of NIST is uniquely suited to find solutions to these research gaps due to facilities, expertise, and mission. Experimental data can be generated through collaboration with the National Fire Research Laboratory (NFRL). Due to NFRL’s capabilities, realistic fire conditions can be realized rather than conditions simulated by furnaces where most research on fire-affected concrete is conducted.

The development of a semi-destructive methodology/protocol that describes investigation of concrete structures post-fire exposure is planned. This will be accomplished using petrographic tools and complimentary laboratory techniques to delineate a structure’s temperature history after fire exposure. Accurately diagnosing the affected depth can define a concrete element’s residual strength, which has implications regarding its remaining service life and if a pathway to repair is more suitable than rebuilding. Standard and specialized concrete mixes will be developed and exposed to in coordination with the Performance-based Design for Structures in Fire: Modeling and Validation project within the Engineering Laboratory. The materials will be exposed to a range of fire conditions utilizing capabilities of the NFRL. Two-inch diameter cores will be extracted from intervals across the concrete slab. Large thin sections (2” x 3”) will be sectioned with depth, impregnated with fluorescence epoxy, and polished to <25 microns to enable observation of cementitious microstructure. Through visual examination, temperature isograds will be delineated. Additional techniques, such as SEM, XRD, and TGA can be used to further confine the transition from damaged material to unaffected concrete. Multiple zones are anticipated to be found consisting of spalled, dehydrated, and structurally sound concrete. By correlating temperature-related effects, concrete residual properties can be projected with depth. Investigation into the most effective method(s) will be conducted to result in the most expeditious and informative practices.

To account for the wide variability of concrete mixes and aggregate mineralogies, e.g., ternary vs quaternary mixes, limestone vs siliceous aggregate, normal vs high strength, Portland vs alternative cements, etc. sampling will be related to industry usage and trends. This focused approach to guidance on fire-affected concrete response considers the variation of products used within the concrete industry. Specific ingredients, such as aggregates, are often locally sourced and can respond to heat in a unique manner, providing opportunity to develop guidance based on specific situations. Through examination of concrete mixes and their constituents, that are often informed by region, datasets can be developed that provide the framework for states and/or regions to define fire-related damage more accurately.

Concurrent to the development of the fire-affected concrete assessment protocol, datasets of concrete material properties, both pre and post exposure, will be generated that can be used for modeling and simulation. To advance the ability to rapidly test solutions for research questions and adapt to an evolving world, models provide the more cost-efficient means to do so. Models require data inputs; however, and validation to be reliable. A dearth of data on concrete residual properties after real, rather than simulated, fire exposure exists, limiting the advancement of simulations. This research will generate needed data to enable advancement of computational efforts.